Phenylketonuria
Phenylketonuria
Definition
Phenylketonuria (PKU) can be defined as a rare metabolic disorder caused by a deficiency in the production of the hepatic (liver) enzyme phenylalanine hydroxylase (PAH). PKU is the most serious form of a class of diseases referred to as "hyperphenylalaninemia," all of which involve above normal (elevated) levels of phenylalanine in the blood. The primary symptom of untreated PKU, mental retardation, is the result of consuming foods that contain the amino acid phenylalanine, which is toxic to brain tissue.
PKU is an inherited, autosomal recessive disorder. It is the most common genetic disease involving "amino acid metabolism." PKU is incurable, but early, effective treatment can prevent the development of serious mental incapacity.
Description
PKU is a disease caused by the liver's inability to produce a particular type of PAH enzyme. This enzyme converts (metabolizes) the amino acid called phenylalanine into another amino acid, tyrosine. This is the only role of PAH in the body. A lack of PAH results in the buildup of abnormally high phenylalanine concentrations (or levels) in the blood and brain. Above normal levels of phenylalanine are toxic to the cells that make up the nervous system and causes irreversible abnormalities in brain structure and function in PKU patients. Phenylalanine is a type of teratogen. Teratogens are any substance or organism that can cause birth defects in a developing fetus.
The liver is the body's chief protein processing center. Proteins are one of the major food nutrients. They are generally very large molecules composed of strings of smaller building blocks or molecules called amino acids. About twenty amino acids exist in nature. The body breaks down proteins from food into individual amino acids and then reassembles them into "human" proteins. Proteins are needed for growth and repair of cells and tissues, and are the key components of enzymes, antibodies, and other essential substances.
PKU affects on the human nervous system
The extensive network of nerves in the brain and the rest of the nervous system are made up of nerve cells. Nerve cells have specialized extensions called dendrites and axons. Stimulating a nerve cell triggers nerve impulses, or signals, that speed down the axon. These nerve impulses then stimulate the end of an axon to release chemicals called "neurotransmitters" that spread out and communicate with the dendrites of neighboring nerve cells.
Many nerve cells have long, wire-like axons that are covered by an insulating layer called the myelin sheath. This covering helps speed nerve impulses along the axon. In untreated PKU patients, abnormally high phenylalanine levels in the blood and brain can produce nerve cells with "deformed" axons and dendrites, and cause imperfections in the myelin sheath referred to as hypomyelination and demylenation. This loss of myelin can "short circuit" nerve impulses (messages) and interrupt cell communication. A number of brain scan studies also indicate a degeneration of the "white matter" in the brains of older patients who have not maintained adequate dietary control.
PKU can also affect the production of one of the major neurotransmitters in the brain, called dopamine. The brain makes dopamine from the amino acid tyrosine. PKU patients who do not consume enough tyrosine in their diet cannot produce sufficient amounts of dopamine. Low dopamine levels in the brain disrupt normal communication between nerve cells, which results in impaired cognitive (mental) function.
Some preliminary research suggests that nerve cells of PKU patients also have difficulty absorbing tyrosine. This abnormality may explain why many PKU patients who receive sufficient dietary tyrosine still experience some form of learning disability.
Behavior and academic performance
IQ (intelligence quotient) tests provide a measure of cognitive function. The IQ of PKU patients is generally lower than the IQ of their healthy peers. Students with PKU often find academic tasks difficult and must struggle harder to succeed than their non-PKU peers. They may require special tutoring and need to repeat some of their courses. Even patients undergoing treatment programs may experience problems with typical academic tasks as math, reading, and spelling. Visual perception, visual-motor skills, and critical thinking skills can also be affected. Ten years of age seems to be an important milestone for PKU patients. After age 10, variations in a patient's diet seems to have less influence on their IQ development.
People with PKU tend to avoid contact with others, appear anxious and show signs of depression. However, some patients may be much more expressive and tend to have hyperactive, talkative, and impulsive personalities. It is also interesting to note that people with PKU are less likely to display such "antisocial" habits as lying, teasing, and active disobedience. It should be emphasized that current research findings are still quite preliminary and more extensive research is needed to clearly show how abnormal phenylalanine levels in the blood and brain might affect behavior and academic performance.
One in fifty individuals in the United States have inherited a gene for PKU. About five million Americans are PKU carriers. About one in 15,000 babies test positive for PKU in the United States. Studies indicate that the incidence of this disease in Caucasian and Native American populations is higher than in African-American, Hispanic, and Asian populations.
Causes and symptoms
PKU symptoms are caused by alterations or "mutations" in the genetic code for the PAH enzyme. Mutations in the PAH gene prevent the liver from producing adequate levels of the PAH enzyme needed to break down phenylalanine. The PAH gene and its PKU mutations are found on chromosome 12 in the human genome. In more detail, PKU mutations can involve many different types of changes, such as deletions and insertions, in the DNA of the gene that codes for the PAH enzyme.
PKU is described as an inherited, autosomal recessive disorder. The term autosomal means that the gene for PKU is not located on either the X or Y sex chromosome. The normal PAH gene is dominant to recessive PKU mutations. A recessive genetic trait, such as PKU, is one that is expressed—or shows up—only when two copies are inherited (one from each parent).
A person with one normal and one PKU gene is called a carrier. A carrier does not display any symptoms of the disease because their liver produces normal quantities of the PAH enzyme. However, PKU carriers can pass the PKU genetic mutation onto their children. Two carrier parents have a 25% chance of producing a baby with PKU symptoms, and a 50% chance having a baby that is a carrier for the disease. Although PKU conforms to these basic genetic patterns of inheritance, the actual expression, or phenotype, of the disease is not strictly an "either/or" situation. This is because there are at least 400 different types of PKU mutations. Although some PKU mutations cause rather mild forms of the disease, others can initiate much more severe symptoms in untreated individuals. The more severe the PKU mutation, the greater the effect on cognitive development and performance (mental ability).
Untreated PKU patients develop a broad range of symptoms related to severely impaired cognitive function, sometimes referred to as mental retardation. Other symptoms can include extreme patterns of behavior, delayed speech development, seizures, a characteristic body odor, and light body pigmentation. The light pigmentation is due to a lack of melanin, which normally colors the hair, skin and eyes. Melanin is made from the amino acid tyrosine, which is lacking in untreated cases of PKU. Physiologically, PKU patients show high levels of phenylalanine and low levels of tyrosine in the blood. Babies do not show any visible symptoms of the disease for the first few months of life. However, typical PKU symptoms usually do show up by a baby's first birthday.
Diagnosis
The primary diagnostic test for PKU is the measurement of phenylalanine levels in a drop of blood taken from the heel of a newborn baby's foot. This screening procedure is referred to as the Guthrie test (Guthrie bacterial inhibition assay). In this test, PKU is confirmed by the appearance of bacteria growing around high concentrations of phenylalanine in the blood spot. PKU testing was introduced in the early 1960s and is the largest genetic screening program in the United States. It is required by law in all 50 states. Early diagnosis is critical. It ensures early the treatment PKU babies need to develop normally and avoid the ravages of PKU.
The American Academy of Pediatrics recommends that this test should be performed on infants between 24 hours and seven days after birth. The preferred time for testing is after the baby's first feeding. If the initial PKU test produces a positive result, then follow-up tests are performed to confirm the diagnosis and to determine if the elevated phenylalanine levels may be caused by some medical condition other than PKU. Treatment for PKU is recommended for babies that show a blood phenylalanine level of 7-10 mg/dL or higher for more than a few consecutive days. Another, more accurate test procedure for PKU measures the ratio (comparison) of the amount of phenylalanine to the amount of tyrosine in the blood.
Newer diagnostic procedures (called mutation analysis and genotype determination) can actually identify the specific types of PAH gene mutations inherited by PKU infants. Large-scale studies have helped to clarify how various mutations affect the ability of patients to process phenylalanine. This information can help doctors develop more effective customized treatment plans for each of their PKU patients.
Treatment
The severity of the PKU symptoms experienced by people with this disease is determined by both lifestyle as well as genetic factors. In the early 1950s, researchers first demonstrated that phenylalanine restricted diets could eliminate most of the typical PKU symptoms—except for mental retardation. Today, dietary therapy (also called nutrition therapy) is the most common form of treatment for PKU patients. PKU patients who receive early and consistent dietary therapy can develop fairly normal mental capacity to within about five IQ points of their healthy peers. By comparison, untreated PKU patients generally have IQ scores below 50.
Infants with PKU should be put on a specialized diet as soon as they are diagnosed to avoid progressive brain damage and other problems caused by an accumulation of phenylalanine in the body. A PKU diet helps patients maintain very low blood levels of phenylalanine by restricting the intake of natural foods that contain this amino acid. Even breast milk is a problem for PKU babies. Special PKU dietary mixtures or formulas are usually obtained from medical clinics or pharmacies.
Phenylalanine is actually an essential amino acid. This means that it has to be obtained from food because the body cannot produce this substance on its own. Typical diets prescribed for PKU patients provide very small amounts of phenylalanine and higher quantities of other amino acids, including tyrosine. The amount of allowable phenylalanine can be increased slightly as a child becomes older.
In addition, PKU diets include all the nutrients normally required for good health and normal growth, such as carbohydrates, fats, vitamins, and minerals. High protein foods like meat, fish, chicken, eggs, nuts, beans, milk, and other dairy products are banned from PKU diets. Small amounts of moderate protein foods (such as grains and potatoes) and low protein foods (some fruits and vegetables, low protein breads and pastas) are allowed. Sugar-free foods, such as diet soda, which contain the artificial sweetener aspartame, are also prohibited foods for PKU patients. That is because aspartame contains the amino acid phenylalanine.
Ideally, school-age children with PKU should be taught to assume responsibility for managing their diet, recording food intake, and for performing simple blood tests to monitor their phenylalanine levels. Blood tests should be done in the early morning when phenylalanine levels are highest. Infants and young children require more frequent blood tests than older children and adults. The amount of natural foods allowed in a diet could be adjusted to ensure that the level of phenylalanine in the blood is kept within a safe range—two to 6 mg/dL before 12 years of age and 2-15 mg/dL for PKU patients over 12 years old.
A specialized PKU diet can cause abnormal fluctuations in tyrosine levels throughout the day. Thus, some health professionals recommend adding time released tyrosine that can provide a more constant supply of this amino acid to the body. It should be noted that some PKU patients show signs of learning disabilities even with a special diet containing extra tyrosine. Research studies suggests that these PKU patients may not be able to process tyrosine normally.
For PKU caregivers, providing a diet that is appealing as well as healthy and nutritious is a constant challenge. Many PKU patients, especially teenagers, find it difficult to stick to the relatively bland PKU diet for extended periods of time. Some older patients decide to go off their diet plan simply because they feel healthy. However, many patients who abandon careful nutritional management develop cognitive problems, such as difficulties remembering, maintaining focus, and paying attention. Many PKU health professionals contend that all PKU patients should adhere to a strictly controlled diet for life.
One promising line of PKU research involves the synthesis (manufacturing) of a new type of enzyme that can break down phenylalanine in food consumed by the patient. This medication would be taken orally and could prevent the absorption of digested phenylalanine into the patient's bloodstream.
In general, medical researchers express concern about the great variation in treatment programs currently available to PKU patients around the world. They have highlighted the urgent need for new, consistent international standards for proper management of PKU patients, which should emphasize comprehensive psychological as well as physiological monitoring and assessment.
PKU and Pregnancy
Women with PKU must be especially careful with their diets if they want to have children. They should ensure that phenylalanine blood levels are under control before conception and throughout her pregnancy. Mothers with elevated (higher than normal) phenylalanine levels are high risk for having babies with significant birth defects, such as microencephaly (smaller than normal head size), and congenital heart disease (abnormal heart structure and function), stunted growth, mental retardation, and psychomotor (coordination) difficulties. This condition is referred to as maternal PKU and can even affect babies who do not have the PKU disease.
Prognosis
Early newborn screening, careful monitoring, and a life-long strict dietary management can help PKU patients to live normal, healthy, and long lives.
Resources
BOOKS
Brust, John C. M. The Practice Of Neural Science: From Synapses To Symptoms. New York: McGraw-Hill, 2000.
Gilroy, John. Basic Neurology. 3rd ed. New York: McGraw-Hill, 2000.
Ratey, John J. A User's Guide To The Brain: Perception, Attention, And The Four Theaters Of The Brain. 1st ed. New York: Pantheon Books, 2001.
Weiner, William J., and Christopher G. Goetz, editors. Neurology For The Non-Neurologist. 4th ed. Philadelphia: Lippincott, Williams & Wilkins, 1999.
PERIODICALS
Burgard, P. "Development of intelligence in early treated phenylketonuria." European Journal of Pediatrics 159, Supplement 2 (October 2000): S74-9.
Chang, Pi-Nian, Robert M. Gray, and Lisa Lehn O'Brien. "Review: Patterns of academic achievement among patients treated early with phenylketonuria." European Journal of Pediatrics 159, no.14 (2000): S96-9.
Eastman, J.W., J.E. Sherwin, R. Wong, C.L. Liao, R.J. Currier, F. Lorey, and G. Cunningham. "Use of the phenylalanine:tyrosine ratio to test newborns for phenylketonuria in a large public health screening programme." Journal of Medical Screening 7, no. 3 (2000): 131-5.
MacDonald, A. "Diet and compliance in phenylketonuria." European Journal of Pediatrics 159, Supplement 2 (October 2000): S136-41.
Smith, Isabel, and Julie Knowles. "Behaviour in early treated phenylketonuria: a systematic review." European Journal of Pediatrics 159, no. 14 (2000): S89-93.
Stemerdink, B.A., A.F. Kalverboer, J.J. van der Meere, M.W. van der Molen, J. Huisman, L.W. de Jong, F.M. Slijper, P.H. Verkerk, and F.J. van Spronsen. "Behaviour and school achievement in patients with early and continuously treated phenylketonuria." Journal of Inherited Metabolic Disorders 23, no. 6 (2000): 548-62.
van Spronsen, F.J.F., M.M. van Rijn, J. Bekhof, R. Koch, and P.G. Smit. "Phenylketonuria: tyrosine supplementation in phenylalanine-restricted diets." American Journal of Clinical Nutrition 73, no. 2 (2001): 153-7.
Wappner, Rebecca, Sechin Cho, Richard A. Kronmal, Virginia Schuett, and Margretta Reed Seashore. "Management of Phenylketonuria for Optimal Outcome: A Review of Guidelines for Phenylketonuria Management and a Report of Surveys of Parents, Patients, and Clinic Directors." Pediatrics 104, no. 6 (December 1999): e68.
ORGANIZATIONS
American Academy of Allergy, Asthma & Immunology. 611 E. Wells St, Milwaukee, WI 53202. (414) 272-6071. Fax: (414) 272-6070. 〈http://www.aaaai.org/default.stm〉.
Centers for Disease Control. GDP Office, 4770 Buford Highway NE, Atlanta, GA 30341-3724. (770) 488-3235. 〈http://www.cdc.gov/genetics〉.
Children's PKU Network. 1520 State St., Suite 111, San Diego, CA 92101-2930. (619) 233-3202. Fax: (619) 233 0838. [email protected].
March of Dimes Birth Defects Foundation. 1275 Mamaroneck Ave., White Plains, NY 10605. (888) 663-4637. resourcecenter@modimes. 〈http://www.modimes.org〉.
National PKU News. Virginia Schuett, editor/dietician. 6869 Woodlawn Avenue NE #116, Seattle, WA 98115-5469. (206) 525-8140. Fax: (206) 525-5023. 〈http://www.pkunews.org〉.
University of Washington PKU Clinic. CHDD, Box 357920, University of Washington, Seattle, WA. (206) 685-3015. Within Washington State: (877) 685-3015. Clinic Coordinator: [email protected]. 〈http://depts.washington.edu/pku/contact.html〉.
KEY TERMS
Amino acid— Organic compounds that form the building blocks of protein. There are 20 types of amino acids (eight are "essential amino acids" which the body cannot make and must therefore be obtained from food).
Axon— Skinny, wire-like extension of nerve cells.
Enzyme— A protein that catalyzes a biochemical reaction or change without changing its own structure or function.
Gene— A building block of inheritance, which contains the instructions for the production of a particular protein, and is made up of a molecular sequence found on a section of DNA. Each gene is found on a precise location on a chromosome.
Genetic disease— A disease that is (partly or completely) the result of the abnormal function or expression of a gene; a disease caused by the inheritance and expression of a genetic mutation.
IQ— Abbreviation for Intelligence Quotient. Compares an individual's mental age to his/her true or chronological age and multiplies that ratio by 100.
Metabolism— The total combination of all of the chemical processes that occur within cells and tissues of a living body.
Mutation— A permanent change in the genetic material that may alter a trait or characteristic of an individual, or manifest as disease, and can be transmitted to offspring.
Myelin— A fatty sheath surrounding nerves in the peripheral nervous system, which help them conduct impulses more quickly.
Nervous system— The complete network of nerves, sense organs, and brain in the body.
Phenylalanine— An essential amino acid that must be obtained from food since the human body cannot manufacture it.
Protein— Important building blocks of the body, composed of amino acids, involved in the formation of body structures and controlling the basic functions of the human body.
Recessive— Genetic trait expressed only when present on both members of a pair of chromosomes, one inherited from each parent.
OTHER
Allergy and Asthma Network. Mothers of Asthmatics, Inc. 2751 Prosperity Ave., Suite 150, Fairfax, VA 22031. (800) 878-4403. Fax: (703)573-7794.
Consensus Development Conference on Phenylketonuria (PKU): Screening and Management, October 16-18, 2000. 〈http://odp.od.nih.gov/consensus/news/upcoming/pku/pku_info.htm#overview〉.
Genetics and Public Health in the 21st Century. Using Genetic Information to Improve Health and Prevent Disease. 〈http://www.cdc.gov/genetics/_archive/publications/Table〉.
Phenylketonuria
Phenylketonuria
Definition
Phenylketonuria (PKU) is a rare metabolic disorder caused by a deficiency in the production of the hepatic (liver) enzyme phenylalanine hydroxylase (PAH).
Description
PKU is the most serious form of a class of diseases referred to as hyperphenylalaninemia, all of which involve above normal (elevated) levels of phenylalanine in the blood. The primary symptom of untreated PKU, mental retardation , is the result of consuming foods that contain the amino acid phenylalanine, which is toxic to brain tissue.
PKU is an inherited, autosomal recessive disorder. It is the most common genetic disease involving amino acid metabolism. As of 2004, PKU was incurable, but early, effective treatment can prevent the development of serious mental incapacity.
PKU is caused by the liver's inability to produce a particular type of PAH enzyme. This enzyme converts (metabolizes) the amino acid called phenylalanine into another amino acid, tyrosine. This is the only role of PAH in the body. A lack of PAH results in the buildup of abnormally high phenylalanine concentrations (or levels) in the blood and brain. Above normal levels of phenylalanine are toxic to the cells that make up the nervous system and cause irreversible abnormalities in brain structure and function in PKU patients. Phenylalanine is a type of teratogen (any substance or organism that can cause birth defects in a developing fetus).
The liver is the body's chief protein-processing center. Proteins are one of the major food nutrients. They are generally very large molecules composed of strings of smaller building blocks or molecules called amino acids. About twenty amino acids exist in nature. The body breaks down proteins from food into individual amino acids and then reassembles them into human proteins. Proteins are needed for growth and repair of cells and tissues and are the key components of enzymes, antibodies, and other essential substances.
PKU effects on the human nervous system
The extensive network of nerves in the brain and the rest of the nervous system are made up of nerve cells. Nerve cells have specialized extensions called dendrites and axons. Stimulating a nerve cell triggers nerve impulses (signals) that speed down the axon. These nerve impulses then stimulate the end of an axon to release chemicals called neurotransmitters that spread out and communicate with the dendrites of neighboring nerve cells.
Many nerve cells have long, wire-like axons that are covered by an insulating layer called the myelin sheath. This covering helps speed nerve impulses along the axon. In untreated PKU patients, abnormally high phenylalanine levels in the blood and brain can produce nerve cells with deformed axons and dendrites and cause imperfections in the myelin sheath referred to as hypomyelination and demylenation. This loss of myelin can short circuit nerve impulses (messages) and interrupt cell communication. A number of brain scan studies also indicate a degeneration of the white matter in the brains of older patients who have not maintained adequate dietary control.
PKU can also affect the production of one of the major neurotransmitters in the brain, called dopamine. The brain makes dopamine from the amino acid tyrosine. PKU patients who do not consume enough tyrosine in their diets cannot produce sufficient amounts of dopamine. Low dopamine levels in the brain disrupt normal communication between nerve cells, which results in impaired cognitive (mental) function.
Some research suggests that nerve cells of PKU patients also have difficulty absorbing tyrosine. This abnormality may explain why many PKU patients who receive sufficient dietary tyrosine still experience some form of learning disability.
Behavior and academic performance
IQ (intelligence quotient) tests provide a measure of cognitive function. The IQ of PKU patients is generally lower than the IQ of their healthy peers. Students with PKU often find academic tasks difficult and must struggle harder to succeed than their non-PKU peers. They may require special tutoring and need to repeat some of their courses. Even patients undergoing treatment programs may experience problems with typical academic tasks as math, reading, and spelling. Visual perception, visual-motor skills, and critical thinking skills can also be affected. Ten years of age seems to be an important milestone for PKU patients. After these individuals reach age 10, variations in their diets seem to have less influence on their IQ development.
People with PKU tend to avoid contact with others, appear anxious, and show signs of depression. However, some patients may be much more expressive and tend to have hyperactive, talkative, and impulsive personalities. It is also interesting to note that people with PKU are less likely to display such antisocial behaviors as lying , teasing, and active disobedience. It should be emphasized that, as of 2004, research findings were still quite preliminary and more extensive research is needed to clearly show how abnormal phenylalanine levels in the blood and brain might affect behavior and academic performance.
Demographics
One in 50 individuals in the United States has inherited a gene for PKU. About 5 million Americans are PKU carriers. About one in 15,000 babies tests positive for PKU in the United States. Studies indicate that the incidence of this disease in Caucasian and Native American populations is higher than in African-American, Hispanic, and Asian populations.
Causes and symptoms
PKU symptoms are caused by alterations or mutations in the genetic code for the PAH enzyme. Mutations in the PAH gene prevent the liver from producing adequate levels of the PAH enzyme needed to break down phenylalanine. The PAH gene and its PKU mutations are found on chromosome 12 in the human genome. In more detail, PKU mutations can involve many different types of changes, such as deletions and insertions, in the DNA of the gene that codes for the PAH enzyme.
PKU is described as an inherited, autosomal recessive disorder. The term autosomal means that the gene for PKU is not located on either the X or Y sex chromosome. The normal PAH gene is dominant to recessive PKU mutations. A recessive genetic trait, such as PKU, is one that is expressed—or shows up—only when two copies are inherited (one from each parent).
A person with one normal and one PKU gene is called a carrier. A carrier does not display any symptoms of the disease because the carrier's liver produces normal quantities of the PAH enzyme. However, PKU carriers can pass the PKU genetic mutation on to their children. Two carrier parents have a 25 percent chance of producing a baby with PKU symptoms, and a 50 percent chance having a baby that is a carrier for the disease. Although PKU conforms to these basic genetic patterns of inheritance, the actual expression, or phenotype, of the disease is not strictly an either/or situation. This is because there are at least 400 different types of PKU mutations. Although some PKU mutations cause rather mild forms of the disease, others can initiate much more severe symptoms in untreated individuals. The more severe the PKU mutation, the greater the effect on cognitive development and performance (mental ability).
Untreated PKU patients develop a broad range of symptoms related to severely impaired cognitive function, sometimes referred to as mental retardation. Other symptoms can include extreme patterns of behavior, delayed speech development, seizures, a characteristic body odor, and light body pigmentation. The light pigmentation is due to a lack of melanin, which normally colors the hair, skin, and eyes. Melanin is made from the amino acid tyrosine, which is lacking in untreated cases of PKU. Physiologically, PKU patients show high levels of phenylalanine and low levels of tyrosine in the blood. Babies do not show any visible symptoms of the disease for the first few months of life. However, typical PKU symptoms usually do show up by a baby's first birthday.
Diagnosis
The primary diagnostic test for PKU is the measurement of phenylalanine levels in a drop of blood taken from the heel of a newborn baby's foot. This screening procedure is referred to as the Guthrie test (Guthrie bacterial inhibition assay). In this test, PKU is confirmed by the appearance of bacteria growing around high concentrations of phenylalanine in the blood spot. PKU testing was introduced in the early 1960s and is the largest genetic screening program in the United States. It is required by law in all 50 states. Early diagnosis is critical. It ensures early the treatment PKU babies need to develop normally and avoid the ravages of PKU.
The American Academy of Pediatrics recommends that this test be performed on infants between 24 hours and seven days after birth. The preferred time for testing is after the baby's first feeding. If the initial PKU test produces a positive result, then follow-up tests are performed to confirm the diagnosis and to determine if the elevated phenylalanine levels may be caused by some medical condition other than PKU. Treatment for PKU is recommended for babies that show a blood phenylalanine level of 7 to 10 mg/dL or higher for more than a few consecutive days. Another, more accurate test procedure for PKU measures the ratio (comparison) of the amount of phenylalanine to the amount of tyrosine in the blood.
Subsequent diagnostic procedures (called mutation analysis and genotype determination) can actually identify the specific types of PAH gene mutations inherited by PKU infants. Large-scale studies have helped to clarify how various mutations affect the ability of patients to process phenylalanine. This information can help doctors develop more effective customized treatment plans for each of their PKU patients.
Treatment
The severity of the PKU symptoms experienced by people with this disease is determined by both lifestyle and genetic factors. In the early 1950s, researchers first demonstrated that phenylalanine-restricted diets could eliminate most of the typical PKU symptoms—except for mental retardation. As of 2004, dietary therapy (also called nutrition therapy) is the most common form of treatment for PKU patients. PKU patients who receive early and consistent dietary therapy can develop fairly normal mental capacity to within about five IQ points of their healthy peers. By comparison, untreated PKU patients generally have IQ scores below 50.
Infants with PKU should be put on a specialized diet as soon as they are diagnosed to avoid progressive brain damage and other problems caused by an accumulation of phenylalanine in the body. A PKU diet helps patients maintain very low blood levels of phenylalanine by restricting the intake of natural foods that contain this amino acid. Even breast milk is a problem for PKU babies. Special PKU dietary mixtures or formulas are usually obtained from medical clinics or pharmacies.
Phenylalanine is actually an essential amino acid. This means that it has to be obtained from food because the body cannot produce this substance on its own. Typical diets prescribed for PKU patients provide very small amounts of phenylalanine and higher quantities of other amino acids, including tyrosine. The amount of allowable phenylalanine can be increased slightly as a child grows older.
In addition, PKU diets include all the nutrients normally required for good health and normal growth, such as carbohydrates, fats, vitamins , and minerals . High protein foods such as meat, fish, chicken, eggs, nuts, beans, milk, and other dairy products are banned from PKU diets. Small amounts of moderate protein foods (such as grains and potatoes) and low protein foods (some fruits and vegetables and low protein breads and pastas) are allowed. Sugar-free foods, such as diet soda, which contain the artificial sweetener aspartame, are also prohibited foods for PKU patients because aspartame contains the amino acid phenylalanine.
Ideally, school-age children with PKU should be taught to assume responsibility for managing their diets, recording food intake, and for performing simple blood tests to monitor their phenylalanine levels. Blood tests should be done in the early morning when phenylalanine levels are highest. Infants and young children require more frequent blood tests than older children and adults. The amount of natural foods allowed in a diet can be adjusted to ensure that the level of phenylalanine in the blood is kept within a safe range—2 to 6 mg/dL before 12 years of age and 2 to 15 mg/dL for PKU patients over 12 years old.
A specialized PKU diet can cause abnormal fluctuations in tyrosine levels throughout the day. Thus, some health professionals recommend adding time-released tyrosine that can provide a more constant supply of this amino acid to the body. It should be noted that some PKU patients show signs of learning disabilities even with a special diet containing extra tyrosine. Research studies suggests that these PKU patients may not be able to process tyrosine normally.
For PKU caregivers, providing a diet that is appealing as well as healthy and nutritious is a constant challenge. Many PKU patients, especially teenagers, find it difficult to stick to the relatively bland PKU diet for extended periods of time. Some older patients decide to go off their diet plan simply because they feel healthy. However, many patients who abandon careful nutritional management develop cognitive problems, such as difficulties remembering, maintaining focus, and paying attention. Many PKU health professionals contend that all PKU patients should adhere to a strictly controlled diet for life.
One promising line of PKU research involves the synthesis (manufacturing) of a new type of enzyme that can break down phenylalanine in food consumed by the patient. This medication would be taken orally and could prevent the absorption of digested phenylalanine into the patient's bloodstream.
In general, medical researchers express concern about the great variation in treatment programs available in the early 2000s to PKU patients around the world. They have highlighted the urgent need for consistent international standards for proper management of PKU patients, which should emphasize comprehensive psychological as well as physiological monitoring and assessment .
KEY TERMS
Amino acid —An organic compound composed of both an amino group and an acidic carboxyl group. Amino acids are the basic building blocks of proteins. There are 20 types of amino acids (eight are "essential amino acids" which the body cannot make and must therefore be obtained from food).
Axon —A long, threadlike projection that is part of a neuron (nerve cell).
Enzyme —A protein that catalyzes a biochemical reaction without changing its own structure or function.
Gene —A building block of inheritance, which contains the instructions for the production of a particular protein, and is made up of a molecular sequence found on a section of DNA. Each gene is found on a precise location on a chromosome.
Genetic disease —A disease that is (partly or completely) the result of the abnormal function or expression of a gene; a disease caused by the inheritance and expression of a genetic mutation.
Intelligence quotient (IQ) —A measure of somebody's intelligence, obtained through a series of aptitude tests concentrating on different aspects of intellectual functioning.
Metabolism —The sum of all chemical reactions that occur in the body resulting in growth, transformation of foodstuffs into energy, waste elimination, and other bodily functions. These include processes that break down substances to yield energy and processes that build up other substances necessary for life.
Mutation —A permanent change in the genetic material that may alter a trait or characteristic of an individual, or manifest as disease. This change can be transmitted to offspring.
Myelin —A fatty sheath surrounding nerves throughout the body that helps them conduct impulses more quickly.
Nervous system —The system that transmits information, in the form of electrochemical impulses, throughout the body for the purpose of activation, coordination, and control of bodily functions. It is comprised of the brain, spinal cord, and nerves.
Phenylalanine —An essential amino acid that must be obtained from food since the human body cannot manufacture it. It is necessary for normal growth and development and for normal protein metabolism.
Protein —An important building block of the body, a protein is a large, complex organic molecule composed of amino acids. It is involved in the formation of body structures and in controlling the basic functions of the human body.
Recessive —Refers to an inherited trait that is outwardly obvious only when two copies of the gene for that trait are present. An individual displaying a recessive trait must have inherited one copy of the defective gene from each parent.
PKU and pregnancy
Women with PKU must be especially careful with their diets if they want to have children. They should ensure that phenylalanine blood levels are under control before conception and throughout pregnancy. Mothers with elevated (higher than normal) phenylalanine levels are high risk for having babies with significant birth defects, such as microencephaly (smaller than normal head size), congenital heart disease (abnormal heart structure and function), stunted growth, mental retardation, and psychomotor (coordination) difficulties. This condition is referred to as maternal PKU and can even affect babies who do not have the PKU disease.
Prognosis
Early newborn screening, careful monitoring, and life-long strict dietary management can help PKU patients to live normal, healthy, and long lives.
Parental concerns
Every state in the United States has mandatory newborn screening programs in place for phenylketonuria, as well as other diseases. Parents who suspect that other genetic diseases may run in their families should speak to their healthcare providers before their baby's birth to ascertain what other screening tests should be run.
BOOKS
Gascon, Generoso G., and Pinar T. Ozand. "Aminoacidopathies and Organic Acidopathies, Mitochondrial Enzyme Defects, and Other Metabolic Errors." In Textbook of Clinical Neurology. Edited by Christopher G. Goetz. Philadelphia: Saunders, 2003.
Rezvani, Iraj. "Defects in Metabolism of Amino Acids." In Nelson Textbook of Pediatrics. Edited by Richard E. Behrman et al. Philadelphia: Saunders, 2004.
PERIODICALS
Michals-Matalon, K. "Nutrient intake and congenital heart defects in maternal phenylketonuria." American Journal of Obstetrics and Gynecology 187 (August 2002): 441–4.
ORGANIZATIONS
Children's PKU Network. 1520 State St., Suite 111, San Diego, CA 92101–2930. Web site: <www.pkunetwork.org>.
March of Dimes Birth Defects Foundation. 1275 Mamaroneck Ave., White Plains, NY 10605. Web site: <www.modimes.org>.
National PKU News. 6869 Woodlawn Avenue, NE, #116, Seattle, WA 98115–5469. Web site: <www.pkunews.org>.
Marshall G. Letcher, MA Rosalyn Carson-DeWitt, MD
Phenylketonuria
Phenylketonuria
Definition
Phenylketonuria (PKU) can be defined as a rare metabolic disorder caused by a deficiency in the production of the hepatic (liver) enzyme phenylalanine hydroxylase (PAH). PKU is the most serious form of a class of diseases referred to as "hyperphenylalaninemia," all of which involve above normal (elevated) levels of phenylalanine in the blood. The primary symptom of untreated PKU, mental retardation, is the result of consuming foods that contain the amino acid phenylalanine, which is toxic to brain tissue.
PKU is an inherited, autosomal recessive disorder. It is the most common genetic disease involving amino acid metabolism. PKU is incurable, but early, effective treatment can prevent the development of serious mental incapacity.
Description
PKU is a disease caused by the liver's inability to produce a particular type of PAH enzyme. This enzyme converts (metabolizes) the amino acid called phenylalanine into another amino acid, tyrosine. This is the only role of PAH in the body. A lack of PAH results in the build-up of abnormally high phenylalanine concentrations (or levels) in the blood and brain. Above normal levels of phenylalanine are toxic to the cells that make up the nervous system and causes irreversible abnormalities in brain structure and function in PKU patients. Phenylalanine is a type of teratogen . Teratogens are any substance or organism that can cause birth disorders in a developing fetus.
The liver is the body's chief protein processing center. Proteins are one of the major food nutrients. They are generally very large molecules composed of strings of smaller building blocks or molecules called amino acids. About twenty amino acids exist in nature. The body breaks down proteins from food into individual amino acids and then reassembles them into "human" proteins. Proteins are needed for growth and repair of cells and tissues, and are the key components of enzymes, antibodies, and other essential substances.
PKU and the human nervous system
The extensive network of nerves in the brain and the rest of the nervous system are made up of nerve cells. Nerve cells have specialized extensions called dendrites and axons. Stimulating a nerve cell triggers nerve impulses, or signals, that speed down the axon. These nerve impulses then stimulate the end of an axon to release chemicals called neurotransmitters that spread out and communicate with the dendrites of neighboring nerve cells.
Many nerve cells have long, wire-like axons that are covered by an insulating layer called the myelin sheath. This covering helps speed nerve impulses along the axon. In untreated PKU patients, abnormally high phenylalanine levels in the blood and brain can produce nerve cells with abnormal axons and dendrites, and cause imperfections in the myelin sheath referred to as hypomyelination and demylenation. This loss of myelin can "short circuit" nerve impulses (messages) and interrupt cell communication. A number of brain scan studies also indicate a degeneration of the white matter in the brains of older patients who have not maintained adequate dietary control.
PKU can also affect the production of one of the major neurotransmitters in the brain, called dopamine. The brain makes dopamine from the amino acid tyrosine. PKU patients who do not consume enough tyrosine in their diet cannot produce sufficient amounts of dopamine. Low dopamine levels in the brain disrupt normal communication between nerve cells, which results in impaired cognitive (mental) function.
Some preliminary research suggests that nerve cells of PKU patients also have difficulty absorbing tyrosine. This abnormality may explain why many PKU patients who receive sufficient dietary tyrosine still experience some form of learning disability.
Behavior and academic performance
IQ (intelligence quotient) tests provide a measure of cognitive function. The IQ of PKU patients is generally lower than the IQ of their healthy peers. Students with PKU often find academic tasks difficult and must struggle harder to succeed than their non-PKU peers. They may require special tutoring and need to repeat some of their courses. Even patients undergoing treatment programs may experience problems with typical academic tasks such as math, reading, and spelling. Visual perception, visual-motor skills, and critical thinking skills can also be affected. Ten years of age seems to be an important milestone for PKU patients. After age 10, variations in a patient's diet seems to have less influence on their IQ development.
People with PKU tend to avoid contact with others, appear anxious, and show signs of depression . However, some patients may be much more expressive and tend to have hyperactive, talkative, and impulsive personalities. It is also interesting to note that people with PKU are less likely to display such habits as lying, teasing, and active disobedience. It should be emphasized that current research findings are still quite preliminary and more extensive research is needed to clearly show how abnormal phenylalanine levels in the blood and brain might affect behavior and academic performance.
Genetic profile
PKU symptoms are caused by alterations or mutations in the genetic code for the PAH enzyme. Mutations in the PAH gene prevent the liver from producing adequate levels of the PAH enzyme needed to break down phenylalanine. The PAH gene and its PKU mutations are found on chromosome 12 in the human genome. In more detail, PKU mutations can involve many different types of changes, such as deletions and insertions, in the DNA of the gene that codes for the PAH enzyme.
PKU is described as an inherited, autosomal recessive disorder. The term autosomal means that the gene for PKU is not located on either the X or Y sex chromosome. The normal PAH gene is dominant to recessive PKU mutations. A recessive genetic trait, such as PKU, is one that is expressed—or shows up—only when two copies are inherited (one from each parent).
A person with one normal and one PKU gene is called a carrier. A carrier does not display any symptoms of the disease because their liver produces normal quantities of the PAH enzyme. However, PKU carriers can pass the PKU genetic mutation onto their children. Two carrier parents have a 25% chance of producing a baby with PKU symptoms, and a 50% chance having a baby that is a carrier for the disease. Although PKU conforms to these basic genetic patterns of inheritance , the actual expression, or phenotype, of the disease is not strictly an "either/or" situation. This is because there are at least 400 different types of PKU mutations. Although some PKU mutations cause rather mild forms of the disease, others can initiate much more severe symptoms in untreated individuals. The more severe the PKU mutation, the greater the effect on cognitive development and performance (mental ability).
Also, it must be remembered that human cells contain two copies of each type of gene. Different combinations of any two PKU mutations tend to produce a wide spectrum of physiological and psychological symptoms. For example, patients who receive two "severe" PKU mutations from their parents can potentially develop more serious symptoms than people who possess a combination of one severe type and one milder form of mutation. To further complicate the genetic picture of PKU, other types of genes have been identified which seem to be responsible for the abnormal processing of phenylalanine in brain tissue. These abnormalities add to the severity of PKU symptoms experienced by patients who inherit these genes. In more detail, the association of multiple types of genes with a single condition, such as PKU, is referred to as molecular heterogeneity.
Demographics
One in 50 individuals in the United States have inherited a gene for PKU. About five million Americans are PKU carriers. About one in 15,000 babies test positive for PKU in the United States. Studies indicate that the incidence of this disease in Caucasian and Native American populations is higher than in African-American, Hispanic, and Asian populations.
Signs and symptoms
Untreated PKU patients develop a broad range of symptoms related to severely impaired cognitive function, sometimes referred to as mental retardation. Other symptoms can include extreme patterns of behavior, delayed speech development, seizures, a characteristic body odor, and light body pigmentation. The light pigmentation is due to a lack of melanin, which normally colors the hair, skin, and eyes. Melanin is made from the amino acid tyrosine, which is lacking in untreated cases of PKU. Physiologically, PKU patients show high levels of phenylalanine and low levels of tyrosine in the blood. Babies do not show any visible symptoms of the disease for the first few months of life. However, typical PKU symptoms usually do show up by a baby's first birthday.
Diagnosis
The primary diagnostic test for PKU is the measurement of phenylalanine levels in a drop of blood taken from the heel of a newborn baby's foot. This screening procedure is referred to as the Guthrie test (Guthrie bacterial inhibition assay). In this test, PKU is confirmed by the appearance of bacteria growing around high concentrations of phenylalanine in the blood spot. PKU testing was introduced in the early 1960s and is the largest genetic screening program in the United States. It is required by law in all 50 states. Early diagnosis is critical. It ensures the early treatment PKU babies need to develop normally and avoid the complications of PKU.
The American Academy of Pediatrics recommends that this test be performed on infants between 24 hours and seven days after birth. The preferred time for testing is after the baby's first feeding. If the initial PKU test produces a positive result, then follow-up tests are performed to confirm the diagnosis and to determine if the elevated phenylalanine levels may be caused by some medical condition other than PKU. Treatment for PKU is recommended for babies that show a blood phenylalanine level of 7–10 mg/dL or higher for more than a few consecutive days. Another, more accurate test procedure for PKU measures the ratio (comparison) of the amount of phenylalanine to the amount of tyrosine in the blood.
Newer diagnostic procedures (called mutation analysis and genotype determination) can actually identify the specific types of PAH gene mutations inherited by PKU infants. Large-scale studies have helped to clarify how various mutations affect the ability of patients to process phenylalanine. This information can help doctors develop more effective customized treatment plans for each of their PKU patients.
Treatment and management
The severity of the PKU symptoms experienced by people with this disease is determined by both lifestyle and genetic factors. In the early 1950s, researchers first demonstrated that phenylalanine-restricted diets could eliminate most of the typical PKU symptoms—except for mental retardation. Today, dietary therapy (also called nutrition therapy) is the most common form of treatment for PKU patients. PKU patients who receive early and consistent dietary therapy can develop fairly normal mental capacity to within about five IQ points of their healthy peers. By comparison, untreated PKU patients generally have IQ scores below 50.
Infants with PKU should be put on a specialized diet as soon as they are diagnosed to avoid progressive brain damage and other problems caused by an accumulation of phenylalanine in the body. A PKU diet helps patients maintain very low blood levels of phenylalanine by restricting the intake of natural foods that contain this amino acid. Even breast milk is a problem for PKU babies. Special PKU dietary mixtures or formulas are usually obtained from medical clinics or pharmacies.
Phenylalanine is actually an essential amino acid. This means that it has to be obtained from food because the body cannot produce this substance on its own. Typical diets prescribed for PKU patients provide very small amounts of phenylalanine and higher quantities of other amino acids, including tyrosine. The amount of allowable phenylalanine can be increased slightly as a child becomes older.
In addition, PKU diets include all the nutrients normally required for good health and normal growth, such as carbohydrates, fats, vitamins, and minerals. High protein foods like meat, fish, chicken, eggs, nuts, beans, milk, and other dairy products are banned from PKU diets. Small amounts of moderate protein foods (such as grains and potatoes) and low protein foods (some fruits and vegetables, low protein breads and pastas) are allowed. Sugar-free foods, such as diet soda, which contain the artificial sweetener aspartame, are also prohibited foods for patients with PKU. That is because aspartame contains the amino acid phenylalanine.
Ideally, school-age children with PKU should be taught to assume responsibility for managing their diet, recording food intake, and for performing simple blood tests to monitor their phenylalanine levels. Blood tests should be done in the early morning when phenylalanine levels are highest. Infants and young children require more frequent blood tests than older children and adults. The amount of natural foods allowed in a diet could be adjusted to ensure that the level of phenylalanine in the blood is kept within a safe range—two to 6 mg/dL before 12 years of age and 2–15 mg/dL for PKU patients over 12 years old.
A specialized PKU diet can cause abnormal fluctuations in tyrosine levels throughout the day. Thus, some health professionals recommend adding time released tyrosine that can provide a more constant supply of this amino acid to the body. It should be noted that some PKU patients show signs of learning disabilities even with a special diet containing extra tyrosine. Research studies suggest that these patients may not be able to process tyrosine normally.
For PKU caregivers, providing a diet that is appealing as well as healthy and nutritious is a constant challenge. Many patients with PKU, especially teenagers, find it difficult to stick to the relatively bland PKU diet for extended periods of time. Some older patients decide to go off their diet plan simply because they feel healthy. However, many patients who abandon careful nutritional management develop cognitive problems, such as difficulties remembering, maintaining focus, and paying attention. Many PKU health professionals contend that all patients with PKU should adhere to a strictly controlled diet for life.
One promising line of PKU research involves the synthesis (manufacturing) of a new type of enzyme that can break down phenylalanine in food consumed by the patient. This medication would be taken orally and could prevent the absorption of digested phenylalanine into the patient's bloodstream.
In general, medical researchers express concern about the great variation in treatment programs currently available to PKU patients around the world. They have highlighted the urgent need for new, consistent international standards for proper management of PKU patients, which should emphasize comprehensive psychological as well as physiological monitoring and assessment.
PKU and Pregnancy
Women with PKU must be especially careful with their diets if they want to have children. They should ensure that phenylalanine blood levels are under control before conception and throughout pregnancy. Mothers with elevated (higher than normal) phenylalanine levels are high risk for having babies with significant birth disorders, such as microencephaly (smaller than normal head size), and congenital heart disease (abnormal heart structure and function), stunted growth, mental impairment, and psychomotor (coordination) difficulties. This condition is referred to as maternal PKU and can even affect babies who do not have the PKU disease.
Prognosis
Early newborn screening, careful monitoring, and a life-long strict dietary management can help PKU patients to live normal, healthy, and long lives.
Resources
BOOKS
Brust, John C. M. The Practice Of Neural Science: From Synapses To Symptoms. New York: McGraw-Hill, 2000.
Gilroy, John. Basic Neurology. 3rd ed. New York: McGraw-Hill, 2000.
Koch, Jean Holt. Robert Guthrie—The PKU Story: Crusade Against Mental Retardation. Pasadena, CA: Hope Publishing House, 1997.
Ratey, John J. A User's Guide To The Brain: Perception, Attention, And The Four Theaters Of The Brain. 1st ed. New York: Pantheon Books, 2001.
Schuett, Virginia E. Low Protein Cookery For Phenylketonuria. 3rd ed. Madison: University of Wisconsin Press, 1997.
Walker, John M. Genetics and You. Totowa, N.J.: Humana Press, 1996.
Weiner, William J., Christopher G. Goetz, eds. Neurology For The Non-Neurologist. 4th ed. Philadelphia: Lippincott, Williams & Wilkins, 1999.
PERIODICALS
Burgard, P. "Development of intelligence in early treated phenylketonuria." European Journal of Pediatrics 159, Suppl. 2 (October 2000): S74–9.
Chang, Pi-Nian, Robert M. Gray, and Lisa Lehn O'Brien. "Review: Patterns of academic achievement among patients treated early with phenylketonuria." European Journal of Pediatrics 159, no.14 (2000): S96–9.
Eastman, J.W., et al. "Use of the phenylalanine:tyrosine ratio to test newborns for phenylketonuria in a large public health screening programme." Journal of Medical Screening 7, no. 3 (2000): 131–5.
MacDonald, A. "Diet and compliance in phenylketonuria." European Journal of Pediatrics 159, Suppl. 2 (Oct. 2000): S136–41.
Smith, Isabel, and Julie Knowles. "Behaviour in early treated phenylketonuria: a systematic review." European Journal of Pediatrics 159, no. 14 (2000): S89–93.
Stemerdink, B.A., et al. "Behaviour and school achievement in patients with early and continuously treated phenylketonuria." Journal of Inherited Metabolic Disorders 23, no. 6 (2000): 548–62.
van Spronsen, F.J.F., et al. "Phenylketonuria: tyrosine supplementation in phenylalanine-restricted diets." American Journal of Clinical Nutrition 73, no. 2 (2001): 153–7.
Wappner, Rebecca, et al. "Management of Phenylketonuria for Optimal Outcome: A Review of Guidelines for Phenylketonuria Management and a Report of Surveys of Parents, Patients, and Clinic Directors." Pediatrics 104, no. 6 (December 1999): e68.
ORGANIZATIONS
Allergy and Asthma Network. Mothers of Asthmatics, Inc. 2751 Prosperity Ave., Suite 150, Fairfax, VA 22031. (800) 878-4403. Fax: (703)573-7794.
American Academy of Allergy, Asthma & Immunology. 611 E. Wells St, Milwaukee, WI 53202. (414) 272-6071. Fax: (414) 272-6070. <http://www.aaaai.org/default.stm>.
Centers for Disease Control. GDP Office, 4770 Buford Highway NE, Atlanta, GA 30341-3724. (770) 488-3235. <http://www.cdc.gov/genetics>.
Children's PKU Network. 1520 State St., Suite 111, San Diego, CA 92101-2930. (619) 233-3202. Fax: (619) 233 0838. [email protected].
March of Dimes Birth Defects Foundation. 1275 Mamaroneck Ave., White Plains, NY 10605. (888) 663-4637. [email protected]. <http://www.modimes.org>.
National PKU News. Virginia Schuett, editor/dietician. 6869 Woodlawn Avenue NE #116, Seattle, WA 98115-5469. (206) 525-8140. Fax: (206) 525-5023. <http://www.pkunews.org>.
University of Washington PKU Clinic. CHDD, Box 357920, University of Washington, Seattle, WA. (206) 685-3015. Within Washington State: (877) 685-3015. Clinic Coordinator: [email protected]. <http://depts.washington.edu/pku/contact.html.>.
WEBSITES
Consensus Development Conference on Phenylketonuria (PKU): Screening and Management, October 16–18, 2000. <http://odp.od.nih.gov/consensus/news/upcoming/pku/pku_info.htm#overview>.
Genetics and Public Health in the 21st Century. Using Genetic Information to Improve Health and Prevent Disease. <http://www.cdc.gov/genetics/_archive/publications/Table>.
Marshall G. Letcher, MA
Phenylketonuria
Phenylketonuria
Definition
Phenylketonuria (PKU) can be defined as a rare metabolic disorder caused by a deficiency in the production of the hepatic (liver) enzyme phenylalanine hydroxylase (PAH). PKU is the most serious form of a class of diseases referred to as "hyperphenylalaninemia," all of which involve above normal (elevated) levels of phenylalanine in the blood. The primary symptom of untreated PKU, mental retardation, is the result of consuming foods that contain the amino acid phenylalanine, which is toxic to brain tissue.
PKU is an inherited, autosomal recessive disorder. It is the most common genetic disease involving amino acid metabolism. PKU is incurable, but early, effective treatment can prevent the development of serious mental incapacity.
Description
PKU is a disease caused by the liver's inability to produce a particular type of PAH enzyme. This enzyme converts (metabolizes) the amino acid called phenylalanine into another amino acid, tyrosine. This is the only role of PAH in the body. A lack of PAH results in the build–up of abnormally high phenylalanine concentrations (or levels) in the blood and brain. Above normal levels of phenylalanine are toxic to the cells that make up the nervous system and causes irreversible abnormalities in brain structure and function in PKU patients. Phenylalanine is a type of teratogen . Teratogens are any substance or organism that can cause birth disorders in a developing fetus.
The liver is the body's chief protein processing center. Proteins are one of the major food nutrients. They are generally very large molecules composed of strings of smaller building blocks or molecules called amino acids. About twenty amino acids exist in nature. The body breaks down proteins from food into individual amino acids and then reassembles them into "human" proteins. Proteins are needed for growth and repair of cells and tissues, and are the key components of enzymes, antibodies, and other essential substances.
PKU and the human nervous system
The extensive network of nerves in the brain and the rest of the nervous system are made up of nerve cells. Nerve cells have specialized extensions called dendrites and axons. Stimulating a nerve cell triggers nerve impulses, or signals, that speed down the axon. These nerve impulses then stimulate the end of an axon to release chemicals called neurotransmitters that spread out and communicate with the dendrites of neighboring nerve cells.
Many nerve cells have long, wire-like axons that are covered by an insulating layer called the myelin sheath. This covering helps speed nerve impulses along the axon. In untreated PKU patients, abnormally high phenylalanine levels in the blood and brain can produce nerve cells with abnormal axons and dendrites, and cause imperfections in the myelin sheath referred to as hypomyelination and demylenation. This loss of myelin can "short circuit" nerve impulses (messages) and interrupt cell communication. A number of brain scan studies also indicate a degeneration of the white matter in the brains of older patients who have not maintained adequate dietary control.
PKU can also affect the production of one of the major neurotransmitters in the brain, called dopamine. The brain makes dopamine from the amino acid tyrosine. PKU patients who do not consume enough tyrosine in their diet cannot produce sufficient amounts of dopamine. Low dopamine levels in the brain disrupt normal communication between nerve cells, which results in impaired cognitive (mental) function.
Some preliminary research suggests that nerve cells of PKU patients also have difficulty absorbing tyrosine. This abnormality may explain why many PKU patients who receive sufficient dietary tyrosine still experience some form of learning disability.
Behavior and academic performance
IQ (intelligence quotient) tests provide a measure of cognitive function. The IQ of PKU patients is generally lower than the IQ of their healthy peers. Students with PKU often find academic tasks difficult and must struggle harder to succeed than their non-PKU peers. They may require special tutoring and need to repeat some of their courses. Even patients undergoing treatment programs may experience problems with typical academic tasks such as math, reading, and spelling. Visual perception, visual-motor skills, and critical thinking skills can also be affected. Ten years of age seems to be an important milestone for PKU patients. After age 10, variations in a patient's diet seems to have less influence on their IQ development.
People with PKU tend to avoid contact with others, appear anxious, and show signs of depression . However, some patients may be much more expressive and tend to have hyperactive, talkative, and impulsive personalities. It is also interesting to note that people with PKU are less likely to display such habits as lying, teasing, and active disobedience. It should be emphasized that current research findings are still quite preliminary and more extensive research is needed to clearly show how abnormal phenylalanine levels in the blood and brain might affect behavior and academic performance.
Genetic profile
PKU symptoms are caused by alterations or mutations in the genetic code for the PAH enzyme. Mutations in the PAH gene prevent the liver from producing adequate levels of the PAH enzyme needed to break down phenylalanine. The PAH gene and its PKU mutations are found on chromosome 12 in the human genome. In more detail, PKU mutations can involve many different types of changes, such as deletions and insertions, in the DNA of the gene that codes for the PAH enzyme.
PKU is described as an inherited, autosomal recessive disorder. The term autosomal means that the gene for PKU is not located on either the X or Y sex chromosome. The normal PAH gene is dominant to recessive PKU mutations. A recessive genetic trait, such as PKU, is one that is expressed—or shows up—only when two copies are inherited (one from each parent).
A person with one normal and one PKU gene is called a carrier. A carrier does not display any symptoms of the disease because their liver produces normal quantities of the PAH enzyme. However, PKU carriers can pass the PKU genetic mutation onto their children. Two carrier parents have a 25% chance of producing a baby with PKU symptoms, and a 50% chance having a baby that is a carrier for the disease. Although PKU conforms to these basic genetic patterns of inheritance , the actual expression, or phenotype , of the disease is not strictly an "either/or" situation. This is because there are at least 400 different types of PKU mutations. Although some PKU mutations cause rather mild forms of the disease, others can initiate much more severe symptoms in untreated individuals. The more severe the PKU mutation, the greater the effect on cognitive development and performance (mental ability).
Also, it must be remembered that human cells contain two copies of each type of gene. Different combinations of any two PKU mutations tend to produce a wide spectrum of physiological and psychological symptoms. For example, patients who receive two "severe" PKU mutations from their parents can potentially develop more serious symptoms than people who possess a combination of one severe type and one milder form of mutation. To further complicate the genetic picture of PKU, other types of genes have been identified which seem to be responsible for the abnormal processing of phenylalanine in brain tissue. These abnormalities add to the severity of PKU symptoms experienced by patients who inherit these genes. In more detail, the association of multiple types of genes with a single condition, such as PKU, is referred to as molecular heterogeneity.
Demographics
One in 50 individuals in the United States have inherited a gene for PKU. About five million Americans are PKU carriers. About one in 15,000 babies test positive for PKU in the United States. Studies indicate that the incidence of this disease in Caucasian and Native American populations is higher than in African-American, Hispanic, and Asian populations.
Signs and symptoms
Untreated PKU patients develop a broad range of symptoms related to severely impaired cognitive function, sometimes referred to as mental retardation. Other symptoms can include extreme patterns of behavior, delayed speech development, seizures, a characteristic body odor, and light body pigmentation. The light pigmentation is due to a lack of melanin, which normally colors the hair, skin and eyes. Melanin is made from the amino acid tyrosine, which is lacking in untreated cases of PKU. Physiologically, PKU patients show high levels of phenylalanine and low levels of tyrosine in the blood. Babies do not show any visible symptoms of the disease for the first few months of life. However, typical PKU symptoms usually do show up by a baby's first birthday.
Diagnosis
The primary diagnostic test for PKU is the measurement of phenylalanine levels in a drop of blood taken from the heel of a newborn baby's foot. This screening procedure is referred to as the Guthrie test (Guthrie bacterial inhibition assay). In this test, PKU is confirmed by the appearance of bacteria growing around high concentrations of phenylalanine in the blood spot. PKU testing was introduced in the early 1960s and is the largest genetic screening program in the United States. It is
required by law in all 50 states. Early diagnosis is critical. It ensures the early treatment PKU babies need to develop normally and avoid the complications of PKU.
The American Academy of Pediatrics recommends that this test be performed on infants between 24 hours and seven days after birth. The preferred time for testing is after the baby's first feeding. If the initial PKU test produces a positive result, then follow-up tests are performed to confirm the diagnosis and to determine if the elevated phenylalanine levels may be caused by some medical condition other than PKU. Treatment for PKU is recommended for babies that show a blood phenylalanine level of 7–10 mg/dL or higher for more than a few consecutive days. Another, more accurate test procedure for PKU measures the ratio (comparison) of the amount of phenylalanine to the amount of tyrosine in the blood.
Newer diagnostic procedures (called mutation analysis and genotype determination) can actually identify the specific types of PAH gene mutations inherited by PKU infants. Large-scale studies have helped to clarify how various mutations affect the ability of patients to process phenylalanine. This information can help doctors develop more effective customized treatment plans for each of their PKU patients.
Treatment and management
The severity of the PKU symptoms experienced by people with this disease is determined by both lifestyle and genetic factors. In the early 1950s, researchers first demonstrated that phenylalanine-restricted diets could eliminate most of the typical PKU symptoms—except for mental retardation. Today, dietary therapy (also called nutrition therapy) is the most common form of treatment for PKU patients. PKU patients who receive early and consistent dietary therapy can develop fairly normal mental capacity to within about five IQ points of their healthy peers. By comparison, untreated PKU patients generally have IQ scores below 50.
Infants with PKU should be put on a specialized diet as soon as they are diagnosed to avoid progressive brain damage and other problems caused by an accumulation of phenylalanine in the body. A PKU diet helps patients maintain very low blood levels of phenylalanine by restricting the intake of natural foods that contain this amino acid. Even breast milk is a problem for PKU babies. Special PKU dietary mixtures or formulas are usually obtained from medical clinics or pharmacies.
Phenylalanine is actually an essential amino acid. This means that it has to be obtained from food because the body cannot produce this substance on its own. Typical diets prescribed for PKU patients provide very small amounts of phenylalanine and higher quantities of other amino acids, including tyrosine. The amount of allowable phenylalanine can be increased slightly as a child becomes older.
In addition, PKU diets include all the nutrients normally required for good health and normal growth, such as carbohydrates, fats, vitamins, and minerals. High protein foods like meat, fish, chicken, eggs, nuts, beans, milk, and other dairy products are banned from PKU diets. Small amounts of moderate protein foods (such as grains and potatoes) and low protein foods (some fruits and vegetables, low protein breads and pastas) are allowed. Sugar-free foods, such as diet soda, which contain the artificial sweetener aspartame, are also prohibited foods for patients with PKU. That is because aspartame contains the amino acid phenylalanine.
Ideally, school-age children with PKU should be taught to assume responsibility for managing their diet, recording food intake, and for performing simple blood tests to monitor their phenylalanine levels. Blood tests should be done in the early morning when phenylalanine levels are highest. Infants and young children require more frequent blood tests than older children and adults. The amount of natural foods allowed in a diet could be adjusted to ensure that the level of phenylalanine in the blood is kept within a safe range—two to 6 mg/dL before 12 years of age and 2–15 mg/dL for PKU patients over 12 years old.
A specialized PKU diet can cause abnormal fluctuations in tyrosine levels throughout the day. Thus, some health professionals recommend adding time released tyrosine that can provide a more constant supply of this amino acid to the body. It should be noted that some PKU patients show signs of learning disabilities even with a special diet containing extra tyrosine. Research studies suggest that these patients may not be able to process tyrosine normally.
For PKU caregivers, providing a diet that is appealing as well as healthy and nutritious is a constant challenge. Many patients with PKU, especially teenagers, find it difficult to stick to the relatively bland PKU diet for extended periods of time. Some older patients decide to go off their diet plan simply because they feel healthy. However, many patients who abandon careful nutritional management develop cognitive problems, such as difficulties remembering, maintaining focus, and paying attention. Many PKU health professionals contend that all patients with PKU should adhere to a strictly controlled diet for life.
One promising line of PKU research involves the synthesis (manufacturing) of a new type of enzyme that can break down phenylalanine in food consumed by the patient. This medication would be taken orally and could prevent the absorption of digested phenylalanine into the patient's bloodstream.
In general, medical researchers express concern about the great variation in treatment programs currently available to PKU patients around the world. They have highlighted the urgent need for new, consistent international standards for proper management of PKU patients, which should emphasize comprehensive psychological as well as physiological monitoring and assessment.
PKU and Pregnancy
Women with PKU must be especially careful with their diets if they want to have children. They should ensure that phenylalanine blood levels are under control before conception and throughout pregnancy. Mothers with elevated (higher than normal) phenylalanine levels are high risk for having babies with significant birth disorders, such as microencephaly (smaller than normal head size), and congenital heart disease (abnormal heart structure and function), stunted growth, mental impairment, and psychomotor (coordination) difficulties. This condition is referred to as maternal PKU and can even affect babies who do not have the PKU disease.
Prognosis
Early newborn screening, careful monitoring, and a life-long strict dietary management can help PKU patients to live normal, healthy, and long lives.
Resources
BOOKS
Brust, John C. M. The Practice Of Neural Science: From Synapses To Symptoms. New York: McGraw-Hill, 2000.
Gilroy, John. Basic Neurology. 3rd ed. New York: McGraw-Hill, 2000.
Koch, Jean Holt. Robert Guthrie—The PKU Story: Crusade Against Mental Retardation. Pasadena, CA: Hope Publishing House, 1997.
Ratey, John J. A User's Guide To The Brain: Perception, Attention, And The Four Theaters Of The Brain. 1st ed. New York: Pantheon Books, 2001.
Schuett, Virginia E. Low Protein Cookery For Phenylketonuria. 3rd ed. Madison: University of Wisconsin Press, 1997.
Walker, John M. Genetics and You. Totowa, NJ: Humana Press, 1996.
Weiner, William J., Christopher G. Goetz, eds. Neurology For The Non-Neurologist. 4th ed. Philadelphia: Lippincott, Williams & Wilkins, 1999.
PERIODICALS
Burgard, P. "Development of intelligence in early treated phenylketonuria." European Journal of Pediatrics 159, Suppl. 2 (October 2000): S74–9.
Chang, Pi-Nian, Robert M. Gray, and Lisa Lehn O'Brien. "Review: Patterns of academic achievement among patients treated early with phenylketonuria." European Journal of Pediatrics 159, no.14 (2000): S96–9.
Eastman, J.W., et al. "Use of the phenylalanine: Tyrosine ratio to test newborns for phenylketonuria in a large public health screening programme." Journal of Medical Screening 7, no. 3 (2000): 131–5.
MacDonald, A. "Diet and compliance in phenylketonuria." European Journal of Pediatrics 159, Suppl. 2 (Oct. 2000): S136–41.
Smith, Isabel, and Julie Knowles. "Behaviour in early treated phenylketonuria: A systematic review." European Journal of Pediatrics 159, no. 14 (2000): S89–93.
Stemerdink, B.A., et al. "Behaviour and school achievement in patients with early and continuously treated phenylketonuria." Journal of Inherited Metabolic Disorders 23, no. 6 (2000): 548–62.
van Spronsen, F.J.F., et al. "Phenylketonuria: Tyrosine supplementation in phenylalanine-restricted diets." American Journal of Clinical Nutrition 73, no. 2 (2001): 153–7.
Wappner, Rebecca, et al. "Management of Phenylketonuria for Optimal Outcome: A Review of Guidelines for Phenylketonuria Management and a Report of Surveys of Parents, Patients, and Clinic Directors." Pediatrics 104, no. 6 (December 1999): e68.
ORGANIZATIONS
Allergy and Asthma Network. Mothers of Asthmatics, Inc. 2751 Prosperity Ave., Suite 150, Fairfax, VA 22031. (800) 878-4403. Fax: (703)573-7794.
American Academy of Allergy, Asthma & Immunology. 611 E. Wells St, Milwaukee, WI 53202. (414) 272-6071. Fax: (414) 272-6070. <http://www.aaaai.org/default.stm>.
Centers for Disease Control. GDP Office, 4770 Buford Highway NE, Atlanta, GA 30341-3724. (770) 488-3235. <http://www.cdc.gov/genetics>.
Children's PKU Network. 1520 State St., Suite 111, San Diego, CA 92101-2930. (619) 233-3202. Fax: (619) 233 0838. [email protected].
March of Dimes Birth Defects Foundation. 1275 Mamaroneck Ave., White Plains, NY 10605. (888) 663-4637. [email protected]. <http://www.modimes.org>.
National PKU News. Virginia Schuett, editor/dietician. 6869 Woodlawn Avenue NE #116, Seattle, WA 98115-5469. (206) 525-8140. Fax: (206) 525-5023. <http://www.pkunews.org>.
University of Washington PKU Clinic. CHDD, Box 357920, University of Washington, Seattle, WA. (206) 685-3015. Within Washington State: (877) 685-3015. Clinic Coordinator: [email protected]. <http://depts.washington.edu/pku/contact.html.>.
WEBSITES
Consensus Development Conference on Phenylketonuria (PKU): Screening and Management, October 16–18, 2000. <http://odp.od.nih.gov/consensus/news/upcoming/pku/pku_info.htm#overview>.
Genetics and Public Health in the 21st Century. Using Genetic Information to Improve Health and Prevent Disease. <http://www.cdc.gov/genetics/_archive/publications/Table>.
Marshall G. Letcher, MA
Phenylketonuria
Phenylketonuria
Phenylketonuria (PKU) is an inherited disorder in which an enzyme (usually phenylalanine hydroxylase) crucial to the appropriate processing of the amino acid, phenylalanine is totally absent or drastically deficient. The result is that phenylalanine cannot be broken down, and it accumulates in large quantities throughout the body. PKU is sometimes called Folling’s disease (especially in Norway) in honor of Norwegian physician Ivar Asbjorn Folling (1888-1973) who first described it in 1934.
Phenylalanine
Phenylalanine is an essential amino acid. These substances are called essential because the body must get them from food to build the proteins that make up its tissues and keep them working. Therefore, phenylalanine is required for normal development. Phenylalanine is a common amino acid and is found in all natural foods. However, natural foods contain more phenylalanine than required for normal development. This level is too high for patients with PKU, making a special low-phenylalanine diet a requirement.
Normally, phenylalanine is converted to tyrosine. Because tyrosine is involved in the production of melanin (pigment), people with PKU usually have lighter skin and hair than other family members. Without treatment, phenylalanine accumulation in the brain causes severe mental retardation. Treatment is usually started during babyhood; delaying such treatment results in a significantly lowered IQ (intelligence quotient) by age one year. Treatment involves a diet low in phenylalanine (look for warnings aimed at people with PKU on cans of diet drinks containing the artificial sweetener aspartame, which is made from phenylalanine).
Statistics
PKU strikes about one out of every 15,000 to 20,000 newborns. There are areas in the world where the incidence is much higher, particularly Ireland and western Scotland. In Ireland the incidence of PKU is one in 4,500 births. This is the highest incidence in the world and supports a theory that the genetic defect is very old and of Celtic origin. Countries with very little immigration from Ireland or western Scotland tend to have low rates of PKU. In Finland, the incidence is less than one in 100,000 births. Caucasians in the United States have a PKU incidence of one in 8,000, whereas Blacks have an incidence of one in 50,000. Because it is so important to start treatment immediately, many states require that all infants be tested for the disease within the first week of life.
Types
There are a number of specific types of PKU. Maternal phenylketonuria is a condition in which a high level of phenylalanine in a mother’s blood causes mental retardation in her child when in the uterus. A woman who has PKU and is not using a special lowphenylalanine diet will have high levels of phenylala-nine in her blood. Her high phenylalanine levels will cross the placenta and affect the development of her child. Most children born from these pregnancies are mentally retarded and have physical problems, including small head size (microcephaly) and congenital heart disease. The majority of these children do not have PKU. There is no treatment for maternal phenylketonuria. Control of maternal phenylalanine levels is thought to limit the effects of maternal phenylketonuria.
Hyperphenylalaninemia is a condition in which patients have high levels of phenylalanine in their blood, but not as high as seen in patients with classical PKU. There are two forms of hyperphenylalaninemia: mild and severe.
Tyrosinemia is characterized by a high levels of two amino acids in the blood, phenylalanine and tyrosine. Patients with this disease have many of the same symptoms as seen in classical PKU, including mental retardation. Treatment consists of a special diet similar to the diet for PKU. The main difference between the two diets is that patients with tyrosinemia must eat a diet that is low in both phenylalanine and tyrosine.
Cause and heredity
The underlying cause of PKU is mutation in the gene that tells the body to make the enzyme phenylalanine hydroxylase. This enzyme allows the body to break down phenylalanine and ultimately use it to build proteins. Normally, the first step in phenylala-nine metabolism is conversion to tyrosine, another amino acid. The genetic mutations result in no enzyme or poor quality enzyme being made. As a consequence, phenylalanine is not converted and builds up in the body. The high levels of phenylalanine can be detected in the blood and urine.
PKU is a genetic disease. A child must inherit defective genes from both parents to develop PKU. A person with one defective gene and one good gene will develop normally because the good gene will make sufficient phenylalanine hydroxylase. People with one good gene are called carriers because they do not have the disease, but are capable of passing the defective gene on to their children.
If both parents are carriers of defective phenylalanine hydroxylase genes, then the chances of their child having PKU is one in four, or 25%. The chance that their child will be a carrier is two in four, or 50%. These percentages hold for each pregnancy.
Symptoms
Children with PKU appear normal at birth, but develop irreversible mental retardation unless treated early. Treatment consists of a special diet that contains very little phenylalanine. This diet must be used throughout the patient’s life. Untreated newborns develop disease symptoms at age three to five months. At first they appear to be less attentive and may have problems eating. By one year of age, they are considered mentally retarded.
Patients with PKU tend to have lighter colored skin, hair, and eyes than other family members. They are also likely to have eczema and seizures. PKU patients have a variety of neurologic symptoms. Approximately 75 to 90% of PKU patients have abnormal electrocardiograms (ECGs), which measure the activity of their heart. Their sweat and urine may have a mousy smell that is caused by phenylacetic acid, a byproduct of phenylalanine metabolism. Untreated PKU children tend to be hyperactive and demonstrate loss of contact with reality (psychosis).
Detection
PKU must be detected shortly after birth. Although children with PKU appear normal at birth, they already have high phenylalanine levels. Screening is the only way to detect PKU before symptoms start to develop. In many areas of the world, screening newborns for PKU is performed routinely. The test is typically performed between one and seven days after birth. Blood is obtained by pricking the heel of the newborn and analyzing it for phenylalanine concentration.
Treatment
The only treatment for persons with PKU is to limit the amount of phenylalanine in their diet. PKU patients should eat a special diet that is low in phenylalanine. The diet has small amounts of phenylalanine because it is essential for normal growth and development. The diet should be started before the fourth week of life to prevent mental retardation. If started early enough, the diet is 75% effective in preventing severe mental retardation. Many natural foods, including breast milk, must be avoided because they contain more phenylalanine than PKU patients can tolerate. However, low protein, natural foods, including fruits, vegetables, and some cereals, are acceptable on the diet. Monitoring of blood phenylalanine levels must be done to ensure that normal levels are maintained.
Patients who make a small amount of phenylalanine hydroxylase can eat a limited amount of regular food if their phenylalanine levels remain within an acceptable range. Low-phenylalanine and phenylalanine-free foods are available commercially. The special diet must be used throughout the patient’s life.
Phenylketonuria (PKU)
Phenylketonuria (PKU)
What Are the Signs and Symptoms of PKU?
Phenylketonuria (fen-il-kee-to-NU-ree-a) or PKU is an inherited metabolic disease in which the body cannot change one essential amino acid phenylalanine (fen-il-AL-a-neen), into another needed amino acid, tyrosine (TY-ro-seen). Untreated, PKU often results in severe mental retardation, but if it is detected at birth and the children are put on a special diet, they can lead normal lives.
KEYWORDS
for searching the Internet and other reference sources
Mental retardation
Metabolism
Inherited genetic disorders
Children with a Strange Smell
In 1934, a mother with two children with mental retardation went to a Norwegian medical doctor, Asbjørn Føiling. Dr. Foiling became interested in the children’s condition when the mother described the children’s odd musty odor. The children, ages 4 and 6, had severe mental retardation. The younger child could not speak or walk, and still wore diapers. The older child could speak only a few words and had problems walking.
Dr. Føiling examined the children’s urine and found no protein or glucose in it, but he was puzzled when their urine turned a deep green, instead of staying brownish, after he added the chemical compound ferric chloride to it. Dr. Foiling expanded his study to other children whose urine had the same kind of reaction and concluded that the unusual quality of the urine seemed to be connected with mental retardation. Thus, he discovered PKU, which he concluded was a genetic error in the children’s metabolism*.
- * metabolism
- (me-TAB-o-liz-um) is the process in the body that converts food into energy and waste products.
What Causes PKU?
PKU is a disruption in normal metabolism. Normally, an enzyme* in the body called phenylalanine hydroxylase (fen-il-AL-a-neen hy-DROK-si-lase) changes the essential amino acid* phenylalanine to another needed amino acid called tyrosine (TY-ro-seen). If phenylalanine hydroxylase is missing, as it is in PKU, phenylalanine builds up in the blood and passes out of the body in urine.
- * enzymes
- (EN-zymz) are natural substances that speed up specific chemical reactions in the body.
- * amino acids
- (a-MEE-no acids) are the chief building blocks of proteins. In humans, certain amino acids are required to sustain life.
PKU affects on average one out of every 10,000-15,000 babies in the United States. Since PKU is an inherited disorder, there is a good bit of variation among different ethnic and racial groups. The condition is found less often in people of African descent (1 in every 50,000 babies) and in Ashkenazic Jews.
What Are the Signs and Symptoms of PKU?
Infants with PKU usually have lighter skin, hair, and eyes than the rest of their families, but symptoms do not appear until they are about 3 to 6 months old. These may include:
- an eczema-like rash
- seizures
- hyperactivity
- an unpleasant musty or mousy body odor (caused by phenylacetic acid in the urine and sweat)
- mental retardation
How is PKU Diagnosed?
Almost all babies born in hospitals in the United States are screened for PKU within 48 hours of birth with a blood test that measures their levels of phenylalanine. In families with a history of PKU, the disease can usually be diagnosed in the fetus during pregnancy. A pregnant woman who has untreated PKU herself has a much greater chance of having a baby with (often severe) birth defects.
Living with PKU
Jennifer is a happy 12-year-old with fair skin, blue eyes, and a very normal life except for her diet. Jennifer has PKU, which was diagnosed at birth through hospital screening. She cannot go out for pizza, drink diet sodas, or eat many of the common foods that most people take for granted.
Infants with PKU must begin treatment within days of being born to prevent their becoming mentally retarded. They must eat a special diet that restricts their intake of phenylalanine and provides the tyrosine that the body cannot make. Babies drink special formula that is low in phenylalanine. People with PKU cannot have high-protein foods such as meat, poultry, fish, milk, eggs, cheese, ice cream, and nuts; products containing regular flour; or products that contain the artificial sweetener aspartame (also known by the brand name NutraSweet). They can eat low-protein foods such as fruits, vegetables, and certain cereals.
Not adhering to the diet may cause serious problems like drops in IQ and problems with learning and behavior. Experts are not sure whether and at what age it is safe to stop treatment. Most recommend that people with PKU stay on the special diet for life.
Once destined to become mentally retarded, people like Jennifer are now growing up normally. Most people with untreated PKU, however, have severe mental retardation and are unable to live independently.
See also
Mental Retardation
Resources
Book
Wessel, Kenneth. A Journey into the World of PKU, Baltimore, MD: Pediatric Genetics, 1991.
Organization
Children’s PKU Network, 1520 State St., Suite 240, San Diego, CA 92111. Telephone 619-233-3202
PKU News posts information about PKU on its website. http://www.pkunews.org/
Phenylketonuria (PKU)
Phenylketonuria (PKU)
Phenylketonuria (fee-nyl-key-ton-uria), or PKU, is an inherited metabolic disease that results in severe developmental delay and neurological problems when treatment is not started very early and maintained throughout life. The disease is caused by the absence of the enzyme phenylalanine hydroxylase, which normally converts the amino acid phenylalanine to another amino acid, tyrosine. This results in a build-up of phenylalanine and a low level of tyrosine, which causes a variety of problems, including cognitive decline, learning disabilities, behavior or neurological problems, and skin disorders.
PKU occurs in about 1 in 10,000 births. It is an autosomal recessive disorder, meaning the affected person inherits two copies of the defective gene, one from each parent. Newborn screening for PKU began in the mid-1960s and is now carried out in every state in the United States, as well as in many other countries.
The treatment for PKU consists of a special phenylalanine-restricted diet designed to maintain levels of phenylalanine in the blood between 2 and 6 mg/dl (milligrams per deciliter). All proteins are made up of amino acids; therefore, the diet for PKU consists of foods that contain only enough protein to provide the amount of phenylalanine necessary for growth and development . Foods allowed are primarily vegetables, fruits, and some cereals and grains. A synthetic formula containing all the amino acids except phenylalanine provides the remaining protein and calories for individuals with PKU.
see also Amino Acids; Artificial Sweeteners; Inborn Errors of Metabolism.
Patricia D. Thomas
Internet Resources
National PKU News. Available from <http://www.pkunews.org>
Phenylketonuria
PHENYLKETONURIA
Phenylketonuria (PKU) is an autosomal recessive disorder that results from phenylalanine hydroxylase (PAH) deficiency. If uncontrolled, PKU leads to mental retardation. The prevalence is approximately 1 in 10,000 in temperate climates and varies by race, with a frequency of 1 in 8,000 in U.S. Caucasians, and 1 in 50,000 in African Americans. Many mutant alleles are present in the population. The high frequency of defective genes suggests that there is an advantage to being a carrier, perhaps due to resistance to natural toxins. Newborn screening (with the Guthrie test) is mandated by law and is critical because the mental retardation is treatable by restricting dietary intake of phenylalanine and/or ingesting a form of phenylalanine ammonia lysase, a plant enzyme. Unfortunately, the offspring of affected mothers are typically mentally retarded, even when the child's endogenous PAH activity is adequate.
Harry W. Schroeder, Jr.
(see also: Congenital Anomalies; Genetic Disorders; Genetics and Health; Newborn Screening )
Bibliography
Scriver, C. R.; Eisensmith, R. C.; Woo, S. L. C.; and Kaufman, S. (1994). "The Hyperphenylalaninemias of Man and Mouse." Annual Review of Genetics 28:141–165.
Phenylketonuria
PHENYLKETONURIA
Phenylketonuria (pku) is due to a deficiency of phenylalanine hydroxylase, the enzyme that converts phenylalanine to tyrosine. High levels of phenylala-nine accumulate in the body resulting in severe mental retardation, seizures, and a musty odor. PKU is an autosomal recessive genetic disorder. The incidence is 1 in 10,000 to 1 in 12,000 live births. PKU is detected on newborn screens, which are most sensitive when performed between forty-eight and seventy-two hours of life, and after the infant has been fed a diet containing protein. Phenylalanine levels are easily detected by a few drops of blood, and early treatment prevents complications making it a model disease for screening. Treatment includes a diet low in phenylalanine and frequent monitoring of blood levels. Dietary therapy should begin immediately in the newborn period. The duration of therapy is controversial. It is recommended that children maintain a very restrictive diet through the age of six, and they are encouraged to maintain some restriction throughout life.
See also:BIRTH DEFECTS; NUTRITION
Bibliography
Berry, Gerard. "Inborn Errors of Carbohydrate Ammonia, Amino Acid, and Organic Acid Metabolism." In H. William Taeusch and Roberta Ballard eds., Avery's Diseases of the Newborn. Philadelphia: W. B. Saunders, 1998.
Goodman, Stephen, and Carol Greene. "Metabolic Disorders of the Newborn." Pediatrics in Review 15 (1994):359-365.
Rezvani, Iraj. "Defects in Metabolism of Amino Acids." In RichardE. Behrman, Robert M. Kliegman, and Ann M. Arvin eds., Nelson Textbook of Pediatrics. Philadelphia: W. B. Saunders Company, 1996.
Meica M.Efird
phenylketonuria
The very high plasma concentration of phenylalanine causes disruption of brain development, and if untreated there is severe mental retardation. All infants are screened for phenylketonuria shortly after birth (by measurement of plasma phenylalanine); treatment is by very strict limitation of phenylalanine intake, only providing sufficient to meet requirements for protein synthesis. Once brain development is complete (between the ages of 8 and 12 years) dietary restriction can be relaxed to a considerable extent, since high concentrations of phenylalanine seem to have little adverse effect on the developed brain. There may, however, be benefits from continuing dietary restriction into adult life, and phenylketonuric women require extremely careful dietary control through pregnancy to avoid damage to the fetus's developing brain.