Genetic: Turner Syndrome

Definition
Description
Demographics
Causes and Symptoms
Diagnosis
Treatment
Prognosis
Prevention
The Future
For more information

Definition

Turner syndrome is a genetic disorder in which the cells in a girl's body are missing all or part of one of the two X chromosomes. The syndrome occurs only in girls. It is named for Henry Turner (1892–1970), a doctor in Oklahoma who first described the syndrome at a conference in 1938.

Description

Turner syndrome is a genetic disorder in which a girl is missing one or part of one of the two X chromosomes that define a person's sex as female. Women with Turner syndrome are shorter than average and cannot have children because their ovaries do not develop normally. They may also have some abnormal physical features, such as extra skin around the neck that gives the neck a webbed appearance; swollen hands

and feet; heart or kidney problems; and scoliosis (abnormal sideways curvature of the spine). They are also at increased risk of high blood pressure and osteoporosis (brittle bones) in later life.

Turner syndrome may result either from a complete lack of the second X chromosome—a condition called monosomy X—or from damage to or partial rearrangement of the second X chromosome. About half of all women with Turner syndrome have monosomy X; two-thirds of these patients lack the father's X chromosome and the remaining third lack the mother's X. The other half of patients with Turner syndrome have a damaged or incomplete second X chromosome.

Although geneticists are not certain how many genes on the X chromosome are responsible for some of the characteristics of the syndrome, they have identified one gene known as SHOX, which is responsible for the production of a protein involved in bone growth. A missing or abnormal SHOX gene is thought to cause the short stature and skeletal abnormalities of women with Turner syndrome.

Demographics

Turner syndrome occurs in one in every 2,000–2,500 newborn girls in the United States. Most female fetuses with Turner syndrome, however, do not survive until birth; doctors estimate that 98 percent die before birth or are miscarried.

The rate of Turner syndrome appears to be the same in all countries around the world and in all races and ethnic groups. No risk factors have been identified that increase the likelihood of parents having a daughter with the syndrome; as far as is known, the genetic errors that cause the disorder occur at random.

Causes and Symptoms

Turner syndrome involves genetic errors that occur during the formation of germ cells (eggs and sperm) or during cell division shortly after the egg is fertilized by the sperm. The more common form of the syndrome occurs when an egg or sperm lacking an X chromosome is involved in conception. The reason for the missing copy in the abnormal germ cell is a genetic error called nondisjunction. During the normal process of germ cell formation, the paired chromosomes in the cell divide so that each daughter cell has one member of the pair. In nondis-junction, one daughter cell gets both members of the chromosome pair and the other cell has none. If a germ cell lacking an X chromo-some is fertilized by a normal germ cell from the other parent, the child will have only one X chromosome. This genetic error is called monosomy X.

Some cases of Turner syndrome occur in girls who have some body cells with two X chromosomes and some body cells without the secondX. This condition is called mosaic Turner's. It is thought to result from random errors in cell division during the early stages of fetal development.

An Academy Award Winner

Linda Hunt (1945–), is a film and television actress who won an Oscar in 1983 for her portrayal of a male Chinese-Australian photographer in a film called The Year of Living Dangerously. She is the first star to win an Academy Award for playing a character of the opposite sex.

Diagnosed with Turner syndrome in adolescence, Hunt is short for an adult woman at 4 feet 9 inches (1.45 meters). She has a rich and deep speaking voice that has made her a popular choice as a narrator for such television series as American Experience and California and the American Dream. One of Hunt's best-known film roles is the voice of Grandmother Willow in the 1997 Disney animated feature Pocahontas.

The symptoms of Turner syndrome include:

• Short height in adult life. The average height of an adult woman with Turner syndrome is 4 feet 7 inches (1.4 meters).
• Flat, shield-shaped chest with widely spaced nipples.
• Wide neck with webbing (extra skin) at the base.
• Swelling of the hands and feet.

• Kidney abnormalities, including a single horseshoe-shaped kidney on one side of the body or poor blood flow to the kidneys.
• Low hairline and low-set ears.
• Drooping eyelids.
• Turned-out elbows.
• Frequent ear infections and hearing loss.
• Abnormalities in the structure of the heart. These may include damaged heart valves or a weakened aorta, which is the large artery that leaves the left side of the heart and carries oxygenated blood to all parts of the body. Women with Turner syndrome are at increased risk of a ruptured aorta.
• Hypothyroidism (low levels of thyroid hormone). About a third of women with Turner syndrome develop Hashimoto's disease.
• Increased risk of both type 1 and type 2 diabetes.
• Absence of menstrual periods and inability to have children.
• Specific learning difficulties, most commonly with mathematics and with spatial perception.

Diagnosis

Turner syndrome can be detected by amniocentesis before birth. Amniocentesis is a procedure in which the doctor withdraws a small amount of fluid from the sac that surrounds the baby in the mother's uterus and examines cells from the baby's tissues for genetic abnormalities. It may also be diagnosed after the baby is born when the doctor and parents notice such features as a webbed neck, broad flat chest, swollen hands and feet, and heart defects. The diagnosis can be confirmed by a karyo-type, which is a blood test used to obtain cells for examination of their chromosomes under a microscope.

In some cases Turner syndrome is not diagnosed until the girl's growth slows down in early childhood or until she fails to reach puberty. At that time that doctor may order a karyotype (a photomicrograph of the chromosomes in a single human cell).

Treatment

Turner syndrome cannot be cured because it is a genetic condition. However, growth hormone started early and estrogen replacement treatment in adolescence can help girls with the disorder grow to a normal height

and achieve normal sexual development. Early diagnosis of Turner syndrome is important because growth hormone treatment should be started in childhood. Estrogen replacement therapy is not started until the girl is twelve to fifteen years old, as it will slow down her growth in height if given earlier. Girls with Turner syndrome should continue to receive estrogen until they reach the normal age of menopause in order to prevent osteoporosis.

Patients with Turner syndrome require careful monitoring throughout life for such possible complications as recurrent ear infections and possible hearing loss; kidney or thyroid disorders; heart problems; diabetes; and high blood pressure. They should have annual blood tests as part of routine physical checkups. In addition, they should watch their weight carefully because obesity increases their already high risk of diabetes.

Although women with Turner syndrome are infertile because their ovaries do not develop normally, they can become pregnant with the help of assisted reproductive techniques. They should, however, consult a heart specialist before considering pregnancy because of the risk of aortic rupture or other heart complications.

Most girls with Turner syndrome have normal intelligence, and many are intellectually gifted. Those who have difficulty with mathematics or related areas of study may need special tutoring in those fields. In general, however, TS should not interfere with patients' ability to complete schooling and be successful in a wide variety of occupations. Several studies have shown that many women with Turner syndrome have higher-than-average educational achievements.

Prognosis

The prognosis of Turner syndrome depends partly on whether the patient has monosomy X, a damaged second X chromosome, or mosaic Turner syndrome. In general, patients with monosomy X have a greater chance of developing heart or kidney problems than those with other forms of Turner syndrome and require more careful monitoring of their health. Most patients have an overall good prognosis with a slightly shorter than average life expectancy.

Prevention

Since Turner syndrome is thought to be caused by a random genetic mutation rather than an inherited genetic defect, there is no way to

prevent it. Parents of a daughter with Turner syndrome are unlikely to have other daughters with the disorder.

The Future

Turner syndrome is not likely to become more common in the general population in the future. In terms of treatments, as of 2008 the National Institutes of Health (NIH) was recruiting patients with Turner syndrome for clinical trials to evaluate the long-term effects of growth hormone therapy and estrogen replacement therapy. Research is ongoing to look for other genes on the X chromosome that may be involved in Turner syndrome.

SEE ALSO Hashimoto disease; Osteoporosis; Scoliosis; Triple X syndrome

WORDS TO KNOW

Karyotype: A photomicrograph of the chromosomes in a single human cell. Making a karyotype is one way to test for genetic disorders.

Monosomy: A type of genetic disorder in which a cell contains only one copy of a particular chromosome instead of the normal two.

Mosaicism: A condition in which a person has some body cells containing an abnormal number of chromosomes and other cells containing the normal number. Mosaicism results from random errors during the process of cell division that follows conception.

Nondisjunction: A genetic error in which one or more pairs of chromosomes fail to separate during the formation of germ cells, with the result that both chromosomes are carried to one daughter cell and none to the other.

For more information

BOOKS

Gizel, Kayli. All about Me: Growing Up with Turner Syndrome and Nonverbal Learning Disabilities. Wallingford, VT: Maple Leaf Center, 2004.

WEB SITES

Genetics Home Reference. Turner Syndrome. Available online at http://www.ghr.nlm.nih.gov/condition=turnersyndrome (updated May 2008; accessed June 21, 2008).

National Human Genome Research Institute (NHGRI). Learning about Turner Syndrome. Available online at http://www.genome.gov/19519119 (updated May 29, 2008; accessed June 21, 2008).

National Institute of Child Health and Human Development (NICHD). Turner Syndrome. Available online at http://www.nichd.nih.gov/health/topics/Turner_Syndrome.cfm (updated February 21, 2007; accessed June 21, 2008).

TeensHealth. Turner Syndrome. Available online at http://www.kidshealth.org/teen/diseases_conditions/genetic/turner.html (updated January 2005; accessed June 21, 2008).