Neuraminidase deficiency with beta-galactosidase deficiency

Definition

Neuraminidase deficiency with beta-galactosidase deficiency, commonly-known as galactosialidosis, is a rare inherited metabolic disorder with multiple symptoms that can include skeletal abnormalities, mental retardation, and progressive neurological degeneration.

Description

Neuraminidase deficiency with beta-galactosidase deficiency, or galactosialidosis, is a very rare genetic disorder with progressive signs and symptoms that are almost identical to those of neuraminidase deficiency alone, a disorder that is often called sialidosis. These symptoms can include skeletal and facial abnormalities, seizures, vision and hearing loss, cardiac and kidney problems, and mental retardation. However, as with sialidosis, the severity of the symptoms of galactosialidosis vary greatly.

Galactosialidosis is also known as Goldberg syndrome, after M. F. Goldberg and colleagues who first described the disorder in 1971. The disorder is also sometimes called protective protein/cathepsin A (or PPCA) deficiency, deficiency of lysosomal protective protein, or deficiency of cathepsin A.

Galactosialidosis is caused by a mutation, or change, in the gene encoding an enzyme called protective protein/cathepsin A (PPCA). PPCA forms a very large multi-enzyme complex with three other enzymes: beta-galactosidase, N-acetylgalactosamine-6-sulfate sulfatase (GALNS), and alpha-N-acetylneuraminidase. The latter enzyme is commonly referred to as neuraminidase or sialidase. Whereas sialidosis is caused by a mutation in the gene encoding neuraminidase, a mutation in the gene encoding PPCA can affect the activities of all of the enzymes in the complex. However neuraminidase is the enzyme that is most dependent on PPCA. Without functional PPCA, there is little or no neuraminidase activity. Although beta-galactosidase activity is reduced, a significant amount of active enzyme remains. Therefore, the symptoms of neuraminidase deficiency with beta-galactosidase deficiency are more similar to those of sialidosis than to those of beta-galactosidase deficiency. Mutations in the gene encoding beta-galactosidase can result in the disorders known as GM1-gangliosidosis (beta-galactosidosis) or Morquio B disease.

Galactosialidosis is subdivided into three types, depending on the age of onset: severe, neonatal or early-infantile; milder, late-infantile; and juvenile/adult. The juvenile/adult form is the most common. There also is an atypical form of galactosialidosis. The type and severity of the disorder depends on the specific mutation(s) present in the genes encoding PPCA.

Lysosomal storage diseases

Neuraminidase, beta-galactosidase, PPCA, and GALNS are all enzymes that function inside lysosomes. Lysosomes are membrane-bound spherical compartments or vesicles within the cytosol (fluid part) of cells. Lysosomes contain more than 50 different enzymes that are responsible for digesting, or hydrolyzing, large molecules and cellular components. These include proteins, polysaccharides (long, linear or branched chains of sugars), and lipids, which are large, insoluble biomolecules that are usually built from fatty acids. The smaller breakdown products from the lysosome are recycled back to the cytosol.

Galactosialidosis is one of at least 41 genetically distinct lysosomal storage diseases. In these disorders, some of the macromolecules in the lysosome cannot be degraded. Instead, these large molecules, or their partial-breakdown products, accumulate, and the lysosomes swell to the point that cellular function is disrupted.

Neuraminidase deficiency

Neuraminidase removes sialic acid from the ends of oligosaccharides, which are relatively short chains of sugars. Sialic acid, also known as N-acetylneuraminic acid, is a type of sugar molecule that often is at an end of an oligosaccharide. These oligosaccharides with terminal sialic acid residues may be attached to proteins, called glycoproteins.

Neuraminidase deficiency prevents the breakdown of oligosaccharides and glycoproteins that contain sialic acid and leads to the accumulation and excretion of these substances. It also can lead to the production of abnormal proteins. Following protein synthesis, some lysosomal enzymes reach the lysosome in an inactive form and require further processing for activation. One such processing step is the neuraminidase-catalyzed removal of sialic acid residues from oligosaccharides on enzymes. Thus, under conditions of neuraminidase deficiency, other lysosomal enzymes may not behave properly.

Protective protein/cathepsin A

PPCA is required for the transport of neuraminidase to the lysosome. Once inside the lysosome, the enzymatic activity of PPCA may be involved in the activation of neuraminidase. Furthermore, PPCA mediates the association of multiple molecules of neuraminidase and beta-galactosidase, as well as GALNS. In the absence of PPCA, all three enzymes are rapidly degraded in the lysosome. Thus, PPCA protects and stabilizes these enzyme activities. In the absence of PPCA, substrates for these enzymes may accumulate to dangerous levels.

Gangliosides are very complex components of cell membranes. They are made up of a long-chain amino alcohol called sphingosine, a long-chain fatty acid, and a very complex oligosaccharide that contains sialic acid. The lysosomal beta-galactosidase is responsible for hydrolyzing gangliosides.

GALNS catalyzes the first step in the lysosomal breakdown of a special type of sugar called keratan sulfate. Both gangliosides and keratan sulfate may accumulate in galactosialidosis.

In addition to its protective functions, PPCA has at least three enzymatic activities of its own, including the ability to cleave (break apart), or hydrolyze, other proteins. Some of the neurological abnormalities that develop with galactosialidosis may be due to the loss of this activity, particularly PPCAs ability to cleave endothelin-1. This peptide is overabundant and abnormally distributed in the neurons and glial cells of the brain and spinal cord of individuals with galactosialidosis.

Genetic profile

Galactosialidosis is an autosomal recessive disorder that can be caused by any one of a number of different mutations in the gene encoding PPCA. This gene is known as PPGB, for beta-galactosidase protective protein. The disorder is autosomal since the PPGB gene is located on chromosome 20, rather than on the X or Y sex chromosomes. The disorder is recessive because it only develops when both genes encoding PPCA, one inherited from each parent, are abnormal. However, the two defective genes do not need to carry the same mutations. If the two mutations are identical, the individual is a homozygote. If the two mutations are different, the affected individual is called a compound heterozygote.

PPCA mutations

The type of galactosialidosis and the severity of the symptoms depend on the specific mutations that are present. In general, the higher the level of PPCA activity in the lysosomes, the milder the characteristics of galactosialidosis, and the later the onset of disease.

With some mutations of the PPGB gene, very little of the precursor protein to PPCA is produced and there is no mature PPCA in the lysosome. With other mutations, the precursor protein may not be correctly processed into mature protein. Some individuals with severe early-infantile galactosialidosis carry mutations that prevent precursor PPCA from being targeted to the lysosome. The lysosomes of these individuals have no PPCA.

In contrast, individuals with the late-infantile form of galactosialidosis carry at least one mutant PPGB gene whose product can reach the lysosome. However, there may be only a small amount of PPCA in the lysosome; the PPCA may lack enzymatic activity; the PPCA chains may be unable to combine to form the normal twochained form; or the PPCA may be degraded rapidly. Nevertheless, with these mutations, the symptoms of galactosialidosis are mild and progress very slowly with no mental retardation.

Other identified mutations prevent the PPCA molecules from folding properly or shorten the PPCA protein so that it cannot form a complex with the other enzymes.

Compound heterozygotes, with different mutations in their PPGB genes, usually have symptoms that are intermediate in severity between those of homozygotes for each of the two mutations. Occasionally, the symptoms of a compound heterozygote may be more mild than those of either homozygote, because the two mutant PPCA proteins can complement, or compensate, for each other's abnormalities.

Demographics

As an autosomal recessive disorder, neuraminidase deficiency with beta-galactosidase deficiency occurs with equal frequency among males and females. Since it requires two defective copies of the PPGB gene, one inherited from each parent, it is much more common in the offspring of couples who are related to each other (consanguineous marriages), such as first or second cousins.

Galactosialidosis appears to occur with the highest frequency among Japanese. The juvenile/adult form is particularly common among Japanese and specific mutations in the PPGB gene occur with a high frequency in this population.

Signs and symptoms

Although the features of galactosialidosis vary greatly, they are very similar to those of neuraminidase deficiency (sialidosis). These progressive symptoms include red spots in the eyes, known as cherry-red macules. Eventually, the corneas may be become cloudy and cataracts and blindness may develop. Hearing loss is also common with galactosialidosis.

Myoclonus are sudden involuntary muscle contractions, which may eventually develop into myoclonic seizures. The myoclonus may become debilitating. Tremors and various other neurological conditions may develop. There may be a progressive loss of muscle coordination, called ataxia, and walking and standing may become increasingly difficult.

Small red skin lesions called angiokeratoma are signs of galactosialidosis. Swollen liver and spleen (hepatosplenomegaly) may develop. Cardiac disease can be one of the major consequences of the disorder.

Symptoms of the more severe forms of galactosialidosis include coarse or malformed facial features and a variety of skeletal malformations (dysostosis multiplex), including short stature. Mental retardation also may be present. Galactosialidosis is one cause of nonimmune hydrops fetalis , the excessive accumulation of fluid in the fetus.

Diagnosis

Early-infantile onset

Some findings of the disorder, including facial and skeletal abnormalities, may be apparent at birth. Skeletal x rays may be used to diagnose dysostosis multiplex. Magnetic resonance imaging (MRI) or computer tomography (CT) scans may be used to determine brain atrophy. An electroencephalogram (EEG) may indicate epileptic activity.

Neuraminidase activity

Typically, neuraminidase deficiency is diagnosed by measuring the activity of the enzyme in cultures of fibroblast cells (connective tissue cells) that have been grown from cells obtained by a skin biopsy. Neuraminidase activity usually is measured by testing the ability of fibroblast cell preparations to hydrolyze, or cleave, a synthetic compound such as 4-methylumbelliferyl-D-N-acetylneuraminic acid. Hydrolysis by neuraminidase liberates 4-methylumbelliferone, which is a compound with a fluorescence that can be measured accurately. The normal range of neuraminidase activity in fibroblasts is 95–653 picomoles per min per mg of protein. With galactosialidosis, neuraminidase activity in fibroblasts may be less than 4% of normal.

Beta-galactosidase activity

Beta-galactosidase activity in blood cells is measured in much the same way as neuraminidase activity in fibroblasts. Using the substrate 4-methylumbelliferyl-alpha-D-galactopyranoside, the fluorescence of 4-methylumbelliferone that is liberated through the action of beta-galactosidase is measured.

In severe forms of galactosialidosis, beta-galactosidase activity is less than 15% of normal and neuraminidase activity is less than 1% of normal. The combination of low beta-galactosidase and low neuraminidase in fibroblasts, with normal levels of other lysosomal enzymes, is diagnostic for galactosialidosis.

PPCA activity

The enzymatic activity of PPCA also can be measured in fibroblasts. In the early-infantile form of galactosialidosis, PPCA activity may be completely lacking. A small amount of PPCA activity (2–5% of normal) usually is present in the lysosomes of individuals with other forms of galactosialidosis. The highest levels of PPCA activity are associated with the least severe and later-onset forms of the disorder. Carriers with a single mutated PPGB gene may have only half of the normal level of PPCA activity, although they are without symptoms of the disorder.

Histology

In neuraminidase deficiency with beta-galactosidase deficiency, the lysosomes fill with sialyloligosaccharides and sialylglycopeptides (partially degraded proteins with sialyloligosaccharides still attached). These swollen lysosomes may form inclusion bodies and give cells a vacuolated appearance that is diagnostic of lysosomal storage disease.

Neuraminidase deficiency may be diagnosed by histological, or microscopic, examination of a number of different types of cells that may show this cytosolic vacuolation. These cells include the Kupffer cells of the liver, lymphocytes (white blood cells that produce antibodies), bone marrow cells, epithelial skin cells, fibroblasts, and Schwann cells, which form the myelin sheaths of nerve fibers.

Urine tests

Neuraminidase deficiency may be diagnosed by screening the urine for the presence of sialyloligosaccharides, using chromatography to separate the components of the urine on the basis of size and charge. In unaffected individuals, sialyloligosaccharides are cleaved by neuraminidase and, therefore, are present in the urine in only very low amounts. With neuraminidase deficiency, urine levels of sialyloligosaccharides may be three to five times higher than normal.

Sialylglycopeptides can be detected in the urine under conditions of neuraminidase deficiency. In neuraminidase deficiency with beta-galactosidase deficiency, keratan sulfate, which accumulates because of the low activity of GALNS, also can be identified in the urine.

Prenatal diagnosis

Galactosialidosis may be diagnosed prenatally. In at-risk fetuses, cultured fetal cells from the amniotic fluid (amniocytes), obtained by amniocentesis , or cultured chorionic villi cells, obtained by chorionic villi sampling (CVS) in the early weeks of pregnancy, may be tested for neuraminidase and beta-galactosidase activities. Furthermore, the enzymatic activities of PPCA can be measured in amniocytes and chorionic villi. PPCA activity is normally very high in these cells and low activity is an indication of an affected fetus. However, since carriers of a single mutated PPGB gene do not have symptoms of galactosialidosis, it may be difficult to recognize an at-risk fetus unless there is a family history of the disorder.

Treatment and management

At present, there is no treatment for neuraminidase deficiency with beta-galactosidase deficiency. Rather, attempts are made to manage individual symptoms. Myoclonic seizures, in particular, are very difficult to control. Bone marrow transplantation is being studied as a treatment for severe galactosialidosis.

KEY TERMS

Galactosialidosis

—The inherited disorder known as neuraminidase deficiency with beta-galactosidase deficiency.

Sialidosis

—An inherited disorder known as neuraminidase deficiency.

Prognosis

The prognosis for individuals with this disorder varies greatly depending on the specific genetic mutation, which determines the age of onset and severity of the disease. Individuals with mild forms of galactosialidosis may have nearly normal life expectancies. However, the early-infantile form of galactosialidosis usually results in death shortly after birth.

Resources

BOOKS

Saito, M., and R. K. Yu. "Biochemistry and Function of Sialidases." In Biology of the Sialic Acids. Edited by A. Rosenberg, 7–67. New York: Plenum Press, 1995.

Thomas, G. H., and A. L. Beaudet. "Disorders of Glycoprotein Degradation and Structure: Alpha-mannosidosis, Beta-mannosidosis, Fucosidosis, Sialidosis, Aspartylglucosaminuria and Carbohydrate-deficient Glycoprotein Syndrome." In The Metabolic andMolecular Bases of Inherited Disease. Edited by C. R. Scriver, A. L. Beaudet, W. S. Sly, and D. Valle, 2529–61. New York: McGraw Hill, Inc., 1995.

PERIODICALS

Hiraiwa, M. "Cathepsin A/Protective Protein: An Unusual Lysosomal Multifunctional Protein." Cellular and Molecular Life Sciences 56 (December 1999): 894–907.

ORGANIZATIONS

The International Society for Mannosidosis and Related Diseases. 3210 Batavia Avenue, Baltimore, MD 21214. (410) 254-4903. [email protected]. <http://www.mannosidosis.org>.

United Leukodystrophy Foundation. 2304 Highland Drive, Sycamore, IL 60178. (815) 895-3211. (800) 728-5483. [email protected]. <http://www.ulf.org/>.

WEBSITES

Murphy, Paul. "Lysosomal Storage Diseases: A Family Sourcebook." Human Genetic Disease: A Layman's Approach. <http://mcrcr2.med.nyu.edu/murphp01/lysosome/bill1a.htm>.

Margaret Alic, PhD