Neuraminidase deficiency, or sialidosis, is a rare inherited metabolic disorder with multiple symptoms that can include skeletal abnormalities and progressive neurological degeneration.
Neuraminidase deficiency is caused by a mutation, or change, in the NEU1 gene that codes for the lysosomal enzyme alpha-N-acetylneuraminidase, or neuraminidase for short. This enzyme sometimes is referred to as sialidase. It is also sometimes called N-acetyl-neuraminic acid hydrolase. The disorder is manifested in one of two forms, known as sialidosis types I and II. Sialidosis type I is the milder form of the disorder, with symptoms typically appearing during adolescence. It is known as the non-dysmorphic or normophormic form of sialidosis. Sialidosis type II is the more severe form of neuraminidase deficiency, with symptoms developing in the fetus, at birth, or during infancy or early childhood. It is known as the dysmorphic form of sialidosis.
Over the years, this disorder has been called by a number of different names, in addition to neuraminidase deficiency, alpha-neuraminidase deficiency, sialidase deficiency, and sialidosis. It sometimes is known as cherry-red spot and myoclonus syndrome, cherry-red spot myoclonus epilepsy syndrome, or myoclonus and cherry-red spot syndrome, in reference to characteristic symptoms of the disorder. Other names include glycoprotein neuraminidase deficiency, NEUG deficiency, NEU or NEU1 deficiency, and neuraminidase 1 deficiency. Sialidosis type I sometimes is referred to as juvenile sialidosis and type II as infantile sialidosis, in reference to the age of onset.
Lysosomal storage diseases
Lysosomes are membrane-bound spherical compartments or vesicles within the cytosol, the semi-fluid areas 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, which are 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 lysosomes are recycled to the cytosol.
Neuraminidase deficiency is one of at least 41 genetically-distinct lysosomal storage diseases. These disorders result from mutations in the genes encoding the hydrolytic enzymes of the lysosome. In these disorders, some of the macromolecules in the lysosomes cannot be degraded and they, or their partial-breakdown products, accumulate there. The lysosomes swell to the point where cellular function is disrupted.
Neuraminidase deficiency, particularly sialidosis type II, commonly has been classified as the lysosomal storage disease called mucolipidosis type I (ML I), formerly lipomucopolysaccharidosis. This is because the symptoms of neuraminidase deficiency are similar to various mucolipidosis disorders. However mucolipidoses are characterized by the accumulation of large and complex lipid-polysaccharides. In contrast, neuraminidase deficiency leads to the accumulation of specific types of short chains of sugar called oligosaccharides and of certain proteins with oligosaccharides attached to them, called glycoproteins. Thus, it may be more appropriate to classify neuraminidase deficiency as an oligosaccharide storage disease, since it leads to the accumulation of excess oligosaccharides in various tissues throughout the body and the excretion of oligosaccharides.
Neuraminidase, or sialidase, is a type of enzyme known as an exoglycosidase because it cleaves terminal sugar units, or residues, off oligosaccharides. Specifically, neuraminidase cleaves, or hydrolyzes, terminal sialic acid residues. Sialic acid, also known as N-acetylneuraminic acid, is a type of sugar molecule that often is at an end of an oligosaccharide. The oligosaccharides with sialic acid residues may be attached to proteins (glycoproteins). Therefore, neuraminidase deficiency prevents the proper breakdown of oligosaccharides and glycoproteins that contain sialic acid and the disorder is characterized by the accumulation and excretion of these substances.
In addition to interfering with the lysosomal breakdown of sialic acid compounds, neuraminidase deficiency can lead to 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 the enzymes. Lysosomal hydrolases that require further processing by neuraminidase include acid phosphatase, alpha-mannosidase, arylsulfatase B, and alpha-glucosidase.
Under conditions of neuraminidase deficiency, sialyloligosaccharides accumulate in various cells, including lymphocytes (white blood cells that produce antibodies), fibroblasts (connective tissue cells), bone marrow cells, Kupffer cells of the liver, and Schwann cells, which form the myelin sheaths of nerve fibers. Furthermore, proteins with sialic acid attachments accumulate and can be detected in fibroblasts and in the urine.
Neuraminidase exists in the lysosome in a high-molecular-weight complex with three other proteins: the enzyme beta-galactosidase, the enzyme N-acetylgalactosamine-6-sulfate sulfatase (GALNS), and a multi-functional enzyme called protective protein/cathepsin A (PPCA). Neuraminidase must be associated with PPCA in order for the neuraminidase to reach the lysosome. Once inside the lysosome, PPCA mediates the association of as many as 24 neuraminidase molecules to form active neuraminidase. The active enzyme remains associated with PPCA and beta-galactosidase, which appear to be necessary for protecting and stabilizing the neuraminidase activity. A distinct lysosomal storage disease, neuraminidase deficiency with beta-galactosidase deficiency, or galactosialidosis, results from mutations in the gene encoding PPCA. In this disorder, both neuraminidase and beta-galactosidase are deficient.
Inheritance of neuraminidase deficiency
Neuraminidase deficiency is an autosomal recessive disorder that can be caused by any one of a number of different mutations in the NEU1 gene encoding the lysosomal neuraminidase. The disorder is autosomal because the NEU1 gene is located on chromosome 6, rather than on the X or Y sex chromosomes . The disorder is recessive because it only develops when both genes encoding neuraminidase, one inherited from each parent, are defective; however, the two defective NEU1 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. Individuals with one defective gene and one normal gene encoding neuraminidase may have reduced levels of the active enzyme, but they do not have symptoms of neuraminidase deficiency.
All of the offspring of two parents with neuraminidase deficiency will inherit the disorder. All of the offspring of one parent with neuraminidase deficiency and one parent with a single defective NEU1 gene will inherit at least one defective NEU1 gene. They will have a 50% chance of inheriting two defective genes and, therefore, developing neuraminidase deficiency. The offspring of one parent with neuraminidase deficiency and one parent with normal NEU1 genes will inherit a defective gene from the affected parent, but will not develop neuraminidase deficiency. The offspring of parents who both carry one defective NEU1 gene have a 50% chance of inheriting one defective NEU1 gene and a 25% chance of inheriting two genes and developing neuraminidase deficiency. Finally, the children of one parent with a single defective NEU1 gene and one parent with normal NEU1 genes will have a 50% chance of inheriting the defective gene, but will not develop neuraminidase deficiency.
Mutations in the NEU1 gene
A number of different mutations that can cause neuraminidase deficiency have been identified in the NEU1 gene. The type of neuraminidase deficiency, sialidoses types I or II, as well as the severity of the symptoms, depends on the specific mutation(s) that are present. Some mutations change one amino acid out of the 415 amino acids that compose a single neuraminidase molecule. Other identified mutations result in a shortened enzyme. Many of the identified mutations are clustered in one region on the surface of the protein. These mutations result in a sharp reduction in the activity of the enzyme and lead to the rapid degradation of neuraminidase inside the lysosome.
Some mutations in the NEU1 gene lead to a complete absence of neuraminidase activity, with little or no neuraminidase enzyme present in the lysosomes. These mutations usually result in the severe, infantile-onset, type II sialidosis. Other mutations result in an inactive protein that is present in the lysosome. These mutations generally result in juvenile-onset, type II sialidosis, with symptoms of intermediate severity. Some mutations significantly reduce, but do not obliterate, neuraminidase activity in the lysosome. Individuals with at least one mutated gene of this type are not completely neuraminidase-deficient and have mild, type I sialidosis. Occasionally, individuals have multiple mutations in the NEU1 gene, leading to more severe forms of neuraminidase deficiency.
Neuraminidase deficiency is an extremely rare disorder. Because of its similarities to many other disorders, it has been difficult to determine its frequency. In the United States, it is estimated to occur in one out of every 250,000 live births. In Australia, the estimate is one out of 4.2 million. Since neuraminidase deficiency is an autosomal rather than a sex-linked disorder, it occurs equally in males and females.
As an autosomal recessive disorder, neuraminidase deficiency requires two copies of the defective gene, one inherited from each parent. Thus, neuraminidase deficiency is much more common in the offspring of couples who are related to each other (consanguineous marriages), such as first or second cousins.
Sialidosis type I appears to be more common among Italians. Type 2 sialidosis seems to occur more frequently among Japanese.
Signs and symptoms
The clinical symptoms of neuraminidase deficiency are similar to the symptoms of the mucolipidoses, including I-cell disease (mucolipidosis II) and pseudoHurler polydystrophy (mucolipidosis III). Furthermore, the clinical distinctions between sialidoses types I and II may not be clearly defined.
Sialidosis type I
The symptoms of sialidosis type I do not appear until the second decade of life. Infants and children with this form of neuraminidase deficiency may have a normal appearance and grow normally until adolescence. At that time, the appearance of red spots in both eyes, known as cherry-red macules or cherry-red macular spots, may be one of the first symptoms of neuraminidase deficiency. Eventually, color and/or night blindness may develop. Cataracts may occur and vision may deteriorate gradually into blindness.
Other symptoms of sialidosis type I include myoclonus. These are sudden involuntary muscle contractions, which may eventually develop into myoclonic seizures. The myoclonus may become debilitating, even in sialidosis type I. Individuals with this form of neuraminidase deficiency may have increased deep tendon reflexes and may develop tremors and various other neurological conditions. There may be a progressive loss of muscle coordination, called ataxia, and walking and standing may become increasingly difficult. Speech problems, such as slurring, may develop.
The above symptoms also may occur in sialidosis type II. However, in addition to the age of onset, type I can be distinguished from type II by the absence of skeletal and facial abnormalities. Furthermore, individuals with this form of neuraminidase deficiency have normal intelligence.
Sialidosis type II
Sialidosis type II has three forms: congenital or neonatal, with symptoms present at or before birth; infantile, with symptoms developing at birth or during the first year of life; and juvenile, with symptoms developing between the ages of two and twenty.
Symptoms of sialidosis type II vary from mild to severe, but are always more severe than in type I sialidosis. With neonatal onset, infants may be born with ascites (accumulation of fluid in the abdominal cavity), swollen liver and spleen (hepatosplenomegaly), hernia of the umbilicus or the groin, and other abnormalities. With severe forms of the disorder, children may die in infancy. With milder forms, they may show no symptoms for the first ten years of life. Thus, ascites, hepatosplenomegaly, and hernias may develop later. Children with neuraminidase deficiency may grow abnormally fast. Cherry-red macules, myoclonus, and other neurological abnormalities, including tremors, may be present. The myoclonus may progress into a form of epilepsy. These children may have mild to severe mental retardation.
Sialidosis type II is characterized by a variety of skeletal malformations (dysostosis multiplex). Obvious symptoms may include distinctive, coarse facial features (called coarse facies), a short trunk with relatively long legs and arms, and a prominent breast bone (pectus carinatum). In addition, there may be a lack of muscle tone and strength (hypotonia) and the progressive wasting of muscular tissue.
The hearing may be affected in sialidosis type II. Individuals may have difficulty breathing (dyspnea). Cardiac problems may develop and severe congenital sialidosis type II apparently can result in severely-dilated coronary arteries. Loose bowel movements are common with this form of neuraminidase deficiency.
Typically, neuraminidase deficiency is diagnosed by measuring the activity of the enzyme in cultures of fibroblast cells that have been grown from cells obtained via a skin biopsy. Lysosomal neuraminidase also can be measured in leukocytes (white blood cells). However, human cells have two other types of neuraminidase, encoded by different genes. One of these enzymes is present in the cell membrane and the other is in the cytosol of various cells, including leukocytes. These enzymes are not deficient in sialidosis and their activities can interfere with the measurement of lysosomal neuraminidase.
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. Neuraminidase is an unstable enzyme and special precautions are needed to test for its activity. The normal range of neuraminidase activity in fibroblasts is 95-653 picomoles per minute per milligram of protein. In leukocytes, the normal range is 6-60 picomoles per minute per milligram of protein. Levels of active neuraminidase are much lower in sialidosis type II as compared with type I.
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, or partiallydegraded proteins with sialyloligosaccharides still attached, also can be detected in the urine under conditions of neuraminidase deficiency.
Neuraminidase deficiency and other lysosomal storage diseases interfere with the normal lysosomal breakdown of cellular components. As a result, the lysosomes may fill up with large molecules that are only partially digested. In the case of neuraminidase deficiency, the lysosomes fill up with sialyloligosaccharides and sialylglycopeptides. 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, bone marrow cells, epithelial skin cells, and fibroblasts.
Sialidosis type II
Infants with sialidosis type II often have visual symptoms of the disorder at birth, including facial and skeletal abnormalities. Skeletal x rays may be used to diagnose the dysostosis multiplex of this type of neuraminidase deficiency. Magnetic resonance imaging (MRI) may be used to determine brain atrophy.
Neuraminidase deficiency may be diagnosed prenatally. In at-risk fetuses, cultured fetal cells from the amniotic fluid, obtained by amniocentesis , or cultured chorionic villi cells, obtained by chorionic villi sampling in the early weeks of pregnancy, may be tested for neuraminidase activity. Since carriers of a single mutated NEU1 gene do not have symptoms of neuraminidase deficiency, 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. Rather, attempts are made to manage individual symptoms. Myoclonic seizures, in particular, are very difficult to control.
Individuals with sialidosis type I may have a near-normal life expectancy. However, the myoclonus may be progressively debilitating and myoclonic seizures can be fatal. Children with neonatal-onset sialidosis type II usually are stillborn or die at a young age. Those with infantile-onset sialidosis type II rarely survive through adolescence.
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Canadian Society for Mucopolysaccharide and Related Diseases. PO Box 64714, Unionville, ONT L3R OM9. Canada (905) 479-8701 or (800) 667-1846. <http://www.mpssociety.ca>.
National MPS Society. 102 Aspen Dr., Downingtown, PA 19335. (610) 942-0100. Fax: (610) 942-7188. info @mpssociety.org. <http://www.mpssociety.org>.
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Margaret Alic, PhD