Mucopolysaccharides (i..XaMe..3.0-4 ) are constituents of connective tissue composed of alternating units of polymerized hexuronic acids and hexose amines. As a group, mucopolysaccharidoses are characterized by somatic dysplasia, slowly deteriorating mentation and motor control, storage of mucopolysaccharide within cellular lysosomes, and urinary excretion of heparin sulfate, dermatan sulfate, or related compounds. It is generally agreed that the causative defect in the mucopolysaccharidoses (MPS) involves lysosomal hydrolysis. Because of faulty breakdown of glycosaminoglycans, these products are stored within the lysosomes, distending the lysosome and the cell body. Mucopolysaccharides represent the major component of the lysosomal storage material, particularly in fibroblasts. Because the brain is also a prime storage site, mental retardation is a prominent feature of all MPS. Ihere are 13 subclasses of MPS (see Table..30:4: ).[2z'

Ihe mucopolysaccharidoses are inherited as autosomal recessive diseases, with the exception of Hunter's syndrome, which is sex-linked. Ihe global frequency of the varieties of MPS has not been clearly established. Population studies in the Netherlands indicated that Sanfilippo's syndrome type A is the most common form, with an estimated frequency of 1:24,000 in that population. y In British Columbia, the incidence of Hunter's syndrome was estimated at 1:78,000, y but in Israel its frequency may be higher. Sanfilippo's syndrome type B is seen particularly often in Greece, y and in the Cayman Islands a high frequency of Sanfilippo's syndrome type A has been found, with an estimated carrier frequency of 1:10.

As a group, these diseases are marked by striking somatic dysplasia, slowly deteriorating neurological and systemic symptoms, storage of mucopolysaccharides in the lysosomes, and excretion of mucopolysaccharides in the urine (see Iabje.30-4 ). Although each type has a specific enzyme deficit, the similar spectrum of clinical manifestations in all MPS disorders makes biochemical differentiation essential. For many reasons, Hurler's disease has become the prototypical MPS. Patients who have this syndrome in early infancy may appear reasonably normal, but by 6 months of age it is obvious that a severe disorder is present. Ihe abnormal facial appearance is one of the first signs noted, and hepatosplenomegaly and umbilical and inguinal hernias are soon detected. Affected infants may have chronic rhinorrhea associated with frequent colds, recurrent airway infections, and otitis media. When children with Hurler's disease attempt to sit, a characteristic kyphoscoliosis is often observed, which progresses with time. Ultimately, a frank gibbus deformity, one of the earliest described clinical features of the disease, develops. Vision is often impaired in Hurler's disease because of corneal clouding.

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