Mucopolysaccharidosis type III is a lysosomal disorder caused by an impaired ability of lysosomes to degrade heparan sulphate and heparin owing to deficiency of one of the four enzymes normally involved in this process. Each of the four clinically quite similar but biochemically distinguished subtypes, A, B, C, and D, are inherited in an autosomal recessive manner. Symptoms, that become apparent between 2 and 6 years of age, include delayed speech development, sleep disturbance, and behavioral abnormalities like hyperactivity and aggressiveness. The disease leads to severe central nervous system degeneration with death occurring usually between the second and third decade. Mucopolysaccharidosis type III differs from other mucopolysaccharidoses in that patients usually exhibit only mild somatic changes, especially skeletal changes being generally minimal.
Mucopolysaccharidosis type IIIB is caused by deficiency of the lysosomal enzyme a-N-acetylglucosaminidase, NAG. This enzyme catalyses the removal of terminal a-N-acetylglucosamine residues from heparan sulphate. In the absence of NAG, partially degraded heparan sulphate accumulates in tissues and is excreted in the urine.