Mucopolysaccharidosis II (Hunter Syndrome)

Mucopolysaccharidosis type II or Hunter syndrome is the only mucopolysaccharide storage disease to be inherited as an X-linked recessive disorder. Deficiency of the lysosomal enzyme iduronate-2-sulfatase (IDS; EC 3.1.6.13) is the primary defect in MPS I. As is seen with most X-linked potentially lethal conditions, allelic heterogeneity is significant and thus most families have individual or “private” mutations at the iduronate-2-sulfatase locus. In addition, unlike other LSDs, there is considerable heterogeneity in the molecular mechanisms underlying iduronate-2-sulfatase gene alteration in this condition. As such, this disorder, as with most of the MPSs, shows considerable clinical heterogeneity with the clinical phenotype spanning a spectrum of disease severity from early-onset childhood lethal disease to attenuated disease leading to a near normal life expectancy (Neufeld and Muenzer, 2001). In addition, patients with severe MPS II can have atypical symptomatology which likely relates to more complex genomic rearrangements underlying the disease pathogenesis. The incidence of MPS II appears to be variable in populations which have been studied, and range from 1 in 165,000 (Western Australia) to 1 in 34,000 (Israel) male live births; issues related to incomplete ascertainment in populations is noted in most studies (Lowry and Renwick, 1971; Schaap and Bach, 1980; Lowry et al., 1990; Poorthuis et al., 1999; Nelson, 1997; Meikle et al., 1999; Nelson et al., 2003). Therefore it is likely that MPS II is a panethnic disorder with an incidence in the order of 1 in 75,000 male live births. Increased incidence is expected to be seen in small populations where a founder effect would be anticipated.