Structural basis of the GM2 gangliosidosis B variant.

To study the structural basis of the GM2 gangliosidosis B variant, we constructed the three-dimensional structures of the human beta-hexosaminidase alpha-subunit and the heterodimer of the alpha- and beta-subunits, Hex A, by homology modeling. The alpha-subunit is composed of two domains, domains I and II. Nine mutant models due to specific missense mutations were constructed as well and compared with the wild type to determine structural defects. These nine mutations were divided into five groups according to structural defects. R178H is deduced to affect the active site directly, because R178 is important for binding to the substrate. C458Y and W420C are predicted to cause drastic structural changes in the barrel structure carrying the active site pocket. R504C/H is deduced to introduce a disruption of an essential binding with D494 in the beta-subunit for dimerization. R499C/H, located in an extra-helix, is deduced to disrupt hydrogen bonds with domain I and the barrel. R170W and L484P are deduced to affect the interface between domains I and II, causing destabilization. The structural defects reflect the biochemical abnormalities of the disease.

Molecular analysis of the alpha-N-acetylglucosaminidase gene in seven Japanese patients from six unrelated families with mucopolysaccharidosis IIIB (Sanfilippo type B), including two novel mutations.

Molecular analysis of the alpha-N-acetylglucosaminidase gene in seven Japanese patients with Sanfilippo syndrome type B from six unrelated families was carried out, and six disease-causing mutations were found. The parents of Patient 2 had a consanguinous marriage, but other families did not have any record of consanguinity. Two families were from Okinawa Island, where more patients with Sanfilippo syndrome were found than in other areas in Japan. Patients 1 and 6 showed the most severe phenotype with rapid progression. Patients 2, 5, and 7 were moderate. Patients 3 and 4 (sib cases) showed an attenuated form compared with other patients. Patients 1, 2, and 6 were homozygous for R482W, R565W, and R565P, respectively. Patients 3 and 4 were compound heterozygous for F314L and R565P. Patient 5 had delTG2171-2172 in exon 6 in one allele, and the other allele was unknown. Patient 7 was compound heterozygous for V241M and R482W. The family of Patients 3 and 4 and that of Patient 6 are unrelated, although both families are from Okinawa Island, and the patients have the same mutation, R565P; thus, R565P might be a common mutation in the Okinawa district. F314L and V241M are novel mutations.