OBJECTIVEMucopolysaccharidosis (MPS) types IIIA, B, C, D are a group of autosomal recessive lysosomal storage disorders caused by mutations in one of four genes which encode enzyme activities required for the lysosomal degradation of heparan sulfate. MPSIIIA and MPSIIIB involve deficiencies of heparan N-sulfatase (SGSH) and alpha-N-acetylglucosaminidase (NAGLU). MPS IIIA and MPS IIIB are more common than MPS IIIC and IIID. The present study aimed to establish two enzyme assay methods for SGSH and NAGLU activities for carrying out postnatal and prenatal diagnosis of MPSIIIA and IIIB by means of SGSH and NAGLU activity assay on plasma, leukocyte, uncultured chorionic villi (CV) and cultured amniotic fluid cells (AF cell) using two newly synthesized substrates. Mutation analysis of SGSH gene was also performed.

METHODSTwo fluorigenic substrate (4-methylumbelliferyl-alpha-D-N-sulphoglucosaminide.Na and 4-methylumbelliferyl-alpha-N-acetylglucosaminide) were used for the assay of SGSH and NAGLU activity. SGSH activity in leukocyte was determined for diagnosis MPSIIIA proband. NAGLU activity was determined in plasma for diagnosis of MPSIIIB proband. Twelve cases with MPS III were enrolled in this study, 4 were female and 8 were male, age 3 - 10 years and were from 10 unrelated families. Eight exons of SGSH gene were amplified by PCR. The mutations of the patients were characterized by direct sequencing of the amplified DNA fragments. Prenatal diagnosis in 3 pregnancies at risk was carried out according to NAGLU activity on uncultured CV at 11th week or on cultured AF cell at 18th week of gestation.

RESULTSThe SGSH activities in leukocyte of normal controls were 4.4 - 8.1 nmol/(17 h.mg protein). The NAGLU activity in plasma of normal controls was 33.3 - 62.4 nmol/(4 h.ml). The NAGLU activities were 44.9 - 91.7 nmol/(17 h.mg protein) and 53.2 - 82.2 nmol/(17 h.mg protein) in CV and cultured AF cells respectively. Five cases of MPS IIIB and 7 cases of MPS IIIA were diagnosed. The mutation analysis of SGSH gene showed 6 mutations (G191R, D235N, R377C, E447K, R233X and D219Wfs264X), only one of which (D219Wfs264X) has not been previously reported. Prenatal diagnosis was performed on 3 pregnancies at risk. NAGLU activity of one affected fetus was 1.5 nmol/(17 h.mg protein) in AF cell.

CONCLUSIONSThe method using synthesized fluorigenic 4-methylumbelliferyl-substrates were sensitive, rapid and convenient assay of SGSH and NAGLU activity and were reliable for early prenatal diagnosis. Mutation analysis on MPS IIIA patients suggests new possibilities for molecular diagnosis of the disease.