GENOTYPE-PHENOTYPE CORRELATIONS IN MUCOPOLYSACCHARIDOSIS TYPE-I USINGENZYME-KINETICS, IMMUNOQUANTIFICATION AND IN-VITRO TURNOVER STUDIES

Fibroblasts from 16 patients with known alpha-L-iduronidase gene mutat ions and different clinical phenotypes of mucopolysaccharidosis type I (MPS I) were investigated in order to establish genotype/phenotype cor relations. Enzyme kinetic studies were performed using the specific al pha-L-iduronidase substrate iduronosyl anhydro[1-H-3]mannitol-6-sulfat e. Specific residual enzyme activities were estimated using the kineti c parameters and an immunoquantification assay which determines levels of alpha-L-iduronidase protein. Cells were cultured in the presence o f [S-35]sulfate and the in vivo degradation of accumulated labelled gl ycosaminoglycans measured after different chase times. Residual enzyme activity and different amounts of residual enzyme protein were presen t in extracts from 9 of 16 cell lines covering a wide spectrum of clin ical severity. Catalytic capacity, calculated as the product of k(cat) /K-m and ng iduronidase protein per mg cell protein, was shown in most cases to be directly related to the severity of clinical phenotype, w ith up to 7% of normal values for patients with the attenuated form of MPS I (Scheie) and less than 0.13% for severely affected patients (Hu rler). In vitro turnover studies allowed further refinement of correla tions between genotype and phenotype. Scheie disease compared to Hurle r disease patients were shown to accumulate smaller amounts of glycosa minoglycans that were also turned over faster. A combination of turnov er and residual enzyme data established a correlation between the geno type, the biochemical phenotype and the clinical course of this lysoso mal storage disorder. (C) 1998 Elsevier Science B.V. All rights reserv ed.