MOLECULAR AND BIOCHEMICAL BASIS OF GALACTOSEMIA

Galactosemia is a clinically heterogeneous autosomal recessive inborn error of metabolism caused by deficiency of galactose-l-phosphate urid ylyltransferase (GALT). Despite the numerous point mutations identifie d in the GALT gene, the prevalence of these mutations in different eth nic groups has not been studied. Reports on genotype/phenotype correla tion are not consistent due to the small sample sizes studied and the lack of a sensitive enzyme assay. We applied multiplex PCR/ASO dot blo t analysis to screen 293 galactosemic patients for 17 known point muta tions in exons 5, 6, and 10. Our data demonstrate that only 7 of these mutations were detected in our patients, accounting for 65% of the GA LT mutant alleles. Although Q188R is the most common mutation in Cauca sian and Hispanic patients, the S135L mutation is most common in Afric an-Americans. Another mutation, F171S, was observed only among African -American patients. An improved, sensitive, and accurate method was us ed to measure GALT activity in patient's red blood cells. The results indicated that patients homozygous for Q188R have no enzyme activity w hile those homozygous for S135L had residual enzyme activity. Interest ingly, both Q188R/S135L and S135L/F171S compound heterozygotes demonst rated zero enzyme activity. Overall, 85% of Q188R compound heterozygot es also did not have any enzyme activity, whereas the remaining Q188R and the majority of S135L compound heterozygotes expressed variable am ounts of GALT activity. We speculate that heterodimeric subunit intera ction plays an important role in determining the overall enzymatic act ivity. Various genotypes thus result in biochemical and clinical heter ogeneity among the patients. (C) 1998 Academic Press.