ARG538 TO CYS MUTATION IN A CPG DINUCLEOTIDE OF THE HUMAN BIOTINIDASEGENE IS THE 2ND MOST COMMON-CAUSE OF PROFOUND BIOTINIDASE DEFICIENCY IN SYMPTOMATIC CHILDREN

Biotinidase deficiency is an autosomal recessively inherited disorder in the recycling of the vitamin biotin, The most common mutation that causes profound biotinidase deficiency in symptomatic individuals is a deletion/insertion (G(98):d7i3) that occurs in exon B of the biotinid ase gene. We now report the second most common mutation, a C-to-T subs titution (position 1612) in a CpG dinucleotide in exon D of the biotin idase gene. This mutation results in the substitution of a cysteine fo r arginine538 (designated R538C) and was found in 10 of 30 symptomatic children with profound biotinidase deficiency, 5 of whom also have th e G(98):d7i3 mutation. This mutation was not found in DNA samples from 32 individuals with normal biotinidase activity, but was found in one individual with enzyme activity in the heterozygous range. This mutat ion;was not detected in 371 randomly selected, normal individuals usin g allele-specific oligonucleotide hybridization analysis. Aberrant bio tinidase protein was not detectable in extracts of fibroblasts from a child who is homozygous for the R538C mutation, but was present in les s than normal concentration in identical extracts treated with beta-me rcaptoethanol. Because there is no detectable biotinidase protein in s era of children who are homozygous for the R538C mutation and in combi nation with the deletion/insertion mutation, the R538C mutation likely results in inappropriate intra- or intermolecular disulfide bond form ation, more rapid degradation of the aberrant enzyme, and failure to s ecrete the residual aberrant enzyme from the cells into blood.