CLINICAL FINDINGS AND BIOCHEMICAL AND MOLECULAR ANALYSIS OF FOUR PATIENTS WITH HOLOCARBOXYLASE SYNTHETASE DEFICIENCY

Holocarboxylase synthetase (HLCS) deficiency (HLCSD) is a rare autosomal recessive disorder of biotin metabolism. HLCS catalyzes the biotinylation of the four human biotin-dependent carboxylases. Using the newly available human genomic sequence, we report the map of HLCS genomic structure and the predicted exon/intron boundaries. Moreover, the molecular studies of four patients (two Italians, one Iranian, and one Australian) affected by HLCS deficiency are here reported. The clinical findings, the age of onset, and response to biotin treatment differed between our patients. The diagnosis was made by organic acid analysis and confirmed by enzymatic analysis in three patients. Six mutations in the HLCS gene were identified, including two novel (N511K and G582R) and four known missense mutations (L216R, R508W, V550M, and G581S). Five of the mutations are localized within the HLCS biotin-binding domain, whereas the L216R amino acid change is located in the N-terminal region outside of the putative biotin-binding domain. This mutation, previously reported in a heterozygous state, was detected for the first time in a patient with homozygous status. The patient's severe clinical phenotype and partial responsiveness to biotin support a genotype-phenotype correlation through the involvement of residues of the N-terminal region in a substrate specificity recognition or regulation of the HLCS enzyme.