Maple Syrup Urine Disease in Cypriot Families: Identification of Three Novel Mutations and Biochemical Characterization of the p.Thr211 Met Mutation in the E1α Subunit

摘要

We report five mutations, three of them novel, responsible for maple syrup urine disease in four unrelated Cypriot families. The five children studied are the first cases of classic maple syrup urine disease to be reported among Cypriots. The first novel mutation identified is a single-base deletion in exon 6 of the Elα gene (c.718delG), which leads to a frameshift after Ala240 and to a stop codon 89 residues further downstream. The other two novel mutations identified are in the Elβ subunit: a two-base deletion in exon 6, c.662_663delCC, which leads to a frameshift after Ala221 and creates a stop codon 17 residues further downstream, as well as a splice mutation, IVS3[+3]delA, which results in the skipping of exon 3. The two known mutations identified are in the Elα gene: the G > C transversion at the 3'-splice acceptor site, (IVS5-1G > C), which results in the deletion of the entire exon 6, and the missense mutation in exon 5 (c.632C>T), which corresponds to a p.Thr211Met substitution. The p.Thr211Met substitution is located in a potassium-ion pocket in the El component required for stability of the bound cofactor thiamine diphosphate. The mutant El protein harboring the p.Thr211Met substitution was shown unable to bind thiamine diphosphate, leading to undetectable El activity.