Molecular, biochemical, and structural analysis of a novel mutation in patients with methylmalonyl-CoA mutase deficiency.

BACKGROUND: Methylmalonic aciduria (MMA) is an inborn error of metabolism resulting from genetic defects in methylmalonyl-CoA mutase (MCM). This enzyme is encoded by the MUT gene and is required for the degradation of odd-chain fatty acids, the amino acids valine, isoleucine, methionine, and threonine, and cholesterol.

METHOD: Three unrelated affected patients with isolated MMA and their parents were studied. The MUT gene was analyzed by PCR and sequencing of its entire coding region and the highly conserved exon-intron splice junctions. The homology modeling of the novel mutation found in the MUT gene was performed using the online Swiss-Prot server for automated modeling and then analyzed with special bioinformatics software to better study the structural effects caused by the mutation.

RESULT: We found one homozygous nucleotide change in intron 12 of the MUT gene (c.2125-3 C>G). The variant is located near the highly conserved acceptor splice site of intron 12. A region at the C-terminus of the protein from ASP709 to GLN748 has been deleted by the alteration of c.2125-3 C>G in intron 12 of the MUT gene. Further studies of the novel mutation in the MUT gene by means of homology modeling revealed abnormalities in the protein's structure, which causes the protein to act malfunctioning and also the mRNA expression analysis of MUT gene confirmed these results.

CONCLUSION: We report this novel mutation, including its clinical and biochemical features and genetic defects, in the MUT gene of three patients affected with isolated MMA. Structural analyses of the mutated protein identified changes in the energy and stereochemical features of the protein that unfortunately altered the protein's functionalities. Therefore, we demonstrate that a novel splice site mutation in intron 12 of the MUT gene is a potential highly pathogenic allele via inhibition of alternative splicing.