Mutations in theMMAAgene in patients with thecblAdisorder of vitamin B₁₂metabolism

Abstract

Communicated by Arnold MunnichMutations in the MMAA gene on human chromosome 4q31.21 result in vitamin B 12 -responsive methylmalonic aciduria (cblA complementation group) due to deficiency in the synthesis of adenosylcobalamin. Genomic DNA from 37 cblA patients, diagnosed on the basis of cellular adenosylcobalamin synthesis, methylmalonylcoenzyme A (CoA) mutase function, and complementation analysis, was analyzed for deleterious mutations in the MMAA gene by DNA sequencing of exons and flanking sequences. A total of 18 novel mutations were identified, bringing the total number of mutations identified in 37 cblA patients to 22. A total of 13 mutations result in premature stop codons; three are splice site defects; and six are missense mutations that occur at highly conserved residues. Eight of these mutations were common to two or more individuals. One mutation, c.433C4T (R145X), represents 43% of pathogenic alleles and a common haplotype was identified. Restriction endonuclease or heteroduplex diagnostic tests were designed to confirm mutations. None of the sequence changes identified in cblA patients were found in 100 alleles from unrelated control individuals.

Crystal Structure and Mutagenesis of the Metallochaperone MeaB

“…6A). This rather unusual arrangement of 4 arginine residues, which are charge-neutralized by forming salt bridges with nearby acidic amino acids, may be critical in stabilizing interactions between the G domain and the C-terminal dimerization region, and might explain why mutation of either residue to glutamine is pathogenic (34). Expression of the R57Q and R272Q proteins appear to confirm this as both mutants are expressed in inclusion bodies (data not shown).…”

“…The significance of the auxiliary function of MMAA is evident in that mutations in this protein lead to the birth defect, type A MMA (13,34). Recent studies indicate that MeaB function is to promote the activity of MCM (7,9,10), perhaps by assisting in the loading of coenzyme B 12 into MCM, and/or protecting the B 12 cofactor from oxidative damage during MCM turnover (7,9).…”

In vitamin B 12 deficiency, higher serum folate is associated with increased total homocysteine and methylmalonic acid concentrations

“…This is then associated with the mutase by the action of adenosyltransferase. A mitochondrial protein, methylmalonic aciduria cblA type protein (MMAA), associates with the mutase to protect the mutase from inactivation (26).…”

Structures of the Human GTPase MMAA and Vitamin B12-dependent Methylmalonyl-CoA Mutase and Insight into Their Complex Formation

“…To date, eight hMMAA missense mutations have been reported (6,29,38,39), all of which were characterized biochemically in MeaB (15) except the recently identified homozygous G188R mutation (29). Unlike other hMMAA mutations shown to affect protein stability (15), hMMAA G188R can be expressed recombinantly as a soluble homodimer, and exhibits near wild-type GTPase activity, suggesting the mutation has no deleterious effects on its structure and stability.…”