The molecular basis of human 3-methylcrotonyl-CoA carboxylase deficiency
Matthias R. Baumgartner, Shlomo Almashanu, Terttu Suormala, Cassandra Obie, Robert N. Cole, Seymour Packman, E. Regula Baumgartner, David Valle
Matthias R. Baumgartner, Shlomo Almashanu, Terttu Suormala, Cassandra Obie, Robert N. Cole, Seymour Packman, E. Regula Baumgartner, David Valle
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The molecular basis of human 3-methylcrotonyl-CoA carboxylase deficiency

Abstract

Isolated biotin-resistant 3-methylcrotonyl-CoA carboxylase (MCC) deficiency is an autosomal recessive disorder of leucine catabolism that appears to be the most frequent organic aciduria detected in tandem mass spectrometry–based neonatal screening programs. The phenotype is variable, ranging from neonatal onset with severe neurological involvement to asymptomatic adults. MCC is a heteromeric mitochondrial enzyme composed of biotin-containing α subunits and smaller β subunits. Here, we report cloning of MCCA and MCCB cDNAs and the organization of their structural genes. We show that a series of 14 MCC-deficient probands defines two complementation groups, CG1 and 2, resulting from mutations in MCCB and MCCA, respectively. We identify five MCCA and nine MCCB mutant alleles and show that missense mutations in each result in loss of function.

Authors

Matthias R. Baumgartner, Shlomo Almashanu, Terttu Suormala, Cassandra Obie, Robert N. Cole, Seymour Packman, E. Regula Baumgartner, David Valle

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MCCA and B missense mutations that alter splicing. (a) MCCA D532H. The 1...
MCCA and B missense mutations that alter splicing. (a) MCCA D532H. The 1594G→C transversion of the last bp of exon 13 results in the missense mutation D532H. The 3′ base of an exon also contributes to donor splice site recognition and, as shown in the lower panel, RT-PCR of MCCA cDNA in this patient with primers corresponding to the 5′ and 3′ UTR resulted in a product smaller than in wild-type. Sequence analysis of this product showed that exon 13 (217 bp) is skipped, which shifts the reading frame. Thus, the deleterious consequences of this missense mutation appear to be entirely due to the splicing defect. (b) MCCB I437V. As shown in the upper panel, the 1309A→G transition in exon 14 results in the replacement of isoleucine by valine, a conservative change. However, the mutation also activates a cryptic splice donor. Use of this new donor splice site deletes the last 64 bp of exon 14 from the mature transcript. As shown in the lower panel, direct sequencing of the RT-PCR product shows that virtually all the transcript present uses the new, more 5′ splice donor. The second allele of this compound heterozygous patient does not produce detectable RNA. wt, wild-type; mut, mutant.