Cloning of the human MCCA and MCCB genes and mutations therein reveal the molecular cause of 3-methylcrotonyl-CoA: carboxylase deficiency

3-Methylcrotonyl-CoA: carboxylase (EC 6.4.1.4; MCC) deficiency is an inborn error of the leucine degradation pathway (MIM *210200) characterized by in creased urinary excretion of 3-hydroxyisovaleric acid and 3-methylcrotonylg lycine. The clinical phenotypes are highly variable ranging from asymptomat ic to profound metabolic acidosis and death in infancy, Sequence similarity with Glycine max and Arabidopsis thaliana genes encoding the two subunits of MCC permitted us to clone the cDNAs encoding the alpha- and beta -subuni ts of human MCC, The 2580 bp MCCA cDNA encodes the 725 amino acid biotin-co ntaining or-subunit, The MCCA gene is located on chromosome 3q26-q28 and co nsists of 19 exons, The 2304 bp MCCB cDNA encodes the non-biotin-containing P-subunit of 563 amino acids, The MCCB gene is located on chromosome 5q13 and consists of 17 exons, We have sequenced both genes in four patients wit h isolated biotin-unresponsive deficiency of MCC, In two of them we found m utations in the MCCA gene. Compound heterozygosity for a missense mutation (S535F) and a nonsense mutation (V694X) were identified in one patient, One heterozygous mutation (S535F) was found in another patient. The remaining two patients had mutations in the MCCB gene, One consanguineous patient was homozygous for a missense mutation (R268T), In the other we identified a m issense mutation in one allele (E99Q) and allelic loss of the other. Mutati ons were correlated with an almost total lack of enzyme activity in fibrobl asts, These data provide evidence that human MCC deficiency is caused by mu tations in either the MCCA or MCCS gene.