Isolated 3-Methylcrotonyl-CoA Carboxylase Deficiency: Evidence for an Allele-Specific Dominant Negative Effect and Responsiveness to Biotin Therapy

Abstract

Deficiency of 3-methylcrotonyl-CoA carboxylase (MCC) results in elevated excretion of 3-methylcrotonylglycine (3-MCG) and 3-hydroxyisovaleric acid (3-HIVA). MCC is a heteromeric mitochondrial enzyme comprising biotin-containing alpha subunits and smaller beta subunits, encoded by MCCA and MCCB, respectively. Mutations in these genes cause isolated MCC deficiency, an autosomal recessive disorder with a variable phenotype that ranges from severe neonatal to asymptomatic adult forms. No reported patients have responded to biotin therapy. Here, we describe two patients with a biochemical and, in one case, clinical phenotype of MCC deficiency, both of whom were responsive to biotin. The first patient presented at 3 months with seizures and progressive psychomotor retardation. Metabolic investigation at 2 years revealed elevated excretion of 3-MCG and 3-HIVA, suggesting MCC deficiency. High-dose biotin therapy was associated with a dramatic reduction in seizures, normalization of the electroencephalogram, and correction of the organic aciduria, within 4 weeks. MCC activity in fibroblasts was 25% of normal levels. The second patient, a newborn detected by tandem-mass-spectrometry newborn screening, displayed the same biochemical phenotype and remained asymptomatic with biotin up to the age of 18 months. In both patients, sequence analysis of the complete open reading frames of MCCA and MCCB revealed heterozygosity for MCCA-R385S and for the known polymorphic variant MCCA-P464H but revealed no other coding alterations. MCCA-R385S is unusual, in that it has a normal amount of MCC alpha protein but confers no MCC activity. We show that MCCA-R385S, but not other MCCA missense alleles, reduces the MCC activity of cotransfected MCCA-wild-type allele. Our results suggest that MCCA-R385S is a dominant negative allele and is biotin responsive in vivo.

Is l-Carnitine Supplementation Beneficial in 3-Methylcrotonyl-CoA Carboxylase Deficiency?

“…A variety of other symptoms, mostly neurological, have been reported but most patients are asymptomatic. The clinical picture may show extensive intrafamiliar variation (Visser et al 2000;Baumgartner et al 2004;Darin et al 2007;Dirik et al 2008;Eminoglu et al 2009;Grunert et al 2012). However, most children detected through newborn screening remain asymptomatic (Stadler et al 2006;Lam et al 2013;Koeberl et al 2003).…”

Molecular mechanism of dominant expression in 3‐methylcrotonyl‐CoA carboxylase deficiency

“…High-dose biotin therapy was associated with correction of the organic aciduria within 4 weeks, a dramatic reduction in seizures and normalization of the EEG. The second patient, a newborn detected by tandem mass spectrometry newborn screening, displayed the same biochemical phenotype and remained asymptomatic with biotin up to the present age of 18 months (Baumgartner et al 2004).…”

“…In two separate experiments, cotransfection of MCCA-wild type allele with vector without insert restored MCC activity to 55% of untransfected control ¢broblasts while cotransfection of MCCA-wild type with MCCA-R385S restored MCC activity only to 25%, i.e. to about 50% of that obtained with wild type coexpressed with vector without insert (Figure 2) (Baumgartner et al 2004). In contrast, co-transfection of MCCA-wild type with either MCCA-A289V or MCCA-L437P restored MCC activity to the same level as that obtained with wild type coexpressed with vector without insert (Figure 2).…”

“…Despite an extensive search for a functional mutation on the second allele by RT-PCR of mRNA of the entire MCCA and MCCB ORF as well as by ampli¢cation and sequencing of all MCCA exons and £anking intronic splice sites from genomic DNA, no other coding alterations were identi¢ed (Baumgartner et al 2004). In earlier studies, MCCA-R385S conferred no MCC activity when transfected into reference MCCa-de¢cient cells (Baumgartner et al 2001).…”

“…No correlation between the level of residual enzyme activity measured in vitro and clinical presentation has been observed. Here, two patients with a biochemical and, in one case, a clinical phenotype of MCC de¢ciency, both responsive to biotin, are discussed (Baumgartner et al 2004).…”