There is much debate as to the clinical significance of 3-methylcrotonyl-CoA carboxylase deficiency (3MCC deficiency). In thinking about this issue, I recently read a paper by Arnold et al, (Mol Genet Metab. 2012 Aug;106(4):439-41) that describes their retrospective analysis of 35 cases of 3-MCC deficiency identified by newborn screening and confirmed by enzyme and/or molecular analysis.
One of the most interesting findings of this paper is that they found was a significant inverse correlation between C5OH level and residual enzyme activity. However, there was not a relationship between clinical symptoms and residual enzyme activity, even with enzyme activities ranging from 0.2% of normal to 58% of normal. This lends weight to the idea that 3MCC deficiency may represent a susceptibility factor for metabolic decompensation during illness, but may not be, in and of itself, enough to cause clinical disease.
Hilary Vernon, MD PhD