A single mutation in MCCC1 or MCCC2 as a potential cause of positive screening for 3-methylcrotonyl-CoA carboxylase deficiency

Isolated 3-Methylcrotonyl-CoA carboxylase deficiency (MCC deficiency) is an organic aciduria presenting with a highly variable phenotype and has been part of newborn screening programs in various countries, in particular in the US. Here we present enzymatic and genetic characterisation of 22 individuals with increased 3-hydroxyisovalerylcarnitine and/or 3-methylcrotonylglycine suggesting MCC deficiency, but only partially reduced 3-methylcrotonyl-CoA carboxylase activity. Among these, 21 carried a single mutant allele in either MCCC1 (n = 20) or MCCC2 (n = 1). Our results suggest that heterozygosity for such a single deleterious mutation may lead to misdiagnosis of MCC deficiency.

► A single mutation as a potential cause of positive screening for 3-MCC deficiency.
► Diagnostic approach based only on C5-OH, 3-MCG and 3-HIVA may lead to misdiagnosis.
► False positive confirmatory results in newborn screening programs should be avoided.