Partial Methylcrotonyl-Coenzyme A Carboxylase Deficiency in an Infant With Failure to Thrive, Gastrointestinal Dysfunction, and Hypertonia
Analysis of urinary organic acids will occasionally detect an unexpected metabolic abnormality reflecting an enzyme deficiency in a child with a nonspecific phenotype. We recently found an infant with failure to thrive, gastroesophageal reflux and vomiting, diarrhea, oral candidiasis and lower extremity hypertonia to have a mild organic aciduria of methylcrotonylglycine and 3-hydroxyisovaleric acid unresponsive to pharmacological doses of biotin. Most patients with methylcrotonylglycinuria have defects in biotin metabolism affecting biotin-dependent carboxylase enzymes such as holocarboxylase synthase or biotinidase deficiency and are responsive to biotin administration.1 Enzymatic investigations in the patient described below revealed an isolated partial deficiency of 3-methylcrotonyl-coenzyme A (CoA) carboxylase (MCC) (EC 6.4.1.4) (McKusick 21020) which is biotin resistant, a very rare disorder of leucine degradation.