Three affected members of one family, each with a different clinical presentation of isolated biotin-resistant 3-methylcrotonyl-CoA carboxylase (MCC) deficiency are described. The index patient presented at 7 weeks of age with feeding difficulties, sweating and tachypnoea. Echocardiography showed a severely dilated left ventricle with minimal contractility. MCC deficiency was suspected on the basis of elevated urinary excretion of 3-hydroxyisovalerate and 3-methylcrotonylglycine. Deficiency of MCC activity was found in lymphocytes and fibroblasts (ca. 2% of mean normal). Serum carnitine was low (free 10 micromol/l). Some other possible causes of cardiomyopathy were excluded. Cardiomyopathy was not improved by carnitine therapy. The healthy father and a developmentally delayed brother also had MCC deficiency. Both also had decreased serum carnitine concentrations, but without cardiac involvement. Dilatative cardiomyopathy as predominant symptom in isolated MCC deficiency has not been described before, although severe carnitine deficiency is a common finding in MCC deficiency. It is not clear whether this is a coincidental association.
Conclusion: In order to understand the phenotypic spectrum of this rare disorder, cardiac evaluation should be made in patients with 3-methylcrotonyl-CoA carboxylase deficiency. Biochemical and clinical investigations have also to be performed in their parents and siblings. In addition, 3-methylcrotonyl-CoA carboxylase deficiency should be included in the differential diagnosis of dilatative cardiomyopathy.