Familial hypotonia of childhood caused by isolated 3-methylcrotonyl-coenzyme A carboxylase deficiency.

Hypotonia was the initial symptom in four siblings from a nonconsanguineous Tunisian-Jewish family. Plasma carnitine was severely deficient, and urinary organic acid analysis revealed increased excretion of 3-hydroxyisovaleric acid and 3-methylcrotonylglycine. 3-Methylcrotonyl-coenzyme A carboxylase activity was reduced in skin fibroblasts; pyruvate carboxylase and serum biotinidase activities were normal. We conclude that 3-methylcrotonyl-coenzyme A carboxylase deficiency should be added to the list of metabolic causes of familial hypotonia of childhood.