Rett syndrome revisited: a patient with biotin dependency

A patient with Rett syndrome (cerebral atrophy associated with hyperammonemia) was studied. Primary defects of urea cycle enzymes were excluded as causes of the disorder. The analysis of urinary organic acids showed a moderate increase of lactate, methylcitrate, tiglyglycine and 3-hydroxisovalerate, indicating an abnormality of multiple carboxylases. Biotin supplementation reversed the urinary abnormalities. In fibroblasts grown with a low biotin medium propionylCoA and 3-methylcrotonylCoA carboxylase activities were reduced. Holocarboxylase synthetase activity was normal (Vmax and Km). Surprisingly the biotinidase in fibroblasts was not decreased. The data indicate that some patients with Rett syndrome might suffer from a biotin-dependent defect of unknown nature.