Late-onset holocarboxylase synthetase deficiency with homologous R508W mutation

Holocarboxylase synthetase (HCS) is responsible for the biotinylation of pyruvate carboxylase, propionyl coenzyme A (CoA) carboxylase, β- methylcrotonoyl CoA carboxylase, and acetyl CoA carboxylase. We report on a patient with HCS deficiency resulting in a rare metabolic disease. The patient, a 2-year-old boy, presented with vomiting, consciousness disturbance, and dyspnea. Laboratory, examinations showed hyperglycemia, hyperammonemia, lactic acidosis, and excretion of large amounts of β- hydroxyisovalerate and β-methylcrotonylglycine in the urine. After 10 days of treatment with biotin 5 mg · kg^-1 · day^-1, the abnormal organic acids in his urine had almost completely disappeared. There were no subsequent attacks, and his growth and development remained normal during 1 year of follow- up. Nucleotide sequence analysis of the HCS cDNA of the patient revealed a homozygous 1809C→T (R508W) mutation. The R508W mutation is found worldwide, and might be associated with higher residual HCS activity than other mutations. Late-onset HCS deficiency cannot be differentiated clinically from biotinidase deficiency. Prompt and correct diagnosis is important for these biotin- responsive disorders.