Biotin-Responsive Disorders

Abstract

Two inherited defects in biotin metabolism are known: holocarboxylase synthetase (HCS) deficiency and biotinidase deficiency. Both lead to deficiency of all biotindependent carboxylases, i.e. to multiple carboxylase deficiency (MCD). In HCS deficiency, the binding of biotin to apocarboxylases is impaired. In biotinidase deficiency, biotin depletion ensues from the inability to recycle endogenous biotin and to utilize protein-bound biotin from the diet. As the carboxylases play an essential role in the catabolism of several amino acids, in gluconeogenesis and in fatty-acid synthesis, their deficiency provokes multiple, life-threatening metabolic derangements, eliciting characteristic organic aciduria and neurological symptoms. The clinical presentation is extremely variable in both disorders. Characteristic symptoms include metabolic acidosis, hypotonia, seizures, ataxia, impaired consciousness and cutaneous symptoms, such as skin rash and alopecia. All patients with biotinidase and a majority of patients with HCS deficiency respond dramatically to oral therapy with pharmacological doses of biotin. Delayed diagnosis and treatment in biotinidase deficiency may result in irreversible neurological damage. A few patients with HCS deficiency show a partial or even no response to biotin and seem to have an impaired long-term outcome. Acquired biotin deficiency, which also causes MCD, is extremely rare. A defect in biotin transport has been reported in a single child; however the genetic defect remains unresolved to date. Biotin-Responsive Basal Ganglia Disease (BRBGD) is a recently described subacute encephalopathy which disappears within a few days without neurological sequelae if biotin is administered early.