Remethylation defects: guidelines for clinical diagnosis and treatment.

The main remethylation defects include disorders which all have defective methionine synthesis in common. Methylenetetrahydrofolate reductase deficiency impairs methyltetrahydrofolate synthesis, defects in cytosolic reduction of hydroxocobalamin (CblC/D) impair the synthesis of both methyl- and adenosyl cobalamin and deficiencies of methionine synthase (CblE/G) are associated with defective methyl cobalamin synthesis. The clinical presentation is characterized by acute neurological distress in early infancy. In childhood, patients present with progressive encephalopathy with an end-stage which has many signs in common with the adult onset form. In fact, both have more or less severe signs of subacute degeneration of the cord. Cobalamin defective patients must be treated with parenteral supplementation of hydroxocobalamin (1-2 mg per dose). Some methylenetetrahydrofolate patients could be folate responsive and must have a high-dosage folate trial. In addition, oral betaine supplementation (2-9 g per day depending on age) appears an effective means to prevent further neurological deterioration.