First Revision of the Guidelines for the Diagnosis and Management of Remethylation Disorders.

This guideline summarizes diagnostic and therapeutic approaches based on a systematic literature review and evidence evaluation using the GRADE methodology. Given the limited high-quality data, expert consensus was additionally obtained through a modified Delphi process. Remethylation disorders are rare inherited conditions that disrupt the methionine-homocysteine cycle and consecutively impair essential methylation dependent metabolic pathways. Remethylation disorders are caused by defects in the cobalamin or folate metabolism. The disorders typically result in elevated homocysteine and often low methionine; combined cobalamin-related defects also affect mitochondrial methylmalonic acid clearance. The cblC-MMACHC defect is the most common cobalamin-related remethylation disorder. Early-onset patients usually present with severe neurological and eye symptoms. Late-onset cases show variable symptoms (e.g., psychiatric, renal, thromboembolic events). Plasma total homocysteine, methionine, methylmalonic acid, serum vitamin B12 (and folates) should be assessed in suspected cases. Early detection through newborn screening is associated with improved clinical outcomes. Betaine as first-line therapy for methylenetetrahydrofolate reductase deficiency and parenteral hydroxocobalamin for cobalamin-related defects have reduced mortality and morbidity. Total homocysteine, methionine (and methylmalonic acid) should be kept as close to normal values as achievable. Emerging evidence suggests that early use of high-dose hydroxocobalamin (> 0.35 mg/kg/day) may improve neurocognitive impairment and may ameliorate eye disease in severe cobalamin-related defects. A major limitation in current practice is the lack of availability of high concentration hydroxocobalamin formulations for parenteral administration.