Methylmalonic aciduria due to transcobalamin receptor defect

Methylmalonic aciduria due to transcobalamin receptor defect (also known as methylmalonic aciduria due to TCblR defect or AMNR deficiency) is a rare inherited metabolic disorder caused by mutations in the CD320 gene, which encodes the transcobalamin receptor (TCblR/CD320). This receptor is responsible for the cellular uptake of transcobalamin-bound cobalamin (vitamin B12). When the receptor is defective, cells cannot efficiently internalize vitamin B12, leading to impaired intracellular cobalamin metabolism and elevated methylmalonic acid levels in blood and urine. The condition primarily affects metabolic pathways dependent on vitamin B12, particularly the conversion of methylmalonyl-CoA to succinyl-CoA. Clinically, the condition is often detected through newborn screening programs that identify elevated levels of methylmalonic acid or propionylcarnitine (C3). Many affected individuals appear to be clinically asymptomatic or mildly affected, which has raised questions about the full clinical significance of this genetic defect. Some patients may present with mildly elevated methylmalonic acid levels without overt symptoms, while others may develop subtle neurological or developmental concerns. The metabolic abnormality is generally considered mild compared to other forms of methylmalonic aciduria. Treatment, when deemed necessary, typically involves intramuscular or oral vitamin B12 (hydroxocobalamin or cyanocobalamin) supplementation, which can help normalize or reduce methylmalonic acid levels by bypassing the receptor defect through alternative uptake mechanisms. The long-term prognosis appears favorable, though ongoing monitoring of metabolic markers and developmental milestones is recommended. The condition is increasingly recognized due to expanded newborn screening, and further research is needed to fully characterize its natural history and determine which patients require treatment.

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