Transcobalamin I deficiency

Transcobalamin I deficiency, also known as haptocorrin deficiency or R-binder deficiency, is a rare condition involving a deficiency of transcobalamin I (also called haptocorrin or TC I), one of the vitamin B12 (cobalamin) binding proteins in the blood. Transcobalamin I (encoded by the TCN1 gene) is a glycoprotein produced primarily by granulocytes and mucosal cells that binds the majority of circulating cobalamin in the serum. Unlike transcobalamin II deficiency, which causes severe megaloblastic anemia and neurological complications, transcobalamin I deficiency is generally considered a benign condition. Individuals with transcobalamin I deficiency typically have very low measured serum vitamin B12 levels on standard laboratory testing because most circulating cobalamin is normally bound to haptocorrin. However, the metabolically active fraction of cobalamin — that bound to transcobalamin II (holotranscobalamin) — remains normal, and cellular delivery of vitamin B12 is unimpaired. As a result, affected individuals are usually asymptomatic and do not develop the clinical manifestations of true vitamin B12 deficiency such as megaloblastic anemia or neurological dysfunction. The condition is most often discovered incidentally when routine blood work reveals unexpectedly low serum cobalamin levels in an otherwise healthy person. No specific treatment is required for transcobalamin I deficiency, as it does not cause functional cobalamin deficiency. The key clinical importance of recognizing this condition lies in avoiding unnecessary vitamin B12 supplementation or invasive diagnostic workups for low serum B12 levels. Measurement of holotranscobalamin II levels and metabolic markers such as methylmalonic acid and homocysteine can help confirm that cellular B12 status is adequate. The condition follows an autosomal recessive inheritance pattern and is considered extremely rare, though it may be underdiagnosed due to its benign nature.

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