Does TCN2 rs1801198 CG modestly affect biologically active B12 delivery to cells?

The TCN2 rs1801198 (776C>G, Pro259Arg) missense polymorphism alters transcobalamin II, the primary transport protein responsible for delivering vitamin B12 to tissues.

Cellular delivery and transport mechanisms

• Reduced binding affinity: The G allele (present in the CG genotype) encodes an arginine residue that lowers transcobalamin II’s binding affinity for vitamin B12.
• Decreased holotranscobalamin: This reduced affinity impairs the formation of circulating holotranscobalamin (holo-TC). Holo-TC represents the biologically active fraction of vitamin B12 that must bind to cellular transcobalamin receptors to be taken up by cells.
• Paradoxical serum levels: Because cellular uptake is compromised, untransported cobalamin can accumulate in the extracellular circulation. This results in high total serum vitamin B12 levels that can paradoxically coexist with, and mask, a functional intracellular deficiency.

Hematological implications

• Elevated MCV: Insufficient intracellular cobalamin in erythroid progenitor cells disrupts DNA synthesis during erythropoiesis. This impairment leads to macrocytosis, clinically observed as an above-optimal mean corpuscular volume (MCV), particularly under marginal B12 conditions.
• Elevating RDW: The disruption in normal red blood cell maturation also promotes anisocytosis (increased variation in red blood cell size), which characteristically manifests as an above-optimal red cell distribution width (RDW).

Bottom line

• The TCN2 rs1801198 CG genotype modestly impairs cellular B12 delivery by reducing active holotranscobalamin formation. This functional deficit can drive macrocytic changes—such as elevated MCV and RDW—even when total serum B12 levels appear high.