Can TCN2 rs1801198 limit tissue vitamin B12 delivery despite normal serum B12?

Vitamin B12 transport relies on specialized carrier proteins to cross cell membranes, meaning systemic blood levels do not always reflect intracellular availability.

Cellular transport mechanisms

• Protein alteration: The TCN2 gene encodes transcobalamin II, the primary transport protein that binds vitamin B12 to form active holotranscobalamin (holoTC). The common TCN2 rs1801198 (776C>G, Pro259Arg) genetic polymorphism alters the physical structure of transcobalamin II.
• Reduced delivery: This structural change modifies carrier binding and transport behavior, leading to lower levels of circulating active holoTC. Consequently, receptor-mediated internalization of B12 into target tissues is impaired, limiting intracellular availability.

Clinical and biomarker implications

• Masked deficiency: Because standard serum tests measure total vitamin B12 rather than active cellular transport, individuals with the rs1801198 variant can experience intracellular B12 deficiency despite presenting with completely normal total serum B12 levels.
• Metabolic indicators: This functional tissue-level deficiency impairs B12-dependent metabolic pathways, which is clinically indicated by elevations in downstream functional biomarkers like homocysteine or methylmalonic acid in some populations.

Bottom line

• The TCN2 rs1801198 variation alters transcobalamin II structure and lowers active holotranscobalamin levels, restricting intracellular B12 delivery and causing functional, tissue-level deficiency despite normal total serum B12.