Does TCN2 rs1801198 affect vitamin B12 delivery and status?

The transcobalamin II (TCN2) rs1801198 (776C>G, Pro259Arg) genetic variant is a key modulator of vitamin B12 transport and cellular bioavailability, representing a critical determinant of functional B12 status.

Mechanistic pathways

• Reduced protein stability: The rs1801198 variant alters the local secondary structure, folding, or stability of the transcobalamin II protein, rather than directly disrupting its physical binding affinity to cell-surface receptors.
• Impaired cellular uptake: The resulting Arg259 isoform reduces circulating holotranscobalamin (holo-TC) levels. This lower ligand abundance directly limits the substrate available for CD320 receptor-mediated endocytosis, restricting B12 delivery into target tissues.

Biomarker and clinical evidence

• Altered active B12 markers: Individuals homozygous for the G allele (GG genotype) exhibit significantly lower active holo-TC concentrations compared to CC homozygotes, even when total serum B12 levels remain within normal clinical reference ranges.
• Intracellular deficiency indicators: This transport impairment leads to functional intracellular B12 deficiency. Consequently, GG carriers—especially those with marginal baseline B12 status—show elevated levels of homocysteine and methylmalonic acid (MMA), alongside potential secondary hematologic shifts like elevated red blood cell mean corpuscular volume (MCV).

Bottom line

• The TCN2 rs1801198 GG genotype compromises cellular vitamin B12 delivery by reducing the stability and availability of active holotranscobalamin, resulting in functional intracellular B12 deficiency marked by elevated homocysteine and MMA.