Can CUBN rs1801222 and TCN2 rs1801198 variants impair vitamin B12 status?

Genetic variations in absorption and transport pathways can significantly impair vitamin B12 homeostasis, leading to functional tissue deficiency.

Genetic determinants of absorption and transport

• CUBN rs1801222 (p.Ser253Phe): This missense mutation impairs the functionality of cubilin, a key extracellular receptor component of the ileal intrinsic factor-cobalamin (IF-B12) complex. This defect reduces intestinal B12 absorption, lowering overall serum levels. Notably, this variant acts as a pharmacogenetic sensitizer that significantly amplifies metformin-induced B12 malabsorption.
• TCN2 rs1801198 (p.Arg259Pro): This missense polymorphism in the transcobalamin II gene alters protein folding, transcription, or binding affinity. This leads to reduced circulating levels of holotranscobalamin—the biologically active fraction of B12 destined for cellular uptake—thereby compromising systemic tissue delivery and elevating homocysteine levels.

Mechanistic impact on intracellular metabolism

• Enzyme dysfunction: Vitamin B12 is an essential cofactor for the mitochondrial enzyme methylmalonyl-CoA mutase, which converts methylmalonyl-CoA to succinyl-CoA.
• Intracellular deprivation: When intestinal uptake (via cubilin receptor defects) or systemic tissue delivery (via altered transcobalamin-mediated transport) is defective, cells experience a severe cobalamin deficit. This underactivation of methylmalonyl-CoA mutase causes methylmalonyl-CoA to accumulate and hydrolyze into methylmalonic acid (MMA), leading to elevated systemic MMA levels.

Bottom line

• The metabolic profile of low serum B12 and elevated MMA points directly to intracellular cobalamin deficiency, a state highly consistent with genetic impairment of intestinal absorption via the CUBN rs1801222 variant or compromised systemic tissue delivery via the TCN2 rs1801198 variant.