rs5753231 — TCN2 TCN2 2KB Upstream Variant

TCN2 Upstream Variant — A Genetic Boost to B12 Transport

Most people know vitamin B12 deficiency from its classic blood tests — low serum B12, megaloblastic anemia. But the real question for cellular function is not how much B12 circulates in total, but how much reaches your cells. That depends almost entirely on one transport protein: transcobalamin II¹¹ transcobalamin II
The only B12-binding protein able to deliver cobalamin into cells via the CD320 receptor on cell surfaces; about 20–25% of circulating B12 is bound to this protein, forming "holotranscobalamin" (holoTC), the active fraction (TCN2). The rs5753231 variant sits approximately 2 kilobases upstream of the TCN2 gene — in the promoter-proximal region that governs how much transcobalamin II your liver and other tissues produce.

Unlike the well-studied TCN2 coding variant rs1801198 (Pro259Arg), which impairs the protein's ability to bind and release B12, rs5753231 affects a different level of regulation: the amount of TCN2 protein your body makes in the first place. The rare T allele is associated with higher circulating transcobalamin II levels — potentially translating to a modest advantage in B12 delivery capacity.

The Mechanism

rs5753231 falls in a 2KB upstream region²² 2KB upstream region
By convention, "2KB upstream" means within 2,000 base pairs of the gene's transcription start site — a region rich in regulatory elements including promoter sequences, enhancer-binding sites, and transcription factor recognition motifs of TCN2 on chromosome 22q12.2. Variants in this class of regulatory elements can alter gene expression by affecting transcription factor binding³³ transcription factor binding
Proteins that attach to specific DNA sequences upstream of a gene and control how frequently that gene is transcribed into mRNA, RNA stability, or promoter efficiency without changing the amino acid sequence of the resulting protein.

The T allele is associated with higher serum TCN2 protein — a pattern consistent with increased transcriptional activity at this locus. More TCN2 protein in circulation means a greater capacity to load and deliver cobalamin to peripheral tissues. The C allele (carried by most people) represents the population-average expression level.

Because this is a promoter-region variant rather than a missense change, its functional impact is probabilistic rather than structural: it shifts the average abundance of a fully functional protein rather than reducing that protein's per-molecule effectiveness.

The Evidence

Two independent large-scale protein GWAS have identified rs5753231-T as a cis-pQTL for serum TCN2 levels. The Sun et al. 2018 Genomic Atlas⁴⁴ Sun et al. 2018 Genomic Atlas
Sun BB et al. Genomic atlas of the human plasma proteome. Nature, 2018 — a landmark analysis of 1,478 plasma proteins in 3,301 healthy blood donors from the INTERVAL cohort — detected the rs5753231-T association with TCN2 protein at genome-wide significance (beta = 0.21, p = 8 × 10⁻¹³). This effect survived strict multiple-testing correction across the entire plasma proteome.

The Gudjonsson et al. 2022 serum protein GWAS⁵⁵ Gudjonsson et al. 2022 serum protein GWAS
Gudjonsson A et al. A genome-wide association study of serum proteins reveals shared loci with common diseases. Nature Communications, 2022 independently replicated the association in 5,368 participants from the AGES-Reykjavik cohort (beta = 0.117, p = 3 × 10⁻⁷), confirming that the effect is not cohort-specific.

Both studies detect TCN2 protein quantity (total transcobalamin II in circulation) rather than functional holotranscobalamin (B12-loaded TCN2). Higher total TCN2 theoretically increases the pool available to be loaded with cobalamin, but the downstream functional significance — whether T allele carriers have meaningfully better cellular B12 delivery — has not been directly tested in dietary or clinical intervention studies. This places the clinical relevance at an emerging evidence level⁶⁶ emerging evidence level
Single studies or pQTL associations without clinical outcomes data; the biological direction is clear but the practical effect size for individuals has not been quantified.

ClinVar classifies rs5753231-T as benign/likely-benign for transcobalamin II deficiency, consistent with its protective (rather than loss-of-function) direction.

Practical Actions

For TT homozygotes, the pQTL data suggests a modestly elevated baseline transcobalamin II pool. The most practical implication is that standard B12 monitoring is appropriate, and holotranscobalamin (holoTC) testing — which directly measures the active B12 fraction — would give the most relevant picture of actual transport capacity.

For CC homozygotes (the majority), this is a neutral finding at this evidence level; no corrective action is indicated. The actionable variants for TCN2 are the coding changes in rs1801198 and rs1131603, which directly alter the protein's B12-binding function.

Interactions

rs5753231 is an independent secondary association signal at the TCN2 locus, distinct from the coding variant rs1801198 (Pro259Arg). The two variants capture different aspects of TCN2 biology: rs1801198 affects protein function (B12-binding efficiency), while rs5753231 affects protein abundance (expression level). Individuals carrying the GG genotype at rs1801198 (reduced binding efficiency) alongside the CC genotype at rs5753231 (lower expression) represent a double hit on transcobalamin function — although direct interaction studies are lacking.

The broader one-carbon metabolism pathway context is the same as for rs1801198: adequate cellular B12 is needed for methionine synthase (MTR, rs1805087) to recycle homocysteine, and methionine synthase reductase (MTRR, rs1801394) to keep that enzyme active. TCN2 variants upstream or coding both feed into the same pathway limitation when B12 delivery to cells is suboptimal.