rs228921 — TMPRSS6 TMPRSS6 iron regulation variant

TMPRSS6 Upstream Variant — A Second Iron Gate

The TMPRSS6 gene produces matriptase-2¹¹ matriptase-2
A type II transmembrane serine protease expressed primarily in liver cells that acts as the body's main brake on hepcidin production, the enzyme that keeps hepcidin — the master iron-regulatory hormone — in check. Most genetic research on TMPRSS6 has focused on the Ala736Val variant (rs855791), which sits in the enzyme's catalytic domain. But rs228921, located roughly 2 kilobases upstream of the TMPRSS6 transcription start site, tags a second, independent signal at this locus. This upstream variant operates through a different mechanism: instead of altering the enzyme's activity, it likely affects how much matriptase-2 protein the liver produces. The result, however, is similar — lower matriptase-2 output means less hepcidin suppression, higher hepcidin, and reduced iron absorption from the gut.

The Mechanism

Matriptase-2 normally cleaves hemojuvelin²² hemojuvelin
A membrane-bound co-receptor that activates the BMP/SMAD signaling cascade, which drives hepcidin gene transcription in hepatocytes from the surface of liver cells. By removing this coreceptor, matriptase-2 blocks the BMP/SMAD pathway³³ BMP/SMAD pathway
Bone morphogenetic protein / son of mothers against decapentaplegic — a signaling cascade that upregulates hepcidin transcription and reduces hepcidin secretion. When matriptase-2 is produced at lower levels — as may occur with the G allele at rs228921 — this suppression is less effective. Hepcidin levels rise, ferroportin on gut enterocytes is internalized and degraded, and less dietary iron crosses the gut wall into the bloodstream.

rs228921 sits in low linkage disequilibrium with rs855791 (r² < 0.1 in European and Indian Asian populations), confirming that it is an independent genetic signal rather than a proxy for the Ala736Val variant. This means the two variants can be studied and act together as additive genetic risk⁴⁴ additive genetic risk
When two independent variants at the same locus both predispose to lower iron status, carrying both can compound the effect. An individual who is GG at rs228921 and also AA at rs855791 carries risk at two independent locations in the TMPRSS6 gene.

The Evidence

The landmark genome-wide association study by Chambers et al.⁵⁵ Chambers et al.
Chambers JC et al. Genome-wide association study identifies variants in TMPRSS6 associated with hemoglobin levels. Nat Genet, 2009 — conducted in 16,001 individuals of European and Indian Asian ancestry — identified rs228921 as independently associated with hemoglobin levels at genome-wide significance (combined P = 1.9 × 10⁻¹⁰). The variant clustered with rs228918 and rs228919 in a separate haplotype block from the primary rs855791/rs4820268 cluster, confirming its independent contribution to iron phenotype variation.

A systematic review⁶⁶ systematic review
Gichohi-Wainaina WN et al. Inter-ethnic differences in genetic variants within the transmembrane protease, serine 6 (TMPRSS6) gene associated with iron status indicators. Genes Nutr, 2015 documented comparable minor allele frequencies for rs228921 across Caucasian (MAF ~0.41) and Indian Asian (MAF ~0.48) populations, with this variant included among the eight TMPRSS6 SNPs showing inter-ethnic differences in association with iron status indicators.

A study in female Black South African populations found that the rs228918/rs228921 GG haplotype was associated with lower odds of elevated soluble transferrin receptor (sTfR > 8.3 mg/L; OR: 0.79, 95% CI: 0.63–0.98), illustrating how the biological effect of this upstream variant depends on haplotype context and may differ across ancestries — a reminder that iron-related genetic associations are influenced by population-specific haplotype backgrounds.

Practical Actions

For carriers of one or two G alleles, the implications parallel those of rs855791 but are additive: reduced TMPRSS6 activity → elevated hepcidin → lower iron absorption efficiency. The practical response focuses on the same strategies that improve iron uptake in the face of high hepcidin: pairing non-heme iron sources with vitamin C, choosing heme iron when possible, avoiding absorption inhibitors at iron-containing meals, and monitoring iron stores with periodic ferritin and transferrin saturation testing.

If a supplement is needed, iron bisglycinate⁷⁷ iron bisglycinate
A chelated amino acid form of iron that is absorbed partly through peptide transporters (PEPT1), bypassing the ferroportin bottleneck that hepcidin controls. Also labeled chelated iron or gentle iron is preferable to ferrous sulfate because its absorption is less dependent on ferroportin. Every-other-day dosing maximizes fractional absorption by allowing hepcidin to reset between doses.

The combined genetic picture matters: if you also carry the risk allele at rs855791 (or rs4820268), the two independent TMPRSS6 signals compound your predisposition toward lower iron absorption, making monitoring more important.

Interactions

rs228921 and rs855791 are in low LD (r² < 0.1) and represent independent signals at the TMPRSS6 locus — they can co-occur in the same individual and their effects are additive. Carrying both risk alleles compounds the predisposition to lower iron status. rs228918, rs228919, and rs575620 form a tight haplotype cluster with rs228921 and are likely in near-complete LD with each other.

TMPRSS6 variants also interact with HFE variants⁸⁸ HFE variants
HFE encodes a protein involved in hepcidin signaling. Loss-of-function variants C282Y (rs1800562) and H63D (rs1799945) reduce hepcidin and cause iron overload in homozygotes. See rs1800562. In individuals with HFE hemochromatosis variants, a hepcidin-reducing TMPRSS6 variant would amplify iron loading; conversely, in HFE carriers the higher hepcidin from TMPRSS6 risk alleles partially offsets the HFE-driven reduction. These opposing effects mean the clinical interpretation depends on what HFE variants are co-present.