rs2235321 — TMPRSS6 TMPRSS6 synonymous variant (hepcidin modulator)

TMPRSS6 Tyr730= — A Synonymous Variant That Still Moves the Iron Dial

TMPRSS6¹¹ TMPRSS6
Transmembrane serine protease 6, also called matriptase-2 — a type II transmembrane serine protease expressed predominantly in the liver, whose key function is to cleave hemojuvelin from hepatocyte surfaces and thereby suppress hepcidin production is the body's primary brake on the iron-regulatory hormone hepcidin²² hepcidin
A 25-amino-acid peptide produced by the liver that controls iron homeostasis by targeting ferroportin, the sole iron export channel on gut enterocytes and macrophages, for degradation. Common TMPRSS6 variants consistently rank among the strongest genetic determinants of serum iron, transferrin saturation, and hemoglobin in genome-wide association studies across diverse populations.

rs2235321 sits in exon 15 of TMPRSS6 at chromosome 22q12.3. The G-to-A change on the plus strand corresponds to a synonymous substitution in the coding sequence — both codons encode tyrosine at position 730 (TAC→TAT, p.Tyr730=). Because the amino acid is unchanged, rs2235321 is not itself the functional variant. Instead, it acts as a haplotype tag³³ haplotype tag
A variant in linkage disequilibrium with one or more nearby functional variants; its alleles reliably predict which haplotype a chromosome belongs to even though the tag variant itself may have no direct functional effect, marking a haplotype that carries altered TMPRSS6 regulatory or splicing activity at a nearby position. The downstream consequence — measured directly in human studies — is a difference in circulating hepcidin levels between GG and AA homozygotes.

The Mechanism

Because rs2235321 is synonymous at the protein level, its association with hepcidin must be mediated through linkage with a nearby functional variant — one that influences TMPRSS6 expression level, mRNA splicing efficiency, or a second coding position in partial LD. Whatever the functional driver, the biological consequence fits the established TMPRSS6–hepcidin–iron axis: the G allele tags reduced effective matriptase-2 activity, leaving more hemojuvelin⁴⁴ hemojuvelin
A GPI-anchored co-receptor that amplifies BMP/SMAD signaling in hepatocytes, driving hepcidin transcription when intact on the cell surface; matriptase-2 cleaves it to dampen this pathway intact on hepatocyte surfaces. More intact hemojuvelin amplifies the BMP/SMAD signaling pathway⁵⁵ BMP/SMAD signaling pathway
Bone morphogenetic protein receptor / SMAD transcription factor cascade that is the primary positive regulator of hepcidin gene expression in the liver, driving hepcidin up. Higher hepcidin degrades ferroportin on duodenal enterocytes and recycling macrophages, tightening the outflow of iron into the bloodstream.

The A allele, by contrast, tags a haplotype with relatively higher matriptase-2 activity: more hemojuvelin is cleaved, BMP/SMAD signaling is damped, hepcidin production falls, and iron absorption and recycling improve.

The Evidence

Two complementary studies from the same Gambian research group provide the primary evidence for rs2235321's association with hepcidin.

A recall-by-genotype study⁶⁶ recall-by-genotype study
Jallow MW et al. Common Variants in the TMPRSS6 Gene Alter Hepcidin but not Plasma Iron in Response to Oral Iron in Healthy Gambian Adults. Curr Dev Nutr, 2021 recruited 80 healthy adults stratified by TMPRSS6 genotype and administered a single oral dose of ferrous sulfate (130 mg elemental iron). At baseline, GG homozygotes had significantly higher plasma hepcidin than AA homozygotes (9.50 vs 6.60 ng/mL, P = 0.035) — a 44% difference. The G allele showed a clear additive dose-response, with heterozygotes (AG) intermediate between the two homozygote groups.

A larger cross-sectional analysis⁷⁷ larger cross-sectional analysis
Jallow MW et al. Association of common TMPRSS6 and TF gene variants with hepcidin and iron status in healthy rural Gambians. Sci Rep, 2021 in 1,316 healthy Gambians confirmed the association: rs2235321 significantly affected plasma hepcidin concentrations (AA genotype having lower hepcidin; F = 3.7, P = 0.014). Notably, rs2235321 was the only TMPRSS6 variant in that cohort that independently affected hepcidin — neither rs855791 nor other genotyped TMPRSS6 variants reached significance for hepcidin in this African population, suggesting rs2235321 tags a distinct functional element at the TMPRSS6 locus that is informative beyond the primary Ala736Val coding variant.

In a prospective pregnancy cohort⁸⁸ prospective pregnancy cohort
Hayashi I et al. Association of a pro-inflammatory diet and gestational diabetes mellitus with maternal anemia and hemoglobin levels during pregnancy. Nutr Res, 2023, rs2235321 was associated with hemoglobin levels during the third trimester — a period of high iron demand when even modest absorption inefficiencies become clinically relevant.

ClinVar classifies this variant as benign, reflecting that it does not cause Mendelian disease on its own. The iron-status effects are modest, population-wide, and relevant primarily under conditions of increased iron demand rather than in nutritionally adequate populations. The evidence level is rated moderate: the hepcidin association is replicated across two independent analyses in African adults, with pregnancy-cohort replication in a separate population, but large-scale multi-ethnic conditional GWAS data specifically for hepcidin remain limited.

Practical Actions

The practical relevance of rs2235321 scales with iron demand. Under conditions of adequate dietary iron and no additional stressors, even GG homozygotes typically maintain normal hemoglobin. The gap between GG and AA becomes meaningful when iron demand is chronically elevated: heavy menstrual cycles, pregnancy, regular blood donation, endurance athletics, or vegetarian and vegan diets where only non-heme iron is available.

For GG individuals in these situations, the actionable steps mirror those for other TMPRSS6 iron-reducing variants: maximize dietary iron absorption through meal composition (vitamin C pairing, avoiding inhibitors), monitor serum ferritin periodically, and choose iron bisglycinate over ferrous sulfate if supplementation is needed. AA individuals can use their genetically lower hepcidin as reassurance that iron deficiency is less likely — though high-demand states can deplete anyone's stores.

Interactions

rs2235321 belongs to the same TMPRSS6 locus as the primary coding variant rs855791⁹⁹ rs855791
Ala736Val missense variant; the strongest common genetic determinant of iron status, with the A allele reducing matriptase-2 enzyme activity directly, the intronic variant rs2413450¹⁰¹⁰ rs2413450
TMPRSS6 intronic tag variant independently associated with MCV, MCH, and hemoglobin in GWAS of over 700,000 individuals, and the upstream regulatory variants rs228918 and rs228921. These variants are partially correlated through linkage disequilibrium¹¹¹¹ linkage disequilibrium
Non-random co-inheritance of alleles at nearby loci because they sit close together on the chromosome and are rarely separated by recombination in the population but capture partially distinct functional signals. In the Jallow 2021 Gambian cohort, rs2235321 showed independent hepcidin effects even when rs855791 did not — suggesting it tags a haplotype element active in African ancestry populations where the LD structure around rs855791 differs from European populations.

Carriers of multiple TMPRSS6 risk alleles across these variants may have a compounded hepcidin-elevating effect. In practice, ferritin monitoring resolves the question of whether the combined genetic architecture translates into meaningful iron depletion for a specific individual.