rs2206949 — ESR1 ESR1 Intronic Endometriosis Variant

ESR1 rs2206949 — The 6q25.1 Endometriosis Signal and Estrogen Receptor Regulation

The 6q25.1 chromosomal region is one of the most important genetic loci in female reproductive biology. It contains the ESR1 gene (estrogen receptor alpha), which orchestrates estrogen signaling across the endometrium, ovary, bone, brain, and cardiovascular system. Nearby sits ARMT1 (Acidic Residue Methyltransferase 1), a gene co-located at 6q25.1 that shows coordinated expression with estrogen-sensitive tissues. The rs2206949 variant — a C-to-T substitution in an intron of ESR1 — is one of several independent signals at this locus linking estrogen receptor biology to endometriosis risk.

The Mechanism

rs2206949 lies within an intronic region of ESR1 at chromosome 6, position 151,716,421 (GRCh38). Because it is intronic, it does not alter the amino acid sequence of estrogen receptor alpha. Instead, intronic variants at this locus are thought to act through regulatory mechanisms¹¹ regulatory mechanisms
intronic variants can affect gene expression by altering enhancer activity, splicing efficiency, or chromatin organization that modulate ERα expression levels or isoform ratios in hormone-responsive tissues.

The 6q25.1 region contains several genes — SYNE1, ESR1, CCDC170, and ARMT1 — that show chromatin interactions and coordinated expression in the endometrium. Fine-mapping studies using Summary-based Mendelian Randomization²² Summary-based Mendelian Randomization
SMR tests whether a GWAS signal co-localizes with a cis-eQTL, suggesting the GWAS variant acts through gene expression rather than being a coding variant have identified ESR1 expression in blood and endometrial tissue as a mediator of endometriosis susceptibility at this locus, with variants in strong LD acting as eQTLs for ESR1. The proximity of rs2206949 to ARMT1 (~264 kb) also places it within a regulatory domain where chromatin interactions between the two genes have been detected.

The Evidence

The clearest evidence for rs2206949 comes from a large international GWAS meta-analysis in 17,045 endometriosis cases and 191,596 controls³³ GWAS meta-analysis in 17,045 endometriosis cases and 191,596 controls
Sapkota et al. Meta-analysis identifies five novel loci associated with endometriosis highlighting key genes involved in hormone metabolism. Nature Communications, 2017 that identified the ESR1/6q25.1 region as harboring two independent association signals, with the ESR1 locus emerging as one of five novel genome-wide significant findings. The T allele at rs2206949 carries a modest but replicated effect (OR ~1.10, 95% CI 1.06–1.14, p=3×10⁻⁷) on endometriosis risk.

The 6q25.1 locus has since been substantially refined. In the largest endometriosis GWAS to date, a meta-analysis of 60,674 cases and 701,926 controls of European and East Asian ancestry⁴⁴ meta-analysis of 60,674 cases and 701,926 controls of European and East Asian ancestry
Rahmioglu et al. The genetic basis of endometriosis and comorbidity with other pain and inflammatory conditions. Nature Genetics, 2023, conditional analysis revealed five distinct independent signals at the SYNE1/6q25.1 locus — more than any other locus in the genome — with rs73625113 within ESR1 intron identified as a high-confidence causal variant (posterior probability 0.506). These signals collectively underscore the critical role of this region in endometriosis pathogenesis.

Beyond reproductive disease, rs2206949 was identified in a longitudinal GWAS of bone mineral density in 141,261 UK Biobank participants⁵⁵ longitudinal GWAS of bone mineral density in 141,261 UK Biobank participants
PMID 37500982, 2023 as a genome-wide significant (p=2×10⁻⁸) determinant of BMD trajectory. This mirrors the well-known pleiotropic effects of ESR1 variants on both reproductive and skeletal outcomes, as estrogen receptor alpha mediates estrogen's bone-protective actions alongside its role in endometrial function.

The T allele is substantially more common in African populations (~43%) and substantially less common in East and South Asian populations (~13%), making ancestral background an important consideration when interpreting this result.

Practical Actions

The effect of rs2206949 on endometriosis risk is modest — an OR of ~1.10 per T allele means TT homozygotes carry roughly 1.21-fold elevated baseline risk compared to CC homozygotes. This is not a diagnostic marker for endometriosis, and many factors beyond this single variant determine individual susceptibility. However, the T allele sits within a broader pattern of ESR1-region variation that, when combined with other 6q25.1 signals, contributes meaningfully to the genetic architecture of the disease.

For T-allele carriers — especially TT homozygotes — the most actionable implication is heightened awareness of endometriosis symptoms and avoiding the diagnostic delays (averaging 4–11 years in clinical practice) that are common for this condition. Symptoms warranting specialist evaluation include severe dysmenorrhea, chronic pelvic pain, deep dyspareunia, and unexplained infertility.

The bone mineral density association adds a secondary consideration: ESR1-region variants track estrogen sensitivity in skeletal tissue, making proactive bone density monitoring relevant for TT homozygotes — particularly as estrogen levels decline perimenopausally.

Interactions

rs9340799 and rs2234693 (ESR1 XbaI and PvuII): These are two other independent ESR1 intronic variants already well-characterized for endometriosis and reproductive outcomes. rs2206949 represents a third, conditionally independent signal at the same locus — meaning it adds information beyond what rs9340799 and rs2234693 capture. Women carrying risk alleles at multiple ESR1 loci may have more pronounced estrogen-signaling dysregulation, though formal combined-effect studies are needed.

rs12700667 (near HOXA10/HOXA11): The HOXA homeobox genes regulate endometrial development and uterine receptivity. Risk alleles at both the ESR1 (rs2206949) and HOXA loci may compound endometriosis susceptibility through independent pathways — ERα-mediated signaling and HOX-mediated endometrial patterning respectively.

rs7521902 (near WNT4): WNT4 signaling is critical for female sex determination and suppression of androgen biosynthesis. The WNT4 endometriosis locus is among the most robustly replicated signals genome-wide. Carrying risk alleles at both ESR1 (rs2206949) and WNT4 loci may confer additive endometriosis susceptibility through convergent hormonal dysregulation, though epistatic interaction testing has not been published for this pair.