rs2881766 — ESR1 ESR1 intron variant

Intronic variant in estrogen receptor alpha that alters ESR1 expression and has been associated with gestational hypertension risk and breast cancer susceptibility

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Estrogen Receptor Alpha — A Genetic Influence on Vascular and Hormonal Health

Estrogen receptor alpha (ERα), encoded by the ESR1 gene on chromosome 6q25.1, is one of the body's most important hormone receptors. It mediates estrogen's protective effects across multiple systems — from promoting vasodilation and suppressing vascular inflammation in the cardiovascular system, to regulating cell proliferation and differentiation in breast tissue. The rs2881766 polymorphism sits within an intron of ESR1 and has been associated with two distinct but mechanistically connected phenotypes: risk of pregnancy-induced hypertension (PIH/gestational hypertension) and breast cancer susceptibility. Both reflect the broader consequence of altered estrogen receptor signaling across different physiological contexts.

The Mechanism

rs2881766 is located at chr6:151,797,984 on the GRCh38 plus strand, within an intron of the ESR1 gene. As an intronic variant, it does not change the ERα protein sequence directly. Instead, intronic variants in ESR1 are thought to act through regulatory mechanisms — altering [splicing efficiency | intronic variants can affect how exons are joined together during mRNA processing], modifying transcription factor binding within intron-derived regulatory elements, or serving as [tag SNPs | a tag SNP is in linkage disequilibrium with a nearby functional variant and marks that variant in association studies] for nearby functional variants that have not yet been individually characterized.

The T allele (GRCh38 reference) appears to be the risk-associated allele. In the context of pregnancy-induced hypertension, estrogen's role in vascular adaptation is essential: during normal pregnancy, rising estrogen levels promote nitric oxide synthesis, endothelial relaxation, and the dramatic increase in uterine blood flow required to sustain fetal growth. Impaired ESR1 signaling — whether through reduced expression or altered receptor function — could blunt these vascular adaptations, contributing to hypertensive complications.

In breast tissue, ERα is the primary driver of estrogen-stimulated cell proliferation. Variants that alter ESR1 expression levels or receptor activity can shift the balance between proliferative and protective estrogen signaling, influencing breast cancer susceptibility.

The Evidence

The cardiovascular association was established in a haplotype-based case-control study of 95 Japanese women with PIH and 200 matched controls11 haplotype-based case-control study of 95 Japanese women with PIH and 200 matched controls
Tamura M et al. Hypertens Res 2008;31(2):221-8. The T allele of rs2881766 was significantly more prevalent in the PIH group. Haplotype analysis revealed that the G-A combination at rs2881766-rs1643821 and the G-A-T combination at rs2881766-rs1643821-rs988328 were protective against PIH — confirming that the G allele tags the protective haplotype and the T allele marks the susceptibility configuration. This finding is biologically coherent with estrogen's known role in maintaining gestational vascular homeostasis.

The breast cancer associations have been documented across multiple Asian populations. In a Chinese Han cohort of 459 cases and 549 controls, the GG genotype was associated with decreased breast cancer risk (OR=0.63, 95% CI 0.44–0.91) compared to TT22 the GG genotype was associated with decreased breast cancer risk (OR=0.63, 95% CI 0.44–0.91) compared to TT
Dai Z et al. Cancer Cell Int 2019;19:9. The GG genotype also correlated with lymph node metastasis status and estrogen receptor expression in tumors, suggesting rs2881766 influences the estrogen-driven biology of breast cancer.

A larger Korean study of 830 breast cancer cases and 390 controls found that rs2881766 increased breast cancer risk across all three age groups examined — premenopausal women under 35, premenopausal women over 35, and postmenopausal women — and across multiple tumor subtypes33 rs2881766 increased breast cancer risk across all three age groups examined — premenopausal women under 35, premenopausal women over 35, and postmenopausal women — and across multiple tumor subtypes
Son BH et al. J Cancer Res Clin Oncol 2015;141(4):633-45. A smaller Han Chinese study of 198 cases and 218 controls additionally found that the GG genotype elevated risk in women with later menarche (>13 years), suggesting the variant's effect may interact with cumulative estrogen exposure44 suggesting the variant's effect may interact with cumulative estrogen exposure
Chen L et al. Gynecol Endocrinol 2016;32(7):553-6.

Evidence is currently at the moderate level — associations are replicated in multiple independent Asian cohorts, with a plausible biological mechanism, but large European or multi-ancestry meta-analyses specifically examining rs2881766 are lacking.

Practical Implications

The strongest actionable signal from rs2881766 is in the context of pregnancy. Women with the TT genotype who are pregnant or planning pregnancy face a modestly elevated risk of gestational hypertension and should ensure their blood pressure is monitored throughout pregnancy, particularly in the second and third trimesters. The cardiovascular protective role of estrogen signaling is most critical during pregnancy and in the years around menopause, when estrogen levels shift dramatically.

The breast cancer association identifies TT carriers as having relatively higher estrogen-driven breast cancer risk. This makes awareness of estrogen-amplifying exposures — including combined oral contraceptives and menopausal hormone therapy — particularly relevant for risk-benefit discussions with a healthcare provider.

Interactions

rs2881766 exists within a haplotype context with other ESR1 variants. The PIH study demonstrated that the protective effect is carried by the G-A haplotype across rs2881766 and rs1643821, meaning the combination of alleles matters more than rs2881766 in isolation. The classic ESR1 functional polymorphisms rs2234693 (PvuII, intron 1) and rs9340799 (XbaI, intron 1) are the most studied variants in this gene and may interact with rs2881766 effects through haplotype structure. Women carrying risk alleles at both rs2881766 and rs9340799 may face compounded gestational hypertension risk, as rs9340799 GG has independently been associated with severe preeclampsia in multiple studies.

Genotype Interpretations

What each possible genotype means for this variant.

GG Protective Genotype Beneficial

Lower estrogen receptor variant risk — reduced susceptibility to gestational hypertension and breast cancer

You carry two copies of the G allele at rs2881766, the genotype associated with lower risk of pregnancy-induced hypertension and decreased breast cancer susceptibility in multiple Asian cohort studies. This genotype is relatively uncommon in European populations (about 4% of European-ancestry individuals are GG homozygotes), but more frequent in African (about 41%) and East Asian (about 20%) populations. The G allele tags the protective haplotype in the ESR1 gene, suggesting more favorable estrogen receptor signaling patterns in vascular and breast tissue contexts.

GT Intermediate Risk Intermediate

One copy of each allele — intermediate gestational hypertension and breast cancer risk

You carry one G allele and one T allele at rs2881766. This is the most common genotype globally (approximately 46% of people). You have an intermediate risk profile compared to GG homozygotes (lower risk) and TT homozygotes (higher risk). Because the association follows an additive pattern — each T allele adds some risk — your single T allele confers a moderate elevation above the GG baseline.

TT Elevated Risk Genotype High Risk

Higher estrogen receptor risk variant — elevated gestational hypertension and breast cancer susceptibility

The T allele at rs2881766 is both the GRCh38 reference allele and the population-major allele in most ancestries — which means TT is statistically "normal" from a population frequency standpoint, even though it confers higher relative risk compared to GG. This illustrates an important principle: common variants can still confer meaningful disease risk when the protective allele (G) is rarer.

For pregnancy-induced hypertension, the biological plausibility is strong: estrogen receptor alpha is essential for the vascular adaptations of pregnancy, including endothelial relaxation and uterine blood flow increases. Any reduction in ERα signaling efficiency could impair these adaptations and contribute to hypertensive disorders. The Tamura et al. 2008 study (PMID 18360040) provides direct genetic evidence that T-bearing ESR1 haplotypes confer susceptibility to PIH.

For breast cancer, the TT data from Korean and Chinese studies shows consistent elevation of risk across different age groups and tumor subtypes. The GG genotype shows OR=0.63 in the Dai et al. 2019 study, which translates to approximately 59% higher odds of breast cancer for TT compared to GG (1/0.63 ≈ 1.59). These associations are strongest in Asian populations where the G allele is more frequent; European-specific data is limited.