rs4775936 — CYP19A1 CYP19A1 Promoter Region Variant (Aro1)

The Aromatase Switch — How rs4775936 Shapes Your Estrogen Blueprint

Aromatase is the enzyme that converts androgens (testosterone, androstenedione) into estrogens (estradiol, estrone). Without it, no estrogen is made. The CYP19A1 gene encodes aromatase and is expressed in multiple tissues — ovaries, bone, breast, brain, and adipose. rs4775936, located in a negative regulatory region¹¹ negative regulatory region
a promoter region that normally suppresses gene transcription of CYP19A1 (specifically exon I.6), influences how much aromatase is made in those tissues. Even modest changes in aromatase activity reshape the androgen-to-estrogen ratio across the body, with downstream effects on bone density, fertility, breast tissue, and how well aromatase-blocking drugs work.

The Mechanism

CYP19A1 is expressed from multiple tissue-specific promoters. The rs4775936 variant sits within the I.6 promoter region — a regulatory element that governs aromatase expression in bone and gonadal tissue. CYP19A1 lies on the minus strand of chromosome 15; the variant is C→T on the genomic plus strand (corresponding to G→A on the coding strand, which is why older papers describe it as an A/G polymorphism). The T allele appears to alter binding affinity for transcriptional repressors, modifying aromatase output in a tissue-specific manner. This promoter-level regulation — rather than a change in the aromatase protein itself — explains why the variant's effects are context-dependent, varying by tissue type, sex, age, and hormonal environment.

The Evidence

Bone mineral density. Enjuanes et al. studied 256 postmenopausal Spanish women²² Enjuanes et al. studied 256 postmenopausal Spanish women
Enjuanes A et al. A new SNP in a negative regulatory region of the CYP19A1 gene is associated with lumbar spine BMD in postmenopausal women. Bone, 2006 and found that TT homozygotes had significantly higher lumbar spine bone mineral density compared to CC or CT individuals (p=0.029). Higher local aromatase activity in bone increases estradiol at the tissue level, which suppresses osteoclast-driven bone resorption — the likely mechanism.

Aromatase inhibitor response. Park et al. genotyped 46 CYP19A1 variants in 109 patients with hormone receptor-positive metastatic breast cancer on letrozole³³ Park et al. genotyped 46 CYP19A1 variants in 109 patients with hormone receptor-positive metastatic breast cancer on letrozole
Park IH et al. Single nucleotide polymorphisms of CYP19A1 predict clinical outcomes associated with letrozole. Cancer Chemother Pharmacol, 2011. The T allele of rs4775936 was associated with higher clinical benefit rate (OR 2.60, 95% CI 1.12–6.02, p=0.026). A larger retrospective study of 308 patients by Ferraldeschi et al. confirmed a trend⁴⁴ Ferraldeschi et al. confirmed a trend
Ferraldeschi R et al. Polymorphisms of CYP19A1 and response to aromatase inhibitors in metastatic breast cancer. Breast Cancer Res Treat, 2012 (HR 0.79 per T allele, 95% CI 0.66–0.95, p=0.012), though the effect was attenuated after adjusting for disease extent and other prognostic factors.

Aromatase inhibitor-induced arthralgia. Borrie et al. prospectively followed 196 women initiating letrozole or anastrozole⁵⁵ Borrie et al. prospectively followed 196 women initiating letrozole or anastrozole
Borrie AE et al. Genetic and clinical predictors of arthralgia during letrozole or anastrozole therapy. Breast Cancer Res Treat, 2020. More than 50% of the group experienced arthralgia, and rs4775936 was significantly associated with both developing arthralgia (adjusted p=0.016) and with discontinuing therapy due to intolerable joint pain. The same variant that may improve tumor response appears also to predispose to this debilitating side effect — likely through altered estrogen withdrawal kinetics in joint tissue.

Lipid effects. In 303 women on adjuvant aromatase inhibitors, Santa-Maria et al. identified rs4775936 as one of seven CYP19A1 variants linked to triglyceride reductions of 20.2–39.3 mg/dL⁶⁶ Santa-Maria et al. identified rs4775936 as one of seven CYP19A1 variants linked to triglyceride reductions of 20.2–39.3 mg/dL
Santa-Maria CA et al. Association of variants in candidate genes with lipid profiles in women on adjuvant aromatase inhibitor therapy. Clin Cancer Res, 2016 (p<0.00053), reflecting aromatase's role in modulating sex-hormone-sensitive lipid metabolism.

Cancer risk. A meta-analysis of 25,446 endometrial cancer cases and 41,106 controls found CYP19A1 rs4775936 associated with increased endometrial cancer risk⁷⁷ CYP19A1 rs4775936 associated with increased endometrial cancer risk
Das AP et al. Meta-analysis of 49 SNPs identifies polymorphisms in hormone regulation genes associated with endometrial cancer risk. Genes, 2023. In prostate cancer, Kanda et al. reported⁸⁸ Kanda et al. reported
Kanda S et al. Functional genetic polymorphisms in CYP19A1 and prostate cancer risk and survival. Int J Cancer, 2015 that the variant allele reduced prostate cancer susceptibility but correlated with shorter survival in metastatic disease — possibly through effects on the estrone/androstenedione ratio in the tumor microenvironment.

Practical Actions

People carrying one or two T alleles face a nuanced picture: potential protection in bone density, possible benefit in aromatase inhibitor treatment response, but elevated risk of AI-induced joint pain and need for proactive monitoring of hormone-sensitive tissue (endometrium, prostate). Given that this variant acts through tissue-specific promoter modulation, the downstream effects depend heavily on hormonal context — most pronounced in postmenopausal women (low circulating estrogens, maximal dependence on local aromatase) and during AI therapy (aromatase pharmacologically suppressed).

Women with breast cancer being considered for aromatase inhibitor therapy should ensure their oncology team is aware of this variant: it may inform both expected response and the likelihood of joint-pain-driven discontinuation. For postmenopausal women not on AI therapy, higher local aromatase activity in bone may confer modest skeletal protection — though this requires adequate calcium and vitamin D to be realized.

Interactions

rs4775936 sits within a CYP19A1 haplotype block with several other functional variants, including rs10046, rs700518, rs1062033, and rs767199. Combined haplotype analyses consistently show stronger associations than any single SNP alone, particularly for bone density and hormone levels. The variant also interacts with estrogen receptor alpha variants (ESR1) — ESR1 polymorphisms modulate sensitivity to aromatase-derived estrogens, so the combination of altered aromatase output (CYP19A1) and altered estrogen signaling (ESR1) can amplify or dampen phenotypic effects. Gene-environment interactions with fracture risk probability scores have been documented (PMID 41929826), suggesting clinical context mediates genetic risk substantially.