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Abstract Background High levels of estrogen are associated with increased risk of breast and endometrial cancer and have been suggested to also play a role in the development of ovarian cancer. Cancerogenic effects of estradiol, the most prominent form of estrogen, have been highlighted as a side effect of estrogen-only menopausal hormone therapy. However, whether high levels of endogenous estrogens, produced within the body, promote cancer development, has not been fully established. Objective We aimed to examine causal effects of estradiol on breast, endometrial, and ovarian cancer. Methods Here we performed a two-sample Mendelian randomization (MR) to estimate the effect of endogenous estradiol on the risk of developing breast, endometrial, and ovarian cancer, using the UK Biobank as well as 3 independent cancer cohorts. Results Using 3 independent instrumental variables, we showed that higher estradiol levels significantly increase the risk for ovarian cancer (OR = 3.18 95% CI, 1.47-6.87, P = 0.003). We also identified a nominally significant effect for ER-positive breast cancer (OR = 2.16 95% CI, 1.09-4.26, P = 0.027). However, we could not establish a clear link to the risk of endometrial cancer (OR = 1.93 95% CI, 0.77-4.80, P = 0.160). Conclusion Our results suggest that high estradiol levels promote the development of ovarian and ER-positive breast cancer.
Johansson et al. (Wed,) studied this question.