Endometriosis risk in relation to naevi, freckles and skin sensitivity to sun exposure: the French E3N cohort

BACKGROUND: Endometriosis is an important women's health issue, however its aetiology remains unknown. An association between endometriosis and cutaneous melanoma was described, possibly explained through common genetic features. To further investigate this association, we assessed the link between phenotypic traits predisposing to melanoma and the risk of endometriosis. METHODS: Using a case-control design, we analysed data from 97,215 women of the Etude Epidémiologique auprès de femmes de l'Education Nationale study, a cohort of 98,995 French women insured by a national health scheme mostly covering teachers, and aged 40-65 years at inclusion in 1990. Risk estimates were computed using unconditional logistic regression models. RESULTS: After adjustment for potential confounding factors, there was a positive dose-effect relationship between risk of endometriosis (reported as surgically ascertained, n = 4241) and skin sensitivity to sun exposure [moderate: odds ratio (OR) 1.09, 95% confidence interval (CI) 0.99-1.19; high: OR 1.22; 95% CI 1.10-1.36; compared with none; Ptrend < 0.0001], number of naevi (few: OR 1.19, 95% CI 1.05-1.35; many: OR 1.37, 95% CI 1.21-1.55; very many: OR 1.59, 95% CI 1.37-1.83; compared with none; Ptrend < 0.0001) and number of freckles (few: OR 1.08, 95% CI 1.00-1.17; very many/many: OR 1.11, 95% CI 1.03-1.20; compared with none; Ptrend = 0.005). CONCLUSION: This study is, to our knowledge, the first to report a positive dose-effect relationship between the risk of endometriosis and skin sensitivity to sun exposure, and number of naevi and freckles. These data suggest that endometriosis and melanoma may share some genetic features.