Reproductive and hormonal risk factors for luminal, HER2-overexpressing, and triple-negative breast cancer in Japanese women.

Although the clinical relevance of the molecular subtypes of breast cancer is evident, etiological differences among subtypes have not been well established, especially among Asian. Here, we evaluated the hypothesis that the etiologic impact of reproductive and hormonal features differs among molecular subtypes. We conducted a case-control study in pre- and postmenopausal Japanese. We examined 706 breast cancer patients and 1412 age- and menopausal status-matched noncancer controls. Immunohistochemical stains for estrogen receptor, progesterone receptor, and human epidermal growth factor receptor 2 (HER2) were used to classify the cases into 554 luminal (hormone receptor positive), 84 HER2-overexpressing (hormone receptor negative, HER2 positive), and 68 triple-negative cases (hormone receptor negative, HER2 negative). Associations were evaluated using multivariate polytomous logistic regression models. A significant association was observed between early age at menarche and risk of luminal disease (odds ratios = 1.67, 95% confidence interval: 1.22-2.29; P trend = 0.001). No significant differences in association with parity, age at first live birth, breastfeeding history, age at menopause, or synthetic hormonal use were seen across molecular subtypes of breast cancer. These findings indicate that reproductive events in adolescence have differential impact on the risk of breast cancer molecular subtypes in Japanese.