Background: Molecular profiling studies have identified subtypes of breast cancer that can be approximately classified by estrogen receptor (ER), progesterone receptor (PR), and HER-2/neu (HER-2) expression. These molecular subtypes are prognostically significant, but to the authors' knowledge, differences in their etiologic profiles have not been established. Reproductive factors may plausibly be differentially correlated with the risk of different breast cancer subtypes because these factors are presumed to impact exposure to endogenous sex hormones.
Methods: The authors pooled 2 population-based, case-control studies of breast cancer in women ages 55 to 79 years for an analysis including 1476 controls and 1023 cases of luminal breast cancer, 39 cases of HER-2-overexpressing breast cancer, and 78 cases of triple-negative breast cancer. Polytomous logistic regression was used to compare each case group with controls.
Results: Associations varied by molecular subtype. Early age at menarche was only found to be associated with risk of HER-2-overexpressing disease (odds ratio [OR] of 2.7; 95% confidence interval [95% CI], 1.4-5.5), whereas breastfeeding for > or =6 months was only found to be protective for luminal and triple-negative disease (OR of 0.8 [95% CI, 0.6-1.0] and OR of 0.5 [95% CI, 0.3-0.9], respectively). Both late age at menopause and the use of estrogen plus progestin hormone therapy were only found to be associated with risk of luminal disease (OR of 1.6 [95% CI, 1.1-2.2] and OR of 1.7 [95% CI, 1.3-2.1], respectively). No differences in risks associated with parity or age at first live birth were observed by subtype.
Conclusions: Certain reproductive factors may have a greater impact on the risk of certain molecular subtypes of disease compared with others. Future studies that further define the etiology of breast cancer subtypes will add to the biologic understanding of this disease.