Epidemiology of basal-like and luminal breast cancers among black women in the AMBER consortium.

BACKGROUND: Evidence suggests etiologic heterogeneity among breast cancer subtypes. Previous studies with six-marker immunohistochemical classification of intrinsic subtypes included small numbers of black women.

METHODS: Using centralized laboratory results for estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor 2 (HER2), proliferation marker Ki-67, epidermal growth factor receptor (EGFR), and cytokeratin (CK)5/6, we estimated case-only and case-control odds ratios (ORs) for established breast cancer risk factors among cases (n=2,354) and controls (n =2,932) in the African American Breast Cancer Epidemiology and Risk (AMBER) consortium. ORs were estimated by ER status and intrinsic subtype using adjusted logistic regression.

RESULTS: Case-only analyses by ER status showed etiologic heterogeneity by age at menarche, parity (versus nulliparity), and age at first birth. In case-control analyses for intrinsic subtype, increased body mass index (BMI) and waist-to-hip (WHR) ratio were associated with increased risk of luminal A subtype, while older age at menarche and parity, regardless of breastfeeding, were associated with reduced risk. For basal-like cancers, parity without breastfeeding and increasing WHR were associated with increased risk, whereas breastfeeding and age ≥ 25 years at first birth were associated with reduced risk among parous women. Basal-like and ER-/HER2+ subtypes had earlier age-at-incidence distribution relative to luminal subtypes.

CONCLUSIONS: Breast cancer subtypes show distinct etiologic profiles in the AMBER consortium, a study of over 5,000 black women with centrally assessed tumor biospecimens.

IMPACT: Among black women, high WHR and parity without breastfeeding are emerging as important intervention points to reduce the incidence of basal-like breast cancer.