Context: Gene expression analysis has identified several breast cancer subtypes, including
basal-like, human epidermal growth factor receptor-2 positive/estrogen receptor
negative (HER2+/ER–), luminal A, and luminal B.
Objectives: To determine population-based distributions and clinical associations for
breast cancer subtypes.
Design, Setting, and Participants: Immunohistochemical surrogates for each subtype
were applied to 496 incident cases of invasive breast cancer from the Carolina
Breast Cancer Study (ascertained between May 1993 and December 1996), a population based,
case-control study that oversampled premenopausal and African American
women. Subtype definitions were as follows: luminal A (ER+ and/or progesterone receptor
positive [PR+], HER2−), luminal B (ER+ and/or PR+, HER2+), basal-like (ER−,
PR−, HER2−, cytokeratin 5/6 positive, and/or HER1+), HER2+/ER− (ER−, PR−, and
HER2+), and unclassified (negative for all 5 markers).
Main Outcome Measures: We examined the prevalence of breast cancer subtypes
within racial and menopausal subsets and determined their associations with tumor
size, axillary nodal status, mitotic index, nuclear pleomorphism, combined grade,
p53 mutation status, and breast cancer–specific survival.
Results The basal-like breast cancer subtype was more prevalent among premenopausal
African American women (39%) compared with postmenopausal African
American women (14%) and non–African American women (16%) of any age
(P<.001), whereas the luminal A subtype was less prevalent (36% vs 59% and
54%, respectively). The HER2+/ER− subtype did not vary with race or menopausal
status (6%-9%). Compared with luminal A, basal-like tumors had more TP53
mutations (44% vs 15%, P<.001), higher mitotic index (odds ratio [OR], 11.0;
95% confidence interval [CI], 5.6-21.7), more marked nuclear pleomorphism (OR,
9.7; 95% CI, 5.3-18.0), and higher combined grade (OR, 8.3; 95% CI, 4.4-15.6).
Breast cancer–specific survival differed by subtype (P<.001), with shortest survival
among HER2+/ER− and basal-like subtypes.
Conclusions: Basal-like breast tumors occurred at a higher prevalence among premenopausal
African American patients compared with postmenopausal African
American and non–African American patients in this population-based study. A
higher prevalence of basal-like breast tumors and a lower prevalence of luminal A
tumors could contribute to the poor prognosis of young African American women
with breast cancer
