Most breast cancers that occur in women with germline BRCA1 mutations are estrogen receptor-negative (ER-) and typically lack expression of progesterone receptor (PR) and human epidermal growth factor receptor (HER) 2 overexpression (so-called 'triple-negative' breast cancers) [1–8]. These BRCA1-associated ER-tumors are most often high-grade invasive ductal carcinomas with a high mitotic rate that frequently exhibit other characteristic pathologic features including a prominent lymphocytic infiltrate, pushing or circumscribed margins, and geographic areas of necrosis or a central fibrotic focus [3, 9, 10]. In addition, these tumors often express 'basal' biomarkers and most cluster within the 'basal-like' group in gene expression profiling studies [7, 11–13].

Although the combination of the triple-negative phenotype and the pathologic features described above have come to be considered characteristic of BRCA1-associated breast cancers, approximately 10 to 36% of breast cancers that occur in BRCA1 mutation carriers (BRCA1 carriers) are, in fact, ER-positive (ER+) [4, 6, 8, 14, 15]. Relatively little is known about these BRCA1-associated ER+ breast cancers or about the factors that predict for the ER status of breast cancers that develop in these women.

As BRCA1 cancers are so often ER-, it has been suggested that ER negativity is intrinsic to BRCA1 cancers and reflects the cell of origin of these tumors [16]. Supporting this theory, Hosey and colleagues [17] have shown that transfection of the wild-type BRCA1 gene into HCC1937 cells, an ER- breast cancer cell line homozygous for the BRCA1 mutation, restores ER production. Likewise, knockdown of BRCA1 expression in the ER+ cell lines MCF-7 and T47D eliminates expression of ER. These investigators further showed that BRCA1 protein regulates the synthesis of ER through binding to the estrogen receptor-alpha gene promoter, ESR1. Liu and colleagues [18] proposed that BRCA1 may actually be required in the differentiation of ER- stem/progenitor cells to ER+ luminal cells. In prophylactic mastectomy specimens from women with germline BRCA1 mutations, breast tissue was found that appeared to be histologically normal, yet displayed loss of heterozygosity (LOH) for BRCA1 and was positive for the expression of the cancer stem cell marker, ALDH1 and negative for the expression of ER. This finding suggested that loss of BRCA1 may result in the accumulation of ER- breast stem cells, which are genetically unstable and more likely to undergo carcinogenesis.

If, in fact, ER negativity is intrinsic to BRCA1 cancers, this would raise the possibility that at least some BRCA1 ER+ cancers may be 'incidental', and not caused by a complete loss of BRCA1 function in the cancer cells.

It has been reported that ER+ breast cancers may be more common as BRCA1 carriers age [16]. If so, the frequency with which these ER+ breast cancers are encountered in clinical practice may increase as strategies for both prevention and treatment of the more common ER- breast cancers improve and mutation carriers live longer.

Given the paucity of information regarding ER+ breast cancers in BRCA1 mutation carriers, we undertook a study to: determine the clinical factors that predict for ER+ breast cancers in this population; compare the pathologic features of ER+ BRCA1-associated breast cancers with those of ER- BRCA1-associated breast cancers; and perform a case-control analysis to compare the pathologic features of ER+ BRCA1-associated breast cancers with those of ER+ sporadic breast cancers.

The results of this study suggest that BRCA1 carriers who are older at the time of diagnosis of their first invasive breast cancer are more likely to have an ER+ breast cancer than are BRCA1 carriers who are younger at diagnosis. Menopausal status was also a predictor of ER positivity, with ER+ breast cancers being more common in post-menopausal carriers. However, this difference was not significant in multiple covariate analysis, perhaps because of the confounding between menopausal status and age. In particular, only 14% of BRCA1 carriers younger than age 50 years in our study were post-menopausal. As mutation carriers increasingly become surgically menopausal at younger ages it will be important to determine the relative contributions of age and menopausal status for predicting ER status of the breast cancers that develop in this population. Our data are consistent with those of Foulkes and colleagues [16] who also found an increase in ER+ breast cancers with increasing age among BRCA1 mutation carriers. These investigators noted that this increase in ER positivity paralleled that seen in breast cancers that develop in non-mutation carriers. They did not study the effect of menopausal status on ER status of these cancers. The observation that BRCA1 mutation carriers who are older or post-menopausal at the time of diagnosis of breast cancer are more likely to have an ER+ breast cancer may help to define a population of BRCA1 mutation carriers for whom estrogen-modifying agents will be particularly effective.

Of the BRCA1 cancers in this series, 34% were ER+. This is consistent with the 31% frequency of ER+ BRCA1 breast cancers recently reported in the retrospective series by Atchley and colleagues [8].

Our comparison of the pathologic features of ER+ and ER- BRCA1 cancers revealed that the ER+ cancers less often had features typically associated with BRCA1 cancers, such as high mitotic rate, pushing margins, marked lymphocytic infiltrate, and geographic necrosis/fibrotic focus. These differences were not due to differences in histologic grade, because most remained significant when only high-grade ER+ and ER- cancers were compared. Although previous studies have noted that ER- BRCA1 cancers are more likely to be high-grade invasive ductal carcinomas compared with ER+ BRCA1 cancers, this is the first report to our knowledge analyzing the relation of ER status to other pathologic features that have come to be considered to be BRCA-related.

The differences in pathologic features between ER+ and ER- BRCA1 cancers raise the possibility that at least some BRCA1 ER+ cancers may be 'incidental', and not caused by a complete loss of BRCA1 function in the cancer cells. In order to address the issue of whether ER+ BRCA1 cancers are more akin to sporadic ER+ breast cancers than to ER- BRCA1 cancers, we performed a case-control analysis in which the pathologic features of these tumors were compared with those of a control group of ER+ sporadic breast cancers. We found that BRCA1-associated ER+ cancers had a much more limited distribution of histologic types and were significantly more often pure invasive ductal carcinomas with a high mitotic rate than ER+ sporadic cancers.

There are several possible explanations for our observation that the histopathology of ER+ BRCA1 breast cancers differs significantly from both ER- BRCA1 cancers as well as ER+ sporadic breast cancers. First, it may be that although some ER+ BRCA1 breast cancers develop from complete loss of BRCA1 function, others still have intact BRCA1 function resulting in tumors that as a group have phenotypic features that are intermediate between ER- BRCA1 and ER+ sporadic breast cancers. The issue of whether ER+ BRCA1-associated breast cancers demonstrate LOH for the wild-type (wt) BRCA1 allele has been investigated. In this regard, Manié and colleagues recently found 4 of 19 ER+ BRCA1-associated breast cancers did not show loss of the wt BRCA1 allele [19]. King and colleagues [20] demonstrated that 11 of 22 BRCA1-associated invasive breast cancers did not show LOH for wt BRCA1; no mention of ER status was included in their study. The results of these studies are difficult to compare because of differences in patient populations and molecular methodology. Nonetheless, taken together the results of these two studies raise the possibility that not all BRCA1-associated breast cancers exhibit complete loss of BRCA1 function. However, the frequency of this phenomenon, particularly for ER+ BRCA1 cancers, remains to be more clearly defined.

It is also possible that no breast cancer that develops in a BRCA1 mutation carrier is really 'incidental' or sporadic, even if LOH of wt BRCA1 does not exist. Haploinsufficiency of BRCA1, which exists in the BRCA1 heterozygous state, has been shown to have demonstrable effects on the breast tissue of BRCA1 carriers. Normal breast tissue from BRCA1 carriers has been shown to grow abnormally in three-dimensional mammosphere cultures (even though 75% of cells show retention of BRCA1 heterozygosity) [21], and express increased aromatase [22] compared with reduction mammoplasty specimens from non-mutation carriers. Likewise, MCF-7 cells with BRCA1 haploinsuffiency demonstrate decreased efficiency in homologous recombination [23]. Haploinsufficiency of BRCA1 may predispose both to the development of breast cancer as well as to a more limited histopathologic profile. Finally, if loss of BRCA1 function does exist in the majority of ER+ BRCA1 breast cancers, it is possible that ER+ and ER- BRCA1 cancers originate from different cells of origin (e.g. early progenitor cell vs stem cell) leading to different phenotypic expressions. The cell of origin for BRCA1-associated breast cancers is still being determined [24, 25].

One of the strengths of this study is that the pathologic features of all cancers in this study, both BRCA1 and control, were reviewed by two dedicated breast pathologists (SJS and LCC). It should be noted that more of the BRCA1 ER- breast cancers identified were unavailable for pathologic review. Although 34% of the cases in the clinical analysis were ER+, 42% of the cancers reviewed pathologically were ER+. Given the uniformity of many of the pathologic features of the BRCA1 ER- breast cancers in this study, we think it is unlikely that this substantially affected the major findings of our study.

One potential limitation of the case-control study is that BRCA1 and 2 genetic testing information was not available for all of the women with sporadic cancers. However, as BRCA1 and BRCA2 cancers comprise only 5 to 10% of all cancers and potential controls were excluded if a family history of breast or ovarian cancer was noted in the medical record, it seems very unlikely that more than a few of the 'control' cases had germline BRCA1 mutations. We intentionally chose controls from the general hospital population rather than from those who tested negative for BRCA mutations through the genetic testing clinic. Thus, our group of controls is more likely to represent sporadic breast cancers than those identified through a genetic testing program, many of whom may have inherited breast cancers, although not through a germline BRCA1 or 2 mutation.

In conclusion, the results of this study indicate that BRCA1 carriers who are older at the time of invasive breast cancer diagnosis are more likely to have ER+ breast cancers than younger BRCA1 carriers. Furthermore, ER+ BRCA1 breast cancers appear to be pathologically 'intermediate' between ER- BRCA1 cancers and ER+ sporadic cancers, thus comprising a unique group. These observations raise the possibility that either some of the ER+ BRCA1 cancers are incidental (i.e. not BRCA1-related), or that there is a unique mechanism by which they develop. Given the availability of new and effective therapies that exploit the defect in homologous recombination, which exists in BRCA1-related cancers such as poly (ADP-ribose) polymerase (PARP) inhibitors [26] and Cisplatin [27], it will become increasingly important to determine whether the pathways leading to ER+ BRCA1 breast cancers are similar to those that result in ER- BRCA1 cancers and whether these new therapies are likely to be effective in ER+ BRCA1 cancers. Toward this end, a detailed immunophenotypic and molecular analysis of the ER+ BRCA1 cancers is currently underway.