In this study of over 2,000 breast cancer cases, we found significant differences in the association between breast cancer risk factors and molecular subtypes of tumors. As expected many reproductive risk factors including age at menarche, parity, age at first birth, age at menopause, and postmenopausal hormone use were associated with luminal A tumors, the most common type of breast of cancer. In general, hormonally related risk factors in later adult life demonstrated the most heterogeneity. The association between postmenopausal hormone use and luminal A tumors was significantly stronger than the relation with luminal B tumors (the other ER+ subtype). Interestingly, weight gain since age 18 was more strongly associated with luminal B tumors than luminal A tumors.
Unexpectedly, we found that some hormonal factors were associated with hormone receptor negative subtypes. For example, age at menopause was significantly associated with the HER2-type and unclassified subtype, and current estrogen plus progestin hormone use was strongly associated with both basal-like and unclassified tumor types.
A number of other studies have evaluated the association between breast cancer risk factors and tumor subtypes, although only a handful have evaluated markers beyond ER, PR and HER2. The Polish Breast Cancer Study (n=804 breast cancer cases) also found that most established breast cancer risk factors were associated with luminal A tumors [16
]. However, they reported that age at menarche was more strongly inversely associated with basal-like tumors than luminal A tumors (p-heterogeneity =0.0009), which we did not observe in the current study. Similar to our study, the Polish Breast Cancer Study found that having a family history of breast cancer was a risk factor for almost all subtypes although the magnitude of the effect was greatest for basal-like and HER2-type breast cancers.
The Carolina Breast Cancer Study (CBCS) (n=1424 breast cancer cases) also examined the association between risk factors for molecular subtypes of breast cancer in a case-control study of both Caucasian and African American women[17
]. Millikan et al.
found that increasing parity was associated with reduced risk of luminal A tumors and an increased risk of basal-like tumors. Our results with respect to parity are consistent with this finding. In addition, The CBCS reported an inverse association between lactation and basal-like tumors. Although there was no significant heterogeneity between lactation and subtype in our study, we did find a strong inverse association between lactation and basal-like tumors. For women with total breast feeding of 4+ months, we found a 40% reduced risk of basal-like breast cancer, which is in line with the 30% reduced risk observed in the CBCS.
In addition, studies examining risk factors in relation to tumors classified with information on ER, PR and HER2 only have also been conducted. A combined study of the LACE and Pathways studies within Kaiser Permanente Northern California examined breast cancer risk factors in relation to subtypes defined by ER, PR, and HER2. In this study of 2544 invasive breast cancer cases, Kwan et al [22
]found that relative to luminal A cases (ER+ and/or PR+/HER2-), luminal B cases (ER+ and/or PR+/HER2+) were less likely to consume alcohol and use HRT. Breast feeding for at least four months was associated with a lower risk of triple negative cases (ER-/PR-/HER2-) compared with luminal A. Similarly, two other studies Phipps et al [23
](n=1130 total cases) and Gaudet et al [24
](n=890 total cases), also reported an inverse association between breastfeeding for 6 or more months and triple negative breast tumors.
Of interest, a number of risk factors in our study did not demonstrate heterogeneity across tumor subtypes including age at menarche, BMI at age 18, previous BBD, and alcohol consumption. It is possible that these factors are having a similar effect on risk across the different subtypes and this may be indicating how these factors are affecting breast cancer etiology. For example, having a prior BBD may indicate having early proliferative lesions which could have developed through a number of different pathways. BBD is believed to be a general marker of breast cancer risk, and thus may reflect the culmination of many risk factors and not be specific to any one pathway. It is also possible that we may not have had enough power to detect the difference across subtypes for some exposures.
Our classification of tumor subtypes was similar although not identical to those used in previous epidemiologic studies [16
]. Both of the prior studies utilized immunohistochemical markers to define molecular subtypes, while we also incorporated histologic grade. Others have shown that the distinction between luminal A and B tumors can be refined by adding the proliferation marker Ki67 to ER, PR, and HER2[25
]. Given that Ki67 data were not available for our cases, we used histologic grade as a surrogate for proliferation rate given the close correlation between proliferation rate and histologic grade. Thus, our definitions for luminal A and B are different than the two previous studies, but more in keeping with the most recently proposed classification scheme[25
]. This may limit our ability to compare across studies and explain some of the differences observed.
The results of the current study taken together with the previous studies suggest that many of the traditional breast cancer risk factors are associated with luminal A tumors, and that there may be some differences with other subtypes. The associations with luminal A tumors is not surprising since these are the most common tumor type and may also reflect the selection of exposures we have focused in the current study. We have chosen to examine traditional breast cancer risk factors; these were initially identified because they are the risk factors shown to be most commonly associated with breast cancer. The majority of these risk factors have also been shown to be associated with the most common type of breast cancer, namely ER+ breast cancers. While it is reassuring that these risk factors are associated with the luminal A subtype, it does not help us to further our understanding of risk factors associated with ER- subtypes. Adding the additional markers and dividing the cases into smaller subsets limits our power to detect associations for the rare subtypes. One of the most consistent findings from this study and other studies is the inverse association between breast feeding and triple negative or basal like tumors. This finding supports the hypothesis that molecular classification of tumors may help us to better understand etiology and/or provide insights into the mechanisms by which these less common molecular subtypes develop.
This study has a number of strengths including the large study population with over 2,000 invasive breast cancer cases, the prospectively collected nature of the exposure variables, and uniform staining and scoring of molecular markers. Despite the large number of cases, we were still limited by the number of less common subtypes in particular the HER2-type and the unclassified tumors. Thus, our power to detect significant differences by these subtypes was limited. In addition, we lacked adequate power to examine these associations among premenopausal breast cancer cases. It is worth noting that the frequency of receptor status positivity and molecular subtype frequency among invasive tumors in our study population was very similar to other populations suggesting that samples included in this study are representative of the overall US population.
In conclusion, in this study we found that traditional breast cancer risk factors demonstrated different relations with the molecular subtypes of breast cancer. In general, many of the reproductive factors were most strongly associated with the luminal A subtype. In addition, we confirmed a previously reported strong inverse association between lactation and basal-like tumors. It is unclear whether classifying breast tumors according to the molecular phenotypes (ie. subtypes that are known to have prognostic importance) permits identification of differences in risk factor profiles. Additional work to determine if the differences that have been observed are due to single markers or to the molecular phenotype is necessary. It remains to be seen whether non-traditional breast cancer risk factors may exist that are associated with less common tumor subtypes. Identifying risk factors for these less common subtypes such as HER2 and basal-like tumors, which also have a poorer prognosis, has important implications for prevention of these tumor subtypes.