MBCs are aggressive, chemoresistant tumors associated with poor outcomes (2
). Although uncommon, MBCs account for several hundred new breast cancer cases every year in the United States, thus representing a therapeutic dilemma for oncologists. With only retrospective case reviews as a basis for making recommendations, it has not been possible to define therapy guidelines. Thus, we sought to determine the relationship of MBCs to common breast cancers, particularly basal-like breast cancers given the common assumption that MBCs are basal-like cancers. We also sought to determine whether the underlying pathophysiology of MBCs would result in the identification of new drug targets.
Supplementary Fig. S8 summarizes the features of MBC defined in this study that have potential clinical and therapeutic utility. Due to low expression of hormone receptors and HER-2 as well as expression of some basal epithelial markers, MBCs have been proposed to represent a form of basal-like breast cancer (4
). However, based on the integrated analyses herein, most MBCs likely represent an independent subtype that is distinct from basal-like cancers. Their transcriptional profiles are most closely related to claudin-low cancers, a novel subgroup of receptor-negative breast cancers that are clearly different from basal-like cancers (Supplementary Fig. S3). Comparative genomic hybridization profiles, their enrichment for stem cell-like markers, and their PI3K/AKT pathway activation status also differentiate MBCs from basal-like cancers. MBCs, like claudin-low cancers, express high levels of EMT markers and show elevated CD44/CD24 and CD29/CD24 ratios, which have been proposed to represent breast cancer stem cell-like markers (46
). Indeed, a recent study detected a direct and causative link between EMT and the gain of epithelial stem cell properties (51
). These features likely contribute to the lineage plasticity of MBCs on light microscopy and to their limited chemoresponsiveness (2
). Claudin-low features, including EMT and stem cell-like properties, also potentially contribute to the aggressive phenotype of MBCs. This is supported by the significant overlap between the MBC signature and the “tumorigenic” signature,13
with overlapping genes including five of the claudin-low genes (Supplementary Fig. S3C). The MBC, the “tumorigenic,” and the claudin-low signatures are all enriched in residual post-treatment chemoresistant breast tumors. Thus, MBCs and claudin-low breast tumors may arise from a more primitive and chemoresistant “stem” cell than luminal or basal-like tumors.
The pattern of chromosomal gains and losses in MBCs is distinct from that in other breast cancers including basal-like cancers. This unique pattern suggests that the processes underlying metaplastic carcinogenesis are distinct from those associated with other breast cancer subtypes. MBCs show a high frequency of mutation, amplification, and activation of PI3K/AKT pathway components. This is markedly different from basal-like breast cancers, where we and others have shown that PI3K/AKT pathway genomic mutations are uncommon (12
We also did not detect PIK3CA
mutations in 14 claudin-low breast cancers pointing to differences between claudin-low breast cancers and MBCs. The frequency of PIK3CA
mutations combined with amplification of AKT
suggests that PI3K/AKT pathway activation is critical to metaplastic carcinogenesis. Activation of this pathway, along with enrichment for tumor-initiating/stem cells, may underlie the chemoresistance and poor outcomes associated with MBC (28
). MBCs also show a high frequency of amplification, overexpression, and activation of MAPK pathway components. As particularly important genes and targets in cancer are likely to be aberrant at the level of the genome, transcriptome, and proteome, the PI3K/AKT and MAPK pathways are therefore potentially attractive therapy targets in MBC. As inhibitors of the PI3K/AKT and MAPK pathways are now in clinical trials, it will be of interest to determine whether these inhibitors will sensitize MBCs to cytotoxic drugs (52
). As there are no MBC cell lines or animal models available, it will be necessary to develop a consortium approach to test this hypothesis in patients.
In contrast to a recently published study, mutations in exon 3 of CTNNB1
were not identified in 19 MBCs in our study (53
). This may in part relate to the fact that our study was restricted to high-grade MBCs, whereas the previous study included a significant proportion of lower-grade MBCs, tumors that behave in a less aggressive fashion than high-grade MBCs.
This study has several potential limitations. Although a subset (6 of 12, 50%) of MBCs constitute a significantly related group of tumors as defined by SigClust, it is clear that MBCs form a somewhat heterogeneous group of receptor-negative breast cancers in terms of their molecular characteristics. Just as breast tumors that are defined as ductal are clearly heterogeneous based on receptor status and transcriptional profiling, it is not surprising that MBCs represent a molecularly heterogeneous group. MBCs are microscopically heterogeneous, with this study being limited to tumors with squamous and sarcomatoid metaplasia (1
). The rarity of MBC precluded analysis of other histologic variants. In addition, it is possible that MBC represents multiple different diseases. However, there were no clear correlation between the pattern of gene copy number change and the histologic appearance of the tumors analyzed.
The major conclusions of this article are (a) MBCs are molecularly distinct from other breast cancers; (b) despite their relative histologic uniformity, MBCs are molecularly heterogeneous; and (c) claudin-low breast cancers are likely the most closely related ductal breast cancer subset to MBCs. The molecular mechanisms underlying metaplastic carcinogenesis are likely different from those associated with other breast cancer subtypes including basal-like cancers. By gene expression analysis, MBCs and claudin-low tumors share common features that suggest related cellular origins, potentially from a more primitive cell than that implicated as a precursor to luminal or basal-like tumors. It is likely that MBCs, and potentially claudin-low tumors, define a novel chemoresistant triple-negative breast cancer subgroup that exhibits a signature similar to that of breast tumor-initiating cells and of residual common breast tumor cells isolated after patient treatment. The frequency of PI3K/AKT pathway aberrations argues that this pathway should be explored as a therapeutic target in MBC. A challenge to advancing therapy for MBC patients is the infrequency of this disease. However, a centralized clinical trial effort is a feasible venture that will improve patient outcomes.