Effects of PIK3CA mutations on mammary cell fate and cancer

The adult mouse mammary epithelium contains self-sustained cell lineages that form the inner luminal and outer basal cell layers, with stem and progenitor cells contributing to its proliferative and regenerative potential. A key issue in breast cancer biology is the effect of genomic lesions in specific mammary cell lineages on tumor heterogeneity and progression. The impact of transforming events on fate conversion in cancer cells-of-origins and thus their contribution to tumor heterogeneity remains largely elusive. The phosphoinositide 3-kinase (PI3K) signaling pathway is crucial for cell growth, proliferation, metabolism and survival and is frequently deregulated in human cancer, including ~70% of breast tumors. The gene encoding for the alpha catalytic subunit of PI3K (PIK3CA) is mutated and/or amplified in over 30% of breast cancers. Mutations in either the kinase domain (H1047R) or the helical domain (E545K) are most common and result in a constitutively active enzyme with oncogenic capacity. PIK3CA H1047R was previously shown to induce heterogeneous mammary tumors in transgenic mouse models. Whether overexpression of the PIK3CA E545K mutant is sufficient to induce mammary tumors in transgenic mice has not been defined. Moreover, the origin of PIK3CA H1047R-evoked tumor heterogeneity and the influence of the cell-of-origin on aggressiveness of breast cancer have remained elusive. In my PhD studies, we demonstrate that expression of PIK3CA E545K in the mouse mammary gland induces heterogeneous mammary carcinomas but with a longer latency than PIK3CA H1047R suggesting that the helical domain mutant PIK3CA E545K is a less potent inducer of mammary tumors. Furthermore, by using in situ genetic lineage tracing, gene expression analyses and limiting dilution transplantation, we have unraveled the potential of PIK3CA H1047R to induce multipotency during tumorigenesis in the mammary gland. We show that expression of PIK3CA H1047R in lineage-committed basal Lgr5-positive and luminal keratin 8-positive cells of the adult mouse mammary gland evokes cell dedifferentiation into a multipotent stem-like state, suggesting this to be a mechanism involved in the formation of heterogeneous, multi-lineage mammary tumors. Moreover, we show that the tumor cell-of-origin influences the frequency of malignant mammary tumors. Our results define a key effect of PIK3CA H1047R on mammary cell fate in the pre-neoplastic mammary gland and show that the cell-of-origin of PIK3CA H1047R tumors dictates their malignancy, thus revealing a mechanism underlying tumor heterogeneity and aggressiveness.