Phenotypic heterogeneity drives differential disease outcome

The triple-negative breast cancer (TNBC) subtype is one of the most aggressive forms of 14 breast cancer that has poor clinical outcome and still remains as an unmet clinical challenge. Accu15 mulating evidence suggests that intratumoral heterogeneity or the presence of phenotypically het16 erogeneous cell populations within a tumor plays a crucial role in chemoresistance, tumor progres17 sion and metastasis. Increased understanding of the molecular regulators of intratumoral heteroge18 neity will enable the development of effective therapeutic strategies in TNBC. We have identified a 19 molecular mediator involved in intratumoral heterogeneity in breast cancer using an unbiased ap20 proach. We isolated two heterogeneous tumor cell populations from the 4T1 TNBC tumor model 21 and phenotypic characterization revealed that the cells are distinct in terms of their morphology, 22 proliferation and self-renewal ability in vitro; as well as primary tumor formation and metastatic 23 potential in vivo. Further, RNA sequencing on both cell populations was performed to identify the 24 molecular mediators underlying this heterogeneity. Bioinformatic analysis performed on the differ25 entially expressed genes along with the Kaplan-Meier survival analysis in TNBC patients identified 26 Metastasis associated colon cancer 1 (Macc1) as the top candidate gene mediating the aggressive 27 phenotype. The role of Macc1 in regulating the proliferative phenotype was validated using siRNA 28 mediated gene knockdown. The role of Macc1 in the aggressive cancer cell phenotypes and disease 29 progression is being studied further using a small molecule transcriptional inhibitor of Macc1 in cell 30 line and animal models, thus increasing our understanding of the molecular underpinnings of in31 tratumoral heterogeneity in breast cancer that is critical to the improvement in treatment of women 32 currently living with the highly aggressive TNBC subtype. 33