Despite the expansion of the therapeutic arsenal available to oncologists, resistance to chemo and targeted therapies remains a critical issue. Recent studies interrogating the intratumor heterogeneity of solid tumors suggest that phenotypic heterogeneity between the cells of a single tumor can provide both de novo and acquired resistance. Genetic, epigenetic and allelic differences provide unique phenotypes to tumor subpopulations, creating a landscape of differential sensitivity to therapeutics throughout the tumor.

Through examining the expression of markers of the luminal, basal, and mesenchymal differentiation phenotypes of normal breast tissue, we discovered that breast cancer tumors can display dramatic intratumor cellular heterogeneity of these differentiation state markers. We hypothesized that this phenotypic heterogeneity of differentiation state was also present within breast cancer cell lines; Interrogation of the RNAseq data of 50+ breast cancer cell lines did in fact suggest that some cell lines exhibit simultaneous expression luminal, basal, and mesenchymal differentiation state markers. Further analysis of the differentiation markers through quantified immunofluorescence confirmed the presence of heterogeneity of marker expression amongst subpopulations of many of the cell lines. Our results reveal that this differentiation state heterogeneity is prominent in basal-like tumors and cell lines, and that the heterogeneity persists in 3D culture, 3D-bioprinted tumors, and in vivo.

Comparison of our findings with results of previous large-scale drug studies revealed that heterogeneous lines display increased insensitivity to targeted therapeutics compared to less heterogeneous lines, suggesting that the differentiation state heterogeneity affords these lines some resistance. To determine the therapeutic consequences of this observed heterogeneity within the breast cancer cell lines, top ranked heterogeneous lines were run through high-throughput screens to assess the effects of >120 drugs on cell phenotypes including proliferation and differentiation-marker expression. We observed that certain therapeutics had significant effects on population heterogeneity. Importantly, we have found that small-molecule kinase inhibitors, in a target dependent manner, can shift the population to homogeneity in marker expression. It is an intriguing possibility that by reducing the intratumoral heterogeneity, we could increase the sensitivity of these cells to a second therapeutic. Therefore, our ongoing efforts are focused on identifying therapies that effectively eliminate the homogenous population we can now drive, as well as on understanding the mechanisms by which these therapies alter the differentiation phenotypes.

Citation Format: Tyler Risom, Juha Rantala, Ellen Langer, Nicholas Wang, Laura Heiser, Megan Troxell, Joe Gray, Rosalie Sears. Differentiation state heterogeneity in breast cancer and its role in therapeutic response. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr LB-222. doi:10.1158/1538-7445.AM2014-LB-222