Abstract 2913: The SOX11 transcription factor is critical for basal-like breast cancer growth and migration and is associated with poor survival

Introduction: Basal-like breast cancers (BLBCs) are aggressive breast cancers that are associated with poor survival and have distinctive mRNA expression profiles compared to non-BLBC tumors. Through an integrated screen of transcription factors (TFs) we identified SOX11 as uniquely critical for growth in BLBC, but not essential in estrogen-receptor (ER)-positive breast cancer cells. In this study we examine the role of SOX11 regulating growth and other BLBC phenotypes, including increased migration, invasion and poor survival. Methods: SOX11 was identified through an integrated primary screen comparing mRNA expression of TFs, DNA sequences of differentially expressed BLBC genes, and differential binding of nuclear proteins to oligonucleotide sequences of transcription factor binding sites, and a secondary siRNA screen of growth in a panel of breast cancer cell lines. Further growth studies were performed using siRNA to SOX11 and measuring 2D-growth, or 3D-growth of cell lines grown in soft-agar. Immuno-fluorescent staining of phosphorylated histone H3 (pH3) was used to measure the effect of SOX11 depletion on proliferation. Migration and invasion assays were performed with membranes coated in Matrigel (invasion) or uncoated (migration). Survival analyses were performed with patients dichotomized by median SOX11 expression level, and log rank statistical comparisons as well as a multivariable Cox Proportional Hazards model to investigate the prognostic value of SOX11. Results: SOX11 depletion resulted in reduced growth of ER-negative breast cancer cell lines, including ER-negative/ Her2-positive cells; whereas, growth of ER-positive breast cancer cells or ER-negative, non-transformed breast cells was not affected. In MDA-MB-468 cells, SOX11 depletion resulted in a 60% reduction of pH3-positive cells. Additionally, depletion of SOX11 in MDA-MB-231 or MDA-MB-468 cells reduced migration and invasion 40-60%, while over-expression of SOX11 in MCF7 cells increased migration nearly 12-fold. High SOX11 is correlated with worse prognosis, whether analyzed for all patients or subsets of patients divided by ER or HER2 status. In the multivariable analysis accounting for tumor size, grade, lymph node status, and PAM50 class, high SOX11 expression was associated with increased risk of breast cancer related death with a hazard ratio of 1.42 and p-value Conclusions: SOX11 is a critical regulator of multiple BLBC phenotypes, including growth, migration, invasion, and expression of signature BLBC genes. Growth reduction of BLBC cells following SOX11 depletion is a result of decreased proliferation. High SOX11 expression was also found to be an independent prognostic indicator of poor survival in women with breast cancer. These results identify SOX11 as a potential target for the treatment of BLBC, the most aggressive form of breast cancer. Citation Format: Jonathan Shepherd, Abhijit Mazumdar, Anna Tsimelzon, Susan G. Hilsenbeck, Powel H. Brown. The SOX11 transcription factor is critical for basal-like breast cancer growth and migration and is associated with poor survival. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2913.