MED12 Mutations Play a Pivotal Role in Breast Cancer Oncogenesis

Abstract

There is a critical need for superior breast cancer treatment since this disease remains the second leading cause of cancer deaths among American women. Though radiation and chemotherapy have been the frontline choice of treatment for cancer over the past few decades, due to the detrimental side effects, personalized treatment is rapidly rising as a superior treatment method. This is largely due to recent advances in genome‐wide DNA sequencing that have allowed for the identification of cancer‐related mutational landscapes. Through these technologies, MED12, a subunit of Mediator protein, has been found to be commonly mutated in a significant number of breast cancer patients. Therefore, uncovering the mechanistic basis behind MED12 mutant breast cancer is critical in order to find a personalized treatment regimen for this subclass of breast cancer. Previous work has shown that MED12 restricts GLI3‐dependent Sonic Hedgehog (SHH) signaling, a pathway that is often hyper activated in advanced breast cancer. We therefore hypothesize that mutated MED12 promotes GLI3‐dependent SHH signaling in breast cancer cells thereby promoting tumorigenesis and that this signaling pathway could be an effective therapeutic target in MED12 mutant breast cancer. To study the effect of mutant MED12, a lentivirus carrying an shRNA against MED12 was generated and infected into MCF‐7 cells. Since the vast majority of breast cancer‐associated MED12 mutations lead to loss of protein function, the knockdown strategy through lentiviral shRNA is assumed to mimic the MED12 mutant setting. Proliferation assays were utilized to confirm that downregulated MED12 plays a role in the increased proliferation of breast cancer cells. Next, quantitative PCR was performed to determine the effect of downregulated MED12 on the expression of genes that are known to be regulated by GLI3. Finally, a screening strategy was employed by using a natural compound library to find potential novel treatment strategies for MED12 mutant breast cancer. Our findings confirmed that downregulation of MED12 increases proliferation of breast cancer cells in vitro thereby providing a strong argument that mutant MED12 would have the same effect. Through quantitative PCR it was confirmed that GLI3 target genes are upregulated in cancer cells when MED12 expression is low. Importantly, our natural compound screen identified several novel therapeutic compounds that specifically target MED12 downregulated breast cancer cells through a mechanism that involves SHH signaling. We therefore uncovered that MED12 mutations promote GLI3‐dependent SHH signaling in breast cancer and we identified potential novel therapeutic strategies for MED12 mutant breast cancer patients.