The addition of CDK4/6 inhibitors to endocrine therapy has been the most significant advancement for patients with estrogen receptor positive (ER+) breast cancer in the last decade. Combination endocrine therapy + CDK4/6 inhibition has doubled the progression-free survival of patients with advanced/metastatic ER+ breast cancer, and is now standard-of-care therapy in many countries, including Australia. Unfortunately, resistance to endocrine therapy + CDK4/6 inhibition in ER+ breast cancer is almost inevitable after 1-2 years of treatment, and CDK4/6 inhibitor resistant disease is not yet well understood.
To identify novel drivers of resistance to endocrine therapy and palbociclib (a CDK4/6 inhibitor), we performed 4 genome-wide CRISPR/Cas9 loss-of-function screens. These screens identified MKK7 amongst other apoptotic JNK pathway targets as the major determinant of resistance to endocrine therapy and CDK4/6 inhibition in ER+ breast cancer cells. Using primary and metastatic ER+ breast cancer datasets, we correlated MKK7 loss to an increased metastatic burden following endocrine therapy, and a poor anti-proliferative response to CDK4/6 inhibition in advance ER+ breast cancer patients. Multiple CRISPR/Cas9 knockout cell lines in ER+ MCF-7 and T-47D breast cancer cells validated that MKK7 loss prevented an anti-proliferative and pro-senescent response to endocrine therapy + CDK4/6 inhibition. Moreover, these cells have impaired JNK mediated stress and apoptotic responses. Importantly, we found that cells with MKK7 loss have maintained sensitivity to BH3 mimetics, drugs that induce apoptosis by targeting pro-survival proteins.
Overall, our data provides evidence that MKK7 acts as a tumour suppressor for ER+ breast cancers treated with endocrine therapy + CDK4/6 inhibition. We also provide the pre-clinical rationale for combining BH3 mimetics with endocrine therapy + CDK4/6 inhibition to improve the survival of patients with deficient apoptotic JNK signalling.