Rationale: Metastatic estrogen receptor positive (ER+) breast cancer is responsible for >50% of breast cancer related deaths. The standard-of-care for metastatic ER+ breast cancer is selective inhibitors of CDK4/6 kinases (CDK4/6i) in combination with endocrine therapy. While this treatment doubles the progression-free survival of patients, resistance to combination therapy is inevitable after 1-2 years of treatment. Unfortunately no clear treatment strategy exists for patients who stop responding to CDK4/6i so there is an urgent need to understand the mechanisms of resistance to combination therapy and develop novel therapeutic strategies for this cohort of patients.
Methods: To identify novel drivers of resistance to endocrine therapy and palbociclib (a CDK4/6i), we performed the first genome-wide CRISPR/Cas9 screens of combination resistance. These screens identified inactivation of JNK signalling, including MKK7 loss, as a major driver of resistance.
Results: We identify that ~15% of metastatic ER+ breast cancer patients refractory to endocrine therapy+CDK4/6i have low expression of both pJNK and MKK7, suggesting these cancers have defective JNK signalling. Our data demonstrates that patients with low pJNK and low MKK7 have a significantly poorer outcome following endocrine+CDK4/6i therapy. We specifically correlated MKK7 loss to an increased metastatic burden and reduced survival following endocrine therapy, and poor response to CDK4/6i in advanced ER+ BC patients. Multiple CRISPR/Cas9 knockout ER+ breast cancer cell lines validated that MKK7 loss prevents a pro-senescent response to endocrine therapy+CDK4/6i, resulting in increased cell survival. 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.
Conclusions: We have identified a significant proportion of patients treated with endocrine therapy+CDK4/6i have defective JNK pathway signalling, identifying that JNK signalling is tumour suppressive in drug refractory ER+ breast cancer. We also provide the rationale to screen advanced ER+ breast cancers for pJNK and MKK7 expression and propose the triplet combination of endocrine therapy+CDK4/6i+BH3 mimetic to improve the survival of patients with JNK deficient-treatment refractory ER+ breast cancer.