Breast cancer mutations HER2V777Land PIK3CAH1047Ractivate the p21-CDK4/6 –Cyclin D1 axis driving tumorigenesis and drug resistance

In metastatic breast cancer, HER2 activating mutations frequently co-occur with mutations in thePIK3CA,TP53, or E-cadherin genes. Of these co-occurring mutations,HER2andPIK3CAmutations are the most prevalent gene pair, with approximately 40% ofHER2mutated breast cancers also having activating mutations inPIK3CA. To study the effects of co-occurringHER2andPIK3CAmutations, we bred genetically engineered mice with theHER2V777L;PIK3CAH1047Rtransgenes (HP mice) and studied the resulting breast cancers bothin vivoas well asex vivousing cancer organoids. HP breast cancers show accelerated tumor formationin vivoand increased invasion and migration inin vitroassays. HP breast cancers have resistance to the pan-HER tyrosine kinase inhibitor, neratinib, but are effectively treated by neratinib plus trastuzumab deruxtecan. Proteomic and RNA-Seq analysis of HP breast cancers showed increased gene expression of Cyclin D1 and p21WAF1/Cip1 and changes in cell cycle markers. Combining neratinib with CDK4/6 inhibitors was another effective strategy for HP breast cancers with neratinib plus palbociclib showing a statistically significant reduction in mouse HP tumors as compared to either drug alone. We validated both the neratinib plus trastuzumab deruxtecan and neratinib plus palbociclib combinations using a human breast cancer patient-derived xenograft that has very similar HER2 andPIK3CAmutations. Both of these drug combinations are being tested in phase 1 clinical trials and this study provides valuable preclinical evidence for them.