Abstract 5778: Breast cancer mutations HER2V777L and PIK3CAH1047R activate the p21-CDK4/6 -Cyclin D1 axis driving tumorigenesis and drug resistance

Abstract In metastatic breast cancer, HER2 activating mutations frequently co-occur with mutations in the PIK3CA, TP53, or E-cadherin genes. Of these co-occurring mutations, HER2 and PIK3CA mutations are the most prevalent gene pair, with approximately 40% of HER2 mutated breast cancers also having activating mutations in PIK3CA. To study the effects of co-occurring HER2 and PIK3CA mutations, we bred genetically engineered mice with the (loxP-STOP-loxP) HER2V777L; PIK3CAH1047R transgenes (HP mice) and studied the resulting breast cancers both in vivo as well as ex vivo using breast cancer organoids. HP mice rapidly developed invasive mammary adenocarcinoma at a median time of 2.1 weeks after adenoviral Cre injection into the mammary gland. Organoids from these breast cancers showed increased number of buddings in branching morphogenesis assay and increased migration and invasion in vitro. In vivo, HP breast cancers are resistance to the pan-HER tyrosine kinase inhibitor, neratinib, but are effectively treated by the combination of neratinib plus trastuzumab deruxtecan (T-DXd). Ex vivo, we found strong synergy between neratinib and T-DXd in HP organoids. Proteomic and RNA-seq analysis of HP breast cancers showed increased gene expression of CCND1 (cyclin D1) and CDKN1A (which encodes p21WAF1/Cip1) and changes in cell cycle markers. An increase in p-p53, p-p27, and p-PDK1 in HP organoids was seen. The GSEA analysis showed that the mTOR pathway and the MYC target signature were significantly upregulated in the HP organoid group. As p21 stabilizes the cyclin D1-CDK4/6 complex to further activate CDK4/6, we found CDK4/6 inhibitors inhibit cell proliferation in HP mice-derived organoids. 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 T-DXd and neratinib plus palbociclib combinations using a human breast cancer patient-derived xenograft that has HER2 and PIK3CA mutations very similar to our transgenic mouse. This study provides valuable preclinical evidence for these drug combinations, which are being tested in phase 1 clinical trials. Citation Format: Xiaoqing Cheng, Yirui Sun, Maureen Highkin, Nagalaxmi Vemalapally, Xiaohua Jin, Brandon Zhou, Julie L. Prior, Ashley R. Tipton, Shunqiang Li, Anton Iliuk, Samuel Achilefu, Ian S. Hagemann, John Edwards, Ron Bose. Breast cancer mutations HER2V777L and PIK3CAH1047R activate the p21-CDK4/6 -Cyclin D1 axis driving tumorigenesis and drug resistance. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5778.