Abstract A007: Mutant PIK3CA accelerates HER2-driven transgenic mammary tumor progression, enhances cancer stem cell features, and induces resistance to combinations of anti-HER2 therapies
Authors: Hanker Ariella B. , Pfefferle Adam , Balko Justin M. , Kuba Maria Gabriela , Young Christian D. , Sanchez Violeta , Sutton Cammie R. , Cheng Hailing , Perou Charles M. , Zhao Jean J. , Cook Rebecca S. , Arteaga Carlos L. 
Published: May 2015

Abstract
The HER2 (ERBB2) oncogene is amplified in 20-25% of breast cancers and is associated with poor patient outcome. Mutational activation of PIK3CA, the gene encoding the p110α catalytic subunit of phosphatidylinositol 3-kinase (PI3K), occurs in ~30% of breast cancers. HER2 amplification and PIK3CA mutations often co-occur in breast cancer. Aberrant activation of the PI3K pathway has been shown to correlate with a diminished response to HER2-directed therapies. We sought to directly determine whether mutant PI3K enhances HER2-induced tumor progression and promotes resistance to HER2 inhibition in vivo. For this purpose, we generated a conditional transgenic mouse model of HER2-overexpressing (HER2+), PIK3CAH1047R-mutant breast cancer. Mice expressing both human HER2 and mutant PIK3CA in the mammary epithelium developed tumors with shorter latencies compared to mice expressing PIK3CA alone (p=0.0012) or HER2 alone (p&lt;0.0001). HER2 and mutant PIK3CA also strongly cooperated to promote lung metastases following tail vein injection of cells derived from transgenic tumors (p&lt;0.01). PIK3CA and HER2+/PIK3CA tumors were histologically diverse (i.e., adenocarcinoma, adenomyoepithelioma, adenocarcinoma with squamous metaplasia, and metaplastic carcinoma), expressed luminal and basal cytokeratins by IHC, and were associated with the claudin-low subtype of human breast cancers by microarray analysis. Accordingly, HER2+/PIK3CA tumors expressed elevated transcripts encoding EMT and stem cell markers, such as Snail, Slug, Twist, Bmi1, ITGB1 (CD29), and Procr. Further, single cells from HER2+/PIK3CA tumors formed more and larger mammospheres compared to cells from HER2-driven tumors. HER2+/PIK3CA mammospheres and tumors were completely resistant to the HER2 monocolonal antibody trastuzumab, alone and in combination with the HER2 tyrosine kinase inhibitor lapatinib or with pertuzumab, an antibody that inhibits ligand-induced HER2 dimerization. Both drug resistance and enhanced mammosphere formation were reversed by treatment with the pan-PI3K inhibitor BKM120. BKM120 treatment strongly inhibited T308 and S473 pAKT, whereas pAKT levels were unaffected by trastuzumab in combination with lapatinib or with pertuzumab. We further generated a model of resistance to the combination of trastuzumab, pertuzumab, and BKM120 by long-term treatment of HER2+/PIK3CA tumor transplants. Tumors initially responded, but a subset (3/11) resumed growth in the presence of drugs within 2 weeks of treatment initiation. Resistance was maintained following passaging in mice. We are currently performing whole-exome sequencing on resistant vs. untreated tumors in order to determine the mechanism(s) of resistance. In sum, PIK3CAH1047R accelerates HER2-mediated breast epithelial transformation and metastatic progression, alters the intrinsic phenotype of HER2-overexpressing cancers and generates resistance to FDA-approved combinations of anti-HER2 therapies. We propose the mouse model presented herein will be a valuable tool to investigate the underlying biology of HER2+/PI3K-mutant tumors and for the preclinical testing of therapeutic strategies against this subtype of breast cancer.
Citation Format: Ariella B. Hanker, Adam Pfefferle, Justin M. Balko, Maria Gabriela Kuba, Christian D. Young, Violeta Sanchez, Cammie R. Sutton, Hailing Cheng, Charles M. Perou, Jean J. Zhao, Rebecca S. Cook, Carlos L. Arteaga. Mutant PIK3CA accelerates HER2-driven transgenic mammary tumor progression, enhances cancer stem cell features, and induces resistance to combinations of anti-HER2 therapies. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Breast Cancer Research: Genetics, Biology, and Clinical Applications; Oct 3-6, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2013;11(10 Suppl):Abstract nr A007.