Abstract 4875: Human HER2 and PI3K H1047R cooperate to promote mammary tumorigenesis in vivo

Abstract

Abstract The HER2 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 PI3K, occurs in ∼30% of breast cancers. HER2 gene amplification and PIK3CA mutations often co-occur in breast cancer, suggesting that these two oncogenes cooperate to promote tumor growth. Whether mutant PI3K enhances HER2-induced tumorigenesis and cancer progression in vivo has not yet been examined. In addition, aberrant activation of the PI3K pathway has been implicated in resistance to the HER2 inhibitors trastuzumab and lapatinib. We have created a conditional mouse model of HER2-overexpressing (HER2+), PI3K-mutant breast cancer bearing the following three transgenes: MMTV-HER2 (human HER2), MMTV-rtTA, and TetOp-HA-PIK3CA-H1047R-IRES-Luciferase (referred to as HER2.rtTA.PIK3CA mice). The latter two transgenes allow for mammary-specific, doxycycline (DOX)-inducible expression of PI3K with the ‘hotspot’ H1047R kinase domain mutation. We induced PI3K H1047R expression with DOX in female HER2.rtTA.PIK3CA and rtTA.PIK3CA mice beginning at 4 weeks of age. HER2.rtTA.PIK3CA + DOX mice developed mammary tumors with a significantly shorter latency compared to mice expressing PI3K (p=0.0012) or HER2 (p<0.0001) alone. We confirmed that triple-transgenic tumors expressed high levels of HER2 and HA-PI3K as well as downstream signal transducers such as P-HER3, P-Akt, P-ERK, and P-S6 by immunoblot analysis. Tumors expressing both oncogenes are histologically diverse and display distinct histological phenotypes from mice expressing either oncogene alone. Whereas HER2-driven tumors primarily expressed the luminal cytokeratin (CK) 18, HER2.rtTA.PIK3CA + DOX and rtTA.PIK3CA + DOX tumors expressed both basal CK 14 and luminal CK 18. Interestingly, by microarray analysis, PIK3CA-driven tumors were associated with the claudin-low subtype, HER2-driven tumors clustered with the luminal subtype, and HER2.rtTA.PIK3CA + DOX tumors shared features of both subtypes. Co-staining with trastuzumab (to detect only human HER2) and an anti-HA antibody revealed that the majority of triple-transgenic, DOX-treated tumors expressed both human HER2 and HA-PI3K. Of note, HER2 and PI3K colocalization in the same tumor cell was increased in metastatic lesions. Experiments are underway to determine whether DOX-treated HER2.rtTA.PIK3CA tumors are resistant to the HER2 antibody trastuzumab. Taken together, these studies suggest that the co-expression of HER2 and PI3K H1047R in the mouse mammary gland accelerates tumorigenesis and promotes formation of tumors with claudin-low and luminal features. We anticipate that this mouse model will be an invaluable tool to investigate the underlying biology of HER2+, PI3K-mutant breast cancers and for preclinical testing of therapeutic strategies for this subtype of breast cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4875. doi:1538-7445.AM2012-4875