Abstract 1200: HER-2 Isoform Interaction in Mammary Carcinoma Onset and Progression

Abstract Human breast cancer cells express full-length HER-2 along with proteins resulting from mutation, alternative splicing, alternative initiation of translation and post-translational modification. The Delta16 splice variant, lacking exon 16, has the properties of an activated oncogene, but it could also play beneficial roles in the response to targeted therapeutic agents. To study mammary carcinogenesis in a mouse model that mimics human coexpression of full-length HER-2 and Delta16 isoforms, we produced hybrid mice bearing heterozygous copies of both human transgenes (F1 mice), and we compared them to parental mice (Delta16 and HER-2 transgenic mice, respectively). Tumor onset in F1 and Delta16 mice was much faster than in HER-2 mice (30 vs >80 weeks), but the growth of established tumors and metastatic spread were not enhanced. Each mammary carcinoma of F1 mice expressed both isoforms at variable ratios. Most (∼80%) expressed high levels of Delta16 and low levels of full length HER-2, a few (∼5%) expressed full-length HER-2 and little, if any, Delta 16, and the remainder (∼15%) coexpressed at high level both isoforms. The study of tumor vascularization showed that full-length HER-2 tumors mainly contained few large vessels or vascular lacunae, whereas Delta16 tumors were perfused by numerous endothelium-lined small vessels. F1 tumors with high full-length HER-2 expression made few large vessels, whereas tumors with low full-length HER-2 and high Delta16 contained numerous small vessels and expressed high levels of both VEGF and VEGFR2. Administration of trastuzumab to young F1 mice effectively prevented mammary carcinoma onset in ∼85% of mice at 1 year of age. The preventive effect of trastuzumab was stronger in F1 mice than in both parental strain. To analyze the intrinsic sensitivity of F1 mammary carcinoma cells to targeted drugs in 3D, we established cell lines expressing different HER-2 isoform ratios. High Delta16 expression caused high sensitivity to HER-2 inhibitors (trastuzumab, neratinib, lapatinib), whereas high full-length HER-2 was associated with a lower sensitivity. Interestingly, high levels of both isoforms was associated with resistance to trastuzumab, but sensitivity to small molecule inhibitors. In conclusion, the coexpression of full-length HER-2 and Delta16 controls several determinants of mammary carcinoma development, progression and sensitivity to targeted agents, as revealed by the study of F1 mice. Citation Format: Arianna Palladini, Massimiliano Dall’Ora, Tania Balboni, Giordano Nicoletti, Marianna Ianzano, Roberta Laranga, Lorena Landuzzi, Veronica Giusti, Alessia Lamolinara, Carla De Giovanni, Augusto Amici, Serenella M. Pupa, Manuela Iezzi, Patrizia Nanni, Pier-Luigi Lollini. HER-2 isoform interaction in mammary carcinoma onset and progression. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1200.