Abstract 2314: d16HER2 splice variant regulates the activity of HER2-positive breast cancer-initiating cells

Abstract

Abstract The transmembrane tyrosine kinase receptor HER2, overexpressed in ∼20% of human breast cancers (BCs), identifies an aggressive tumor subtype and is reportedly an important regulator of breast cancer-initiating cell activity (BCIC). We found that ∼90% of HER2+ BC patients constitutively express a splice isoform of wild-type HER2 (WTHER2) gene characterized by the lack of exon 16 (d16HER2), a deletion that promotes the generation of a particularly aggressive HER2 isoform and that forms stable and constitutively activated d16HER2 homodimers. Our comparison of the tumorigenic potential of the human d16HER2 and WTHER2 genes in the corresponding transgenic mouse models revealed a significantly shorter tumor latency period (p< 0.001) and a higher tumor incidence in the d16HER2 mice (p<0.001), suggesting a role for this variant in HER2-driven activation of BCICs. In this context, our preliminary analyses of HER2-positive primary mammary tumor cell lines MI6 and MI7 derived from spontaneous transgenic d16HER2 mice showed a significantly higher mammosphere-forming efficiency and higher levels of stem cell marker transcripts, including CD44, Wnt, Notch and Bmi1, compared to transgenic mouse WTHER2 tumor cells (WTHER2_1 and WTHER2_2). Mammospheres generated from human HER2-overexpressing breast cancer cell lines BT474 and MDA-MB-361, which also express the d16HER2 variant, exhibited an increase in the relative abundance of d16HER2 mRNA compared with that in the same parental cells cultured in adhesion conditions, as indicated by qPCR analyses. Experiments in mice injected into the mammary fat pad with the d16HER2- and WTHER2-positive cell lines at different serial dilutions indicate a consistently higher “stemness potential” of MI6 and MI7 cells compared to WTHER2_1 and WTHER2_2 cells, strongly suggesting that the d16HER2 variant plays a greater role than WTHER2 in regulating BCICs of HER2-driven mammary tumors. Supported by Minister of Health and AIRC Citation Format: Lorenzo Castagnoli, Ada Koschorke, Gaia C. Ghedini, Lorenzo Galvani, Valentina Ciravolo, Cristina Ghirelli, Arianna Palladini, Alessia Lamolinara, Manuela Iezzi, Pier Luigi Lollini, Tiziana Triulzi, Patrizia Nanni, Elda Tagliabue, Serenella M. Pupa. d16HER2 splice variant regulates the activity of HER2-positive breast cancer-initiating cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2314. doi:10.1158/1538-7445.AM2015-2314