Uncovering the role of oncogenic <i>HER2</i> variants as drivers of pancreatic cancer.

772 Background: The therapeutic resistance of pancreatic ductal adenocarcinoma (PDAC) to conventional cytotoxic therapy underscores the need for advances in targeted molecular therapeutics. Previous studies have reported approximately 2% of PDAC patients with HER2 gene amplification. While anti- HER2 cancer therapy has yielded success in other disease sites, it has had minimal reported benefit in PDAC. Multiple HER2 isoforms exist that are generated either by loss of exon 16 ( d16 HER2 ), or through N-terminal truncations ( p95 HER2 ). These isoforms are implicated in differential tumor behavior and response to anti-HER2 therapy in the breast but have not been investigated in PDAC. Herein, we hypothesize that these isoforms may underlie the observed resistance to HER2 therapy in PDAC. Methods: To test for the presence of HER2 structural variants within human PDAC, a tissue microarray was constructed of fifty-one primary tumor samples and two metastatic samples. Multiplex immunohistochemistry (mIHC) was performed for staining of the extracellular N-terminal and intracellular C-terminal HER2 domains. We then utilized our previously described HER2 + cancer rainbow (Crainbow) mouse model in which human wild-type HER2 ( WT HER2 ), d16 HER2 , and p95 HER2 were expressed in the same mouse along with fluorescent protein reporters. HER2 Crainbow mice were crossed with PDX1-Cre recombinase to initiate recombination and expression of each fluorescently labelled HER2 isoform during pancreas development. PDX1-Cre / HER2 Crainbow mice were followed for up to one year and sacrificed to lineage trace each isoform and register HER2 lineages with histopathology. Results: In our human data, four of fifty-three patient samples (three primary and one metastatic) had moderate-to-strong HER2 staining. All samples identified on mIHC as HER2 positive stained primarily for the p95 variant of HER2 , with absence of the N-terminal domain and preservation of the intracellular domain. In our transgenic mice, 65% of the mouse cohort developed pre-malignant low grade pancreatic intraepithelial neoplasia (PanIN) by eight weeks. At one year, 58.3% of mice demonstrated advanced high grade PanIN, and 25% developed invasive disease. Lineage tracing revealed that while early low grade PanIN began as polyclonal lesions, comprised of WT- , d16- , and p95 HER2 expressing cells, advanced PanIN and invasive disease were unanimously monoclonal with one dominant isoform arising. WT HER2 was represented abundantly in low grade disease, but high grade and invasive disease were predominantly driven by d16- and p95-HER2. Conclusions: Our data implicates the oncogenic d16- and p95 HER2 variants, rather than WT HER2 , as the primary drivers of HER2+ PDAC tumorigenesis. In breast p95 HER2 has been described as a known cause of Trastuzumab resistance. Therefore, future efforts will be aimed at characterizing the mechanisms driving therapeutic resistance in p95-HER2 PDAC tumors.