Uncovering the role of oncogenic
HER2
variants as drivers of pancreatic cancer.
Authors: Kanu Elishama , Sills Joelle , Shelley Rebecca , Ginzel Joshua , Fernandes Joseph , Agritelley Ethan , Fletcher Ash , Borowsky Alexander D. , Shi Chanjuan , Cardiff Robert D. , Nussbaum Daniel , Allen Peter J , Snyder Joshua 
Published: January 2025

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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.