Abstract B013: Loss of the extracellular domain of human epidermal growth factor receptor 2 (HER2) p95 isoform is common in large DCIS lesions

Background/Aim: Testing for human epidermal growth factor receptor 2 (HER2) positivity in women diagnosed with ductal carcinoma in situ (DCIS) is not performed routinely, although the frequency of HER2+ DCIS is at least 30%. HER2+ DCIS lesions may be at higher risk of progression to invasive carcinoma, and specific isoforms of HER2 may lead to different malignant potential. Loss of the HER2 extracellular domain engineered in mouse models demonstrates rapid progression to invasive behavior. Evaluating isoforms of HER2 may aid in classifying DCIS lesions as higher or lower risk, and may provide insight for novel treatment and prevention approaches. Methods: A cohort of high risk DCIS cases were assembled, enriched for the presence of high risk features, including large size (>5 cm) and palpable. Immunohistochemistry (IHC) and FISH were used to identify cases that were HER2 positive. MultiplexIHC was used to detect extracellular and intracellular domains of HER2, with imaging analysis performed to identify HER2 isoforms of HER2+ specimens. Isoforms were characterized as 1) full-length/extracellular domain (ECD) intact, 2) pure/complete loss of ECD (p95 isoform), 3) subclonal populations of both full-length ECD and p95 and 4) gradient/representing partial loss of ECD per cell (reflecting post-translational cleavage). Results: Out of 51 cases, 34 (67%) cases were HER2+. In multiplexIHC analysis for HER2, 51% displayed gradient (partial) or focal (subclonal) loss of the extracellular domain. 33% displayed pure p95 isoform (complete loss of ECD). Only 16% of the cohort displayed full-length HER2 extracellular domain. Conclusions: HER2 expression is much more common in large in situ lesions than in invasive lesions. This suggests that HER2 positivity may play a role in protecting against invasion. Investigation of HER2 Isoform testing using multiplex staining reveals heterogeneity with partial or complete loss of the HER2 extracellular domain being most common. Loss of the extracellular domain of human epidermal growth factor receptor 2 (HER2) p95 isoform is predominant among patients diagnosed with HER2 positive ductal carcinoma in situ (DCIS). Citation Format: Nicole F. Schindler, Alexa Glencer, Hidetoshi Mori, Gillian Hirst, Cristian Maldonado Rodas, Michael Campbell, Laura Esserman, Alexander Borowsky. Loss of the extracellular domain of human epidermal growth factor receptor 2 (HER2) p95 isoform is common in large DCIS lesions [abstract]. In: Proceedings of the AACR Special Conference on Rethinking DCIS: An Opportunity for Prevention?; 2022 Sep 8-11; Philadelphia, PA. Philadelphia (PA): AACR; Can Prev Res 2022;15(12 Suppl_1): Abstract nr B013.