Abstract P1-05-01: The tumor immune microenvironment and HER2 landscape of high-risk ductal carcinoma in situ: The DEFENSE study
Authors: Glencer Alexa , Borowsky Alexander , Mori Hidetoshi , Campbell Michael , Harismendy Olivier , Stein Janet , Ghule Prachi , Evans Mark , West Robert , Hirst Gillian , Schindler Nicole , Miller Phoebe , Lee Kyra , Weaver Donald , Esserman Laura 
Published: March 2022

Introduction: Ductal carcinoma in situ (DCIS) of the breast is a premalignant lesion representing a spectrum of biology from indolent to aggressive. A minority of women present with clinically high-risk features associated with poor outcome. Yet even in patients with biologically aggressive DCIS, the risk of breast cancer mortality is only 3.3% compared to 30-40% in patients with biologically aggressive invasive cancer of the same size.1 We hypothesize that the tumor immune microenvironment could play a proactive role in preventing invasion in high grade clinically high-risk DCIS and that HER2 status, including specific HER2 isoform expression and post-translational modification of HER2, could impact progression. Methods: DCIS: Elaboration of Factors from Enlarged lesions that Nevertheless remain Stage 0 Entities (DEFENSE) is a study of high-risk DCIS, defined as having at least two of the following characteristics: large (&gt;5cm), high grade, hormone receptor-negative status and/or HER2+ status. Slides obtained from FFPE tissue blocks were stained with fluorescence-based multiplex immunohistochemistry (mIHC) panels and imaged to characterize immune infiltrate within the ducts and the stromal compartments. mIHC was also 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). Clinical characteristics were correlated with molecular profile, tumor immune infiltrates, and HER2 isoform. Finally, expression profiling with a 44k array was conducted by Agendia, and MammaPrint and BluePrint results were generated. Results: Of 92 total patients, median age is 46 years. The average DCIS lesion size is 8.2cm, and 33% are hormone receptor negative (HR-). Based upon initial analysis, mIHC demonstrates significant heterogeneity in immune infiltrate populations of pathologically identical DCIS specimens and within regions of the same specimen. High-grade HR- disease has highly reactive stroma, characterized by dense CD3+, CD34+, and CD68+ infiltrate within the stromal compartment. HER2 testing of the first 51 cases demonstrates a high rate of positivity of 67% (34/51). Of those tested for HER2 isoform expression (n=21), none had homogeneous intact full-length HER2. Six (29%) demonstrate the pure p95 isoform, 3 (14%) demonstrate the subclonal isoform, and 12 (57%) demonstrate a gradient HER2 isoform phenotype. Preliminary data from expression profiling shows that the HER2+ cases are also HER2 intrinsic sub-type by BluePrint. Across all of the high-risk DCIS cases, all were scored as MammaPrint high risk, either Luminal B or HER2-type, with only 1 basal and no Luminal A. Additional analyses are ongoing, including completion of testing for the whole data set as well whole exome DNA sequencing and SMART-3SEQ RNA sequencing. Conclusions: Clinically high-risk and pathologically homogenous DCIS lesions demonstrate significant immune infiltrate heterogeneity. Nearly 70% of these large clinically high-risk DCIS lesions are HER2+ with HER2 isoforms most commonly representing either partial or complete loss of the HER2 ECD. This is significantly higher than what is reported for invasive HER2+ breast cancer. A comparison to size and molecularly matched invasive cancers, including from the I-SPY 2 trial, is underway in an effort to elucidate how molecularly aggressive lesions remain in situ despite their large size. References:1 Narod SA et al (2015). Breast Cancer Mortality After a Diagnosis of Ductal Carcinoma in Situ. JAMAOncol. 1(7): 888-96.

Citation Format: Alexa Glencer, Alexander Borowsky, Hidetoshi Mori, Michael Campbell, Olivier Harismendy, Janet Stein, Prachi Ghule, Mark Evans, Robert West, Gillian Hirst, Nicole Schindler, Phoebe Miller, Kyra Lee, Donald Weaver, Laura Esserman. The tumor immune microenvironment and HER2 landscape of high-risk ductal carcinoma in situ: The DEFENSE study [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P1-05-01.