Abstract 1790: PDJ amplicon heterogeneity in triple negative breast cancers

Background: Amplification of chromosome 9p24.1 targeting PD-L1, PD-L2, and JAK2 (PDJ amplicon) is present in ~15% of TNBCs and is associated with activation of the JAK/STAT pathway and poor clinical outcomes. In preclinical studies, PD-L1 expression is markedly and rapidly inducible by low-dose IFN-γ in a PDJ amplicon copy-number dependent manner, mimicking an in situ inflammatory response. This suggests PDJ amplification could identify those TNBCs that would respond to ICIs and/or JAK2 targeted-therapies. The goals in this study were to determine the PDJ status of a well annotated cohort of TNBC PDXs with a 3 color FISH assay and to further define the genomes and transcriptomes of PDJ+ and PDJ- TNBCs. Methods: Tissue microarrays (TMAs) of 49 cores from 27 breast cancer PDX samples, including 21 TNBCs were obtained from the Baylor College of Medicine. The TMAs were interrogated with a 3 color FISH assay consisting of three probes that simultaneously target the PDJ amplicon, 9q22, and 9q34.1. The ratios of PDJ to 9q22 and PDJ to 9q34.1 were calculated to distinguish PDJ amplification from polyploidy. In each case, 50 cells were scored. Samples with PDJ signal >4.0, PDJ/9q22 ≥2 and/or PDJ/9q34.1 ≥2 were called positive by FISH. A subset of PDJ+ PDXs was then flow sorted by DNA content and interrogated with 400k whole genome CNV arrays. These results were integrated with existing clinical, exome, and transcriptome data for the TMAs (pdxportal.research.bcm.edu). Results: PDJ amplification was identified in 6/27 PDX samples including 3/21 (14%) TNBCs. Notably the number of PDJ+ cells in each sample ranged from 2-18. Single cells with ≥10 copies were observed in each PDJ+ sample as well as two PDJ- TNBCs. PDJ+ samples had increased expression of PD-L1 and JAK2 in their RNAseq data. Whole genome CNV analysis of flow sorted PDJ+ samples confirmed the presence of the PDJ amplicon and identified co-occurring amplifications and deletions. A similar pattern of PDJ amplicon heterogeneity was seen in a Mayo Clinic Arizona cohort of 70 surgically resected TNBCs. Previous whole genome CNV analysis of flow sorted samples identified 13/70 (18%) PDJ+ cases. FISH analysis of 14 TNBCs from this cohort (PDJ+ and PDJ- cases) validated these results and identified PDJ+ tumors with cells containing >20 copies of the amplicon. Conclusion: Our multicolor FISH assay of PDX and primary resected samples confirms a prevalence of ~15% PDJ+ TNBCs. Previous studies have reported an enrichment of JAK2 amplification in neoadjuvant treated TNBCs. Given the heterogeneous FISH patterns in both PDJ+ and PDJ- samples in our cohorts, we hypothesize that neoadjuvant therapy could enrich the presence of PDJ+ cells and thereby sensitize TNBCs to ICIs and JAK2 targeted therapies. Future work will explore the role of PDJ amplification and the presence of PDJ+ cells in TNBCs in response to neoadjuvant therapy. Citation Format: Alexander S. Roesler, Smriti Malasi, Elizabeth Lenkiewicz, Barbara A. Pockaj, Michael T. Lewis, Lacey E. Dobrolecki, Lori Koslosky, Peter Hartmayer, Karen S. Anderson, Michael T. Barrett. PDJ amplicon heterogeneity in triple negative breast cancers [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1790.