Abstract P4-04-20: Subtype specific differential expression and immunogenicity of endogenous retrovirus elements in breast cancer

Background: Endogenous retroviruses (ERVs) are germline encoded DNA sequences that entered the human genome millions of years ago. While they are mostly inactivated due to accumulated termination codons and deletions, previous studies have demonstrated overexpression and antibody-targeted immunotherapeutic potential, of ERV-related env proteins in breast cancer. We sought to elucidate subtype specificity, immunogenicity, and correlation with innate immunity related gene signatures of ERVs in breast cancer. Methods: We utilized publically available RNASeq gene expression data of breast cancer samples along and corresponding matched normals from TCGA. The dataset included 191 ER-/HER2- (TN), 197 HER2+, and 627 ER+/HER2- (Luminal) breast cancers. ERV expression was obtained by mapping bowtie2-aligned reads of recently annotated to be transcriptionally active to the RNAseq bam files (Rooney et al 2015, Mayer et al 2011). ERVs preferentially expressed in tumors compared normal tissue were identified as those for which the 5th percentile of ERV expression in the tumors exceeded the 95th percentile of ERV expression in the normal samples. A gene signature involving GZMB, PRF1, CXCL13, IRF1, IKZF1, and HLA-E was used as a measure of immune activity. To assess the immunogenic potential of the tumor-specific ERVs, we compared the expression level of the ERV within the lower and higher immune signature tertiles using the Wilcoxon signed-rank test. To elucidate mechanism of potential immune response, ERVs found to be significantly associated with immune response at a false discovery rate of Results: Out of the 66 original annotated ERVs, 47 were found to be expressed at significantly higher levels in breast cancer compared to normal tissue and 22 were immunogenic. Examples include members of the ERV-K family, as have also been previously detected by flow cytometry and IHC. Subtype-specific immunogenic potential was demonstrated in 4 ERVs in TNBC (ERVK10, ERVK17, ERVFRD.1, ERVPABLB.1) and in 7 ERVs in the luminal subtype (ERV3.1, ERVE.4, ERVFRD.2, ERVK.15, ERVK.19, ERVK.20, ERVK.25, ERVW.3). Twelve of the 22 immunogenic ERVs were significantly correlated with expression of all ten TLR evaluated, while four ERVs showed more specific correlation patterns with TLRs. High ERV3.1 expression was associated with high TLR3, TLR8, TLR9 that specifically target double stranded or single stranded RNA, suggesting a potential mechanism for mediation of ERV related immune response. Conclusion: Our results suggest breast cancer subtype specific ERV dysregulation and immunogenicity. The potentially immunogenic ERVs were generally not self-correlated or located in the same amplicon as HLA genes, suggesting an independent immune response pathway. Furthermore, ERV expression correlates with specific endosomal nucleic-acid recognizing toll-like receptors, which may prompt further investigation into subtype-specific TLR-targeted therapy. Citation Format: Safonov A, Bianchini G, Jiang T, Pusztai L, Hatzis C. Subtype specific differential expression and immunogenicity of endogenous retrovirus elements in breast cancer. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P4-04-20.