LAUSR

Overexpression of repetitive RNA is an emerging hallmark of human cancers. These diverse repetitive elements, including SINEs (Short Interspersed Nuclear Elements) and LINEs (Long Interspersed Nuclear Elements), can mimic viruses by replicating in the genome through retrotransposition and displaying pathogen-associated molecular patterns, potentially triggering inflammatory responses. However, our knowledge of how specific repeats affect tumor evolution and shape the tumor immune microenvironment in a pro- or anti-tumorigenic manner remains incomplete. We integrated evolutionary analysis on whole genome and total RNA sequencing data from a unique cohort of multiregion-sampled human primary and metastatic pancreatic ductal adenocarcinoma (PDAC). We find that the more recently evolved SINEs are more likely to form double-strand RNAs (dsRNAs), a structure known to stimulate innate immune response. Consistently, these younger SINEs are highly associated with a RIG-I associated type-I interferon signature. LINE-1 (L1) retrotransposition activity in these samples tracks with tumor phylogenetic branching and is depended on the status of TP53 mutations, supporting active differential surveillance of L1 mobility by mutant TP53 during tumor evolution. Lastly, we discovered that immunostimulatory SINE expression is anti-correlated with both ADAR1 expressions, exonuclease DIS3 expression, and L1 retrotransposons activity, suggesting a cooperative relationship between ADAR1 and L1 activity in regulating immunogenic SINE expression. We propose that pancreatic tumors modulate immunogenic SINE abundance to their pro-tumorigenic inflammatory advantage through hyperactive L1 activity via accelerated exonuclease degradation when ADAR1 is not activated. Our analysis illustrates for the first time the evolutionary role of dark matter genomic repeats in allowing tumors to actively leverage viral-like innate immune responses to manipulate their immune microenvironment for selective advantage. Citation Format: Siyu Sun. Repetitive elements and viral mimicry co-evolve with the immune microenvironment in pancreatic ductal adenocarcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 665.