Abstract 7480: Human LINE-1 ORF1p retrotransposition independent function in pancreatic cancer
Authors: You Eunae , Patel Bidish , Rojas Alexandra , Ho Natalie , Sun Siyu , Raabe Michael , Song Yuhui , Phillips Ildiko , Xu Katherine , Richieri Peter , Nieman Linda , Greenbaum Benjamin , Ting David 
Published: March 2024

Abstract
The aberrant expression of repeat elements is a common feature found in both mouse models and human pancreatic ductal adenocarcinoma (PDAC). The majority of these are non-coding RNAs that have shown the ability to stimulate a viral-like innate immune response. The long interspersed nuclear element 1 (LINE-1) retrotransposon is one of the few protein coding repeat elements in the human genome. LINE-1 has two open reading frames, ORF1 and ORF2. The former (ORF1p) encodes a ribonucleotide protein and the latter (ORF2p) a reverse transcriptase and endonuclease. Although nuclear ORF1p and ORF2p is required for retrotransposition, ORF1p is primarily localized in the cytoplasm, which suggests the possibility of ORF1p function independent of LINE-1 retrotransposition. Here, we show that ORF1p has a function to shield repeat RNAs from activating pathogen recognition receptor (PRR) mediated antiviral response in PDAC cells. LINE-1 ORF1p-targeting shRNA significantly inhibits patient-derived PDAC tumor growth both in vitro and in vivo. Total RNA sequencing (RNA-seq) reveals distinct transcriptional profiles with ORF1p deficiency. Gene set enrichment analysis (GSEA) identified high enrichment of innate immune response-related pathways including IL6-JAK-STAT3 signaling, inflammatory response, interferon response, and apoptosis in ORF1 knockdown PDAC cell lines. These immune related pathways are suppressed by deletion of RIG-I and MAVS, two components of viral RNA sensing. Mechanistically, we demonstrated that ORF1p is associated with repeat RNAs including LINE-1 RNA (L1HS) in processing bodies (PBs), which are cytoplasmic membraneless ribonucleoprotein (RNP) granules. In the PBs, ORF1p interacts with MOV10 RNA helicase, an essential enzyme for dsRNA unwinding. Using MOV10 truncated mutants, we found that the ORF1p-MOV10 interaction is important in attenuation of PRR sensing and apoptosis upon repeat RNA exposure. Finally, we evalulated the activation of the innate immune response in the tumor microenvironment by ORF1p depletion. These results suggest that ORF1p loss induces a desmoplastic reaction driven by Acta2+ myofibroblastic CAFs in response to inflammatory cytokines. Collectively, our results demonstrate a critical and previously uncharacterized role of ORF1p in evasion of endogenous repeat RNA-triggered antiviral immune response in PDAC cells with implications in the tumor microenvironment.
Citation Format: Eunae You, Bidish Patel, Alexandra Rojas, Natalie Ho, Siyu Sun, Michael Raabe, Yuhui Song, Ildiko Phillips, Katherine Xu, Peter Richieri, Linda Nieman, Benjamin Greenbaum, David Ting. Human LINE-1 ORF1p retrotransposition independent function in pancreatic cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 7480.