Abstract A013: Transcription initiation and its regulation in mouse L1s

Abstract Long interspersed elements type 1 (LINE-1s or L1s) are highly abundant non-LTR retrotransposons in mammalian genomes. Apart from an evolutionary role in altering the genomic landscape and gene expression, recent discoveries have revealed the scale and impact of active movement by L1, both in the germline and in somatic niches (e.g., the brain and many cancers). As transcriptional activation is a prerequisite for L1 retrotransposition, it is important to understand how L1 transcription is regulated. L1 transcription is controlled by an internal promoter contained in the 5’ untranslated region (5’UTR) of a full-length element. It has been shown that transcription start sites (TSSs) of human L1 are clustered at the beginning of its 5’UTR and the precision of transcription initiation is controlled by transcription factor YY1. The laboratory mouse is an essential model for investigating L1 retrotransposition and the associated consequences in vivo. However, mouse L1 5’UTRs are distinctly different from human L1’s: the former are organized into tandem repeats called monomers and separated from the open reading frames by a nonmonomeric sequence called tether. Unlike the human L1 promoter, transcription initiation and its regulation in mouse L1 subfamilies remain unknown. In this study, we streamlined the cloning of different types of mouse L1 promoters. TSSs were determined using RNA ligation-mediated 5’ rapid amplification of cDNA ends (RLM-5’RACE) incorporating a highly efficient reverse transcriptase enzyme followed by Nanopore sequencing. In the context of two monomer promoter reporter construct, transcription could initiate from either monomer or the tether sequence. While mouse L1 TSSs are distributive, a higher number of transcripts were initiated from the inner monomer. Surprisingly, a significant fraction of transcripts was also initiated from the tether sequence. In addition, we showed that YY1 transcriptionally activates mouse L1 5’UTR and is responsible for the synergy between the monomers. As transcription initiation is a major determinant for the proportion of active L1 loci in the next round of retrotransposition, these findings have important implications on L1 biology in the mouse model. Citation Format: Karabi Saha, Wenfeng An. Transcription initiation and its regulation in mouse L1s [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Tumor Immunology and Immunotherapy; 2024 Oct 18-21; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2024;12(10 Suppl):Abstract nr A013.