LINE-1 ORF1p RIP-seq reveals widespread association with p-body enriched mRNAs

Background Long INterspersed Element-1 (LINE-1) is an autonomous retroelement able to “copy-and-paste” itself into new loci of the host genome through a process called retrotransposition. The LINE-1 bicistronic mRNA codes for two proteins, ORF1p, a nucleic acid chaperone, and ORF2p, a protein with endonuclease and reverse transcriptase activity. Both proteins bind LINE-1 mRNA in cis and are necessary for retrotransposition. While LINE-1 transcription is usually repressed in most healthy somatic cells through a plethora of mechanisms, ORF1p expression has been observed in nearly 50% of tumors, and new LINE-1 insertions have been documented in a similar fraction of tumors, including prostate cancer. Results Here, we utilized RNA ImmunoPrecipitation (RIP) and the L1EM analysis software to identify ORF1p bound RNA in prostate cancer cells. We identified LINE-1 loci that were expressed in androgen sensitive and androgen independent cells, that we show are representative of LINE-1 copies expressed in prostate cancer before and after treatment. In all androgen independent cells, we found higher levels of LINE-1 RNA, as well as unique expression patterns of LINE-1 loci. Interestingly, we observed that ORF1p bound many non-LINE-1 mRNA in all prostate cancer cell lines evaluated, and polyA RNA, and RNA localized in p-bodies were especially enriched. Furthermore, the expression levels of many of the identified ORF1p bound mRNAs also correlated with expression of LINE-1 RNA in prostate tumors from The Cancer Genome Atlas (TCGA). Conclusion Our results show a significant remodeling of LINE-1 loci expression in androgen independent cell lines when compared to parental androgen dependent cells, suggesting an evolution of LINE-1 expression during prostate cancer progression. Additionally, our finding that ORF1p bound a significant amount of non-LINE-1 mRNA, and that the enriched ORF1p bound mRNAs are also amplified in LINE-1 expressing TCGA prostate tumors, suggest that ORF1p may play a role in non-LINE-1 RNA processing and regulation of specific transcripts in prostate tumors.