Different alterations in mRNP composition trigger an increase in harmful R-loops

Understanding how the assembly of nascent mRNA into a ribonucleoprotein (mRNP) influences R-loop homeostasis is crucial for gaining insight into the cellular mechanisms that prevent genome instability. Here, we identify three RNA-binding proteins, Rie1, Rim4 and She2, whose stoichiometry is important to limit R-loop accumulation and, thus, to prevent DNA damage. Interestingly, Rim4 and She2 are overrepresented in mRNPs assembled in the absence of THO. In addition, we found that an excess of Dis3 impairs exosome function, promoting R-loops, particularly from non-coding RNAs, which cause genomic instability. Our results indicate that mRNP assembly is a precise process that, when perturbed by changes in the availability of different RBPs or RNAs, causes R-loop-mediated DNA damage in the cell. These results may help to understand the mechanism that promotes cancer, as several RBPs are overexpressed in different types of tumors. Author summary R-loops are three-stranded structures formed by a DNA-RNA hybrid and the displaced ssDNA. The accumulation of R-loops damages the genome, highlighting the need for their tight regulation. One of the mechanisms that prevents R-loop formation is the co-transcriptional assembly of the nascent RNA into a messenger ribonucleoprotein (mRNP). Our study reveals that mRNP biogenesis is highly sensitive to perturbations. Not only the absence of mRNP components, but also the overexpression of specific RNA-binding proteins (RBPs) or the persistence of unstable transcripts, significantly stresses mRNP assembly, resulting in R-loop accumulation and consequent DNA damage. R-loops are involved in a number of neurodegenerative diseases and cancer, highlighting the importance of understanding the cellular processes that mitigate their accumulation.