R-Loop-Associated Genomic Instability and Implication of WRN and WRNIP1.

Eva Malacaria,Giorgia Lillo,Veronica Marabitti,Pietro Pichierri,Valentina Palermo,Pasquale Valenzisi,Annapaola Franchitto

doi:10.3390/ijms23031547

Abstract

Maintenance of genome stability is crucial for cell survival and relies on accurate DNA replication. However, replication fork progression is under constant attack from different exogenous and endogenous factors that can give rise to replication stress, a source of genomic instability and a notable hallmark of pre-cancerous and cancerous cells. Notably, one of the major natural threats for DNA replication is transcription. Encounters or conflicts between replication and transcription are unavoidable, as they compete for the same DNA template, so that collisions occur quite frequently. The main harmful transcription-associated structures are R-loops. These are DNA structures consisting of a DNA–RNA hybrid and a displaced single-stranded DNA, which play important physiological roles. However, if their homeostasis is altered, they become a potent source of replication stress and genome instability giving rise to several human diseases, including cancer. To combat the deleterious consequences of pathological R-loop persistence, cells have evolved multiple mechanisms, and an ever growing number of replication fork protection factors have been implicated in preventing/removing these harmful structures; however, many others are perhaps still unknown. In this review, we report the current knowledge on how aberrant R-loops affect genome integrity and how they are handled, and we discuss our recent findings on the role played by two fork protection factors, the Werner syndrome protein (WRN) and the Werner helicase-interacting protein 1 (WRNIP1) in response to R-loop-induced genome instability.

Highlights

DNA replication is a fundamental process of the cell, and it is not surprising that defects in its execution or regulation can give rise to severe human diseases
We report the current knowledge on how aberrant R-loops affect genome integrity and how they are handled, and we discuss our recent findings on the role played by two fork protection factors, the Werner syndrome protein (WRN) and the Werner helicase-interacting protein 1 (WRNIP1) in response to R-loop-induced genome instability
R-loop accumulation could contribute significantly to genomic instability in Werner syndrome (WS) cells, as revealed by experiments performed by overexpressing an ectopic GFP-RNaseH1, a ribonuclease that degrades the RNA moiety of DNA–RNA hybrids in the nucleus, or by transcription inhibition, which strongly reduced the levels of DNA damage and the frequency of chromosomal aberrations in WS cells

Summary

Introduction

DNA replication is a fundamental process of the cell, and it is not surprising that defects in its execution or regulation can give rise to severe human diseases. The main transcription-associated structures detrimental to fork progression are R-loops [3,4], transient and reversible structures with physiological functions [2]. If their turnover is deregulated, they can cause a co-directional or head-on clash between the replisome and the RNA polymerase, leading to R-loop-driven replication stress [5,6]. We will report an involvement of the Werner syndrome protein (WRN) and the Werner syndrome helicase-interacting protein 1 (WRNIP1), two fork protection factors, in response to R-loop-driven genome instability, providing new clues to understanding the way replication–transcription conflicts could be handled

Replication Stress

Transcription as a Source of Replication Stress

Mechanisms of Resolution and Removal of R-Loops

A Role of WRN in Limiting R-Loop-Associated Genomic Instability

Conclusions