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R-loops are non-B DNA structures with intriguing dual consequences for gene expression and genome stability. In addition to their recognized roles in triggering DNA double-strand breaks (DSBs), R-loops have recently been demonstrated to accumulate in cis to DSBs, especially those induced in transcriptionally active loci. In this Review, we discuss whether R-loops actively participate in DSB repair or are detrimental by-products that must be removed to avoid genome instability. Legube and Marnef review the association between R-loops and DNA repair. They discuss how R-loops are formed near DNA double-strand breaks (DSBs) and how R-loops affect transcription near DSBs and DSB repair processes.

The DNA damage response is essential to safeguard genome integrity. Although the contribution of chromatin in DNA repair has been investigated 1 , 2 , the contribution of chromosome folding to these processes remains unclear 3 . Here we report that, after the production of double-stranded breaks (DSBs) in mammalian cells, ATM drives the formation of a new chromatin compartment (D compartment) through the clustering of damaged topologically associating domains, decorated with γH2AX and 53BP1. This compartment forms by a mechanism that is consistent with polymer–polymer phase separation rather than liquid–liquid phase separation. The D compartment arises mostly in G1 phase, is independent of cohesin and is enhanced after pharmacological inhibition of DNA-dependent protein kinase (DNA-PK) or R-loop accumulation. Importantly, R-loop-enriched DNA-damage-responsive genes physically localize to the D compartment, and this contributes to their optimal activation, providing a function for DSB clustering in the DNA damage response. However, DSB-induced chromosome reorganization comes at the expense of an increased rate of translocations, also observed in cancer genomes. Overall, we characterize how DSB-induced compartmentalization orchestrates the DNA damage response and highlight the critical impact of chromosome architecture in genomic instability. After the production of double-stranded breaks in mammalian cells, ATM drives the formation of the D compartment, which regulates DNA damage-responsive genes, through the clustering of damaged topologically associating domains, with a mechanism that is consistent with polymer–polymer phase separation.

Programmable nucleases have enabled rapid and accessible genome engineering in eukaryotic cells and living organisms. However, their delivery into target cells can be technically challenging when working with primary cells or in vivo. Here, we use engineered murine leukemia virus-like particles loaded with Cas9-sgRNA ribonucleoproteins (Nanoblades) to induce efficient genome-editing in cell lines and primary cells including human induced pluripotent stem cells, human hematopoietic stem cells and mouse bone-marrow cells. Transgene-free Nanoblades are also capable of in vivo genome-editing in mouse embryos and in the liver of injected mice. Nanoblades can be complexed with donor DNA for “all-in-one” homology-directed repair or programmed with modified Cas9 variants to mediate transcriptional up-regulation of target genes. Nanoblades preparation process is simple, relatively inexpensive and can be easily implemented in any laboratory equipped for cellular biology. A current challenge in genome editing is delivering Cas9 and sgRNA into target cells. Here the authors engineer a delivery system based on murine leukemia virus-like particles loaded with Cas9-sgRNA ribonucleoproteins to induce efficient genome editing in both cell culture and in vivo in mouse.

R-loops are non-B DNA structures with intriguing dual consequences for gene expression and genome stability. In addition to their recognized roles in triggering DNA double-strand breaks (DSBs), R-loops have recently been demonstrated to accumulate in response to DSBs, especially when induced in transcriptionally active loci. In this review, we discuss whether Rloops actively participate in DSB repair or are detrimental by-products that must be removed to avoid genome instability.

RNA:DNA hybrids that form across genomes control a wide range of biological processes. A new study shows that N.sup.6-methyladenosine (m.sup.6A) modification on the RNA moieties regulates the formation and genome integrity of these hybrids. This finding opens a new avenue of research on how RNA modifications (the 'epitranscriptome') can help control genome maintenance.

The list of physiological processes affected by m6A is expanding and includes DNA-damage6 and viral-infection responses, cell differentiation, brain development, cell growth and tumor progression5,7,8. m6A regulates R-loop stability In their study, Abakir et al. adapted the antibody-based DNA: Genome-stability consequences and beyond One of the most remarkable findings of this study is that depletion of YTHDF2 and METTL3 (the writer that deposits m6A) increases levels of yH2AX, a marker of DNA double-strand breaks, thus suggesting that pathological R-loop accumulation in the absence of the m6A RNA-methylation pathway challenges genome integrity. [...]dysregulation of R-loops is emerging as a critical factor driving genome instability in a large variety of pathological contexts, including after oncogenic stress12, in cells infected with Kaposi's sarcoma-associated herpesvirus, in neurological disorders associated with trinucleotide-repeat expansion (such as Huntington's disease and fragile X syndrome) and in multiple other inherited ataxias (for example, ataxia with oculomotor apraxia 2)2.

RNA:DNA hybrids that form across genomes control a wide range of biological processes. A new study shows that N 6 -methyladenosine (m 6 A) modification on the RNA moieties regulates the formation and genome integrity of these hybrids. This finding opens a new avenue of research on how RNA modifications (the ‘epitranscriptome’) can help control genome maintenance.

Abstract Programmable nucleases have enabled rapid and accessible genome engineering in eukaryotic cells and living organisms. However, their delivery into target cells can be technically challenging when working with primary cells or in vivo. Here, we use engineered murine leukemia virus-like particles loaded with Cas9-sgRNA ribonucleoproteins (Nanoblades) to induce efficient genome-editing in cell lines and primary cells including human induced pluripotent stem cells, human hematopoietic stem cells and mouse bone-marrow cells. Transgene-free Nanoblades are also capable of in vivo genome-editing in mouse embryos and in the liver of injected mice. Nanoblades can be complexed with donor DNA for “all-in-one” homology-directed repair or programmed with modified Cas9 variants to mediate transcriptional up-regulation of target genes. Nanoblades preparation process is simple, relatively inexpensive and can be easily implemented in any laboratory equipped for cellular biology.