Mechanisms of Ubiquitin-Nucleosome Recognition and Regulation of 53BP1 Chromatin Recruitment by RNF168/169 and RAD18

Mechanism of nucleosomal H2A K13/15 monoubiquitination and adjacent dual monoubiquitination by RNF168

The DNA damage repair regulatory protein RNF168, a monomeric RING-type E3 ligase, has a crucial role in regulating cell fate and DNA repair by specific and efficient ubiquitination of the adjacent K13 and K15 (K13/15) sites at the H2A N-terminal tail. However, understanding how RNF168 coordinates with its cognate E2 enzyme UbcH5c to site-specifically ubiquitinate H2A K13/15 has long been hampered by the lack of high-resolution structures of RNF168 and UbcH5c~Ub (ubiquitin) in complex with nucleosomes. Here we developed chemical strategies and determined the cryo-electron microscopy structures of the RNF168-UbcH5c~Ub-nucleosome complex captured in transient H2A K13/15 monoubiquitination and adjacent dual monoubiquitination reactions, providing a 'helix-anchoring' mode for monomeric E3 ligase RNF168 on nucleosome in contrast to the 'compass-binding' mode of dimeric E3 ligases. Our work not only provides structural snapshots of H2A K13/15 site-specific monoubiquitination and adjacent dual monoubiquitination but also offers a near-atomic-resolution structural framework for understanding pathogenic amino acid substitutions and physiological modifications of RNF168.

PIN1-SUMO2/3 motif suppresses excessive RNF168 chromatin accumulation and ubiquitin signaling to promote IR resistance

RNF168 is an E3 ubiquitin ligase critical to the mammalian DNA double-strand break repair response. The protein is recruited to and amplifies ubiquitin signals at damaged chromatin and, if not properly regulated, can drive an uncontrolled ubiquitin cascade potentially harmful to repair outcomes. Several indirect mechanisms restrict RNF168 positive feedback, and a longstanding question has been whether these alone suppress excessive RNF168 signaling or whether mechanisms to remove RNF168 from damaged chromatin exist. Here, we reveal a cascade of post-translational modifications which act at three adjacent amino acids, threonine-208, proline-209 and lysine-210, to process RNF168 actively. Phosphorylation at threonine-208 by CDK1/2 induces interaction with the peptidyl-prolyl isomerase PIN1. PIN1 promotes RNF168 SUMOylation at lysine-210, resulting in p97/VCP mediated removal. These actions promote RNF168 clearance and limit RNF168 chromatin build-up. Thus, single amino acid substitutions of the regulatory motif (SUMO-PIN1-assisted Chromatin Regulator, SPaCR) that restrict PIN1 interaction or SUMOylation are sufficient to drive supraphysiological accumulation of RNF168, increased ubiquitin signaling, excessive 53BP1 recruitment and radiosensitivity. Our findings define a mechanism of direct RNF168 regulation that is part of the normal damage response, promoting RNF168 dissociation from chromatin and limiting deleterious ubiquitin signaling.

The Emerging Role of the Histone H2AK13/15 Ubiquitination: Mechanisms of Writing, Reading, and Erasing in DNA Damage Repair and Disease

Histone modifications serve as molecular switches controlling critical cellular processes. The ubiquitination of histone H2A at lysines 13 and 15 (H2AK13/15ub) is a crucial epigenetic modification that coordinates DNA repair and genome stability during the DNA damage response (DDR). This epigenetic mark is dynamically regulated by three functional protein groups: "writer" enzymes (e.g., E3 ubiquitin ligase RNF168 that catalyzes H2AK13/15ub formation), "reader" proteins (including 53BP1 and BRCA1-BARD1 that recognize the mark to guide DNA repair), and "eraser" deubiquitinases (such as USP3 and USP16 that remove the modification). Dysregulation of the precisely coordinated network of H2AK13/15ub is strongly associated with various diseases, including RIDDLE syndrome, neurodegenerative disorders, immune deficiencies, and breast cancer. This review systematically analyzes the dynamic regulation of H2AK13/15ub in DDR and explores its therapeutic potential for disease intervention.

A Site-Specific Click Chemistry Approach to Di-Ubiquitylate H1 Variants Reveals Position-Dependent Stimulation of the DNA Repair Protein RNF168

Ubiquitylation of histone H2A at lysines 13 and 15 by the E3 ligase RNF168 plays a key role in orchestrating DNA double-strand break (DSB) repair, which is often deregulated in cancer. RNF168 activity is triggered by DSB signaling cascades, reportedly through K63-linked poly-ubiquitylation of linker histone H1. However, direct experimental evidence of this mechanism has been elusive, primarily due to the lack of methods to specifically poly-ubiquitylate H1. Here, we developed a versatile click chemistry approach to covalently link multiple proteins in a site-specific, controlled, and stepwise manner. Applying this method, we synthesized H1 constructs bearing triazole-linked di-ubiquitin on four DNA repair-associated ubiquitylation hotspots (H1KxUb2, at K17, 46, 64 and 96). Integrated into nucleosome arrays, the H1KxUb2 variants stimulated H2A ubiquitylation by RNF168 in a position-dependent manner, with H1K17Ub2 showing the strongest RNF168 activation effect. Moreover, we show that di-ubiquitin binding is the driving force underlying RNF168 recruitment, introducing H1K17Ub2 into living U-2 OS cells. Together, our results support the hypothesis of poly-ubiquitylated H1 guiding RNF168 recruitment to DSB sites. Moreover, we demonstrate how the streamlined synthesis of H1KxUb2 variants enables mechanistic studies into RNF168 regulation, with potential implications for its inhibition in susceptible cancers.

PCNA-binding activity separates RNF168 functions in DNA replication and DNA double-stranded break signaling

RNF168 orchestrates a ubiquitin-dependent DNA damage response to regulate the recruitment of repair factors, such as 53BP1 to DNA double-strand breaks (DSBs). In addition to its canonical functions in DSB signaling, RNF168 may facilitate DNA replication fork progression. However, the precise role of RNF168 in DNA replication remains unclear. Here, we demonstrate that RNF168 is recruited to DNA replication factories in a manner that is independent of the canonical DSB response pathway regulated by Ataxia-Telangiectasia Mutated (ATM) and RNF8. We identify a degenerate Proliferating Cell Nuclear Antigen (PCNA)-interacting peptide (DPIP) motif in the C-terminus of RNF168, which together with its Motif Interacting with Ubiquitin (MIU) domain mediates binding to mono-ubiquitylated PCNA at replication factories. An RNF168 mutant harboring inactivating substitutions in its DPIP box and MIU1 domain (termed RNF168 ΔDPIP/ΔMIU1) is not recruited to sites of DNA synthesis and fails to support ongoing DNA replication. Notably, the PCNA interaction-deficient RNF168 ΔDPIP/ΔMIU1 mutant fully rescues the ability of RNF168-/- cells to form 53BP1 foci in response to DNA DSBs. Therefore, RNF168 functions in DNA replication and DSB signaling are fully separable. Our results define a new mechanism by which RNF168 promotes DNA replication independently of its canonical functions in DSB signaling.

Structural mechanisms of SLF1 interactions with Histone H4 and RAD18 at the stalled replication fork

DNA damage that obstructs the replication machinery poses a significant threat to genome stability. Replication-coupled repair mechanisms safeguard stalled replication forks by coordinating proteins involved in the DNA damage response (DDR) and replication. SLF1 (SMC5-SMC6 complex localization factor 1) is crucial for facilitating the recruitment of the SMC5/6 complex to damage sites through interactions with SLF2, RAD18, and nucleosomes. However, the structural mechanisms of SLF1's interactions are unclear. In this study, we determined the crystal structure of SLF1's ankyrin repeat domain bound to an unmethylated histone H4 tail, illustrating how SLF1 reads nascent nucleosomes. Using structure-based mutagenesis, we confirmed a phosphorylation-dependent interaction necessary for a stable complex between SLF1's tandem BRCA1 C-Terminal domain (tBRCT) and the phosphorylated C-terminal region (S442 and S444) of RAD18. We validated a functional role of conserved phosphate-binding residues in SLF1, and hydrophobic residues in RAD18 that are adjacent to phosphorylation sites, both of which contribute to the strong interaction. Interestingly, we discovered a DNA-binding property of this RAD18-binding interface, providing an additional domain of SLF1 to enhance binding to nucleosomes. Our results provide critical structural insights into SLF1's interactions with post-replicative chromatin and phosphorylation-dependent DDR signalling, enhancing our understanding of SMC5/6 recruitment and/or activity during replication-coupled DNA repair.

Histone ubiquitination: Role in genome integrity and chromatin organization

Maintenance of genome integrity is a precise but tedious and complex job for the cell. Several post-translational modifications (PTMs) play vital roles in maintaining the genome integrity. Although ubiquitination is one of the most crucial PTMs, which regulates the localization and stability of the nonhistone proteins in various cellular and developmental processes, ubiquitination of the histones is a pivotal epigenetic event critically regulating chromatin architecture. In addition to genome integrity, importance of ubiquitination of core histones (H2A, H2A, H3, and H4) and linker histone (H1) have been reported in several cellular processes. However, the complex interplay of histone ubiquitination and other PTMs, as well as the intricate chromatin architecture and dynamics, pose a significant challenge to unravel how histone ubiquitination safeguards genome stability. Therefore, further studies are needed to elucidate the interactions between histone ubiquitination and other PTMs, and their role in preserving genome integrity. Here, we review all types of histone ubiquitinations known till date in maintaining genomic integrity during transcription, replication, cell cycle, and DNA damage response processes. In addition, we have also discussed the role of histone ubiquitination in regulating other histone PTMs emphasizing methylation and acetylation as well as their potential implications in chromatin architecture. Further, we have also discussed the involvement of deubiquitination enzymes (DUBs) in controlling histone ubiquitination in modulating cellular processes.

To Ub or not to Ub: The epic dilemma of histones that regulate gene expression and epigenetic cross-talk

A dynamic array of histone post-translational modifications (PTMs) regulate diverse cellular processes in the eukaryotic chromatin. Among them, histone ubiquitination is particularly complex as it alters nucleosome surface area fostering intricate cross-talk with other chromatin modifications. Ubiquitin signaling profoundly impacts DNA replication, repair, and transcription. Histones can undergo varied extent of ubiquitination such as mono, multi-mono, and polyubiquitination, which brings about distinct cellular fates. Mechanistic studies of the ubiquitin landscape in chromatin have unveiled a fascinating tapestry of events that orchestrate gene regulation. In this review, we summarize the key contributors involved in mediating different histone ubiquitination and deubiquitination events, and discuss their mechanism which impacts cell transcriptional identity and DNA damage response. We also focus on the proteins bearing epigenetic reader modules critical in discerning site-specific histone ubiquitination, pivotal for establishing complex epigenetic crosstalk. Moreover, we highlight the role of histone ubiquitination in different human diseases including neurodevelopmental disorders and cancer. Overall the review elucidates the intricate orchestration of histone ubiquitination impacting diverse cellular functions and disease pathogenesis, and provides insights into the current challenges of targeting them for therapeutic interventions.

An E3 ubiquitin ligase localization screen uncovers DTX2 as a novel ADP-ribosylation-dependent regulator of DNA double-strand break repair

DNA double-strand breaks (DSBs) elicit an elaborate response to signal damage and trigger repair via two major pathways: nonhomologous end-joining (NHEJ), which functions throughout the interphase, and homologous recombination (HR), restricted to S/G2 phases. The DNA damage response relies, on post-translational modifications of nuclear factors to coordinate the mending of breaks. Ubiquitylation of histones and chromatin-associated factors regulates DSB repair and numerous E3 ubiquitin ligases are involved in this process. Despite significant progress, our understanding of ubiquitin-mediated DNA damage response regulation remains incomplete. Here, we have performed a localization screen to identify RING/U-box E3 ligases involved in genome maintenance. Our approach uncovered 7 novel E3 ligases that are recruited to microirradiation stripes, suggesting potential roles in DNA damage signaling and repair. Among these factors, the DELTEX family E3 ligase DTX2 is rapidly mobilized to lesions in a poly ADP-ribosylation-dependent manner. DTX2 is recruited and retained at DSBs via its WWE and DELTEX conserved C-terminal domains. In cells, both domains are required for optimal binding to mono and poly ADP-ribosylated proteins with WWEs playing a prominent role in this process. Supporting its involvement in DSB repair, DTX2 depletion decreases HR efficiency and moderately enhances NHEJ. Furthermore, DTX2 depletion impeded BRCA1 foci formation and increased 53BP1 accumulation at DSBs, suggesting a fine-tuning role for this E3 ligase in repair pathway choice. Finally, DTX2 depletion sensitized cancer cells to X-rays and PARP inhibition and these susceptibilities could be rescued by DTX2 reexpression. Altogether, our work identifies DTX2 as a novel ADP-ribosylation-dependent regulator of HR-mediated DSB repair.

RAD18 directs DNA double-strand break repair by homologous recombination to post-replicative chromatin

RAD18 is an E3 ubiquitin ligase that prevents replication fork collapse by promoting DNA translesion synthesis and template switching. Besides this classical role, RAD18 has been implicated in homologous recombination; however, this function is incompletely understood. Here, we show that RAD18 is recruited to DNA lesions by monoubiquitination of histone H2A at K15 and counteracts accumulation of 53BP1. Super-resolution microscopy revealed that RAD18 localizes to the proximity of DNA double strand breaks and limits the distribution of 53BP1 to the peripheral chromatin nanodomains. Whereas auto-ubiquitination of RAD18 mediated by RAD6 inhibits its recruitment to DNA breaks, interaction with SLF1 promotes RAD18 accumulation at DNA breaks in the post-replicative chromatin by recognition of histone H4K20me0. Surprisingly, suppression of 53BP1 function by RAD18 is not involved in homologous recombination and rather leads to reduction of non-homologous end joining. Instead, we provide evidence that RAD18 promotes HR repair by recruiting the SMC5/6 complex to DNA breaks. Finally, we identified several new loss-of-function mutations in RAD18 in cancer patients suggesting that RAD18 could be involved in cancer development.

Epigenetic orchestration of the DNA damage response: Insights into the regulatory mechanisms

The DNA damage response (DDR) is a critical cellular mechanism that safeguards genome integrity and prevents the accumulation of harmful DNA lesions. Increasing evidence highlights the intersection between DDR signaling and epigenetic regulation, offering profound insights into various aspects of cellular function including oncogenesis. This comprehensive review explores the intricate relationship between the epigenetic modifications and DDR activation, with a specific focus on the impact of viral infections. Oncogenic viruses, such as human papillomavirus, hepatitis virus (HBV or HCV), and Epstein-Barr virus have been shown to activate the DDR. Consequently, these DNA damage events trigger a cascade of epigenetic alterations, including changes in DNA methylation patterns, histone modifications and the expression of noncoding RNAs. These epigenetic changes exert profound effects on chromatin structure, gene expression, and maintenance of genome stability. Importantly, elucidation of the viral-induced epigenetic alterations in the context of DDR holds significant implications for comprehending the complexity of cancer and provides potential targets for therapeutic interventions.

Structural basis for RAD18 regulation by MAGEA4 and its implications for RING ubiquitin ligase binding by MAGE family proteins

MAGEA4 is a cancer-testis antigen primarily expressed in the testes but aberrantly overexpressed in several cancers. MAGEA4 interacts with the RING ubiquitin ligase RAD18 and activates trans-lesion DNA synthesis (TLS), potentially favouring tumour evolution. Here, we employed NMR and AlphaFold2 (AF) to elucidate the interaction mode between RAD18 and MAGEA4, and reveal that the RAD6-binding domain (R6BD) of RAD18 occupies a groove in the C-terminal winged-helix subdomain of MAGEA4. We found that MAGEA4 partially displaces RAD6 from the RAD18 R6BD and inhibits degradative RAD18 autoubiquitination, which could be countered by a competing peptide of the RAD18 R6BD. AlphaFold2 and cross-linking mass spectrometry (XL-MS) also revealed an evolutionary invariant intramolecular interaction between the catalytic RING and the DNA-binding SAP domains of RAD18, which is essential for PCNA mono-ubiquitination. Using interaction proteomics, we found that another Type-I MAGE, MAGE-C2, interacts with the RING ubiquitin ligase TRIM28 in a manner similar to the MAGEA4/RAD18 complex, suggesting that the MAGEA4 peptide-binding groove also serves as a ligase-binding cleft in other type-I MAGEs. Our data provide new insights into the mechanism and regulation of RAD18-mediated PCNA mono-ubiquitination.

Mechanisms of RNF168 nucleosome recognition and ubiquitylation

RNF168 plays a central role in the DNA damage response (DDR) by ubiquitylating histone H2A at K13 and K15. These modifications direct BRCA1-BARD1 and 53BP1 foci formation in chromatin, essential for cell-cycle-dependent DNA double-strand break (DSB) repair pathway selection. The mechanism by which RNF168 catalyzes the targeted accumulation of H2A ubiquitin conjugates to form repair foci around DSBs remains unclear. Here, using cryoelectron microscopy (cryo-EM), nuclear magnetic resonance (NMR) spectroscopy, and functional assays, we provide a molecular description of the reaction cycle and dynamics of RNF168 as it modifies the nucleosome and recognizes its ubiquitylation products. We demonstrate an interaction of a canonical ubiquitin-binding domain within full-length RNF168, which not only engages ubiquitin but also the nucleosome surface, clarifying how such site-specific ubiquitin recognition propels a signal amplification loop. Beyond offering mechanistic insights into a key DDR protein, our study aids in understanding site specificity in both generating and interpreting chromatin ubiquitylation.

Insights from the protein interaction Universe of the multifunctional "Goldilocks" kinase DYRK1A

Human Dual specificity tyrosine (Y)-Regulated Kinase 1A (DYRK1A) is encoded by a dosage-dependent gene located in the Down syndrome critical region of human chromosome 21. The known substrates of DYRK1A include proteins involved in transcription, cell cycle control, DNA repair and other processes. However, the function and regulation of this kinase is not fully understood, and the current knowledge does not fully explain the dosage-dependent function of this kinase. Several recent proteomic studies identified DYRK1A interacting proteins in several human cell lines. Interestingly, several of known protein substrates of DYRK1A were undetectable in these studies, likely due to a transient nature of the kinase-substrate interaction. It is possible that the stronger-binding DYRK1A interacting proteins, many of which are poorly characterized, are involved in regulatory functions by recruiting DYRK1A to the specific subcellular compartments or distinct signaling pathways. Better understanding of these DYRK1A-interacting proteins could help to decode the cellular processes regulated by this important protein kinase during embryonic development and in the adult organism. Here, we review the current knowledge of the biochemical and functional characterization of the DYRK1A protein-protein interaction network and discuss its involvement in human disease.

UBA80 and UBA52 fine-tune RNF168-dependent histone ubiquitination and DNA repair

The ubiquitin signaling pathway is crucial for the DNA damage response pathway. More specifically, RNF168 is integral in regulating DNA repair proteins at damaged chromatin. However, the detailed mechanism by which RNF168 is regulated in cells is not fully understood. Here, we identify the ubiquitin-ribosomal fusion proteins UBA80 (also known as RPS27A) and UBA52 (also known as RPL40) as interacting proteins for H2A/H2AX histones and RNF168. Both UBA80 and UBA52 are recruited to laser-induced micro-irradiation DNA damage sites and are required for DNA repair. Ectopic expression of UBA80 and UBA52 inhibits RNF168-mediated H2A/H2AX ubiquitination at K13/15 and impairs 53BP1 recruitment to DNA lesions. Mechanistically, the C-terminal ribosomal fragments of UBA80 and UBA52, S27A and L40, respectively, limit RNF168-nucleosome engagement by masking the regulatory acidic residues at E143/E144 and the nucleosome acidic patch. Together, our results reveal that UBA80 and UBA52 antagonize the ubiquitination signaling pathway and fine-tune the spatiotemporal regulation of DNA repair proteins at DNA damage sites.

TRABID overexpression enables synthetic lethality to PARP inhibitor via prolonging 53BP1 retention at double-strand breaks

53BP1 promotes nonhomologous end joining (NHEJ) over homologous recombination (HR) repair by mediating inactivation of DNA end resection. Ubiquitination plays an important role in regulating dissociation of 53BP1 from DNA double-strand breaks (DSBs). However, how this process is regulated remains poorly understood. Here, we demonstrate that TRABID deubiquitinase binds to 53BP1 at endogenous level and regulates 53BP1 retention at DSB sites. TRABID deubiquitinates K29-linked polyubiquitination of 53BP1 mediated by E3 ubiquitin ligase SPOP and prevents 53BP1 dissociation from DSBs, consequently inducing HR defects and chromosomal instability. Prostate cancer cells with TRABID overexpression exhibit a high sensitivity to poly (ADP-ribose) polymerase (PARP) inhibitors. Our work shows that TRABID facilitates NHEJ repair over HR during DNA repair by inducing prolonged 53BP1 retention at DSB sites, suggesting that TRABID overexpression may predict HR deficiency and the potential therapeutic use of PARP inhibitors in prostate cancer.

53BP1: Keeping It under Control, Even at a Distance from DNA Damage

Double-strand breaks (DSBs) are toxic lesions that can be generated by exposure to genotoxic agents or during physiological processes, such as during V(D)J recombination. The repair of these DSBs is crucial to prevent genomic instability and to maintain cellular homeostasis. Two main pathways participate in repairing DSBs, namely, non-homologous end joining (NHEJ) and homologous recombination (HR). The P53-binding protein 1 (53BP1) plays a pivotal role in the choice of DSB repair mechanism, promotes checkpoint activation and preserves genome stability upon DSBs. By preventing DSB end resection, 53BP1 promotes NHEJ over HR. Nonetheless, the balance between DSB repair pathways remains crucial, as unscheduled NHEJ or HR events at different phases of the cell cycle may lead to genomic instability. Therefore, the recruitment of 53BP1 to chromatin is tightly regulated and has been widely studied. However, less is known about the mechanism regulating 53BP1 recruitment at a distance from the DNA damage. The present review focuses on the mechanism of 53BP1 recruitment to damage and on recent studies describing novel mechanisms keeping 53BP1 at a distance from DSBs.

Decoding histone ubiquitylation

Histone ubiquitylation is a critical part of both active and repressed transcriptional states, and lies at the heart of DNA damage repair signaling. The histone residues targeted for ubiquitylation are often highly conserved through evolution, and extensive functional studies of the enzymes that catalyze the ubiquitylation and de-ubiquitylation of histones have revealed key roles linked to cell growth and division, development, and disease in model systems ranging from yeast to human cells. Nonetheless, the downstream consequences of these modifications have only recently begun to be appreciated on a molecular level. Here we review the structure and function of proteins that act as effectors or “readers” of histone ubiquitylation. We highlight lessons learned about how ubiquitin recognition lends specificity and function to intermolecular interactions in the context of transcription and DNA repair, as well as what this might mean for how we think about histone modifications more broadly.

DNA Damage Response Regulation by Histone Ubiquitination

Cells are constantly exposed to numerous genotoxic stresses that induce DNA damage. DNA double-strand breaks (DSBs) are among the most serious damages and should be systematically repaired to preserve genomic integrity. The efficiency of repair is closely associated with chromatin structure, which is regulated by posttranslational modifications of histones, including ubiquitination. Recent evidence shows crosstalk between histone ubiquitination and DNA damage responses, suggesting an integrated model for the systematic regulation of DNA repair. There are two major pathways for DSB repair, viz., nonhomologous end joining and homologous recombination, and the choice of the pathway is partially controlled by posttranslational modifications of histones, including ubiquitination. Histone ubiquitination changes chromatin structure in the vicinity of DSBs and serves as a platform to select and recruit repair proteins; the removal of these modifications by deubiquitinating enzymes suppresses the recruitment of repair proteins and promotes the convergence of repair reactions. This article provides a comprehensive overview of the DNA damage response regulated by histone ubiquitination in response to DSBs.

The Chromatin Landscape Channels DNA Double-Strand Breaks to Distinct Repair Pathways

DNA double-strand breaks (DSBs), the most deleterious DNA lesions, are primarily repaired by two pathways, namely homologous recombination (HR) and non-homologous end joining (NHEJ), the choice of which is largely dependent on cell cycle phase and the local chromatin landscape. Recent studies have revealed that post-translational modifications on histones play pivotal roles in regulating DSB repair pathways including repair pathway choice. In this review, we present our current understanding of how these DSB repair pathways are employed in various chromatin landscapes to safeguard genomic integrity. We place an emphasis on the impact of different histone post-translational modifications, characteristic of euchromatin or heterochromatin regions, on DSB repair pathway choice. We discuss the potential roles of damage-induced chromatin modifications in the maintenance of genome and epigenome integrity. Finally, we discuss how RNA transcripts from the vicinity of DSBs at actively transcribed regions also regulate DSB repair pathway choice.

Multifaceted regulation and functions of 53BP1 in NHEJ‑mediated DSB repair (Review)

The repair of DNA double-strand breaks (DSBs) is crucial for the preservation of genomic integrity and the maintenance of cellular homeostasis. Non-homologous DNA end joining (NHEJ) is the predominant repair mechanism for any type of DNA DSB during the majority of the cell cycle. NHEJ defects regulate tumor sensitivity to ionizing radiation and anti-neoplastic agents, resulting in immunodeficiencies and developmental abnormalities in malignant cells. p53-binding protein 1 (53BP1) is a key mediator involved in DSB repair, which functions to maintain a balance in the repair pathway choices and in preserving genomic stability. 53BP1 promotes DSB repair via NHEJ and antagonizes DNA end overhang resection. At present, novel lines of evidence have revealed the molecular mechanisms underlying the recruitment of 53BP1 and DNA break-responsive effectors to DSB sites, and the promotion of NHEJ-mediated DSB repair via 53BP1, while preventing homologous recombination. In the present review article, recent advances made in the elucidation of the structural and functional characteristics of 53BP1, the mechanisms of 53BP1 recruitment and interaction with the reshaping of the chromatin architecture around DSB sites, the post-transcriptional modifications of 53BP1, and the up- and downstream pathways of 53BP1 are discussed. The present review article also focuses on the application perspectives, current challenges and future directions of 53BP1 research.

New answers to the old RIDDLE: RNF168 and the DNA damage response pathway

The chromatin-based DNA damage response pathway is tightly orchestrated by histone post-translational modifications, including histone H2A ubiquitination. Ubiquitination plays an integral role in regulating cellular processes including DNA damage signaling and repair. The ubiquitin E3 ligase RNF168 is essential in assembling a cohort of DNA repair proteins at the damaged chromatin via its enzymatic activity. RNF168 ubiquitinates histone H2A(X) at the N terminus and generates a specific docking scaffold for ubiquitin-binding motif-containing proteins. The regulation of RNF168 at damaged chromatin and the mechanistic implication in the recruitment of DNA repair proteins to the damaged sites remain an area of active investigation. Here, we review the function and regulation of RNF168 in the context of ubiquitin-mediated DNA damage signaling and repair. We will also discuss the unanswered questions that require further investigation and how understanding RNF168 targeting specificity could benefit the therapeutic development for cancer treatment.

New insights into the DNA repair pathway choice with NuA4/TIP60

In eukaryotic cells, DNA double-strand breaks (DSBs) can be repaired through two main pathways, non-homologous end-joining (NHEJ) or homologous recombination (HR). The selection of the repair pathway choice is governed by an antagonistic relationship between repair factors specific to each pathway, in a cell cycle-dependent manner. The molecular mechanisms of this decision implicate post-translational modifications of chromatin surrounding the break. Here, we discuss the recent advances regarding the function of the NuA4/TIP60 histone acetyltransferase/chromatin remodeling complex during DSBs repair. In particular, we emphasise the contribution of NuA4/TIP60 in repair pathway choice, in collaboration with the SAGA acetyltransferase complex, and how they regulate chromatin dynamics, modify non-histone substrates to allow DNA end resection and recombination.

Visualizing Conformational Ensembles of the Nucleosome by NMR

The formation of chromatin not only compacts the eukaryotic genome into the nucleus but also provides a mechanism for the regulation of all DNA templated processes. Spatial and temporal modulation of the chromatin structure is critical in such regulation and involves fine-tuned functioning of the basic subunit of chromatin, the nucleosome. It has become apparent that the nucleosome is an inherently dynamic system, but characterization of these dynamics at the atomic level has remained challenging. NMR spectroscopy is a powerful tool for investigating the conformational ensemble and dynamics of proteins and protein complexes, and recent advances have made the study of large systems possible. Here, we review recent studies which utilize NMR spectroscopy to uncover the atomic level conformation and dynamics of the nucleosome and provide a better understanding of the importance of these dynamics in key regulatory events.

Laser Microirradiation and Real-time Recruitment Assays Using an Engineered Biosensor

Double-strand breaks (DSBs) are lesions in DNA that, if not properly repaired, can cause genomic instability, oncogenesis, and cell death. Multiple chromatin posttranslational modifications (PTMs) play a role in the DNA damage response to DSBs. Among these, RNF168-mediated ubiquitination of lysines 13 or 15 at the N-terminal tail of histone H2A (H2AK13/15Ub) is essential for the recruitment of effectors of both the non-homologous end joining (NHEJ) and the homologous recombination (HR) repair pathways. Thus, tools and techniques to track the spatiotemporal dynamics of H2AK13/15 ubiquitination at DNA DSBs are important to facilitate studies of DNA repair. Previous work from other groups used the minimal focus-forming region (FFR) of the NHEJ effector 53BP1 to detect H2AK15Ub generated upon damage induced by gamma or laser irradiation in live cells. However, 53BP1-FFR only binds nucleosomes modified with both H2AK15Ub and dimethylation of lysine 20 on histone H4 (H4K20me2); thus, 53BP1-FFR does not recognize H2AK13Ub-nucleosomes or nucleosomes that contain H2AK15Ub but lack methylation of H4K20 (H4K20me0). To overcome this limitation, we developed an avidity-based sensor that binds H2AK13/15Ub without dependence on the methylation status of histone H4K20. This sensor, called Reader1.0, detects DNA damage-associated H2AK13/15Ub in live cells with high sensitivity and selectivity. Here, we present a protocol to detect the formation of H2AK13/15Ub at laser-induced DSBs using Reader1.0 as a live-cell reporter for this histone PTM. Graphic abstract.

Defective repair of topoisomerase I induced chromosomal damage in Huntington's disease

Topoisomerase1 (TOP1)-mediated chromosomal breaks are endogenous sources of DNA damage that affect neuronal genome stability. Whether TOP1 DNA breaks are sources of genomic instability in Huntington's disease (HD) is unknown. Here, we report defective 53BP1 recruitment in multiple HD cell models, including striatal neurons derived from HD patients. Defective 53BP1 recruitment is due to reduced H2A ubiquitination caused by the limited RNF168 activity. The reduced availability of RNF168 is caused by an increased interaction with p62, a protein involved in selective autophagy. Depletion of p62 or disruption of the interaction between RNAF168 and p62 was sufficient to restore 53BP1 enrichment and subsequent DNA repair in HD models, providing new opportunities for therapeutic interventions. These findings are reminiscent to what was described for p62 accumulation caused by C9orf72 expansion in ALS/FTD and suggest a common mechanism by which protein aggregation perturb DNA repair signaling.

DNA Double Strand Break Repair and Its Control by Nucleosome Remodeling

DNA double strand breaks (DSBs) are repaired in eukaryotes by one of several cellular mechanisms. The decision-making process controlling DSB repair takes place at the step of DNA end resection, the nucleolytic processing of DNA ends, which generates single-stranded DNA overhangs. Dependent on the length of the overhang, a corresponding DSB repair mechanism is engaged. Interestingly, nucleosomes-the fundamental unit of chromatin-influence the activity of resection nucleases and nucleosome remodelers have emerged as key regulators of DSB repair. Nucleosome remodelers share a common enzymatic mechanism, but for global genome organization specific remodelers have been shown to exert distinct activities. Specifically, different remodelers have been found to slide and evict, position or edit nucleosomes. It is an open question whether the same remodelers exert the same function also in the context of DSBs. Here, we will review recent advances in our understanding of nucleosome remodelers at DSBs: to what extent nucleosome sliding, eviction, positioning and editing can be observed at DSBs and how these activities affect the DSB repair decision.

Structural Insight into Chromatin Recognition by Multiple Domains of the Tumor Suppressor RBBP1

Retinoblastoma-binding protein 1 (RBBP1) is involved in gene regulation, epigenetic regulation, and disease processes. RBBP1 contains five domains with DNA-binding or histone-binding activities, but how RBBP1 specifically recognizes chromatin is still unknown. An AT-rich interaction domain (ARID) in RBBP1 was proposed to be the key region for DNA-binding and gene suppression. Here, we first determined the solution structure of a tandem PWWP-ARID domain mutant of RBBP1 after deletion of a long flexible acidic loop L12 in the ARID domain. NMR titration results indicated that the ARID domain interacts with DNA with no GC- or AT-rich preference. Surprisingly, we found that the loop L12 binds to the DNA-binding region of the ARID domain as a DNA mimic and inhibits DNA binding. The loop L12 can also bind weakly to the Tudor and chromobarrel domains of RBBP1, but binds more strongly to the DNA-binding region of the histone H2A-H2B heterodimer. Furthermore, both the loop L12 and DNA can enhance the binding of the chromobarrel domain to H3K4me3 and H4K20me3. Based on these results, we propose a model of chromatin recognition by RBBP1, which highlights the unexpected multiple key roles of the disordered acidic loop L12 in the specific binding of RBBP1 to chromatin.

Design of genetically encoded sensors to detect nucleosome ubiquitination in live cells

Histone posttranslational modifications (PTMs) are dynamic, context-dependent signals that modulate chromatin structure and function. Ubiquitin (Ub) conjugation to different lysines of histones H2A and H2B is used to regulate diverse processes such as gene silencing, transcriptional elongation, and DNA repair. Despite considerable progress made to elucidate the players and mechanisms involved in histone ubiquitination, there remains a lack of tools to monitor these PTMs, especially in live cells. To address this, we combined an avidity-based strategy with in silico approaches to design sensors for specifically ubiquitinated nucleosomes. By linking Ub-binding domains to nucleosome-binding peptides, we engineered proteins that target H2AK13/15Ub and H2BK120Ub with K_d values from 10⁻⁸ to 10⁻⁶ M; when fused to fluorescent proteins, they work as PTM sensors in cells. The H2AK13/15Ub-specific sensor, employed to monitor signaling from endogenous DNA damage through the cell cycle, identified and differentiated roles for 53BP1 and BARD1 as mediators of this histone PTM.

BARD1 reads H2A lysine 15 ubiquitination to direct homologous recombination

Protein ubiquitination at sites of DNA double-strand breaks (DSBs) by RNF168 recruits BRCA1 and 53BP11,2, which are mediators of the homologous recombination and non-homologous end joining DSB repair pathways, respectively3. Non-homologous end joining relies on 53BP1 binding directly to ubiquitinated lysine 15 on H2A-type histones (H2AK15ub)4,5 (which is an RNF168-dependent modification6), but how RNF168 promotes BRCA1 recruitment and function remains unclear. Here we identify a tandem BRCT-domain-associated ubiquitin-dependent recruitment motif (BUDR) in BRCA1-associated RING domain protein 1 (BARD1) (the obligate partner protein of BRCA1) that, by engaging H2AK15ub, recruits BRCA1 to DSBs. Disruption of the BUDR of BARD1 compromises homologous recombination and renders cells hypersensitive to PARP inhibition and cisplatin. We further show that BARD1 binds nucleosomes through multivalent interactions: coordinated binding of H2AK15ub and unmethylated H4 lysine 20 by its adjacent BUDR and ankyrin repeat domains, respectively, provides high-affinity recognition of DNA lesions in replicated chromatin and promotes the homologous recombination activities of the BRCA1-BARD1 complex. Finally, our genetic epistasis experiments confirm that the need for BARD1 chromatin-binding activities can be entirely relieved upon deletion of RNF168 or 53BP1. Thus, our results demonstrate that by sensing DNA-damage-dependent and post-replication histone post-translation modification states, BRCA1-BARD1 complexes coordinate the antagonization of the 53BP1 pathway with promotion of homologous recombination, establishing a simple paradigm for the governance of the choice of DSB repair pathway.

Mechanisms of BRCA1-BARD1 nucleosome recognition and ubiquitylation

The BRCA1-BARD1 tumour suppressor is an E3 ubiquitin ligase necessary for the repair of DNA double-strand breaks by homologous recombination^1-10. The BRCA1-BARD1 complex localizes to damaged chromatin after DNA replication and catalyses the ubiquitylation of histone H2A and other cellular targets^11-14. The molecular bases for the recruitment to double-strand breaks and target recognition of BRCA1-BARD1 remain unknown. Here we use cryo-electron microscopy to show that the ankyrin repeat and tandem BRCT domains in BARD1 adopt a compact fold and bind to nucleosomal histones, DNA and monoubiquitin attached to H2A amino-terminal K13 or K15, two signals known to be specific for double-strand breaks^15,16. We further show that RING domains¹⁷ in BRCA1-BARD1 orient an E2 ubiquitin-conjugating enzyme atop the nucleosome in a dynamic conformation, primed for ubiquitin transfer to the flexible carboxy-terminal tails of H2A and variant H2AX. Our work reveals a regulatory crosstalk in which recognition of monoubiquitin by BRCA1-BARD1 at the N terminus of H2A blocks the formation of polyubiquitin chains and cooperatively promotes ubiquitylation at the C terminus of H2A. These findings elucidate the mechanisms of BRCA1-BARD1 chromatin recruitment and ubiquitylation specificity, highlight key functions of BARD1 in both processes and explain how BRCA1-BARD1 promotes homologous recombination by opposing the DNA repair protein 53BP1 in post-replicative chromatin^18-22. These data provide a structural framework to evaluate BARD1 variants and help to identify mutations that drive the development of cancer.

Structural insight into BRCA1-BARD1 complex recruitment to damaged chromatin

The BRCA1-BARD1 complex directs the DNA double-strand break (DSB) repair pathway choice to error-free homologous recombination (HR) during the S-G2 stages. Targeting BRCA1-BARD1 to DSB-proximal sites requires BARD1-mediated nucleosome interaction and histone mark recognition. Here, we report the cryo-EM structure of BARD1 bound to a ubiquitinated nucleosome core particle (NCP^Ub) at 3.1 Å resolution and illustrate how BARD1 simultaneously recognizes the DNA damage-induced mark H2AK15ub and DNA replication-associated mark H4K20me0 on the nucleosome. In vitro and in vivo analyses reveal that the BARD1-NCP^Ub complex is stabilized by BARD1-nucleosome interaction, BARD1-ubiquitin interaction, and BARD1 ARD domain-BARD1 BRCT domain interaction, and abrogating these interactions is detrimental to HR activity. We further identify multiple disease-causing BARD1 mutations that disrupt BARD1-NCP^Ub interactions and hence impair HR. Together, this study elucidates the mechanism of BRCA1-BARD1 complex recruitment and retention by DSB-flanking nucleosomes and sheds important light on cancer therapeutic avenues.

Versatile control of the CDC48 segregase by the plant UBX-containing (PUX) proteins

In plants, AAA-adenosine triphosphatase (ATPase) Cell Division Control Protein 48 (CDC48) uses the force generated through ATP hydrolysis to pull, extract, and unfold ubiquitylated or sumoylated proteins from the membrane, chromatin, or protein complexes. The resulting changes in protein or RNA content are an important means for plants to control protein homeostasis and thereby adapt to shifting environmental conditions. The activity and targeting of CDC48 are controlled by adaptor proteins, of which the plant ubiquitin regulatory X (UBX) domain-containing (PUX) proteins constitute the largest family. Emerging knowledge on the structure and function of PUX proteins highlights that these proteins are versatile factors for plant homeostasis and adaptation that might inspire biotechnological applications.

Ubiquitylation in DNA double-strand break repair

Genome integrity is constantly challenged by various DNA lesions with DNA double-strand breaks (DSBs) as the most cytotoxic lesions. In order to faithfully repair DSBs, DNA damage response (DDR) signaling networks have evolved, which organize many multi-protein complexes to deal with the encountered DNA damage. Spatiotemporal dynamics of these protein complexes at DSBs are mainly modulated by post-translational modifications (PTMs). One of the most well-studied PTMs in DDR is ubiquitylation which can orchestrate cellular responses to DSBs, promote accurate DNA repair, and maintain genome integrity. Here, we summarize the recent advances of ubiquitin-dependent signaling in DDR and discuss how ubiquitylation crosstalks with other PTMs to control fundamental biological processes in DSB repair.

Structural basis of nucleosome dynamics modulation by histone variants H2A.B and H2A.Z.2.2

Nucleosomes are dynamic entities with wide-ranging compositional variations. Human histone variants H2A.B and H2A.Z.2.2 play critical roles in multiple biological processes by forming unstable nucleosomes and open chromatin structures, but how H2A.B and H2A.Z.2.2 confer these dynamic features to nucleosomes remains unclear. Here, we report cryo-EM structures of nucleosome core particles containing human H2A.B (H2A.B-NCP) at atomic resolution, identifying large-scale structural rearrangements in the histone octamer in H2A.B-NCP. H2A.B-NCP compacts approximately 103 bp of DNA wrapping around the core histones in approximately 1.2 left-handed superhelical turns, in sharp contrast to canonical nucleosome encompassing approximately 1.7 turns of DNA. Micrococcal nuclease digestion assay reveals that nineteen H2A.B-specific residues, including a ROF ("regulating-octamer-folding") sequence of six consecutive residues, are responsible for loosening of H2A.B-NCPs. Unlike H2A.B-NCP, the H2A.Z.2.2-containing nucleosome (Z.2.2-NCP) adopts a less-extended structure and compacts around 125 bp of DNA. Further investigation uncovers a crucial role for the H2A.Z.2.2-specific ROF in both H2A.Z.2.2-NCP opening and SWR1-dependent histone replacement. Taken together, these first high-resolution structure of unstable nucleosomes induced by histone H2A variants elucidate specific functions of H2A.B and H2A.Z.2.2 in enhancing chromatin dynamics.

Chromatin Regulation through Ubiquitin and Ubiquitin-like Histone Modifications

Chromatin functions are influenced by the addition, removal, and recognition of histone post-translational modifications (PTMs). Ubiquitin and ubiquitin-like (UBL) PTMs on histone proteins can function as signaling molecules by mediating protein-protein interactions. Fueled by the identification of novel ubiquitin and UBL sites and the characterization of the writers, erasers, and readers, the breadth of chromatin functions associated with ubiquitin signaling is emerging. Here, we highlight recently appreciated roles for histone ubiquitination in DNA methylation control, PTM crosstalk, nucleosome structure, and phase separation. We also discuss the expanding diversity and functions associated with histone UBL modifications. We conclude with a look toward the future and pose key questions that will drive continued discovery at the interface of epigenetics and ubiquitin signaling.

Accelerating the Field of Epigenetic Histone Modification Through Mass Spectrometry-Based Approaches

Histone post-translational modifications (PTMs) are one of the main mechanisms of epigenetic regulation. Dysregulation of histone PTMs leads to many human diseases, such as cancer. Because of its high throughput, accuracy, and flexibility, mass spectrometry (MS) has emerged as a powerful tool in the epigenetic histone modification field, allowing the comprehensive and unbiased analysis of histone PTMs and chromatin-associated factors. Coupled with various techniques from molecular biology, biochemistry, chemical biology, and biophysics, MS has been used to characterize distinct aspects of histone PTMs in the epigenetic regulation of chromatin functions. In this review, we will describe advancements in the field of MS that have facilitated the analysis of histone PTMs and chromatin biology.

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Middle–down is the most suitable to study histone combinatorial post-translational modifications.

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Crosslinking MS has a variety of potential applications in exploring histone post-translational modifications.

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Hydrogen–deuterium exchange MS holds great promise to study the compaction of nucleosome.

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Multi-omics approaches are useful to study complex regulatory networks.

Enhancement of CRISPR-Cas9 induced precise gene editing by targeting histone H2A-K15 ubiquitination

BACKGROUND

Precise genetic modifications are preferred products of CRISPR-Cas9 mediated gene editing in mammalian cells but require the repair of induced double-strand breaks (DSB) through homology directed repair (HDR). Since HDR competes with the prevailing non-homologous end joining (NHEJ) pathway and depends on the presence of repair templates its efficiency is often limited and demands optimized methodology.

RESULTS

For the enhancement of HDR we redirect the DSB repair pathway choice by targeting the Ubiquitin mark for damaged chromatin at Histone H2A-K15. We used fusions of the Ubiquitin binding domain (UBD) of Rad18 or RNF169 with BRCA1 to promote HDR initiation and UBD fusions with DNA binding domains to attract donor templates and facilitate HDR processing. Using a traffic light reporter system in human HEK293 cells we found that the coexpression of both types of UBD fusion proteins promotes HDR, reduces NHEJ and shifts the HDR/NHEJ balance up to 6-fold. The HDR enhancing effect of UBD fusion proteins was confirmed at multiple endogenous loci.

CONCLUSIONS

Our findings provide a novel efficient approach to promote precise gene editing in human cells.

Ubiquitin Phosphorylation at Thr12 Modulates the DNA Damage Response

The ubiquitin system regulates the DNA damage response (DDR) by modifying histone H2A at Lys15 (H2AK15ub) and triggering downstream signaling events. Here, we find that phosphorylation of ubiquitin at Thr12 (pUbT12) controls the DDR by inhibiting the function of 53BP1, a key factor for DNA double-strand break repair by non-homologous end joining (NHEJ). Detectable as a chromatin modification on H2AK15ub, pUbT12 accumulates in nuclear foci and is increased upon DNA damage. Mutating Thr12 prevents the removal of ubiquitin from H2AK15ub by USP51 deubiquitinating enzyme, leading to a pronounced accumulation of ubiquitinated chromatin. Chromatin modified by pUbT12 is inaccessible to 53BP1 but permissive to the homologous recombination (HR) proteins RNF169, RAD51, and the BRCA1/BARD1 complex. Phosphorylation of ubiquitin at Thr12 in the chromatin context is a new histone mark, H2AK15pUbT12, that regulates the DDR by hampering the activity of 53BP1 at damaged chromosomes.

Comprehensive nucleosome interactome screen establishes fundamental principles of nucleosome binding

Nuclear proteins bind chromatin to execute and regulate genome-templated processes. While studies of individual nucleosome interactions have suggested that an acidic patch on the nucleosome disk may be a common site for recruitment to chromatin, the pervasiveness of acidic patch binding and whether other nucleosome binding hot-spots exist remain unclear. Here, we use nucleosome affinity proteomics with a library of nucleosomes that disrupts all exposed histone surfaces to comprehensively assess how proteins recognize nucleosomes. We find that the acidic patch and two adjacent surfaces are the primary hot-spots for nucleosome disk interactions, whereas nearly half of the nucleosome disk participates only minimally in protein binding. Our screen defines nucleosome surface requirements of nearly 300 nucleosome interacting proteins implicated in diverse nuclear processes including transcription, DNA damage repair, cell cycle regulation and nuclear architecture. Building from our screen, we demonstrate that the Anaphase-Promoting Complex/Cyclosome directly engages the acidic patch, and we elucidate a redundant mechanism of acidic patch binding by nuclear pore protein ELYS. Overall, our interactome screen illuminates a highly competitive nucleosome binding hub and establishes universal principles of nucleosome recognition.

RNF168-Mediated Ubiquitin Signaling Inhibits the Viability of BRCA1-Null Cancers

BRCA1 gene mutations impair homologous recombination (HR) DNA repair, resulting in cellular senescence and embryonic lethality in mice. Therefore, BRCA1-deficient cancers require adaptations that prevent excessive genomic alterations from triggering cell death. RNF168-mediated ubiquitination of γH2AX at K13/15 (ub-H2AX) serves as a recruitment module for the localization of 53BP1 to DNA break sites. Here, we found multiple BRCA1-mutant cancer cell lines and primary tumors with low levels of RNF168 protein expression. Overexpression of ectopic RNF168 or a ub-H2AX fusion protein induced cell death and delayed BRCA1-mutant tumor formation. Cell death resulted from the recruitment of 53BP1 to DNA break sites and inhibition of DNA end resection. Strikingly, reintroduction of BRCA1 or 53BP1 depletion restored HR and rescued the ability of cells to maintain RNF168 and ub-H2AX overexpression. Thus, downregulation of RNF168 protein expression is a mechanism for providing BRCA1-null cancer cell lines with a residual level of HR that is essential for viability. Overall, our work identifies loss of RNF168 ubiquitin signaling as a proteomic alteration that supports BRCA1-mutant carcinogenesis. We propose that restoring RNF168-ub-H2AX signaling, potentially through inhibition of deubiquitinases, could represent a new therapeutic approach. SIGNIFICANCE: This study explores the concept that homologous recombination DNA repair is not an all-or-nothing concept, but a spectrum, and that where a tumor stands on this spectrum may have therapeutic relevance.See related commentary by Wang and Wulf, p. 2720.

Ubiquitylation-Mediated Fine-Tuning of DNA Double-Strand Break Repair

The proper function of DNA repair is indispensable for eukaryotic cells since accumulation of DNA damages leads to genome instability and is a major cause of oncogenesis. Ubiquitylation and deubiquitylation play a pivotal role in the precise regulation of DNA repair pathways by coordinating the recruitment and removal of repair proteins at the damaged site. Here, we summarize the most important post-translational modifications (PTMs) involved in DNA double-strand break repair. Although we highlight the most relevant PTMs, we focus principally on ubiquitylation-related processes since these are the most robust regulatory pathways among those of DNA repair.

A role of the 53BP1 protein in genome protection: structural and functional characteristics of 53BP1-dependent DNA repair

Nuclear architecture plays a significant role in DNA repair mechanisms. It is evident that proteins involved in DNA repair are compartmentalized in not only spontaneously occurring DNA lesions or ionizing radiation-induced foci (IRIF), but a specific clustering of these proteins can also be observed within the whole cell nucleus. For example, 53BP1-positive and BRCA1-positive DNA repair foci decorate chromocenters and can appear close to nuclear speckles. Both 53BP1 and BRCA1 are well-described factors that play an essential role in double-strand break (DSB) repair. These proteins are members of two protein complexes: 53BP1-RIF1-PTIP and BRCA1-CtIP, which make a “decision” determining whether canonical nonhomologous end joining (NHEJ) or homology-directed repair (HDR) is activated. It is generally accepted that 53BP1 mediates the NHEJ mechanism, while HDR is activated via a BRCA1-dependent signaling pathway. Interestingly, the 53BP1 protein appears relatively quickly at DSB sites, while BRCA1 is functional at later stages of DNA repair, as soon as the Mre11-Rad50-Nbs1 complex is recruited to the DNA lesions. A function of the 53BP1 protein is also linked to a specific histone signature, including phosphorylation of histone H2AX (γH2AX) or methylation of histone H4 at the lysine 20 position (H4K20me); therefore, we also discuss an epigenetic landscape of 53BP1-positive DNA lesions.

Histone H2A variants alpha1-extension helix directs RNF168-mediated ubiquitination

Histone ubiquitination plays an important role in the DNA damage response (DDR) pathway. RNF168 catalyzes H2A and H2AX ubiquitination on lysine 13/15 (K13/K15) upon DNA damage and promotes the accrual of downstream repair factors at damaged chromatin. Here, we report that RNF168 ubiquitinates the non-canonical H2A variants H2AZ and macroH2A1/2 at the divergent N-terminal tail lysine residue. In addition to their evolutionarily conserved nucleosome acidic patch, we identify the positively charged alpha1-extension helix as essential for RNF168-mediated ubiquitination of H2A variants. Moreover, mutation of the RNF168 UMI (UIM- and MIU-related UBD) hydrophilic acidic residues abolishes RNF168-mediated ubiquitination as well as 53BP1 and BRCA1 ionizing radiation-induced foci formation. Our results reveal a juxtaposed bipartite electrostatic interaction utilized by the nucleosome to direct RNF168 orientation towards the target lysine residues in proximity to the H2A alpha1-extension helix, which plays an important role in the DDR pathway.

Ectopic RNF168 expression promotes break-induced replication-like DNA synthesis at stalled replication forks

The RNF168 E3 ubiquitin ligase is activated in response to double stranded DNA breaks (DSBs) where it mono-ubiquitinates γH2AX (ub-H2AX). RNF168 protein expression and ubiquitin signaling are finely regulated during the sensing, repair and resolution of DNA damage in order to avoid excessive spreading of ubiquitinated chromatin. Supra-physiological RNF168 protein expression levels have been shown to block DNA end resection at DSBs and increase PARP inhibitor (PARPi) sensitivity. In this study, we examined the impact of ectopic RNF168 overexpression on hydroxyurea (HU)-induced stalled replication forks in the setting of BRCA1 deficiency. Surprisingly, RNF168 overexpression resulted in the extension of DNA fibers, despite the presence of HU, in BRCA1 deficient cells. Mechanistically, RNF168 overexpression recruited RAD18 to ub-H2AX at HU-induced DNA breaks. Subsequently, a RAD18-SLF1 axis was responsible for initiating DNA synthesis in a manner that also required the break-induced replication (BIR) factors RAD52 and POLD3. Strikingly, the presence of wild-type BRCA1 blocked RNF168-induced DNA synthesis. Notably, BIR-like repair has previously been linked with tandem duplication events found in BRCA1-mutated genomes. Thus, in the absence of BRCA1, excessive RNF168 expression may drive BIR, and contribute to the mutational signatures observed in BRCA1-mutated cancers.

Ubiquitin and ubiquitin-like molecules in DNA double strand break repair

Both environmental and endogenous factors induce various forms of DNA damage. DNA double strand break (DSB) is the most deleterious DNA lesion. The swift initiation of a complexed network of interconnected pathways to repair the DNA lesion is essential for cell survival. In the past years, the roles of ubiquitin and ubiquitin-like proteins in DNA damage response and DNA repair has been explored. These findings help us better understand the complicated mechanism of DSB signaling pathways.

53BP1 Enforces Distinct Pre- and Post-resection Blocks on Homologous Recombination

53BP1 activity drives genome instability and lethality in BRCA1-deficient mice by inhibiting homologous recombination (HR). The anti-recombinogenic functions of 53BP1 require phosphorylation-dependent interactions with PTIP and RIF1/shieldin effector complexes. While RIF1/shieldin blocks 5'-3' nucleolytic processing of DNA ends, it remains unclear how PTIP antagonizes HR. Here, we show that mutation of the PTIP interaction site in 53BP1 (S25A) allows sufficient DNA2-dependent end resection to rescue the lethality of BRCA1Δ11 mice, despite increasing RIF1 "end-blocking" at DNA damage sites. However, double-mutant cells fail to complete HR, as excessive shieldin activity also inhibits RNF168-mediated loading of PALB2/RAD51. As a result, BRCA1Δ1153BP1S25A mice exhibit hallmark features of HR insufficiency, including premature aging and hypersensitivity to PARPi. Disruption of shieldin or forced targeting of PALB2 to ssDNA in BRCA1D1153BP1S25A cells restores RNF168 recruitment, RAD51 nucleofilament formation, and PARPi resistance. Our study therefore reveals a critical function of shieldin post-resection that limits the loading of RAD51.