RAD18-BRCTx interaction is required for efficient repair of UV-induced DNA damage

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.

Structural insights into Rad18 targeting by the SLF1 BRCT domains

Rad18 interacts with the SMC5/6 localization factor 1 (SLF1) to recruit the SMC5/6 complex to DNA damage sites for repair. The mechanism of the specific Rad18 recognition by SLF1 is unclear. Here, we present the crystal structure of the tandem BRCT repeat (tBRCT) in SLF1 (SLF1tBRCT) bound with the interacting Rad18 peptide. Our structure and biochemical studies demonstrate that SLF1tBRCT interacts with two phosphoserines and adjacent residues in Rad18 for high-affinity and specificity Rad18 recognition. We found that SLF1tBRCT utilizes mechanisms common among tBRCTs as well as unique ones for Rad18 binding, the latter include interactions with an α-helical structure in Rad18 that has not been observed in other tBRCT-bound ligand proteins. Our work provides structural insights into Rad18 targeting by SLF1 and expands the understanding of BRCT-mediated complex assembly.

Genetic connections and convergent evolution of tropical indigenous peoples in Asia

Tropical indigenous peoples in Asia (TIA) attract much attention for their unique appearance, whereas their genetic history and adaptive evolution remain mysteries. We conducted a comprehensive study to characterize the genetic distinction and connection of broad geographical TIAs. Despite the diverse genetic makeup and large interarea genetic differentiation between the TIA groups, we identified a basal Asian ancestry (bASN) specifically shared by these populations. The bASN ancestry was relatively enriched in ancient Asian human genomes dated as early as ∼50,000 years before the present and diminished in more recent history. Notably, the bASN ancestry is unlikely to be derived from archaic hominins. Instead, we suggest it may be better modeled as a survived lineage of the initial peopling of Asia. Shared adaptations inherited from the ancient Asian ancestry were detected among the TIA groups (e.g., LIMS1 for hair morphology, and COL24A1 for bone formation), and they are enriched in neurological functions either at an identical locus (e.g., NKAIN3), or different loci in an identical gene (e.g., TENM4). The bASN ancestry could also have formed the substrate of the genetic architecture of the dark pigmentation observed in the TIA peoples. We hypothesize that phenotypic convergence of the dark pigmentation in TIAs could have resulted from parallel (e.g., DDB1/DAK) or genetic convergence driven by admixture (e.g., MTHFD1 and RAD18), new mutations (e.g., STK11), or notably purifying selection (e.g., MC1R). Our results provide new insights into the initial peopling of Asia and an advanced understanding of the phenotypic convergence of the TIA peoples.

High RAD18 Expression is Associated with Disease Progression and Poor Prognosis in Patients with Gastric Cancer

BACKGROUND

RAD18 plays an important role in DNA damage repair by inducing monoubiquitinated PCNA (mUB-PCNA) in both cancer and normal tissues. Previous studies have not determined the significance of RAD18 expression in clinical gastric cancer (GC) samples. Thus, this study aimed to clarify the expression and functional significance of RAD18 in GC.

METHODS

Overall, 96 resected GC samples were subjected to an immunohistochemical analysis of RAD18. GC cell lines were also subjected to functional RNA interference analyses of RAD18.

RESULTS

RAD18 expression was predominantly nuclear and was observed at higher levels in GC tissues than in normal tissues. In GC tissues, strong RAD18 expression was associated with progression of lymph node metastasis (p = 0.0001), lymphatic invasion (p = 0.0255), venous invasion (p < 0.0001), recurrence (p = 0.028), and disease stage (p = 0.0253). Moreover, GC patients with high tumor RAD18 expression had shorter overall survival (p = 0.0061) and recurrence-free survival durations (p = 0.035) than those with low tumor RAD18 expression. RAD18 knockdown inhibited GC proliferation and invasiveness and increased chemosensitivity by suppressing mUB-PCNA.

CONCLUSIONS

RAD18 expression may be a useful marker of progression and poor prognosis of GC. Moreover, therapeutic strategies that target RAD18 might be a novel chemosensitizer to eradicate the refractory GC.

Brc1 Promotes the Focal Accumulation and SUMO Ligase Activity of Smc5-Smc6 during Replication Stress

As genetic instability drives disease or loss of cell fitness, cellular safeguards have evolved to protect the genome, especially during sensitive cell cycle phases, such as DNA replication. Fission yeast Brc1 has emerged as a key factor in promoting cell survival when replication forks are stalled or collapsed. Brc1 is a multi-BRCT protein that is structurally related to the budding yeast Rtt107 and human PTIP DNA damage response factors, but functional similarities appear limited. Brc1 is a dosage suppressor of a mutation in the essential Smc5-Smc6 genome stability complex and is thought to act in a bypass pathway. In this study, we reveal an unexpectedly intimate connection between Brc1 and Smc5-Smc6 function. Brc1 is required for the accumulation of the Smc5-Smc6 genome stability complex in foci during replication stress and for activation of the intrinsic SUMO ligase activity of the complex by collapsed replication forks. Moreover, we show that the chromatin association and SUMO ligase activity of Smc5-Smc6 require the Nse5-Nse6 heterodimer, explaining how this nonessential cofactor critically supports the DNA repair roles of Smc5-Smc6. We also found that Brc1 interacts with Nse5-Nse6, as well as gamma-H2A, so it can tether Smc5-Smc6 at replicative DNA lesions to promote survival.

Multi-BRCT Domain Protein Brc1 Links Rhp18/Rad18 and γH2A To Maintain Genome Stability during S Phase

DNA replication involves the inherent risk of genome instability, since replisomes invariably encounter DNA lesions or other structures that stall or collapse replication forks during the S phase. In the fission yeast Schizosaccharomyces pombe, the multi-BRCT domain protein Brc1, which is related to budding yeast Rtt107 and mammalian PTIP, plays an important role in maintaining genome integrity and cell viability when cells experience replication stress. The C-terminal pair of BRCT domains in Brc1 were previously shown to bind phosphohistone H2A (γH2A) formed by Rad3/ATR checkpoint kinase at DNA lesions; however, the putative scaffold interactions involving the N-terminal BRCT domains 1 to 4 of Brc1 have remained obscure. Here, we show that these domains bind Rhp18/Rad18, which is an E3 ubiquitin protein ligase that has crucial functions in postreplication repair. A missense allele in BRCT domain 4 of Brc1 disrupts binding to Rhp18 and causes sensitivity to replication stress. Brc1 binding to Rhp18 and γH2A are required for the Brc1 overexpression suppression of smc6-74, a mutation that impairs the Smc5/6 structural maintenance of chromosomes complex required for chromosome integrity and repair of collapsed replication forks. From these findings, we propose that Brc1 provides scaffolding functions linking γH2A, Rhp18, and Smc5/6 complex at damaged replication forks.

DNA repair. Proteomics reveals dynamic assembly of repair complexes during bypass of DNA cross-links

DNA interstrand cross-links (ICLs) block replication fork progression by inhibiting DNA strand separation. Repair of ICLs requires sequential incisions, translesion DNA synthesis, and homologous recombination, but the full set of factors involved in these transactions remains unknown. We devised a technique called chromatin mass spectrometry (CHROMASS) to study protein recruitment dynamics during perturbed DNA replication in Xenopus egg extracts. Using CHROMASS, we systematically monitored protein assembly and disassembly on ICL-containing chromatin. Among numerous prospective DNA repair factors, we identified SLF1 and SLF2, which form a complex with RAD18 and together define a pathway that suppresses genome instability by recruiting the SMC5/6 cohesion complex to DNA lesions. Our study provides a global analysis of an entire DNA repair pathway and reveals the mechanism of SMC5/6 relocalization to damaged DNA in vertebrate cells.

RAD18 activates the G2/M checkpoint through DNA damage signaling to maintain genome integrity after ionizing radiation exposure

The ubiquitin ligase RAD18 is involved in post replication repair pathways via its recruitment to stalled replication forks, and its role in the ubiquitylation of proliferating cell nuclear antigen (PCNA). Recently, it has been reported that RAD18 is also recruited to DNA double strand break (DSB) sites, where it plays novel functions in the DNA damage response induced by ionizing radiation (IR). This new role is independent of PCNA ubiquitylation, but little is known about how RAD18 functions after IR exposure. Here, we describe a role for RAD18 in the IR-induced DNA damage signaling pathway at G2/M phase in the cell cycle. Depleting cells of RAD18 reduced the recruitment of the DNA damage signaling factors ATM, γH2AX, and 53BP1 to foci in cells at the G2/M phase after IR exposure, and attenuated activation of the G2/M checkpoint. Furthermore, depletion of RAD18 increased micronuclei formation and cell death following IR exposure, both in vitro and in vivo. Our data suggest that RAD18 can function as a mediator for DNA damage response signals to activate the G2/M checkpoint in order to maintain genome integrity and cell survival after IR exposure.

The ubiquitin specific protease USP34 promotes ubiquitin signaling at DNA double-strand breaks

Ubiquitylation plays key roles in DNA damage signal transduction. The current model envisions that lysine63-linked ubiquitin chains, via the concerted action of E3 ubiquitin ligases RNF8-RNF168, are built at DNA double-strand breaks (DSBs) to effectively assemble DNA damage-repair factors for proper checkpoint control and DNA repair. We found that RNF168 is a short-lived protein that is stabilized by the deubiquitylating enzyme USP34 in response to DNA damage. In the absence of USP34, RNF168 is rapidly degraded, resulting in attenuated DSB-associated ubiquitylation, defective recruitment of BRCA1 and 53BP1 and compromised cell survival after ionizing radiation. We propose that USP34 promotes a feed-forward loop to enforce ubiquitin signaling at DSBs and highlight critical roles of ubiquitin dynamics in genome stability maintenance.