Tripartite DNA Lesion Recognition and Verification by XPC, TFIIH, and XPA in Nucleotide Excision Repair

XPD stalled on cross-linked DNA provides insight into damage verification

The superfamily 2 helicase XPD is a central component of the general transcription factor II H (TFIIH), which is essential for transcription and nucleotide excision DNA repair (NER). Within these two processes, the helicase function of XPD is vital for NER but not for transcription initiation, where XPD acts only as a scaffold for other factors. Using cryo-EM, we deciphered one of the most enigmatic steps in XPD helicase action: the active separation of double-stranded DNA (dsDNA) and its stalling upon approaching a DNA interstrand cross-link, a highly toxic form of DNA damage. The structure shows how dsDNA is separated and reveals a highly unusual involvement of the Arch domain in active dsDNA separation. Combined with mutagenesis and biochemical analyses, we identified distinct functional regions important for helicase activity. Surprisingly, those areas also affect core TFIIH translocase activity, revealing a yet unencountered function of XPD within the TFIIH scaffold. In summary, our data provide a universal basis for NER bubble formation, XPD damage verification and XPG incision.

Persistent TFIIH binding to non-excised DNA damage causes cell and developmental failure

Congenital nucleotide excision repair (NER) deficiency gives rise to several cancer-prone and/or progeroid disorders. It is not understood how defects in the same DNA repair pathway cause different disease features and severity. Here, we show that the absence of functional ERCC1-XPF or XPG endonucleases leads to stable and prolonged binding of the transcription/DNA repair factor TFIIH to DNA damage, which correlates with disease severity and induces senescence features in human cells. In vivo, in C. elegans, this prolonged TFIIH binding to non-excised DNA damage causes developmental arrest and neuronal dysfunction, in a manner dependent on transcription-coupled NER. NER factors XPA and TTDA both promote stable TFIIH DNA binding and their depletion therefore suppresses these severe phenotypical consequences. These results identify stalled NER intermediates as pathogenic to cell functionality and organismal development, which can in part explain why mutations in XPF or XPG cause different disease features than mutations in XPA or TTDA.

Elf1 promotes Rad26's interaction with lesion-arrested Pol II for transcription-coupled repair

Transcription-coupled nucleotide excision repair (TC-NER) is a highly conserved DNA repair pathway that removes bulky lesions in the transcribed genome. Cockayne syndrome B protein (CSB), or its yeast ortholog Rad26, has been known for decades to play important roles in the lesion-recognition steps of TC-NER. Another conserved protein ELOF1, or its yeast ortholog Elf1, was recently identified as a core transcription-coupled repair factor. How Rad26 distinguishes between RNA polymerase II (Pol II) stalled at a DNA lesion or other obstacles and what role Elf1 plays in this process remains unknown. Here, we present cryo-EM structures of Pol II-Rad26 complexes stalled at different obstacles that show that Rad26 uses a common mechanism to recognize a stalled Pol II, with additional interactions when Pol II is arrested at a lesion. A cryo-EM structure of lesion-arrested Pol II-Rad26 bound to Elf1 revealed that Elf1 induces further interactions between Rad26 and a lesion-arrested Pol II. Biochemical and genetic data support the importance of the interplay between Elf1 and Rad26 in TC-NER initiation. Together, our results provide important mechanistic insights into how two conserved transcription-coupled repair factors, Rad26/CSB and Elf1/ELOF1, work together at the initial lesion recognition steps of transcription-coupled repair.

Characterization of human XPD helicase activity with single-molecule magnetic tweezers

XPD helicase is a DNA-unwinding enzyme involved in DNA repair. As part of TFIIH, XPD opens a repair bubble in DNA for access by proteins in the nucleotide excision repair pathway. XPD uses the energy from ATP hydrolysis to translocate in the 5' to 3' direction on one strand of duplex DNA, displacing the opposite strand in the process. We used magnetic tweezers assays to measure the double-stranded DNA unwinding and single-stranded DNA translocation activities of human XPD in isolation. In our experimental setup, hXPD exhibited low unwinding processivity of ∼14 bp and slow unwinding rate of ∼0.3 bp/s. Measurements of the ssDNA translocation activity demonstrated that hXPD translocated on ssDNA at a similar rate as unwinding, revealing that slow rate was an intrinsic property of the hXPD translocation. Individual unwinding and translocation events were composed of pauses and runs with a distribution of lengths and rates. Analysis of these events unveiled similar mean run lengths and rates for unwinding and translocation, indicating that the unwinding behavior was a direct reflection of the translocation activity. The analysis also revealed that hXPD spent similar time stalling and unwinding/translocating. The detailed basal activity of hXPD reported here provides a baseline for future studies on how hXPD activity is regulated by other TFIIH components.

Differing structures and dynamics of two photolesions portray verification differences by the human XPD helicase

Ultraviolet light generates cyclobutane pyrimidine dimer (CPD) and pyrimidine 6-4 pyrimidone (6-4PP) photoproducts that cause skin malignancies if not repaired by nucleotide excision repair (NER). While the faster repair of the more distorting 6-4PPs is attributed mainly to more efficient recognition by XPC, the XPD lesion verification helicase may play a role, as it directly scans the damaged DNA strand. With extensive molecular dynamics simulations of XPD-bound single-strand DNA containing each lesion outside the entry pore of XPD, we elucidate strikingly different verification processes for these two lesions that have very different topologies. The open book-like CPD thymines are sterically blocked from pore entry and preferably entrapped by sensors that are outside the pore; however, the near-perpendicular 6-4PP thymines can enter, accompanied by a displacement of the Arch domain toward the lesion, which is thereby tightly accommodated within the pore. This trapped 6-4PP may inhibit XPD helicase activity to foster lesion verification by locking the Arch to other domains. Furthermore, the movement of the Arch domain, only in the case of 6-4PP, may trigger signaling to the XPG nuclease for subsequent lesion incision by fostering direct contact between the Arch domain and XPG, and thereby facilitating repair of 6-4PP.

TFIIH central activity in nucleotide excision repair to prevent disease

The heterodecameric transcription factor IIH (TFIIH) functions in multiple cellular processes, foremost in nucleotide excision repair (NER) and transcription initiation by RNA polymerase II. TFIIH is essential for life and hereditary mutations in TFIIH cause the devastating human syndromes xeroderma pigmentosum, Cockayne syndrome or trichothiodystrophy, or combinations of these. In NER, TFIIH binds to DNA after DNA damage is detected and, using its translocase and helicase subunits XPB and XPD, opens up the DNA and checks for the presence of DNA damage. This central activity leads to dual incision and removal of the DNA strand containing the damage, after which the resulting DNA gap is restored. In this review, we discuss new structural and mechanistic insights into the central function of TFIIH in NER. Moreover, we provide an elaborate overview of all currently known patients and diseases associated with inherited TFIIH mutations and describe how our understanding of TFIIH function in NER and transcription can explain the different disease features caused by TFIIH deficiency.

Methods for Assessment of Nucleotide Excision Repair Efficiency

Nucleotide excision repair (NER) is responsible for removing a wide variety of bulky adducts from DNA, thus contributing to the maintenance of genome stability. The efficiency with which proteins of the NER system recognize and remove bulky adducts depends on many factors and is of great clinical and diagnostic significance. The review examines current concepts of the NER system molecular basis in eukaryotic cells and analyzes methods for the assessment of the NER-mediated DNA repair efficiency both in vitro and ex vivo.

Poly (ADP-ribose) polymerase 1 and neurodegenerative diseases: Past, present, and future

Poly (ADP-ribose) polymerase 1 (PARP1) is a first responder that recognizes DNA damage and facilitates its repair. Neurodegenerative diseases, characterized by progressive neuron loss driven by various risk factors, including DNA damage, have increasingly shed light on the pivotal involvement of PARP1. During the early phases of neurodegenerative diseases, PARP1 experiences controlled activation to swiftly address mild DNA damage, thereby contributing to maintain brain homeostasis. However, in late stages, exacerbated PARP1 activation precipitated by severe DNA damage exacerbates the disease condition. Consequently, inhibition of PARP1 overactivation emerges as a promising therapeutic approach for neurodegenerative diseases. In this review, we comprehensively synthesize and explore the multifaceted role of PARP1 in neurodegenerative diseases, with a particular emphasis on its over-activation in the aggregation of misfolded proteins, dysfunction of the autophagy-lysosome pathway, mitochondrial dysfunction, neuroinflammation, and blood-brain barrier (BBB) injury. Additionally, we encapsulate the therapeutic applications and limitations intrinsic of PARP1 inhibitors, mainly including limited specificity, intricate pathway dynamics, constrained clinical translation, and the heterogeneity of patient cohorts. We also explore and discuss the potential synergistic implementation of these inhibitors alongside other agents targeting DNA damage cascades within neurodegenerative diseases. Simultaneously, we propose several recommendations for the utilization of PARP1 inhibitors within the realm of neurodegenerative disorders, encompassing factors like the disease-specific roles of PARP1, combinatorial therapeutic strategies, and personalized medical interventions. Lastly, the encompassing review presents a forward-looking perspective along with strategic recommendations that could guide future research endeavors in this field.

Live cell transcription-coupled nucleotide excision repair dynamics revisited

Transcription-blocking lesions are specifically targeted by transcription-coupled nucleotide excision repair (TC-NER), which prevents DNA damage-induced cellular toxicity and maintains proper transcriptional processes. TC-NER is initiated by the stalling of RNA polymerase II (RNAPII), which triggers the assembly of TC-NER-specific proteins, namely CSB, CSA and UVSSA, which collectively control and drive TC-NER progression. Previous research has revealed molecular functions for these proteins, however, exact mechanisms governing the initiation and regulation of TC-NER, particularly at low UV doses have remained elusive, partly due to technical constraints. In this study, we employ knock-in cell lines designed to target the endogenous CSB gene locus with mClover, a GFP variant. Through live cell imaging, we uncover the intricate molecular dynamics of CSB in response to physiologically relevant UV doses. We showed that the DNA damage-induced association of CSB with chromatin is tightly regulated by the CSA-containing ubiquitin-ligase CRL complex (CRL4CSA). Combining the CSB-mClover knock-in cell line with SILAC-based GFP-mediated complex isolation and mass-spectrometry-based proteomics, revealed novel putative CSB interactors as well as discernible variations in complex composition during distinct stages of TC-NER progression. Our work not only provides molecular insight into TC-NER, but also illustrates the versatility of endogenously tagging fluorescent and affinity tags.

Characterization of human XPD helicase activity with Single Molecule Magnetic Tweezers

XPD helicase is a DNA unwinding enzyme involved in multiple cellular processes. As part of TFIIH, XPD opens a repair bubble in DNA for access by proteins in the nucleotide excision repair pathway. XPD uses the energy from ATP hydrolysis to translocate in the 5-prime to 3-prime direction on one strand of duplex DNA, displacing the opposite strand in the process. We used magnetic tweezers assays to measure the double-stranded DNA (dsDNA) unwinding and single-stranded DNA (ssDNA) translocation activities of human XPD by itself. In our experimental setup, hXPD exhibits low unwinding processivity of ~14 bp and slow overall unwinding rate of ~0.3 bp/s. Individual unwinding and translocation events were composed of fast and slow runs and pauses. Analysis of these events gave similar mean run sizes and rates for unwinding and translocation, suggesting that unwinding is a reflection of translocation. The analysis also revealed that hXPD spent similar time stalling and unwinding. hXPD translocated on ssDNA at a similar overall rate as that of unwinding, pointing to an active helicase. However, we observed modest effects of DNA sequence on stalling and unwinding initiation position. Considering the slow unwinding rate, high probability of base pair separation at the ssDNA/dsDNA fork, and the observed DNA sequence dependences, we propose that hXPD is most likely a partially active helicase. Our results provide detailed information on the basal activity of hXPD which enhances our mechanistic understanding of hXPD activity.

ASH1L-MRG15 methyltransferase deposits H3K4me3 and FACT for damage verification in nucleotide excision repair

To recognize DNA adducts, nucleotide excision repair (NER) deploys the XPC sensor, which detects damage-induced helical distortions, followed by engagement of TFIIH for lesion verification. Accessory players ensure that this factor handover takes place in chromatin where DNA is tightly wrapped around histones. Here, we describe how the histone methyltransferase ASH1L, once activated by MRG15, helps XPC and TFIIH to navigate through chromatin and induce global-genome NER hotspots. Upon UV irradiation, ASH1L adds H3K4me3 all over the genome (except in active gene promoters), thus priming chromatin for XPC relocations from native to damaged DNA. The ASH1L-MRG15 complex further recruits the histone chaperone FACT to DNA lesions. In the absence of ASH1L, MRG15 or FACT, XPC is misplaced and persists on damaged DNA without being able to deliver the lesions to TFIIH. We conclude that ASH1L-MRG15 makes damage verifiable by the NER machinery through the sequential deposition of H3K4me3 and FACT.

Helicases required for nucleotide excision repair: structure, function and mechanism

Nucleotide excision repair (NER) is a major DNA repair pathway conserved from bacteria to humans. Various DNA helicases, a group of enzymes capable of separating DNA duplex into two strands through ATP binding and hydrolysis, are required by NER to unwind the DNA duplex around the lesion to create a repair bubble and for damage verification and removal. In prokaryotes, UvrB helicase is required for repair bubble formation and damage verification, while UvrD helicase is responsible for the removal of the excised damage containing single-strand (ss) DNA fragment. In addition, UvrD facilitates transcription-coupled repair (TCR) by backtracking RNA polymerase stalled at the lesion. In eukaryotes, two helicases XPB and XPD from the transcription factor TFIIH complex fulfill the helicase requirements of NER. Interestingly, homologs of all these four helicases UvrB, UvrD, XPB, and XPD have been identified in archaea. This review summarizes our current understanding about the structure, function, and mechanism of these four helicases.

Dynamic conformational switching underlies TFIIH function in transcription and DNA repair and impacts genetic diseases

Transcription factor IIH (TFIIH) is a protein assembly essential for transcription initiation and nucleotide excision repair (NER). Yet, understanding of the conformational switching underpinning these diverse TFIIH functions remains fragmentary. TFIIH mechanisms critically depend on two translocase subunits, XPB and XPD. To unravel their functions and regulation, we build cryo-EM based TFIIH models in transcription- and NER-competent states. Using simulations and graph-theoretical analysis methods, we reveal TFIIH's global motions, define TFIIH partitioning into dynamic communities and show how TFIIH reshapes itself and self-regulates depending on functional context. Our study uncovers an internal regulatory mechanism that switches XPB and XPD activities making them mutually exclusive between NER and transcription initiation. By sequentially coordinating the XPB and XPD DNA-unwinding activities, the switch ensures precise DNA incision in NER. Mapping TFIIH disease mutations onto network models reveals clustering into distinct mechanistic classes, affecting translocase functions, protein interactions and interface dynamics.

At the core of nucleotide excision repair

Nucleotide excision repair (NER) is unique in its ability to identify and remove vastly different lesions from DNA. Recent advances in the structural characterization of complexes involved in detection, verification, and excision of damaged DNA have reshaped our understanding of the molecular architecture of this efficient and accurate machinery. Initial damage recognition achieved through transcription coupled repair (TC-NER) or global genome repair (GG-NER) has been addressed by complexes of RNA Pol II with different TC-NER factors and XPC/RAD23B/Centrin-2 with TFIIH, respectively. Moreover, transcription factor IIH (TFIIH), one of the core repair factors and a central NER hub was resolved in different states, providing important insights how this complex facilitates DNA opening and damage verification. Combined, these recent advances led to a highly improved understanding of the molecular landscape of NER core processes, sharpening our view on how NER is successfully achieved.

Lesion recognition by XPC, TFIIH and XPA in DNA excision repair

Nucleotide excision repair removes DNA lesions caused by ultraviolet light, cisplatin-like compounds and bulky adducts1. After initial recognition by XPC in global genome repair or a stalled RNA polymerase in transcription-coupled repair, damaged DNA is transferred to the seven-subunit TFIIH core complex (Core7) for verification and dual incisions by the XPF and XPG nucleases2. Structures capturing lesion recognition by the yeast XPC homologue Rad4 and TFIIH in transcription initiation or DNA repair have been separately reported3-7. How two different lesion recognition pathways converge and how the XPB and XPD helicases of Core7 move the DNA lesion for verification are unclear. Here we report on structures revealing DNA lesion recognition by human XPC and DNA lesion hand-off from XPC to Core7 and XPA. XPA, which binds between XPB and XPD, kinks the DNA duplex and shifts XPC and the DNA lesion by nearly a helical turn relative to Core7. The DNA lesion is thus positioned outside of Core7, as would occur with RNA polymerase. XPB and XPD, which track the lesion-containing strand but translocate DNA in opposite directions, push and pull the lesion-containing strand into XPD for verification.

Structural modeling and analyses of genetic variations in the human XPC nucleotide excision repair protein

Xeroderma pigmentosum C (XPC) is a key initiator in the global genome nucleotide excision repair pathway in mammalian cells. Inherited mutations in the XPC gene can cause xeroderma pigmentosum (XP) cancer predisposition syndrome that dramatically increases the susceptibility to sunlight-induced cancers. Various genetic variants and mutations of the protein have been reported in cancer databases and literature. The current lack of a high-resolution 3-D structure of human XPC makes it difficult to assess the structural impact of the mutations/genetic variations. Using the available high-resolution crystal structure of its yeast ortholog, Rad4, we built a homology model of human XPC protein and compared it with a model generated by AlphaFold. The two models are largely consistent with each other in the structured domains. We have also assessed the degree of conservation for each residue using 966 sequences of XPC orthologs. Our structure- and sequence conservation-based assessments largely agree with the variant's impact on the protein's structural stability, computed by FoldX and SDM. Known XP missense mutations such as Y585C, W690S, and C771Y are consistently predicted to destabilize the protein's structure. Our analyses also reveal several highly conserved hydrophobic regions that are surface-exposed, which may indicate novel intermolecular interfaces that are yet to be characterized.Communicated by Ramaswamy H. Sarma.

The genome of a hadal sea cucumber reveals novel adaptive strategies to deep-sea environments

How organisms cope with coldness and high pressure in the hadal zone remains poorly understood. Here, we sequenced and assembled the genome of hadal sea cucumber Paelopatides sp. Yap with high quality and explored its potential mechanisms for deep-sea adaptation. First, the expansion of ACOX1 for rate-limiting enzyme in the DHA synthesis pathway, increased DHA content in the phospholipid bilayer, and positive selection of EPT1 may maintain cell membrane fluidity. Second, three genes for translation initiation factors and two for ribosomal proteins underwent expansion, and three ribosomal protein genes were positively selected, which may ameliorate the protein synthesis inhibition or ribosome dissociation in the hadal zone. Third, expansion and positive selection of genes associated with stalled replication fork recovery and DNA repair suggest improvements in DNA protection. This is the first genome sequence of a hadal invertebrate. Our results provide insights into the genetic adaptations used by invertebrate in deep oceans.

A scanning-to-incision switch in TFIIH-XPG induced by DNA damage licenses nucleotide excision repair

Nucleotide excision repair (NER) is critical for removing bulky DNA base lesions and avoiding diseases. NER couples lesion recognition by XPC to strand separation by XPB and XPD ATPases, followed by lesion excision by XPF and XPG nucleases. Here, we describe key regulatory mechanisms and roles of XPG for and beyond its cleavage activity. Strikingly, by combing single-molecule imaging and bulk cleavage assays, we found that XPG binding to the 7-subunit TFIIH core (coreTFIIH) stimulates coreTFIIH-dependent double-strand (ds)DNA unwinding 10-fold, and XPG-dependent DNA cleavage by up to 700-fold. Simultaneous monitoring of rates for coreTFIIH single-stranded (ss)DNA translocation and dsDNA unwinding showed XPG acts by switching ssDNA translocation to dsDNA unwinding as a likely committed step. Pertinent to the NER pathway regulation, XPG incision activity is suppressed during coreTFIIH translocation on DNA but is licensed when coreTFIIH stalls at the lesion or when ATP hydrolysis is blocked. Moreover, ≥15 nucleotides of 5'-ssDNA is a prerequisite for efficient translocation and incision. Our results unveil a paired coordination mechanism in which key lesion scanning and DNA incision steps are sequentially coordinated, and damaged patch removal is only licensed after generation of ≥15 nucleotides of 5'-ssDNA, ensuring the correct ssDNA bubble size before cleavage.

Microarray and in silico analysis of DNA repair genes between human testis of patients with nonobstructive azoospermia and normal cells

DNA repair processes are critical to maintaining genomic integrity. As a result, dysregulation of repair genes is likely to be linked with health implications, such as an increased prevalence of infertility and an accelerated rate of aging. We evaluated all the DNA repair genes (322 genes) by microarray. This study has provided insight into the connection between DNA repair genes, including RAD23B, OBFC2A, PMS1, UBE2V1, ERCC5, SMUG1, RFC4, PMS2L5, MMS19, SHFM1, INO80, PMS2L1, CHEK2, TRIP13, and POLD4. The microarray analysis of six human cases with different nonobstructive azoospermia revealed that RAD23B, OBFC2A, PMS1, UBE2V1, ERCC5, SMUG1, RFC4, PMS2L5, MMS19, SHFM1, and INO80 were upregulated, and expression of PMS2L1, CHEK2, TRIP13, and POLD4 was downregulated versus the normal case. For this purpose, Enrich Shiny GO, STRING, and Cytoscape online evaluation was applied to predict proteins' functional and molecular interactions and then performed to recognize the master pathways. Functional enrichment analysis revealed that the biological process (BP) terms "base-excision repair, AP site formation," "nucleotide-excision repair, DNA gap filling," and "nucleotide-excision repair, preincision complex assembly" was significantly overexpressed in upregulated differentially expressed genes (DEGs). BP analysis of downregulated DEGs highlighted "histone phosphorylation," "DNA damage response, detection DNA response," "mitotic cell cycle checkpoint signaling," and "double-strand break repair." Overrepresented molecular function (MF) terms in upregulated DEGs included "Oxidized base lesion DNA N-glycosylase activity," "uracil DNA N-glycosylase activity," "bubble DNA binding" and "DNA clamp loader activity." Interestingly, MF investigation of downregulated DEGs showed overexpression in "heterotrimeric G-protein complex," "5'-deoxyribose-5-phosphate lyase activity," "minor groove of adenine-thymine-rich DNA binding," and "histone kinase activity." Our findings suggest that these genes and their interacting hub proteins could help determine the pathophysiology of germ cell abnormalities and infertility.

A nuclease-mimetic platinum nanozyme induces concurrent DNA platination and oxidative cleavage to overcome cancer drug resistance

Platinum (Pt) resistance in cancer almost inevitably occurs during clinical Pt-based chemotherapy. The spontaneous nucleotide-excision repair of cancer cells is a representative process that leads to Pt resistance, which involves the local DNA bending to facilitate the recruitment of nucleotide-excision repair proteins and subsequent elimination of Pt-DNA adducts. By exploiting the structural vulnerability of this process, we herein report a nuclease-mimetic Pt nanozyme that can target cancer cell nuclei and induce concurrent DNA platination and oxidative cleavage to overcome Pt drug resistance. We show that the Pt nanozyme, unlike cisplatin and conventional Pt nanoparticles, specifically induces the nanozyme-catalyzed cleavage of the formed Pt-DNA adducts by generating in situ reactive oxygen species, which impairs the damage recognition factors-induced DNA bending prerequisite for nucleotide-excision repair. The recruitment of downstream effectors of nucleotide-excision repair to DNA lesion sites, including xeroderma pigmentosum groups A and F, is disrupted by the Pt nanozyme in cisplatin-resistant cancer cells, allowing excessive accumulation of the Pt-DNA adducts for highly efficient cancer therapy. Our study highlights the potential benefits of applying enzymatic activities to the use of the Pt nanomedicines, providing a paradigm shift in DNA damaging chemotherapy.

The XPA Protein-Life under Precise Control

Nucleotide excision repair (NER) is a central DNA repair pathway responsible for removing a wide variety of DNA-distorting lesions from the genome. The highly choreographed cascade of core NER reactions requires more than 30 polypeptides. The xeroderma pigmentosum group A (XPA) protein plays an essential role in the NER process. XPA interacts with almost all NER participants and organizes the correct NER repair complex. In the absence of XPA's scaffolding function, no repair process occurs. In this review, we briefly summarize our current knowledge about the XPA protein structure and analyze the formation of contact with its protein partners during NER complex assembling. We focus on different ways of regulation of the XPA protein's activity and expression and pay special attention to the network of post-translational modifications. We also discuss the data that is not in line with the currently accepted hypothesis about the functioning of the XPA protein.

Chromatin-remodeling factor BAZ1A/ACF1 targets UV damage sites in an MLL1-dependent manner to facilitate nucleotide excision repair

Ultraviolet (UV) light irradiation generates pyrimidine dimers on DNA, such as cyclobutane pyrimidine dimers (CPDs) and (6-4) photoproducts. Such dimers distort the high-order DNA structure and prevent transcription and replication. The nucleotide excision repair (NER) system contributes to resolving this type of DNA lesion. There are two pathways that recognize pyrimidine dimers. One acts on transcribed strands of DNA (transcription-coupled NER), and the other acts on the whole genome (global genome-NER; GG-NER). In the latter case, DNA damage-binding protein 2 (DDB2) senses pyrimidine dimers with several histone modification enzymes. We previously reported that histone acetyltransferase binding to ORC1 (HBO1) interacts with DDB2 and facilitates recruitment of the imitation switch chromatin remodeler at UV-irradiated sites via an unknown methyltransferase. Here, we found that the phosphorylated histone methyltransferase mixed lineage leukemia 1 (MLL1) was maintained at UV-irradiated sites in an HBO1-dependent manner. Furthermore, MLL1 catalyzed histone H3K4 methylation and recruited the chromatin remodeler bromodomain adjacent to zinc finger domain 1A (BAZ1A)/ATP-utilizing chromatin assembly and remodeling factor 1 (ACF1). Depletion of MLL1 suppressed BAZ1A accumulation at UV-irradiated sites and inhibited the removal of CPDs. These data indicate that the DDB2-HBO1-MLL1 axis is essential for the recruitment of BAZ1A to facilitate GG-NER.

Bulky Adducts in Clustered DNA Lesions: Causes of Resistance to the NER System

The nucleotide excision repair (NER) system removes a wide range of bulky DNA lesions that cause significant distortions of the regular double helix structure. These lesions, mainly bulky covalent DNA adducts, are induced by ultraviolet and ionizing radiation or the interaction between exogenous/endogenous chemically active substances and nitrogenous DNA bases. As the number of DNA lesions increases, e.g., due to intensive chemotherapy and combination therapy of various diseases or DNA repair impairment, clustered lesions containing bulky adducts may occur. Clustered lesions are two or more lesions located within one or two turns of the DNA helix. Despite the fact that repair of single DNA lesions by the NER system in eukaryotic cells has been studied quite thoroughly, the repair mechanism of these lesions in clusters remains obscure. Identification of the structural features of the DNA regions containing irreparable clustered lesions is of considerable interest, in particular due to a relationship between the efficiency of some antitumor drugs and the activity of cellular repair systems. In this review, we analyzed data on the induction of clustered lesions containing bulky adducts, the potential biological significance of these lesions, and methods for quantification of DNA lesions and considered the causes for the inhibition of NER-catalyzed excision of clustered bulky lesions.

Systematic mutagenesis of TFIIH subunit p52/Tfb2 identifies residues required for XPB/Ssl2 subunit function and genetic interactions with TFB6

TFIIH is an evolutionarily conserved complex that plays central roles in both RNA polymerase II (pol II) transcription and DNA repair. As an integral component of the pol II preinitiation complex, TFIIH regulates pol II enzyme activity in numerous ways. The TFIIH subunit XPB/Ssl2 is an ATP-dependent DNA translocase that stimulates promoter opening prior to transcription initiation. Crosslinking-mass spectrometry and cryo-EM results have shown a conserved interaction network involving XPB/Ssl2 and the C-terminal Hub region of the TFIIH p52/Tfb2 subunit, but the functional significance of specific residues is unclear. Here, we systematically mutagenized the HubA region of Tfb2 and screened for growth phenotypes in a TFB6 deletion background in Saccharomyces cerevisiae. We identified six lethal and 12 conditional mutants. Slow growth phenotypes of all but three conditional mutants were relieved in the presence of TFB6, thus identifying a functional interaction between Tfb2 HubA mutants and Tfb6, a protein that dissociates Ssl2 from TFIIH. Our biochemical analysis of Tfb2 mutants with severe growth phenotypes revealed defects in Ssl2 association, with similar results in human cells. Further characterization of these tfb2 mutant cells revealed defects in GAL gene induction, and reduced occupancy of TFIIH and pol II at GAL gene promoters, suggesting that functionally competent TFIIH is required for proper pol II recruitment to preinitiation complexes in vivo. Consistent with recent structural models of TFIIH, our results identify key residues in the p52/Tfb2 HubA domain that are required for stable incorporation of XPB/Ssl2 into TFIIH and for pol II transcription.

Two interaction surfaces between XPA and RPA organize the preincision complex in nucleotide excision repair

The xeroderma pigmentosum protein A (XPA) and replication protein A (RPA) proteins fulfill essential roles in the assembly of the preincision complex in the nucleotide excision repair (NER) pathway. We have previously characterized the two interaction sites, one between the XPA N-terminal (XPA-N) disordered domain and the RPA32 C-terminal domain (RPA32C), and the other with the XPA DNA binding domain (DBD) and the RPA70AB DBDs. Here, we show that XPA mutations that inhibit the physical interaction in either site reduce NER activity in biochemical and cellular systems. Combining mutations in the two sites leads to an additive inhibition of NER, implying that they fulfill distinct roles. Our data suggest a model in which the interaction between XPA-N and RPA32C is important for the initial association of XPA with NER complexes, while the interaction between XPA DBD and RPA70AB is needed for structural organization of the complex to license the dual incision reaction. Integrative structural models of complexes of XPA and RPA bound to single-stranded/double-stranded DNA (ss/dsDNA) junction substrates that mimic the NER bubble reveal key features of the architecture of XPA and RPA in the preincision complex. Most critical among these is that the shape of the NER bubble is far from colinear as depicted in current models, but rather the two strands of unwound DNA must assume a U-shape with the two ss/dsDNA junctions localized in close proximity. Our data suggest that the interaction between XPA and RPA70 is key for the organization of the NER preincision complex.

Mechanism of lesion verification by the human XPD helicase in nucleotide excision repair

In nucleotide excision repair (NER), the xeroderma pigmentosum D helicase (XPD) scans DNA searching for bulky lesions, stalls when encountering such damage to verify its presence, and allows repair to proceed. Structural studies have shown XPD bound to its single-stranded DNA substrate, but molecular and dynamic characterization of how XPD translocates on undamaged DNA and how it stalls to verify lesions remains poorly understood. Here, we have performed extensive all-atom MD simulations of human XPD bound to undamaged and damaged ssDNA, containing a mutagenic pyrimidine (6-4) pyrimidone UV photoproduct (6-4PP), near the XPD pore entrance. We characterize how XPD responds to the presence of the DNA lesion, delineating the atomistic-scale mechanism that it utilizes to discriminate between damaged and undamaged nucleotides. We identify key amino acid residues, including FeS residues R112, R196, H135, K128, Arch residues E377 and R380, and ATPase lobe 1 residues 215-221, that are involved in damage verification and show how movements of Arch and ATPase lobe 1 domains relative to the FeS domain modulate these interactions. These structural and dynamic molecular depictions of XPD helicase activity with unmodified DNA and its inhibition by the lesion elucidate how the lesion is verified by inducing XPD stalling.

Nucleotide excision repair: a versatile and smart toolkit

Nucleotide excision repair (NER) is a major pathway to deal with bulky adducts induced by various environmental toxins in all cellular organisms. The two sub-pathways of NER, global genome repair (GGR) and transcription-coupled repair (TCR), differ in the damage recognition modes. In this review, we describe the molecular mechanism of NER in mammalian cells, especially the details of damage recognition steps in both sub-pathways. We also introduce new sequencing methods for genome-wide mapping of NER, as well as recent advances about NER in chromatin by these methods. Finally, the roles of NER factors in repairing oxidative damages and resolving R-loops are discussed.

Active DNA damage eviction by HLTF stimulates nucleotide excision repair

Nucleotide excision repair (NER) counteracts the onset of cancer and aging by removing helix-distorting DNA lesions via a "cut-and-patch"-type reaction. The regulatory mechanisms that drive NER through its successive damage recognition, verification, incision, and gap restoration reaction steps remain elusive. Here, we show that the RAD5-related translocase HLTF facilitates repair through active eviction of incised damaged DNA together with associated repair proteins. Our data show a dual-incision-dependent recruitment of HLTF to the NER incision complex, which is mediated by HLTF's HIRAN domain that binds 3'-OH single-stranded DNA ends. HLTF's translocase motor subsequently promotes the dissociation of the stably damage-bound incision complex together with the incised oligonucleotide, allowing for an efficient PCNA loading and initiation of repair synthesis. Our findings uncover HLTF as an important NER factor that actively evicts DNA damage, thereby providing additional quality control by coordinating the transition between the excision and DNA synthesis steps to safeguard genome integrity.

XPG: a multitasking genome caretaker

The XPG/ERCC5 endonuclease was originally identified as the causative gene for Xeroderma Pigmentosum complementation group G. Ever since its discovery, in depth biochemical, structural and cell biological studies have provided detailed mechanistic insight into its function in excising DNA damage in nucleotide excision repair, together with the ERCC1–XPF endonuclease. In recent years, it has become evident that XPG has additional important roles in genome maintenance that are independent of its function in NER, as XPG has been implicated in protecting replication forks by promoting homologous recombination as well as in resolving R-loops. Here, we provide an overview of the multitasking of XPG in genome maintenance, by describing in detail how its activity in NER is regulated and the evidence that points to important functions outside of NER. Furthermore, we present the various disease phenotypes associated with inherited XPG deficiency and discuss current ideas on how XPG deficiency leads to these different types of disease.

Mechanism of action of nucleotide excision repair machinery

Nucleotide excision repair (NER) is a versatile DNA repair pathway essential for the removal of a broad spectrum of structurally diverse DNA lesions arising from a variety of sources, including UV irradiation and environmental toxins. Although the core factors and basic stages involved in NER have been identified, the mechanisms of the NER machinery are not well understood. This review summarizes our current understanding of the mechanisms and order of assembly in the core global genome (GG-NER) pathway.

The interplay of supercoiling and thymine dimers in DNA

Thymine dimers are a major mutagenic photoproduct induced by UV radiation. While they have been the subject of extensive theoretical and experimental investigations, questions of how DNA supercoiling affects local defect properties, or, conversely, how the presence of such defects changes global supercoiled structure, are largely unexplored. Here, we introduce a model of thymine dimers in the oxDNA forcefield, parametrized by comparison to melting experiments and structural measurements of the thymine dimer induced bend angle. We performed extensive molecular dynamics simulations of double-stranded DNA as a function of external twist and force. Compared to undamaged DNA, the presence of a thymine dimer lowers the supercoiling densities at which plectonemes and bubbles occur. For biologically relevant supercoiling densities and forces, thymine dimers can preferentially segregate to the tips of the plectonemes, where they enhance the probability of a localized tip-bubble. This mechanism increases the probability of highly bent and denatured states at the thymine dimer site, which may facilitate repair enzyme binding. Thymine dimer-induced tip-bubbles also pin plectonemes, which may help repair enzymes to locate damage. We hypothesize that the interplay of supercoiling and local defects plays an important role for a wider set of DNA damage repair systems.

Revisiting the structural features of the xeroderma pigmentosum proteins: Focus on mutations and knowledge gaps

The nucleotide excision repair pathway is a broadly studied DNA repair mechanism because impairments of its key players, the xeroderma pigmentosum proteins (XPA to XPG), are associated with multiple hereditary diseases. Due to the massive number of novel mutations reported for these proteins and new structural data published every year, proper categorization and discussion of relevant observations is needed to organize this extensive inflow of knowledge. This review aims to revisit the structural data of all XP proteins while updating it with the information developed in of the past six years. Discussions and interpretations of mutation outcomes, mechanisms of action, and knowledge gaps regarding their structures are provided, as well as new perspectives based on recent research.

The Relevance of G-Quadruplexes for DNA Repair

DNA molecules can adopt a variety of alternative structures. Among these structures are G-quadruplex DNA structures (G4s), which support cellular function by affecting transcription, translation, and telomere maintenance. These structures can also induce genome instability by stalling replication, increasing DNA damage, and recombination events. G-quadruplex-driven genome instability is connected to tumorigenesis and other genetic disorders. In recent years, the connection between genome stability, DNA repair and G4 formation was further underlined by the identification of multiple DNA repair proteins and ligands which bind and stabilize said G4 structures to block specific DNA repair pathways. The relevance of G4s for different DNA repair pathways is complex and depends on the repair pathway itself. G4 structures can induce DNA damage and block efficient DNA repair, but they can also support the activity and function of certain repair pathways. In this review, we highlight the roles and consequences of G4 DNA structures for DNA repair initiation, processing, and the efficiency of various DNA repair pathways.

DNA Repair in Haploid Context

DNA repair is a well-covered topic as alteration of genetic integrity underlies many pathological conditions and important transgenerational consequences. Surprisingly, the ploidy status is rarely considered although the presence of homologous chromosomes dramatically impacts the repair capacities of cells. This is especially important for the haploid gametes as they must transfer genetic information to the offspring. An understanding of the different mechanisms monitoring genetic integrity in this context is, therefore, essential as differences in repair pathways exist that differentiate the gamete’s role in transgenerational inheritance. Hence, the oocyte must have the most reliable repair capacity while sperm, produced in large numbers and from many differentiation steps, are expected to carry de novo variations. This review describes the main DNA repair pathways with a special emphasis on ploidy. Differences between Saccharomyces cerevisiae and Schizosaccharomyces pombe are especially useful to this aim as they can maintain a diploid and haploid life cycle respectively.

Recognition and removal of clustered DNA lesions via nucleotide excision repair

Clustered damage of DNA consists of two or more lesions located within one or two turns of the DNA helix. Clusters consisting of lesions of various structures can arise under the influence of strong damaging factors, especially if the cells have a compromised repair status. In this work, we analyzed how the presence of an analog of the apurinic/apyrimidinic site - a non-nucleoside residue consisting of diethylene glycol phosphodiester (DEG) - affects the recognition and removal of a bulky lesion (a non-nucleoside site of the modified DNA strand containing a fluorescein residue, nFlu) from DNA by a mammalian nucleotide excision repair system. Here we demonstrated that the efficiency of nFlu removal decreases in the presence of DEG in the complementary strand and is completely suppressed when the DEG is located opposite the nFlu. By contrast, protein factor XPC-RAD23B, which initiates global genomic nucleotide excision repair, has higher affinity for DNA containing clustered damage as compared to DNA containing a single bulky lesion; the affinity of XPC strengthens as the positions of DEG and nFlu become closer. The changes in the double-stranded DNA's geometry caused by the presence of clustered damage were also assessed. The obtained experimental data together with the results of molecular dynamics simulations make it possible to get insight into the structural features of DNA containing clustered lesions that determine the efficiency of repair. Speaking more broadly, this study should help to understand the probable fate of bulky adduct-containing clusters of various topologies in the mammalian cell.

Every protagonist has a sidekick: Structural aspects of human xeroderma pigmentosum-binding proteins in nucleotide excision repair

The seven xeroderma pigmentosum proteins (XPps), XPA-XPG, coordinate the nucleotide excision repair (NER) pathway, promoting the excision of DNA lesions caused by exposition to ionizing radiation, majorly from ultraviolet light. Significant efforts are made to investigate NER since mutations in any of the seven XPps may cause the xeroderma pigmentosum and trichothiodystrophy diseases. However, these proteins collaborate with other pivotal players in all known NER steps to accurately exert their purposes. Therefore, in the old and ever-evolving field of DNA repair, it is imperative to reexamine and describe their structures to understand NER properly. This work provides an up-to-date review of the protein structural aspects of the closest partners that directly interact and influence XPps: RAD23B, CETN2, DDB1, RPA (RPA70, 32, and 14), p8 (GTF2H5), and ERCC1. Structurally and functionally vital domains, regions, and critical residues are reexamined, providing structural lessons and perspectives about these indispensable proteins in the NER and other DNA repair pathways. By gathering all data related to the major human xeroderma pigmentosum-interacting proteins, this review will aid newcomers on the subject and guide structural and functional future studies.

Transcription-coupled repair and the transcriptional response to UV-Irradiation

Lesions in genes that result in RNA polymerase II (RNAPII) stalling or arrest are particularly toxic as they are a focal point of genome instability and potently block further transcription of the affected gene. Thus, cells have evolved the transcription-coupled nucleotide excision repair (TC-NER) pathway to identify damage-stalled RNAPIIs, so that the lesion can be rapidly repaired and transcription can continue. However, despite the identification of several factors required for TC-NER, how RNAPII is remodelled, modified, removed, or whether this is even necessary for repair remains enigmatic, and theories are intensely contested. Recent studies have further detailed the cellular response to UV-induced ubiquitylation and degradation of RNAPII and its consequences for transcription and repair. These advances make it pertinent to revisit the TC-NER process in general and with specific discussion of the fate of RNAPII stalled at DNA lesions.

Nucleotide Excision Repair: From Molecular Defects to Neurological Abnormalities

Nucleotide excision repair (NER) is the most versatile DNA repair pathway, which can remove diverse bulky DNA lesions destabilizing a DNA duplex. NER defects cause several autosomal recessive genetic disorders. Xeroderma pigmentosum (XP) is one of the NER-associated syndromes characterized by low efficiency of the removal of bulky DNA adducts generated by ultraviolet radiation. XP patients have extremely high ultraviolet-light sensitivity of sun-exposed tissues, often resulting in multiple skin and eye cancers. Some XP patients develop characteristic neurodegeneration that is believed to derive from their inability to repair neuronal DNA damaged by endogenous metabolites. A specific class of oxidatively induced DNA lesions, 8,5′-cyclopurine-2′-deoxynucleosides, is considered endogenous DNA lesions mainly responsible for neurological problems in XP. Growing evidence suggests that XP is accompanied by defective mitophagy, as in primary mitochondrial disorders. Moreover, NER pathway is absent in mitochondria, implying that the mitochondrial dysfunction is secondary to nuclear NER defects. In this review, we discuss the current understanding of the NER molecular mechanism and focuses on the NER linkage with the neurological degeneration in patients with XP. We also present recent research advances regarding NER involvement in oxidative DNA lesion repair. Finally, we highlight how mitochondrial dysfunction may be associated with XP.

Cryo-EM structure of TFIIH/Rad4-Rad23-Rad33 in damaged DNA opening in nucleotide excision repair

The versatile nucleotide excision repair (NER) pathway initiates as the XPC-RAD23B-CETN2 complex first recognizes DNA lesions from the genomic DNA and recruits the general transcription factor complex, TFIIH, for subsequent lesion verification. Here, we present a cryo-EM structure of an NER initiation complex containing Rad4-Rad23-Rad33 (yeast homologue of XPC-RAD23B-CETN2) and 7-subunit coreTFIIH assembled on a carcinogen-DNA adduct lesion at 3.9-9.2 Å resolution. A ~30-bp DNA duplex could be mapped as it straddles between Rad4 and the Ssl2 (XPB) subunit of TFIIH on the 3' and 5' side of the lesion, respectively. The simultaneous binding with Rad4 and TFIIH was permitted by an unwinding of DNA at the lesion. Translocation coupled with torque generation by Ssl2 and Rad4 would extend the DNA unwinding at the lesion and deliver the damaged strand to Rad3 (XPD) in an open form suitable for subsequent lesion scanning and verification.

Three targets in one complex: A molecular perspective of TFIIH in cancer therapy

The general transcription factor II H (TFIIH) plays an essential role in transcription and nucleotide excision DNA repair (NER). TFIIH is a complex 10 subunit containing molecular machine that harbors three enzymatic activities while the remaining subunits assume regulatory and/or structural functions. Intriguingly, the three enzymatic activities of the CDK7 kinase, the XPB translocase, and the XPD helicase exert different impacts on the overall activities of TFIIH. While the enzymatic function of the XPD helicase is exclusively required in NER, the CDK7 kinase is deeply involved in transcription, whereas XPB is essential to both processes. Recent structural and biochemical endeavors enabled unprecedented details towards the molecular basis of these different TFIIH functions and how the enzymatic activities are regulated within the entire complex. Due to its involvement in two fundamental processes, TFIIH has become increasingly important as a target in cancer therapy and two of the three enzymes have already been addressed successfully. Here we explore the possibilities of recent high resolution structures in the context of TFIIH druggability and shed light on the functional consequences of the different approaches towards TFIIH inhibition.

Timely upstream events regulating nucleotide excision repair by ubiquitin-proteasome system: ubiquitin guides the way

The ubiquitin-proteasome system (UPS) plays crucial roles in regulation of multiple DNA repair pathways, including nucleotide excision repair (NER), which eliminates a broad variety of helix-distorting DNA lesions that can otherwise cause deleterious mutations and genomic instability. In mammalian NER, DNA damage sensors, DDB and XPC acting in global genomic NER (GG-NER), and, CSB and RNAPII acting in transcription-coupled NER (TC-NER) sub-pathways, undergo an array of post-translational ubiquitination at the DNA lesion sites. Accumulating evidence indicates that ubiquitination orchestrates the productive assembly of NER preincision complex by driving well-timed compositional changes in DNA damage-assembled sensor complexes. Conversely, the deubiquitination is also intimately involved in regulating the damage sensing aftermath, via removal of degradative ubiquitin modification on XPC and CSB to prevent their proteolysis for the factor recycling. This review summaries the relevant research efforts and latest findings in our understanding of ubiquitin-mediated regulation of NER and active participation by new regulators of NER, e.g., Cullin-Ring ubiquitin ligases (CRLs), ubiquitin-specific proteases (USPs) and ubiquitin-dependent segregase, valosin-containing protein (VCP)/p97. We project hypothetical step-by-step models in which VCP/p97-mediated timely extraction of damage sensors is integral to overall productive NER. The USPs and proteasome subtly counteract in fine-tuning the vital stability and function of NER damage sensors.

Transcription-coupled nucleotide excision repair: New insights revealed by genomic approaches

Elongation of RNA polymerase II (Pol II) is affected by many factors including DNA damage. Bulky damage, such as lesions caused by ultraviolet (UV) radiation, arrests Pol II and inhibits gene transcription, and may lead to genome instability and cell death. Cells activate transcription-coupled nucleotide excision repair (TC-NER) to remove Pol II-impeding damage and allow transcription resumption. TC-NER initiation in humans is mediated by Cockayne syndrome group B (CSB) protein, which binds to the stalled Pol II and promotes assembly of the repair machinery. Given the complex nature of the TC-NER pathway and its unique function at the interface between transcription and repair, new approaches are required to gain in-depth understanding of the mechanism. Advances in genomic approaches provide an important opportunity to investigate how TC-NER is initiated upon damage-induced Pol II stalling and what factors are involved in this process. In this Review, we discuss new mechanisms of TC-NER revealed by genome-wide DNA damage mapping and new TC-NER factors identified by high-throughput screening. As TC-NER conducts strand-specific repair of mutagenic damage, we also discuss how this repair pathway causes mutational strand asymmetry in the cancer genome.

How to limit the speed of a motor: the intricate regulation of the XPB ATPase and translocase in TFIIH

The superfamily 2 helicase XPB is an integral part of the general transcription factor TFIIH and assumes essential catalytic functions in transcription initiation and nucleotide excision repair. The ATPase activity of XPB is required in both processes. We investigated the interaction network that regulates XPB via the p52 and p8 subunits with functional mutagenesis based on our crystal structure of the p52/p8 complex and current cryo-EM structures. Importantly, we show that XPB’s ATPase can be activated either by DNA or by the interaction with the p52/p8 proteins. Intriguingly, we observe that the ATPase activation by p52/p8 is significantly weaker than the activation by DNA and when both p52/p8 and DNA are present, p52/p8 dominates the maximum activation. We therefore define p52/p8 as the master regulator of XPB acting as an activator and speed limiter at the same time. A correlative analysis of the ATPase and translocase activities of XPB shows that XPB only acts as a translocase within the context of complete core TFIIH and that XPA increases the processivity of the translocase complex without altering XPB’s ATPase activity. Our data define an intricate network that tightly controls the activity of XPB during transcription and nucleotide excision repair.

Tethering-facilitated DNA 'opening' and complementary roles of β-hairpin motifs in the Rad4/XPC DNA damage sensor protein

XPC/Rad4 initiates eukaryotic nucleotide excision repair on structurally diverse helix-destabilizing/distorting DNA lesions by selectively 'opening' these sites while rapidly diffusing along undamaged DNA. Previous structural studies showed that Rad4, when tethered to DNA, could also open undamaged DNA, suggesting a 'kinetic gating' mechanism whereby lesion discrimination relied on efficient opening versus diffusion. However, solution studies in support of such a mechanism were lacking and how 'opening' is brought about remained unclear. Here, we present crystal structures and fluorescence-based conformational analyses on tethered complexes, showing that Rad4 can indeed 'open' undamaged DNA in solution and that such 'opening' can largely occur without one or the other of the β-hairpin motifs in the BHD2 or BHD3 domains. Notably, the Rad4-bound 'open' DNA adopts multiple conformations in solution notwithstanding the DNA's original structure or the β-hairpins. Molecular dynamics simulations reveal compensatory roles of the β-hairpins, which may render robustness in dealing with and opening diverse lesions. Our study showcases how fluorescence-based studies can be used to obtain information complementary to ensemble structural studies. The tethering-facilitated DNA 'opening' of undamaged sites and the dynamic nature of 'open' DNA may shed light on how the protein functions within and beyond nucleotide excision repair in cells.

The Dark Side of UV-Induced DNA Lesion Repair

In their life cycle, plants are exposed to various unfavorable environmental factors including ultraviolet (UV) radiation emitted by the Sun. UV-A and UV-B, which are partially absorbed by the ozone layer, reach the surface of the Earth causing harmful effects among the others on plant genetic material. The energy of UV light is sufficient to induce mutations in DNA. Some examples of DNA damage induced by UV are pyrimidine dimers, oxidized nucleotides as well as single and double-strand breaks. When exposed to light, plants can repair major UV-induced DNA lesions, i.e., pyrimidine dimers using photoreactivation. However, this highly efficient light-dependent DNA repair system is ineffective in dim light or at night. Moreover, it is helpless when it comes to the repair of DNA lesions other than pyrimidine dimers. In this review, we have focused on how plants cope with deleterious DNA damage that cannot be repaired by photoreactivation. The current understanding of light-independent mechanisms, classified as dark DNA repair, indispensable for the maintenance of plant genetic material integrity has been presented.

Molecular basis of transcriptional pausing, stalling, and transcription-coupled repair initiation

Transcription elongation by RNA polymerase II (Pol II) is constantly challenged by numerous types of obstacles that lead to transcriptional pausing or stalling. These obstacles include DNA lesions, DNA epigenetic modifications, DNA binding proteins, and non-B form DNA structures. In particular, lesion-induced prolonged transcriptional blockage or stalling leads to genome instability, cellular dysfunction, and cell death. Transcription-coupled nucleotide excision repair (TC-NER) pathway is the first line of defense that detects and repairs these transcription-blocking DNA lesions. In this review, we will first summarize the recent research progress toward understanding the molecular basis of transcriptional pausing and stalling by different kinds of obstacles. We will then discuss new insights into Pol II-mediated lesion recognition and the roles of CSB in TC-NER.

Structural basis of the XPB helicase-Bax1 nuclease complex interacting with the repair bubble DNA

Nucleotide excision repair (NER) removes various DNA lesions caused by UV light and chemical carcinogens. The DNA helicase XPB plays a key role in DNA opening and coordinating damage incision by nucleases during NER, but the underlying mechanisms remain unclear. Here, we report crystal structures of XPB from Sulfurisphaera tokodaii (St) bound to the nuclease Bax1 and their complex with a bubble DNA having one arm unwound in the crystal. StXPB and Bax1 together spirally encircle 10 base pairs of duplex DNA at the double-/single-stranded (ds–ss) junction. Furthermore, StXPB has its ThM motif intruding between the two DNA strands and gripping the 3′-overhang while Bax1 interacts with the 5′-overhang. This ternary complex likely reflects the state of repair bubble extension by the XPB and nuclease machine. ATP binding and hydrolysis by StXPB could lead to a spiral translocation along dsDNA and DNA strand separation by the ThM motif, revealing an unconventional DNA unwinding mechanism. Interestingly, the DNA is kept away from the nuclease domain of Bax1, potentially preventing DNA incision by Bax1 during repair bubble extension.

DNA helicases and their roles in cancer

DNA helicases, known for their fundamentally important roles in genomic stability, are high profile players in cancer. Not only are there monogenic helicase disorders with a strong disposition to cancer, it is well appreciated that helicase variants are associated with specific cancers (e.g., breast cancer). Flipping the coin, DNA helicases are frequently overexpressed in cancerous tissues and reduction in helicase gene expression results in reduced proliferation and growth capacity, as well as DNA damage induction and apoptosis of cancer cells. The seminal roles of helicases in the DNA damage and replication stress responses, as well as DNA repair pathways, validate their vital importance in cancer biology and suggest their potential values as targets in anti-cancer therapy. In recent years, many laboratories have characterized the specialized roles of helicase to resolve transcription-replication conflicts, maintain telomeres, mediate cell cycle checkpoints, remodel stalled replication forks, and regulate transcription. In vivo models, particularly mice, have been used to interrogate helicase function and serve as a bridge for preclinical studies that may lead to novel therapeutic approaches. In this review, we will summarize our current knowledge of DNA helicases and their roles in cancer, emphasizing the latest developments.

Genetic Polymorphisms and the Efficacy of Platinum-Based Chemotherapy: Review

Previous studies have indicated that genetic variations in individuals may result in changes in gene expression and amino acids. The effect of these changes may lead to different responses to platinum-based chemotherapy. A vast response rate interval and a short survival rate indicate that the efficacy and efficiency of the selection of chemotherapy have not been optimized. This article aims to illustrate the potential relationship of various genetic polymorphisms in response to platinum-based chemotherapy for several types of cancer. This review was conducted using articles from the last three- and five-year periods (2014-2019) that use gene polymorphism and its relationship to the efficacy of platinum-based chemotherapy as their theme. A total of 26 out of 488 relevant articles were included based on specific criteria. Through various mechanisms, genes, including ERCC1, ERCC2/XPD, XPC, XPA, XRCC1, APE-1, PARP1, OGG1, ABCC2, MRP, GSTP1, GSTM1, GSTT1, MATE1, and OCT2, have been associated with patient response to platinum-based chemotherapy. We conclude that genetic polymorphism analysis is recommended for the management of cancer so that each patient can be administered therapy based on his or her genetic profile to achieve an effective and efficient outcome.

Ubiquitin and TFIIH-stimulated DDB2 dissociation drives DNA damage handover in nucleotide excision repair

DNA damage sensors DDB2 and XPC initiate global genome nucleotide excision repair (NER) to protect DNA from mutagenesis caused by helix-distorting lesions. XPC recognizes helical distortions by binding to unpaired ssDNA opposite DNA lesions. DDB2 binds to UV-induced lesions directly and facilitates efficient recognition by XPC. We show that not only lesion-binding but also timely DDB2 dissociation is required for DNA damage handover to XPC and swift progression of the multistep repair reaction. DNA-binding-induced DDB2 ubiquitylation and ensuing degradation regulate its homeostasis to prevent excessive lesion (re)binding. Additionally, damage handover from DDB2 to XPC coincides with the arrival of the TFIIH complex, which further promotes DDB2 dissociation and formation of a stable XPC-TFIIH damage verification complex. Our results reveal a reciprocal coordination between DNA damage recognition and verification within NER and illustrate that timely repair factor dissociation is vital for correct spatiotemporal control of a multistep repair process.