SNM1A is crucial for efficient repair of complex DNA breaks in human cells.

DNA double-strand breaks (DSBs), such as those produced by radiation and radiomimetics, are amongst the most toxic forms of cellular damage, in part because they involve extensive oxidative modifications at the break termini. Prior to completion of DSB repair, the chemically modified termini must be removed. Various DNA processing enzymes have been implicated in the processing of these dirty ends, but molecular knowledge of this process is limited. Here, we demonstrate a role for the metallo-ß-lactamase fold 5'-3' exonuclease SNM1A in this vital process. Cells disrupted for SNM1A manifest increased sensitivity to radiation and radiomimetic agents and show defects in DSB damage repair. SNM1A is recruited and is retained at the sites of DSB damage via the concerted action of its three highly conserved PBZ, PIP box and UBZ interaction domains, which mediate interactions with poly-ADP-ribose chains, PCNA and the ubiquitinated form of PCNA, respectively. SNM1A can resect DNA containing oxidative lesions induced by radiation damage at break termini. The combined results reveal a crucial role for SNM1A to digest chemically modified DNA during the repair of DSBs and imply that the catalytic domain of SNM1A is an attractive target for potentiation of radiotherapy.

Cell-active small molecule inhibitors validate the SNM1A DNA repair nuclease as a cancer target.

The three human SNM1 metallo-ß-lactamase fold nucleases (SNM1A-C) play key roles in DNA damage repair and in maintaining telomere integrity. Genetic studies indicate that they are attractive targets for cancer treatment and to potentiate chemo- and radiation-therapy. A high-throughput screen for SNM1A inhibitors identified diverse pharmacophores, some of which were shown by crystallography to coordinate to the di-metal ion centre at the SNM1A active site. Structure and turnover assay-guided optimization enabled the identification of potent quinazoline-hydroxamic acid containing inhibitors, which bind in a manner where the hydroxamic acid displaces the hydrolytic water and the quinazoline ring occupies a substrate nucleobase binding site. Cellular assays reveal that SNM1A inhibitors cause sensitisation to, and defects in the resolution of, cisplatin-induced DNA damage, validating the tractability of MBL fold nucleases as cancer drug targets.

Novobiocin blocks nucleic acid binding to Polθ and inhibits stimulation of its ATPase activity.

Polymerase theta (Polθ) acts in DNA replication and repair, and its inhibition is synthetic lethal in BRCA1 and BRCA2-deficient tumor cells. Novobiocin (NVB) is a first-in-class inhibitor of the Polθ ATPase activity, and it is currently being tested in clinical trials as an anti-cancer drug. Here, we investigated the molecular mechanism of NVB-mediated Polθ inhibition. Using hydrogen deuterium exchange-mass spectrometry (HX-MS), biophysical, biochemical, computational and cellular assays, we found NVB is a non-competitive inhibitor of ATP hydrolysis. NVB sugar group deletion resulted in decreased potency and reduced HX-MS interactions, supporting a specific NVB binding orientation. Collective results revealed that NVB binds to an allosteric site to block DNA binding, both in vitro and in cells. Comparisons of The Cancer Genome Atlas (TCGA) tumors and matched controls implied that POLQ upregulation in tumors stems from its role in replication stress responses to increased cell proliferation: this can now be tested in fifteen tumor types by NVB blocking ssDNA-stimulation of ATPase activity, required for Polθ function at replication forks and DNA damage sites. Structural and functional insights provided in this study suggest a path for developing NVB derivatives with improved potency for Polθ inhibition by targeting ssDNA binding with entropically constrained small molecules.

Crystal structures and fragment screening of SARS-CoV-2 NSP14 reveal details of exoribonuclease activation and mRNA capping and provide starting points for antiviral drug development.

NSP14 is a dual function enzyme containing an N-terminal exonuclease domain (ExoN) and C-terminal Guanine-N7-methyltransferase (N7-MTase) domain. Both activities are essential for the viral life cycle and may be targeted for anti-viral therapeutics. NSP14 forms a complex with NSP10, and this interaction enhances the nuclease but not the methyltransferase activity. We have determined the structure of SARS-CoV-2 NSP14 in the absence of NSP10 to 1.7 Å resolution. Comparisons with NSP14/NSP10 complexes reveal significant conformational changes that occur within the NSP14 ExoN domain upon binding of NSP10, including helix to coil transitions that facilitate the formation of the ExoN active site and provide an explanation of the stimulation of nuclease activity by NSP10. We have determined the structure of NSP14 in complex with cap analogue 7MeGpppG, and observe conformational changes within a SAM/SAH interacting loop that plays a key role in viral mRNA capping offering new insights into MTase activity. We perform an X-ray fragment screen on NSP14, revealing 72 hits bound to sites of inhibition in the ExoN and MTase domains. These fragments serve as excellent starting point tools for structure guided development of NSP14 inhibitors that may be used to treat COVID-19 and potentially other future viral threats.

FOXN1 forms higher-order nuclear condensates displaced by mutations causing immunodeficiency.

The transcription factor FOXN1 is a master regulator of thymic epithelial cell (TEC) development and function. Here, we demonstrate that FOXN1 expression is differentially regulated during organogenesis and participates in multimolecular nuclear condensates essential for the factor's transcriptional activity. FOXN1's C-terminal sequence regulates the diffusion velocity within these aggregates and modulates the binding to proximal gene regulatory regions. These dynamics are altered in a patient with a mutant FOXN1 that is modified in its C-terminal sequence. This mutant is transcriptionally inactive and acts as a dominant negative factor displacing wild-type FOXN1 from condensates and causing athymia and severe lymphopenia in heterozygotes. Expression of the mutated mouse ortholog selectively impairs mouse TEC differentiation, revealing a gene dose dependency for individual TEC subtypes. We have therefore identified the cause for a primary immunodeficiency disease and determined the mechanism by which this FOXN1 gain-of-function mutant mediates its dominant negative effect.

Structure, mechanism and crystallographic fragment screening of the SARS-CoV-2 NSP13 helicase.

There is currently a lack of effective drugs to treat people infected with SARS-CoV-2, the cause of the global COVID-19 pandemic. The SARS-CoV-2 Non-structural protein 13 (NSP13) has been identified as a target for anti-virals due to its high sequence conservation and essential role in viral replication. Structural analysis reveals two "druggable" pockets on NSP13 that are among the most conserved sites in the entire SARS-CoV-2 proteome. Here we present crystal structures of SARS-CoV-2 NSP13 solved in the APO form and in the presence of both phosphate and a non-hydrolysable ATP analog. Comparisons of these structures reveal details of conformational changes that provide insights into the helicase mechanism and possible modes of inhibition. To identify starting points for drug development we have performed a crystallographic fragment screen against NSP13. The screen reveals 65 fragment hits across 52 datasets opening the way to structure guided development of novel antiviral agents.

A phosphate binding pocket is a key determinant of exo- versus endo-nucleolytic activity in the SNM1 nuclease family.

The SNM1 nucleases which help maintain genome integrity are members of the metallo-ß-lactamase (MBL) structural superfamily. Their conserved MBL-ß-CASP-fold SNM1 core provides a molecular scaffold forming an active site which coordinates the metal ions required for catalysis. The features that determine SNM1 endo- versus exonuclease activity, and which control substrate selectivity and binding are poorly understood. We describe a structure of SNM1B/Apollo with two nucleotides bound to its active site, resembling the product state of its exonuclease reaction. The structure enables definition of key SNM1B residues that form contacts with DNA and identifies a 5' phosphate binding pocket, which we demonstrate is important in catalysis and which has a key role in determining endo- versus exonucleolytic activity across the SNM1 family. We probed the capacity of SNM1B to digest past sites of common endogenous DNA lesions and find that base modifications planar to the nucleobase can be accommodated due to the open architecture of the active site, but lesions axial to the plane of the nucleobase are not well tolerated due to constriction around the altered base. We propose that SNM1B/Apollo might employ its activity to help remove common oxidative lesions from telomeres.

Structural and mechanistic insights into the Artemis endonuclease and strategies for its inhibition.

Artemis (SNM1C/DCLRE1C) is an endonuclease that plays a key role in development of B- and T-lymphocytes and in dsDNA break repair by non-homologous end-joining (NHEJ). Artemis is phosphorylated by DNA-PKcs and acts to open DNA hairpin intermediates generated during V(D)J and class-switch recombination. Artemis deficiency leads to congenital radiosensitive severe acquired immune deficiency (RS-SCID). Artemis belongs to a superfamily of nucleases containing metallo-ß-lactamase (MBL) and ß-CASP (CPSF-Artemis-SNM1-Pso2) domains. We present crystal structures of the catalytic domain of wildtype and variant forms of Artemis, including one causing RS-SCID Omenn syndrome. The catalytic domain of the Artemis has similar endonuclease activity to the phosphorylated full-length protein. Our structures help explain the predominantly endonucleolytic activity of Artemis, which contrasts with the predominantly exonuclease activity of the closely related SNM1A and SNM1B MBL fold nucleases. The structures reveal a second metal binding site in its ß-CASP domain unique to Artemis, which is amenable to inhibition by compounds including ebselen. By combining our structural data with that from a recently reported Artemis structure, we were able model the interaction of Artemis with DNA substrates. The structures, including one of Artemis with the cephalosporin ceftriaxone, will help enable the rational development of selective SNM1 nuclease inhibitors.

SPRTN protease-cleaved MRE11 decreases DNA repair and radiosensitises cancer cells.

The human MRE11/RAD50/NBS1 (MRN) complex plays a crucial role in sensing and repairing DNA DSB. MRE11 possesses dual 3'-5' exonuclease and endonuclease activity and forms the core of the multifunctional MRN complex. We previously identified a C-terminally truncated form of MRE11 (TR-MRE11) associated with post-translational MRE11 degradation. Here we identified SPRTN as the essential protease for the formation of TR-MRE11 and characterised the role of this MRE11 form in its DNA damage response (DDR). Using tandem mass spectrometry and site-directed mutagenesis, the SPRTN-dependent cleavage site for MRE11 was identified between 559 and 580 amino acids. Despite the intact interaction of TR-MRE11 with its constitutive core complex proteins RAD50 and NBS1, both nuclease activities of truncated MRE11 were dramatically reduced due to its deficient binding to DNA. Furthermore, lack of the MRE11 C-terminal decreased HR repair efficiency, very likely due to abolished recruitment of TR-MRE11 to the sites of DNA damage, which consequently led to increased cellular radiosensitivity. The presence of this DNA repair-defective TR-MRE11 could explain our previous finding that the high MRE11 protein expression by immunohistochemistry correlates with improved survival following radical radiotherapy in bladder cancer patients.

Structure of the helicase core of Werner helicase, a key target in microsatellite instability cancers.

Loss of WRN, a DNA repair helicase, was identified as a strong vulnerability of microsatellite instable (MSI) cancers, making WRN a promising drug target. We show that ATP binding and hydrolysis are required for genome integrity and viability of MSI cancer cells. We report a 2.2-Å crystal structure of the WRN helicase core (517-1,093), comprising the two helicase subdomains and winged helix domain but not the HRDC domain or nuclease domains. The structure highlights unusual features. First, an atypical mode of nucleotide binding that results in unusual relative positioning of the two helicase subdomains. Second, an additional ß-hairpin in the second helicase subdomain and an unusual helical hairpin in the Zn2+ binding domain. Modelling of the WRN helicase in complex with DNA suggests roles for these features in the binding of alternative DNA structures. NMR analysis shows a weak interaction between the HRDC domain and the helicase core, indicating a possible biological role for this association. Together, this study will facilitate the structure-based development of inhibitors against WRN helicase.

RecQ helicases in DNA repair and cancer targets.

Helicases are enzymes that use the energy derived from ATP hydrolysis to catalyze the unwinding of DNA or RNA. The RecQ family of helicases is conserved through evolution from prokaryotes to higher eukaryotes and plays important roles in various DNA repair pathways, contributing to the maintenance of genome integrity. Despite their roles as general tumor suppressors, there is now considerable interest in exploiting RecQ helicases as synthetic lethal targets for the development of new cancer therapeutics. In this review, we summarize the latest developments in the structural and mechanistic study of RecQ helicases and discuss their roles in various DNA repair pathways. Finally, we consider the potential to exploit RecQ helicases as therapeutic targets and review the recent progress towards the development of small molecules targeting RecQ helicases as cancer therapeutics.

The SNM1A DNA repair nuclease.

Unrepaired, or misrepaired, DNA damage can contribute to the pathogenesis of a number of conditions, or disease states; thus, DNA damage repair pathways, and the proteins within them, are required for the safeguarding of the genome. Human SNM1A is a 5'-to-3' exonuclease that plays a role in multiple DNA damage repair processes. To date, most data suggest a role of SNM1A in primarily ICL repair: SNM1A deficient cells exhibit hypersensitivity to ICL-inducing agents (e.g. mitomycin C and cisplatin); and both in vivo and in vitro experiments demonstrate SNM1A and XPF-ERCC1 can function together in the 'unhooking' step of ICL repair. SNM1A further interacts with a number of other proteins that contribute to genome integrity outside canonical ICL repair (e.g. PCNA and CSB), and these may play a role in regulating SNM1As function, subcellular localisation, and post-translational modification state. These data also provide further insight into other DNA repair pathways to which SNM1A may contribute. This review aims to discuss all aspects of the exonuclease, SNM1A, and its contribution to DNA damage tolerance.

STAG1 vulnerabilities for exploiting cohesin synthetic lethality in STAG2-deficient cancers.

The cohesin subunit STAG2 has emerged as a recurrently inactivated tumor suppressor in human cancers. Using candidate approaches, recent studies have revealed a synthetic lethal interaction between STAG2 and its paralog STAG1 To systematically probe genetic vulnerabilities in the absence of STAG2, we have performed genome-wide CRISPR screens in isogenic cell lines and identified STAG1 as the most prominent and selective dependency of STAG2-deficient cells. Using an inducible degron system, we show that chemical genetic degradation of STAG1 protein results in the loss of sister chromatid cohesion and rapid cell death in STAG2-deficient cells, while sparing STAG2-wild-type cells. Biochemical assays and X-ray crystallography identify STAG1 regions that interact with the RAD21 subunit of the cohesin complex. STAG1 mutations that abrogate this interaction selectively compromise the viability of STAG2-deficient cells. Our work highlights the degradation of STAG1 and inhibition of its interaction with RAD21 as promising therapeutic strategies. These findings lay the groundwork for the development of STAG1-directed small molecules to exploit synthetic lethality in STAG2-mutated tumors.

The crystal structure of human forkhead box N1 in complex with DNA reveals the structural basis for forkhead box family specificity.

Forkhead box N1 (FOXN1) is a member of the forkhead box family of transcription factors and plays an important role in thymic epithelial cell differentiation and development. FOXN1 mutations in humans and mice give rise to the "nude" phenotype, which is marked by athymia. FOXN1 belongs to a subset of the FOX family that recognizes an alternative forkhead-like (FHL) consensus sequence (GACGC) that is different from the more widely recognized forkhead (FKH) sequence RYAAAYA (where R is purine, and Y is pyrimidine). Here, we present the FOXN1 structure in complex with DNA containing an FHL motif at 1.6 Å resolution, in which the DNA sequence is recognized by a mixture of direct and water-mediated contacts provided by residues in an α-helix inserted in the DNA major groove (the recognition helix). Comparisons with the structure of other FOX family members revealed that the FKH and FHL DNA sequences are bound in two distinct modes, with partially different registers for the protein DNA contacts. We identified a single alternative rotamer within the recognition helix itself as an important determinant of DNA specificity and found protein sequence features in the recognition helix that could be used to predict the specificity of other FOX family members. Finally, we demonstrate that the C-terminal region of FOXN1 is required for high-affinity DNA binding and that FOXN1 has a significantly reduced affinity for DNA that contains 5'-methylcytosine, which may have implications for the role of FOXN1 in thymic involution.

A Ubiquitin-Binding Domain that Binds a Structural Fold Distinct from that of Ubiquitin.

Ubiquitylation, the posttranslational linkage of ubiquitin moieties to lysines in target proteins, helps regulate a myriad of biological processes. Ubiquitin, and sometimes ubiquitin-homology domains, are recognized by ubiquitin-binding domains, including CUE domains. CUE domains are thus generally thought to function by mediating interactions with ubiquitylated proteins. The chromatin remodeler, SMARCAD1, interacts with KAP1, a transcriptional corepressor. The SMARCAD1-KAP1 interaction is direct and involves the first SMARCAD1 CUE domain (CUE1) and the RBCC domain of KAP1. Here, we present a structural model of the KAP1 RBCC-SMARCAD1 CUE1 complex based on X-ray crystallography. Remarkably, CUE1, a canonical CUE domain, recognizes a cluster of exposed hydrophobic and surrounding charged/amphipathic residues on KAP1, which are presented in the context of a coiled-coil domain, not in a structure resembling ubiquitin. Together, these data suggest that CUE domains may have a wider function than simply recognizing ubiquitin and the ubiquitin-fold.

RECQL5: Another DNA helicase potentially involved in hereditary breast cancer susceptibility.

There is still around 50% of the familial breast cancer (BC) cases with an undefined genetic cause, here we have used next-generation sequencing (NGS) technology to identify new BC susceptibility genes. This approach has led to the identification of RECQL5, a member of RECQL-helicases family, as a new BC susceptibility candidate, which deserves further study. We have used a combination of whole exome sequencing in a family negative for mutations in BRCA1/2 throughout (BRCAX), in which we found a probably deleterious variant in RECQL5, and targeted NGS of the complete coding regions and exon-intron boundaries of the candidate gene in 699 BC Spanish BRCAX families and 665 controls. Functional characterization and in silico inference of pathogenicity were performed to evaluate the deleterious effect of detected variants. We found at least seven deleterious or likely deleterious variants among the cases and only one in controls. These results prompt us to propose RECQL5 as a gene that would be worth to analyze in larger studies to explore its possible implication in BC susceptibility.

Interactome Rewiring Following Pharmacological Targeting of BET Bromodomains.

Targeting bromodomains (BRDs) of the bromo-and-extra-terminal (BET) family offers opportunities for therapeutic intervention in cancer and other diseases. Here, we profile the interactomes of BRD2, BRD3, BRD4, and BRDT following treatment with the pan-BET BRD inhibitor JQ1, revealing broad rewiring of the interaction landscape, with three distinct classes of behavior for the 603 unique interactors identified. A group of proteins associate in a JQ1-sensitive manner with BET BRDs through canonical and new binding modes, while two classes of extra-terminal (ET)-domain binding motifs mediate acetylation-independent interactions. Last, we identify an unexpected increase in several interactions following JQ1 treatment that define negative functions for BRD3 in the regulation of rRNA synthesis and potentially RNAPII-dependent gene expression that result in decreased cell proliferation. Together, our data highlight the contributions of BET protein modules to their interactomes allowing for a better understanding of pharmacological rewiring in response to JQ1.

Structural insight into the human mitochondrial tRNA purine N1-methyltransferase and ribonuclease P complexes.

Mitochondrial tRNAs are transcribed as long polycistronic transcripts of precursor tRNAs and undergo posttranscriptional modifications such as endonucleolytic processing and methylation required for their correct structure and function. Among them, 5'-end processing and purine 9 N1-methylation of mitochondrial tRNA are catalyzed by two proteinaceous complexes with overlapping subunit composition. The Mg2+-dependent RNase P complex for 5'-end cleavage comprises the methyltransferase domain-containing protein tRNA methyltransferase 10C, mitochondrial RNase P subunit (TRMT10C/MRPP1), short-chain oxidoreductase hydroxysteroid 17ß-dehydrogenase 10 (HSD17B10/MRPP2), and metallonuclease KIAA0391/MRPP3. An MRPP1-MRPP2 subcomplex also catalyzes the formation of 1-methyladenosine/1-methylguanosine at position 9 using S-adenosyl-l-methionine as methyl donor. However, a lack of structural information has precluded insights into how these complexes methylate and process mitochondrial tRNA. Here, we used a combination of X-ray crystallography, interaction and activity assays, and small angle X-ray scattering (SAXS) to gain structural insight into the two tRNA modification complexes and their components. The MRPP1 N terminus is involved in tRNA binding and monomer-monomer self-interaction, whereas the C-terminal SPOUT fold contains key residues for S-adenosyl-l-methionine binding and N1-methylation. The entirety of MRPP1 interacts with MRPP2 to form the N1-methylation complex, whereas the MRPP1-MRPP2-MRPP3 RNase P complex only assembles in the presence of precursor tRNA. This study proposes low-resolution models of the MRPP1-MRPP2 and MRPP1-MRPP2-MRPP3 complexes that suggest the overall architecture, stoichiometry, and orientation of subunits and tRNA substrates.

Insights into the RecQ helicase mechanism revealed by the structure of the helicase domain of human RECQL5.

RecQ helicases are important maintainers of genome integrity with distinct roles in almost every cellular process requiring access to DNA. RECQL5 is one of five human RecQ proteins and is particularly versatile in this regard, forming protein complexes with a diverse set of cellular partners in order to coordinate its helicase activity to various processes including replication, recombination and DNA repair. In this study, we have determined crystal structures of the core helicase domain of RECQL5 both with and without the nucleotide ADP in two distinctly different ('Open' and 'Closed') conformations. Small angle X-ray scattering studies show that the 'Open' form of the protein predominates in solution and we discuss implications of this with regards to the RECQL5 mechanism and conformational changes. We have measured the ATPase, helicase and DNA binding properties of various RECQL5 constructs and variants and discuss the role of these regions and residues in the various RECQL5 activities. Finally, we have performed a systematic comparison of the RECQL5 structures with other RecQ family structures and based on these comparisons we have constructed a model for the mechano-chemical cycle of the common catalytic core of these helicases.

Metalloprotease SPRTN/DVC1 Orchestrates Replication-Coupled DNA-Protein Crosslink Repair.

The cytotoxicity of DNA-protein crosslinks (DPCs) is largely ascribed to their ability to block the progression of DNA replication. DPCs frequently occur in cells, either as a consequence of metabolism or exogenous agents, but the mechanism of DPC repair is not completely understood. Here, we characterize SPRTN as a specialized DNA-dependent and DNA replication-coupled metalloprotease for DPC repair. SPRTN cleaves various DNA binding substrates during S-phase progression and thus protects proliferative cells from DPC toxicity. Ruijs-Aalfs syndrome (RJALS) patient cells with monogenic and biallelic mutations in SPRTN are hypersensitive to DPC-inducing agents due to a defect in DNA replication fork progression and the inability to eliminate DPCs. We propose that SPRTN protease represents a specialized DNA replication-coupled DPC repair pathway essential for DNA replication progression and genome stability. Defective SPRTN-dependent clearance of DPCs is the molecular mechanism underlying RJALS, and DPCs are contributing to accelerated aging and cancer.