A senataxin-associated exonuclease SAN1 is required for resistance to DNA interstrand cross-links

Function and inhibition of the DNA repair enzyme SNM1A

SNM1A is an enzyme involved in several important biological pathways. To date, most investigations have focused on its role in repairing interstrand crosslinks, a highly cytotoxic form of DNA damage. SNM1A acts as a 5'-3' exonuclease, displaying an unusual capability to digest DNA past the site of a crosslink lesion. Recently, additional functions of this enzyme in the repair of DNA double-strand breaks and critically shortened telomeres have been uncovered. Furthermore, SNM1A is involved in two cell cycle checkpoints that arrest cell division in response to DNA damage. Inhibition of both DNA repair enzymes and cell cycle checkpoint proteins are effective strategies for cancer treatment, and SNM1A is therefore of significant interest as a potential therapeutic target. As a member of the metallo-β-lactamase superfamily, SNM1A is postulated to contain two metal ions in the active site that catalyse hydrolysis of the phosphodiester backbone of DNA. Substrate-mimic probes based on a nucleoside or oligonucleotide scaffold appended with a metal-binding group have proven effective in vitro. High-throughput screening campaigns have identified potent inhibitors, some of which were successful in sensitising cells to DNA-damaging cancer drugs. This review discusses the biological role, structure, and mechanism of action of SNM1A, and the development of SNM1A inhibitors for cancer therapy.

Repair of genomic interstrand crosslinks

Genomic interstrand crosslinks (ICLs) are formed by reactive species generated during normal cellular metabolism, produced by the microbiome, and employed in cancer chemotherapy. While there are multiple options for replication dependent and independent ICL repair, the crucial step for each is unhooking one DNA strand from the other. Much of our insight into mechanisms of unhooking comes from powerful model systems based on plasmids with defined ICLs introduced into cells or cell free extracts. Here we describe the properties of exogenous and endogenous ICL forming compounds and provide an historical perspective on early work on ICL repair. We discuss the modes of unhooking elucidated in the model systems, the concordance or lack thereof in drug resistant tumors, and the evolving view of DNA adducts, including ICLs, formed by metabolic aldehydes.

Role of senataxin in R-loop-mediated neurodegeneration

Senataxin is an RNA:DNA helicase that plays an important role in the resolution of RNA:DNA hybrids (R-loops) formed during transcription. R-loops are involved in the regulation of biological processes such as immunoglobulin class switching, gene expression and DNA repair. Excessive accumulation of R-loops results in DNA damage and loss of genomic integrity. Senataxin is critical for maintaining optimal levels of R-loops to prevent DNA damage and acts as a genome guardian. Within the nucleus, senataxin interacts with various RNA processing factors and DNA damage response and repair proteins. Senataxin interactors include survival motor neuron and zinc finger protein 1, with whom it co-localizes in sub-nuclear bodies. Despite its ubiquitous expression, mutations in senataxin specifically affect neurons and result in distinct neurodegenerative diseases such as amyotrophic lateral sclerosis type 4 and ataxia with oculomotor apraxia type 2, which are attributed to the gain-of-function and the loss-of-function mutations in senataxin, respectively. In addition, low levels of senataxin (loss-of-function) in spinal muscular atrophy result in the accumulation of R-loops causing DNA damage and motor neuron degeneration. Senataxin may play multiple functions in diverse cellular processes; however, its emerging role in R-loop resolution and maintenance of genomic integrity is gaining attention in the field of neurodegenerative diseases. In this review, we highlight the role of senataxin in R-loop resolution and its potential as a therapeutic target to treat neurodegenerative diseases.

Replication-Independent ICL Repair: From Chemotherapy to Cell Homeostasis

Interstrand crosslinks (ICLs) are a type of covalent lesion that can prevent transcription and replication by inhibiting DNA strand separation and instead trigger cell death. ICL inducing compounds are commonly used as chemotherapies due to their effectiveness in inhibiting cell proliferation. Naturally occurring crosslinking agents formed from metabolic processes can also pose a challenge to genome stability especially in slowly or non-dividing cells. Cells maintain a variety of ICL repair mechanisms to cope with this stressor within and outside the S phase of the cell cycle. Here, we discuss the mechanisms of various replication-independent ICL repair pathways and how crosslink repair efficiency is tied to aging and disease.

Role of Senataxin in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) is a progressive, uncurable neurodegenerative disorder characterized by the degradation of motor neurons leading to muscle impairment, failure, and death. Senataxin, encoded by the SETX gene, is a human helicase protein whose mutations have been linked with ALS onset, particularly in its juvenile ALS4 form. Using senataxin's yeast homolog Sen1 as a model for study, it is suggested that senataxin's N-terminus interacts with RNA polymerase II, whilst its C-terminus engages in helicase activity. Senataxin is heavily involved in transcription regulation, termination, and R-loop resolution, enabled by recruitment and interactions with enzymes such as ubiquitin protein ligase SAN1 and ribonuclease H (RNase H). Senataxin also engages in DNA damage response (DDR), primarily interacting with the exosome subunit Rrp45. The Sen1 mutation E1597K, alongside the L389S and R2136H gain-of-function mutations to senataxin, is shown to cause negative structural and thus functional effects to the protein, thus contributing to a disruption in WT functions, motor neuron (MN) degeneration, and the manifestation of ALS clinical symptoms. This review corroborates and summarizes published papers concerning the structure and function of senataxin as well as the effects of their mutations in ALS pathology in order to compile current knowledge and provide a reference for future research. The findings compiled in this review are indicative of the experimental and therapeutic potential of senataxin and its mutations as a target in future ALS treatment/cure discovery, with some potential therapeutic routes also being discussed in the review.

Senataxin and R-loops homeostasis: multifaced implications in carcinogenesis

R-loops are inherent byproducts of transcription consisting of an RNA:DNA hybrid and a displaced single-stranded DNA. These structures are of key importance in controlling numerous physiological processes and their homeostasis is tightly controlled by the activities of several enzymes deputed to process R-loops and prevent their unproper accumulation. Senataxin (SETX) is an RNA/DNA helicase which catalyzes the unwinding of RNA:DNA hybrid portion of the R-loops, promoting thus their resolution. The key importance of SETX in R-loops homeostasis and its relevance with pathophysiological events is highlighted by the evidence that gain or loss of function SETX mutations underlie the pathogenesis of two distinct neurological disorders. Here, we aim to describe the potential impact of SETX on tumor onset and progression, trying to emphasize how dysregulation of this enzyme observed in human tumors might impact tumorigenesis. To this aim, we will describe the functional relevance of SETX in regulating gene expression, genome integrity, and inflammation response and discuss how cancer-associated SETX mutations might affect these pathways, contributing thus to tumor development.

Emerging role for R-loop formation in hepatocellular carcinoma

The pathophysiological characteristics of hepatocellular carcinoma (HCC) is closely associated with genomic instability. Genomic instability has long been considered to be a hallmark of both human genetic disease and cancers. It is now well accepted that regulating R-loop formation to minimized levels is one of critical modulation to maintain genome integrity, and that improper regulation of R-loop metabolism causes genomic instability via DNA breakage, ultimately resulting in replicative senescence and even tumorigenesis. Given that R-loop is natural by-product formed during normal transcription condition, and that several types of cancer have defense mechanism against the genomic instability resulted from R-loop formation, modulating functional implication of proteins involved in the intrinsic and specific mechanisms of abnormal R-loop formation in cancers therefore could play an important part in appropriated therapeutic strategies for HCC cohorts. In this review, we highlight the latest understanding on how R-loops promote genomic instability and address how alterations in these pathways link to human HCC.

FANCD2 promotes mitotic rescue from transcription-mediated replication stress in SETX-deficient cancer cells

Replication stress (RS) is a leading cause of genome instability and cancer development. A substantial source of endogenous RS originates from the encounter between the transcription and replication machineries operating on the same DNA template. This occurs predominantly under specific contexts, such as oncogene activation, metabolic stress, or a deficiency in proteins that specifically act to prevent or resolve those transcription-replication conflicts (TRCs). One such protein is Senataxin (SETX), an RNA:DNA helicase involved in resolution of TRCs and R-loops. Here we identify a synthetic lethal interaction between SETX and proteins of the Fanconi anemia (FA) pathway. Depletion of SETX induces spontaneous under-replication and chromosome fragility due to active transcription and R-loops that persist in mitosis. These fragile loci are targeted by the Fanconi anemia protein, FANCD2, to facilitate the resolution of under-replicated DNA, thus preventing chromosome mis-segregation and allowing cells to proliferate. Mechanistically, we show that FANCD2 promotes mitotic DNA synthesis that is dependent on XPF and MUS81 endonucleases. Importantly, co-depleting FANCD2 together with SETX impairs cancer cell proliferation, without significantly affecting non-cancerous cells. Therefore, we uncovered a synthetic lethality between SETX and FA proteins for tolerance of transcription-mediated RS that may be exploited for cancer therapy.

And-1 Coordinates with the FANCM Complex to Regulate Fanconi Anemia Signaling and Cisplatin Resistance

The Fanconi anemia (FA) pathway is essential for repairing DNA interstrand crosslinks (ICL). ICLs induce stalled DNA replication forks and trigger activation of the FA pathway by promoting recruitment of the FANCM/FAAP24/MHF complex to ICL sites. Given that stalled replication forks are proximal to ICL sites, fork-associated proteins may coordinate with FA factors to rapidly sense ICLs for activation of FA signaling. Here we report that And-1, a replisome protein, is critical for activation of the FA pathway by sensing ICL-stalled forks and recruiting the FANCM/FAAP24 complex to ICLs. In response to ICLs, And-1 rapidly accumulated at ICL-stalled forks in a manner dependent on ataxia telangiectasia and Rad3-related protein-induced phosphorylation at T826. And-1 phosphorylation triggered an intramolecular change that promoted the interaction of And-1 with FANCM/FAAP24, resulting in recruitment of the FANCM/FAAP24 complex to ICLs. Furthermore, p-T826 And-1 was elevated in cisplatin-resistant ovarian cancer cells, and activated And-1 contributed to cisplatin resistance. Collectively, these studies elucidate a mechanism by which And-1 regulates FA signaling and identify And-1 as a potential target for developing therapeutic approaches to treat platinum-resistant ovarian cancer.

SIGNIFICANCE

This work shows that phosphorylation of And-1 by ATR activates Fanconi anemia signaling at interstrand crosslink-stalled replication forks by recruiting the FANCM/FAAP24 complex, revealing And-1 as a potential therapeutic target in cancer.

DNA damage and repair in the hematopoietic system

Although hematopoietic stem cells (HSCs) in the bone marrow are in a state of quiescence, they harbor the self-renewal capacity and the pluripotency to differentiate into mature blood cells when needed, which is key to maintain hematopoietic homeostasis. Importantly, HSCs are characterized by their long lifespan ( e. g., up to 60 months for mice), display characteristics of aging, and are vulnerable to various endogenous and exogenous genotoxic stresses. Generally, DNA damage in HSCs is endogenous, which is typically induced by reactive oxygen species (ROS), aldehydes, and replication stress. Mammalian cells have evolved a complex and efficient DNA repair system to cope with various DNA lesions to maintain genomic stability. The repair machinery for DNA damage in HSCs has its own characteristics. For instance, the Fanconi anemia (FA)/BRCA pathway is particularly important for the hematopoietic system, as it can limit the damage caused by DNA inter-strand crosslinks, oxidative stress, and replication stress to HSCs to prevent FA occurrence. In addition, HSCs prefer to utilize the classical non-homologous end-joining pathway, which is essential for the V(D)J rearrangement in developing lymphocytes and is involved in double-strand break repair to maintain genomic stability in the long-term quiescent state. In contrast, the base excision repair pathway is less involved in the hematopoietic system. In this review, we summarize the impact of various types of DNA damage on HSC function and review our knowledge of the corresponding repair mechanisms and related human genetic diseases.

Integrated genome and transcriptome analyses reveal the mechanism of genome instability in ataxia with oculomotor apraxia 2

Mutations in the SETX gene, which encodes Senataxin, are associated with the progressive neurodegenerative diseases ataxia with oculomotor apraxia 2 (AOA2) and amyotrophic lateral sclerosis 4 (ALS4). To identify the causal defect in AOA2, patient-derived cells and SETX knockouts (human and mouse) were analyzed using integrated genomic and transcriptomic approaches. A genome-wide increase in chromosome instability (gains and losses) within genes and at chromosome fragile sites was observed, resulting in changes to gene-expression profiles. Transcription stress near promoters correlated with high GCskew and the accumulation of R-loops at promoter-proximal regions, which localized with chromosomal regions where gains and losses were observed. In the absence of Senataxin, the Cockayne syndrome protein CSB was required for the recruitment of the transcription-coupled repair endonucleases (XPG and XPF) and RAD52 recombination protein to target and resolve transcription bubbles containing R-loops, leading to genomic instability. These results show that transcription stress is an important contributor to SETX mutation-associated chromosome fragility and AOA2.

Structure and Function of SNM1 Family Nucleases

Three human nucleases, SNM1A, SNM1B/Apollo, and SNM1C/Artemis, belong to the SNM1 gene family. These nucleases are involved in various cellular functions, including homologous recombination, nonhomologous end-joining, cell cycle regulation, and telomere maintenance. These three proteins share a similar catalytic domain, which is characterized as a fused metallo-β-lactamase and a CPSF-Artemis-SNM1-PSO2 domain. SNM1A and SNM1B/Apollo are exonucleases, whereas SNM1C/Artemis is an endonuclease. This review contains a summary of recent research on SNM1's cellular and biochemical functions, as well as structural biology studies. In addition, protein structure prediction by the artificial intelligence program AlphaFold provides a different view of the proteins' non-catalytic domain features, which may be used in combination with current results from X-ray crystallography and cryo-EM to understand their mechanism more clearly.

San1 deficiency leads to cardiomyopathy due to excessive R-loop-associated DNA damage and cardiomyocyte hypoplasia

R-loops are naturally occurring transcriptional intermediates containing RNA/DNA hybrids. Excessive R-loops cause genomic instability, DNA damage, and replication stress. Senataxin-associated exonuclease (San1) is a protein that interacts with Senataxin (SETX), a helicase resolving R-loops. It remains unknown if R-loops-induced DNA damage plays a role in the heart, especially in the proliferative neonatal cardiomyocytes (CMs). San1-/- mice were generated using the CRISPR/Cas9 technique. The newborn San1-/- mice show no overt phenotype, but their hearts were smaller with larger, yet fewer CMs. CM proliferation was impaired with reduced cell cycle-related transcripts and proteins. S9.6 staining revealed that excessive R-loops accumulated in the nucleus of neonatal San1-/- CMs. Increased γH2AX staining on newborn and adult heart sections exhibited increased DNA damage. Similarly, San1-/- AC16-cardiomyocytes showed cumulative R-loops and DNA damage, leading to the activation of cell cycle checkpoint kinase ATR and PARP1 hyperactivity, arresting G2/M cell-cycle and CM proliferation. Together, the present study uncovers an essential role of San1 in resolving excessive R-loops that lead to DNA damage and repressing CM proliferation, providing new insights into a novel biological function of San1 in the neonatal heart. San1 may serve as a novel therapeutic target for the treatment of hypoplastic cardiac disorders.

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.

Fanconi anemia-independent DNA inter-strand crosslink repair in eukaryotes

DNA inter-strand crosslinks (ICLs) are dangerous lesions that can be caused by a variety of endogenous and exogenous bifunctional compounds. Because covalently linking both strands of the double helix locally disrupts DNA replication and transcription, failure to remove even a single ICL can be fatal to the cell. Thus, multiple ICL repair pathways have evolved, with the best studied being the canonical Fanconi anemia (FA) pathway. However, recent research demonstrates that different types of ICLs (e.g., backbone distorting vs. non-distorting) can be discriminated by the cell, which then mounts a specific repair response using the FA pathway or one of a variety of FA-independent ICL repair pathways. This review focuses on the latter, covering current work on the transcription-coupled, base excision, acetaldehyde-induced, and SNM1A/RecQ4 ICL repair pathways and highlighting unanswered questions in the field. Answering these questions will provide mechanistic insight into the various pathways of ICL repair and enable ICL-inducing agents to be more effectively used as chemotherapeutics.

The yeast Hrq1 helicase stimulates Pso2 translesion nuclease activity and thereby promotes DNA interstrand crosslink repair

DNA interstrand crosslink (ICL) repair requires a complex network of DNA damage response pathways. Removal of the ICL lesions is vital, as they are physical barriers to essential DNA processes that require the separation of duplex DNA, such as replication and transcription. The Fanconi anemia (FA) pathway is the principal mechanism for ICL repair in metazoans and is coupled to DNA replication. In Saccharomyces cerevisiae, a vestigial FA pathway is present, but ICLs are predominantly repaired by a pathway involving the Pso2 nuclease, which is hypothesized to use its exonuclease activity to digest through the lesion to provide access for translesion polymerases. However, Pso2 lacks translesion nuclease activity in vitro, and mechanistic details of this pathway are lacking, especially relative to FA. We recently identified the Hrq1 helicase, a homolog of the disease-linked enzyme RecQ-like helicase 4 (RECQL4), as a component of Pso2-mediated ICL repair. Here, using genetic, biochemical, and biophysical approaches, including single-molecule FRET (smFRET)- and gel-based nuclease assays, we show that Hrq1 stimulates the Pso2 nuclease through a mechanism that requires Hrq1 catalytic activity. Importantly, Hrq1 also stimulated Pso2 translesion nuclease activity through a site-specific ICL in vitro We noted that stimulation of Pso2 nuclease activity is specific to eukaryotic RecQ4 subfamily helicases, and genetic and biochemical data suggest that Hrq1 likely interacts with Pso2 through their N-terminal domains. These results advance our understanding of FA-independent ICL repair and establish a role for the RecQ4 helicases in the repair of these detrimental DNA lesions.

The dark side of RNA:DNA hybrids

RNA:DNA hybrids form when nascent transcripts anneal to the DNA template strand or any homologous DNA region. Co-transcriptional RNA:DNA hybrids, organized in R-loop structures together with the displaced non-transcribed strand, assist gene expression, DNA repair and other physiological cellular functions. A dark side of the matter is that RNA:DNA hybrids are also a cause of DNA damage and human diseases. In this review, we summarize recent advances in the understanding of the mechanisms by which the impairment of hybrid turnover promotes DNA damage and genome instability via the interference with DNA replication and DNA double-strand break repair. We also discuss how hybrids could contribute to cancer, neurodegeneration and susceptibility to viral infections, focusing on dysfunctions associated with the anti-R-loop helicase Senataxin.

Senataxin homologue Sen1 is required for efficient termination of RNA polymerase III transcription

R-loop disassembly by the human helicase Senataxin contributes to genome integrity and to proper transcription termination at a subset of RNA polymerase II genes. Whether Senataxin also contributes to transcription termination at other classes of genes has remained unclear. Here, we show that Sen1, one of two fission yeast homologues of Senataxin, promotes efficient termination of RNA polymerase III (RNAP3) transcription in vivo. In the absence of Sen1, RNAP3 accumulates downstream of RNAP3-transcribed genes and produces long exosome-sensitive 3'-extended transcripts. Importantly, neither of these defects was affected by the removal of R-loops. The finding that Sen1 acts as an ancillary factor for RNAP3 transcription termination in vivo challenges the pre-existing view that RNAP3 terminates transcription autonomously. We propose that Sen1 is a cofactor for transcription termination that has been co-opted by different RNA polymerases in the course of evolution.