The atm-1 gene is required for genome stability in Caenorhabditis elegans

Phosphoregulation of DSB-1 mediates control of meiotic double-strand break activity

In the first meiotic cell division, proper segregation of chromosomes in most organisms depends on chiasmata, exchanges of continuity between homologous chromosomes that originate from the repair of programmed double-strand breaks (DSBs) catalyzed by the Spo11 endonuclease. Since DSBs can lead to irreparable damage in germ cells, while chromosomes lacking DSBs also lack chiasmata, the number of DSBs must be carefully regulated to be neither too high nor too low. Here, we show that in Caenorhabditis elegans, meiotic DSB levels are controlled by the phosphoregulation of DSB-1, a homolog of the yeast Spo11 cofactor Rec114, by the opposing activities of PP4^PPH-4.1 phosphatase and ATR^ATL-1 kinase. Increased DSB-1 phosphorylation in pph-4.1 mutants correlates with reduction in DSB formation, while prevention of DSB-1 phosphorylation drastically increases the number of meiotic DSBs both in pph-4.1 mutants and in the wild-type background. C. elegans and its close relatives also possess a diverged paralog of DSB-1, called DSB-2, and loss of dsb-2 is known to reduce DSB formation in oocytes with increasing age. We show that the proportion of the phosphorylated, and thus inactivated, form of DSB-1 increases with age and upon loss of DSB-2, while non-phosphorylatable DSB-1 rescues the age-dependent decrease in DSBs in dsb-2 mutants. These results suggest that DSB-2 evolved in part to compensate for the inactivation of DSB-1 through phosphorylation, to maintain levels of DSBs in older animals. Our work shows that PP4^PPH-4.1, ATR^ATL-1, and DSB-2 act in concert with DSB-1 to promote optimal DSB levels throughout the reproductive lifespan.

Inhibition of ATR Reverses a Mitochondrial Respiratory Insufficiency

Diseases that affect the mitochondrial electron transport chain (ETC) often manifest as threshold effect disorders, meaning patients only become symptomatic once a certain level of ETC dysfunction is reached. Cells can invoke mechanisms to circumvent reaching their critical ETC threshold, but it is an ongoing challenge to identify such processes. In the nematode Caenorhabditis elegans, severe reduction of mitochondrial ETC activity shortens life, but mild reduction actually extends it, providing an opportunity to identify threshold circumvention mechanisms. Here, we show that removal of ATL-1, but not ATM-1, worm orthologs of ATR and ATM, respectively, key nuclear DNA damage checkpoint proteins in human cells, unexpectedly lessens the severity of ETC dysfunction. Multiple genetic and biochemical tests show no evidence for increased mutation or DNA breakage in animals exposed to ETC disruption. Reduced ETC function instead alters nucleotide ratios within both the ribo- and deoxyribo-nucleotide pools, and causes stalling of RNA polymerase, which is also known to activate ATR. Unexpectedly, atl-1 mutants confronted with mitochondrial ETC disruption maintain normal levels of oxygen consumption, and have an increased abundance of translating ribosomes. This suggests checkpoint signaling by ATL-1 normally dampens cytoplasmic translation. Taken together, our data suggest a model whereby ETC insufficiency in C. elegans results in nucleotide imbalances leading to the stalling of RNA polymerase, activation of ATL-1, dampening of global translation, and magnification of ETC dysfunction. The loss of ATL-1 effectively reverses the severity of ETC disruption so that animals become phenotypically closer to wild type.

Protection of the C. elegans germ cell genome depends on diverse DNA repair pathways during normal proliferation

Maintaining genome integrity is particularly important in germ cells to ensure faithful transmission of genetic information across generations. Here we systematically describe germ cell mutagenesis in wild-type and 61 DNA repair mutants cultivated over multiple generations. ~44% of the DNA repair mutants analysed showed a >2-fold increased mutagenesis with a broad spectrum of mutational outcomes. Nucleotide excision repair deficiency led to higher base substitution rates, whereas polh-1(Polη) and rev-3(Polζ) translesion synthesis polymerase mutants resulted in 50-400 bp deletions. Signatures associated with defective homologous recombination fall into two classes: 1) brc-1/BRCA1 and rad-51/RAD51 paralog mutants showed increased mutations across all mutation classes, 2) mus-81/MUS81 and slx-1/SLX1 nuclease, and him-6/BLM, helq-1/HELQ or rtel-1/RTEL1 helicase mutants primarily accumulated structural variants. Repetitive and G-quadruplex sequence-containing loci were more frequently mutated in specific DNA repair backgrounds. Tandem duplications embedded in inverted repeats were observed in helq-1 helicase mutants, and a unique pattern of 'translocations' involving homeologous sequences occurred in rip-1 recombination mutants. atm-1/ATM checkpoint mutants harboured structural variants specifically enriched in subtelomeric regions. Interestingly, locally clustered mutagenesis was only observed for combined brc-1 and cep-1/p53 deficiency. Our study provides a global view of how different DNA repair pathways contribute to prevent germ cell mutagenesis.

Efficient collection of a large number of mutations by mutagenesis of DNA damage response defective animals

With the development of massive parallel sequencing technology, it has become easier to establish new model organisms that are ideally suited to the specific biological phenomena of interest. Considering the history of research using classical model organisms, we believe that the efficient construction and sharing of gene mutation libraries will facilitate the progress of studies using these new model organisms. Using C. elegans, we applied the TMP/UV mutagenesis method to animals lacking function in the DNA damage response genes atm-1 and xpc-1. This method produces genetic mutations three times more efficiently than mutagenesis of wild-type animals. Furthermore, we confirmed that the use of next-generation sequencing and the elimination of false positives through machine learning could automate the process of mutation identification with an accuracy of over 95%. Eventually, we sequenced the whole genomes of 488 strains and isolated 981 novel mutations generated by the present method; these strains have been made available to anyone who wants to use them. Since the targeted DNA damage response genes are well conserved and the mutagens used in this study are also effective in a variety of species, we believe that our method is generally applicable to a wide range of animal species.

DNA repair, recombination, and damage signaling

DNA must be accurately copied and propagated from one cell division to the next, and from one generation to the next. To ensure the faithful transmission of the genome, a plethora of distinct as well as overlapping DNA repair and recombination pathways have evolved. These pathways repair a large variety of lesions, including alterations to single nucleotides and DNA single and double-strand breaks, that are generated as a consequence of normal cellular function or by external DNA damaging agents. In addition to the proteins that mediate DNA repair, checkpoint pathways have also evolved to monitor the genome and coordinate the action of various repair pathways. Checkpoints facilitate repair by mediating a transient cell cycle arrest, or through initiation of cell suicide if DNA damage has overwhelmed repair capacity. In this chapter, we describe the attributes of Caenorhabditis elegans that facilitate analyses of DNA repair, recombination, and checkpoint signaling in the context of a whole animal. We review the current knowledge of C. elegans DNA repair, recombination, and DNA damage response pathways, and their role during development, growth, and in the germ line. We also discuss how the analysis of mutational signatures in C. elegans is helping to inform cancer mutational signatures in humans.

ATM Promotes RAD51-Mediated Meiotic DSB Repair by Inter-Sister-Chromatid Recombination in Arabidopsis

Meiotic recombination ensures accurate homologous chromosome segregation during meiosis and generates novel allelic combinations among gametes. During meiosis, DNA double strand breaks (DSBs) are generated to facilitate recombination. To maintain genome integrity, meiotic DSBs must be repaired using appropriate DNA templates. Although the DNA damage response protein kinase Ataxia-telangiectasia mutated (ATM) has been shown to be involved in meiotic recombination in Arabidopsis, its mechanistic role is still unclear. In this study, we performed cytological analysis in Arabidopsis atm mutant, we show that there are fewer γH2AX foci, but more RAD51 and DMC1 foci on atm meiotic chromosomes. Furthermore, we observed an increase in meiotic Type I crossovers (COs) in atm. Our genetic analysis shows that the meiotic phenotype of atm rad51 double mutants is similar to the rad51 single mutant. Whereas, the atm dmc1 double mutant has a more severe chromosome fragmentation phenotype compared to both single mutants, suggesting that ATM functions in concert with RAD51, but in parallel to DMC1. Lastly, we show that atm asy1 double mutants also have more severe meiotic recombination defects. These data lead us to propose a model wherein ATM promotes RAD51-mediated meiotic DSB repair by inter-sister-chromatid (IS) recombination in Arabidopsis.

ATM and ATR Influence Meiotic Crossover Formation Through Antagonistic and Overlapping Functions in Caenorhabditis elegans

During meiosis, formation of double-strand breaks (DSBs) and repair by homologous recombination between homologs creates crossovers (COs) that facilitate chromosome segregation. CO formation is tightly regulated to ensure the integrity of this process. The DNA damage response kinases, Ataxia-telangiectasia mutated (ATM) and RAD3-related (ATR) have emerged as key regulators of CO formation in yeast, flies, and mice, influencing DSB formation, repair pathway choice, and cell cycle progression. The molecular networks that ATM and ATR influence during meiosis are still being resolved in other organisms. Here, we show that Caenorhabditis elegans ATM and ATR homologs, ATM-1 and ATL-1 respectively, act at multiple steps in CO formation to ultimately ensure that COs are formed on all chromosomes. We show a role for ATM-1 in regulating the choice of repair template, biasing use of the homologous chromosome instead of the sister chromatid. Our data suggest a model in which ATM-1 and ATL-1 have antagonistic roles in very early repair processing, but are redundantly required for accumulation of the RAD-51 recombinase at DSB sites. We propose that these features of ATM-1 and ATL-1 ensure both CO formation on all chromosomes and accurate repair of additional DSBs.

Rhizobium induces DNA damage in Caenorhabditis elegans intestinal cells

In their natural habitat of rotting fruit, the nematode Caenorhabditis elegans feeds on the complex bacterial communities that thrive in this rich growth medium. Hundreds of diverse bacterial strains cultured from such rotting fruit allow C. elegans growth and reproduction when tested individually. In screens for C. elegans responses to single bacterial strains associated with nematodes in fruit, we found that Rhizobium causes a genome instability phenotype; we observed abnormally long or fragmented intestinal nuclei due to aberrant nuclear division, or defective karyokinesis. The karyokinesis defects were restricted to intestinal cells and required close proximity between bacteria and the worm. A genetic screen for C. elegans mutations that cause the same intestinal karyokinesis defect followed by genome sequencing of the isolated mutant strains identified mutations that disrupt DNA damage repair pathways, suggesting that Rhizobium may cause DNA damage in C. elegans intestinal cells. We hypothesized that such DNA damage is caused by reactive oxygen species produced by Rhizobium and found that hydrogen peroxide added to benign Escherichia coli can cause the same intestinal karyokinesis defects in WT C. elegans Supporting this model, free radical scavengers suppressed the Rhizobium-induced C. elegans DNA damage. Thus, Rhizobium may signal to eukaryotic hosts via reactive oxygen species, and the host may respond with DNA damage repair pathways.

ATM in breast and brain tumors: a comprehensive review

The ATM gene is mutated in the syndrome, ataxia-telangiectasia (AT), which is characterized by predisposition to cancer. Patients with AT have an elevated risk of breast and brain tumors Carrying mutations in ATM, patients with AT have an elevated risk of breast and brain tumors. An increased frequency of ATM mutations has also been reported in patients with breast and brain tumors; however, the magnitude of this risk remains uncertain. With the exception of a few common mutations, the spectrum of ATM alterations is heterogeneous in diverse populations, and appears to be remarkably dependent on the ethnicity of patients. This review aims to provide an easily accessible summary of common variants in different populations which could be useful in ATM screening programs. In addition, we have summarized previous research on ATM, including its molecular functions. We attempt to demonstrate the significance of ATM in exploration of breast and brain tumors and its potential as a therapeutic target.

ATM Induces Cell Death with Autophagy in Response to H2O2 Specifically in Caenorhabditis elegans Nondividing Cells

Introduction

Ataxia-telangiectasia-mutated (ATM) kinase is a master regulator of the DNA damage response and is directly activated by reactive oxygen species (ROSs) in addition to DNA double-stranded breaks. However, the physiological function of the response to ROSs is not understood.

Purpose

In the present study, we investigated how ATM responds to ROSs in Caenorhabditis elegans (C. elegans).

Materials and Methods

First, we measured sensitivities of larvae to DNA-damaging agents and ROSs. Next, we analyzed the drug sensitivities of fully matured adult worms, which consist of nondividing somatic cells. Dead cell staining with acridine orange was performed to visualize the dead cells. In addition, we performed GFP reporter assays of lgg-1, an autophagy-related gene, to determine the types of cell death.

Results

atm-1(tm5027) larvae showed a wide range of sensitivities to both DNA-damaging agents and ROSs. In contrast, fully matured adult worms, which consist of nondividing somatic cells, showed sensitivity to DNA-damaging agent, NaHSO₃, but they showed resistance to H₂O₂. Dead cell staining and GFP reporter assays of lgg-1 suggest that C. elegans ATM-1 induces the cell death with autophagy in intestinal cells in response to H₂O₂.

Conclusion

We revealed that ATM induces cell death in response to H₂O₂.

JMJD-5/KDM8 regulates H3K36me2 and is required for late steps of homologous recombination and genome integrity

The eukaryotic genome is organized in a three-dimensional structure called chromatin, constituted by DNA and associated proteins, the majority of which are histones. Post-translational modifications of histone proteins greatly influence chromatin structure and regulate many DNA-based biological processes. Methylation of lysine 36 of histone 3 (H3K36) is a post-translational modification functionally relevant during early steps of DNA damage repair. Here, we show that the JMJD-5 regulates H3K36 di-methylation and it is required at late stages of double strand break repair mediated by homologous recombination. Loss of jmjd-5 results in hypersensitivity to ionizing radiation and in meiotic defects, and it is associated with aberrant retention of RAD-51 at sites of double strand breaks. Analyses of jmjd-5 genetic interactions with genes required for resolving recombination intermediates (rtel-1) or promoting the resolution of RAD-51 double stranded DNA filaments (rfs-1 and helq-1) suggest that jmjd-5 prevents the formation of stalled postsynaptic recombination intermediates and favors RAD-51 removal. As these phenotypes are all recapitulated by a catalytically inactive jmjd-5 mutant, we propose a novel role for H3K36me2 regulation during late steps of homologous recombination critical to preserve genome integrity.

X Chromosome Crossover Formation and Genome Stability in Caenorhabditis elegans Are Independently Regulated by xnd-1

The germ line efficiently combats numerous genotoxic insults to ensure the high fidelity propagation of unaltered genomic information across generations. Yet, germ cells in most metazoans also intentionally create double-strand breaks (DSBs) to promote DNA exchange between parental chromosomes, a process known as crossing over. Homologous recombination is employed in the repair of both genotoxic lesions and programmed DSBs, and many of the core DNA repair proteins function in both processes. In addition, DNA repair efficiency and crossover (CO) distribution are both influenced by local and global differences in chromatin structure, yet the interplay between chromatin structure, genome integrity, and meiotic fidelity is still poorly understood. We have used the xnd-1 mutant of Caenorhabditis elegans to explore the relationship between genome integrity and crossover formation. Known for its role in ensuring X chromosome CO formation and germ line development, we show that xnd-1 also regulates genome stability. xnd-1 mutants exhibited a mortal germ line, high embryonic lethality, high incidence of males, and sensitivity to ionizing radiation. We discovered that a hypomorphic allele of mys-1 suppressed these genome instability phenotypes of xnd-1, but did not suppress the CO defects, suggesting it serves as a separation-of-function allele. mys-1 encodes a histone acetyltransferase, whose homolog Tip60 acetylates H2AK5, a histone mark associated with transcriptional activation that is increased in xnd-1 mutant germ lines, raising the possibility that thresholds of H2AK5ac may differentially influence distinct germ line repair events. We also show that xnd-1 regulated him-5 transcriptionally, independently of mys-1, and that ectopic expression of him-5 suppressed the CO defects of xnd-1 Our work provides xnd-1 as a model in which to study the link between chromatin factors, gene expression, and genome stability.

NAD+ Replenishment Improves Lifespan and Healthspan in Ataxia Telangiectasia Models via Mitophagy and DNA Repair

Ataxia telangiectasia (A-T) is a rare autosomal recessive disease characterized by progressive neurodegeneration and cerebellar ataxia. A-T is causally linked to defects in ATM, a master regulator of the response to and repair of DNA double-strand breaks. The molecular basis of cerebellar atrophy and neurodegeneration in A-T patients is unclear. Here we report and examine the significance of increased PARylation, low NAD+, and mitochondrial dysfunction in ATM-deficient neurons, mice, and worms. Treatments that replenish intracellular NAD+ reduce the severity of A-T neuropathology, normalize neuromuscular function, delay memory loss, and extend lifespan in both animal models. Mechanistically, treatments that increase intracellular NAD+ also stimulate neuronal DNA repair and improve mitochondrial quality via mitophagy. This work links two major theories on aging, DNA damage accumulation, and mitochondrial dysfunction through nuclear DNA damage-induced nuclear-mitochondrial signaling, and demonstrates that they are important pathophysiological determinants in premature aging of A-T, pointing to therapeutic interventions.

A novel DNA damage response mediated by DNA mismatch repair in Caenorhabditis elegans: induction of programmed autophagic cell death in non-dividing cells

DNA mismatch repair (MMR) contributes to genome integrity by correcting errors of DNA polymerase and inducing cell death in response to DNA damage. Dysfunction of MMR results in increased mutation frequency and cancer risk. Clinical researches revealed that MMR abnormalities induce cancers of non-dividing tissues, such as kidney and liver. However, how MMR suppresses cancer in non-dividing tissues is not understood.

To address that mechanism, we analyzed the roles of MMR in non-dividing cells using Caenorhabditis elegans (C. elegans), in which all somatic cells are non-dividing in the adult stage. In this study, we used stable MMR-mutant lines with a balancer chromosome. First, we confirmed that deficiency of MMR leads to resistance to various mutagens in C. elegans dividing cells. Next, we performed drug resistance assays, and found that MMR-deficient adult worms were resistant to SN1-type alkylating and oxidizing agents. In addition, dead cell staining and reporter assays of an autophagy-related gene demonstrated that the cell death was autophagic cell death. Interestingly, this autophagic cell death was not suppressed by caffeine, implying that MMR induces death of non-dividing cells in an atl-1-independent manner. Hence, we propose the hypothesis that MMR prevents cancers in non-dividing tissues by directly inducing cell death.

Zygotic Genome Activation Triggers Chromosome Damage and Checkpoint Signaling in C. elegans Primordial Germ Cells

Recent findings have identified highly transcribed genes as a source of genome instability; however, the degree to which large-scale shifts in transcriptional activity cause DNA damage was not known. One example of a large-scale shift in transcriptional activity occurs during development, when maternal regulators are destroyed and zygotic genome activation (ZGA) occurs. Here, we show that ZGA triggers widespread chromosome damage in the primordial germ cells of the nematode C. elegans. We show that ZGA-induced DNA damage activates a checkpoint response, the damage is repaired by factors required for inter-sister homologous recombination, and topoisomerase II plays a role in generating the damage. These findings identify ZGA as a source of intrinsic genome instability in the germline and suggest that genome destabilization may be a general consequence of extreme shifts in cellular transcriptional load.

Synthetic cytotoxicity: digenic interactions with TEL1/ATM mutations reveal sensitivity to low doses of camptothecin

Many tumors contain mutations that confer defects in the DNA-damage response and genome stability. DNA-damaging agents are powerful therapeutic tools that can differentially kill cells with an impaired DNA-damage response. The response to DNA damage is complex and composed of a network of coordinated pathways, often with a degree of redundancy. Tumor-specific somatic mutations in DNA-damage response genes could be exploited by inhibiting the function of a second gene product to increase the sensitivity of tumor cells to a sublethal concentration of a DNA-damaging therapeutic agent, resulting in a class of conditional synthetic lethality we call synthetic cytotoxicity. We used the Saccharomyces cerevisiae nonessential gene-deletion collection to screen for synthetic cytotoxic interactions with camptothecin, a topoisomerase I inhibitor, and a null mutation in TEL1, the S. cerevisiae ortholog of the mammalian tumor-suppressor gene, ATM. We found and validated 14 synthetic cytotoxic interactions that define at least five epistasis groups. One class of synthetic cytotoxic interaction was due to telomere defects. We also found that at least one synthetic cytotoxic interaction was conserved in Caenorhabditis elegans. We have demonstrated that synthetic cytotoxicity could be a useful strategy for expanding the sensitivity of certain tumors to DNA-damaging therapeutics.

The TP53 signaling network in mammals and worms

The nematode worm Caenorhabditis elegans has been an invaluable model organism for studying the molecular mechanisms that govern cell fate, from fundamental aspects of multicellular development to programmed cell death (apoptosis). The transparency of this organism permits visualization of cells in living animals at high resolution. The powerful genetics and functional genomics tools available in C. elegans allow for detailed analysis of gene function, including genes that are frequently deregulated in human diseases such as cancer. The TP53 protein is a critical suppressor of tumor formation in vertebrates, and the TP53 gene is mutated in over 50% of human cancers. TP53 suppresses malignancy by integrating a variety of cellular stresses that direct it to activate transcription of genes that help to repair the damage or trigger apoptotic death if the damage is beyond repair. The TP53 paralogs, TP63 and TP73, have distinct roles in development as well as overlapping functions with TP53 in apoptosis and repair, which complicates their analysis in vertebrates. C. elegans contains a single TP53 family member, cep-1, that shares properties of all three vertebrate genes and thus offers a simple system in which to study the biological functions of this important gene family. This review summarizes major advances in our understanding of the TP53 family using C. elegans as a model organism.