The fidelity of DNA synthesis by eukaryotic replicative and translesion synthesis polymerases

Molecular dependencies and genomic consequences of a global DNA damage tolerance defect

BACKGROUND

DNA damage tolerance (DDT) enables replication to continue in the presence of fork stalling lesions. In mammalian cells, DDT is regulated by two independent pathways, controlled by the polymerase REV1 and ubiquitinated PCNA, respectively.

RESULTS

To determine the molecular and genomic impact of a global DDT defect, we studied PcnaK164R/-;Rev1-/- compound mutants in mouse cells. Double-mutant cells display increased replication stress, hypersensitivity to genotoxic agents, replication speed, and repriming. A whole-genome CRISPR-Cas9 screen revealed a strict reliance of double-mutant cells on the CST complex, where CST promotes fork stability. Whole-genome sequencing indicated that this double-mutant DDT defect favors the generation of large, replication-stress inducible deletions of 0.4-4.0 kbp, defined as type 3 deletions. Junction break sites of these deletions reveal microhomology preferences of 1-2 base pairs, differing from the smaller type 1 and type 2 deletions. These differential characteristics suggest the existence of molecularly distinct deletion pathways. Type 3 deletions are abundant in human tumors, can dominate the deletion landscape, and are associated with DNA damage response status and treatment modality.

CONCLUSIONS

Our data highlight the essential contribution of the DDT system to genome maintenance and type 3 deletions as mutational signature of replication stress. The unique characteristics of type 3 deletions implicate the existence of a novel deletion pathway in mice and humans that is counteracted by DDT.

Enhancing Cellular and Enzymatic Properties Through In Vivo Continuous Evolution

Directed evolution seeks to evolve target genes at a rate far exceeding the natural mutation rate, thereby endowing cellular and enzymatic properties with desired traits. In vivo continuous directed evolution achieves these purposes by generating libraries within living cells, enabling a continuous cycle of mutant generation and selection, enhancing the exploration of gene variants. Continuous evolution has become powerful tools for unraveling evolution mechanism and improving cellular and enzymatic properties. This review categorizes current continuous evolution into three distinct classes: non-targeted chromosomal, targeted chromosomal, and extra-chromosomal hypermutation approaches. It also compares various continuous evolution strategies based on different principles, providing a reference for selecting suitable methods for specific evolutionary goals. Furthermore, this review discusses the two primary limitations for further widespread application of in vivo continuous evolution, which are lack of general applicability and insufficient mutagenic capability. We envision that developing generally applicable mutagenic components and methods to enhance mutation rates for in vivo continuous evolution are promising future directions for wide range applications of continuous evolution.

ATAD5-BAZ1B interaction modulates PCNA ubiquitination during DNA repair

Mono-ubiquitinated PCNA (mono-Ub-PCNA) is generated when replication forks encounter obstacles, enabling the bypass of DNA lesions. After resolving stalled forks, Ub-PCNA must be de-ubiquitinated to resume high-fidelity DNA synthesis. ATAD5, in cooperation with the UAF1-USP1 complex, is responsible for this de-ubiquitination. However, the precise regulation of timely Ub-PCNA de-ubiquitination remains unclear. Our research reveals that BAZ1B, a regulatory subunit of the BAZ1B-SMARCA5 chromatin-remodeling complex (also known as the WICH complex), plays a crucial role in fine-tuning the de-ubiquitination process of Ub-PCNA. The BAZ1B binding region of ATAD5 encompasses the UAF1-binding domain of ATAD5. Disruption of the ATAD5-BAZ1B interaction results in premature de-ubiquitination of Ub-PCNA following treatment with hydrogen peroxide. Cells with impaired BAZ1B binding to ATAD5 display increased sensitivity to oxidative stress compared to wild-type cells. These findings suggest that BAZ1B prevents premature Ub-PCNA de-ubiquitination, thereby safeguarding genome integrity.

Widespread mutagenesis and chromosomal instability shape somatic genomes in systemic sclerosis

Systemic sclerosis is a connective tissue disorder characterized by excessive fibrosis that primarily affects women, and can present as a multisystem pathology. Roughly 4-22% of patients with systemic sclerosis develop cancer, which drastically worsens prognosis. However, the mechanisms underlying systemic sclerosis initiation, propagation, and cancer development are poorly understood. We hypothesize that the inflammation and immune response associated with systemic sclerosis can trigger DNA damage, leading to elevated somatic mutagenesis, a hallmark of pre-cancerous tissues. To test our hypothesis, we culture clonal lineages of fibroblasts from the lung tissues of controls and systemic sclerosis patients and compare their mutation burdens and spectra. We find an overall increase in all major mutation types in systemic sclerosis samples compared to control lung samples, from small-scale events such as single base substitutions and insertions/deletions, to chromosome-level changes, including copy-number changes and structural variants. In the genomes of patients with systemic sclerosis, we find evidence of somatic hypermutation or kategis (typically only seen in cancer genomes), we identify mutation signatures closely resembling the error-prone translesion polymerase Polη activity, and observe an activation-induced deaminase-like mutation signature, which overlaps with genomic regions displaying kataegis.

DNA polymerase η is regulated by mutually exclusive mono-ubiquitination and mono-NEDDylation

DNA polymerase eta (Pol η) is a Y-family translesion polymerase responsible for synthesizing new DNA across UV-damaged templates. It is recruited to replication forks following mono-ubiquitination of the PCNA DNA clamp. This interaction is mediated by PCNA-interacting protein (PIP) motifs within Pol η, as well as by its C-terminal ubiquitin-binding zinc finger (UBZ) domain. Previous work has suggested that Pol η itself is mono-ubiquitinated at four C-terminal lysine residues, which is dependent on prior ubiquitin-binding by its UBZ domain. Here, we show that Pol η can be modified at the same lysine residues by the ubiquitin-like protein, NEDD8. Like ubiquitination, this modification is driven by non-covalent interactions between NEDD8 and the UBZ domain. While only a small proportion of Pol η is mono-NEDDylated under normal conditions, these levels rapidly increase by inhibiting the COP9 signalosome, suggesting that mono-NEDDylation is maintained under strong negative regulation. Finally, we provide data to support that mono-ubiquitination is important for Pol η foci formation and suggest that NEDDylation disrupts this process. These results reveal a new mechanism of Pol η regulation by ubiquitin-like proteins.

Glycosylase-based base editors for efficient T-to-G and C-to-G editing in mammalian cells

Base editors show promise for treating human genetic diseases, but most current systems use deaminases, which cause off-target effects and are limited in editing type. In this study, we constructed deaminase-free base editors for cytosine (DAF-CBE) and thymine (DAF-TBE), which contain only a cytosine-DNA or a thymine-DNA glycosylase (CDG/TDG) variant, respectively, tethered to a Cas9 nickase. Multiple rounds of mutagenesis by directed evolution in Escherichia coli generated two variants with enhanced base-converting activity-CDG-nCas9 and TDG-nCas9-with efficiencies of up to 58.7% for C-to-A and 54.3% for T-to-A. DAF-BEs achieve C-to-G/T-to-G editing in mammalian cells with minimal Cas9-dependent and Cas9-independent off-target effects as well as minimal RNA off-target effects. Additional engineering resulted in DAF-CBE2/DAF-TBE2, which exhibit altered editing windows from the 5' end to the middle of the protospacer and increased C-to-G/T-to-G editing efficiency of 3.5-fold and 1.2-fold, respectively. Compared to prime editing or CGBEs, DAF-BEs expand conversion types of base editors with similar efficiencies, smaller sizes and lower off-target effects.

Exploring the molecular aspect and updating evolutionary approaches to the DNA polymerase enzymes for biotechnological needs: A comprehensive review

DNA polymerases are essential enzymes that play a key role in living organisms, as they participate in the synthesis and maintenance of the DNA molecule. The intrinsic properties of these enzymes have been widely observed and studied to understand their functions, activities, and behavior, which has allowed their natural power in DNA synthesis to be exploited in modern biotechnology, to the point of making them true pillars of the field. In this context, the laboratory evolution of these enzymes, either by directed evolution or rational design, has led to the generation of a wide range of new DNA polymerases with novel properties, suitable for a variety of biotechnological needs. In this review, we examine DNA polymerases at the molecular level, their biotechnological use, and their evolutionary methods in relation to the novel properties sought, providing a chronological selection of evolved DNA polymerases cited in the literature that we consider to be of great interest. To our knowledge, this work is the first to bring together the molecular, functional and evolutionary aspects of the DNA polymerase enzyme. We believe it will be of great interest to researchers whose aim is to produce new lines of evolved DNA polymerases.

(Single-stranded DNA) gaps in understanding BRCAness

The tumour-suppressive roles of BRCA1 and 2 have been attributed to three seemingly distinct functions - homologous recombination, replication fork protection, and single-stranded (ss)DNA gap suppression - and their relative importance is under debate. In this review, we examine the origin and resolution of ssDNA gaps and discuss the recent advances in understanding the role of BRCA1/2 in gap suppression. There are ample data showing that gap accumulation in BRCA1/2-deficient cells is linked to genomic instability and chemosensitivity. However, it remains unclear whether there is a causative role and the function of BRCA1/2 in gap suppression cannot unambiguously be dissected from their other functions. We therefore conclude that the three functions of BRCA1 and 2 are closely intertwined and not mutually exclusive.

Strand-specific PCR-competitive replication and adduct bypass assay for assessing how DNA adducts perturb DNA replication in mammalian cells

Human genomes are susceptible to damage by a variety of endogenous and exogenous agents. If not repaired, the resulting DNA lesions can potentially lead to mutations, genome instability, and cell death. While existing in vitro experiments allow for characterizing replication outcomes from the use of purified translesion synthesis (TLS) DNA polymerases, such studies often lack the sophistication and dynamic nature of cellular contexts. Here, we present a strand-specific PCR-based Competitive Replication and Adduct Bypass (ssPCR-CRAB) assay designed to investigate quantitatively the impact of DNA lesions on replication efficiency and fidelity in mammalian cells. Combined with genetic manipulation, this approach facilitates the revelation of diverse functions of TLS polymerases in replication across DNA lesions.

Copy number signatures and CCNE1 amplification reveal the involvement of replication stress in high-grade endometrial tumors oncogenesis

PURPOSE

Managing high-grade endometrial cancer in Martinique poses significant challenges. The diversity of copy number alterations in high-grade endometrial tumors, often associated with a TP53 mutation, is a key factor complicating treatment. Due to the high incidence of high-grade tumors with poor prognosis, our study aimed to characterize the molecular signature of these tumors within a cohort of 25 high-grade endometrial cases.

METHODS

We conducted a comprehensive pangenomic analysis to categorize the copy number alterations involved in these tumors. Whole-Exome Sequencing (WES) and Homologous Recombination (HR) analysis were performed. The alterations obtained from the WES were classified into various signatures using the Copy Number Signatures tool available in COSMIC.

RESULTS

We identified several signatures that correlated with tumor stage and disctinct prognoses. These signatures all seem to be linked to replication stress, with CCNE1 amplification identified as the primary driver of oncogenesis in over 70% of tumors analyzed.

CONCLUSION

The identification of CCNE1 amplification, which is currently being explored as a therapeutic target in clinical trials, suggests new treatment strategies for high-grade endometrial cancer. This finding holds particular significance for Martinique, where access to care is challenging.

Induced genetic diversity through mutagenesis in wheat gene pool and significant use of SCoT markers to underpin key agronomic traits

BACKGROUND

This research explores the efficacy of mutagenesis, specifically using sodium azide (SA) and hydrazine hydrate (HZ) treatments, to introduce genetic diversity and enhance traits in three wheat (Triticum aestivum L.) genotypes. The experiment entails subjecting the seeds to different doses of SA and HZ and cultivating them in the field for two consecutive generations: M1 (first generation) and M2 (second generation). We then employed selective breeding techniques with Start Codon Targeted (SCoT) markers to select traits within the wheat gene pool. Also, the correlation between SCoT markers and specific agronomic traits provides insights into the genetic mechanisms underlying mutagenesis-induced changes in wheat.

RESULTS

In the study, eleven genotypes were derived from parent varieties Sids1, Sids12, and Giza 168, and eight mutant genotypes were selected from the M1 generation and further cultivated to establish the M2 generation. The results revealed that various morphological and agronomical characteristics, such as plant height, spikes per plant, spike length, spikelet per spike, grains per spikelet, and 100-grain weight, showed increases in different genotypes from M1 to M2. SCoT markers were employed to assess genetic diversity among the eleven genotypes. The bioinformatics analysis identified a correlation between SCoT markers and the transcription factors ABSCISIC ACID INSENSITIVE3 (ABI3) and VIVIPAROUS1 (VP1), crucial for plant development, growth, and stress adaptation. A comprehensive examination of genetic distance and the function identification of gene-associated SCoT markers may provide valuable insights into the mechanisms by which SA and HZ act as mutagens, enhancing wheat agronomic qualities.

CONCLUSIONS

This study demonstrates the effective use of SA and HZ treatments to induce gene diversity through mutagenesis in the wheat gene pool, resulting in the enhancement of agronomic traits, as revealed by SCoT markers. The significant improvements in morphological and agronomical characteristics highlight the potential of mutagenesis techniques for crop improvement. These findings offer valuable information for breeders to develop effective breeding programs to enhance wheat quality and resilience through increased genetic diversity.

Functional characterization of single nucleotide polymorphic variants of DNA repair enzyme NEIL1 in South Asian populations

The base excision repair (BER) pathway is a precise and versatile mechanism of DNA repair that is initiated by DNA glycosylases. Endonuclease VIII-like 1 (NEIL1) is a bifunctional glycosylase/abasic site (AP) lyase that excises a damaged base and subsequently cleaves the phosphodiester backbone. NEIL1 is able to recognize and hydrolyze a broad range of oxidatively-induced base lesions and substituted ring-fragmented guanines, including aflatoxin-induced 8,9-dihydro-8-(2,6-diamino-4-oxo-3,4-dihydropyrimid-5-yl-formamido)-9-hydroxyaflatoxin B1 (AFB1-FapyGua). Due to NEIL1's protective role against these and other pro-mutagenic lesions, it was hypothesized that naturally occurring single nucleotide polymorphic (SNP) variants of NEIL1 could increase human risk for aflatoxin-induced hepatocellular carcinoma (HCC). Given that populations in South Asia experience high levels of dietary aflatoxin exposures and hepatitis B viral infections that induce oxidative stress, investigations on SNP variants of NEIL1 that occur in this region may have clinical implications. In this study, the most common South Asian variants of NEIL1 were expressed, purified, and functionally characterized. All tested variants exhibited activities and substrate specificities similar to wild type (wt)-NEIL1 on high-molecular weight DNA containing an array of oxidatively-induced base lesions. On short oligodeoxynucleotides (17-mers) containing either a site-specific apurinic/apyrimidinic (AP) site, thymine glycol (ThyGly), or AFB1-FapyGua, P206L-NEIL1 was catalytically comparable to wt-NEIL1, while the activities of NEIL1 variants Q67K and T278I on these substrates were ≈2-fold reduced. Variant T103A had a greatly diminished ability to bind to 17-mer DNAs, limiting the subsequent glycosylase and lyase reactions. Consistent with this observation, the rate of excision by T103A on 17-mer oligodeoxynucleotides containing ThyGly or AFB1-FapyGua could not be measured. However, the ability of T103A to excise ThyGly was improved on longer oligodeoxynucleotides (51-mers), with ≈7-fold reduced activity compared to wt-NEIL1. Our studies suggest that NEIL1 variant T103A may present a pathogenic phenotype that is limited in damage recognition, potentially increasing human risk for HCC.

Protein Assemblies in Translesion Synthesis

Translesion synthesis (TLS) is a mechanism of DNA damage tolerance utilized by eukaryotic cells to replicate DNA across lesions that impede the high-fidelity replication machinery. In TLS, a series of specialized DNA polymerases are employed, which recognize specific DNA lesions, insert nucleotides across the damage, and extend the distorted primer-template. This allows cells to preserve genetic integrity at the cost of mutations. In humans, TLS enzymes include the Y-family, inserter polymerases, Polη, Polι, Polκ, Rev1, and the B-family extender polymerase Polζ, while in S. cerevisiae only Polη, Rev1, and Polζ are present. To bypass DNA lesions, TLS polymerases cooperate, assembling into a complex on the eukaryotic sliding clamp, PCNA, termed the TLS mutasome. The mutasome assembly is contingent on protein-protein interactions (PPIs) between the modular domains and subunits of TLS enzymes, and their interactions with PCNA and DNA. While the structural mechanisms of DNA lesion bypass by the TLS polymerases and PPIs of their individual modules are well understood, the mechanisms by which they cooperate in the context of TLS complexes have remained elusive. This review focuses on structural studies of TLS polymerases and describes the case of TLS holoenzyme assemblies in action emerging from recent high-resolution Cryo-EM studies.

Pan-cancer analysis of the interplay between mutational signatures and cellular signaling

Cancer is a multi-faceted disease with intricate relationships between mutagenic processes, alterations in cellular signaling, and the tissue microenvironment. To date, these processes have been largely studied in isolation. A systematic understanding of how they interact and influence each other is lacking. Here, we present a framework for systematically characterizing the interaction between pairs of mutational signatures and between signatures and signaling pathway alterations. We applied this framework to large-scale data from TCGA and PCAWG and identified multiple positive and negative interactions, both cross֊tissue and tissue֊specific, that provide new insights into the molecular routes observed in tumorigenesis and their respective drivers. This framework allows for a more fine-grained dissection of common and distinct etiology of mutational signatures. We further identified several interactions with both positive and negative impacts on patient survival, demonstrating their clinical relevance and potential for improving personalized cancer care.

An interaction network in the polymerase active site is a prerequisite for Watson-Crick base pairing in Pol γ

The replication accuracy of DNA polymerase gamma (Pol γ) is essential for mitochondrial genome integrity. Mutation of human Pol γ arginine-853 has been linked to neurological diseases. Although not a catalytic residue, Pol γ arginine-853 mutants are void of polymerase activity. To identify the structural basis for the disease, we determined a crystal structure of the Pol γ mutant ternary complex with correct incoming nucleotide 2'-deoxycytidine 5'-triphosphate (dCTP). Opposite to the wild type that undergoes open-to-closed conformational changes when bound to a correct nucleotide that is essential for forming a catalytically competent active site, the mutant complex failed to undergo the conformational change, and the dCTP did not base pair with its Watson-Crick complementary templating residue. Our studies revealed that arginine-853 coordinates an interaction network that aligns the 3'-end of primer and dCTP with the catalytic residues. Disruption of the network precludes the formation of Watson-Crick base pairing and closing of the active site, resulting in an inactive polymerase.

Plant Organellar MSH1 Is a Displacement Loop-Specific Endonuclease

MutS HOMOLOG 1 (MSH1) is an organellar-targeted protein that obstructs ectopic recombination and the accumulation of mutations in plant organellar genomes. MSH1 also modulates the epigenetic status of nuclear DNA, and its absence induces a variety of phenotypic responses. MSH1 is a member of the MutS family of DNA mismatch repair proteins but harbors an additional GIY-YIG nuclease domain that distinguishes it from the rest of this family. How MSH1 hampers recombination and promotes fidelity in organellar DNA inheritance is unknown. Here, we elucidate its enzymatic activities by recombinantly expressing and purifying full-length MSH1 from Arabidopsis thaliana (AtMSH1). AtMSH1 is a metalloenzyme that shows a strong binding affinity for displacement loops (D-loops). The DNA-binding abilities of AtMSH1 reside in its MutS domain and not in its GIY-YIG domain, which is the ancillary nickase of AtMSH1. In the presence of divalent metal ions, AtMSH1 selectively executes multiple incisions at D-loops, but not other DNA structures including Holliday junctions or dsDNA, regardless of the presence or absence of mismatches. The selectivity of AtMSH1 to dismantle D-loops supports the role of this enzyme in preventing recombination between short repeats. Our results suggest that plant organelles have evolved novel DNA repair routes centered around the anti-recombinogenic activity of MSH1.

Myxococcus xanthus translesion DNA synthesis protein ImuA is an ATPase enhanced by DNA

Translesion DNA synthesis pathways are necessary to ensure bacterial replication in the presence of DNA damage. Translesion DNA synthesis carried out by the PolV mutasome is well-studied in Escherichia coli, but ~one third of bacteria use a functionally homologous protein complex, consisting of ImuA, ImuB, and ImuC (also called DnaE2). Numerous in vivo studies have shown that all three proteins are required for translesion DNA synthesis and that ImuC is the error-prone polymerase, but the roles of ImuA and ImuB are unclear. Here we carry out biochemical characterization of ImuA and a truncation of ImuB from Myxococcus xanthus. We find that ImuA is an ATPase, with ATPase activity enhanced in the presence of DNA. The ATPase activity is likely regulated by the C-terminus, as loss of the ImuA C-terminus results in DNA-independent ATP hydrolysis. We also find that ImuA binds a variety of DNA substrates, with DNA binding affinity affected by the addition of ADP or adenylyl-imidodiphosphate. An ImuB truncation also binds DNA, with lower affinity than ImuA. In the absence of DNA, ImuA directly binds ImuB with moderate affinity. Finally, we show that ImuA and ImuB self-interact, but that ImuA is predominantly a monomer, while truncated ImuB is a trimer in vitro. Together, with our findings and the current literature in the field, we suggest a model for translesion DNA synthesis, where a trimeric ImuB would provide sufficient binding sites for DNA, the β-clamp, ImuC, and ImuA, and where ImuA ATPase activity may regulate assembly and disassembly of the translesion DNA synthesis complex.

Mismatch Repair Protein Msh6Tt Is Necessary for Nuclear Division and Gametogenesis in Tetrahymena thermophila

DNA mismatch repair (MMR) improves replication accuracy by up to three orders of magnitude. The MutS protein in E. coli or its eukaryotic homolog, the MutSα (Msh2-Msh6) complex, recognizes base mismatches and initiates the mismatch repair mechanism. Msh6 is an essential protein for assembling the heterodimeric complex. However, the function of the Msh6 subunit remains elusive. Tetrahymena undergoes multiple DNA replication and nuclear division processes, including mitosis, amitosis, and meiosis. Here, we found that Msh6Tt localized in the macronucleus (MAC) and the micronucleus (MIC) during the vegetative growth stage and starvation. During the conjugation stage, Msh6Tt only localized in MICs and newly developing MACs. MSH6Tt knockout led to aberrant nuclear division during vegetative growth. The MSH6TtKO mutants were resistant to treatment with the DNA alkylating agent methyl methanesulfonate (MMS) compared to wild type cells. MSH6Tt knockout affected micronuclear meiosis and gametogenesis during the conjugation stage. Furthermore, Msh6Tt interacted with Msh2Tt and MMR-independent factors. Downregulation of MSH2Tt expression affected the stability of Msh6Tt. In addition, MSH6Tt knockout led to the upregulated expression of several MSH6Tt homologs at different developmental stages. Msh6Tt is involved in macronuclear amitosis, micronuclear mitosis, micronuclear meiosis, and gametogenesis in Tetrahymena.

A Novel Interaction Between RAD23A/B and Y-family DNA Polymerases

The Y-family DNA polymerases - Pol ι, Pol η, Pol κ and Rev1 - are most well-known for their roles in the DNA damage tolerance pathway of translesion synthesis (TLS). They function to overcome replication barriers by bypassing DNA damage lesions that cannot be normally replicated, allowing replication forks to continue without stalling. In this work, we demonstrate a novel interaction between each Y-family polymerase and the nucleotide excision repair (NER) proteins, RAD23A and RAD23B. We initially focus on the interaction between RAD23A and Pol ι, and through a series of biochemical, cell-based, and structural assays, find that the RAD23A ubiquitin-binding domains (UBA1 and UBA2) interact with separate sites within the Pol ι catalytic domain. While this interaction involves the ubiquitin-binding cleft of UBA2, Pol ι interacts with a distinct surface on UBA1. We further find that mutating or deleting either UBA domain disrupts the RAD23A-Pol ι interaction, demonstrating that both interactions are necessary for stable binding. We also provide evidence that both RAD23 proteins interact with Pol ι in a similar manner, as well as with each of the Y-family polymerases. These results shed light on the interplay between the different functions of the RAD23 proteins and reveal novel binding partners for the Y-family TLS polymerases.

Elevated MSH2 MSH3 expression interferes with DNA metabolism in vivo

The Msh2-Msh3 mismatch repair (MMR) complex in Saccharomyces cerevisiae recognizes and directs repair of insertion/deletion loops (IDLs) up to ∼17 nucleotides. Msh2-Msh3 also recognizes and binds distinct looped and branched DNA structures with varying affinities, thereby contributing to genome stability outside post-replicative MMR through homologous recombination, double-strand break repair (DSBR) and the DNA damage response. In contrast, Msh2-Msh3 promotes genome instability through trinucleotide repeat (TNR) expansions, presumably by binding structures that form from single-stranded (ss) TNR sequences. We previously demonstrated that Msh2-Msh3 binding to 5' ssDNA flap structures interfered with Rad27 (Fen1 in humans)-mediated Okazaki fragment maturation (OFM) in vitro. Here we demonstrate that elevated Msh2-Msh3 levels interfere with DNA replication and base excision repair in vivo. Elevated Msh2-Msh3 also induced a cell cycle arrest that was dependent on RAD9 and ELG1 and led to PCNA modification. These phenotypes also required Msh2-Msh3 ATPase activity and downstream MMR proteins, indicating an active mechanism that is not simply a result of Msh2-Msh3 DNA-binding activity. This study provides new mechanistic details regarding how excess Msh2-Msh3 can disrupt DNA replication and repair and highlights the role of Msh2-Msh3 protein abundance in Msh2-Msh3-mediated genomic instability.

Meiotic DNA breaks drive multifaceted mutagenesis in the human germ line

Meiotic recombination commences with hundreds of programmed DNA breaks; however, the degree to which they are accurately repaired remains poorly understood. We report that meiotic break repair is eightfold more mutagenic for single-base substitutions than was previously understood, leading to de novo mutation in one in four sperm and one in 12 eggs. Its impact on indels and structural variants is even higher, with 100- to 1300-fold increases in rates per break. We uncovered new mutational signatures and footprints relative to break sites, which implicate unexpected biochemical processes and error-prone DNA repair mechanisms, including translesion synthesis and end joining in meiotic break repair. We provide evidence that these mechanisms drive mutagenesis in human germ lines and lead to disruption of hundreds of genes genome wide.

Two independent DNA repair pathways cause mutagenesis in template switching deficient Saccharomyces cerevisiae

Upon DNA replication stress, cells utilize the postreplication repair pathway to repair single-stranded DNA and maintain genome integrity. Postreplication repair is divided into 2 branches: error-prone translesion synthesis, signaled by proliferating cell nuclear antigen (PCNA) monoubiquitination, and error-free template switching, signaled by PCNA polyubiquitination. In Saccharomyces cerevisiae, Rad5 is involved in both branches of repair during DNA replication stress. When the PCNA polyubiquitination function of Rad5 s disrupted, Rad5 recruits translesion synthesis polymerases to stalled replication forks, resulting in mutagenic repair. Details of how mutagenic repair is carried out, as well as the relationship between Rad5-mediated mutagenic repair and the canonical PCNA-mediated mutagenic repair, remain to be understood. We find that Rad5-mediated mutagenic repair requires the translesion synthesis polymerase ζ but does not require other yeast translesion polymerase activities. Furthermore, we show that Rad5-mediated mutagenic repair is independent of PCNA binding by Rev1 and so is separable from canonical mutagenic repair. In the absence of error-free template switching, both modes of mutagenic repair contribute additively to replication stress response in a replication timing-independent manner. Cellular contexts where error-free template switching is compromised are not simply laboratory phenomena, as we find that a natural variant in RAD5 is defective in PCNA polyubiquitination and therefore defective in error-free repair, resulting in Rad5- and PCNA-mediated mutagenic repair. Our results highlight the importance of Rad5 in regulating spontaneous mutagenesis and genetic diversity in S. cerevisiae through different modes of postreplication repair.

Recommendations for the classification of germline variants in the exonuclease domain of POLE and POLD1

BACKGROUND

Germline variants affecting the proofreading activity of polymerases epsilon and delta cause a hereditary cancer and adenomatous polyposis syndrome characterized by tumors with a high mutational burden and a specific mutational spectrum. In addition to the implementation of multiple pieces of evidence for the classification of gene variants, POLE and POLD1 variant classification is particularly challenging given that non-disruptive variants affecting the proofreading activity of the corresponding polymerase are the ones associated with cancer. In response to an evident need in the field, we have developed gene-specific variant classification recommendations, based on the ACMG/AMP (American College of Medical Genetics and Genomics/Association for Molecular Pathology) criteria, for the assessment of non-disruptive variants located in the sequence coding for the exonuclease domain of the polymerases.

METHODS

A training set of 23 variants considered pathogenic or benign was used to define the usability and strength of the ACMG/AMP criteria. Population frequencies, computational predictions, co-segregation data, phenotypic and tumor data, and functional results, among other features, were considered.

RESULTS

Gene-specific variant classification recommendations for non-disruptive variants located in the exonuclease domain of POLE and POLD1 were defined. The resulting recommendations were applied to 128 exonuclease domain variants reported in the literature and/or public databases. A total of 17 variants were classified as pathogenic or likely pathogenic, and 17 as benign or likely benign.

CONCLUSIONS

Our recommendations, with room for improvement in the coming years as more information become available on carrier families, tumor molecular characteristics and functional assays, are intended to serve the clinical and scientific communities and help improve diagnostic performance, avoiding variant misclassifications.

Implications of Translesion DNA Synthesis Polymerases on Genomic Stability and Human Health

Replication fork arrest-induced DNA double strand breaks (DSBs) caused by lesions are effectively suppressed in cells due to the presence of a specialized mechanism, commonly referred to as DNA damage tolerance (DDT). In eukaryotic cells, DDT is facilitated through translesion DNA synthesis (TLS) carried out by a set of DNA polymerases known as TLS polymerases. Another parallel mechanism, referred to as homology-directed DDT, is error-free and involves either template switching or fork reversal. The significance of the DDT pathway is well established. Several diseases have been attributed to defects in the TLS pathway, caused either by mutations in the TLS polymerase genes or dysregulation. In the event of a replication fork encountering a DNA lesion, cells switch from high-fidelity replicative polymerases to low-fidelity TLS polymerases, which are associated with genomic instability linked with several human diseases including, cancer. The role of TLS polymerases in chemoresistance has been recognized in recent years. In addition to their roles in the DDT pathway, understanding noncanonical functions of TLS polymerases is also a key to unraveling their importance in maintaining genomic stability. Here we summarize the current understanding of TLS pathway in DDT and its implication for human health.

Changes in Metabolite Profiling and Expression Levels of Key Genes Involved in the Terpenoid Biosynthesis Pathway in Garden Sage (Salvia officinalis) under the Effect of Hydrazine Hydrate

Mutagenesis is a highly efficient tool for establishing genetic variation and is widely used for genetic enhancement in various plants. The key benefit of mutation breeding is the prospect of enhancing one or several characteristics of a variety without altering the genetic background. In this study, we exposed the seeds of Salvia officinalis to four concentrations of hydrazine hydrate (HZ), i.e., (0%, 0.1%, 0.2%, and 0.3%) for 6 h. The contents of terpenoid compounds in the S. officinalis plantlets driven from the HZ-treated seeds were determined by GC-MS, which resulted in the identification of a total of 340 phytochemical compounds; 163 (87.48%), 145 (84.49%), 65 (97.45%), and 62 (98.32%), from the four concentrations of HZ (0%, 0.1%, 0.2%, and 0.3%), respectively. Furthermore, we used the qRT-PCR system to disclose the "transcriptional control" for twelve TPS genes related to terpenoid and terpene biosynthesis, namely, SoGPS, SoMYRS, SoNEOD, SoCINS, SoSABS, SoLINS, SoFPPS, SoHUMS, SoTPS6, SoSQUS, SoGGPS, and SoGA2. Altogether, results are likely to ensure some positive relationship between the concentrations of the chemical mutagen HZ used for treating the seeds, the type and amount of the produced terpenes, and the expression of their corresponding genes.

Saccharomyces cerevisiae DNA polymerase IV overcomes Rad51 inhibition of DNA polymerase δ in Rad52-mediated direct-repeat recombination

Saccharomyces cerevisiae DNA polymerase IV (Pol4) like its homolog, human DNA polymerase lambda (Polλ), is involved in Non-Homologous End-Joining and Microhomology-Mediated Repair. Using genetic analysis, we identified an additional role of Pol4 also in homology-directed DNA repair, specifically in Rad52-dependent/Rad51-independent direct-repeat recombination. Our results reveal that the requirement for Pol4 in repeat recombination was suppressed by the absence of Rad51, suggesting that Pol4 counteracts the Rad51 inhibition of Rad52-mediated repeat recombination events. Using purified proteins and model substrates, we reconstituted in vitro reactions emulating DNA synthesis during direct-repeat recombination and show that Rad51 directly inhibits Polδ DNA synthesis. Interestingly, although Pol4 was not capable of performing extensive DNA synthesis by itself, it aided Polδ in overcoming the DNA synthesis inhibition by Rad51. In addition, Pol4 dependency and stimulation of Polδ DNA synthesis in the presence of Rad51 occurred in reactions containing Rad52 and RPA where DNA strand-annealing was necessary. Mechanistically, yeast Pol4 displaces Rad51 from ssDNA independent of DNA synthesis. Together our in vitro and in vivo data suggest that Rad51 suppresses Rad52-dependent/Rad51-independent direct-repeat recombination by binding to the primer-template and that Rad51 removal by Pol4 is critical for strand-annealing dependent DNA synthesis.

Biological evolution requires an emergent, self-organizing principle

In this perspective review, we assess fundamental flaws in Darwinian evolution, including its modern versions. Fixed mutations 'explain' microevolution but not macroevolution including speciation events and the origination of all the major body plans of the Cambrian explosion. Complex, multifactorial change is required for speciation events and inevitably requires self-organization beyond what is accomplished by known mechanisms. The assembly of ribosomes and ATP synthase are specific examples. We propose their origin is a model for what is unexplained in biological evolution. Probability of evolution is modeled in Section 9 and values are absurdly improbable. Speciation and higher taxonomic changes become exponentially less probable as the number of required, genetically-based events increase. Also, the power required of the proposed selection mechanism (survival of the fittest) is nil for any biological advance requiring multiple changes, because they regularly occur in multiple generations (different genomes) and would not be selectively conserved by the concept survival of the fittest (a concept ultimately centered on the individual). Thus, survival of the fittest cannot 'explain' the origin of the millions of current and extinct species. We also focus on the inadequacies of laboratory chemistry to explain the complex, required biological self-organization seen in cells. We propose that a 'bioelectromagnetic' field/principle emerges in living cells. Synthesis by self-organization of massive molecular complexes involves biochemical responses to this emergent field/principle. There are ramifications for philosophy, science, and religion. Physics and mathematics must be more strongly integrated with biology and integration should receive dedicated funding with special emphasis for medical applications; treatment of cancer and genetic diseases are examples.

Evaluation of toxicity and mutagenicity of oxaliplatin on germ cells in an alternative in vivo model Caenorhabditis elegans

The platinum compound oxaliplatin is a widely used chemotherapeutic drug that shows a broad spectrum of activity in various human tumors. While the treatment-related side effects of oxaliplatin on directly treated individuals have been well-documented, little is known about the influence of oxaliplatin on germ cells and non-exposed progenies. Here we investigated the reproductive toxicity of oxaliplatin in a 3R-compliant in vivo model Caenorhabditis elegans, and evaluated the germ cell mutagenicity of oxaliplatin by using whole genome sequencing. Our results indicated that oxaliplatin treatment significantly disrupts development of spermatids and oocytes. By treating parental worms with oxaliplatin for three successive generations, sequencing data unveiled the clear mutagenic effects of oxaliplatin on germ cells. Analysis of genome-wide mutation spectra showed the preferentially induction of indels by oxaliplatin. In addition, we uncovered the involvement of translesion synthesis polymerase ζ in modulating mutagenic effects of oxaliplatin. These findings suggest that germ cell mutagenicity is worthy of consideration for the health risk assessment of chemotherapeutic drugs, while the combined use of alternative in vivo models and next generation sequencing technology appears to be a promising way for the preliminary safety assessment of various drugs.

GCNA is a histone binding protein required for spermatogonial stem cell maintenance

Recycling and de-novo deposition of histones during DNA replication is a critical challenge faced by eukaryotic cells and is coordinated by histone chaperones. Spermatogenesis is highly regulated sophisticated process necessitating not only histone modification but loading of testis specific histone variants. Here, we show that Germ Cell Nuclear Acidic protein (GCNA), a germ cell specific protein in adult mice, can bind histones and purified GCNA exhibits histone chaperone activity. GCNA associates with the DNA replication machinery and supports progression through S-phase in murine undifferentiated spermatogonia (USGs). Whilst GCNA is dispensable for embryonic germ cell development, it is required for the maintenance of the USG pool and for long-term production of sperm. Our work describes the role of a germ cell specific histone chaperone in USGs maintenance in mice. These findings provide a mechanistic basis for the male infertility observed in patients carrying GCNA mutations.

DNA Polymerases for Whole Genome Amplification: Considerations and Future Directions

In the same way that specialized DNA polymerases (DNAPs) replicate cellular and viral genomes, only a handful of dedicated proteins from various natural origins as well as engineered versions are appropriate for competent exponential amplification of whole genomes and metagenomes (WGA). Different applications have led to the development of diverse protocols, based on various DNAPs. Isothermal WGA is currently widely used due to the high performance of Φ29 DNA polymerase, but PCR-based methods are also available and can provide competent amplification of certain samples. Replication fidelity and processivity must be considered when selecting a suitable enzyme for WGA. However, other properties, such as thermostability, capacity to couple replication, and double helix unwinding, or the ability to maintain DNA replication opposite to damaged bases, are also very relevant for some applications. In this review, we provide an overview of the different properties of DNAPs widely used in WGA and discuss their limitations and future research directions.

DNA polymerase κ suppresses inflammation and inflammation-induced mutagenesis and carcinogenic potential in the colon of mice

BACKGROUND

Chronic inflammation induces DNA damage and promotes cell proliferation, thereby increasing the risk of cancer. DNA polymerase κ (Pol κ), involved in translesion DNA synthesis, counteracts mutagenesis induced by inflammation in the colon of mice. In the present study, we examined whether Pol κ suppressed inflammation-induced colon tumorigenesis by treating inactivated Polk knock-in (Polk^-/-) mice with dextran sulfate sodium (DSS), an inducer of colon inflammation.

RESULTS

Male and female Polk^-/- and Polk^+/+ mice were administered 2% DSS in drinking water for six consecutive days, succeeded via a recovery period of 16 days, followed by 2% DSS for another two days. DSS treatment strongly induced colitis, and the severity of colitis was higher in Polk^-/- mice than in Polk^+/+ mice. The mice were sacrificed after 19 weeks from the initiation of the first DSS treatment and subjected to pathological examination and mutation analysis. DSS treatment induced colonic dysplasia, and the multiplicity of dysplasia was higher in Polk^-/- mice than in Polk^+/+mice. Some of the dysplasias in Polk^-/- mice exhibited β-catenin-stained nucleus and/or cytoplasm. Mutation frequencies in the gpt reporter gene were increased by DSS treatment in Polk^-/- mice, and were higher than those in Polk^+/+ mice.

CONCLUSIONS

Pol κ suppresses inflammation and inflammation-induced dysplasia as well as inflammation-induced mutagenesis. The possible mechanisms by which Pol κ suppresses colitis- and colitis-induced dysplasia are discussed.

Analysis of genome instability and implications for the consequent phenotype in Plasmodium falciparum containing mutated MSH2-1 (P513T)

Malarial parasites exhibit extensive genomic plasticity, which induces the antigen diversification and the development of antimalarial drug resistance. Only a few studies have examined the genome maintenance mechanisms of parasites. The study aimed at elucidating the impact of a mutation in a DNA mismatch repair gene on genome stability by maintaining the mutant and wild-type parasites through serial in vitro cultures for approximately 400 days and analysing the subsequent spontaneous mutations. A P513T mutant of the DNA mismatch repair protein PfMSH2-1 from Plasmodium falciparum 3D7 was created. The mutation did not influence the base substitution rate but significantly increased the insertion/deletion (indel) mutation rate in short tandem repeats (STRs) and minisatellite loci. STR mutability was affected by allele size, genomic category and certain repeat motifs. In the mutants, significant telomere healing and homologous recombination at chromosomal ends caused extensive gene loss and generation of chimeric genes, resulting in large-scale chromosomal alteration. Additionally, the mutant showed increased tolerance to N-methyl-N'-nitro-N-nitrosoguanidine, suggesting that PfMSH2-1 was involved in recognizing DNA methylation damage. This work provides valuable insights into the role of PfMSH2-1 in genome stability and demonstrates that the genomic destabilization caused by its dysfunction may lead to antigen diversification.

Revisiting mutagenesis at non-B DNA motifs in the human genome

Non-B DNA structures formed by repetitive sequence motifs are known instigators of mutagenesis in experimental systems. Analyzing this phenomenon computationally in the human genome requires careful disentangling of intrinsic confounding factors, including overlapping and interrupted motifs and recurrent sequencing errors. Here, we show that accounting for these factors eliminates all signals of repeat-induced mutagenesis that extend beyond the motif boundary, and eliminates or dramatically shrinks the magnitude of mutagenesis within some motifs, contradicting previous reports. Mutagenesis not attributable to artifacts revealed several biological mechanisms. Polymerase slippage generates frequent indels within every variety of short tandem repeat motif, implicating slipped-strand structures. Interruption-correcting single nucleotide variants within short tandem repeats may originate from error-prone polymerases. Secondary-structure formation promotes single nucleotide variants within palindromic repeats and duplications within direct repeats. G-quadruplex motifs cause recurrent sequencing errors, whereas mutagenesis at Z-DNAs is conspicuously absent.

Mathematical and physical considerations indicating that the cell genome is a read-write memory

The molecular mechanisms that govern biological evolution have not been fully elucidated so far. Recent studies indicate that regulatory proteins, acting as decision-making complex devices, can accelerate or retard the evolution of cells. Such biochemically controlled evolution may be considered as an optimization process of logical nature aimed at developing fitter species that can better survive in a specific environment. Therefore, we may assume that new genetic information can be stored in the cell memory (i.e., genome) by a sophisticated biomolecular process that resembles writing in computer memory. Such a hypothesis is theoretically supported by a recent work showing that logic is a necessary component of life, so living systems process information in the same way as computers. The current study summarizes existing evidence showing that cells can intentionally modify their stored data by biochemical processes resembling stochastic algorithms to avoid environmental stress and increase their chances of survival. Furthermore, the mathematical and physical considerations that render a read-write memory a necessary component of biological systems are presented.

Mammalian life depends on two distinct pathways of DNA damage tolerance

DNA damage threatens genomic integrity and instigates stem cell failure. To bypass genotoxic lesions during replication, cells employ DNA damage tolerance (DDT), which is regulated via PCNA ubiquitination and REV1. DDT is conserved in all domains of life, yet its relevance in mammals remains unclear. Here, we show that inactivation of both PCNA-ubiquitination and REV1 results in embryonic and adult lethality, and the accumulation of DNA damage in hematopoietic stem and progenitor cells (HSPCs) that ultimately resulted in their depletion. Our results reveal the crucial relevance of DDT in the maintenance of stem cell compartments and mammalian life in unperturbed conditions.

Learning Context-Dependent DNA Mutation Patterns in Error-Prone Polymerase Chain Reaction

We present a novel statistical learning method for studying context-dependent error rates in error-prone polymerase chain reaction (PCR) experiments. We demonstrate the method by applying it to error-prone PCR sequencing data and show how it may be exploited to improve the evolvability of genes in protein engineering.

Leveraging the replication stress response to optimize cancer therapy

High-fidelity DNA replication is critical for the faithful transmission of genetic information to daughter cells. Following genotoxic stress, specialized DNA damage tolerance pathways are activated to ensure replication fork progression. These pathways include translesion DNA synthesis, template switching and repriming. In this Review, we describe how DNA damage tolerance pathways impact genome stability, their connection with tumorigenesis and their effects on cancer therapy response. We discuss recent findings that single-strand DNA gap accumulation impacts chemoresponse and explore a growing body of evidence that suggests that different DNA damage tolerance factors, including translesion synthesis polymerases, template switching proteins and enzymes affecting single-stranded DNA gaps, represent useful cancer targets. We further outline how the consequences of DNA damage tolerance mechanisms could inform the discovery of new biomarkers to refine cancer therapies.

Optimizing dynamical functions for speed with stochastic paths

Living systems are built from microscopic components that function dynamically; they generate work with molecular motors, assemble and disassemble structures such as microtubules, keep time with circadian clocks, and catalyze the replication of DNA. How do we implement these functions in synthetic nanostructured materials to execute them before the onset of dissipative losses? Answering this question requires a quantitative understanding of when we can improve performance and speed while minimizing the dissipative losses associated with operating in a fluctuating environment. Here, we show that there are four modalities for optimizing dynamical functions that can guide the design of nanoscale systems. We analyze Markov models that span the design space: a clock, ratchet, replicator, and self-assembling system. Using stochastic thermodynamics and an exact expression for path probabilities, we classify these models of dynamical functions based on the correlation of speed with dissipation and with the chosen performance metric. We also analyze random networks to identify the model features that affect their classification and the optimization of their functionality. Overall, our results show that the possible nonequilibrium paths can determine our ability to optimize the performance of dynamical functions, despite ever-present dissipation, when there is a need for speed.

New insights into DNA polymerase mechanisms provided by time-lapse crystallography

DNA polymerases play central roles in DNA replication and repair by catalyzing template-directed nucleotide incorporation. Recently time-lapse X-ray crystallography, which allows one to observe reaction intermediates, has revealed numerous and unexpected mechanistic features of DNA polymerases. In this article, we will examine recent new discoveries that have come from time-lapse crystallography that are currently transforming our understanding of the structural mechanisms used by DNA polymerases. Among these new discoveries are the binding of a third metal ion within the polymerase active site, the mechanisms of translocation along the DNA, the presence of new fidelity checkpoints, a novel pyrophosphatase activity within the active site, and the mechanisms of pyrophosphorolysis.

The nonequilibrium cost of accurate information processing

Accurate information processing is crucial both in technology and in nature. To achieve it, any information processing system needs an initial supply of resources away from thermal equilibrium. Here we establish a fundamental limit on the accuracy achievable with a given amount of nonequilibrium resources. The limit applies to arbitrary information processing tasks and arbitrary information processing systems subject to the laws of quantum mechanics. It is easily computable and is expressed in terms of an entropic quantity, which we name the reverse entropy, associated to a time reversal of the information processing task under consideration. The limit is achievable for all deterministic classical computations and for all their quantum extensions. As an application, we establish the optimal tradeoff between nonequilibrium and accuracy for the fundamental tasks of storing, transmitting, cloning, and erasing information. Our results set a target for the design of new devices approaching the ultimate efficiency limit, and provide a framework for demonstrating thermodynamical advantages of quantum devices over their classical counterparts.

Response of the Urothelial Carcinoma Cell Lines to Cisplatin

Cisplatin (CDDP)-based chemotherapy is the standard of care in patients with muscle-invasive bladder cancer. However, in a large number of cases, the disease becomes resistant or does not respond to CDDP, and thus progresses and disseminates. In such cases, prognosis of patients is very poor. CDDP manifests its cytotoxic effects mainly through DNA damage induction. Hence, response to CDDP is mainly dependent on DNA damage repair and tolerance mechanisms. Herein, we have examined CDDP response in a panel of the urothelial carcinoma cell (UCC) lines. We characterized these cell lines with regard to viability after CDDP treatment, as well as kinetics of induction and repair of CDDP-induced DNA damage. We demonstrate that repair of CDDP-induced DNA lesions correlates, at least to some extent, with CDDP sensitivity. Furthermore, we monitored expression of the key genes involved in selected DNA repair and tolerance mechanisms, nucleotide excision repair, homologous recombination and translesion DNA synthesis, and show that it differs in the UCC lines and positively correlates with CDDP resistance. Our data indicate that CDDP response in the UCC lines is dependent on DNA damage repair and tolerance factors, which may, therefore, represent valuable therapeutic targets in this malignancy.

The mismatch recognition protein MutSα promotes nascent strand degradation at stalled replication forks

DNA mismatch repair (MMR) is well known for its role in maintaining replication fidelity by correcting mispairs generated during replication. Here, we identify an unusual MMR function to promote genome instability in the replication stress response. Under replication stress, binding of the mismatch recognition protein MutSα to replication forks blocks the loading of fork protection factors FANCD2 and BRCA1 to replication forks and promotes the recruitment of exonuclease MRE11 onto DNA to nascent strand degradation. This MutSα-dependent MRE11-catalyzed DNA degradation causes DNA breaks and chromosome abnormalities, contributing to an ultramutator phenotype.

Mismatch repair (MMR) is a replication-coupled DNA repair mechanism and plays multiple roles at the replication fork. The well-established MMR functions include correcting misincorporated nucleotides that have escaped the proofreading activity of DNA polymerases, recognizing nonmismatched DNA adducts, and triggering a DNA damage response. In an attempt to determine whether MMR regulates replication progression in cells expressing an ultramutable DNA polymerase ɛ (Polɛ), carrying a proline-to-arginine substitution at amino acid 286 (Polɛ-P286R), we identified an unusual MMR function in response to hydroxyurea (HU)-induced replication stress. Polɛ-P286R cells treated with hydroxyurea exhibit increased MRE11-catalyzed nascent strand degradation. This degradation by MRE11 depends on the mismatch recognition protein MutSα and its binding to stalled replication forks. Increased MutSα binding at replication forks is also associated with decreased loading of replication fork protection factors FANCD2 and BRCA1, suggesting blockage of these fork protection factors from loading to replication forks by MutSα. We find that the MutSα-dependent MRE11-catalyzed fork degradation induces DNA breaks and various chromosome abnormalities. Therefore, unlike the well-known MMR functions of ensuring replication fidelity, the newly identified MMR activity of promoting genome instability may also play a role in cancer avoidance by eliminating rogue cells.

The advantage of sex: Reinserting fluctuating selection in the pluralist approach

The advantage of sex, and its fixation in some clades and species all over the eukaryote tree of life, is considered an evolutionary enigma, especially regarding its assumed two-fold cost. Several likely hypotheses have been proposed such as (1) a better response to the negative frequency-dependent selection imposed by the “Red Queen” hypothesis; (2) the competition between siblings induced by the Tangled Bank hypothesis; (3) the existence of genetic and of (4) ecological factors that can diminish the cost of sex to less than the standard assumed two-fold; and (5) a better maintenance of genetic diversity and its resulting phenotypic variation, providing a selective advantage in randomly fluctuating environments. While these hypotheses have mostly been studied separately, they can also act simultaneously. This was advocated by several studies which presented a pluralist point of view. Only three among the five causes cited above were considered yet in such a framework: the Red Queen hypothesis, the Tangled Bank and the genetic factors lowering the cost of sex. We thus simulated the evolution of a finite mutating population undergoing negative frequency-dependent selection on phenotypes and a two-fold (or less) cost of sexuality, experiencing randomly fluctuating selection along generations. The individuals inherited their reproductive modes, either clonal or sexual. We found that exclusive sexuality begins to fix in populations exposed to environmental variation that exceeds the width of one ecological niche (twice the standard deviation of a Gaussian response to environment). This threshold was lowered by increasing negative frequency-dependent selection and when reducing the two-fold cost of sex. It contributes advocating that the different processes involved in a short-term advantage of sex and recombination can act in combination to favor the fixation of sexual reproduction in populations.

Predicting the Prognostic Value of POLI Expression in Different Cancers via a Machine Learning Approach

Translesion synthesis (TLS) is a cell signaling pathway that facilitates the tolerance of replication stress. Increased TLS activity, the particularly elevated expression of TLS polymerases, has been linked to resistance to cancer chemotherapeutics and significantly altered patient outcomes. Building upon current knowledge, we found that the expression of one of these TLS polymerases (POLI) is associated with significant differences in cervical and pancreatic cancer survival. These data led us to hypothesize that POLI expression is associated with cancer survival more broadly. However, when cancers were grouped cancer type, POLI expression did not have a significant prognostic value. We presented a binary cancer random forest classifier using 396 genes that influence the prognostic characteristics of POLI in cervical and pancreatic cancer selected via graphical least absolute shrinkage and selection operator. The classifier was then used to cluster patients with bladder, breast, colorectal, head and neck, liver, lung, ovary, melanoma, stomach, and uterus cancer when high POLI expression was associated with worsened survival (Group I) or with improved survival (Group II). This approach allowed us to identify cancers where POLI expression is a significant prognostic factor for survival (p = 0.028 in Group I and p = 0.0059 in Group II). Multiple independent validation approaches, including the gene ontology enrichment analysis and visualization tool and network visualization support the classification scheme. The functions of the selected genes involving mitochondrial translational elongation, Wnt signaling pathway, and tumor necrosis factor-mediated signaling pathway support their association with TLS and replication stress. Our multidisciplinary approach provides a novel way of identifying tumors where increased TLS polymerase expression is associated with significant differences in cancer survival.

Post-Translational Modifications of PCNA: Guiding for the Best DNA Damage Tolerance Choice

The sliding clamp PCNA is a multifunctional homotrimer mainly linked to DNA replication. During this process, cells must ensure an accurate and complete genome replication when constantly challenged by the presence of DNA lesions. Post-translational modifications of PCNA play a crucial role in channeling DNA damage tolerance (DDT) and repair mechanisms to bypass unrepaired lesions and promote optimal fork replication restart. PCNA ubiquitination processes trigger the following two main DDT sub-pathways: Rad6/Rad18-dependent PCNA monoubiquitination and Ubc13-Mms2/Rad5-mediated PCNA polyubiquitination, promoting error-prone translation synthesis (TLS) or error-free template switch (TS) pathways, respectively. However, the fork protection mechanism leading to TS during fork reversal is still poorly understood. In contrast, PCNA sumoylation impedes the homologous recombination (HR)-mediated salvage recombination (SR) repair pathway. Focusing on Saccharomyces cerevisiae budding yeast, we summarized PCNA related-DDT and repair mechanisms that coordinately sustain genome stability and cell survival. In addition, we compared PCNA sequences from various fungal pathogens, considering recent advances in structural features. Importantly, the identification of PCNA epitopes may lead to potential fungal targets for antifungal drug development.

Complex mutation profiles in mismatch repair and ribonucleotide reductase mutants reveal novel repair substrate specificity of MutS homolog (MSH) complexes

Determining mutation signatures is standard for understanding the etiology of human tumors and informing cancer treatment. Multiple determinants of DNA replication fidelity prevent mutagenesis that leads to carcinogenesis, including the regulation of free deoxyribonucleoside triphosphate pools by ribonucleotide reductase and repair of replication errors by the mismatch repair system. We identified genetic interactions between rnr1 alleles that skew and/or elevate deoxyribonucleoside triphosphate levels and mismatch repair gene deletions. These defects indicate that the rnr1 alleles lead to increased mutation loads that are normally acted upon by mismatch repair. We then utilized a targeted deep-sequencing approach to determine mutational profiles associated with mismatch repair pathway defects. By combining rnr1 and msh mutations to alter and/or increase deoxyribonucleoside triphosphate levels and alter the mutational load, we uncovered previously unreported specificities of Msh2-Msh3 and Msh2-Msh6. Msh2-Msh3 is uniquely able to direct the repair of G/C single-base deletions in GC runs, while Msh2-Msh6 specifically directs the repair of substitutions that occur at G/C dinucleotides. We also identified broader sequence contexts that influence variant profiles in different genetic backgrounds. Finally, we observed that the mutation profiles in double mutants were not necessarily an additive relationship of mutation profiles in single mutants. Our results have implications for interpreting mutation signatures from human tumors, particularly when mismatch repair is defective.

Mitochondrial DNA Instability in Mammalian Cells

Significance: The small, multicopy mitochondrial genome (mitochondrial DNA [mtDNA]) is essential for efficient energy production, as alterations in its coding information or a decrease in its copy number disrupt mitochondrial ATP synthesis. However, the mitochondrial replication machinery encounters numerous challenges that may limit its ability to duplicate this important genome and that jeopardize mtDNA stability, including various lesions in the DNA template, topological stress, and an insufficient nucleotide supply. Recent Advances: An ever-growing array of DNA repair or maintenance factors are being reported to localize to the mitochondria. We review current knowledge regarding the mitochondrial factors that may contribute to the tolerance or repair of various types of changes in the mitochondrial genome, such as base damage, incorporated ribonucleotides, and strand breaks. We also discuss the newly discovered link between mtDNA instability and activation of the innate immune response. Critical Issues: By which mechanisms do mitochondria respond to challenges that threaten mtDNA maintenance? What types of mtDNA damage are repaired, and when are the affected molecules degraded instead? And, finally, which forms of mtDNA instability trigger an immune response, and how? Future Directions: Further work is required to understand the contribution of the DNA repair and damage-tolerance factors present in the mitochondrial compartment, as well as the balance between mtDNA repair and degradation. Finally, efforts to understand the events underlying mtDNA release into the cytosol are warranted. Pursuing these and many related avenues can improve our understanding of what goes wrong in mitochondrial disease. Antioxid. Redox Signal. 36, 885-905.

The nuclease activity of DNA2 promotes exonuclease 1-independent mismatch repair

The DNA mismatch repair (MMR) system is a major DNA repair system that corrects DNA replication errors. In eukaryotes, the MMR system functions via mechanisms both dependent on and independent of exonuclease 1 (EXO1), an enzyme that has multiple roles in DNA metabolism. Although the mechanism of EXO1-dependent MMR is well understood, less is known about EXO1-independent MMR. Here, we provide genetic and biochemical evidence that the DNA2 nuclease/helicase has a role in EXO1-independent MMR. Biochemical reactions reconstituted with purified human proteins demonstrated that the nuclease activity of DNA2 promotes an EXO1-independent MMR reaction via a mismatch excision-independent mechanism that involves DNA polymerase δ. We show that DNA polymerase ε is not able to replace DNA polymerase δ in the DNA2-promoted MMR reaction. Unlike its nuclease activity, the helicase activity of DNA2 is dispensable for the ability of the protein to enhance the MMR reaction. Further examination established that DNA2 acts in the EXO1-independent MMR reaction by increasing the strand-displacement activity of DNA polymerase δ. These data reveal a mechanism for EXO1-independent mismatch repair.

Kinetic Proofreading

Biochemistry and molecular biology rely on the recognition of structural complementarity between molecules. Molecular interactions must be both quickly reversible, i.e., tenuous, and specific. How the cell reconciles these conflicting demands is the subject of this article. The problem and its theoretical solution are discussed within the wider theoretical context of the thermodynamics of stochastic processes (stochastic thermodynamics). The solution-an irreversible reaction cycle that decreases internal error at the expense of entropy export into the environment-is shown to be widely employed by biological processes that transmit genetic and regulatory information. Expected final online publication date for the Annual Review of Biochemistry, Volume 91 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Investigating the Conformational Dynamics of a Y-Family DNA Polymerase during Its Folding and Binding to DNA and a Nucleotide

During DNA polymerization, the Y-family DNA polymerases are capable of bypassing various DNA damage, which can stall the replication fork progression. It has been well acknowledged that the structures of the Y-family DNA polymerases have been naturally evolved to undertake this vital task. However, the mechanisms of how these proteins utilize their unique structural and conformational dynamical features to perform the translesion DNA synthesis are less understood. Here, we developed structure-based models to study the precatalytic DNA polymerization process, including DNA and nucleotide binding to DPO4, a paradigmatic Y-family polymerase from Sulfolobus solfataricus. We studied the interplay between the folding and the conformational dynamics of DPO4 and found that DPO4 undergoes first unraveling (unfolding) and then folding for accomplishing the functional "open-to-closed" conformational transition. DNA binding dynamically modulates the conformational equilibrium in DPO4 during the stepwise binding through different types of interactions, leading to different conformational distributions of DPO4 at different DNA binding stages. We observed that nucleotide binding induces modulation of a few contacts surrounding the active site of the DPO4-DNA complex associated with a high free energy barrier. Our simulation results resonate with the experimental evidence that the conformational change at the active site led by nucleotide is the rate-limiting step of nucleotide incorporation. In combination with localized frustration analyses, we underlined the importance of DPO4 conformational dynamics and fluctuations in facilitating DNA and nucleotide binding. Our findings offer mechanistic insights into the processes of DPO4 conformational dynamics associated with the substrate binding and contribute to the understanding of the "structure-dynamics-function" relationship in the Y-family DNA polymerases.