Defective DNA polymerase-delta proofreading causes cancer susceptibility in mice

Immune Checkpoint Blockade Delays Cancer and Extends Survival in Murine DNA Polymerase Mutator Syndromes

Mutations in polymerases Pold1 and Pole exonuclease domains in humans are associated with increased cancer incidence, elevated tumor mutation burden (TMB) and response to immune checkpoint blockade (ICB). Although ICB is approved for treatment of several cancers, not all tumors with elevated TMB respond. Here we generated Pold1 and Pole proofreading mutator mice and show that ICB treatment of mice with high TMB tumors did not improve survival as only a subset of tumors responded. Similarly, introducing the mutator alleles into mice with Kras/p53 lung cancer did not improve survival, however, passaging mutator tumor cells in vitro without immune editing caused rejection in immune-competent hosts, demonstrating the efficiency by which cells with antigenic mutations are eliminated. Finally, ICB treatment of mutator mice earlier, before observable tumors delayed cancer onset, improved survival, and selected for tumors without aneuploidy, suggesting the use of ICB in individuals at high risk for cancer prevention.

Highlights

Germline somatic and conditional Pold1 and Pole exonuclease domain mutations in mice produce a mutator phenotype. Spontaneous cancers arise in mutator mice that have genomic features comparable to human tumors with these mutations.ICB treatment of mutator mice with tumors did not improve survival as only a subset of tumors respond. Introduction of the mutator alleles into an autochthonous mouse lung cancer model also did not produce immunogenic tumors, whereas passaging mutator tumor cells in vitro caused immune rejection indicating efficient selection against antigenic mutations in vivo . Prophylactic ICB treatment delayed cancer onset, improved survival, and selected for tumors with no aneuploidy.

Replication DNA polymerases, genome instability and cancer therapies

It has been over a decade since the initial identification of exonuclease domain mutations in the genes encoding the catalytic subunits of replication DNA polymerases ϵ and δ (POLE and POLD1) in tumors from highly mutated endometrial and colorectal cancers. Interest in studying POLE and POLD1 has increased significantly since then. Prior to those landmark cancer genome sequencing studies, it was well documented that mutations in replication DNA polymerases that reduced their DNA synthesis accuracy, their exonuclease activity or their interactions with other factors could lead to increased mutagenesis, DNA damage and even tumorigenesis in mice. There are several recent, well-written reviews of replication DNA polymerases. The aim of this review is to gather and review in some detail recent studies of DNA polymerases ϵ and δ as they pertain to genome instability, cancer and potential therapeutic treatments. The focus here is primarily on recent informative studies on the significance of mutations in genes encoding their catalytic subunits (POLE and POLD1), mutational signatures, mutations in associated genes, model organisms, and the utility of chemotherapy and immune checkpoint inhibition in polymerase mutant tumors.

Non-canonical binding of the Chaetomium thermophilum PolD4 N-terminal PIP motif to PCNA involves Q-pocket and compact 2-fork plug interactions but no 310 helix

DNA polymerase δ (Pol δ) is a key enzyme for the maintenance of genome integrity in eukaryotic cells, acting in concert with the sliding clamp processivity factor PCNA (proliferating cell nuclear antigen). Three of the four subunits of human Pol δ interact directly with the PCNA homotrimer via a short, conserved protein sequence known as a PCNA interacting protein (PIP) motif. Here, we describe the identification of a PIP motif located towards the N terminus of the PolD4 subunit of Pol δ (equivalent to human p12) from the thermophilic filamentous fungus Chaetomium thermophilum and present the X-ray crystal structure of the corresponding peptide bound to PCNA at 2.45 Å. Like human p12, the fungal PolD4 PIP motif displays non-canonical binding to PCNA. However, the structures of the human p12 and fungal PolD4 PIP motif peptides are quite distinct, with the fungal PolD4 PIP motif lacking the 3₁₀ helical segment that characterises most previously identified PIP motifs. Instead, the fungal PolD4 PIP motif binds PCNA via conserved glutamine that inserts into the Q-pocket on the surface of PCNA and with conserved leucine and phenylalanine sidechains forming a compact 2-fork plug that inserts into the hydrophobic pocket on PCNA. Despite the unusual binding mode of the fungal PolD4, isothermal calorimetry (ITC) measurements show that its affinity for PCNA is similar to that of its human orthologue. These observations add to a growing body of information on how diverse proteins interact with PCNA and highlight how binding modes can vary significantly between orthologous PCNA partner proteins.

Unusual suspects in hereditary melanoma: POT1, POLE, BAP1

Systematic literature searches on POT1/POLE/BAP1 found that limited skin phenotypic characteristics have been documented in mutation carriers; 248 variants were annotated, and high-cluster variant regions associated with cutaneous melanoma were found in all three genes. Genotype-phenotype correlations can be used to identify patient disease predisposition based on mutation position and cluster regions.

Increased somatic mutation burdens in normal human cells due to defective DNA polymerases

Mutation accumulation in somatic cells contributes to cancer development and is proposed as a cause of aging. DNA polymerases Pol ε and Pol δ replicate DNA during cell division. However, in some cancers, defective proofreading due to acquired POLE/POLD1 exonuclease domain mutations causes markedly elevated somatic mutation burdens with distinctive mutational signatures. Germline POLE/POLD1 mutations cause familial cancer predisposition. Here, we sequenced normal tissue and tumor DNA from individuals with germline POLE/POLD1 mutations. Increased mutation burdens with characteristic mutational signatures were found in normal adult somatic cell types, during early embryogenesis and in sperm. Thus human physiology can tolerate ubiquitously elevated mutation burdens. Except for increased cancer risk, individuals with germline POLE/POLD1 mutations do not exhibit overt features of premature aging. These results do not support a model in which all features of aging are attributable to widespread cell malfunction directly resulting from somatic mutation burdens accrued during life.

The stabilized Pol31-Pol3 interface counteracts Pol32 ablation with differential effects on repair

DNA polymerase δ, which contains the catalytic subunit, Pol3, Pol31, and Pol32, contributes both to DNA replication and repair. The deletion of pol31 is lethal, and compromising the Pol3-Pol31 interaction domains confers hypersensitivity to cold, hydroxyurea (HU), and methyl methanesulfonate, phenocopying pol32Δ. We have identified alanine-substitutions in pol31 that suppress these deficiencies in pol32Δ cells. We characterize two mutants, pol31-T415A and pol31-W417A, which map to a solvent-exposed loop that mediates Pol31-Pol3 and Pol31-Rev3 interactions. The pol31-T415A substitution compromises binding to the Pol3 CysB domain, whereas Pol31-W417A improves it. Importantly, loss of Pol32, such as pol31-T415A, leads to reduced Pol3 and Pol31 protein levels, which are restored by pol31-W417A. The mutations have differential effects on recovery from acute HU, break-induced replication and trans-lesion synthesis repair pathways. Unlike trans-lesion synthesis and growth on HU, the loss of break-induced replication in pol32Δ cells is not restored by pol31-W417A, highlighting pathway-specific roles for Pol32 in fork-related repair. Intriguingly, CHIP analyses of replication forks on HU showed that pol32Δ and pol31-T415A indirectly destabilize DNA pol α and pol ε at stalled forks.

Unraveling the genomic landscape of colorectal cancer through mutational signatures

Colorectal cancer, along with most other cancer types, is driven by somatic mutations. Characteristic patterns of somatic mutations, known as mutational signatures, arise as a result of the activities of different mutational processes. Mutational signatures have diverse origins, including exogenous and endogenous sources. In the case of colorectal cancer, the analysis of mutational signatures has elucidated specific signatures for classically associated DNA repair deficiencies, namely mismatch repair (leading to microsatellite instability), base excision repair (due to MUTYH or NTHL1 mutations), and polymerase proofreading (due to POLE and POLD1 exonuclease domain mutations). Additional signatures also play a role in colorectal cancer, including those related to normal aging and those associated with gut microbiota, as well as a number of signatures with unknown etiologies. This chapter provides an overview of the current knowledge of mutational signatures, with a focus on colorectal cancer and on the recently reported signatures in physiologically normal and inflammatory bowel disease-affected somatic colon tissues.

Underappreciated Roles of DNA Polymerase δ in Replication Stress Survival

Recent structural analysis of Fe-S centers in replication proteins and insights into the structure and function of DNA polymerase δ (DNA Pol δ) subunits have shed light on the key role played by this polymerase at replication forks under stress. The sequencing of cancer genomes reveals multiple point mutations that compromise the activity of POLD1, the DNA Pol δ catalytic subunit, whereas the loci encoding the accessory subunits POLD2 and POLD3 are amplified in a very high proportion of human tumors. Consistently, DNA Pol δ is key for the survival of replication stress and is involved in multiple long-patch repair pathways. Synthetic lethality arises from compromising the function and availability of the noncatalytic subunits of DNA Pol δ under conditions of replication stress, opening the door to novel therapies.

The double-edged sword of cancer mutations: exploiting neoepitopes for the fight against cancer

Defects in DNA repair have been linked to the accumulation of somatic mutations in tumours. These mutations can promote oncogenesis; however, recent developments have indicated that they may also lead to a targeted immune response against the tumour. This response is initiated by the development of new antigenic epitopes (neoepitopes) arising from mutations in protein-coding genes that are processed and then presented on the surface of tumour cells. These neoepitopes are unique to the tumour, thus enabling lymphocytes to launch an immune response against the cancer cells. Immunotherapies, such as checkpoint inhibitors (CPIs) and tumour-derived vaccines, have been shown to enhance the immunogenic response to cancers and have led to complete remission in some cancer patients. There are tumours that are not responsive to immunotherapy or conventional tumour therapeutics; therefore, there is a push for new treatments to combat these unresponsive cancers. Recently, combinatorial treatments have been developed to further utilise the immune system in the fight against cancer. These treatments have the potential to exploit the defects in DNA repair by inducing more DNA damage and mutations. This can potentially lead to the expression of high levels of neoepitopes on the surface of tumour cells that will stimulate an immunological response. Overall, exploiting DNA repair defects in tumours may provide an edge in this long fight against cancer.

POLE and POLD1 pathogenic variants in the proofreading domain in papillary thyroid cancer

Thyroid cancer is the most frequent endocrine cancer with an increasing incidence rate worldwide and is the second most common malignancy among females in Saudi Arabia. Papillary thyroid cancer (PTC) is the most common subtype. Germline pathogenic variants in the proofreading domain of the POLE and POLD1 genes predispose to several types of cancers. However, the role of pathogenic variants of these two genes in PTC remains unknown. Capture sequencing, Sanger sequencing and immunohistochemistry were performed on 300 PTC cases from the Middle Eastern region. One germline pathogenic variant each of POLE (1/300, 0.33%) and POLD1 (1/300, 0.33%) genes was identified. Low expression of POLD1 was detected in 46.5% (133/286) of cases and was significantly associated with the follicular variant of PTC (P = 0.0006), distant metastasis (P = 0.0033) and stage IV tumours (P = 0.0081). However, no somatic pathogenic variant was detected in POLE gene. Furthermore, low expression of POLE was noted in 61.7% (175/284) of cases with no significant clinicopathological associations. Our study shows that pathogenic variant in the POLE and POLD1 proofreading domain is a cause of PTC and low expression of POLD1 is associated with poor prognostic markers in the Middle Eastern population. Further studies from different geographic populations are needed to determine the frequency and spectrum of proofreading domain pathogenic variants in POLE and POLD1 genes and in PTC from different ethnicities.

Spontaneous Polyploids and Antimutators Compete During the Evolution of Saccharomyces cerevisiae Mutator Cells

Mutations affecting DNA polymerase exonuclease domains or mismatch repair (MMR) generate "mutator" phenotypes capable of driving tumorigenesis. Cancers with both defects exhibit an explosive increase in mutation burden that appears to reach a threshold, consistent with selection acting against further mutation accumulation. In Saccharomyces cerevisiae haploid yeast, simultaneous defects in polymerase proofreading and MMR select for "antimutator" mutants that suppress the mutator phenotype. We report here that spontaneous polyploids also escape this "error-induced extinction" and routinely outcompete antimutators in evolved haploid cultures. We performed similar experiments to explore how diploid yeast adapt to the mutator phenotype. We first evolved cells with homozygous mutations affecting polymerase δ proofreading and MMR, which we anticipated would favor tetraploid emergence. While tetraploids arose with a low frequency, in most cultures, a single antimutator clone rose to prominence carrying biallelic mutations affecting the polymerase mutator alleles. Variation in mutation rate between subclones from the same culture suggests that there exists continued selection pressure for additional antimutator alleles. We then evolved diploid yeast modeling MMR-deficient cancers with the most common heterozygous exonuclease domain mutation (POLE-P286R). Although these cells grew robustly, within 120 generations, all subclones carried truncating or nonsynonymous mutations in the POLE-P286R homologous allele (pol2-P301R) that suppressed the mutator phenotype as much as 100-fold. Independent adaptive events in the same culture were common. Our findings suggest that analogous tumor cell populations may adapt to the threat of extinction by polyclonal mutations that neutralize the POLE mutator allele and preserve intratumoral genetic diversity for future adaptation.

POLE Mutation Spectra Are Shaped by the Mutant Allele Identity, Its Abundance, and Mismatch Repair Status

Human tumors with exonuclease domain mutations in the gene encoding DNA polymerase ε (POLE) have incredibly high mutation burdens. These errors arise in four unique mutation signatures occurring in different relative amounts, the etiologies of which remain poorly understood. We used CRISPR-Cas9 to engineer human cell lines expressing POLE tumor variants, with and without mismatch repair (MMR). Whole-exome sequencing of these cells after defined numbers of population doublings permitted analysis of nascent mutation accumulation. Unlike an exonuclease active site mutant that we previously characterized, POLE cancer mutants readily drive signature mutagenesis in the presence of functional MMR. Comparison of cell line and human patient data suggests that the relative abundance of mutation signatures partitions POLE tumors into distinct subgroups dependent on the nature of the POLE allele, its expression level, and MMR status. These results suggest that different POLE mutants have previously unappreciated differences in replication fidelity and mutagenesis.

Identification of PCNA-interacting protein motifs in human DNA polymerase δ

DNA polymerase δ (Polδ) is a highly processive essential replicative DNA polymerase. In humans, the Polδ holoenzyme consists of p125, p50, p68 and p12 subunits and recently, we showed that the p12 subunit exists as a dimer. Extensive biochemical studies suggest that all the subunits of Polδ interact with the processivity factor proliferating cell nuclear antigen (PCNA) to carry out a pivotal role in genomic DNA replication. While PCNA-interacting protein motif (PIP) motifs in p68, p50 and p12 have been mapped, same in p125, the catalytic subunit of the holoenzyme, remains elusive. Therefore, in the present study by using multiple approaches we have conclusively mapped a non-canonical PIP motif from residues 999VGGLLAFA1008 in p125, which binds to the inter-domain-connecting loop (IDCL) of PCNA with high affinity. Collectively, including previous studies, we conclude that similar to Saccharomyces cerevisiae Polδ, each of the human Polδ subunits possesses motif to interact with PCNA and significantly contributes toward the processive nature of this replicative DNA polymerase.

DNA polymerase δ proofreads errors made by DNA polymerase ε

Polδ and Polε are the two major replicative polymerases in eukaryotes, but their precise roles at the replication fork remain a subject of debate. A bulk of data supports a model where Polε and Polδ synthesize leading and lagging DNA strands, respectively. However, this model has been difficult to reconcile with the fact that mutations in Polδ have much stronger consequences for genome stability than equivalent mutations in Polε. We provide direct evidence for a long-entertained idea that Polδ can proofread errors made by Polε in addition to its own errors, thus, making a more prominent contribution to mutation avoidance. This paper provides an essential advance in the understanding of the mechanism of eukaryotic DNA replication.

During eukaryotic replication, DNA polymerases ε (Polε) and δ (Polδ) synthesize the leading and lagging strands, respectively. In a long-known contradiction to this model, defects in the fidelity of Polε have a much weaker impact on mutagenesis than analogous Polδ defects. It has been previously proposed that Polδ contributes more to mutation avoidance because it proofreads mismatches created by Polε in addition to its own errors. However, direct evidence for this model was missing. We show that, in yeast, the mutation rate increases synergistically when a Polε nucleotide selectivity defect is combined with a Polδ proofreading defect, demonstrating extrinsic proofreading of Polε errors by Polδ. In contrast, combining Polδ nucleotide selectivity and Polε proofreading defects produces no synergy, indicating that Polε cannot correct errors made by Polδ. We further show that Polδ can remove errors made by exonuclease-deficient Polε in vitro. These findings illustrate the complexity of the one-strand–one-polymerase model where synthesis appears to be largely divided, but Polδ proofreading operates on both strands.

POLD1 variants leading to reduced polymerase activity can cause hearing loss without syndromic features

DNA polymerase δ, whose catalytic subunit is encoded by POLD1, is responsible for synthesizing the lagging strand of DNA. Single heterozygous POLD1 mutations in domains with polymerase and exonuclease activities have been reported to cause syndromic deafness as a part of multisystem metabolic disorder or predisposition to cancer. However, the phenotypes of diverse combinations of POLD1 genotypes have not been elucidated in humans. We found that five members of a multiplex family segregating autosomal recessive nonsyndromic sensorineural hearing loss (NS-SNHL) have revealed novel compound heterozygous POLD1 variants (p.Gly1100Arg and a presumptive null function variant, p.Ser197Hisfs*54). The recombinant p.Gly1100Arg polymerase δ showed a reduced polymerase activity by 30-40%, but exhibited normal exonuclease activity. The polymerase activity in cell extracts from the affected subject carrying the two POLD1 mutant alleles was about 33% of normal controls. We suggest that significantly decreased polymerase δ activity, but not a complete absence, with normal exonuclease activity could lead to NS-SNHL.

Human DNA polymerase delta requires an iron-sulfur cluster for high-fidelity DNA synthesis

The iron–sulfur cluster in human DNA polymerase delta has an impact on DNA polymerase and exonuclease activities and can hence influence the fidelity of DNA synthesis.

Replication of eukaryotic genomes relies on the family B DNA polymerases Pol α, Pol δ, and Pol ε. All of these enzymes coordinate an iron–sulfur (FeS) cluster, but the function of this cofactor has remained largely unclear. Here, we show that the FeS cluster in the catalytic subunit of human Pol δ is coordinated by four invariant cysteines of the C-terminal CysB motif. FeS cluster loss causes a partial destabilisation of the four-subunit enzyme, a defect in double-stranded DNA binding, and compromised polymerase and exonuclease activities. Importantly, complex stability, DNA binding, and enzymatic activities are restored in the presence of proliferating cell nuclear antigen. We further show that also more subtle changes to the FeS cluster-binding pocket that do not abolish FeS cluster binding can have repercussions on the distant exonuclease domain and render the enzyme error-prone. Our data hence suggest that the FeS cluster in human Pol δ is an important co-factor that despite its C-terminal location has an impact on both DNA polymerase and exonuclease activities, and can influence the fidelity of DNA synthesis.

POLE proofreading defects: Contributions to mutagenesis and cancer

DNA polymerases are uniquely poised to contribute to the elevated mutation burdens seen in many human tumors. These mutations can arise through a number of different polymerase-dependent mechanisms, including intrinsic errors made using template DNA and precursor dNTPs free from chemical modifications, misinsertion events opposite chemically damaged template DNA or insertion events using modified nucleotides. While specific DNA repair polymerases have been known to contribute to tumorigenesis, the role of replication polymerases in mutagenesis in human disease has come into sharp focus over the last decade. This review describes how mutations in these replication DNA polymerases help to drive mutagenesis and tumor development, with particular attention to DNA polymerase epsilon. Recent studies using cancer genome sequencing, mutational signature analyses, yeast and mouse models, and the influence of mismatch repair on tumors with DNA polymerase mutations are discussed.

Molecular profiles and tumor mutational burden analysis in Chinese patients with gynecologic cancers

The goal of this work was to investigate the tumor mutational burden (TMB) in Chinese patients with gynecologic cancer. In total, 117 patients with gynecologic cancers were included in this study. Both tumor DNA and paired blood cell genomic DNA were isolated from formalin-fixed paraffin-embedded (FFPE) specimens and blood samples, and next-generation sequencing was performed to identify somatic mutations. TP53, PTEN, ARID1A, and PIK3CA alterations were significantly different in various types of gynecologic cancers (p = 0.001, 1.15E-07, 0.004, and 0.009, respectively). The median TMB of all 117 gynecologic tumor specimens was 0.37 mutations/Mb, with a range of 0-41.45 mutations/Mb. Despite the lack of significant difference, endometrial cancer cases had a higher median TMB than cervical and ovarian cancer cases. Younger gynecologic cancer patients (age <40 years) had a significantly lower TMB than older patients (age ≥40 years) (p = 0.04). In addition, TMB was significantly increased with increasing clinical stage of disease (p = 0.001). PTEN alterations were commonly observed in patients with a moderate to high TMB (n = 8, 38.10%, p = 9.95E-04). Although limited by sample size, all of the patients with TSC2 (n = 3, p = 3.83E-11) or POLE (n = 2, p = 0.005) mutations had a moderate to high TMB. Further large-scale, prospective studies are needed to validate our findings.

Fidelity of DNA replication-a matter of proofreading

DNA that is transmitted to daughter cells must be accurately duplicated to maintain genetic integrity and to promote genetic continuity. A major function of replicative DNA polymerases is to replicate DNA with the very high accuracy. The fidelity of DNA replication relies on nucleotide selectivity of replicative DNA polymerase, exonucleolytic proofreading, and postreplicative DNA mismatch repair (MMR). Proofreading activity that assists most of the replicative polymerases is responsible for removal of incorrectly incorporated nucleotides from the primer terminus before further primer extension. It is estimated that proofreading improves the fidelity by a 2–3 orders of magnitude. The primer with the incorrect terminal nucleotide has to be moved to exonuclease active site, and after removal of the wrong nucleotide must be transferred back to polymerase active site. The mechanism that allows the transfer of the primer between pol and exo site is not well understood. While defects in MMR are well known to be linked with increased cancer incidence only recently, the replicative polymerases that have alterations in the exonuclease domain have been associated with some sporadic and hereditary human cancers. In this review, we would like to emphasize the importance of proofreading (3′-5′ exonuclease activity) in the fidelity of DNA replication and to highlight what is known about switching from polymerase to exonuclease active site.

Functional Analysis of Cancer-Associated DNA Polymerase ε Variants in Saccharomyces cerevisiae

DNA replication fidelity relies on base selectivity of the replicative DNA polymerases, exonucleolytic proofreading, and postreplicative DNA mismatch repair (MMR). Ultramutated human cancers without MMR defects carry alterations in the exonuclease domain of DNA polymerase ε (Polε). They have been hypothesized to result from defective proofreading. However, modeling of the most common variant, Polε-P286R, in yeast produced an unexpectedly strong mutator effect that exceeded the effect of proofreading deficiency by two orders of magnitude and indicated the involvement of other infidelity factors. The in vivo consequences of many additional Polε mutations reported in cancers remain poorly understood. Here, we genetically characterized 13 cancer-associated Polε variants in the yeast system. Only variants directly altering the DNA binding cleft in the exonuclease domain elevated the mutation rate. Among these, frequently recurring variants were stronger mutators than rare variants, in agreement with the idea that mutator phenotype has a causative role in tumorigenesis. In nearly all cases, the mutator effects exceeded those of an exonuclease-null allele, suggesting that mechanisms distinct from loss of proofreading may drive the genome instability in most ultramutated tumors. All mutator alleles were semidominant, supporting the view that heterozygosity for the polymerase mutations is sufficient for tumor development. In contrast to the DNA binding cleft alterations, peripherally located variants, including a highly recurrent V411L, did not significantly elevate mutagenesis. Finally, the analysis of Polε variants found in MMR-deficient tumors suggested that the majority cause no mutator phenotype alone but some can synergize with MMR deficiency to increase the mutation rate.

Explosive mutation accumulation triggered by heterozygous human Pol ε proofreading-deficiency is driven by suppression of mismatch repair

Tumors defective for DNA polymerase (Pol) ε proofreading have the highest tumor mutation burden identified. A major unanswered question is whether loss of Pol ε proofreading by itself is sufficient to drive this mutagenesis, or whether additional factors are necessary. To address this, we used a combination of next generation sequencing and in vitro biochemistry on human cell lines engineered to have defects in Pol ε proofreading and mismatch repair. Absent mismatch repair, monoallelic Pol ε proofreading deficiency caused a rapid increase in a unique mutation signature, similar to that observed in tumors from patients with biallelic mismatch repair deficiency and heterozygous Pol ε mutations. Restoring mismatch repair was sufficient to suppress the explosive mutation accumulation. These results strongly suggest that concomitant suppression of mismatch repair, a hallmark of colorectal and other aggressive cancers, is a critical force for driving the explosive mutagenesis seen in tumors expressing exonuclease-deficient Pol ε.

Prognostic Significance of POLE Exonuclease Domain Mutations in High-Grade Endometrioid Endometrial Cancer on Survival and Recurrence: A Subanalysis

OBJECTIVE

POLE mutations in high-grade endometrioid endometrial cancer (EEC) have been associated with improved survival. We sought to investigate the prevalence of POLE tumor mutation and its prognostic significance on outcomes and clinical applications in a subanalysis of women with high-grade EEC from a previously described cohort of 544 EEC patients in which POLE mutation status and survival outcomes were assessed.

METHODS

Polymerase chain reaction amplification and Sanger sequencing were used to test for POLE mutations in 72 tumors. Associations between POLE mutation, demographic and clinicopathologic features, and survival were investigated with Cox proportional hazard models.

RESULTS

POLE mutations were identified in 7 (9.7%) of 72 grade 3 EECs. No significant differences in the clinicopathologic features between those with POLE mutations and those without were identified. Adjusted for age, a decreased risk of recurrence was suggested in patients with a POLE mutation (adjusted hazard ratio, 0.37; 95% confidence interval, 0.09-1.55), as well as decreased risk of death (adjusted hazard ratio, 0.19; 95% confidence interval, 0.03-1.42).

CONCLUSIONS

POLE mutations in tumors of women with grade 3 EEC are associated with a lower risk of recurrence and death, although not statistically significant because of high variability in these estimates. These findings, consistent with recently published combined analyses, support POLE mutation status as a noteworthy prognostic marker and may favor a change in the treatment of women with grade 3 EECs, particularly in those with early-stage disease, in which omission of adjuvant therapy and decreased surveillance could possibly be appropriate.

Replicative DNA polymerase defects in human cancers: Consequences, mechanisms, and implications for therapy

The fidelity of DNA replication relies on three error avoidance mechanisms acting in series: nucleotide selectivity of replicative DNA polymerases, exonucleolytic proofreading, and post-replicative DNA mismatch repair (MMR). MMR defects are well known to be associated with increased cancer incidence. Due to advances in DNA sequencing technologies, the past several years have witnessed a long-predicted discovery of replicative DNA polymerase defects in sporadic and hereditary human cancers. The polymerase mutations preferentially affect conserved amino acid residues in the exonuclease domain and occur in tumors with an extremely high mutation load. Thus, a concept has formed that defective proofreading of replication errors triggers the development of these tumors. Recent studies of the most common DNA polymerase variants, however, suggested that their pathogenicity may be determined by functional alterations other than loss of proofreading. In this review, we summarize our current understanding of the consequences of DNA polymerase mutations in cancers and the mechanisms of their mutator effects. We also discuss likely explanations for a high recurrence of some but not other polymerase variants and new ideas for therapeutic interventions emerging from the mechanistic studies.

Normally lethal amino acid substitutions suppress an ultramutator DNA Polymerase δ variant

In yeast, the pol3-01,L612M double mutant allele, which causes defects in DNA polymerase delta (Pol δ) proofreading (pol3-01) and nucleotide selectivity (pol3-L612M), confers an “ultramutator” phenotype that rapidly drives extinction of haploid and diploid MMR-proficient cells. Here, we investigate antimutator mutations that encode amino acid substitutions in Pol δ that suppress this lethal phenotype. We find that most of the antimutator mutations individually suppress the pol3-01 and pol3-L612M mutator phenotypes. The locations of many of the amino acid substitutions in Pol δ resemble those of previously identified antimutator substitutions; however, two novel mutations encode substitutions (R674G and Q697R) of amino acids in the fingers domain that coordinate the incoming dNTP. These mutations are lethal without pol3-L612M and markedly change the mutation spectra produced by the pol3-01,L612M mutator allele, suggesting that they alter nucleotide selection to offset the pol3-L612M mutator phenotype. Consistent with this hypothesis, mutations and drug treatments that perturb dNTP pool levels disproportionately influence the viability of pol3-L612M,R674G and pol3-L612M,Q697R cells. Taken together, our findings suggest that mutation rate can evolve through genetic changes that alter the balance of dNTP binding and dissociation from DNA polymerases.

Update on immune checkpoint inhibitors in gynecological cancers

In recent years, progress in our understanding of immune-modulatory signaling pathways in immune cells and the tumor microenvironment (TME) has led to rejuvenated interest in cancer immunotherapy. In particular, immunotherapy targeting the immune checkpoint receptors such as cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), programmed cell-death 1 (PD-1), and programmed cell-death ligand 1 (PD-L1) have demonstrated clinical activity in a wide variety of tumors, including gynecological cancers. This review will focus on the emerging clinical data on the therapeutic role of immune checkpoint inhibitors, and potential strategies to enhance the efficacy of this class of compounds, in the context of gynecological cancers. It is anticipated that future biomarker-directed clinical trials will provide further insights into the mechanisms underlying response and resistance to immunotherapy, and help guide our approach to designing therapeutic combinations that have the potential to enhance the benefit of immunotherapy in patients with gynecologic cancers.

Changes in mitochondrial stability during the progression of the Barrett's esophagus disease sequence

Background

Barrett’s esophagus follows the classic step-wise progression of metaplasia-dysplasia-adenocarcinoma. While Barrett’s esophagus is a leading known risk factor for esophageal adenocarcinoma, the pathogenesis of this disease sequence is poorly understood. Mitochondria are highly susceptible to mutations due to high levels of reactive oxygen species (ROS) coupled with low levels of DNA repair. The timing and levels of mitochondria instability and dysfunction across the Barrett’s disease progression is under studied.

Methods

Using an in-vitro model representing the Barrett’s esophagus disease sequence of normal squamous epithelium (HET1A), metaplasia (QH), dysplasia (Go), and esophageal adenocarcinoma (OE33), random mitochondrial mutations, deletions and surrogate markers of mitochondrial function were assessed. In-vivo and ex-vivo tissues were also assessed for instability profiles.

Results

Barrett’s metaplastic cells demonstrated increased levels of ROS (p < 0.005) and increased levels of random mitochondrial mutations (p < 0.05) compared with all other stages of the Barrett’s disease sequence in-vitro. Using patient in-vivo samples, Barrett’s metaplasia tissue demonstrated significantly increased levels of random mitochondrial deletions (p = 0.043) compared with esophageal adenocarcinoma tissue, along with increased expression of cytoglobin (CYGB) (p < 0.05), a gene linked to oxidative stress, compared with all other points across the disease sequence. Using ex-vivo Barrett’s metaplastic and matched normal patient tissue explants, higher levels of cytochrome c (p = 0.003), SMAC/Diablo (p = 0.008) and four inflammatory cytokines (all p values <0.05) were secreted from Barrett’s metaplastic tissue compared with matched normal squamous epithelium.

Conclusions

We have demonstrated that increased mitochondrial instability and markers of cellular and mitochondrial stress are early events in the Barrett’s disease sequence.

POLD1: Central mediator of DNA replication and repair, and implication in cancer and other pathologies

The evolutionarily conserved human polymerase delta (POLD1) gene encodes the large p125 subunit which provides the essential catalytic activities of polymerase δ (Polδ), mediated by 5′–3′ DNA polymerase and 3′–5′ exonuclease moieties. POLD1 associates with three smaller subunits (POLD2, POLD3, POLD4), which together with Replication Factor C and Proliferating Nuclear Cell Antigen constitute the polymerase holoenzyme. Polδ function is essential for replication, with a primary role as the replicase for the lagging strand. Polδ also has an important proofreading ability conferred by the exonuclease activity, which is critical for ensuring replicative fidelity, but also serves to repair DNA lesions arising as a result of exposure to mutagens. Polδ has been shown to be important for multiple forms of DNA repair, including nucleotide excision repair, double strand break repair, base excision repair, and mismatch repair. A growing number of studies in the past decade have linked germline and sporadic mutations in POLD1 and the other subunits of Polδ with human pathologies. Mutations in Polδ in mice and humans lead to genomic instability, mutator phenotype and tumorigenesis. The advent of genome sequencing techniques has identified damaging mutations in the proofreading domain of POLD1 as the underlying cause of some inherited cancers, and suggested that mutations in POLD1 may influence therapeutic management. In addition, mutations in POLD1 have been identified in the developmental disorders of mandibular hypoplasia, deafness, progeroid features and lipodystrophy and atypical Werner syndrome, while changes in expression or activity of POLD1 have been linked to senescence and aging. Intriguingly, some recent evidence suggests that POLD1 function may also be altered in diabetes. We provide an overview of critical Polδ activities in the context of these pathologic conditions.

A panoply of errors: polymerase proofreading domain mutations in cancer

Although it has long been recognized that the exonucleolytic proofreading activity intrinsic to the replicative DNA polymerases Pol δ and Pol ε is essential for faithful replication of DNA, evidence that defective DNA polymerase proofreading contributes to human malignancy has been limited. However, recent studies have shown that germline mutations in the proofreading domains of Pol δ and Pol ε predispose to cancer, and that somatic Pol ε proofreading domain mutations occur in multiple sporadic tumours, where they underlie a phenotype of 'ultramutation' and favourable prognosis. In this Review, we summarize the current understanding of the mechanisms and consequences of polymerase proofreading domain mutations in human malignancies, and highlight the potential utility of these variants as novel cancer biomarkers and therapeutic targets.

Meta-analysis of genome-wide association studies identifies common susceptibility polymorphisms for colorectal and endometrial cancer near SH2B3 and TSHZ1

High-risk mutations in several genes predispose to both colorectal cancer (CRC) and endometrial cancer (EC). We therefore hypothesised that some lower-risk genetic variants might also predispose to both CRC and EC. Using CRC and EC genome-wide association series, totalling 13,265 cancer cases and 40,245 controls, we found that the protective allele [G] at one previously-identified CRC polymorphism, rs2736100 near TERT, was associated with EC risk (odds ratio (OR) = 1.08, P = 0.000167); this polymorphism influences the risk of several other cancers. A further CRC polymorphism near TERC also showed evidence of association with EC (OR = 0.92; P = 0.03). Overall, however, there was no good evidence that the set of CRC polymorphisms was associated with EC risk, and neither of two previously-reported EC polymorphisms was associated with CRC risk. A combined analysis revealed one genome-wide significant polymorphism, rs3184504, on chromosome 12q24 (OR = 1.10, P = 7.23 × 10(-9)) with shared effects on CRC and EC risk. This polymorphism, a missense variant in the gene SH2B3, is also associated with haematological and autoimmune disorders, suggesting that it influences cancer risk through the immune response. Another polymorphism, rs12970291 near gene TSHZ1, was associated with both CRC and EC (OR = 1.26, P = 4.82 × 10(-8)), with the alleles showing opposite effects on the risks of the two cancers.

Volatility of Mutator Phenotypes at Single Cell Resolution

Mutator phenotypes accelerate the evolutionary process of neoplastic transformation. Historically, the measurement of mutation rates has relied on scoring the occurrence of rare mutations in target genes in large populations of cells. Averaging mutation rates over large cell populations assumes that new mutations arise at a constant rate during each cell division. If the mutation rate is not constant, an expanding mutator population may contain subclones with widely divergent rates of evolution. Here, we report mutation rate measurements of individual cell divisions of mutator yeast deficient in DNA polymerase ε proofreading and base-base mismatch repair. Our data are best fit by a model in which cells can assume one of two distinct mutator states, with mutation rates that differ by an order of magnitude. In error-prone cell divisions, mutations occurred on the same chromosome more frequently than expected by chance, often in DNA with similar predicted replication timing, consistent with a spatiotemporal dimension to the hypermutator state. Mapping of mutations onto predicted replicons revealed that mutations were enriched in the first half of the replicon as well as near termination zones. Taken together, our findings show that individual genome replication events exhibit an unexpected volatility that may deepen our understanding of the evolution of mutator-driven malignancies.

Mutations fuel microbial evolution and cancer. Cells with an increased rate of mutation are said to have a “mutator phenotype” and adapt more rapidly than non-mutator cells. Our study utilizes a novel way of measuring mutation rates of individual cell divisions to show that mutator cells can adopt one of two mutation rates that differ tenfold in magnitude. This mutator volatility suggests that the rates of mutation accumulation may vary widely within the same clone of mutator cells. Understanding how to modulate the mutator state may provide an avenue to treat certain cancers.

dNTP pool levels modulate mutator phenotypes of error-prone DNA polymerase ε variants

Mutator phenotypes create genetic diversity that fuels tumor evolution. DNA polymerase (Pol) ε mediates leading strand DNA replication. Proofreading defects in this enzyme drive a number of human malignancies. Here, using budding yeast, we show that mutator variants of Pol ε depend on damage uninducible (Dun)1, an S-phase checkpoint kinase that maintains dNTP levels during a normal cell cycle and up-regulates dNTP synthesis upon checkpoint activation. Deletion of DUN1 (dun1Δ) suppresses the mutator phenotype of pol2-4 (encoding Pol ε proofreading deficiency) and is synthetically lethal with pol2-M644G (encoding altered Pol ε base selectivity). Although pol2-4 cells cycle normally, pol2-M644G cells progress slowly through S-phase. The pol2-M644G cells tolerate deletions of mediator of the replication checkpoint (MRC) 1 (mrc1Δ) and radiation sensitive (Rad) 9 (rad9Δ), which encode mediators of checkpoint responses to replication stress and DNA damage, respectively. The pol2-M644G mutator phenotype is partially suppressed by mrc1Δ but not rad9Δ; neither deletion suppresses the pol2-4 mutator phenotype. Thus, checkpoint activation augments the Dun1 effect on replication fidelity but is not required for it. Deletions of genes encoding key Dun1 targets that negatively regulate dNTP synthesis, suppress the dun1Δ pol2-M644G synthetic lethality and restore the mutator phenotype of pol2-4 in dun1Δ cells. DUN1 pol2-M644G cells have constitutively high dNTP levels, consistent with checkpoint activation. In contrast, pol2-4 and POL2 cells have similar dNTP levels, which decline in the absence of Dun1 and rise in the absence of the negative regulators of dNTP synthesis. Thus, dNTP pool levels correlate with Pol ε mutator severity, suggesting that treatments targeting dNTP pools could modulate mutator phenotypes for therapy.

Colon cancer-associated mutator DNA polymerase δ variant causes expansion of dNTP pools increasing its own infidelity

Defects in DNA polymerases δ (Polδ) and ε (Polε) cause hereditary colorectal cancer and have been implicated in the etiology of some sporadic colorectal and endometrial tumors. We previously reported that the yeast pol3-R696W allele mimicking a human cancer-associated variant, POLD1-R689W, causes a catastrophic increase in spontaneous mutagenesis. Here, we describe the mechanism of this extraordinary mutator effect. We found that the mutation rate increased synergistically when the R696W mutation was combined with defects in Polδ proofreading or mismatch repair, indicating that pathways correcting DNA replication errors are not compromised in pol3-R696W mutants. DNA synthesis by purified Polδ-R696W was error-prone, but not to the extent that could account for the unprecedented mutator phenotype of pol3-R696W strains. In a search for cellular factors that augment the mutagenic potential of Polδ-R696W, we discovered that pol3-R696W causes S-phase checkpoint-dependent elevation of dNTP pools. Abrogating this elevation by strategic mutations in dNTP metabolism genes eliminated the mutator effect of pol3-R696W, whereas restoration of high intracellular dNTP levels restored the mutator phenotype. Further, the use of dNTP concentrations present in pol3-R696W cells for in vitro DNA synthesis greatly decreased the fidelity of Polδ-R696W and produced a mutation spectrum strikingly similar to the spectrum observed in vivo. The results support a model in which (i) faulty synthesis by Polδ-R696W leads to a checkpoint-dependent increase in dNTP levels and (ii) this increase mediates the hypermutator effect of Polδ-R696W by facilitating the extension of mismatched primer termini it creates and by promoting further errors that continue to fuel the mutagenic pathway.

Mispairs with Watson-Crick base-pair geometry observed in ternary complexes of an RB69 DNA polymerase variant

Recent structures of DNA polymerase complexes with dGMPCPP/dT and dCTP/dA mispairs at the insertion site have shown that they adopt Watson-Crick geometry in the presence of Mn(2+) indicating that the tautomeric or ionization state of the base has changed. To see whether the tautomeric or ionization state of base-pair could be affected by its microenvironment, we determined 10 structures of an RB69 DNA polymerase quadruple mutant with dG/dT or dT/dG mispairs at position n-1 to n-5 of the Primer/Template duplex. Different shapes of the mispairs, including Watson-Crick geometry, have been observed, strongly suggesting that the local environment of base-pairs plays an important role in their tautomeric or ionization states.

Somatic mosaicism in the human genome

Somatic mosaicism refers to the occurrence of two genetically distinct populations of cells within an individual, derived from a postzygotic mutation. In contrast to inherited mutations, somatic mosaic mutations may affect only a portion of the body and are not transmitted to progeny. These mutations affect varying genomic sizes ranging from single nucleotides to entire chromosomes and have been implicated in disease, most prominently cancer. The phenotypic consequences of somatic mosaicism are dependent upon many factors including the developmental time at which the mutation occurs, the areas of the body that are affected, and the pathophysiological effect(s) of the mutation. The advent of second-generation sequencing technologies has augmented existing array-based and cytogenetic approaches for the identification of somatic mutations. We outline the strengths and weaknesses of these techniques and highlight recent insights into the role of somatic mosaicism in causing cancer, neurodegenerative, monogenic, and complex disease.

Phenotypic characterization of missense polymerase-δ mutations using an inducible protein-replacement system

Next-generation sequencing has revolutionized the search for disease-causing genetic alterations. Unfortunately, the task of distinguishing the handful of causative mutations from rare variants remains daunting. We now describe an assay that permits the analysis of all types of mutations in any gene of choice through the generation of stable human cell lines, in which the endogenous protein has been inducibly replaced with its genetic variant. Here we studied the phenotype of variants of the essential replicative polymerase-δ carrying missense mutations in its active site, similar to those recently identified in familial colon cancer patients. We show that expression of the mutants but not the wild-type protein endows the engineered cells with a mutator phenotype and that the mutations affect the fidelity and/or the exonuclease activity of the isolated enzyme in vitro. This proof-of-principle study demonstrates the general applicability of this experimental approach in the study of genotype-phenotype correlations.

Polymerase ɛ (POLE) mutations in endometrial cancer: clinical outcomes and implications for Lynch syndrome testing

BACKGROUND

DNA polymerase ɛ (POLE) exonuclease domain mutations characterize a subtype of endometrial cancer (EC) with a markedly increased somatic mutational burden. POLE-mutant tumors were described as a molecular subtype with improved progression-free survival by The Cancer Genome Atlas. In this study, the frequency, spectrum, prognostic significance, and potential clinical application of POLE mutations were investigated in patients with endometrioid EC.

METHODS

Polymerase chain reaction amplification and Sanger sequencing were used to test for POLE mutations in 544 tumors. Correlations between demographic, survival, clinicopathologic, and molecular features were investigated. Statistical tests were 2-sided.

RESULTS

Thirty POLE mutations (5.6%) were identified. Mutations were associated with younger age (<60 years; P=.001). POLE mutations were detected in tumors with microsatellite stability (MSS) and microsatellite instability (MSI) at similar frequencies (5.9% and 5.2%, respectively) and were most common in tumors with MSI that lacked mutL homolog 1 (MLH1) methylation (P<.001). There was no association with progression-free survival (hazard ratio, 0.22; P=.127).

CONCLUSIONS

The discovery that mutations occur with equal frequency in MSS and MSI tumors and are most frequent in MSI tumors lacking MLH1 methylation has implications for Lynch syndrome screening and mutation testing. The current results indicate that POLE mutations are associated with somatic mutation in DNA mismatch repair genes in a subset of tumors. The absence of an association between POLE mutation and progression-free survival indicates that POLE mutation status is unlikely to be a clinically useful prognostic marker. However, POLE testing in MSI ECs could serve as a marker of somatic disease origin. Therefore, POLE tumor testing may be a valuable exclusionary criterion for Lynch syndrome gene testing.

DNA polymerase ε and its roles in genome stability

DNA Polymerase Epsilon (Pol ε) is one of three DNA Polymerases (along with Pol δ and Pol α) required for nuclear DNA replication in eukaryotes. Pol ε is comprised of four subunits, the largest of which is encoded by the POLE gene and contains the catalytic polymerase and exonuclease activities. The 3'-5' exonuclease proofreading activity is able to correct DNA synthesis errors and helps protect against genome instability. Recent cancer genome sequencing efforts have shown that 3% of colorectal and 7% of endometrial cancers contain mutations within the exonuclease domain of POLE and are associated with significantly elevated levels of single nucleotide substitutions (15-500 per Mb) and microsatellite stability. POLE mutations have also been found in other tumor types, though at lower frequency, suggesting roles in tumorigenesis more broadly in different tissue types. In addition to its proofreading activity, Pol ε contributes to genome stability through multiple mechanisms that are discussed in this review.

Emerging roles of Jab1/CSN5 in DNA damage response, DNA repair, and cancer

Jab1/CSN5 is a multifunctional protein that plays an important role in integrin signaling, cell proliferation, apoptosis, and the regulation of genomic instability and DNA repair. Dysregulation of Jab1/CSN5 activity has been shown to contribute to oncogenesis by functionally inactivating several key negative regulatory proteins and tumor suppressors. In this review, we discuss our current understanding of the relationship between Jab1/CSN5 and DNA damage and summarize recent findings regarding opportunities for and challenges to therapeutic intervention.

DNA replication error-induced extinction of diploid yeast

Genetic defects in DNA polymerase accuracy, proofreading, or mismatch repair (MMR) induce mutator phenotypes that accelerate adaptation of microbes and tumor cells. Certain combinations of mutator alleles synergistically increase mutation rates to levels that drive extinction of haploid cells. The maximum tolerated mutation rate of diploid cells is unknown. Here, we define the threshold for replication error-induced extinction (EEX) of diploid Saccharomyces cerevisiae. Double-mutant pol3 alleles that carry mutations for defective DNA polymerase-δ proofreading (pol3-01) and accuracy (pol3-L612M or pol3-L612G) induce strong mutator phenotypes in heterozygous diploids (POL3/pol3-01,L612M or POL3/pol3-01,L612G). Both pol3-01,L612M and pol3-01,L612G alleles are lethal in the homozygous state; cells with pol3-01,L612M divide up to 10 times before arresting at random stages in the cell cycle. Antimutator eex mutations in the pol3 alleles suppress this lethality (pol3-01,L612M,eex or pol3-01,L612G,eex). MMR defects synergize with pol3-01,L612M,eex and pol3-01,L612G,eex alleles, increasing mutation rates and impairing growth. Conversely, inactivation of the Dun1 S-phase checkpoint kinase suppresses strong pol3-01,L612M,eex and pol3-01,L612G,eex mutator phenotypes as well as the lethal pol3-01,L612M phenotype. Our results reveal that the lethal error threshold in diploids is 10 times higher than in haploids and likely determined by homozygous inactivation of essential genes. Pronounced loss of fitness occurs at mutation rates well below the lethal threshold, suggesting that mutator-driven cancers may be susceptible to drugs that exacerbate replication errors.

Human Pol ε-dependent replication errors and the influence of mismatch repair on their correction

Mutations in human DNA polymerase (Pol) ε, one of three eukaryotic Pols required for DNA replication, have recently been found associated with an ultramutator phenotype in tumors from somatic colorectal and endometrial cancers and in a familial colorectal cancer. Possibly, Pol ε mutations reduce the accuracy of DNA synthesis, thereby increasing the mutational burden and contributing to tumor development. To test this possibility in vivo, we characterized an active site mutant allele of human Pol ε that exhibits a strong mutator phenotype in vitro when the proofreading exonuclease activity of the enzyme is inactive. This mutant has a strong bias toward mispairs opposite template pyrimidine bases, particularly T • dTTP mispairs. Expression of mutant Pol ε in human cells lacking functional mismatch repair caused an increase in mutation rate primarily due to T • dTTP mispairs. Functional mismatch repair eliminated the increased mutagenesis. The results indicate that the mutant Pol ε causes replication errors in vivo, and is at least partially dominant over the endogenous, wild type Pol ε. Since tumors from familial and somatic colorectal patients arise with Pol ε mutations in a single allele, are microsatellite stable and have a large increase in base pair substitutions, our data are consistent with a Pol ε mutation requiring additional factors to promote tumor development.

Role of DNA damage response pathways in preventing carcinogenesis caused by intrinsic replication stress

Defective DNA replication can result in genomic instability, cancer, and developmental defects. To understand the roles of DNA damage response (DDR) genes on carcinogenesis in mutants defective for core DNA replication components, we utilized the Mcm4^{Chaos3/Chaos3} (“Chaos3”) mouse model which, by virtue of an amino acid alteration in MCM4 that destabilizes the MCM2-7 DNA replicative helicase, has fewer dormant replication origins and an increased number of stalled replication forks. This leads to genomic instability and cancer in most Chaos3 mice. We found that animals doubly mutant for Chaos3 and components of the ATM double strand break response pathway (Atm, p21/Cdkn1a, Chk2/Chek2) had decreased tumor latency and/or increased tumor susceptibility. Tumor latency and susceptibility differed between genetic backgrounds and genders, with females demonstrating an overall greater cancer susceptibility to Atm and p21 deficiency than males. ATM deficiency was semilethal in the Chaos3 background and impaired embryonic fibroblast proliferation, suggesting that ATM drug inhibitors might be useful against tumors with DNA replication defects. Hypomorphism for the 9-1-1 component Hus1 did not affect tumor latency or susceptibility in Chaos3 animals, and tumors in these mice did not exhibit impaired ATR pathway signaling. These and other data indicate that under conditions of systemic replication stress, the ATM pathway is particularly important both for cancer suppression and viability during development.

Alteration in the cavity size adjacent to the active site of RB69 DNA polymerase changes its conformational dynamics

Internal cavities are a common feature of many proteins, often having profound effects on the dynamics of their interactions with substrate and binding partners. RB69 DNA polymerase (pol) has a hydrophobic cavity right below the nucleotide binding pocket at the tip of highly conserved L415 side chain. Replacement of this residue with Gly or Met in other B family pols resulted in higher mutation rates. When similar substitutions for L415 were introduced into RB69pol, only L415A and L415G had dramatic effects on pre-steady-state kinetic parameters, reducing base selectivity by several hundred fold. On the other hand, the L415M variant behaved like the wild-type. Using a novel tC^o-tC_nitro Förster Resonance Energy Transfer (FRET) assay, we were able to show that the partition of the primer terminus between pol and exonuclease (exo) domains was compromised with the L415A and L415G mutants, but not with the L415M variant. These results could be rationalized by changes in their structures as determined by high resolution X-ray crystallography.

DNA polymerase ε and δ exonuclease domain mutations in endometrial cancer

Accurate duplication of DNA prior to cell division is essential to suppress mutagenesis and tumour development. The high fidelity of eukaryotic DNA replication is due to a combination of accurate incorporation of nucleotides into the nascent DNA strand by DNA polymerases, the recognition and removal of mispaired nucleotides (proofreading) by the exonuclease activity of DNA polymerases δ and ε, and post-replication surveillance and repair of newly synthesized DNA by the mismatch repair (MMR) apparatus. While the contribution of defective MMR to neoplasia is well recognized, evidence that faulty DNA polymerase activity is important in cancer development has been limited. We have recently shown that germline POLE and POLD1 exonuclease domain mutations (EDMs) predispose to colorectal cancer (CRC) and, in the latter case, to endometrial cancer (EC). Somatic POLE mutations also occur in 5-10% of sporadic CRCs and underlie a hypermutator, microsatellite-stable molecular phenotype. We hypothesized that sporadic ECs might also acquire somatic POLE and/or POLD1 mutations. Here, we have found that missense POLE EDMs with good evidence of pathogenic effects are present in 7% of a set of 173 endometrial cancers, although POLD1 EDMs are uncommon. The POLE mutations localized to highly conserved residues and were strongly predicted to affect proofreading. Consistent with this, POLE-mutant tumours were hypermutated, with a high frequency of base substitutions, and an especially large relative excess of G:C>T:A transversions. All POLE EDM tumours were microsatellite stable, suggesting that defects in either DNA proofreading or MMR provide alternative mechanisms to achieve genomic instability and tumourigenesis.

Germline mutations affecting the proofreading domains of POLE and POLD1 predispose to colorectal adenomas and carcinomas

Many individuals with multiple or large colorectal adenomas or early-onset colorectal cancer (CRC) have no detectable germline mutations in the known cancer predisposition genes. Using whole-genome sequencing, supplemented by linkage and association analysis, we identified specific heterozygous POLE or POLD1 germline variants in several multiple-adenoma and/or CRC cases but in no controls. The variants associated with susceptibility, POLE p.Leu424Val and POLD1 p.Ser478Asn, have high penetrance, and POLD1 mutation was also associated with endometrial cancer predisposition. The mutations map to equivalent sites in the proofreading (exonuclease) domain of DNA polymerases ɛ and δ and are predicted to cause a defect in the correction of mispaired bases inserted during DNA replication. In agreement with this prediction, the tumors from mutation carriers were microsatellite stable but tended to acquire base substitution mutations, as confirmed by yeast functional assays. Further analysis of published data showed that the recently described group of hypermutant, microsatellite-stable CRCs is likely to be caused by somatic POLE mutations affecting the exonuclease domain.

DNA mismatch synthesis complexes provide insights into base selectivity of a B family DNA polymerase

Current hypotheses that attempt to rationalize the high degree of base selectivity exhibited by replicative DNA polymerases (pols) concur that ternary complexes formed with incorrect dNTPs are destabilized. Knowing what accounts for this destabilization is likely to be the key to understanding base discrimination. To address this issue, we have determined crystal structures of ternary complexes with all 12 mismatches using an engineered RB69 pol quadruple mutant (qm, L415A/L561A/S565G/Y567A) that enabled us to capture nascent mispaired dNTPs. These structures show that mismatches in the nascent base-pair binding pocket (NBP) of the qm pol differ markedly from mismatches embedded in binary pol-DNA complexes. Surprisingly, only 3 of 12 mismatches clash with the NBP when they are modeled into the wild-type (wt) pol. The remaining can fit into a wt pol ternary complex but deviate from normal Watson-Crick base-pairs. Repositioning of the templating nucleotide residue and the enlarged NBP in qm ternary complex play important roles in accommodating incorrect incoming dNTPs. From these structures, we propose additional reasons as to why incorrect dNTPs are incorporated so inefficiently by wt RB69 pol: (i) steric clashes with side chains in the NBP after Fingers closing; (ii) weak interactions or large gaps between the incoming dNTP and the templating base; and (iii) burying a protonated base in the hydrophobic environment of the NBP. All of these possibilities would be expected to destabilize the closed ternary complex so that incorporation of incorrect dNTP would be a rare event.

The role of DNA exonucleases in protecting genome stability and their impact on ageing

Exonucleases are key enzymes involved in many aspects of cellular metabolism and maintenance and are essential to genome stability, acting to cleave DNA from free ends. Exonucleases can act as proof-readers during DNA polymerisation in DNA replication, to remove unusual DNA structures that arise from problems with DNA replication fork progression, and they can be directly involved in repairing damaged DNA. Several exonucleases have been recently discovered, with potentially critical roles in genome stability and ageing. Here we discuss how both intrinsic and extrinsic exonuclease activities contribute to the fidelity of DNA polymerases in DNA replication. The action of exonucleases in processing DNA intermediates during normal and aberrant DNA replication is then assessed, as is the importance of exonucleases in repair of double-strand breaks and interstrand crosslinks. Finally we examine how exonucleases are involved in maintenance of mitochondrial genome stability. Throughout the review, we assess how nuclease mutation or loss predisposes to a range of clinical diseases and particularly ageing.

DNA polymerase delta in DNA replication and genome maintenance

The eukaryotic genome is in a constant state of modification and repair. Faithful transmission of the genomic information from parent to daughter cells depends upon an extensive system of surveillance, signaling, and DNA repair, as well as accurate synthesis of DNA during replication. Often, replicative synthesis occurs over regions of DNA that have not yet been repaired, presenting further challenges to genomic stability. DNA polymerase δ (pol δ) occupies a central role in all of these processes: catalyzing the accurate replication of a majority of the genome, participating in several DNA repair synthetic pathways, and contributing structurally to the accurate bypass of problematic lesions during translesion synthesis. The concerted actions of pol δ on the lagging strand, pol ϵ on the leading strand, associated replicative factors, and the mismatch repair (MMR) proteins results in a mutation rate of less than one misincorporation per genome per replication cycle. This low mutation rate provides a high level of protection against genetic defects during development and may prevent the initiation of malignancies in somatic cells. This review explores the role of pol δ in replication fidelity and genome maintenance.

Mouse models of DNA polymerases

In 1956, Arthur Kornberg discovered the mechanism of the biological synthesis of DNA and was awarded the Nobel Prize in Physiology or Medicine in 1959 for this contribution, which included the isolation and characterization of Escherichia coli DNA polymerase I. Now there are 15 known DNA polymerases in mammalian cells that belong to four different families. These DNA polymerases function in many different cellular processes including DNA replication, DNA repair, and damage tolerance. Several biochemical and cell biological studies have provoked a further investigation of DNA polymerase function using mouse models in which polymerase genes have been altered using gene-targeting techniques. The phenotypes of mice harboring mutant alleles reveal the prominent role of DNA polymerases in embryogenesis, prevention of premature aging, and cancer suppression.