Identification of the first small-molecule inhibitor of the REV7 DNA repair protein interaction

Antiviral peptides inhibiting the main protease of SARS-CoV-2 investigated by computational screening and in vitro protease assay

The main protease (Mpro) of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) plays an important role in viral replication and transcription and received great attention as a vital target for drug/peptide development. Therapeutic agents such as small-molecule drugs or peptides that interact with the Cys-His present in the catalytic site of Mpro are an efficient way to inhibit the protease. Although several emergency-approved vaccines showed good efficacy and drastically dropped the infection rate, evolving variants are still infecting and killing millions of people globally. While a small-molecule drug (Paxlovid) received emergency approval, small-molecule drugs have low target specificity and higher toxicity. Besides small-molecule drugs, peptide therapeutics are thus gaining increasing popularity as they are easy to synthesize and highly selective and have limited side effects. In this study, we investigated the therapeutic value of 67 peptides targeting Mpro using molecular docking. Subsequently, molecular dynamics (MD) simulations were implemented on eight protein-peptide complexes to obtain molecular-level information on the interaction between these peptides and the Mpro active site, which revealed that temporin L, indolicidin, and lymphocytic choriomeningitis virus (LCMV) GP1 are the best candidates in terms of stability, interaction, and structural compactness. These peptides were synthesized using the solid-phase peptide synthesis protocol, purified by reversed-phase high-performance liquid chromatography (RP-HPLC), and authenticated by mass spectrometry (MS). The in vitro fluorometric Mpro activity assay was used to validate the computational results, where temporin L and indolicidin were observed to be very active against SARS-CoV-2 Mpro with IC50 values of 38.80 and 87.23 μM, respectively. A liquid chromatography-MS (LC-MS) assay was developed, and the IC50 value of temporin L was measured at 23.8 μM. The solution-state nuclear magnetic resonance (NMR) structure of temporin L was determined in the absence of sodium dodecyl sulfate (SDS) micelles and was compared to previous temporin structures. This combined investigation provides critical insights and assists us to further develop peptide inhibitors of SARS-CoV-2 Mpro through structural guided investigation.

REV7 is involved in outcomes of platinum-based chemotherapy in pancreatic cancer by controlling the DNA damage response

REV7 is a multifunctional protein implicated in various biological processes, including DNA damage response. REV7 expression in human cancer cells affects their sensitivity to DNA-damaging agents. In the present study, we investigated the significance of REV7 in pancreatic ductal adenocarcinoma (PDAC). REV7 expression was immunohistochemically examined in 92 resected PDAC specimens and 60 endoscopic ultrasound-guided fine-needle aspiration biopsy (EUS-FNAB) specimens of unresectable PDAC treated with platinum-based chemotherapy, and its association with clinicopathologic features was analyzed. Although REV7 expression was not significantly associated with the progression of primary tumors (T-factor and Stage) in either resected or unresectable PDAC, decreased levels of REV7 expression in EUS-FNAB specimens of unresectable PDAC were significantly associated with better outcomes of platinum-based chemotherapy and a favorable prognosis. REV7-deficient PDAC cell lines showed suppressed cell growth and enhanced sensitivity to cisplatin in vitro. Tumor-bearing mice generated using REV7-deficient PDAC cell lines also showed enhanced sensitivity to cisplatin in vivo. RNA sequencing analysis using WT and REV7-deficient PDAC cell lines revealed that REV7 inactivation promoted the downregulation of genes involved in the DNA repair and the upregulation of genes involved in apoptosis. Our results indicate that decreased expression of REV7 is associated with better outcomes of platinum-based chemotherapy in PDAC by suppressing the DNA damage response. It is also suggested that REV7 is a useful biomarker for predicting the outcome of platinum-based chemotherapy and the prognosis of unresectable PDAC and is a potential target for PDAC treatment.

Peptiligase, an enzyme for efficient chemo-enzymatic synthesis of aviptadil

Increasing attention to peptides as prospective therapeutics has created a renaissance in searching for new alternatives to the current peptide synthetic approaches as well as their modification. In this context, it is necessary to develop different approaches for peptide ligation. Using enzymes as a novel strategy and powerful tool for the peptide and protein ligation has recently received a lot of attention. We here designed a fully convergent chemo-enzymatic peptide synthesis (CEPS) process for the synthesis of aviptadil a 28-mer therapeutic peptide with potential therapeutic effects in various medical contexts specially in the treatment of acute respiratory distress syndrome (ARDS) by coupling two peptide segments with four different peptiligase variants in aqueous environments. Our study reveals that peptiligase variants are capable of ligation reaction in 15 min. The overall time of ligation is shorter than those peptides with similar lengths and hinderance to aviptadil which reported for conventional synthesis by full solid-phase peptide synthesis. Yields ranged from 54 % to 76 %.

REV7 in Cancer Biology and Management

DNA repair and cell cycle regulation are potential biological fields to develop molecular targeting therapies for cancer. Human REV7 was originally discovered as a homologous molecule to yeast Rev7, which is involved in DNA damage response and mutagenesis, and as the second homolog of yeast Mad2, involved in the spindle assembly checkpoint. Although REV7 principally functions in the fields of DNA repair and cell cycle regulation, many binding partners of REV7 have been identified using comprehensive analyses in the past decade, and the significance of REV7 is expanding in various other biological fields, such as gene transcription, epigenetics, primordial germ cell survival, neurogenesis, intracellular signaling, and microbial infection. In addition, the clinical significance of REV7 has been demonstrated in studies using human cancer tissues, and investigations in cancer cell lines and animal models have revealed the greater impacts of REV7 in cancer biology, which makes it an attractive target molecule for cancer management. This review focuses on the functions of REV7 in human cancer and discusses the utility of REV7 for cancer management with a summary of the recent development of inhibitors targeting REV7.

Roles of trans-lesion synthesis (TLS) DNA polymerases in tumorigenesis and cancer therapy

DNA damage tolerance and mutagenesis are hallmarks and enabling characteristics of neoplastic cells that drive tumorigenesis and allow cancer cells to resist therapy. The 'Y-family' trans-lesion synthesis (TLS) DNA polymerases enable cells to replicate damaged genomes, thereby conferring DNA damage tolerance. Moreover, Y-family DNA polymerases are inherently error-prone and cause mutations. Therefore, TLS DNA polymerases are potential mediators of important tumorigenic phenotypes. The skin cancer-propensity syndrome xeroderma pigmentosum-variant (XPV) results from defects in the Y-family DNA Polymerase Pol eta (Polη) and compensatory deployment of alternative inappropriate DNA polymerases. However, the extent to which dysregulated TLS contributes to the underlying etiology of other human cancers is unclear. Here we consider the broad impact of TLS polymerases on tumorigenesis and cancer therapy. We survey the ways in which TLS DNA polymerases are pathologically altered in cancer. We summarize evidence that TLS polymerases shape cancer genomes, and review studies implicating dysregulated TLS as a driver of carcinogenesis. Because many cancer treatment regimens comprise DNA-damaging agents, pharmacological inhibition of TLS is an attractive strategy for sensitizing tumors to genotoxic therapies. Therefore, we discuss the pharmacological tractability of the TLS pathway and summarize recent progress on development of TLS inhibitors for therapeutic purposes.

N-(4-acetamidophenyl)-5-acetylfuran-2-carboxamide as a novel orally available diuretic that targets urea transporters with improved PD and PK properties

Urea transporters (UTs) have been identified as new targets for diuretics. Functional deletion of UTs led to urea-selective urinary concentrating defects with relative salt sparing. In our previous study, a UT inhibitor with a diarylamide scaffold, which is denoted as 11a, was demonstrated as the first orally available UT inhibitor. However, the oral bioavailability of 11a was only 4.38%, which obstructed its clinical application. In this work, by replacing the nitro group of 11a with an acetyl group, 25a was obtained. Compared with 11a, 25a showed a 10 times stronger inhibitory effect on UT-B (0.14 μM vs. 1.41 μM in rats, and 0.48 μM vs. 5.82 μM in mice) and a much higher inhibition rate on UT-A1. Moreover, the metabolic stability both in vitro and in vivo and the drug-like properties (permeability and solubility) of 25a were obviously improved compared with those of 11a. Moreover, the bioavailability of 25a was 15.18%, which was 3 times higher than that of 11a, thereby resulting in significant enhancement of the diuretic activities in rats and mice. 25a showed excellent potential for development as a promising clinical diuretic candidate for targeting UTs to treat diseases that require long-term usage of diuretics, such as hyponatremia.

Design, synthesis, and evaluation of the self-assembled antimicrobial peptides based on the ovalbumin-derived peptide TK913

The preparation, self-assembly, and antimicrobial activity of peptides based on TK913 is described. TK9Z4 incorporating a Pro-Pro motif exhibited self-assembly but no cytotoxicity. However, peptide TKZ3 (obtained by changing the amino acid sequence of TK9Z4) showed morphological changes at different concentrations, potent antimicrobial activity, low cytotoxicity, and trypsin resistance. Accordingly, TKZ3 is proposed as new AMP derived from ovalbumin-derived peptides.

Protein-Protein Interactions in Translesion Synthesis

Translesion synthesis (TLS) is an error-prone DNA damage tolerance mechanism used by actively replicating cells to copy past DNA lesions and extend the primer strand. TLS ensures that cells continue replication in the presence of damaged DNA bases, albeit at the expense of an increased mutation rate. Recent studies have demonstrated a clear role for TLS in rescuing cancer cells treated with first-line genotoxic agents by allowing them to replicate and survive in the presence of chemotherapy-induced DNA lesions. The importance of TLS in both the initial response to chemotherapy and the long-term development of acquired resistance has allowed it to emerge as an interesting target for small molecule drug discovery. Proper TLS function is a complicated process involving a heteroprotein complex that mediates multiple attachment and switching steps through several protein–protein interactions (PPIs). In this review, we briefly describe the importance of TLS in cancer and provide an in-depth analysis of key TLS PPIs, focusing on key structural features at the PPI interface while also exploring the potential druggability of each key PPI.

Targeting protein-protein interactions in the DNA damage response pathways for cancer chemotherapy

Cellular DNA damage response (DDR) is an extensive signaling network that orchestrates DNA damage recognition, repair and avoidance, cell cycle progression and cell death. DDR alteration is a hallmark of cancer, with the deficiency in one DDR capability often compensated by a dependency on alternative pathways endowing cancer cells with survival and growth advantage. Targeting these DDR pathways has provided multiple opportunities for the development of cancer therapies. Traditional drug discovery has mainly focused on catalytic inhibitors that block enzyme active sites, which limits the number of potential drug targets within the DDR pathways. This review article describes the emerging approach to the development of cancer therapeutics targeting essential protein-protein interactions (PPIs) in the DDR network. The overall strategy for the structure-based design of small molecule PPI inhibitors is discussed, followed by an overview of the major DNA damage sensing, DNA repair, and DNA damage tolerance pathways with a specific focus on PPI targets for anti-cancer drug design. The existing small molecule inhibitors of DDR PPIs are summarized that selectively kill cancer cells and/or sensitize cancers to front-line genotoxic therapies, and a range of new PPI targets are proposed that may lead to the development of novel chemotherapeutics.

REV7: Jack of many trades

The HORMA domain protein REV7, also known as MAD2L2, interacts with a variety of proteins and thereby contributes to the establishment of different complexes. With doing so, REV7 impacts a diverse range of cellular processes and gained increasing interest as more of its activities became uncovered. REV7 has important roles in translesion synthesis and mitotic progression, and acts as a central component in the recently discovered shieldin complex that operates in DNA double-strand break repair. Here we discuss the roles of REV7 in its various complexes, focusing on its activity in genome integrity maintenance. Moreover, we will describe current insights on REV7 structural features that allow it to be such a versatile protein.

Mitotic syndicates Aurora Kinase B (AURKB) and mitotic arrest deficient 2 like 2 (MAD2L2) in cohorts of DNA damage response (DDR) and tumorigenesis

Aurora Kinase B (AURKB) and Mitotic Arrest Deficient 2 Like 2 (MAD2L2) are emerging anticancer therapeutic targets. AURKB and MAD2L2 are the least well studied members of their protein families, compared to AURKA and MAD2L1. Both AURKB and MAD2L2 play a critical role in mitosis, cell cycle checkpoint, DNA damage response (DDR) and normal physiological processes. However, the oncogenic roles of AURKB and MAD2L2 in tumorigenesis and genomic instability have also been reported. DDR acts as an arbitrator for cell fate by either repairing the damage or directing the cell to self-destruction. While there is strong evidence of interphase DDR, evidence of mitotic DDR is just emerging and remains largely unelucidated. To date, inhibitors of the DDR components show effective anti-cancer roles. Contrarily, long-term resistance towards drugs that target only one DDR target is becoming a challenge. Targeting interactions between protein-protein or protein-DNA holds prominent therapeutic potential. Both AURKB and MAD2L2 play critical roles in the success of mitosis and their emerging roles in mitotic DDR cannot be ignored. Small molecule inhibitors for AURKB are in clinical trials. A few lead compounds towards MAD2L2 inhibition have been discovered. Targeting mitotic DDR components and their interaction is emerging as a potent next generation anti-cancer therapeutic target. This can be done by developing small molecule inhibitors for AURKB and MAD2L2, thereby targeting DDR components as anti-cancer therapeutic targets and/or targeting mitotic DDR. This review focuses on AURKB and MAD2L2 prospective synergy to deregulate the p53 DDR pathway and promote favourable conditions for uncontrolled cell proliferation.

Translesion DNA Synthesis and Reinitiation of DNA Synthesis in Chemotherapy Resistance

Many chemotherapy drugs block tumor cell division by damaging DNA. DNA polymerases eta (Pol η), iota (Pol ι), kappa (Pol κ), REV1 of the Y-family and zeta (Pol ζ) of the B-family efficiently incorporate nucleotides opposite a number of DNA lesions during translesion DNA synthesis. Primase-polymerase PrimPol and the Pol α-primase complex reinitiate DNA synthesis downstream of the damaged sites using their DNA primase activity. These enzymes can decrease the efficacy of chemotherapy drugs, contribute to the survival of tumor cells and to the progression of malignant diseases. DNA polymerases are promising targets for increasing the effectiveness of chemotherapy, and mutations and polymorphisms in some DNA polymerases can serve as additional prognostic markers in a number of oncological disorders.

Translesion synthesis inhibitors as a new class of cancer chemotherapeutics

Introduction: Translesion synthesis (TLS) is a DNA damage tolerance mechanism that replaces the replicative DNA polymerase with a specialized, low-fidelity TLS DNA polymerase that can copy past DNA lesions during active replication. Recent studies have demonstrated a primary role for TLS in replicating past DNA lesions induced by first-line genotoxic agents, resulting in decreased efficacy and acquired chemoresistance. With this in mind, targeting TLS as a combination strategy with first-line genotoxic agents has emerged as a promising approach to develop a new class of anti-cancer adjuvant agents. Areas covered: In this review, we provide a brief background on TLS and its role in cancer. We also discuss the identification and development of inhibitors that target various TLS DNA polymerases or key protein-protein interactions (PPIs) in the TLS machinery. Expert opinion: TLS inhibitors have demonstrated initial promise; however, their continued study is essential to more fully understand the clinical potential of this emerging class of anti-cancer chemotherapeutics. It will be important to determine whether a specific protein involved in TLS is an optimal target. In addition, an expanded understanding of what current genotoxic chemotherapies synergize with TLS inhibitors will guide the clinical strategies for devising combination therapies.

Beyond Kinases: Targeting Replication Stress Proteins in Cancer Therapy

DNA replication stress describes a state of impaired replication fork progress that triggers a cellular stress response to maintain genome stability and complete DNA synthesis. Replication stress is a common state that must be tolerated in many cancers. One promising therapeutic approach is targeting replication stress response factors such as the ataxia telangiectasia and rad 3-related kinase (ATR) or checkpoint kinase 1 (CHK1) kinases that some cancers depend upon to survive endogenous replication stress. However, research revealing the complexity of the replication stress response suggests new genetic interactions and candidate therapeutic targets. Many of these candidates regulate DNA transactions around reversed replication forks, including helicases, nucleases and alternative polymerases that promote fork stability and restart. Here we review emerging strategies to exploit replication stress for cancer therapy.

Structure and mechanism of B-family DNA polymerase ζ specialized for translesion DNA synthesis

DNA polymerase ζ (Polζ) belongs to the same B-family as high-fidelity replicative polymerases, yet is specialized for the extension reaction in translesion DNA synthesis (TLS). Despite its importance in TLS, the structure of Polζ is unknown. We present cryo-EM structures of the Saccharomyces cerevisiae Polζ holoenzyme in the act of DNA synthesis (3.1 Å) and without DNA (4.1 Å). Polζ displays a pentameric ring-like architecture, with catalytic Rev3, accessory Pol31' Pol32 and two Rev7 subunits forming an uninterrupted daisy chain of protein-protein interactions. We also uncover the features that impose high fidelity during the nucleotide-incorporation step and those that accommodate mismatches and lesions during the extension reaction. Collectively, we decrypt the molecular underpinnings of Polζ's role in TLS and provide a framework for new cancer therapeutics.

Inactivation of REV7 enhances chemosensitivity and overcomes acquired chemoresistance in testicular germ cell tumors

REV7 is a multitasking protein involved in replication past DNA lesions, cell cycle regulation, and gene expression. REV7 is highly expressed in the adult testis and plays an essential role in primordial germ cell maintenance in mice. In this study, we analyzed whether REV7 can be a molecular target for the treatment of testicular germ cell tumors (TGCTs), in which acquired chemoresistance is a major cause of treatment failure. Strong expression of REV7 was detected in human TGCT tissues by immunohistochemistry. REV7 depletion in the TGCT cell lines suppressed cell proliferation and increased sensitivity to cisplatin and doxorubicin. cDNA microarray analysis revealed that REV7 depletion downregulated genes in the DNA repair gene set and upregulated genes in the apoptosis gene set. REV7 depletion-provoked chemosensitivity was associated with DNA double-strand break accumulation and apoptosis activation. In addition, inactivation of REV7 in cisplatin-resistant TGCT cells recovered chemosensitivity at almost equal levels as parental cells in vitro and in vivo. Our results indicate that inactivation of REV7 enhances chemosensitivity and overcomes chemoresistance in TGCT cells, suggesting REV7 as a potential therapeutic target in chemoresistant TGCTs.

CDK-Independent and PCNA-Dependent Functions of p21 in DNA Replication

p21^Waf/CIP1 is a small unstructured protein that binds and inactivates cyclin-dependent kinases (CDKs). To this end, p21 levels increase following the activation of the p53 tumor suppressor. CDK inhibition by p21 triggers cell-cycle arrest in the G1 and G2 phases of the cell cycle. In the absence of exogenous insults causing replication stress, only residual p21 levels are prevalent that are insufficient to inhibit CDKs. However, research from different laboratories has demonstrated that these residual p21 levels in the S phase control DNA replication speed and origin firing to preserve genomic stability. Such an S-phase function of p21 depends fully on its ability to displace partners from chromatin-bound proliferating cell nuclear antigen (PCNA). Vice versa, PCNA also regulates p21 by preventing its upregulation in the S phase, even in the context of robust p21 induction by irradiation. Such a tight regulation of p21 in the S phase unveils the potential that CDK-independent functions of p21 may have for the improvement of cancer treatments.

Replication Fork Remodeling and Therapy Escape in DNA Damage Response-Deficient Cancers

Most cancers have lost a critical DNA damage response (DDR) pathway during tumor evolution. These alterations provide a useful explanation for the initial sensitivity of tumors to DNA-targeting chemotherapy. A striking example is dysfunctional homology-directed repair (HDR), e.g., due to inactivating mutations in BRCA1 and BRCA2 genes. Extensive efforts are being made to develop novel targeted therapies exploiting such an HDR defect. Inhibitors of poly(ADP-ribose) polymerase (PARP) are an instructive example of this approach. Despite the success of PARP inhibitors, the presence of primary or acquired therapy resistance remains a major challenge in clinical oncology. To move the field of precision medicine forward, we need to understand the precise mechanisms causing therapy resistance. Using preclinical models, various mechanisms underlying chemotherapy resistance have been identified. Restoration of HDR seems to be a prevalent mechanism but this does not explain resistance in all cases. Interestingly, some factors involved in DNA damage response (DDR) have independent functions in replication fork (RF) biology and their loss causes RF instability and therapy sensitivity. However, in BRCA-deficient tumors, loss of these factors leads to restored stability of RFs and acquired drug resistance. In this review we discuss the recent advances in the field of RF biology and its potential implications for chemotherapy response in DDR-defective cancers. Additionally, we review the role of DNA damage tolerance (DDT) pathways in maintenance of genome integrity and their alterations in cancer. Furthermore, we refer to novel tools that, combined with a better understanding of drug resistance mechanisms, may constitute a great advance in personalized diagnosis and therapeutic strategies for patients with HDR-deficient tumors.

Fanconi anemia pathway as a prospective target for cancer intervention

Fanconi anemia (FA) is a recessive genetic disorder caused by biallelic mutations in at least one of 22 FA genes. Beyond its pathological presentation of bone marrow failure and congenital abnormalities, FA is associated with chromosomal abnormality and genomic instability, and thus represents a genetic vulnerability for cancer predisposition. The cancer relevance of the FA pathway is further established with the pervasive occurrence of FA gene alterations in somatic cancers and observations of FA pathway activation-associated chemotherapy resistance. In this article we describe the role of the FA pathway in canonical interstrand crosslink (ICL) repair and possible contributions of FA gene alterations to cancer development. We also discuss the perspectives and potential of targeting the FA pathway for cancer intervention.

Inhibitors of the Fanconi anaemia pathway as potential antitumour agents for ovarian cancer

The Fanconi anaemia (FA) pathway is an important mechanism for cellular DNA damage repair, which functions to remove toxic DNA interstrand crosslinks. This is particularly relevant in the context of ovarian and other cancers which rely extensively on interstrand cross-link generating platinum chemotherapy as standard of care treatment. These cancers often respond well to initial treatment, but reoccur with resistant disease and upregulation of DNA damage repair pathways. The FA pathway is therefore of great interest as a target for therapies that aim to improve the efficacy of platinum chemotherapies, and reverse tumour resistance to these. In this review, we discuss recent advances in understanding the mechanism of interstrand cross-link repair by the FA pathway, and the potential of the component parts as targets for therapeutic agents. We then focus on the current state of play of inhibitor development, covering both the characterisation of broad spectrum inhibitors and high throughput screening approaches to identify novel small molecule inhibitors. We also consider synthetic lethality between the FA pathway and other DNA damage repair pathways as a therapeutic approach.

[Structural Basis of the Multifunctional Hub Protein and Identification of a Small-molecule Compound for Drug Discovery]

Translesion DNA synthesis (TLS) is an emergency system activated to inhibit cell death caused by DNA damage-induced replication arrest. Thus, TLS enables cancer cells to acquire resistance to alkylate anticancer drugs. REV7 functions as the hub protein that interacts with both the inserter DNA polymerase REV1 and the extender DNA polymerase REV3 in TLS. REV7-mediated protein-protein interactions (PPIs) are essential for the activation of TLS, and are therefore attractive targets for anticancer drug development. To clarify the REV7-REV3 and REV7-REV1 PPIs, we determined the structures of REV7-REV3 and REV7-REV3-REV1 complexes. In the structures of REV7-REV3 and REV7-REV3-REV1 complexes, REV7 wraps around the REV3 fragment, and the REV1-binding interface is distinct from the REV3-binding site of REV7. We also identified a novel REV7 binding protein, transcription factor II-I (TFII-I), which is required for TLS. Of note, TFII-I binds the REV7-REV3-REV1 complex, suggesting that REV7-TFII-I PPIs are independent of other REV7-mediated PPIs. Furthermore, we found a small-molecule compound that inhibits TLS by targeting the REV7-REV3 PPIs. Lastly, we determined the structure of REV7 in complex with chromosome alignment maintaining phosphoprotein (CAMP), a known kinetochore-microtubule attachment protein. The overall structure of the REV7-CAMP complex is similar to that of the REV7-REV3 complex, but the REV7-CAMP PPIs are markedly different from the REV7-REV3 PPIs. These findings improve our understanding of multifunctional hub proteins, and are helpful for designing small-molecule compounds for novel anticancer drug development.

Inhibition of Human DNA Polymerases Eta and Kappa by Indole-Derived Molecules Occurs through Distinct Mechanisms

Overexpression of human DNA polymerase kappa (hpol κ) in glioblastoma is associated with shorter survival time and resistance to the alkylating agent temozolomide (TMZ), making it an attractive target for the development of small-molecule inhibitors. We previously reported on the development and characterization of indole barbituric acid-derived (IBA) inhibitors of translesion DNA synthesis polymerases (TLS pols). We have now identified a potent and selective inhibitor of hpol κ based on the indole-aminoguanidine (IAG) chemical scaffold. The most promising IAG analogue, IAG-10, exhibited greater inhibitory action against hpol κ than any other human Y-family member, as well as pols from the A-, B-, and X-families. Inhibition of hpol κ by IAG analogues appears to proceed through a mechanism that is distinct from inhibition of hpol η based on changes in DNA binding affinity and nucleotide insertion kinetics. By way of comparison, both IAG and IBA analogues inhibited binary complex formation by hpol κ and ternary complex formation by hpol η. Decreasing the concentration of enzyme and DNA in the reaction mixture lowered the IC₅₀ value of IAG-10 to submicromolar values, consistent with inhibition of binary complex formation for hpol κ. Chemical footprinting experiments revealed that IAG-10 binds to a cleft between the finger, little finger, and N-clasp domains on hpol κ and that this likely disrupts the interaction between the N-clasp and the TLS pol core. In cell culture, IAG-10 potentiated the antiproliferative activity and DNA damaging effects of TMZ in hpol κ-proficient cells but not in hpol κ-deficient cells, indicative of a target-dependent effect. Mutagenic replication across alkylation damage increased in hpol κ-proficient cells treated with IAG-10, while no change in mutation frequency was observed for hpol κ-deficient cells. In summary, we developed a potent and selective small-molecule inhibitor of hpol κ that takes advantage of structural features unique to this TLS enzyme to potentiate TMZ, a standard-of-care drug used in the treatment of malignant brain tumors. Furthermore, the IAG scaffold represents a new chemical space for the exploration of TLS pol inhibitors, which could prove useful as a strategy for improving patient response to genotoxic drugs.

A Small Molecule Targeting Mutagenic Translesion Synthesis Improves Chemotherapy

Intrinsic and acquired drug resistance and induction of secondary malignancies limit successful chemotherapy. Because mutagenic translesion synthesis (TLS) contributes to chemoresistance as well as treatment-induced mutations, targeting TLS is an attractive avenue for improving chemotherapeutics. However, development of small molecules with high specificity and in vivo efficacy for mutagenic TLS has been challenging. Here, we report the discovery of a small-molecule inhibitor, JH-RE-06, that disrupts mutagenic TLS by preventing recruitment of mutagenic POL ζ. Remarkably, JH-RE-06 targets a nearly featureless surface of REV1 that interacts with the REV7 subunit of POL ζ. Binding of JH-RE-06 induces REV1 dimerization, which blocks the REV1-REV7 interaction and POL ζ recruitment. JH-RE-06 inhibits mutagenic TLS and enhances cisplatin-induced toxicity in cultured human and mouse cell lines. Co-administration of JH-RE-06 with cisplatin suppresses the growth of xenograft human melanomas in mice, establishing a framework for developing TLS inhibitors as a novel class of chemotherapy adjuvants.

Post-replication repair: Rad5/HLTF regulation, activity on undamaged templates, and relationship to cancer

The eukaryotic post-replication repair (PRR) pathway allows completion of DNA replication when replication forks encounter lesions on the DNA template and are mediated by post-translational ubiquitination of the DNA sliding clamp proliferating cell nuclear antigen (PCNA). Monoubiquitinated PCNA recruits translesion synthesis (TLS) polymerases to replicate past DNA lesions in an error-prone manner while addition of K63-linked polyubiquitin chains signals for error-free template switching to the sister chromatid. Central to both branches is the E3 ubiquitin ligase and DNA helicase Rad5/helicase-like transcription factor (HLTF). Mutations in PRR pathway components lead to genomic rearrangements, cancer predisposition, and cancer progression. Recent studies have challenged the notion that the PRR pathway is involved only in DNA lesion tolerance and have shed new light on its roles in cancer progression. Molecular details of Rad5/HLTF recruitment and function at replication forks have emerged. Mounting evidence indicates that PRR is required during lesion-less replication stress, leading to TLS polymerase activity on undamaged templates. Analysis of PRR mutation status in human cancers and PRR function in cancer models indicates that down regulation of PRR activity is a viable strategy to inhibit cancer cell growth and reduce chemoresistance. Here, we review these findings, discuss how they change our views of current PRR models, and look forward to targeting the PRR pathway in the clinic.

One-Step Synthesis of Thieno[2,3-d]pyrimidin-4(3H)-ones via a Catalytic Four-Component Reaction of Ketones, Ethyl Cyanoacetate, S8 and Formamide

Thieno[2,3-d]pyrimidin-4(3H)-ones are important pharmacophores that previously required a three step synthesis with two chromatography steps. We herein report a green approach to the synthesis of this pharmacologically important class of compounds via a catalytic four-component reaction using a ketone, ethyl cyanoacetate, S₈ and formamide. The reported reaction is characterized by step economy, reduced catalyst loading and easy purification.

Small-molecules that bind to the ubiquitin-binding motif of REV1 inhibit REV1 interaction with K164-monoubiquitinated PCNA and suppress DNA damage tolerance

REV1 protein is a mutagenic DNA damage tolerance (DDT) mediator and encodes two ubiquitin-binding motifs (i.e., UBM1 and UBM2) that are essential for the DDT function. REV1 interacts with K164-monoubiquitinated PCNA (UbPCNA) in cells upon DNA-damaging stress. By using AlphaScreen assays to detect inhibition of REV1 and UbPCNA protein interactions along with an NMR-based strategy, we identified small-molecule compounds that inhibit the REV1/UbPCNA interaction and that directly bind to REV1 UBM2. In cells, one of the compound prevented recruitment of REV1 to PCNA foci on chromatin upon cisplatin treatment, delayed removal of UV-induced cyclopyrimidine dimers from nuclei, prevented UV-induced mutation of HPRT gene, and diminished clonogenic survival of cells that were challenged by cyclophosphamide or cisplatin. This study demonstrates the potential utility of a small-molecule REV1 UBM2 inhibitor for preventing DDT.

Translesion DNA Synthesis in Cancer: Molecular Mechanisms and Therapeutic Opportunities

The genomic landscape of cancer is one marred by instability, but the mechanisms that underlie these alterations are multifaceted and remain a topic of intense research. Cellular responses to DNA damage and/or replication stress can affect genome stability in tumors and influence the response of patients to therapy. In addition to direct repair, DNA damage tolerance (DDT) is an element of genomic maintenance programs that contributes to the etiology of several types of cancer. DDT mechanisms primarily act to resolve replication stress, and this can influence the effectiveness of genotoxic drugs. Translesion DNA synthesis (TLS) is an important component of DDT that facilitates direct bypass of DNA adducts and other barriers to replication. The central role of TLS in the bypass of drug-induced DNA lesions, the promotion of tumor heterogeneity, and the involvement of these enzymes in the maintenance of the cancer stem cell niche presents an opportunity to leverage inhibition of TLS as a way of improving existing therapies. In the review that follows, we summarize mechanisms of DDT, misregulation of TLS in cancer, and discuss the potential for targeting these pathways as a means of improving cancer therapies.

Potential Strategies to Target Protein-Protein Interactions in the DNA Damage Response and Repair Pathways

This review article discusses some insights about generating novel mechanistic inhibitors of the DNA damage response and repair (DDR) pathways by focusing on protein-protein interactions (PPIs) of the key DDR components. General requirements for PPI strategies, such as selecting the target PPI site on the basis of its functionality, are discussed first. Next, on the basis of functional rationale and biochemical feasibility to identify a PPI inhibitor, 26 PPIs in DDR pathways (BER, MMR, NER, NHEJ, HR, TLS, and ICL repair) are specifically discussed for inhibitor discovery to benefit cancer therapies using a DNA-damaging agent.

Targeting DNA repair and replication stress in the treatment of ovarian cancer

Approximately half of high-grade serous epithelial ovarian cancers incur alterations in genes of homologous recombination (BRCA1, BRCA2, RAD51C, Fanconi anemia genes), and the rest incur alterations in other DNA repair pathways at high frequencies. Such cancer-specific gene alterations can confer selective sensitivity to DNA damaging agents such as cisplatin and carboplatin, topotecan, etoposide, doxorubicin, and gemcitabine. Originally presumed to inhibit DNA repair, PARP inhibitors that have recently been approved by the FDA for the treatment of advanced ovarian cancer also act as DNA damaging agents by inducing PARP-DNA complexes. These DNA damaging agents induce different types of DNA lesions that require various DNA repair genes for the repair, but commonly induce replication fork slowing or stalling, also referred to as replication stress. Replication stress activates DNA repair checkpoint proteins (ATR, CHK1), which prevent further DNA damage. Hence, targeting DNA repair genes or DNA repair checkpoint genes augments the anti-tumor activity of DNA damaging agents. This review describes the rational basis for using DNA repair and DNA repair checkpoint inhibitors as single agents. The review also presents the strategies combining these inhibitors with DNA damaging agents for ovarian cancer therapy based on specific gene alterations.

Structural insight into inhibition of REV7 protein interaction revealed by docking, molecular dynamics and MM/PBSA studies

The inhibitors of the REV7/REV3L protein interaction bind to the two pockets of REV7 divided by the ‘safety-belt’ structure, as revealed by computational modeling.