DNA Damage Signalling and Repair Inhibitors: The Long-Sought-After Achilles' Heel of Cancer

Preclinical Evaluation of the ATR Inhibitor BAY 1895344 as a Radiosensitizer for Head and Neck Squamous Cell Carcinoma

PURPOSE

Despite aggressive multimodal treatment that typically includes definitive or adjuvant radiation therapy (RT), locoregional recurrence rates approach 50% for patients with locally advanced human papillomavirus (HPV)-negative head and neck squamous cell carcinoma (HNSCC). Thus, more effective therapeutics are needed to improve patient outcomes. We evaluated the radiosensitizing effects of ataxia telangiectasia and RAD3-related (ATR) inhibitor (ATRi) BAY 1895344 in preclinical models of HNSCC.

METHODS AND MATERIALS

Murine and human HPV-negative HNSCC cells (MOC2, MOC1, JHU-012) were treated with vehicle or ATRi with or without 4 Gy. Checkpoint kinase 1 phosphorylation and DNA damage (γH2AX) were evaluated by Western blot, and ATRi half-maximal inhibitory concentration was determined by MTT assay for HNSCC cells and immortalized murine oral keratinocytes. In vitro radiosensitization was tested by clonogenic assay. Cell cycle distribution and mitotic catastrophe were evaluated by flow cytometry. Mitotic aberrations were quantified by fluorescent microscopy. Tumor growth delay and survival were assessed in mice bearing MOC2 or JHU-012 transplant tumors treated with vehicle, ATRi, RT (10 Gy × 1 or 8 Gy × 3), or combined ATRi + RT.

RESULTS

ATRi caused dose-dependent reduction in checkpoint kinase 1 phosphorylation at 1 hour post-RT (4 Gy) and dose-dependent increase in γH2AX at 18 hours post-RT. Addition of RT to ATRi led to decreased BAY 1895344 half-maximal inhibitory concentration in HNSCC cell lines but not in normal tissue surrogate immortalized murine oral keratinocytes. Clonogenic assays demonstrated radiosensitization in the HNSCC cell lines. ATRi abrogated the RT-induced G2/M checkpoint, leading to mitosis with unrepaired DNA damage and increased mitotic aberrations (multinucleated cells, micronuclei, nuclear buds, nucleoplasmic bridges). ATRi and RT significantly delayed tumor growth in MOC2 and JHU-012 in vivo models, with improved overall survival in the MOC2 model.

CONCLUSIONS

These findings demonstrated that BAY 1895344 increased in vitro and in vivo radiosensitivity in HPV-negative HNSCC preclinical models, suggesting therapeutic potential warranting evaluation in clinical trials for patients with locally advanced or recurrent HPV-negative HNSCC.

Anticancer effects of phytol against Sarcoma (S-180) and Human Leukemic (HL-60) cancer cells

Phytol (Pyt), a diterpenoid, possesses many important bioactivities. This study evaluates the anticancer effects of Pyt on sarcoma 180 (S-180) and human leukemia (HL-60) cell lines. For this purpose, cells were treated with Pyt (4.72, 7.08, or 14.16 μM) and a cell viability assay was performed. Additionally, the alkaline comet assay and micronucleus test with cytokinesis were also performed using doxorubicin (6 μM) and hydrogen peroxide (10 mM) as positive controls and stressors, respectively. Results revealed that Pyt significantly reduced the viability and rate of division in S-180 and HL-60 cells with IC50 values of 18.98 ± 3.79 and 1.17 ± 0.34 μM, respectively. Pyt at 14.16 μM exerted aneugenic and/or clastogenic effects in S-180 and HL-60 cells, where the number of micronuclei and other nuclear abnormalities (e.g., nucleoplasmic bridges and nuclear buds) were frequently observed. Moreover, Pyt at all concentrations induced apoptosis and showed necrosis at 14.16 μM, suggesting its anticancer effects on the tested cancer cell lines. Taken together, Pyt showed promising anticancer effects, possibly through inducing apoptosis and necrosis mechanisms, and it exerted aneugenic and/or clastogenic effects on the S-180 and HL-60 cell lines.

Combination Therapy as a Promising Way to Fight Oral Cancer

Oral cancer is a highly aggressive tumor with invasive properties that can lead to metastasis and high mortality rates. Conventional treatment strategies, such as surgery, chemotherapy, and radiation therapy, alone or in combination, are associated with significant side effects. Currently, combination therapy has become the standard practice for the treatment of locally advanced oral cancer, emerging as an effective approach in improving outcomes. In this review, we present an in-depth analysis of the current advancements in combination therapies for oral cancer. The review explores the current therapeutic options and highlights the limitations of monotherapy approaches. It then focuses on combinatorial approaches that target microtubules, as well as various signaling pathway components implicated in oral cancer progression, namely, DNA repair players, the epidermal growth factor receptor, cyclin-dependent kinases, epigenetic readers, and immune checkpoint proteins. The review discusses the rationale behind combining different agents and examines the preclinical and clinical evidence supporting the effectiveness of these combinations, emphasizing their ability to enhance treatment response and overcome drug resistance. Challenges and limitations associated with combination therapy are discussed, including potential toxicity and the need for personalized treatment approaches. A future perspective is also provided to highlight the existing challenges and possible resolutions toward the clinical translation of current oral cancer therapies.

Anatomical and surgical characteristics correlate with pachymeningeal failure in patients with brain metastases after neurosurgical resection and adjuvant stereotactic radiosurgery

PURPOSE

Neurosurgery (NS) is an essential modality for large brain metastases (BM). Postoperative stereotactic radiosurgery (SRS) is the standard of care adjuvant treatment. Pachymeningeal failure (PMF) is a newly described entity, distinct from classical leptomeningeal failure (LMF), that is uniquely observed in postoperative patients treated with adjuvant SRS. We sought to identify risk factors for PMF in patients treated with NS + SRS.

METHODS

From a prospective registry (2009 to 2021), we identified all patients treated with NS + SRS. Clinical, imaging, pathological, and treatment factors were analyzed. PMF incidence was evaluated using a competing risks model.

RESULTS

144 Patients were identified. The median age was 62 (23-90). PMF occurred in 21.5% (31/144). Female gender [Hazard Ratio (HR) 2.65, p = 0.013], higher Graded Prognostic Assessment (GPA) index (HR 2.4, p < 0.001), absence of prior radiation therapy (HR N/A, p = 0.018), controlled extracranial disease (CED) (HR 3.46, p = 0.0038), and pia/dura contact (PDC) (HR 3.30, p = 0.0053) were associated with increased risk for PMF on univariate analysis. In patients with PDC, wider target volumes correlated with reduced risk of PMF. Multivariate analysis indicated PDC (HR 3.51, p = 0.0053), piecemeal resection (HR 2.38, p = 0.027), and CED (HR 3.97, p = 0.0016) independently correlated with PMF risk. PMF correlated with reduced OS (HR 2.90, p < 0.001) at a lower rate compared to LMF (HR 10.15, p < 0.001).

CONCLUSION

PMF correlates with tumor PDC and piecemeal resection in patients treated with NS + SRS. For unclear reasons, it is also associated with CED. In tumors with PDC, wider dural radiotherapy coverage was associated with a lower risk of PMF.

Sensitization of cervical cancer cells to radiation by the cyclin-dependent kinase inhibitor dinaciclib

Dinaciclib is a selective cyclin-dependent kinase inhibitor, but its radiosensitizing effect remains unclear. The aim of this study is to investigate the radiosensitizing effect of Dinaciclib on cervical cancer cells. Two cervical cancer cell lines, Hela and Siha, were selected, and the IC50 was determined by CCK8. The radiosensitizing effect of Dinaciclib was verified by plate cloning assay, and the G2/M phase arrest and apoptosis of IR cells were verified by flow cytometry. Immunofluorescence assay was used to verify the formation of γH2AX foci following DNA damage. Western blot was performed to detect cell cycle, apoptosis, autophagy, and DNA damage-related pathways. Dinaciclib increased the cell sensitivity to IR. IR induced G2/M phase arrest and apoptosis, and Dinaciclib enhanced this effect. Further, Dinaciclib delayed DNA repair, including non-homologous end joining repair and homologous recombination repair, and reduced the expression of DNA repair proteins Ku80 (SiHa cells), Ku70, and RAD51, as well as the expression of apoptotic marker Bcl-2. The expression of autophagy marker Beclin1 induced tumor cell death and increased the formation of DNA damage marker γH2AX foci. Dinaciclib improves the sensitivity of cervical cancer cells to IR by inducing cell cycle arrest, delaying DNA repair, and increasing apoptosis. However, further research is needed to unravel the complexity of DNA repair pathways.

The EMT transcription factor ZEB1 governs a fitness-promoting but vulnerable DNA replication stress response

The DNA damage response (DDR) and epithelial-to-mesenchymal transition (EMT) are two crucial cellular programs in cancer biology. While the DDR orchestrates cell-cycle progression, DNA repair, and cell death, EMT promotes invasiveness, cellular plasticity, and intratumor heterogeneity. Therapeutic targeting of EMT transcription factors, such as ZEB1, remains challenging, but tumor-promoting DDR alterations elicit specific vulnerabilities. Using multi-omics, inhibitors, and high-content microscopy, we discover a chemoresistant ZEB1-high-expressing sub-population (ZEB1^hi) with co-rewired cell-cycle progression and proficient DDR across tumor entities. ZEB1 stimulates accelerated S-phase entry via CDK6, inflicting endogenous DNA replication stress. However, DDR buildups involving constitutive MRE11-dependent fork resection allow homeostatic cycling and enrichment of ZEB1^hi cells during transforming growth factor β (TGF-β)-induced EMT and chemotherapy. Thus, ZEB1 promotes G1/S transition to launch a progressive DDR benefitting stress tolerance, which concurrently manifests a targetable vulnerability in chemoresistant ZEB1^hi cells. Our study thus highlights the translationally relevant intercept of the DDR and EMT.

Histological analysis of sleep and circadian brain circuitry in cranial radiation-induced hypersomnolence (C-RIH) mouse model

Disrupted sleep, including daytime hypersomnolence, is a core symptom reported by primary brain tumor patients and often manifests after radiotherapy. The biological mechanisms driving the onset of sleep disturbances after cranial radiation remains unclear but may result from treatment-induced injury to neural circuits controlling sleep behavior, both circadian and homeostatic. Here, we develop a mouse model of cranial radiation-induced hypersomnolence which recapitulates the human experience. Additionally, we used the model to explore the impact of radiation on the brain. We demonstrated that the DNA damage response following radiation varies across the brain, with homeostatic sleep and cognitive regions expressing higher levels of γH2AX, a marker of DNA damage, than the circadian suprachiasmatic nucleus (SCN). These findings were supported by in vitro studies comparing radiation effects in SCN and cortical astrocytes. Moreover, in our mouse model, MRI identified structural effects in cognitive and homeostatic sleep regions two-months post-treatment. While the findings are preliminary, they suggest that homeostatic sleep and cognitive circuits are vulnerable to radiation and these findings may be relevant to optimizing treatment plans for patients.

Homologous Recombination Deficiency in Ovarian, Breast, Colorectal, Pancreatic, Non-Small Cell Lung and Prostate Cancers, and the Mechanisms of Resistance to PARP Inhibitors

Homologous recombination (HR) is a highly conserved DNA repair mechanism that protects cells from exogenous and endogenous DNA damage. Breast cancer 1 (BRCA1) and breast cancer 2 (BRCA2) play an important role in the HR repair pathway by interacting with other DNA repair proteins such as Fanconi anemia (FA) proteins, ATM, RAD51, PALB2, MRE11A, RAD50, and NBN. These pathways are frequently aberrant in cancer, leading to the accumulation of DNA damage and genomic instability known as homologous recombination deficiency (HRD). HRD can be caused by chromosomal and subchromosomal aberrations, as well as by epigenetic inactivation of tumor suppressor gene promoters. Deficiency in one or more HR genes increases the risk of many malignancies. Another conserved mechanism involved in the repair of DNA single-strand breaks (SSBs) is base excision repair, in which poly (ADP-ribose) polymerase (PARP) enzymes play an important role. PARP inhibitors (PARPIs) convert SSBs to more cytotoxic double-strand breaks, which are repaired in HR-proficient cells, but remain unrepaired in HRD. The blockade of both HR and base excision repair pathways is the basis of PARPI therapy. The use of PARPIs can be expanded to sporadic cancers displaying the “BRCAness” phenotype. Although PARPIs are effective in many cancers, their efficacy is limited by the development of resistance. In this review, we summarize the prevalence of HRD due to mutation, loss of heterozygosity, and promoter hypermethylation of 35 DNA repair genes in ovarian, breast, colorectal, pancreatic, non-small cell lung cancer, and prostate cancer. The underlying mechanisms and strategies to overcome PARPI resistance are also discussed.

Recent Advances in the Development of Non-PIKKs Targeting Small Molecule Inhibitors of DNA Double-Strand Break Repair

The vast majority of cancer patients receive DNA-damaging drugs or ionizing radiation (IR) during their course of treatment, yet the efficacy of these therapies is tempered by DNA repair and DNA damage response (DDR) pathways. Aberrations in DNA repair and the DDR are observed in many cancer subtypes and can promote de novo carcinogenesis, genomic instability, and ensuing resistance to current cancer therapy. Additionally, stalled or collapsed DNA replication forks present a unique challenge to the double-strand DNA break (DSB) repair system. Of the various inducible DNA lesions, DSBs are the most lethal and thus desirable in the setting of cancer treatment. In mammalian cells, DSBs are typically repaired by the error prone non-homologous end joining pathway (NHEJ) or the high-fidelity homology directed repair (HDR) pathway. Targeting DSB repair pathways using small molecular inhibitors offers a promising mechanism to synergize DNA-damaging drugs and IR while selective inhibition of the NHEJ pathway can induce synthetic lethality in HDR-deficient cancer subtypes. Selective inhibitors of the NHEJ pathway and alternative DSB-repair pathways may also see future use in precision genome editing to direct repair of resulting DSBs created by the HDR pathway. In this review, we highlight the recent advances in the development of inhibitors of the non-phosphatidylinositol 3-kinase-related kinases (non-PIKKs) members of the NHEJ, HDR and minor backup SSA and alt-NHEJ DSB-repair pathways. The inhibitors described within this review target the non-PIKKs mediators of DSB repair including Ku70/80, Artemis, DNA Ligase IV, XRCC4, MRN complex, RPA, RAD51, RAD52, ERCC1-XPF, helicases, and DNA polymerase θ. While the DDR PIKKs remain intensely pursued as therapeutic targets, small molecule inhibition of non-PIKKs represents an emerging opportunity in drug discovery that offers considerable potential to impact cancer treatment.

Perspective on the Use of DNA Repair Inhibitors as a Tool for Imaging and Radionuclide Therapy of Glioblastoma

Simple Summary

The current routine treatment for glioblastoma (GB), the most lethal high-grade brain tumor in adults, aims to induce DNA damage in the tumor. However, the tumor cells might be able to repair that damage, which leads to therapy resistance. Fortunately, DNA repair defects are common in GB cells, and their survival is often based on a sole backup repair pathway. Hence, targeted drugs inhibiting essential proteins of the DNA damage response have gained momentum and are being introduced in the clinic. This review gives a perspective on the use of radiopharmaceuticals targeting DDR kinases for imaging in order to determine the DNA repair phenotype of GB, as well as for effective radionuclide therapy. Finally, four new promising radiopharmaceuticals are suggested with the potential to lead to a more personalized GB therapy.

Abstract

Despite numerous innovative treatment strategies, the treatment of glioblastoma (GB) remains challenging. With the current state-of-the-art therapy, most GB patients succumb after about a year. In the evolution of personalized medicine, targeted radionuclide therapy (TRT) is gaining momentum, for example, to stratify patients based on specific biomarkers. One of these biomarkers is deficiencies in DNA damage repair (DDR), which give rise to genomic instability and cancer initiation. However, these deficiencies also provide targets to specifically kill cancer cells following the synthetic lethality principle. This led to the increased interest in targeted drugs that inhibit essential DDR kinases (DDRi), of which multiple are undergoing clinical validation. In this review, the current status of DDRi for the treatment of GB is given for selected targets: ATM/ATR, CHK1/2, DNA-PK, and PARP. Furthermore, this review provides a perspective on the use of radiopharmaceuticals targeting these DDR kinases to (1) evaluate the DNA repair phenotype of GB before treatment decisions are made and (2) induce DNA damage via TRT. Finally, by applying in-house selection criteria and analyzing the structural characteristics of the DDRi, four drugs with the potential to become new therapeutic GB radiopharmaceuticals are suggested.

The effect of Anzer honey on X-ray induced genotoxicity in human lymphocytes: An in vitro study

Anzer honey is well known in Turkey and used for its medicinal properties, especially for pharyngitis, tonsillitis, ulcers and cancer. In this study, we investigated whether Anzer honey, which is shown to have antioxidant, anti-tumoral, and anti-inflammatory properties, has a protective effect against X-ray induced genotoxic damage by cytogenetic methods. Peripheral blood lymphocytes isolated from 20 healthy volunteers were divided into two groups and cultivated by conventional methods. Study group lymphocytes were treated with 10% diluted honey while those in the control group were not. Both groups were exposed to a high dose (2 Gy) X-ray at the 48th hour of culture. Conventional cytogenetic staining and Giemsa banding methods were applied to evaluate chromosomal breakage and ring formation. Micronucleus frequencies were determined by the cytokinesis-block micronucleus (CBMN) assay. Paired sample t test was used to compare groups. Anzer honey, which was analyzed melissopalynologically, was used. Micronucleus frequency was significantly decreased in the study group (CI = 348.75 ± 31, median 326, min. 98, max. 704) compared to the control group (CI = 489.10 ± 27, median 500, min. 216, max. 645) (p = .001). Chromosomal breakage was also significantly decreased in the study group (CI = 118.70 ± 16, median 109, min. 12, max. 316) compared to the control group (CI = 233.60 ± 25, median 225, min. 65, max. 492) (p < .0001). This is the first study indicating that genotoxic damage in the peripheral blood lymphocytes of healthy volunteers induced by X-radiation may be prevented or alleviated by adding Anzer honey in vitro. These results encourage further research about the protective effects of honey.

WRN helicase safeguards deprotected replication forks in BRCA2-mutated cancer cells

The tumor suppressor BRCA2 protects stalled forks from degradation to maintain genome stability. However, the molecular mechanism(s) whereby unprotected forks are stabilized remains to be fully characterized. Here, we demonstrate that WRN helicase ensures efficient restart and limits excessive degradation of stalled forks in BRCA2-deficient cancer cells. In vitro, WRN ATPase/helicase catalyzes fork restoration and curtails MRE11 nuclease activity on regressed forks. We show that WRN helicase inhibitor traps WRN on chromatin leading to rapid fork stalling and nucleolytic degradation of unprotected forks by MRE11, resulting in MUS81-dependent double-strand breaks, elevated non-homologous end-joining and chromosomal instability. WRN helicase inhibition reduces viability of BRCA2-deficient cells and potentiates cytotoxicity of a poly (ADP)ribose polymerase (PARP) inhibitor. Furthermore, BRCA2-deficient xenograft tumors in mice exhibited increased DNA damage and growth inhibition when treated with WRN helicase inhibitor. This work provides mechanistic insight into stalled fork stabilization by WRN helicase when BRCA2 is deficient.

DNA adduct formation and reduced EIF4A3expression contributes to benzo[a]pyrene-induced DNA damage in human bronchial epithelial BEAS-2B cells

Benzo[a]pyrene(B[a]P) is a known human carcinogen. The ability of B[a]P to form stable DNA adducts has been repeatedly demonstrated. However, the relationship between DNA adduct formation and cell damage and its underlying molecular mechanisms are less well understood. In this study, we determined the cytotoxicity of benzo[a]pyrenediolepoxide, a metabolite of B[a]P, in human bronchial epithelial cells (BEAS-2B). The formation of BPDE-DNA adducts was quantified using a dot blot. DNA damage resulting from the formation of BPDE-DNA adducts was detected by chromatin immuneprecipitation sequencing (ChIP-Seq), with minor modifications, using specific antibodies against BPDE. In total, 1846 differentially expressed gene loci were detected between the treatment and control groups. The distribution of the BPDE-bound regions indicated that BPDE could covalently bind with both coding and non-coding regions to cause DNA damage. However, the majority of binding occurred at protein-coding genes. Furthermore, among the BPDE-bound genes, we found 16 protein-coding genes related to DNA damage repair. We explored the response to BPDE exposure at the transcriptional level using qRT-PCR and observed a strong inhibition of EIF4A3. We then established an EIF4A3 overexpression cell model and performed comet assays, which revealed that the levels of DNA damage in EIF4A3-overexpressing cells were lower than those in normal cells following BPDE exposure. This suggests that the BPDE-DNA adduct-induced reduction in EIF4A3 expression contributed to the DNA damage induced by BPDE exposure in BEAS-2B cells. These novel findings indicate that ChIP-Seq combined with BPDE specific antibody may be used for exploring the underlying mechanism of DNA adduct-induced genomic damage.

Molecular Determinant of DIDS Analogs Targeting RAD51 Activity

RAD51 is the central protein in DNA repair by homologous recombination (HR), involved in several steps of this process. It is shown that overexpression of the RAD51 protein is correlated with increased survival of cancer cells to cancer treatments. For the past decade, RAD51 overexpression-mediated resistance has justified the development of targeted inhibitors. One of the first molecules described to inhibit RAD51 was the 4,4′-diisothiocyanato-stilbene-2,2′-disulfonic acid (DIDS) molecule. This small molecule is effective in inhibiting different functions of RAD51, however its mode of action and the chemical functions involved in this inhibition have not been identified. In this work, we used several commercial molecules derived from DIDS to characterize the structural determinants involved in modulating the activity of RAD51. By combining biochemical and biophysical approaches, we have shown that DIDS and two analogs were able to inhibit the binding of RAD51 to ssDNA and prevent the formation of D-loop by RAD51. Both isothiocyanate substituents of DIDS appear to be essential in the inhibition of RAD51. These results open the way to the synthesis of new molecules derived from DIDS that should be greater modulators of RAD51 and more efficient for HR inhibition.

Genome-wide Screens Implicate Loss of Cullin Ring Ligase 3 in Persistent Proliferation and Genome Instability in TP53-Deficient Cells

TP53 deficiency is the most common alteration in cancer; however, this alone is typically insufficient to drive tumorigenesis. To identify genes promoting tumorigenesis in combination with TP53 deficiency, we perform genome-wide CRISPR-Cas9 knockout screens coupled with proliferation and transformation assays in isogenic cell lines. Loss of several known tumor suppressors enhances cellular proliferation and transformation. Loss of neddylation pathway genes promotes uncontrolled proliferation exclusively in TP53-deficient cells. Combined loss of CUL3 and TP53 activates an oncogenic transcriptional program governed by the nuclear factor κB (NF-κB), AP-1, and transforming growth factor β (TGF-β) pathways. This program maintains persistent cellular proliferation, induces partial epithelial to mesenchymal transition, and increases DNA damage, genomic instability, and chromosomal rearrangements. Our findings reveal CUL3 loss as a key event stimulating persistent proliferation in TP53-deficient cells. These findings may be clinically relevant, since TP53-CUL3-deficient cells are highly sensitive to ataxia telangiectasia mutated (ATM) inhibition, exposing a vulnerability that could be exploited for cancer treatment.

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Mixed-effect models with MEMcrispR applied to CRISPR screen analyses

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Knockout of neddylation genes increases persistent proliferation in TP53^−/− cells

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TP53^−/−,CUL3^−/− cells exhibit persistent proliferation and partial EMT phenotype

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TP53^−/−,CUL3^−/− cells show increased DNA damage and display sensitivity to ATM inhibition

Molecular Docking and Molecular Dynamics Simulation Studies of Quinoline-3-Carboxamide Derivatives with DDR Kinases–Selectivity Studies towards ATM Kinase

Quinoline-3-carboxamides are an essential class of drug-like small molecules that are known to inhibit the phosphatidylinositol 3-kinase-related kinases (PIKK) family kinases. The quinoline nitrogen is shown to bind to the hinge region of the kinases, making them competitive inhibitors of adenosine triphosphate (ATP). We have previously designed and synthesized quinoline-3-carboxamides as potential ataxia telangiectasia mutated (ATM) kinase inhibitors to function as an adjuvant treatment with DNA damaging agents. This article discusses the molecular docking studies performed with these derivatives with the DNA damage and response (DDR) kinases-ATM, ataxia telangiectasia and rad3 related (ATR), and DNA dependent protein kinase catalytic subunit (DNA-PKcs) and highlights their selectivity towards ATM kinase. Docking studies were also performed with mTOR and PI3Kγ, which are close homologs of the DDR kinases. Molecular dynamics simulations were performed for one of the inhibitors against all the enzymes to establish the stability of the interactions involved. Finally, the absorption, distribution, metabolism, and excretion (ADME) properties of the inhibitors were predicted using the QikProp manual in Maestro. In conclusion, the molecules synthesized showed high selectivity towards the ATM kinase in comparison with the other kinases, though the sequence similarity between them was relatively high.

Effect of ATR Inhibition in RT Response of HPV-Negative and HPV-Positive Head and Neck Cancers

Radiotherapy (RT) has a central role in head and neck squamous cell carcinoma (HNSCC) treatment. Targeted therapies modulating DNA damage response (DDR) and more specific cell cycle checkpoints can improve the radiotherapeutic response. Here, we assessed the influence of ataxia-telangiectasia mutated and Rad3-related (ATR) inhibition with the ATR inhibitor AZD6738 on RT response in both human papillomavirus (HPV)-negative and HPV-positive HNSCC. We found that ATR inhibition enhanced RT response in HPV-negative and HPV-positive cell lines independent of HPV status. The radiosensitizing effect of AZD6738 was correlated with checkpoint kinase 1 (CHK1)-mediated abrogation of G2/M-arrest. This resulted in the inhibition of RT-induced DNA repair and in an increase in the percentage of micronucleated cells. We validated the enhanced RT response in HPV-negative and HPV-positive xenograft models. These data demonstrate the potential use of ATR inhibition in combination with RT as a treatment option for both HPV-negative and HPV-positive HNSCC patients.

FEN1 inhibitor synergizes with low-dose camptothecin to induce increased cell killing via the mitochondria mediated apoptotic pathway

Camptothecin has been used in tumor therapy for a long time but its antitumor effect is rather limited due to the side effect and the drug resistance. FEN1, a major component of DNA repair systems, plays important roles in maintaining genomic stability via DNA replication and repair. Here we found that FEN1 inhibitor greatly sensitizes cancer cells to low-dose camptothecin. The combinative treatment of FEN1 inhibitor and 1 nM camptothecin induced a synthetic lethal effect, which synergistically suppressed cancer cell proliferation and significantly mediated apoptosis both in vitro and in vivo. Our study suggested that targeting FEN1 could be a potent strategy for tumor-targeting cancer therapy.

Combining PARP Inhibition with Platinum, Ruthenium or Gold Complexes for Cancer Therapy

Platinum drugs are heavily used first-line chemotherapeutic agents for many solid tumours and have stimulated substantial interest in the biological activity of DNA-binding metal complexes. These complexes generate DNA lesions which trigger the activation of DNA damage response (DDR) pathways that are essential to maintain genomic integrity. Cancer cells exploit this intrinsic DNA repair network to counteract many types of chemotherapies. Now, advances in the molecular biology of cancer has paved the way for the combination of DDR inhibitors such as poly (ADP-ribose) polymerase (PARP) inhibitors (PARPi) and agents that induce high levels of DNA replication stress or single-strand break damage for synergistic cancer cell killing. In this review, we summarise early-stage, preclinical and clinical findings exploring platinum and emerging ruthenium anti-cancer complexes alongside PARPi in combination therapy for cancer and also describe emerging work on the ability of ruthenium and gold complexes to directly inhibit PARP activity.

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.

Amentoflavone triggers cell cycle G2/M arrest by interfering with microtubule dynamics and inducing DNA damage in SKOV3 cells

Ovarian cancer is the seventh most common cancer and the second most common cause of cancer-associated mortality among gynecological malignancies worldwide. The combination of antimitotic agents, such as taxanes, and the DNA-damaging agents, such as platinum compounds, is the standard treatment for ovarian cancer. However, due to chemoresistance, development of novel therapeutic strategies for the treatment of ovarian cancer remains critical. Amentoflavone (AMF) is a biflavonoid derived from the extracts of Selaginella tamariscina, which has been used as a Chinese herb for thousands of years. A previous study demonstrated that AMF inhibits angiogenesis of endothelial cells and induces apoptosis in hypertrophic scar fibroblasts. In order to check the influence of AMF on cell proliferation, the effects of AMF on cell cycle and DNA damage were measured by cell viability, flow cytometry, immunofluorescence and western blotting assays in SKOV3 cells, an ovarian cell line. In the present study, treatment with AMF inhibited ovarian cell proliferation, increased P21 expression, decreased CDK1/2 expression, interrupted the balance of microtubule dynamics and arrested cells at the G2 phase. Furthermore, treatment with AMF increased the expression levels of phospho-Histone H2AX (γ-H2AX; a variant of histone 2A, that belongs to the histone 2A family member X) and the DNA repair protein RAD51 homolog 1 (Rad51), indicating the occurrence of DNA damage since γ-H2AX and Rad51 are both key markers of DNA damage. Consistent with previous findings, the results of the present study suggest that AMF is a potential therapeutic agent for the treatment of ovarian cancer. In addition, the effects of AMF on cell cycle arrest and DNA damage induction may be the molecular mechanisms by which AMF might exert its potential therapeutic benefits in ovarian cancer.

DNA damaging agents in ovarian cancer

Epithelial ovarian cancer (EOC) is very sensitive to upfront chemotherapy. This condition is attributable to defects in the DNA damage repair system. Agents that damage DNA are the main drugs used for its treatment. Many EOC cells have DNA repair deficiencies that confer susceptibility to these agents. Platinum is the most important agent for first-line and also for relapses, together with other drugs that can be given as monotherapy or along with platinum or other drugs. Lately, the emerging role of PARP inhibitors has changed the landscape of opportunities for patients with EOC. All these strategies will be reviewed in this article.

A scalable CRISPR/Cas9-based fluorescent reporter assay to study DNA double-strand break repair choice

Double-strand breaks (DSBs) are the most toxic type of DNA lesions. Cells repair these lesions using either end protection- or end resection-coupled mechanisms. To study DSB repair choice, we present the Color Assay Tracing-Repair (CAT-R) to simultaneously quantify DSB repair via end protection and end resection pathways. CAT-R introduces DSBs using CRISPR/Cas9 in a tandem fluorescent reporter, whose repair distinguishes small insertions/deletions from large deletions. We demonstrate CAT-R applications in chemical and genetic screens. First, we evaluate 21 compounds currently in clinical trials which target the DNA damage response. Second, we examine how 417 factors involved in DNA damage response influence the choice between end protection and end resection. Finally, we show that impairing nucleotide excision repair favors error-free repair, providing an alternative way for improving CRISPR/Cas9-based knock-ins. CAT-R is a high-throughput, versatile assay to assess DSB repair choice, which facilitates comprehensive studies of DNA repair and drug efficiency testing.

Synthesis and Characterization of Quinoline-3-Carboxamide Derivatives as Inhibitors of the ATM Kinase

BACKGROUND

The importance of inhibiting the kinases of the DDR pathway for radiosensitizing cancer cells is well established. Cancer cells exploit these kinases for their survival, which leads to the development of resistance towards DNA damaging therapeutics.

OBJECTIVE

In this article, the focus is on targeting the key mediator of the DDR pathway, the ATM kinase. A new set of quinoline-3-carboxamides, as potential inhibitors of ATM, is reported.

METHODS

Quinoline-3-carboxamide derivatives were synthesized and cytotoxicity assay was performed to analyze the effect of molecules on different cancer cell lines like HCT116, MDA-MB-468, and MDA-MB-231.

RESULTS

Three of the synthesized compounds showed promising cytotoxicity towards a selected set of cancer cell lines. Western Blot analysis was also performed by pre-treating the cells with quercetin, a known ATM upregulator, by causing DNA double-strand breaks. SAR studies suggested the importance of the electron-donating nature of the R group for the molecule to be toxic. Finally, Western-Blot analysis confirmed the down-regulation of ATM in the cells. Additionally, the PTEN negative cell line, MDA-MB-468, was more sensitive towards the compounds in comparison with the PTEN positive cell line, MDA-MB-231. Cytotoxicity studies against 293T cells showed that the compounds were at least three times less toxic when compared with HCT116.

CONCLUSION

In conclusion, these experiments will lay the groundwork for the evolution of potent and selective ATM inhibitors for the radio- and chemo-sensitization of cancer cells.

Emerging Therapeutic Targets Against Toxoplasma gondii: Update on DNA Repair Response Inhibitors and Genotoxic Drugs

Toxoplasma gondii is the causative agent of toxoplasmosis in animals and humans. This infection is transmitted to humans through oocysts released in the feces of the felines into the environment or by ingestion of undercooked meat. This implies that toxoplasmosis is a zoonotic disease and T. gondii is a foodborne pathogen. In addition, chronic toxoplasmosis in goats and sheep is the cause of recurrent abortions with economic losses in the sector. It is also a health problem in pets such as cats and dogs. Although there are therapies against this infection in its acute stage, they are not able to permanently eliminate the parasite and sometimes they are not well tolerated. To develop better, safer drugs, we need to elucidate key aspects of the biology of T. gondii. In this review, we will discuss the importance of the homologous recombination repair (HRR) pathway in the parasite's lytic cycle and how components of these processes can be potential molecular targets for new drug development programs. In that sense, the effect of different DNA damage agents or HHR inhibitors on the growth and replication of T. gondii will be described. Multitarget drugs that were either associated with other targets or were part of general screenings are included in the list, providing a thorough revision of the drugs that can be tested in other scenarios.

ATR-CHK1 pathway as a therapeutic target for acute and chronic leukemias

Progress in cancer therapy changed the outcome of many patients and moved therapy from chemotherapy agents to targeted drugs. Targeted drugs already changed the clinical practice in treatment of leukemias, such as imatinib (BCR/ABL inhibitor) in chronic myeloid leukemia (CML) and acute lymphoblastic leukemia (ALL), ibrutinib (Bruton's tyrosine kinase inhibitor) in chronic lymphocytic leukemia (CLL), venetoclax (BCL2 inhibitor) in CLL and acute myeloid leukemia (AML) or midostaurin (FLT3 inhibitor) in AML. In this review, we focused on DNA damage response (DDR) inhibition, specifically on inhibition of ATR-CHK1 pathway. Cancer cells harbor often defects in different DDR pathways, which render them vulnerable to DDR inhibition. Some DDR inhibitors showed interesting single-agent activity even in the absence of cytotoxic drug especially in cancers with underlying defects in DDR or DNA replication. Almost no mutations were found in ATR and CHEK1 genes in leukemia patients. Together with the fact that ATR-CHK1 pathway is essential for cell development and survival of leukemia cells, it represents a promising therapeutic target for treatment of leukemia. ATR-CHK1 inhibition showed excellent results in preclinical testing in acute and chronic leukemias. However, results in clinical trials are so far insufficient. Therefore, the ongoing and future clinical trials will decide on the success of ATR/CHK1 inhibitors in clinical practice of leukemia treatment.

MDC1 depletion promotes cisplatin induced cell death in cervical cancer cells

OBJECTIVE

Cisplatin, the most common chemotherapeutic drug for the treatment of advanced stage cervical cancers has limitations in terms of drugs resistance observed in patients partly due to functional DNA damage repair (DDR) processes in the cell. Mediator of DNA damage checkpoint 1 (MDC1) is an important protein in the Ataxia telangiectasia mutated (ATM) mediated double stranded DNA break (DSB) repair pathway. In this regard, we investigated the effect of MDC1 change in expression on the cisplatin sensitivity in cervical cancer cells.

RESULTS

Through modulation of MDC1 expression in the cervical cancer cell lines; Hela, SiHa and Caski, we found that all the three cell lines silenced for MDC1 exhibited higher sensitivity to cisplatin treatment with inefficiency in accumulation of p γH2AX, Ser 139 foci and increased accumulation of pChk2 Thr 68 at the damaged chromatin followed by enhanced apoptosis. Further, we observed the increased p53 Ser 15 phosphorylation in the MDC1 depleted cells. Our studies suggest that MDC1 expression could be a key determinant in cervical cancer prognosis and its depletion in combination with cisplatin has the potential to be explored for the sensitisation of chemo-resistant cervical cancer cells.

Role of Rad51 and DNA repair in cancer: A molecular perspective

The maintenance of genome integrity is essential for any organism survival and for the inheritance of traits to offspring. To the purpose, cells have developed a complex DNA repair system to defend the genetic information against both endogenous and exogenous sources of damage. Accordingly, multiple repair pathways can be aroused from the diverse forms of DNA lesions, which can be effective per se or via crosstalk with others to complete the whole DNA repair process. Deficiencies in DNA healing resulting in faulty repair and/or prolonged DNA damage can lead to genes mutations, chromosome rearrangements, genomic instability, and finally carcinogenesis and/or cancer progression. Although it might seem paradoxical, at the same time such defects in DNA repair pathways may have therapeutic implications for potential clinical practice. Here we provide an overview of the main DNA repair pathways, with special focus on the role played by homologous repair and the RAD51 recombinase protein in the cellular DNA damage response. We next discuss the recombinase structure and function per se and in combination with all its principal mediators and regulators. Finally, we conclude with an analysis of the manifold roles that RAD51 plays in carcinogenesis, cancer progression and anticancer drug resistance, and conclude this work with a survey of the most promising therapeutic strategies aimed at targeting RAD51 in experimental oncology.

The N'-Substituted Derivatives of 5-Chloro-3-Methylisothiazole-4-Carboxylic Acid Hydrazide with Antiproliferative Activity

Thanks to the progress in oncology, pharmacological treatment of cancer is gaining in importance and in the near future anti-cancer chemotherapeutics are expected to be the main method of treatment for cancer diseases. What is more, the search for new anti-cancer compounds with the desired application properties is constantly underway. As a result of designed syntheses, we obtained some new N’-substituted 5-chloro-3-methylisothiazole-4-carboxylic acid hydrazide derivatives with anticancer activity. The structure of new compounds was determined by mass spectrometry (MS), elemental analysis, proton nuclear magnetic resonance spectroscopy (¹H-NMR), carbon nuclear magnetic resonance spectroscopy (¹³C-NMR), ¹H-¹³C NMR correlations and infrared spectroscopy (IR). Moreover, the structures of the compounds were confirmed by crystallographic examination. The antiproliferative MTT tests for 11 prepared compounds was conducted towards human biphenotypic B cell myelomonocytic leukemia MV4-11. SRB test was used to examine their potential anticancer activity towards human colon adenocarcinoma cell lines sensitive LoVo, resistant to doxorubicin LoVo/DX, breast adenocarcinoma MCF-7 and normal non-tumorigenic epithelial cell line derived from mammary gland MCF-10A. The most active compound was 5-chloro-3-methyl-N′-[(1E,2E)-(3-phenyloprop-2-en-1-ylidene]isothiazole-4-carbohydrazide, which showed the highest antiproliferative activity against all tested cell lines.

FEN1 inhibitor increases sensitivity of radiotherapy in cervical cancer cells

Background

Cervical cancer is one of the most common causes of cancer‐associated mortality among affected women in the world. At present, treatment with weekly cisplatin plus ionizing radiation (IR) therapy is the standard regimen for cervical cancer, especially for locally advanced cervical cancer. The purpose of this study is to determine whether FEN1 inhibitors could enhance the therapeutic effect of IR therapy.

Methods

Western blot was applied to determine the expression of FEN1‐ and apoptosis‐related proteins. Cell growth inhibition assay and colony formation assay were used to determine the effects of FEN1 inhibitor and IR exposure for Hela cells in vitro. CRISPR technology was used to knockdown FEN1 expression level of 293T cells, and tumor xenograft in nude mice was employed to determine the effects of FEN1 inhibitor and IR exposure on tumor growth in vivo.

Results

Our data revealed that FEN1 is overexpressed in HeLa cell and can be upregulated further by IR. We also demonstrated that FEN1 inhibitor enhances IR sensitivity of cervical cancer in vitro and in vivo.

Conclusion

FEN1 inhibitor SC13 could sensitize radiotherapy of cervical cancer cell.

Interactions of the Rad51 inhibitor DIDS with human and bovine serum albumins: Optical spectroscopy and isothermal calorimetry approaches

Rad51 is a key protein in DNA repair by homologous recombination and an important target for development of drugs in cancer therapy. 4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) has been used in clinic during the past 30 years as an inhibitor of anion transporters and channels. Recently DIDS has been demonstrated to affect Rad51-mediated homologous pairing and strand exchange, key processes in homologous recombination. Consequently, DIDS has been considered as a potential revertant of radio- and chemo-resistance of cancer cells, the major causes of therapy failure. Here, we have investigated the behavior of DIDS towards serum albumins. The effects of environmental factors, primarily, solvent polarity, on DIDS stability were evaluated, and the mechanisms of interaction of DIDS with human or bovine serum albumin were analyzed using isothermal calorimetry, circular dichroism and fluorescence spectroscopies. DIDS interaction with both serum albumins have been demonstrated, and the interaction characteristics have been determined. By comparing these characteristics for several DIDS derivatives, we have identified the DIDS moiety essential for the interaction. Furthermore, site competition data indicate that human albumin has two DIDS-binding sites: a high-affinity site in the IIIA subdomain and a low-affinity one in the IB subdomain. Molecular docking has revealed the key molecular moieties of DIDS responsible for its interactions in each site and shown that the IB site can bind two ligands. These findings show that binding of DIDS to serum albumin may change the balance between the free and bound DIDS forms, thereby affecting its bioavailability and efficacy against Rad51.

The oncoprotein DEK affects the outcome of PARP1/2 inhibition during mild replication stress

DNA replication stress is a major source of genomic instability and is closely linked to tumor formation and progression. Poly(ADP-ribose)polymerases1/2 (PARP1/2) enzymes are activated in response to replication stress resulting in poly(ADP-ribose) (PAR) synthesis. PARylation plays an important role in the remodelling and repair of impaired replication forks, providing a rationale for targeting highly replicative cancer cells with PARP1/2 inhibitors. The human oncoprotein DEK is a unique, non-histone chromatin architectural protein whose deregulated expression is associated with the development of a wide variety of human cancers. Recently, we showed that DEK is a high-affinity target of PARylation and that it promotes the progression of impaired replication forks. Here, we investigated a potential functional link between PAR and DEK in the context of replication stress. Under conditions of mild replication stress induced either by topoisomerase1 inhibition with camptothecin or nucleotide depletion by hydroxyurea, we found that the effect of acute PARP1/2 inhibition on replication fork progression is dependent on DEK expression. Reducing DEK protein levels also overcomes the restart impairment of stalled forks provoked by blocking PARylation. Non-covalent DEK-PAR interaction via the central PAR-binding domain of DEK is crucial for counteracting PARP1/2 inhibition as shown for the formation of RPA positive foci in hydroxyurea treated cells. Finally, we show by iPOND and super resolved microscopy that DEK is not directly associated with the replisome since it binds to DNA at the stage of chromatin formation. Our report sheds new light on the still enigmatic molecular functions of DEK and suggests that DEK expression levels may influence the sensitivity of cancer cells to PARP1/2 inhibitors.

Differential Expression of DNA Repair Genes in Prognostically-Favorable versus Unfavorable Uveal Melanoma

Expression of DNA repair genes was studied in uveal melanoma (UM) in order to identify genes that may play a role in metastases formation. We searched for genes that are differentially expressed between tumors with a favorable and unfavorable prognosis. Gene-expression profiling was performed on 64 primary UM from the Leiden University Medical Center (LUMC), Leiden, The Netherlands. The expression of 121 genes encoding proteins involved in DNA repair pathways was analyzed: a total of 44 genes differed between disomy 3 and monosomy 3 tumors. Results were validated in a cohort from Genoa and Paris and the The Cancer Genome Atlas (TCGA) cohort. Expression of the PRKDC, WDR48, XPC, and BAP1 genes was significantly associated with clinical outcome after validation. PRKDC was highly expressed in metastasizing UM (p < 0.001), whereas WDR48, XPC, and BAP1 were lowly expressed (p < 0.001, p = 0.006, p = 0.003, respectively). Low expression of WDR48 and XPC was related to a large tumor diameter (p = 0.01 and p = 0.004, respectively), and a mixed/epithelioid cell type (p = 0.007 and p = 0.03, respectively). We conclude that the expression of WDR48, XPC, and BAP1 is significantly lower in UM with an unfavorable prognosis, while these tumors have a significantly higher expression of PRKDC. Pharmacological inhibition of DNA-PKcs resulted in decreased survival of UM cells. PRKDC may be involved in proliferation, invasion and metastasis of UM cells. Unraveling the role of DNA repair genes may enhance our understanding of UM biology and result in the identification of new therapeutic targets.

Lipopolysaccharide-induced DNA damage response activates nuclear factor κB signalling pathway via GATA4 in dental pulp cells

AIM

To investigate the role of GATA-binding protein 4 (GATA4) in the inflammatory response induced by DNA double-strand breaks (DSBs) in human dental pulp cells (hDPCs).

METHODOLOGY

Lipopolysaccharide (LPS) was used for stimulating inflammation in dental pulp tissue in vivo and hDPCs in vitro. Expression levels of GATA4 and γ-H2A.X (a marker for DSBs) were detected at different stages of pulpitis in a rat model and human pulp tissues by immunohistochemistry. Real-time quantitative polymerase chain reaction and Western blot were performed to assess expression of GATA4 and γ-H2A.X and the activation of nuclear factor κB (NF-κB) in hDPCs stimulated by LPS. The comet assay was used for detecting the extent of DSBs in hDPCs. Immunocytochemistry and Western blot were utilized to evaluate expression of γ-H2A.X and GATA4 and activation of NF-κB in hDPCs pre-treated with inhibitors of DNA damage response or transfected with GATA4 small interfering RNA before the treatment of LPS. Data were analysed statistically using one-way anova or Kruskal-Wallis tests.

RESULTS

The expression of GATA4 and activation of DNA damage response and NF-κB in inflamed pulp tissue and LPS-treated hDPCs were identified. Significantly decreased expression of GATA4 and significantly decreased inflammatory processes in hDPCs were demonstrated via suppression of DNA damage response (P < 0.05). In GATA4-knockdown cells, the expression of γ-H2A.X did not change, but nuclear translocation of p65 was significantly suppressed (P < 0.05) upon induction by LPS.

CONCLUSIONS

Lipopolysaccharide-induced DSBs activated the NF-κB signalling pathway in hDPCs, and GATA4 acts as a positive moderator of the progress. The involvement of GATA4 in this pathology may serve as a therapeutic target in pulpitis.

Breaking the DNA Damage Response via Serine/Threonine Kinase Inhibitors to Improve Cancer Treatment

Multiple, both endogenous and exogenous, sources may induce DNA damage and DNA replication stress. Cells have developed DNA damage response (DDR) signaling pathways to maintain genomic stability and effectively detect and repair DNA lesions. Serine/ threonine kinases such as Ataxia-telangiectasia mutated (ATM) and Ataxia-telangiectasia and Rad3-Related (ATR) are the major regulators of DDR, since after sensing stalled DNA replication forks, DNA double- or single-strand breaks, may directly phosphorylate and activate their downstream targets, that play a key role in DNA repair, cell cycle arrest and apoptotic cell death. Interestingly, key components of DDR signaling networks may constitute an attractive target for anti-cancer therapy through two distinct potential approaches: as chemoand radiosensitizers to enhance the effectiveness of currently used genotoxic treatment or as single agents to exploit defects in DDR in cancer cells via synthetic lethal approach. Moreover, the newest data reported that serine/threonine protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK) is also closely associated with cancer development and progression. Thereby, utilization of small-molecule, serine/threonine kinase inhibitors may provide a novel, groundbreaking, anti-cancer treatment strategy. Currently, a range of potent, highlyselective toward ATM, ATR and PERK inhibitors has been discovered, but after foregoing study, additional investigations are necessary for their future clinical use.

New Phosphorylation Sites of Rad51 by c-Met Modulates Presynaptic Filament Stability

Genomic instability through deregulation of DNA repair pathways can initiate cancer and subsequently result in resistance to chemo and radiotherapy. Understanding these biological mechanisms is therefore essential to overcome cancer. RAD51 is the central protein of the Homologous Recombination (HR) DNA repair pathway, which leads to faithful DNA repair of DSBs. The recombinase activity of RAD51 requires nucleofilament formation and is regulated by post-translational modifications such as phosphorylation. In the last decade, studies have suggested the existence of a relationship between receptor tyrosine kinases (RTK) and Homologous Recombination DNA repair. Among these RTK the c-MET receptor is often overexpressed or constitutively activated in many cancer types and its inhibition induces the decrease of HR. In this study, we show for the first time that c-MET is able to phosphorylate the RAD51 protein. We demonstrate in vitro that c-MET phosphorylates four tyrosine residues localized mainly in the subunit-subunit interface of RAD51. Whereas these post-translational modifications do not affect the presynaptic filament formation, they strengthen its stability against the inhibitor effect of the BRC peptide obtained from BRCA2. Taken together, these results confirm the role of these modifications in the regulation of the BRCA2-RAD51 interaction and underline the importance of c-MET in DNA damage response.

Evaluation of ATM Kinase Inhibitor KU-55933 as Potential Anti-Toxoplasma gondii Agent

Toxoplasma gondii is an apicomplexan protozoan parasite with a complex life cycle composed of multiple stages that infect mammals and birds. Tachyzoites rapidly replicate within host cells to produce acute infection during which the parasite disseminates to tissues and organs. Highly replicative cells are subject to Double Strand Breaks (DSBs) by replication fork collapse and ATM, a member of the phosphatidylinositol 3-kinase (PI3K) family, is a key factor that initiates DNA repair and activates cell cycle checkpoints. Here we demonstrate that the treatment of intracellular tachyzoites with the PI3K inhibitor caffeine or ATM kinase-inhibitor KU-55933 affects parasite replication rate in a dose-dependent manner. KU-55933 affects intracellular tachyzoite growth and induces G1-phase arrest. Addition of KU-55933 to extracellular tachyzoites also leads to a significant reduction of tachyzoite replication upon infection of host cells. ATM kinase phosphorylates H2A.X (γH2AX) to promote DSB damage repair. The level of γH2AX increases in tachyzoites treated with camptothecin (CPT), a drug that generates fork collapse, but this increase was not observed when co-administered with KU-55933. These findings support that KU-55933 is affecting the Toxoplasma ATM-like kinase (TgATM). The combination of KU-55933 and other DNA damaging agents such as methyl methane sulfonate (MMS) and CPT produce a synergic effect, suggesting that TgATM kinase inhibition sensitizes the parasite to damaged DNA. By contrast, hydroxyurea (HU) did not further inhibit tachyzoite replication when combined with KU-55933.

BLM's balancing act and the involvement of FANCJ in DNA repair

Timely recruitment of DNA damage response proteins to sites of genomic structural lesions is very important for signaling mechanisms to activate appropriate cell cycle checkpoints but also repair the altered DNA sequence to suppress mutagenesis. The eukaryotic cell is characterized by a complex cadre of players and pathways to ensure genomic stability in the face of replication stress or outright genomic insult by endogenous metabolites or environmental agents. Among the key performers are molecular motor DNA unwinding enzymes known as helicases that sense genomic perturbations and separate structured DNA strands so that replacement of a damaged base or sugar-phosphate backbone lesion can occur efficiently. Mutations in the BLM gene encoding the DNA helicase BLM leads to a rare chromosomal instability disorder known as Bloom's syndrome. In a recent paper by the Sengupta lab, BLM's role in the correction of double-strand breaks (DSB), a particularly dangerous form of DNA damage, was investigated. Adding to the complexity, BLM appears to be a key ringmaster of DSB repair as it acts both positively and negatively to regulate correction pathways of high or low fidelity. The FANCJ DNA helicase, mutated in another chromosomal instability disorder known as Fanconi Anemia, is an important player that likely coordinates with BLM in the balancing act. Further studies to dissect the roles of DNA helicases like FANCJ and BLM in DSB repair are warranted.

The evolving role of DNA inter-strand crosslinks in chemotherapy

DNA crosslinking agents make up a broad class of chemotherapy agents that target rapidly dividing cancer cells by disrupting DNA synthesis. These drugs differ widely in both chemical structure and biological effect. In cells, crosslinking agents can form multiple types of DNA lesions with varying efficiencies. Inter-strand crosslinks (ICLs) are considered to be the most cytotoxic lesion, creating a covalent roadblock to replication and transcription. Despite over 50 years in the clinic, the use of crosslinking agents that specialize in the formation of ICLs remains limited, largely due to high toxicity in patients. Current ICL-based therapeutics have focused on late-stage and drug-resistant tumors, or localized treatments that limit exposure. In this article, we review the development of clinical crosslinking agents, our understanding of how cells respond to different lesions, and the potential to improve ICL-based chemotherapeutics in the future.

Assessment of DNA-PKcs kinase activity by quantum dot-based microarray

Therapeutic efficacy against cancer is often based on a variety of DNA lesions, including DNA double-strand breaks (DSBs) which are repaired by homologous recombination and non-homologous end joining (NHEJ) pathways. In the past decade, the functions of the DNA repair proteins have been described as a potential mechanism of resistance in tumor cells. Therefore, the DNA repair proteins have become targets to improve the efficacy of anticancer therapy. Given the central role of DNA-PKcs in NHEJ, the therapeutic efficacy of targeting DNA-PKcs is frequently described as a strategy to prevent repair of treatment-induced DNA damage in cancer cells. The screening of a new inhibitor acting as a sensitizer requires the development of a high-throughput tool in order to identify and assess the most effective molecule. Here, we describe the elaboration of an antibody microarray dedicated to the NHEJ pathway that we used to evaluate the DNA-PKcs kinase activity in response to DNA damage. By combining a protein microarray with Quantum-Dot detection, we show that it is possible to follow the modification of phosphoproteomic cellular profiles induced by inhibitors during the response to DNA damage. Finally, we discuss the promising tool for screening kinase inhibitors and targeting DSB repair to improve cancer treatment.

Biological effects of radiation on cancer cells

With the development of radiotherapeutic oncology, computer technology and medical imaging technology, radiation therapy has made great progress. Research on the impact and the specific mechanism of radiation on tumors has become a central topic in cancer therapy. According to the traditional view, radiation can directly affect the structure of the DNA double helix, which in turn activates DNA damage sensors to induce apoptosis, necrosis, and aging or affects normal mitosis events and ultimately rewires various biological characteristics of neoplasm cells. In addition, irradiation damages subcellular structures, such as the cytoplasmic membrane, endoplasmic reticulum, ribosome, mitochondria, and lysosome of cancer cells to regulate various biological activities of tumor cells. Recent studies have shown that radiation can also change the tumor cell phenotype, immunogenicity and microenvironment, thereby globally altering the biological behavior of cancer cells. In this review, we focus on the effects of therapeutic radiation on the biological features of tumor cells to provide a theoretical basis for combinational therapy and inaugurate a new era in oncology.

New Insights Into DNA Helicases as Druggable Targets for Cancer Therapy

Small molecules that deter the functions of DNA damage response machinery are postulated to be useful for enhancing the DNA damaging effects of chemotherapy or ionizing radiation treatments to combat cancer by impairing the proliferative capacity of rapidly dividing cells that accumulate replicative lesions. Chemically induced or genetic synthetic lethality is a promising area in personalized medicine, but it remains to be optimized. A new target in cancer therapy is DNA unwinding enzymes known as helicases. Helicases play critical roles in all aspects of nucleic acid metabolism. We and others have investigated small molecule targeted inhibition of helicase function by compound screens using biochemical and cell-based approaches. Small molecule-induced trapping of DNA helicases may represent a generalized mechanism exemplified by certain topoisomerase and PARP inhibitors that exert poisonous consequences, especially in rapidly dividing cancer cells. Taking the lead from the broader field of DNA repair inhibitors and new information gleaned from structural and biochemical studies of DNA helicases, we predict that an emerging strategy to identify useful helicase-interacting compounds will be structure-based molecular docking interfaced with a computational approach. Potency, specificity, drug resistance, and bioavailability of helicase inhibitor drugs and targeting such compounds to subcellular compartments where the respective helicases operate must be addressed. Beyond cancer therapy, continued and new developments in this area may lead to the discovery of helicase-interacting compounds that chemically rescue clinically relevant helicase missense mutant proteins or activate the catalytic function of wild-type DNA helicases, which may have novel therapeutic application.

Clinically Applicable Inhibitors Impacting Genome Stability

Advances in technology have facilitated the molecular profiling (genomic and transcriptomic) of tumours, and has led to improved stratification of patients and the individualisation of treatment regimes. To fully realize the potential of truly personalised treatment options, we need targeted therapies that precisely disrupt the compensatory pathways identified by profiling which allow tumours to survive or gain resistance to treatments. Here, we discuss recent advances in novel therapies that impact the genome (chromosomes and chromatin), pathways targeted and the stage of the pathways targeted. The current state of research will be discussed, with a focus on compounds that have advanced into trials (clinical and pre-clinical). We will discuss inhibitors of specific DNA damage responses and other genome stability pathways, including those in development, which are likely to synergistically combine with current therapeutic options. Tumour profiling data, combined with the knowledge of new treatments that affect the regulation of essential tumour signalling pathways, is revealing fundamental insights into cancer progression and resistance mechanisms. This is the forefront of the next evolution of advanced oncology medicine that will ultimately lead to improved survival and may, one day, result in many cancers becoming chronic conditions, rather than fatal diseases.

Mechanism underlying the effect of long-term exposure to low dose of pesticides on DNA integrity

Pesticides, including herbicides, insecticides and fungicides, are widely used in intensive agriculture. Recently, the long-term effects of pesticide exposure were found to be associated with many diseases. In this study, we evaluated the long-term effect of low-level exposure to a mixture of pesticides on DNA damage response (DDR) in relation to individual detoxifying variability. A residential population chronically exposed to pesticides was enrolled, biological/environmental pesticide levels; paroxonase 1 (PON-1) activity and 192 Q/R polymorphism and DDR were evaluated at three different periods of pesticide exposure. OGG1-dependent DNA repair activity was decreased in relation to pesticide exposure. The increase of DNA lesions and pesticide levels in the intensive pesticide-spraying period was independent on PON-1 activity. Next, human bronchial epithelial and neuronal cells were used as a model for in vitro evaluation of the mechanistic effect of pesticides. Pesticides induced mitochondrial dysfunction leading to ROS formation. ROS from mitochondria induced DNA damage, which in turn induced OGG1-dependent DNA repair activity through 8-oxoguanine DNA glycosylase 1 (OGG1) expression and activation. Even though OGG1 was overexpressed, an inhibition of its activity, associated with DNA lesion accumulation, was found at prolonged pesticide-exposure. A post-translational regulation of OGG1 by pesticide may be postulated. Taken together, long-term exposure to low-levels of pesticides affects DDR resulting in accumulation of DNA lesions that eventually may lead to cancer or neurological disorders.

Targeting Allostery with Avatars to Design Inhibitors Assessed by Cell Activity: Dissecting MRE11 Endo- and Exonuclease Activities

For inhibitor design, as in most research, the best system is question dependent. We suggest structurally defined allostery to design specific inhibitors that target regions beyond active sites. We choose systems allowing efficient quality structures with conformational changes as optimal for structure-based design to optimize inhibitors. We maintain that evolutionarily related targets logically provide molecular avatars, where this Sanskrit term for descent includes ideas of functional relationships and of being a physical embodiment of the target's essential features without requiring high sequence identity. Appropriate biochemical and cell assays provide quantitative measurements, and for biomedical impacts, any inhibitor's activity should be validated in human cells. Specificity is effectively shown empirically by testing if mutations blocking target activity remove cellular inhibitor impact. We propose this approach to be superior to experiments testing for lack of cross-reactivity among possible related enzymes, which is a challenging negative experiment. As an exemplary avatar system for protein and DNA allosteric conformational controls, we focus here on developing separation-of-function inhibitors for meiotic recombination 11 nuclease activities. This was achieved not by targeting the active site but rather by geometrically impacting loop motifs analogously to ribosome antibiotics. These loops are neighboring the dimer interface and active site act in sculpting dsDNA and ssDNA into catalytically competent complexes. One of our design constraints is to preserve DNA substrate binding to geometrically block competing enzymes and pathways from the damaged site. We validate our allosteric approach to controlling outcomes in human cells by reversing the radiation sensitivity and genomic instability in BRCA mutant cells.

Association of DNA repair genes polymorphisms and mutations with increased risk of head and neck cancer: a review

DNA repair mechanisms allow maintain genomic stability and proper functioning within the cells. Any aberrations may cause an increased risk of many diseases such as cancer. The most crucial risk factors for head and neck squamous cell carcinoma are behavioral factors, predominantly chronic exposure to tobacco, alcohol addiction, and infection with human papillomavirus or Epstein–Barr virus. These agents can induce DNA damage; therefore, cells must activate appropriate mechanisms in order to function correctly. Cancer cells are marked with genomic instability, which is associated with a greater tendency for the accumulation of a DNA damage and increased chemo- and radioresistance. Multiple studies have assessed the correlation of increased head and neck cancer (HNC) risk with polymorphism in the DNA repair genes. However, they suggest that interaction of DNA repair genes mutations with susceptibility to HNC depends on a patient’s race and risk factors, especially tobacco smoking. Further identification of these sequence variations must be performed. In this review, we discuss the current knowledge about the DNA repair genes mutations and polymorphisms associated with the high risk of head and neck treatment.