RI-1: a chemical inhibitor of RAD51 that disrupts homologous recombination in human cells

Changes in repair pathways of radiation-induced DNA double-strand breaks at the midblastula transition in Xenopus embryo

Ionizing radiation (IR) causes DNA damage, particularly DNA double-strand breaks (DSBs), which have significant implications for genome stability. The major pathways of repairing DSBs are homologous recombination (HR) and nonhomologous end joining (NHEJ). However, the repair mechanism of IR-induced DSBs in embryos is not well understood, despite extensive research in somatic cells. The externally developing aquatic organism, Xenopus tropicalis, serves as a valuable model for studying embryo development. A significant increase in zygotic transcription occurs at the midblastula transition (MBT), resulting in a longer cell cycle and asynchronous cell divisions. This study examines the impact of X-ray irradiation on Xenopus embryos before and after the MBT. The findings reveal a heightened X-ray sensitivity in embryos prior to the MBT, indicating a distinct shift in the DNA repair pathway during embryo development. Importantly, we show a transition in the dominant DSB repair pathway from NHEJ to HR before and after the MBT. These results suggest that the MBT plays a crucial role in altering DSB repair mechanisms, thereby influencing the IR sensitivity of developing embryos.

RNF4 sustains Myc-driven tumorigenesis by facilitating DNA replication

The mammalian SUMO-targeted E3 ubiquitin ligase Rnf4 has been reported to act as a regulator of DNA repair, but the importance of RNF4 as a tumor suppressor has not been tested. Using a conditional-knockout mouse model, we deleted Rnf4 in the B cell lineage to test the importance of RNF4 for growth of somatic cells. Although Rnf4-conditional-knockout B cells exhibited substantial genomic instability, Rnf4 deletion caused no increase in tumor susceptibility. In contrast, Rnf4 deletion extended the healthy lifespan of mice expressing an oncogenic c-myc transgene. Rnf4 activity is essential for normal DNA replication, and in its absence, there was a failure in ATR-CHK1 signaling of replication stress. Factors that normally mediate replication fork stability, including members of the Fanconi anemia gene family and the helicases PIF1 and RECQL5, showed reduced accumulation at replication forks in the absence of RNF4. RNF4 deficiency also resulted in an accumulation of hyper-SUMOylated proteins in chromatin, including members of the SMC5/6 complex, which contributes to replication failure by a mechanism dependent on RAD51. These findings indicate that RNF4, which shows increased expression in multiple human tumor types, is a potential target for anticancer therapy, especially in tumors expressing c-myc.

ATR signaling controls the bystander responses of human chondrosarcoma cells by promoting RAD51-dependent DNA repair

PURPOSE

Radiation-induced bystander effect (RIBE) frequently is seen as DNA damage in unirradiated bystander cells, but the repair processes initiated in response to that DNA damage are not well understood. RIBE-mediated formation of micronuclei (MN), a biomarker of persistent DNA damage, was previously observed in bystander normal fibroblast (AG01522) cells, but not in bystander human chondrosarcoma (HTB94) cells. The molecular mechanisms causing this disparity are not clear. Herein, we investigate the role of DNA repair in the bystander responses of the two cell lines.

METHODS

Cells were irradiated with X-rays and immediately co-cultured with un-irradiated cells using a trans-well insert system in which they share the same medium. The activation of DNA damage response (DDR) proteins was detected by immunofluorescence staining or Western blotting. MN formation was examined by the cytokinesis-block MN assay, which is a robust method to detect persistent DNA damage.

RESULTS

Immunofluorescent foci of γH2AX and 53BP1, biomarkers of DNA damage and repair, revealed a greater capacity for DNA repair in HTB94 cells than in AG01522 cells in both irradiated and bystander populations. Autophosphorylation of ATR at the threonine 1989 site was expressed at a greater level in HTB94 cells compared to AG01522 cells at the baseline and in response to hydroxyurea treatment or exposure to 1 Gy of X-rays. An inhibitor of ATR, but not of ATM, promoted MN formation in bystander HTB94 cells. In contrast, no effect of either inhibitor was observed in bystander AG01522 cells, indicating that ATR signaling might be a pivotal pathway to preventing the MN formation in bystander HTB94 cells. Supporting this idea, we found an ATR-dependent increase in the fractions of bystander HTB94 cells with pRPA2 S33 and RAD51 foci. A blocker of RAD51 facilitated MN formation in bystander HTB94 cells.

CONCLUSION

Our results indicate that HTB94 cells were likely more efficient in DNA repair than AG01522 cells, specifically via ATR signaling, which inhibited the bystander signal-induced MN formation. This study highlights the significance of DNA repair efficiency in bystander cell responses.

Selective inhibition of DNA ligase IV provides additional efficacy to the treatment of anaplastic thyroid cancer

Background

Although the incidence of anaplastic thyroid carcinoma (ATC) is low (2.5% of thyroid cancer cases), this cancer has a very poor prognosis (survival rates < 5 months) and accounts for 14-39% of deaths. Conventional therapies based on surgery in combination with radiotherapy or chemotherapy showed limited effectiveness primarily due to the robust and protective DNA damage response in thyroid cancer cells.

Methods

We used single-cell transcriptomic data from patients with different subtypes of thyroid cancer to study expression of genes involved in homologous recombination (HR) and non-homologous end joining (NHEJ) pathways. Then, we investigated the mechanisms of DNA damage and repair in anaplastic (C643 and Hth74) and papillary (TPC-1) thyroid cancer cell lines. The effect of caffeine (inhibitor of ATM and ATR) and UCN-01 (CHK1 inhibitor) was evaluated in cell cycle progression of thyroid cancer cells after γ-radiation or doxorubicin treatment. The DNA damage response was monitored after staining of phosphorylated γ-H2AX and 53BP1. Reporter plasmids were used to determine the efficacy of double-strand DNA breaks (DSBs) repair by HR and NHEJ in thyroid cancer cells. We evaluated the combination of selective inhibition of the DNA ligase IV by SCR7 and doxorubicin on cellular apoptosis and tumor growth in xenograft murine models of anaplastic thyroid cancer.

Results

Single-cell RNA-Seq showed that NHEJ- and HR-related genes are expressed in ATC and PTC patients. We showed that ATC cells undergo mitosis in the presence of unrepaired DNA damage caused by γ-radiation and doxorubicin treatment. To proliferate and survive, these cells efficiently repair DNA lesions using homologous recombination (HR) and non-homologous end joining (NHEJ). The combination of SCR7 with doxorubicin, significantly increased apoptosis and impaired ATC tumor growth in a xenograft mouse model compared to doxorubicin monotherapy.

Conclusion

This study shows the therapeutic value of the combination of a DNA ligase IV inhibitor and DNA-damaging agents (doxorubicin and/or γ-radiation) for the treatment of anaplastic thyroid cancer.

SARS-CoV-2 exploits cellular RAD51 to promote viral propagation: implication of RAD51 inhibitor as a potential drug candidate against COVID-19

IMPORTANCE

Viruses are constantly evolving to promote propagation in the host. Here, we show that severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) utilizes host RAD51 for replication. Silencing of RAD51 impaired SARS-CoV-2 propagation. Viral RNA colocalized with RAD51 in the cytoplasm of SARS-CoV-2-infected cells, suggesting that both viral RNA and RAD51 may form a replication complex. We, therefore, evaluated RAD51 inhibitors as possible therapeutic agents against SARS-CoV-2. Indeed, RAD51 inhibitors exerted antiviral activities against not only Wuhan but also variants of SARS-CoV-2. Molecular docking model shows that RAD51 inhibitors impede SARS-CoV-2 propagation by interfering with dimerization of RAD51. These data suggest that RAD51 may represent a novel host-based drug target for coronavirus disease 2019 treatment.

The impact of DNA double-strand break repair pathways throughout the carbon ion spread-out Bragg peak beam

Following carbon ion beam irradiation in mammalian cells, such as used in carbon ion radiotherapy (CIRT), it has been suggested that the balance between whether nonhomologous end joining (NHEJ) or homologous recombination (HR) is utilized depends on the DNA double-strand break (DSB) complexity. Here, we quantified DSB distribution and identified the importance of each DSB repair pathway at increasing depths within the carbon ion spread-out Bragg peak (SOBP) beam range. Chinese hamster ovary (CHO) cell lines were irradiated in a single biological system capable of incorporating the full carbon ion SOBP beam range. Cytotoxicity and DSB distribution/repair kinetics were examined at increasing beam depths using cell survival as an endpoint and γ-H2AX as a surrogate marker for DSBs. We observed that proximal SOBP had the highest number of total foci/cell and lowest survival, while distal SOBP had the most dense tracks. Both NHEJ- and HR-deficient CHO cells portrayed an increase in radiosensitivity throughout the full carbon beam range, although NHEJ-deficient cells were the most radiosensitive cell line from beam entrance up to proximal SOBP and demonstrated a dose-dependent decrease in ability to repair DSBs. In contrast, HR-deficient cells had the greatest ratio of survival fraction at entrance depth to the lowest survival fraction within the SOBP and demonstrated a linear energy transfer (LET)-dependent decrease in ability to repair DSBs. Collectively, our results provide insight into treatment planning and potential targets to inhibit, as HR was a more beneficial pathway to inhibit than NHEJ to enhance the cell killing effect of CIRT in targeted tumor cells within the SOBP while maintaining limited unwanted damage to surrounding healthy cells.

High-efficiency and multilocus targeted integration in CHO cells using CRISPR-mediated donor nicking and DNA repair inhibitors

Efforts to leverage clustered regularly interspaced short palindromic repeats/CRISPR-associated protein 9 (CRISPR/Cas9) for targeted genomic modifications in mammalian cells are limited by low efficiencies and heterogeneous outcomes. To aid method optimization, we developed an all-in-one reporter system, including a novel superfolder orange fluorescent protein (sfOrange), to simultaneously quantify gene disruption, site-specific integration (SSI), and random integration (RI). SSI strategies that utilize different donor plasmid formats and Cas9 nuclease variants were evaluated for targeting accuracy and efficiency in Chinese hamster ovary cells. Double-cut and double-nick donor formats significantly improved targeting accuracy by 2.3-8.3-fold and 19-22-fold, respectively, compared to standard circular donors. Notably, Cas9-mediated donor linearization was associated with increased RI events, whereas donor nicking minimized RI without sacrificing SSI efficiency and avoided low-fidelity outcomes. A screen of 10 molecules that modulate the major mammalian DNA repair pathways identified two inhibitors that further enhance targeting accuracy and efficiency to achieve SSI in 25% of transfected cells without selection. The optimized methods integrated transgene expression cassettes with 96% efficiency at a single locus and with 53%-55% efficiency at two loci simultaneously in selected clones. The CRISPR-based tools and methods developed here could inform the use of CRISPR/Cas9 in mammalian cell lines, accelerate mammalian cell line engineering, and support advanced recombinant protein production applications.

DNA repair pathways in breast cancer: from mechanisms to clinical applications

BACKGROUND

Breast cancer (BC) is a complex disease with various subtypes and genetic alterations that impact DNA repair pathways. Understanding these pathways is essential for developing effective treatments and improving patient outcomes.

AREA COVERED

This study investigates the significance of DNA repair pathways in breast cancer, specifically focusing on various pathways such as nucleotide excision repair, base excision repair, mismatch repair, homologous recombination repair, non-homologous end joining, fanconi anemia pathway, translesion synthesis, direct repair, and DNA damage tolerance. The study also examines the role of these pathways in breast cancer resistance and explores their potential as targets for cancer treatment.

CONCLUSION

Recent advances in targeted therapies have shown promise in exploiting DNA repair pathways for BC treatment. However, much research is needed to improve the efficacy of these therapies and identify new targets. Additionally, personalized treatments that target specific DNA repair pathways based on tumor subtype or genetic profile are being developed. Advances in genomics and imaging technologies can potentially improve patient stratification and identify biomarkers of treatment response. However, many challenges remain, including toxicity, resistance, and the need for more personalized treatments. Continued research and development in this field could significantly improve BC treatment.

Synthesis and Biological Evaluation of DIDS Analogues as Efficient Inhibitors of RAD51 Involved in Homologous Recombination

RAD51 is a pivotal protein of the homologous recombination DNA repair pathway, and is overexpressed in some cancer cells, disrupting then the efficiency of cancer-treatments. The development of RAD51 inhibitors appears as a promising solution to restore these cancer cells sensitization to radio- or chemotherapy. From a small molecule identified as a modulator of RAD51, the 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), two series of analogues with small or bulky substituents on the aromatic parts of the stilbene moiety were prepared for a structure-activity relationship study. Three compounds, the cyano analogue (12), and benzamide (23) or phenylcarbamate (29) analogues of DIDS were characterized as novel potent RAD51 inhibitors with HR inhibition in the micromolar range.

Heat Shock Protein Family A Member 1 Promotes Intracellular Amplification of Hepatitis B Virus Covalently Closed Circular DNA

Hepatitis B virus (HBV) contains a partially double-stranded relaxed circular DNA (rcDNA) genome that is converted into a covalently closed circular DNA (cccDNA) in the nucleus of the infected hepatocyte by cellular DNA repair machinery. cccDNA associates with nucleosomes to form a minichromosome that transcribes RNA to support the expression of viral proteins and reverse transcriptional replication of viral DNA. In addition to the de novo synthesis from incoming virion rcDNA, cccDNA can also be synthesized from rcDNA in the progeny nucleocapsids within the cytoplasm of infected hepatocytes via the intracellular amplification pathway. In our efforts to identify cellular DNA repair proteins required for cccDNA synthesis using a chemogenetic screen, we found that B02, a small-molecule inhibitor of DNA homologous recombination repair protein RAD51, significantly enhanced the synthesis of cccDNA via the intracellular amplification pathway in human hepatoma cells. Ironically, neither small interfering RNA (siRNA) knockdown of RAD51 expression nor treatment with another structurally distinct RAD51 inhibitor or activator altered cccDNA amplification. Instead, it was found that B02 treatment significantly elevated the levels of multiple heat shock protein mRNA, and siRNA knockdown of HSPA1 expression or treatment with HSPA1 inhibitors significantly attenuated B02 enhancement of cccDNA amplification. Moreover, B02-enhanced cccDNA amplification was efficiently inhibited by compounds that selectively inhibit DNA polymerase α or topoisomerase II, the enzymes required for cccDNA intracellular amplification. Our results thus indicate that B02 treatment induces a heat shock protein-mediated cellular response that positively regulates the conversion of rcDNA into cccDNA via the authentic intracellular amplification pathway. IMPORTANCE Elimination or functional inactivation of cccDNA minichromosomes in HBV-infected hepatocytes is essential for the cure of chronic hepatitis B virus (HBV) infection. However, lack of knowledge of the molecular mechanisms of cccDNA metabolism and regulation hampers the development of antiviral drugs to achieve this therapeutic goal. Our findings reported here imply that enhanced cccDNA amplification may occur under selected pathobiological conditions, such as cellular stress, to subvert the dilution or elimination of cccDNA and maintain the persistence of HBV infection. Therapeutic inhibition of HSPA1-enhanced cccDNA amplification under these pathobiological conditions should facilitate the elimination of cccDNA and cure of chronic hepatitis B.

Targeting RAD51-Mediated Homologous Recombination as a Treatment for Advanced Solid and Hematologic Malignancies: Opportunities and Challenges Ahead

RAD51 is integral in homologous recombination DNA damage repair and has garnered much interest as both a biomarker and potential therapeutic target in oncology. Multiple in vitro and in vivo studies have demonstrated its role as a predictive marker, particularly in the context of platinum-based therapies and poly ADP-ribose polymerase (PARP) inhibitors. In this review, we highlight the development of RAD51 inhibitors, with a focus on novel molecules and ongoing clinical trials. Despite many efforts to develop effective and tolerable direct RAD51 inhibitors, identification of these agents remains challenging. Clinically, however, there may be a role of pharmacological indirect RAD51 inhibition.

The BLM helicase is a new therapeutic target in multiple myeloma involved in replication stress survival and drug resistance

Multiple myeloma (MM) is a hematologic cancer characterized by accumulation of malignant plasma cells in the bone marrow. To date, no definitive cure exists for MM and resistance to current treatments is one of the major challenges of this disease. The DNA helicase BLM, whose depletion or mutation causes the cancer-prone Bloom's syndrome (BS), is a central factor of DNA damage repair by homologous recombination (HR) and genomic stability maintenance. Using independent cohorts of MM patients, we identified that high expression of BLM is associated with a poor outcome with a significant enrichment in replication stress signature. We provide evidence that chemical inhibition of BLM by the small molecule ML216 in HMCLs (human myeloma cell lines) leads to cell cycle arrest and increases apoptosis, likely by accumulation of DNA damage. BLM inhibition synergizes with the alkylating agent melphalan to efficiently inhibit growth and promote cell death in HMCLs. Moreover, ML216 treatment re-sensitizes melphalan-resistant cell lines to this conventional therapeutic agent. Altogether, these data suggest that inhibition of BLM in combination with DNA damaging agents could be of therapeutic interest in the treatment of MM, especially in those patients with high BLM expression and/or resistance to melphalan.

Abrogation of Cellular Senescence Induced by Temozolomide in Glioblastoma Cells: Search for Senolytics

A first-line therapeutic for high-grade glioma, notably glioblastoma (GBM), is the DNA methylating drug temozolomide (TMZ). Previously, we showed that TMZ induces not only apoptosis and autophagy, but also cellular senescence (CSEN). We presented the hypothesis that GBM cells may escape from CSEN, giving rise to recurrent tumors. Furthermore, the inflammatory phenotype associated with CSEN may attenuate chemotherapy and drive tumor progression. Therefore, treatments that specifically target senescent cells, i.e., senolytic drugs, may lead to a better outcome of GBM therapy by preventing recurrences and tumor inflammation. Here, we tested Bcl-2 targeting drugs including ABT-737, ABT-263 (navitoclax), several natural substances such as artesunate, fisetin and curcumin as well as lomustine (CCNU) and ionizing radiation (IR) for their senolytic capacity in GBM cells. Additionally, several proteins involved in the DNA damage response (DDR), ATM, ATR, Chk1/2, p53, p21, NF-kB, Rad51, PARP, IAPs and autophagy, a pathway involved in CSEN induction, were tested for their impact in maintaining CSEN. Treatment of GBM cells with a low dose of TMZ for 8-10 days resulted in &gt;80% CSEN, confirming CSEN to be the major trait induced by TMZ. To identify senolytics, we treated the senescent population with the compounds of interest and found that ABT-737, navitoclax, chloroquine, ATMi, ATRi, BV-6, PX-866 and the natural compounds fisetin and artesunate exhibit senolytic activity, inducing death in senescent cells more efficiently than in proliferating cells. Curcumin showed the opposite effect. No specific effect on CSEN cells was observed by inhibition of Chk1/Chk2, p21, NF-kB, Rad51 and PARP. We conclude that these factors neither play a critical role in maintaining TMZ-induced CSEN nor can their inhibitors be considered as senolytics. Since IR and CCNU did not exhibit senolytic activity, radio- and chemotherapy with alkylating drugs is not designed to eliminate TMZ-induced senescent cancer cells.

Targeting Replication Stress Response Pathways to Enhance Genotoxic Chemo- and Radiotherapy

Proliferating cells regularly experience replication stress caused by spontaneous DNA damage that results from endogenous reactive oxygen species (ROS), DNA sequences that can assume secondary and tertiary structures, and collisions between opposing transcription and replication machineries. Cancer cells face additional replication stress, including oncogenic stress that results from the dysregulation of fork progression and origin firing, and from DNA damage induced by radiotherapy and most cancer chemotherapeutic agents. Cells respond to such stress by activating a complex network of sensor, signaling and effector pathways that protect genome integrity. These responses include slowing or stopping active replication forks, protecting stalled replication forks from collapse, preventing late origin replication firing, stimulating DNA repair pathways that promote the repair and restart of stalled or collapsed replication forks, and activating dormant origins to rescue adjacent stressed forks. Currently, most cancer patients are treated with genotoxic chemotherapeutics and/or ionizing radiation, and cancer cells can gain resistance to the resulting replication stress by activating pro-survival replication stress pathways. Thus, there has been substantial effort to develop small molecule inhibitors of key replication stress proteins to enhance tumor cell killing by these agents. Replication stress targets include ATR, the master kinase that regulates both normal replication and replication stress responses; the downstream signaling kinase Chk1; nucleases that process stressed replication forks (MUS81, EEPD1, Metnase); the homologous recombination catalyst RAD51; and other factors including ATM, DNA-PKcs, and PARP1. This review provides an overview of replication stress response pathways and discusses recent pre-clinical studies and clinical trials aimed at improving cancer therapy by targeting replication stress response factors.

Noncanonical NF-κB factor p100/p52 regulates homologous recombination and modulates sensitivity to DNA-damaging therapy

Homologous recombination (HR) serves multiple roles in DNA repair that are essential for maintaining genomic stability, including double-strand DNA break (DSB) repair. The central HR protein, RAD51, is frequently overexpressed in human malignancies, thereby elevating HR proficiency and promoting resistance to DNA-damaging therapies. Here, we find that the non-canonical NF-κB factors p100/52, but not RelB, control the expression of RAD51 in various human cancer subtypes. While p100/p52 depletion inhibits HR function in human tumor cells, it does not significantly influence the proficiency of non-homologous end joining, the other key mechanism of DSB repair. Clonogenic survival assays were performed using a pair DLD-1 cell lines that differ only in their expression of the key HR protein BRCA2. Targeted silencing of p100/p52 sensitizes the HR-competent cells to camptothecin, while sensitization is absent in HR-deficient control cells. These results suggest that p100/p52-dependent signaling specifically controls HR activity in cancer cells. Since non-canonical NF-κB signaling is known to be activated after various forms of genomic crisis, compensatory HR upregulation may represent a natural consequence of DNA damage. We propose that p100/p52-dependent signaling represents a promising oncologic target in combination with DNA-damaging treatments.

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.

dCas9-based gene editing for cleavage-free genomic knock-in of long sequences

Gene editing is a powerful tool for genome and cell engineering. Exemplified by CRISPR–Cas, gene editing could cause DNA damage and trigger DNA repair processes that are often error-prone. Such unwanted mutations and safety concerns can be exacerbated when altering long sequences. Here we couple microbial single-strand annealing proteins (SSAPs) with catalytically inactive dCas9 for gene editing. This cleavage-free gene editor, dCas9–SSAP, promotes the knock-in of long sequences in mammalian cells. The dCas9–SSAP editor has low on-target errors and minimal off-target effects, showing higher accuracy than canonical Cas9 methods. It is effective for inserting kilobase-scale sequences, with an efficiency of up to approximately 20% and robust performance across donor designs and cell types, including human stem cells. We show that dCas9–SSAP is less sensitive to inhibition of DNA repair enzymes than Cas9 references. We further performed truncation and aptamer engineering to minimize its size to fit into a single adeno-associated-virus vector for future application. Together, this tool opens opportunities towards safer long-sequence genome engineering.

Wang, Qu et al. developed a genome-editing system, utilizing catalytically inactive Cas9 fused to microbial single-strand annealing proteins, for kilobase-scale insertion in human cells without introducing DNA nicks or breaks.

The trans cell cycle effects of PARP inhibitors underlie their selectivity toward BRCA1/2-deficient cells

In this study, Simoneau et al. investigated why PARPi is more effective than other DNA-damaging drugs when used to treat BRCA1/2-deficient tumors. They show that PARPi induces DSBs progressively through trans-cell-cycle ssDNA gaps, and BRCA1/2-deficient cells fail to slow down and repair DSBs over multiple cell cycles, explaining the unique efficacy of PARPi in BRCA1/2-deficient cells.

PARP inhibitor (PARPi) is widely used to treat BRCA1/2-deficient tumors, but why PARPi is more effective than other DNA-damaging drugs is unclear. Here, we show that PARPi generates DNA double-strand breaks (DSBs) predominantly in a trans cell cycle manner. During the first S phase after PARPi exposure, PARPi induces single-stranded DNA (ssDNA) gaps behind DNA replication forks. By trapping PARP on DNA, PARPi prevents the completion of gap repair until the next S phase, leading to collisions of replication forks with ssDNA gaps and a surge of DSBs. In the second S phase, BRCA1/2-deficient cells are unable to suppress origin firing through ATR, resulting in continuous DNA synthesis and more DSBs. Furthermore, BRCA1/2-deficient cells cannot recruit RAD51 to repair collapsed forks. Thus, PARPi induces DSBs progressively through trans cell cycle ssDNA gaps, and BRCA1/2-deficient cells fail to slow down and repair DSBs over multiple cell cycles, explaining the unique efficacy of PARPi in BRCA1/2-deficient cells.

Inhibition of the DSB repair protein RAD51 potentiates the cytotoxic efficacy of doxorubicin via promoting apoptosis-related death pathways

The anthracycline derivative doxorubicin (Doxo) induces DNA double-strand breaks (DSBs) by inhibition of DNA topoisomerase type II. Defective mismatch repair (MMR) contributes to Doxo resistance and has been reported for colon and mammary carcinomas. Here, we investigated the outcome of pharmacological inhibition of various DNA repair-related mechanisms on Doxo-induced cytotoxicity employing MMR-deficient HCT-116 colon carcinoma cells. Out of different inhibitors tested (i.e. HDACi, PARPi, MRE11i, RAD52i, RAD51i), we identified the RAD51-inhibitor B02 as the most powerful compound to synergistically increase Doxo-induced cytotoxicity. B02-mediated synergism rests on pleiotropic mechanisms, including pronounced G2/M arrest, damage to mitochondria and caspase-driven apoptosis. Of note, B02 also promotes the cytotoxicity of oxaliplatin and 5-fluoruracil (5-FU) in HCT-116 cells and, furthermore, also increases Doxo-induced cytotoxicity in MMR-proficient colon and mammary carcinoma cells. Summarizing, pharmacological inhibition of RAD51 is suggested to synergistically increase the cytotoxic efficacy of various types of conventional anticancer drugs in different tumor entities. Hence, pre-clinical in vivo studies are preferable to determine the therapeutic window of B02 in a clinically oriented therapeutic regimen.

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.

Inhibiting homologous recombination by targeting RAD51 protein

Homologous recombination (HR) is involved in repairing DNA double-strand breaks (DSB), the most harmful for the cell. Regulating HR is essential for maintaining genomic stability. In many forms of cancer, overactivation of HR increases tumor resistance to DNA-damaging treatments. RAD51, HR's core protein, is very often over-expressed in these cancers and plays a critical role in cancer cell development and survival. Targeting RAD51 directly to reduce its activity and its expression is therefore one strategy to sensitize and overcome resistance cancer cells to existing DNA-damaging therapies which remains the limiting factor for the success of targeted therapy. This review describes the structure and biological roles of RAD51, summarizes the different targeted sites of RAD51 and its inhibitory compounds discovered and described in the last decade.

Exploiting DNA Damage Repair in Precision Cancer Therapy: BRCA1 as a Prime Therapeutic Target

Simple Summary

Chemical inhibition of central DNA damage repair (DDR) proteins has become a promising approach in precision cancer therapy. In particular, BRCA1 and its DDR-associated proteins constitute important targets for developing DNA repair inhibiting drugs. This review provides relevant insights on DDR biology and pharmacology, aiming to boost the development of more effective DDR targeted therapies.

Abstract

Precision medicine aims to identify specific molecular alterations, such as driver mutations, allowing tailored and effective anticancer therapies. Poly(ADP)-ribose polymerase inhibitors (PARPi) are the prototypical example of targeted therapy, exploiting the inability of cancer cells to repair DNA damage. Following the concept of synthetic lethality, PARPi have gained great relevance, particularly in BRCA1 dysfunctional cancer cells. In fact, BRCA1 mutations culminate in DNA repair defects that can render cancer cells more vulnerable to therapy. However, the efficacy of these drugs has been greatly affected by the occurrence of resistance due to multi-connected DNA repair pathways that may compensate for each other. Hence, the search for additional effective agents targeting DNA damage repair (DDR) is of crucial importance. In this context, BRCA1 has assumed a central role in developing drugs aimed at inhibiting DNA repair activity. Collectively, this review provides an in-depth understanding of the biology and regulatory mechanisms of DDR pathways, highlighting the potential of DDR-associated molecules, particularly BRCA1 and its interconnected partners, in precision cancer medicine. It also affords an overview about what we have achieved and a reflection on how much remains to be done in this field, further addressing encouraging clues for the advance of DDR targeted therapy.

New RAD51 Inhibitors to Target Homologous Recombination in Human Cells

Targeting DNA repair proteins with small-molecule inhibitors became a proven anti-cancer strategy. Previously, we identified an inhibitor of a major protein of homologous recombination (HR) RAD51, named B02. B02 inhibited HR in human cells and sensitized them to chemotherapeutic drugs in vitro and in vivo. Here, using a medicinal chemistry approach, we aimed to improve the potency of B02. We identified the B02 analog, B02-isomer, which inhibits HR in human cells with significantly higher efficiency. We also show that B02-iso sensitizes triple-negative breast cancer MDA-MB-231 cells to the PARP inhibitor (PARPi) olaparib.

Regulation of RAD51 at the Transcriptional and Functional Levels: What Prospects for Cancer Therapy?

The RAD51 recombinase is a critical effector of Homologous Recombination (HR), which is an essential DNA repair mechanism for double-strand breaks. The RAD51 protein is recruited onto the DNA break by BRCA2 and forms homopolymeric filaments that invade the homologous chromatid and use it as a template for repair. RAD51 filaments are detectable by immunofluorescence as distinct foci in the cell nucleus, and their presence is a read out of HR proficiency. RAD51 is an essential gene, protecting cells from genetic instability. Its expression is low and tightly regulated in normal cells and, contrastingly, elevated in a large fraction of cancers, where its level of expression and activity have been linked with sensitivity to genotoxic treatment. In particular, BRCA-deficient tumors show reduced or obliterated RAD51 foci formation and increased sensitivity to platinum salt or PARP inhibitors. However, resistance to treatment sets in rapidly and is frequently based on a complete or partial restoration of RAD51 foci formation. Consequently, RAD51 could be a highly valuable therapeutic target. Here, we review the multiple levels of regulation that impact the transcription of the RAD51 gene, as well as the post-translational modifications that determine its expression level, recruitment on DNA damage sites and the efficient formation of homofilaments. Some of these regulation levels may be targeted and their impact on cancer cell survival discussed.

Recent Advances in Therapeutic Application of DNA Damage Response Inhibitors against Cancer

DNA integrity is continuously challenged by intrinsic cellular processes and environmental agents. To overcome this genomic damage, cells have developed multiple signaling pathways collectively named as DNA damage response (DDR) and composed of three components: (i) sensor proteins, which detect DNA damage, (ii) mediators that relay the signal downstream and recruit the repair machinery, and (iii) the repair proteins, which restore the damaged DNA. A flawed DDR and failure to repair the damage lead to the accumulation of genetic lesions and increased genomic instability, which is recognized as a hallmark of cancer. Cancer cells tend to harbor increased mutations in DDR genes and often have fewer DDR pathways than normal cells. This makes cancer cells more dependent on particular DDR pathways and thus become more susceptible to compounds inhibiting those pathways compared to normal cells, which have all the DDR pathways intact. Understanding the roles of different DDR proteins in the DNA damage response and repair pathways and identification of their structures have paved the way for the development of their inhibitors as targeted cancer therapy. In this review, we describe the major participants of various DDR pathways, their significance in carcinogenesis, and focus on the inhibitors developed against several key DDR proteins.

Microbial single-strand annealing proteins enable CRISPR gene-editing tools with improved knock-in efficiencies and reduced off-target effects

Several existing technologies enable short genomic alterations including generating indels and short nucleotide variants, however, engineering more significant genomic changes is more challenging due to reduced efficiency and precision. Here, we developed RecT Editor via Designer-Cas9-Initiated Targeting (REDIT), which leverages phage single-stranded DNA-annealing proteins (SSAP) RecT for mammalian genome engineering. Relative to Cas9-mediated homology-directed repair (HDR), REDIT yielded up to a 5-fold increase of efficiency to insert kilobase-scale exogenous sequences at defined genomic regions. We validated our REDIT approach using different formats and lengths of knock-in templates. We further demonstrated that REDIT tools using Cas9 nickase have efficient gene-editing activities and reduced off-target errors, measured using a combination of targeted sequencing, genome-wide indel, and insertion mapping assays. Our experiments inhibiting repair enzyme activities suggested that REDIT has the potential to overcome limitations of endogenous DNA repair steps. Finally, our REDIT method is applicable across cell types including human stem cells, and is generalizable to different Cas9 enzymes.

XRCC2 repairs mitochondrial DNA damage and fuels malignant behavior in hepatocellular carcinoma

The effects of DNA damage repair (DDR) and mitochondrial dysfunction associated with HCC have been investigated, but the functional role of mitochondrial DDR in HCC remains elusive. We studied the DDR genes and identified XRCC2 as a potential prognostic marker for HCC. XRCC2 overexpression was detected in HCC cells and shown to promote the malignant behavior of cancer cells. XRCC2 depletion in HCC cells led to DNA damage accumulation at the replication site in the nucleus. Additionally, XRCC2-depleted HCC cells exhibited impaired mitochondrial respiration and reduced complex I (CI) activity as XRCC2 was responsible for elimination of mitochondrial DNA (mtDNA) damage and maintenance of mtDNA-encoded CI-related genes' transcription in a RAD51-dependent manner. We showed that tunicamycin (Tm)-activated sXBP1 bound to the TGTCAT domain and suppressed XRCC2 expression. In HCC patients, we observed a negative correlation between XBP1 and XRCC2 expression. Moreover, XRCC2 inhibition by Tm led to genomic and mtDNA damage, which impaired the transcription of mtDNA-encoded CI-related genes and prevented tumor proliferation in vivo. We described the role of XRCC2 in mtDNA damage repair and HCC progression while unveiling the potential anti-tumor effect of Tm.

Small Molecule Inhibitors Targeting Key Proteins in the DNA Damage Response for Cancer Therapy

DNA damage response (DDR) is a complicated interactional pathway. Defects that occur in subordinate pathways of the DDR pathway can lead to genomic instability and cancer susceptibility. Abnormal expression of some proteins in DDR, especially in the DNA repair pathway, are associated with the subsistence and resistance of cancer cells. Therefore, the development of small molecule inhibitors targeting the chief proteins in the DDR pathway is an effective strategy for cancer therapy. In this review, we summarize the development of small molecule inhibitors targeting chief proteins in the DDR pathway, particularly focusing on their implications for cancer therapy. We present the action mode of DDR molecule inhibitors in preclinical studies and clinical cancer therapy, including monotherapy and combination therapy with chemotherapeutic drugs or checkpoint suppression therapy.

The Valproate Mediates Radio-Bidirectional Regulation Through RFWD3-Dependent Ubiquitination on Rad51

Ionizing radiation (IR) can induce DNA double-strand breaks (DSBs) in tumor cells during radiotherapy (RT), but the efficiency of RT is limited because of the toxicity to normal cells. Locating an adjuvant treatment to alleviate damage in normal cells while sensitizing tumor cells to IR has attracted much attention. Here, using the 7,12-dimethylbenz[α]anthracene (DMBA)-induced malignant transformed MCF10A cells, we found that valproate (VPA), a histone deacetylase inhibitor (HDACi), radiosensitized transformed cells while alleviated IR-induced damage in normal cells at a safe dose (0.5 mM). We further demonstrated the decrease of homologous recombination (HR)-associated Rad51 in the transformed cells was related to the increase of its ubiquitination regulated by E3 ligase RFWD3 for the radiosensitization, which was opposite to normal cells, indicating that RFWD3-dependent ubiquitination on Rad51 was involved in the VPA-mediated radio-bidirectional effect. Through DMBA-transformed breast cancer rat model, VPA at 200 mg/kg radiosensitized tumor tissue cells by increasing RFWD3 and inhibited Rad51, while radioprotected normal tissue cells by decreasing RFWD3 and enhanced Rad51. In addition, we found high-level Rad51 was associated with tumorigenesis and poor prognosis in breast cancer patients. Our findings uncovered RFWD3-dependent Rad51 ubiquitination was the novel mechanism of VPA-mediated radio-bidirectional effect, VPA is a potential adjuvant treatment for tumor RT.

A small-molecule inhibitor of the BRCA2-RAD51 interaction modulates RAD51 assembly and potentiates DNA damage-induced cell death

BRCA2 controls RAD51 recombinase during homologous DNA recombination (HDR) through eight evolutionarily conserved BRC repeats, which individually engage RAD51 via the motif Phe-x-x-Ala. Using structure-guided molecular design, templated on a monomeric thermostable chimera between human RAD51 and archaeal RadA, we identify CAM833, a 529 Da orthosteric inhibitor of RAD51:BRC with a K_d of 366 nM. The quinoline of CAM833 occupies a hotspot, the Phe-binding pocket on RAD51 and the methyl of the substituted α-methylbenzyl group occupies the Ala-binding pocket. In cells, CAM833 diminishes formation of damage-induced RAD51 nuclear foci; inhibits RAD51 molecular clustering, suppressing extended RAD51 filament assembly; potentiates cytotoxicity by ionizing radiation, augmenting 4N cell-cycle arrest and apoptotic cell death and works with poly-ADP ribose polymerase (PARP)1 inhibitors to suppress growth in BRCA2-wildtype cells. Thus, chemical inhibition of the protein-protein interaction between BRCA2 and RAD51 disrupts HDR and potentiates DNA damage-induced cell death, with implications for cancer therapy.

Exploiting DNA repair pathways for tumor sensitization, mitigation of resistance, and normal tissue protection in radiotherapy

More than half of cancer patients are treated with radiotherapy, which kills tumor cells by directly and indirectly inducing DNA damage, including cytotoxic DNA double-strand breaks (DSBs). Tumor cells respond to these threats by activating a complex signaling network termed the DNA damage response (DDR). The DDR arrests the cell cycle, upregulates DNA repair, and triggers apoptosis when damage is excessive. The DDR signaling and DNA repair pathways are fertile terrain for therapeutic intervention. This review highlights strategies to improve therapeutic gain by targeting DDR and DNA repair pathways to radiosensitize tumor cells, overcome intrinsic and acquired tumor radioresistance, and protect normal tissue. Many biological and environmental factors determine tumor and normal cell responses to ionizing radiation and genotoxic chemotherapeutics. These include cell type and cell cycle phase distribution; tissue/tumor microenvironment and oxygen levels; DNA damage load and quality; DNA repair capacity; and susceptibility to apoptosis or other active or passive cell death pathways. We provide an overview of radiobiological parameters associated with X-ray, proton, and carbon ion radiotherapy; DNA repair and DNA damage signaling pathways; and other factors that regulate tumor and normal cell responses to radiation. We then focus on recent studies exploiting DSB repair pathways to enhance radiotherapy therapeutic gain.

Synthetic Lethality through the Lens of Medicinal Chemistry

Personalized medicine and therapies represent the goal of modern medicine, as drug discovery strives to move away from one-cure-for-all and makes use of the various targets and biomarkers within differing disease areas. This approach, especially in oncology, is often undermined when the cells make use of alternative survival pathways. As such, acquired resistance is unfortunately common. In order to combat this phenomenon, synthetic lethality is being investigated, making use of existing genetic fragilities within the cancer cell. This Perspective highlights exciting targets within synthetic lethality, (PARP, ATR, ATM, DNA-PKcs, WEE1, CDK12, RAD51, RAD52, and PD-1) and discusses the medicinal chemistry programs being used to interrogate them, the challenges these programs face, and what the future holds for this promising field.

Predictive biomarkers for sacituzumab govitecan efficacy in Trop-2-expressing triple-negative breast cancer

Sacituzumab govitecan (SG) is an antibody-drug conjugate composed of a humanized anti-Trop-2 IgG antibody conjugated via a hydrolysable linker to SN-38, the topoisomerase I-inhibitory active component of irinotecan. We investigated whether Trop-2-expression and homologous recombination repair (HRR) of SN-38-mediated double-strand DNA (dsDNA) breaks play a role in the sensitivity of triple-negative breast cancer (TNBC) to SG. Activation of HRR pathways, as evidenced by Rad51 expression, was assessed in SG-sensitive cell lines with low and moderate Trop-2-expression (SK-MES-1 squamous cell lung carcinoma and HCC1806 TNBC, respectively), compared to a low Trop-2-expressing, less SG-sensitive TNBC cell line (MDA-MB-231). Further, two Trop-2-transfectants of MDA-MB-231, C13 and C39 (4- and 25-fold higher Trop-2, respectively), were treated in mice with SG to determine whether increasing Trop-2 expression improves SG efficacy. SG mediated >2-fold increase in Rad51 in MDA-MB-231 but had no effect in SK-MES-1 or HCC1806, resulting in lower levels of dsDNA breaks in MDA-MB-231. SG and saline produced similar effects in parental MDA-MB-231 tumor-bearing mice (median survival time (MST) = 21d and 19.5d, respectively). However, in mice bearing higher Trop-2-expressing C13 and C39 tumors after Trop-2 transfection, SG provided a significant survival benefit, even compared to irinotecan (MST = 97d vs. 35d for C13, and 81d vs. 28d for C39, respectively; P < 0.0007). These results suggest that SG could provide better clinical benefit than irinotecan in patients with HRR-proficient tumors expressing high levels of Trop-2, as well as to patients with HRR-deficient tumors expressing low/moderate levels of Trop-2.

Regulation and pharmacological targeting of RAD51 in cancer

Regulation of homologous recombination (HR) is central for cancer prevention. However, too little HR can increase cancer incidence, whereas too much HR can drive cancer resistance to therapy. Importantly, therapeutics targeting HR deficiency have demonstrated a profound efficacy in the clinic improving patient outcomes, particularly for breast and ovarian cancer. RAD51 is central to DNA damage repair in the HR pathway. As such, understanding the function and regulation of RAD51 is essential for cancer biology. This review will focus on the role of RAD51 in cancer and beyond and how modulation of its function can be exploited as a cancer therapeutic.

Canonical non-homologous end-joining promotes genome mutagenesis and translocations induced by transcription-associated DNA topoisomerase 2 activity

DNA topoisomerase II (TOP2) is a major DNA metabolic enzyme, with important roles in replication, transcription, chromosome segregation and spatial organisation of the genome. TOP2 is the target of a class of anticancer drugs that poison the DNA-TOP2 transient complex to generate TOP2-linked DNA double-strand breaks (DSBs). The accumulation of DSBs kills tumour cells but can also result in genome instability. The way in which topoisomerase activity contributes to transcription remains unclear. In this work we have investigated how transcription contributes to TOP2-dependent DSB formation, genome instability and cell death. Our results demonstrate that gene transcription is an important source of abortive TOP2 activity. However, transcription does not contribute significantly to apoptosis or cell death promoted by TOP2-induced DSBs. On the contrary: transcription-dependent breaks greatly contribute to deleterious mutations and translocations, and can promote oncogenic rearrangements. Importantly, we show that TOP2-induced genome instability is mediated by mutagenic canonical non-homologous end joining whereas homologous recombination protects cells against these insults. Collectively, these results uncover mechanisms behind deleterious effects of TOP2 abortive activity during transcription, with relevant implications for chemotherapy.

Targeting DNA Double-Strand Break (DSB) Repair to Counteract Tumor Radio-resistance

During the last decade, advances of radiotherapy (RT) have been made in the clinical practice of cancer treatment. RT exerts its anticancer effect mainly via leading to the DNA Double-Strand Break (DSB), which is one of the most toxic DNA damages. Non-Homologous End Joining (NHEJ) and Homologous Recombination (HR) are two major DSB repair pathways in human cells. It is known that dysregulations of DSB repair elicit a predisposition to cancer and probably result in resistance to cancer therapies including RT. Therefore, targeting the DSB repair presents an attractive strategy to counteract radio-resistance. In this review, we describe the latest knowledge of the two DSB repair pathways, focusing on several key proteins contributing to the repair, such as DNA-PKcs, RAD51, MRN and PARP1. Most importantly, we discuss the possibility of overcoming radiation resistance by targeting these proteins for therapeutic inhibition. Recent tests of DSB repair inhibitors in the laboratory and their translations into clinical studies are also addressed.

Targeting Rad51 as a strategy for the treatment of melanoma cells resistant to MAPK pathway inhibition

Rad51 is an essential factor of the homologous recombination DNA repair pathway and therefore plays an important role in maintaining genomic stability. We show that RAD51 and other homologous recombination repair genes are overexpressed in metastatic melanoma cell lines and in melanoma patient samples, which correlates with reduced survival of melanoma patients. In addition, Rad51 expression in melanoma cells was regulated on a transcriptional level by the MAPK signaling pathway with Elk1 as the main downstream transcriptional effector. Most strikingly, melanoma cells which developed resistance towards MAPK inhibitors could be efficiently targeted by Rad51 inhibitors similar to their sensitive counterparts, leading to DNA damage, G2/M arrest and apoptosis. Furthermore, the treatment of MAPK inhibitor resistant cells with Rad51 inhibitors enhances the susceptibility of these cells for MAPK inhibitor treatment in vitro and in vivo. These data indicate that Rad51 plays a critical role in the survival of metastatic melanoma cells and is a promising target for the therapy of melanoma irrespective of its MAPK inhibitor resistance status.

Protein expression profiling identifies differential modulation of homologous recombination by platinum-based antitumor agents

PURPOSE

Oxaliplatin and satraplatin demonstrate activity against cisplatin-resistant tumor cells. Although the two platinum analogs are structurally-related, oxaliplatin is more active. Therefore, studies focusing on protein expression profiling were undertaken to identify the molecular mechanism for the difference in antitumor activity.

METHODS

We included cisplatin as reference and DAP as a Pt(IV)-prodrug of oxaliplatin to offset Pt(IV) status of satraplatin, and utilized A2780, cisplatin-resistant 2780CP/Cl-16, U2OS, and HCT-116 tumor cells in the investigation. Protein expressions following drug exposures were examined by reverse-phase protein array and ingenuity pathway analysis. Cell cycle was assessed by flow cytometry, cytotoxicity by growth inhibition assay, and homologous recombination (HR) by a GFP reporter assay.

RESULTS

Clustering analysis paired oxaliplatin with DAP and, surprisingly, satraplatin with cisplatin. This correlated with differential upregulation of p53/p21 pathway, with S and G2/M arrests by cisplatin and satraplatin in contrast to G1 arrest by oxaliplatin and DAP. Moreover, Rad51 and BRCA1 were severely downregulated by oxaliplatin and DAP, but not cisplatin and satraplatin. As a result, HR was inhibited only by oxaliplatin and DAP and this also contributed to their greater drug activity over cisplatin and satraplatin.

CONCLUSIONS

Oxaliplatin and DAP robustly activate p53 and p21, which downregulate HR proteins to enhance drug activity. More significantly, since oxaliplatin induces a BRCAness state, it may have potential against BRCA-proficient cancers. Satraplatin, on the other hand, resembled cisplatin in its protein expression profile, which indicates that small changes in chemical structure can substantially alter signal transduction pathways to modulate drug activity.

The repair of endo/exogenous DNA double-strand breaks and its effects on meiotic chromosome segregation in oocytes

Germ cell-derived genomic structure variants not only drive the evolution of species but also induce developmental defects in offspring. The genomic structure variants have different types, but most of them are originated from DNA double-strand breaks (DSBs). It is still not well known whether DNA DSBs exist in adult mammalian oocytes and how the growing and fully grown oocytes repair their DNA DSBs induced by endogenous or exogenous factors. In this study, we detected the endogenous DNA DSBs in the growing and fully grown mouse oocytes and found that the DNA DSBs mainly localized at the centromere-adjacent regions, which are also copy number variation hotspots. When the exogenous DNA DSBs were introduced by Etoposide, we found that Rad51-mediated homologous recombination (HR) was used to repair the broken DNA. However, the HR repair caused the chromatin intertwined and impaired the homologous chromosome segregation in oocytes. Although we had not detected the indication about HR repair of endogenous centromere-adjacent DNA DSBs, we found that Rad52 and RNA:DNA hybrids colocalized with these DNA DSBs, indicating that a Rad52-dependent DNA repair might exist in oocytes. In summary, our results not only demonstrated an association between endogenous DNA DSBs with genomic structure variants but also revealed one specific DNA DSB repair manner in oocytes.

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.

GLI1 Inhibitor SRI-38832 Attenuates Chemotherapeutic Resistance by Downregulating NBS1 Transcription in BRAFV600E Colorectal Cancer

Resistance to radiation and chemotherapy in colorectal cancer (CRC) patients contribute significantly to refractory disease and disease progression. Herein, we provide mechanistic rationale for acquired or inherent chemotherapeutic resistance to the anti-tumor effects of 5-fluorouracil (5-FU) that is linked to oncogenic GLI1 transcription activity and NBS1 overexpression. Patients with high levels of GLI1 also expressed high levels of NBS1. Non-canonical activation of GLI1 is driven through oncogenic pathways in CRC, like the BRAFV600E mutation. GLI1 was identified as a novel regulator of NBS1 and discovered that by knocking down GLI1 levels in vitro, diminished NBS1 expression, increased DNA damage/apoptosis, and re-sensitization of 5-FU resistant cancer to treatment was observed. Furthermore, a novel GLI1 inhibitor, SRI-38832, which exhibited pharmacokinetic properties suitable for in vivo testing, was identified. GLI1 inhibition in a murine BRAFV600E variant xenograft model of CRC resulted in the same down-regulation of NBS1 observed in vitro as well as significant reduction of tumor growth/burden. GLI1 inhibition could therefore be a therapeutic option for 5-FU resistant and BRAFV600E variant CRC patients.

Chromosome breaks generated by low doses of ionizing radiation in G2-phase are processed exclusively by gene conversion

Four repair pathways process DNA double-strand breaks (DSBs). Among these pathways the homologous recombination repair (HRR) subpathway of gene conversion (GC) affords error-free processing, but functions only in S- and G₂-phases of the cell cycle. Classical non-homologous end-joining (c-NHEJ) operates throughout the cell cycle, but causes small deletions and translocations. Similar deficiencies in exaggerated form, combined with reduced efficiency, are associated with alternative end-joining (alt-EJ). Finally, single-strand annealing (SSA) causes large deletions and possibly translocations. Thus, processing of a DSB by any pathway, except GC, poses significant risks to the genome, making the mechanisms navigating pathway-engagement critical to genome stability. Logically, the cell ought to attempt engagement of the pathway ensuring preservation of the genome, while accommodating necessities generated by the types of DSBs induced. Thereby, inception of DNA end-resection will be key determinant for GC, SSA and alt-EJ engagement. We reported that during G₂-phase, where all pathways are active, GC engages in the processing of almost 50 % of DSBs, at low DSB-loads in the genome, and that this contribution rapidly drops to nearly zero with increasing DSB-loads. At the transition between these two extremes, SSA and alt-EJ compensate, but at extremely high DSB-loads resection-dependent pathways are suppressed and c-NHEJ remains mainly active. We inquired whether in this processing framework all DSBs have similar fates. Here, we analyze in G₂-phase the processing of a subset of DSBs defined by their ability to break chromosomes. Our results reveal an absolute requirement for GC in the processing of chromatid breaks at doses in the range of 1 Gy. Defects in c-NHEJ delay significantly the inception of processing by GC, but leave processing kinetics unchanged. These results delineate the essential role of GC in chromatid break repair before mitosis and classify DSBs that underpin this breakage as the exclusive substrate of GC.

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.

Harnessing DNA Double-Strand Break Repair for Cancer Treatment

DNA double-strand breaks (DSBs) are highly deleterious, with a single unrepaired DSB being sufficient to trigger cell death. Compared to healthy cells, cancer cells have a higher DSB burden due to oncogene-induced replication stress and acquired defects in DNA damage response (DDR) mechanisms. Consequently, hyperproliferating cancer cells rely on efficient DSB repair for their survival. Moreover, augmented DSB repair capacity is a major cause of radio- and chemoresistance and, ultimately, cancer recurrence. Although inherited DDR defects can predispose individuals to develop certain cancers, the very same vulnerability may be therapeutically exploited to preferentially kill tumor cells. A paradigm for DNA repair targeted therapy has emerged in cancers that exhibit mutations in BRCA1 or BRCA2 tumor suppressor genes, conferring a strong defect in homologous recombination, a major and error-free DSB repair pathway. Clinical validation of such approaches, commonly described as synthetic lethality (SL), has been provided by the regulatory approval of poly(ADP-ribose) polymerase 1 inhibitors (PARPi) as monotherapy for BRCA1/2-mutated breast and ovarian tumors. In this review, we will describe the different DSB repair mechanisms and discuss how their specific features could be exploited for cancer therapy. A major emphasis is put on advances in combinatorial treatment modalities and SL approaches arising from DSB repair pathway interdependencies.

Hematopoietic stem cell gene therapy: The optimal use of lentivirus and gene editing approaches

Due to pioneering in vitro investigations on gene modification, gene engineering platforms have incredibly improved to a safer and more powerful tool for the treatment of multiple blood and immune disorders. Likewise, several clinical trials have been initiated combining autologous hematopoietic stem cell transplantation (auto-HSCT) with gene therapy (GT) tools. As several GT modalities such as lentivirus and gene editing tools have a long developmental path ahead to diminish its negative side effects, it is hard to decide which modality is optimal for treating a specific disease. Gene transfer by lentiviruses is the platform of choice for loss-of-mutation diseases, whereas gene correction/addition or gene disruption by gene editing tools, mainly CRISPR/Cas9, is likely to be more efficient in diseases where tight regulation is needed. Therefore, in this review, we compiled pertinent information about lentiviral gene transfer and CRISPR/Cas9 gene editing, their evolution to a safer platform for HSCT, and their applications on other types of gene disorders based on the etiology of the disease and cell fitness.

A Novel Cell-Penetrating Antibody Fragment Inhibits the DNA Repair Protein RAD51

DNA damaging chemotherapies are successful in cancer therapy, however, the damage can be reversed by DNA repair mechanisms that may be up-regulated in cancer cells. We hypothesized that inhibiting RAD51, a protein involved in homologous recombination DNA repair, would block DNA repair and restore the effectiveness of DNA damaging chemotherapy. We used phage-display to generate a novel synthetic antibody fragment that bound human RAD51 with high affinity (KD = 8.1 nM) and inhibited RAD51 ssDNA binding in vitro. As RAD51 is an intracellular target, we created a corresponding intrabody fragment that caused a strong growth inhibitory phenotype on human cells in culture. We then used a novel cell-penetrating peptide "iPTD" fusion to generate a therapeutically relevant antibody fragment that effectively entered living cells and enhanced the cell-killing effect of a DNA alkylating agent. The iPTD may be similarly useful as a cell-penetrating peptide for other antibody fragments and open the door to numerous intracellular targets previously off-limits in living cells.

The VEGF receptor neuropilin 2 promotes homologous recombination by stimulating YAP/TAZ-mediated Rad51 expression

Vascular endothelial growth factor (VEGF) signaling in tumor cells mediated by neuropilins (NRPs) contributes to the aggressive nature of several cancers, including triple-negative breast cancer (TNBC), independently of its role in angiogenesis. Understanding the mechanisms by which VEGF-NRP signaling contributes to the phenotype of such cancers is a significant and timely problem. We report that VEGF-NRP2 promote homologous recombination (HR) in BRCA1 wild-type TNBC cells by contributing to the expression and function of Rad51, an essential enzyme in the HR pathway that mediates efficient DNA double-strand break repair. Mechanistically, we provide evidence that VEGF-NRP2 stimulates YAP/TAZ-dependent Rad51 expression and that Rad51 is a direct YAP/TAZ-TEAD transcriptional target. We also discovered that VEGF-NRP2-YAP/TAZ signaling contributes to the resistance of TNBC cells to cisplatin and that Rad51 rescues the defects in DNA repair upon inhibition of either VEGF-NRP2 or YAP/TAZ. These findings reveal roles for VEGF-NRP2 and YAP/TAZ in DNA repair, and they indicate a unified mechanism involving VEGF-NRP2, YAP/TAZ, and Rad51 that contributes to resistance to platinum chemotherapy.

DNA Double Strand Breaks Repair Inhibitors: Relevance as Potential New Anticancer Therapeutics

DNA double-strand breaks are considered one of the most lethal forms of DNA damage. Many effective anticancer therapeutic approaches used chemical and physical methods to generate DNA double-strand breaks in the cancer cells. They include: IR and drugs which mimetic its action, topoisomerase poisons, some alkylating agents or drugs which affected DNA replication process. On the other hand, cancer cells are mostly characterized by highly effective systems of DNA damage repair. There are two main DNA repair pathways used to fix double-strand breaks: NHEJ and HRR. Their activity leads to a decreased effect of chemotherapy. Targeting directly or indirectly the DNA double-strand breaks response by inhibitors seems to be an exciting option for anticancer therapy and is a part of novel trends that arise after the clinical success of PARP inhibitors. These trends will provide great opportunities for the development of DNA repair inhibitors as new potential anticancer drugs. The main objective of this article is to address these new promising advances.