Dual disruption of DNA repair and telomere maintenance for the treatment of head and neck cancer

PTEN inhibition enhances sensitivity of ovarian cancer cells to the poly (ADP-ribose) polymerase inhibitor by suppressing the MRE11-RAD50-NBN complex

BACKGROUND

Poly (ADP-ribose) polymerase inhibitors (PARPis) can effectively treat ovarian cancer patients with defective homologous recombination (HR). Loss or dysfunction of PTEN, a typical tumour suppressor, impairs double-strand break (DSB) repair. Hence, we explored the possibility of inhibiting PTEN to induce HR deficiency (HRD) for PARPi application.

METHODS

Functional studies using PTEN inhibitor VO-OHpic and PARPi olaparib were performed to explore the molecular mechanisms in vitro and in vivo.

RESULTS

In this study, the combination of VO-OHpic with olaparib exhibited synergistic inhibitory effects on ovarian cancer cells was demonstrated. Furthermore, VO-OHpic was shown to enhance DSBs by reducing nuclear expression of PTEN and inhibiting HR repair through the modulation of MRE11-RAD50-NBN (MRN) complex, critical for DSB repair. TCGA and GTEx analysis revealed a strong correlation between PTEN and MRN in ovarian cancer. Mechanistic studies indicated that VO-OHpic reduced expression of MRN, likely by decreasing PTEN/E2F1-mediated transcription. Moreover, PTEN-knockdown inhibited expression of MRN, increased sensitivities to olaparib, and induced DSBs. In vivo experiments showed that the combination of VO-OHpic with olaparib exhibited enhanced inhibitory effects on tumour growth.

CONCLUSIONS

Collectively, this study highlights the potential of PTEN inhibitors in combination therapy with PARPis to create HRD for HRD-negative ovarian cancers.

The Multiple Faces of the MRN Complex: Roles in Medulloblastoma and Beyond

Hypomorphic mutations in MRN complex genes are frequently found in cancer, supporting their role as oncosuppressors. However, unlike canonical oncosuppressors, MRN proteins are often overexpressed in tumor tissues, where they actively work to counteract DSBs induced by both oncogene-dependent RS and radio-chemotherapy. Moreover, at the same time, MRN genes are also essential genes, since the constitutive KO of each component leads to embryonic lethality. Therefore, even though it is paradoxical, MRN genes may work as oncosuppressive, oncopromoting, and essential genes. In this review, we discussed how alterations in the MRN complex impact the physiopathology of cancer, in light of our recent discoveries on the gene-dosage-dependent effect of NBS1 in Medulloblastoma. These updates aim to understand whether MRN complex can be realistically used as a prognostic/predictive marker and/or as a therapeutic target for the treatment of cancer patients in the future.

DNA Damage Response Mechanisms in Head and Neck Cancer: Significant Implications for Therapy and Survival

Head and neck cancer (HNC) is a term collectively used to describe a heterogeneous group of tumors that arise in the oral cavity, larynx, nasopharynx, oropharynx, and hypopharynx, and represents the sixth most common type of malignancy worldwide. Despite advances in multimodality treatment, the disease has a recurrence rate of around 50%, and the prognosis of metastatic patients remains poor. HNCs are characterized by a high degree of genomic instability, which involves a vicious circle of accumulating DNA damage, defective DNA damage repair (DDR), and replication stress. Nonetheless, the damage that is induced on tumor cells by chemo and radiotherapy relies on defective DDR processes for a successful response to treatment, and may play an important role in the development of novel and more effective therapies. This review summarizes the current knowledge on the genes and proteins that appear to be deregulated in DDR pathways, their implication in HNC pathogenesis, and the rationale behind targeting these genes and pathways for the development of new therapies. We give particular emphasis on the therapeutic targets that have shown promising results at the pre-clinical stage and on those that have so far been associated with a therapeutic advantage in the clinical setting.

DNA Repair Mechanisms, Protein Interactions and Therapeutic Targeting of the MRN Complex

Repair of a DNA double-strand break relies upon a pathway of proteins to identify damage, regulate cell cycle checkpoints, and repair the damage. This process is initiated by a sensor protein complex, the MRN complex, comprised of three proteins-MRE11, RAD50, and NBS1. After a double-stranded break, the MRN complex recruits and activates ATM, in-turn activating other proteins such as BRCA1/2, ATR, CHEK1/2, PALB2 and RAD51. These proteins have been the focus of many studies for their individual roles in hereditary cancer syndromes and are included on several genetic testing panels. These panels have enabled us to acquire large amounts of genetic data, much of which remains a challenge to interpret due to the presence of variants of uncertain significance (VUS). While the primary aim of clinical testing is to accurately and confidently classify variants in order to inform medical management, the presence of VUSs has led to ambiguity in genetic counseling. Pathogenic variants within MRN complex genes have been implicated in breast, ovarian, prostate, colon cancers and gliomas; however, the hundreds of VUSs within MRE11, RAD50, and NBS1 precludes the application of these data in genetic guidance of carriers. In this review, we discuss the MRN complex's role in DNA double-strand break repair, its interactions with other cancer predisposing genes, the variants that can be found within the three MRN complex genes, and the MRN complex's potential as an anti-cancer therapeutic target.

Homologous Recombination Repair in Biliary Tract Cancers: A Prime Target for PARP Inhibition?

Biliary tract cancers (BTCs) are a heterogeneous group of malignancies that make up ~7% of all gastrointestinal tumors. It is notably aggressive and difficult to treat; in fact, >70% of patients with BTC are diagnosed at an advanced, unresectable stage and are not amenable to curative therapy. For these patients, chemotherapy has been the mainstay treatment, providing an inadequate overall survival of less than one year. Despite the boom in targeted therapies over the past decade, only a few targeted agents have been approved in BTCs (i.e., IDH1 and FGFR inhibitors), perhaps in part due to its relatively low incidence. This review will explore current data on PARP inhibitors (PARPi) used in homologous recombination deficiency (HRD), particularly with respect to BTCs. Greater than 28% of BTC cases harbor mutations in genes involved in homologous recombination repair (HRR). We will summarize the mechanisms for PARPi and its role in synthetic lethality and describe select genes in the HRR pathway contributing to HRD. We will provide our rationale for expanding patient eligibility for PARPi use based on literature and anecdotal evidence pertaining to mutations in HRR genes, such as RAD51C, and the potential use of reliable surrogate markers of HRD.

PARP5B is required for nonhomologous end joining during tumorigenesis in vivo

Poly(ADP-ribose) polymerases (PARP) act as DNA damage sensors that produce poly(ADP-ribose) (PAR) chains at double-strand breaks, facilitating the recruitment of repair factors. Cancers with homologous recombination defects are sensitive to small molecule PARP inhibitors. Despite PARP5B gene copy number changes in many cancers, the effects of this genetic alteration on tumor phenotype are largely unknown. To better understand this clinical finding, we characterized a PARP5B null mutation in a carcinogen-induced in vivo head and neck squamous cell carcinoma (SCC) model. Reduced PARP5B expression inhibited tumor growth, induced primary tumor differentiation and apoptosis, and inhibited cell proliferation and metastasis. Loss of PARP5B expression-induced ataxia telangiectasia and Rad3 related (ATR) activation and depleted the cancer stem cell fraction. PARP5B null tumor cells lacked 53BP1+ double-strand break foci, ATM activation, and p53 induction compared to PARP5B+/+ cancers. PARP5B null SCC expresses a multiprotein complex containing PML, pRPA, Rad50, Rad51, XRCC1, proliferating cell nuclear antigen (PCNA), and Mcm2, suggesting an HR-mediated repair mechanism at DNA replication foci. Low doses of etoposide combined with the PARP5B inhibitor XAV939 induced senescence and apoptosis in human SCC lines. NBS1 overexpression in these cells inhibited the effects of low-dose etoposide/XAV939 treatment. Our results indicate that PARP5B inhibition is new targeted cancer therapy.

RAD50 Loss of Function Variants in the Zinc Hook Domain Associated with Higher Risk of Familial Esophageal Squamous Cell Carcinoma

Simple Summary

Two deleterious RAD50 loss-of-function germline mutations were identified from the blood DNA of a cohort of 3289 Henan individuals by next-generation sequencing. These rare loss-of-function RAD50 variants were associated with a substantial increased risk of familial esophageal squamous cell carcinoma in high-risk Northern China. A functional study suggested that the RAD50 mutations may affect DNA repair and cell survival upon replication stress. Our preliminary functional study provided novel insight and the potential clinical implication that patients with heterozygous RAD50^L1264F and RAD50^Q672X status may have a potential synthetic lethal therapeutic option with CHK1 inhibitors. Further study is warranted for validation of the implicated genetic susceptibility role of the RAD50 Zinc Hook mutants.

Abstract

Unbiased whole-exome sequencing approaches in familial esophageal squamous cell carcinoma (ESCC) initially prioritized RAD50 as a candidate cancer predisposition gene. The combined study with 3289 Henan individuals from Northern China identified two pathogenic RAD50 protein truncation variants, p.Q672X and a recurrent p.K722fs variant at the zinc hook domain significantly conferring increased familial ESCC risk. Effects of ~10-fold higher familial ESCC risk were observed, when compared to East Asians from the gnomAD database. Functional characterization suggested that the RAD50^Q672X mutation contributes a dominant-negative effect in DNA repair of double-stranded breaks. Overexpression of the RAD50^Q672X and RAD50^L1264F missense mutation also sensitized cell death upon replication stress stimuli induced by formaldehyde treatment and the CHK1 inhibitor, AZD7762. Our study suggested the novel insight of the potential for synthetic lethal therapeutic options for RAD50^Q672X and the East-Asian-specific RAD50^L1264F variants and CHK1 inhibitors. Our study also suggested the association of RAD50 LOF variants in the zinc hook domain with a higher risk of familial ESCC in Chinese.

DNA damage repair: historical perspectives, mechanistic pathways and clinical translation for targeted cancer therapy

Genomic instability is the hallmark of various cancers with the increasing accumulation of DNA damage. The application of radiotherapy and chemotherapy in cancer treatment is typically based on this property of cancers. However, the adverse effects including normal tissues injury are also accompanied by the radiotherapy and chemotherapy. Targeted cancer therapy has the potential to suppress cancer cells' DNA damage response through tailoring therapy to cancer patients lacking specific DNA damage response functions. Obviously, understanding the broader role of DNA damage repair in cancers has became a basic and attractive strategy for targeted cancer therapy, in particular, raising novel hypothesis or theory in this field on the basis of previous scientists' findings would be important for future promising druggable emerging targets. In this review, we first illustrate the timeline steps for the understanding the roles of DNA damage repair in the promotion of cancer and cancer therapy developed, then we summarize the mechanisms regarding DNA damage repair associated with targeted cancer therapy, highlighting the specific proteins behind targeting DNA damage repair that initiate functioning abnormally duo to extrinsic harm by environmental DNA damage factors, also, the DNA damage baseline drift leads to the harmful intrinsic targeted cancer therapy. In addition, clinical therapeutic drugs for DNA damage and repair including therapeutic effects, as well as the strategy and scheme of relative clinical trials were intensive discussed. Based on this background, we suggest two hypotheses, namely "environmental gear selection" to describe DNA damage repair pathway evolution, and "DNA damage baseline drift", which may play a magnified role in mediating repair during cancer treatment. This two new hypothesis would shed new light on targeted cancer therapy, provide a much better or more comprehensive holistic view and also promote the development of new research direction and new overcoming strategies for patients.

MRE11-RAD50-NBS1 complex alterations and DNA damage response: implications for cancer treatment

Genome instability is a hallmark of cancer cells and can be accelerated by defects in cellular responses to DNA damage. This feature of malignant cells opens new avenues for tumor targeted therapy. MRE11-RAD50-NBS1 complex plays a crucial role in sensing and repair of DNA damage. Through interacting with other important players of DNA damage response, MRE11-RAD50-NBS1 complex is engaged in various DNA damage repair pathways. Mutations in any member of this complex may lead to hypersensitivity to genotoxic agents and predisposition to malignancy. It is assumed that the defects in the complex may contribute to tumorigenesis and that treatments targeting the defect may be beneficial to cancer patients. Here, we summarized the recent research findings of the role of MRE11-RAD50-NBS1 complex in tumorigenesis, cancer treatment and discussed the potential approaches of targeting this complex to treat cancer.

Synthetic Lethality of PARP Inhibition and Ionizing Radiation is p53-dependent

PARP inhibitors (PARPi) are potentially effective therapeutic agents capable of inducing synthetic lethality in tumors with deficiencies in homologous recombination (HR)-mediated DNA repair such as those carrying BRCA1 mutations. However, BRCA mutations are rare, the majority of tumors are proficient in HR repair, and thus most tumors are resistant to PARPi. Previously, we observed that ionizing radiation (IR) initiates cytoplasmic translocation of BRCA1 leading to suppression of HR-mediated DNA repair and induction of synthetic PARPi lethality in wild-type BRCA1 and HR-proficient tumor cells. The tumor suppressor p53 was identified as a key factor that regulates DNA damage-induced BRCA1 cytoplasmic sequestration following IR. However, the role of p53 in IR-induced PARPi sensitization remains unclear. This study elucidates the role of p53 in IR-induced PARPi cytotoxicity in HR-proficient cancer cells and suggests p53 status may help define a patient population that might benefit from this treatment strategy. Sensitization to PARPi following IR was determined in vitro and in vivo utilizing human breast and glioma tumor cells carrying wild-type BRCA1 and p53, and in associated cells in which p53 function was modified by knockdown or mutation. In breast and glioma cells with proficient HR repair, IR-induced BRCA1 cytoplasmic sequestration, HR repair inhibition, and subsequent PARPi sensitization in vitro and in vivo was dependent upon functional p53.Implications: Implications: p53 status determines PARP inhibitor sensitization by ionizing radiation in multiple BRCA1 and HR-proficient tumor types and may predict which patients are most likely to benefit from combination therapy. Mol Cancer Res; 16(7); 1092-102. ©2018 AACR.

The emerging role of homologous recombination repair and PARP inhibitors in genitourinary malignancies

As cells age and are exposed to genotoxic stress, preservation of the genomic code requires multiple DNA repair pathways to remove single-strand or double-strand breaks. Loss of function somatic genomic aberrations or germline deficiency in genes involved in DNA repair can result in acute cell death or, after a latency period, cellular transformation. Therapeutic exploitation of DNA repair by inhibition of poly (adenosine diphosphate [ADP]) ribose polymerases (PARP), a family of enzymes involved in the repair of single-strand and in some cases double-strand breaks, has become a novel cancer treatment. Although the application of PARP inhibitors (PARPis) initially focused on tumors with BRCA1 or BRCA2 deficiencies, synthetic susceptibilities to PARPis have been expanded due to the identification of tumors with mutations pathways involved in DNA damage repair, in particular those that repair double-strand breaks using homologous recombination (HR). There is an increasing appreciation that genitourinary (GU) malignancies, including bladder cancer and especially prostate cancer, contain subsets of patients with germline and somatic alterations in HR genes that may reflect an increased response to PARPis. In this review, the authors describe the mechanisms and rationale of the use of PARPis in patients with GU cancers, summarize previously reported preclinical and clinical trials, and identify ongoing trials to determine how PARPis and strategies targeted at HR repair can have widespread application in patients with GU cancers. Cancer 2017;123:1912-1924. © 2017 American Cancer Society.

Predictors and Modulators of Synthetic Lethality: An Update on PARP Inhibitors and Personalized Medicine

Poly(ADP-ribose) polymerase (PARP) inhibitors have proven to be successful agents in inducing synthetic lethality in several malignancies. Several PARP inhibitors have reached clinical trial testing for treatment in different cancers, and, recently, Olaparib (AZD2281) has gained both United States Food and Drug Administration (USFDA) and the European Commission (EC) approval for use in BRCA-mutated advanced ovarian cancer treatment. The need to identify biomarkers, their interactions in DNA damage repair pathways, and their potential utility in identifying patients who are candidates for PARP inhibitor treatment is well recognized. In this review, we detail many of the biomarkers that have been investigated for their ability to predict both PARP inhibitor sensitivity and resistance in preclinical studies as well as the results of several clinical trials that have tested the safety and efficacy of different PARP inhibitor agents in BRCA and non-BRCA-mutated cancers.

Effective Biomarkers and Radiation Treatment in Head and Neck Cancer

CONTEXT

Radiation is a key arm in the multidisciplinary treatment of patients with head and neck squamous cell carcinoma. During the past 2 decades, significant changes in the way radiation therapy is planned and delivered have improved efficacy and decreased toxicity. Refined approaches in the application of radiation and chemoradiation have led to organ-sparing treatment regimens for laryngeal and pharyngeal cancers and have improved local and regional control rates in the postoperative, adjuvant setting. The molecular and genetic determinants of tumor cell response to radiation have been studied, and several potential biomarkers are emerging that could further improve application and efficacy of radiation treatment in head and neck squamous cell carcinoma.

OBJECTIVE

To discuss the current understanding of potential biomarkers related to radiation response in head and neck squamous cell carcinoma.

DATA SOURCES

Existing published literature.

CONCLUSIONS

Several potential biomarkers are actively being studied as predictors and targets to improve the use and efficacy of radiation therapy to treat head and neck squamous cell carcinoma. Several promising candidates have been defined, and new markers are on the horizon.