DNA Repair Capacity in Multiple Pathways Predicts Chemoresistance in Glioblastoma Multiforme

Fluorescence Sheds Light on DNA Damage, DNA Repair, and Mutations

DNA damage can lead to carcinogenic mutations and toxicity that promotes diseases. Therefore, having rapid assays to quantify DNA damage, DNA repair, mutations, and cytotoxicity is broadly relevant to health. For example, DNA damage assays can be used to screen chemicals for genotoxicity, and knowledge about DNA repair capacity has applications in precision prevention and in personalized medicine. Furthermore, knowledge of mutation frequency has predictive power for downstream cancer, and assays for cytotoxicity can predict deleterious health effects. Tests for all of these purposes have been rendered faster and more effective via adoption of fluorescent readouts. Here, we provide an overview of established and emerging cell-based assays that exploit fluorescence for studies of DNA damage and its consequences.

Fisetin inhibits proliferation of pancreatic adenocarcinoma by inducing DNA damage via RFXAP/KDM4A-dependent histone H3K36 demethylation

Pancreatic adenocarcinoma (PDAC) is an extremely malignant tumor that is associated with low survival rates. Fisetin is a natural flavonoid that shows diverse antitumor effects, including DNA damage, in various cancers. Increasing studies have demonstrated that epigenetic modifications play critical roles in DNA-damage response. However, the epigenetic regulation mechanism of fisetin in cancers is hardly studied. RFXAP is a critical transcription factor for MHC II molecules, however, its transcriptional role in PDAC is poorly understood. The anti-PDAC effect of fisetin was measured by CCK-8, flow cytometry, xenograft tumor nude mice model. DNA-damage levels were examined by immunofluorescence. Bioinformatics analysis was used to examine the expression of RFXAP and other genes involved in DNA-damage response. ChIP sequencing was used to explore the transcriptional role of RFXAP. The expression of target gene KDM4A was measured by qRT-PCR and western blots. KDM4A promoter activity was analyzed using dual-luciferase reporter assay. RFXAP overexpressing or silencing of PDAC cells was used to explore the effect of RFXAP in DNA damage induced by fisetin. We found that fisetin inhibited cell proliferation and induced DNA damage and S-phase arrest in PDAC. Expression of RFXAP and other DNA-damage response genes were upregulated by fisetin. We revealed that RFXAP expression was relatively low in PDAC and correlated with tumor stage and poor prognosis. Then we explored the transcriptional role of RFXAP and found that RFXAP targeted KDM4A, a special demethylase specific for tri- and dimethylated histone H3K36. We found that overexpression of RFXAP upregulated KDM4A and attenuated methylation of H3K36, thereby impairing DNA repair and enhancing the DNA damage induced by fisetin, while RFXAP silencing showed the opposite effect. We also found the function of fisetin in enhancing the effect of chemotherapy on pancreatic cancer cells. Our findings revealed that fisetin induced DNA damage via RFXAP/KDM4A-dependent histone H3K36 demethylation, thus causing inhibition of proliferation in PDAC.

The Roles of ceRNAs-Mediated Autophagy in Cancer Chemoresistance and Metastasis

Simple Summary

Chemoresistance and metastasis are the main causes of treatment failure in cancers. Autophagy contribute to the survival and metastasis of cancer cells. Competing endogenous RNA (ceRNA), particularly long non-coding RNAs and circular RNA (circRNA), can bridge the interplay between autophagy and chemoresistance or metastasis in cancers via sponging miRNAs. This review aims to discuss on the function of ceRNA-mediated autophagy in the process of metastasis and chemoresistance in cancers. ceRNA network can sequester the targeted miRNA expression to indirectly upregulate the expression of autophagy-related genes, and thereof participate in autophagy-mediated chemoresistance and metastasis. Our clarification of the mechanism of autophagy regulation in metastasis and chemoresistance may greatly improve the efficacy of chemotherapy and survival in cancer patients. The combination of the tissue-specific miRNA delivery and selective autophagy inhibitors, such as hydroxychloroquine, is attractive to treat cancer patients in the future.

Abstract

Chemoresistance and metastasis are the main causes of treatment failure and unfavorable outcome in cancers. There is a pressing need to reveal their mechanisms and to discover novel therapy targets. Autophagy is composed of a cascade of steps controlled by different autophagy-related genes (ATGs). Accumulating evidence suggests that dysregulated autophagy contributes to chemoresistance and metastasis via competing endogenous RNA (ceRNA) networks including lncRNAs and circRNAs. ceRNAs sequester the targeted miRNA expression to indirectly upregulate ATGs expression, and thereof participate in autophagy-mediated chemoresistance and metastasis. Here, we attempt to summarize the roles of ceRNAs in cancer chemoresistance and metastasis through autophagy regulation.

Biological Basis for Threshold Responses to Methylating Agents

The cellular outcomes of chemical exposure are as much about the cellular response to the chemical as it is an effect of the chemical. We are growing in our understanding of the genotoxic interaction between chemistry and biology. For example, recent data has revealed the biological basis for mutation induction curves for a methylating chemical, which has been shown to be dependent on the repair capacity of the cells. However, this is just one end point in the toxicity pathway from chemical exposure to cell death. Much remains to be known in order for us to predict how cells will respond to a certain dose. Methylating agents, a subset of alkylating agents, are of particular interest, because of the variety of adverse genetic end points that can result, not only at increasing doses, but also over time. For instance, methylating agents are mutagenic, their potency, for this end point, is determined by the cellular repair capacity of an enzyme called methylguanine DNA-methyltransferase (MGMT) and its ability to repair the induceed methyl adducts. However, methyl adducts can become clastogenic. Erroneous biological processing will convert mutagenic adducts to clastogenic events in the form of double strand breaks (DSBs). How the cell responds to DSBs is via a cascade of protein kinases, which is called the DNA damage response (DDR), which will determine if the damage is repaired effectively, via homologous recombination, or with errors, via nonhomologous end joining, or whether the cell dies via apoptosis or enters senescence. The fate of cells may be determined by the extent of damage and the resulting strength of DDR signaling. Therefore, thresholds of damage may exist that determine cell fate. Such thresholds would be dependent on each of the repair and response mechanisms that these methyl adducts stimulate. The molecular mechanism of how methyl adducts kill cells is still to be fully resolved. If we are able to quantify each of these thresholds of damage for a given cell, then we can ascertain, of the many adducts that are induced, what proportion of them are mutagenic, what proportion are clastogenic, and how many of these clastogenic events are toxic. This review examines the possibility of dose and damage thresholds for methylating agents, from the perspective of the underlying evolutionary mechanisms that may be accountable.

Exploiting DNA repair defects in triple negative breast cancer to improve cell killing

Background:

The lack of molecular targets for triple negative breast cancer (TNBC) has limited treatment options and reduced survivorship. Identifying new molecular targets may help improve patient survival and decrease recurrence and metastasis. As DNA repair defects are prevalent in breast cancer, we evaluated the expression and repair capacities of DNA repair proteins in preclinical models.

Methods:

DNA repair capacity was analyzed in four TNBC cell lines, MDA-MB-157 (MDA-157), MDA-MB-231 (MDA-231), MDA-MB-468 (MDA-468), and HCC1806, using fluorescence multiplex host cell reactivation (FM-HCR) assays. Expression of DNA repair genes was analyzed with RNA-seq, and protein expression was evaluated with immunoblot. Responses to the combination of DNA damage response inhibitors and primary chemotherapy drugs doxorubicin or carboplatin were evaluated in the cell lines.

Results:

Defects in base excision and nucleotide excision repair were observed in preclinical TNBC models. Gene expression analysis showed a limited correlation between these defects. Loss in protein expression was a better indicator of these DNA repair defects. Over-expression of PARP1, XRCC1, RPA, DDB1, and ERCC1 was observed in TNBC preclinical models, and likely contributed to altered sensitivity to chemotherapy and DNA damage response (DDR) inhibitors. Improved cell killing was achieved when primary therapy was combined with DDR inhibitors for ATM, ATR, or CHK1.

Conclusion:

Base excision and nucleotide excision repair pathways may offer new molecular targets for TNBC. The functional status of DNA repair pathways should be considered when evaluating new therapies and may improve the targeting for primary and combination therapies with DDR inhibitors.

Methodologies for detecting environmentally induced DNA damage and repair

Environmental DNA damaging agents continuously challenge the integrity of the genome by introducing a variety of DNA lesions. The DNA damage caused by environmental factors will lead to mutagenesis and subsequent carcinogenesis if they are not removed efficiently by repair pathways. Methods for detection of DNA damage and repair can be applied to identify, visualize, and quantify the DNA damage formation and repair events, and they enable us to illustrate the molecular mechanisms of DNA damage formation, DNA repair pathways, mutagenesis, and carcinogenesis. Ever since the discovery of the double helical structure of DNA in 1953, a great number of methods have been developed to detect various types of DNA damage and repair. Rapid advances in sequencing technologies have facilitated the emergence of a variety of novel methods for detecting environmentally induced DNA damage and repair at the genome-wide scale during the last decade. In this review, we provide a historical overview of the development of various damage detection methods. We also highlight the current methodologies to detect DNA damage and repair, especially some next generation sequencing-based methods.

Acquired temozolomide resistance in MGMT-deficient glioblastoma cells is associated with regulation of DNA repair by DHC2

The acquisition of temozolomide resistance is a major clinical challenge for glioblastoma treatment. Chemoresistance in glioblastoma is largely attributed to repair of temozolomide-induced DNA lesions by O6-methylguanine-DNA methyltransferase (MGMT). However, some MGMT-deficient glioblastomas are still resistant to temozolomide, and the underlying molecular mechanisms remain unclear. We found that DYNC2H1 (DHC2) was expressed more in MGMT-deficient recurrent glioblastoma specimens and its expression strongly correlated to poor progression-free survival in MGMT promotor methylated glioblastoma patients. Furthermore, silencing DHC2, both in vitro and in vivo, enhanced temozolomide-induced DNA damage and significantly improved the efficiency of temozolomide treatment in MGMT-deficient glioblastoma. Using a combination of subcellular proteomics and in vitro analyses, we showed that DHC2 was involved in nuclear localization of the DNA repair proteins, namely XPC and CBX5, and knockdown of either XPC or CBX5 resulted in increased temozolomide-induced DNA damage. In summary, we identified the nuclear transportation of DNA repair proteins by DHC2 as a critical regulator of acquired temozolomide resistance in MGMT-deficient glioblastoma. Our study offers novel insights for improving therapeutic management of MGMT-deficient glioblastoma.

Pathways of 4-Hydroxy-2-Nonenal Detoxification in a Human Astrocytoma Cell Line

One of the consequences of the increased level of oxidative stress that often characterizes the cancer cell environment is the abnormal generation of lipid peroxidation products, above all 4-hydroxynonenal. The contribution of this aldehyde to the pathogenesis of several diseases is well known. In this study, we characterized the ADF astrocytoma cell line both in terms of its pattern of enzymatic activities devoted to 4-hydroxynonenal removal and its resistance to oxidative stress induced by exposure to hydrogen peroxide. A comparison with lens cell lines, which, due to the ocular function, are normally exposed to oxidative conditions is reported. Our results show that, overall, ADF cells counteract oxidative stress conditions better than normal cells, thus confirming the redox adaptation demonstrated for several cancer cells. In addition, the markedly high level of NADP⁺-dependent dehydrogenase activity acting on the glutahionyl-hydroxynonanal adduct detected in ADF cells may promote, at the same time, the detoxification and recovery of cell-reducing power in these cells.

MGMT Status as a Clinical Biomarker in Glioblastoma

Glioblastoma is the most common primary malignant brain tumor. Although current standard therapy extends median survival to ~15 months, most patients do not have a sustained response to treatment. While O⁶-methylguanine (O⁶-MeG)-DNA methyltransferase (MGMT) promoter methylation status is accepted as a prognostic and promising predictive biomarker in glioblastoma, its value in informing treatment decisions for glioblastoma patients remains debatable. Discrepancies between MGMT promoter methylation status and treatment response in some patients may stem from inconsistencies between MGMT methylation and expression levels in glioblastoma. Here, we discuss MGMT as a biomarker and elucidate the discordance between MGMT methylation, expression, and patient outcome, which currently challenges the implementation of this biomarker in clinical practice.

Fanconi anemia pathway as a prospective target for cancer intervention

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

Natural compounds as potential adjuvants to cancer therapy: Preclinical evidence

Traditional chemotherapy is being considered due to hindrances caused by systemic toxicity. Currently, the administration of multiple chemotherapeutic drugs with different biochemical/molecular targets, known as combination chemotherapy, has attained numerous benefits like efficacy enhancement and amelioration of adverse effects that has been broadly applied to various cancer types. Additionally, seeking natural-based alternatives with less toxicity has become more important. Experimental evidence suggests that herbal extracts such as Solanum nigrum and Claviceps purpurea and isolated herbal compounds (e.g., curcumin, resveratrol, and matairesinol) combined with antitumoral drugs have the potential to attenuate resistance against cancer therapy and to exert chemoprotective actions. Plant products are not free of risks: Herb adverse effects, including herb-drug interactions, should be carefully considered. LINKED ARTICLES: This article is part of a themed section on The Pharmacology of Nutraceuticals. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v177.6/issuetoc.

Lipid Peroxidation Plays an Important Role in Chemotherapeutic Effects of Temozolomide and the Development of Therapy Resistance in Human Glioblastoma

BACKGROUND

Glioblastoma (GBM) is the most malignant primary brain tumor. Relapse occurs regularly, and the clinical behavior seems to be due to a therapy-resistant subpopulation of glioma-initiating cells that belong to the group of cancer stem cells. Aldehyde dehydrogenase (ALDH) has been identified as a marker for this cell population, and we have shown previously that ALDH1A3-positive GBM cells are more resistant against temozolomide (TMZ) treatment. However, it is still unclear how ALDH expression mediates chemoresistance.

MATERIALS AND METHODS

ALDH1A3 expression was analyzed in 112 specimens from primary and secondary surgical resections of 56 patients with GBM (WHO grade IV). All patients received combined adjuvant radiochemotherapy. For experimental analysis, CRISPR-Cas9-induced knockout cells from three established GBM cell lines (LN229, U87MG, T98G) and two glioma stem-like cell lines were investigated after TMZ treatment.

RESULTS

ALDH1A3 knockout cells were more sensitive to TMZ, and oxidative stress seemed to be the molecular process where ALDH1A3 exerts its role in resistance against TMZ. Oxidative stress led to lipid peroxidation, yielding active aldehydes that were detoxified by ALDH enzymatic activity. During the metabolic process, autophagy was induced leading to downregulation of the enzyme, but ALDH1A3 is upregulated to even higher expression levels after finishing the TMZ therapy in vitro. Recurrent GBMs show significantly higher ALDH1A3 expression than the respective samples from the primary tumor, and patients suffering from GBM with high ALDH1A3 expression showed a shorter median survival time (12 months vs 21 months, P < .05).

CONCLUSION

Oxidative stress is an important and clinically relevant component of TMZ-induced therapeutic effects. Cytotoxicity seems to be mediated by aldehydes resulting from lipid peroxidation, and ALDH1A3 is able to reduce the number of toxic aldehydes. Therefore, we present a molecular explanation of the role of ALDH1A3 in therapeutic resistance of human GBM cells.

Targeting Cancer Metabolism to Resensitize Chemotherapy: Potential Development of Cancer Chemosensitizers from Traditional Chinese Medicines

Cancer is a common and complex disease with high incidence and mortality rates, which causes a severe public health problem worldwide. As one of the standard therapeutic approaches for cancer therapy, the prognosis and outcome of chemotherapy are still far from satisfactory due to the severe side effects and increasingly acquired resistance. The development of novel and effective treatment strategies to overcome chemoresistance is urgent for cancer therapy. Metabolic reprogramming is one of the hallmarks of cancer. Cancer cells could rewire metabolic pathways to facilitate tumorigenesis, tumor progression, and metastasis, as well as chemoresistance. The metabolic reprogramming may serve as a promising therapeutic strategy and rekindle the research enthusiasm for overcoming chemoresistance. This review focuses on emerging mechanisms underlying rewired metabolic pathways for cancer chemoresistance in terms of glucose and energy, lipid, amino acid, and nucleotide metabolisms, as well as other related metabolisms. In particular, we highlight the potential of traditional Chinese medicine as a chemosensitizer for cancer chemotherapy from the metabolic perspective. The perspectives of metabolic targeting to chemoresistance are also discussed. In conclusion, the elucidation of the underlying metabolic reprogramming mechanisms by which cancer cells develop chemoresistance and traditional Chinese medicines resensitize chemotherapy would provide us a new insight into developing promising therapeutics and scientific evidence for clinical use of traditional Chinese medicine as a chemosensitizer for cancer therapy.

Longitudinal assessment of tumor development using cancer avatars derived from genetically engineered pluripotent stem cells

Many cellular models aimed at elucidating cancer biology do not recapitulate pathobiology including tumor heterogeneity, an inherent feature of cancer that underlies treatment resistance. Here we introduce a cancer modeling paradigm using genetically engineered human pluripotent stem cells (hiPSCs) that captures authentic cancer pathobiology. Orthotopic engraftment of the neural progenitor cells derived from hiPSCs that have been genome-edited to contain tumor-associated genetic driver mutations revealed by The Cancer Genome Atlas project for glioblastoma (GBM) results in formation of high-grade gliomas. Similar to patient-derived GBM, these models harbor inter-tumor heterogeneity resembling different GBM molecular subtypes, intra-tumor heterogeneity, and extrachromosomal DNA amplification. Re-engraftment of these primary tumor neurospheres generates secondary tumors with features characteristic of patient samples and present mutation-dependent patterns of tumor evolution. These cancer avatar models provide a platform for comprehensive longitudinal assessment of human tumor development as governed by molecular subtype mutations and lineage-restricted differentiation.

Sensitivity of Mesothelioma Cells to PARP Inhibitors Is Not Dependent on BAP1 but Is Enhanced by Temozolomide in Cells With High-Schlafen 11 and Low-O6-methylguanine-DNA Methyltransferase Expression

INTRODUCTION

BRCA1-associated protein-1 (BAP1), a nuclear deubiquitinase thought to be involved in DNA double-strand break repair, is frequently mutated in mesothelioma. Because poly(adenosine diphosphate-ribose) polymerase inhibitors (PARPIs) induce synthetic lethality in BRCA1/2 mutant cancers, we evaluated whether BAP1 inactivating mutations confer sensitivity to PARPIs in mesothelioma and if combination therapy with temozolomide (TMZ) would be beneficial.

METHODS

A total of 10 patient-derived mesothelioma cell lines were generated and characterized for BAP1 mutation status, protein expression, nuclear localization, and sensitivity to the PARPIs, olaparib, and talazoparib, alone or in combination with TMZ. BAP1 deubiquitinase (DUB) activity was evaluated by ubiquitin with 7-amido-4-methylcoumarin assay. BAP1 knockout mesothelioma cell lines were generated by CRISPR-Cas9. Because Schlafen 11 (SLFN11) and O⁶-methylguanine-DNA methyltransferase also drive response to TMZ and PARPIs, we tested their expression and relationship with drug response.

RESULTS

BAP1 mutations or copy-number alterations, or both were present in all 10 cell lines. Nonetheless, four cell lines exhibited intact DUB activity and two had nuclear BAP1 localization. Half maximal-inhibitory concentrations of olaparib and talazoparib ranged from 4.8 μM to greater than 50 μM and 0.039 μM to greater than 5 μM, respectively, classifying them into sensitive (two) or resistant (seven) cells, independent of their BAP1 status. Cell lines with BAP1 knockout resulted in the loss of BAP1 DUB activity but did not increase sensitivity to talazoparib. Response to PARPI tended to be associated with high SLFN11 expression, and combination with temozolomide increased sensitivity of cells with low or no MGMT expression.

CONCLUSIONS

BAP1 status does not determine sensitivity to PARPIs in patient-derived mesothelioma cell lines. Combination of PARPI with TMZ may be beneficial for patients whose tumors have high SLFN11 and low or no MGMT expression.

MicroRNA-Based Combinatorial Cancer Therapy: Effects of MicroRNAs on the Efficacy of Anti-Cancer Therapies

The susceptibility of cancer cells to different types of treatments can be restricted by intrinsic and acquired therapeutic resistance, leading to the failure of cancer regression and remission. To overcome this problem, a combination therapy has been proposed as a fundamental strategy to improve therapeutic responses; however, resistance is still unavoidable. MicroRNA (miRNAs) are associated with cancer therapeutic resistance. The modulation of dysregulated miRNA levels through miRNA-based therapy comprising a replacement or inhibition approach has been proposed to sensitize cancer cells to other anti-cancer therapies. The combination of miRNA-based therapy with other anti-cancer therapies (miRNA-based combinatorial cancer therapy) is attractive, due to the ability of miRNAs to target multiple genes associated with the signaling pathways controlling therapeutic resistance. In this article, we present an overview of recent findings on the role of therapeutic resistance-related miRNAs in different types of cancer. We review the feasibility of utilizing dysregulated miRNAs in cancer cells and extracellular vesicles as potential candidates for miRNA-based combinatorial cancer therapy. We also discuss innate properties of miRNAs that need to be considered for more effective combinatorial cancer therapy.

DNA repair genes in astrocytoma tumorigenesis, progression and therapy resistance

Glioblastoma (GBM) is the most common and malignant type of primary brain tumor, showing rapid development and resistance to therapies. On average, patients survive 14.6 months after diagnosis and less than 5% survive five years or more. Several pieces of evidence have suggested that the DNA damage signaling and repair activities are directly correlated with GBM phenotype and exhibit opposite functions in cancer establishment and progression. The functions of these pathways appear to present a dual role in tumorigenesis and cancer progression. Activation and/or overexpression of ATRX, ATM and RAD51 genes were extensively characterized as barriers for GBM initiation, but paradoxically the exacerbated activity of these genes was further associated with cancer progression to more aggressive stages. Excessive amounts of other DNA repair proteins, namely HJURP, EXO1, NEIL3, BRCA2, and BRIP, have also been connected to proliferative competence, resistance and poor prognosis. This scenario suggests that these networks help tumor cells to manage replicative stress and treatment-induced damage, diminishing genome instability and conferring therapy resistance. Finally, in this review we address promising new drugs and therapeutic approaches with potential to improve patient survival. However, despite all technological advances, the prognosis is still dismal and further research is needed to dissect such complex mechanisms.

Constitutional mismatch repair deficiency-associated brain tumors: report from the European C4CMMRD consortium

Background

Malignant brain tumors (BT) are among the cancers most frequently associated with constitutional mismatch repair deficiency (CMMRD), a rare childhood cancer predisposition syndrome resulting from biallelic germline mutations in mismatch repair genes. This study analyzed data from the European “Care for CMMRD” (C4CMMRD) database to describe their clinical characteristics, treatments, and outcome with the aim of improving its diagnosis/treatment.

Methods

Retrospective analysis of data on patients with CMMRD and malignant BT from the C4CMMRD database up to July 2017.

Results

Among the 87 registered patients, 49 developed 56 malignant BTs: 50 high-grade gliomas (HGG) (with giant multinucleated cells in 16/21 histologically reviewed tumors) and 6 embryonal tumors. The median age at first BT was 9.2 years [1.1–40.6], with nine patients older than 18. Twenty-seven patients developed multiple malignancies (including16 before the BT). Most patients received standard treatment, and eight patients immunotherapy for relapsed HGG. The 3- and 5-year overall survival (OS) rates were 30% (95% CI: 19–45) and 22% (95% CI: 12–37) after the first BT, with worse prognosis for HGG (3-year OS = 20.5%). Six patients were alive (median follow-up 2.5 years) and 43 dead (38 deaths, 88%, were BT-related). Other CMMRD-specific features were café-au-lait macules (40/41), multiple BTs (5/15), developmental brain anomalies (11/15), and consanguinity (20/38 families).

Conclusions

Several characteristics could help suspecting CMMRD in pediatric malignant BTs: giant cells on histology, previous malignancies, parental consanguinity, café-au-lait macules, multiple BTs, and developmental brain anomalies. The prognosis of CMMRD-associated BT treated with standard therapies is poor requiring new therapeutic up-front approaches.

DNA repair capacity and response to treatment of colon cancer

DNA repair, a complex biological process, ensures genomic integrity. Alterations in DNA repair, occurring in many cancers, contribute to the accumulation of mutations in the genome, resulting in genomic instability and cancer progression. DNA repair also plays a substantial role in response to chemotherapeutics: rapidly dividing colon cancer cells, vulnerable to DNA-damaging agents and overcoming DNA repair, undergo cell death. DNA repair capacity represents a complex biomarker, integrating gene variants, gene expressions, the stability of gene products, the effect of inhibitors/stimulators, lifestyle and environmental factors. Here, we discuss DNA repair capacity in sporadic colon cancer, a frequent malignancy worldwide, in relation to tumor heterogeneity, prognosis and prediction, measurements in surrogate and target tissues and suggest important tasks to be addressed.

Enrichment of superoxide dismutase 2 in glioblastoma confers to acquisition of temozolomide resistance that is associated with tumor-initiating cell subsets

Background

Intratumor subsets with tumor-initiating features in glioblastoma are likely to survive treatment. Our goal is to identify the key factor in the process by which cells develop temozolomide (TMZ) resistance.

Methods

Resistant cell lines derived from U87MG and A172 were established through long-term co-incubation of TMZ. Primary tumors obtained from patients were maintained as patient-derived xenograft for studies of tumor-initating cell (TIC) features. The cell manifestations were assessed in the gene modulated cells for relevance to drug resistance.

Results

Among the mitochondria-related genes in the gene expression databases, superoxide dismutase 2 (SOD2) was a significant factor in resistance and patient survival. SOD2 in the resistant cells functionally determined the cell fate by limiting TMZ-stimulated superoxide reaction and cleavage of caspase-3. Genetic inhibition of the protein led to retrieval of drug effect in mouse study. SOD2 was also associated with the TIC features, which enriched in the resistant cells. The CD133⁺ specific subsets in the resistant cells exhibited superior superoxide regulation and the SOD2-related caspase-3 reaction. Experiments applying SOD2 modulation showed a positive correlation between the TIC features and the protein expression. Finally, co-treatment with TMZ and the SOD inhibitor sodium diethyldithiocarbamate trihydrate in xenograft mouse models with the TMZ-resistant primary tumor resulted in lower tumor proliferation, longer survival, and less CD133, Bmi-1, and SOD2 expression.

Conclusion

SOD2 plays crucial roles in the tumor-initiating features that are related to TMZ resistance. Inhibition of the protein is a potential therapeutic strategy that can be used to enhance the effects of chemotherapy.

Graphical abstract

ZNF830 mediates cancer chemoresistance through promoting homologous-recombination repair

Homologous recombination (HR), which mediates the repair of DNA double-strand breaks (DSB), is crucial for maintaining genomic integrity and enhancing survival in response to chemotherapy and radiotherapy in human cancers. However, the mechanisms of HR repair in treatment resistance for the improvement of cancer therapy remains unclear. Here, we report that the zinc finger protein 830 (ZNF830) promotes HR repair and the survival of cancer cells in response to DNA damage. Mechanistically, ZNF830 directly participates in DNA end resection via interacting with CtIP and regulating CtIP recruitment to DNA damage sites. Moreover, the recruitment of ZNF830 at DNA damage sites is dependent on its phosphorylation at serine 362 by ATR. ZNF830 directly and preferentially binds to double-strand DNA with its 3′ or 5′ overhang through the Zinc finger (Znf) domain, facilitating HR repair and maintaining genome stability. Thus, our study identified a novel function of ZNF830 as a HR repair regulator in DNA end resection, conferring the chemoresistance to genotoxic therapy for cancers those that overexpress ZNF830.

MiR-7-5p-mediated downregulation of PARP1 impacts DNA homologous recombination repair and resistance to doxorubicin in small cell lung cancer

Background

Chemo-resistance is one of the major challenges in the therapy of small cell lung cancer (SCLC). Multiple mechanisms are thought to be involved in chemo-resistance during SCLC treatment, but unfortunately, these mechanisms have not been well elucidated. Herein, we investigated the role of miRNA in the resistance of SCLC cells to doxorubicin (Dox).

Methods

MiRNA microarray analysis revealed that several miRNAs, including miR-7-5p, were specifically decreased in Dox-resistant SCLC cells (H69AR) compared to parental cells (H69). The expression level of miR-7-5p was confirmed by qRT-PCR in Dox-resistant cells (H69AR and H446AR cells) and their parental cells. Bioinformatic analysis indicated that poly ADP-ribose polymerase 1 (PARP1) is a direct target of miR-7-5p. The binding sites of miR-7-5p in the PARP1 3′ UTR were verified by luciferase reporter and Western blot assays. To investigate the role of miR-7-5p in the chemo-resistance of SCLC cells to doxorubicin, mimic or inhibitor of miR-7-5p was transfected into SCLC cells, and the effect of miR-7-5p on homologous recombination (HR) repair was analyzed by HR reporter assays. Furthermore, the expression of HR repair factors (Rad51 and BRCA1) induced by doxorubicin was detected by Western blot and immunofluorescent staining in H446AR cells transfected with miR-7-5p mimic.

Results

The expression level of miR-7-5p was remarkably reduced (4-fold) in Dox-resistant SCLC cells (H69AR and H446AR cells) compared with that in parental cells (H69 and H446 cells). Poly ADP-ribose polymerase 1 (PARP1) is a direct target of miR-7-5p, and PARP1 expression was downregulated by miR-7-5p. MiR-7-5p impeded Dox-induced HR repair by inhibiting the expression of HR repair factors (Rad51 and BRCA1) that resulted in resensitizing SCLC cells to doxorubicin.

Conclusions

Our findings provide evidence that miR-7-5p targets PARP1 to exert its suppressive effects on HR repair, indicating that the alteration of the expression of miR-7-5p may be a promising strategy for overcoming chemo-resistance in SCLC therapy.

Electronic supplementary material

The online version of this article (10.1186/s12885-019-5798-7) contains supplementary material, which is available to authorized users.

DNA repair in personalized brain cancer therapy with temozolomide and nitrosoureas

Alkylating agents have been used since the 60ties in brain cancer chemotherapy. Their target is the DNA and, although the DNA of normal and cancer cells is damaged unselectively, they exert tumor-specific killing effects because of downregulation of some DNA repair activities in cancer cells. Agents exhibiting methylating properties (temozolomide, procarbazine, dacarbazine, streptozotocine) induce at least 12 different DNA lesions. These are repaired by damage reversal mechanisms involving the alkyltransferase MGMT and the alkB homologous protein ALKBH2, and through base excision repair (BER). There is a strong correlation between the MGMT expression level and therapeutic response in high-grade malignant glioma, supporting the notion that O⁶-methylguanine and, for nitrosoureas, O⁶-chloroethylguanine are the most relevant toxic damages at therapeutically relevant doses. Since MGMT has a significant impact on the outcome of anti-cancer therapy, it is a predictive marker of the effectiveness of methylating anticancer drugs, and clinical trials are underway aimed at assessing the influence of MGMT inhibition on the therapeutic success. Other DNA repair factors involved in methylating drug resistance are mismatch repair, DNA double-strand break (DSB) repair by homologous recombination (HR) and DSB signaling. Base excision repair and ALKBH2 might also contribute to alkylating drug resistance and their downregulation may have an impact on drug sensitivity notably in cells expressing a high amount of MGMT and at high doses of temozolomide, but the importance in a therapeutic setting remains to be shown. MGMT is frequently downregulated in cancer cells (up to 40% in glioblastomas), which is due to CpG promoter methylation. Astrocytoma (grade III) are frequently mutated in isocitrate dehydrogenase (IDH1). These tumors show a surprisingly good therapeutic response. IDH1 mutation has an impact on ALKBH2 activity thus influencing DNA repair. A master switch between survival and death is p53, which often retains transactivation activity (wildtype) in malignant glioma. The role of p53 in regulating survival via DNA repair and the routes of death are discussed and conclusions as to cancer therapeutic options were drawn.

Fluorescent reporter assays provide direct, accurate, quantitative measurements of MGMT status in human cells

The DNA repair protein O6-methylguanine DNA methyltransferase (MGMT) strongly influences the effectiveness of cancer treatment with chemotherapeutic alkylating agents, and MGMT status in cancer cells could potentially contribute to tailored therapies for individual patients. However, the promoter methylation and immunohistochemical assays presently used for measuring MGMT in clinical samples are indirect, cumbersome and sometimes do not accurately report MGMT activity. Here we directly compare the accuracy of 6 analytical methods, including two fluorescent reporter assays, against the in vitro MGMT activity assay that is considered the gold standard for measuring MGMT DNA repair capacity. We discuss the relative advantages of each method. Our data indicate that two recently developed fluorescence-based assays measure MGMT activity accurately and efficiently, and could provide a functional dimension to clinical efforts to identify patients who are likely to benefit from alkylating chemotherapy.

Modeling the interplay between DNA-PK, Artemis, and ATM in non-homologous end-joining repair in G1 phase of the cell cycle

Modeling a biological process equips us with more comprehensive insight into the process and a more advantageous experimental design. Non-homologous end joining (NHEJ) is a major double-strand break (DSB) repair pathway that occurs throughout the cell cycle. The objective of the current work is to model the fast and slow phases of NHEJ in G1 phase of the cell cycle following exposure to ionizing radiation (IR). The fast phase contains the major components of NHEJ; Ku70/80 complex, DNA-dependent protein kinase catalytic subunit (DNA-PKcs), and XLF/XRCC4/ligase IV complex (XXL). The slow phase in G1 phase of the cell cycle is associated with more complex lesions and involves ATM and Artemis proteins in addition to the major components. Parameters are mainly obtained from experimental data. The model is successful in predicting the kinetics of DSB foci in 13 normal, ATM-deficient, and Artemis-deficient mammalian fibroblast cell lines in G1 phase of the cell cycle after exposure to low doses of IR. The involvement of ATM provides the model with the potency to be connected to different signaling pathways. Ku70/80 concentration and DNA-binding rate as well as XXL concentration and enzymatic activity are introduced as the best targets for affecting NHEJ DSB repair process. On the basis of the current model, decreasing concentration and DNA binding rate of DNA-PKcs is more effective than inhibiting its activity towards the Artemis protein.

PARP Inhibition Induces Enrichment of DNA Repair-Proficient CD133 and CD117 Positive Ovarian Cancer Stem Cells

PARP inhibitors (PARPi) are FDA-approved monotherapy agents for the treatment of recurrent ovarian cancer in patients with and without a BRCA mutation. Despite promising response rates, not all patients derive benefit, and the majority develop resistance. PARPi treatment in vitro and in vivo induced an enrichment of CD133⁺ and CD117⁺ ovarian cancer stem cells (CSC). This effect was not affected by BRCA mutation status. In the CSC fractions, PARPi induced cell-cycle arrest in G₂-M with a consequent accumulation of γH2AX, RAD51, and uniquely DMC1 foci. DNA damage and repair monitoring assays demonstrated that CSCs display more efficient DNA repair due, in part, to activation of embryonic repair mechanisms which involved the RAD51 homologue, DMC1 recombinase. Preserved and induced homologous repair (HR) could be a mechanism of an inherent resistance of CSCs to the synthetic lethality of PARPi that likely promotes disease recurrence. IMPLICATIONS: Treatment with PARPi fails to significantly affect ovarian cancer CSC populations, likely contributing to recurrent disease. Ovarian cancer CSCs stabilize genomic integrity after PARPi treatment, due to a more efficient inherent DNA repair capacity. PARPi-induced DMC1 recombinase and HR proficiency provide CSCs the opportunity to repair DNA damage more efficiently.Visual Overview: http://mcr.aacrjournals.org/content/molcanres/17/2/431/F1.large.jpg.

Potential Strategies Overcoming the Temozolomide Resistance for Glioblastoma

Glioblastoma (GBM) is a highly malignant type of primary brain tumor with a high mortality rate. Although the current standard therapy consists of surgery followed by radiation and temozolomide (TMZ), chemotherapy can extend patient's post-operative survival but most cases eventually demonstrate resistance to TMZ. O6-methylguanine-DNA methyltransferase (MGMT) repairs the main cytotoxic lesion, as O6-methylguanine, generated by TMZ, can be the main mechanism of the drug resistance. In addition, mismatch repair and BER also contribute to TMZ resistance. TMZ treatment can induce self-protective autophagy, a mechanism by which tumor cells resist TMZ treatment. Emerging evidence also demonstrated that a small population of cells expressing stem cell markers, also identified as GBM stem cells (GSCs), contributes to drug resistance and tumor recurrence owing to their ability for self-renewal and invasion into neighboring tissue. Some molecules maintain stem cell properties. Other molecules or signaling pathways regulate stemness and influence MGMT activity, making these GCSs attractive therapeutic targets. Treatments targeting these molecules and pathways result in suppression of GSCs stemness and, in highly resistant cases, a decrease in MGMT activity. Recently, some novel therapeutic strategies, targeted molecules, immunotherapies, and microRNAs have provided new potential treatments for highly resistant GBM cases. In this review, we summarize the current knowledge of different resistance mechanisms, novel strategies for enhancing the effect of TMZ, and emerging therapeutic approaches to eliminate GSCs, all with the aim to produce a successful GBM treatment and discuss future directions for basic and clinical research to achieve this end.

Saponin-rich fraction from Agave sisalana: effect against malignant astrocytic cells and its chemical characterisation by ESI-MS/MS

Astrocytic tumour cells derived from human (GL-15) and rat (C6) gliomas, as well as non-tumoural astrocytic cells, were exposed to the saponin-rich fraction (SF) from Agave sisalana waste and the cytotoxic effects were evaluated. Cytotoxicity assays revealed a reduction of cell viability that was more intensive in glioma than in non-tumoural cells. The SF induced morphological changes in C6 cells. They were characterised by cytoplasmic vacuole formation associated with increase in the formation of acidic lysosomes. The SF was subjected to purification on Sephadex LH-20, which characterised three probable steroidal saponins (sisalins) by electrospray ionisation mass spectrometry multistage (ESI-MSⁿ). Sisalins from sisal may be responsible for the cytotoxicity, which involves cytoplasmatic vacuole formation and selective action for glioma cells.

Up-regulation of MSH6 is associated with temozolomide resistance in human glioblastoma

The impact of DNA mismatch repair (MMR) on resistance to temozolomide (TMZ) therapy in patients with glioblastoma (GBM) is recently reported but the mechanisms are not understood. We aim to analyze the correlation between MMR function and the acquired TMZ resistance in GBM using both relevant clinical samples and TMZ resistant cells. First we found increased expression of MSH6, one of key components of MMR, in recurrent GBM patients' samples who underwent TMZ chemotherapy, comparing with those matched samples collected at the time of diagnosis. Using the cellular models of acquired resistance to TMZ, we further confirmed the up-regulation of MSH6 in TMZ resistant cells. Moreover, a TCGA dataset contains a large cohort of GBM clinical samples with or without TMZ treatment reinforced the increased expression of MSH6 and other MMR genes after long-term TMZ chemotherapy, which may resulted in MMR dysfunction and acquired TMZ resistance. Our results suggest that increased expression of MSH6, or other MMR, may be a new mechanism contributing to the acquired resistance during TMZ therapy; and may serve as an indicator to the resistance in GBM.

Synthesis and growth-inhibitory activities of imidazo[5,1-d]-1,2,3,5-tetrazine-8-carboxamides related to the anti-tumour drug temozolomide, with appended silicon, benzyl and heteromethyl groups at the 3-position

A series of 3-(benzyl-substituted)-imidazo[5,1-d]-1,2,3,5-tetrazines (13) and related derivatives with 3-heteromethyl groups has been synthesised and screened for growth-inhibitory activity in vitro against two pairs of glioma cell lines with temozolomide-sensitive and -resistant phenotypes dependent on the absence/presence of the DNA repair protein O⁶-methylguanine-DNA methyltransferase (MGMT). In general the compounds had low inhibitory activity with GI₅₀ values >50 μM against both sets of cell lines. Two silicon-containing derivatives, the TMS-methylimidazotetrazine (9) and the SEM-analogue (10), showed interesting differences: compound (9) had a profile very similar to that of temozolomide with the MGMT+ cell lines being 5 to 10-fold more resistant than MGMT- isogenic partners; the SEM-substituted compound (10) showed potency across all cell lines irrespective of their MGMT status.

Cdc20 overexpression is involved in temozolomide-resistant glioma cells with epithelial-mesenchymal transition

Glioma remains one of the most aggressive and lethal cancers in central nervous system. Temozolomide (TMZ) is the most commonly used chemotherapeutic agent in gliomas. However, therapeutic benefits of TMZ could be very limited and all patients would finally suffer from tumor progression as the tumors develop resistance to TMZ. In this study, we aim to investigate the underlying mechanism of chemoresistance in glioma cell line and to identify whether there is still a close link between epithelial-mesenchymal transition (EMT) and TMZ resistance in gliomas. The real-time RT-PCR and Western blotting were used to measure the expression of EMT markers in TMZ-resistant cells. The migration and invasion assays were conducted to detect the cell motility activity in TMZ-resistant cells. The transfection was used to down-regulate the Cdc20 expression. The student t-test was applied for data analysis. We established stable TMZ-resistant glioma cells and designated as TR. Our results revealed that TR cells exhibited a significantly increased resistance to TMZ compared with their parental cells. Moreover, TMZ-resistant cells had acquired EMT-like changes. For the mechanism study, we measured a significant increased expression of CDC20 and decreased expression of Bim in TR cells. Moreover, upon suppression of CDC20 by shRNA transfection, TR cells underwent a reverse of EMT features. Importantly, knockdown of CDC20 enhanced the drug sensitivity of TR cells to TMZ. Our results suggested that inactivation of CDC20 could contribute to the future therapy that possibly overcomes drug resistance in human cancers.

In vivo measurements of interindividual differences in DNA glycosylases and APE1 activities

The integrity of our DNA is challenged with at least 100,000 lesions per cell on a daily basis. Failure to repair DNA damage efficiently can lead to cancer, immunodeficiency, and neurodegenerative disease. Base excision repair (BER) recognizes and repairs minimally helix-distorting DNA base lesions induced by both endogenous and exogenous DNA damaging agents. Levels of BER-initiating DNA glycosylases can vary between individuals, suggesting that quantitating and understanding interindividual differences in DNA repair capacity (DRC) may enable us to predict and prevent disease in a personalized manner. However, population studies of BER capacity have been limited because most methods used to measure BER activity are cumbersome, time consuming and, for the most part, only allow for the analysis of one DNA glycosylase at a time. We have developed a fluorescence-based multiplex flow-cytometric host cell reactivation assay wherein the activity of several enzymes [four BER-initiating DNA glycosylases and the downstream processing apurinic/apyrimidinic endonuclease 1 (APE1)] can be tested simultaneously, at single-cell resolution, in vivo. Taking advantage of the transcriptional properties of several DNA lesions, we have engineered specific fluorescent reporter plasmids for quantitative measurements of 8-oxoguanine DNA glycosylase, alkyl-adenine DNA glycosylase, MutY DNA glycosylase, uracil DNA glycosylase, and APE1 activity. We have used these reporters to measure differences in BER capacity across a panel of cell lines collected from healthy individuals, and to generate mathematical models that predict cellular sensitivity to methylmethane sulfonate, H₂O₂, and 5-FU from DRC. Moreover, we demonstrate the suitability of these reporters to measure differences in DRC in multiple pathways using primary lymphocytes from two individuals.

Mechanisms of Drug Resistance in Cancer: The Role of Extracellular Vesicles

Drug resistance remains a major barrier to the successful treatment of cancer. The mechanisms by which therapeutic resistance arises are multifactorial. Recent evidence has shown that extracellular vesicles (EVs) play a role in mediating drug resistance. EVs are small vesicles carrying a variety of macromolecular cargo released by cells into the extracellular space and can be taken up into recipient cells, resulting in transfer of cellular material. EVs can mediate drug resistance by several mechanisms. They can serve as a pathway for sequestration of cytotoxic drugs, reducing the effective concentration at target sites. They can act as decoys carrying membrane proteins and capturing monoclonal antibodies intended to target receptors at the cell surface. EVs from resistant tumor cells can deliver mRNA, miRNA, long noncoding RNA, and protein inducing resistance in sensitive cells. This provides a new model for how resistance that arises can then spread through a heterogeneous tumor. EVs also mediate cross-talk between cancer cells and stromal cells in the tumor microenvironment, leading to tumor progression and acquisition of therapeutic resistance. In this review, we will describe what is known about how EVs can induce drug resistance, and discuss the ways in which EVs could be used as therapeutic targets or diagnostic markers for managing cancer treatment. While further characterization of the vesiculome and the mechanisms of EV function are still required, EVs offer an exciting opportunity in the fight against cancer.

Precision Oncology: The Road Ahead

Current efforts in precision oncology largely focus on the benefit of genomics-guided therapy. Yet, advances in sequencing techniques provide an unprecedented view of the complex genetic and nongenetic heterogeneity within individual tumors. Herein, we outline the benefits of integrating genomic and transcriptomic analyses for advanced precision oncology. We summarize relevant computational approaches to detect novel drivers and genetic vulnerabilities, suitable for therapeutic exploration. Clinically relevant platforms to functionally test predicted drugs/drug combinations for individual patients are reviewed. Finally, we highlight the technological advances in single cell analysis of tumor specimens. These may ultimately lead to the development of next-generation cancer drugs, capable of tackling the hurdles imposed by genetic and phenotypic heterogeneity on current anticancer therapies.

Restricted Delivery of Talazoparib Across the Blood-Brain Barrier Limits the Sensitizing Effects of PARP Inhibition on Temozolomide Therapy in Glioblastoma

Poly ADP-ribose polymerase (PARP) inhibitors, including talazoparib, potentiate temozolomide efficacy in multiple tumor types; however, talazoparib-mediated sensitization has not been evaluated in orthotopic glioblastoma (GBM) models. This study evaluates talazoparib ± temozolomide in clinically relevant GBM models. Talazoparib at 1-3 nmol/L sensitized T98G, U251, and GBM12 cells to temozolomide, and enhanced DNA damage signaling and G₂-M arrest in vitroIn vivo cyclical therapy with talazoparib (0.15 mg/kg twice daily) combined with low-dose temozolomide (5 mg/kg daily) was well tolerated. This talazoparib/temozolomide regimen prolonged tumor stasis more than temozolomide alone in heterotopic GBM12 xenografts [median time to endpoint: 76 days versus 50 days temozolomide (P = 0.005), 11 days placebo (P < 0.001)]. However, talazoparib/temozolomide did not accentuate survival beyond that of temozolomide alone in corresponding orthotopic xenografts [median survival 37 vs. 30 days with temozolomide (P = 0.93), 14 days with placebo, P < 0.001]. Average brain and plasma talazoparib concentrations at 2 hours after a single dose (0.15 mg/kg) were 0.49 ± 0.07 ng/g and 25.5±4.1 ng/mL, respectively. The brain/plasma distribution of talazoparib in Bcrp^-/- versus wild-type (WT) mice did not differ, whereas the brain/plasma ratio in Mdr1a/b^-/- mice was higher than WT mice (0.23 vs. 0.02, P < 0.001). Consistent with the in vivo brain distribution, overexpression of MDR1 decreased talazoparib accumulation in MDCKII cells. These results indicate that talazoparib has significant MDR1 efflux liability that may restrict delivery across the blood-brain barrier, and this may explain the loss of talazoparib-mediated temozolomide sensitization in orthotopic versus heterotopic GBM xenografts. Mol Cancer Ther; 16(12); 2735-46. ©2017 AACR.

The molecular mechanisms of chemoresistance in cancers

Overcoming intrinsic and acquired drug resistance is a major challenge in treating cancer patients because chemoresistance causes recurrence, cancer dissemination and death. This review summarizes numerous molecular aspects of multi-resistance, including transporter pumps, oncogenes (EGFR, PI3K/Akt, Erk and NF-κB), tumor suppressor gene (p53), mitochondrial alteration, DNA repair, autophagy, epithelial-mesenchymal transition (EMT), cancer stemness, and exosome. The chemoresistance-related proteins are localized to extracellular ligand, membrane receptor, cytosolic signal messenger, and nuclear transcription factors for various events, including proliferation, apoptosis, EMT, autophagy and exosome. Their cross-talk frequently appears, such as the regulatory effects of EGFR-Akt-NF-κB signal pathway on the transcription of Bcl-2, Bcl-xL and survivin or EMT-related stemness. It is essential for the realization of the target, individualized and combine therapy to clarify these molecular mechanisms, explore the therapy target, screen chemosensitive population, and determine the efficacy of chemoreagents by cell culture and orthotopic model.

MicroRNA-mediated drug resistance in ovarian cancer

The development of intrinsic or acquired resistance to chemotherapeutic agents used in the treatment of various human cancers is a major obstacle for the successful abolishment of cancer. The accumulated efforts in the understanding the exact mechanisms of development of multidrug resistance (MDR) have led to the introduction of several unique and common mechanisms. Recent studies demonstrate the regulatory role of small noncoding RNA or miRNA in the several parts of cancer biology. Practically all aspects of cell physiology under normal and disease conditions are reported to be controlled by miRNAs. In this review, we discuss how the miRNA profile is changed upon MDR development and the pivotal regulatory role played by miRNAs in overcoming resistance to chemotherapeutic agents. It is hoped that further studies will support the use of these differentially expressed miRNAs as prognostic and predictive markers, as well as novel therapeutic targets to overcome resistance in ovarian cancer.

Chemotherapeutic Drugs: DNA Damage and Repair in Glioblastoma

Despite improvements in therapeutic strategies, glioblastoma (GB) remains one of the most lethal cancers. The presence of the blood-brain barrier, the infiltrative nature of the tumor and several resistance mechanisms account for the failure of current treatments. Distinct DNA repair pathways can neutralize the cytotoxicity of chemo- and radio-therapeutic agents, driving resistance and tumor relapse. It seems that a subpopulation of stem-like cells, indicated as glioma stem cells (GSCs), is responsible for tumor initiation, maintenance and recurrence and they appear to be more resistant owing to their enhanced DNA repair capacity. Recently, attention has been focused on the pivotal role of the DNA damage response (DDR) in tumorigenesis and in the modulation of therapeutic treatment effects. In this review, we try to summarize the knowledge concerning the main molecular mechanisms involved in the removal of genotoxic lesions caused by alkylating agents, emphasizing the role of GSCs. Beside their increased DNA repair capacity in comparison with non-stem tumor cells, GSCs show a constitutive checkpoint expression that enables them to survive to treatments in a quiescent, non-proliferative state. The targeted inhibition of checkpoint/repair factors of DDR can contribute to eradicate the GSC population and can have a great potential therapeutic impact aiming at sensitizing malignant gliomas to treatments, improving the overall survival of patients.

Temozolomide in the Era of Precision Medicine

In the January 1, 2017, issue of Cancer Research, Nagel and colleagues demonstrate the value of assays that determine the DNA repair capacity of cancers in predicting response to temozolomide. Using a fluorescence-based multiplex flow cytometric host cell reactivation assay that provides simultaneous readout of DNA repair capacity across multiple pathways, they show that the multivariate drug response models derived from cell line data were applicable to patient-derived xenograft models of glioblastoma. In this commentary, we first outline the mechanism of activity and current clinical application of temozolomide, which, until now, has been largely limited to glioblastoma. Given the challenges of clinical application of functional assays, we argue that functional readouts be approximated by genomic signatures. In this context, a combination of MGMT activity and mismatch repair (MMR) status of the tumor are important parameters that determine sensitivity to temozolomide. More reliable methods are needed to determine MGMT activity as DNA methylation, the current standard, does not accurately reflect the expression of MGMT. Also, genomics for MMR are warranted. Furthermore, based on patterns of MGMT expression across different solid tumors, we make a case for revisiting temozolomide use in a broader spectrum of cancers based on our current understanding of its molecular basis of activity. Cancer Res; 77(4); 823-6. ©2017 AACR.