RAD51 Mediates Resistance of Cancer Stem Cells to PARP Inhibition in Triple-Negative Breast Cancer

Screening of DNA Damage Repair Genes Involved in the Prognosis of Triple-Negative Breast Cancer Patients Based on Bioinformatics

Background: Triple-negative breast cancer (TNBC) is a special subtype of breast cancer with poor prognosis. DNA damage response (DDR) is one of the hallmarks of this cancer. However, the association of DDR genes with the prognosis of TNBC is still unclear.

Methods: We identified differentially expressed genes (DEGs) between normal and TNBC samples from The Cancer Genome Atlas (TCGA). DDR genes were obtained from the Molecular Signatures Database through six DDR gene sets. After the expression of six differential genes were verified by quantitative real-time polymerase chain reaction (qRT-PCR), we then overlapped the DEGs with DDR genes. Based on univariate and LASSO Cox regression analyses, a prognostic model was constructed to predict overall survival (OS). Kaplan–Meier analysis and receiver operating characteristic curve were used to assess the performance of the prognostic model. Cox regression analysis was applied to identify independent prognostic factors in TNBC. The Human Protein Atlas was used to study the immunohistochemical data of six DEGs. The prognostic model was validated using an independent dataset. Gene Ontology and the Kyoto Encyclopedia of Genes and Genomes analysis were performed by using gene set enrichment analysis (GSEA). Single-sample gene set enrichment analysis was employed to estimate immune cells related to this prognostic model. Finally, we constructed a transcriptional factor (TF) network and a competing endogenous RNA regulatory network.

Results: Twenty-three differentially expressed DDR genes were detected between TNBC and normal samples. The six-gene prognostic model we developed was shown to be related to OS in TNBC using univariate and LASSO Cox regression analyses. All the six DEGs were identified as significantly up-regulated in the tumor samples compared to the normal samples in qRT-PCR. The GSEA analysis indicated that the genes in the high-risk group were mainly correlated with leukocyte migration, cytokine interaction, oxidative phosphorylation, autoimmune diseases, and coagulation cascade. The mutation data revealed the mutated genes were different. The gene-TF regulatory network showed that Replication Factor C subunit 4 occupied the dominant position.

Conclusion: We identified six gene markers related to DDR, which can predict prognosis and serve as an independent biomarker for TNBC patients.

Messing Up the Cancer Stem Cell Chemoresistance Mechanisms Supported by Tumor Microenvironment

Despite the recent advances in cancer patient management and in the development of targeted therapies, systemic chemotherapy is currently used as a first-line treatment for many cancer types. After an initial partial response, patients become refractory to standard therapy fostering rapid tumor progression. Compelling evidence highlights that the resistance to chemotherapeutic regimens is a peculiarity of a subpopulation of cancer cells within tumor mass, known as cancer stem cells (CSCs). This cellular compartment is endowed with tumor-initiating and metastasis formation capabilities. CSC chemoresistance is sustained by a plethora of grow factors and cytokines released by neighboring tumor microenvironment (TME), which is mainly composed by adipocytes, cancer-associated fibroblasts (CAFs), immune and endothelial cells. TME strengthens CSC refractoriness to standard and targeted therapies by enhancing survival signaling pathways, DNA repair machinery, expression of drug efflux transporters and anti-apoptotic proteins. In the last years many efforts have been made to understand CSC-TME crosstalk and develop therapeutic strategy halting this interplay. Here, we report the combinatorial approaches, which perturb the interaction network between CSCs and the different component of TME.

Cation Lipid-Assisted PEG6-PLGA Polymer Nanoparticles Encapsulated Knocking Down Long ncRNAs Reverse Non-Coding RNA of Xist Through the Support Vector Machine Model to Regulate the Molecular Mechanisms of Gastric Cancer Cell Apoptosis

Gastric adenocarcinoma (GAC) is one kind of gastric cancer with a high incidence rate and mortality. It is essential to study the etiology of GAC and provide theoretical guidance for the prevention and treatment of GAC. Bioinformatics was used via differential expression analysis, weighted gene co-expression network analysis, gene set enrichment analysis, and a training support vector machine (SVM) model to construct a TSIX/mir-320a/Rad51 network as the research index of GAC disease. On the basis of CRISPR/Cas9 gene editing technology, the present study utilizes the Cation lipid-assisted PEG-6-PLGA polymer nanoparticle (CLAN) drug carrier system to prepare the target knock-out TSIX drug with CRISPR/CaS9 nucleic acid. Knocking down lncRNA TSIX restored the suppression role of miR-320a on Rad51 and inhibited the Rad51 expression. Simultaneously, this ceRNA network activated the ATF6 signaling pathway after endoplasmic reticulum stress to promote GAC cells' apoptosis and inhibit the disease. TSIX/miR-320a/Rad51 network may be a potential biological target of GAC disease and provides a new strategy for treating GAC disease.

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.

Targeting Genome Stability in Melanoma-A New Approach to an Old Field

Despite recent groundbreaking advances in the treatment of cutaneous melanoma, it remains one of the most treatment-resistant malignancies. Due to resistance to conventional chemotherapy, the therapeutic focus has shifted away from aiming at melanoma genome stability in favor of molecularly targeted therapies. Inhibitors of the RAS/RAF/MEK/ERK (MAPK) pathway significantly slow disease progression. However, long-term clinical benefit is rare due to rapid development of drug resistance. In contrast, immune checkpoint inhibitors provide exceptionally durable responses, but only in a limited number of patients. It has been increasingly recognized that melanoma cells rely on efficient DNA repair for survival upon drug treatment, and that genome instability increases the efficacy of both MAPK inhibitors and immunotherapy. In this review, we discuss recent developments in the field of melanoma research which indicate that targeting genome stability of melanoma cells may serve as a powerful strategy to maximize the efficacy of currently available therapeutics.

Novel molecular regulators of breast cancer stem cell plasticity and heterogeneity

Tumors consist of heterogeneous cell populations, and tumor heterogeneity plays key roles in regulating tumorigenesis, metastasis, recurrence and resistance to anti-tumor therapies. More and more studies suggest that cancer stem cells (CSCs) promote tumorigenesis, metastasis, recurrence and drug resistance as well as are the major source for heterogeneity of cancer cells. CD24^-CD44⁺ and ALDH⁺ are the most common markers for breast cancer stem cells (BCSCs). Previous studies showed that different BCSC markers label different BCSC populations, indicating the heterogeneity of BCSCs. Therefore, defining the regulation mechanisms of heterogeneous BCSCs is essential for precisely targeting BCSCs and treating breast cancer. In this review, we summarized the novel regulators existed in BCSCs and their niches for BCSC heterogeneity which has been discovered in recent years, and discussed their regulation mechanisms and the latest corresponding cancer treatments, which will extend our understanding on BCSC heterogeneity and plasticity, and provide better prognosis prediction and more efficient novel therapeutic strategies for breast cancer.

The CD44high Subpopulation of Multifraction Irradiation-Surviving NSCLC Cells Exhibits Partial EMT-Program Activation and DNA Damage Response Depending on Their p53 Status

Ionizing radiation (IR) is used for patients diagnosed with unresectable non-small cell lung cancer (NSCLC). However, radiotherapy remains largely palliative due to the survival of specific cell subpopulations. In the present study, the sublines of NSCLC cells, A549IR (p53wt) and H1299IR (p53null) survived multifraction X-ray radiation exposure (MFR) at a total dose of 60 Gy were investigated three weeks after the MFR course. We compared radiosensitivity (colony formation), expression of epithelial-mesenchymal transition (EMT) markers, migration activity, autophagy, and HR-dependent DNA double-strand break (DSB) repair in the bulk and entire CD44high/CD166high CSC-like populations of both parental and MFR survived NSCLC cells. We demonstrated that the p53 status affected: the pattern of expression of N-cadherin, E-cadherin, Vimentin, witnessing the appearance of EMT-like phenotype of MFR-surviving sublines; 1D confined migratory behavior (wound healing); the capability of an irradiated cell to continue to divide and form a colony of NSCLC cells before and after MFR; influencing the CD44/CD166 expression level in MFR-surviving NSCLC cells after additional single irradiation. Our data further emphasize the impact of p53 status on the decay of γH2AX foci and the associated efficacy of the DSB repair in NSCLC cells survived after MFR. We revealed that Rad51 protein might play a principal role in MFR-surviving of p53 null NSCLC cells promoting DNA DSB repair by homologous recombination (HR) pathway. The proportion of Rad51 + cells elevated in CD44high/CD166high population in MFR-surviving p53wt and p53null sublines and their parental cells. The p53wt ensures DNA-PK-mediated DSB repair for both parental and MFR-surviving cells irrespectively of a subsequent additional single irradiation. Whereas in the absence of p53, a dose-dependent increase of DNA-PK-mediated non-homologous end joining (NHEJ) occurred as an early post-irradiation response is more intensive in the CSC-like population MFR-surviving H1299IR, compared to their parental H1299 cells. Our study strictly observed a significantly higher content of LC3 + cells in the CD44high/CD166high populations of p53wt MFR-surviving cells, which enriched the CSC-like cells in contrast to their p53null counterparts. The additional 2 Gy and 5 Gy X-ray exposure leads to the dose-dependent increase in the proportion of LC3 + cells in CD44high/CD166high population of both parental p53wt and p53null, but not MFR-surviving NSCLC sublines. Our data indicated that autophagy is not necessarily associated with CSC-like cells’ radiosensitivity, emphasizing that careful assessment of other milestone processes (such as senescence and autophagy-p53-Zeb1 axis) of primary radiation responses may provide new potential targets modulated for therapeutic benefit through radiosensitizing cancer cells while rescuing normal tissue. Our findings also shed light on the intricate crosstalk between autophagy and the p53-related EMT, by which MFR-surviving cells might obtain an invasive phenotype and metastatic potential.

Clinical Perspectives in Addressing Unsolved Issues in (Neo)Adjuvant Therapy for Primary Breast Cancer

The treatment of primary breast cancer has evolved over the past 50 years based on the concept that breast cancer is a systemic disease, with the escalation of adjuvant and neoadjuvant therapies and de-escalation of breast cancer surgery. Despite the development of these therapies, recurrence with distant metastasis during the 10 years after surgical treatment is observed, albeit infrequently. Recent advances in genomic analysis based on circulating tumor cells and circulating tumor DNA have enabled the development of targeted therapies based on genetic mutations in residual tumor cells. A paradigm shift involving the application of neoadjuvant chemotherapy (NAC) has enabled the prediction of treatment response and long-term prognoses; additional adjuvant chemotherapy targeting remaining tumor cells after NAC improves survival. The activation of antitumor immunity by anticancer agents may be involved in the eradication of residual tumor cells. Elucidation of the manner in which antitumor immunity is induced by anticancer agents and unknown factors, and the overcoming of drug resistance via the targeted eradication of residual tumor cells based on genomic profiles, will inevitably lead to the achievement of 0% distant recurrence and a complete cure for primary breast cancer.

DNA damage response inhibitors: An avenue for TNBC treatment

The DNA damage response (DDR) is critical for the maintenance of genomic stability by sensing DNA damage, regulating cell cycle and initiating DNA repair. Drugs targeting DDR pathways have been increasingly exploited in treating various tumors. Triple negative breast cancer (TNBC) is a highly heterogeneous and aggressive tumor with constitutive activation of oncogenes, inducing replication stress and DNA damage, which require the DDR for survival. In addition, emerging studies have demonstrated that TNBC harbors aberrant genetic alterations in DDR pathways, such as a high frequency of p53 dysfunction and BRCA1/2 mutations. DDR alterations force TNBC to rely on the existing DDR pathways for survival, and make TNBC particularly sensitive to specific DDR inhibitors, such as high sensitivity of TNBC with BRCA1/2 mutations to PARP inhibitors. This review first and comprehensively covers the current status of the development of DDR inhibitors and discusses the mechanism of targeting the DDR in TNBC. Preclinical and clinical studies on inhibitors of the ATR-CHK1-WEE1 pathway and PARP inhibitors, the most studied inhibitors, and some other DDR inhibitors as monotherapy or combination therapy in TNBC are summarized. We also highlight the possible predictive biomarkers for these DDR inhibitors and their potential combination strategies with chemotherapy, radiotherapy or other targeted agents to optimize the efficacy of DDR inhibitors in TNBC treatment. In conclusion, this review discussed the recent considerations related to the use of DDR inhibitors for TNBC and provides a perspective to address future directions and potential therapeutic strategies for patients with TNBC.

Clinical use and mechanisms of resistance for PARP inhibitors in homologous recombination-deficient cancers

Cells are continuously subjected to DNA damaging agents. DNA damages are repaired by one of the many pathways guarding genomic integrity. When one or several DNA damage pathways are rendered inefficient, cells can accumulate mutations, which modify normal cellular pathways, favoring abnormal cell growth. This supports malignant transformation, which can occur when cells acquire resistance to cell cycle checkpoints, apoptosis, or growth inhibition signals. Mutations in genes involved in the repair of DNA double strand breaks (DSBs), such as BRCA1, BRCA2, or PALB2, significantly increase the risk of developing cancer of the breast, ovaries, pancreas, or prostate. Fortunately, the inability of these tumors to repair DNA breaks makes them sensitive to genotoxic chemotherapies, allowing for the development of therapies precisely tailored to individuals' genetic backgrounds. Unfortunately, as with many anti-cancer agents, drugs used to treat patients carrying a BRCA1 or BRCA2 mutation create a selective pressure, and over time tumors can become drug resistant. Here, we detail the cellular function of tumor suppressors essential in DNA damage repair pathways, present the mechanisms of action of inhibitors used to create synthetic lethality in BRCA carriers, and review the major molecular sources of drug resistance. Finally, we present examples of the many strategies being developed to circumvent drug resistance.

PARP Inhibition Increases the Reliance on ATR/CHK1 Checkpoint Signaling Leading to Synthetic Lethality-An Alternative Treatment Strategy for Epithelial Ovarian Cancer Cells Independent from HR Effectiveness

Poly (ADP-ribose) polymerase inhibitor (PARPi, olaparib) impairs the repair of DNA single-strand breaks (SSBs), resulting in double-strand breaks (DSBs) that cannot be repaired efficiently in homologous recombination repair (HRR)-deficient cancers such as BRCA1/2-mutant cancers, leading to synthetic lethality. Despite the efficacy of olaparib in the treatment of BRCA1/2 deficient tumors, PARPi resistance is common. We hypothesized that the combination of olaparib with anticancer agents that disrupt HRR by targeting ataxia telangiectasia and Rad3-related protein (ATR) or checkpoint kinase 1 (CHK1) may be an effective strategy to reverse ovarian cancer resistance to olaparib. Here, we evaluated the effect of olaparib, the ATR inhibitor AZD6738, and the CHK1 inhibitor MK8776 alone and in combination on cell survival, colony formation, replication stress response (RSR) protein expression, DNA damage, and apoptotic changes in BRCA2 mutated (PEO-1) and HRR-proficient BRCA wild-type (SKOV-3 and OV-90) cells. Combined treatment caused the accumulation of DNA DSBs. PARP expression was associated with sensitivity to olaparib or inhibitors of RSR. Synergistic effects were weaker when olaparib was combined with CHK1i and occurred regardless of the BRCA2 status of tumor cells. Because PARPi increases the reliance on ATR/CHK1 for genome stability, the combination of PARPi with ATR inhibition suppressed ovarian cancer cell growth independently of the efficacy of HRR. The present results were obtained at sub-lethal doses, suggesting the potential of these inhibitors as monotherapy as well as in combination with olaparib.

Breast cancer stem cells, heterogeneity, targeting therapies and therapeutic implications

Both hereditary and sporadic breast cancer are suggested to develop from a stem cell subcomponent retaining most key stem cell properties but with dysregulation of self-renewal pathways, which drives tumorigenic differentiation and cellular heterogeneity. Cancer stem cells (CSCs), characterized by their self-renewal and differentiation potential, have been reported to contribute to chemo-/radio-resistance and tumor initiation and to be the main reason for the failure of current therapies in breast cancer and other CSC-bearing cancers. Thus, CSC-targeted therapies, such as those inducing CSC apoptosis and differentiation, inhibiting CSC self-renewal and division, and targeting the CSC niche to combat CSC activity, are needed and may become an important component of multimodal treatment. To date, the understanding of breast cancer has been extended by advances in CSC biology, providing more accurate prognostic and predictive information upon diagnosis. Recent improvements have enhanced the prospect of targeting breast cancer stem cells (BCSCs), which has shown promise for increasing the breast cancer remission rate. However, targeted therapy for breast cancer remains challenging due to tumor heterogeneity. One major challenge is determining the CSC properties that can be exploited as therapeutic targets. Another challenge is identifying suitable BCSC biomarkers to assess the efficacy of novel BCSC-targeted therapies. This review focuses mainly on the characteristics of BCSCs and the roles of BCSCs in the formation, maintenance and recurrence of breast cancer; self-renewal signaling pathways in BCSCs; the BCSC microenvironment; potential therapeutic targets related to BCSCs; and current therapies and clinical trials targeting BCSCs.

Development of synthetic lethality in cancer: molecular and cellular classification

Recently, genetically targeted cancer therapies have been a topic of great interest. Synthetic lethality provides a new approach for the treatment of mutated genes that were previously considered unable to be targeted in traditional genotype-targeted treatments. The increasing researches and applications in the clinical setting made synthetic lethality a promising anticancer treatment option. However, the current understandings on different conditions of synthetic lethality have not been systematically assessed and the application of synthetic lethality in clinical practice still faces many challenges. Here, we propose a novel and systematic classification of synthetic lethality divided into gene level, pathway level, organelle level, and conditional synthetic lethality, according to the degree of specificity into its biological mechanism. Multiple preclinical findings of synthetic lethality in recent years will be reviewed and classified under these different categories. Moreover, synthetic lethality targeted drugs in clinical practice will be briefly discussed. Finally, we will explore the essential implications of this classification as well as its prospects in eliminating existing challenges and the future directions of synthetic lethality.

Triple‑negative breast cancer therapy: Current and future perspectives (Review)

Triple-negative breast cancer (TNBC) accounts for 10-15% of all breast cancer cases. TNBCs lack estrogen and progesterone receptors and express low levels of HER2, and therefore do not respond to hormonal or anti-HER2 therapies. TNBC is a particularly aggressive form of breast cancer that generally displays poorer prognosis compared to other breast cancer subtypes. TNBC is chemotherapy sensitive, and this treatment remains the standard of care despite its limited benefit. Recent advances with novel agents have been made for specific subgroups with PD-L1⁺ tumors or germline Brca-mutated tumors. However, only a fraction of these patients responds to immune checkpoint or PARP inhibitors and even those who do respond often develop resistance and relapse. Various new agents and combination strategies have been explored to further understand molecular and immunological aspects of TNBC. In this review, we discuss clinical trials in the management of TNBC as well as perspectives for potential future treatments.

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.

PARP Inhibitor Resistance Mechanisms and Implications for Post-Progression Combination Therapies

The use of PARP inhibitors (PARPi) is growing widely as FDA approvals have shifted its use from the recurrence setting to the frontline setting. In parallel, the population developing PARPi resistance is increasing. Here we review the role of PARP, DNA damage repair, and synthetic lethality. We discuss mechanisms of resistance to PARP inhibition and how this informs on novel combinations to re-sensitize cancer cells to PARPi.

ONECUT2 Accelerates Tumor Proliferation Through Activating ROCK1 Expression in Gastric Cancer

Background

Transcription factors (TFs) are key regulators which control gene expression during cancer initiation and progression. In the current study, we aimed to explore the proliferative function and clinical significance of TFs in gastric cancer (GC).

Methods

Differential analysis was used to investigate the overall expression difference between normal and tumor tissues of each TF in TCGA-STAD cohort. The quantitative real-time polymerase chain reaction (qRT-PCR) was performed to confirm the mRNA expression of one cut homeobox 2 (ONECUT2) in GC tissues. Western blot analysis was conducted to confirm the protein knockdown efficiency. Cell counting, colony formation, and GC xenograft model assays were performed to confirm the proliferative function of ONECUT2 in GC cells. Gene set enrichment analysis (GESA) and qRT-PCR were conducted to confirm the affected signaling pathways and downstream targets of ONECUT2.

Results

Our data indicated that a TF named ONECUT2 was highly expressed in GC and correlated with patients’ poor prognosis. Importantly, knockdown of ONECUT2 dramatically decreased GC cells proliferation, whereas overexpression of ONECUT2 promoted carcinogenesis in GC. Kyoto encyclopedia of genes and genomes (KEGG) analysis revealed that the upregulating ONECUT2 induced the activation of Wnt signaling pathway and cell cycle regulation pathway. We further identified that ONECUT2 boosted gastric cancer cell proliferation through enhancing ROCK1 (Rho associated coiled-coil containing protein kinase 1) mRNA expression. High level of ROCK1 expression rescued proliferative behavior of ONECUT2-deficient GC cells.

Conclusion

Our findings demonstrated that ONECUT2 promoted GC cells proliferation through activating ROCK1 expression at the DNA level, suggesting that ONECUT2-ROCK1 axis might be a potential therapeutic target in GC.

Efficacy and pharmacodynamics of niraparib in BRCA-mutant and wild-type intracranial triple-negative breast cancer murine models

Background

Despite the poor prognosis of triple-negative breast cancer (TNBC) brain metastases, there are no approved systemic therapies. We explored the DNA-damaging poly(ADP-ribose) polymerase inhibitor (PARPi) niraparib in intracranial mouse models of breast cancer susceptibility protein (BRCA)-mutant TNBC.

Methods

Mice bearing intracranial human-derived TNBC cell lines (SUM149, MDA-MB-231Br, or MDA-MB-436) were treated with niraparib and monitored for survival; intracranial tissues were analyzed for PAR levels and niraparib concentration by mass spectrometry. RNASeq data of primary breast cancers using The Cancer Genome Atlas were analyzed for DNA damage signatures. Combined RAD51 and PARP inhibition in TNBC cell lines was assessed in vitro by colony-forming assays.

Results

Daily niraparib increased median survival and decreased tumor burden in the BRCA-mutant MDA-MB-436 model, but not in the BRCA-mutant SUM149 or BRCA-wild-type MDA-MB-231Br models despite high concentrations in intracranial tumors. RAD51 inhibitor B02 was shown to sensitize all cell lines to PARP inhibition (PARPi). In the analysis of BRCA-mutant primary human TNBCs, gene expression predictors of PARPi sensitivity and DNA repair signatures demonstrate widespread heterogeneity, which may explain the differential response to PARPi. Interestingly, these signatures are significantly correlated to RAD51 expression including PARPi sensitivity (R² = 0.602, R²= 0.758).

Conclusions

Niraparib penetrates intracranial tumor tissues in mouse models of TNBC with impressive single-agent efficacy in BRCA-mutant MDA-MB-436. Clinical evaluation of niraparib to treat TNBC brain metastases, an unmet clinical need desperate for improved therapies, is warranted. Further compromising DNA repair through RAD51 inhibition may further augment TNBC’s response to PARPi.

Oxidized ATM promotes breast cancer stem cell enrichment through energy metabolism reprogram-mediated acetyl-CoA accumulation

Cancer stem cell (CSC) is a challenge in the therapy of triple-negative breast cancer (TNBC). Intratumoral hypoxia is a common feature of solid tumor. Hypoxia may contribute to the maintenance of CSC, resulting in a poor efficacy of traditional treatment and recurrence of TNBC cases. However, the underlying molecular mechanism involved in hypoxia-induced CSC stemness maintenance remains unclear. Here, we report that hypoxia stimulated DNA double-strand breaks independent of ATM kinase activation (called oxidized ATM in this paper) play a crucial role in TNBC mammosphere formation and stemness maintenance by governing a specific energy metabolism reprogramming (EMR). Oxidized ATM up-regulates GLUT1, PKM2, and PDHa expressions to enhance the uptake of glucose and production of pyruvate rather than lactate products, which facilitates glycolytic flux to mitochondrial pyruvate and citrate, thus resulting in accumulation of cytoplasmic acetyl-CoA instead of the tricarboxylic acid (TCA) cycle by regulating ATP-citrate lyase (ACLY) activity. Our findings unravel a novel model of TNBC-CSC glucose metabolism and its functional role in maintenance of hypoxic TNBC-CSC stemness. This work may help us to develop new therapeutic strategies for TNBC treatment.

PARP inhibitor resistance: the underlying mechanisms and clinical implications

Due to the DNA repair defect, BRCA1/2 deficient tumor cells are more sensitive to PARP inhibitors (PARPi) through the mechanism of synthetic lethality. At present, several PAPRi targeting poly (ADP-ribose) polymerase (PARP) have been approved for ovarian cancer and breast cancer indications. However, PARPi resistance is ubiquitous in clinic. More than 40% BRCA1/2-deficient patients fail to respond to PARPi. In addition, lots of patients acquire PARPi resistance with prolonged oral administration of PARPi. Homologous recombination repair deficient (HRD), as an essential prerequisite of synthetic lethality, plays a vital role in killing tumor cells. Therefore, Homologous recombination repair restoration (HRR) becomes the predominant reason of PARPi resistance. Recently, it was reported that DNA replication fork protection also contributed to PARPi resistance in BRCA1/2-deficient cells and patients. Moreover, various factors, such as reversion mutations, epigenetic modification, restoration of ADP-ribosylation (PARylation) and pharmacological alteration lead to PARPi resistance as well. In this review, we reviewed the underlying mechanisms of PARP inhibitor resistance in detail and summarized the potential strategies to overcome PARPi resistance and increase PARPi sensitivity.

Nutlin-3a suppresses poly (ADP-ribose) polymerase 1 by mechanisms different from conventional PARP1 suppressors in a human breast cancer cell line

Poly (ADP-ribose) polymerase 1 (PARP1) plays important roles in single strand DNA repair. PARP1 inhibitors enhance the effects of DNA damaging drugs in homologous recombination-deficient tumors including tumors with breast cancer susceptibility gene (BRCA1) mutation. Nutlin-3a, an analog of cis-imidazoline, inhibits degradation of murine double minute 2 (MDM2) and stabilizes p53. We previously reported that nutlin-3a induces PARP1 degradation in p53-dependent manner in mouse fibroblasts, suggesting nutlin-3a may be a PARP1 suppressor. Here, we investigated the effects of nutlin-3a on PARP1 in MCF-7, a human breast cancer cell line. Consistent with our previous results, nutlin-3a reduced PARP1 levels in dose- and time-dependent manners in MCF-7 cells, but this reduction was suppressed in p53 knockdown cells. RITA, a p53 stabilizer that binds to p53 itself, failed to reduce PARP1 protein levels. Moreover, transient MDM2 knockdown repressed nutlin-3a-mediated PARP1 reduction. The MG132 proteasome inhibitor, and knockdown of checkpoint with forkhead and ring finger domains (CHFR) and ring finger protein 146 (RNF146), E3 ubiquitin ligases targeting PARP1, suppressed nutlin-3a-induced PARP1 reduction. Short-term nutlin-3a treatment elevated the levels of PARylated PARP1, suggesting nutlin-3a promoted PARylation of PARP1, thereby inducing its proteasomal degradation. Furthermore, nutlin-3a-induced PARP1 degradation enhanced DNA-damaging effects of cisplatin in BRCA1 knockdown cells. Our study revealed that nutlin-3a is a PARP1 suppressor that induces PARP1 proteasomal degradation by binding to MDM2 and promoting autoPARylation of PARP1. Further analysis of the mechanisms in nutlin-3a-induced PARP1 degradation may lead to the development of novel PARP1 suppressors applicable for cancers with BRCA1 mutation.

Expression of RAD51 and Its Clinical Impact in Oral Squamous Cell Carcinoma

Purpose

To examine the expression of RAD51 in oral squamous cell carcinoma (OSCC) and analyze its connection with pathological grade, clinical stage, and lymphatic metastasis potential.

Methods

For this study, 74 OSCC samples, 15 normal mucosa tissues, and 11 normal skin tissue samples were collected. RAD51 expression was investigated using immunohistochemistry. A follow-up visit was used to assess the prognosis of each patient. We compared RAD51 expression in oral mucosa epithelial cells (OMECs), keratinocytes, and tongue squamous cell carcinoma cells (TSCCs) by Western blot analysis.

Results

RAD51 expression was higher in tumor cells than in normal mucosal tissues. In addition, RAD51 expression was associated with higher tumor differentiation (P < 0.05). Also, RAD51 expression was higher (P < 0.05). Also, RAD51 expression was higher (P < 0.05). Also, RAD51 expression was higher (

Conclusion

A strong positive correlation was found between RAD51 expression and the degree of malignancy in OSCC patients, suggesting that RAD51 could be an excellent prognostic indicator for OSCC patients.

miR-155 increases stemness and decitabine resistance in triple-negative breast cancer cells by inhibiting TSPAN5

Effective therapeutic targets for triple-negative breast cancer (TNBC), a special type of breast cancer (BC) with rapid metastasis and poor prognosis, are lacking, especially for patients with chemotherapy resistance. Decitabine (DCA) is a Food and Drug Administration-approved DNA methyltransferase inhibitor that has been proven effective for the treatment of tumors. However, its antitumor effect in cancer cells is limited by multidrug resistance. Cancer stem cells (CSCs), which are thought to act as seeds during tumor formation, regulate tumorigenesis, metastasis, and drug resistance through complex signaling. Our previous study found that miR-155 is upregulated in BC, but whether and how miR-155 regulates DCA resistance is unclear. In this study, we demonstrated that miR-155 was upregulated in CD24^- CD44⁺ BC stem cells (BCSCs). In addition, the overexpression of miR-155 increased the number of CD24^- CD44⁺ CSCs, DCA resistance and tumor clone formation in MDA-231 and BT-549 BC cells, and knockdown of miR-155 inhibited DCA resistance and stemness in BCSCs in vitro. Moreover, miR-155 induced stemness and DCA resistance by inhibiting the direct target gene tetraspanin-5 (TSPAN5). We further confirmed that overexpression of TSPAN5 abrogated the effect of miR-155 in promoting stemness and DCA resistance in BC cells. Our data show that miR-155 increases stemness and DCA resistance in BC cells by targeting TSPAN5. These data provide a therapeutic strategy and mechanistic basis for future possible clinical applications targeting the miR-155/TSPAN5 signaling axis in the treatment of TNBC.

Tumor microenvironment: Challenges and opportunities in targeting metastasis of triple negative breast cancer

Triple negative breast cancer (TNBC) is most aggressive subtype of breast cancers with high probability of metastasis as well as lack of specific targets and targeted therapeutics. TNBC is characterized with unique tumor microenvironment (TME), which differs from other subtypes. TME is associated with induction of proliferation, angiogenesis, inhibition of apoptosis and immune system suppression, and drug resistance. Exosomes are promising nanovesicles, which orchestrate the TME by communicating with different cells within TME. The components of TME including transformed ECM, soluble factors, immune suppressive cells, epigenetic modifications and re-programmed fibroblasts together hamper antitumor response and helps progression and metastasis of TNBCs. Therefore, TME could be a therapeutic target of TNBC. The current review presents latest updates on the role of exosomes in modulation of TME, approaches for targeting TME and combination of immune checkpoint inhibitors and target chemotherapeutics. Finally, we also discussed various phytochemicals that alter genetic, transcriptomic and proteomic profiles of TME along with current challenges and future implications. Thus, as TME is associated with the hallmarks of TNBC, the understanding of the impact of different components can improve the clinical benefits of TNBC patients.

PARP and PARG inhibitors in cancer treatment

In this review, Slade provides an overview of the molecular mechanisms and cellular consequences of PARP and PARG inhibition. The author also highlights the clinical performance of four PARP inhibitors used in cancer therapy (olaparib, rucaparib, niraparib, and talazoparib) and discusses the predictive biomarkers of inhibitor sensitivity and mechanisms of resistance as well as the means of overcoming them through combination therapy.

Oxidative and replication stress underlie genomic instability of cancer cells. Amplifying genomic instability through radiotherapy and chemotherapy has been a powerful but nonselective means of killing cancer cells. Precision medicine has revolutionized cancer therapy by putting forth the concept of selective targeting of cancer cells. Poly(ADP-ribose) polymerase (PARP) inhibitors represent a successful example of precision medicine as the first drugs targeting DNA damage response to have entered the clinic. PARP inhibitors act through synthetic lethality with mutations in DNA repair genes and were approved for the treatment of BRCA mutated ovarian and breast cancer. PARP inhibitors destabilize replication forks through PARP DNA entrapment and induce cell death through replication stress-induced mitotic catastrophe. Inhibitors of poly(ADP-ribose) glycohydrolase (PARG) exploit and exacerbate replication deficiencies of cancer cells and may complement PARP inhibitors in targeting a broad range of cancer types with different sources of genomic instability. Here I provide an overview of the molecular mechanisms and cellular consequences of PARP and PARG inhibition. I highlight clinical performance of four PARP inhibitors used in cancer therapy (olaparib, rucaparib, niraparib, and talazoparib) and discuss the predictive biomarkers of inhibitor sensitivity, mechanisms of resistance as well as the means of overcoming them through combination therapy.

Insights into new mechanisms and models of cancer stem cell multidrug resistance

The acquisition of genetic alterations, clonal evolution, and the tumor microenvironment promote cancer progression, metastasis and therapy resistance. These events correspond to the establishment of the great phenotypic heterogeneity and plasticity of cancer cells that contribute to tumor progression and resistant disease. Targeting resistant cancers is a major challenge in oncology; however, the underlying processes are not yet fully understood. Even though current treatments can reduce tumor size and increase life expectancy, relapse and multidrug resistance (MDR) ultimately remain the second cause of death in developed countries. Recent evidence points toward stem-like phenotypes in cancer cells, promoted by cancer stem cells (CSCs), as the main culprit of cancer relapse, resistance (radiotherapy, hormone therapy, and/or chemotherapy) and metastasis. Many mechanisms have been proposed for CSC resistance, such as drug efflux through ABC transporters, overactivation of the DNA damage response (DDR), apoptosis evasion, prosurvival pathways activation, cell cycle promotion and/or cell metabolic alterations. Nonetheless, targeted therapy toward these specific CSC mechanisms is only partially effective to prevent or abolish resistance, suggesting underlying additional causes for CSC resilience. This article aims to provide an integrated picture of the MDR mechanisms that operate in CSCs' behavior and to propose a novel model of tumor evolution during chemotherapy. Targeting the pathways mentioned here might hold promise and reveal new strategies for future clinical therapeutic approaches.

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.

DNA damage response and resistance of cancer stem cells

The cancer stem cell (CSC) model defines tumors as hierarchically organized entities, containing a small population of tumorigenic CSC, or tumour-initiating cells, placed at the apex of this hierarchy. These cells may share common qualities with chemo- and radio-resistant cancer cells and contribute to self-renewal activities resulting in tumour formation, maintenance, growth and metastasis. Yet, it remains obscure what self-defense mechanisms are utilized by these cells against the chemotherapeutic drugs or radiotherapy. Recently, attention has been focused on the pivotal role of the DNA damage response (DDR) in tumorigenesis. In line with this note, an increased DDR that prevents CSC and chemoresistant cells from genotoxic pressure of chemotherapeutic drugs or radiation may be responsible for cancer metastasis. In this review, we focus on the current knowledge concerning the role of DDR in CSC and resistant cancer cells and describe the existing opportunities of re-sensitizing such cells to modulate therapeutic treatment effects.

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.

Ovarian Cancer Stem Cells: Role in Metastasis and Opportunity for Therapeutic Targeting

Ovarian cancer (OC) is a heterogeneous disease usually diagnosed at a late stage. Cancer stem cells (CSCs) that exist within the bulk tumor survive first-line chemotherapy and contribute to resistant disease with metastasis. Understanding the key features of CSC biology provides valuable opportunities to develop OCSC-directed therapeutics, which will eventually improve the clinical outcomes of patients. Although significant developments have occurred since OCSCs were first described, the involvement of CSCs in ovarian tumor metastasis is not fully understood. Here, we discuss putative CSC markers and the fundamental role of CSCs in facilitating tumor dissemination in OC. Additionally, we focus on promising CSC-targeting strategies in preclinical and clinical studies of OC and discuss potential challenges in CSC research.

Cancer Stem Cells and Radioresistance: DNA Repair and Beyond

The current preclinical and clinical findings demonstrate that, in addition to the conventional clinical and pathological indicators that have a prognostic value in radiation oncology, the number of cancer stem cells (CSCs) and their inherent radioresistance are important parameters for local control after radiotherapy. In this review, we discuss the molecular mechanisms of CSC radioresistance attributable to DNA repair mechanisms and the development of CSC-targeted therapies for tumor radiosensitization. We also discuss the current challenges in preclinical and translational CSC research including the high inter- and intratumoral heterogeneity, plasticity of CSCs, and microenvironment-stimulated tumor cell reprogramming.

Therapeutic considerations of PARP in stem cell biology: Relevance in cancer and beyond

Cancer stem cells (CSCs) are of fundamental importance in tumor progression because of their tumor-initiating properties, their resistance to radio- and chemotherapy, their invasive properties and their propensity to escape immune responses that together contribute to tumor relapse. These highly aggressive features underscore the importance of constantly identifying new and innovative therapeutic solutions to eradicate these cells. In this narrative review we discuss recent findings on the involvement of PARP family members in cancer stem cell biology and the benefit of their inhibition. Nonetheless, an important limitation in the use of PARP inhibitors is the emergence of a prominent function of PARP1 in non-cancer stem cell biology including stem cell maintenance and differentiation during development, neurogenesis or adipogenesis. Thus, we also summarize the dominant discoveries revealing the importance of PARP1 in normal stem cell biology.

DNA double-strand break repair pathway choice - from basic biology to clinical exploitation

Mutations in genes encoding components of the DNA damage response (DDR) are among the most frequent aberrations in human tumors. Moreover, a large array of human syndromes is caused by mutations in genes involved in DDR pathways. Among others, homologous recombination repair (HR) of DNA double-strand breaks (DSB) is frequently affected by disabling mutations. While impaired HR is clearly promoting tumorigenesis, it is also associated with an actionable sensitivity against PARP inhibitors. PARP inhibitors have recently received FDA approval for the treatment of breast- and ovarian cancer. However, as with all molecularly targeted agents, acquired resistance limits its use. Both pharmaco-genomic approaches and the study of human genome instability syndromes have led to a profound understanding of PARP inhibitor resistance. These experiments have revealed new insights into the molecular mechanisms that drive mammalian DSB repair. Here, we review recent discoveries in the field and provide a clinical perspective.

BRCA Genes: The Role in Genome Stability, Cancer Stemness and Therapy Resistance

Carcinogenesis is a multistep process, and tumors frequently harbor multiple mutations regulating genome integrity, cell division and death. The integrity of cellular genome is closely controlled by the mechanisms of DNA damage signaling and DNA repair. The association of breast cancer susceptibility genes BRCA1 and BRCA2 with breast and ovarian cancer development was first demonstrated over 20 years ago. Since then the germline mutations within these genes were linked to genomic instability and increased risk of many other cancer types. Genomic instability is an engine of the oncogenic transformation of non-tumorigenic cells into tumor-initiating cells and further tumor evolution. In this review we discuss the biological functions of BRCA1 and BRCA2 genes and the role of BRCA mutations in tumor initiation, regulation of cancer stemness, therapy resistance and tumor progression.

The Prognostic and Clinical Value of CD44 in Colorectal Cancer: A Meta-Analysis

Background: CD44 is widely used as a putative cancer stem cells (CSCs) marker for colorectal cancer (CRC). However, the prognostic role of CD44 in CRC remains controversial. Methods: We performed a systematic review and meta-analysis to evaluate the association of various CD44 isoforms and overall survival (OS) and clinicopathological features of CRC patients. Results: A total of 48 studies were included in the meta-analysis. Total CD44 isoforms overexpression was significantly correlated with worse OS of patients with CRC (HR = 1.32, 95% CI = 1.08-1.61, P = 0.007). In a stratified analysis, a higher level of either CD44v6 or CD44v2 had an unfavorable impact on OS (HR_CD44v6 = 1.50, 95% CI = 1.10-2.14, P = 0.010; HR_CD44v2 = 2.93, 95% CI = 1.49-5.77, P = 0.002). Additionally, CD44 was shown to be associated with some clinicopathological features, such as lymph node metastasis (OR_CD44 = 1.56, 95% CI = 1.01-2.41, P = 0.044; OR_CD44v6 = 1.97, 95% CI = 1.19-3.26, P = 0.008; OR_{Total CD44 isoforms} = 1.57, 95% CI = 1.15-2.14, P = 0.004), distant metastasis (OR_CD44 = 2.90, 95% CI = 1.08-7.83, P = 0.035; OR_{Total CD44 isoforms} = 1.89, 95% CI = 1.02-3.53, P = 0.044). Moreover, a high level of CD44 showed a possible correlation with poor differentiation (OR_{Total CD44 isoforms} = 1.44, 95% CI = 1.00-2.08, P = 0.051), elevated level of CD44v6 tend to be correlated with tumor size (OR = 1.71, 95% CI = 0.99-2.96, P = 0.056). Conclusions: This meta-analysis demonstrated that CD44 overexpression might be an unfavorable prognostic factor for CRC patients and could be used to predict poor differentiation, lymph node metastasis and distant metastasis.

Activation of the Unfolded Protein Response via Inhibition of Protein Disulfide Isomerase Decreases the Capacity for DNA Repair to Sensitize Glioblastoma to Radiotherapy

Patients with glioblastoma multiforme (GBM) survive on average 12 to 14 months after diagnosis despite surgical resection followed by radiotheraphy and temozolomide therapy. Intrinsic or acquired resistance to chemo- and radiotherapy is common and contributes to a high rate of recurrence. To investigate the therapeutic potential of protein disulfide isomerase (PDI) as a target to overcome resistance to chemoradiation, we developed a GBM tumor model wherein conditional genetic ablation of prolyl 4-hydroxylase subunit beta (P4HB), the gene that encodes PDI, can be accomplished. Loss of PDI expression induced the unfolded protein response (UPR) and decreased cell survival in two independent GBM models. Nascent RNA Bru-seq analysis of PDI-depleted cells revealed a decrease in transcription of genes involved in DNA repair and cell-cycle regulation. Activation of the UPR also led to a robust decrease in RAD51 protein expression as a result of its ubiquitination-mediated proteosomal degradation. Clonogenic survival assays demonstrated enhanced killing of GBM cells in response to a combination of PDI knockdown and ionizing radiation (IR) compared with either modality alone, which correlated with a decreased capacity to repair IR-induced DNA damage. Synergistic tumor control was also observed with the combination of PDI inhibition and IR in a mouse xenograft model compared with either single agent alone. These findings provide a strong rationale for the development of PDI inhibitors and their use in combination with DNA damage-inducing, standard-of-care therapies such as IR. SIGNIFICANCE: These findings identify PDIA1 as a therapeutic target in GBM by demonstrating efficacy of its inhibition in combination with radiotherapy through a novel mechanism involving downregulation of DNA repair genes.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/79/11/2923/F1.large.jpg.

PARP Inhibitors as a Therapeutic Agent for Homologous Recombination Deficiency in Breast Cancers

Poly (ADP-ribose) polymerases (PARPs) play an important role in various cellular processes, such as replication, recombination, chromatin remodeling, and DNA repair. Emphasizing PARP's role in facilitating DNA repair, the PARP pathway has been a target for cancer researchers in developing compounds which selectively target cancer cells and increase sensitivity of cancer cells to other anticancer agents, but which also leave normal cells unaffected. Since certain tumors (BRCA1/2 mutants) have deficient homologous recombination repair pathways, they depend on PARP-mediated base excision repair for survival. Thus, inhibition of PARP is a promising strategy to selectively kill cancer cells by inactivating complementary DNA repair pathways. Although PARP inhibitor therapy has predominantly targeted BRCA-mutated cancers, this review also highlights the growing conversation around PARP inhibitor treatment for non-BRCA-mutant tumors, those which exhibit BRCAness and homologous recombination deficiency. We provide an update on the field's progress by considering PARP inhibitor mechanisms, predictive biomarkers, and clinical trials of PARP inhibitors in development. Bringing light to these findings would provide a basis for expanding the use of PARP inhibitors beyond BRCA-mutant breast tumors.

The F-Box Domain-Dependent Activity of EMI1 Regulates PARPi Sensitivity in Triple-Negative Breast Cancers

The BRCA1-BRCA2-RAD51 axis is essential for homologous recombination repair (HRR) and is frequently disrupted in breast cancers. PARP inhibitors (PARPis) are used clinically to treat BRCA-mutated breast tumors. Using a genetic screen, we identified EMI1 as a modulator of PARPi sensitivity in triple-negative breast cancer (TNBC) cells. This function requires the F-box domain of EMI1, through which EMI1 assembles a canonical SCF ubiquitin ligase complex that constitutively targets RAD51 for degradation. In response to genotoxic stress, CHK1-mediated phosphorylation of RAD51 counteracts EMI1-dependent degradation by enhancing RAD51's affinity for BRCA2, leading to RAD51 accumulation. Inhibition of RAD51 degradation restores HRR in BRCA1-depleted cells. Human breast cancer samples display an inverse correlation between EMI1 and RAD51 protein levels. A subset of BRCA1-deficient TNBC cells develop resistance to PARPi by downregulating EMI1 and restoring RAD51-dependent HRR. Notably, reconstitution of EMI1 expression reestablishes PARPi sensitivity both in cellular systems and in an orthotopic mouse model.

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.

Optimizing poly (ADP-ribose) polymerase inhibition through combined epigenetic and immunotherapy

Triple‐negative breast cancer (TNBC) is an aggressive breast cancer subtype with poor survival outcomes. Currently, there are no targeted therapies available for TNBCs despite remarkable progress in targeted and immune‐directed therapies for other solid organ malignancies. Poly (ADP‐ribose) polymerase inhibitors (PARPi) are effective anticancer drugs that produce good initial clinical responses, especially in homologous recombination DNA repair‐deficient cancers. However, resistance is the rule rather than the exception, and recurrent tumors tend to have an aggressive phenotype associated with poor survival. Many efforts have been made to overcome PARPi resistance, mostly by targeting genes and effector proteins participating in homologous recombination that are overexpressed during PARPi therapy. Due to many known and unknown compensatory pathways, genes, and effector proteins, overlap and shared resistance are common. Overexpression of programmed cell death‐ligand 1 (PD‐L1) and cancer stem cell (CSC) sparing are novel PARPi resistance hypotheses. Although adding programmed cell death‐1 (PD‐1)/PD‐L1 inhibitors to PARPi might improve immunogenic cell death and be crucial for durable responses, they are less likely to target the CSC population that drives recurrent tumor growth. Lysine‐specific histone demethylase‐1A and histone deacetylase inhibitors have shown promising activity against CSCs. Combining epigenetic drugs such as lysine‐specific histone demethylase‐1A inhibitors or histone deacetylase inhibitors with PARPi/anti‐PD‐1/PD‐L1 is a novel, potentially synergistic strategy for priming tumors and overcoming resistance. Furthermore, such an approach could pave the way for the identification of new upstream epigenetic and genetic signatures.

Chromosome 19 miRNA cluster and CEBPB expression specifically mark and potentially drive triple negative breast cancers

Triple negative breast cancers (TNBCs) are known to express low PGR, ESR1, and ERBB2, and high KRT5, KRT14, and KRT17. However, the reasons behind the increased expressions of KRT5, KRT14, KRT17 and decreased expressions of PGR, ESR1, and ERBB2 in TNBCs are not fully understood. Here we show that, expression of chromosome 19 miRNA cluster (C19MC) specifically marks human TNBCs. Low REST and high CEBPB correlate with expression of C19MC, KRT5, KRT14, and KRT17 and enhancers of these genes/cluster are regulated by CEBPB and REST binding sites. The C19MC miRNAs in turn can potentially target REST to offer a positive feedback loop, and might target PGR, ESR1, ERBB2, GATA3, SCUBE2, TFF3 mRNAs to contribute towards TNBC phenotype. Thus our study demonstrates that C19MC miRNA expression marks TNBCs and that C19MC miRNAs and CEBPB might together determine the TNBC marker expression pattern.

CAPER as a therapeutic target for triple negative breast cancer

Breast cancers (BCas) that lack expression of the estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) are referred to as triple negative breast cancers (TNBCs) and have the poorest clinical outcome. Once these aggressive tumors progress to distant organs, the median survival decreases to 12 months. With endocrine therapies being ineffective in this BCa subtype, highly toxic chemo- and radiation therapies are the only options. A better understanding of the functional role(s) of molecular targets contributing to TNBC progression could help in the design and development of new treatments that are more targeted with less toxicity. CAPER (Co-activator of AP-1 and ER) is a nuclear transcriptional co-activator that was recently involved in ER-positive BCa progression, however its role in hormone-independent cancers remains unknown. Our current report demonstrates that CAPER expression is upregulated in human TNBC specimens compared to normal breast tissue and that its selective downregulation through a lentiviral-mediated shRNA knockdown approach resulted in decreased cell numbers in MDA-MB-231 and BT549 TNBC cell lines without affecting the growth of non-tumorigenic cell line MCF-10A. Concordant with these observations, CAPER knockdown was also associated with a decrease in DNA repair proteins leading to a marked increase in apoptosis, through caspase-3/7 activation without any changes in cell cycle. Collectively, we propose CAPER as an important signaling molecule in the development of TNBC linked to DNA repair mechanisms, which could lead to new therapeutic modalities for the treatment of this aggressive cancer.

Final Report of a Phase I Trial of Olaparib with Cetuximab and Radiation for Heavy Smoker Patients with Locally Advanced Head and Neck Cancer

Purpose: Our goal was to evaluate the safety and toxicity of combining a PARP inhibitor, olaparib, with cetuximab and fractionated intensity-modulated radiotherapy for patients with locally advanced head and neck cancer and heavy smoking histories.Patients and Methods: Patients with ≥10 packs/year history of smoking were treated with olaparib at doses ranging from 25-200 mg orally twice daily beginning approximately 10 days prior to initiation of and with concurrent radiation (69.3 Gy in 33 fractions) using a time-to-event continual reassessment method model. Cetuximab was administered starting approximately 5 days prior to radiation per standard of care.Results: A total of 16 patients were entered onto the study, with 15 evaluable for acute toxicity. The most common treatment-related grade 3-4 side effects were radiation dermatitis and mucositis (38% and 69%, respectively). The MTD was determined to be 50 mg orally twice daily, but the recommended phase II dose was deemed to be 25 mg orally twice daily. At a median follow-up of 26 months, the actuarial median overall survival was 37 months, but was not reached for other endpoints. Two-year overall survival, progression-free survival, local control, and distant control rates were 72%, 63%, 72%, and 79%, respectively. Patients who continued to smoke during therapy experienced higher recurrence rates. MYC and KMT2A were identified as potential correlatives of response on gene amplification and mutational analysis.Conclusions: Olaparib at 25 mg orally twice daily with concurrent cetuximab and radiation was well tolerated with reduced dermatitis within the radiation field. Response rates were promising for this high-risk population. Clin Cancer Res; 24(20); 4949-59. ©2018 AACR.

Clinically Applicable Inhibitors Impacting Genome Stability

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

Reactive Oxygen Species-Mediated Tumor Microenvironment Transformation: The Mechanism of Radioresistant Gastric Cancer

Radioresistance is one of the primary causes responsible for therapeutic failure and recurrence of cancer. It is well documented that reactive oxygen species (ROS) contribute to the initiation and development of gastric cancer (GC), and the levels of ROS are significantly increased in patients with GC accompanied with abnormal expressions of multiple inflammatory factors. It is also well documented that ROS can activate cancer cells and inflammatory cells, stimulating the release of a variety of inflammatory cytokines, which subsequently mediates the tumor microenvironment (TME) and promotes cancer stem cell (CSC) maintenance as well as renewal and epithelial-mesenchymal transition (EMT), ultimately resulting in radioresistance and recurrence of GC.

Targeting cancer stem cells in the clinic: Current status and perspectives

Resistance to chemotherapy and cancer relapse are major clinical challenges attributed to a sub population of cancer stem cells (CSCs). The concept of CSCs has been the subject of intense research by the oncology community since evidence for their existence was first published over twenty years ago. Emerging data indicates that they are also able to evade novel therapies such as targeted agents, immunotherapies and anti-angiogenics. The inability to appropriately identify and isolate CSCs is a major hindrance to the field and novel technologies are now being utilized. Agents that target CSC-associated cell surface receptors and signaling pathways have generated promising pre-clinical results and are now entering clinical trial. Here we discuss and evaluate current therapeutic strategies to target CSCs.