Interaction between DNA Polymerase β and BRCA1

Emerging roles of lamins and DNA damage repair mechanisms in ovarian cancer

Lamins are type V intermediate filament proteins which are ubiquitously present in all metazoan cells providing a platform for binding of chromatin and related proteins, thereby serving a wide range of nuclear functions including DNA damage repair. Altered expression of lamins in different subtypes of cancer is evident from researches worldwide. But whether cancer is a consequence of this change or this change is a consequence of cancer is a matter of future investigation. However changes in the expression levels of lamins is reported to have direct or indirect association with cancer progression or have regulatory roles in common neoplastic symptoms like higher nuclear deformability, increased genomic instability and reduced susceptibility to DNA damaging agents. It has already been proved that loss of A type lamin positively regulates cathepsin L, eventually leading to degradation of several DNA damage repair proteins, hence impairing DNA damage repair pathways and increasing genomic instability. It is established in ovarian cancer, that the extent of alteration in nuclear morphology can determine the degree of genetic changes and thus can be utilized to detect low to high form of serous carcinoma. In this review, we have focused on ovarian cancer which is largely caused by genomic alterations in the DNA damage response pathways utilizing proteins like RAD51, BRCA1, 53BP1 which are regulated by lamins. We have elucidated the current understanding of lamin expression in ovarian cancer and its implications in the regulation of DNA damage response pathways that ultimately result in telomere deformation and genomic instability.

Quaternary structural diversity in eukaryotic DNA polymerases: monomeric to multimeric form

Sixteen eukaryotic DNA polymerases have been identified and studied so far. Based on the sequence similarity of the catalytic subunits of DNA polymerases, these have been classified into four A, B, X and Y families except PrimPol, which belongs to the AEP family. The quaternary structure of these polymerases also varies depending upon whether they are composed of one or more subunits. Therefore, in this review, we used a quaternary structure-based classification approach to group DNA polymerases as either monomeric or multimeric and highlighted functional significance of their accessory subunits. Additionally, we have briefly summarized various DNA polymerase discoveries from a historical perspective, emphasized unique catalytic mechanism of each DNA polymerase and highlighted recent advances in understanding their cellular functions.

DNA damage response and repair in ovarian cancer: Potential targets for therapeutic strategies

Ovarian cancer is among the most lethal gynecologic malignancies with a poor survival prognosis. The current therapeutic strategies involve surgery and chemotherapy. Research is now focused on novel agents especially those targeting DNA damage response (DDR) pathways. Understanding the DDR process in ovarian cancer necessitates having a detailed knowledge on a series of signaling mediators at the cellular and molecular levels. The complexity of the DDR process in ovarian cancer and how this process works in metastatic conditions is comprehensively reviewed. For evaluating the efficacy of therapeutic agents targeting DNA damage in ovarian cancer, we will discuss the components of this system including DDR sensors, DDR transducers, DDR mediators, and DDR effectors. The constituent pathways include DNA repair machinery, cell cycle checkpoints, and apoptotic pathways. We also will assess the potential of active mediators involved in the DDR process such as therapeutic and prognostic candidates that may facilitate future studies.

DNA damage repair in breast cancer and its therapeutic implications

The DNA damage response (DDR) involves the activation of numerous cellular activities that repair DNA lesions and maintain genomic integrity, and is critical in preventing tumorigenesis. Inherited or acquired mutations in specific genes involved in the DNA damage response, for example the breast cancer susceptibility genes 1/2 (BRCA1/2), phosphatase and tensin homolog (PTEN) and P53 are associated with various subtypes of breast cancer. Such changes can render breast cancer cells particularly sensitive to specific DNA damage response inhibitors, for example BRCA1/2 germline mutated cells are sensitive to poly (ADP-ribose) polymerase (PARP) inhibitors. The aims of this review are to discuss specific DNA damage response defects in breast cancer and to present the current stage of development of various DDR inhibitors (namely PARP, ATM/ATR, DNA-PK, PARG, RECQL5, FEN1 and APE1) for breast cancer mono- and combination therapy.

DNA repair prognostic index modelling reveals an essential role for base excision repair in influencing clinical outcomes in ER negative and triple negative breast cancers

Stratification of oestrogen receptor (ER) negative and triple negative breast cancers (TNBCs) is urgently needed. In the current study, a cohort of 880 ER- (including 635 TNBCs) was immuno-profiled for a panel of DNA repair proteins including: Pol β, FEN1, APE1, XRCC1, SMUG1, PARP1, BRCA1, ATR, ATM, DNA-PKcs, Chk1, Chk2, p53, and TOPO2. Multivariate Cox proportional hazards models (with backward stepwise exclusion of these factors, using a criterion of p < 0.05 for retention of factors in the model) were used to identify factors that were independently associated with clinical outcomes. XRCC1 (p = 0.002), pol β (p = 0.032) FEN1 (p = 0.001) and BRCA1 (p = 0.040) levels were independently associated with poor BCSS. Subsequently, DNA repair index prognostic (DRPI) scores for breast cancer specific survival (BCSS) were calculated and two prognostic groups (DRPI-PGs) were identified. Patients in prognostic group 2 (DRPI-PG2) have higher risk of death (p < 0.001). Furthermore, in DRPI-PG2 patients, exposure to anthracycline reduced the risk of death [(HR (95% CI) = 0.79 (0.64–0.98), p = 0.032) by 21–26%. In addition, DRPI-PG2 patients have adverse clinicopathological features including higher grade, lympho-vascular invasion, Her-2 positive phenotype, compared to those in DRPI-PG1 (p < 0.01). Receiver operating characteristic (ROC) curves indicated that the DRPI outperformed the currently used prognostic factors and adding DRPI to lymph node stage significantly improved their performance as a predictor for BCSS [p < 0.00001, area under curve (AUC) = 0.70]. BER strongly influences pathogenesis of ER- and TNBCs. The DRPI accurately predicts BCSS and can also serve as a valuable prognostic and predictive tool for TNBCs.

Base excision repair-mediated resistance to cisplatin in KRAS(G12C) mutant NSCLC cells

KRAS mutations in NSCLC are supposed to indicate a poor prognosis and poor response to anticancer treatments but this feature lacks a mechanistic basis so far. In tumors, KRAS was found to be mutated mostly at codons 12 and 13 and a pool of mutations differing in the base alteration and the amino acid substitution have been described. The different KRAS mutations may differently impact on cancerogenesis and drug sensitivity. On this basis, we hypothesized that a different KRAS mutational status in NSCLC patients determines a different profile in the tumor response to treatments. In this paper, isogenic NSCLC cell clones expressing mutated forms of KRAS were used to determine the response to cisplatin, the main drug used in the clinic against NSCLC. Cells expressing the KRAS(G12C) mutation were found to be less sensitive to treatment both in vitro and in vivo. Systematic analysis of drug uptake, DNA adduct formation and DNA damage responses implicated in cisplatin adducts removal revealed that the KRAS(G12C) mutation might be particular because it stimulates Base Excision Repair to rapidly remove platinum from DNA even before the formation of cross-links. The presented results suggest a different pattern of sensitivity/resistance to cisplatin depending on the KRAS mutational status and these data might provide proof of principle for further investigations on the role of the KRAS status as a predictor of NSCLC response.

Micro-irradiation tools to visualize base excision repair and single-strand break repair

Microscopy and micro-irradiation imaging techniques have significantly advanced our knowledge of DNA damage tolerance and the assembly of DNA repair proteins at the sites of damage. While these tools have been extensively applied to the study of nucleotide excision repair and double-strand break repair, their application to the repair of oxidatively-induced base lesions and single-strand breaks is just beginning to yield new insights. This review will focus on examining micro-irradiation techniques reported to create base lesions and single-strand breaks; these lesions are considered to be primarily addressed by proteins involved in the base excision repair (BER) pathway. By examining conditions for generating these DNA lesions and reviewing information on the assembly and dissociation of repair complexes at the induced lesion sites, we hope to promote further investigations into BER and to stimulate further development and enhancement of these techniques for the study of BER.

Bisphenol a promotes cell survival following oxidative DNA damage in mouse fibroblasts

Bisphenol A (BPA) is a biologically active industrial chemical used in production of consumer products. BPA has become a target of intense public scrutiny following concerns about its association with human diseases such as obesity, diabetes, reproductive disorders, and cancer. Recent studies link BPA with the generation of reactive oxygen species, and base excision repair (BER) is responsible for removing oxidatively induced DNA lesions. Yet, the relationship between BPA and BER has yet to be examined. Further, the ubiquitous nature of BPA allows continuous exposure of the human genome concurrent with the normal endogenous and exogenous insults to the genome, and this co-exposure may impact the DNA damage response and repair. To determine the effect of BPA exposure on base excision repair of oxidatively induced DNA damage, cells compromised in double-strand break repair were treated with BPA alone or co-exposed with either potassium bromate (KBrO₃) or laser irradiation as oxidative damaging agents. In experiments with KBrO₃, co-treatment with BPA partially reversed the KBrO₃-induced cytotoxicity observed in these cells, and this was coincident with an increase in guanine base lesions in genomic DNA. The improvement in cell survival and the increase in oxidatively induced DNA base lesions were reminiscent of previous results with alkyl adenine DNA glycosylase-deficient cells, suggesting that BPA may prevent initiation of repair of oxidized base lesions. With laser irradiation-induced DNA damage, treatment with BPA suppressed DNA repair as revealed by several indicators. These results are consistent with the hypothesis that BPA can induce a suppression of oxidized base lesion DNA repair by the base excision repair pathway.

Targeting BRCA1-BER deficient breast cancer by ATM or DNA-PKcs blockade either alone or in combination with cisplatin for personalized therapy

BRCA1, a key factor in homologous recombination (HR) repair may also regulate base excision repair (BER). Targeting BRCA1-BER deficient cells by blockade of ATM and DNA-PKcs could be a promising strategy in breast cancer. We investigated BRCA1, XRCC1 and pol β protein expression in two cohorts (n = 1602 sporadic and n = 50 germ-line BRCA1 mutated) and mRNA expression in two cohorts (n = 1952 and n = 249). Artificial neural network analysis for BRCA1-DNA repair interacting genes was conducted in 249 tumours. Pre-clinically, BRCA1 proficient and deficient cells were DNA repair expression profiled and evaluated for synthetic lethality using ATM and DNA-PKcs inhibitors either alone or in combination with cisplatin. In human tumours, BRCA1 negativity was strongly associated with low XRCC1, and low pol β at mRNA and protein levels (p < 0.0001). In patients with BRCA1 negative tumours, low XRCC1 or low pol β expression was significantly associated with poor survival in univariate and multivariate analysis compared to high XRCC1 or high pol β expressing BRCA1 negative tumours (ps < 0.05). Pre-clinically, BRCA1 negative cancer cells exhibit low mRNA and low protein expression of XRCC1 and pol β. BRCA1-BER deficient cells were sensitive to ATM and DNA-PKcs inhibitor treatment either alone or in combination with cisplatin and synthetic lethality was evidenced by DNA double strand breaks accumulation, cell cycle arrest and apoptosis. We conclude that XRCC1 and pol β expression status in BRCA1 negative tumours may have prognostic significance. BRCA1-BER deficient cells could be targeted by ATM or DNA-PKcs inhibitors for personalized therapy.

A portable BRCA1-HAC (human artificial chromosome) module for analysis of BRCA1 tumor suppressor function

BRCA1 is involved in many disparate cellular functions, including DNA damage repair, cell-cycle checkpoint activation, gene transcriptional regulation, DNA replication, centrosome function and others. The majority of evidence strongly favors the maintenance of genomic integrity as a principal tumor suppressor activity of BRCA1. At the same time some functional aspects of BRCA1 are not fully understood. Here, a HAC (human artificial chromosome) module with a regulated centromere was constructed for delivery and expression of the 90 kb genomic copy of the BRCA1 gene into BRCA1-deficient human cells. A battery of functional tests was carried out to demonstrate functionality of the exogenous BRCA1. In separate experiments, we investigated the role of BRCA1 in maintenance of heterochromatin integrity within a human functional kinetochore. We demonstrated that BRCA1 deficiency results in a specific activation of transcription of higher-order alpha-satellite repeats (HORs) assembled into heterochromatin domains flanking the kinetochore. At the same time no detectable elevation of transcription was observed within HORs assembled into centrochromatin domains. Thus, we demonstrated a link between BRCA1 deficiency and kinetochore dysfunction and extended previous observations that BRCA1 is required to silence transcription in heterochromatin in specific genomic loci. This supports the hypothesis that epigenetic alterations of the kinetochore initiated in the absence of BRCA1 may contribute to cellular transformation.