Nucleotide Excision Repair Proteins Research Articles

Exploring the Roles of Different DNA Repair Proteins in Short Inverted Repeat Mediated Genomic Instability: A Pilot Study

Repetitive DNA sequences are abundant in the human genome and can adopt alternative (i.e., non-B) DNA structures. These sequences contribute to diverse biological functions, including genomic instability. Previously, we found that Z-DNA-, H-DNA- and cruciform DNA-forming sequences are mutagenic, implicating them in cancer etiology. These sequences can stimulate the formation of DNA double-strand breaks (DSBs), causing deletions via cleavage by the endonuclease ERCC1-XPF. Interestingly, the activity of ERCC1-XPF in H-DNA-induced mutagenesis is nucleotide excision repair (NER)-dependent, but its role in Z-DNA-induced mutagenesis is NER-independent. Instead, Z-DNA is processed by ERCC1-XPF in a mechanism dependent on the mismatch repair (MMR) complex, MSH2-MSH3. These observations indicate distinct mechanisms of non-B-induced genomic instability. However, the roles of NER and MMR proteins, as well as additional nucleases (CtIP and MRE11), in the processing of cruciform DNA remain unknown. Here, we present data on the processing of cruciform-forming short inverted repeats (IRs) by DNA repair proteins using mammalian cell-based systems. From this pilot study, we show that, in contrast to H-DNA and Z-DNA, short IRs are processed in a NER- and MMR-independent manner, and the nucleases CtIP and MRE11 suppress short IR-induced genomic instability in mammalian cells.

Abstract 2034: The NAE1 inhibitor MLN4924 sensitizes both BRCA1 wild type and mutant triple negative breast cancers to cisplatin by reducing the nucleotide excision repair protein XPC level

Abstract Background: Triple-negative breast cancer (TNBC) shows poor survival and prognosis, high recurrence, and poor response to conventional therapeutics. To develop novel therapeutics for TNBC, we explored the efficacy of the neddylation inhibitor, MLN4924. We showed that MLN4924 displayed increased sensitivity against TNBC compared to non-TNBC subtypes. Mechanistically, MLN4924 induced extensive re-replication and DNA damage leading to cell death. Furthermore, MLN4924 improved cisplatin efficacy by elevating the DNA damage levels. To demonstrate the mechanisms of MLN4924+cisplatin-mediated enhanced cell death, we examined the XPC level, a key factor in the nucleotide excision repair (NER) pathway, playing a central role in cisplatin damage repair. Neddylation of CUL4A promotes XPC ubiquitination which influences XPC binding to the DNA damage sites for NER. Methods: We examined the XPC protein level by Western blot assay and RNA level by Real Time PCR using both BRCA1-wild type (MDA-MB-231) and BRCA1-mutated (MDA-MB-436) cell lines and mouse xenograft tumor samples. We evaluated if the proteosome inhibitor MG132 stabilizes XPC ubiquitination and protein levels by treating these cells with MG132 for various times. We also overexpressed XPC in these cell lines and examined the effect on survival upon MLN4924 and MLN4924+cisplatin treatments. Results: Our results demonstrate that MLN4924 reduces XPC ubiquitination upon shorter drug treatment but reduces the protein level upon longer drug treatment in both BRCA1-wild type and BRCA1-mutated cell lines. MG132 treatment did not stabilize the XPC ubiquitination level upon shorter drug treatment but stabilized the protein level upon longer drug treatment. We also observed a reduction in the XPC protein level in tumor samples which correlated with the increase in the DNA damage level. Surprisingly, XPC overexpression improved the cell survival in both MLN4924- and MLN4924+cisplatin-treated BRCA1-wild type and mutated TNBC cells. Conclusion: We hypothesize that a reduction in the XPC ubiquitination and protein levels plays a role in cisplatin-induced DNA damage repair reducing NER efficiency and consequently increasing the DNA damage upon MLN4924+cisplatin treatment. Since overexpression of XPC results in improved survival of both MLN4924-and MLN4924+cisplatin-treated cells, we anticipate that XPC not only regulates cisplatin damage repair through NER but may have a broader role in MLN4924 sensitization beyond the NER pathway. Importantly, MLN4924 treatment does not affect the XPC mRNA level emphasizing that neddylation influences the XPC protein level. In summary, our research uncovers novel insights into the role of XPC in both MLN4924 and MLN4924+cisplatin sensitization, shedding light on potential avenues for innovative therapeutic strategies for TNBC patients. Citation Format: Shrilekha Misra, Mateusz Opyrchal, Alo Ray. The NAE1 inhibitor MLN4924 sensitizes both BRCA1 wild type and mutant triple negative breast cancers to cisplatin by reducing the nucleotide excision repair protein XPC level [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 2034.

Comparing Mfd- and UvrD-dependent models of transcription coupled DNA repair in live Escherichia coli using single-molecule tracking

During transcription-coupled DNA repair (TCR) the detection of DNA damage and initiation of nucleotide excision repair (NER) is performed by translocating RNA polymerases (RNAP), which are arrested upon encountering bulky DNA lesions. Two opposing models of the subsequent steps of TCR in bacteria exist. In the first model, stalled RNAPs are removed from the damage site by recruitment of Mfd which dislodges RNAP by pushing it forwards before recruitment of UvrA and UvrB. In the second model, UvrD helicase backtracks RNAP from the lesion site. Recent studies have proposed that both UvrD and UvrA continuously associate with RNAP before damage occurs, which forms the primary damage sensor for NER. To test these two models of TCR in living E. coli, we applied super-resolution microscopy (PALM) combined with single particle tracking to directly measure the mobility and recruitment of Mfd, UvrD, UvrA, and UvrB to DNA during ultraviolet-induced DNA damage. The intracellular mobilities of NER proteins in the absence of DNA damage showed that most UvrA molecules could in principle be complexed with RNAP, however, this was not the case for UvrD. Upon DNA damage, Mfd recruitment to DNA was independent of the presence of UvrA, in agreement with its role upstream of this protein in the TCR pathway. In contrast, UvrD recruitment to DNA was strongly dependent on the presence of UvrA. Inhibiting transcription with rifampicin abolished Mfd DNA-recruitment following DNA damage, whereas significant UvrD, UvrA, and UvrB recruitment remained, consistent with a UvrD and UvrA performing their NER functions independently of transcribing RNAP. Together, although we find that up to ∼8 UvrD-RNAP-UvrA complexes per cell could potentially form in the absence of DNA damage, our live-cell data is not consistent with this complex being the primary DNA damage sensor for NER.

ERCC1 abundance is an indicator of DNA repair-apoptosis decision upon DNA damage

DNA repair is essential for successful propagation of genetic material and fidelity of transcription. Nucleotide excision repair (NER) is one of the earliest DNA repair mechanisms, functionally conserved from bacteria to human. The fact that number of NER genes vary significantly between prokaryotes and metazoans gives the insight that NER proteins have evolved to acquire additional functions to combat challenges associated with a diploid genome, including being involved in the decision between DNA repair and apoptosis. However, no direct association between apoptosis and NER proteins has been shown to date. In this study, we induced apoptosis with a variety of agents, including oxaliplatin, doxorubicin and TRAIL, and observed changes in the abundance and molecular weight of NER complex proteins. Our results showed that XPA, XPC and ERCC1 protein levels change during DNA damage-induced apoptosis. Among these, ERCC1 decrease was observed as a pre-mitochondria depolarisation event which marks the “point of no return” in apoptosis signalling. ERCC1 decrease was due to proteasomal degradation upon lethal doses of oxaliplatin exposure. When ERCC1 protein was stabilised using proteasome inhibitors, the pro-apoptotic activity of oxaliplatin was attenuated. These results explain why clinical trials using proteasome inhibitors and platinum derivatives showed limited efficacy in carcinoma treatment and also the importance of how deep understanding of DNA repair mechanisms can improve cancer therapy.

Decoding the Link Between XPC and Lung Cancer Susceptibility: A Study of Cigarette Smoke-Induced DNA Damage in the Setting of XPC Deficiency

Background/Objective:
 Despite the known mutagenicity of cigarette smoke, only 10-15% of smokers will develop lung cancer in their lifetimes. What determines a smoker’s susceptibility to lung cancer is poorly understood. We identify the nucleotide excision repair (NER) protein Xeroderma pigmentosum Complementation Group C (XPC) as a tumor suppressor that may contribute to lungtumorigenesis when mutated and combined with cigarette smoke. Micronuclei, which are chromosome fragments and/or lagging chromosomes separated from the main nucleus, occur in many cancers, and indicate genomic instability. We hypothesize that cigarette smoke and XPC knockdown will cause genomic instability that will activate the DNA Damage Response (DDR) and increase the frequency of micronuclei and nuclear aberrancies.
 Methods:
 A human bronchial epithelial cell line (Beas-2B), and two lung adenocarcinoma cell lines (H1299 and A549) with stable XPC lentiviral knock-down (shXPC) or control shRNA (shCtrl) were treated with cigarette smoke extract (CSE) or air control (AC). DNA Damage Response (DDR) proteins were analyzed via immunoblot (Western). Micronuclei and nuclear aberrancies were quantified through cytokinesis-block micronucleus assay (CBMT) using immunofluorescence microscopy (DAPI).
 Results:
 Both CSE and XPC knockdown independently amplify expression of phospho-ATM (pATM), a DDR protein, in H1299 cells. Nuclear aberrancies increased significantly (p<0.05) with CSE in all three cell lines. Micronucleus frequency increased significantly with CSE in H1299 and Beas-2B cells (p<0.05) and with XPC knockdown in Beas-2B cells compared to shCtrl (p<0.001).
 Scientific Impact and Implications:
 We identified a previously uncharacterized role of XPC deficiency in augmenting cigarette smoke induced chromosomal breaks manifesting as micronuclei, particularly in non-cancerous Beas-2B cells. These findings offer insight into tumorigenesis in cigarette smoking and shed light on mechanisms of continued DNA damage in cancer cells. Future research should clarify the mechanisms of micronucleus formation in human translational specimens and pinpoint additional functions of XPC beyond NER, including in replication repair.

Human NTHL1 expression and subcellular distribution determines cisplatin sensitivity in human lung epithelial and non-small cell lung cancer cells.

Base excision repair is critical for maintaining genomic stability and for preventing malignant transformation. NTHL1 is a bifunctional DNA glycosylase/AP lyase that initiates repair of oxidatively damaged pyrimidines. Our recent work established that transient over-expression of NTHL1 leads to acquisition of several hallmarks of cancer in non-tumorigenic immortalized cells likely through interaction with nucleotide excision repair protein XPG. Here, we investigate how NTHL1 expression levels impact cellular sensitivity to cisplatin in non-tumorigenic immortalized cells and five non-small cell lung carcinomas cell lines. The cell line with lowest expression of NTHL1 (H522) shows the highest resistance to cisplatin indicating that decrease in NTHL1 levels may modulate resistance to crosslinking agents in NSCLC tumors. In a complementation study, overexpression of NTHL1 in H522 cell line sensitized it to cisplatin. Using NTHL1 N-terminal deletion mutants defective in nuclear localization we show that cisplatin treatment can alter NTHL1 subcellular localization possibly leading to altered protein-protein interactions and affecting cisplatin sensitivity. Experiments presented in this study reveal a previously unknown link between NTHL1 expression levels and cisplatin sensitivity of NSCLC tumor cells. These findings provide an opportunity to understand how altered NTHL1 expression levels and subcellular distribution can impact cisplatin sensitivity in NSCLC tumor cells.

XPA tumor variant leads to defects in NER that sensitize cells to cisplatin.

Nucleotide excision repair (NER) reduces efficacy of treatment with platinum (Pt)-based chemotherapy by removing Pt lesions from DNA. Previous study has identified that missense mutation or loss of the NER genes Excision Repair Cross Complementation Group 1 and 2 (ERCC1 and ERCC2) leads to improved patient outcomes after treatment with Pt-based chemotherapies. Although most NER gene alterations found in patient tumors are missense mutations, the impact of mutations in the remaining nearly 20 NER genes is unknown. Towards this goal, we previously developed a machine learning strategy to predict genetic variants in an essential NER protein, Xeroderma Pigmentosum Complementation Group A (XPA), that disrupt repair. In this study, we report in-depth analyses of a subset of the predicted variants, including in vitro analyses of purified recombinant protein and cell-based assays to test Pt agent sensitivity in cells and determine mechanisms of NER dysfunction. The most NER deficient variant Y148D had reduced protein stability, weaker DNA binding, disrupted recruitment to damage, and degradation. Our findings demonstrate that tumor mutations in XPA impact cell survival after cisplatin treatment and provide valuable mechanistic insights to improve variant effect prediction. Broadly, these findings suggest XPA tumor variants should be considered when predicting chemotherapy response.

A Novel Interaction Between RAD23A/B and Y-family DNA Polymerases

The Y-family DNA polymerases – Pol ι, Pol η, Pol κ and Rev1 – are most well-known for their roles in the DNA damage tolerance pathway of translesion synthesis (TLS). They function to overcome replication barriers by bypassing DNA damage lesions that cannot be normally replicated, allowing replication forks to continue without stalling. In this work, we demonstrate a novel interaction between each Y-family polymerase and the nucleotide excision repair (NER) proteins, RAD23A and RAD23B. We initially focus on the interaction between RAD23A and Pol ι, and through a series of biochemical, cell-based, and structural assays, find that the RAD23A ubiquitin-binding domains (UBA1 and UBA2) interact with separate sites within the Pol ι catalytic domain. While this interaction involves the ubiquitin-binding cleft of UBA2, Pol ι interacts with a distinct surface on UBA1. We further find that mutating or deleting either UBA domain disrupts the RAD23A-Pol ι interaction, demonstrating that both interactions are necessary for stable binding. We also provide evidence that both RAD23 proteins interact with Pol ι in a similar manner, as well as with each of the Y-family polymerases. These results shed light on the interplay between the different functions of the RAD23 proteins and reveal novel binding partners for the Y-family TLS polymerases.

1H, 15N, 13C resonance assignments for proteasome shuttle factor hHR23a.

hHR23a (human homolog of Rad23 a) functions in nucleotide excision repair and proteasome-mediated protein degradation. It contains an N-terminal ubiquitin-like (UBL) domain, an xeroderma pigmentosum C (XPC)-binding domain, and a ubiquitin-associated (UBA) domain preceding and following the XPC-binding domain. Each of the four structural domains are connected by flexible linker regions. We report in this NMR study, the 1H, 15N and 13C resonance assignments for the backbone and sidechain atoms of the hHR23a full-length protein with BioMagResBank accession number 52059. Assignments are 97% and 87% for the backbone (NH, N, C', Cα, and Hα) and sidechain atoms of the hHR23a structured regions. The secondary structural elements predicted from the NMR data fit well to the hHR23a NMR structure. The assignments described in this manuscript can be used to apply NMR for studies of hHR23a with its binding partners.

Conditional Dependency of LP-184 on Prostaglandin Reductase 1 is Synthetic Lethal in Pancreatic Cancers with DNA Damage Repair Deficiencies.

The greater efficacy of DNA-damaging drugs for pancreatic adenocarcinoma (PDAC) relies on targeting cancer-specific vulnerabilities while sparing normal organs and tissues due to their inherent toxicities. We tested LP-184, a novel acylfulvene analog, for its activity in preclinical models of PDAC carrying mutations in the DNA damage repair (DDR) pathways. Cytotoxicity of LP-184 is solely dependent on prostaglandin reductase 1 (PTGR1), so that PTGR1 expression robustly correlates with LP-184 cytotoxicity in vitro and in vivo. Low-passage patient-derived PDAC xenografts with DDR deficiencies treated ex vivo are more sensitive to LP-184 compared with DDR-proficient tumors. Additional in vivo testing of PDAC xenografts for their sensitivity to LP-184 demonstrates marked tumor growth inhibition in models harboring pathogenic mutations in ATR, BRCA1, and BRCA2. Depletion of PTGR1, however, completely abrogates the antitumor effect of LP-184. Testing combinatorial strategies for LP-184 aimed at deregulation of nucleotide excision repair proteins ERCC3 and ERCC4 established synergy. Our results provide valuable biomarkers for clinical testing of LP-184 in a large subset of genetically defined characterized refractory carcinomas. High PTGR1 expression and deleterious DDR mutations are present in approximately one third of PDAC making these patients ideal candidates for clinical trials of LP-184.

EXPRESSION OF NUCLEOTIDE EXCISION REPAIR PROTEIN ERCC1 IN TUMOR TISSUE AS A PROGNOSTIC FACTOR IN COLORECTAL CANCER

At present, biomarkers of tumor resistance to platinum drugs in colorectal cancer are of much concern. Relapses and metastases occur in 35 % of patients with colorectal cancer (stages I–III) after adjuvant polychemotherapy with platinum-based anticancer drugs within the first 5 years. Resistance to platinum-based drugs is determined by a number of factors, including increased DNA repair capacity.
 The aim of the study is to evaluate the prognostic role of ERCC1 expression in tumor tissue in colorectal cancer.
 Materials and Methods. The authors studied the expression of the nucleotide excision repair protein ERCC1 in tumor tissue as a prognostic marker for adjuvant chemotherapy FOLFOX/XELOX in patients with colorectal cancer (stages II–IV). The correlation between ERCC1 expression in tumor tissue and chemotherapy effectiveness, clinical stage, age, survival, differentiation, and EGFR pathway mutations (NRAS, KRAS, BRAF) was assessed.
 Results. ERCC1 expression was detected in 46.9 % of tumor tissue samples and in 88.9 % of resection line tissue samples. In patients with ERCC1 overexpression, the median survival was significantly lower than in patients with low ERCC1 expression. In tumors with NRAS and BRAF mutations, no ERCC1 expression was observed. No differences were determined in patient according to the disease stage, tumor differentiation, and mutations.
 Cancer tumors is an independent predictor of FOLFOX/XELOX chemotherapy resistance.

Structural characterization of transcription-coupled repair protein UVSSA and its interaction with TFIIH protein

UV-stimulated scaffold protein A (UVSSA) is a key protein in the Transcription-Coupled Nucleotide Excision Repair (TC-NER) pathway. UVSSA, an intrinsically disordered protein, interacts with multiple members of the pathway, tethering them into the complex. Several studies have reported that UVSSA recruits Transcription Factor IIH (TFIIH) via direct interaction, following which CSB is degraded and the lesion recognition TC-NER complex dissociates from the damage site to facilitate the DNA repair. Structural insights into these events remain largely unknown. Herein, we have investigated the interaction of human UVSSA with the Pleckstrin-Homology-domain of p62 subunit of TFIIH (p62-PHD) using biophysical techniques. We observed that UVSSA forms a stable complex with the p62-PHD in vitro. Small-angle scattering measurements using X-rays and neutrons revealed a significant change in pair-distance distribution function for UVSSA662/p62-PHD complex compared to UVSSA alone. Additionally, a significant decrease was observed in the radius of gyration of the complex. Our findings suggest that TFIIH binding to UVSSA causes significant conformational changes in UVSSA. We hypothesize that these conformational changes play an important role in the dissociation of the lesion recognition TC-NER complex.

Oxidative stress drives mutagenesis through transcription-coupled repair in bacteria

In bacteria, mutations lead to the evolution of antibiotic resistance, which is one of the main public health problems of the twenty-first century. Therefore, determining which cellular processes most frequently contribute to mutagenesis, especially in cells that have not been exposed to exogenous DNA damage, is critical. Here, we show that endogenous oxidative stress is a key driver of mutagenesis and the subsequent development of antibiotic resistance. This is the case for all classes of antibiotics and highly divergent species tested, including patient-derived strains. We show that the transcription-coupled repair pathway, which uses the nucleotide excision repair proteins (TC-NER), is responsible for endogenous oxidative stress-dependent mutagenesis and subsequent evolution. This suggests that a majority of mutations arise through transcription-associated processes rather than the replication fork. In addition to determining that the NER proteins play a critical role in mutagenesis and evolution, we also identify the DNA polymerases responsible for this process. Our data strongly suggest that cooperation between three different mutagenic DNA polymerases, likely at the last step of TC-NER, is responsible for mutagenesis and evolution. Overall, our work identifies a highly conserved pathway that drives mutagenesis due to endogenous oxidative stress, which has broad implications for all diseases of evolution, including antibiotic resistance development.

Molecular Mechanisms of Transcription-Coupled Repair.

Transcription-coupled repair (TCR), discovered as preferential nucleotide excision repair of UV-induced cyclobutane pyrimidine dimers located in transcribed mammalian genes compared to those in nontranscribed regions of the genome, is defined as faster repair of the transcribed strand versus the nontranscribed strand in transcribed genes. The phenomenon, universal in model organisms including Escherichia coli, yeast, Arabidopsis, mice, and humans, involves a translocase that interacts with both RNA polymerase stalled at damage in the transcribed strand and nucleotide excision repair proteins to accelerate repair. Drosophila, a notable exception, exhibits TCR but lacks an obvious TCR translocase. Mutations inactivating TCR genes cause increased damage-induced mutagenesis in E. coli and severe neurological and UV sensitivity syndromes in humans. To date, only E. coli TCR has been reconstituted in vitro with purified proteins. Detailed investigations of TCR using genome-wide next-generation sequencing methods, cryo-electron microscopy, single-molecule analysis, and other approaches have revealed fascinating mechanisms.

Phylogenetic distribution, structural analysis and interaction of nucleotide excision repair proteins in cyanobacteria

Cyanobacteria are photosynthetic Gram-negative, oxygen evolving prokaryotes with cosmopolitan distribution. Ultraviolet radiation (UVR) and other abiotic stresses result in DNA lesions in cyanobacteria. Nucleotide excision repair (NER) pathway removes the DNA lesions produced by UVR to normal DNA sequence. In cyanobacteria, detailed knowledge about NER proteins is poorly studied. Therefore, we have studied the NER proteins in cyanobacteria. Analyses of 289 amino acids sequence from 77 cyanobacterial species have revealed the presence of a minimum of one copy of NER protein in their genome. Phylogenetic analysis of NER protein shows that UvrD has maximal rate of amino acid substitutions which resulted in increased branch length. The motif analysis shows that UvrABC proteins is more conserved than UvrD, Further, UvrA with UvrB protein interacts with each other and form stable complex which have DNA binding domain on the surface of the complex. UvrB also have DNA binding domain. Positive electrostatic potential was found in the DNA binding region, which is followed by negative and neutral electrostatic potential. Additionally, the surface accessibility values at the DNA strands of T5-T6 dimer binding site were maximal. Protein nucleotide interaction shows the strong binding of T5-T6 dimer with NER proteins of Synechocystis sp. PCC 6803. This process repairs the UV-induced DNA lesions in dark when photoreactivation is inactive. Regulation of NER proteins protect cyanobacterial genome and maintain the fitness of organism under different abiotic stresses.

Structural modeling and analyses of genetic variations in the human XPC nucleotide excision repair protein

Xeroderma pigmentosum C (XPC) is a key initiator in the global genome nucleotide excision repair pathway in mammalian cells. Inherited mutations in the XPC gene can cause xeroderma pigmentosum (XP) cancer predisposition syndrome that dramatically increases the susceptibility to sunlight-induced cancers. Various genetic variants and mutations of the protein have been reported in cancer databases and literature. The current lack of a high-resolution 3-D structure of human XPC makes it difficult to assess the structural impact of the mutations/genetic variations. Using the available high-resolution crystal structure of its yeast ortholog, Rad4, we built a homology model of human XPC protein and compared it with a model generated by AlphaFold. The two models are largely consistent with each other in the structured domains. We have also assessed the degree of conservation for each residue using 966 sequences of XPC orthologs. Our structure- and sequence conservation-based assessments largely agree with the variant’s impact on the protein’s structural stability, computed by FoldX and SDM. Known XP missense mutations such as Y585C, W690S, and C771Y are consistently predicted to destabilize the protein’s structure. Our analyses also reveal several highly conserved hydrophobic regions that are surface-exposed, which may indicate novel intermolecular interfaces that are yet to be characterized.Communicated by Ramaswamy H. Sarma

Bioinformatics and expression analysis of the Xeroderma Pigmentosum complementation group C (XPC) of Trypanosoma evansi in Trypanosoma cruzi cells.

Nucleotide excision repair (NER) acts repairing damages in DNA, such as lesions caused by cisplatin. Xeroderma Pigmentosum complementation group C (XPC) protein is involved in recognition of global genome DNA damages during NER (GG-NER) and it has been studied in different organisms due to its importance in other cellular processes. In this work, we studied NER proteins in Trypanosoma cruzi and Trypanosoma evansi, parasites of humans and animals respectively. We performed three-dimensional models of XPC proteins from T. cruzi and T. evansi and observed few structural differences between these proteins. In our tests, insertion of XPC gene from T. evansi (TevXPC) in T. cruzi resulted in slower cell growth under normal conditions. After cisplatin treatment, T. cruzi overexpressing its own XPC gene (TcXPC) was able to recover cell division rates faster than T. cruzi expressing TevXPC gene. Based on these tests, it is suggested that TevXPC (being an exogenous protein in T. cruzi) interferes negatively in cellular processes where TcXPC (the endogenous protein) is involved. This probably occurred due interaction of TevXPC with some endogenous molecules or proteins from T.cruzi but incapacity of interaction with others. This reinforces the importance of correctly XPC functioning within the cell.

Metformin Sensitizes Cisplatin-induced Apoptosis Through Regulating Nucleotide Excision Repair Pathway In Cisplatin-resistant Human Lung Cancer Cells

Background: Lung cancer is a leading cause of cancer death globally. Platinum-based chemotherapeutic medications are essential for treating advanced NSCLC, despite that drug resistance severely limits its effectiveness. Objective: In this study, we investigated the cytotoxic effect of metformin on cisplatin-resistant NSCLC cells (A549/DDP) and its potential mechanisms. Methods: Anti-lung cancer efficacy of metformin, cisplatin, and metformin combined with cisplatin was examined in A549 and A549/DDP cells. The cell counting kit-8 (CCK-8) assay was applied for measuring cell proliferation. CalcuSyn software was used to calculate the combination index and estimate the synergistic effect of metformin and cisplatin on cell proliferation. The cell apoptosis was analyzed by flow cytometry and the expression of apoptosis-related proteins, Bcl-2, Bax and caspase-3 were analyzed using Western blot. Futhermore, the expression of key nucleotide excision repair (NER) proteins, ERCC1, XPF, and XPA, was also analyzed using Western blot. Results: We found that metformin had dose-dependent antiproliferative effects on A549/DDP and A549 cells. The combination of metformin and cisplatin had higher effectiveness in inhibiting A549/DDP and A549 cell growth than either of the two drugs alone. Flow cytometry analysis indicated that the combined treatment could cause more cell apoptosis than the single-drug treatment. Consistently, the combined treatment decreased the expression of Bcl-2 protein and elevated the expression of Bax, and cleaved caspase-3 proteins. The expression level of ERCC1, XPF, and XPA proteins were lower in the combined treatment than in either of metformin and cisplatin treatment alone. Conclusions: Our study suggested that metformin and cisplatin had synergistic antitumorigenic effects in A549/DDP cells. The combination of cisplatin and metformin could be promising drug candidates to sensitize cisplatin-induced apoptosis through regulating nucleotide excision repair pathways in lung cancer.

Combining Phenotypes of Nucleotide Excision Repair Pathway to Predict the Risk of Head and Neck Squamous Cell Carcinomas in a Chinese Population

Background Nucleotide excision repair (NER) is pivotal in the development of smoking-related malignancies. Nine core genes (XPA, XPB, XPC, XPD, XPF, XPG, ERCC1, DDB1, and DDB2) are highly involved in the NER process. We combined two phenotypes of NER pathway (NER protein and NER gene mRNA expression) and evaluated their associations with the risks of the head and neck squamous cell carcinomas (HNSCCs) in a Chinese population. Methods We conducted a case-control study of 337 HNSCC patients and 285 cancer-free controls by measuring the expression levels of nine core NER proteins and NER gene mRNA in cultured peripheral lymphocytes. Results Compared with the controls, cases had statistically significantly lower protein expression levels of XPA (P < 0.001) and lower mRNA expression levels of XPA and XPB (P = 0.005 and 0.001, respectively). After dividing the subjects by controls' medians of expression levels, we found an association between increased risks of HNSCCs and low XPA protein level (Ptrend = 0.031), as well as low mRNA levels of XPA and XPB (Ptrend = 0.024 and 0.001, respectively). Subsequently, we correlated the two phenotypes and found associations between the NER mRNA and protein levels. Finally, the sensitivity of the expanded model with protein and mRNA expression levels, in addition to demographic variables, on HNSCCs risk was significantly improved. Conclusions Combining two phenotypes of NER pathway may be more effective than the model only including one single phenotype for the assessment of risks of HNSCCs.

Involvement of Xeroderma Pigmentosum Complementation Group G (XPG) in epigenetic regulation of T-Helper (TH) cell differentiation during breast cancer

TNFα and IFN-γ secreted by CD4+T-Helper (TH) cells have antitumor activity followed by polarisation of TH1 phenotype in response to IL-12 secreted by dendritic cells, inducing expression of XPG, Nucleotide-Excision Repair (NER) complex component, which is downregulated in breast cancer. Therefore, we investigated the involvement of XPG in TH-cell differentiation in breast cancer. XPG knock-out (KO) PBMC and TH1 polarised CD4+ TH-cells isolated from breast cancer and control subjects blood samples were used to observe mRNA expressions of associated genes, % enrichment of corresponding epigenetic markers, and m6A RNA methylation levels to study the molecular mechanisms involved. Assays to investigate Cytotoxic T Lymphocyte (CTL) activity after cross-checking extracellular secretion levels. Our XPGKO results indicated upregulation of TH2 and Treg, downregulation of TH1, and negligible change for TH17; reduced expression of genes associated with tumour suppression (TP53, BRCA1) and DNA repair (H2AFX, ATM) for breast cancer TH-cells. CTCF associated TH1 specific function, reduced %enrichment of XPG, CSA, and ERCC1, increased %enrichment of γH2A.X, and altered histone modifications (methylation, deacetylation) at the IFN-γ gene locus in XPGKO breast cancer TH1-cells. Increased m6A RNA methylation mediated by XPG leads to TH1 cell specificity, further inducing CTL activity by releasing extracellular IFG-γ, which activates CD8+ CTLs. This article explores the association of the vital NER protein, XPG with the epigenetic modifications behind TH1 cell differentiation, augmenting the expressions of TH1-network genes to evoke protective immunity in breast cancer.