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Kommajosyula N, Moreau L, D'Andrea AD, Shapiro GI. Abstract 2540: Identification of factors governing sensitivity of NSCLC cell lines to inhibition of Poly (ADP-Ribose) Polymerase (PARP). Cancer Res 2012. [DOI: 10.1158/1538-7445.am2012-2540] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Abstract
Recently, Poly (ADP Ribose) Polymerase-1 (PARP-1) has been shown to be an important target in subsets of breast and ovarian cancers. PARP inhibitors are emerging as important new anticancer agents, especially in cancers defective in homologous recombination (HR) repair, such as those lacking BRCA1 or BRCA2. To date, the potential role of PARP inhibition in NSCLC has not been extensively studied. BRCA1 or BRCA2 promoter hypermethylation and ATM mutation have been described in primary NSCLCs, alterations that may sensitize cells to PARP inhibition. We have examined a panel of 26 genotypically-defined NSCLC cell lines for sensitivity to the PARP-1 inhibitor AG014699 using colony formation assays. We have identified 5cell lines that are exquisitely sensitive (IC50 ≤ 50nM), 9 cell lines that are moderately sensitive (IC50 ≤ 60-500nM) and 12 cell lines that are resistant (IC50 > 5 µM). To determine whether sensitive cell lines have defective HR compared to resistant cell lines, we examined the formation of Rad51 foci following exposure to PARP inhibition, as well as to gamma-irradiation and cisplatin. Both sensitive and resistant cell lines demonstrated efficient formation of Rad51 foci as well as gamma-H2AX foci in response to DNA damaging treatments, suggesting correct initiation of HR. In resistant cell lines, the repair process following PARP inhibition is completed within 24 hours, with resolution of Rad51 and gamma H2AX foci. No alterations in cell cycle distribution were detected in these cell lines, suggesting that the efficiency of repair allows proliferation to continue largely unabated. However, in sensitive cell lines, these foci persist greater than 30 hours following DNA damaging treatments. These results suggest that although sensitive lines are able to initiate HR repair, they are unable to complete the process, so that Rad51 foci are not resolved. The persistence of DNA damage in these cell lines is confirmed by persistent gamma H2AX focus formation as well as S/G2 arrest, the latter representing intact checkpoint control. We are currently investigating whether PARP inhibitor-sensitive cell lines have a deficiency in one of the helicases or resolvases required for resolution of the complex Holliday junctions formed during HR repair. Our results may define a novel subset of NSCLCs with a late-step defect in HR that can be approached with PARP inhibition as a novel treatment strategy.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2540. doi:1538-7445.AM2012-2540
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Zhu K, Kushwaha D, Cron K, Hsieh G, Merzon D, Evans D, Ng K, Chen CC, D'Andrea AD, Kozono D. Abstract 1448: Whole genome RNAi screen identifies proteasome inhibition as a strategy for NSCLC radiosensitization. Cancer Res 2012. [DOI: 10.1158/1538-7445.am2012-1448] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Abstract
Background: Each year, approximately 60,000 patients in the US are diagnosed with locally advanced non-small cell lung carcinoma (NSCLC). The majority is treated with a combination of radiation therapy (RT), chemotherapy +/− surgery. Despite best available therapy, over a third of patients fail locoregionally. The addition of concurrent chemotherapy to RT adds only about a 5% absolute survival benefit. Novel radiosensitizers may improve the therapeutic index of RT. Materials and Methods: We performed whole genome pooled RNAi screens using the Hannon-Elledge library of 74,705 distinct shRNAs directed against over 18,000 genes, in two NSCLC cell lines A549 and NCI-H460. After stable transduction, we passaged the lines for 12 population doublings, and extracted DNA both before and after passaging to compare the representation of each shRNA at each time point. We defined top hits as those genes for which multiple shRNAs decreased in representation by at least 50% during passaging, signifying silencings that were detrimental to proliferation. Results: The top hit of our screen was PSMA1, a subunit of the 20S proteasome; out of six shRNAs, between four and six scored in the two lines in screens performed in triplicate. Bortezomib, an inhibitor of the 20S proteasome, has shown activity against NSCLC in preclinical studies and Phase I/II clinical trials. We observed that bortezomib results in synergistic cytotoxicity in cells treated with fractionated ionizing radiation (IR) in vitro. 10 nM bortezomib resulted in 46% clonogenic survival of unirradiated A549 cells, but only 29% survival of cells treated with 1 Gy x 3 daily fractions compared to cells treated with IR alone. Similar results were observed with MG-132 and with NCI-H460. In soft agar anchorage-independent growth assays, bortezomib resulted in less than 1% colony formation following 1 Gy x 3 daily fractions compared to cells treated with IR alone. In vivo experiments are underway with subcutaneous xenografts in nude mice treated with concurrent CT-guided conformal RT. A retrospective analysis of 443 patients with profiled lung adenocarcinomas showed worse overall survival among patients with tumors showing high proteasome gene expression (median OS 4.2 vs. 6.2 y, 5-year OS 47.5 vs. 61.0%, log rank p = 0.04). Conclusions: Proteasome inhibition emerged as the top drug target of an RNAi screen designed to identify targets with sustained activity over multiple cell generations. Its synergistic impact on NSCLC viability when coupled with fractionated RT should be further explored in preclinical studies and early clinical trials.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1448. doi:1538-7445.AM2012-1448
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Kang J, D'Andrea AD, Kozono D. A DNA repair pathway-focused score for prediction of outcomes in ovarian cancer treated with platinum-based chemotherapy. J Natl Cancer Inst 2012; 104:670-81. [PMID: 22505474 PMCID: PMC3341307 DOI: 10.1093/jnci/djs177] [Citation(s) in RCA: 128] [Impact Index Per Article: 10.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022] Open
Abstract
Background New tools are needed to predict outcomes of ovarian cancer patients treated with platinum-based chemotherapy. We hypothesized that a molecular score based on expression of genes that are involved in platinum-induced DNA damage repair could provide such prognostic information. Methods Gene expression data was extracted from The Cancer Genome Atlas (TCGA) database for 151 DNA repair genes from tumors of serous ovarian cystadenocarcinoma patients (n = 511). A molecular score was generated based on the expression of 23 genes involved in platinum-induced DNA damage repair pathways. Patients were divided into low (scores 0–10) and high (scores 11–20) score groups, and overall survival (OS) was analyzed by Kaplan–Meier method. Results were validated in two gene expression microarray datasets. Association of the score with OS was compared with known clinical factors (age, stage, grade, and extent of surgical debulking) using univariate and multivariable Cox proportional hazards models. Score performance was evaluated by receiver operating characteristic (ROC) curve analysis. Correlations between the score and likelihood of complete response, recurrence-free survival, and progression-free survival were assessed. Statistical tests were two-sided. Results Improved survival was associated with being in the high-scoring group (high vs low scores: 5-year OS, 40% vs 17%, P < .001), and results were reproduced in the validation datasets (P < .05). The score was the only pretreatment factor that showed a statistically significant association with OS (high vs low scores, hazard ratio of death = 0.40, 95% confidence interval = 0.32 to 0.66, P < .001). ROC curves indicated that the score outperformed the known clinical factors (score in a validation dataset vs clinical factors, area under the curve = 0.65 vs 0.52). The score positively correlated with complete response rate, recurrence-free survival, and progression-free survival (Pearson correlation coefficient [r2] = 0.60, 0.84, and 0.80, respectively; P < .001 for all). Conclusion The DNA repair pathway–focused score can be used to predict outcomes and response to platinum therapy in ovarian cancer patients.
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Kim H, Yang K, Dejsuphong D, D'Andrea AD. Regulation of Rev1 by the Fanconi anemia core complex. Nat Struct Mol Biol 2012; 19:164-70. [PMID: 22266823 PMCID: PMC3280818 DOI: 10.1038/nsmb.2222] [Citation(s) in RCA: 117] [Impact Index Per Article: 9.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/10/2011] [Accepted: 12/05/2011] [Indexed: 01/21/2023]
Abstract
The fifteen known Fanconi Anemia (FA) proteins cooperate in a pathway which regulates DNA interstrand crosslink repair. Recent studies indicate that the FA pathway also controls Rev1-mediated translesion DNA synthesis (TLS). Here we identify a novel protein FAAP20, which is an integral subunit of the multisubunit FA core complex. FAAP20 binds to FANCA subunit and is required for complex stability and monoubiquitination of FANCD2. FAAP20 contains a UBZ4 (Ubiquitin Binding Zinc finger 4) domain and binds to the monoubiquitinated form of Rev1. FAAP20 binding stabilizes Rev1 nuclear foci and promotes the interaction of the FA core with PCNA/Rev1 DNA damage bypass complexes. FAAP20 therefore provides a critical link between the FA pathway and TLS polymerase activity. We propose that the FA core complex regulates crosslink repair, by channeling lesions to damage bypass pathways and preventing large DNA insertions and deletions.
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Kee Y, Huang M, Chang S, Moreau LA, Park E, Smith PG, D'Andrea AD. Inhibition of the Nedd8 system sensitizes cells to DNA interstrand cross-linking agents. Mol Cancer Res 2012; 10:369-77. [PMID: 22219386 DOI: 10.1158/1541-7786.mcr-11-0497] [Citation(s) in RCA: 59] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
The Fanconi anemia pathway is required for repair of DNA interstrand cross-links (ICL). Fanconi anemia pathway-deficient cells are hypersensitive to DNA ICL-inducing drugs such as cisplatin. Conversely, hyperactivation of the Fanconi anemia pathway is a mechanism that may underlie cellular resistance to DNA ICL agents. Modulating FANCD2 monoubiquitination, a key step in the Fanconi anemia pathway, may be an effective therapeutic approach to conferring cellular sensitivity to ICL agents. Here, we show that inhibition of the Nedd8 conjugation system increases cellular sensitivity to DNA ICL-inducing agents. Mechanistically, the Nedd8 inhibition, either by siRNA-mediated knockdown of Nedd8-conjugating enzymes or treatment with a Nedd8-activating enzyme inhibitor MLN4924, suppressed DNA damage-induced FANCD2 monoubiquitination and CHK1 phosphorylation. Our data indicate that inhibition of the Fanconi anemia pathway is largely responsible for the heightened cellular sensitivity to DNA ICLs upon Nedd8 inhibition. These results suggest that a combination of Nedd8 inhibition with ICL-inducing agents may be an effective strategy for sensitizing a subset of drug-resistant cancer cells.
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Yang K, Moldovan GL, Vinciguerra P, Murai J, Takeda S, D'Andrea AD. Regulation of the Fanconi anemia pathway by a SUMO-like delivery network. Genes Dev 2011; 25:1847-58. [PMID: 21896657 DOI: 10.1101/gad.17020911] [Citation(s) in RCA: 86] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
Abstract
The USP1/UAF1 complex deubiquitinates the Fanconi anemia protein FANCD2, thereby promoting homologous recombination and DNA cross-link repair. How USP1/UAF1 is targeted to the FANCD2/FANCI heterodimer has remained unknown. Here we show that UAF1 contains a tandem repeat of SUMO-like domains in its C terminus (SLD1 and SLD2). SLD2 binds directly to a SUMO-like domain-interacting motif (SIM) on FANCI. Deletion of the SLD2 sequence of UAF1 or mutation of the SIM on FANCI disrupts UAF1/FANCI binding and inhibits FANCD2 deubiquitination and DNA repair. The USP1/UAF1 complex also deubiquitinates PCNA-Ub, and deubiquitination requires the PCNA-binding protein hELG1. The SLD2 sequence of UAF1 binds to a SIM on hELG1, thus targeting the USP1/UAF1 complex to its PCNA-Ub substrate. We propose that the regulated targeting of USP1/UAF1 to its DNA repair substrates, FANCD2-Ub and PCNA-Ub, by SLD-SIM interactions coordinates homologous recombination and translesion DNA synthesis.
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Ayrapetov MK, Xu C, Sun Y, Zhu K, Parmar K, D'Andrea AD, Price BD. Activation of Hif1α by the prolylhydroxylase inhibitor dimethyoxalyglycine decreases radiosensitivity. PLoS One 2011; 6:e26064. [PMID: 22016813 PMCID: PMC3189247 DOI: 10.1371/journal.pone.0026064] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/13/2011] [Accepted: 09/19/2011] [Indexed: 12/15/2022] Open
Abstract
Hypoxia inducible factor 1α (Hif1α) is a stress responsive transcription factor, which regulates the expression of genes required for adaption to hypoxia. Hif1α is normally hydroxylated by an oxygen-dependent prolylhydroxylase, leading to degradation and clearance of Hif1α from the cell. Under hypoxic conditions, the activity of the prolylhydroxylase is reduced and Hif1α accumulates. Hif1α is also constitutively expressed in tumor cells, where it is associated with resistance to ionizing radiation. Activation of the Hif1α transcriptional regulatory pathway may therefore function to protect normal cells from DNA damage caused by ionizing radiation. Here, we utilized the prolylhydroxylase inhibitor dimethyloxalylglycine (DMOG) to elevate Hif1α levels in mouse embryonic fibroblasts (MEFs) to determine if DMOG could function as a radioprotector. The results demonstrate that DMOG increased Hif1α protein levels and decreased the sensitivity of MEFs to ionizing radiation. Further, the ability of DMOG to function as a radioprotector required Hif1α, indicating a key role for Hif1α's transcriptional activity. DMOG also induced the Hif1α -dependent accumulation of several DNA damage response proteins, including CHD4 and MTA3 (sub-units of the NuRD deacetylase complex) and the Suv39h1 histone H3 methyltransferase. Depletion of Suv39h1, but not CHD4 or MTA3, reduced the ability of DMOG to protect cells from radiation damage, implicating increased histone H3 methylation in the radioprotection of cells. Finally, treatment of mice with DMOG prior to total body irradiation resulted in significant radioprotection of the mice, demonstrating the utility of DMOG and related prolylhydroxylase inhibitors to protect whole organisms from ionizing radiation. Activation of Hif1α through prolylhydroxylase inhibition therefore identifies a new pathway for the development of novel radiation protectors.
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D'Andrea AD. Abstract IA9: Targeting the Fanconi anemia/BRCA pathway in cancer therapy. Cancer Res 2011. [DOI: 10.1158/1538-7445.fbcr11-ia9] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Abstract
Cancer cells have genomic instability resulting from acquired defects in DNA repair. One DNA repair pathway, the Fanconi anemia/BRCA homologous recombination pathway (Kennedy and D'Andrea. Genes & Development. 2005;19:2925), is defective in many human cancers, including breast, ovarian, pancreatic, and lung neoplasms. Disruption of the FA/BRCA pathway results in the characteristic chromosome instability and radiation/crosslinker hypersensitivity of these tumors. In general, loss of one DNA repair pathway often leads to hyperdependence on another pathway for tumor cell survival. This hyperdependence offers a unique opportunity for the development of anticancer therapeutics. For instance, FA/BRCA pathway deficient tumors are hyperdependent on base excision repair (BER) and, accordingly, these tumors are hypersensitive to single agent treatment with PARP1 inhibitors which block BER. Our laboratory efforts are focused on profiling the FA/BRCA pathway and the other five major DNA repair pathways in tumor cells (Kennedy and D'Andrea, JCO. 2006;24:3799). Each DNA repair pathway has a characteristic protein biomarker and repairs a specific type of DNA lesion. By profiling these pathways in primary tumor samples with activation-specific antibodies to DNA repair proteins, we are developing methods 1) to predict the sensitivity of tumors to conventional chemotherapy and radiation (personalized medicine); 2) to subset tumors for their sensitivity to novel classes of DNA repair inhibitors (i.e., PARP1, Chk1, and ATM inhibitors); and 3) to screen for new small molecule inhibitors of other DNA repair pathways. This combination of novel DNA repair inhibitors, conventional DNA damaging agents, and DNA repair biomarkers offers new opportunities for developing more effective anticancer therapy.
The FA/BRCA pathway was elucidated through the systematic study of the rare inherited chromosome breakage disorder, Fanconi anemia (Moldovan and D'Andrea. Ann Rev Genet. 2009;43:223). FA is an autosomal recessive disease characterized by bone marrow failure, congenital malformations, and cellular sensitivity to cisplatin, mitomycin C, and other DNA inter Strand crosslinking agents. Patients with FA develop hematopoietic malignancies and squamous cell carcinomas. Based on somatic cell fusion studies, there are 15 FA complementation groups (A, B, C, D1, D2, E, F, G, I, J, L, M, N, O, and P), and the corresponding gene for each of these complementation groups has been identified. Interestingly, the 15 FA proteins cooperate in a common cellular pathway in normal human cells, referred to as the Fanconi anemia/BRCA pathway. Eight of the FA proteins (A, B, C, E, F, G, L, and M) are assembled in a core complex (the FA core complex), which is an active ubiquitin E3 ligase. In response to DNA damage, the FA core complex modifies (monoubiquitinates) the downstream FANCD2 protein. Monoubiquitinated FANCD2 translocates to nuclear foci where it interacts with the FANCD1/BRCA2 protein and participates in the process of homologous recombination DNA repair. Additional FA proteins (namely, FANCJ/BRIP1 and FANCN/PALB2) function downstream of FANCD2 monoubiquitination. At least three of the FA genes (FANCD1, FANCJ, and FANCN) are inherited breast cancer susceptibility genes. Disruption of any step in the FA/BRCA pathway results in the common clinical and cellular phenotype of FA patients.
Human tumor cells derived from cancer patients from the general (non-FA) population often exhibit genomic instability. Genomic instability of tumors has important clinical consequences. On the one hand, genomic instability gives the tumor the ability to break and fuse chromosomes, inactivate tumor suppressor genes, form novel oncogene fusions, and amplify drug-resistance genes. Thus, the tumor with genomic instability may become more malignant and drug resistant over time. On the other hand, in order to achieve a state of genomic instability, a tumor cell must inactivate one of its major DNA repair pathways. This inactivation appears to account, at least in part, for the selective hypersensitivity of cancer cells to the cytotoxic effects of conventional anticancer radiation and chemotherapy.
Recent studies indicate that some human tumors inactivate the FA/BRCA pathway. Pathway inactivation may result from somatic mutation of genes in the FA/BRCA pathway or by epigenetic silencing. For instance, methylation of one of the FA genes, FANCF, has been implicated as a mechanism for genomic instability in a wide variety of cancers, including ovarian, breast, lung, cervical, and head and neck squamous cell carcinomas. Somatic inactivation of the FA/BRCA pathway appears to account for the genomic instability and the cisplatin hypersensitivity of many of these cancers.
A wide array of biomarkers is available for measuring the activity of the FA/BRCA pathway in human tumors or for measuring the activity of the other human DNA repair pathways. Since many of the genes in the FA/BRCA pathway (i.e., the 15 cloned FA genes) have been identified, tumors can be screened for germline or somatic mutations in these genes. Furthermore, tumors can be analyzed for function of the pathway by following various biochemical events in the pathway. For instance, the full function of the pathway requires monoubiquitination of the FANCD2 protein, ATR and CHK1 dependent phosphorylation of subunits of the pathway, and assembly of FANCD2 and FANCE nuclear foci. A tumor which has defects in any of these genetic or biochemical biomarkers has a defect in the FA/BRCA pathway and may therefore have a hypersensitivity to crosslinking drugs such as cisplatin. Thus, biochemical monitoring of DNA repair pathways may be a means for predicting drug sensitivity of individual tumors.
Loss of DNA repair pathways can lead to hyperdependence on other survival pathways. Inhibition of these alternative pathways may represent a therapeutic approach which is selectively toxic to repair pathway deficient tumor cells. For instance, breast and ovarian tumor cells which are deficient in the homologous recombination (HR) pathway are hypersensitive to drugs (PARP1 inhibitors), which selectively inactivate a compensatory DNA Repair pathway, the base excision repair (BER) pathway. We recently used a high-throughput siRNA screening approach to identify DNA damage response genes that were critical for the survival of FA/BRCA pathway deficient tumor cells (Kennedy et al. J Clin Invest. 2007;117:1440). Using this approach we identified the DNA damage response kinase, ATM, as being important for the survival of cells deficient in the FA pathway. Accordingly, we found that the Atm −/− Fancg −/− mouse genotype was deleterious when Fancg +/- Atm +/- mice were interbred. We also demonstrated constitutive activation of ATM in FA pathway deficient cells, which was abrogated by reconstitution of the pathway. Furthermore, inhibition of ATM using siRNA oligonucleotides or the specific ATM inhibitor KU55933 resulted in DNA breakage and cell death specifically in cells with a nonfunctioning FA pathway.
These data suggest that ATM and the FA pathway function in parallel and compensatory roles following endogenous DNA damage. Moreover, pharmaceutical inhibition of ATM may be selectively toxic to cancer cells that have lost function of the FA pathway. We are currently exploring other therapeutic strategies to selectively target tumors with a defect in the FA/BRCA pathway or with a defect in one of the other human DNA repair pathways. For instance, we recently determined that tumors which are deficient in the FA/BRCA pathway are hypersensitive to CHK1 inhibitors (Chen et al. Mol Cancer. 2009;8:24). In my presentation, I will review the features of the six major DNA repair pathways in human cells, and how knowledge of the disruption of these pathways in cancers can lead to the prediction of drug response.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the Second AACR International Conference on Frontiers in Basic Cancer Research; 2011 Sep 14-18; San Francisco, CA. Philadelphia (PA): AACR; Cancer Res 2011;71(18 Suppl):Abstract nr IA9.
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Vaezi A, Wang X, Buch S, Gooding W, Wang L, Seethala RR, Weaver DT, D'Andrea AD, Argiris A, Romkes M, Niedernhofer LJ, Grandis JR. XPF expression correlates with clinical outcome in squamous cell carcinoma of the head and neck. Clin Cancer Res 2011; 17:5513-22. [PMID: 21737503 DOI: 10.1158/1078-0432.ccr-11-0086] [Citation(s) in RCA: 41] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/12/2023]
Abstract
PURPOSE Tumor-specific biomarkers that predict resistance to DNA damaging agents may improve therapeutic outcomes by guiding the selection of effective therapies and limiting morbidity related to ineffective approaches. XPF (ERCC4) is an essential component of several DNA repair pathways and XPF-deficient cells are exquisitely sensitive to DNA damaging agents. The purpose of this study was to determine whether XPF expression levels predict clinical response to DNA damaging agents in head and neck squamous cell carcinoma (HNSCC). EXPERIMENTAL DESIGN Quantitative immunohistochemistry was used to measure XPF expression in tumors from a cohort of 80 patients with newly diagnosed HNSCC treated with radiation therapy with or without platinum-based chemotherapy; samples were collected prospectively. Genomic DNA isolated from blood samples was analyzed for nine single nucleotide polymorphisms (SNP) in the XPF gene by using a custom array. The primary endpoint was progression-free survival (PFS). RESULTS XPF expression was higher in tumors from the oral cavity than from the other sites (P < 0.01). High XPF expression correlated with early time to progression both by univariate (HR = 1.87, P = 0.03) and multivariate analysis (HR = 1.83, P = 0.05). The one year PFS for high expressers was 47% (95% CI = 31-62) compared with 72% (95% CI = 55-83) for low expressers. In addition, we identified four XPF SNPs that showed marginal association with treatment failure. CONCLUSIONS Expression level of XPF in HNSCC tumors correlates with clinical response to DNA damaging agents. XPF has potential to guide next generation personalized cancer therapy.
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Kao WH, Riker AI, Kushwaha DS, Ng K, Enkemann SA, Jove R, Buettner R, Zinn PO, Sánchez NP, Villa JL, D'Andrea AD, Sánchez JL, Kennedy RD, Chen CC, Matta JL. Upregulation of Fanconi anemia DNA repair genes in melanoma compared with non-melanoma skin cancer. J Invest Dermatol 2011; 131:2139-42. [PMID: 21697891 PMCID: PMC3174338 DOI: 10.1038/jid.2011.181] [Citation(s) in RCA: 14] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022]
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Moldovan GL, D'Andrea AD. DNA damage discrimination at stalled replication forks by the Rad5 homologs HLTF and SHPRH. Mol Cell 2011; 42:141-3. [PMID: 21504827 DOI: 10.1016/j.molcel.2011.03.018] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
In this issue of Molecular Cell, Lin et al. (2011) describe how HLTF and SHPRH, the human homologs of yeast Rad5, can discriminate between MMS-induced versus UV-induced DNA damage. The results have important implications for the suppression of damage-specific mutagenesis and for the maintenance of genomic stability.
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Johnson N, Li YC, Walton Z, Li D, Moreau L, Rodig SJ, D'Andrea AD, Thomas H, Newell DR, Curtin NJ, Wong KK, Shapiro GI. Abstract 2979: Combined cdk1 and PARP inhibition results in tumor regression in a KrasG12D p53L/L murine lung adenocarcinoma model. Cancer Res 2011. [DOI: 10.1158/1538-7445.am2011-2979] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Abstract
Kras is one of the most commonly mutated oncogenes in non-small cell lung cancer (NSCLC), and targeted therapies are not available for this lung cancer subset. Consequently, there is an unmet need for improved therapies for patients with NSCLCs harboring Kras mutation. We previously demonstrated that cdk1 phosphorylates BRCA1, and is essential for efficient BRCA1 focus formation and activation of the S phase checkpoint in response to DNA damage in RAS mutant NSCLC cell lines (Johnson et al., Mol Cell 2009; 35: 327-39). PARP inhibition has been shown to selectively kill cancer cells that are unable to perform homologous recombination (HR)-mediated DNA repair. Furthermore, PARP inhibitors have provided clinical responses in patients with BRCA1 and BRCA2 mutations. We hypothesized that small molecule mediated cdk1 inhibition would disrupt BRCA1 function and sensitize BRCA-proficient, RAS mutant NSCLC cells to PARP inhibitors.
Using a gene conversion assay to directly measure HR, in which GFP expression indicates the occurrence of HR repair, we demonstrated that the small molecule cdk1 inhibitors RO-3306 and AG024322 reduced GFP expression 7.7-fold (p = 0.013) and 3.4 fold (p = 0.016), respectively, compared to vehicle. In colony forming assays, RO-3306 and AG024322 reduced the LC50 value of the PARP inhibitor AG014699 6.6-fold and 18.7-fold in Nras mutant, TP53-deleted NCI-H1299 cells. Similar results were observed with Kras mutant A549 cells. In contrast to transformed cells, non-transformed Retinal Pigment Epithelial (RPE) cells were less sensitive to combined RO-3306 and AG014699 treatment. Similarly, when NCI-H1299 xenograft-bearing mice were treated with both AG024322 and AG014699, tumor growth was significantly delayed compared to vehicle exposure or treatment with either compound alone. We further assessed the therapeutic efficacy of combined AG024322 and AG014699 treatment in the KrasG12Dp53L/L murine lung adenocarcinoma model. Mice treated with vehicle, AG024322 or AG014699 alone all demonstrated tumor growth by MRI. In contrast, 87.5% (14/16) treated with combined AG024322 and AG014699 had up to 70% reduction in tumor volume. Furthermore, combined AG024322 and AG014699 treatment resulted in a marked reduction in Ki67 staining and increased TUNEL staining in residual tumor compared to vehicle or individual treatments. No toxicity or damage to normal mouse tissues and organs was found after combination treatment by pathologic assessment. This approach provides the potential to extend well-tolerated PARP inhibition to treatment for BRCA-proficient Kras mutant NSCLC.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2979. doi:10.1158/1538-7445.AM2011-2979
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Alexander BM, Sprott K, Farrow DA, Wang X, D'Andrea AD, Schnitt SJ, Collins LC, Weaver DT, Garber JE. DNA repair protein biomarkers associated with time to recurrence in triple-negative breast cancer. Clin Cancer Res 2011; 16:5796-804. [PMID: 21138871 DOI: 10.1158/1078-0432.ccr-10-0292] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
PURPOSE To evaluate the prognostic utility of immunohistochemical assessment of key proteins in multiple DNA repair pathways in triple-negative breast cancer (TNBC; estrogen receptor negative, progesterone receptor negative, and HER2/neu negative by immunohistochemistry). EXPERIMENTAL DESIGN Archived clinically annotated tumor specimens from 112 women with TNBC were immunostained with antibodies against DNA repair proteins and scored using digital image analysis. The cohort was divided into training and test sets for development of a multiantibody model. Scores were combined with clinical data to assess association with outcome. RESULTS Low XPF (P = 0.005), pMK2 (P = 0.01), MLH; P = 0.002), and FANCD2 (P = 0.001) were each associated with shorter time to recurrence (TTR) in univariate analysis. A 4-antibody model could segregate high-risk and low-risk groups on the basis of TTR in both the training (relative risk [RR] = 3.52; P = 9.05E-07) and test (RR 2.67; P = 0.019) cohorts. CONCLUSIONS DNA repair proteins may be useful as prognostic markers in TNBC. Further study in larger, uniformly treated cohorts with additional clinical parameters is warranted.
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Vinciguerra P, Godinho SA, Parmar K, Pellman D, D'Andrea AD. Cytokinesis failure occurs in Fanconi anemia pathway-deficient murine and human bone marrow hematopoietic cells. J Clin Invest 2010; 120:3834-42. [PMID: 20921626 DOI: 10.1172/jci43391] [Citation(s) in RCA: 92] [Impact Index Per Article: 6.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/18/2010] [Accepted: 08/18/2010] [Indexed: 01/18/2023] Open
Abstract
Fanconi anemia (FA) is a genomic instability disorder characterized by bone marrow failure and cancer predisposition. FA is caused by mutations in any one of several genes that encode proteins cooperating in a repair pathway and is required for cellular resistance to DNA crosslinking agents. Recent studies suggest that the FA pathway may also play a role in mitosis, since FANCD2 and FANCI, the 2 key FA proteins, are localized to the extremities of ultrafine DNA bridges (UFBs), which link sister chromatids during cell division. However, whether FA proteins regulate cell division remains unclear. Here we have shown that FA pathway-deficient cells display an increased number of UFBs compared with FA pathway-proficient cells. The UFBs were coated by BLM (the RecQ helicase mutated in Bloom syndrome) in early mitosis. In contrast, the FA protein FANCM was recruited to the UFBs at a later stage. The increased number of bridges in FA pathway-deficient cells correlated with a higher rate of cytokinesis failure resulting in binucleated cells. Binucleated cells were also detectable in primary murine FA pathway-deficient hematopoietic stem cells (HSCs) and bone marrow stromal cells from human patients with FA. Based on these observations, we suggest that cytokinesis failure followed by apoptosis may contribute to bone marrow failure in patients with FA.
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Kee Y, D'Andrea AD. Expanded roles of the Fanconi anemia pathway in preserving genomic stability. Genes Dev 2010; 24:1680-94. [PMID: 20713514 DOI: 10.1101/gad.1955310] [Citation(s) in RCA: 246] [Impact Index Per Article: 17.6] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
Studying rare human genetic diseases often leads to a better understanding of normal cellular functions. Fanconi anemia (FA), for example, has elucidated a novel DNA repair mechanism required for maintaining genomic stability and preventing cancer. The FA pathway, an essential tumor-suppressive pathway, is required for protecting the human genome from a specific type of DNA damage; namely, DNA interstrand cross-links (ICLs). In this review, we discuss the recent progress in the study of the FA pathway, such as the identification of new FANCM-binding partners and the identification of RAD51C and FAN1 (Fanconi-associated nuclease 1) as new FA pathway-related proteins. We also focus on the role of the FA pathway as a potential regulator of DNA repair choices in response to double-strand breaks, and its novel functions during the mitotic phase of the cell cycle.
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Abstract
To cope with the life-threatening crisis of a DNA interstrand cross-link (ICL), human cells must invoke the Fanconi anemia (FA) DNA repair pathway. The FA pathway is a multistep repair process, requiring multiple nucleolytic incisions and translesion DNA synthesis. Recent work from four laboratories has identified a novel FA-associated nuclease, FAN1, that binds directly to monoubiquitinated FANCD2, resolving a decade-long puzzle regarding the function of this FANCD2 modification.
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Huang M, Kim JM, Shiotani B, Yang K, Zou L, D'Andrea AD. The FANCM/FAAP24 complex is required for the DNA interstrand crosslink-induced checkpoint response. Mol Cell 2010; 39:259-68. [PMID: 20670894 DOI: 10.1016/j.molcel.2010.07.005] [Citation(s) in RCA: 99] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/05/2010] [Revised: 04/22/2010] [Accepted: 06/16/2010] [Indexed: 11/18/2022]
Abstract
Cells from Fanconi anemia (FA) patients are extremely sensitive to DNA interstrand crosslinking (ICL) agents, but the molecular basis of the hypersensitivity remains to be explored. FANCM (FA complementation group M), and its binding partner, FAAP24, anchor the multisubunit FA core complex to chromatin after DNA damage and may contribute to ICL-specific cellular response. Here we show that the FANCM/FAAP24 complex is specifically required for the recruitment of replication protein A (RPA) to ICL-stalled replication forks. ICL-induced RPA foci formation requires the DNA-binding activity of FAAP24 but not the DNA translocase activity of FANCM. Furthermore, FANCM/FAAP24-dependent RPA foci formation is required for efficient ATR-mediated checkpoint activation in response to ICL. Therefore, we propose that FANCM/FAAP24 plays a role in ICL-induced checkpoint activation through regulating RPA recruiment at ICL-stalled replication forks.
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Abstract
The study of rare genetic diseases can lead to insights into the cause and treatment of common diseases. An example is the rare chromosomal instability disorder, Fanconi Anemia (FA). Studies of this disease have elucidated general mechanisms of bone marrow failure, cancer pathogenesis, and resistance to chemotherapy. The principal features of FA are aplastic anemia in childhood, susceptibility to cancer or leukemia, and hypersensitivity of FA cells to DNA cross-linking agents. There are thirteen FA genes, and one of these genes is identical to the well known breast cancer susceptibility gene, BRCA2. The corresponding FA proteins cooperate in the recognition and repair of damaged DNA. Inactivation of FA genes occurs not only in FA patients but also in a variety of cancers in the general population. These findings have broad implications for predicting the sensitivity and resistance of tumors to conventional anti-cancer agents, to inhibitors of poly-ADP ribose polymerase 1, an enzyme involved in DNA repair, and to other inhibitors of DNA repair.
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D'Andrea AD. Abstract SY28-01: Targeting DNA repair pathways in cancer therapy. Cancer Res 2010. [DOI: 10.1158/1538-7445.am10-sy28-01] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Abstract
Cancer cells have genomic instability resulting from acquired defects in DNA repair. One DNA repair pathway_the Fanconi Anemia/BRCA homologous recombination pathway (Kennedy and D'Andrea, Genes&Development 19:2925, 2005) _ is defective in many human cancers, including breast, ovarian, pancreatic, and lung neoplasms. Disruption of the FA/BRCA Pathway results in the characteristic chromosome instability and radiation/crosslinker hypersensitivity of these tumors. In general, loss of one DNA repair pathway often leads to hyperdependence on another pathway for tumor cell survival. This hyperdependence offers a unique opportunity for the development of anticancer therapeutics. For instance, FA/BRCA pathway deficient tumors are hyperdependent on Base Excision Repair (BER) and, accordingly, these tumors are hypersensitive to single agent treatment with PARP1 inhibitors which block BER. Our laboratory efforts are focused on profiling the FA/BRCA pathway and the other five major DNA repair pathways in tumor cells (Kennedy and D'Andrea, JCO 24: 3799, 2006). Each DNA repair pathway has a characteristic protein biomarker and repairs a specific type of DNA lesion. By profiling these pathways in primary tumor samples with activation-specific antibodies to DNA repair proteins, we are developing methods (1) to predict the sensitivity of tumors to conventional chemotherapy and radiation (Personalized Medicine) (2) to subset tumors for their sensitivity to novel classes of DNA repair inhibitors, (i.e., PARP1, Chk1, and ATM inhibitors) and (3) to screen for new small molecule inhibitors of other DNA repair pathways. This combination of novel DNA repair inhibitors, conventional DNA damaging agents, and DNA repair biomarkers offers new opportunities for developing more effective anticancer therapy.
The FA/BRCA pathway was elucidated through the systematic study of the rare inherited chromosome breakage disorder, Fanconi Anemia (Moldovan and D'Andrea, Ann Rev Genet 43:223, 2009). FA is an autosomal recessive disease characterized by bone marrow failure, congenital malformations, and cellular sensitivity to Cisplatin, Mitomycin C, and other DNA interstrand crosslinking agents. Patients with FA develop hematopoietic malignancies and squamous cell carcinomas. Based on somatic cell fusion studies, there are thirteen FA complementation groups (A, B, C, D1, D2, E, F, G, I, J, L, M, N), and the corresponding gene for each of these complementation groups has been identified. Interestingly, the thirteen FA proteins cooperate in a common cellular pathway in normal human cells, referred to as the Fannoni Anemia/BRCA pathway. Eight of the FA proteins (A, B, C, E, F, G, L, M) are assembled in a core complex (the FA core complex) which is an active ubiquitin E3 ligase. In response to DNA damage, the FA core complex modifies (monoubiquitinates) the downstream FANCD2 protein. Monoubiquitinated FANCD2 translocates to nuclear foci where it interacts with the FANCD1/BRCA2 protein and participates in the process of homologous recombination DNA repair. Additional FA proteins (namely, FANCJ/BRIP1 and FANCN/PALB2) function downstream of FANCD2 monoubiquitination. At least three of the FA genes (FANCD1, FANCJ, and FANCN) are inherited breast cancer susceptibility genes. Disruption of any step in the FA/BRCA pathway results in the common clinical and cellular phenotype of FA patients.
Human tumor cells, derived from cancer patients from the general (non-FA) population, often exhibit genomic instability. Genomic instability of tumors has important clinical consequences. On the one hand, genomic instability gives the tumor the ability to break and fuse chromosomes, inactivate tumor suppressor genes, form novel oncogene fusions, and amplify drug resistance genes. Thus, the tumor with genomic instability may become more malignant and drug resistant over time. On the other hand, in order to achieve a state of genomic instability, a tumor cell must inactivate one of its major DNA repair pathways. This inactivation appears to account, at least in part, for the selective hypersensitivity of cancer cells to the cytotoxic effects of conventional anti-cancer radiation and chemotherapy.
Recent studies indicate that some human tumors inactivate the FA/BRCA pathway. Pathway inactivation may result from somatic mutation of genes in the FA/BRCA pathway or by epigenetic silencing. For instance, methylation of one of the FA genes, FANCF, has been implicated as a mechanism for genomic instability in a wide variety of cancers, including ovarian, breast, lung, cervical, and head and neck squamous cell carcinomas. Somatic inactivation of the FA/BRCA pathway appears to account for the genomic instability and the cisplatin hypersensitivity of many of these cancers.
A wide array of biomarkers is available for measuring the activity of the FA/BRCA pathway in human tumors, or for measuring the activity of the other human DNA repair pathways. Since many of the genes in the FA/BRCA pathway (i.e., the thirteen cloned FA genes) have been identified, tumors can be screened for germline or somatic mutations in these genes. Furthermore, tumors can be analyzed for function of the pathway by following various biochemical events in the pathway. For instance, the full function of the pathway requires monoubiquitination of the FANCD2 protein, ATR and CHK1 dependent phosphorylation of subunits of the pathway, and assembly of FANCD2 and FANCE nuclear foci. A tumor which has defects in any of these genetic or biochemical biomarkers has a defect in the FA/BRCA pathway and may therefore have a hypersensitivity to crosslinking drugs such as cisplatin. Thus, biochemical monitoring of DNA repair pathways may be a means for predicting drug sensitivity of individual tumors.
Loss of DNA repair pathways can lead to hyperdependence on other survival pathways. Inhibition of these alternative pathways may represent a therapeutic approach which is selectively toxic to repair pathway deficient tumor cells. For instance, breast and ovarian tumor cells which are deficient in the homologous recombination (HR) pathway are hypersensitive to drugs (PARP1 inhibitors) which selectively inactivate a compensatory DNA Repair pathway, the Base Excision Repair (BER) pathway. We recently used a high throughput siRNA screening approach to identify DNA damage response genes that were critical for the survival of FA/BRCA pathway deficient tumor cells (Kennedy et al, J. Clin Invest 117:1440, 2007). Using this approach we identified the DNA damage response kinase, ATM, as being important for the survival of cells deficient in the FA pathway. Accordingly, we found that the Atm -/- Fancg -/- mouse genotype was deleterious when Fancg +/- Atm +/- mice were interbred. We also demonstrated constitutive activation of ATM in FA pathway deficient cells, which was abrogated by reconstitution of the pathway. Furthermore, inhibition of ATM using siRNA oligonucleotides or the specific ATM inhibitor KU55933 resulted DNA breakage and cell death specifically in cells with a non functioning FA pathway.
These data suggest that ATM and the FA pathway function in parallel and compensatory roles following endogenous DNA Damage. Moreover, pharmaceutical inhibition of ATM may be selectively toxic to cancer cells that have lost function of the FA pathway. We are currently exploring other therapeutic strategies to selectively target tumors with a defect in the FA/BRCA pathway or with a defect in one of the other human DNA repair pathways. For instance, we recently determined that tumors which are deficient in the FA/BRCA pathway are hypersensitive to CHK1 inhibitors (Chen et al, Mol Cancer 8: 24, 2009). In my presentation, I will review the features of the six major DNA repair pathways in human cells, and how knowledge of the disruption of these pathways in cancers can lead to the prediction of drug response.
Citation Format: Alan D. D'Andrea. Targeting DNA repair pathways in cancer therapy [abstract]. In: Proceedings of the AACR 101st Annual Meeting 2010; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr SY28-01
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Yang K, Moldovan GL, D'Andrea AD. RAD18-dependent recruitment of SNM1A to DNA repair complexes by a ubiquitin-binding zinc finger. J Biol Chem 2010; 285:19085-91. [PMID: 20385554 DOI: 10.1074/jbc.m109.100032] [Citation(s) in RCA: 45] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
SNM1A is a member of the SNM1 family of nucleases required for cellular processing of interstrand DNA crosslinks (ICLs). Little is known about the molecular function of SNM1A, in terms of its recruitment to ICL lesions or its DNA damage processing activity. Here we show that SNM1A contains a functional PIP box (PCNA-interacting protein box) and a UBZ (ubiquitin binding zinc finger), required for assembly of SNM1A into nuclear focus. Moreover, RAD18-dependent monoubiquitination of PCNA is required for Mitomycin C and Ultraviolet Light inducible SNM1A nuclear focus assembly. Taken together, our results identify a novel RAD18-PCNA(Ub)-SNM1A pathway required for nuclear focus formation and ICL resistance.
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Kee Y, Yang K, Cohn MA, Haas W, Gygi SP, D'Andrea AD. WDR20 regulates activity of the USP12 x UAF1 deubiquitinating enzyme complex. J Biol Chem 2010; 285:11252-7. [PMID: 20147737 DOI: 10.1074/jbc.m109.095141] [Citation(s) in RCA: 73] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The UAF1 (Usp1-associated factor 1) protein binds and stimulates three deubiquitinating enzymes: USP1, USP12, and USP46. Although the USP1 x UAF1 complex is required for regulation of the Fanconi anemia (FA) DNA repair pathway, less is known about the USP12 x UAF1 and the USP46 x UAF1 complexes. To understand further the nature of the USP12 and USP46 complexes, we attempted to identify proteins that interact with the USP12 and USP46 deubiquitinating enzyme complexes. We identified WDR20, a WD40-repeat containing protein, as a common binding partner of UAF1, USP12, and USP46. Further analysis showed that WDR20 associates exclusively with USP12 and USP46, not with USP1. Furthermore, we demonstrate the purification of a ternary USP12 x UAF1 x WDR20 complex. Interestingly, and consistent with the binding assays, WDR20 stimulated the enzymatic activity of USP12 x UAF1, but not of USP1 x UAF1. Consistent with our previous report that USP12 and USP46 do not regulate the FA pathway, small interference RNA-mediated depletion of WDR20 protein did not affect the FA pathway or DNA damage responses. We provide a model in which WDR20 serves as a stimulatory subunit for preserving and regulating the activity of the subset of the UAF1 x USP complexes.
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Lee KY, Yang K, Cohn MA, Sikdar N, D'Andrea AD, Myung K. Human ELG1 regulates the level of ubiquitinated proliferating cell nuclear antigen (PCNA) through Its interactions with PCNA and USP1. J Biol Chem 2010; 285:10362-9. [PMID: 20147293 DOI: 10.1074/jbc.m109.092544] [Citation(s) in RCA: 99] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022] Open
Abstract
The level of monoubiquitinated proliferating cell nuclear antigen (PCNA) is closely linked with DNA damage bypass to protect cells from a high level of mutagenesis. However, it remains unclear how the level of monoubiquitinated PCNA is regulated. Here, we demonstrate that human ELG1 protein, which comprises an alternative replication factor C (RFC) complex and plays an important role in preserving genomic stability, as an interacting partner for the USP1 (ubiquitin-specific protease 1)-UAF1 (USP1-associated factor 1) complex, a deubiquitinating enzyme complex for PCNA and FANCD2. ELG1 protein interacts with PCNAs that are localized at stalled replication forks. ELG1 knockdown specifically resulted in an increase in the level of PCNA monoubiquitination without affecting the level of FANCD2 ubiquitination. It is a novel function of ELG1 distinct from its role as an alternative RFC complex because knockdowns of any other RFC subunits or other alternative RFCs did not affect PCNA monoubiquitination. Lastly, we identified a highly conserved N-terminal domain in ELG1 that was responsible for the USP1-UAF1 interaction as well as the activity to down-regulate PCNA monoubiquitination. Taken together, ELG1 specifically directs USP1-UAF1 complex for PCNA deubiquitination.
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Abstract
Left unrepaired, DNA interstrand crosslinks represent impassable hurdles for DNA replication, and their removal is a complicated stepwise process involving a variety of enzymes. In a recent paper in Science, Knipscheer et al. (2009) demonstrate that the Fanconi Anemia protein FANCD2 promotes multiple steps of the crosslink repair process.
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Abstract
In this issue of Molecular Cell, Deans and West (2009) reveal the molecular basis of the phenotypic similarities between Fanconi Anemia (FA) and Bloom's Syndrome, identifying FANCM as the anchor for both FA and Bloom's complexes at the site of the DNA interstrand crosslink.
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Abstract
Fanconi Anemia (FA) is an inherited genomic instability disorder, caused by mutations in genes regulating replication-dependent removal of interstrand DNA crosslinks. The Fanconi Anemia pathway is thought to coordinate a complex mechanism that enlists elements of three classic DNA repair pathways, namely homologous recombination, nucleotide excision repair, and mutagenic translesion synthesis, in response to genotoxic insults. To this end, the Fanconi Anemia pathway employs a unique nuclear protein complex that ubiquitinates FANCD2 and FANCI, leading to formation of DNA repair structures. Lack of obvious enzymatic activities among most FA members has made it challenging to unravel its precise modus operandi. Here we review the current understanding of how the Fanconi Anemia pathway components participate in DNA repair and discuss the mechanisms that regulate this pathway to ensure timely, efficient, and correct restoration of chromosomal integrity.
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