New insights into the Fanconi anemia pathway from an isogenic FancG hamster CHO mutant

Sulfoquinovosyl acylpropanediol (SQAP): Inhibition of poly(ADP-ribose) metabolism and enhanced cytotoxicity in homologous recombination repair-deficient Chinese hamster-derived cells

Sulfoquinovosyl acylpropanediol (SQAP; a synthetic derivative of the sulfoglycolipid natural product sulfoquinovosyl acylglycerol, SQAG), has anti-tumor and radiosensitizing activities in tumor xenograft mouse models. Here, we have studied the PARP inhibitory activity of SQAP and synthetic lethality in BRCA2-deficient cells. In initial screening studies with DNA repair-deficient Chinese hamster ovary cells, homologous recombination repair-deficient cell lines showed increased sensitivity to SQAP, compared to wild-type cells or other DNA repair-deficient mutants. Chinese hamster lung V79 cells and the derivative cell lines V-C8 (BRCA2-deficient) and V-C8 + BRCA2 gene corrections were used to test the role of BRCA2 in SQAP cytotoxicity. The findings were confirmed in studies of the human colon cancer cell lines DLD-1 and its BRCA2-knockout derivative. SQAP inhibited the enzymes poly(ADP-ribose) polymerase (PARP) and poly(ADP-ribose) glycohydrolase (PARG). SQAP pretreatment decreased H2O2induced poly(ADP-ribose) formation in V79 cells. SQAP caused DNA double-strand breaks and chromosome aberrations in V79 BRCA2-mutated cells but did not affect cells in the G2 phase. We have demonstrated that SQAP induces synthetic lethality in BRCA2-deficient Chinese hamster-derived cells via its effects on poly(ADP-ribose) metabolism, motivating further examination of its therapeutic potential, especially against tumors that are deficient in homologous recombination repair due to mutations in BRCA2 or other genes.

A clickable melphalan for monitoring DNA interstrand crosslink accumulation and detecting ICL repair defects in Fanconi anemia patient cells

Fanconi anemia (FA) is a genetic disorder associated with developmental defects, bone marrow failure and cancer. The FA pathway is crucial for the repair of DNA interstrand crosslinks (ICLs). In this study, we have developed and characterized a new tool to investigate ICL repair: a clickable version of the crosslinking agent melphalan which we name click-melphalan. Our results demonstrate that click-melphalan is as effective as its unmodified counterpart in generating ICLs and associated toxicity. The lesions induced by click-melphalan can be detected in cells by post-labelling with a fluorescent reporter and quantified using flow cytometry. Since click-melphalan induces both ICLs and monoadducts, we generated click-mono-melphalan, which only induces monoadducts, in order to distinguish between the two types of DNA repair. By using both molecules, we show that FANCD2 knock-out cells are deficient in removing click-melphalan-induced lesions. We also found that these cells display a delay in repairing click-mono-melphalan-induced monoadducts. Our data further revealed that the presence of unrepaired ICLs inhibits monoadduct repair. Finally, our study demonstrates that these clickable molecules can differentiate intrinsic DNA repair deficiencies in primary FA patient cells from those in primary xeroderma pigmentosum patient cells. As such, these molecules may have potential for developing diagnostic tests.

PARP deficiency causes hypersensitivity to Taxol through oxidative stress induced DNA damage

Taxol is an antitumor drug derived from the bark of the Pacific Yew tree that inhibits microtubule disassembly, resulting in cell cycle arrest in late G2 and M phases. Additionally, Taxol increases cellular oxidative stress by generating reactive oxygen species. We hypothesized that the inhibition of specific DNA repair machinery/mechanisms would increase cellular sensitivity to the oxidative stress capacity of Taxol. Initial screening using Chinese hamster ovary (CHO) cell lines demonstrated that base excision repair deficiency, especially PARP deficiency, caused cellular Taxol hypersensitivity. Taxane diterpenes-containing Taxus yunnanensis extract also showed hypertoxicity in PARP deficient cells, which was consistent with other microtubule inhibitors like colcemid, vinblastine, and vincristine. Acute exposure of 50 nM Taxol treatment induced both significant cytotoxicity and M-phase arrest in PARP deficient cells, but caused neither significant cytotoxicity nor late G2-M cell cycle arrest in wild type cells. Acute exposure of 50 nM Taxol treatment induced oxidative stress and DNA damage. The antioxidant Ascorbic acid 2 glucoside partially reduced the cytotoxicity of Taxol in PARP deficient cell lines. Finally, the PARP inhibitor Olaparib increased cytotoxicity of Taxol in wild type CHO cells and two human cancer cell lines. Our study clearly demonstrates that cytotoxicity of Taxol would be enhanced by inhibiting PARP function as an enzyme implicated in DNA repair for oxidative stress.

Cytotoxicity and genotoxicity of blue LED light and protective effects of AA2G in mammalian cells and associated DNA repair deficient cell lines

Light emitting diode (LED) devices emit narrow bands of the blue, green, and red light spectrum rather than the continuous spectrum emitted from sunlight and fluorescent light bulbs. LED devices have become considerably common in society, and the fluence of blue light from LED devices is more intense than other light sources. Previous studies presented that the blue light spectrum may harness potentially inimical genotoxicity. Therefore, the aim of this study was to investigate this potential cytotoxicity and genotoxicity, as well as identify the mechanism of the cellular effects induced by blue LED light exposure in mammalian cell lines with their DNA repair deficient mutants. Our results demonstrated that blue LED light induced both oxidative stress to cells and cytotoxic and genotoxic effects including reduction of clonogenicity, cell cycle arrest, induction of sister chromatid exchanges, endoreduplicated chromosomes, and increased frequency of HPRT locus mutations. In DNA repair deficient cells, particularly those involving double strand break repair deficiency, cells presented hypersensitivity to blue LED light exposure. Blue LED light also induced chromosome aberrations more in DNA repair deficient cells than wild type cells. The cytotoxicity of blue LED light was reduced by an effective antioxidant, ascorbic acid 2-glucoside, which can suppress blue LED light induced oxidative stress. These results indicated that prolonged, high intensity exposure to blue LED light induces genotoxic stress to cells, and oxidative stress induced by blue LED light is targeting DNA to induce these biological effects.

Evaluating the Genotoxic and Cytotoxic Effects of Thymidine Analogs, 5-Ethynyl-2'-Deoxyuridine and 5-Bromo-2'-Deoxyurdine to Mammalian Cells

BrdU (bromodeoxyuridine) and EdU (ethynyldeoxyuridine) have been largely utilized as the means of monitoring DNA replication and cellular division. Although BrdU induces gene and chromosomal mutations and induces sensitization to photons, EdU's effects have not been extensively studied yet. Therefore, we investigated EdU's potential cytotoxic and mutagenic effects and its related underlying mechanisms when administered to Chinese hamster ovary (CHO) wild type and DNA repair-deficient cells. EdU treatment displayed a higher cytotoxicity and genotoxicity than BrdU treatment. Cells with defective homologous recombination repair displayed a greater growth delay and severe inhibition of clonogenicity with EdU compared to wild type and other DNA repair-deficient cells. Inductions of sister chromatid exchange and hypoxanthine phosphorybosyl transferase (HPRT) mutation were observed in EdU-incorporated cells as well. Interestingly, on the other hand, EdU did not induce sensitization to photons to the same degree as BrdU. Our results demonstrate that elevated concentrations (similar to manufacturers suggested concentration; >5-10 μM) of EdU treatment were toxic to the cell cultures, particularly in cells with a defect in homologous recombination repair. Therefore, EdU should be administered with additional precautions.

DNA Repair Deficient Chinese Hamster Ovary Cells Exhibiting Differential Sensitivity to Charged Particle Radiation under Aerobic and Hypoxic Conditions

It has been well established that hypoxia significantly increases both cellular and tumor resistance to ionizing radiation. Hypoxia associated radiation resistance has been known for some time but there has been limited success in sensitizing cells to radiation under hypoxic conditions. These studies show that, when irradiated with low linear energy transfer (LET) gamma-rays, poly (ADP-ribose), polymerase (PARP), Fanconi Anemia (FANC), and mutant Chinese Hamster Ovary (CHO) cells respond similarly to the non-homologous end joining (NHEJ) and the homologous recombination (HR) repair mutant CHO cells. Comparable results were observed in cells exposed to 13 keV/μm carbon ions. However, when irradiated with higher LET spread out Bragg peak (SOBP) carbon ions, we observed a decrease in the oxygen enhancement ratio (OER) in all the DNA of repair mutant cell lines. Interestingly, PARP mutant cells were observed as having the largest decrease in OER. Finally, these studies show a significant increase in the relative biological effectiveness (RBE) of high LET SOBP carbon and iron ions in HR and PARP mutants. There was also an increase in the RBE of NHEJ mutants when irradiated to SOBP carbon and iron ions. However, this increase was lower than in other mutant cell lines. These findings indicate that high LET radiation produces unique types of DNA damage under hypoxic conditions and PARP and HR repair pathways play a role in repairing this damage.

Oxidatively-generated damage to DNA and proteins mediated by photosensitized UVA

UVA accounts for about 95% of the solar ultraviolet (UV) radiation that reaches Earth and most likely contributes to human skin cancer risk. In contrast to UVB, which comprises the remaining 5% and is absorbed by DNA nucleobases to cause direct photodamage, UVA damages DNA indirectly. It does this largely through its interactions with cellular chromophores that act as photosensitisers to generate reactive oxygen species. Exogenously supplied chemicals, including some widely-prescribed medicines, may also act as photosensitisers and these drugs are associated with an increased risk of sun-related cancer. Because they amplify the effects of UVA on cells, they provide a means to investigate the mechanisms and effects of UVA-induced photodamage. Here, we describe some of the major lesions induced by two groups of UVA photosensitisers, the DNA thionucleotides and the fluoroquinolone antibiotics. In thionucleotides, replacement of the oxygen atoms of canonical nucleobases by sulfur converts them into strong UVA chromophores that can be incorporated into DNA. The fluoroquinolones are also UVA chromophores. They are not incorporated into DNA and induce a different range of DNA damages. We also draw attention to the potentially important contribution of photochemical protein damage to the cellular effects of photosensitised UVA. Proteins targeted for oxidation damage include DNA repair factors and we suggest that UVA-mediated protein damage may contribute to sunlight-induced cancer risk.

Rad18 confers hematopoietic progenitor cell DNA damage tolerance independently of the Fanconi Anemia pathway in vivo

In cultured cancer cells the E3 ubiquitin ligase Rad18 activates Trans-Lesion Synthesis (TLS) and the Fanconi Anemia (FA) pathway. However, physiological roles of Rad18 in DNA damage tolerance and carcinogenesis are unknown and were investigated here. Primary hematopoietic stem and progenitor cells (HSPC) co-expressed RAD18 and FANCD2 proteins, potentially consistent with a role for Rad18 in FA pathway function during hematopoiesis. However, hematopoietic defects typically associated with fanc-deficiency (decreased HSPC numbers, reduced engraftment potential of HSPC, and Mitomycin C (MMC) -sensitive hematopoiesis), were absent in Rad18^−/− mice. Moreover, primary Rad18^−/− mouse embryonic fibroblasts (MEF) retained robust Fancd2 mono-ubiquitination following MMC treatment. Therefore, Rad18 is dispensable for FA pathway activation in untransformed cells and the Rad18 and FA pathways are separable in hematopoietic cells. In contrast with responses to crosslinking agents, Rad18^−/− HSPC were sensitive to in vivo treatment with the myelosuppressive agent 7,12 Dimethylbenz[a]anthracene (DMBA). Rad18-deficient fibroblasts aberrantly accumulated DNA damage markers after DMBA treatment. Moreover, in vivo DMBA treatment led to increased incidence of B cell malignancy in Rad18^−/− mice. These results identify novel hematopoietic functions for Rad18 and provide the first demonstration that Rad18 confers DNA damage tolerance and tumor-suppression in a physiological setting.

Role of various DNA repair pathways in chromosomal inversion formation in CHO mutants

PURPOSE

In an effort to better understand the formation of chromosomal inversions, we investigated the role of various DNA repair pathways, including the non-homologous end joining (NHEJ), homologous recombination (HR), and Fanconi Anemia (FA) repair pathways for the formation of radiation induced chromosomal inversions.

MATERIALS AND METHODS

CHO10B2 wild type, CHO DNA repair-deficient, and CHO DNA repair-deficient corrected mutant cells were synchronized into G1 phase and exposed to gamma-rays. First post-irradiation metaphase cells were analyzed for chromosomal inversions by a differential chromatid staining technique involving a single cycle pre-irradiation ethynyl-uridine treatment and statistic calculations.

RESULTS

It was observed that inhibition of the NHEJ pathway resulted in an overall decrease in the number of radiation-induced inversions, roughly a 50% decrease when compared to the CHO wild type. Interestingly, inhibition of the FA pathway resulted in an increase in both the number of spontaneous inversions and the number of radiation-induced inversions observed after exposure to 2 Gy of ionizing radiation. It was observed that FA-deficient cells contained roughly 330% (1.24 inversions per cell) more spontaneous inversions and 20% (0.4 inversions per cell) more radiation-induced inversions than the wild-type CHO cell lines. The HR mutants, defective in Rad51 foci, showed similar number of spontaneous and radiation-induced inversion as the wild-type cells. Gene complementation resulted in both spontaneous and radiation-induced inversions resembling the CHO wild-type cells.

CONCLUSIONS

We have concluded that the NHEJ repair pathway contributes to the formation of radiation-induced inversions. Additionally, through an unknown molecular mechanism it appears that the FA signal pathway prevents the formation of both spontaneous and radiation induced inversions.

DNA Damage Response Assessments in Human Tumor Samples Provide Functional Biomarkers of Radiosensitivity

Predictive biomarkers are urgently needed for individualization of radiation therapy and treatment with radiosensitizing anticancer agents. Genomic profiling of human cancers provides us with unprecedented insight into the mutational landscape of genes directly or indirectly involved in the response to radiation-induced DNA damage. However, to what extent this wealth of structural information about the cancer genome produces biomarkers of sensitivity to radiation remains to be seen. Investigators are increasingly studying the subnuclear accumulation (ie, foci) of proteins in the DNA damage response (DDR), such as gamma-H2AX, 53BP1, or RAD51, as a surrogate of treatment sensitivity. Recent findings from preclinical studies have demonstrated the predictive potential of DDR foci by correlating foci with clinically relevant end points such as tumor control probability. Therefore, preclinical investigations of DDR foci responses are increasingly moving into cells and tissues from patients, which is the major focus of this review. The advantage of using DDR foci as functional biomarkers is that they can detect alterations in DNA repair due to various mechanisms. Moreover, they provide a global measurement of DDR network function without needing to know the identities of all the components, many of which remain unknown. Foci assays are thus expected to yield functional insight that may complement or supersede genomic information, thereby giving radiation oncologists unique opportunities to individualize cancer treatments in the near future.

PF, Brogan JR, Kurimasa A, Chen DJ, Bedford JS, Chen BPC. Differential radiosensitivity phenotypes of DNA-PKcs mutations affecting NHEJ and HRR systems following irradiation with gamma-rays or very low fluences of alpha particles

We have examined cell-cycle dependence of chromosomal aberration induction and cell killing after high or low dose-rate γ irradiation in cells bearing DNA-PKcs mutations in the S2056 cluster, the T2609 cluster, or the kinase domain. We also compared sister chromatid exchanges (SCE) production by very low fluences of α-particles in DNA-PKcs mutant cells, and in homologous recombination repair (HRR) mutant cells including Rad51C, Rad51D, and Fancg/xrcc9. Generally, chromosomal aberrations and cell killing by γ-rays were similarly affected by mutations in DNA-PKcs, and these mutant cells were more sensitive in G1 than in S/G2 phase. In G1-irradiated DNA-PKcs mutant cells, both chromosome- and chromatid-type breaks and exchanges were in excess than wild-type cells. For cells irradiated in late S/G2 phase, mutant cells showed very high yields of chromatid breaks compared to wild-type cells. Few exchanges were seen in DNA-PKcs-null, Ku80-null, or DNA-PKcs kinase dead mutants, but exchanges in excess were detected in the S2506 or T2609 cluster mutants. SCE induction by very low doses of α-particles is resulted from bystander effects in cells not traversed by α-particles. SCE seen in wild-type cells was completely abolished in Rad51C- or Rad51D-deficient cells, but near normal in Fancg/xrcc9 cells. In marked contrast, very high levels of SCEs were observed in DNA-PKcs-null, DNA-PKcs kinase-dead and Ku80-null mutants. SCE induction was also abolished in T2609 cluster mutant cells, but was only slightly reduced in the S2056 cluster mutant cells. Since both non-homologous end-joining (NHEJ) and HRR systems utilize initial DNA lesions as a substrate, these results suggest the possibility of a competitive interference phenomenon operating between NHEJ and at least the Rad51C/D components of HRR; the level of interaction between damaged DNA and a particular DNA-PK component may determine the level of interaction of such DNA with a relevant HRR component.

The DNA translocase FANCM/MHF promotes replication traverse of DNA interstrand crosslinks

The replicative machinery encounters many impediments, some of which can be overcome by lesion bypass or replication restart pathways, leaving repair for a later time. However, interstrand crosslinks (ICLs), which preclude DNA unwinding, are considered absolute blocks to replication. Current models suggest that fork collisions, either from one or both sides of an ICL, initiate repair processes required for resumption of replication. To test these proposals, we developed a single-molecule technique for visualizing encounters of replication forks with ICLs as they occur in living cells. Surprisingly, the most frequent patterns were consistent with replication traverse of an ICL, without lesion repair. The traverse frequency was strongly reduced by inactivation of the translocase and DNA binding activities of the FANCM/MHF complex. The results indicate that translocase-based mechanisms enable DNA synthesis to continue past ICLs and that these lesions are not always absolute blocks to replication.

5-Aza-2'-deoxycytidine causes replication lesions that require Fanconi anemia-dependent homologous recombination for repair

5-Aza-2′-deoxycytidine (5-azadC) is a DNA methyltransferase (DNMT) inhibitor increasingly used in treatments of hematological diseases and works by being incorporated into DNA and trapping DNMT. It is unclear what DNA lesions are caused by 5-azadC and if such are substrates for DNA repair. Here, we identify that 5-azadC induces DNA damage as measured by γ-H2AX and 53BP1 foci. Furthermore, 5-azadC induces radial chromosomes and chromatid breaks that depend on active replication, which altogether suggest that trapped DNMT collapses oncoming replication forks into double-strand breaks. We demonstrate that RAD51-mediated homologous recombination (HR) is activated to repair 5-azadC collapsed replication forks. Fanconi anemia (FA) is a rare autosomal recessive disorder, and deaths are often associated with leukemia. Here, we show that FANCG-deficient cells fail to trigger HR-mediated repair of 5-azadC-induced lesions, leading to accumulation of chromatid breaks and inter-chromosomal radial fusions as well as hypersensitivity to the cytotoxic effects of 5-azadC. These data demonstrate that the FA pathway is important to protect from 5-azadC-induced toxicity. Altogether, our data demonstrate that cytotoxicity of the epigenetic drug 5-azadC can, at least in part, be explained by collapsed replication forks requiring FA-mediated HR for repair.

Evidence that BRCA1- or BRCA2-associated cancers are not inevitable

Inheriting a BRCA1 or BRCA2 gene mutation can cause a deficiency in repairing complex DNA damage. This step leads to genomic instability and probably contributes to an inherited predisposition to breast and ovarian cancer. Complex DNA damage has been viewed as an integral part of DNA replication before cell division. It causes temporary replication blocks, replication fork collapse, chromosome breaks and sister chromatid exchanges (SCEs). Chemical modification of DNA may also occur spontaneously as a byproduct of normal processes. Pathways containing BRCA1 and BRCA2 gene products are essential to repair spontaneous complex DNA damage or to carry out SCEs if repair is not possible. This scenario creates a theoretical limit that effectively means there are spontaneous BRCA1/2-associated cancers that cannot be prevented or delayed. However, much evidence for high rates of spontaneous DNA mutation is based on measuring SCEs by using bromodeoxyuridine (BrdU). Here we find that the routine use of BrdU has probably led to overestimating spontaneous DNA damage and SCEs because BrdU is itself a mutagen. Evidence based on spontaneous chromosome abnormalities and epidemiologic data indicates strong effects from exogenous mutagens and does not support the inevitability of cancer in all BRCA1/2 mutation carriers. We therefore remove a theoretical argument that has limited efforts to develop chemoprevention strategies to delay or prevent cancers in BRCA1/2 mutation carriers.

Oxidation-mediated DNA cross-linking contributes to the toxicity of 6-thioguanine in human cells

The thiopurines azathioprine and 6-mercaptopurine have been extensively prescribed as immunosuppressant and anticancer agents for several decades. A third member of the thiopurine family, 6-thioguanine (6-TG), has been used less widely. Although known to be partly dependent on DNA mismatch repair (MMR), the cytotoxicity of 6-TG remains incompletely understood. Here, we describe a novel MMR-independent pathway of 6-TG toxicity. Cell killing depended on two properties of 6-TG: its incorporation into DNA and its ability to act as a source of reactive oxygen species (ROS). ROS targeted DNA 6-TG to generate potentially lethal replication-arresting DNA lesions including interstrand cross-links. These triggered processing by the Fanconi anemia and homologous recombination DNA repair pathways. Allopurinol protected against 6-TG toxicity by acting as a ROS scavenger and preventing DNA damage. Together, our findings provide mechanistic evidence to support the proposed use of thiopurines to treat HR-defective tumors and for the coadministration of 6-TG and allopurinol as an immunomodulation strategy in inflammatory disorders.

Use of gene targeting to study recombination in mammalian cell DNA repair mutants

The study of gene function has been greatly facilitated by the development of strategies to modify genomic DNA. Gene targeting is one of the most successfully applied techniques used to examine the roles of specific genes in a wide variety of model systems from yeast to mammals. Our laboratory has pioneered the use of the Chinese hamster ovary (CHO) cell culture model system to study pathways of DNA repair and recombination at the hemizygous CHO APRT locus. By using a simple and effective gene targeting method, we have generated a number of DNA repair-deficient cell lines that have been used in targeted recombination experiments to investigate pathways of recombinational repair in somatic mammalian cells. These methods can be readily customized to generate a variety of cell lines deficient in specific genes of interest and can be applied to study the roles of other DNA repair proteins in pathways of mammalian recombinational repair.

Homologous recombination repair-dependent cytotoxicity of the benzotriazine di-N-oxide CEN-209: comparison with other hypoxia-activated prodrugs

CEN-209 (SN30000) is a second-generation benzotriazine di-N-oxide currently in advanced preclinical development as a hypoxia-activated prodrug (HAP). Herein we describe the DNA repair-, hypoxia- and one-electron reductase-dependence of CEN-209 cytotoxicity. We deployed mutant CHO cell lines to generate DNA repair profiles for CEN-209, and compared the profiles with those for other HAPs. Hypoxic selectivity of CEN-209 was significantly greater than PR-104A and the nitro-chloromethylbenzindoline (nCBI/SN29428) and comparable to tirapazamine and TH-302. CEN-209 was selective for homologous recombination (HR) repair-deficient cells (Rad51d⁻/⁻), but less so than nitrogen mustard prodrugs TH-302 and PR-104A. Further, DNA repair profiles for CEN-209 differed under oxic and hypoxic conditions, with oxic cytotoxicity more dependent on HR. This feature was conserved across all three members of the benzotriazine di-N-oxide class examined (tirapazamine, CEN-209 and CEN-309/SN29751). Enhancing one-electron reduction of CEN-209 by forced expression of a soluble form of NADPH:cytochrome P450 oxidoreductase (sPOR) increased CEN-209 cytotoxicity more markedly under oxic than hypoxic conditions. Comparison of oxygen consumption, H₂O₂ production and metabolism of CEN-209 to the corresponding 1-oxide and nor-oxide reduced metabolites suggested that enhanced oxic cytotoxicity in cells with high one-electron reductase activity is due to futile redox cycling. This study supports the hypothesis that both oxic and hypoxic cell killing by CEN-209 is mechanistically analogous to tirapazamine and is dependent on oxidative DNA damage repaired via multiple pathways. However, HAPs that generate DNA interstrand cross-links, such as TH-302 and PR-104, may be more suitable than benzotriazine di-N-oxides for exploiting reported HR repair defects in hypoxic tumour cells.

Efficient DNA interstrand crosslinking by 6-thioguanine and UVA radiation

Patients taking the immunosuppressant and anticancer thiopurines 6-mercaptopurine, azathioprine or 6-thioguanine (6-TG), develop skin cancer at a very high frequency. Their DNA contains 6-TG which absorbs ultraviolet A (UVA) radiation, and their skin is UVA hypersensitive, consistent with the formation of DNA photodamage. Here we demonstrate that UVA irradiation of 6-TG-containing DNA causes DNA interstrand crosslinking. In synthetic duplex oligodeoxynucleotides, the interstrand crosslinks (ICLs) can form between closely opposed 6-TG bases and, in a less favoured reaction, between 6-TG and normal bases on the opposite strand. In vivo, UVA irradiation of cultured cells containing 6-TG-substituted DNA also causes ICL formation and induces the chromosome aberrations that are characteristically associated with this type of DNA lesion. 6-TG/UVA activates the Fanconi anemia (FA) pathway via monoubiquitination of the FANCD2 protein. Cells defective in the FA pathway or other factors involved in ICL processing, such as XPF and DNA Polζ, are all hypersensitive to killing by 6-TG/UVA-consistent with a significant contribution of photochemical ICLs to the cytotoxicity of this treatment. Our findings suggest that sunlight-exposed skin of thiopurine treated patients may experience chronic photochemical DNA damage that requires constant intervention of the FA pathway.

FANCD2 but not FANCA promotes cellular resistance to type II topoisomerase poisons

Genetic or epigenetic inactivation of the pathway formed by the Fanconi Anemia (FA) and BRCA proteins occurs in several cancer types, including lung and breast cancer, rendering the affected tumors potentially hypersensitive to DNA crosslinking agents. However, the cytotoxicity of other commonly used cancer therapeutics in cells with FA/BRCA pathway defects remains to be defined. Building on earlier data that implicated BRCA1 and BRCA2 in the repair of DNA damage caused by the topoisomerase II poison etoposide, we studied the role of FANCD2 in mediating resistance to several topoisomerase II poisons. We establish that the loss of FANCD2 increases cell death in response to etoposide. FANCD2 promotes homologous recombination repair (HRR) and prevents DNA double-strand break formation and chromosomal aberrations in etoposide-treated cells. Strikingly, this function of FANCD2 is independent of FANCD2 foci formation and of FANCA, which is a member of the FA core complex upstream of FANCD2 mono-ubiquitination. Thus, FANCD2 appears to promote HRR in a mono-ubiquitination-independent manner in conjunction with BRCA1/2. These data add to an emerging body of evidence indicating that the FA pathway is not linear and that several protein subcomplexes with different functions exist. Our findings are potentially relevant for predicting the sensitivity of lung and breast cancers to etoposide and doxorubicin, respectively.

FANCD1/BRCA2 plays predominant role in the repair of DNA damage induced by ACNU or TMZ

Nimustine (ACNU) and temozolomide (TMZ) are DNA alkylating agents which are commonly used in chemotherapy for glioblastomas. ACNU is a DNA cross-linking agent and TMZ is a methylating agent. The therapeutic efficacy of these agents is limited by the development of resistance. In this work, the role of the Fanconi anemia (FA) repair pathway for DNA damage induced by ACNU or TMZ was examined. Cultured mouse embryonic fibroblasts were used: FANCA(-/-), FANCC(-/-), FANCA(-/-)C(-/-), FANCD2(-/-) cells and their parental cells, and Chinese hamster ovary and lung fibroblast cells were used: FANCD1/BRCA2mt, FANCG(-/-) and their parental cells. Cell survival was examined after a 3 h ACNU or TMZ treatment by using colony formation assays. All FA repair pathways were involved in ACNU-induced DNA damage. However, FANCG and FANCD1/BRCA2 played notably important roles in the repair of TMZ-induced DNA damage. The most effective molecular target correlating with cellular sensitivity to both ACNU and TMZ was FANCD1/BRCA2. In addition, it was found that FANCD1/BRCA2 small interference RNA efficiently enhanced cellular sensitivity toward ACNU and TMZ in human glioblastoma A172 cells. These findings suggest that the down-regulation of FANCD1/BRCA2 might be an effective strategy to increase cellular chemo-sensitization towards ACNU and TMZ.

The homologous recombination protein RAD51D mediates the processing of 6-thioguanine lesions downstream of mismatch repair

Thiopurines are extensively used as immunosuppressants and in the treatment of childhood cancers, even though there is concern about therapy-induced leukemias and myelodysplastic syndromes resulting from thiopurine use. Following metabolic activation, thiopurines are incorporated into DNA and invoke mismatch repair (MMR). Recognition of 6-thioguanine (6-thioG) in DNA by key MMR proteins results in cell death rather than repair. There are suggestions that homologous recombination (HR) is involved downstream of MMR following thiopurine treatment, but the precise role of HR is poorly understood. In this study, we demonstrate that cells deficient in RAD51D (a RAD51 paralogue) are extremely sensitive to 6-thioG. This sensitivity is almost completely rescued by the deletion of Mlh1, which suggests that HR is involved in the repair of the 6-thioG-induced recombinogenic lesions generated by MMR. Furthermore, 6-thioG induces chromosome aberrations in the Rad51d-deficient cells. Interestingly, Rad51d-deficient cells show a striking increase in the frequency of triradial and quadriradial chromosomes in response to 6-thioG therapy. The presence of these chromatid exchange-type aberrations indicates that the deficiency in RAD51D-dependent HR results in profound chromosomal damage precipitated by the processing of 6-thioG by MMR. The radials are notable as an important source of chromosomal translocations, which are the most common class of mutations found in hematologic malignancies. This study thus suggests that HR insufficiency could be a potential risk factor for the development of secondary cancers that result from long-term use of thiopurines in patients.

Repair pathways independent of the Fanconi anemia nuclear core complex play a predominant role in mitigating formaldehyde-induced DNA damage

The role of the Fanconi anemia (FA) repair pathway for DNA damage induced by formaldehyde was examined in the work described here. The following cell types were used: mouse embryonic fibroblast cell lines FANCA(-/-), FANCC(-/-), FANCA(-/-)C(-/-), FANCD2(-/-) and their parental cells, the Chinese hamster cell lines FANCD1 mutant (mt), FANCGmt, their revertant cells, and the corresponding wild-type (wt) cells. Cell survival rates were determined with colony formation assays after formaldehyde treatment. DNA double strand breaks (DSBs) were detected with an immunocytochemical γH2AX-staining assay. Although the sensitivity of FANCA(-/-), FANCC(-/-) and FANCA(-/-)C(-/-) cells to formaldehyde was comparable to that of proficient cells, FANCD1mt, FANCGmt and FANCD2(-/-) cells were more sensitive to formaldehyde than the corresponding proficient cells. It was found that homologous recombination (HR) repair was induced by formaldehyde. In addition, γH2AX foci in FANCD1mt cells persisted for longer times than in FANCD1wt cells. These findings suggest that formaldehyde-induced DSBs are repaired by HR through the FA repair pathway which is independent of the FA nuclear core complex.

Rad18-mediated translesion synthesis of bulky DNA adducts is coupled to activation of the Fanconi anemia DNA repair pathway

Fanconi anemia (FA) is a cancer susceptibility syndrome characterized by sensitivity to DNA-damaging agents. The FA proteins (FANCs) are implicated in DNA repair, although the precise mechanisms by which FANCs process DNA lesions are not fully understood. An epistatic relationship between the FA pathway and translesion synthesis (TLS, a post-replication DNA repair mechanism) has been suggested, but the basis for cross-talk between the FA and TLS pathways is poorly understood. We show here that ectopic overexpression of the E3 ubiquitin ligase Rad18 (a central regulator of TLS) induces DNA damage-independent mono-ubiquitination of proliferating cell nuclear antigen (PCNA) (a known Rad18 substrate) and FANCD2. Conversely, DNA damage-induced mono-ubiquitination of both PCNA and FANCD2 is attenuated in Rad18-deficient cells, demonstrating that Rad18 contributes to activation of the FA pathway. WT Rad18 but not an E3 ubiquitin ligase-deficient Rad18 C28F mutant fully complements both PCNA ubiquitination and FANCD2 activation in Rad18-depleted cells. Rad18-induced mono-ubiquitination of FANCD2 is not observed in FA core complex-deficient cells, demonstrating that Rad18 E3 ligase activity alone is insufficient for FANCD2 ubiquitylation. Instead, Rad18 promotes FA core complex-dependent FANCD2 ubiquitination in a manner that is secondary to PCNA mono-ubiquitination. Taken together, these results demonstrate a novel Rad18-dependent mechanism that couples activation of the FA pathway with TLS.

Role of homologous recombination in DNA interstrand crosslink repair

Homologous recombination repair (HRR) encompasses mechanisms that employ homologous DNA sequences as templates for repair or tolerance of a wide range of DNA lesions that inhibit DNA replication in S phase. Arguably the most imposing of these DNA lesions is that of the interstrand crosslink (ICL), consisting of a covalently attached chemical bridge between opposing DNA strands. ICL repair requires the coordinated activities of HRR and a number of proteins from other DNA repair and damage response systems, including nucleotide excision repair, base excision repair, mismatch repair, and translesion DNA synthesis (TLS). Interestingly, different organisms favor alternative methods of HRR in the ICL repair process. E. coli perform ICL repair using a homology-driven damage bypass mechanism analogous to daughter strand gap repair. Eukaryotes from yeast to humans initiate ICL repair primarily during DNA replication, relying on HRR activity to restart broken replication forks associated with double-strand break intermediates induced by nucleolytic activities of other excision repair factors. Higher eukaryotes also employ several additional factors, including members of the Fanconi anemia damage-response network, which further promote replication-associated ICL repair through the activation and coordination of various DNA excision repair, TLS, and HRR proteins. This review focuses on the proteins and general mechanisms of HRR associated with ICL repair in different model organisms.

Sequence conversion by single strand oligonucleotide donors via non-homologous end joining in mammalian cells

Double strand breaks (DSBs) can be repaired by homology independent nonhomologous end joining (NHEJ) pathways involving proteins such as Ku70/80, DNAPKcs, Xrcc4/Ligase 4, and the Mre11/Rad50/Nbs1 (MRN) complex. DSBs can also be repaired by homology-dependent pathways (HDR), in which the MRN and CtIP nucleases produce single strand ends that engage homologous sequences either by strand invasion or strand annealing. The entry of ends into HDR pathways underlies protocols for genomic manipulation that combine site-specific DSBs with appropriate informational donors. Most strategies utilize long duplex donors that participate by strand invasion. Work in yeast indicates that single strand oligonucleotide (SSO) donors are also active, over considerable distance, via a single strand annealing pathway. We examined the activity of SSO donors in mammalian cells at DSBs induced either by a restriction nuclease or by a targeted interstrand cross-link. SSO donors were effective immediately adjacent to the break, but activity declined sharply beyond approximately 100 nucleotides. Overexpression of the resection nuclease CtIP increased the frequency of SSO-mediated sequence modulation distal to the break site, but had no effect on the activity of an SSO donor adjacent to the break. Genetic and in vivo competition experiments showed that sequence conversion by SSOs in the immediate vicinity of the break was not by strand invasion or strand annealing pathways. Instead these donors competed for ends that would have otherwise entered NHEJ pathways.

Several tetratricopeptide repeat (TPR) motifs of FANCG are required for assembly of the BRCA2/D1-D2-G-X3 complex, FANCD2 monoubiquitylation and phleomycin resistance

The Fanconi anaemia (FA) FANCG protein is an integral component of the FA nuclear core complex that is required for monoubiquitylation of FANCD2. FANCG is also part of another protein complex termed D1-D2-G-X3 that contains FANCD2 and the homologous recombination repair proteins BRCA2 (FANCD1) and XRCC3. Formation of the D1-D2-G-X3 complex is mediated by serine-7 phosphorylation of FANCG and occurs independently of the FA core complex and FANCD2 monoubiquitylation. FANCG contains seven tetratricopeptide repeat (TPR) motifs that mediate protein-protein interactions and here we show that mutation of several of the TPR motifs at a conserved consensus residue ablates the in vivo binding activity of FANCG. Expression of mutated TPR1, TPR2, TPR5 and TPR6 in Chinese hamster fancg mutant NM3 fails to functionally complement its hypersensitivities to mitomycin C (MMC) and phleomycin and fails to restore FANCD2 monoubiquitylation. Using co-immunoprecipitation analysis, we demonstrate that these TPR-mutated FANCG proteins fail to interact with BRCA2, XRCC3, FANCA or FANCF. The interactions of other proteins in the D1-D2-G-X3 complex are also absent, including the interaction of BRCA2 with both the monoubiquitylated (FANCD2-L) and non-ubiquitylated (FANCD2-S) isoforms of FANCD2. Interestingly, a mutation of TPR7 (R563E), that complements the MMC and phleomycin hypersensitivity of human FA-G EUFA316 cells, fails to complement NM3, despite the mutated FANCG protein co-precipitating with FANCA, BRCA2 and XRCC3. Whilst interaction of TPR7-mutated FANCG with FANCF does appear to be reduced in NM3, FANCD2 is monoubiquitylated suggesting that sub-optimal interactions of FANCG in the core complex and the D1-D2-G-X3 complex are responsible for the observed MMC- and phleomycin-hypersensitivity, rather than a defect in FANCD2 monoubiquitylation. Our data demonstrate that FANCG functions as a mediator of protein-protein interactions and is vital for the assembly of multi-protein complexes including the FA core complex and the D1-D2-G-X3 complex.

A histone-fold complex and FANCM form a conserved DNA-remodeling complex to maintain genome stability

FANCM remodels branched DNA structures and plays essential roles in the cellular response to DNA replication stress. Here, we show that FANCM forms a conserved DNA-remodeling complex with a histone-fold heterodimer, MHF. We find that MHF stimulates DNA binding and replication fork remodeling by FANCM. In the cell, FANCM and MHF are rapidly recruited to forks stalled by DNA interstrand crosslinks, and both are required for cellular resistance to such lesions. In vertebrates, FANCM-MHF associates with the Fanconi anemia (FA) core complex, promotes FANCD2 monoubiquitination in response to DNA damage, and suppresses sister-chromatid exchanges. Yeast orthologs of these proteins function together to resist MMS-induced DNA damage and promote gene conversion at blocked replication forks. Thus, FANCM-MHF is an essential DNA-remodeling complex that protects replication forks from yeast to human.

The role of the Fanconi anemia network in the response to DNA replication stress

Fanconi anemia is a genetically heterogeneous disorder associated with chromosome instability and a highly elevated risk for developing cancer. The mutated genes encode proteins involved in the cellular response to DNA replication stress. Fanconi anemia proteins are extensively connected with DNA caretaker proteins, and appear to function as a hub for the coordination of DNA repair with DNA replication and cell cycle progression. At a molecular level, however, the raison d'être of Fanconi anemia proteins still remains largely elusive. The thirteen Fanconi anemia proteins identified to date have not been embraced into a single and defined biological process. To help put the Fanconi anemia puzzle into perspective, we begin this review with a summary of the strategies employed by prokaryotes and eukaryotes to tolerate obstacles to the progression of replication forks. We then summarize what we know about Fanconi anemia with an emphasis on biochemical aspects, and discuss how the Fanconi anemia network, a late acquisition in evolution, may function to permit the faithful and complete duplication of our very large vertebrate chromosomes.

FANCD2-deficient human fibroblasts are hypersensitive to ionising radiation at oxygen concentrations of 0% and 3% but not under normoxic conditions

PURPOSE

Individuals suffering from Fanconi Anemia (FA) exhibit a pronounced hypersensitivity to agents that cause DNA inter-strand crosslinks and frequently also to ionising radiation. However, fibroblast lines derived from FA patients generally show little or no radiosensitivity in vitro. Here, we sought to elucidate the role of the central FA protein D2 (FANCD2) in determining cellular radioresistance.

MATERIAL AND METHODS

Clonogenic radiation survival was assessed in an isogenic pair of human fibroblasts with or without wild-type FANCD2 under varying oxygen concentrations. Additional endpoints included single-cell gel electrophoresis, RAD51 foci formation, and apoptosis.

RESULTS

At 20% oxygen, there was no reduction in the survival of FANCD2-deficient fibroblasts compared to wild-type complemented cells. However, at 0% oxygen FANCD2-deficient cells were more radiosensitive than wild-type cells. Interestingly, at 3% oxygen, which more closely resembles the physiological environment in human tissues, the difference in radiosensitivity was maintained. Our data also suggest that the increased radiosensitivity of FANCD2-deficient cells seen under conditions of reduced oxygen is associated with apoptotic cell death, but not secondary to a defect in the homologous recombination repair pathway that is required for crosslink repair.

CONCLUSIONS

Our data may help explain the previously described discrepancy between the clinical and cellular radiosensitivity of FA patients.

Cellular and molecular consequences of defective Fanconi anemia proteins in replication-coupled DNA repair: mechanistic insights

The Fanconi anemia (FA) molecular network consists of 15 "FANC" proteins, of which 13 are associated with mutations in patients with this cancer-prone chromosome instability disorder. Whereas historically the common phenotype associated with FA mutations is marked sensitivity to DNA interstrand crosslinking agents, the literature supports a more global role for FANC proteins in coping with diverse stresses encountered by replicative polymerases. We have attempted to reconcile and integrate numerous observations into a model in which FANC proteins coordinate the following physiological events during DNA crosslink repair: (a) activating a FANCM-ATR-dependent S-phase checkpoint, (b) mediating enzymatic replication-fork breakage and crosslink unhooking, (c) filling the resulting gap by translesion synthesis (TLS) by error-prone polymerase(s), and (d) restoring the resulting one-ended double-strand break by homologous recombination repair (HRR). The FANC core subcomplex (FANCA, B, C, E, F, G, L, FAAP100) promotes TLS for both crosslink and non-crosslink damage such as spontaneous oxidative base damage, UV-C photoproducts, and alkylated bases. TLS likely helps prevent stalled replication forks from breaking, thereby maintaining chromosome continuity. Diverse DNA damages and replication inhibitors result in monoubiquitination of the FANCD2-FANCI complex by the FANCL ubiquitin ligase activity of the core subcomplex upon its recruitment to chromatin by the FANCM-FAAP24 heterodimeric translocase. We speculate that this translocase activity acts as the primary damage sensor and helps remodel blocked replication forks to facilitate checkpoint activation and repair. Monoubiquitination of FANCD2-FANCI is needed for promoting HRR, in which the FANCD1/BRCA2 and FANCN/PALB2 proteins act at an early step. We conclude that the core subcomplex is required for both TLS and HRR occurring separately for non-crosslink damages and for both events during crosslink repair. The FANCJ/BRIP1/BACH1 helicase functions in association with BRCA1 and may remove structural barriers to replication, such as guanine quadruplex structures, and/or assist in crosslink unhooking.

The Walker B motif in avian FANCM is required to limit sister chromatid exchanges but is dispensable for DNA crosslink repair

FANCM, the most highly conserved component of the Fanconi Anaemia (FA) pathway can resolve recombination intermediates and remodel synthetic replication forks. However, it is not known if these activities are relevant to how this conserved protein activates the FA pathway and promotes DNA crosslink repair. Here we use chicken DT40 cells to systematically dissect the function of the helicase and nuclease domains of FANCM. Our studies reveal that these domains contribute distinct roles in the tolerance of crosslinker, UV light and camptothecin-induced DNA damage. Although the complete helicase domain is critical for crosslink repair, a predicted inactivating mutation of the Walker B box domain has no impact on FA pathway associated functions. However, this mutation does result in elevated sister chromatid exchanges (SCE). Furthermore, our genetic dissection indicates that FANCM functions with the Blm helicase to suppress spontaneous SCE events. Overall our results lead us to reappraise the role of helicase domain associated activities of FANCM with respect to the activation of the FA pathway, crosslink repair and in the resolution of recombination intermediates.

RAD51D- and FANCG-dependent base substitution mutagenesis at the ATP1A1 locus in mammalian cells

Elaborate processes act at the DNA replication fork to minimize the generation of chromatid discontinuity when lesions are encountered. To prevent collapse of stalled replication forks, mutagenic translesion synthesis (TLS) polymerases are recruited temporarily to bypass DNA lesions. When a replication-associated (one-ended) double-strand break occurs, homologous recombination repair (HRR) can restore chromatid continuity in what has traditionally been regarded as an "error-free" process. Our previous mutagenesis studies show an important role for HRR in preventing deletions and rearrangements that would otherwise result from error-prone nonhomologous end joining (NHEJ) after fork breakage. An analogous, but distinct, role in minimizing mutations is attributed to the proteins defective in the cancer predisposition disease Fanconi anemia (FA). Cells from FA patients and model systems show an increased proportion of gene-disrupting deletions at the hprt locus as well as decreased mutation rates in the hprt assay, suggesting a role for the FANC proteins in promoting TLS, HRR, and possibly also NHEJ. It remains unclear whether HRR, like the FANC pathway, impacts the rate of base substitution mutagenesis. Therefore, we measured, in isogenic rad51d and fancg CHO mutants, mutation rates at the Na(+)/K(+)-ATPase alpha-subunit (ATP1A1) locus using ouabain resistance, which specifically detects base substitution mutations. Surprisingly, we found that the spontaneous mutation rate was reduced approximately 2.5-fold in rad51d knockout cells, an even greater extent than observed in fancg cells, when compared with parental and isogenic gene-complemented control lines. A approximately 2-fold reduction in induced mutations in rad51d cells was seen after treatment with the DNA alkylating agent ethylnitrosurea while a lesser reduction occurred in fancg cells. Should the model ATP1A1 locus be representative of the genome, we conclude that at least 50% of base substitution mutations in this mammalian system arise through error-prone polymerase(s) acting during HRR-mediated restart of broken replication forks.

Relationship between DNA lesions, DNA repair and chromosomal damage induced by acetaldehyde

Acetaldehyde (AA) was tested along with two other crosslinking agents: formaldehyde (FA), an inducer of DNA-protein crosslinks (DPCs) and mitomycin C (MMC), an inducer of interstrand crosslinks (ICLs), to find out whether the mechanism of action of AA resembles more MMC or FA. Using a modification of the standard protocol for comet assay we demonstrate that AA induces crosslinks. Using a combination of alkaline comet version and proteinase-K, a clear abrogation of AA-induced reduction in DNA migration, like after FA treatment, was observed demonstrating that both agents induce DPCs, whereas MMC induces predominantly ICLs. A possible correlation between the types of induced crosslink and the induction chromosome damage in different repair deficient mutant Chinese hamster ovary cell lines treated with AA, MMC and FA was investigated. TCR/NER pathways are involved in repairing FA induced DPCs, but less in AA-induced DPCs. Our preliminary data suggest that DPCs are weaker inducers of SCEs in comparison with ICLs.

Biomarkers and mechanisms of FANCD2 function

Genetic or epigenetic inactivation of the pathway formed by the Fanconi anemia (FA) and BRCA1 proteins occurs in several cancer types, making the affected tumors potentially hypersensitive to DNA cross-linkers and other chemotherapeutic agents. It has been proposed that the inability of FA/BRCA-defective cells to form subnuclear foci of effector proteins, such as FANCD2, can be used as a biomarker to aid individualization of chemotherapy. We show that FANCD2 inactivation not only renders cells sensitive to cross-links, but also oxidative stress, a common effect of cancer therapeutics. Oxidative stress sensitivity does not correlate with FANCD2 or RAD51 foci formation, but associates with increased γH2AX foci levels and apoptosis. Therefore, FANCD2 may protect cells against cross-links and oxidative stress through distinct mechanisms, consistent with the growing notion that the pathway is not linear. Our data emphasize the need for multiple biomarkers, such as γH2AX, FANCD2, and RAD51, to capture all pathway activities.

The Fanconi anemia protein FANCM can promote branch migration of Holliday junctions and replication forks

Fanconi anemia (FA) is a genetically heterogeneous cancer-prone disorder associated with chromosomal instability and cellular hypersensitivity to DNA crosslinking agents. The FA pathway is suspected to play a crucial role in the cellular response to DNA replication stress. At a molecular level, however, the function of most of the FA proteins is unknown. FANCM displays DNA-dependent ATPase activity and promotes the dissociation of DNA triplexes, but the physiological significance of this activity remains elusive. Here we show that purified FANCM binds to Holliday junctions and replication forks with high specificity and promotes migration of their junction point in an ATPase-dependent manner. Furthermore, we provide evidence that FANCM can dissociate large recombination intermediates, via branch migration of Holliday junctions through 2.6 kb of DNA. Our data suggest a direct role for FANCM in DNA processing, consistent with the current view that FA proteins coordinate DNA repair at stalled replication forks.

FANCG promotes formation of a newly identified protein complex containing BRCA2, FANCD2 and XRCC3

Fanconi anemia (FA) is a human disorder characterized by cancer susceptibility and cellular sensitivity to DNA crosslinks and other damages. Thirteen complementation groups and genes are identified, including BRCA2, which is defective in the FA-D1 group. Eight of the FA proteins, including FANCG, participate in a nuclear core complex that is required for the monoubiquitylation of FANCD2 and FANCI. FANCD2, like FANCD1/BRCA2, is not part of the core complex, and we previously showed direct BRCA2-FANCD2 interaction using yeast two-hybrid analysis. We now show in human and hamster cells that expression of FANCG protein, but not the other core complex proteins, is required for co-precipitation of BRCA2 and FANCD2. We also show that phosphorylation of FANCG serine 7 is required for its co-precipitation with BRCA2, XRCC3 and FANCD2, as well as the direct interaction of BRCA2-FANCD2. These results argue that FANCG has a role independent of the FA core complex, and we propose that phosphorylation of serine 7 is the signalling event required for forming a discrete complex comprising FANCD1/BRCA2-FANCD2-FANCG-XRCC3 (D1-D2-G-X3). Cells that fail to express either phospho-Ser7-FANCG, or full length BRCA2 protein, lack the interactions amongst the four component proteins. A role for D1-D2-G-X3 in homologous recombination repair (HRR) is supported by our finding that FANCG and the RAD51-paralog XRCC3 are epistatic for sensitivity to DNA crosslinking compounds in DT40 chicken cells. Our findings further define the intricate interface between FANC and HRR proteins in maintaining chromosome stability.

Contributions of DNA interstrand cross-links to aging of cells and organisms

Impaired DNA damage repair, especially deficient transcription-coupled nucleotide excision repair, leads to segmental progeroid syndromes in human patients as well as in rodent models. Furthermore, DNA double-strand break signalling has been pinpointed as a key inducer of cellular senescence. Several recent findings suggest that another DNA repair pathway, interstrand cross-link (ICL) repair, might also contribute to cell and organism aging. Therefore, we summarize and discuss here that (i) systemic administration of anti-cancer chemotherapeutics, in many cases DNA cross-linking drugs, induces premature progeroid frailty in long-term survivors; (ii) that ICL-inducing 8-methoxy-psoralen/UVA phototherapy leads to signs of premature skin aging as prominent long-term side effect and (iii) that mutated factors involved in ICL repair like ERCC1/XPF, the Fanconi anaemia proteins, WRN and SNEV lead to reduced replicative life span in vitro and segmental progeroid syndromes in vivo. However, since ICL-inducing drugs cause damage different from ICL and since all currently known ICL repair factors work in more than one pathway, further work will be needed to dissect the actual contribution of ICL damage to aging.

Identification, developmental expression and regulation of the Xenopus ortholog of human FANCG/XRCC9

Fanconi anemia (FA) is associated with variable developmental abnormalities, bone marrow failure and cancer susceptibility. FANCG/XRCC9 is member of the FA core complex, a group of proteins that control the monoubiquitylation of FANCD2, an event that plays a critical role in maintaining genomic stability. Here we report the identification of the Xenopus laevis ortholog of human FANCG (xFANCG), its expression during development, and its molecular interactions with a partner protein, xFANCA. The xFANCG protein sequence is 47% similar to its human ortholog, with highest conservation in the two putative N-terminal leucine zippers and the tetratricopeptide repeat (TPR) motifs. xFANCG is maternally and zygotically transcribed. Prior to the midblastula stage, a single xFANCG transcript is observed but two additional alternatively spliced mRNAs are detected after the midblastula transition. One of the variants is predicted to encode a novel isoform of xFANCG lacking exon 2. The mutual association between FANCG and FANCA required for their nuclear import is conserved in Xenopus egg extracts. Our data demonstrate that interactions between FANCA and FANCG occur at the earliest stage of vertebrate development and raise the possibility that functionally different isoforms of xFANCG may play a role in early development.

Disparate contributions of the Fanconi anemia pathway and homologous recombination in preventing spontaneous mutagenesis

Fanconi anemia (FA) is a chromosomal instability disorder in which DNA-damage processing defects are reported for translesion synthesis (TLS), non-homologous end joining (NHEJ) and homologous recombination (HR; both increased and decreased). To reconcile these diverse findings, we compared spontaneous mutagenesis in FA and HR mutants of hamster CHO cells. In the fancg mutant we find a reduced mutation rate accompanied by an increased proportion of deletions within the hprt gene. Moreover, in fancg cells gene amplification at the CAD and dhfr loci is elevated, another manifestation of inappropriate processing of damage during DNA replication. In contrast, the rad51d HR mutant has a greatly elevated rate of hprt mutations, >85% of which are deletions. Our analysis supports the concept that HR faithfully restores broken replication forks, whereas the FA pathway acts more globally to ensure chromosome stability by promoting efficient end joining of replication-derived breaks, as well as TLS and HR.

Role of the Fancg gene in protecting cells from particulate chromate-induced chromosome instability

Particulate hexavalent chromium (Cr(VI)) is a known human lung carcinogen. Cr(VI)-induced tumors exhibit chromosome instability (CIN), but the mechanisms underlying these effects are unknown. We investigated a possible role for the Fanconi anemia (FA) pathway in particulate Cr(VI)-induced chromosomal damage by focusing on the Fancg gene, which plays an important role in cellular resistance to DNA interstrand crosslinks. We used the isogenic Chinese hamster ovary (CHO) KO40 fancg mutant compared with parental and gene-complemented cells. We found that fancg cells treated with lead chromate had lower intracellular Cr ion levels than control cell lines. Accounting for differences of Cr ion levels between cell lines, we discovered that fancg cells treated with lead chromate had increased cytotoxicity and chromosomal aberrations, which was not observed after restoring the Fancg gene. Chromosomal damage was manifest as increased total chromosome damage and percent metaphases with damage, specifically an increase in chromatid and isochromatid breaks. We conclude that Fancg protects cells from particulate Cr(VI)-induced cytotoxicity and chromosome damage, which is consistent with the known sensitivity of fancg cells to crosslinking damage and the ability of Cr(VI) to produce crosslinks.

Molecular mechanisms of sister-chromatid exchange

Sister-chromatid exchange (SCE) is the process whereby, during DNA replication, two sister chromatids break and rejoin with one another, physically exchanging regions of the parental strands in the duplicated chromosomes. This process is considered to be conservative and error-free, since no information is generally altered during reciprocal interchange by homologous recombination. Upon the advent of non-radiolabel detection methods for SCE, such events were used as genetic indicators for potential genotoxins/mutagens in laboratory toxicology tests, since, as we now know, most forms of DNA damage induce chromatid exchange upon replication fork collapse. Much of our present understanding of the mechanisms of SCE stems from studies involving nonhuman vertebrate cell lines that are defective in processes of DNA repair and/or recombination. In this article, we present a historical perspective of studies spearheaded by Dr. Anthony V. Carrano and colleagues focusing on SCE as a genetic outcome, and the role of the single-strand break DNA repair protein XRCC1 in suppressing SCE. A more general overview of the cellular processes and key protein "effectors" that regulate the manifestation of SCE is also presented.

Four human FANCG polymorphic variants show normal biological function in hamster CHO cells

Fanconi anemia (FA) is a rare cancer predisposition disease caused by mutations in at least 12 genes encoding proteins that cooperate to maintain genomic integrity. Variants of FA genes, including FANCG, have been identified in human population screening, but their potential reduction in protein function and role in cancer susceptibility is unclear. To test for possible dysfunction, we constructed plasmids containing four FANCG polymorphisms found in the human population and introduced them in the Fancg-deficient (fancg) KO40 line derived from AA8 hamster CHO cells. Expression of wild-type human FANCG provided fancg cells with complete phenotypic correction as assessed by resistance to the DNA crosslinking agent mitomycin C (MMC), thus providing a sensitive test for detecting the degree of complementation activity for the FANCG variants. We found that all four variants conferred levels of mitomycin C resistance as well as restoration of monoubiquitination of Fancd2, a key indicator of a functional FA protein pathway, similar to those observed in wild-type transfectants. Under the same conditions, the L71P amino acid substitution mutant, identified in an FA patient, gave no complementation. Using this novel system for determining FANCG functionality, we detect no decrement in function of the human FANCG polymorphic variants examined.

The Fanconi anemia pathway limits the severity of mutagenesis

Fanconi anemia (FA) is a developmental and cancer predisposition disorder in which key, yet unknown, physiological events promoting chromosome stability are compromised. FA cells exhibit excess metaphase chromatid breaks and are universally hypersensitive to DNA interstrand crosslinking agents. Published mutagenesis data from single-gene mutation assays show both increased and decreased mutation frequencies in FA cells. In this review we discuss the data from the literature and from our isogenic fancg knockout hamster CHO cells, and interpret these data within the framework of a molecular model that accommodates these seemingly divergent observations. In FA cells, reduced rates of recovery of viable X-linked hypoxanthine phosphoribosyltransferase (hprt) mutants are characteristically observed for diverse mutagenic agents, but also in untreated cultures, indicating the relevance of the FA pathway for processing assorted DNA lesions. We ascribe these reductions to: (1) impaired mutagenic translesion synthesis within hprt during DNA replication and (2) lethality of mutant cells following replication fork breakage on the X chromosome, caused by unrepaired double-strand breaks or large deletions/translocations encompassing essential genes flanking hprt. These findings, along with studies showing increased spontaneous mutability of FA cells at two autosomal loci, support a model in which FA proteins promote both translesion synthesis at replication-blocking lesions and repair of broken replication forks by homologous recombination and DNA end joining. The essence of this model is that the FANC protein pathway serves to restrict the severity of mutational outcome by favoring base substitutions and small deletions over larger deletions and chromosomal rearrangements.

Targeted disruption of FANCC and FANCG in human cancer provides a preclinical model for specific therapeutic options

BACKGROUND & AIMS

How specifically to treat pancreatic and other cancers harboring Fanconi anemia gene mutations has raised great interest recently, yet preclinical studies have been hampered by the lack of well-controlled human cancer models.

METHODS

We endogenously disrupted FANCC and FANCG in a human adenocarcinoma cell line and determined the impact of these genes on drug sensitivity, irradiation sensitivity, and genome maintenance.

RESULTS

FANCC and FANCG disruption abrogated FANCD2 monoubiquitination, confirming an impaired Fanconi anemia pathway function. On treatment with DNA interstrand-cross-linking agents, FANCC and FANCG disruption caused increased clastogenic damage, G2/M arrest, and decreased proliferation. The extent of hypersensitivity varied among agents, with ratios of inhibitory concentration 50% ranging from 2-fold for oxaliplatin to 14-fold for melphalan, a drug infrequently used in solid tumors. No hypersensitivity was observed on gemcitabine, etoposide, 3-aminobenzamide, NU1025, or hydrogen peroxide. FANCC and FANCG disruption also resulted in increased clastogenic damage on irradiation, but only FANCG disruption caused a subsequent decrease in relative survival. Finally, FANCC and FANCG disruption increased spontaneous chromosomal breakage, supporting the role of these genes in genome maintenance and likely explaining why they are mutated in sporadic cancer.

CONCLUSIONS

Our human cancer cell model provides optimal controls to elucidate fundamental biologic features of individual Fanconi anemia gene defects and facilitates preclinical studies of therapeutic options. The impact of Fanconi gene defects on drug and irradiation sensitivity renders these genes promising targets for a specific, genotype-based therapy for individual cancer patients, providing a strong rationale for clinical trials.

Functional interplay between BRCA2/FancD1 and FancC in DNA repair

A rare hereditary disorder, Fanconi anemia (FA), is caused by mutations in an array of genes, which interact in a common FA pathway/network. These genes encode components of the FA "core" complex, a key factor FancD2, the familial breast cancer suppressor BRCA2/FancD1, and Brip1/FancJ helicase. Although BRCA2 is known to play a pivotal role in homologous recombination repair by regulating Rad51 recombinase, the precise functional relationship between BRCA2 and the other FA genes is unclear. Here we show that BRCA2-dependent chromatin loading of Rad51 after mitomycin C treatment was not compromised by disruption of FANCC or FANCD2. Rad51 and FancD2 form colocalizing subnuclear foci independently of each other. Furthermore, we created a conditional BRCA2 truncating mutation lacking the C-terminal conserved domain (CTD) (brca2DeltaCTD), and disrupted the FANCC gene in this background. The fancc/brca2DeltaCTD double mutant revealed an epistatic relationship between FANCC and BRCA2 CTD in terms of x-ray sensitivity. In contrast, levels of cisplatin sensitivity and mitomycin C-induced chromosomal aberrations were increased in fancc/brca2DeltaCTD cells relative to either single mutant. Taken together, these results indicate that FA proteins work together with BRCA2/Rad51-mediated homologous recombination in double strand break repair, whereas the FA pathway plays a role that is independent of the CTD of BRCA2 in interstrand cross-link repair. These results provide insights into the functional interplay between the classical FA pathway and BRCA2.

Tetratricopeptide-motif-mediated interaction of FANCG with recombination proteins XRCC3 and BRCA2

Fanconi anaemia is an inherited chromosomal instability disorder characterised by cellular sensitivity to DNA interstrand crosslinkers, bone-marrow failure and a high risk of cancer. Eleven FA genes have been identified, one of which, FANCD1, is the breast cancer susceptibility gene BRCA2. At least eight FA proteins form a nuclear core complex required for monoubiquitination of FANCD2. The BRCA2/FANCD1 protein is connected to the FA pathway by interactions with the FANCG and FANCD2 proteins, both of which co-localise with the RAD51 recombinase, which is regulated by BRCA2. These connections raise the question of whether any of the FANC proteins of the core complex might also participate in other complexes involved in homologous recombination repair. We therefore tested known FA proteins for direct interaction with RAD51 and its paralogs XRCC2 and XRCC3. FANCG was found to interact with XRCC3, and this interaction was disrupted by the FA-G patient derived mutation L71P. FANCG was co-immunoprecipitated with both XRCC3 and BRCA2 from extracts of human and hamster cells. The FANCG-XRCC3 and FANCG-BRCA2 interactions did not require the presence of other FA proteins from the core complex, suggesting that FANCG also participates in a DNA repair complex that is downstream and independent of FANCD2 monoubiquitination. Additionally, XRCC3 and BRCA2 proteins co-precipitate in both human and hamster cells and this interaction requires FANCG. The FANCG protein contains multiple tetratricopeptide repeat motifs (TPRs), which function as scaffolds to mediate protein-protein interactions. Mutation of one or more of these motifs disrupted all of the known interactions of FANCG. We propose that FANCG, in addition to stabilising the FA core complex, may have a role in building multiprotein complexes that facilitate homologous recombination repair.

Resistance of hypoxic cells to ionizing radiation is influenced by homologous recombination status

PURPOSE

To determine the role of DNA repair in hypoxic radioresistance.

METHODS AND MATERIALS

Chinese hamster cell lines with mutations in homologous recombination (XRCC2, XRCC3, BRAC2, RAD51C) or nonhomologous end-joining (DNA-PKcs) genes were irradiated under normoxic (20% oxygen) and hypoxic (<0.1% oxygen) conditions, and the oxygen enhancement ratio (OER) was calculated. In addition, Fanconi anemia fibroblasts (complementation groups C and G) were compared with fibroblasts from nonsyndrome patients. RAD51 foci were studied using immunofluorescence.

RESULTS

All hamster cell lines deficient in homologous recombination showed a decrease in OER (1.5-2.0 vs. 2.6-3.0 for wild-types). In contrast, the OER for the DNA-PKcs-deficient line was comparable to wild-type controls. The two Fanconi anemia cell strains also showed a significant reduction in OER. The OER for RAD51 foci formation at late times after irradiation was considerably lower than that for survival in wild-type cells.

CONCLUSION

Homologous recombination plays an important role in determining hypoxic cell radiosensitivity. Lower OERs have also been reported in cells deficient in XPF and ERCC1, which, similar to homologous recombination genes, are known to play a role in cross-link repair. Because Fanconi anemia cells are also sensitive to cross-linking agents, this strengthens the notion that the capacity to repair cross-links determines hypoxic radiosensitivity.

Influence of double-strand-break repair pathways on radiosensitivity throughout the cell cycle in CHO cells

Unrepaired DNA double-strand breaks (DSBs) produced by ionizing radiation (IR) are a major determinant of cell killing. To determine the contribution of DNA repair pathways to the well-established cell cycle variation in IR sensitivity, we compared the radiosensitivity of wild-type CHO cells to mutant lines defective in nonhomologous end joining (NHEJ), homologous recombination repair (HRR), and the Fanconi anemia pathway. Cells were irradiated with IR doses that killed approximately 90% of each asynchronous population, separated into synchronous fractions by centrifugal elutriation, and assayed for survival (colony formation). Wild-type cells had lowest resistance in early G1 and highest resistance in S phase, followed by declining resistance as cells move into G2/M. In contrast, HR-defective cells (xrcc3 mutation) were most resistant in early G1 and became progressively less resistant in S and G2/M, indicating that the S-phase resistance in wild-type cells requires HRR. Cells defective in NHEJ (dna-pk(cs) mutation) were exquisitely sensitive in early G1, most resistant in S phase, and then somewhat less resistant in G2/M. Fancg mutant cells had almost normal IR sensitivity and normal cell cycle dependence, suggesting that Fancg contributes modestly to survival and in a manner that is independent of cell cycle position.