Fanconi Anemia (Cross)linked to DNA Repair
2005; Cell Press; Volume: 123; Issue: 7 Linguagem: Inglês
10.1016/j.cell.2005.12.009
ISSN1097-4172
AutoresLaura J. Niedernhofer, Astrid S. Lalai, Jan H.J. Hoeijmakers,
Tópico(s)Microtubule and mitosis dynamics
ResumoFanconi anemia is characterized by hypersensitivity to DNA interstrand crosslinks (ICLs) and susceptibility to tumor formation. Despite the identification of numerous Fanconi anemia (FANC) genes, the mechanism by which proteins encoded by these genes protect a cell from DNA interstrand crosslinks remains unclear. The recent discovery of two DNA helicases that, when defective, cause Fanconi anemia tips the balance in favor of the direct involvement of the FANC proteins in DNA repair and the bypass of DNA lesions. Fanconi anemia is characterized by hypersensitivity to DNA interstrand crosslinks (ICLs) and susceptibility to tumor formation. Despite the identification of numerous Fanconi anemia (FANC) genes, the mechanism by which proteins encoded by these genes protect a cell from DNA interstrand crosslinks remains unclear. The recent discovery of two DNA helicases that, when defective, cause Fanconi anemia tips the balance in favor of the direct involvement of the FANC proteins in DNA repair and the bypass of DNA lesions. Fanconi anemia (FA) is a rare, recessive (autosomal or X-linked) chromosomal-instability disorder, characterized by a striking hypersensitivity to DNA interstrand crosslinks (ICLs). The disease is clinically highly heterogeneous, even between monozygotic twins (Auerbach and Allen, 1991Auerbach A.D. Allen R.G. Leukemia and preleukemia in Fanconi anemia patients. A review of the literature and report of the International Fanconi Anemia Registry.Cancer Genet. Cytogenet. 1991; 51: 1-12Abstract Full Text PDF PubMed Scopus (247) Google Scholar, Auerbach et al., 2001Auerbach A.D. Buchwald M. Joenje H. Fanconi anemia.in: Scriver C.R. Beaudet A.L. Sly W.S. Valle D. The Metabolic and Molecular Basis of Inherited Disease. McGraw-Hill, New York2001: 753-768Google Scholar, Kook, 2005Kook H. Fanconi anemia: Current management.Hematology (Am Soc Hematol Educ Program). 2005; 10: 108-110Google Scholar), which dramatically complicates diagnosis. FA may be evident at birth due to congenital anomalies including aplasia of the thumb or radii and abnormal facies. Given that FA is linked with an increase in spontaneous chromosomal aberrations and cell death, these congenital anomalies are probably caused by such stochastic events in critical progenitor cells early in development. In some cases, FA may only become evident in early childhood due to renal dysfunction, abnormal pigmentation, and short stature, or the diagnosis may be made in adulthood due to the early onset of cancer. A hallmark feature of FA is pancytopenia (median age at onset 7 years) due to increased apoptosis of hematopoietic cells. The genomic instability of the surviving cells leads to myelodysplastic syndrome and acute myeloid leukemia (800-fold increased risk), with 14 years as the median age of onset (Kook, 2005Kook H. Fanconi anemia: Current management.Hematology (Am Soc Hematol Educ Program). 2005; 10: 108-110Google Scholar). The risk of developing solid tumors is also increased, in particular squamous cell carcinomas of the mucosal epithelia and liver tumors (Kook, 2005Kook H. Fanconi anemia: Current management.Hematology (Am Soc Hematol Educ Program). 2005; 10: 108-110Google Scholar). Although the median survival of FA patients is 16 years, the survival range is wide. Complications of bone-marrow failure are the most common cause of death (Alter et al., 1991Alter B.P. Knobloch M.E. Weinberg R.S. Erythropoiesis in Fanconi's anemia.Blood. 1991; 78: 602-608PubMed Google Scholar, Kook, 2005Kook H. Fanconi anemia: Current management.Hematology (Am Soc Hematol Educ Program). 2005; 10: 108-110Google Scholar). The early onset of cancer is a common feature of FA and is attributable to chromosomal instability. In addition, there is likely to be an accelerated aging component of FA caused by increased cell death in the proliferative compartments of the body, which leads to, for example, depletion of hematopoietic reserves. The hypersensitivity of cells from FA patients to DNA ICL agents (such as mitomycin C or diepoxybutane) is the basis for the diagnosis of FA. ICLs are a particularly deleterious type of DNA lesion because they covalently tether both strands of the DNA helix together. This prevents strand separation, an obvious prerequisite for vital cellular processes such as DNA replication and transcription. As a result, ICLs are extremely cytotoxic, particularly for proliferating cells. Hence, crosslinking agents are among the most effective cytotoxic drugs used in cancer chemotherapy. The fact that FA cells are hypersensitive to this type of DNA damage suggests that individuals with FA have defects in ICL repair. However, increased sensitivity could also arise from abnormalities in (1) complex mechanisms to tolerate ICLs rather than repairing them (such as replication bypass), (2) the cell-cycle-control machinery, (3) mechanisms of apoptosis, or (4) metabolic inactivation of crosslinking agents. In addition, cells from FA patients are modestly sensitive to other types of DNA damage (such as ionizing radiation and oxidative stress), and FA proteins are activated in S phase of the cell cycle (Taniguchi et al., 2002Taniguchi T. Garcia-Higuera I. Andreassen P.R. Gregory R.C. Grompe M. D'Andrea A.D. S-phase-specific interaction of the Fanconi anemia protein, FANCD2, with BRCA1 and RAD51.Blood. 2002; 100: 2414-2420Crossref PubMed Scopus (385) Google Scholar), suggesting a more general role in the DNA-damage response. Thus, the precise nature of the cellular processes affected in FA is unresolved. In all cells, replication of DNA containing ICLs induces double-strand breaks (DSBs) (De Silva et al., 2000De Silva I.U. McHugh P.J. Clingen P.H. Hartley J.A. Defining the roles of nucleotide excision repair and recombination in the repair of DNA interstrand cross-links in mammalian cells.Mol. Cell. Biol. 2000; 20: 7980-7990Crossref PubMed Scopus (380) Google Scholar, Niedernhofer et al., 2004Niedernhofer L.J. Odijk H. Budzowska M. van Drunen E. Maas A. Theil A.F. de Wit J. Jaspers N.G. Beverloo H.B. Hoeijmakers J.H. Kanaar R. The structure-specific endonuclease Ercc1-Xpf is required to resolve DNA interstrand cross-link-induced double-strand breaks.Mol. Cell. Biol. 2004; 24: 5776-5787Crossref PubMed Scopus (417) Google Scholar). ICLs are also potent inducers of sister-chromatid exchange (SCE), suggesting that these replication-dependent DSBs are frequently resolved through the production of SCEs (Figure 1). SCE represents the reciprocal exchange of the DNA strands between identical sister chromatids and thus only occurs during or after DNA replication. Not surprisingly, SCE requires the homologous-recombination machinery (Sonoda et al., 1999Sonoda E. Sasaki M.S. Morrison C. Yamaguchi-Iwai Y. Takata M. Takeda S. Sister chromatid exchanges are mediated by homologous recombination in vertebrate cells.Mol. Cell. Biol. 1999; 19: 5166-5169Crossref PubMed Scopus (358) Google Scholar). During DNA repair, homologous recombination allows the DNA at a broken sister-chromatid end to pair with the identical sequence of the other sister chromatid, using this sister as a template for the synthesis of lost genetic information (Helleday, 2003Helleday T. Pathways for mitotic homologous recombination in mammalian cells.Mutat. Res. 2003; 532: 103-115Crossref PubMed Scopus (254) Google Scholar). During this intricate DNA gymnastics, crossover can occur, resulting in the exchange of DNA strands between the two sister chromatids. Importantly, DSBs in DNA that yield two broken ends (such as those caused by ionizing radiation), which can be directly recombined with one another, generally do not induce SCEs (Helleday, 2003Helleday T. Pathways for mitotic homologous recombination in mammalian cells.Mutat. Res. 2003; 532: 103-115Crossref PubMed Scopus (254) Google Scholar). However, collapse of a replication fork results in only one broken double-strand end (for structure, see Figure 2). These single DSBs that arise in S phase are more likely to stimulate SCE (Helleday, 2003Helleday T. Pathways for mitotic homologous recombination in mammalian cells.Mutat. Res. 2003; 532: 103-115Crossref PubMed Scopus (254) Google Scholar). These observations imply a role for DNA replication, replication-induced DSBs, and the subsequent repair of these DSBs by homologous recombination in the repair of ICLs.Figure 2A Speculative Model of the DNA ICL Repair Pathway Highlighting the Involvement of the FANC ProteinsShow full caption(1) DNA-damaging agents such as mitomycin C but also endogenous metabolic processes such as lipid peroxidation can cause interstrand crosslinks (ICLs). (2) ICLs physically block DNA replication, leading to activation of the ATR kinase, which phosphorylates a number of proteins, including FANCD2 (inset). Phosphorylated FANCD2 is monoubiquitinated by the FANCL subunit of the FA core complex (circled). This triggers the creation of nuclear foci at sites of replication stress consisting of numerous proteins involved in homologous recombination and translesion synthesis. (3) A second consequence of blocked replication is the creation of a DSB, which occurs by incision of one of the template strands upstream of the lesion by an endonuclease (depicted as scissors). (4) To separate the two template strands, a second incision is required, probably facilitated by the opening of the crosslinked substrate DNA by a helicase such as FANCM. (5) To unhook the ICL from the template strand, the second incision must occur on the same strand as the first incision but on the other side of the ICL. A likely endonuclease for this incision is ERCC1-XPF, facilitated by the single-stranded binding protein RPA, which dictates which strand is cut. (6) The second incision creates a gap and a new 3′ end that can be used to prime DNA synthesis. (7) Gap-filling DNA synthesis, probably by an error-prone translesion polymerase, is necessary to restore the damaged template strand. (8) Although these specialized polymerases permit bypass of lesions, they frequently insert incorrect bases, creating a mutation at the site of damage. (9) Once the ICL is unhooked and strand integrity is restored, DSB repair can occur by template switching, requiring the homologous-recombination machinery. This is initiated by the resection of the broken end to yield a 3′ single-strand overhang able to pair with its sister chromatid. (10–11) Repair requires the homologous-recombination machinery, including RAD51, BRCA1, BRCA2, PCNA, and RPA. The FANCJ helicase is probably involved at this step. (12) The resolution of Holliday junction recombination intermediates permits replication to continue and may or may not result in the formation of a SCE (Figure 1). The unhooked ICL carrying a short oligonucleotide is a likely substrate for removal by the nucleotide excision repair system, or, alternatively, spontaneous hydrolysis of the glycosidic bond may detach it from the DNA, leaving a mutagenic abasic site. Deubiquitination at the end of S phase turns the pathway off (center). See text for a more detailed explanation.View Large Image Figure ViewerDownload Hi-res image Download (PPT) (1) DNA-damaging agents such as mitomycin C but also endogenous metabolic processes such as lipid peroxidation can cause interstrand crosslinks (ICLs). (2) ICLs physically block DNA replication, leading to activation of the ATR kinase, which phosphorylates a number of proteins, including FANCD2 (inset). Phosphorylated FANCD2 is monoubiquitinated by the FANCL subunit of the FA core complex (circled). This triggers the creation of nuclear foci at sites of replication stress consisting of numerous proteins involved in homologous recombination and translesion synthesis. (3) A second consequence of blocked replication is the creation of a DSB, which occurs by incision of one of the template strands upstream of the lesion by an endonuclease (depicted as scissors). (4) To separate the two template strands, a second incision is required, probably facilitated by the opening of the crosslinked substrate DNA by a helicase such as FANCM. (5) To unhook the ICL from the template strand, the second incision must occur on the same strand as the first incision but on the other side of the ICL. A likely endonuclease for this incision is ERCC1-XPF, facilitated by the single-stranded binding protein RPA, which dictates which strand is cut. (6) The second incision creates a gap and a new 3′ end that can be used to prime DNA synthesis. (7) Gap-filling DNA synthesis, probably by an error-prone translesion polymerase, is necessary to restore the damaged template strand. (8) Although these specialized polymerases permit bypass of lesions, they frequently insert incorrect bases, creating a mutation at the site of damage. (9) Once the ICL is unhooked and strand integrity is restored, DSB repair can occur by template switching, requiring the homologous-recombination machinery. This is initiated by the resection of the broken end to yield a 3′ single-strand overhang able to pair with its sister chromatid. (10–11) Repair requires the homologous-recombination machinery, including RAD51, BRCA1, BRCA2, PCNA, and RPA. The FANCJ helicase is probably involved at this step. (12) The resolution of Holliday junction recombination intermediates permits replication to continue and may or may not result in the formation of a SCE (Figure 1). The unhooked ICL carrying a short oligonucleotide is a likely substrate for removal by the nucleotide excision repair system, or, alternatively, spontaneous hydrolysis of the glycosidic bond may detach it from the DNA, leaving a mutagenic abasic site. Deubiquitination at the end of S phase turns the pathway off (center). See text for a more detailed explanation. In addition to SCEs, a small fraction (<5%) of cells treated with a crosslinking agent also contain a radial structure (Figure 1). Radial structures are created when broken sister chromatids of disparate chromosomes become erroneously fused to one another, creating a tandem array of chromosomes connected via recombined sister chromatids (Figure 1). Radial-structure formation, like SCE, requires DNA replication and DSBs. Apparently, they arise if homologous-recombination-mediated repair of the DSBs does not occur and instead the DSBs are repaired through illegitimate recombination—for example by single-strand annealing, in which broken ends with only small regions of sequence homology are joined. The frequency of SCE in response to crosslinking agents is normal in FA (Mosedale et al., 2005Mosedale G. Niedzwiedz W. Alpi A. Perrina F. Pereira-Leal J.B. Johnson M. Langevin F. Pace P. Patel K.J. The vertebrate Hef ortholog is a component of the Fanconi anemia tumor-suppressor pathway.Nat. Struct. Mol. Biol. 2005; 12: 763-771Crossref PubMed Scopus (164) Google Scholar, Niedzwiedz et al., 2004Niedzwiedz W. Mosedale G. Johnson M. Ong C.Y. Pace P. Patel K.J. The Fanconi anaemia gene FANCC promotes homologous recombination and error-prone DNA repair.Mol. Cell. 2004; 15: 607-620Abstract Full Text Full Text PDF PubMed Scopus (255) Google Scholar); however, the number of radial structures increases dramatically. Multiple radial structures occur in 30%–100% of FA cells treated with crosslinking agents, which is the diagnostic criteria for FA (Kook, 2005Kook H. Fanconi anemia: Current management.Hematology (Am Soc Hematol Educ Program). 2005; 10: 108-110Google Scholar). This implies that FA cells are defective in homologous-recombination-mediated repair of DSBs induced by ICLs (Figure 1). Alternatively, the cells may be defective in a cell-cycle checkpoint that permits safe repair of ICLs before reentry into the cell cycle or in some other mechanism of handling ICLs during replication. With respect to the last possibility, FA cells intriguingly show a lower frequency of point mutations (single-base-pair changes) than normal cells, in striking contrast to their elevated chromosomal instability. This indicates that, normally, the intact FA pathway induces point mutations while preventing chromosome aberrations. This is consistent with a mechanism that involves the bypass of a DNA lesion (which has a high chance of creating point mutations) in the FA pathway to repair ICLs. This is supported by genetic data demonstrating that error-prone translesion polymerases act in the FA pathway (Niedzwiedz et al., 2004Niedzwiedz W. Mosedale G. Johnson M. Ong C.Y. Pace P. Patel K.J. The Fanconi anaemia gene FANCC promotes homologous recombination and error-prone DNA repair.Mol. Cell. 2004; 15: 607-620Abstract Full Text Full Text PDF PubMed Scopus (255) Google Scholar). Confusingly, cells from FA patients are not inherently defective in homologous recombination, as demonstrated by their capacity to trigger SCE (Mosedale et al., 2005Mosedale G. Niedzwiedz W. Alpi A. Perrina F. Pereira-Leal J.B. Johnson M. Langevin F. Pace P. Patel K.J. The vertebrate Hef ortholog is a component of the Fanconi anemia tumor-suppressor pathway.Nat. Struct. Mol. Biol. 2005; 12: 763-771Crossref PubMed Scopus (164) Google Scholar, Niedzwiedz et al., 2004Niedzwiedz W. Mosedale G. Johnson M. Ong C.Y. Pace P. Patel K.J. The Fanconi anaemia gene FANCC promotes homologous recombination and error-prone DNA repair.Mol. Cell. 2004; 15: 607-620Abstract Full Text Full Text PDF PubMed Scopus (255) Google Scholar), their relatively mild sensitivity to ionizing radiation (Bridge et al., 2005Bridge W.L. Vandenberg C.J. Franklin R.J. Hiom K. The BRIP1 helicase functions independently of BRCA1 in the Fanconi anemia pathway for DNA crosslink repair.Nat. Genet. 2005; 37: 953-957Crossref PubMed Scopus (164) Google Scholar, Niedzwiedz et al., 2004Niedzwiedz W. Mosedale G. Johnson M. Ong C.Y. Pace P. Patel K.J. The Fanconi anaemia gene FANCC promotes homologous recombination and error-prone DNA repair.Mol. Cell. 2004; 15: 607-620Abstract Full Text Full Text PDF PubMed Scopus (255) Google Scholar), and their ability to accurately repair a DSB created by a restriction endonuclease (Nakanishi et al., 2005Nakanishi K. Yang Y.G. Pierce A.J. Taniguchi T. Digweed M. D'Andrea A.D. Wang Z.Q. Jasin M. Human Fanconi anemia monoubiquitination pathway promotes homologous DNA repair.Proc. Natl. Acad. Sci. USA. 2005; 102: 1110-1115Crossref PubMed Scopus (308) Google Scholar). Thus, if DNA repair is affected in FA, the defect is likely limited to a special subset of lesions (most notably ICLs) that induce replication stress. Alternatively, other findings point to impaired cell-cycle control in FA. Rather than halting DNA replication during S phase to permit ICL repair, FA cells continue DNA synthesis (Sala-Trepat et al., 2000Sala-Trepat M. Rouillard D. Escarceller M. Laquerbe A. Moustacchi E. Papadopoulo D. Arrest of S-phase progression is impaired in Fanconi anemia cells.Exp. Cell Res. 2000; 260: 208-215Crossref PubMed Scopus (71) Google Scholar), risking creation of DSBs at sites of damage. In support of this, spontaneous chromosomal breaks are more common in FA cells than in wild-type cells. This is highly reminiscent of another chromosomal-instability disorder, ataxia telangiectasia, in which DNA replication is not delayed in response to radiation-induced DNA damage. ATM, the protein defective in this disorder, is a transducer in a complex signaling cascade in response to DSBs (Kurz and Lees-Miller, 2004Kurz E.U. Lees-Miller S.P. DNA damage-induced activation of ATM and ATM-dependent signaling pathways.DNA Repair (Amst.). 2004; 3: 889-900Crossref PubMed Scopus (385) Google Scholar). In fact, one of the FA proteins (FANCD2) is an effector of ATM required for the DNA-damage checkpoint in S phase (Taniguchi et al., 2002Taniguchi T. Garcia-Higuera I. Andreassen P.R. Gregory R.C. Grompe M. D'Andrea A.D. S-phase-specific interaction of the Fanconi anemia protein, FANCD2, with BRCA1 and RAD51.Blood. 2002; 100: 2414-2420Crossref PubMed Scopus (385) Google Scholar). In contrast to S phase, the G2 checkpoint is intact in FA cells (Sala-Trepat et al., 2000Sala-Trepat M. Rouillard D. Escarceller M. Laquerbe A. Moustacchi E. Papadopoulo D. Arrest of S-phase progression is impaired in Fanconi anemia cells.Exp. Cell Res. 2000; 260: 208-215Crossref PubMed Scopus (71) Google Scholar). Thus, after exposure of FA cells to genotoxic stress, the majority of the cells collect in G2. In fact, G2 is prolonged in FA cells that have not been exposed to stress compared to normal cells, apparently in response to persistent endogenous DNA damage. The phenotypic heterogeneity of this syndrome spawned a search for multiple genetic causes of FA that could be responsible for the extreme clinical variation. This search was facilitated by the exquisite sensitivity of FA cells to ICLs, which allowed for the identification of distinct complementation groups (each complementation group represents FA patients with defects in the same gene). Such complementation groups can be identified when cells (such as fibroblasts or lymphoblastoid cells) from different patients are fused together and the resulting cell population is screened for wild-type resistance to ICL damage by the complementation of each other's defect. This endeavor proved highly rewarding. The total number of FA complementation groups has now risen to 12 (FANCA, B, C, D1, D2, E, F, G, I, J, L, and very recently M; see below), with more to come. Curiously, this genetic heterogeneity does not explain the clinical heterogeneity, although a systematic analysis of a possible link between individual complementation groups and specific clinical manifestations is warranted. The expectation that cloning the genes defective in each of the complementation groups would reveal an FA pathway turned out to be false. Sequence analysis of the first series of FA genes isolated did not yield any meaningful homology to other proteins or hints about protein function. With FANCI still awaiting identification, only 4 of the 11 cloned FA genes have provided clues about the functions of the proteins they encode. This includes FANCD1, which is identical to BRCA2, the breast cancer susceptibility gene implicated in homologous recombination (Howlett et al., 2002Howlett N.G. Taniguchi T. Olson S. Cox B. Waisfisz Q. De Die-Smulders C. Persky N. Grompe M. Joenje H. Pals G. et al.Biallelic inactivation of BRCA2 in Fanconi anemia.Science. 2002; 297: 606-609Crossref PubMed Scopus (935) Google Scholar); FANCL, which encodes an E3 ubiquitin ligase implicated in the monoubiquitination of FANCD2, an essential step in the FA pathway (Meetei et al., 2003aMeetei A.R. de Winter J.P. Medhurst A.L. Wallisch M. Waisfisz Q. van de Vrugt H.J. Oostra A.B. Yan Z. Ling C. Bishop C.E. et al.A novel ubiquitin ligase is deficient in Fanconi anemia.Nat. Genet. 2003; 35: 165-170Crossref PubMed Scopus (462) Google Scholar); and the latest additions: FANCJ and M, both encoding DNA helicases. Nearly all of the FA proteins physically interact in the cell, comprising an FA core complex (Garcia-Higuera et al., 2001Garcia-Higuera I. Taniguchi T. Ganesan S. Meyn M.S. Timmers C. Hejna J. Grompe M. D'Andrea A.D. Interaction of the Fanconi anemia proteins and BRCA1 in a common pathway.Mol. Cell. 2001; 7: 249-262Abstract Full Text Full Text PDF PubMed Scopus (1002) Google Scholar, Meetei et al., 2003aMeetei A.R. de Winter J.P. Medhurst A.L. Wallisch M. Waisfisz Q. van de Vrugt H.J. Oostra A.B. Yan Z. Ling C. Bishop C.E. et al.A novel ubiquitin ligase is deficient in Fanconi anemia.Nat. Genet. 2003; 35: 165-170Crossref PubMed Scopus (462) Google Scholar, Meetei et al., 2004Meetei A.R. Levitus M. Xue Y. Medhurst A.L. Zwaan M. Ling C. Rooimans M.A. Bier P. Hoatlin M. Pals G. et al.X-linked inheritance of Fanconi anemia complementation group B.Nat. Genet. 2004; 36: 1219-1224Crossref PubMed Scopus (235) Google Scholar). This complex includes FANCA, B, C, E, F, G, and L, together with as yet uncharacterized additional components (Meetei et al., 2005Meetei A.R. Medhurst A.L. Ling C. Xue Y. Singh T.R. Bier P. Steltenpool J. Stone S. Dokal I. Mathew C.G. et al.A human ortholog of archaeal DNA repair protein Hef is defective in Fanconi anemia complementation group M.Nat. Genet. 2005; 37: 958-963Crossref PubMed Scopus (353) Google Scholar). In S phase, FANCL monoubiquitinates FANCD2, a process that also requires FANCI (which is not part of the core FA complex). This in turn triggers association of FANCD2 with chromatin and its accumulation in nuclear foci. These foci are thought to demarcate the sites where DSB repair and replication bypass of DNA damage occurs. Activated FANCD2 colocalizes with factors involved in homologous-recombination-mediated DSB repair such as BRCA1, BRCA2/FANCD1, and RAD51 (Taniguchi et al., 2002Taniguchi T. Garcia-Higuera I. Andreassen P.R. Gregory R.C. Grompe M. D'Andrea A.D. S-phase-specific interaction of the Fanconi anemia protein, FANCD2, with BRCA1 and RAD51.Blood. 2002; 100: 2414-2420Crossref PubMed Scopus (385) Google Scholar). RAD51 forms a nucleoprotein filament on broken DNA ends, searches for the homologous sequence in the sister chromatid, and catalyzes strand exchange required for homologous recombination. Also, replication factors such as PCNA and RPA are present in these nuclear foci. The FA core complex itself is required for translocation of FANCD2 to these nuclear foci, even after monoubiquitination is complete, and subsequently for the nuclear foci to restore replication and cell-cycle progression (Matsushita et al., 2005Matsushita N. Kitao H. Ishiai M. Nagashima N. Hirano S. Okawa K. Ohta T. Yu D.S. McHugh P.J. Hickson I.D. et al.A FancD2-monoubiquitin fusion reveals hidden functions of Fanconi anemia core complex in DNA repair.Mol. Cell. 2005; 19: 841-847Abstract Full Text Full Text PDF PubMed Scopus (114) Google Scholar). The FA core complex also interacts with the BLM-TOPOIIIα complex (Meetei et al., 2003bMeetei A.R. Sechi S. Wallisch M. Yang D. Young M.K. Joenje H. Hoatlin M.E. Wang W. A multiprotein nuclear complex connects Fanconi anemia and Bloom syndrome.Mol. Cell. Biol. 2003; 23: 3417-3426Crossref PubMed Scopus (287) Google Scholar). This complex is involved in the proper dissolution of double Holliday junctions, intermediates of homologous recombination (Wu et al., 2005Wu L. Lung Chan K. Ralf C. Bernstein D.A. Garcia P.L. Bohr V.A. Vindigni A. Janscak P. Keck J.L. Hickson I.D. The HRDC domain of BLM is required for the dissolution of double Holliday junctions.EMBO J. 2005; 24: 2679-2687Crossref PubMed Scopus (134) Google Scholar). Now, the newest FA proteins, FANCJ and FANCM, can be added to the FA pathway. Single-nucleotide polymorphism analysis of FANCJ families pinpointed a region on chromosome 17q22 containing the culprit gene. Sequencing of candidate loci identified biallelic mutations in a gene called BRIP1 (BRCA1-interacting protein 1) or BACH1 (BRCA1-associated C-terminal helicase 1) in multiple families from the FANCJ complementation group (Levitus et al., 2005Levitus M. Waisfisz Q. Godthelp B.C. Vries Y. Hussain S. Wiegant W.W. Elghalbzouri-Maghrani E. Steltenpool J. Rooimans M.A. Pals G. et al.The DNA helicase BRIP1 is defective in Fanconi anemia complementation group J.Nat. Genet. 2005; 37: 934-935Crossref PubMed Scopus (365) Google Scholar, Levran et al., 2005Levran O. Attwooll C. Henry R.T. Milton K.L. Neveling K. Rio P. Batish S.D. Kalb R. Velleuer E. Barral S. et al.The BRCA1-interacting helicase BRIP1 is deficient in Fanconi anemia.Nat. Genet. 2005; 37: 931-933Crossref PubMed Scopus (299) Google Scholar). The link with FA was independently made in chicken lymphoblast DT40 cells, in which inactivation of the chicken ortholog of BRIP1 yielded an FA-like phenotype with extreme sensitivity to ICL damage (but not to UV, ionizing radiation, hydrogen peroxide, or alkylation damage) and arrest in late S/G2 in response to ICL damage (Bridge et al., 2005Bridge W.L. Vandenberg C.J. Franklin R.J. Hiom K. The BRIP1 helicase functions independently of BRCA1 in the Fanconi anemia pathway for DNA crosslink repair.Nat. Genet. 2005; 37: 953-957Crossref PubMed Scopus (164) Google Scholar). Ubiquitination of FANCD2 is normal in these cells, indicating that FANCJ acts downstream of this key event in the FA pathway. Cells deficient in both FANCJ and FANCC are more sensitive to ICL damage than cells with a mutation in just one of these FANC proteins, suggesting that the two proteins may participate in redundant DNA-repair pathways (Bridge et al., 2005Bridge W.L. Vandenberg C.J. Franklin R.J. Hiom K. The BRIP1 helicase functions independently of BRCA1 in the Fanconi anemia pathway for DNA crosslink repair.Nat. Genet. 2005; 37: 953-957Crossref PubMed Scopus (164) Google Scholar). Similar results were obtained in human cells using small interfering RNAs (siRNAs) to knock down expression of BRIP1/BACH1. These cells showed accumulation of ICL-induced chromatid breaks and radial structures, as well as γH2AX and RAD51 foci (indicative of DSBs), but normal FANCD2 ubiquitination in response to ICL damage (Litman et al., 2005Litman R. Peng M. Jin Z. Zhang F. Zhang J. Powell S. Andreassen P.R. Cantor S.B. BACH1 is critical for homologous recombination and appears to be the Fanconi anemia gene product FANCJ.Cancer Cell. 2005; 8: 255-265Abstract Full Text Full Text PDF PubMed Scopus (316) Google Scholar). FANCJ encodes a 5′→3′ helicase that interacts with the BRCT repeats of BRCA1, which mediates colocalization of the two proteins in late S/G2 (Cantor et al., 2001Cantor S.B. Bell D.W. Ganesan S. Kass E.M. Drapkin R. Grossman S. Wahrer D.C. Sgroi D.C. Lane W.S. Haber D.A. Livingston D.M. BACH1, a novel helicase-like protein, interacts directly with BRCA1 and contributes to its DNA repair function.Cell. 2001; 105: 149-160Abstract Full Text Full
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