FANCI functions as a repair/apoptosis switch in response to DNA crosslinks

Cells counter DNA damage through repair or apoptosis, yet a direct mechanism for this choice has remained elusive. When facing interstrand crosslinks (ICLs), the ICL-repair protein FANCI heterodimerizes with FANCD2 to initiate ICL excision. We found that FANCI alternatively interacts with a pro-apoptotic factor, PIDD1, to enable PIDDosome (PIDD1-RAIDD-caspase-2) formation and apoptotic death. FANCI switches from FANCD2/repair to PIDD1/apoptosis signaling in the event of ICL-repair failure. Specifically, removing key endonucleases downstream of FANCI/FANCD2, increasing ICL levels, or allowing damaged cells into mitosis (when repair is suppressed) all suffice for switching. Reciprocally, apoptosis-committed FANCI reverts from PIDD1 to FANCD2 after a failed attempt to assemble the PIDDosome. Monoubiquitination and deubiquitination at FANCI K523 impact interactor selection. These data unveil a repair-or-apoptosis switch in eukaryotes. Beyond ensuring the removal of unrepaired genomes, the switch's bidirectionality reveals that damaged cells can offset apoptotic defects via de novo attempts at lesion repair.

Identification of UHRF2 as a novel DNA interstrand crosslink sensor protein

The Fanconi Anemia (FA) pathway is important for repairing interstrand crosslinks (ICLs) between the Watson-Crick strands of the DNA double helix. An initial and essential stage in the repair process is the detection of the ICL. Here, we report the identification of UHRF2, a paralogue of UHRF1, as an ICL sensor protein. UHRF2 is recruited to ICLs in the genome within seconds of their appearance. We show that UHRF2 cooperates with UHRF1, to ensure recruitment of FANCD2 to ICLs. A direct protein-protein interaction is formed between UHRF1 and UHRF2, and between either UHRF1 and UHRF2, and FANCD2. Importantly, we demonstrate that the essential monoubiquitination of FANCD2 is stimulated by UHRF1/UHRF2. The stimulation is mediating by a retention of FANCD2 on chromatin, allowing for its monoubiquitination by the FA core complex. Taken together, we uncover a mechanism of ICL sensing by UHRF2, leading to FANCD2 recruitment and retention at ICLs, in turn facilitating activation of FANCD2 by monoubiquitination.

FANCD2 limits replication stress and genome instability in cells lacking BRCA2

The tumor suppressor BRCA2 plays a key role in genome integrity by promoting replication-fork stability and homologous recombination (HR) DNA repair. Here we report that human cancer cells lacking BRCA2 rely on the Fanconi anemia protein FANCD2 to limit replication-fork progression and genomic instability. Our results identify a new role of FANCD2 in limiting constitutive replication stress in BRCA2-deficient cells, thereby affecting cell survival and treatment responses.

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.

Fanconi anemia D2 protein confers chemoresistance in response to the anticancer agent, irofulven

The Fanconi anemia-BRCA pathway of genes are frequently mutated or epigenetically repressed in human cancer. The proteins of this pathway play pivotal roles in DNA damage signaling and repair. Irofulven is one of a new class of anticancer agents that are analogues of mushroom-derived illudin toxins. Preclinical studies and clinical trials have shown that irofulven is effective against several tumor cell types. The exact nature of irofulven-induced DNA damage is not completely understood. Previously, we have shown that irofulven activates ATM and its targets, NBS1, SMC1, CHK2, and p53. In this study, we hypothesize that irofulven induces DNA double-strand breaks and FANCD2 may play an important role in modulating cellular responses and chemosensitivity in response to irofulven treatment. By using cells that are proficient or deficient for FANCD2, ATR, or ATM, we showed that irofulven induces FANCD2 monoubiquitination and nuclear foci formation. ATR is important in mediating irofulven-induced FANCD2 monoubiquitination. Furthermore, we showed that FANCD2 plays a critical role in maintaining chromosome integrity and modulating chemosensitivity in response to irofulven-induced DNA damage. Therefore, this study suggests that it might be clinically significant to target irofulven therapy to cancers defective for proteins of the Fanconi anemia-BRCA pathway.

Loss of FANCC function is associated with failure to inhibit late firing replication origins after DNA cross-linking

Fanconi anemia (FA) cells are abnormally sensitive to DNA cross-linking agents with increased levels of apoptosis and chromosomal instability. Defects in eight FA complementation groups inhibit monoubiquitination of FANCD2, and subsequent recruitment of FANCD2 to DNA damage and S-phase-associated nuclear foci. The specific functional defect in repair or response to DNA damage in FA cells remains unknown. Damage-resistant DNA synthesis is present 2.5-5 h after cross-linker treatment of FANCC, FANCA and FANCD2-deficient cells. Analysis of the size distribution of labeled DNA replication strands revealed that diepoxybutane treatment suppressed labeling of early but not late-firing replicons in FANCC-deficient cells. In contrast, normal responses to ionizing radiation were observed in FANCC-deficient cells. Absence of this late S-phase response in FANCC-deficient cells leads to activation of secondary checkpoint responses.

Histone H2AX and Fanconi anemia FANCD2 function in the same pathway to maintain chromosome stability

Fanconi anemia (FA) is a chromosome fragility syndrome characterized by bone marrow failure and cancer susceptibility. The central FA protein FANCD2 is known to relocate to chromatin upon DNA damage in a poorly understood process. Here, we have induced subnuclear accumulation of DNA damage to prove that histone H2AX is a novel component of the FA/BRCA pathway in response to stalled replication forks. Analyses of cells from H2AX knockout mice or expressing a nonphosphorylable H2AX (H2AX(S136A/S139A)) indicate that phosphorylated H2AX (gammaH2AX) is required for recruiting FANCD2 to chromatin at stalled replication forks. FANCD2 binding to gammaH2AX is BRCA1-dependent and cells deficient or depleted of H2AX show an FA-like phenotype, including an excess of chromatid-type chromosomal aberrations and hypersensitivity to MMC. This MMC hypersensitivity of H2AX-deficient cells is not further increased by depleting FANCD2, indicating that H2AX and FANCD2 function in the same pathway in response to DNA damage-induced replication blockage. Consequently, histone H2AX is functionally connected to the FA/BRCA pathway to resolve stalled replication forks and prevent chromosome instability.

Developmental stage- and DNA damage-specific functions of C. elegans FANCD2

In this study, we set out to investigate the role of Fanconi anemia complementation group D2 protein (FANCD2) in developmental stage-specific DNA damage responses in Caenorhabditis elegans. A mutant C. elegans strain containing a deletion in the gene encoding the FANCD2 homolog, FCD-2, exhibited egg-laying defects, precocious oogenesis, and partial defects in fertilization. The mutant strain also had a lower hatching rate than the wild-type after gamma-irradiation of embryos, but not after the irradiation of pachytene stage germ cells. This mutation sensitized pachytene stage germ cells to the genotoxic effects of photoactivated psoralen, as seen by a greatly reduced hatching rate and increased chromosomal aberrations. This mutation also enhanced physiological M-phase arrest and apoptosis. Taken together, our data reveal that the C. elegans FANCD2 homolog participates in the repair of spontaneous DNA damage and DNA crosslinks, not only in proliferating cells but also in pachytene stage cells, and it may have an additional role in double-stranded DNA break repair during embryogenesis.

Activation of fanconi anemia pathway in cells with re-replicated DNA

To maintain genome stability, the cell has to limit initiation of DNA replication to once per cell cycle. Loss of this control leads to DNA rereplication with repeated firing of replication origins in the same cell cycle. Rereplication generates both ssDNA and double strand breaks, as well as activation of the DNA damage checkpoint. In rereplicated cells, activation of the checkpoint is critical to arrest cells in G(2) resulting in accumulation of cells with rereplicated DNA. Abrogation of this checkpoint suppresses the progressive accumulation of cells with excess DNA and causes apoptosis. Recently, the Fanconi Anemia pathway was reported to be activated in rereplicating cells. Interestingly, FA core complexes but not FANCD2, is required for checkpoint activation in rereplicated cells, suggesting that the pathway to checkpoint activation requires the ubiquitination of substrates other than FANCD2. In addition, FANCD2 is required for recruitment of Rad51 to foci in rereplicated cells, so that the repair pathways activated after small degrees of rereplication are expected to be compromised in cells with mutations in the FA pathway.

C. elegans FANCD2 responds to replication stress and functions in interstrand cross-link repair

One of the least well understood DNA repair processes in cells is the repair of DNA interstrand cross-links (ICLs) which present a major obstacle to DNA replication and must be repaired or bypassed to allow fork progression. Fanconi anemia (FA) is an inherited genome instability syndrome characterized by hypersensitivity to ICL damage. Central to the FA repair pathway is FANCD2 that is mono-ubiquitylated in response to replication stress and ICL damage through the action of the FA core complex and its E3-ubiquitin ligase subunit, FANCL. In its mono-ubiquitylated form FANCD2 is recruited to repair foci where it is believed to somehow coordinate ICL repair and restart of impeded replication forks. However, the precise mechanism through which the FA pathway and mono-ubiquitylation of FANCD2 promotes ICL repair remains unclear. Here we report on a functional homologue of FANCD2 in C. elegans (FCD-2). Although fcd-2 mutants are homozygous viable, they are exquisitely sensitive to ICL-inducing agents, but insensitive to ionizing radiation (IR). fcd-2 is dispensable for meiotic recombination and activation of the S-phase checkpoint, indicating that ICL sensitivity is likely due to a repair rather than a signalling defect. Indeed, we show that FCD-2 is mono-ubiquitylated in response to ICL damage and is recruited to nuclear repair foci. Consistent with the sensitivity of fcd-2 mutants, FCD-2 focus formation is induced in response to ICL damage and replication stress, but not following IR, suggesting that FCD-2 responds to lesions that block DNA replication and not DNA double strand breaks per se. The realization that the FA pathway is conserved in a genetically tractable model system will permit the comprehensive analysis of the interplay between the FA, homologous recombination (HR), translesion synthesis (TLS) and nucleotide excision repair (NER) pathways, critical to the understanding of ICL repair.

Drosophila homologs of FANCD2 and FANCL function in DNA repair

Fanconi anemia (FA) is a genetically heterogeneous disease characterized by developmental defects, progressive bone marrow failure and cancer susceptibility. Cells derived from patients with FA show spontaneous chromosomal aberrations and hypersensitivity to cross-linking agents, indicating a cellular defect in DNA repair. Among the 12 FA genes, only FANCD2, FANCL and FANCM have Drosophila homologs. Given this difference between the human and Drosophila FA pathways, it is unknown whether the fly homologs function in DNA repair. Here, we report that knockdown of Drosophila FANCD2 or FANCL leads to specific hypersensitivity to cross-linking agents. Further analysis revealed that FANCD2 and FANCL function in a linear pathway with FANCL being necessary for the monoubiquitination of FANCD2. FANCD2 mutants also exhibited the same defect in the ionizing radiation-inducible S-phase checkpoint that is seen in mammalian cells deficient for this gene. Finally, in an assay for inactivating mutations, FANCD2 mutants have an elevated mutation rate in response to nitrogen mustard, indicating that these flies are hypermutable. Taken together, these data demonstrate that Drosophila FANCD2 and FANCL play a critical role in DNA repair. Because of the lack of other FA genes, further studies will determine whether the conserved FA genes function as the minimal machinery or whether additional genes are involved in the Drosophila FA pathway.

FANCD2 associated with sporadic breast cancer risk

Several components of the Fanconi anaemia (FA) family of proteins allow the formation of the DNA repair complex foci formed by proteins such as BRCA1/2 and RAD51. Because the genes that participate in the DNA repair pathway have been described as low-penetrance breast cancer susceptibility genes, we postulated that variants in FA genes could also be associated with sporadic breast cancer risk. We studied seven SNPs in FANCA, FANCL and FANCD2 in a total of 897 consecutive and non-related sporadic breast cancer cases and 1033 unaffected controls from the Spanish population. We observed a statistically significant association with sporadic breast cancer for the variant rs2272125 (L1366L) located on FANCD2 (OR per allele=1.35; 95% C.I. 1.09-1.67; P=0.005). Both haplotype and diplotype analyses confirmed this association, where one haplotype and pooled diplotypes carrying it were associated with more than 4-fold risk (P=0.007 and P=0.006, respectively). Screening for potential causal variants in FANCD2 was performed, detecting one in the putative promoter region, which is located in a phylogenetically conserved motif with consensus binding sites for some transcriptional factors, suggesting a functional implication. Our data indicate that a relationship between FANCD2 and sporadic breast cancer risk may exist.

Non-homologous end-joining defect in fanconi anemia hematopoietic cells exposed to ionizing radiation

Fanconi anemia is a genetically heterogeneous recessive disease characterized mainly by bone marrow failure and cancer predisposition. Although it is accepted that Fanconi cells are highly sensitive to DNA crosslinking agents, their response to ionizing radiation is still unclear. Using pulsed-field gel electrophoresis, we have observed that radiation generates a similar number of DNA double-strand breaks in normal and Fanconi cells from three (FA-A, FA-C and FA-F) of the 11 complementation groups identified. Nonsynchronized as well as nonproliferating Fanconi anemia cells showed an evident defect in rejoining the double-strand breaks generated by ionizing radiation, indicating defective non-homologous end-joining repair. At the cellular level, no difference in the radiosensitivity of normal and FA-A lymphoblast cells was noted, and a modest increase in the radiosensitivity of Fanca-/- hematopoietic progenitor cells was observed compared to Fanca+/+ cells. Finally, when animals were exposed to a fractionated total-body irradiation of 5 Gy, a similar hematopoietic syndrome was observed in wild-type and Fanca-/- mice. Taken together, our observations suggest that Fanconi cells, in particular those having nonfunctional Fanconi proteins upstream of FANCD2, have a defect in the non-homologous end-joining repair of double-strand breaks produced by ionizing radiation, and that compensatory mechanisms of DNA repair and/or stem cell regeneration should limit the impact of this defect in irradiated organisms.

FANCC, FANCE, and FANCD2 form a ternary complex essential to the integrity of the Fanconi anemia DNA damage response pathway

Fanconi anemia (FA) is a genetically heterogeneous disorder characterized by bone marrow failure, cancer predisposition, and increased cellular sensitivity to DNA-cross-linking agents. The products of seven of the nine identified FA genes participate in a protein complex required for monoubiquitination of the FANCD2 protein. Direct interaction of the FANCE protein with both fellow FA complex component FANCC and the downstream FANCD2 protein has been observed in the yeast two-hybrid system. Here, we demonstrate the ability of FANCE to mediate the interaction between FANCC and FANCD2 in the yeast three-hybrid system and confirm the FANCE-mediated association of FANCC with FANCD2 in human cells. A yeast two-hybrid system-based screen was devised to identify randomly mutagenized FANCE proteins capable of interaction with FANCC but not with FANCD2. Exogenous expression of these mutants in an FA-E cell line and subsequent evaluation of FANCD2 monoubiquitination and DNA cross-linker sensitivity indicated a critical role for the FANCE/FANCD2 interaction in maintaining FA pathway integrity. Three-hybrid experiments also demonstrated the ability of FANCE to mediate the interaction between FA core complex components FANCC and FANCF, indicating an additional role for FANCE in complex assembly. Thus, FANCE is shown to be a key mediator of protein interactions both in the architecture of the FA protein complex and in the connection of complex components to the putative downstream targets of complex activity.