Functional activity of the fanconi anemia protein FAA requires FAC binding and nuclear localization

HROB Is Implicated in DNA Replication

DNA replication represents a series of precisely regulated events performed by a complex protein machinery that guarantees accurate duplication of the genetic information. Since DNA replication is permanently faced by a variety of exogenous and endogenous stressors, DNA damage response, repair and replication must be closely coordinated to maintain genomic integrity. HROB has been identified recently as a binding partner and activator of the Mcm8/9 helicase involved in DNA interstrand crosslink (ICL) repair. We identified HROB independently as a nuclear protein whose expression is co-regulated with various DNA replication factors. Accordingly, the HROB protein level showed a maximum in S phase and a downregulation in quiescence. Structural prediction and homology searches revealed that HROB is a largely intrinsically disordered protein bearing a helix-rich region and a canonical oligonucleotide/oligosaccharide-binding-fold motif that originated early in eukaryotic evolution. Employing a flow cytometry Förster resonance energy transfer (FRET) assay, we detected associations between HROB and proteins of the DNA replication machinery. Moreover, ectopic expression of HROB protein led to an almost complete shutdown of DNA replication. The available data imply a function for HROB during DNA replication across barriers such as ICLs.

Reversion from basal histone H4 hypoacetylation at the replication fork increases DNA damage in FANCA deficient cells

The FA/BRCA pathway safeguards DNA replication by repairing interstrand crosslinks (ICL) and maintaining replication fork stability. Chromatin structure, which is in part regulated by histones posttranslational modifications (PTMs), has a role in maintaining genomic integrity through stabilization of the DNA replication fork and promotion of DNA repair. An appropriate balance of PTMs, especially acetylation of histones H4 in nascent chromatin, is required to preserve a stable DNA replication fork. To evaluate the acetylation status of histone H4 at the replication fork of FANCA deficient cells, we compared histone acetylation status at the DNA replication fork of isogenic FANCA deficient and FANCA proficient cell lines by using accelerated native immunoprecipitation of nascent DNA (aniPOND) and in situ protein interactions in the replication fork (SIRF) assays. We found basal hypoacetylation of multiple residues of histone H4 in FA replication forks, together with increased levels of Histone Deacetylase 1 (HDAC1). Interestingly, high-dose short-term treatment with mitomycin C (MMC) had no effect over H4 acetylation abundance at the replication fork. However, chemical inhibition of histone deacetylases (HDAC) with Suberoylanilide hydroxamic acid (SAHA) induced acetylation of the FANCA deficient DNA replication forks to levels comparable to their isogenic control counterparts. This forced permanence of acetylation impacted FA cells homeostasis by inducing DNA damage and promoting G2 cell cycle arrest. Altogether, this caused reduced RAD51 foci formation and increased markers of replication stress, including phospho-RPA-S33. Hypoacetylation of the FANCA deficient replication fork, is part of the cellular phenotype, the perturbation of this feature by agents that prevent deacetylation, such as SAHA, have a deleterious effect over the delicate equilibrium they have reached to perdure despite a defective FA/BRCA pathway.

Histone deacetylation regulates nucleotide excision repair through an interaction with the XPC protein

The XPC protein complex plays a central role in DNA lesion recognition for global genome nucleotide excision repair (GG-NER). Lesion recognition can be accomplished in either a UV-DDB-dependent or -independent manner; however, it is unclear how these sub-pathways are regulated in chromatin. Here, we show that histone deacetylases 1 and 2 facilitate UV-DDB-independent recruitment of XPC to DNA damage by inducing histone deacetylation. XPC localizes to hypoacetylated chromatin domains in a DNA damage-independent manner, mediated by its structurally disordered middle (M) region. The M region interacts directly with the N-terminal tail of histone H3, an interaction compromised by H3 acetylation. Although the M region is dispensable for in vitro NER, it promotes DNA damage removal by GG-NER in vivo, particularly in the absence of UV-DDB. We propose that histone deacetylation around DNA damage facilitates the recruitment of XPC through the M region, contributing to efficient lesion recognition and initiation of GG-NER.

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Histone deacetylation by HDAC1/2 promotes the DNA lesion recognition by XPC

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The HDAC1/2 activators, MTA proteins, also promote the recruitment of XPC

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XPC tends to localize in hypoacetylated chromatin independently of DNA damage

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Disordered middle region of XPC interacts with histone H3 tail and promotes GG-NER

MRE11-dependent instability in mitochondrial DNA fork protection activates a cGAS immune signaling pathway

Mitochondrial DNA (mtDNA) instability activates cGAS-dependent innate immune signaling by unknown mechanisms. Here, we find that Fanconi anemia suppressor genes are acting in the mitochondria to protect mtDNA replication forks from instability. Specifically, Fanconi anemia patient cells show a loss of nascent mtDNA through MRE11 nuclease degradation. In contrast to DNA replication fork stability, which requires pathway activation by FANCD2-FANCI monoubiquitination and upstream FANC core complex genes, mitochondrial replication fork protection does not, revealing a mechanistic and genetic separation between mitochondrial and nuclear genome stability pathways. The degraded mtDNA causes hyperactivation of cGAS-dependent immune signaling resembling the unphosphorylated ISG3 response. Chemical inhibition of MRE11 suppresses this innate immune signaling, identifying MRE11 as a nuclease responsible for activating the mtDNA-dependent cGAS/STING response. Collective results establish a previously unknown molecular pathway for mtDNA replication stability and reveal a molecular handle to control mtDNA-dependent cGAS activation by inhibiting MRE11 nuclease.

In silico study of missense variants of FANCA, FANCC and FANCG genes reveals high risk deleterious alleles predisposing to Fanconi anemia pathogenesis

Among the 22 Fanconi anemia (FA) reported genes, 90% of mutational spectra were found in three genes, namely FANCA (64%), FANCC (12%) and FANCG (8%). Therefore, this study aimed to identify the high-risk deleterious variants in three selected genes (FANCA, FANCC, and FANCG) through various computational approaches. The missense variant datasets retrieved from the UCSC genome browser were analyzed for their pathogenicity, stability, and phylogenetic conservancy. A total of 23 alterations, of which 16 in FANCA, 6 in FANCC and one variant in FANCG, were found to be highly deleterious. The native and mutant structures were generated, which demonstrated a profound impact on the respective proteins. Besides, their pathway analysis predicted many other pathways in addition to the Fanconi anemia pathway, homologous recombination, and mismatch repair pathways. Hence, this is the first comprehensive study that can be useful for understanding the genetic signatures in the development of FA.

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.

Esophageal cancer as initial presentation of Fanconi anemia in patients with a hypomorphic FANCA variant

Fanconi anemia (FA) is a clinically heterogenous and genetically diverse disease with 22 known complementation groups (FA-A to FA-W), resulting from the inability to repair DNA interstrand cross-links. This rare disorder is characterized by congenital defects, bone marrow failure, and cancer predisposition. FANCA is the most commonly mutated gene in FA and a variety of mostly private mutations have been documented, including small and large indels and point and splicing variants. Genotype-phenotype associations in FA are complex, and a relationship between particular FANCA variants and the observed cellular phenotype or illness severity remains unclear. In this study, we describe two siblings with compound heterozygous FANCA variants (c.3788_3790delTCT and c.4199G > A) who both presented with esophageal squamous cell carcinoma at the age of 51. The proband came to medical attention when he developed pancytopenia after a single cycle of low-dose chemotherapy including platinum-based therapy. Other than a minor thumb abnormality, neither patient had prior findings to suggest FA, including normal blood counts and intact fertility. Patient fibroblasts from both siblings display increased chromosomal breakage and hypersensitivity to interstrand cross-linking agents as seen in typical FA. Based on our functional data demonstrating that the c.4199G > A/p.R1400H variant represents a hypomorphic FANCA allele, we conclude that the residual activity of the Fanconi anemia repair pathway accounts for lack of spontaneous bone marrow failure or infertility with the late presentation of malignancy as the initial disease manifestation. This and similar cases of adult-onset esophageal cancer stress the need for chromosome breakage testing in patients with early onset of aerodigestive tract squamous cell carcinomas before platinum-based therapy is initiated.

Structural basis of the fanconi anemia-associated mutations within the FANCA and FANCG complex

Monoubiquitination of the Fanconi anemia complementation group D2 (FANCD2) protein by the FA core ubiquitin ligase complex is the central event in the FA pathway. FANCA and FANCG play major roles in the nuclear localization of the FA core complex. Mutations of these two genes are the most frequently observed genetic alterations in FA patients, and most point mutations in FANCA are clustered in the C-terminal domain (CTD). To understand the basis of the FA-associated FANCA mutations, we determined the cryo-electron microscopy (EM) structures of Xenopus laevis FANCA alone at 3.35 Å and 3.46 Å resolution and two distinct FANCA–FANCG complexes at 4.59 and 4.84 Å resolution, respectively. The FANCA CTD adopts an arc-shaped solenoid structure that forms a pseudo-symmetric dimer through its outer surface. FA- and cancer-associated point mutations are widely distributed over the CTD. The two different complex structures capture independent interactions of FANCG with either FANCA C-terminal HEAT repeats, or the N-terminal region. We show that mutations that disturb either of these two interactions prevent the nuclear localization of FANCA, thereby leading to an FA pathway defect. The structure provides insights into the function of FANCA CTD, and provides a framework for understanding FA- and cancer-associated mutations.

Fanconi-BRCA pathway mutations in childhood T-cell acute lymphoblastic leukemia

BRCA2 (also known as FANCD1) is a core component of the Fanconi pathway and suppresses transformation of immature T-cells in mice. However, the contribution of Fanconi-BRCA pathway deficiency to human T-cell acute lymphoblastic leukemia (T-ALL) remains undefined. We identified point mutations in 9 (23%) of 40 human T-ALL cases analyzed, with variant allele fractions consistent with heterozygous mutations early in tumor evolution. Two of these mutations were present in remission bone marrow specimens, suggesting germline alterations. BRCA2 was the most commonly mutated gene. The identified Fanconi-BRCA mutations encode hypomorphic or null alleles, as evidenced by their inability to fully rescue Fanconi-deficient cells from chromosome breakage, cytotoxicity and/or G2/M arrest upon treatment with DNA cross-linking agents. Disabling the tumor suppressor activity of the Fanconi-BRCA pathway is generally thought to require biallelic gene mutations. However, all mutations identified were monoallelic, and most cases appeared to retain expression of the wild-type allele. Using isogenic T-ALL cells, we found that BRCA2 haploinsufficiency induces selective hypersensitivity to ATR inhibition, in vitro and in vivo. These findings implicate Fanconi-BRCA pathway haploinsufficiency in the molecular pathogenesis of T-ALL, and provide a therapeutic rationale for inhibition of ATR or other druggable effectors of homologous recombination.

Identification of rare heterozygous missense mutations in FANCA in esophageal atresia patients using next-generation sequencing

Esophageal atresia and tracheoesophageal fistula (EA/TEF) are relatively common malformations in newborns, but the etiology of EA/TEF remains unknown. Fanconi anemia (FA) complementation group A (FANCA) is a key component of the FA core complex and is essential for the activation of the DNA repair pathway. The middle region (amino acids 674-1208) of FANCA is required for its interaction with FAAP20. We performed targeted sequencing of this binding region of FANCA (exons 23-36) in 40 EA/TEF patients. We also investigated the effect of the p.A958V mutation on the protein-protein interaction between FANCA and FAAP20 using an in vitro binding assay and co-immunoprecipitation. Immunolocalization analysis was performed to investigate the subcellular localization of FANCA, and tissue sections and immunohistochemistry were used to explore the expression of FANCA. We identified four rare missense variants in the FANCA binding region. FANCA mutations were significantly overrepresented in EA/TEF patients compared with 4300 control subjects from the NHLBI-ESP project (Fisher's exact p = 2.17 × 10^-5, odds ratio = 31.75). p.A958V, a novel de novo mutation in the FANCA gene, was identified in one patient with EA/TEF. We provide further evidence that the p.A958V mutation reduces the binding affinity of FANCA for FAAP20. Interestingly, the p.A958V mutation impaired the nuclear localization of the FANCA protein expressed in HeLa cells. We found that FANCA was more highly expressed in stratified squamous epithelium than in smooth muscle. In conclusion, mutations in the FANCA gene are associated with EA/TEF in humans.

Amelioration of Head and Neck Radiation-Induced Mucositis and Distant Marrow Suppression in Fanca-/- and Fancg-/- Mice by Intraoral Administration of GS-Nitroxide (JP4-039)

Squamous cell carcinomas of the head and neck are appearing with increased frequency in both marrow transplanted and non-transplanted Fanconi anemia (FA) patients. FA patients commonly display radiosensitivity of epithelial tissues, complicating effective radiotherapy. Fancd2^-/- mice (C57BL/6J and 129/Sv background) demonstrate epithelial tissue sensitivity to single-fraction or fractionated irradiation to the head and neck and distant marrow suppression (abscopal effect), both ameliorated by intraoral administration of the mitochondrial-targeted antioxidant, GS-nitroxide, JP4-039. We now report that mice of two other FA genotypes, Fancg^-/- (B6) and the most prevalent human genotype Fanca^-/- (129/Sv), also demonstrate: 1. reduced longevity of hematopoiesis in long-term bone marrow cultures; 2. radiosensitivity of bone marrow stromal cell lines; and 3. head and neck radiation-induced severe mucositis and abscopal suppression of distant marrow hematopoiesis. Intraoral administration of JP4-039/F15, but not non-mitochondrial-targeted 4-amino-Tempo/F15 or F15 alone, prior to each radiation treatment ameliorated both local and abscopal radiation effects. Head and neck irradiated TGF-β-resistant SMAD3^-/- (129/Sv) mice and double-knockout SMAD3^-/- Fancd2^-/- (129/Sv) mice treated daily with TGF-β receptor antagonist, LY364947, still displayed abscopal bone marrow suppression, implicating a non-TGF-β mechanism. Thus, amelioration of both local normal tissue radiosensitivity and distant marrow suppression by intraoral administration of JP4-039 in Fancg^-/- and Fanca^-/- mice supports a clinical trial of this locally administered normal tissue radioprotector and mitigator during head and neck irradiation in FA patients.

A comprehensive approach to identification of pathogenic FANCA variants in Fanconi anemia patients and their families

Fanconi anemia (FA) is a rare recessive DNA repair deficiency resulting from mutations in one of at least 22 genes. Two-thirds of FA families harbor mutations in FANCA. To genotype patients in the International Fanconi Anemia Registry (IFAR) we employed multiple methodologies, screening 216 families for FANCA mutations. We describe identification of 57 large deletions and 261 sequence variants, in 159 families. All but seven families harbored distinct combinations of two mutations demonstrating high heterogeneity. Pathogenicity of the 18 novel missense variants was analyzed functionally by determining the ability of the mutant cDNA to improve the survival of a FANCA-null cell line when treated with MMC. Overexpressed pathogenic missense variants were found to reside in the cytoplasm, and nonpathogenic in the nucleus. RNA analysis demonstrated that two variants (c.522G > C and c.1565A > G), predicted to encode missense variants, which were determined to be nonpathogenic by a functional assay, caused skipping of exons 5 and 16, respectively, and are most likely pathogenic. We report 48 novel FANCA sequence variants. Defining both variants in a large patient cohort is a major step toward cataloging all FANCA variants, and permitting studies of genotype-phenotype correlations.

A case report and literature review of Fanconi Anemia (FA) diagnosed by genetic testing

Fanconi anemia (FA) is a genetically heterogeneous rare autosomal recessive disorder characterized by congenital malformations, hematological problems and predisposition to malignancies. The genes that have been found to be mutated in FA patients are called FANC. To date 16 distinct FANC genes have been reported. Among these, mutations in FANCA are the most frequent among FA patients worldwide which account for 60- 65%. In this study, a nine years old male child was brought to our hospital one year ago for opinion and advice. He was the third child born to consanguineous parents. The mutation analyses were performed for proband, parents, elder sibling and the relatives [maternal aunt and maternal aunt’s son (cousin)]. Molecular genetic testing [targeted next-generation sequencing (MiSeq, Illumina method)] was performed by mutation analysis in 15 genes involved. Entire coding exons and their flanking regions of the genes were analysed. Sanger sequencing [(ABI 3730 analyzer by Applied Biosystems)] was performed using primers specific for 43 coding exons of the FANCA gene. A novel splice site mutation, c.3066 + 1G > T, (IVS31 + 1G > T), homozygote was detected by sequencing in the patient. The above sequence variant was identified in heterozygous state in his parents. Further, the above sequence variant was not identified in other family members (elder sibling, maternal aunt and cousin). It is concluded that genetic study should be done if possible in all the cases of suspected FA, including siblings, parents and close blood relatives. It will help us to plan appropriate treatment and also to select suitable donor for hematopoietic stem cell transplantation and to plan for genetic counseling. In addition to the case report, the main focus of this manuscript was to review literature on role of FANCA gene in FA since large number of FANCA mutations and polymorphisms have been identified.

Regulation of the Fanconi anemia pathway by a CUE ubiquitin-binding domain in the FANCD2 protein

The Fanconi anemia (FA)-BRCA pathway is critical for the repair of DNA interstrand crosslinks (ICLs) and the maintenance of chromosome stability. A key step in FA-BRCA pathway activation is the covalent attachment of monoubiquitin to FANCD2 and FANCI. Monoubiquitinated FANCD2 and FANCI localize in chromatin-associated nuclear foci where they interact with several well-characterized DNA repair proteins. Importantly, very little is known about the structure, function, and regulation of FANCD2. Herein, we describe the identification and characterization of a CUE (coupling of ubiquitin conjugation to endoplasmic reticulum degradation) ubiquitin-binding domain (UBD) in FANCD2, and demonstrate that the CUE domain mediates noncovalent binding to ubiquitin in vitro. We show that although mutation of the CUE domain destabilizes FANCD2, the protein remains competent for DNA damage-inducible monoubiquitination and phosphorylation. Importantly, we demonstrate that the CUE domain is required for interaction with FANCI, retention of monoubiquitinated FANCD2, and FANCI in chromatin, and for efficient ICL repair. Our results suggest a model by which heterodimerization of monoubiquitinated FANCD2 and FANCI in chromatin is mediated in part through a noncovalent interaction between the FANCD2 CUE domain and monoubiquitin covalently attached to FANCI, and that this interaction shields monoubiquitinated FANCD2 from polyubiquitination and proteasomal degradation.

A distinct replication fork protection pathway connects Fanconi anemia tumor suppressors to RAD51-BRCA1/2

Genes mutated in patients with Fanconi anemia (FA) interact with the DNA repair genes BRCA1 and BRCA2/FANCD1 to suppress tumorigenesis, but the molecular functions ascribed to them cannot fully explain all of their cellular roles. Here, we show a repair-independent requirement for FA genes, including FANCD2, and BRCA1 in protecting stalled replication forks from degradation. Fork protection is surprisingly rescued in FANCD2-deficient cells by elevated RAD51 levels or stabilized RAD51 filaments. Moreover, FANCD2-mediated fork protection is epistatic with RAD51 functions, revealing an unanticipated fork protection pathway that connects FA genes to RAD51 and the BRCA1/2 breast cancer suppressors. Collective results imply a unified molecular mechanism for repair-independent functions of FA, RAD51, and BRCA1/2 proteins in preventing genomic instability and suppressing tumorigenesis.

Towards a molecular understanding of the fanconi anemia core complex

Fanconi Anemia (FA) is a genetic disorder characterized by the inability of patient cells to repair DNA damage caused by interstrand crosslinking agents. There are currently 14 verified FA genes, where mutation of any single gene prevents repair of DNA interstrand crosslinks (ICLs). The accumulation of ICL damage results in genome instability and patients having a high predisposition to cancers. The key event of the FA pathway is dependent on an eight-protein core complex (CC), required for the monoubiquitination of each member of the FANCD2-FANCI complex. Interestingly, the majority of patient mutations reside in the CC. The molecular mechanisms underlying the requirement for such a large complex to carry out a monoubiquitination event remain a mystery. This paper documents the extensive efforts of researchers so far to understand the molecular roles of the CC proteins with regard to its main function in the FA pathway, the monoubiquitination of FANCD2 and FANCI.

Fanconi anemia proteins and their interacting partners: a molecular puzzle

In recent years, Fanconi anemia (FA) has been the subject of intense investigations, primarily in the DNA repair research field. Many discoveries have led to the notion of a canonical pathway, termed the FA pathway, where all FA proteins function sequentially in different protein complexes to repair DNA cross-link damages. Although a detailed architecture of this DNA cross-link repair pathway is emerging, the question of how a defective DNA cross-link repair process translates into the disease phenotype is unresolved. Other areas of research including oxidative metabolism, cell cycle progression, apoptosis, and transcriptional regulation have been studied in the context of FA, and some of these areas were investigated before the fervent enthusiasm in the DNA repair field. These other molecular mechanisms may also play an important role in the pathogenesis of this disease. In addition, several FA-interacting proteins have been identified with roles in these “other” nonrepair molecular functions. Thus, the goal of this paper is to revisit old ideas and to discuss protein-protein interactions related to other FA-related molecular functions to try to give the reader a wider perspective of the FA molecular puzzle.

Fanconi anemia complementation group A (FANCA) protein has intrinsic affinity for nucleic acids with preference for single-stranded forms

The Fanconi anemia complementation group A (FANCA) gene is one of 15 disease-causing genes and has been found to be mutated in ∼60% of Fanconi anemia patients. Using purified protein, we report that human FANCA has intrinsic affinity for nucleic acids. FANCA binds to both single-stranded (ssDNA) and double-stranded (dsDNA) DNAs; however, its affinity for ssDNA is significantly higher than for dsDNA in an electrophoretic mobility shift assay. FANCA also binds to RNA with an intriguingly higher affinity than its DNA counterpart. FANCA requires a certain length of nucleic acids for optimal binding. Using DNA and RNA ladders, we determined that the minimum number of nucleotides required for FANCA recognition is ∼30 for both DNA and RNA. By testing the affinity between FANCA and a variety of DNA structures, we found that a 5'-flap or 5'-tail on DNA facilitates its interaction with FANCA. A patient-derived FANCA truncation mutant (Q772X) has diminished affinity for both DNA and RNA. In contrast, the complementing C-terminal fragment of Q772X, C772-1455, retains the differentiated nucleic acid-binding activity (RNA > ssDNA > dsDNA), indicating that the nucleic acid-binding domain of FANCA is located primarily at its C terminus, where most disease-causing mutations are found.

GS-nitroxide (JP4-039)-mediated radioprotection of human Fanconi anemia cell lines

Fanconi anemia (FA) is an inherited disorder characterized by defective DNA repair and cellular sensitivity to DNA crosslinking agents. Clinically, FA is associated with high risk for marrow failure, leukemia and head and neck squamous cell carcinoma (HNSCC). Radiosensitivity in FA patients compromises the use of total-body irradiation for hematopoietic stem cell transplantation and radiation therapy for HNSCC. A radioprotector for the surrounding tissue would therefore be very valuable during radiotherapy for HNSCC. Clonogenic radiation survival curves were determined for pre- or postirradiation treatment with the parent nitroxide Tempol or JP4-039 in cells of four FA patient-derived cell lines and two transgene-corrected subclonal lines. FancG(-/-) (PD326) and FancD2(-/-) (PD20F) patient lines were more sensitive to the DNA crosslinking agent mitomycin C (MMC) than their transgene-restored subclonal cell lines (both P < 0.0001). FancD2(-/-) cells were more radiosensitive than the transgene restored subclonal cell line (ñ = 2.0 ± 0.7 and 4.7 ± 2.2, respectively, P = 0.03). In contrast, FancG(-/-) cells were radioresistant relative to the transgene-restored subclonal cell line (ñ = 9.4 ± 1.5 and 2.2 ± 05, respectively, P = 0.001). DNA strand breaks measured by the comet assay correlated with radiosensitivity. Cell lines from a Fanc-C and Fanc-A patients showed radiosensitivity similar to that of Fanc-D2(-/-) cells. A fluorophore-tagged JP4-039 (BODIPY-FL) analog targeted the mitochondria of the cell lines. Preirradiation or postirradiation treatment with JP4-039 at a lower concentration than Tempol significantly increased the radioresistance and stabilized the antioxidant stores of all cell lines. Tempol increased the toxicity of MMC in FancD2(-/-) cells. These data provide support for the potential clinical use of JP4-039 for normal tissue radioprotection during chemoradiotherapy in FA patients.

Numerical chromosomal changes and risk of development of myelodysplastic syndrome--acute myeloid leukemia in patients with Fanconi anemia

Fanconi Anemia (FA) is an inherited bone marrow failure syndrome characterized by congenital abnormalities, progressive marrow failure and predisposition to myelodysplastic syndrome (MDS), acute myeloid leukemia (AML), and solid tumors. The most common acquired chromosomal aberrations in FA patients are trisomy of 1q and monosomy of chromosome 7; the latter is known to be associated with poor prognosis. A few reports also suggest that gains of 3q are associated with progression to MDS-AML and overall poor prognosis. It is not uncommon for patients with Fanconi anemia to have easily detectable (oligoclonal) chromosomal alterations in their still normal (nonmalignant) marrow, which makes it even more challenging to determine the import of such alterations. We conducted a retrospective longitudinal analysis of fluorescent in situ hybridization (FISH) analysis for gains in 1q and 3q and for monosomy 7 and 7q deletions on 212 bone marrow samples from 77 children with FA treated at our institution between 1987 and 2007. Given the baseline increased chromosomal instability and defective DNA repair in patients with FA, which leads to unbalanced chromosomal aberrations such as deletions, insertions, and translocations, for the purpose of this analysis an abnormal clone was defined as ≥10% abnormal cells. Chromosome 3 and 7 aberrations were associated with increased risk of developing MDS-AML (P = 0.019 and P < 0.001 respectively), although the significance of chromosome 3 aberrations disappeared when different observation times were accounted for. Gain of 1q alone did not predict development of MDS-AML. In conclusion, children with FA should be followed closely with FISH analyses, because some of the clonal chromosomal abnormalities may be early indicators of progression toward MDS-AML and thus also of the need for hematopoietic stem cell transplantation.

Patient-derived C-terminal mutation of FANCI causes protein mislocalization and reveals putative EDGE motif function in DNA repair

Fanconi anemia (FA) is a rare familial genome instability syndrome caused by mutations in FA genes that results in defective DNA crosslink repair. Activation of the FA pathway requires the FA core ubiquitin ligase complex-dependent monoubiquitination of 2 interacting FA proteins, FANCI and FANCD2. Although loss of either FANCI or FANCD2 is known to prevent monoubiquitination of its respective partner, it is unclear whether FANCI has any additional domains that may be important in promoting DNA repair, independent of its monoubiquitination. Here, we focus on an FA-I patient-derived FANCI mutant protein, R1299X (deletion of 30 residues from its C-terminus), to characterize important structural region(s) in FANCI that is required to activate the FA pathway. We show that, within this short 30 amino acid stretch contains 2 separable functional signatures, a nuclear localization signal and a putative EDGE motif, that is critical for the ability of FANCI to properly monoubiquitinate FANCD2 and promote DNA crosslink resistance. Our study enable us to conclude that, although proper nuclear localization of FANCI is crucial for robust FANCD2 monoubiquitination, the putative FANCI EDGE motif is important for DNA crosslink repair.

Bone marrow transplantation for inherited bone marrow failure syndromes

The inherited bone marrow failure (BMF) syndromes are characterized by impaired hematopoiesis and cancer predisposition. Most inherited BMF syndromes are also associated with a range of congenital anomalies. Progress in improving the outcomes for children with inherited BMF syndromes has been limited by the rarity of these disorders, as well as disease-specific genetic, molecular, cellular, and clinical characteristics that increase the risks of complications associated with hematopoietic stem cell transplantation (HSCT). As a result, the ability to develop innovative transplant approaches to circumvent these problems has been limited. Recent progress has been made, as best evidenced in studies adding fludarabine to the preparative regimen for children undergoing unrelated donor HSCT for Fanconi anemia. The rarity of these diseases coupled with the far more likely incremental improvements that will result from ongoing research will require prospective international clinical trials to improve the outcome for these children.

Functional interaction between the Fanconi Anemia D2 protein and proliferating cell nuclear antigen (PCNA) via a conserved putative PCNA interaction motif

Fanconi Anemia (FA) is a rare recessive disease characterized by congenital abnormalities, bone marrow failure, and cancer susceptibility. The FA proteins and the familial breast cancer susceptibility gene products, BRCA1 and FANCD1/BRCA2, function cooperatively in the FA-BRCA pathway to repair damaged DNA and to prevent cellular transformation. Activation of this pathway occurs via the mono-ubiquitination of the FANCD2 protein, targeting it to nuclear foci where it co-localizes with FANCD1/BRCA2, RAD51, and PCNA. The regulation of the mono-ubiquitination of FANCD2, as well as its function in DNA repair remain poorly understood. In this study, we have further characterized the interaction between the FANCD2 and PCNA proteins. We have identified a highly conserved, putative FANCD2 PCNA interaction motif (PIP-box), and demonstrate that mutation of this motif disrupts FANCD2-PCNA binding and precludes the mono-ubiquitination of FANCD2. Consequently, the FANCD2 PIP-box mutant protein fails to correct the mitomycin C hypersensitivity of FA-D2 patient cells. Our results suggest that PCNA may function as a molecular platform to facilitate the mono-ubiquitination of FANCD2 and activation of the FA-BRCA pathway.

CHK1 inhibition as a strategy for targeting Fanconi Anemia (FA) DNA repair pathway deficient tumors

Background

DNA repair deficient tumor cells have been shown to accumulate high levels of DNA damage. Consequently, these cells become hyper-dependent on DNA damage response pathways, including the CHK1-kinase-mediated response. These observations suggest that DNA repair deficient tumors should exhibit increased sensitivity to CHK1 inhibition. Here we offer experimental evidence in support of this hypothesis.

Results

Using isogenic pairs of cell lines differing only in the Fanconi Anemia (FA) DNA repair pathway, we showed that FA deficient cell lines were hypersensitive to CHK1 silencing by independent siRNAs as well as CHK1 pharmacologic inhibition by Gö6976 and UCN-01. In parallel, an siRNA screen designed to identify gene silencings synthetically lethal with CHK1 inhibition identified genes required for FA pathway function. To confirm these findings in vivo, we demonstrated that whole zebrafish embryos, depleted for FANCD2 by a morpholino approach, were hypersensitive to Gö6976. Silencing of FA genes led to hyper-activation of CHK1 and vice versa. Furthermore, inactivation of CHK1 in FA deficient cell lines caused increased accumulation of DNA strand and chromosomal breakages. These results suggest that the functions subserved by CHK1 and the FA pathway mutually compensate in maintaining genome integrity. As CHK1 inhibition has been under clinical trial in combination with cisplatin, we showed that the FA specific tumoricidal effect of CHK1 inhibition and cisplatin was synergistic.

Conclusion

Taken together, these results suggest CHK1 inhibition as a strategy for targeting FA deficient tumors.

ATR-dependent phosphorylation of FANCA on serine 1449 after DNA damage is important for FA pathway function

Previous work has shown several proteins defective in Fanconi anemia (FA) are phosphorylated in a functionally critical manner. FANCA is phosphorylated after DNA damage and localized to chromatin, but the site and significance of this phosphorylation are unknown. Mass spectrometry of FANCA revealed one phosphopeptide, phosphorylated on serine 1449. Serine 1449 phosphorylation was induced after DNA damage but not during S phase, in contrast to other posttranslational modifications of FA proteins. Furthermore, the S1449A mutant failed to completely correct a variety of FA-associated phenotypes. The DNA damage response is coordinated by phosphorylation events initiated by apical kinases ATM (ataxia telangectasia mutated) and ATR (ATM and Rad3-related), and ATR is essential for proper FA pathway function. Serine 1449 is in a consensus ATM/ATR site, phosphorylation in vivo is dependent on ATR, and ATR phosphorylated FANCA on serine 1449 in vitro. Phosphorylation of FANCA on serine 1449 is a DNA damage-specific event that is downstream of ATR and is functionally important in the FA pathway.

Hepatitis B virus X protein disrupts DNA interstrand crosslinking agent mitomycin C induced ATR dependent intra-S-phase checkpoint

Chronic infection of hepatitis B virus (HBV) is one of the major causes of hepatocellular carcinoma (HCC) in the world. The hepatitis B virus X protein (HBx) is implicated in HCC development, although its oncogenic role remains controversial. HBx is a multifunctional regulator that modulates transcription, signal transduction, cell cycle progress, and DNA repair by directly or indirectly interacting with host factors. We constructed the HBx stably expressing HepG2 cell line to investigate the impact of HBx on intra-S-phase checkpoint induced by mitomycin C (MMC). The HBx transformed HepG2 cells are more sensitive to MMC treatment and showed defective radioresistant DNA synthesis compared to the control cell line transformed with empty vector. With DNA content assay, HBx transformed cells showed defective S phase arrest and a consequent G2/M arrest after MMC treatment. HBx impaired the ATR dependent phosphorylation of Chk1 and monoubiquitination of FANCD2. Overexpression of ATR reverted the MMC induced phenotype of Chk1 and FANCD2 in HBx transformed cells. The defect of intra-S-phase checkpoint resulted in accumulation of genomic instability. In conclusion, HBx disrupts intra-S-phase checkpoint induced by MMC through ATR-Chk1 and ATR-FANCD2 pathways.

Secondary mutations as a mechanism of cisplatin resistance in BRCA2-mutated cancers

Ovarian carcinomas with mutations in the tumour suppressor BRCA2 are particularly sensitive to platinum compounds. However, such carcinomas ultimately develop cisplatin resistance. The mechanism of that resistance is largely unknown. Here we show that acquired resistance to cisplatin can be mediated by secondary intragenic mutations in BRCA2 that restore the wild-type BRCA2 reading frame. First, in a cisplatin-resistant BRCA2-mutated breast-cancer cell line, HCC1428, a secondary genetic change in BRCA2 rescued BRCA2 function. Second, cisplatin selection of a BRCA2-mutated pancreatic cancer cell line, Capan-1 (refs 3, 4), led to five different secondary mutations that restored the wild-type BRCA2 reading frame. All clones with secondary mutations were resistant both to cisplatin and to a poly(ADP-ribose) polymerase (PARP) inhibitor (AG14361). Finally, we evaluated recurrent cancers from patients whose primary BRCA2-mutated ovarian carcinomas were treated with cisplatin. The recurrent tumour that acquired cisplatin resistance had undergone reversion of its BRCA2 mutation. Our results suggest that secondary mutations that restore the wild-type BRCA2 reading frame may be a major clinical mediator of acquired resistance to platinum-based chemotherapy.

Fanconi anemia pathway-deficient tumor cells are hypersensitive to inhibition of ataxia telangiectasia mutated

The Fanconi anemia (FA) pathway maintains genomic stability in replicating cells. Some sporadic breast, ovarian, pancreatic, and hematological tumors are deficient in FA pathway function, resulting in sensitivity to DNA-damaging agents. FA pathway dysfunction in these tumors may result in hyperdependence on alternative DNA repair pathways that could be targeted as a treatment strategy. We used a high-throughput siRNA screening approach that identified ataxia telangiectasia mutated (ATM) as a critical kinase for FA pathway-deficient human fibroblasts. Human fibroblasts and murine embryonic fibroblasts deficient for the FA pathway were observed to have constitutive ATM activation and Fancg(-/-)Atm(-/-) mice were found to be nonviable. Abrogation of ATM function in FA pathway-deficient cells resulted in DNA breakage, cell cycle arrest, and apoptotic cell death. Moreover, Fanconi anemia complementation group G- (FANCG-) and FANCC-deficient pancreatic tumor lines were more sensitive to the ATM inhibitor KU-55933 than isogenic corrected lines. These data suggest that ATM and FA genes function in parallel and compensatory roles to maintain genomic integrity and cell viability. Pharmaceutical inhibition of ATM may have a role in the treatment of FA pathway-deficient human cancers.

Hsp90 regulates the Fanconi anemia DNA damage response pathway

Heat shock protein 90 (Hsp90) regulates diverse signaling pathways. Emerging evidence suggests that Hsp90 inhibitors, such as 17-allylamino-17-demethoxygeldanamycin (17-AAG), enhance DNA damage-induced cell death, suggesting that Hsp90 may regulate cellular responses to genotoxic stress. However, the underlying mechanisms are poorly understood. Here, we show that the Fanconi anemia (FA) pathway is involved in the Hsp90-mediated regulation of genotoxic stress response. In the FA pathway, assembly of 8 FA proteins including FANCA into a nuclear multiprotein complex, and the complex-dependent activation of FANCD2 are critical events for cellular tolerance against DNA cross-linkers. Hsp90 associates with FANCA, in vivo and in vitro, in a 17-AAG-sensitive manner. Disruption of the FANCA/Hsp90 association by cellular treatment with 17-AAG induces rapid proteasomal degradation and cytoplasmic relocalization of FANCA, leading to impaired activation of FANCD2. Furthermore, 17-AAG promotes DNA cross-linker-induced cytotoxicity, but this effect is much less pronounced in FA pathway-defective cells. Notably, 17-AAG enhances DNA cross-linker-induced chromosome aberrations. In conclusion, our results identify FANCA as a novel client of Hsp90, suggesting that Hsp90 promotes activation of the FA pathway through regulation of intracellular turnover and trafficking of FANCA, which is critical for cellular tolerance against genotoxic stress.

Fanconi anemia A is a nucleocytoplasmic shuttling molecule required for gonadotropin-releasing hormone (GnRH) transduction of the GnRH receptor

GnRH binds its cognate G protein-coupled GnRH receptor (GnRHR) located on pituitary gonadotropes and drives expression of gonadotropin hormones. There are two gonadotropin hormones, comprised of a common alpha- and hormone-specific beta-subunit, which are required for gonadal function. Recently we identified that Fanconi anemia a (Fanca), a DNA damage repair gene, is differentially expressed within the LbetaT2 gonadotrope cell line in response to stimulation with GnRH. FANCA is mutated in more than 60% of cases of Fanconi anemia (FA), a rare genetically heterogeneous autosomal recessive disorder characterized by bone marrow failure, endocrine tissue cancer susceptibility, and infertility. Here we show that induction of FANCA protein is mediated by the GnRHR and that the protein constitutively adopts a nucleocytoplasmic intracellular distribution pattern. Using inhibitors to block nuclear import and export and a GnRHR antagonist, we demonstrated that GnRH induces nuclear accumulation of FANCA and green fluorescent protein (GFP)-FANCA before exporting back to the cytoplasm using the nuclear export receptor CRM1. Using FANCA point mutations that locate GFP-FANCA to the cytoplasm (H1110P) or functionally uncouple GFP-FANCA (Q1128E) from the wild-type nucleocytoplasmic distribution pattern, we demonstrated that wild-type FANCA was required for GnRH-induced activation of gonadotrope cell markers. Cotransfection of H1110P and Q1128E blocked GnRH activation of the alphaGsu and GnRHR but not the beta-subunit gene promoters. We conclude that nucleocytoplasmic shuttling of FANCA is required for GnRH transduction of the alphaGSU and GnRHR gene promoters and propose that FANCA functions as a GnRH-induced signal transducer.

Cytogenetic instability in ovarian epithelial cells from women at risk of ovarian cancer

Fanconi anemia is an inherited cancer predisposition disease characterized by cytogenetic and cellular hypersensitivity to cross-linking agents. Seeking evidence of Fanconi anemia protein dysfunction in women at risk of ovarian cancer, we screened ovarian surface epithelial cells from 25 primary cultures established from 22 patients using cross-linker hypersensitivity assays. Samples were obtained from (a) women at high risk for ovarian cancer with histologically normal ovaries, (b) ovarian cancer patients, and (c) a control group with no family history of breast or ovarian cancer. In chromosomal breakage assays, all control cells were mitomycin C (MMC) resistant, but eight samples (five of the six high-risk and three of the eight ovarian cancer) were hypersensitive. Lymphocytes from all eight patients were MMC resistant. Only one of the eight patients had a BRCA1 germ-line mutation and none had BRCA2 mutations, but FANCD2 was reduced in five of the eight. Ectopic expression of normal FANCD2 cDNA increased FANCD2 protein and induced MMC resistance in both hypersensitive lines tested. No FANCD2 coding region or promoter mutations were found, and there was no genomic loss or promoter methylation in any Fanconi anemia genes. Therefore, in high-risk women with no BRCA1 or BRCA2 mutations, tissue-restricted hypersensitivity to cross-linking agents is a frequent finding, and chromosomal breakage responses to MMC may be a sensitive screening strategy because cytogenetic instability identified in this way antedates the onset of carcinoma. Inherited mutations that result in tissue-specific FANCD2 gene suppression may represent a cause of familial ovarian cancer.

Structural determinants of human FANCF protein that function in the assembly of a DNA damage signaling complex

Fanconi anemia (FA) is a rare autosomal recessive and X-linked chromosomal instability disorder. At least eight FA proteins (FANCA, B, C, E, F, G, L, and M) form a nuclear core complex required for monoubiquitination of a downstream protein, FANCD2. The human FANCF protein reportedly functions as a molecular adaptor within the FA nuclear complex, bridging between the subcomplexes A:G and C:E. Our x-ray crystallographic studies of the C-terminal domain of FANCF reveal a helical repeat structure similar to the Cand1 regulator of the Cul1-Rbx1-Skp1-Fbox(Skp2) ubiquitin ligase complex. Two C-terminal loops of FANCF are essential for monoubiquitination of FANCD2 and normal cellular resistance to the DNA cross-linking agent mitomycin C. FANCF mutants bearing amino acid substitutions in this C-terminal surface fail to interact with other components of the FA complex, indicating that this surface is critical for the proper assembly of the FA core complex.

Chemosensitization to cisplatin by inhibitors of the Fanconi anemia/BRCA pathway

Cisplatin resistance occurs, at least in part, through the function of the Fanconi anemia (FA)/BRCA pathway, a DNA-damage response pathway required for repair of cisplatin cross-links. In the current study, we designed a cell-based screening strategy to identify small-molecule inhibitors of the FA/BRCA pathway with the hypothesis that such molecules could restore sensitivity to platinum agents. We identified four inhibitors, including three protein kinase inhibitors (wortmannin, H-9, and alsterpaullone) and one natural compound (curcumin) that inhibit the FA/BRCA pathway. We show that curcumin, a compound that is generally regarded as safe, inhibits the monoubiquitination of the FANCD2 protein as predicted by the screen and consequently sensitizes ovarian and breast tumor cell lines to cisplatin through apoptotic cell death. We believe that this study shows an efficient, high-throughput method for identifying new compounds that may sensitize cancer cells to DNA-damaging chemotherapy.

Phosphorylation of FANCD2 on two novel sites is required for mitomycin C resistance

The Fanconi anemia (FA) pathway is a DNA damage-activated signaling pathway which regulates cellular resistance to DNA cross-linking agents. Cloned FA genes and proteins cooperate in this pathway, and monoubiquitination of FANCD2 is a critical downstream event. The cell cycle checkpoint kinase ATR is required for the efficient monoubiquitination of FANCD2, while another checkpoint kinase, ATM, directly phosphorylates FANCD2 and controls the ionizing radiation (IR)-inducible intra-S-phase checkpoint. In the present study, we identify two novel DNA damage-inducible phosphorylation sites on FANCD2, threonine 691 and serine 717. ATR phosphorylates FANCD2 on these two sites, thereby promoting FANCD2 monoubiquitination and enhancing cellular resistance to DNA cross-linking agents. Phosphorylation of the sites is required for establishment of the intra-S-phase checkpoint response. IR-inducible phosphorylation of threonine 691 and serine 717 is also dependent on ATM and is more strongly impaired when both ATM and ATR are knocked down. Threonine 691 is phosphorylated during normal S-phase progression in an ATM-dependent manner. These findings further support the functional connection of ATM/ATR kinases and FANCD2 in the DNA damage response and support a role for the FA pathway in the coordination of the S phase of the cell cycle.

Fanconi Anemia

Fanconi anemia (FA) is a rare hereditary disease characterized by bone marrow failure and developmental anomalies; a high incidence of myelodysplasia (MDS), acute nonlymphocytic leukemia (AML), and solid tumors; and cellular hypersensitivity to cross-linking agents. The genetic basis of FA is mutations in any one of the known FA genes. The function of the proteins is largely unknown, but many form complexes with each other, and in one canonical "pathway," eight of the known FA proteins bind together in a complex and monoubiquitinate FANCD2, a protein not present in the core complex. Monoubiquitinated FANCD2 translocates to damage-induced nuclear foci containing BRCA1, BRCA2, and Rad51, thereby protecting the genome. Because hypersensitivity to genotoxic stress is a feature of all somatic cells, this aspect of FA protein function cannot account for the nearly universal development of bone marrow failure. There is strong in vitro and in vivo evidence that at least some of the FA proteins promote survival signaling pathways in hematopoietic cells by forming complexes with signaling molecules. Because associations with heat shock proteins occur in this context, we suggest that these proteins function as co-chaperones and scaffolds that organize proper responses to a wide variety of extracellular cues, some global, and some specific for hematopoietic cells.

The WD40 Repeats of FANCL Are Required for Fanconi Anemia Core Complex Assembly

Fanconi anemia (FA) is an autosomal recessive disorder characterized by aplastic anemia, cancer susceptibility, and cellular sensitivity to mitomycin C. Eight of the 11 cloned Fanconi anemia gene products (FANCA, -B, -C, -E, -F, -G, -L, and -M) form a multisubunit nuclear complex (FA core complex) required for monoubiquitination of a downstream FA protein, FANCD2. FANCL, which possesses three WD40 repeats and a plant homeodomain (PHD), is the putative E3 ubiquitin ligase subunit of the FA complex. Here, we demonstrate that the WD40 repeats of FANCL are required for interaction with other subunits of the FA complex. The PHD is dispensable for this interaction, although it is required for FANCD2 mono-ubiquitination. The PHD of FANCL also shares sequence similarity to the canonical RING finger of c-CBL, including a conserved tryptophan required for E2 binding by c-CBL. Mutation of this tryptophan in the FANCL PHD significantly impairs in vivo mono-ubiquitination of FANCD2 and in vitro auto-ubiquitination activity, and partially impairs restoration of mitomycin C resistance. We propose a model in which FANCL, via its WD40 region, binds the FA complex and, via its PHD, recruits an as-yet-unidentified E2 for mono-ubiquitination of FANCD2.

Natural gene therapy in monozygotic twins with Fanconi anemia

Monozygotic twin sisters, with nonhematologic symptoms of Fanconi anemia (FA), were discovered to be somatic mosaics for mutations in the FANCA gene. Skin fibroblasts, but not lymphocytes or committed hematopoietic progenitors, were sensitive to DNA cross-linking agents. Molecular analysis revealed, in skin cells of both twins, a frameshift causing deletion in exon 27 (2555deltaT) and an exon 28 missense mutation (2670G>A/R880Q). The latter resulted in primarily cytoplasmic expression and reduced function of the mutant FANCA (R880Q) protein. Surprisingly, the same acquired exon 30 missense change (2927G>A/E966K) was detected in the hematopoietic cells of both sisters, but not in their fibroblasts, nor in either parent. This compensatory mutation existed in cis with the maternal exon 28 mutation, and it restored function and nuclear localization of the resulting protein. Both sisters have been free of hematologic symptoms for more than 2 decades, suggesting that this de novo mutation occurred prenatally in a single hematopoietic stem cell (HSC) in one twin and that descendants of this functionally corrected HSC, via intra-uterine circulation, repopulated the blood lineages of both sisters. This finding suggests that treating FA patients with gene therapy might require transduction of only a few hematopoietic stem cells.

Molecular Pathogenesis of Fanconi Anemia

Fanconi anemia (FA) is a rare inherited disorder characterized clinically by aplastic anemia, developmental defects, and a susceptibility to cancer. Eleven complementation groups have been identified (FA-A, -B, -C, -D1, -D2, -E, -F, -G, -I, -J, and -L), and the genes responsible for 9 groups (FANCA, B, C, D1, D2, E, F, G, and L) have been cloned. The proteins involved in FA act coordinately in the cellular response to DNA cross-links in a pathway that has been shown to interact physically or functionally with a variety of other proteins involved in DNA repair or cell cycle control, notably BRCA1, Rad51,ATM,ATR, and Nbs1. Considerable advances in the identification and description of proteins involved in FA have been recorded, but the precise biochemical function of the FA pathway remains elusive. As research continues to improve our understanding of FA, insight will be gained into what is a pivotal process in cancer biology.

Fancd2 functions in a double strand break repair pathway that is distinct from non-homologous end joining

Fanconi anemia (FA) is a multigenic recessive disease resulting in bone marrow failure and increased cancer susceptibility. Cells from FA patients and mouse models are sensitive to DNA interstrand crosslinks (ICLs) and FA mice are moderately sensitive to ionizing radiation (IR). Both kinds of damage induce DNA double strand breaks (DSBs). To date, nine genes in 11 complementation groups have been identified; however, the precise function of the FA pathway remains unclear. Many of the proteins form a nuclear complex necessary for the mono-ubiquitination of the downstream protein, Fancd2. To further investigate the role of the FA pathway in repair of DSBs, we generated Fancd2(-/-)/Prkdc(sc/sc) double mutant mice. Prkdc(sc/sc) mutant mice have a defect in non-homologous end joining (NHEJ) and are sensitive to IR-induced DNA damage. Double mutant animals and primary cells were more sensitive to IR than either single mutant, suggesting that Fancd2 operates in DSB repair pathway distinct from NHEJ. Fancd2(-/-)/Prkdc(sc/sc) double mutant cells were also more sensitive to DSBs generated by a restriction endonuclease. The role of Fancd2 in DSB repair may account for the moderate sensitivity of FA cells to irradiation and FA cells sensitivity to ICLs that are repaired via a DSB intermediate.

Identification of multiple nuclear export sequences in Fanconi anemia group A protein that contribute to CRM1-dependent nuclear export

The Fanconi anemia (FA) pathway plays an important role in maintaining genomic stability, and defects in this pathway cause cancer susceptibility. The FA proteins have been found to function primarily in a nuclear complex, although a cytoplasmic localization and function for several FA proteins has also been reported. In this study, we investigated the possibility that FANCA, FANCC and FANCG are subjected to active export out of the nucleus. After treatment with leptomycin B, a specific inhibitor of CRM1-mediated nuclear export, the accumulation of epitope-tagged FANCA in the nucleus increased, whereas FANCC was affected to a lesser extent and FANCG showed no response. CRM1-mediated export of FANCA was further confirmed using CRM1 cotransfection, which led to a dramatic relocalization of FANCA to the cytoplasm. Five functional leucine-rich nuclear export sequences (NESs) distributed throughout the FANCA sequence were identified and characterized using an in vivo export assay. Simultaneous inactivation of three of these NESs resulted in a discrete but reproducible increase of FANCA nuclear accumulation. However, these NES mutations did not affect the ability of FANCA to complement the mitomycin C or cisplatin sensitivity of FA-A lymphoblasts. Surprisingly, mutations in the other two NESs resulted in an almost complete relocation of the protein to cytoplasm, suggesting that these motifs overlap with domains that are crucial for nuclear import. Taken together, these findings indicate that FANCA can be actively exported out of the nucleus by CRM1, revealing a new mechanism to regulate the function of the FA protein complex.

FANCG is phosphorylated at serines 383 and 387 during mitosis

Fanconi anemia (FA) is an autosomal recessive disease marked by congenital defects, bone marrow failure, and high incidence of leukemia and solid tumors. Eight genes have been cloned, with the accompanying protein products participating in at least two complexes, which appear to be functionally dependent upon one another. Previous studies have described chromatin localization of the FA core complex, except at mitosis, which is associated with phosphorylation of the FANCG protein (F. Qiao, A. Moss, and G. M. Kupfer, J. Biol. Chem. 276:23391-23396, 2001). The phosphorylation of FANCG at serine 7 by using mass spectrometry was previously mapped. The purpose of this study was to map the phosphorylation sites of FANCG at mitosis and to assess their functional importance. Reasoning that a potential kinase might be cdc2, which was previously reported to bind to FANCC, we showed that cdc2 chiefly phosphorylated a 14-kDa fragment of the C-terminal half of FANCG. Mass spectrometry analysis demonstrated that this fragment contains amino acids 374 to 504. Kinase motif analysis demonstrated that three amino acids in this fragment were leading candidates for phosphorylation. By using PCR-directed in vitro mutagenesis we mutated S383, S387, and T487 to alanine. Mutation of S383 and S387 abolished the phosphorylation of FANCG at mitosis. These results were confirmed by use of phosphospecific antibodies directed against phosphoserine 383 and phosphoserine 387. Furthermore, the ability to correct FA-G mutant cells of human or hamster (where S383 and S387 are conserved) origin was also impaired by these mutations, demonstrating the functional importance of these amino acids. S387A mutant abolished FANCG fusion protein phosphorylation by cdc2. The FA pathway, of which FANCG is a part, is highly regulated by a series of phosphorylation steps that are important to its overall function.

Regulated interaction of the Fanconi anemia protein, FANCD2, with chromatin

DNA damage activates the monoubiquitination of the Fanconi anemia (FA) protein, FANCD2, resulting in the assembly of FANCD2 nuclear foci. In the current study, we characterize structural features of FANCD2 required for this intranuclear translocation. We have previously identified 2 normal mRNA splice variants of FANCD2, one containing exon 44 sequence at the 3' end (FANCD2-44) and one containing exon 43 sequence (FANCD2-43). The 2 predicted FANCD2 proteins differ in their carboxy terminal 24 amino acids. In stably transfected FANCD2(-/-) fibroblasts, FANCD2-44 and FANCD2-43 proteins were monoubiquitinated on K561. Only FANCD2-44 corrected the mitomycin C (MMC) sensitivity of the transfected cells. We find that monoubiquitinated FANCD2-44 was translocated from the soluble nuclear compartment into chromatin. A mutant form of FANCD2-44 (FANCD2-K561R) was not monoubiquitinated and failed to bind chromatin. A truncated FANCD2 protein (Exon44-T), lacking the carboxy terminal 24 amino acids encoded by exon 44 but retaining K561, and another mutant FANCD2 protein, with a single amino acid substitution at a conserved residue within the C-terminal 24 amino acids (D1428A), were monoubiquitinated. Both mutants were targeted to chromatin but failed to correct MMC sensitivity. Taken together, our results indicate that monoubiquitination of FANCD2 regulates chromatin binding and that D1428 within the carboxy terminal acidic sequence encoded by exon 44 is independently required for functional complementation of FA-D2 cells. We hypothesize that the carboxy terminus of FANCD2-44 plays a critical role in sensing or repairing DNA damage.

Functional defects in the fanconi anemia pathway in pancreatic cancer cells

Biallelic BRCA2-mutations can cause Fanconi anemia and are found in approximately 7% of pancreatic cancers. Recently, several sequence changes in FANCC and FANCG were reported in pancreatic cancer. Functional defects in the Fanconi pathway can result in a marked hypersensitivity to interstrand crosslinking agents, such as mitomycin C. The functional implications of mutations in the Fanconi pathway in cancer have not been fully studied yet; these studies are needed to pave the way for clinical trials of treatment with crosslinking agents of Fanconi-defective cancers. The competence of the proximal Fanconi pathway was screened in 21 pancreatic cancer cell lines by an assay of Fancd2 monoubiquitination using a Fancd2 immunoblot. The pancreatic cancer cell lines Hs766T and PL11 were defective in Fancd2 monoubiquitination. In PL11, this defect led to the identification of a large homozygous deletion in FANCC, the first cancer cell line found to be FANCC-null. The Fanconi-defective cell lines Hs766T, PL11, and CAPAN1 were hypersensitive to the crosslinking agent mitomycin C and some to cis-platin, as measured by cell survival assays and G(2)/M cell-cycle arrest. These results support the practical exploration of crosslinking agents for non-Fanconi anemia patients that have tumors defective in the Fanconi pathway.

Phosphorylation of fanconi anemia (FA) complementation group G protein, FANCG, at serine 7 is important for function of the FA pathway

Fanconi anemia (FA) is an autosomal recessive disease of cancer susceptibility. FA cells exhibit a characteristic hypersensitivity to DNA cross-linking agents. The molecular mechanism for the disease is unknown as few of the FA proteins have functional motifs. Several post-translational modifications of the proteins have been described. We and others have reported that the FANCG protein (Fanconi complementation group G) is phosphorylated. We show that in an in vitro kinase reaction FANCG is radioactively labeled. Mass spectrometry analysis detected a peptide containing phosphorylation of serine 7. Using PCR-mediated site-directed mutagenesis we mutated serine 7 to alanine. Only wild-type FANCG cDNA fully corrected FA-G mutant cells. We also tested the effect of human wild-type FANCG in Chinese hamster ovary cells in which the FANCG homologue is mutant. Human FANCG complemented these cells, whereas human FANCG(S7A) did not. Unexpectedly, FANCG(S7A) bound to and stabilized the endogenous forms of the FANCA and FANCC proteins in the FA-G cells. FANCG(S7A) aberrantly localized to globules in chromatin and did not abrogate the internuclear bridges seen in the FA-G mutant cells. Phosphorylation of serine 7 in FANCG is functionally important in the FA pathway.

The Fanconi anemia core complex associates with chromatin during S phase

Fanconi anemia (FA) is an autosomal recessive disease marked by bone marrow failure, birth defects, and cancer. The FA proteins FANCA, FANCC, FANCE, FANCF, FANCG, and FANCL participate in a core complex. We previously have shown that several members of this complex bind to chromatin until mitosis and that this binding increases after DNA damage. The purpose of the present study was to determine the dynamics of complex movement between cytoplasm and nuclear compartments. Fluorescent-tagged versions of FANCA, FANCC, and FANCG colocalize in cytoplasm and nucleus, chiefly in chromatin. At the G1-S border, the FA core complex exists as foci on chromatin, progressively diffusing and migrating to the nuclear periphery and becoming completely excluded from condensed chromosomes by mitosis. Chromatin fiber analysis shows FA proteins diffusely staining along chromatin fibers during G1-S and S phase. Treatment with the DNA cross-linker mitomycin C results in a diffusion of foci and increased binding of complex proteins to chromatin, as well as diffuse and increased complex binding to chromatin fibers. These data are consistent with the idea that the FA proteins function at the level of chromatin during S phase to regulate and maintain genomic stability.

Oxidative stress/damage induces multimerization and interaction of Fanconi anemia proteins

Fanconi anemia (FANC) is a heterogeneous genetic disorder characterized by a hypersensitivity to DNA-damaging agents, chromosomal instability, and defective DNA repair. Eight FANC genes have been identified so far, and five of them (FANCA, -C, -E, -F, and -G) assemble in a multinuclear complex and function at least in part in a complex to activate FANCD2 by monoubiquitination. Here we show that FANCA and FANCG are redox-sensitive proteins that are multimerized and/or form a nuclear complex in response to oxidative stress/damage. Both FANCA and FANCG proteins exist as monomers under non-oxidizing conditions, whereas they become multimers following H2O2 treatment. Treatment of cells with oxidizing agent not only triggers the multimeric complex of FANCA and FANCG in vivo but also induces the interaction between FANCA and FANCG. N-Ethylmaleimide treatment abolishes multimerization and interaction of FANCA and FANCG in vitro. Taken together, our results lead us to conclude that FANCA and FANCG uniquely respond to oxidative damage by forming complex(es) via intermolecular disulfide linkage(s), which may be crucial in forming such complexes and in determining their function.

The Fanconi anemia core complex forms four complexes of different sizes in different subcellular compartments

Fanconi anemia (FA) is an autosomal recessive disease marked by congenital defects, bone marrow failure, and cancer susceptibility. FA cells exhibit a characteristic hypersensitivity to DNA crosslinking agents such as mitomycin C. The molecular mechanism for the disease remains elusive, but at least 6 FA proteins are known to be part of what is termed the FA core complex. We used affinity pulldown of FLAG-FANCA to pull down the FA complex from whole-cell extracts. Mass spectroscopy detected previously reported FA-binding proteins, including FANCA, FANCC, FANCG, cdc2, and GRP94, thus validating the approach. We further describe a method of purification of the FA core complex in an effort to find novel complex components and biochemical activity to define the function of the complex. By using conventional chromatographic fractionation of subcellular preparations, we report: (i) the FA core complex exists in a cytoplasmic form at 500-600 kDa; (ii) a larger, 750-kDa cytoplasmic form is seen only at mitosis; (iii) a nuclear form achieves a size of 2 megaDaltons; and (iv) a distinct 1-megaDalton FA core complex exists bound to chromatin that contains phosphorylated FANCA after undergoing DNA damage. We are continuing our analysis using mass spectroscopy in an effort to characterize novel binding proteins. These data will help define the biochemical role of the FA core complex in normal cell physiology as well as in the development of the FA disease state.

Fanconi anemia protein complex: mapping protein interactions in the yeast 2- and 3-hybrid systems

Fanconi anemia (FA) is an autosomal recessive syndrome characterized by progressive bone marrow failure and cancer predisposition. Eight FA complementation groups have been identified. The FANCA, FANCC, FANCE, FANCF, and FANCG proteins form a nuclear complex required for the monoubiquination of the FANCD2 protein. To investigate the architecture of the FA protein complex, the yeast 2-hybrid system was used to map contact points of the FANCA/FANCG, FANCC/FANCE, and FANCF/FANCG interactions. FANCG was shown to interact with both the amino-terminus of FANCA and the carboxyl-terminal region of FANCF. A FANCG mutant truncated at the carboxyl-terminus retained the ability to interact with FANCA. The interaction between FANCG and FANCF was ablated by a Leu71Pro mutant of FANCG. A central region of FANCE was sufficient for FANCC binding. A Leu554Pro mutant of FANCC failed to interact with FANCE. To further examine complex assembly, the yeast 3-hybrid system was used to investigate the ability of FANCG to act as a molecular bridge in mediating interaction between other FA proteins. FANCG was able to mediate interaction between FANCA and FANCF, as well as between monomers of FANCA. Direct interaction between FANCE and FANCD2 was also demonstrated in the yeast 2-hybrid system. This interaction involving an amino-terminal region of FANCD2 may provide a link between the FA protein complex and its downstream targets.