Impaired DNA double strand break repair in cells from Nijmegen breakage syndrome patients

Nijmegen breakage syndrome, caused by mutations in the NBS1 gene, is an autosomal recessive chromosomal instability disorder characterized by cancer predisposition. Cells isolated from Nijmegen breakage syndrome patients display increased levels of spontaneous chromosome aberrations and sensitivity to ionizing radiation. Here, we have investigated DNA double strand break repair pathways of homologous recombination, including single strand annealing, and non-homologous end-joining in Nijmegen breakage syndrome patient cells. We used recently developed GFP-YFP-based plasmid substrates to measure the efficiency of DNA double strand break repair. Both single strand annealing and non-homologous end-joining processes were markedly impaired in NBS1-deficient cells, and repair proficiency was restored upon re-introduction of full length NBS1 cDNA. Despite the observed defects in the repair efficiency, no apparent differences in homologous recombination or non-homologous end-joining effector proteins RAD51, KU70, KU86, or DNA-PK(CS) were observed. Furthermore, comparative analysis of junction sequences of plasmids recovered from NBS1-deficient and NBS1-complemented cells revealed increased dependence on microhomology-mediated end-joining DNA repair process in NBS1-complemented cells.

Direct DNA binding activity of the Fanconi anemia D2 protein

It is known that the Fanconi anemia D2 protein is vital for protecting the genome from DNA damage, but what activities this protein has are unknown. In these experiments we purified full-length Fanconi anemia protein D2 (FANCD2), and we found that FANCD2 bound to DNA with specificity for certain structures: double strand DNA ends and Holliday junctions. Proteins containing patient-derived mutations or artificial variants of the FANCD2 protein were similarly expressed and purified, and each variant bound to the Holliday junction DNA with similar affinity as did the wild-type protein. There was no single discrete domain of FANCD2 protein that bound to DNA, but rather the full-length protein was required for structure-specific DNA binding. This finding of DNA binding is the first biochemical activity identified for this key protein in the Fanconi anemia pathway.

The deubiquitinating enzyme USP1 regulates the Fanconi anemia pathway

Protein ubiquitination and deubiquitination are dynamic processes implicated in the regulation of numerous cellular pathways. Monoubiquitination of the Fanconi anemia (FA) protein FANCD2 appears to be critical in the repair of DNA damage because many of the proteins that are mutated in FA are required for FANCD2 ubiquitination. By screening a gene family RNAi library, we identify the deubiquitinating enzyme USP1 as a novel component of the Fanconi anemia pathway. Inhibition of USP1 leads to hyperaccumulation of monoubiquitinated FANCD2. Furthermore, USP1 physically associates with FANCD2, and the proteins colocalize in chromatin after DNA damage. Finally, analysis of crosslinker-induced chromosomal aberrations in USP1 knockdown cells suggests a role in DNA repair. We propose that USP1 deubiquitinates FANCD2 when cells exit S phase or recommence cycling after a DNA damage insult and may play a critical role in the FA pathway by recycling FANCD2.

The Fanconi anemia pathway is required for the DNA replication stress response and for the regulation of common fragile site stability

Fanconi anemia (FA) is a rare multi-genic, autosomal and X-linked recessive disorder characterized by hematological abnormalities, developmental defects and increased cancer susceptibility. Patient-derived FA cells display heightened sensitivity to DNA cross-linking agents such as mitomycin C (MMC). In response to DNA damaging agents, and during S-phase of the cell cycle, the FA pathway is activated via the mono-ubiquitination of FANCD2 (FANCD2-Ub), signaling its translocation to discrete nuclear foci, where it co-localizes with the central DNA repair proteins BRCA1 and RAD51. However, the exact function of activated FANCD2-Ub remains unclear. Here, we have characterized the role of the FA pathway in response to DNA replicative stress by aphidicolin (APH) and hydroxyurea (HU). The FA pathway is strongly activated in response to both agents. In addition, using patient-derived FA cell lines and siRNA targeting FANCD2, we demonstrate a functional requirement for the FA pathway in response to low doses of APH: a replicative stress treatment known to result in chromosome breakage at common fragile sites. Both the total number of chromosome gaps and breaks and breaks at the specific common fragile sites FRA3B and FRA16D were significantly elevated in the absence of an intact FA pathway. Furthermore, we demonstrate that APH activates the mono-ubiquitination of both FANCD2 and PCNA and the phosphorylation of RPA2, signaling processive DNA replication arrest. Following APH treatment, FANCD2-Ub co-localizes with PCNA (early) and RPA2 (late) in discrete nuclear foci. Our results demonstrate an integral role for the FA pathway in the DNA replication stress response.

Human Fanconi anemia monoubiquitination pathway promotes homologous DNA repair

Fanconi anemia (FA) is a recessive disorder characterized by congenital abnormalities, progressive bone-marrow failure, and cancer susceptibility. Cells from FA patients are hypersensitive to agents that produce DNA crosslinks and, after treatment with these agents, have pronounced chromosome breakage and other cytogenetic abnormalities. Eight FANC genes have been cloned, and the encoded proteins interact in a common cellular pathway. DNA-damaging agents activate the monoubiquitination of FANCD2, resulting in its targeting to nuclear foci that also contain BRCA1 and BRCA2/FANCD1, proteins involved in homology-directed DNA repair. Given the interaction of the FANC proteins with BRCA1 and BRCA2, we tested whether cells from FA patients (groups A, G, and D2) and mouse Fanca-/- cells with a targeted mutation are impaired for this repair pathway. We find that both the upstream (FANCA and FANCG) and downstream (FANCD2) FA pathway components promote homology-directed repair of chromosomal double-strand breaks (DSBs). The FANCD2 monoubiquitination site is critical for normal levels of repair, whereas the ATM phosphorylation site is not. The defect in these cells, however, is mild, differentiating them from BRCA1 and BRCA2 mutant cells. Surprisingly, we provide evidence that these proteins, like BRCA1 but unlike BRCA2, promote a second DSB repair pathway involving homology, i.e., single-strand annealing. These results suggest an early role for the FANC proteins in homologous DSB repair pathway choice.

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.

Detection of somatic mosaicism and classification of Fanconi anemia patients by analysis of the FA/BRCA pathway

Fanconi anemia (FA) is characterized by congenital abnormalities, bone marrow failure, chromosome fragility, and cancer susceptibility. Eight FA-associated genes have been identified so far, the products of which function in the FA/BRCA pathway. A key event in the pathway is the monoubiquitination of the FANCD2 protein, which depends on a multiprotein FA core complex. In a number of patients, spontaneous genetic reversion can correct FA mutations, leading to somatic mosaicism. We analyzed the FA/BRCA pathway in 53 FA patients by FANCD2 immunoblots and chromosome breakage tests. Strikingly, FANCD2 monoubiquitination was detected in peripheral blood lymphocytes (PBLs) in 8 (15%) patients. FA reversion was further shown in these patients by comparison of primary fibro-blasts and PBLs. Reversion was associated with higher blood counts and clinical stability or improvement. Once constitutional FANCD2 patterns were determined, patients could be classified based on the level of FA/BRCA pathway disruption, as "FA core" (upstream inactivation; n = 47, 89%), FA-D2 (n = 4, 8%), and an unidentified downstream group (n = 2, 4%). FA-D2 and unidentified group patients were therefore relatively common, and they had more severe congenital phenotypes. These results show that specific analysis of the FA/BRCA pathway, combined with clinical and chromosome breakage data, allows a comprehensive characterization of FA patients.

ATR couples FANCD2 monoubiquitination to the DNA-damage response

Fanconi anemia (FA) is a multigenic autosomal recessive cancer susceptibility syndrome. The FA pathway regulates the monoubiquitination of FANCD2 and the assembly of damage-associated FANCD2 nuclear foci. How FANCD2 monoubiquitination is coupled to the DNA-damage response has remained undetermined. Here, we demonstrate that the ATR checkpoint kinase and RPA1 are required for efficient FANCD2 monoubiquitination. Deficiency of ATR function, either in Seckel syndrome, which clinically resembles Fanconi anemia, or by siRNA silencing, results in the formation of radial chromosomes in response to the DNA cross-linker, mitomycin C (MMC), thus mimicking the chromosome instability of FA cells.

Functional interaction of monoubiquitinated FANCD2 and BRCA2/FANCD1 in chromatin

Fanconi anemia (FA) is an autosomal recessive cancer susceptibility syndrome with at least 11 complementation groups (A, B, C, D1, D2, E, F, G, I, J, and L), and eight FA genes have been cloned. The FANCD1 gene is identical to the breast cancer susceptibility gene, BRCA2. The FA proteins cooperate in a common pathway, but the function of BRCA2/FANCD1 in this pathway remains unknown. Here we show that monoubiquitination of FANCD2, which is activated by DNA damage, is required for targeting of FANCD2 to chromatin, where it interacts with BRCA2. FANCD2-Ub then promotes BRCA2 loading into a chromatin complex. FANCD2(-/-) cells are deficient in the assembly of DNA damage-inducible BRCA2 foci and in chromatin loading of BRCA2. Functional complementation with the FANCD2 cDNA restores BRCA2 foci and its chromatin loading following DNA damage. BRCA2(-/-) cells expressing a carboxy-terminal truncated BRCA2 protein form IR-inducible BRCA2 and FANCD2 foci, but these foci fail to colocalize. Functional complementation of these cells with wild-type BRCA2 restores the interaction of BRCA2 and FANCD2. The C terminus of BRCA2 is therefore required for the functional interaction of BRCA2 and FANCD2 in chromatin. Taken together, our results demonstrate that monoubiquitination of FANCD2, which is regulated by the FA pathway, promotes BRCA2 loading into chromatin complexes. These complexes appear to be required for normal homology-directed DNA repair.

The interplay of Fanconi anemia proteins in the DNA damage response

Fanconi anemia (FA) is a rare autosomal recessive disease characterized by chromosome instability and cancer predisposition. At least 11 complementation groups for FA have been identified, and eight FA genes have been cloned. Interestingly, the eight known FA proteins cooperate in a common pathway leading to the interaction of monoubiquitinated FANCD2 and BRCA2 in damaged chromatin. Disruption of this pathway results in the clinical and cellular abnormalities common to all FA subtypes. This review will examine the interaction of the cloned FA proteins with each other and with other DNA damage response proteins (i.e., ATM, ATR, and NBS1). Also, somatic (acquired) disruption of the FA pathway in human tumors appears to account for their chromosome instability and crosslinker hypersensitivity.

Knockdown of zebrafish Fancd2 causes developmental abnormalities via p53-dependent apoptosis

Mechanisms underlying the multiple developmental defects observed in Fanconi anemia (FA) patients are not well defined. We have identified the zebrafish homolog of human FANCD2, which encodes a nuclear effector protein that is monoubiquitinated in response to DNA damage, targeting it to nuclear foci where it preserves chromosomal integrity. Fancd2-deficient zebrafish embryos develop defects similar to those found in children with FA, including shortened body length, microcephaly, and microophthalmia, which are due to extensive cellular apoptosis. Developmental defects and increased apoptosis in Fancd2-deficient zebrafish were corrected by injection of human FANCD2 or zebrafish bcl2 mRNA, or by knockdown of p53, indicating that in the absence of Fancd2, developing tissues spontaneously undergo p53-dependent apoptosis. Thus, Fancd2 is essential during embryogenesis to prevent inappropriate apoptosis in neural cells and other tissues undergoing high levels of proliferative expansion, implicating this mechanism in the congenital abnormalities observed in human infants with FA.

Association of biallelic BRCA2/FANCD1 mutations with spontaneous chromosomal instability and solid tumors of childhood

The clinical, cytogenetic, and molecular findings of 2 Fanconi anemia (FA) subtype D1 kindreds, initially identified through a young child with a solid tumor (medullobastoma, Wilms tumor), are described. Each kindred subsequently had a second affected child; one developed Wilms tumor followed by a medulloblastoma, and the other developed T-lineage acute lymphoblastic leukemia. Cytogenetic studies revealed an unusually high spontaneous chromosome aberration rate, contrasting with other FA subtypes. Molecular analysis revealed biallelic BRCA2/FANCD1 mutations. The patients did not exhibit bone marrow failure. Our studies suggest that the D1 subtype represents a severe end of the cytogenetic spectrum within FA, consistent with a critical downstream role of BRCA2 in the FA pathway. Furthermore, this FA subgroup may be preferentially associated with an increased predisposition to solid tumors in early childhood. Recognition of this constellation of findings has significant implications for medical management and genetic counseling of FA families.

Menin associates with FANCD2, a protein involved in repair of DNA damage

Multiple endocrine neoplasia type I (MEN1) is an inherited tumor syndrome characterized by tumors in multiple endocrine organs including the parathyroids, pancreatic islets, and the pituitary. The gene mutated in MEN1 patients, Men1, encodes a protein of 610 amino acid residues, menin, and mutations in the Men1 gene lead to the MEN1 syndrome. Although the chromosomal instability in the peripheral lymphocytes from the MEN1 patients has been reported previously, it is not clear whether menin is involved in repair of DNA damage. Here we show that menin specifically interacts with FANCD2, a protein encoded by a gene involved in DNA repair and mutated in patients with an inherited cancer-prone syndrome, Fanconi anemia. The interaction between menin and FANCD2 is enhanced by gamma-irradiation. Moreover, loss of menin expression in mouse embryonic fibroblasts leads to increased sensitivity to DNA damage. Furthermore, menin is localized to chromatin and nuclear matrix, and the association with nuclear matrix is enhanced by gamma-irradiation. Together, these results suggest that menin plays a critical role in repair of DNA damage in concert with FANCD2.

The Fanconi Anemia/BRCA signaling pathway: disruption in cisplatin-sensitive ovarian cancers

Ovarian tumors often exhibit chromosome instability and hypersensitivity to the chemotherapeutic agent cisplatin. Recently, we have shown that this cellular phenotype may result from an acquired disruption of the Fanconi Anemia/BRCA (FA/BRCA) signaling pathway. Disruption results from methylation and silencing of one of the FA genes (FANCF), leading to cisplatin sensitivity. Restoration of this pathway is associated with demethylation of FANCF, leading to acquired cisplatinum resistance. The serial inactivation and reactivation of the FA/BRCA pathway has important implications for the diagnosis and treatment of ovarian cancers and related cancers.

Genetic reversion in an acute myelogenous leukemia cell line from a Fanconi anemia patient with biallelic mutations in BRCA2

A 2-year old boy was diagnosed with Fanconi anemia (FA) and acute myeloid leukemia (AML). A cell line (termed FA-AML1) was established from blast cells obtained after a second relapse after a successful bone marrow transplant. Histochemical and surface marker analysis confirmed that the cells were derived from the myeloid lineage. Cytogenetic analysis revealed multiple chromosomal aberrations, including a ring 7. Stable proliferation of the cultured cells was absolutely dependent on the presence of granulocyte macrophage colony-stimulating factor or interleukin 3. This is the first AML cell line successfully established from a FA patient. Remarkably, FA-AML1 cells appeared to lack the characteristic cellular FA phenotype, i.e., a hypersensitivity to growth inhibition and chromosomal breakage by the cross-linking agent mitomycin C. Genomic DNA from the patient showed biallelic mutations [8415G>T (K2729N)and 8732C>A (S2835STOP)] in the breast cancer susceptibility gene FANCD1/BRCA2 [N. Howlett et al., Science (Wash. DC), 297: 606-609, 2002]. In the AML cells, however, the 8732C>A nonsense mutation was changed into a missense mutation by a secondary alteration, 8731T>G, resulting in 2835E, which restored the open-reading frame of the gene and could explain the reverted phenotype of these cells. Loss of the FA phenotype by genetic correction of a FA gene mutation during AML progression may be a common late event in the pathogenesis of AML in FA patients, which may be treatment related. This finding suggests a novel mechanistic principle of tumor progression based on the genetic correction of an early caretaker gene defect.

Disruption of the Fanconi anemia-BRCA pathway in cisplatin-sensitive ovarian tumors

Ovarian tumor cells are often genomically unstable and hypersensitive to cisplatin. To understand the molecular basis for this phenotype, we examined the integrity of the Fanconi anemia-BRCA (FANC-BRCA) pathway in those cells. This pathway regulates cisplatin sensitivity and is governed by the coordinate activity of six genes associated with Fanconi anemia (FANCA, FANCC, FANCD2, FANCE, FANCF and FANCG) as well as BRCA1 and BRCA2 (FANCD1). Here we show that the FANC-BRCA pathway is disrupted in a subset of ovarian tumor lines. Mono-ubiquitination of FANCD2, a measure of the function of this pathway, and cisplatin resistance were restored by functional complementation with FANCF, a gene that is upstream in this pathway. FANCF inactivation in ovarian tumors resulted from methylation of its CpG island, and acquired cisplatin resistance correlated with demethylation of FANCF. We propose a model for ovarian tumor progression in which the initial methylation of FANCF is followed by FANCF demethylation and ultimately results in cisplatin resistance.

Regulation of the Fanconi anemia pathway by monoubiquitination

Fanconi anemia (FA) is an autosomal recessive cancer susceptibility syndrome characterized by multiple congenital anomalies, bone marrow failure, and cellular sensitivity to mitomycin C (MMC). To date, six FA genes have been cloned, and the encoded proteins function in a novel pathway. The FA pathway is required for the normal cellular response to DNA damage. Following DNA damage, the pathway is activated, leading to monoubiquitination of the FA protein, FANCD2, and its targeting to subnuclear foci. Disruption of the FA pathway results in the absence of FANCD2 nuclear foci, leading to the cellular and clinical abnormalities of FA. Here, we review the recent studies describing the regulated monoubiquitination of the FANCD2 protein and discuss the interaction of the FA pathway with other DNA damage response pathways.

The Fanconi anaemia/BRCA pathway

Fanconi anaemia (FA) is a rare genetic cancer-susceptibility syndrome that is characterized by congenital abnormalities, bone-marrow failure and cellular sensitivity to DNA crosslinking agents. Seven FA-associated genes have recently been cloned, and their products were found to interact with well-known DNA-damage-response proteins, including BRCA1, ATM and NBS1. The FA proteins could therefore be involved in the cell-cycle checkpoint and DNA-repair pathways. Recent studies implicate the FA proteins in the process of repairing chromosome defects that occur during homologous recombination, and disruption of the FA genes results in chromosome instability--a common feature of many human cancers.

A novel diagnostic screen for defects in the Fanconi anemia pathway

Fanconi anemia (FA) is an autosomal recessive chromosomal instability syndrome characterized by congenital abnormalities, progressive bone marrow failure, and cancer predisposition. Although patients with FA are candidates for bone marrow transplantation or gene therapy, their phenotypic heterogeneity can delay or obscure diagnosis. The current diagnostic test for FA consists of cytogenetic quantitation of chromosomal breakage in response to diepoxybutane (DEB) or mitomycin C (MMC). Recent studies have elucidated a biochemical pathway for Fanconi anemia that culminates in the monoubiquitination of the FANCD2 protein. In the current study, we develop a new rapid diagnostic and subtyping FA assay amenable for screening broad populations at risk of FA. Primary lymphocytes were assayed for FANCD2 monoubiquitination by immunoblot. The absence of the monoubiquitinated FANCD2 isoform correlated with the diagnosis of FA by DEB testing in 11 known patients with FA, 37 patients referred for possible FA, and 29 healthy control subjects. Monoubiquitination of FANCD2 was normal in other bone marrow failure syndromes and chromosomal breakage syndromes. A combination of retroviral gene transfer and FANCD2 immunoblotting provides a rapid subtyping assay for patients newly diagnosed with FA. These new FA screening assays would allow efficient testing of broad populations at risk.

Heterogeneous activation of the Fanconi anemia pathway by patient-derived FANCA mutants

Fanconi anemia (FA) is an autosomal recessive disorder of hematopoiesis characterized by hypersensitivity to DNA crosslinkers such as mitomycin C (MMC). There is growing evidence for a model of the FA pathway, wherein a nuclear multiprotein complex of five FA proteins (FANCA, C, E, F and G) regulates activation of FANCD2 into a monoubiquitinated form, which, collaborating with the BRCA1 machinery, affects cellular response to DNA damage. However, the role of the FA pathway in defective DNA damage response caused by various mutant forms of FA proteins has not been fully assessed. In the present study, 21 patient-derived FANCA mutants with a missense or a small in-frame deletion were expressed in FANCA-deficient fibroblasts and examined for complementation of MMC sensitivity and for reconstitution of the FA pathway: FANCA phosphorylation, interaction with FANCC, FANCF and FANCG and nuclear localization and FANCD2 monoubiquitination. The altered FANCA proteins complemented MMC sensitivity at different grades: five proteins (group I) behaved like wild-type FANCA, whereas the other proteins were either mildly (group II, n=4) or severely (group III, n=12) impaired. Group I proteins showed an apparently normal reconstitution of the FA pathway, thus they may be pathogenic by reducing endogenous expression or possibly benign polymorphisms. Reconstitution of the FA pathway by group II and III mutants closely correlated with cellular sensitivity to MMC. The different activation of the FA pathway may partly account for the phenotypic variation seen in FA patients.

Interaction of FANCD2 and NBS1 in the DNA damage response

Fanconi anaemia (FA) and Nijmegen breakage syndrome (NBS) are autosomal recessive chromosome instability syndromes with distinct clinical phenotypes. Cells from individuals affected with FA are hypersensitive to mitomycin C (MMC), and cells from those with NBS are hypersensitive to ionizing radiation. Here we report that both NBS cell lines and individuals with NBS are hypersensitive to MMC, indicating that there may be functional linkage between FA and NBS. In wild-type cells, MMC activates the colocalization of the FA subtype D2 protein (FANCD2) and NBS1 protein in subnuclear foci. Ionizing radiation activates the ataxia telangiectasia kinase (ATM)-dependent and NBS1-dependent phosphorylation of FANCD2, resulting in an S-phase checkpoint. NBS1 and FANCD2 therefore cooperate in two distinct cellular functions, one involved in the DNA crosslink response and one involved in the S-phase checkpoint response.

S-phase-specific interaction of the Fanconi anemia protein, FANCD2, with BRCA1 and RAD51

Fanconi anemia (FA) is a human autosomal recessive cancer susceptibility disorder characterized by cellular sensitivity to mitomycin C and defective cell-cycle progression. Six FA genes (corresponding to subtypes A, C, D2, E, F, and G) have been cloned, and the encoded FA proteins interact in a common pathway. DNA damage activates this pathway, leading to monoubiquitination of the downstream FANCD2 protein and targeting to nuclear foci containing BRCA1. In the current study, we demonstrate that FANCD2 also undergoes monoubiquitination during S phase of the cell cycle. Monoubiquitinated FANCD2 colocalizes with BRCA1 and RAD51 in S-phase-specific nuclear foci. Monoubiquitination of FANCD2 is required for normal cell-cycle progression following cellular exposure to mitomycin C. Our data indicate that the monoubiquitination of FANCD2 is highly regulated, and they suggest that FANCD2/BRCA1 complexes and FANCD2/RAD51 complexes participate in an S-phase-specific cellular process, such as DNA repair by homologous recombination.

The Fanconi anemia protein, FANCE, promotes the nuclear accumulation of FANCC

Fanconi anemia is an autosomal recessive disorder characterized by aplastic anemia, cancer susceptibility, and cellular sensitivity to mitomycin C. The 6 known Fanconi anemia gene products (FANCA, FANCC, FANCD2, FANCE, FANCF, and FANCG proteins) interact in a common pathway. The monoubiquitination and nuclear foci formation of FANCD2 are essential for the function of this pathway. FANCA, FANCC, FANCG, and FANCF proteins form a multisubunit nuclear complex (FA complex) required for FANCD2 monoubiquitination. Because FANCE and FANCC interact in vitro and FANCE is required for FANCD2 monoubiquitination, we reasoned that FANCE is a component of the FA complex in vivo. Here we demonstrate that retroviral transduction of Fanconi anemia subtype E (FA-E) cells with the FANCE cDNA restores the nuclear accumulation of FANCC protein, FANCA-FANCC complex formation, monoubiquitination and nuclear foci formation of FANCD2, and mitomycin C resistance. Hemagglutinin (HA)-tagged FANCE protein localizes diffusely in the nucleus. In normal cells, HA-tagged FANCE protein coimmunoprecipitates with FANCA, FANCC, and FANCG but not with FANCD2. Our data indicate that FANCE is a component of the nuclear FA complex in vivo and is required for the monoubiquitination of FANCD2 and the downstream events in the FA pathway.