Fanconi anemia proteins FANCA, FANCC, and FANCG/XRCC9 interact in a functional nuclear complex

Nonerythroid alphaII spectrin is required for recruitment of FANCA and XPF to nuclear foci induced by DNA interstrand cross-links

The events responsible for repair of DNA interstrand cross-links in mammalian cells, the proteins involved and their interactions with each other are poorly understood. The present study demonstrates that the structural protein nonerythroid alpha spectrin (alphaSpIISigma*), present in normal human cell nuclei, plays an important role in repair of DNA interstrand cross-links. These results show that alphaSpIISigma* relocalizes to nuclear foci after damage of normal human cells with the DNA interstrand cross-linking agent 8-methoxypsoralen plus ultraviolet A (UVA) light and that FANCA and the known DNA repair protein XPF localize to the same nuclear foci. That alphaSpIISigma* is essential for this re-localization is demonstrated by the finding that in cells from patients with Fanconi anemia complementation group A (FA-A), which have decreased ability to repair DNA interstrand cross-links and decreased levels of alphaSpIISigma*, there is a significant reduction in formation of damage-induced XPF as well as alphaSpIISigma* nuclear foci, even though levels of XPF are normal in these cells. In corrected FA-A cells, in which levels of alphaSpIISigma* are restored to normal, numbers of damage-induced nuclear foci are also returned to normal. Co-immunoprecipitation studies show that alphaSpIISigma*, FANCA and XPF co-immunoprecipitate with each other from normal human nuclear proteins. These results demonstrate that alphaSpIISigma*, FANCA and XPF interact with each other in the nucleus and indicate that there is a close functional relationship between these proteins. These studies suggest that an important role for alphaSpIISigma* in the nucleus is to act as a scaffold, aiding in recruitment and alignment of repair proteins at sites of damage.

Fanconi anemia protein complex is a novel target of the IKK signalsome

Fanconi anemia (FA), a genetic disorder predisposing to aplastic anemia and cancer, is characterized by hypersensitivity to DNA-damaging agents and oxidative stress. Five of the cloned FA proteins (FANCA, FANCC, FANCE, FANCF, FANCG) appear to be involved in a common functional pathway that is required for the monoubiquitination of a sixth gene product, FANCD2. Here, we report that FANCA associates with the IkappaB kinase (IKK) signalsome via interaction with IKK2. Components of the FANCA complex undergo rapid, stimulus-dependent changes in phosphorylation, which are blocked by kinase-inactive IKK2 (IKK2 K > M). When exposed to mitomycin C, cells expressing IKK2 K > M develop a cell cycle abnormality characteristic of FA. Thus, FANCA may function to recruit IKK2, thus providing the cell a means of rapidly responding to stress.

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.

Spectrum of sequence variation in the FANCG gene: an International Fanconi Anemia Registry (IFAR) study

Fanconi anemia (FA) is a genetically heterogeneous autosomal recessive syndrome associated with chromosomal instability, hypersensitivity to DNA cross-linking agents, and predisposition to malignancy. The gene for FA complementation group G (FANCG) was the third FA gene to be cloned, and was found to be identical with human XRCC9, which maps to 9p13. The cDNA is predicted to encode a polypeptide of 622 amino acids, with no sequence similarities to any other known protein or motifs that could point to a molecular function for FANCG/XRCC9. We used single strand conformational polymorphism analysis (SSCP) to screen genomic DNA from a panel of 307 racially and ethnically diverse unrelated FA patients from the International Fanconi Anemia Registry (IFAR) for variants in FANCG. Twenty-seven abnormal SSCP patterns were found; 18 of these variants appear to be pathogenic mutations while nine are likely to be nonpathogenic polymorphisms. Direct sequencing of genomic DNA from seven FA-G probands with one mutant allele not detected in the SSCP study and three additional probands assigned to the FA-G complementation group by retroviral correction with FANCG resulted in the detection of nine additional pathogenic mutations and two common SNPs. Conditions for rapid screening for these mutations by DHPLC for use in a clinical laboratory setting were established. The most common FANCG mutations in the IFAR population were: IVS8-2A>G (seven Portuguese-Brazilian probands), IVS11+1G>C (seven French-Acadian probands), 1794_1803del10 (seven European probands), and IVS3+1G>C (five Korean or Japanese probands). Our data suggest that the Portuguese-Brazilian, French-Acadian, and Korean/Japanese mutations were likely to have been present in a founding member of each of these populations.

Genetic basis of Fanconi anemia

Fanconi anemia is a rare autosomal recessive disease characterized by bone marrow failure, developmental anomalies, a high incidence of myelodysplasia and acute nonlymphocytic leukemia, and cellular hypersensitivity to cross linking agents. Five of the seven known Fanconi anemia proteins bind together in a complex and influence the function of a sixth, FANCD2, which colocalizes with BRCA1 in nuclear foci after genotoxic stress. Carboxy-terminal truncating mutations of the seventh Fanconi anemia gene, BRCA2, are hypomorphic and lead to FA-D1 and possibly FA-B. Because the Fanconi anemia alleles of BRCA2 fail to bind to Rad51 in response to genotoxic stress and Rad51 therefore fails to localize to nuclear damage foci, many investigators in the field suspect that the Fanconi anemia pathway supports the integrity of the Rad51 and BRCA1 and BRCA2 pathways as they function in homologous recombination repair. Because these abnormalities are common to all somatic cells, it is unlikely that dysfunction of this particular pathway results in tissue-specific apoptosis of hematopoietic cells. Indeed, at least one of the Fanconi anemia proteins, FANCC, exhibits functions in hematopoietic cells in addition to its role in the complex. Because FANCC protects hematopoietic cells from apoptotic cues in ways that do not require an intact heteromeric Fanconi anemia complex, it is reasonable to expect that the other Fanconi anemia gene products will have independent cytoplasmic and nuclear functions, particularly in hematopoietic and germ cells that seem to rely so substantially on an intact portfolio of Fanconi anemia proteins.

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.

Fanconi anaemia

Fanconi anaemia (FA) is an autosomal recessive disease characterised by congenital abnormalities, defective haemopoiesis, and a high risk of developing acute myeloid leukaemia and certain solid tumours. Chromosomal instability, especially on exposure to alkylating agents, may be shown in affected subjects and is the basis for a diagnostic test. FA can be caused by mutations in at least seven different genes. Interaction pathways have been established, both between the FA proteins and other proteins involved in DNA damage repair, such as ATM, BRCA1 and BRCA2, thereby providing a link with other disorders in which defective DNA damage repair is a feature. This review summarises the clinical features of FA and the natural history of the disease, discusses diagnosis and management, and puts the recent molecular advances into the context of the cellular and clinical FA phenotype.

The anti-apoptotic function of Hsp70 in the interferon-inducible double-stranded RNA-dependent protein kinase-mediated death signaling pathway requires the Fanconi anemia protein, FANCC

Proteins encoded by five of the six known Fanconi anemia (FA) genes form a heteromeric complex that facilitates repair of DNA damage induced by cross-linking agents. A certain number of these proteins, notably FANCC, also function independently to modulate apoptotic signaling, at least in part, by suppressing ground state activation of the pro-apoptotic interferon-inducible double-stranded RNA-dependent protein kinase (PKR). Because certain FANCC mutations interdict its anti-apoptotic function without interfering with the capacity of FANCC to participate functionally in the FA multimeric complex, we suspected that FANCC enhances cell survival independent of its participation in the complex. By investigating this function in both mammalian cells and in yeast, an organism with no FA orthologs, we show that FANCC inhibited the kinase activity of PKR both in vivo and in vitro, and this effect depended upon a physical interaction between FANCC and Hsp70 but not on interactions of FANCC with other Fanconi proteins. Hsp70, FANCC, and PKR form a ternary complex in lymphoblasts and in yeast expressing PKR. We conclude that Hsp70 requires the cooperation of FANCC to suppress PKR activity and support survival of hematopoietic cells and that FANCC does not require the multimeric FA complex to exert this function.

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.

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.

Recombinational DNA repair and human disease

We review the genes and proteins related to the homologous recombinational repair (HRR) pathway that are implicated in cancer through either genetic disorders that predispose to cancer through chromosome instability or the occurrence of somatic mutations that contribute to carcinogenesis. Ataxia telangiectasia (AT), Nijmegen breakage syndrome (NBS), and an ataxia-like disorder (ATLD), are chromosome instability disorders that are defective in the ataxia telangiectasia mutated (ATM), NBS, and Mre11 genes, respectively. These genes are critical in maintaining cellular resistance to ionizing radiation (IR), which kills largely by the production of double-strand breaks (DSBs). Bloom syndrome involves a defect in the BLM helicase, which seems to play a role in restarting DNA replication forks that are blocked at lesions, thereby promoting chromosome stability. The Werner syndrome gene (WRN) helicase, another member of the RecQ family like BLM, has very recently been found to help mediate homologous recombination. Fanconi anemia (FA) is a genetically complex chromosomal instability disorder involving seven or more genes, one of which is BRCA2. FA may be at least partially caused by the aberrant production of reactive oxidative species. The breast cancer-associated BRCA1 and BRCA2 proteins are strongly implicated in HRR; BRCA2 associates with Rad51 and appears to regulate its activity. We discuss in detail the phenotypes of the various mutant cell lines and the signaling pathways mediated by the ATM kinase. ATM's phosphorylation targets can be grouped into oxidative stress-mediated transcriptional changes, cell cycle checkpoints, and recombinational repair. We present the DNA damage response pathways by using the DSB as the prototype lesion, whose incorrect repair can initiate and augment karyotypic abnormalities.

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.

Fanconi anemia group A and C double-mutant mice: functional evidence for a multi-protein Fanconi anemia complex

OBJECTIVE

Fanconi anemia (FA) is a genetically heterogeneous disorder associated with defects in at least eight genes. The biochemical function(s) of the FA proteins are unknown, but together they define the FA pathway, which is involved in cellular responses to DNA damage and in other cellular processes. It is currently unknown whether all FA proteins are involved in controlling a single function or whether some of the FA proteins have additional roles. The aim of this study was 1) to determine whether the FA group A and group C genes have identical or partially distinct functions, and 2) to have a better model for human FA.

MATERIALS AND METHODS

We generated mice with a targeted mutation in fanca and crossed them with fancc disrupted animals. Several phenotypes including sensitivity to DNA cross linkers and ionizing radiation, hematopoietic colony growth, and germ cell loss were analyzed in fanca-/-, fancc-/-, fanca/fancc double -/-, and controls.

RESULTS

Fibroblast cells and hematopoietic precursors from fanca/fancc double-mutant mice were not more sensitive to MMC than those of either single mutant. fanca/fancc double mutants had no evidence for an additive phenotype at the cellular or organismal level.

CONCLUSIONS

These results support a model where both FANCA and FANCC are part of a multi-protein nuclear FA complex with identical function in cellular responses to DNA damage and germ cell survival.

FANCE: the link between Fanconi anaemia complex assembly and activity

The Fanconi anaemia (FA) nuclear complex (composed of the FA proteins A, C, G and F) is essential for protection against chromosome breakage. It activates the downstream protein FANCD2 by monoubiquitylation; this then forges an association with the BRCA1 protein at sites of DNA damage. Here we show that the recently identified FANCE protein is part of this nuclear complex, binding both FANCC and FANCD2. Indeed, FANCE is required for the nuclear accumulation of FANCC and provides a critical bridge between the FA complex and FANCD2. Disease-associated FANCC mutants do not bind to FANCE, cannot accumulate in the nucleus and are unable to prevent chromosome breakage.

The two faces of BRCA2, a FANCtastic discovery

The molecular defects responsible for the cancer predisposition syndrome Fanconi's Anemia (FA) have been elusive. A recent study reports that the FANC-B and -D1 subgroups result from hypomorphic mutations in BRCA2. Given that BRCA2 protein participates in homologous recombination, this finding connects at least a subset of the FA phenotypes to defective DNA repair.

Convergence of the fanconi anemia and ataxia telangiectasia signaling pathways

Fanconi anemia (FA) and ataxia telangiectasia (AT) are clinically distinct autosomal recessive disorders characterized by spontaneous chromosome breakage and hematological cancers. FA cells are hypersensitive to mitomycin C (MMC), while AT cells are hypersensitive to ionizing radiation (IR). Here, we identify the Fanconi anemia protein, FANCD2, as a link between the FA and ATM damage response pathways. ATM phosphorylates FANCD2 on serine 222 in vitro. This site is also phosphorylated in vivo in an ATM-dependent manner following IR. Phosphorylation of FANCD2 is required for activation of an S phase checkpoint. The ATM-dependent phosphorylation of FANCD2 on S222 and the FA pathway-dependent monoubiquitination of FANCD2 on K561 are independent posttranslational modifications regulating discrete cellular signaling pathways. Biallelic disruption of FANCD2 results in both MMC and IR hypersensitivity.

Molecular biology of Fanconi anaemia--an old problem, a new insight

Fanconi anaemia (FA) comprises a group of autosomal recessive disorders resulting from mutations in one of eight genes (FANCA, FANCB, FANCC, FANCD1, FANCD2, FANCE, FANCF and FANCG). Although caused by relatively simple mutations, the disease shows a complex phenotype, with a variety of features including developmental abnormalities and ultimately severe anaemia and/or leukemia leading to death in the mid teens. Since 1992 all but two of the genes have been identified, and molecular analysis of their products has revealed a complex mode of action. Many of the proteins form a nuclear multisubunit complex that appears to be involved in the repair of double-strand DNA breaks. Additionally, at least one of the proteins, FANCC, influences apoptotic pathways in response to oxidative damage. Further analysis of the FANC proteins will provide vital information on normal cell responses to damage and allow therapeutic strategies to be developed that will hopefully supplant bone marrow transplantation.

Phenotypic correction of primary Fanconi anemia T cells with retroviral vectors as a diagnostic tool

OBJECTIVE

The aim of this study was to develop a rapid laboratory procedure that is capable of subtyping Fanconi anemia (FA) complementation groups FA-A, FA-C, FA-G, and FA-nonACG patients from a small amount of peripheral blood.

MATERIALS AND METHODS

For this test, primary peripheral blood-derived FA T cells were transduced with oncoretroviral vectors that expressed FANCA, FANCC, or FANCG cDNA. We achieved a high efficiency of gene transfer into primary FA T cells by using the fibronectin fragment CH296 during transduction. Transduced cells were analyzed for correction of the characteristic DNA cross-linker hypersensitivity by cell survival or by metaphase analyses.

RESULTS

Retroviral vectors containing the cDNA for FA-A, FA-C, and FA-G, the most frequent complementation groups in North America, allowed rapid identification of the defective gene by complementation of primary T cells from 12 FA patients.

CONCLUSION

Phenotypic correction of FA T cells using retroviral vectors can be used successfully to determine the FA complementation group immediately after diagnosis of the disease.

Phosphorylation of Fanconi anemia protein, FANCA, is regulated by Akt kinase

Phosphorylation of the Fanconi anemia complementation group A (FANCA) protein is thought to be important for the function of the FA pathway. However, the kinase for FANCA (so-called FANCA-PK) remains to be identified. FANCA has a consensus sequence for Akt kinase near serine 1149 (Ser1149), suggesting that Akt can phosphorylate FANCA. We performed in vitro kinase assays using as substrate either a GST-fusion wild-type (WT) FANCA fragment or a GST-fusion FANCA fragment containing a mutation from serine to alanine at 1149 (FANCA-S1149A). These experiments confirmed that FANCA is phosphorylated at Ser 1149, in vitro. However, (32)P-orthophosphate labeling experiments revealed that FANCA-S1149A was more efficiently phosphorylated than WT-FANCA. Furthermore, phosphorylation of wild-type FANCA was blocked by coexpression of a constitutively active (CA)-Akt and enhanced by a dominant-negative (DN) Akt. Our results suggest that Akt is a negative regulator of FANCA phosphorylation.

Characterization, expression and complex formation of the murine Fanconi anaemia gene product Fancg

BACKGROUND

Fanconi anaemia (FA) is an autosomal recessive chromosomal instability disorder. Six distinct FA disease genes have been identified, the products of which function in an integrated pathway that is thought to support a nuclear caretaker function. Comparison of FA gene characteristics in different species may help to unravel the molecular function of the FA pathway.

RESULTS

We have cloned the murine homologue of the Fanconi anaemia complementation group G gene, FANCG/XRCC9. The murine Fancg protein shows an 83% similarity to the human protein sequence, and has a predicted molecular weight of 68.5 kDa. Expression of mouse Fancg in human FA-G lymphoblasts fully corrects their cross-linker hypersensitivity. At mRNA and protein levels we detected the co-expression of Fancg and Fanca in murine tissues. In addition, mouse Fancg and Fanca proteins co-purify by immunoprecipitation. Upon transfection into Fanca-deficient mouse embryonic fibroblasts EGFP-Fancg chimeric protein was detectable in the nucleus.

CONCLUSIONS

We identified a murine cDNA, Fancg, which cross-complements the cellular defect of human FA-G cells and thus represents a true homologue of human FANCG. Spleen, thymus and testis showed the highest Fancg expression levels. Although Fancg and Fanca are able to form a complex, this interaction is not required for Fancg to accumulate in the nuclear compartment.

Molecular pathogenesis of fanconi anemia

Fanconi anemia (FA) is a rare autosomal recessive chromosomal breakage disorder characterized by the childhood onset of aplastic anemia, developmental defects, cancer susceptibility, and cellular hypersensitivity to DNA-cross-linking agents. FA patients can be divided into at least 8 complementation groups (FA-A, FA-B, FA-C, FA-D1, FA-D2, FA-E, FA-F, and FA-G). FA proteins encoded by 6 cloned FA genes (FANCA, FANCC, FANCD2, FANCE, FANCF, and FANCG) cooperate in a common pathway, culminating in the monoubiquitination of FANCD2 protein and colocalization of FANCD2 and BRCA1 proteins in nuclear foci. These BRCA1 foci have been implicated in the process of homologous recombination-mediated DNA repair. In this review, we will summarize the current progress in the field of FA research and highlight some of the potential functions of the FA pathway in DNA-damage response.

DNA damage processing defects and disease

Inherited defects in DNA repair or the processing of DNA damage can lead to disease. Both autosomal recessive and autosomal dominant modes of inheritance are represented. The diseases as a group are characterized by genomic instability, with eventual appearance of cancer. The inherited defects frequently have a specific DNA damage sensitivity, with cells from affected individuals showing normal resistance to other genotoxic agents. The known defects are subtle alterations in transcription, replication, or recombination, with alternate pathways of processing permitting cellular viability. Distinct diseases may arise from different mutations in one gene; thus, clinical phenotypes may reflect the loss of different partial functions of a gene. The findings indicate that partial defects in transcription or recombination lead to genomic instability, cancer, and characteristic disease phenotypes.

A cytoplasmic serine protein kinase binds and may regulate the Fanconi anemia protein FANCA

Fanconi anemia (FA) is an autosomal recessive disease with congenital anomalies, bone marrow failure, and susceptibility to leukemia. Patient cells show chromosome instability and hypersensitivity to DNA cross-linking agents. At least 8 complementation groups (A-G) have been identified and 6 FA genes (for subtypes A, C, D2, E, F, and G) have been cloned. Increasing evidence indicates that a protein complex assembly of multiple FA proteins, including FANCA and FANCG, plays a crucial role in the FA pathway. Previously, it was reported that FANCA was phosphorylated in lymphoblasts from normal controls, whereas the phosphorylation was defective in those derived from patients with FA of multiple complementation groups. The present study examined phosphorylation of FANCA ectopically expressed in FANCA(-) cells. Several patient-derived mutations abrogated in vivo phosphorylation of FANCA in this system, suggesting that FANCA phosphorylation is associated with its function. In vitro phosphorylation studies indicated that a physiologic protein kinase for FANCA (FANCA-PK) forms a complex with the substrate. Furthermore, at least a part of FANCA-PK as well as phosphorylated FANCA were included in the FANCA/FANCG complex. Thus, FANCA-PK appears to be another component of the FA protein complex and may regulate function of FANCA. FANCA-PK was characterized as a cytoplasmic serine kinase sensitive to wortmannin. Identification of the protein kinase is expected to elucidate regulatory mechanisms that control the FA pathway.

The Chinese hamster FANCG/XRCC9 mutant NM3 fails to express the monoubiquitinated form of the FANCD2 protein, is hypersensitive to a range of DNA damaging agents and exhibits a normal level of spontaneous sister chromatid exchange

Fanconi anemia (FA) is a human autosomal disorder characterized by cancer susceptibility and cellular sensitivity to DNA crosslinking agents such as mitomycin C and diepoxybutane. Six FA genes have been cloned including a gene designated XRCC9 (for X-ray Repair Cross Complementing), isolated using a mitomycin C-hypersensitive Chinese hamster cell mutant termed UV40, and subsequently found to be identical to FANCG. A nuclear complex containing the FANCA, FANCC, FANCE, FANCF and FANCG proteins is needed for the activation of a sixth FA protein FANCD2. When monoubiquitinated, the FANCD2 protein co-localizes with the breast cancer susceptibility protein BRCA1 in DNA damage induced foci. In this study, we have assigned NM3, a nitrogen mustard-hypersensitive Chinese hamster mutant to the same genetic complementation group as UV40. NM3, like human FA cell lines (but unlike UV40) exhibits a normal spontaneous level of sister chromatid exchange. We show that both NM3 and UV40 are also hypersensitive to other DNA crosslinking agents (including diepoxybutane and chlorambucil) and to non-crosslinking DNA damaging agents (including bleomycin, streptonigrin and EMS), and that all these sensitivities are all corrected upon transfection of the human FANCG/XRCC9 cDNA. Using immunoblotting, NM3 and UV40 were found not to express the active monoubiquitinated isoform of the FANCD2 protein, although expression of the FANCD-L isoform was restored in the FANCG cDNA transformants, correlating with the correction of mutagen-sensitivity. These data indicate that cellular resistance to these DNA damaging agents requires FANCG and that the FA gene pathway, via its activation of FANCD2 and that protein's subsequent interaction with BRCA1, is involved in maintaining genomic stability in response not only to DNA interstrand crosslinks but also a range of other DNA damages including DNA strand breaks. NM3 and other "FA-like" Chinese hamster mutants should provide an important resource for the study of these processes in mammalian cells.

The 4N cell cycle delay in Fanconi anemia reflects growth arrest in late S phase

Fanconi anemia (FA) is a human genetic disorder characterized by hypersensitivity to DNA crosslinking agents. Its cellular phenotypes include increased chromosome breakage and a marked cell-cycle delay with 4N DNA content after introduction of interstrand DNA crosslinks (ICL). To further understand the nature of this delay previously described as a G2/M arrest, we introduced ICL specifically during G2 and monitored the cells for passage into mitosis. Our results showed that, even at the highest doses, postreplication ICL produced neither G2/M arrest nor chromosome breakage in FA-A or FA-C cells. This suggests that, similar to wild-type cells, DNA replication is required to trigger both responses. Therefore, the 4N cell DNA content observed in FA cells after ICL treatment also represents incomplete DNA replication and arrest in late S phase. FA fibroblasts from complementation groups A and C were able to recover from the ICL-induced cell-cycle arrest, but took approximately 3 times longer than controls. These results indicate that the FA pathway is required for the efficient resolution of ICL-induced S-phase arrest.

Function of the Fanconi anemia pathway in Fanconi anemia complementation group F and D1 cells

OBJECTIVE

Fanconi anemia (FA) is a human autosomal-recessive cancer susceptibility disorder characterized by multiple congenital abnormalities, progressive bone marrow failure, and cellular sensitivity to mitomycin C (MMC). FA has at least eight complementation groups (A, B, C, D1, D2, E, F, G), and six of the FA genes have been cloned. Several FA proteins, including FANCA, FANCC, FANCF, and FANCG, interact in a nuclear complex, and this complex is required for the activation (monoubiquitination) of the downstream FANCD2 protein. Activation of FANCD2 results in the assembly of FANCD2/BRCA1 foci. The aim of this study was to analyze the FA pathway in several FA patient-derived cell lines.

MATERIALS AND METHODS

We generated an antibody to FANCF and analyzed FANCF expression in human lymphoblasts corresponding to all known FA subtypes. We systematically analyzed the FA pathway (FANCD2 monoubiquitination and assembly of FANCD2 nuclear foci) in patient-derived FA-F and FA-D1 cell lines.

RESULTS

FANCF protein expression is normal in cells derived from all FA complementation groups except FA-F and does not vary during cell cycle progression. FANCF, but not FANCD2, is a component of the nuclear FA protein complex and appears to stabilize other subunits of the complex. FANCF is required for the monoubiquitination of the FANCD2 protein following ionizing radiation. FANCD2 is monoubiquitinated in FA-D1 cells, even though these cells are highly sensitive to MMC.

CONCLUSIONS

The recently cloned FANCF protein is required for FANCD2 activation, and the yet uncloned FANCD1 protein functions further downstream or independently of the FA pathway.

Targeted disruption of the murine Fanconi anemia gene, Fancg/Xrcc9

Fanconi anemia (FA) is a human autosomal recessive cancer susceptibility disorder characterized by cellular sensitivity to mitomycin C and ionizing radiation. 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 cellular pathway. To further understand the in vivo role of one of these human genes (FANCG), we generated a targeted disruption of murine Fancg and bred mice homozygous for the targeted allele. Similar to the phenotype of the previously described Fancc(-/-) and Fanca(-/-) mice, the Fancg(-/-) mice had normal viability and no gross developmental abnormalities. Primary splenic lymphocytes, bone marrow progenitor cells, and murine embryo fibroblasts from the Fancg(-/-) mice demonstrated spontaneous chromosome breakage and increased sensitivity to mitomycin C and, to a lesser extent, ionizing radiation. Fancg(-/-) lymphocytes had a defect in the FA pathway, based on their failure to activate the monoubiquitination of the downstream Fancd2 protein in response to IR. Finally, Fancg(-/-) mice had decreased fertility and abnormal gonadal histology. In conclusion, disruption of the Fancg gene confirms the role of Fancg in the FA pathway. The Fancg(-/-) mouse may be useful as an animal model for future gene therapy and cancer susceptibility studies.

Repair of DNA interstrand cross-links

DNA interstrand cross-links (ICLs) are very toxic to dividing cells, because they induce mutations, chromosomal rearrangements and cell death. Inducers of ICLs are important drugs in cancer treatment. We discuss the main properties of several classes of ICL agents and the types of damage they induce. The current insights in ICL repair in bacteria, yeast and mammalian cells are reviewed. An intriguing aspect of ICLs is that a number of multi-step DNA repair pathways including nucleotide excision repair, homologous recombination and post-replication/translesion repair all impinge on their repair. Furthermore, the breast cancer-associated proteins Brca1 and Brca2, the Fanconi anemia-associated FANC proteins, and cell cycle checkpoint proteins are involved in regulating the cellular response to ICLs. We depict several models that describe possible pathways for the repair or replicational bypass of ICLs.

The Fanconi anemia complementation group C gene product: structural evidence of multifunctionality

The Fanconi anemia (FA) group C gene product (FANCC) functions to protect cells from cytotoxic and genotoxic effects of cross-linking agents. FANCC is also required for optimal activation of STAT1 in response to cytokine and growth factors and for suppressing cytokine-induced apoptosis by modulating the activity of double-stranded RNA-dependent protein kinase. Because not all FANCC mutations affect STAT1 activation, the hypothesis was considered that cross-linker resistance function of FANCC depends on structural elements that differ from those required for the cytokine signaling functions of FANCC. Structure-function studies were designed to test this notion. Six separate alanine-substituted mutations were generated in 3 highly conserved motifs of FANCC. All mutants complemented mitomycin C (MMC) hypersensitive phenotype of FA-C cells and corrected aberrant posttranslational activation of FANCD2 in FA-C mutant cells. However, 2 of the mutants, S249A and E251A, failed to correct defective STAT1 activation. FA-C lymphoblasts carrying these 2 mutants demonstrated a defect in recruitment of STAT1 to the interferon gamma (IFN-gamma) receptor and GST-fusion proteins bearing S249A and E251A mutations were less efficient binding partners for STAT1 in stimulated lymphoblasts. These same mutations failed to complement the characteristic hypersensitive apoptotic responses of FA-C cells to tumor necrosis factor-alpha (TNF-alpha) and IFN-gamma. Cells bearing a naturally occurring FANCC mutation (322delG) that preserves this conserved region showed normal STAT1 activation but remained hypersensitive to MMC. The conclusion is that a central highly conserved domain of FANCC is required for functional interaction with STAT1 and that structural elements required for STAT1-related functions differ from those required for genotoxic responses to cross-linking agents. Preservation of signaling capacity of cells bearing the del322G mutation may account for the reduced severity and later onset of bone marrow failure associated with this mutation.

Chromosome instability syndromes

The chromosome instability syndromes, ataxia telangiectasia (A-T), Fanconi anaemia (FA) and Bloom syndrome (BS) have been known for many years. More recently Nijmegen breakage syndrome (NBS) and ataxia telangiectasia-like disorder (ATLD) have been identified. A-T, ATLD and NBS form a group of disorders all of which show very similar cellular features that result from the consequences of increased sensitivity to ionizing radiation (IR). They also share some clinical features, particularly A-T and ATLD, and all show an immunodeficiency. A-T and NBS both show a predisposition to lymphoid tumours. Fanconi anaemia can be caused by mutations in eight different genes, although the majority of mutations are accounted for by FANCA and FANCC. The very rare Bloom syndrome is caused by mutation in a single gene, BLM. An important feature which all of these disorders have in common is that the genes identified are involved in aspects of recombination repair of DNA damage.

FANCC interacts with Hsp70 to protect hematopoietic cells from IFN-gamma/TNF-alpha-mediated cytotoxicity

The Fanconi anemia (FA) complementation group C gene product (FANCC) functions to protect hematopoietic cells from cytotoxicity induced by interferon-gamma (IFN-gamma), tumor necrosis factor-alpha (TNF-alpha) and double-stranded RNA (dsRNA). Because apoptotic responses of mutant FA-C cells involve activation of interferon-inducible, dsRNA-dependent protein kinase PKR, we sought to identify FANCC-binding cofactors that may modulate PKR activation. We identified the molecular chaperone Hsp70 as an interacting partner of FANCC in lymphoblasts and HeLa cells using 'pull-down' and co-immunoprecipitation experiments. In vitro binding assays showed that the association of FANCC and Hsp70 involves the ATPase domain of Hsp70 and the central 320 residues of FANCC, and that both Hsp40 and ATP/ADP are required. In whole cells, Hsp70-FANCC binding and protection from IFN-gamma/TNF-alpha-induced cytotoxicity were blocked by alanine mutations located in a conserved motif within the Hsp70-interacting domain of FANCC. We therefore conclude that FANCC acts in concert with Hsp70 to prevent apoptosis in hematopoietic cells exposed to IFN-gamma and TNF-alpha.

[Fanconi's anemia and molecular biology research]

Fanconi's anemia is a rare autosomal recessive disease characterized by congenital abnormalities, a progressive pancytopenia and a predisposition to cancer. The diagnosis is based on an abnormal increase of spontaneous chromosome breakage, more specifically on a clear-cut increase of chromosome breakage in the presence of bifunctional alkylating agents. Eight complementation groups (A to H) have been defined, and the genes corresponding to four of these groups have been cloned (FANCA, FANCC, FANCF and FANCG). The function of the proteins encoded by the genes of Fanconi's anemia remains unknown. Numerous studies indicate that different cellular processes are probably involved, including DNA repair pathways, apoptosis, cell cycle regulation and oxygen metabolism. Nevertheless, the exact cellular and molecular mechanisms implicated in Fanconi's anemia remain a challenge for fundamental research. The treatment of Fanconi's anemia is also the subject of intense research, bearing principally upon bone marrow transplantation, which is successful in the case of HLA-identical sibling donors, and gene therapy, which is still at a preliminary stage on the clinical level.

Functional analysis of patient-derived mutations in the Fanconi anemia gene, FANCG/XRCC9

OBJECTIVE

Fanconi anemia (FA) is an autosomal-recessive cancer susceptibility syndrome with seven complementation groups. Six of the FA genes have been cloned (corresponding to subtypes A, C, D2, E, F, and G) and the encoded proteins interact in a common pathway. Patient-derived mutations in FA genes have been helpful in delineating functional domains of FA proteins. The purpose of this work was to subtype FA patient-derived cell lines in our repository and to identify FA gene mutations.

METHODS

We subtyped 62 FA patients as type A, G, C, or non-ACG by using a combination of retroviral gene transfer and immunoblot analysis. Among these FA patients, we identified six FA-G patients for further analysis. We used a strategy involving amplification of FANCG/XRCC9 exons and direct sequencing to identify novel FANCG mutations in cell lines derived from these FA-G patients. We functionally analyzed FANCG mutant alleles by transducing the corresponding cDNAs into a known FA-G indicator cell line and scoring correction of MMC sensitivity.

RESULTS

Our results demonstrate a wide range of mutations in the FANCG gene (splice, nonsense, and missense mutations). Based on this mutational screen, a carboxy terminal functional domain of the FANCG protein appears to be required for complementation of FA-G cells and for normal assembly of the FANCA/FANCG/FANCC protein complex.

CONCLUSION

The identification of patient-derived mutant alleles of FA genes can provide important insights to the function of FA proteins. FA subtyping is also a necessary precondition for gene therapy.

Normal cellular radiosensitivity in an adult Fanconi anaemia patient with marked clinical radiosensitivity

BACKGROUND

Fanconi anaemia is a rare disease associated with cellular sensitivity to chemicals (e.g. mitomycin C and diepoxybutane); variable but mild cellular radiosensitivity has also been reported.

MATERIALS AND METHODS

A 32-year-old patient with Fanconi anaemia and tonsillar carcinoma, treated by radiotherapy, was found to exhibit profound clinical radiosensitivity. Confluent, ulcerating oropharyngeal mucositis developed after a conventionally fractionated dose of 34Gy and healing was incomplete by 2 months after cessation of therapy.

RESULTS

Cellular radiosensitivity assays and RPLD studies from this patient did not suggest any major detectable radiosensitivity.

CONCLUSION

There is a discrepancy between the observed clinical radiosensitivity and the usual "predictive" radiosensitivity assays in this patient with Fanconi anaemia.

Current knowledge on the pathophysiology of Fanconi anemia: from genes to phenotypes

Fanconi anemia (FA) is an autosomal recessive disease characterized by congenital anomalies, bone marrow failure, and leukemia susceptibility. FA cells show chromosome instability and hypersensitivity to DNA cross-linking agents such as mitomycin C. Recent studies indicate that there are at least 8 genetically distinct FA groups (A, B, C, D1, D2, E, F, G). To date, 6 genes (for A, C, D2, E, F, and G) have been cloned. In this review, we describe the structures and functions of FA proteins. Increasing evidence indicates that the multiple FA proteins cooperate in a biochemical pathway and/or a multimer complex. FANCD2, a downstream component of the FA pathway, has recently been shown to be ubiquitinated in response to DNA damage and to translocate to nuclear foci containing BRCA1, a breast cancer susceptibility gene product, suggesting a role for this protein in DNA repair functions. We also describe 2 emerging issues: genotype-phenotype relationships and mosaicism. The FA pathway is likely to play a critical role as a caretaker of genomic integrity in hematopoietic stem cells. Clarifying the molecular basis of this disease may provide new insights into the pathogenesis of bone marrow failure syndromes and myeloid malignancies.

Fanconi anemia proteins localize to chromatin and the nuclear matrix in a DNA damage- and cell cycle-regulated manner

Fanconi anemia (FA) is a genetic disease characterized by congenital defects, bone marrow failure, and cancer susceptibility. Cells from patients with FA exhibit genomic instability and hypersensitivity to DNA cross linking agents such as mitomycin C. Despite the identification of seven complementation groups and the cloning of six genes, the function of the encoded gene products remains elusive. The FancA (Fanconi anemia complementation group A), FancC, and FancG proteins have been detected within a nuclear complex, but no change in level, binding, or localization has been reported as a result of drug treatment or cell cycle. We show that in immunofluorescence studies, FancA appears as a non-nucleolar nuclear protein that is excluded from condensed, mitotic chromosomes. Biochemical fractionation reveals that the FA proteins are found in nuclear matrix and chromatin and that treatment with mitomycin C results in increase of the FA proteins in nuclear matrix and chromatin fractions. This induction occurs in wild-type cells and mutant FA-D (Fanconi complementation group D) cells but not in mutant FA-A cells. Immunoprecipitation of FancA protein in chromatin demonstrates the coprecipitation of FancA, FancC, and FancG, showing that the FA proteins move together as a complex. Also, fractionation of mitotic cells confirms the lack of FA proteins in chromatin or the nuclear matrix. Furthermore, phosphorylation of FancG was found to be temporally correlated with exit of the FA complex from chromosomes at mitosis. Taken together, these findings suggest a role for FA proteins in chromatin and nuclear matrix.

Erythropoiesis: Current Clinical Practice: Advances in the Genetics and Biology of Fanconi Anaemia

The autosomal recessive disorder Fanconi anaemia (FA) has been the subject of intense study for over a decade. The genes mutated in FA patients are being cloned, but so far, the sequences of these genes have not given any clear indication of their function. Various models for the function of the FA proteins have been postulated to explain the spontaneous chromosomal abnormalities and clastogen sensitivity described in FA cells. This review summarises the critical experimental evidence for and against these models, and attempts to give some indication of the possible mechanisms by which mutations in FA genes cause patients to suffer pancytopaenia and acute myeloid leukaemia, as well as an increased risk of other malignancies.

Foci on fanconi

The protein network protecting the stability of the genome is defective in Fanconi anemia (FA). The newest in a series of FA proteins is involved in DNA damage response, but the mechanism is still unclear. Clues may come from yeast two-hybrid experiments, an extraordinarily successful tool for determining molecular function.

The emerging genetic and molecular basis of Fanconi anaemia

The past few years have witnessed a considerable expansion in our understanding of the pathways that maintain chromosome stability in dividing cells through the identification of genes that are mutated in certain human chromosome instability disorders. Cells that are derived from patients with Fanconi anaemia (FA) show spontaneous chromosomal instability and mutagen hypersensitivity, but FA poses a unique challenge as the nature of the DNA-damage-response pathway thought to be affected by the disease has long been a mystery. However, the recent cloning of most of the FA-associated genes, and the characterization of their protein products, has provided tantalizing clues as to the molecular basis of this disease.

Evaluation of growth and hormonal status in patients referred to the International Fanconi Anemia Registry

OBJECTIVES

1) To determine the extent of short stature in patients with Fanconi anemia (FA); 2) to determine the extent and nature of endocrinopathy in FA; 3) to assess the impact on height of any endocrinopathies in these patients; and 4) to study the correlation, if any, between height, endocrinopathy, and FA complementation group.

STUDY DESIGN

Fifty-four patients with FA, 30 males and 24 females from 47 unrelated families, were prospectively evaluated in a Pediatric Clinical Research Center. The patients ranged in age from 0.1-31.9 years, with the mean age at assessment 8.6 years.

RESULTS

Endocrine abnormalities were found in 44 of the 54 FA patients tested (81%), including short stature, growth hormone (GH) insufficiency, hypothyroidism, glucose intolerance, hyperinsulinism, and/or overt diabetes mellitus. Twenty-one of 48 (44%) participants had a subnormal response to GH stimulation; 19 of 53 (36%) had overt or compensated hypothyroidism, while 8 of 40 participants had reduced thyroid-hormone binding. Two patients were diabetic at the time of study; impaired glucose tolerance was found in 8 of 40 patients (25%), but most surprisingly, hyperinsulinemia was present in 28 of 39 (72%) participants tested. Significantly, spontaneous overnight GH secretion was abnormal in all patients tested (n = 13). In addition, participants demonstrated a tendency toward primary hypothyroidism with serum tetraiodothyronine levels at the lower range of normal, while also having thyrotropin (thyroid-stimulating hormone) levels at the high end of normal. Sixteen patients were assigned to FA complementation group A, (FA-A), 12 to FA-C, and 5 to FA-G; 10 of the 12 participants in FA-C were homozygous for a mutation in the intron-4 donor splice site of the FANCC gene. Patients in groups FA-A and FA-G were relatively taller than the group as a whole (but still below the mean for the general population), whereas those in FA-C had a significantly reduced height for age. GH response to stimulation testing was most consistently normal in participants from FA-G, but this did not reach statistical significance. The tendency toward hypothyroidism was more pronounced in participants belonging to complementation groups FA-C and FA-G, whereas insulin resistance was most evident in patients in FA-G, and least evident in those in FA-C. Short stature was a very common finding among the patients with a mean height >2 standard deviations below the reference mean (standard deviation score: -2.35 +/- 0.28). Patients with subnormal GH response and those with overt or compensated hypothyroidism were shorter than the group with no endocrinopathies. The heights of those participants with glucose or insulin abnormalities were less severely affected than those of normoglycemic, normoinsulinemic participants, although all were significantly below the normal mean. The mean height standard deviation score of patients with entirely normal endocrine function was also >2 standard deviations below the normal mean, demonstrating that short stature is an inherent feature of FA.

CONCLUSION

Endocrinopathies are a common feature of FA, primarily manifesting as glucose/insulin abnormalities, GH insufficiency, and hypothyroidism. Although short stature is a well-recognized feature of FA, 23 patients (43%) were within 2 standard deviations, and 5 of these (9% of the total) were actually above the mean for height for the general population. Those patients with endocrine dysfunction are more likely to have short stature. These data indicate that short stature is an integral feature of FA, but that superimposed endocrinopathies further impact on growth. The demonstration of abnormal endogenous GH secretion may demonstrate an underlying hypothalamic-pituitary dysfunction that results in poor growth.

Deficient DNA end joining activity in extracts from fanconi anemia fibroblasts

Fanconi anemia (FA) is a genetic disorder associated with genomic instability and cancer predisposition. Cultured cells from FA patients display a high level of spontaneous chromosome breaks and an increased frequency of intragenic deletions, suggesting that FA cells may have deficiencies in properly processing DNA double strand breaks. In this study, an in vitro plasmid DNA end joining assay was used to characterize the end joining capabilities of nuclear extracts from diploid FA fibroblasts from complementation groups A, C, and D. The Fanconi anemia extracts had 3-9-fold less DNA end joining activity and rejoined substrates with significantly less fidelity than normal extracts. Wild-type end joining activity could be reconstituted by mixing FA-D extracts with FA-A or FA-C extracts, while mixing FA-A and FA-C extracts had no effect on end joining activity. Protein expression levels of the DNA-dependent protein kinase (DNA-PK)/Ku-dependent nonhomologous DNA end-joining proteins Xrcc4, DNA ligase IV, Ku70, and Ku86 in FA and normal extracts were indistinguishable, as were DNA-dependent protein kinase and DNA end binding activities. The end joining activity as measured by the assay was not sensitive to the DNA-PK inhibitor wortmannin or dependent on the nonhomologous DNA end-joining factor Xrcc4. However, when DNA/protein ratios were lowered, the end joining activity became wortmannin-sensitive and no difference in end joining activity was observed between normal and FA extracts. Taken together, these results suggest that the FA fibroblast extracts have a deficiency in a DNA end joining process that is distinct from the DNA-PK/Ku-dependent nonhomologous DNA end joining pathway.

Role of double-stranded RNA-dependent protein kinase in mediating hypersensitivity of Fanconi anemia complementation group C cells to interferon gamma, tumor necrosis factor-alpha, and double-stranded RNA

Hematopoietic cells bearing inactivating mutations of Fanconi anemia group C (FANCC) are excessively apoptotic and demonstrate hypersensitivity not only to cross-linking agents but also to interferon gamma (IFN-gamma) and tumor necrosis factor-alpha. Seeking essential signaling pathways for this phenotype, this study quantified constitutive and induced RNA-dependent protein kinase (PKR) activation in Fanconi anemia cells of the C complementation group (FA-C). PKR was constitutively phosphorylated and exhibited an increased binding affinity for double-stranded RNA (dsRNA) in FANCC(-/-) cells. FANCC(-/-) cells were hypersensitive to both dsRNA and the combination of dsRNA and IFN-gamma in that these agents induced a higher fraction of apoptosis in FANCC(-/-) cells than in normal cells. Overexpression of wild-type PKR-sensitized FANCC(-/-) cells to apoptosis induced by IFN-gamma and dsRNA. Conversely, inhibition of PKR function by enforced expression of a dominant-negative inhibitory mutant of PKR (PKRDelta6) substantially reduced the IFN and dsRNA hypersensitivity of FANCC(-/-) cells. Two PKR target molecules, IkappaB-alpha and IRF-1, were not differentially activated in FANCC(-/-) cells, but enforced expression of a nonphosphorylatable form of eukaryotic translation initiation factor-2alpha reversed the PKR-mediated block of messenger RNA translation and partially abrogated the PKR-mediated apoptosis in FANCC(-/-) cells. Because no evidence was found of a PKR/FANCC complex in normal cells, it was concluded that an essential function of FANCC is to suppress, indirectly, the activity of PKR and that FANCC inactivation results in IFN hypersensitivity, at least in part, because this function of FANCC is abrogated.