Identification of two complementation groups in Fanconi anemia

Correction of Fanconi Anemia Type C Phenotypic Abnormalities Using a Clinically Suitable Retroviral Vector Infection Protocol

Fanconi anemia (FA) is a complex autosomal recessive disease with hematologic manifestations characterized by a progressive hypoplastic anemia, hypersensitivity to clastogenic agents, and an increased incidence of acute myelogenous leukemia. The cDNA that corrects one of four FA complementation subtypes, named Fanconi anemia Type C (FAC) has recently been identified. We constructed a simplified recombinant retrovirus (vMFGFAC) encoding only the FAC cDNA, and tested its ability to correct the FAC defect in a lymphocytic cell line and primary mobilized blood progenitor cells. In addition, the gene transfer efficiency using a clinically applicable gene transfer protocol into normal primitive hematopoietic progenitor cells, high proliferating potential colony forming cells (HPP-CFC), derived from CD34+ purified cord blood cells was examined. The gene transfer efficiency was significantly enhanced when cells were transduced with supernatant while adherent to a 30/35 KD fragment of fibronectin, FN30/35, and was similar to efficiency obtained by coculture with retrovirus packaging cells. Transduction of an FAC deficient lymphoid cell line with vMFGFAC supernatant resulted in an enhanced cell viability, and G-CSF mobilized peripheral blood cells from an FAC-deficient patient transduced with the vMFGFAC virus demonstrated enhanced progenitor cell colony formation. These data indicate that the vMFGFAC virus allows functional complementation of FAC in lymphoblasts and primary hematopoietic progenitors, and that primitive cord blood hematopoietic stem/progenitor cells can be transduced at an efficiency comparable to protocols using cocultivation if adherent to FN 30/35 fragment.

A never-ending story: the steadily growing family of the FA and FA-like genes

Among the chromosome fragility-associated human syndromes that present cancer predisposition, Fanconi anemia (FA) is unique due to its large genetic heterogeneity. To date, mutations in 21 genes have been associated with an FA or an FA-like clinical and cellular phenotype, whose hallmarks are bone marrow failure, predisposition to acute myeloid leukemia and a cellular and chromosomal hypersensitivity to DNA crosslinking agents exposure. The goal of this review is to trace the history of the identification of FA genes, a history that started in the eighties and is not yet over, as indicated by the cloning of a twenty-first FA gene in 2016.

Monoketone analogs of curcumin, a new class of Fanconi anemia pathway inhibitors

BACKGROUND

The Fanconi anemia (FA) pathway is a multigene DNA damage response network implicated in the repair of DNA lesions that arise during replication or after exogenous DNA damage. The FA pathway displays synthetic lethal relationship with certain DNA repair genes such as ATM (Ataxia Telangectasia Mutated) that are frequently mutated in tumors. Thus, inhibition of FANCD2 monoubiquitylation (FANCD2-Ub), a key step in the FA pathway, might target tumor cells defective in ATM through synthetic lethal interaction. Curcumin was previously identified as a weak inhibitor of FANCD2-Ub. The aim of this study is to identify derivatives of curcumin with better activity and specificity.

RESULTS

Using a replication-free assay in Xenopus extracts, we screened monoketone analogs of curcumin for inhibition of FANCD2-Ub and identified analog EF24 as a strong inhibitor. Mechanistic studies suggest that EF24 targets the FA pathway through inhibition of the NF-kB pathway kinase IKK. In HeLa cells, nanomolar concentrations of EF24 inhibited hydroxyurea (HU)-induced FANCD2-Ub and foci in a cell-cycle independent manner. Survival assays revealed that EF24 specifically sensitizes FA-competent cells to the DNA crosslinking agent mitomycin C (MMC). In addition, in contrast with curcumin, ATM-deficient cells are twofold more sensitive to EF24 than matched wild-type cells, consistent with a synthetic lethal effect between FA pathway inhibition and ATM deficiency. An independent screen identified 4H-TTD, a compound structurally related to EF24 that displays similar activity in egg extracts and in cells.

CONCLUSIONS

These results suggest that monoketone analogs of curcumin are potent inhibitors of the FA pathway and constitute a promising new class of targeted anticancer compounds.

L'anémie de Fanconi : aspects cellulaires et moléculaires

Fanconi anemia (FA) is a recessive human cancer prone syndrome featuring bone marrow failure, developmental abnormalities and hypersensitivity to DNA crosslinking agents exposure. 11 among 12 FA gene have been isolated. The biochemical functions of the FANC proteins remain poorly understood. Anyhow, to cope with DNA crosslinks a cell needs a functional FANC pathway. Moreover, the FANC proteins appear to be involved in cell protection against oxidative damage and in the control of TNF-alpha activity. In this review, we describe the current understanding of the FANC pathway and we present how it may be integrated in the complex networks of proteins involved in maintaining the cellular homeostasis.

3R coordination by Fanconi anemia proteins

Fanconi anemia (FA) is a recessive cancer prone syndrome featuring bone marrow failure and hypersensitivity to DNA crosslinks. Nine FA genes have been isolated so far. The biochemical function(s) of the FA proteins remain(s) poorly determined. However, a large consensus exists on the evidence that, to cope with DNA cross-links, a cell needs a functional FA pathway. In this review, we resume current understanding of how the FA pathway works in response to DNA damage and how it is integrated in a complex network of proteins involved in the maintenance of the genetic stability.

Heterogeneity in Fanconi anemia: evidence for 2 new genetic subtypes

Fanconi anemia (FA) is an autosomal recessive syndrome featuring diverse symptoms including progressive bone marrow failure and early occurrence of acute myeloid leukemia. Nine genetic subtypes have been described for FA (A, B, C, D1, D2, E, F, G, and L), all of which have been connected to distinct disease genes, except B. Here we report on 8 unrelated FA patients who were excluded from the known subtypes on the basis of phenotypic correction or genetic data. Four of these cell lines failed to complement each other in somatic cell hybrids and therefore represent a new group, termed FA-I. The remaining cell lines complemented group FA-I but did not complement each other, thus representing a second new group, FA-J. Both FA-I and -J cell lines were capable of forming an FA multiprotein core complex. This complex is required for activation of the FANCD2 protein by mono-ubiquitination, a key downstream event in the FA pathway. In FA-I cells FANCD2 was not mono-ubiquitinated, indicating a defect upstream in the FA pathway, whereas in FA-J cells FANCD2 was mono-ubiquitinated, indicating a downstream defect. Our results suggest that the FA pathway of genome stabilization may be controlled by at least 11 different genes, including FANCI and FANCJ.

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.

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.

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.

Preclinical protocol for in vivo selection of hematopoietic stem cells corrected by gene therapy in Fanconi anemia group C

Fanconi anemia (FA) is an autosomal recessive disorder characterized by birth defects, increased incidence of malignancy, progressive bone marrow failure, and cellular hypersensitivity to DNA cross-linking agents. Bone marrow transplantation is therapeutic and therefore FA is a candidate disease for hematopoietic gene therapy. We have previously used mitomycin C (MMC) to achieve in vivo selection of wild-type hematopoietic stem cells (HSC) transplanted into FANCC knockout mice. However, clinical application of MMC in human FA gene therapy is unlikely because of its unknown toxicity profile in human FA patients. In contrast, cyclophosphamide (CPA) and gamma-irradiation (IR) are already in use with human FA patients and we therefore tested these regimens for their ability to achieve selection of genetically corrected HSCs in vivo. We found that nonmyeloablative doses of CPA or IR or combinations of CPA + IR were highly efficient at achieving in vivo selection of transplanted wild-type HSC. Furthermore, this nontoxic regimen also selected FANCC-mutant HSC corrected by ex vivo retroviral gene therapy. We suggest those nontoxic doses of CPA and/or IR could also be used to enhance gene therapy in human FA patients.

The genetics of Fanconi's anaemia

Fanconi's anaemia (FA) is an inherited bone marrow failure syndrome characterized by considerable clinical and cellular heterogeneity. This has also been recently demonstrated at the genetic and molecular levels following cloning of four out of the seven FA genes. Although this now enables molecular diagnosis in the majority of patients, because of the considerable molecular heterogeneity, the diepoxybutane/mitomycin-C stress test based on the increased chromosomal instability seen in FA cells, compared to normal controls, remains the front-line diagnostic test. This FA cell hallmark has led to the suggestion that FA may represent a defect in DNA repair although the precise function of the cloned FA genes remains unknown. Recent data suggest that they function in a novel cell pathway which has an important role in maintaining chromosome stability. The advances in the genetics of FA have already had some impact on diagnosis--for example, identification of patients with somatic mosaicism who have atypical clinical presentations--but to date they have had little impact on treatment. However, new treatments may now follow; indeed, for a number of reasons, FA may be a good candidate for haemopoietic gene therapy.

Complementation analysis in Fanconi anemia: assignment of the reference FA-H patient to group A

Fanconi anemia (FA) is an autosomal recessive disorder with diverse clinical symptoms and extensive genetic heterogeneity. Of eight FA genes that have been implicated on the basis of complementation studies, four have been identified and two have been mapped to different loci; the status of the genes supposed to be defective in groups B and H is uncertain. Here we present evidence indicating that the patient who has been the sole representative of the eighth complementation group (FA-H) in fact belongs to group FA-A. Previous exclusion from group A was apparently based on phenotypic reversion to wild-type rather than on genuine complementation in fusion hybrids. To avoid the pitfall of reversion, future assignment of patients with FA to new complementation groups should conform with more-stringent criteria. A new group should be based on at least two patients with FA whose cell lines are excluded from all known groups and that fail to complement each other in fusion hybrids, or, if only one such cell line were available, on a new complementing gene that carries pathogenic mutations in this cell line. On the basis of these criteria, the current number of complementation groups in FA is seven.

Genetic disorders associated with cancer predisposition and genomic instability

Genomic instability in its broadest sense is a feature of virtually all neoplastic cells. In addition to the mutations and/or gene amplifications that appear to be a prerequisite for the acquisition of a neoplastic phenotype, human cancers exhibit other "markers" of genomic instability--in particular, a high degree of aneuploidy. Indeed, many studies have shown that aneuploidy is an almost invariant feature of cancer cells, and it has been argued by some that the emergence of aneuploid cells is a necessary step during tumorigenesis. The functional link between genomic instability and cancer is strengthened by the existence of several "genetic instability" disorders of humans that are associated with a moderate to severe increase in the incidence of cancers. These disorders include ataxia telangiectasia, Bloom's syndrome, Fanconi anemia, xeroderma pigmentosum, and Nijmegen breakage syndrome, all of which are very rare and are inherited in a recessive manner. Analysis of the cells from such cancer-prone individuals is clearly a potentially fruitful approach for delineating the genetic basis for instability in the genome. It is assumed that by identifying the underlying cause of genetic instability in these disorders, one can derive valuable information not only about the basis of particular genetic diseases, but also about the underlying causes of genomic instability in sporadic cancers in the general population. In this article, we review the clinical and cellular properties of genetic instability disorders associated with cancer predisposition. In particular, we focus on the rapid advances made in our understanding of these disorders that have derived from the cloning of the genes mutated in each case. Because in many instances the affected genes have analogs in lower eukaryotic species, we shall discuss how studies in yeasts in particular have proved valuable in our understanding of human diseases and predisposition to cancer.

In Vivo Selection of Wild-Type Hematopoietic Stem Cells in a Murine Model of Fanconi Anemia

Fanconi anemia (FA) is an autosomal recessive disorder characterized by birth defects, increased incidence of malignancy, and progressive bone marrow failure. Bone marrow transplantation is therapeutic and, therefore, FA is a candidate disease for hematopoietic gene therapy. The frequent finding of somatic mosaicism in blood of FA patients has raised the question of whether wild-type bone marrow may have a selective growth advantage. To test this hypothesis, a cohort radio-ablated wild-type mice were transplanted with a 1:1 mixture of FA group C knockout (FACKO) and wild-type bone marrow. Analysis of peripheral blood at 1 month posttransplantation showed only a moderate advantage for wild-type cells, but upon serial transplantation, clear selection was observed. Next, a cohort of FACKO mice received a transplant of wild-type marrow cells without prior radio-ablation. No wild-type cells were detected in peripheral blood after transplantation, but a single injection of mitomycin C (MMC) resulted in an increase to greater than 25% of wild-type DNA. Serial transplantation showed that the selection occurred at the level of hematopoietic stem cells. No systemic side effects were observed. Our results show that in vivo selection for wild-type hematopoietic stem cells occurs in FA and that it is enhanced by MMC administration.

In Vivo Selection of Wild-Type Hematopoietic Stem Cells in a Murine Model of Fanconi Anemia

Fanconi anemia (FA) is an autosomal recessive disorder characterized by birth defects, increased incidence of malignancy, and progressive bone marrow failure. Bone marrow transplantation is therapeutic and, therefore, FA is a candidate disease for hematopoietic gene therapy. The frequent finding of somatic mosaicism in blood of FA patients has raised the question of whether wild-type bone marrow may have a selective growth advantage. To test this hypothesis, a cohort radio-ablated wild-type mice were transplanted with a 1:1 mixture of FA group C knockout (FACKO) and wild-type bone marrow. Analysis of peripheral blood at 1 month posttransplantation showed only a moderate advantage for wild-type cells, but upon serial transplantation, clear selection was observed. Next, a cohort of FACKO mice received a transplant of wild-type marrow cells without prior radio-ablation. No wild-type cells were detected in peripheral blood after transplantation, but a single injection of mitomycin C (MMC) resulted in an increase to greater than 25% of wild-type DNA. Serial transplantation showed that the selection occurred at the level of hematopoietic stem cells. No systemic side effects were observed. Our results show that in vivo selection for wild-type hematopoietic stem cells occurs in FA and that it is enhanced by MMC administration.

Drug sensitivity spectra in Fanconi anemia lymphoblastoid cell lines of defined complementation groups

Fanconi anemia (FA) is one of several genetic diseases with characteristic cellular hypersensitivity to DNA crosslinking agents which suggest that FA proteins may function as part of DNA repair processes. At the clinical level, FA is characterized by bone marrow failure that affects children at an early age. The clinical phenotype is heterogeneous and includes various congenital malformations as well as cancer predisposition. FA patients are distributed into eight complementation groups suggesting a complex molecular pathway. Three of the eight possible FA genes have been cloned, although their function(s) have not been identified. FA cells are highly sensitive to DNA crosslinking agents (mitomycin C (MMC) and diepoxybutane), with some variability between cell lines. Sensitivity to monofunctional alkylating agents has been reported in some cases, although these studies were performed with genetically unclassified FA cells. To further analyse and characterize the newly identified FA complementation groups, we tested their sensitivity to UV radiation, monofunctional and bifunctional alkylating agents and to the X-ray mimetic drug bleomycin. We found that FA complementation groups D to H show increased sensitivity to the X-ray mimetic drug bleomycin. Furthermore, the single known FA-H cell line shows increased sensitivity to ethylethane sulfonate (EMS), methylmethane sulfonate (MMS) in addition to the characteristic sensitivity to crosslinking agents, suggesting a broader spectrum of drug sensitivities in FA cells.

The molecular and cellular biology of Fanconi anemia

Fanconi anemia is a rare autosomal recessive disease characterized by multiple congenital abnormalities, bone marrow failure, and cancer susceptibility. The mean age of onset of anemia is 8 years, and the mean survival is 16 years. Death usually results from complications of bone marrow failure. Considerable progress in Fanconi anemia research has resulted from the recent identification and cloning of three Fanconi anemia genes. The current review describes the structure and function of the Fanconi anemia genes and describes the role of the encoded Fanconi anemia proteins in a cellular pathway controlling chromosome stability.

[Fanconi disease: study of 43 cases in southern Tunisia]

BACKGROUND

To report the epidemiologic, clinical, biological features and course of Fanconi's anemia in southern Tunisia.

PATIENTS AND METHODS

During a period of 12 years we observed 43 cases. For each patient, careful clinical, biological (hemogram, myelogram, bone marrow biopsy, hemoglobin electrophoresis, karyotype) and radiological (skeleton X-rays, abdominal echography and intravenous urography) examinations were performed. All the patients who were at a pancytopenia stage were given androgens. None had a bone marrow allograft.

RESULTS

There were 24 girls and 19 boys. The mean age at diagnosis was 10 years and 9 months. The familial character was present in 53% of the cases. The most frequent initial complaint was anemic syndrome (69%). In ten cases (24%), the diagnosis has been established during a familial investigation. Malformations were present in all cases (abnormal pigmentation: 86%; skeletal maturation retardation: 83%; facial dysmorphy: 76%; statural hypotrophy: 65%; bone abnormalities: 53%; renal malformations: 44%). Anemia was present in 88% of the cases, thrombocytopenia and neutropenia in all cases. Bone marrow was hypoplastic or aplastic in all cases on biopsies. Spontaneous chromosomal breaks were found in 79% of the studied cases. Fetal hemoglobin was increased in 80% of the studied cases with a mean level of 20.5%. Actuarial survival rate at 5 years was 48%, but long survival durations were rare (eight out of 43 patients).

DISCUSSION

This disease, rare in the world, seems to be frequent in southern Tunisia. A normal karyotype (with classical techniques), found in five patients, could not discard the diagnosis; for this reason, the use of sensitizing agents should improve the sensitivity of the test. Besides, an increased level of fetal hemoglobin enabled us to suggest the diagnosis in some cases. Androgenotherapy increased the survival duration to more than 5 years in eight patients. However, bone marrow allograft remains the only possibility of cure.

Is Fanconi anemia caused by a defect in the processing of DNA damage?

Fanconi anemia (FA) is an autosomal genetic disease characterized by a complex array of developmental disorders, a high predisposition to bone marrow failure and to acute myelogenous leukemia. The chromosomal instability and the hypersensitivity to DNA cross-linking agents led to its classification with the DNA repair disorders. This review aimed at establishing whether it is still appropriate to consider 1/approximately FA within a DNA repair framework taking into account the recently discovered genetic heterogeneity characteristics of the defect (eight complementation groups). We discuss the possibility that the FA proteins interact to form a complex which may control different functions, including the processing of specific DNA lesions. Such a complex may act as a sensor to initiate protective systems as well as transcription of specific genes specifying, among others proteins, growth factors. Such steps may be organized as a linear cascade or more likely under the form of a web network.

Fanconi anemia C gene product plays a role in the fidelity of blunt DNA end-joining

Mutations in genes controlling the correct functioning of the replicative, repair and recombination machineries may lead to genomic instability. A high level of spontaneous chromosomal aberrations amplified by treatment with DNA cross-linking agents is the hallmark of Fanconi anemia (FA), an inherited chromosomal instability syndrome associated with cancer proneness. Two of the eight FA genes have been cloned (FAA and FAC), but their function has not yet been defined. The lack of homology with known genes suggests the involvement of FA genes in a novel pathway specific to vertebrates. Using a DNA end-joining assay in cultured cells, we studied the processing of both blunt and cohesive-ended double strand breaks (DSB) in normal and FA cells. The results show that: (i) the overall ligation efficiency is normal in FA lymphoblasts; (ii) in FA-C, error-free processing of blunt-ended DSB is markedly decreased, resulting in a higher deletion frequency and larger deletion size; (iii) the fidelity of processing of blunt-DSB is completely restored in FACC cells (complemented with wild-type FAC gene) and the deletion size shifted to values similar to that observed in normal cells; (iv) the fidelity of cohesive end-joining is not affected in FA cells; (v) activities and/or expression of known factors involved in DSB processing, such as the components of the DNA-PK complex and XRCC4, are normal in FA cells. Our results provide strong evidence that the lack of a functional FAC gene results in loss of fidelity of end-joining, which likely accounts for the FA-C phenotype of chromosome instability. We conclude that FAC, and perhaps all FA gene products, are likely to play a role in the fidelity of end-joining of specific DSB.

Applicability of nonsyngeneic cell models for screening of genes in monogenetic diseases via differential display technique

Conventional subtraction library techniques or DNA-transfection studies are standard techniques applied for identification and isolation of genes relevant in monogenetic diseases like Fanconi anemia (FA). The differential display technique (DDT) was developed to compare mRNA expression between a mutant cell line and its syngeneic control and allows comparison of almost all mRNA species within a short time. However, for identification of genes relevant in monogenetic diseases, no syngeneic cell model is available. In this report, we show that the use of nonsyngeneic diploid human fibroblasts does not increase the number of differentially displayed bands due to diversity of untranslated regions. cDNA bands with a length of up to 1000 bp were obtained and applied to DDT. After screening of about 13000 cDNA bands, only 0.5% were found to be differentially expressed between FA and control cells. Finally, three mRNAs were cloned and verified in Northern blot experiments to be differentially expressed in FA fibroblasts. The low number of differentially displayed cDNA bands in DDT indicates the usefulness of this statistical, molecular approach for identification of multiple genes dysregulated in gene regulation cascades potentially relevant for cell cycle disturbances.

MxA overexpression reveals a common genetic link in four Fanconi anemia complementation groups

Fanconi anemia (FA) consists of a group of at least five autosomal recessive disorders that share both clinical (e.g., birth defects and hematopoietic failure) and cellular (e.g., sensitivity to cross-linking agents and predisposition to apoptosis) features with each other. However, a common pathogenetic link among these groups has not been established. To identify genetic pathways that are altered in FA and characterize shared molecular defects, we used mRNA differential display to isolate genes that have altered expression patterns in FA cells. Here, we report that the expression of an interferon-inducible gene, MxA, is highly upregulated in cells of FA complementation groups A, B, C, and D, but it is suppressed in FA group C cells complemented with wild-type FAC cDNA as well as in non-FA cells. A posttranscriptional mechanism rather than transcriptional induction appears to account for MxA overexpression. Forced expression of MxA in Hep3B cells enhances their sensitivity to mitomycin C and induces apoptosis, similar to the FA phenotype. Thus, MxA is a downstream target of FAC and is the first genetic marker to be identified among multiple FA complementation groups. These data suggest that FA subtypes converge onto a final common pathway, which is intimately related to the interferon signaling mechanism. Constitutive activity of this pathway may explain a number of the phenotypic features of FA, particularly the pathogenesis of bone marrow failure.

The fidelity of double strand breaks processing is impaired in complementation groups B and D of Fanconi anemia, a genetic instability syndrome

In mammalian cells, nonhomologous end-joining is the predominant mechanism to eliminate DNA double strand breaks. Such events are at the origin of deletion mutagenesis and chromosomal rearrangements. The hallmark of Fanconi anemia, an inherited cancer prone disorder, is increased chromosomal breakage associated to over-production of deletions. Knowing that double strand breaks are at the origin of deletion mutagenesis, the question arises whether their processing is affected in FA. We set up a "host cell end-joining assay" to analyze the fate of double strand breaks into extrachromosomal substrates transiently replicated in normal and FA-D lymphoblasts. Although no difference in plasmid survival was found, blunt-ended breaks were sealed with significantly lower fidelity in FA cells, resulting in a higher deletion frequency and a larger deletion size. The results suggest that FA-D and FA-B gene products are likely to play a role in end-joining fidelity of specific DNA double strand breaks.

Functional Correction of Fanconi Anemia Group A Hematopoietic Cells by Retroviral Gene Transfer

Fanconi anemia (FA) is an autosomal recessive genetic disorder characterized by a variety of physical anomalies, bone marrow failure, and an increased risk for malignancy. FA cells exhibit chromosomal instability and are hypersensitive to DNA cross-linking agents such as mitomycin C (MMC). FA is a clinically heterogeneous disorder and can be functionally divided into at least five different complementation groups (A-E). We previously described the use of a retroviral vector expressing the FAC cDNA in the complementation of mutant hematopoietic cells from FA-C patients. This vector is currently being tested in a clinical trial of ex vivo hematopoietic progenitor cell transduction. The FA-A group accounts for over 65% of all FA cases, and the FAA cDNA was recently identified by both expression and positional cloning techniques. We report here the transduction and phenotypic correction of lymphoblastoid cell lines from four unrelated FA-A patients, using two amphotropic FAA retroviral vectors. Expression of the FAA transgene was adequate to normalize cell growth, cell-cycle kinetics, and chromosomal breakage in the presence of MMC. We then analyzed the effect of retroviral vector transduction on hematopoietic progenitor cell growth. After FAA transduction of mutant progenitor cells, either colony number or colony size increased in the presence of MMC. In addition, FAA but not FAC retroviral transduction markedly improved colony growth of progenitor cells derived from an unclassified FA patient. FAA retroviral vectors should be useful for both complementation studies and clinical trials of gene transduction.

Cytoplasmic Localization of a Functionally Active Fanconi Anemia Group A–Green Fluorescent Protein Chimera in Human 293 Cells

Hypersensitivity to cross-linking agents and predisposition to malignancy are characteristic of the genetically heterogeneous inherited bone marrow failure syndrome, Fanconi anemia (FA). The protein encoded by the recently cloned FA complementation group A gene, FAA, has been expected to localize in the nucleus as based on the presence of sequences homologous to a bipartite nuclear localization signal (NLS) and a leucine repeat motif. In contrast to this expectation, we show here that a functionally active FAA-green fluorescent protein (GFP) hybrid resides in the cytoplasmic compartment of human kidney 293 cells. In accordance with this finding, disruption of the putative NLS by site-directed mutagenesis failed to affect both subcellular localization and the capacity to complement hypersensitivity to the cross-linking agent mitomycin C in FA-A lymphoblasts. Furthermore, the N-terminal part of FAA with the putative NLS at amino acid position 18 to 35 showed no nuclear translocation activity when fused to GFP, while the first 115 N-terminal amino acids appeared to be indispensable for the complementing activity in FA-A cells. Similarly, mutagenesis studies of the putative leucine repeat showed that, even though this region of the protein is important for complementing activity, this activity does not depend on an intact leucine zipper motif. Finally, fusion of the NLS motif derived from the SV40 large T antigen to FAA could not direct the hybrid protein into the nucleus of 293 cells, suggesting that FAA is somehow maintained in the cytoplasm via currently unknown mechanisms. Thus, like the first identified FA protein, FAC, FAA seems to exert its function in the cytoplasmic compartment suggesting FA proteins to be active in a yet to be elucidated cytoplasmic pathway that governs hematopoiesis and protects against genomic instability.

Evidence for at least eight Fanconi anemia genes

Fanconi anemia (FA) is an autosomal recessive chromosomal breakage disorder with diverse clinical symptoms including progressive bone marrow failure and increased cancer risk. FA cells are hypersensitive to crosslinking agents, which has been exploited to assess genetic heterogeneity through complementation analysis. Five complementation groups (FA-A through FA-E) have so far been distinguished among the first 20 FA patients analyzed. Complementation groups in FA are likely to represent distinct disease genes, two of which (FAC and FAA) have been cloned. Following the identification of the first FA-E patient, additional patients were identified whose cell lines complemented groups A-D. To assess their possible assignment to the E group, we introduced selection markers into the original FA-E cell line and analyzed fusion hybrids with three cell lines classified as non-ABCD. All hybrids were complemented for cross-linker sensitivity, indicating nonidentity with group E. We then marked the three non-ABCDE cell lines and examined all possible hybrid combinations for complementation, which indicated that each individual cell line represented a separate complementation group. These results thus define three new groups, FA-F, FA-G, and FA-H, providing evidence for a minimum of eight distinct FA genes.

The Fanconi Anemia Polypeptide, FAC, Binds to the Cyclin-Dependent Kinase, cdc2

Fanconi anemia (FA) is an autosomal recessive disorder characterized by developmental defects, bone marrow failure, and cancer susceptibility. Cells derived from FA patients are sensitive to crosslinking agents and have a prolonged G2 phase, suggesting a cell cycle abnormality. Although transfection of type-C FA cells with the FAC cDNA corrects these cellular abnormalities, the molecular function of the FAC polypeptide remains unknown. In the current study we show that expression of the FAC polypeptide is regulated during cell cycle progression. In synchronized HeLa cells, FAC protein expression increased during S phase, was maximal at the G2 /M transition, and declined during M phase. In addition, the FAC protein coimmunoprecipitated with the cyclin-dependent kinase, cdc2. We next tested various mutant forms of the FAC polypeptide for binding to cdc2. A patient-derived mutant FAC polypeptide, containing a point mutation at L554P, failed to bind to cdc2. The FAC/cdc2 binding interaction therefore correlated with the functional activity of the FAC protein. Moreover, binding of FAC to cdc2 was mediated by the carboxyl-terminal 50 amino acids of FAC in a region of the protein required for FAC function. Taken together, our results suggest that the binding of FAC and cdc2 is required for normal G2 /M progression in mammalian cells. Absence of a functional interaction between FAC and cdc2 in FA cells may underlie the cell cycle abnormality and clinical abnormalities of FA.

The Fanconi Anemia Polypeptide, FAC, Binds to the Cyclin-Dependent Kinase, cdc2

Fanconi anemia (FA) is an autosomal recessive disorder characterized by developmental defects, bone marrow failure, and cancer susceptibility. Cells derived from FA patients are sensitive to crosslinking agents and have a prolonged G2 phase, suggesting a cell cycle abnormality. Although transfection of type-C FA cells with the FAC cDNA corrects these cellular abnormalities, the molecular function of the FAC polypeptide remains unknown. In the current study we show that expression of the FAC polypeptide is regulated during cell cycle progression. In synchronized HeLa cells, FAC protein expression increased during S phase, was maximal at the G2 /M transition, and declined during M phase. In addition, the FAC protein coimmunoprecipitated with the cyclin-dependent kinase, cdc2. We next tested various mutant forms of the FAC polypeptide for binding to cdc2. A patient-derived mutant FAC polypeptide, containing a point mutation at L554P, failed to bind to cdc2. The FAC/cdc2 binding interaction therefore correlated with the functional activity of the FAC protein. Moreover, binding of FAC to cdc2 was mediated by the carboxyl-terminal 50 amino acids of FAC in a region of the protein required for FAC function. Taken together, our results suggest that the binding of FAC and cdc2 is required for normal G2 /M progression in mammalian cells. Absence of a functional interaction between FAC and cdc2 in FA cells may underlie the cell cycle abnormality and clinical abnormalities of FA.

Complementation group assignments in Fanconi anemia fibroblast cell lines from North America

Fanconi anemia is a rare autosomal recessive disease characterized by developmental defects of the thumb and radius, childhood onset of pancytopenic anemia and increased risk of leukemia. At least five complementation groups (A-E) have been defined but only the FAC gene has been cloned. Cells can be assigned to complementation group C by direct mutation analysis. To facilitate the search for additional FA genes and to measure the frequency of complementation groups, we have established new genetically marked immortalized FA-A and FA-D fibroblast cell lines and show their usefulness as universal fusion donors. These reference FA cell lines facilitated somatic cell fusion analysis and enabled us to assign the complementation group in 16 unrelated FA patients from North America. The majority of patients, belong to FA complementation group A (69%), followed by FA-C (18%), FA-D (4%) and FA-B or FA-E (9%).

Expression cloning of a cDNA for the major Fanconi anaemia gene, FAA

Fanconi anaemia (FA) is an autosomal recessive disorder characterized by a diversity of clinical symptoms including skeletal abnormalities, progressive bone marrow failure and a marked predisposition to cancer. FA cells exhibit chromosomal instability and hyper-responsiveness to the clastogenic and cytotoxic effects of bifunctional alkylating (cross-linking) agents, such as diepoxybutane (DEB) and mitomycin C (MMC). Five complementation groups (A-E) have been distinguished on the basis of somatic cell hybridization experiments, with group FA-A accounting for over 65% of the cases analysed. A cDNA for the group C gene (FAC) was reported and localized to chromosome 9q22.3 (ref.8). Genetic map positions were recently reported for two more FA genes, FAA (16q24.3) and FAD (3p22-26). Here we report the isolation of a cDNA representing the FAA gene, following an expression cloning method similar to the one used to clone the FAC gene. The 5.5-kb cDNA has an open reading frame of 4,368 nucleotides. In contrast to the 63-kD cytosolic protein encoded by the FAC gene, the predicted FAA protein (M(r) 162, 752) contains two overlapping bipartite nuclear localization signals and a partial leucine zipper consensus, which are suggestive of a nuclear localization.

Assessment of the flexed-tail mouse as a possible model for Fanconi anemia: analysis of mitomycin C-induced micronuclei

Fanconi anemia (FA) is a rare, autosomal recessive disorder characterized by elevated frequencies of chromosome aberrations, hypersensitivity to DNA cross-linking agents and predisposition to cancer. At least 5 complementation groups (FA-A to FA-E) underlie FA and the gene defective in FA-C (FAC) has been cloned. The mouse orthologue, Fac, maps in close proximity to the f locus, on chromosome 13, which codes for the flexed-tail mouse phenotype, raising the possibility that f and Fac are synonymous. If this were the case flexed-tail mice could be used as mouse models for FA-C to help determine the basic defect and to evaluate clinical intervention and gene therapy. To further characterize the flexed-tail mouse, the frequency of micronuclei (a measure of chromosomal aberrations) induced by mitomycin C (MMC), an alkylating and DNA cross-linking agent, was analyzed in peripheral blood and bone marrow erythrocytes. Although a higher spontaneous micronucleus frequency was seen in flexed tail mice in comparison to wild-type mice, the sensitivity to MMC was not elevated. This result suggests that f and Fac are different genes and that the flexed-tail mouse is not a model for FA-C.

Fanconi anaemia in Italy: high prevalence of complementation group A in two geographic clusters

Cell fusion studies using lymphoblastoid cell lines from Fanconi anaemia (FA) patients have identified five complementation groups (FA-A to FA-E) among European FA patients. In Italy, of the 45 FA families referred to the Italian Registry of Fanconi Anaemia (RIAF), 15 took part in a project for the identification of complementation groups. Since three immortalized lymphoblast lines were resistant to a cross-linking agent, we analysed only 12 patients by complementation analysis and found that 11 belong to complementation group A. Four and seven families came from two geographic clusters in the Veneto and Campania regions, respectively, which are thought to consist of aggregates of related families in reproductive isolation. The clinical characteristics of the patients showed both intra- and interfamilial heterogeneity, although overall the disease had a relatively mild course. Since the populations in both Veneto and Campania are likely to represent genetic isolates, our finding predicts linkage disequilibrium for markers flanking the FAA gene. DNAs from these FA families may thus be utilized for positional cloning of this gene through haplotype disequilibrium mapping.

Cytoplasmic localization of FAC is essential for the correction of a prerepair defect in Fanconi anemia group C cells

Mutations in the gene defective in Fanconi anemia complementation group C, FAC, are responsible for a subset of Fanconi anemia, a group of autosomal recessive disorders characterized by chromosomal instability, hypersensitivity to cross-linking agents, and cancer susceptibility. Although abnormalities in DNA repair have been suspected, localization of the FAC gene product to the cytoplasm has cast doubt on such a mechanism. Monitoring of interstrand DNA cross-linking shows that the predominant defect in group C cells is in the initial induction of cross-links, not in repair synthesis. Both the cross-linking defect and the enhanced cytotoxicity of cross-linkers on Fanconi anemia group C cells are corrected completely by cytoplasmic isoforms of the FAC protein, but not by an isoform targeted to the nucleus. The ability of FAC to correct these phenotypic abnormalities reaches a maximum threshold despite overexpression leading to higher levels of cytosolic protein. These results demonstrate that cytoplasmic localization is essential for the intracellular activity of the FAC protein. It is proposed that this activity is coupled to a cytoplasmic defense mechanism against a specific class of genotoxic agents.

Fanconi anaemia complementation groups in Germany and The Netherlands. European Fanconi Anaemia Research group

Fanconi anaemia (FA) is an autosomal recessive chromosomal instability disorder with extensive genetic heterogeneity. We determined the genetic subtypes in 28 ethnically and clinically unselected FA patients from Germany and The Netherlands, by complementation analysis. All five currently known complementation analysis. All five currently known complementation groups (FA-A to FA-E) appeared to be represented in the sample studied. The distribution of subtypes differed markedly in the two countries: FA-A patients were most prevalent in Germany (13/22, 59%), whereas in The Netherlands, the majority of patients were FA-C (4/6, 67%). This geographical inhomogeneity has implications for mutation-screening strategies in European FA patients.

Developmental expression of the Fac gene correlates with congenital defects in Fanconi anemia patients

Fanconi anemia (FA) is a genetically heterogeneous, autosomal recessive disorder characterized by a variety of congenital and skeletal malformations, progressive pancytopaenia and predisposition to malignancies. While the basic defect in this disease is not known, the cloning of the gene defective in FA group C patients (FAC) allows analysis of its expression pattern, which may provide clues about the functional properties of the protein. This paper describes the distribution of Fac transcripts during murine development (8-19.5 days p.c.), using RNA in situ hybridization. Fac is initially expressed (8-10 days p.c.) in the mesenchyme and its derivatives with osteogenic potential. The transcript is also apparent at later stages of bone development (13-19.5 days p.c.), localized to cells of the inner perichondrium, periosteum and zone of endochondral ossification. In the latter, Fac transcripts are seen in cells from both osteogenic and hematopoietic lineages. Fac mRNA is also seen in intramembranous cranial and facial bones. In addition, Fac signal is detected in non-skeletal tissues: brain, whisker follicles, lung, kidney, gut and stomach. Fac expression is high in progenitor cell populations but is downregulated in differentiating cells that give rise to connective tissue. The pattern of Fac expression is consistent with the skeletal and non-skeletal congenital abnormalities in FA patients. As well, expression in rapidly dividing progenitors is consistent with hypotheses regarding the nature of the basic defect in FA: a role of the protein in DNA repair or protection from oxygen toxicity.

Localisation of the Fanconi anaemia complementation group A gene to chromosome 16q24.3

Fanconi anaemia (FA) is an autosomal recessive disorder associated with diverse developmental abnormalities, bone-marrow failure and predisposition to cancer. FA cells show increased chromosome breakage and hypersensitivity to DNA cross-linking agents such as diepoxybutane and mitomycin C. Somatic-cell hybridisation analysis of FA cell lines has demonstrated the existence of at least five complementation groups (FA-A to FA-E), the most common of which is FA-A. This genetic heterogeneity has been a major obstacle to the positional cloning of FA genes by classical linkage analysis. The FAC gene was cloned by functional complementation, and localised to chromosome 9q22.3 (ref. 2), but this approach has thus far failed to yield the genes for the other complementation groups. We have established a panel of families classified as FA-A by complementation analysis, and used them to search for the FAA gene by linkage analysis. We excluded the previous assignment by linkage of an FA gene to chromosome 20q, and obtained conclusive evidence for linkage of FAA to microsatellite markers on chromosome 16q24.3. Strong evidence of allelic association with the disease was detected with the marker D16S303 in the Afrikaner population of South Africa, indicating the presence of a founder effect.

Identification of cytosolic proteins that bind to the Fanconi anemia complementation group C polypeptide in vitro. Evidence for a multimeric complex

The oligomeric structure of Fanconi anemia complementation group C (FACC) was investigated in mammalian cell lysates. Using an affinity-purified polyclonal antibody, FACC was immunoprecipitated from radiolabeled cell lysates and shown to form monomers of 63 kDa. Association of FACC with heterologous proteins was investigated by co-precipitation of radiolabeled proteins with a recombinant chimeric FACC molecule fused to the constant portion of the human IgG1 heavy chain (FACC gamma 1). Expression of FACC gamma 1 in FACC-deficient Fanconi anemia (FA) lymphoblasts corrected the hypersensitivity of these cells to mitomycin C. Binding of FACC gamma 1 to protein A-agarose and incubation with radiolabeled cell lysates identified three polypeptides with molecular masses of 65, 50, and 35 kDa that were also detected on immunoblots probed with the purified FACC gamma 1 polypeptide. FACC, as well as the three FACC-binding polypeptides, co-fractionated with cytosolic and membrane extracts. Binding was specific for the FACC moiety of FACC gamma 1 and was detected in cytosolic extracts of a number of FA and non-FA mammalian cells. These results demonstrate that FACC binds directly to a family of ubiquitous cytosolic proteins and is conserved in a wide range of mammalian cells.

Comparison of the effects of DNA topoisomerase inhibitors on lymphoblasts from normal and Fanconi anemia donors

DNA topoisomerases modify supercoiled DNA through concerted breaking and rejoining of the DNA strands and consequently play a key role in DNA biosynthesis and processing. It has been suggested that topoisomerases may facilitate access to damaged sites of excision repair enzymes due to their property to relax supercoiled DNA. We show here that treatment with nalidixic acid and novobiocin, which affects topoisomerase II activity among other targets, impairs the incision of 8-methoxypsoralen photoinduced DNA interstrand cross-links in normal human fibroblasts. Since cells derived from Fanconi anemia (FA) demonstrate hypersensitivity to DNA cross-linking agents associated with a reduced repair efficiency of cross-links, we compared the effects of different topoisomerase I and II inhibitors on FA and normal lymphoblasts. No differences were found in growth inhibition or induction of chromosome aberrations between FA and normal cells. The specificity of inhibitors is questionable and even if topoisomerases are indeed inhibited alternative pathways may be involved. However, our observations provisionally suggested that topoisomerases activities are normal in FA cells.

Comparative study of Fanconi anemia in children of different ethnic origin in South Africa

A comparative study of clinical, hematologic, and cytogenetic findings was made in 40 black and 35 white children with Fanconi anemia. The black children were Bantu-speaking Negroid stock of diverse tribal origin. The white children were predominantly Afrikaans stock of Dutch/German/French Huguenot origin. All of the patients had IFAR scores of 2 to 4+ and over 80% in each group had increased spontaneous and/or mutagen-induced chromosomal breakage (CB-positive). There were no significant clinical differences between black and white patients or between CB-pos and CB-neg patients, with the exception of white children in whom significantly more CB-pos patients had thumb and radial anomalies than the CB-neg patients. The age-at-onset of hematologic manifestations was the same for all groups, but more black than white CB-pos patients were severely anemic at the time of diagnosis. Response to androgen and steroid therapy occurred in only 33% of black children compared with 86-90% of white children; 81% of black patients died during the 18 year study period compared with 30% of white children, but the age at death was similar. More sophisticated studies are required to determine whether these differences are genetically determined or related to cultural, educational, and socio-economic differences between the two ethnic groups.

Localization of Fanconi anemia C protein to the cytoplasm of mammalian cells

Features of chromosomal aberrations, hypersensitivity to DNA crosslinking agents, and predisposition to malignancy have suggested a fundamental anomaly of DNA repair in Fanconi anemia. The function of the recently isolated FACC (Fanconi anemia group C complementing) gene for a subset of this disorder is not yet known. The notion that FACC plays a direct role in DNA repair would predict that the polypeptide should reside in the nucleus. In this study, a polyclonal antiserum raised against FACC was used to determine the subcellular location of the polypeptide. Immunofluorescence and subcellular fractionation studies of human cell lines as well as COS-7 cells transiently expressing human FACC showed that the protein was localized primarily to the cytoplasm under steady-state conditions, transit through the cell cycle, and exposure to crosslinking or cytotoxic agents. However, placement of a nuclear localization signal from the simian virus 40 large tumor antigen at the amino terminus of FACC directed the hybrid protein to the nuclei of transfected COS-7 cells. These observations suggest an indirect role for FACC in regulating DNA repair in this group of Fanconi anemia.

Apoptosis is not involved in the hypersensitivity of Fanconi anemia cells to mitomycin C

A striking feature of Fanconi anemia (FA) cells is their hypersensitivity towards crosslinking agents such as mitomycin C (MMC). In this study, we have shown that treatment of lymphoblastoid cells with MMC resulted in nuclear fragmentation, chromatin condensation, and DNA degradation, which is characteristic of apoptosis. The level of DNA fragmentation 48 hours after MMC treatment reached approximately 33% in both control and FA cells. In addition, 24 hours after drug addition a decrease in the number of cells in the G2/M phase of the cell cycle was seen. This coincided with the appearance of apoptotic cells in the sub-G1 phase, indicating that once the cells had passed through G2/M, apoptosis occurred. The number of apoptotic cells increased to 60% 96 hours after MMC treatment. The number of apoptotic cells increased to 60% 96 hours after MMC treatment. The onset and level of apoptosis was found to be identical in FA and control cell lines, indicating that the FA defect does not lead to abnormal apoptotic cell death.

Mutagenic response and repair of cis-DDP-induced DNA cross-links in the Chinese hamster V79 cell mutant V-H4 which is homologous to Fanconi anemia (group A)

Previously, it has been shown that the V-H4 mutant of Chinese hamster V79 cells is homologous to Fanconi anemia (FA) group A cells. This hamster cell mutant shows a specific sensitivity to DNA cross-linking agents; therefore, the induction and repair of DNA cross-links were studied in V-H4 and wild-type V79 cells after cis-DDP treatment by the DNA alkaline elution technique. A significant difference in repair of these lesions in V-H4 and wild-type cells was observed. After the cis-DDP treatment (24 h) about 3 times more cross-links remained in V-H4 cells in comparison to the parental V79 cells. These results indicate that the process of cross-link repair in V-H4 cells is hampered when compared to that of wild-type cells. To assess the effect of slower removal of DNA cross-links on the mutability of V-H4, the induction of mutants at the hypoxanthine-guanine phosphoribosyltransferase locus (HPRT) by cis-DDP was studied in V-H4 and V79 cells. Despite the increased cytotoxicity of cis-DDP to V-H4 cells, the mutation induction at the HPRT locus was not significantly different in both cell lines, but when the frequency of the hprt mutants was plotted against survival, hypomutability was observed in V-H4 cells after the cis-DDP treatment.

Fanconi anemia revisited: old ideas and new advances

This review summarizes both historical and more recent data on the clinical, cellular and genetic features of Fanconi anemia (FA), a rare autosomal recessive disorder. FA patients are characterized by pancytopenia, congenital malformations, growth delay and an increased susceptibility to the development of malignancies, particularly acute myelogenous leukemia. FA cells show chromosomal fragility, slow growth and increased sensitivity to DNA crosslinking agents. FA can be caused by defects in any one of at least four genes. Two general hypotheses have been proposed to explain the underlying defect: loss of a DNA repair function or of a step in the defense toward oxygen toxicity. After many attempts to clone the FA genes, the first one, that defective in group C, has been cloned by complementation of the increased sensitivity of FA(C) cells to mitomycin C and diepoxybutane. This gene (FACC) codes for a novel protein and is ubiquitously expressed. Mutations in various FA(C) patients that cause loss of function have been identified. The review concludes by suggesting directions for future research in FA.