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

Correction of cross-linker sensitivity of Fanconi anemia group F cells by CD33-mediated protein transfer

Studies have previously described the feasibility of receptor-mediated protein transfer in a cell culture model of Fanconi anemia (FA) group C. This study explores the versatility of this approach by using an antibody single-chain fusion protein to correct the phenotypic defect in FA group F cells. A 68.5-kd chimeric protein (His-M195FANCF) was expressed, consisting of a His tag, a single-chain antibody to the myeloid antigen CD33, and the FANCF protein, as well as a 43-kd His-FANCF fusion protein lacking the antibody motif, in Escherichia coli. The nickel-agarose-purified His-M195FANCF protein bound specifically to the surface of HeLa cells transfected with CD33 and internalized through vesicular structures. The fusion protein, but not CD33, sorted to the nucleus, consistent with the known nuclear localization of FANCF. No similar binding or internalization was observed with His-FANCF. Pretreatment of the transfected cells with chloroquine abolished nuclear accumulation, but there was little change with brefeldin A, indicating a minimal if any role for the Golgi apparatus in mediating transport from endosomes to the cytosol and the nucleus. The intracellular half-life of His-M195FANCF was approximately 160 minutes. Treatment of CD33-transfected FA group F lymphoblastoid cells with 0.1 mg/mL His-M195FANCF conferred resistance to mitomycin C. No similar protection was noted in CD33(-) parental cells or CD33(+) FA cells belonging to groups A and C. These results demonstrate that antibody-directed, receptor-mediated protein transfer is a versatile method for the delivery of biologically active proteins into hematopoietic cells.

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.

Functional analysis of the putative peroxidase domain of FANCA, the Fanconi anemia complementation group A protein

Fanconi anemia (FA) is an autosomal recessive disorder manifested by chromosomal breakage, birth defects, and susceptibility to bone marrow failure and cancer. At least seven complementation groups have been identified, and the genes defective in four groups have been cloned. The most common subtype is complementation group A. Although the normal functions of the gene products defective in FA cells are not completely understood, a clue to the function of the FA group A gene product (FANCA) was provided by the detection of limited homology in the amino terminal region to a class of heme peroxidases. We evaluated this hypothesis by mutagenesis and functional complementation studies. We substituted alanine residues for the most conserved FANCA residues in the putative peroxidase domain and tested their effects on known biochemical and cellular functions of FANCA. While the substitution mutants were comparable to wild-type FANCA with regard to their stability, subcellular localization, and interaction with FANCG, only the Trp(183)-to-Ala substitution (W183A) abolished the ability of FANCA to complement the sensitivity of FA group A cells to mitomycin C. By contrast, TUNEL assays for apoptosis after exposure to H2O2 showed no differences between parental FA group A cells, cells complemented with wild-type FANCA, and cells complemented with the W183A of FANCA. Moreover, semiquantitative RT-PCR analysis for the expression of the peroxide-sensitive heme oxygenase gene showed appropriate induction after H2O2 exposure. Thus, W183A appears to be essential for the in vivo activity of FANCA in a manner independent of its interaction with FANCG. Moreover, neither wild-type FANCA nor the W183A mutation appears to alter the peroxide-induced apoptosisor peroxide-sensing ability of FA group A cells.

Posttranscriptional cell cycle–dependent regulation of human FANCC expression

The Fanconi Anemia (FA) Group C complementation group gene (FANCC) encodes a protein, FANCC, with a predicted Mr of 63000 daltons. FANCC is found in both the cytoplasmic and the nuclear compartments and interacts with certain other FA complementation group proteins as well as with non-FA proteins. Despite intensive investigation, the biologic roles of FANCC and of the other cloned FA gene products (FANCA and FANCG) remain unknown. As an approach to understanding FANCC function, we have studied the molecular regulation of FANCC expression. We found that although FANCCmRNA levels are constant throughout the cell cycle, FANCC is expressed in a cell cycle-dependent manner, with the lowest levels seen in cells synchronized at the G1/S boundary and the highest levels in the M-phase. Cell cycle–dependent regulation occurred despite deletion of the 5′ and 3′ FANCC untranslated regions, indicating that information in the FANCC coding sequence is sufficient to mediate cell cycle–dependent regulation. Moreover, inhibitors of proteasome function blocked the observed regulation. We conclude that FANCC expression is controlled by posttranscriptional mechanisms that are proteasome dependent. Recent work has demonstrated that the functional activity of FA proteins requires the physical interaction of at least FANCA, FANCC, and FANCG, and possibly of other FA and non-FA proteins. Our observation of dynamic control of FANCC expression by the proteasome has important implications for understanding the molecular regulation of the multiprotein complex.

Posttranscriptional cell cycle–dependent regulation of human FANCC expression

The Fanconi Anemia (FA) Group C complementation group gene (FANCC) encodes a protein, FANCC, with a predicted Mr of 63000 daltons. FANCC is found in both the cytoplasmic and the nuclear compartments and interacts with certain other FA complementation group proteins as well as with non-FA proteins. Despite intensive investigation, the biologic roles of FANCC and of the other cloned FA gene products (FANCA and FANCG) remain unknown. As an approach to understanding FANCC function, we have studied the molecular regulation of FANCC expression. We found that although FANCCmRNA levels are constant throughout the cell cycle, FANCC is expressed in a cell cycle-dependent manner, with the lowest levels seen in cells synchronized at the G1/S boundary and the highest levels in the M-phase. Cell cycle–dependent regulation occurred despite deletion of the 5′ and 3′ FANCC untranslated regions, indicating that information in the FANCC coding sequence is sufficient to mediate cell cycle–dependent regulation. Moreover, inhibitors of proteasome function blocked the observed regulation. We conclude that FANCC expression is controlled by posttranscriptional mechanisms that are proteasome dependent. Recent work has demonstrated that the functional activity of FA proteins requires the physical interaction of at least FANCA, FANCC, and FANCG, and possibly of other FA and non-FA proteins. Our observation of dynamic control of FANCC expression by the proteasome has important implications for understanding the molecular regulation of the multiprotein complex.

Effects of cystic fibrosis and congenital bilateral absence of the vas deferens-associated mutations on cystic fibrosis transmembrane conductance regulator-mediated regulation of separate channels

The protein defective in cystic fibrosis (CF), the CF transmembrane-conductance regulator (CFTR), functions as an epithelial chloride channel and as a regulator of separate ion channels. Although the consequences that disease-causing mutations have on the chloride-channel function have been studied extensively, little is known about the effects that mutations have on the regulatory function. To address this issue, we transiently expressed CFTR-bearing mutations associated with CF or its milder phenotype, congenital bilateral absence of the vas deferens, and determined whether mutant CFTR could regulate outwardly rectifying chloride channels (ORCCs). CFTR bearing a CF-associated mutation in the first nucleotide-binding domain (NBD1), DeltaF508, functioned as a chloride channel but did not regulate ORCCs. However, CFTR bearing disease-associated mutations in other domains retained both functions, regardless of the associated phenotype. Thus, a relationship between loss of CFTR regulatory function and disease severity is evident for NBD1, a region of CFTR that appears important for regulation of separate channels.

Investigation of Fanconi anemia protein interactions by yeast two-hybrid analysis

Fanconi anemia is a chromosomal breakage disorder with eight complementation groups (A-H), and three genes (FANCA, FANCC, and FANCG) have been identified. Initial investigations of the interaction between FANCA and FANCC, principally by co-immunoprecipitation, have proved controversial. We used the yeast two-hybrid assay to test for interactions of the FANCA, FANCC, and FANCG proteins. No activation of the reporter gene was observed in yeast co-expressing FANCA and FANCC as hybrid proteins, suggesting that FANCA does not directly interact with FANCC. However, a high level of activation was found when FANCA was co-expressed with FANCG, indicating strong, direct interaction between these proteins. Both FANCA and FANCG show weak but consistent interaction with themselves, suggesting that their function may involve dimerisation. The site of interaction of FANCG with FANCA was investigated by analysis of 12 mutant fragments of FANCG. Although both N- and C-terminal fragments did interact, binding to FANCA was drastically reduced, suggesting that more than one region of the FANCG protein is required for proper interaction with FANCA.

Targeting of a Heterologous Protein to a Regulated Secretion Pathway in Cultured Endothelial Cells

The stimulation of regulated exocytosis in vascular endothelial cells (EC) by a variety of naturally occurring agonists contributes to the interrelated processes of inflammation, thrombosis, and fibrinolysis. The Weibel-Palade body (WPB) is a well-described secretory granule in EC that contains both von Willebrand factor (vWF) and P-selectin, but the mechanisms responsible for the targeting of these proteins into this organelle remain poorly understood. Through adenoviral transduction, we have expressed human growth hormone (GH) as a model of regulated secretory protein sorting in EC. Immunofluorescence microscopy of EC infected with GH-containing recombinant adenovirus (GHrAd) demonstrated a granular distribution of GH that colocalized with vWF. In contrast, EC infected with an rAd expressing the IgG1 heavy chain (IG), a constitutively secreted protein, did not demonstrate colocalization of IG and vWF. In response to phorbol ester, GH as well as endogenously synthesized vWF were rapidly released from GHrAd-infected EC. By immunofluorescence microscopy, granular colocalization of GH with endogenous tissue-type plasminogen activator (tPA) was also demonstrated, and most of the tPA colocalized with vWF. These data indicate that EC are capable of selectively targeting heterologous proteins, such as GH, to the regulated secretory pathway, which suggests that EC and neuroendocrine cells share common protein targeting recognition signals or receptors.

Analysis of epitope-tagged forms of the dyskeratosis congenital protein (dyskerin): identification of a nuclear localization signal

The X-linked form of the bone marrow failure syndrome Dyskeratosis congenital is caused by mutations in dyskerin, a 514 amino acid protein that is presumed to play a role in ribosome biogenesis. Here we report that dyskerin tagged with the human immunoglobulin epitope localizes to nuclei of transfected HeLa and COS-1 cells. A carboxyl-terminal domain consisting of amino acids 467-475 and encoding KKEKKKSKK is both necessary and sufficient to mediate nuclear entry. Immunoglobulin-tagged dyskerin did not interact with the Fanconi anemia group A protein, FANCA. These results suggest a nuclear role for dyskerin. Moreover, hematopoietic failure observed in both Dyskeratosis congenital and the most common type of Fanconi anemia is unlikely to have a common mechanism resulting from abnormal physical interactions between the respective gene products of these disorders.

Fanconi anemia: genetic testing in Ashkenazi Jews

Fanconi anemia (FA) is an autosomal recessive disorder characterized clinically by progressive pancytopenia, diverse congenital abnormalities, and a predisposition to malignancy, particularly acute myelogenous leukemia (AML). Hypersensitivity of FA cells to the clastogenic effect of crosslinking agents such as diepoxybutane (DEB) is used as a diagnostic criterion, because phenotypic heterogeneity makes clinical diagnosis difficult. Studies of genetic heterogeneity have shown that there are at least five different complementation groups, FA-A through FA-E. Overall, FA-A is the most prevalent group, accounting for 60%-65% of all FA. The FAA gene, which maps to chromosome 16q24.3, was recently isolated and methods for molecular diagnosis of FA-A are currently being developed. The first FA gene to be isolated (FAC) maps to chromosome 9q22.3; FA-C accounts for 10%-15% of FA. A variety of mutations and polymorphisms have been described in FAC. The most common of these is IVS4 +4 A-->T, which is the only FAC mutation found in Ashkenazi Jewish FA patients and their families. This mutation has not been found in any affected individual of non-Jewish ancestry. The carrier frequency of the IVS4 mutation was found to be 1 in 89 (1.1%; 95% confidence interval 0.79% to 1.56%) in an Ashkenazi Jewish population, whereas no carriers were identified in an Iraqi Jewish population, which represents the original gene pool of the Jews. We have developed amplification refractory mutation system (ARMS) assays for FAC mutations, which provide a means of rapid, nonradioactive genetic testing. These assays have been used to assign FA patients to Group C, to provide rapid carrier testing and prenatal diagnosis for FA-C families.

Expression of the Fanconi Anemia Group A Gene (Fanca) During Mouse Embryogenesis

About 80% of all cases of Fanconi anemia (FA) can be accounted for by complementation groups A and C. To understand the relationship between these groups, we analyzed the expression pattern of the mouse FA group-A gene (Fanca) during embryogenesis and compared it with the known pattern of the group-C gene (Fancc). Northern analysis of RNA from mouse embryos at embryonic days 7, 11, 15, and 17 showed a predominant 4.5 kb band in all stages. By in situ hybridization, Fanca transcripts were found in the whisker follicles, teeth, brain, retina, kidney, liver, and limbs. There was also stage-specific variation in Fanca expression, particularly within the developing whiskers and the brain. Some tissues known to express Fancc (eg, gut) failed to show Fancaexpression. These observations show that (1) Fanca is under both tissue- and stage-specific regulation in several tissues; (2) the expression pattern of Fanca is consistent with the phenotype of the human disease; and (3) Fanca expression is not necessarily coupled to that of Fancc. The presence of distinct tissue targets for FA genes suggests that some of the variability in the clinical phenotype can be attributed to the complementation group assignment.

Expression of the Fanconi Anemia Group A Gene (Fanca) During Mouse Embryogenesis

About 80% of all cases of Fanconi anemia (FA) can be accounted for by complementation groups A and C. To understand the relationship between these groups, we analyzed the expression pattern of the mouse FA group-A gene (Fanca) during embryogenesis and compared it with the known pattern of the group-C gene (Fancc). Northern analysis of RNA from mouse embryos at embryonic days 7, 11, 15, and 17 showed a predominant 4.5 kb band in all stages. By in situ hybridization, Fanca transcripts were found in the whisker follicles, teeth, brain, retina, kidney, liver, and limbs. There was also stage-specific variation in Fanca expression, particularly within the developing whiskers and the brain. Some tissues known to express Fancc (eg, gut) failed to show Fancaexpression. These observations show that (1) Fanca is under both tissue- and stage-specific regulation in several tissues; (2) the expression pattern of Fanca is consistent with the phenotype of the human disease; and (3) Fanca expression is not necessarily coupled to that of Fancc. The presence of distinct tissue targets for FA genes suggests that some of the variability in the clinical phenotype can be attributed to the complementation group assignment.

Accelerated telomere shortening and telomerase activation in Fanconi's anaemia

Fanconi's anaemia (FA) is an autosomal recessive disorder characterized by progressive bone marrow failure that often evolves towards acute leukaemia. FA also belongs to a group of chromosome instability diseases. Because telomeres are directly involved in chromosomal stability and in cell proliferation capacity, we examined telomere metabolism in peripheral blood mononuclear cells (PBMC). Telomere length was significantly shorter in 54 FA patient samples, compared to 51 controls (P<0.0001). In addition, mean telomere terminal restriction fragment lengths (TRF) in nine heterozygous patient samples did not differ from those of controls. In 14 samples from FA patients with severe aplastic anaemia (SFA), telomere length was significantly shorter than in 22 samples of age-matched FA patients with moderate haematological abnormalities (NSFA) (P<0.001). However, no correlation was found between TRF length and the presence of bone marrow clonal abnormalities in 16 additional, separately analysed, patient samples. Sequential measurement of TRF in six FA patients showed an accelerated rate of telomere shortening. Accordingly, telomere shortening rate was inversely correlated with clinical status. Telomerase, the enzyme that counteracts telomere shortening, was 4.8-fold more active in 25 FA patients than in 15 age-matched healthy controls. A model for the FA disease process is proposed.

Inherited and inducible chromosomal instability: a fragile bridge between genome integrity mechanisms and tumourigenesis

Cancer is a multi-step process evolving as the result of the accumulation of a number of mutational events. The growing body of evidence implicating genetic instability as a key feature of this evolutionary process and the risk of malignancy associated with chromosomal instability syndromes highlight the importance of understanding the mechanisms that cells use to maintain the integrity of their genomes. Classic examples of inherited chromosomal instability with cancer predisposition are Bloom's syndrome, ataxia telangiectasia, and Fanconi anaemia, although the mechanisms involved are far from understood. Selected features of these inherited disorders are reviewed to provide a background to the more recently discovered inducible chromosomal instability, a phenotype in which apparently normal cells that have survived ionizing radiation and certain chemical insults may produce descendants exhibiting a high frequency of de novo chromosome aberrations and gene mutations. The phenotype is induced at frequencies considerably greater than conventional mutation frequencies but little is understood of the underlying mechanism(s). To date, chromosomal instability induced by ionizing radiation has been the most extensively studied phenotype and it is evident that the expression of inducible instability has a strong dependence on the type of radiation exposure, the cell type irradiated, and the genetic 'predisposition' of the irradiated cell.

Protein Replacement by Receptor-Mediated Endocytosis Corrects the Sensitivity of Fanconi Anemia Group C Cells to Mitomycin C

Current methods for direct gene transfer into hematopoietic cells are inefficient. Here we show that functional complementation of Fanconi anemia (FA) group C cells by protein replacement can be as efficacious as by transfection with wild-type FAC cDNA. We expressed a chimeric protein (called His-ILFAC) consisting of the mature coding portion of gibbon interleukin-3 (IL-3) and full-length FAC inEscherichia coli. The purified bacterial protein is internalized by hematopoietic cells via IL-3 receptors. The intracellular half-life of His-ILFAC is approximately 60 minutes, which is comparable to that of the transgene-encoded FAC protein. In this cell-culture model His-ILFAC completely corrects the sensitivity of FA group C cells to mitomycin C, but it has no effect on FA cells that belong to complementation groups A and B. We suggest that receptor-mediated endocytosis of cytokine-fusion proteins may be of general use to deliver macromolecules into hematopoietic progenitor cells.

Protein Replacement by Receptor-Mediated Endocytosis Corrects the Sensitivity of Fanconi Anemia Group C Cells to Mitomycin C

Current methods for direct gene transfer into hematopoietic cells are inefficient. Here we show that functional complementation of Fanconi anemia (FA) group C cells by protein replacement can be as efficacious as by transfection with wild-type FAC cDNA. We expressed a chimeric protein (called His-ILFAC) consisting of the mature coding portion of gibbon interleukin-3 (IL-3) and full-length FAC inEscherichia coli. The purified bacterial protein is internalized by hematopoietic cells via IL-3 receptors. The intracellular half-life of His-ILFAC is approximately 60 minutes, which is comparable to that of the transgene-encoded FAC protein. In this cell-culture model His-ILFAC completely corrects the sensitivity of FA group C cells to mitomycin C, but it has no effect on FA cells that belong to complementation groups A and B. We suggest that receptor-mediated endocytosis of cytokine-fusion proteins may be of general use to deliver macromolecules into hematopoietic progenitor cells.

Involvement of the Fanconi anemia protein FA-C in repair processes of oxidative DNA damages

Fanconi anemia (FA) is an autosomal recessive disorder characterized by skeletal abnormalities, pancytopenia and a marked predisposition to cancer. FA cells exhibit chromosomal instability and hypersensitivity towards oxygen and cross-linking agents such as diepoxybutane and mitomycin C. An increased level of reactive oxygen intermediates and an elevation of 8-oxoguanine in FA cells point to a defective oxygen metabolism in FA cells. We investigated the repair activity of oxidatively damaged DNA in lymphoblastoid cells from FA patients of complementation groups A-E. The repair activity for oxidatively damaged DNA was significantly reduced in lymphoblastoid cell lines of complementation groups B-E. Complementation of the FA-C cell line with the wild type FA-C gene restored the repair activity to normal. This indicates that the FA-C protein participates in the repair of oxidatively damaged DNA.

Abnormal Microsomal Detoxification Implicated in Fanconi Anemia Group C by Interaction of the FAC Protein With NADPH Cytochrome P450 Reductase

The FAC protein encoded by the Fanconi anemia (FA) complementation group C gene is thought to function in the cytoplasm at a step before DNA repair. Because FA cells are susceptible to mitomycin C, we considered the possibility that FAC might interact with enzymes involved in the bioreductive activation of this drug. Here we report that FAC binds to NADPH cytochrome-P450 reductase (RED), a microsomal membrane protein involved in electron transfer, in both transfected COS-1 and normal murine liver cells. FAC-RED interaction requires the amino-terminal region of FAC and the cytosolic, membrane-proximal domain of the reductase. The latter contains a known binding site for flavin mononucleotide (FMN). Addition of FMN to cytosolic lysates disrupts FAC-reductase complexes, while flavin dinucleotide, which binds to a distinct carboxy-terminal domain, fails to alter FAC-RED complexes at concentrations similar to FMN. FAC is also functionally coupled to this enzyme as its expression in COS-1 cells suppresses the ability of RED to reduce cytochrome c in the presence of NADPH. We propose that FAC plays a fundamental role in vivo by attenuating the activity of RED, thereby regulating a major detoxification pathway in mammalian cells.

© 1998 by The American Society of Hematology.

Abnormal Microsomal Detoxification Implicated in Fanconi Anemia Group C by Interaction of the FAC Protein With NADPH Cytochrome P450 Reductase

The FAC protein encoded by the Fanconi anemia (FA) complementation group C gene is thought to function in the cytoplasm at a step before DNA repair. Because FA cells are susceptible to mitomycin C, we considered the possibility that FAC might interact with enzymes involved in the bioreductive activation of this drug. Here we report that FAC binds to NADPH cytochrome-P450 reductase (RED), a microsomal membrane protein involved in electron transfer, in both transfected COS-1 and normal murine liver cells. FAC-RED interaction requires the amino-terminal region of FAC and the cytosolic, membrane-proximal domain of the reductase. The latter contains a known binding site for flavin mononucleotide (FMN). Addition of FMN to cytosolic lysates disrupts FAC-reductase complexes, while flavin dinucleotide, which binds to a distinct carboxy-terminal domain, fails to alter FAC-RED complexes at concentrations similar to FMN. FAC is also functionally coupled to this enzyme as its expression in COS-1 cells suppresses the ability of RED to reduce cytochrome c in the presence of NADPH. We propose that FAC plays a fundamental role in vivo by attenuating the activity of RED, thereby regulating a major detoxification pathway in mammalian cells.

© 1998 by The American Society of Hematology.

The Fanconi Anemia Proteins FAA and FAC Function in Different Cellular Compartments to Protect Against Cross-Linking Agent Cytotoxicity

Fanconi anemia (FA) is an autosomal recessive disease characterized by chromosomal instability, bone marrow failure, and a high risk of developing malignancies. Although the disorder is genetically heterogeneous, all FA cells are defined by their sensitivity to the apoptosis-inducing effect of cross-linking agents, such as mitomycin C (MMC). The cloned FA disease genes, FAC and FAA, encode proteins with no homology to each other or to any known protein. We generated a highly specific antibody against FAA and found the protein in both the cytoplasm and nucleus of mammalian cells. By subcellular fractionation, FAA is also associated with intracellular membranes. To identify the subcellular compartment that is relevant for FAA activity, we appended nuclear export and nuclear localization signals to the carboxy terminus of FAA and enriched its localization in either the cytoplasm or the nucleus. Nuclear localization of FAA was both necessary and sufficient to correct MMC sensitivity in FA-A cells. In addition, we found no evidence for an interaction between FAA and FAC either in vivo or in vitro. Together with a previous finding that FAC is active in the cytoplasm but not in the nucleus, our results indicate that FAA and FAC function in separate subcellular compartments. Thus, FAA and FAC, if functionally linked, are more likely to be in a linear pathway rather than form a macromolecular complex to protect against cross-linker cytotoxicity.

The Fanconi Anemia Proteins FAA and FAC Function in Different Cellular Compartments to Protect Against Cross-Linking Agent Cytotoxicity

Fanconi anemia (FA) is an autosomal recessive disease characterized by chromosomal instability, bone marrow failure, and a high risk of developing malignancies. Although the disorder is genetically heterogeneous, all FA cells are defined by their sensitivity to the apoptosis-inducing effect of cross-linking agents, such as mitomycin C (MMC). The cloned FA disease genes, FAC and FAA, encode proteins with no homology to each other or to any known protein. We generated a highly specific antibody against FAA and found the protein in both the cytoplasm and nucleus of mammalian cells. By subcellular fractionation, FAA is also associated with intracellular membranes. To identify the subcellular compartment that is relevant for FAA activity, we appended nuclear export and nuclear localization signals to the carboxy terminus of FAA and enriched its localization in either the cytoplasm or the nucleus. Nuclear localization of FAA was both necessary and sufficient to correct MMC sensitivity in FA-A cells. In addition, we found no evidence for an interaction between FAA and FAC either in vivo or in vitro. Together with a previous finding that FAC is active in the cytoplasm but not in the nucleus, our results indicate that FAA and FAC function in separate subcellular compartments. Thus, FAA and FAC, if functionally linked, are more likely to be in a linear pathway rather than form a macromolecular complex to protect against cross-linker cytotoxicity.

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.

Development and characterization of immortalized fibroblastoid cell lines from an FA(C) mouse model

Fanconi anemia (FA) is an autosomal recessive disorder, characterised by multiple congenital malformations, bone marrow failure and a predisposition to developing malignancies, especially leukemia. FA cells show increased levels of spontaneous chromosomal aberrations and a hypersensitivity to DNA cross-linking agents such as mitomycin C (MMC) and diepoxybutane (DEB). There are at least eight complementation groups involved in FA, and the genes for two of these groups, FA(A) and FA(C), have been isolated and cloned. Mouse models for FA(C) have been developed by replacing exon 8 or exon 9 of Fac with the neo gene. Mice homozygous for Fac mutations show reduced fertility and hypersensitivity to induction of chromosomal aberrations by MMC and DEB. To facilitate the study of cellular defects in vitro, transformed mouse fibroblast cell lines were established. Cell-killing experiments and cytogenetic analyses were performed on these cells following treatment with MMC and DEB. Fac-/- showed significant hypersensitivity to MMC and DEB as compared with Fac+/+ and +/- for both cellular phenotypes. This is consistent with results obtained from similar studies on human fibroblasts and lymphoblastoid cell lines. Therefore, these isogenic transformed mouse fibroblasts provide as in vitro model for further investigation of the hypersensitivity of Fanconi anemia cells to DNA cross-linking agents.

Molecular Chaperone GRP94 Binds to the Fanconi Anemia Group C Protein and Regulates Its Intracellular Expression

The FAC protein encoded by the gene defective in Fanconi anemia (FA) complementation group C binds to at least three ubiquitous cytoplasmic proteins in vitro. We used here the complete coding sequence ofFAC in a yeast two-hybrid screen to identify interacting proteins. The molecular chaperone GRP94 was isolated twice from a B-lymphocyte cDNA library. Binding was confirmed by coimmunoprecipitation of FAC and GRP94 from cytosolic, but not nuclear, lysates of transfected COS-1 cells, as well as from mouse liver cytoplasmic extracts. Deletion mutants of FAC showed that residues 103-308 were required for interaction with GRP94, and a natural splicing mutation within the IVS-4 of FAC that removes residues 111-148 failed to bind GRP94. Ribozyme-mediated inactivation of GRP94 in the rat NRK cell line led to significantly reduced levels of immunoreactive FAC and concomitant hypersensitivity to mitomycin C, similar to the cellular phenotype of FA. Our results demonstrate that GRP94 interacts with FAC both in vitro and in vivo and regulates its intracellular level in a cell culture model. In addition, the pathogenicity of the IVS-4 splicing mutation in the FAC gene may be mediated in part by its inability to bind to GRP94.

Molecular Chaperone GRP94 Binds to the Fanconi Anemia Group C Protein and Regulates Its Intracellular Expression

The FAC protein encoded by the gene defective in Fanconi anemia (FA) complementation group C binds to at least three ubiquitous cytoplasmic proteins in vitro. We used here the complete coding sequence ofFAC in a yeast two-hybrid screen to identify interacting proteins. The molecular chaperone GRP94 was isolated twice from a B-lymphocyte cDNA library. Binding was confirmed by coimmunoprecipitation of FAC and GRP94 from cytosolic, but not nuclear, lysates of transfected COS-1 cells, as well as from mouse liver cytoplasmic extracts. Deletion mutants of FAC showed that residues 103-308 were required for interaction with GRP94, and a natural splicing mutation within the IVS-4 of FAC that removes residues 111-148 failed to bind GRP94. Ribozyme-mediated inactivation of GRP94 in the rat NRK cell line led to significantly reduced levels of immunoreactive FAC and concomitant hypersensitivity to mitomycin C, similar to the cellular phenotype of FA. Our results demonstrate that GRP94 interacts with FAC both in vitro and in vivo and regulates its intracellular level in a cell culture model. In addition, the pathogenicity of the IVS-4 splicing mutation in the FAC gene may be mediated in part by its inability to bind to GRP94.

Identification of a novel c-DNA overexpressed in Fanconi's anemia fibroblasts partially homologous to a putative L-3-phosphoserine-phosphatase

We applied the cDNA differential display technique (DDT) in a DNA-repair deficient cell model to isolate genes involved in dysregulation of cell proliferation and development of cancer. The comparative analysis of mRNA expression patterns of human diploid fibroblasts from Fanconi's amemia (FA) and normal phenotype led to the identification of a novel cDNA CO9. Northern blot analysis reveals that CO9 is significantly upregulated in FA fibroblasts but downregulated or absent in fibroblasts from normal donors. CO9 was also highly expressed in FA B-cells of complementation group A and in Raji cells. However, CO9 is not expressed in FA complementation groups B, C, D and E. The full-length cDNA is 840 bp long and contains an open reading frame of 216 bp (72 amino acids), which encodes for a 7.6-kDa protein. The lengths of the 5' and 3' untranslated region are 165 and 459 bp, respectively. The N-terminal and C-terminal nucleotide sequence of CO9 shows homology to a putative human L-3-phosphoserine phosphatase identified recently (HSPSPASE, EMBL Accession No. Y10275) but lacks a 476-bp stretch in the open reading frame. The loss of nucleotides within the open reading frame introduces a new termination codon in the CO9 cDNA along with a novel COOH terminus resulting in a new protein product. Database chromosome mapping localized CO9 to chromosome 7q 11.2. We hypothesize that CO9 represents a novel protein being a partial homologue to the L-3-phosphoserine phosphatase but with a different regulatory cell function.

The Fanconi Anemia Group C Gene Product Is Located in Both the Nucleus and Cytoplasm of Human Cells

The Fanconi anemia (FA) complementation group C (FAC) protein gene encodes a cytoplasmic protein with a predicted Mrof 63,000. The protein's function is unknown, but it has been hypothesized that it either mediates resistance to DNA cross-linking agents or facilitates repair after exposure to such factors. The protein also plays a permissive role in the growth of colony-forming unit–granulocyte/macrophage (CFU-GM), burst-forming unit–erythroid (BFU-E), and CFU-erythroid (CFU-E). Attributing a specific function to this protein requires an understanding of its intracellular location. Recognizing that prior study has established the functional importance of its cytoplasmic location, we tested the hypothesis that FAC protein can also be found in the nucleus. Purified recombinant Escherichia coli–derived FAC antigens were used to create antisera able to specifically identify an Mr = 58,000 protein in lysates from human Epstein-Barr virus (EBV)-transformed cell lines by immunoblot analysis. Subcellular fractionation of the cell lysates followed by immunoblot analysis revealed that the majority of the FAC protein was cytoplasmic, as reported previously; however, approximately 10% of FAC protein was reproducibly detected in nuclear fractions. These results were reproducible by two different fractionation methods, and included markers to control for contamination of nuclear fractions by cytoplasmic proteins. Moreover, confocal image analysis of human 293 cells engineered to express FAC clearly demonstrated that FAC protein is located in both cytoplasmic and nuclear compartments, consistent with data obtained from fractionation of the FA cell lines. Finally, complementation of the FAC defect using retroviral-mediated gene transfer resulted in a substantial increase in nuclear FAC protein. Therefore, while cytoplasmic localization of this protein appears to be functionally important, it may also exert some essential nuclear function.

Small deletions in the regulatory 3' UTR of the human alpha-tropomyosin gene identified by differential display

The differential display technique (DDT) was used to compare Fanconi anaemia (FA) fibroblasts with those of normal controls in a screen for genes involved in DNA repair, recognizing and handling damage or indicating cell cycle abnormalities as a result of genetic changes. The DDT revealed two different deletions of 5 and 11 bp at a single locus in the 3' untranslated region (UTR) of a gene known to encode human alpha-tropomyosin (TPM1) in FA cells. These small deletions were detected by analysis of shifted 900-bp long cDNA fragments on polyacrylamide gels. They were characterized as loss of GTTTT or TGTTTTGTTTT, respectively, in a region with five GTTTT tandem repeats. Since it was postulated that the 3' UTR of the TPM1 gene plays a regulatory role in cell differentiation and tumour suppression, the existence and possible patterns of deletions in a variety of normal donors was investigated. The heterogenous distribution of non-deleted, 5- and 11-bp deleted 3' UTR regions indicate a polymorphism of the TPM1 gene in this tandem repeat motif. Therefore the expression pattern of these mutations among FA and non-FA cells rendered any direct relationship to the putative DNA repair defect in FA unlikely. Of note, however, the fact remains that such deletions reportedly facilitate mRNA degradation and may bear significance in the TPM1 gene action. Finally, of further interest is the finding that even small deletions can be identified by DDT in addition to the identification of the differential expression patterns of genes.

DNA Cross-Linker–Induced G2/M Arrest in Group C Fanconi Anemia Lymphoblasts Reflects Normal Checkpoint Function

Cells from individuals with Fanconi anemia (FA) arrest excessively in the G2/M cell cycle compartment after exposure to low doses of DNA cross-linking agents. The relationship of this abnormality to the fundamental genetic defect in such cells is unknown, but many investigators have speculated that the various FA genes directly regulate cell cycle checkpoints. We tested the hypothesis that the protein encoded by the FA group C complementing gene (FAC) functions to control a cell cycle checkpoint and that cells from group C patients (FA[C]) have abnormalities of cell cycle regulation directly related to the genetic mutation. We found that retroviral transduction of FA(C) lymphoblasts with wild-type FAC cDNA resulted in normalization of the cell cycle response to low-dose mitomycin C (MMC). However, when DNA damage was quantified in terms of cytogenetic damage or cellular cytotoxicity, we found similar degrees of G2/M arrest in response to equitoxic amounts of MMC in FA(C) cells as well as in normal lymphoblasts. Similar results were obtained using isogenic pairs of uncorrected, FAC- or mock-corrected (neo only) FA(C) cell lines. To test the function of other checkpoints we examined the effects of hydroxyurea (HU) and ionizing radiation on cell cycle kinetics of FA(C) and normal lymphoblasts as well as with isogenic pairs of uncorrected, FAC-corrected, or mock-corrected FA(C) cell lines. In all cases the cell cycle response of FA(C) and normal lymphoblasts to these two agents were identical. Based on these studies we conclude that the aberrant G2/M arrest that typifies the response of FA(C) cells to low doses of cross-linking agents does not represent an abnormal cell cycle response but instead represents a normal cellular response to the excessive DNA damage that results in FA(C) cells following exposure to low doses of cross-linking agents.

DNA Cross-Linker–Induced G2/M Arrest in Group C Fanconi Anemia Lymphoblasts Reflects Normal Checkpoint Function

Cells from individuals with Fanconi anemia (FA) arrest excessively in the G2/M cell cycle compartment after exposure to low doses of DNA cross-linking agents. The relationship of this abnormality to the fundamental genetic defect in such cells is unknown, but many investigators have speculated that the various FA genes directly regulate cell cycle checkpoints. We tested the hypothesis that the protein encoded by the FA group C complementing gene (FAC) functions to control a cell cycle checkpoint and that cells from group C patients (FA[C]) have abnormalities of cell cycle regulation directly related to the genetic mutation. We found that retroviral transduction of FA(C) lymphoblasts with wild-type FAC cDNA resulted in normalization of the cell cycle response to low-dose mitomycin C (MMC). However, when DNA damage was quantified in terms of cytogenetic damage or cellular cytotoxicity, we found similar degrees of G2/M arrest in response to equitoxic amounts of MMC in FA(C) cells as well as in normal lymphoblasts. Similar results were obtained using isogenic pairs of uncorrected, FAC- or mock-corrected (neo only) FA(C) cell lines. To test the function of other checkpoints we examined the effects of hydroxyurea (HU) and ionizing radiation on cell cycle kinetics of FA(C) and normal lymphoblasts as well as with isogenic pairs of uncorrected, FAC-corrected, or mock-corrected FA(C) cell lines. In all cases the cell cycle response of FA(C) and normal lymphoblasts to these two agents were identical. Based on these studies we conclude that the aberrant G2/M arrest that typifies the response of FA(C) cells to low doses of cross-linking agents does not represent an abnormal cell cycle response but instead represents a normal cellular response to the excessive DNA damage that results in FA(C) cells following exposure to low doses of cross-linking agents.

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.

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.

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.

Phenotypic Consequences of Mutations in the Fanconi Anemia FAC Gene: An International Fanconi Anemia Registry Study

Fanconi anemia (FA) is a genetically and phenotypically heterogeneous disorder defined by cellular hypersensitivity to DNA cross-linking agents; mutations in the gene defective in FA complementation group C, FAC, are responsible for the syndrome in a subset of patients. We have performed an analysis of the clinical effects of specific mutations in the FAC gene. Using the amplification refractory mutation system assays that we developed to rapidly detect FAC mutations, at least one mutated copy of the FAC gene was identified in 59 FA patients from the International Fanconi Anemia Registry (IFAR). This represents 15% of the 397 FA patients tested. FA-C patients were divided into three subgroups based on results of a genotype-phenotype analysis using the Cox proportional hazards model: (1) patients with the IVS4 mutation (n = 26); (2) patients with at least one exon 14 mutation (R548X or L554P) (n = 16); and (3) patients with at least one exon 1 mutation (322delG or Q13X) and no known exon 14 mutation (n = 17). Kaplan-Meier analysis shows that IVS4 or exon 14 mutations define poor risk subgroups, as they are associated with significantly earlier onset of hematologic abnormalities and poorer survival compared to exon 1 patients and to the non-FA-C IFAR population. There was no direct correlation between the degree of cellular hypersensitivity to the clastogenic effect of diepoxybutane and severity of clinical phenotype. Sixteen of the 59 FA-C patients (27%) have developed acute myelogenous leukemia. Thirteen of these patients have died; AML was the cause of death in 46% of the expired FA-C patients. This study enables us to define this clinically heterogeneous disorder genotypically to better predict clinical outcome and aid decision-making regarding major therapeutic modalities for a subset of FA patients.

Phenotypic Consequences of Mutations in the Fanconi Anemia FAC Gene: An International Fanconi Anemia Registry Study

Fanconi anemia (FA) is a genetically and phenotypically heterogeneous disorder defined by cellular hypersensitivity to DNA cross-linking agents; mutations in the gene defective in FA complementation group C, FAC, are responsible for the syndrome in a subset of patients. We have performed an analysis of the clinical effects of specific mutations in the FAC gene. Using the amplification refractory mutation system assays that we developed to rapidly detect FAC mutations, at least one mutated copy of the FAC gene was identified in 59 FA patients from the International Fanconi Anemia Registry (IFAR). This represents 15% of the 397 FA patients tested. FA-C patients were divided into three subgroups based on results of a genotype-phenotype analysis using the Cox proportional hazards model: (1) patients with the IVS4 mutation (n = 26); (2) patients with at least one exon 14 mutation (R548X or L554P) (n = 16); and (3) patients with at least one exon 1 mutation (322delG or Q13X) and no known exon 14 mutation (n = 17). Kaplan-Meier analysis shows that IVS4 or exon 14 mutations define poor risk subgroups, as they are associated with significantly earlier onset of hematologic abnormalities and poorer survival compared to exon 1 patients and to the non-FA-C IFAR population. There was no direct correlation between the degree of cellular hypersensitivity to the clastogenic effect of diepoxybutane and severity of clinical phenotype. Sixteen of the 59 FA-C patients (27%) have developed acute myelogenous leukemia. Thirteen of these patients have died; AML was the cause of death in 46% of the expired FA-C patients. This study enables us to define this clinically heterogeneous disorder genotypically to better predict clinical outcome and aid decision-making regarding major therapeutic modalities for a subset of FA patients.

p53 activates Fanconi anemia group C gene expression

The tumor suppressor protein p53 (wtp53) can bind to specific target sequences and activate transcription of genes adjacent to these DNA elements. Two p53 binding sites are present in the gene coding for the Fanconi anemia complementation group C (FAC), one in the promoter region (from -1295 to -1266) and one in the coding region of FAC (from +1828 to +1848). Gel shift experiments show that wtp53 binds to the p53 target sequence in the promoter region of the FAC gene. We have investigated whether binding of p53 to these target sites may affect expression of the FAC gene. Transfection experiments show that overexpression of wtp53 in human diploid fibroblasts and lymphoblasts augments transcription of the FAC gene up to three-fold. The transfection efficacy was approximately 15% for both cell types. The FAC expression activity per transformed cell was stimulated to an estimated level of 18- to 21-fold upon overexpression of p53. The tumor-derived p53 mutants, His175 and His273, that fail to bind DNA showed only a reduced stimulatory activity on FAC transcription. Luciferase assays demonstrated that interaction of p53 with its target site in the FAC promoter does not modulate the promoter activity. We suggest that the p53 binding site contributes to, but may not be an absolute prerequisite for p53-directed transcriptional activation. We conclude that the FAC gene can be added to the list of genes that interact with p53.

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.

Induction of Fanconi anemia cellular phenotype in human 293 cells by overexpression of a mutant FAC allele

The polypeptide encoded by the Fanconi anemia (FA) complementation group C gene, FAC, binds to a group of cytoplasmic proteins in vitro and may form a multimeric complex. A known mutant allele of FAC resulting from the substitution of Pro for Leu at codon 554 fails to correct the sensitivity of FA group C cells to mitomycin C. We reasoned that overexpression of the mutant protein in a wild-type cellular background might induce the FA phenotype by competing with endogenous FAC for binding to the accessory proteins. After stable transfection of 293 cells with wild-type and a mutant FAC allele containing the L554P substitution, four independent clones that expressed four-to-fifteen fold higher levels of transcript from the mutant transgene relative to the endogenous FAC gene showed hypersensitivity to mitomycin C. By contrast, both parental and FAC-overexpressing cells maintained their relative resistance to mitomycin C. No differences in the biosynthesis, subcellular localization and protein interactions of the normal and mutant proteins were detected. The induction of the FA phenotype in this system is compatible with the competition hypothesis and provides support for a functional role of the FAC-binding proteins in vivo.