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

A Narrative Review on Fanconi Anemia: Genetic and Diagnostic Considerations

Fanconi anemia (FA) is an autosomal recessive disorder, both genetically and phenotypically. It is characterized by chromosomal instability, progressive bone marrow failure, susceptibility to cancer, and various other congenital abnormalities. It involves all the three cell lines of blood. So far, biallelic mutations in 21 genes and one x-linked gene have been detected and found to be associated with FA phenotype. Signs and symptoms start setting in by the age of 4 to 7 years, mainly hematological symptoms. This includes pancytopenia, that is, a reduction in the number of white blood cells (WBCs), red blood cells (RBCs), and platelets. Therefore, the main criteria for diagnosis of FA include skeletal malformations, pancytopenia, hyperpigmentation, short stature, urogenital abnormalities, central nervous system, auditory, renal, ocular, and familial occurrence. Patients showing signs and symptoms of FA should be thoroughly evaluated. A complete blood count will reveal a reduced number of RBC, WBC, and platelets, that is, pancytopenia. Chromosomal breakage study/stress cytogenetics should be done in patients with severe pancytopenia. Momentousness timely diagnosis of current disease, prenatal diagnosis, and genetic counseling should be emphasized.

The Fanconi anemia group C protein interacts with uncoordinated 5A and delays apoptosis

The Fanconi anemia group C protein (FANCC) is one of the several proteins that comprise the Fanconi anemia (FA) network involved in genomic surveillance. FANCC is mainly cytoplasmic and has many functions, including apoptosis suppression through caspase-mediated proteolytic processing. Here, we examined the role of FANCC proteolytic fragments by identifying their binding partners. We performed a yeast two-hybrid screen with caspase-mediated FANCC cleavage products and identified the dependence receptor uncoordinated-5A (UNC5A) protein. Here, we show that FANCC physically interacts with UNC5A, a pro-apoptotic dependence receptor. FANCC interaction occurs through the UNC5A intracellular domain, specifically via its death domain. FANCC modulates cell sensitivity to UNC5A-mediated apoptosis; we observed reduced UNC5A-mediated apoptosis in the presence of FANCC and increased apoptosis in FANCC-depleted cells. Our results show that FANCC interferes with UNC5A's functions in apoptosis and suggest that FANCC may participate in developmental processes through association with the dependence receptor UNC5A.

The Fanconi anemia DNA repair pathway: structural and functional insights into a complex disorder

Mutations in any of at least sixteen FANC genes (FANCA-Q) cause Fanconi anemia, a disorder characterized by sensitivity to DNA interstrand crosslinking agents. The clinical features of cytopenia, developmental defects, and tumor predisposition are similar in each group, suggesting that the gene products participate in a common pathway. The Fanconi anemia DNA repair pathway consists of an anchor complex that recognizes damage caused by interstrand crosslinks, a multisubunit ubiquitin ligase that monoubiquitinates two substrates, and several downstream repair proteins including nucleases and homologous recombination enzymes. We review progress in the use of structural and biochemical approaches to understanding how each FANC protein functions in this pathway.

Fanconi anemia proteins interact with CtBP1 and modulate the expression of the Wnt antagonist Dickkopf-1

Fanconi anemia (FA) is a genetic disorder characterized by congenital abnormalities, bone marrow failure, and increased susceptibility to cancer. Of the fifteen FA proteins, Fanconi anemia group C (FANCC) is one of eight FA core complex components of the FA pathway. Unlike other FA core complex proteins, FANCC is mainly localized in the cytoplasm, where it is thought to function in apoptosis, redox regulation, cytokine signaling, and other processes. Previously, we showed that regulation of FANCC involved proteolytic processing during apoptosis. To elucidate the biological significance of this proteolytic modification, we searched for molecular interacting partners of proteolytic FANCC fragments. Among the candidates obtained, the transcriptional corepressor protein C-terminal binding protein-1 (CtBP1) interacted directly with FANCC and other FA core complex proteins. Although not required for stability of the FA core complex or ubiquitin ligase activity, CtBP1 is essential for proliferation, cell survival, and maintenance of chromosomal integrity. Expression profiling of CtBP1-depleted and FA-depleted cells revealed that several genes were commonly up- and down-regulated, including the Wnt antagonist Dickkopf-1 (DKK1). These findings suggest that FA and Wnt signaling via CtBP1 could share common effectors.

Fanconi anemia proteins and their interacting partners: a molecular puzzle

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

Evidence for complete epistasis of null mutations in murine Fanconi anemia genes Fanca and Fancg

Fanconi anemia (FA) is a heritable disease characterized by bone marrow failure, congenital abnormalities, and cancer predisposition. The 15 identified FA genes operate in a molecular pathway to preserve genomic integrity. Within this pathway the FA core complex operates as an ubiquitin ligase that activates the complex of FANCD2 and FANCI to coordinate DNA repair. The FA core complex is formed by at least 12 proteins. However, only the FANCL subunit displays ubiquitin ligase activity. FANCA and FANCG are members of the FA core complex for which no other functions have been described than to participate in protein interactions. In this study we generated mice with combined null alleles for Fanca and Fancg to identify extended functions for these genes by characterizing the double mutant mice and cells. Double mutant a(-/-)/g(-/-) mice were born at near Mendelian frequencies without apparent developmental abnormalities. Histological analysis of a(-/-)/g(-/-) mice revealed a Leydig cell hyperplasia and frequent vacuolization of Sertoli cells in testes, while ovaries were depleted from developing follicles and displayed an interstitial cell hyperplasia. These gonadal aberrations were associated with a compromised fertility of a(-/-)/g(-/-) males and females. During the first year of life a(-/-)/g(-/-) did not develop malignancies or bone marrow failure. At the cellular level a(-/-)/g(-/-), Fanca(-/-), and Fancg(-/-) cells proved equally compromised in DNA crosslink and homology-directed repair. Overall the phenotype of a(-/-)/g(-/-) double knockout mice and cells appeared highly similar to the phenotype of Fanca or Fancg single knockouts. The lack of an augmented phenotype suggest that null mutations in Fanca or Fancg are fully epistatic, making additional important functions outside of the FA core complex highly unlikely.

Cytoplasmic FANCA-FANCC complex interacts and stabilizes the cytoplasm-dislocalized leukemic nucleophosmin protein (NPMc)

Eight of the Fanconi anemia (FA) proteins form a core complex that activates the FA pathway. Some core complex components also form subcomplexes for yet-to-be-elucidated functions. Here, we have analyzed the interaction between a cytoplasmic FA subcomplex and the leukemic nucleophosmin (NPMc). Exogenous NPMc was degraded rapidly in FA acute myeloid leukemia bone marrow cells. Knockdown of FANCA or FANCC in leukemic OCI/AML3 cells induced rapid degradation of endogenous NPMc. NPMc degradation was mediated by the ubiquitin-proteasome pathway involving the IBR-type RING-finger E3 ubiquitin ligase IBRDC2, and genetic correction of FA-A or FA-C lymphoblasts prevented NPMc ubiquitination. Moreover, cytoplasmic FANCA and FANCC formed a cytoplasmic complex and interacted with NPMc. Using a patient-derived FANCC mutant and a nuclearized FANCC, we demonstrated that the cytoplasmic FANCA-FANCC complex was essential for NPMc stability. Finally, depletion of FANCA and FANCC in NPMc-positive leukemic cells significantly increased inflammation and chemoresistance through NF-κB activation. Our findings not only reveal the molecular mechanism involving cytoplasmic retention of NPMc but also suggest cytoplasmic function of FANCA and FANCC in NPMc-related leukemogenesis.

FLT3 K663Q is a novel AML-associated oncogenic kinase: Determination of biochemical properties and sensitivity to Sunitinib (SU11248)

Somatic mutations of FLT3 resulting in constitutive kinase activation are the most common acquired genomic abnormality found in acute myeloid leukemia (AML). The majority of these mutations are internal tandem duplications (ITD) of the juxtamembrane region (JM). In addition, a minority of cases of AML are associated with mutation of the FLT3 activation loop (AL), typically involving codons D835 and/or I836. We hypothesized that other novel mutations of FLT3 could also contribute to leukemogenesis. We genotyped 109 cases of AML and identified two novel gain-of-function mutations. The first mutation, N841 H, is similar to previously described mutations involving amino-acid substitutions of codon 841. The other novel mutation, FLT3 K663Q, is the first AML-associated gain-of-function mutation located outside the JM and AL domains. Of note, this mutation was potently inhibited by Sunitinib (SU11248), a previously described FLT3 kinase inhibitor. Sunitinib reduced the proliferation and induced apoptosis of transformed Ba/F3 cells expressing FLT3 K663Q. The potency of Sunitinib against FLT3 K663Q was similar to its potency against FLT3 ITD mutations. We conclude that FLT3 mutations in AML can involve novel regions of the TK1. Future studies are needed to define the incidence and prognostic significance of FLT3 mutations outside the well-established JM and AL regions.

Fanconi Anemia

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

The nuclear accumulation of the Fanconi anemia protein FANCE depends on FANCC

The Fanconi anemia (FA) protein FANCE is an essential component of the nuclear FA core complex, which is required for monoubiquitination of the downstream target FANCD2, an important step in the FA pathway of DNA cross-link repair. FANCE is predominantly localized in the nucleus and acts as a molecular bridge between the FA core complex and FANCD2, through direct binding of both FANCC and FANCD2. At present, it is poorly understood how the nuclear accumulation of FANCE is regulated and therefore we investigated the nuclear localization of this FA protein. We found that FANCE has a strong tendency to localize in the nucleus, since the addition of a nuclear export signal does not interfere with the nuclear localization of FANCE. We also demonstrate that the nuclear accumulation of FANCE does not rely solely on its nuclear localization signal motifs, but also on FANCC. The other FA proteins are not involved in the nuclear accumulation of FANCE, indicating a tight relationship between FANCC and FANCE, as suggested from their direct interaction. Finally, we show that the region of FANCE interacting with FANCC appears to be different from the region involved in binding FANCD2. This strengthens the idea that FANCE recruits FANCD2 to the core complex, without interfering with the binding of FANCC.

Intermediate DNA repair activity associated with the 322delG allele of the fanconi anemia complementation group C gene

Fanconi anemia (FA) is an autosomal recessive disorder associated with pancytopenia and cancer susceptibility. The disorder is heterogeneous, with at least nine complementation groups having been identified. Several recent studies have suggested that defective plasmid DNA end-joining is a consistent feature of FA cells. It was therefore surprising to discover a strain of fibroblasts from an FA patient that possessed wild-type plasmid DNA end-joining activity. Unlike other FA strains, these fibroblasts have wild-type levels of homologous DNA recombination activity and are relatively insensitive to restriction endonuclease-induced death. Interestingly, while end-joining in a number of FA fibroblast strains belonging to complementation groups A, C, and D2 was approximately 70% precise, end-joining in this latter strain of fibroblasts was more than 95% imprecise. Analysis revealed that these latter cells harbored an allele of the FA C gene, referred to as 322delG, that encodes an amino-terminal truncated protein. The relative rarity of this allele precluded the analysis of other FA fibroblast strains; however, studies revealed that overexpression of this allele in normal cells recapitulated the DNA end-joining phenotype seen in the 322delG FA fibroblast strain. These results indicate that DNA end-joining in fibroblasts expressing the 322delG allele of the FA-C gene in fibroblasts is highly imprecise; however, the DNA repair efficiency of these cells is more normal than that commonly associated with FA fibroblasts. This conclusion is intriguing, since a number of reports have suggested that patients harboring this allele exhibit a milder clinical course than do individuals with other alleles of the FA-C gene.

Regulation of the Fanconi Anemia Group C Protein through Proteolytic Modification

The function of the Fanconi anemia group C protein (FANCC) is still unknown, though many studies point to a role in damage response signaling. Unlike other known FA proteins, FANCC is mainly localized to the cytoplasm and is thought to act as a messenger of cellular damage rather than an effector of repair. FANCC has been shown to interact with several cytoplasmic and nuclear proteins and to delay the onset of apoptosis through redox regulation of GSTP1. We investigated the fate and function of FANCC during apoptosis. Here we show that FANCC undergoes proteolytic modification by a caspase into a predominant 47-kDa ubiquitinated protein fragment. Lack of proteolytic modification at the putative cleavage site delays apoptosis but does not affect MMC complementation. These results suggest that FANCC function is regulated through proteolytic processing.

A novel ubiquitin ligase is deficient in Fanconi anemia

Fanconi anemia is a recessively inherited disease characterized by congenital defects, bone marrow failure and cancer susceptibility. Cells from individuals with Fanconi anemia are highly sensitive to DNA-crosslinking drugs, such as mitomycin C (MMC). Fanconi anemia proteins function in a DNA damage response pathway involving breast cancer susceptibility gene products, BRCA1 and BRCA2 (refs. 1,2). A key step in this pathway is monoubiquitination of FANCD2, resulting in the redistribution of FANCD2 to nuclear foci containing BRCA1 (ref. 3). The underlying mechanism is unclear because the five Fanconi anemia proteins known to be required for this ubiquitination have no recognizable ubiquitin ligase motifs. Here we report a new component of a Fanconi anemia protein complex, called PHF9, which possesses E3 ubiquitin ligase activity in vitro and is essential for FANCD2 monoubiquitination in vivo. Because PHF9 is defective in a cell line derived from an individual with Fanconi anemia, we conclude that PHF9 (also called FANCL) represents a novel Fanconi anemia complementation group (FA-L). Our data suggest that PHF9 has a crucial role in the Fanconi anemia pathway as the likely catalytic subunit required for monoubiquitination of FANCD2.

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

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

Genetic basis of Fanconi anemia

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

SU5416 and SU5614 inhibit kinase activity of wild-type and mutant FLT3 receptor tyrosine kinase

Internal tandem duplication (ITD) in the juxtamembrane portion of Fms-like tyrosine kinase 3 (FLT3), a type III receptor tyrosine kinase (RTK), is the most common molecular defect associated with acute myeloid leukemia (AML). The high prevalence of this activating mutation makes it a potential target for molecularly based therapy. Indolinone tyrosine kinase inhibitors have known activity against KIT, another member of the type III RTK family. Given the conserved homology between members of this family, we postulated that the activity of some KIT inhibitors would extend to FLT3. We used various leukemic cell lines (BaF3, MV 4-11, RS 4;11) to test the activity of indolinone compounds against the FLT3 kinase activity of both wild-type (WT) and ITD isoforms. Both SU5416 and SU5614 were capable of inhibiting autophosphorylation of ITD and WT FLT3 (SU5416 concentration that inhibits 50% [IC(50)], 100 nM; and SU5614 IC(50) 10 nM). FLT3-dependent activation of the downstream signaling proteins mitogen-activated protein kinase (MAPK) and signal transducer and activator of transcription 5 (STAT5) was also inhibited by treatment in the same concentration ranges. FLT3 inhibition by SU5416 and SU5614 resulted in reduced proliferation (IC(50), 250 nM and 100 nM, respectively) and induction of apoptosis of FLT3 ITD-positive leukemic cell lines. Treatment of these cells with an alternative growth factor (granulocyte-macrophage colony-stimulating factor [GM-CSF]) restored MAPK signaling and cellular proliferation, demonstrating specificity of the observed inhibitory effects. We conclude that SU5416 and SU5614 are potent inhibitors of FLT3. Our finding that inhibition of FLT3 induces apoptosis of leukemic cells supports the feasibility of targeting FLT3 as a novel treatment strategy for AML.

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

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

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

OBJECTIVE

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSIONS

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

Molecular pathogenesis of fanconi anemia

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

DNA damage processing defects and disease

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

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

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

Repair of DNA interstrand cross-links

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

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

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

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.

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

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

The emerging genetic and molecular basis of Fanconi anaemia

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

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

OBJECTIVES

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

STUDY DESIGN

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

RESULTS

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

CONCLUSION

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

Interferon-γ–induced apoptotic responses of Fanconi anemia group C hematopoietic progenitor cells involve caspase 8–dependent activation of caspase 3 family members

Hematopoietic progenitor cells (HPC) from mice nullizygous at the Fanconi anemia (FA) group C locus and children with Fanconi anemia group C (FA-C) are hypersensitive to interferon-gamma (IFN-γ) and tumor necrosis factor-α. This hypersensitivity results, in part, from the capacity of these cytokines to prime the fas pathway. Because fas-mediated programmed cell death in many cells involves sequential activation of specific caspases, we tested the hypothesis that programmed cell death in FA HPC involves the ordered activation of specific caspase molecules. Lysates from lymphoblasts treated with both agonistic anti-fas antibody and IFN-γ contained activated caspase 3 family members (caspases 3, 6, and 7), as well as caspase 8, whereas activation of caspases 1, 2, 4, 9, and 10 was not detected. The apoptotic effects of fas agonists in IFN-γ-treated human and murine FA-C cells were blocked when pretreated with inhibitors (ac-DEVD-cho, CP-DEVD-cho, Z-DEVD-FMK) of the caspase 3 protease. Inhibitors (ac-YVAD-cho, CP-YVAD-cho, Z-YVAD-FMK) of caspase 1 did not block apoptosis or caspase 3 activation. Treatment of FA cells with the fluoromethyl ketone tetrapeptide caspase 8 inhibitor (ac-IETD-FMK) did suppress caspase 3 activation. A 4-fold greater fraction of IFN-induced FA-C cells expressed caspase 3 than FA-C cells complemented by retroviral-mediated transfer of FANCC. Therefore fas-induced apoptosis in Fanconi anemia cells of the C type involves the activation of caspase 8, which controls activation of caspase 3 family members and one direct or indirect function of the FANCC protein is to suppress apoptotic responses to IFN-γ upstream of caspase 3 activation.

Interferon-γ–induced apoptotic responses of Fanconi anemia group C hematopoietic progenitor cells involve caspase 8–dependent activation of caspase 3 family members

Hematopoietic progenitor cells (HPC) from mice nullizygous at the Fanconi anemia (FA) group C locus and children with Fanconi anemia group C (FA-C) are hypersensitive to interferon-gamma (IFN-γ) and tumor necrosis factor-α. This hypersensitivity results, in part, from the capacity of these cytokines to prime the fas pathway. Because fas-mediated programmed cell death in many cells involves sequential activation of specific caspases, we tested the hypothesis that programmed cell death in FA HPC involves the ordered activation of specific caspase molecules. Lysates from lymphoblasts treated with both agonistic anti-fas antibody and IFN-γ contained activated caspase 3 family members (caspases 3, 6, and 7), as well as caspase 8, whereas activation of caspases 1, 2, 4, 9, and 10 was not detected. The apoptotic effects of fas agonists in IFN-γ-treated human and murine FA-C cells were blocked when pretreated with inhibitors (ac-DEVD-cho, CP-DEVD-cho, Z-DEVD-FMK) of the caspase 3 protease. Inhibitors (ac-YVAD-cho, CP-YVAD-cho, Z-YVAD-FMK) of caspase 1 did not block apoptosis or caspase 3 activation. Treatment of FA cells with the fluoromethyl ketone tetrapeptide caspase 8 inhibitor (ac-IETD-FMK) did suppress caspase 3 activation. A 4-fold greater fraction of IFN-induced FA-C cells expressed caspase 3 than FA-C cells complemented by retroviral-mediated transfer of FANCC. Therefore fas-induced apoptosis in Fanconi anemia cells of the C type involves the activation of caspase 8, which controls activation of caspase 3 family members and one direct or indirect function of the FANCC protein is to suppress apoptotic responses to IFN-γ upstream of caspase 3 activation.

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.

Inhibition of c-kit receptor tyrosine kinase activity by STI 571, a selective tyrosine kinase inhibitor

STI 571 (formerly known as CGP 57148B) is a known inhibitor of the c-abl, bcr-abl, and platelet-derived growth-factor receptor (PDGFR) tyrosine kinases. This compound is being evaluated in clinical trials for the treatment of chronic myelogenous leukemia. We sought to extend the activity profile of STI 571 by testing its ability to inhibit the tyrosine kinase activity of c-kit, a receptor structurally similar to PDGFR. We treated a c-kit expressing a human myeloid leukemia cell line, M-07e, with STI 571 before stimulation with Steel factor (SLF). STI 571 inhibited c-kit autophosphorylation, activation of mitogen-activated protein (MAP) kinase, and activation of Akt without altering total protein levels of c-kit, MAP kinase, or Akt. The concentration that produced 50% inhibition for these effects was approximately 100 nmol/L. STI 571 also significantly decreased SLF-dependent growth of M-07e cells in a dose-dependent manner and blocked the antiapoptotic activity of SLF. In contrast, the compound had no effect on MAP kinase activation or cellular proliferation in response to granulocyte-macrophage colony-stimulating factor. We also tested the activity of STI 571 in a human mast cell leukemia cell line (HMC-1), which has an activated mutant form of c-kit. STI 571 had a more potent inhibitory effect on the kinase activity of this mutant receptor than it did on ligand-dependent activation of the wild-type receptor. These findings show that STI 571 selectively inhibits c-kit tyrosine kinase activity and downstream activation of target proteins involved in cellular proliferation and survival. This compound may be useful in treating cancers associated with increased c-kit kinase activity.

Inhibition of c-kit receptor tyrosine kinase activity by STI 571, a selective tyrosine kinase inhibitor

STI 571 (formerly known as CGP 57148B) is a known inhibitor of the c-abl, bcr-abl, and platelet-derived growth-factor receptor (PDGFR) tyrosine kinases. This compound is being evaluated in clinical trials for the treatment of chronic myelogenous leukemia. We sought to extend the activity profile of STI 571 by testing its ability to inhibit the tyrosine kinase activity of c-kit, a receptor structurally similar to PDGFR. We treated a c-kit expressing a human myeloid leukemia cell line, M-07e, with STI 571 before stimulation with Steel factor (SLF). STI 571 inhibited c-kit autophosphorylation, activation of mitogen-activated protein (MAP) kinase, and activation of Akt without altering total protein levels of c-kit, MAP kinase, or Akt. The concentration that produced 50% inhibition for these effects was approximately 100 nmol/L. STI 571 also significantly decreased SLF-dependent growth of M-07e cells in a dose-dependent manner and blocked the antiapoptotic activity of SLF. In contrast, the compound had no effect on MAP kinase activation or cellular proliferation in response to granulocyte-macrophage colony-stimulating factor. We also tested the activity of STI 571 in a human mast cell leukemia cell line (HMC-1), which has an activated mutant form of c-kit. STI 571 had a more potent inhibitory effect on the kinase activity of this mutant receptor than it did on ligand-dependent activation of the wild-type receptor. These findings show that STI 571 selectively inhibits c-kit tyrosine kinase activity and downstream activation of target proteins involved in cellular proliferation and survival. This compound may be useful in treating cancers associated with increased c-kit kinase activity.

The Fanconi anemia protein FANCC binds to and facilitates the activation of STAT1 by gamma interferon and hematopoietic growth factors

Hematopoietic progenitor cells from Fanconi anemia (FA) group C (FA-C) patients display hypersensitivity to the apoptotic effects of gamma interferon (IFN-gamma) and constitutively express a variety of IFN-dependent genes. Paradoxically, however, STAT1 activation is suppressed in IFN-stimulated FA cells, an abnormality corrected by transduction of normal FANCC cDNA. We therefore sought to define the specific role of FANCC protein in signal transduction through receptors that activate STAT1. Expression and phosphorylation of IFN-gamma receptor alpha chain (IFN-gammaRalpha) and JAK1 and JAK2 tyrosine kinases were equivalent in both normal and FA-C cells. However, in coimmunoprecipitation experiments STAT1 did not dock at the IFN-gammaR of FA-C cells, an abnormality corrected by transduction of the FANCC gene. In addition, glutathione S-transferase fusion genes encoding normal FANCC but not a mutant FANCC bearing an inactivating point mutation (L554P) bound to STAT1 in lysates of IFN-gamma-stimulated B cells and IFN-, granulocyte-macrophage colony-stimulating factor- and stem cell factor-stimulated MO7e cells. Kinetic studies revealed that the initial binding of FANCC was to nonphosphorylated STAT1 but that subsequently the complex moved to the receptor docking site, at which point STAT1 became phosphorylated. The STAT1 phosphorylation defect in FA-C cells was functionally significant in that IFN induction of IFN response factor 1 was suppressed and STAT1-DNA complexes were not detected in nuclear extracts of FA-C cells. We also determined that the IFN-gamma hypersensitivity of FA-C hematopoietic progenitor cells does not derive from STAT1 activation defects because granulocyte-macrophage CFU and erythroid burst-forming units from STAT1(-/-) mice were resistant to IFN-gamma. However, BFU-E responses to SCF and erythropoietin were suppressed in STAT(-/-) mice. Consequently, because the FANCC protein is involved in the activation of STAT1 through receptors for at least three hematopoietic growth and survival factor molecules, we reason that FA-C hematopoietic cells are excessively apoptotic because of an imbalance between survival cues (owing to a failure of STAT1 activation in FA-C cells) and apoptotic and mitogenic inhibitory cues (constitutively activated in FA-C cells in a STAT1-independent fashion).

The FANCA gene in Japanese Fanconi anemia: reports of eight novel mutations and analysis of sequence variability

Fanconi anemia (FA), an autosomal recessive disorder characterized by a progressive pancytopenia associated with congenital anomalies and high predisposition to malignancies, is a genetically and clinically heterogeneous disease. At least eight complementation groups (FA-A to FA-H) have been identified with their relative prevalence varying among the ethnical backgrounds. Recently, responsible genes, FANCA and FANCC, have been cloned. This report describes mutations of the FANCA gene, which we studied by direct sequencing of cDNA with confirmation on genomic DNA in 15 unclassified Japanese FA patients. A total of 19 sequence alterations were identified, of which 10 (six missense and four silent alterations) were likely to be nonpathogenic polymorphism. The remaining nine alterations, of which eight were novel mutations, were assumed to be pathogenic and consisted of two missense mutations and seven mutations resulting in truncation of gene product, demonstrating a wide allelic heterogeneity. The pathogenic mutations were found in 12 patients (80%); they were either homozygous or compound heterozygous in 10 patients, apparently heterozygous in two patients and none in three patients. We conclude that the sequence variability is intrinsic to the FANCA gene and that the relative prevalence of the FA-A subtype is unusually high in Japanese FA patients.

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.

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.

Loss of the Fanconi anemia group C protein activity results in an inability to activate caspase-3 after ionizing radiation

Fanconi anemia (FA) is a human genetic disease featuring cancer predisposition, genetic instability and DNA damage hypersensitivity. Although abnormalities in DNA repair and cell cycle checkpoint have been proposed as the underlying defect in this syndrome, these hypotheses did not provide full explanations of the complex phenotype. Although not exclusive of such possibilities, alterations in the control of apoptosis might account for the pleiotropic phenotype of this syndrome. We and others have previously reported a deregulation of the apoptotic response to mitomycin C, suggesting that the products of the Fanconi anemia group C protein (FANCC) contribute to the regulation of apoptosis. To explore the functional importance of the apoptotic alterations in FA we analyzed biochemical steps of the execution phase of apoptosis stimulated by another DNA damaging agent, the gamma-ray using FA cell lines derived from complementation group C (FA-C) independent patients. It is shown that the poly(ADP-ribose) polymerase, a target of caspase-3, is not cleaved in FA-C after ionizing radiation (IR). Moreover, caspase-3 is not processed in its active form and, its activity is not increased by IR in FA-C cells compared to normal cells. Altogether, these results demonstrate that loss of the FANCC activity results in a deficiency of the IR-induced apoptosis which is due to an inability to activate caspase-3. Our work suggests that apoptosis signaling induced by mitomycin C and IR is subject to common regulation involving the FANCC protein.

A Novel BTB/POZ Transcriptional Repressor Protein Interacts With the Fanconi Anemia Group C Protein and PLZF

Fanconi anemia (FA) is an autosomal recessive cancer susceptibility syndrome. The phenotype includes developmental defects, bone marrow failure, and cell cycle abnormalities. At least eight complementation groups (A-H) exist, and although three of the corresponding complementation group genes have been cloned, they lack recognizable motifs, and their functions are unknown. We have isolated a binding partner for the Fanconi anemia group C protein (FANCC) by yeast two-hybrid screening. We show that the novel gene, FAZF, encodes a 486 amino acid protein containing a conserved amino terminal BTB/POZ protein interaction domain and three C-terminal Krüppel-like zinc fingers. FAZF is homologous to the promyelocytic leukemia zinc finger (PLZF) protein, which has been shown to act as a transcriptional repressor by recruitment of nuclear corepressors (N-CoR, Sin3, and HDAC1 complex). Consistent with a role in FA, BTB/POZ-containing proteins have been implicated in oncogenesis, limb morphogenesis, hematopoiesis, and proliferation. We show that FAZF is a transcriptional repressor that is able to bind to the same DNA target sequences as PLZF. Our data suggest that the FAZF/FANCC interaction maps to a region of FANCC deleted in FA patients with a severe disease phenotype. We also show that FAZF and wild-type FANCC can colocalize in nuclear foci, whereas a patient-derived mutant FANCC that is compromised for nuclear localization cannot. These results suggest that the function of FANCC may be linked to a transcriptional repression pathway involved in chromatin remodeling.

A Novel BTB/POZ Transcriptional Repressor Protein Interacts With the Fanconi Anemia Group C Protein and PLZF

Fanconi anemia (FA) is an autosomal recessive cancer susceptibility syndrome. The phenotype includes developmental defects, bone marrow failure, and cell cycle abnormalities. At least eight complementation groups (A-H) exist, and although three of the corresponding complementation group genes have been cloned, they lack recognizable motifs, and their functions are unknown. We have isolated a binding partner for the Fanconi anemia group C protein (FANCC) by yeast two-hybrid screening. We show that the novel gene, FAZF, encodes a 486 amino acid protein containing a conserved amino terminal BTB/POZ protein interaction domain and three C-terminal Krüppel-like zinc fingers. FAZF is homologous to the promyelocytic leukemia zinc finger (PLZF) protein, which has been shown to act as a transcriptional repressor by recruitment of nuclear corepressors (N-CoR, Sin3, and HDAC1 complex). Consistent with a role in FA, BTB/POZ-containing proteins have been implicated in oncogenesis, limb morphogenesis, hematopoiesis, and proliferation. We show that FAZF is a transcriptional repressor that is able to bind to the same DNA target sequences as PLZF. Our data suggest that the FAZF/FANCC interaction maps to a region of FANCC deleted in FA patients with a severe disease phenotype. We also show that FAZF and wild-type FANCC can colocalize in nuclear foci, whereas a patient-derived mutant FANCC that is compromised for nuclear localization cannot. These results suggest that the function of FANCC may be linked to a transcriptional repression pathway involved in chromatin remodeling.

Resistance to mitomycin C requires direct interaction between the Fanconi anemia proteins FANCA and FANCG in the nucleus through an arginine-rich domain

Fanconi anemia (FA) is a genetically heterogeneous disorder characterized by bone marrow failure, birth defects, and chromosomal instability. Because FA cells are sensitive to mitomycin C (MMC), FA gene products could be involved in cellular defense mechanisms. The FANCA and FANCG proteins deficient in FA groups A and G interact directly with each other. We have localized the mutual interaction domains of these proteins to amino acids 18-29 of FANCA and to two noncontiguous carboxyl-terminal domains of FANCG encompassing amino acids 400-475 and 585-622. Site-directed mutagenesis of FANCA residues 18-29 revealed a novel arginine-rich interaction domain (RRRAWAELLAG). By alanine mutagenesis, Arg(1), Arg(2), and Leu(8) but not Arg(3), Trp(5), and Glu(7) appeared to be critical for binding to FANCG. Similar immunolocalization for FANCA and FANCG suggested that these proteins interact in vivo. Moreover, targeting of FANCA to the nucleus or the cytoplasm with nuclear localization and nuclear export signals, respectively, showed concordance between the localization patterns of FANCA and FANCG. The complementation function of FANCA was abolished by mutations in its FANCG-binding domain. Conversely, stable expression of FANCA mutants encoding intact FANCG interaction domains induced hypersensitivity to MMC in HeLa cells. These results demonstrate that FANCA-FANCG complexes are required for cellular resistance to MMC. Because the FANCC protein deficient in FA group C works within the cytoplasm, we suggest that FANCC and the FANCA-FANCG complexes suppress MMC cytotoxicity within distinct cellular compartments.

A physical complex of the Fanconi anemia proteins FANCG/XRCC9 and FANCA

Fanconi anemia (FA) is a recessively inherited disease characterized at the cellular level by spontaneous chromosomal instability and specific hypersensitivity to cross-linking agents. FA is genetically heterogeneous, comprising at least eight complementation groups (A-H). We report that the protein encoded by the gene mutated in complementation group G (FANCG) localizes to the cytoplasm and nucleus of the cell and assembles in a molecular complex with the FANCA protein, both in vivo and in vitro. Endogenous FANCA/FANCG complex was detected in both non-FA cells and in FA cells from groups D and E. By contrast, no complex was detected in specific cell lines belonging to groups A and G, whereas reduced levels were found in cells from groups B, C, F, and H. Wild-type levels of FANCA/FANCG complex were restored upon correction of the cellular phenotype by transfection or cell fusion experiments, suggesting that this complex is of functional significance in the FA pathway. These results indicate that the cellular FA phenotype can be connected to three biochemical subtypes based on the levels of FANCA/FANCG complex. Disruption of the complex may provide an experimental strategy for chemosensitization of neoplastic cells.

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.

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

Fanconi anemia (FA) is an autosomal recessive cancer susceptibility syndrome with at least eight complementation groups (A to H). Three FA genes, corresponding to complementation groups A, C, and G, have been cloned, but their cellular function remains unknown. We have previously demonstrated that the FANCA and FANCC proteins interact and form a nuclear complex in normal cells, suggesting that the proteins cooperate in a nuclear function. In this report, we demonstrate that the recently cloned FANCG/XRCC9 protein is required for binding of the FANCA and FANCC proteins. Moreover, the FANCG protein is a component of a nuclear protein complex containing FANCA and FANCC. The amino-terminal region of the FANCA protein is required for FANCG binding, FANCC binding, nuclear localization, and functional activity of the complex. Our results demonstrate that the three cloned FA proteins cooperate in a large multisubunit complex. Disruption of this complex results in the specific cellular and clinical phenotype common to most FA complementation groups.

Intracellular localization of the Fanconi anemia complementation group A protein

Mutations in the Fanconi anemia (FA) complementation group A (FANCA) gene leads to bone marrow failure, developmental abnormalities and cancer predisposition. To map the intracellular site of FANCA, we constructed a plasmid vector which linked in-frame the enhanced green fluorescent protein (EGFP cDNA) to the 5' end of the FANCA cDNA (pDAS-3). We studied the expression of pDAS-3 in the FANCA mutant fibroblast cell line (GM6914). MMC sensitivity of pDAS-3 transfected cells was comparable to wild-type fibroblasts. The resulting fluorescence pattern in the stable pDAS-3 cell line expressing the fusion protein was primarily nuclear. EGFP-selected cells (lacking FANCA) remain hypersensitive to MMC and maintained a cytoplasmic fluorescence pattern. Using deletion mutants of pDAS-3, a nuclear localization domain was identified at the amino terminus of the polypeptide. Western blot results of FANCA protein confirmed the presence of FANCA in nuclear fractions and FANCA protein levels did not vary during cell cycling. This nuclear trafficking of FANCA should guide future work in defining the function of this protein.