The Fanconi anemia group C gene product: signaling functions in hematopoietic cells

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

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

The ubiquitin family meets the Fanconi anemia proteins

Fanconi anaemia (FA) is a hereditary disorder characterized by bone marrow failure, developmental defects, predisposition to cancer and chromosomal abnormalities. FA is caused by biallelic mutations that inactivate genes encoding proteins involved in replication stress-associated DNA damage responses. The 20 FANC proteins identified to date constitute the FANC pathway. A key event in this pathway involves the monoubiquitination of the FANCD2-FANCI heterodimer by the collective action of at least 10 different proteins assembled in the FANC core complex. The FANC core complex-mediated monoubiquitination of FANCD2-FANCI is essential to assemble the heterodimer in subnuclear, chromatin-associated, foci and to regulate the process of DNA repair as well as the rescue of stalled replication forks. Several recent works have demonstrated that the activity of the FANC pathway is linked to several other protein post-translational modifications from the ubiquitin-like family, including SUMO and NEDD8. These modifications are related to DNA damage responses but may also affect other cellular functions potentially related to the clinical phenotypes of the syndrome. This review summarizes the interplay between the ubiquitin and ubiquitin-like proteins and the FANC proteins that constitute a major pathway for the surveillance of the genomic integrity and addresses the implications of their interactions in maintaining genome stability.

Stress and DNA repair biology of the Fanconi anemia pathway

Fanconi anemia (FA) represents a paradigm of rare genetic diseases, where the quest for cause and cure has led to seminal discoveries in cancer biology. Although a total of 16 FA genes have been identified thus far, the biochemical function of many of the FA proteins remains to be elucidated. FA is rare, yet the fact that 5 FA genes are in fact familial breast cancer genes and FA gene mutations are found frequently in sporadic cancers suggest wider applicability in hematopoiesis and oncology. Establishing the interaction network involving the FA proteins and their associated partners has revealed an intersection of FA with several DNA repair pathways, including homologous recombination, DNA mismatch repair, nucleotide excision repair, and translesion DNA synthesis. Importantly, recent studies have shown a major involvement of the FA pathway in the tolerance of reactive aldehydes. Moreover, despite improved outcomes in stem cell transplantation in the treatment of FA, many challenges remain in patient care.

GS-nitroxide (JP4-039)-mediated radioprotection of human Fanconi anemia cell lines

Fanconi anemia (FA) is an inherited disorder characterized by defective DNA repair and cellular sensitivity to DNA crosslinking agents. Clinically, FA is associated with high risk for marrow failure, leukemia and head and neck squamous cell carcinoma (HNSCC). Radiosensitivity in FA patients compromises the use of total-body irradiation for hematopoietic stem cell transplantation and radiation therapy for HNSCC. A radioprotector for the surrounding tissue would therefore be very valuable during radiotherapy for HNSCC. Clonogenic radiation survival curves were determined for pre- or postirradiation treatment with the parent nitroxide Tempol or JP4-039 in cells of four FA patient-derived cell lines and two transgene-corrected subclonal lines. FancG(-/-) (PD326) and FancD2(-/-) (PD20F) patient lines were more sensitive to the DNA crosslinking agent mitomycin C (MMC) than their transgene-restored subclonal cell lines (both P < 0.0001). FancD2(-/-) cells were more radiosensitive than the transgene restored subclonal cell line (ñ = 2.0 ± 0.7 and 4.7 ± 2.2, respectively, P = 0.03). In contrast, FancG(-/-) cells were radioresistant relative to the transgene-restored subclonal cell line (ñ = 9.4 ± 1.5 and 2.2 ± 05, respectively, P = 0.001). DNA strand breaks measured by the comet assay correlated with radiosensitivity. Cell lines from a Fanc-C and Fanc-A patients showed radiosensitivity similar to that of Fanc-D2(-/-) cells. A fluorophore-tagged JP4-039 (BODIPY-FL) analog targeted the mitochondria of the cell lines. Preirradiation or postirradiation treatment with JP4-039 at a lower concentration than Tempol significantly increased the radioresistance and stabilized the antioxidant stores of all cell lines. Tempol increased the toxicity of MMC in FancD2(-/-) cells. These data provide support for the potential clinical use of JP4-039 for normal tissue radioprotection during chemoradiotherapy in FA patients.

Pathophysiology and management of inherited bone marrow failure syndromes

The inherited marrow failure syndromes are a diverse set of genetic disorders characterized by hematopoietic aplasia and cancer predisposition. The clinical phenotypes are highly variable and much broader than previously recognized. The medical management of the inherited marrow failure syndromes differs from that of acquired aplastic anemia or malignancies arising in the general population. Diagnostic workup, molecular pathogenesis, and clinical treatment are reviewed.

Fanconi anemia

Fanconi anemia (FA) is an autosomal and X-linked recessive disorder characterized by bone marrow failure, acute myelogenous leukemia, solid tumors, and developmental abnormalities. Recent years have seen a dramatic improvement in FA patient treatment, resulting in a greater survival of children into adulthood. These improvements have been made despite the fact that a definitive cellular function for the proteins in the FA pathway has yet to be elucidated. Delineating the cellular functions of the FA pathway could help further improve the treatment options for FA patients and further reduce the probability of succumbing to the disease. This article reviews the current clinical aspects of FA including presentation, diagnosis, and treatment followed by a review of the molecular aspects of FA as they are currently understood.

Oxidative stress in Fanconi anemia hematopoiesis and disease progression

Patients with the genomic instability syndrome Fanconi anemia (FA) commonly develop progressive bone marrow failure and have a high risk of cancer. The prominent role of the FA protein family involves DNA damage response and/or repair. Oxidative stress, defined as an imbalance between the production of reactive oxygen species and antioxidant defense, is considered to be an important pathogenic factor in leukemia-prone bone marrow diseases such as FA. Cellular responses inducing resistance to oxidative stress are important for cellular survival, organism lifespan, and cancer prevention, but until recently, mammalian factors regulating resistance to oxidative stress have not been well characterized. Significant evidence supports excessive apoptosis of hematopoietic stem/progenitor cells, induced by stresses, most significantly oxidative stress, as a critical factor in the pathogenesis of bone marrow failure and leukemia progression in FA. In this brief review, we discuss the functional link between FA proteins and oxidative DNA damage response/repair, with emphasis on the implication of oxidative stress in the pathophysiology and abnormal hematopoiesis in FA.

Aberrant activation of stress-response pathways leads to TNF-alpha oversecretion in Fanconi anemia

Fanconi anemia (FA), an inherited syndrome that associates bone marrow failure, cancer predisposition, and genetic instability, is characterized by an overproduction of the myelosuppressive cytokine TNF-alpha through unknown mechanisms. We demonstrate here that FANC pathway loss-of-function results in the aberrant activation of 2 major stress-signaling pathways: NF-kappaB and MAPKs. These responses are independent on TNF-alpha expression. On the contrary, inhibition of the MAPK pathways normalizes TNF-alpha oversecretion in FA. Moreover, our data show that the overexpression of the matrix metalloproteinase MMP-7 is the key event directly responsible for the high rate of TNF-alpha shedding and release from the cytoplasmic membrane in FA. TNF-alpha overproduction is, indeed, normalized by MMP-7 inhibition. Finally, MAPK inhibition impacts on MMP-7 overexpression. Evidence is provided of the existence of a linear pathway in which FANC mutations activate MAPK signaling that induces MMP-7 overexpression leading, in fine, to TNF-alpha oversecretion. TNF-alpha may, in turn, sustain or amplify both MAPKs and NF-kappaB activation. Aberrant expression or activity of NF-kappaB and/or MAPKs has been already involved in bone marrow failure and leukemia, and their inhibition offered clinical benefit for patients. In conclusion, our data provide a strong rationale for new clinical trials on FA patients.

Bax expression and apoptotic cell death in Fanconi anaemia peripheral blood lymphocytes

OBJECTIVE

Deregulated apoptosis might be involved in some of the features of Fanconi anaemia (FA). The possibility that the pro-apoptotic Bax protein could be involved in an increased susceptibility to apoptosis in FA patients was investigated.

MATERIALS AND METHODS

Intracellular Bax expression, Bcl-2 expression (an anti-apoptotic protein) and cell death were analysed in 26 FA peripheral blood lymphocyte samples.

RESULTS

Most FA samples (69%) displayed increased levels of Bax and were more susceptible to both spontaneous apoptosis and mitogen activation-induced cell death. Two subgroups were identified: one presented elevated levels of Bax (n = 18), whereas the other (n = 8), had Bax levels lower than controls. Two subgroups based on Bcl-2 expression were also identified: one with normal and another with high Bcl-2 expression. No inverse correlation was found between Bcl-2 levels and Bax expression. A clear difference in susceptibility to induced cell death could be observed between control and FA samples. The best correlation was observed between high levels of Bax and mitogen-induced apoptosis of cells; these displayed characteristics of necrosis secondary to apoptosis, suggesting that the intrinsic apoptotic pathway was being activated.

CONCLUSION

Despite increased susceptibility to cell death induction, there was no correlation between Bax levels, chromosome breakage, haematological parameters or androgen therapy. The importance of apoptosis and Bax expression in the clinical development of FA awaits clarification.

L'anémie de Fanconi : aspects cellulaires et moléculaires

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

Fanconi anaemia genes and susceptibility to cancer

Fanconi anaemia (FA) is a rare recessive disorder associated with chromosomal fragility, aplastic anaemia, congenital abnormalities and a high risk of cancer, including acute myeloid leukaemia and squamous cell carcinomas. The identification of 11 different FA genes has revealed a complex web of interacting proteins that are involved in the recognition or repair of DNA interstrand crosslinks and perhaps other forms of DNA damage. Bi-allelic mutations in BRCA2 are associated with a rare and highly cancer-prone form of FA, and the DNA helicase BRIP1 (formerly BACH1) is mutated in FA group J. There is little convincing evidence that FA heterozygotes are at increased risk of cancer, but larger studies are needed to address the possibility of modest risk effects. Somatic inactivation of the FA pathway by mutation or epigenetic silencing has been observed in several different types of sporadic cancer, and this may have important implications for targeted chemotherapy. Inhibition of this pathway represents a possible route to sensitization of tumours to DNA crosslinking drugs such as cisplatin.

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.

3R coordination by Fanconi anemia proteins

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

How Fanconi anemia proteins promote the four Rs: replication, recombination, repair, and recovery

The genetically complex disease Fanconi anemia (FA) comprises cancer predisposition, developmental defects, and bone marrow failure due to elevated apoptosis. The FA cellular phenotype includes universal sensitivity to DNA crosslinking damage, symptoms of oxidative stress, and reduced mutability at the X-linked HPRT gene. In this review article, we present a new heuristic molecular model that accommodates these varied features of FA cells. In our view, the FANCA, -C, and -G proteins, which are both cytoplasmic and nuclear, have an integrated dual role in which they sense and convey information about cytoplasmic oxidative stress to the nucleus, where they participate in the further assembly and functionality of the nuclear core complex (NCCFA= FANCA/B/C/E/F/G/L). In turn, NCCFA facilitates DNA replication at sites of base damage and strand breaks by performing the critical monoubiquitination of FANCD2, an event that somehow helps stabilize blocked and broken replication forks. This stabilization facilitates two kinds of processes: translesion synthesis at sites of blocking lesions (e.g., oxidative base damage), which produces point mutations by error-prone polymerases, and homologous recombination-mediated restart of broken forks, which arise spontaneously and when crosslinks are unhooked by the ERCC1-XPF endonuclease. In the absence of the critical FANCD2 monoubiquitination step, broken replication forks further lose chromatid continuity by collapsing into a configuration that is more difficult to restart through recombination and prone to aberrant repair through nonhomologous end joining. Thus, the FA regulatory pathway promotes chromosome integrity by monitoring oxidative stress and coping efficiently with the accompanying oxidative DNA damage during DNA replication.

Fanconi anemia type C-deficient hematopoietic cells are resistant to TRAIL (TNF-related apoptosis-inducing ligand)-induced cleavage of pro-caspase-8

OBJECTIVE

The pathophysiology of bone marrow failure in Fanconi anemia (FA) patients is thought to involve excessive apoptosis involving signaling triggered by fas ligation and tumor necrosis factor (TNF)-alpha, or interferon (IFN)-gamma exposure. We investigated whether a new member of the TNF family, TRAIL (TNF-related apoptosis-inducing ligand), would similarly trigger preferential apoptotic cell death in FA phenotype cells.

MATERIAL AND METHODS

Hematopoietic cells from FANCC(-/-) transgenic mice and human FA-C lymphoblasts (HSC536N) as well as their phenotypically corrected counterparts (FANCC(+/+), HSC536/FA-Cneo) were compared for their response to apoptosis induction by TRAIL and fas ligation in the presence or absence of IFN-gamma. Cells were also studied for the protein and gene expression of TRAIL-receptors, caspase-8 and its inhibitory protein, FLIP.

RESULTS

TRAIL exposure by itself or in combination with IFN-gamma did not lead to preferential apoptosis induction in human and murine FA-C phenotype hematopoietic cells. This resistance was unrelated to the expression of TRAIL receptors or FLIP isoforms, but correlated with absent cleavage of pro-caspase-8. Results were validated by those from gene expression profiling of relevant genes in the two lymphoblast cell lines.

CONCLUSION

TRAIL, in contrast to fas ligation, does not induce preferential apoptosis in FA-C phenotype cells despite shared downstream signaling described in non-FA models. These data provide further insight into the complexity of FA-C-regulated apoptotic signaling.

Oxidant hypersensitivity of Fanconi anemia type C-deficient cells is dependent on a redox-regulated apoptotic pathway

Fanconi anemia is a genetic disorder characterized by bone marrow failure. Significant evidence supports enhanced apoptosis of hematopoietic stem/progenitor cells as a critical factor in the pathogenesis of bone marrow failure in Fanconi anemia. However, the molecular mechanism(s) responsible for the apoptotic phenotype are incompletely understood. Here, we tested whether alterations in the activation of a redox-dependent pathway may participate in the pro-apoptotic phenotype of primary Fancc -/- cells in response to oxidative stress. Our data indicate that Fancc -/- cells are highly sensitive to oxidant stimuli and undergo enhanced oxidant-mediated apoptosis compared with wild type controls. In addition, antioxidants preferentially enhanced the survival of Fancc -/- cells. Because oxidative stress activates the redox-dependent ASK1 pathway, we assessed whether Fancc -/- cells exhibited increased oxidant-induced ASK1 activation. Our results revealed ASK1 hyperactivation in H2O2-treated Fancc -/- cells. Furthermore, using small interfering RNAs to decrease ASK1 expression and a dominant negative ASK1 mutant to inhibit ASK1 kinase activity, we determined that H2O2-induced apoptosis was ASK1-dependent. Collectively, these data argue that the predisposition of Fancc -/- hematopoietic stem/progenitor cells to apoptosis is mediated in part through altered redox regulation and ASK1 hyperactivation.

Nucleophosmin interacts with and inhibits the catalytic function of eukaryotic initiation factor 2 kinase PKR

In normal cells the protein kinase PKR effects apoptosis in response to various extra and intracellular cues and can also function to suppress the neoplastic phenotype. Because most neoplastic cells are resistant to certain apoptotic cues, we reasoned that an early molecular event in carcinogenesis or leukemogenesis might be the inactivation of PKR by expression or activation of intracellular PKR inhibitors. Seeking novel PKR-modulating proteins we report here that nucleophosmin (NPM), a protein frequently overexpressed in a variety of human malignancies, binds to PKR, and inhibits its activation. Co-immunoprecipitation and in vitro binding experiments showed that NPM associated with PKR. Kinase assays demonstrated that recombinant NPM inhibited PKR activation in a dose-dependent manner. In addition, purified recombinant NPM was phosphorylated by activated PKR. Most importantly, overexpression of NPM suppressed PKR activity, enhanced protein synthesis, and inhibited apoptosis. Lymphoblasts from patients with Fanconi anemia (FA) expressed low levels of NPM, which correlated with high ground-state activation of PKR and cellular hypersensitivity to apoptotic cues, but enforced expression of NPM in these mutant cells reduced aberrant apoptotic responses. Inhibition of PKR by NPM may be one mechanism by which neoplastic clones evolve in sporadic malignancies and in neoplastic cells arising in the context of the cancer predisposition syndrome, Fanconi anemia.

Yeast two-hybrid screens imply involvement of fanconi anemia proteins in transcription regulation, cell signaling, oxidative metabolism, and cellular transport

Mutations in one of at least eight different genes cause bone marrow failure, chromosome instability, and predisposition to cancer associated with the rare genetic syndrome Fanconi anemia (FA). The cloning of seven genes has provided the tools to study the molecular pathway disrupted in Fanconi anemia patients. The structure of the genes and their gene products provided few clues to their functional role. We report here the use of 3 FA proteins, FANCA, FANCC, and FANCG, as "baits" in the hunt for interactors to obtain clues for FA protein functions. Using five different human cDNA libraries we screened 36.5x10(6) clones with the technique of the yeast two-hybrid system. We identified 69 proteins which have not previously been linked to the FA pathway as direct interactors of FANCA, FANCC, or FANCG. Most of these proteins are associated with four functional classes including transcription regulation (21 proteins), signaling (13 proteins), oxidative metabolism (10 proteins), and intracellular transport (11 proteins). Interaction with 6 proteins, DAXX, Ran, IkappaBgamma, USP14, and the previously reported SNX5 and FAZF, was additionally confirmed by coimmunoprecipitation and/or colocalization studies. Taken together, our data strongly support the hypothesis that FA proteins are functionally involved in several complex cellular pathways including transcription regulation, cell signaling, oxidative metabolism, and cellular transport.

Cisplatin triggers apoptotic or nonapoptotic cell death in Fanconi anemia lymphoblasts in a concentration-dependent manner

Cells derived from Fanconi anemia (FA) patients are hypersensitive for cross-linking agents, such as cisplatin, that are potent inducers of programmed cell death (PCD). Here, we studied cisplatin hypersensitivity in FA in relation to the mechanism of PCD in lymphoblastoid cells representing FA groups A and C. In FA cells, a low concentration of cisplatin caused chromatin condensation, phosphatidylserine (PS) externalization, and the expression of an 18-kDa variant of Bax, all indicators of apoptotic cell death, and the latter suggesting the involvement of a mitochondrial route. However, procaspases-3, -8, and -9, and PARP were not cleaved, although small increases in caspase activity could be detected. At a high concentration of cisplatin, both FA and corrected cells showed a robust cleavage of procaspases and PARP. DNA fragmentation was clearly visible under high cisplatin conditions and to some extent at a low concentration in FA-A cells, but not in the FA-C cell line regardless of the presence of functional FANCC, suggesting an unknown deficiency in these cells. We conclude that hypersensitivity in FA cells is associated with a mixture of necrotic and apoptotic features that is best described as apoptotic-like cell death, and that a defective FA pathway does not interfere with the proper activation of caspase-mediated cell death.

Increased sensitivity of Fancc-deficient hematopoietic cells to nitric oxide and evidence that this species mediates growth inhibition by cytokines

Fanconi anemia complementation group C (Fancc)-deficient murine bone marrow progenitors demonstrate increased sensitivity to growth inhibition by interferon gamma (IFNgamma), tumor necrosis factor alpha (TNFalpha), and macrophage inflammatory protein 1alpha (MIP-1alpha). This property has been proposed as a possible pathogenic factor in the marrow failure seen in Fanconi anemia. Supporting our hypothesis that nitric oxide (NO) production might be a common effector in this sensitivity, we found that cytokine-mediated growth inhibition of Fancc(-/-) bone marrow cells was prevented by inhibiting NO synthase activity. Interestingly, Fancc(-/-) hematopoietic cells also exhibited increased growth inhibition on exposure to 2 distinct NO-generating agents, S-nitroso-N-acetyl-D, L-penicillamine (SNAP) and diethylenetriamine nitric oxide adduct (DETA/NO). In keeping with the sensitivity of Fancc(-/-) cells to IFNgamma, inducible nitric oxide synthase (iNOS) levels and nitrite release were both increased following stimulation of Fancc(-/-) macrophages with this cytokine, either alone or in combination with bacterial lipopolysaccharide. Suggesting a plausible mechanism for the increased expression of iNOS, IFNgamma-stimulated Fancc(-/-) macrophages generated higher levels of phospho-Stat1, a positive regulator of inos (nos2) gene expression. These observations, while confined to C57BL/6 Fancc(-/-) hematopoietic cells, raise the possibility that nitric oxide has a role in the pathogenesis of Fanconi anemia.

Constitutional sequence variation in the Fanconi anaemia group C (FANCC) gene in childhood acute myeloid leukaemia

The extent to which genetic susceptibility contributes to the causation of childhood acute myeloid leukaemia (AML) is not known. The inherited bone marrow failure disorder Fanconi anaemia (FA) carries a substantially increased risk of AML, raising the possibility that constitutional variation in the FA (FANC) genes is involved in the aetiology of childhood AML. We have screened genomic DNA extracted from remission blood samples of 97 children with sporadic AML and 91 children with sporadic acute lymphoblastic leukaemia (ALL), together with 104 cord blood DNA samples from newborn children, for variations in the Fanconi anaemia group C (FANCC) gene. We found no evidence of known FANCC pathogenic mutations in children with AML, ALL or in the cord blood samples. However, we detected 12 different FANCC sequence variants, of which five were novel to this study. Among six FANCC variants leading to amino-acid substitutions, one (S26F) was present at a fourfold greater frequency in children with AML than in the cord blood samples (odds ratio: 4.09, P = 0.047; 95% confidence interval 1.08-15.54). Our results thus do not exclude the possibility that this polymorphic variant contributes to the risk of a small proportion of childhood AML.

The role of mitochondrial-mediated apoptosis in a myelodysplastic syndrome secondary to congenital deletion of the short arm of chromosome 4

OBJECTIVE

Myelodysplastic syndromes (MDS) are characterized by peripheral cytopenia and ineffective hematopoiesis. In adult-onset MDS and in certain inherited marrow failure syndromes, apoptosis is increased and is mediated mainly through activation of the Fas pathway. It is unclear whether the various myelodysplastic disorders share the same apoptosis pathways. I investigated apoptosis pathways in a patient with refractory cytopenia with ring sideroblasts associated with congenital 4p deletion to determine the mechanism for bone marrow failure.

METHODS

Marrow cells and lymphoblast cell lines generated from peripheral blood were analyzed for apoptosis and protein expression by flow cytometry, Western blot, and confocal microscopy, either directly or after gamma irradiation (15 G). Cell viability after treatment with inhibitors of specific apoptosis pathways was also determined.

RESULTS

Compared to controls, the patient's marrow and lymphoblastoid cells showed significantly higher apoptosis rates and activation of caspase-3. Investigation of the mitochondrial apoptosis pathway showed a consistent pro-apoptosis profile, namely, upregulation of Bax, Bax-alpha, cytochrome c, and Apaf1, and low bcl-2. Differences between the patient's and the normal cells were further accentuated after irradiation; p53 expression was strikingly higher in the patient only after irradiation. In contrast, Fas and FADD expression on the patient's and the control's cells were comparable. Addition of caspase 3 or caspase 9 inhibitors markedly increased patient cell viablity, but blocking anti-Fas antibody did not.

CONCLUSION

The ineffective hematopoiesis in this case is explained by increased apoptosis and is linked to hyperactivation of the mitochondrial cell death machinery and not to the Fas pathway, which might be secondary to an intramitochondrial defect. This information is crucial because the development of anti-apoptotic agents for the treatment of MDS may not be universally efficacious and should target the specific derangement.

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

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

In vitro phenotypic correction of hematopoietic progenitors from Fanconi anemia group A knockout mice

Fanconi anemia (FA) is a rare autosomal recessive disease, characterized by bone marrow failure and cancer predisposition. So far, 8 complementation groups have been identified, although mutations in FANCA account for the disease in the majority of FA patients. In this study we characterized the hematopoietic phenotype of a Fanca knockout mouse model and corrected the main phenotypic characteristics of the bone marrow (BM) progenitors using retroviral vectors. The hematopoiesis of these animals was characterized by a modest though significant thrombocytopenia, consistent with reduced numbers of BM megakaryocyte progenitors. As observed in other FA models, the hematopoietic progenitors from Fanca−/− mice were highly sensitive to mitomycin C (MMC). In addition, we observed for the first time in a FA mouse model a marked in vitro growth defect ofFanca−/−progenitors, either when total BM or when purified Lin−Sca-1+ cells were subjected to in vitro stimulation. Liquid cultures ofFanca−/−BM that were stimulated with stem cell factor plus interleukin-11 produced low numbers of granulocyte macrophage colony-forming units, contained a high proportion of apoptotic cells, and generated a decreased proportion of granulocyte versus macrophage cells, compared to normal BM cultures. Aiming to correct the phenotype of Fanca−/−progenitors, purified Lin−Sca-1+ cells were transduced with retroviral vectors encoding the enhanced green fluorescent protein (EGFP) gene and human FANCAgenes. Lin−Sca-1+ cells fromFanca−/−mice were transduced with an efficiency similar to that of samples from wild-type mice. More significantly, transductions with FANCA vectors corrected both the MMC hypersensitivity as well as the impaired ex vivo expansion ability that characterized the BM progenitors ofFanca−/−mice.

In vitro phenotypic correction of hematopoietic progenitors from Fanconi anemia group A knockout mice

Fanconi anemia (FA) is a rare autosomal recessive disease, characterized by bone marrow failure and cancer predisposition. So far, 8 complementation groups have been identified, although mutations in FANCA account for the disease in the majority of FA patients. In this study we characterized the hematopoietic phenotype of a Fanca knockout mouse model and corrected the main phenotypic characteristics of the bone marrow (BM) progenitors using retroviral vectors. The hematopoiesis of these animals was characterized by a modest though significant thrombocytopenia, consistent with reduced numbers of BM megakaryocyte progenitors. As observed in other FA models, the hematopoietic progenitors from Fanca−/− mice were highly sensitive to mitomycin C (MMC). In addition, we observed for the first time in a FA mouse model a marked in vitro growth defect ofFanca−/−progenitors, either when total BM or when purified Lin−Sca-1+ cells were subjected to in vitro stimulation. Liquid cultures ofFanca−/−BM that were stimulated with stem cell factor plus interleukin-11 produced low numbers of granulocyte macrophage colony-forming units, contained a high proportion of apoptotic cells, and generated a decreased proportion of granulocyte versus macrophage cells, compared to normal BM cultures. Aiming to correct the phenotype of Fanca−/−progenitors, purified Lin−Sca-1+ cells were transduced with retroviral vectors encoding the enhanced green fluorescent protein (EGFP) gene and human FANCAgenes. Lin−Sca-1+ cells fromFanca−/−mice were transduced with an efficiency similar to that of samples from wild-type mice. More significantly, transductions with FANCA vectors corrected both the MMC hypersensitivity as well as the impaired ex vivo expansion ability that characterized the BM progenitors ofFanca−/−mice.