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

Role of MicroRNA in linking diabetic retinal neurodegeneration and vascular degeneration

Diabetic retinopathy is the major cause of blindness in diabetic patients, with limited treatment options that do not always restore optimal vision. Retinal nerve degeneration and vascular degeneration are two primary pathological processes of diabetic retinopathy. The retinal nervous system and vascular cells have a close coupling relationship. The connection between neurodegeneration and vascular degeneration is not yet fully understood. Recent studies have found that microRNA plays a role in regulating diabetic retinal neurovascular degeneration and can help delay the progression of the disease. This article will review how microRNA acts as a bridge connecting diabetic retinal neurodegeneration and vascular degeneration, focusing on the mechanisms of apoptosis, oxidative stress, inflammation, and endothelial factors. The aim is to identify valuable targets for new research and clinical treatment of diabetic retinopathy.

Innate Immunity: A Balance between Disease and Adaption to Stress

Since first being documented in ancient times, the relation of inflammation with injury and disease has evolved in complexity and causality. Early observations supported a cause (injury) and effect (inflammation) relationship, but the number of pathologies linked to chronic inflammation suggests that inflammation itself acts as a potent promoter of injury and disease. Additionally, results from studies over the last 25 years point to chronic inflammation and innate immune signaling as a critical link between stress (exogenous and endogenous) and adaptation. This brief review looks to highlight the role of the innate immune response in disease pathology, and recent findings indicating the innate immune response to chronic stresses as an influence in driving adaptation.

Identification of Three Novel Mutations in the FANCA, FANCC, and ITGA2B Genes by Whole Exome Sequencing

Background

Various blood diseases are caused by mutations in the FANCA, FANCC, and ITGA2B genes. Exome sequencing is a suitable method for identifying single-gene disease and genetic heterogeneity complaints.

Methods

Among families who were referred to Narges Genetic and PND Laboratory in 2015-2017, five families with a history of blood diseases were analyzed using the whole exome sequencing (WES) method.

Results

We detected two novel mutations (c.190-2A>G and c.2840C>G) in the FANCA gene, c. 1429dupA mutation in the FANCC gene, and c.1392A>G mutation in the ITGA2B gene. The prediction of variant pathogenicity has been done using bioinformatics tools such as Mutation taster PhD-SNP and polyphen2 and were confirmed by Sanger sequencing.

Conclusions

WES could be as a precise tool for identifying the pathologic variants in affected patient and heterozygous carriers among families. This highly successful technique will remain at the forefront of platelet and blood genomic research.

Dental-craniofacial manifestation and treatment of rare diseases

Rare diseases are usually genetic, chronic and incurable disorders with a relatively low incidence. Developments in the diagnosis and management of rare diseases have been relatively slow due to a lack of sufficient profit motivation and market to attract research by companies. However, due to the attention of government and society as well as economic development, rare diseases have been gradually become an increasing concern. As several dental-craniofacial manifestations are associated with rare diseases, we summarize them in this study to help dentists and oral maxillofacial surgeons provide an early diagnosis and subsequent management for patients with these rare diseases.

Recent advances in understanding hematopoiesis in Fanconi Anemia

Fanconi anemia is an inherited disease characterized by genomic instability, hypersensitivity to DNA cross-linking agents, bone marrow failure, short stature, skeletal abnormalities, and a high relative risk of myeloid leukemia and epithelial malignancies. The 21 Fanconi anemia genes encode proteins involved in multiple nuclear biochemical pathways that effect DNA interstrand crosslink repair. In the past, bone marrow failure was attributed solely to the failure of stem cells to repair DNA. Recently, non-canonical functions of many of the Fanconi anemia proteins have been described, including modulating responses to oxidative stress, viral infection, and inflammation as well as facilitating mitophagic responses and enhancing signals that promote stem cell function and survival. Some of these functions take place in non-nuclear sites and do not depend on the DNA damage response functions of the proteins. Dysfunctions of the canonical and non-canonical pathways that drive stem cell exhaustion and neoplastic clonal selection are reviewed, and the potential therapeutic importance of fully investigating the scope and interdependences of the canonical and non-canonical pathways is emphasized.

Identification of the PKR nuclear interactome reveals roles in ribosome biogenesis, mRNA processing and cell division

The double-strand RNA-dependent protein kinase, PKR, plays a central role in inflammatory/chronic stress-mediated pathologies such as cancer, diabetes, and neuro/muscular degenerative diseases. Although a significant amount of research has been conducted to elucidate the role of PKR signaling in the cytosol, only recently has attention been paid to the role of PKR in the nuclear compartment. Previously our group reported that phosphorylated forms of PKR are present in the nucleus of acute leukemic cell lines, representing a reservoir of active kinase that responds to stress. Using the CCRF-CEM acute T-cell leukemia cell line, a PKR-specific inhibitor, co-immunoprecipitation and a proteomics approach, which included affinity purified mass spectrometry analysis (AP/MS), we identified the proteins present in active and inactive PKR nuclear complexes. Of the proteins identified in the PKR complexes, sixty-nine (69) were specific to the active complex, while thirty-eight (38) were specific to the inactive complex. An additional thirteen (13) proteins associated specifically with both complexes. The majority of the proteins identified are involved in, ribosome biogenesis, RNA splicing, mRNA stability, gene expression, cell cycle, or chromatin organization, including several with known significance to normal hematopoiesis and/or hematological disease. In agreement with the AP/MS data, basal- or over-expression of PKR under normal growth conditions favored cell proliferation in the tested cell lines, whereas pharmacological inhibition of PKR or shRNA-mediated knock-down did not. PKR was also found to influence the isoform and the level of expression of the proto-oncogene MYC.

Proteomic analysis reveals a FANCA-modulated neddylation pathway involved in CXCR5 membrane targeting and cell mobility

The aim of this study was to identify novel substrates of the FANCcore complex, the inactivation of which leads to the genetic disorder Fanconi anemia, which is associated with bone marrow failure, developmental abnormalities and a predisposition to cancer. Eight FANC proteins participate in the nuclear FANCcore complex, which functions as an E3 ubiquitin-ligase that monoubiquitylates FANCD2 and FANCI in response to replicative stress. Here, we use mass spectrometry to compare proteins from FANCcore-complex-deficient cells to those of rescued control cells after treatment with hydroxyurea, an inducer of FANCD2 monoubiquitylation. FANCD2 and FANCI appear to be the only targets of the FANCcore complex. We identify other proteins that are post-translationally modified in a FANCA- or FANCC-dependent manner. The majority of these potential targets localize to the cell membrane. Finally, we demonstrate that (a) the chemokine receptor CXCR5 is neddylated; (b) FANCA but not FANCC appears to modulate CXCR5 neddylation through an unknown mechanism; (c) CXCR5 neddylation is involved in targeting the receptor to the cell membrane; and (d) CXCR5 neddylation stimulates cell migration and motility. Our work has uncovered a pathway involving FANCA in neddylation and cell motility.

Oxidative stress-associated protein tyrosine kinases and phosphatases in Fanconi anemia

SIGNIFICANCE

Fanconi anemia (FA) is a genetic disorder featuring chromosomal instability, developmental defects, progressive bone marrow failure, and predisposition to cancer. Besides the predominant role in DNA damage response and/or repair, many studies have linked FA proteins to oxidative stress. Oxidative stress, defined as imbalance in pro-oxidant and antioxidant homeostasis, has been considered to contribute to disease development, including FA.

RECENT ADVANCES

A variety of signaling pathways may be influenced by oxidative stress, particularly the equilibrium between protein kinases and phosphatases, consequently leading to an aberrant phosphorylation state of cellular proteins. Dysfunction of kinases/phosphatases has been implicated in the pathophysiology of human diseases. In FA, evidence is emerging that links abnormal phosphorylation/de-phosphorylation of signaling molecules to clinical complications and malformations.

CRITICAL ISSUES

In this study, we review the recent findings on the oxidative stress-related kinases and phosphatases, particularly tyrosine phosphatases in FA.

FUTURE DIRECTIONS

Understanding the role of oxidative stress-related kinases and phosphatases in FA may provide unique and generic possibilities for the future development of therapeutic strategies by targeting the dysregulated protein kinases and phosphatases in a clinical setting.

Why does the bone marrow fail in Fanconi anemia?

The inherited bone marrow failure (BMF) syndromes are a rare and diverse group of genetic disorders that ultimately result in the loss of blood production. The molecular defects underlying many of these conditions have been elucidated, and great progress has been made toward understanding the normal function of these gene products. This review will focus on perhaps the most well-known and genetically heterogeneous BMF syndrome: Fanconi anemia. More specifically, this account will review the current state of our knowledge on why the bone marrow fails in this illness and what this might tell us about the maintenance of bone marrow function and hematopoiesis.

Fanconi anemia links reactive oxygen species to insulin resistance and obesity

AIMS

Insulin resistance is a hallmark of obesity and type 2 diabetes. Reactive oxygen species (ROS) have been proposed to play a causal role in insulin resistance. However, evidence linking ROS to insulin resistance in disease settings has been scant. Since both oxidative stress and diabetes have been observed in patients with the Fanconi anemia (FA), we sought to investigate the link between ROS and insulin resistance in this unique disease model.

RESULTS

Mice deficient for the Fanconi anemia complementation group A (Fanca) or Fanconi anemia complementation group C (Fancc) gene seem to be diabetes-prone, as manifested by significant hyperglycemia and hyperinsulinemia, and rapid weight gain when fed with a high-fat diet. These phenotypic features of insulin resistance are characterized by two critical events in insulin signaling: a reduction in tyrosine phosphorylation of the insulin receptor (IR) and an increase in inhibitory serine phosphorylation of the IR substrate-1 in the liver, muscle, and fat tissues from the insulin-challenged FA mice. High levels of ROS, spontaneously accumulated or generated by tumor necrosis factor alpha in these insulin-sensitive tissues of FA mice, were shown to underlie the FA insulin resistance. Treatment of FA mice with the natural anti-oxidant Quercetin restores IR signaling and ameliorates the diabetes- and obesity-prone phenotypes. Finally, pairwise screen identifies protein-tyrosine phosphatase (PTP)-α and stress kinase double-stranded RNA-dependent protein kinase (PKR) that mediate the ROS effect on FA insulin resistance.

INNOVATION

These findings establish a pathogenic and mechanistic link between ROS and insulin resistance in a unique human disease setting.

CONCLUSION

ROS accumulation contributes to the insulin resistance in FA deficiency by targeting both PTP-α and PKR.

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.

Impaired function of Fanconi anemia type C-deficient macrophages

FA is a genetic disorder characterized by BM failure, developmental defects, and cancer predisposition. Previous studies suggest that FA patients exhibit alterations in immunologic function. However, it is unclear whether the defects are immune cell-autonomous or secondary to leukopenia from evolving BM failure. Given the central role that macrophages have in the innate immune response, inflammation resolution, and antigen presentation for acquired immunity, we examined whether macrophages from Fancc-/- mice exhibit impaired function. Peritoneal inflammation induced by LPS or sodium periodate resulted in reduced monocyte/macrophage recruitment in Fancc-/- mice compared with WT controls. Fancc-/- mice also had decreased inflammatory monocytes mobilized into the peripheral blood after LPS treatment compared with controls. Furthermore, Fancc-/- peritoneal macrophages displayed cell-autonomous defects in function, including impaired adhesion to FN or endothelial cells, reduced chemoattractant-mediated migration, and decreased phagocytosis. Moreover, dysregulated F-actin rearrangement was detected in Fancc-/- macrophages after adhesion to FN, which was consistent with an observed reduction in RhoA-GTP levels. Importantly, these data suggest that impaired cytoskeletal rearrangements in Fancc-/- macrophages may be the common mechanism responsible for cell-autonomous defects detected in vitro, as well as altered monocyte/macrophage trafficking in vivo.

Direct inhibition of TNF-α promoter activity by Fanconi anemia protein FANCD2

Fanconi anemia (FA), an inherited disease, is associated with progressive bone marrow failure, predisposition to cancer, and genomic instability. Genes corresponding to 15 identified FA complementation groups have been cloned, and each gene product functions in the response to DNA damage induced by cross-linking agents and/or in protection against genome instability. Interestingly, overproduction of inflammatory cytokines such as tumor necrosis factor alpha (TNF-α) and aberrant activation of NF-κB-dependent transcriptional activity have been observed in FA cells. Here we demonstrated that FANCD2 protein inhibits NF-κB activity in its monoubiquitination-dependent manner. Furthermore, we detected a specific association between FANCD2 and an NF-κB consensus element in the TNF-α promoter by electrophoretic mobility shift assays (EMSA) and chromatin immunoprecipitation (ChIP) assay. Therefore, we propose FANCD2 deficiency promotes transcriptional activity of the TNF-α promoter and induces overproduction of TNF-which then sustains prolonged inflammatory responses. These results also suggest that artificial modulation of TNFα production could be a promising therapeutic approach to FA.

A role for PKR in hematologic malignancies

The double-stranded RNA-dependent kinase PKR has been described for many years as strictly a pro-apoptotic kinase. Recent data suggest that the main purpose of this kinase is damage control and repair following stress and, if all else fails, apoptosis. Aberrant activation of PKR has been reported in numerous neurodegenerative diseases and cancer. Although a subset of myelodysplastic syndromes (MDS) and chronic lymphocytic leukemia contain low levels of PKR expression and activity, elevated PKR activity and/or expression have been detected in a wide range of hematologic malignancies, from bone marrow failure disorders to acute leukemia. With the recent findings that cancers containing elevated PKR activity are highly sensitive to PKR inhibition, we explore the role of PKR in hematologic malignancies, signal transduction pathways affected by PKR, and how PKR may contribute to leukemic transformation.

Molecular mechanisms of viral immune evasion proteins to inhibit MHC class I antigen processing and presentation

Viral products inhibit MHC class I antigen processing and presentation via three major pathways: inhibition of major histocompatibility complex (MHC) class I expression on cells, blockade of peptide trafficking and loading on MHC class I molecules, and inhibition of peptide generation in host cells. Viral products also interfere with IFN-gamma -mediated JAK/STAT signal transduction in cells. These results imply that viral proteins probably inhibit the function of IFN-gamma in MHC class I antigen presentation via inactivation of JAK/STAT signal transduction in host cells. Mechanisms of viral products to inhibit IFN-gamma -mediated MHC class I antigen presentation were summarized in this literature review.

TLR8-dependent TNF-(alpha) overexpression in Fanconi anemia group C cells

Tumor necrosis factor alpha (TNF-alpha) production is abnormally high in Fanconi anemia (FA) cells and contributes to the hematopoietic defects seen in FA complementation group C-deficient (Fancc(-/-)) mice. Applying gene expression microarray and proteomic methods to studies on FANCC-deficient cells we found that genes encoding proteins directly involved in ubiquitinylation are overrepresented in the signature of FA bone marrow cells and that ubiquitinylation profiles of FA-C and complemented cells were substantially different. Finding that Toll-like receptor 8 (TLR8) was one of the proteins ubiquitinylated only in mutant cells, we confirmed that TLR8 (or a TLR8-associated protein) is ubiquitinylated in mutant FA-C cells and that TNF-alpha production in mutant cells depended upon TLR8 and the canonical downstream signaling intermediates interleukin 1 receptor-associated kinase (IRAK) and IkappaB kinase-alpha/beta. FANCC-deficient THP-1 cells and macrophages from Fancc(-/-) mice overexpressed TNF-alpha in response to TLR8 agonists but not other TLR agonists. Ectopically expressed FANCC point mutants were capable of fully complementing the mitomycin-C hypersensitivity phenotype of FA-C cells but did not suppress TNF-alpha overproduction. In conclusion, FANCC suppresses TNF-alpha production in mononuclear phagocytes by suppressing TLR8 activity and this particular function of FANCC is independent of its function in protecting the genome from cross-linking agents.

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.

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.

RAX, the PKR activator, sensitizes cells to inflammatory cytokines, serum withdrawal, chemotherapy, and viral infection

While the interferon (IFN)-inducible double-stranded RNA (dsRNA)-dependent protein kinase PKR is reported to initiate apoptosis in some instances, the mechanism by which diverse stress stimuli activate PKR remains unknown. Now we report that RAX, the only known cellular activator for PKR, initiates PKR activation in response to a broad range of stresses including serum deprivation, cytotoxic cytokine or chemotherapy treatment, or viral infection. Thus, knock-down of RAX expression by 80% using small interfering RNA (siRNA) prevents IFNgamma/tumor necrosis factor alpha (TNFalpha)-induced PKR activation and eIF2alpha phosphorylation, IkappaB degradation, IRF-1 expression, and STAT1 phosphorylation, resulting in enhanced murine embryonic fibroblast (MEF) cell survival. In contrast, expression of exogenous RAX, but not of the nonphosphorylatable, dominant-negative RAX(S18A) mutant, sensitizes cells to IFNgamma/TNFalpha, mitomycin C (MMC), or serum deprivation in association with increased PKR activity and apoptosis. Furthermore, RAX(S18A) expression in Fanconi anemia complementation group C-null MEF cells not only prevents PKR activation but also blocks hypersensitivity to IFNgamma/TNFalpha or mitomycin C that results in enhanced apoptosis. In addition, reduced RAX expression facilitates productive viral infection with vesicular stomatitis virus (VSV) and promotes anchorage-independent colony growth of MEF cells. Collectively, these data indicate that RAX may function as a negative regulator of growth that is required to activate PKR in response to a broad range of apoptosis-inducing stress.

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.

GST genotype may modify clinical phenotype in patients with Fanconi anaemia

In the search for genetic modifiers of the Fanconi anaemia (FA) phenotype, we examined the role of polymorphism in three glutathione s-transferase genes (GSTT1, GSTM1 and GSTP1) in 356 FA patients enrolled in the International Fanconi Anaemia Registry (IFAR). Cellular sensitivity to 1,2:3,4 diepoxybutane was significantly increased in GSTT1 deleted compared with GSTT1 positive cases (median chromosomal breaks 11.1 vs. 8.3, P < 0.01) but there was no effect on clinical manifestations of FA. GSTM1 genotype significantly influenced time to bone marrow failure in complementation group FA-C, (median age 3.0 years vs. 7.0 years, P < 0.01). GSTP1 genotype did not influence FA phenotype.

Molecular Pathogenesis of Fanconi Anemia

Fanconi anemia (FA) is a rare inherited disorder characterized clinically by aplastic anemia, developmental defects, and a susceptibility to cancer. Eleven complementation groups have been identified (FA-A, -B, -C, -D1, -D2, -E, -F, -G, -I, -J, and -L), and the genes responsible for 9 groups (FANCA, B, C, D1, D2, E, F, G, and L) have been cloned. The proteins involved in FA act coordinately in the cellular response to DNA cross-links in a pathway that has been shown to interact physically or functionally with a variety of other proteins involved in DNA repair or cell cycle control, notably BRCA1, Rad51,ATM,ATR, and Nbs1. Considerable advances in the identification and description of proteins involved in FA have been recorded, but the precise biochemical function of the FA pathway remains elusive. As research continues to improve our understanding of FA, insight will be gained into what is a pivotal process in cancer biology.

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.

Enhanced TNF-alpha-induced apoptosis in Fanconi anemia type C-deficient cells is dependent on apoptosis signal-regulating kinase 1

Fanconi anemia (FA) is a chromosomal instability disorder characterized by progressive bone marrow failure. Experimental evidence suggests that enhanced oxidant and myelosuppressive cytokine-mediated apoptosis of hematopoietic stem and progenitor cells contributes to the pathogenesis of marrow failure in FA. However, the molecular mechanisms responsible for the apoptotic phenotype in hematopoietic cells are incompletely understood. Recent data in Fancc-/- murine embryonic fibroblasts (MEFs) implicate increased oxidant-induced apoptotic signaling through the redox-dependent protein, apoptosis signal-regulating kinase 1 (Ask1). Here, we examined whether altered Ask1 signaling participated in the proapoptotic phenotype of primary Fancc-/- MEFs and hematopoietic progenitors treated with the myelosuppressive cytokine tumor necrosis factor-alpha (TNF-alpha). Our data indicate that TNF-alpha induces hyperactivation of Ask1 and the downstream effector p38 in Fancc-/- MEFs. In addition,Ask1 inactivation in Fancc-/- MEFs and hematopoietic progenitors restored survival to wild-type (WT) levels in the presence of TNF-alpha. Furthermore, targeting the Ask1 pathway by using either antioxidants or a p38 inhibitor protected Fancc-/- MEFs and c-kit+ cells from TNF-alpha-induced apoptosis. Collectively, these data argue that the predisposition of Fancc-/- hematopoietic progenitors to apoptosis is mediated in part through altered redox regulation and Ask1 hyperactivation.

Hypoxia-reoxygenation induces premature senescence in FA bone marrow hematopoietic cells

Hematopoietic cells are often exposed to transient hypoxia and reoxygenation as they develop and migrate. Given that bone marrow (BM) failure occurred in patients with Fanconi anemia (FA), we reason that hypoxia-then-reoxygenation represents a physiologically relevant stress for FA hematopoietic progenitor/stem cells. Here we show that expansion of Fancc-/- BM cells enriched for progenitor and stem cells was significantly decreased after 2 continuous cycles of hyperoxic-hypoxic-hyperoxic treatments compared with wild-type (WT) BM cells. This inhibition was attributable to a marked decrease of lineage-depleted (Lin-) ScaI- c-kit+ cells and more primitive Lin- ScaI+ c-kit+ cells in Fancc-/- BM cells following reoxygenation. Evaluation of the cell-cycle profile of long-term BM culture (LTBMC) revealed that a vast majority (70.6%) of reoxygenated Fancc-/- LTBMC cells was residing in the G0 and G1 phases compared with 55.8% in WT LTBMC cells. Fancc-/- LTBMC cells stained intensely for SA-beta-galactosidase activity, a biomarker for senescence; this was associated with increased expression of senescence-associated proteins p53 and p21(WAF1/CIP1). Taken together, these results suggest that reoxygenation induces premature senescence in Fancc-/- BM hematopoietic cells by signaling through p53, up-regulating p21, and causing senescent cell-cycle arrest. Thus, reoxygenation-induced premature senescence may be a novel mechanism underlying hematopoietic cell depletion and BM failure in FA.

Double-stranded RNA-dependent protein kinase (PKR) is downregulated by phorbol ester

The double-stranded RNA-dependent protein kinase (PKR) is one of the key mediators of interferon (IFN) action against certain viruses. PKR also plays an important role in signal transduction and immunomodulation. Understanding the regulation of PKR activity is important for the use of PKR as a tool to discover and develop novel therapeutics for viral infections, cancer and immune dysfunction. We found that phorbol 12-myristate 13-acetate (PMA), a potent activator of protein kinase C (PKC), decreased the level of autophosphorylated PKR in a dose- and time-dependent manner in IFN-treated mouse fibroblast cells. Polyinosinic-polycytidylic acid (poly I:C) treatment enhanced the activity of PKR induced by IFN, but did not overcome the PMA-induced reduction of PKR autophosphorylation. Western blot analysis with a monoclonal antibody to mouse PKR revealed that the decrease of PKR autophosphorylation in cells by PMA was a result of PKR protein degradation. Selective PKC inhibitors blocked the degradation of PKR stimulated by PMA, indicating that PKC activity was required for the effect. Furthermore, we also found that proteasome inhibitors prevented PMA-induced down regulation of PKR, indicating that an active proteasome is required. Our results identify a novel mechanism for the post-translational regulation of PKR.

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.

The Fanconi anemia proteins functionally interact with the protein kinase regulated by RNA (PKR)

Protein kinase regulated by RNA (PKR) plays critical roles in cell growth and apoptosis and is implicated as a potential pathogenic factor of Alzheimer's, Parkinson's, and Huntington's diseases. Here we report that this proapoptotic kinase is also involved in Fanconi anemia (FA), a disease characterized by bone marrow (BM) failure and leukemia. We have used a BM extract to show that three FA proteins, FANCA, FANCC, and FANCG, functionally interact with the PKR kinase, which in turn regulates translational control. By using a combined immunoprecipitation and reconstituted kinase assay, in which an active PKR kinase complex was captured from a normal cell extract, we demonstrated functional interactions between the FA proteins and the PKR kinase. In primary human BM cells, mutations in the FANCA, FANCC, and FANCG genes markedly increase the amount of PKR bound to FANCC, and this PKR accumulation is correlated with elevated PKR activation and hypersensitivity of BM progenitor cells to growth repression mediated by the inhibitory cytokines interferon-gamma and tumor necrosis factor-alpha. Specific inhibition of PKR by 2-aminopurine in these FA BM cells attenuates PKR activation and apoptosis induction. In lymphoblasts derived from an FA-C patient, overexpression of a dominant negative mutant PKR (PKRK296R) suppressed PKR activation and apoptosis induced by interferon-gamma and tumor necrosis factor-alpha. Furthermore, by using genetically matched wild-type and PKR-null cells, we demonstrated that forced expression of a patient-derived FA-C mutant (FANCCL554P) augmented double-stranded RNA-induced PKR activation and cell death. Thus, inappropriate activation of PKR as a consequence of certain FA mutations might play a role in bone marrow failure that frequently occurred in FA.

Fanconi anemia C gene product regulates expression of genes involved in differentiation and inflammation

Loss of Fanconi anemia (FA) proteins activity by recessive inherited mutations in one of the FA genes leads to a disease characterized by bone marrow failure, myeloid leukemia and DNA damage hypersensitivity. The aim of this work was to improve our understanding of the FA syndrome defining the transcription profile of the FA complementation group C (FANCC)-deficient cells in comparison to their ectopically corrected counterpart using oligonucleotide microarrays. In this way, 49 RNAs have been isolated, which showed a consistent differential pattern of expression among FANCC mutated and corrected cells. The observed specific changes in gene expression suggest that FANCC regulates specifically myeloid differentiation and unmasks a previously unsuspected anti-inflammatory role for the FA proteins. In spite of the DNA damage hypersensitivity of the syndrome, no gene coding for a protein directly involved in DNA repair/damage response was found to be deregulated in our analysis. This observation suggests that FANCC does not directly control genes involved in DNA repair at the transcriptional level, but does not exclude a regulation at the translational or post-translational level, or by protein/protein interactions. The potential role of the differentially expressed genes in FA phenotype as well as a functional- and cellular-based clustering of the identified genes are presented and discussed.

Continuous in vivo infusion of interferon-gamma (IFN-gamma) preferentially reduces myeloid progenitor numbers and enhances engraftment of syngeneic wild-type cells in Fancc-/- mice

Fanconi anemia (FA) is characterized by bone marrow (BM) failure and cancer susceptibility. Identification of the cDNAs of many FA complementation types allows the potential of using gene transfer technology to introduce functional cDNAs as transgenes into autologous stem cells and provide a cure for the BM failure in FA patients. Previous studies in FA murine models and in a phase 1 clinical trial suggest that myelopreparation is required for significant engraftment of exogenous, genetically corrected stem cells. Since myeloid progenitors from Fancc-/- mice and human Fanconi anemia group C protein (FANCC) patients have increased apoptosis in response to interferon gamma (IFN-gamma) in vitro, we hypothesized that IFN-gamma may be useful as a nongenotoxic, myelopreparative conditioning agent. To test this hypothesis, IFN-gamma was administered as a continuous infusion to Fancc-/- and wild-type (WT) mice for 1 week. Primitive and mature myeloid lineages were preferentially reduced in IFN-gamma-treated Fancc-/- mice. Further, IFN-gamma conditioning of Fancc-/- recipients was sufficient as a myelopreparative regimen to allow consistent engraftment of isogenic WT repopulating stem cells. Collectively, these data demonstrate that Fancc-/- hematopoietic cell populations have increased hypersensitivity to IFN-gamma in vivo and that IFN-gamma conditioning may be useful as a nongenotoxic strategy for myelopreparation in this disorder.

Fanconi anemia type C and p53 cooperate in apoptosis and tumorigenesis

Fanconi anemia (FA) is a recessive genomic instability syndrome characterized by developmental defects, progressive bone marrow failure, and cancer. FA is genetically heterogeneous, however; the proteins encoded by different FA loci interact functionally with each other and with the BRCA1, BRCA2, and ATM gene products. Although patients with FA are highly predisposed to the development of myeloid leukemia and solid tumors, the alterations in biochemical pathways responsible for the progression of tumorigenesis in these patients remain unknown. FA cells are hypersensitive to a range of genotoxic and cellular stresses that activate signaling pathways mediating apoptosis. Here we show that ionizing radiation (IR) induces modestly elevated levels of p53 in cells from FA type C (Fancc) mutant mice and that inactivation of Trp53 rescues tumor necrosis factor alpha-induced apoptosis in myeloid cells from Fancc-/- mice. Further, whereas Fancc-/- mice failed to form hematopoietic or solid malignancies, mice mutant at both Fancc and Trp53 developed tumors more rapidly than mice mutant at Trp53 alone. This shortened latency was associated with the appearance of tumor types that are found in patients with FA but not in mice mutant at Trp53 only. Collectively, these data demonstrate that p53 and Fancc interact functionally to regulate apoptosis and tumorigenesis in Fancc-deficient cells.

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.

Fanconi anemia type C-deficient hematopoietic stem/progenitor cells exhibit aberrant cell cycle control

The pathogenesis of bone marrow failure in Fanconi anemia is poorly understood. Suggested mechanisms include enhanced apoptosis secondary to DNA damage and altered inhibitory cytokine signaling. Recent data determined that disrupted cell cycle control of hematopoietic stem and/or progenitor cells disrupts normal hematopoiesis with increased hematopoietic stem cell cycling resulting in diminished function and increased sensitivity to cell cycle-specific apoptotic stimuli. Here, we used Fanconi anemia complementation type C-deficient (Fancc-/-) mice to demonstrate that Fancc-/- phenotypically defined cell populations enriched for hematopoietic stem and progenitor cells exhibit increased cycling. In addition, we established that the defect in cell cycle regulation is not a compensatory mechanism from enhanced apoptosis occurring in vivo. Collectively, these data provide a previously unrecognized phenotype in Fancc-/- hematopoietic stem/progenitor cells, which may contribute to the progressive bone marrow failure in Fanconi anemia.

Interferons and apoptosis

The interferons (IFNs), in addition to their well-known antiviral activities, have important roles in the control of cell proliferation and are effective agents for the treatment of a limited number of malignant diseases. IFNs not only regulate cell growth and division but also influence cell survival through their effects on apoptosis. This review describes the current state of knowledge about the mechanisms of action of these cytokines on the apoptotic machinery, with particular emphasis on the synergism that exists between the IFNs and other proapoptotic agents, such as members of the tumor necrosis factor (TNF) family. The review also discusses the physiologic and clinical implications of the effects of the IFNs on apoptosis for regulation of viral infection and tumor growth.

An oxidative mechanism of interferon induced priming of the Fas pathway in Fanconi anemia cells

Hematopoietic progenitor cells from children with Fanconi anemia of the C complementation group (FA-C) are excessively apoptotic and hypersensitive to various extracellular cues including Fas-ligand, tumor necrosis factor-alpha and double-stranded RNA. Interferon (IFN)-gamma is known to augment apoptotic responses of these factors. The "priming" effect of IFN-gamma is not fully explained. In view of the strong evidence that FA cells are intolerant of oxidative stress, we tested the notion that IFN-priming involves the induction of reactive oxygen species (ROS) in two FA-C B-lymphocyte cell lines and in peripheral blood neutrophils and mononuclear cells of FA patients. We also investigated whether the combination of IFN-gamma and Fas created an intracellular environment that promoted apoptosis. Significantly lower doses of IFN-gamma induced ROS accumulation in neutrophils and mononuclear cell of FA patients compared to cells of normal individuals. Enhanced ROS accumulation and decreased intracellular glutathione levels were observed in FA-C B-cell lines primed with IFN-gamma and treated with agonistic anti-Fas antibody than in isogenic control cells corrected with FANCC. The above treatment also induced caspase-3 and -8 activation as well as apoptosis. That antioxidants reduced the priming effect of IFN-gamma in Fas and IFN-gamma-treated FA lymphoblast cells, demonstrates that ROS represent a critical effector mechanism for the exaggerated responses to IFN-gamma characteristic of FA-C cells.

Genetic basis of Fanconi anemia

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

The 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 vivo administration of interferon gamma does not cause marrow aplasia in mice with a targeted disruption of FANCC

OBJECTIVE

Hematopoietic cells from patients with Fanconi anemia (FA) and mice carrying a targeted disruption of the gene encoding complementation group C protein (FANCC(-/-)) demonstrate an apoptotic phenotype in response to alkylating agents and cytokines including interferon gamma (IFN-gamma) in vitro. The aim of this study was to explore these apoptosis-inducing effects of IFN-gamma on the bone marrow of FANCC(-/-) mice as a potential strategy to select gene-corrected cells in vivo. Following pharmacokinetic studies to determine if serum concentrations effective in vitro can be achieved in vivo, we injected FANCC(-/-) mice with recombinant murine IFN-gamma. Hematopoietic effects were investigated by serial determinations of blood counts, progenitor colony formation, and marrow cellularity.

RESULTS

Serial weekly intraperitoneal administrations of escalating doses of rmIFN-gamma did not affect peripheral blood counts in FANCC(-/-) mice, even after subsequent antibody-mediated fas ligation. Additionally, prolonged exposure after sequential daily administration of recombinant IFN-gamma did not impair progenitor cell clonogenicity in vitro. Pharmacokinetic data confirmed that the failure of IFN-gamma to induce marrow aplasia occurred in spite of peak serum levels greater than 100-fold in excess of those effective in vitro.

CONCLUSION

We conclude that in spite of the well-documented in vitro apoptotic tendency of FA-phenotype hematopoietic cells, the in vivo administration of IFN-gamma with and without subsequent fas ligation does not induce bone marrow failure in FANCC(-/-) (129SvJ strain) mice. Additional selective pressure may be necessary to achieve targeted ablation of uncorrected, FA-phenotype, marrow cells.

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.

Disease model: Fanconi anemia

Fanconi anemia (FA) is a chromosomal instability syndrome characterized by the presence of pancytopenia, congenital malformations and cancer predisposition. Six genes associated with this disorder have been cloned, and mice with targeted disruptions of several of the FA genes have been generated. These mouse models display the characteristic FA feature of cellular hypersensitivity to DNA cross-linking agents. Although they do not develop hematological or developmental abnormalities spontaneously, they mimic FA patients in their reduced fertility. Studies using these animal models provide valuable insights into the involvement of apoptotic pathways in FA, and help characterize the defects in FA hematopoietic cells. In addition, mouse models are also useful for testing treatments for FA.

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.

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

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

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.

Functional correction of FA-C cells with FANCC suppresses the expression of interferon gamma-inducible genes

Because hematopoietic cells derived from Fanconi anemia (FA) patients of the C-complementation group (FA-C) are hypersensitive to the inhibitory effects of interferon gamma (IFNgamma), the products of certain IFNgamma-inducible genes known to influence hematopoietic cell survival were quantified. High constitutive expression of the IFNgamma-inducible genes, IFN-stimulated gene factor 3 gamma subunit (ISGF3gamma), IFN regulatory factor-1 (IRF-1), and the cyclin-dependent kinase inhibitor p21(WAF1) was found in FANCC mutant B lymphoblasts, low-density bone marrow cells, and murine embryonic fibroblasts. Paradoxically, these cells do not activate signal transducer and activator of transcription (STAT) 1 properly. In an attempt to clarify mechanisms by which FA-C cells overexpress IFNgamma-inducible genes in the face of defective STAT1 phosphorylation, it was reasoned that decreased levels of activated STAT1 might result in reduced expression of a hematopoietic IFNgamma-responsive protein that normally modulates expression of other IFNgamma-responsive genes. Levels of the IFNgamma-inducible factor IFN consensus sequence binding protein (ICSBP), a negative trans-acting regulator of some IFNgamma-inducible genes, were quantified. ICSBP levels were reduced in FA-C B lymphoblasts and MEFs. However, enforced expression of ICSBP failed to down-regulate IRF-1, ISGF3gamma, and p21(WAF1). Thus, the FANCC protein functions to modulate expression of a family of genes that in normal cells are inducible only by specific environmental cues for apoptosis or mitogenic inhibition, but it does so independently of the classic IFN-STAT1 pathway and is not the direct result of reduced ICSBP expression.