Oxidative stress/damage induces multimerization and interaction of Fanconi anemia proteins

A Population-Based Human In Vitro Approach to Quantify Inter-Individual Variability in Responses to Chemical Mixtures

Human cell-based population-wide in vitro models have been proposed as a strategy to derive chemical-specific estimates of inter-individual variability; however, the utility of this approach has not yet been tested for cumulative exposures in mixtures. This study aimed to test defined mixtures and their individual components and determine whether adverse effects of the mixtures were likely to be more variable in a population than those of the individual chemicals. The in vitro model comprised 146 human lymphoblastoid cell lines from four diverse subpopulations of European and African descent. Cells were exposed, in concentration−response, to 42 chemicals from diverse classes of environmental pollutants; in addition, eight defined mixtures were prepared from these chemicals using several exposure- or hazard-based scenarios. Points of departure for cytotoxicity were derived using Bayesian concentration−response modeling and population variability was quantified in the form of a toxicodynamic variability factor (TDVF). We found that 28 chemicals and all mixtures exhibited concentration−response cytotoxicity, enabling calculation of the TDVF. The median TDVF across test substances, for both individual chemicals or defined mixtures, ranged from a default assumption (101/2) of toxicodynamic variability in human population to >10. The data also provide a proof of principle for single-variant genome-wide association mapping for toxicity of the chemicals and mixtures, although replication would be necessary due to statistical power limitations with the current sample size. This study demonstrates the feasibility of using a set of human lymphoblastoid cell lines as an in vitro model to quantify the extent of inter-individual variability in hazardous properties of both individual chemicals and mixtures. The data show that population variability of the mixtures is unlikely to exceed that of the most variable component, and that similarity in genome-wide associations among components may be used to accrue additional evidence for grouping of constituents in a mixture for cumulative assessments.

Head and Neck Cancer Susceptibility and Metabolism in Fanconi Anemia

Fanconi anemia (FA) is a rare inherited, generally autosomal recessive syndrome, but it displays X-linked or dominant negative inheritance for certain genes. FA is characterized by a deficiency in DNA damage repair that results in bone marrow failure, and in an increased risk for various epithelial tumors, most commonly squamous cell carcinomas of the head and neck (HNSCC) and of the esophagus, anogenital tract and skin. Individuals with FA exhibit increased human papilloma virus (HPV) prevalence. Furthermore, a subset of anogenital squamous cell carcinomas (SCCs) in FA harbor HPV sequences and FA-deficient laboratory models reveal molecular crosstalk between HPV and FA proteins. However, a definitive role for HPV in HNSCC development in the FA patient population is unproven. Cellular metabolism plays an integral role in tissue homeostasis, and metabolic deregulation is a known hallmark of cancer progression that supports uncontrolled proliferation, tumor development and metastatic dissemination. The metabolic consequences of FA deficiency in keratinocytes and associated impact on the development of SCC in the FA population is poorly understood. Herein, we review the current literature on the metabolic consequences of FA deficiency and potential effects of resulting metabolic reprogramming on FA cancer phenotypes.

Antioxidant Activity of Fluoxetine and Vortioxetine in a Non-Transgenic Animal Model of Alzheimer's Disease

Depression is a risk factor for the development of Alzheimer’s disease (AD). A neurobiological and clinical continuum exists between AD and depression, with neuroinflammation and oxidative stress being involved in both diseases. Second-generation antidepressants, in particular selective serotonin reuptake inhibitors (SSRIs), are currently investigated as neuroprotective drugs in AD. By employing a non-transgenic AD model, obtained by intracerebroventricular (i.c.v.) injection of amyloid-β (Aβ) oligomers in 2-month-old C57BL/6 mice, we recently demonstrated that the SSRI fluoxetine (FLX) and the multimodal antidepressant vortioxetine (VTX) reversed the depressive-like phenotype and memory deficits induced by Aβ oligomers rescuing the levels of transforming growth factor-β1 (TGF-β1). Aim of our study was to test FLX and VTX for their ability to prevent oxidative stress in the hippocampus of Aβ-injected mice, a brain area strongly affected in both depression and AD. The long-term intraperitoneal (i.p.) administration of FLX (10 mg/kg) or VTX (5 and 10 mg/kg) for 24 days, starting 7 days before Aβ injection, was able to prevent the over-expression of inducible nitric oxide synthase (iNOS) and NADPH oxidase 2 (Nox2) induced by Aβ oligomers. Antidepressant pre-treatment was also able to rescue the mRNA expression of glutathione peroxidase 1 (Gpx1) antioxidant enzyme. FLX and VTX also prevented Aβ-induced neurodegeneration in mixed neuronal cultures treated with Aβ oligomers. Our data represent the first evidence that the long-term treatment with the antidepressants FLX or VTX can prevent the oxidative stress phenomena related to the cognitive deficits and depressive-like phenotype observed in a non-transgenic animal model of AD.

Genotoxic C8-Arylamino-2'-deoxyadenosines Act as Latent Alkylating Agents to Induce DNA Interstrand Cross-Links

DNA interstrand cross-links (ICLs) are extremely deleterious and structurally diverse, driving the evolution of ICL repair pathways. Discovering ICL-inducing agents is, thus, crucial for the characterization of ICL repair pathways and Fanconi anemia, a genetic disease caused by mutations in ICL repair genes. Although several studies point to oxidative stress as a cause of ICLs, oxidative stress-induced cross-linking events remain poorly characterized. Also, polycyclic aromatic amines, potent environmental carcinogens, have been implicated in producing ICLs, but their identities and sequences are unknown. To close this knowledge gap, we tested whether ICLs arise by the oxidation of 8-arylamino-2'-deoxyadenosine (ArNHdA) lesions, adducts produced by arylamino carcinogens. Herein, we report that ArNHdA acts as a latent cross-linking agent to generate ICLs under oxidative conditions. The formation of an ICL from 8-aminoadenine, but not from 8-aminoguanine, highlights the specificity of 8-aminopurine-mediated ICL production. Under the influence of the reactive oxygen species (ROS) nitrosoperoxycarbonate, ArNHdA (Ar = biphenyl, fluorenyl) lesions were selectively oxidized to generate ICLs. The cross-linking reaction may occur between the C2-ArNHdA and N2-dG, presumably via oxidation of ArNHdA into a reactive diiminoadenine intermediate followed by the nucleophilic attack of the N2-dG on the diiminoadenine. Overall, ArNHdA-mediated ICLs represent rare examples of ROS-induced ICLs and polycyclic aromatic amine-mediated ICLs. These results reveal novel cross-linking chemistry and the genotoxic effects of arylamino carcinogens and support the hypothesis that C8-modified adenines with low redox potential can cause ICLs in oxidative stress.

DNA interstrand cross-links induced by the major oxidative adenine lesion 7,8-dihydro-8-oxoadenine

Oxidative damage to DNA generates 7,8-dihydro-8-oxoguanine (oxoG) and 7,8-dihydro-8-oxoadenine (oxoA) as two major lesions. Despite the comparable prevalence of these lesions, the biological effects of oxoA remain poorly characterized. Here we report the discovery of a class of DNA interstrand cross-links (ICLs) involving oxidized nucleobases. Under oxidative conditions, oxoA, but not oxoG, readily reacts with an opposite base to produce ICLs, highlighting a latent alkylating nature of oxoA. Reactive halogen species, one-electron oxidants, and the myeloperoxidase/H₂O₂/Cl⁻ system induce oxoA ICLs, suggesting that oxoA-mediated cross-links may arise endogenously. Nucleobase analog studies suggest C2-oxoA is covalently linked to N2-guanine and N3-adenine for the oxoA-G and oxoA-A ICLs, respectively. The oxoA ICLs presumably form via the oxidative activation of oxoA followed by the nucleophilic attack by an opposite base. Our findings provide insights into oxoA-mediated mutagenesis and contribute towards investigations of oxidative stress-induced ICLs and oxoA-based latent alkylating agents.

7,8-dihydro-8-oxoguanine and 7,8-dihydro-8-oxoadenine (oxoA) are generated upon oxidative damage to DNA, but the biological effects of oxoA are not well known. Here, the authors report that only oxoA forms DNA interstrand crosslinks (ICLs) upon secondary oxidation and that these ICLs can be induced by reactive halogen species, one-electron oxidants and the myeloperoxidase/H₂O₂/Cl^- system.

Structural basis of the fanconi anemia-associated mutations within the FANCA and FANCG complex

Monoubiquitination of the Fanconi anemia complementation group D2 (FANCD2) protein by the FA core ubiquitin ligase complex is the central event in the FA pathway. FANCA and FANCG play major roles in the nuclear localization of the FA core complex. Mutations of these two genes are the most frequently observed genetic alterations in FA patients, and most point mutations in FANCA are clustered in the C-terminal domain (CTD). To understand the basis of the FA-associated FANCA mutations, we determined the cryo-electron microscopy (EM) structures of Xenopus laevis FANCA alone at 3.35 Å and 3.46 Å resolution and two distinct FANCA–FANCG complexes at 4.59 and 4.84 Å resolution, respectively. The FANCA CTD adopts an arc-shaped solenoid structure that forms a pseudo-symmetric dimer through its outer surface. FA- and cancer-associated point mutations are widely distributed over the CTD. The two different complex structures capture independent interactions of FANCG with either FANCA C-terminal HEAT repeats, or the N-terminal region. We show that mutations that disturb either of these two interactions prevent the nuclear localization of FANCA, thereby leading to an FA pathway defect. The structure provides insights into the function of FANCA CTD, and provides a framework for understanding FA- and cancer-associated mutations.

Mitoprotective Clinical Strategies in Type 2 Diabetes and Fanconi Anemia Patients: Suggestions for Clinical Management of Mitochondrial Dysfunction

Oxidative stress (OS) and mitochondrial dysfunction (MDF) occur in a number of disorders, and several clinical studies have attempted to counteract OS and MDF by providing adjuvant treatments against disease progression. The present review is aimed at focusing on two apparently distant diseases, namely type 2 diabetes (T2D) and a rare genetic disease, Fanconi anemia (FA). The pathogenetic links between T2D and FA include the high T2D prevalence among FA patients and the recognized evidence for OS and MDF in both disorders. This latter phenotypic/pathogenetic feature-namely MDF-may be regarded as a mechanistic ground both accounting for the clinical outcomes in both diseases, and as a premise to clinical studies aimed at counteracting MDF. In the case for T2D, the working hypothesis is raised of evaluating any in vivo decrease of mitochondrial cofactors, or mitochondrial nutrients (MNs) such as α-lipoic acid, coenzyme Q10, and l-carnitine, with possibly combined MN-based treatments. As for FA, the established knowledge of MDF, as yet only obtained from in vitro or molecular studies, prompts the requirement to ascertain in vivo MDF, and to design clinical studies aimed at utilizing MNs toward mitigating or delaying FA's clinical progression. Altogether, this paper may contribute to building hypotheses for clinical studies in a number of OS/MDF-related diseases.

Fancd2-deficient hematopoietic stem and progenitor cells depend on augmented mitochondrial translation for survival and proliferation

Members of the Fanconi anemia (FA) protein family are involved in multiple cellular processes including response to DNA damage and oxidative stress. Here we show that a major FA protein, Fancd2, plays a role in mitochondrial biosynthesis through regulation of mitochondrial translation. Fancd2 interacts with Atad3 and Tufm, which are among the most frequently identified components of the mitochondrial nucleoid complex essential for mitochondrion biosynthesis. Deletion of Fancd2 in mouse hematopoietic stem and progenitor cells (HSPCs) leads to increase in mitochondrial number, and enzyme activity of mitochondrion-encoded respiratory complexes. Fancd2 deficiency increases mitochondrial protein synthesis and induces mitonuclear protein imbalance. Furthermore, Fancd2-deficient HSPCs show increased mitochondrial respiration and mitochondrial reactive oxygen species. By using a cell-free assay with mitochondria isolated from WT and Fancd2-KO HSPCs, we demonstrate that the increased mitochondrial protein synthesis observed in Fancd2-KO HSPCs was directly linked to augmented mitochondrial translation. Finally, Fancd2-deficient HSPCs are selectively sensitive to mitochondrial translation inhibition and depend on augmented mitochondrial translation for survival and proliferation. Collectively, these results suggest that Fancd2 restricts mitochondrial activity through regulation of mitochondrial translation, and that augmented mitochondrial translation and mitochondrial respiration may contribute to HSC defect and bone marrow failure in FA.

Impact of carbonylation on glutathione peroxidase-1 activity in human hyperglycemic endothelial cells

AIMS

High levels of glucose and reactive carbonyl intermediates of its degradation pathway such as methylglyoxal (MG) may contribute to diabetic complications partly via increased generation of reactive oxygen species (ROS). This study focused on glutathione peroxidase-1 (GPx1) expression and the impact of carbonylation as an oxidative protein modification on GPx1 abundance and activity in human umbilical vein endothelial cells (HUVEC) under conditions of mild to moderate oxidative stress.

RESULTS

High extracellular glucose and MG enhanced intracellular ROS formation in HUVECs. Protein carbonylation was only transiently augmented pointing to an effective antioxidant defense in these cells. Nitric oxide synthase expression was decreased under hyperglycemic conditions but increased upon exposure to MG, whereas superoxide dismutase expression was not significantly affected. Increased glutathione peroxidase (GPx) activity seemed to compensate for a decrease in GPx1 protein due to enhanced degradation via the proteasome. Mass spectrometry analysis identified Lys-114 as a possible carbonylation target which provides a vestibule for the substrate H2O2 and thus enhances the enzymatic reaction.

INNOVATION

Oxidative protein carbonylation has so far been associated with functional inactivation of modified target proteins mainly contributing to aging and age-related diseases. Here, we demonstrate that mild oxidative stress and subsequent carbonylation seem to activate protective cellular redox signaling pathways whereas severe oxidative stress overwhelms the cellular antioxidant defense leading to cell damage.

CONCLUSIONS

This study may contribute to a better understanding of redox homeostasis and its role in the development of diabetes and related vascular complications.

Persistent response of Fanconi anemia haematopoietic stem and progenitor cells to oxidative stress

Oxidative stress is considered as an important pathogenic factor in many human diseases including Fanconi anemia (FA), an inherited bone marrow failure syndrome with extremely high risk of leukemic transformation. Members of the FA protein family are involved in DNA damage and other cellular stress responses. Loss of FA proteins renders cells hypersensitive to oxidative stress and cancer transformation. However, how FA cells respond to oxidative DNA damage remains unclear. By using an in vivo stress-response mouse strain expressing the Gadd45β-luciferase transgene, we show here that haematopoietic stem and progenitor cells (HSPCs) from mice deficient for the FA gene Fanca or Fancc persistently responded to oxidative stress. Mechanistically, we demonstrated that accumulation of unrepaired DNA damage, particularly in oxidative damage-sensitive genes, was responsible for the long-lasting response in FA HSPCs. Furthermore, genetic correction of Fanca deficiency almost completely abolished the persistent oxidative stress-induced G₂/M arrest and DNA damage response in vivo. Our study suggests that FA pathway is an integral part of a versatile cellular mechanism by which HSPCs respond to oxidative stress.

Maintenance of genome stability by Fanconi anemia proteins

Persistent dysregulation of the DNA damage response and repair in cells causes genomic instability. The resulting genetic changes permit alterations in growth and proliferation observed in virtually all cancers. However, an unstable genome can serve as a double-edged sword by providing survival advantages in the ability to evade checkpoint signaling, but also creating vulnerabilities through dependency on alternative genomic maintenance factors. The Fanconi anemia pathway comprises an intricate network of DNA damage signaling and repair that are critical for protection against genomic instability. The importance of this pathway is underlined by the severity of the cancer predisposing syndrome Fanconi anemia which can be caused by biallelic mutations in any one of the 21 genes known thus far. This review delineates the roles of the Fanconi anemia pathway and the molecular actions of Fanconi anemia proteins in confronting replicative, oxidative, and mitotic stress.

Positive correlation between insulin resistance and iron overload-induced oxidative stress in patients with fanconi anemia (FA)- and non-FA-related bone marrow failure: The results of a multicenter study

This study investigated the relationship between DNA, protein, and lipid oxidations and insulin resistance in patients with Fanconi anemia (FA)- and non-FA-related bone marrow failure. Sixteen patients with FA, 7 non-FA-related aplastic anemia, and 10 controls were included in the study. Fasting blood glucose, simultaneous insulin, hepcidin, ferritin, 8-hydroxy deoxyguanosine (8-OHdG), protein carbonyls, malondialdehyde (MDA), and homeostatic model assessment-insulin resistance (HOMA-IR) were investigated in the patients and controls. Diepoxybutane test-positive (DEB+) patients were diagnosed with FA, whereas DEB-patients were diagnosed as non-FA. 8-OHdG levels in both FA and non-FA patients were significantly higher than those in the controls (P = .001 and P = .005, respectively). Serum ferritin levels were also higher in FA and non-FA patients than in the controls (P = .0001 and P = .005, respectively). Insulin resistance (IR) was significantly higher in FA patients than in non-FA patients and controls (P = .005 and P = .015, respectively). Significant differences were observed between 8-OHdG, ferritin, and MDA levels in patients with or without IR (P = .009, P = .001, and P = .013, respectively). Moderate and strong relations of 44% and 85% were determined between IR and ferritin levels in patients with FA or non-FA (P = .08 and P = .014, respectively). FA and non-FA patients exhibited a tendency to IR. IR was related to ferritin levels, and ferritin levels were also correlated with oxidative stress. These findings suggest that the increased rate of IR in patients with FA and non-FA may derive from increased oxidative stress, which may in turn be due to elevated serum ferritin levels.

Fanconi anemia (FA) and crosslinker sensitivity: Re-appraising the origins of FA definition

The commonly accepted definition of Fanconi anemia (FA) relying on DNA repair deficiency is submitted to a critical review starting from the early reports pointing to mitomycin C bioactivation and to the toxicity mechanisms of diepoxybutane and a group of nitrogen mustards causing DNA crosslinks in FA cells. A critical analysis of the literature prompts revisiting the FA phenotype and crosslinker sensitivity in terms of an oxidative stress (OS) background, redox-related anomalies of FA (FANC) proteins, and mitochondrial dysfunction. This re-appraisal of FA basic defect might lead to innovative approaches both in elucidating FA phenotypes and in clinical management.

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.

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.

Damaged mitochondria in Fanconi anemia - an isolated event or a general phenomenon?

Fanconi anemia (FA) is known as an inherited bone marrow failure syndrome associated with cancer predisposition and susceptibility to a number of DNA damaging stimuli, along with a number of clinical features such as upper limb malformations, increased diabetes incidence and typical anomalies in skin pigmentation. The proteins encoded by FA-defective genes (FANC proteins) display well-established roles in DNA damage and repair pathways. Moreover, some independent studies have revealed that mitochondrial dysfunction (MDF) is also involved in FA phenotype. Unconfined to FA, we have shown that other syndromes featuring DNA damage and repair (such as ataxia-telangiectasia, AT, and Werner syndrome, WS) display MDF-related phenotypes, along with oxidative stress (OS) that, altogether, may play major roles in these diseases. Experimental and clinical studies are warranted in the prospect of future therapies to be focused on compounds scavenging reactive oxygen species (ROS) as well as protecting mitochondrial functions.

Bone marrow cell transcripts from Fanconi anaemia patients reveal in vivo alterations in mitochondrial, redox and DNA repair pathways

Fanconi anaemia (FA) is a genetic cancer predisposition disorder associated with cytogenetic instability, bone marrow failure and a pleiotropic cellular phenotype, including low thresholds of responses to oxidative stress, cross-linking agents and selected cytokines. This study was aimed at defining the scope of abnormalities in gene expression using the publicly available FA Transcriptome Consortium (FTC) database (Gene Expression Omnibus, 2009 and publicly available as GSE16334). We evaluated the data set that included transcriptomal analyses on RNA obtained from low-density bone marrow cells (BMC) from 20 patients with FA and 11 healthy volunteers, by seeking to identify changes in expression of over 22,000 genes, including a set of genes involved in: (i) bioenergetic pathways; (ii) antioxidant activities; (iii) response to stress and metal-chelating proteins; (iv) inflammation-related cytokines and (v) DNA repair. Ontological analysis of genes expressed at magnitudes of 1.5-fold or greater demonstrated significant suppression of genes in the categories of (i) energy metabolism; (ii) antioxidant activities; and (iii) stress and chelating proteins. Enhanced expression was found for 16 of 26 genes encoding inflammatory cytokines. A set of 20 of 21 transcripts for DNA repair activities were down-regulated; four of these transcripts related to type II topoisomerase. The data provide evidence for alterations in gene regulation of bioenergetic activities, redox-related activities, stress and metal-chelating proteins, and of some selected DNA repair activities in patients with FA.

From clinical description, to in vitro and animal studies, and backward to patients: oxidative stress and mitochondrial dysfunction in Fanconi anemia

Fanconi anemia (FA) is a rare genetic disease associated with deficiencies in DNA repair pathways. A body of literature points to a pro-oxidant state in FA patients, along with evidence for oxidative stress (OS) in the FA phenotype reported by in vitro, molecular, and animal studies. A highlight arises from the detection of mitochondrial dysfunction (MDF) in FA cell lines of complementation groups A, C, D2, and G. As yet lacking, in vivo studies should focus on FA-associated MDF, which may help in the understanding of the mitochondrial basis of OS detected in cells and body fluids from FA patients. Beyond the in vitro and animal databases, the available analytical devices may prompt the direct observation of metabolic and mitochondrial alterations in FA patients. These studies should evaluate a set of MDF-related endpoints, to be related to OS endpoints. The working hypothesis is raised that, parallel to OS, nitrosative stress might be another, so far unexplored, hallmark of the FA phenotype. The expected results may shed light on the FA pathogenesis and might provide grounds for pilot chemoprevention trials using mitochondrial nutrients.

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.

Disrupted Signaling through the Fanconi Anemia Pathway Leads to Dysfunctional Hematopoietic Stem Cell Biology: Underlying Mechanisms and Potential Therapeutic Strategies

Fanconi anemia (FA) is the most common inherited bone marrow failure syndrome. FA patients suffer to varying degrees from a heterogeneous range of developmental defects and, in addition, have an increased likelihood of developing cancer. Almost all FA patients develop a severe, progressive bone marrow failure syndrome, which impacts upon the production of all hematopoietic lineages and, hence, is thought to be driven by a defect at the level of the hematopoietic stem cell (HSC). This hypothesis would also correlate with the very high incidence of MDS and AML that is observed in FA patients. In this paper, we discuss the evidence that supports the role of dysfunctional HSC biology in driving the etiology of the disease. Furthermore, we consider the different model systems currently available to study the biology of cells defective in the FA signaling pathway and how they are informative in terms of identifying the physiologic mediators of HSC depletion and dissecting their putative mechanism of action. Finally, we ask whether the insights gained using such disease models can be translated into potential novel therapeutic strategies for the treatment of the hematologic disorders in FA patients.

The FA pathway counteracts oxidative stress through selective protection of antioxidant defense gene promoters

Oxidative stress has been implicated in the pathogenesis of many human diseases including Fanconi anemia (FA), a genetic disorder associated with BM failure and cancer. Here we show that major antioxidant defense genes are down-regulated in FA patients, and that gene down-regulation is selectively associated with increased oxidative DNA damage in the promoters of the antioxidant defense genes. Assessment of promoter activity and DNA damage repair kinetics shows that increased initial damage, rather than a reduced repair rate, contributes to the augmented oxidative DNA damage. Mechanistically, FA proteins act in concert with the chromatin-remodeling factor BRG1 to protect the promoters of antioxidant defense genes from oxidative damage. Specifically, BRG1 binds to the promoters of the antioxidant defense genes at steady state. On challenge with oxidative stress, FA proteins are recruited to promoter DNA, which correlates with significant increase in the binding of BRG1 within promoter regions. In addition, oxidative stress-induced FANCD2 ubiquitination is required for the formation of a FA-BRG1-promoter complex. Taken together, these data identify a role for the FA pathway in cellular antioxidant defense.

Overcoming reprogramming resistance of Fanconi anemia cells

Fanconi anemia (FA) is a recessive syndrome characterized by progressive fatal BM failure and chromosomal instability. FA cells have inactivating mutations in a signaling pathway that is critical for maintaining genomic integrity and protecting cells from the DNA damage caused by cross-linking agents. Transgenic expression of the implicated genes corrects the phenotype of hematopoietic cells, but previous attempts at gene therapy have failed largely because of inadequate numbers of hematopoietic stem cells available for gene correction. Induced pluripotent stem cells (iPSCs) constitute an alternate source of autologous cells that are amenable to ex vivo expansion, genetic correction, and molecular characterization. In the present study, we demonstrate that reprogramming leads to activation of the FA pathway, increased DNA double-strand breaks, and senescence. We also demonstrate that defects in the FA DNA-repair pathway decrease the reprogramming efficiency of murine and human primary cells. FA pathway complementation reduces senescence and restores the reprogramming efficiency of somatic FA cells to normal levels. Disease-specific iPSCs derived in this fashion maintain a normal karyotype and are capable of hematopoietic differentiation. These data define the role of the FA pathway in reprogramming and provide a strategy for future translational applications of patient-specific FA iPSCs.

Fanconi anemia complementation group A (FANCA) protein has intrinsic affinity for nucleic acids with preference for single-stranded forms

The Fanconi anemia complementation group A (FANCA) gene is one of 15 disease-causing genes and has been found to be mutated in ∼60% of Fanconi anemia patients. Using purified protein, we report that human FANCA has intrinsic affinity for nucleic acids. FANCA binds to both single-stranded (ssDNA) and double-stranded (dsDNA) DNAs; however, its affinity for ssDNA is significantly higher than for dsDNA in an electrophoretic mobility shift assay. FANCA also binds to RNA with an intriguingly higher affinity than its DNA counterpart. FANCA requires a certain length of nucleic acids for optimal binding. Using DNA and RNA ladders, we determined that the minimum number of nucleotides required for FANCA recognition is ∼30 for both DNA and RNA. By testing the affinity between FANCA and a variety of DNA structures, we found that a 5'-flap or 5'-tail on DNA facilitates its interaction with FANCA. A patient-derived FANCA truncation mutant (Q772X) has diminished affinity for both DNA and RNA. In contrast, the complementing C-terminal fragment of Q772X, C772-1455, retains the differentiated nucleic acid-binding activity (RNA > ssDNA > dsDNA), indicating that the nucleic acid-binding domain of FANCA is located primarily at its C terminus, where most disease-causing mutations are found.

Fanconi anemia D2 protein is an apoptotic target mediated by caspases

FANCD2, a key factor in the FANC-BRCA1 pathway is monoubiquitinated and targeted to discrete nuclear foci following DNA damage. Since monoubiquitination of FANCD2 is a crucial indicator for cellular response to DNA damage, we monitored the fate of FANCD2 and its monoubiquitination following DNA damage. Disappearance of FANCD2 protein was induced following DNA damage in a dose-dependent manner, which correlated with degradation of BRCA1 and poly-ADP ribose polymerase (PARP), known targets for caspase-mediated apoptosis. Disappearance of FANCD2 was not affected by a proteasome inhibitor but was blocked by a caspase inhibitor. DNA damage-induced disappearance of FANCD2 was also observed in cells lacking FANCA, suggesting that disappearance of FANCD2 does not depend on FANC-BRCA1 pathway and FANCD2 monoubiquitination. In keeping with this, cells treated with TNF-α, an apoptotic stimulus without causing any DNA damage, also induced disappearance of FANCD2 without monoubiquitination. Together, our data suggest that FANCD2 is a target for caspase-mediated apoptotic pathway, which may be an early indicator for apoptotic cell death.

Induction of DNA damage signaling genes in benzidine-treated HepG2 cells

We examined genotoxicity and DNA damage response in HepG2 cells following exposure to benzidine. Using the Comet assay, we showed that benzidine (50-200 μM) induces DNA damage in HepG2 cells. DNA damage signaling pathway-based PCR arrays were used to investigate expression changes in genes involved in cell-cycle arrest, apoptosis, and DNA repair and showed upregulation of 23 genes and downregulation of one gene in benzidine-treated cells. Induction of G2/M arrest and apoptosis was confirmed at the protein level. Real-time PCR and Western blots were used to demonstrate the expression of select DNA repair-associated genes from the PCR array. Upregulation of the p53 protein in benzidine-treated cells suggests the induction of the p53 DNA damage signaling pathway. Collectively, DNA damage response genes induced by benzidine indicate recruitment complex molecular machinery involved in DNA repair, cell-cycle arrest, and potentially, activation of the apoptosis.

Monoubiquitinated Fanconi anemia D2 (FANCD2-Ub) is required for BCR-ABL1 kinase-induced leukemogenesis

Fanconi D2 (FANCD2) is monoubiquitinated on K561 (FANCD2-Ub) in response to DNA double-strand breaks (DSBs) to stimulate repair of these potentially lethal DNA lesions. FANCD2-Ub was upregulated in CD34+ chronic myeloid leukemia (CML) cells and in BCR-ABL1 kinase –positive cell lines in response to elevated levels of reactive oxygen species (ROS) and DNA cross-linking agent mitomycin C. Downregulation of FANCD2 and inhibition of FANCD2-Ub reduced the clonogenic potential of CD34+ CML cells and delayed BCR-ABL1 leukemogenesis in mice. Retarded proliferation of BCR-ABL1 -positive FANCD2−/− leukemia cells could be rescued by FANCD2 expression. BCR-ABL1 –positive FANCD2−/− cells accumulated more ROS-induced DSBs in comparison to BCR-ABL1 –positive FANCD2+/+ cells. Antioxidants diminished the number of DSBs and enhanced proliferation of BCR-ABL1 –positive FANCD2−/− cells. Expression of wild-type FANCD2 and FANCD2(S222A) phosphorylation-defective mutant (deficient in stimulation of intra-S phase checkpoint but proficient in DSB repair), but not FANCD2(K561R) monoubiquitination-defective mutant (proficient in stimulation of intra-S phase checkpoint but deficient in DSB repair) reduced the number of DSBs and facilitated proliferation of BCR-ABL1 –positive FANCD2−/− cells. We hypothesize that FANCD2-Ub plays an important role in BCR-ABL1 leukemogenesis due to its ability to facilitate the repair of numerous ROS-induced DSBs.

Normal red blood cells partially decrease diepoxybutane-induced chromosome breakage in cultured lymphocytes from Fanconi anaemia patients

OBJECTIVES

Fanconi anaemia (FA) is a cancer-prone chromosome instability syndrome characterized by hypersensitivity to DNA cross-linking agents, such as diepoxybutane (DEB). Previous studies have shown that normal red blood cells (RBC) can protect cultured lymphocytes against chromosomal breaks induced by DEB. The present study was designed to analyse influence of RBCs from normal individuals on frequency of DEB-induced chromosome breaks in lymphocyte cultures from FA patients.

MATERIALS AND METHODS

A comparative study was performed between DEB-induced chromosome breaks in cultures of FA lymphocytes with either autologous or heterologous RBCs. A further comparative study was carried out between whole blood cultures from FA patients performed on two occasions, before and 1 week after transfusion of RBCs.

RESULTS

It was observed that normal RBCs compared to FA RBCs, partially reduced chromosome breaks in cultured FA lymphocytes. A significant reduction in DEB-induced breaks was also observed in FA cultured lymphocytes obtained 1 week after transfusion of RBCs, in comparison to those observed in the same patients before RBC transfusion.

CONCLUSIONS

This study shows that DEB-induced chromosome instability in FA lymphocytes is partially reduced by normal RBCs. This effect may have some clinical relevance in vivo, whenever FA patients receive a RBC transfusion.

The Fanconi anemia pathway promotes DNA glycosylase-dependent excision of interstrand DNA crosslinks

Fanconi anemia (FA) is a recessive cancer prone syndrome featuring bone marrow failure and hypersensitivity to DNA interstrand crosslinks (ICLs) and, to a milder extension, to ionizing radiation and oxidative stress. Recently, we reported that human oxidative DNA glycosylase, NEIL1 excises with high efficiency the unhooked crosslinked oligomer within three-stranded DNA repair intermediate induced by photoactivated psoralen exposure. Complete reconstitution of repair of the ICL within a three-stranded DNA structure shows that it is processed in the short-patch base excision repair (BER) pathway. To examine whether the DNA damage hypersensitivity in FA cells follows impaired BER activities, we measured DNA glycosylase and AP endonuclease activities in cell-free extracts from wild-type, FA, and FA-corrected cells. We showed that immortalized lymphoid cells of FA complementation Groups A, C, and D and from control cells from normal donors contain similar BER activities. Intriguingly, the cellular level of NEIL1 protein strongly depends on the intact FA pathway suggesting that the hypersensitivity of FA cells to ICLs may, at least in part, arise from downregulation or degradation of NEIL1. Consistent with this result, plasmid-based expression of the FLAG-tagged NEIL1 protein partially complements the hypersensitivity FA cells to the crosslinking agents exposures, suggesting that NEIL1 specifically complements impaired capability of FA cells to repair ICLs and oxidative DNA damage. These findings shed light to how the FA pathway may regulate DNA repair proteins and bring explanation for the long-time disputed problem of the oxidative stress sensitive phenotype of FA cells.

Constitutive activation of caspase-3 and Poly ADP ribose polymerase cleavage in fanconi anemia cells

Fanconi anemia (FA) is a rare syndrome characterized by developmental abnormalities, progressive bone marrow failure, and cancer predisposition. Cells from FA patients exhibit hypersensitivity to DNA cross-linking agents and oxidative stress that may trigger apoptosis. Damage-induced activation of caspases and poly ADP ribose polymerase (PARP) enzymes have been described for some of the FA complementation groups. Here, we show the constitutive activation of caspase-3 and PARP cleavage in the FA cells without exposure to exogenous DNA-damaging factors. These effects can be reversed in the presence of reactive oxygen species scavenger N-acetylcystein. We also show the accumulation of oxidized proteins in FA cells, which is accompanied by the tumor necrosis factor (TNF)-alpha oversecretion and the upregulation of early stress response kinases pERK1/2 and p-P38. Suppression of TNF-alpha secretion by the extracellular signal-regulated kinase inhibitor PD98059 results in reduction of caspase-3 cleavage, suggesting a possible mechanism of caspases-3 activation in FA cells. Thus, the current study is the first evidence demonstrating the damage-independent activation of caspase-3 and PARP in FA cells, which seems to occur through mitogen-activated protein kinase activation and TNF-alpha oversecretion.

Fanconi anemia proteins and endogenous stresses

Each of the thirteen identified Fanconi anemia (FA) genes is required for resistance to DNA interstrand crosslinking agents, such as mitomycin C, cisplatin, and melphalan. While these agents are excellent tools for understanding the function of FA proteins in DNA repair, it is uncertain whether a defect in the removal of DNA interstrand crosslinks (ICLs) is the basis for the pathophysiology of FA. For example, DNA interstrand crosslinking agents induce other types of DNA damage, in addition to ICLs. Further, other DNA-damaging agents, such as ionizing or ultraviolet radiation, activate the FA pathway, leading to monoubiquitination of FANCD2 and FANCI. Also, FA patients display congenital abnormalities, hematologic deficiencies, and a predisposition to cancer in the absence of an environmental source of ICLs that is external to cells. Here we consider potential sources of endogenous DNA damage, or endogenous stresses, to which FA proteins may respond. These include ICLs formed by products of lipid peroxidation, and other forms of oxidative DNA damage. FA proteins may also potentially respond to telomere shortening or replication stress. Defining these endogenous sources of DNA damage or stresses is critical for understanding the pathogenesis of deficiencies for FA proteins.We propose that FA proteins are centrally involved in the response to replication stress, including replication stress arising from oxidative DNA damage.

Cellular and molecular consequences of defective Fanconi anemia proteins in replication-coupled DNA repair: mechanistic insights

The Fanconi anemia (FA) molecular network consists of 15 "FANC" proteins, of which 13 are associated with mutations in patients with this cancer-prone chromosome instability disorder. Whereas historically the common phenotype associated with FA mutations is marked sensitivity to DNA interstrand crosslinking agents, the literature supports a more global role for FANC proteins in coping with diverse stresses encountered by replicative polymerases. We have attempted to reconcile and integrate numerous observations into a model in which FANC proteins coordinate the following physiological events during DNA crosslink repair: (a) activating a FANCM-ATR-dependent S-phase checkpoint, (b) mediating enzymatic replication-fork breakage and crosslink unhooking, (c) filling the resulting gap by translesion synthesis (TLS) by error-prone polymerase(s), and (d) restoring the resulting one-ended double-strand break by homologous recombination repair (HRR). The FANC core subcomplex (FANCA, B, C, E, F, G, L, FAAP100) promotes TLS for both crosslink and non-crosslink damage such as spontaneous oxidative base damage, UV-C photoproducts, and alkylated bases. TLS likely helps prevent stalled replication forks from breaking, thereby maintaining chromosome continuity. Diverse DNA damages and replication inhibitors result in monoubiquitination of the FANCD2-FANCI complex by the FANCL ubiquitin ligase activity of the core subcomplex upon its recruitment to chromatin by the FANCM-FAAP24 heterodimeric translocase. We speculate that this translocase activity acts as the primary damage sensor and helps remodel blocked replication forks to facilitate checkpoint activation and repair. Monoubiquitination of FANCD2-FANCI is needed for promoting HRR, in which the FANCD1/BRCA2 and FANCN/PALB2 proteins act at an early step. We conclude that the core subcomplex is required for both TLS and HRR occurring separately for non-crosslink damages and for both events during crosslink repair. The FANCJ/BRIP1/BACH1 helicase functions in association with BRCA1 and may remove structural barriers to replication, such as guanine quadruplex structures, and/or assist in crosslink unhooking.

Impact of inorganic nutrients on maintenance of genomic stability

Maintenance of genome stability is of fundamental importance for counteracting carcinogenesis. Many human genome instability syndromes exhibit a predisposition to cancer. An increasing body of epidemiological evidence has suggested a link between nutrient status and risk of cancer. Like other chemicals, nutrients can be toxic when consumed in excess. It has become clear that both nutritional deficiency and toxicity can compromise the integrity of the genome. This article focuses on roles of inorganic trace nutrients, including selenium, copper, zinc, and iron, in the redox regulation of genome stability and how they relate to the pathologies of genomic instability syndromes and cancer.

Impaired synthesis of heme oxygenase-1 in Fanconi anemia cells can be rescued by transfection of Fanconi wild-type cDNA

Fanconi anemia is a fatal, hereditary chromosome instability syndrome of early childhood with progressive pancytopenia and cancer-proneness. Hypersensitivity to alkylating agents points to DNA repair inefficiency. Excess reactive oxygen intermediates and hypersensitivity to oxygen, all features of Fanconi anemia cells, give evidence for a disturbed oxidative metabolism. Here, we report that expression of the inducible heme oxygenase-1, an essential antioxidative defense protein, is impaired in Fanconi anemia cells and can be reinstated with the transfection of Fanconi A wild-type cDNA. A causative interaction of Fanconi anemia proteins with transcription of selected proteins is indicated. The results enlighten the oxygen sensitivity in Fanconi anemia.

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.

Differential p53 engagement in response to oxidative and oncogenic stresses in Fanconi anemia mice

Members of the Fanconi anemia (FA) protein family are involved in repair of genetic damage caused by DNA cross-linkers. It is not clear whether the FA proteins function in oxidative DNA damage and oncogenic stress response. Here, we report that deficiency in the Fanca gene in mice elicits a p53-dependent growth arrest and DNA damage response to oxidative DNA damage and oncogenic stress. Using a Fanca-/-Trp53-/- double knockout model and a functionally switchable p53 retrovirus, we define the kinetics, dependence, and persistence of p53-mediated response to oxidative and oncogenic stresses in Fanca-/- cells. Notably, oxidative stress induces persistent p53 response in Fanca-/- cells, likely due to accumulation of unrepaired DNA damage. On the other hand, whereas wild-type cells exhibit prolonged response to oncogene activation, the p53-activating signals induced by oncogenic ras are short-lived in Fanca-/- cells, suggesting that Fanca may be required for the cell to engage p53 during constitutive ras activation. We propose that the FA proteins protect cells from stress-induced proliferative arrest and tumor evolution by acting as a modulator of the signaling pathways that link FA to p53.

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.

The role of the RB tumour suppressor pathway in oxidative stress responses in the haematopoietic system

Exposure to pro-oxidants and defects in the repair of oxidative base damage are associated with disease and ageing and also contribute to the development of anaemia, bone marrow failure and haematopoietic malignancies. This Review assesses emerging data indicative of a specific role for the RB tumour suppressor pathway in the response of the haematopoietic system to oxidative stress. This is mediated through signalling pathways that involve DNA damage sensors, forkhead box O (Foxo) transcription factors and p38 mitogen-activated protein kinases and has downstream consequences for cell cycle progression, antioxidant capacity, mitochondrial mass and cellular metabolism.

Biomarkers and mechanisms of FANCD2 function

Genetic or epigenetic inactivation of the pathway formed by the Fanconi anemia (FA) and BRCA1 proteins occurs in several cancer types, making the affected tumors potentially hypersensitive to DNA cross-linkers and other chemotherapeutic agents. It has been proposed that the inability of FA/BRCA-defective cells to form subnuclear foci of effector proteins, such as FANCD2, can be used as a biomarker to aid individualization of chemotherapy. We show that FANCD2 inactivation not only renders cells sensitive to cross-links, but also oxidative stress, a common effect of cancer therapeutics. Oxidative stress sensitivity does not correlate with FANCD2 or RAD51 foci formation, but associates with increased γH2AX foci levels and apoptosis. Therefore, FANCD2 may protect cells against cross-links and oxidative stress through distinct mechanisms, consistent with the growing notion that the pathway is not linear. Our data emphasize the need for multiple biomarkers, such as γH2AX, FANCD2, and RAD51, to capture all pathway activities.

Different patterns of in vivo pro-oxidant states in a set of cancer- or aging-related genetic diseases

A comparative evaluation is reported of pro-oxidant states in 82 patients with ataxia telangectasia (AT), Bloom syndrome (BS), Down syndrome (DS), Fanconi anemia (FA), Werner syndrome (WS), and xeroderma pigmentosum (XP) vs 98 control donors. These disorders display cancer proneness, and/or early aging, and/or other clinical features. The measured analytes were: (a) leukocyte and urinary 8-hydroxy-2'-deoxyguanosine (8-OHdG), (b) blood glutathione (GSSG and GSH), (c) plasma glyoxal (Glx) and methylglyoxal (MGlx), and (d) some plasma antioxidants [uric acid (UA) and ascorbic acid (AA)]. Leukocyte 8-OHdG levels ranked as follows: WS>BS approximately FA approximately XP>DS approximately AT approximately controls. Urinary 8-OHdG levels were significantly increased in a total of 22 patients with BS, FA, or XP vs 47 controls. The GSSG:GSH ratio was significantly increased in patients with WS and in young (< or =15 years) patients with DS or with FA and decreased in older patients with DS or FA and in AT, BS, and XP patients. The plasma levels of Glx and/or MGlx were significantly increased in patients with WS, FA, and DS. The UA and AA levels were significantly increased in WS and DS patients, but not in AT, FA, BS, nor XP patients. Rationale for chemoprevention trials is discussed.

Runx3 interacts with DNA repair protein Ku70

Recent studies have suggested that Runt-related transcription factor 3 (Runx3) is associated with genesis and progression of gastric carcinoma. A proteomic approach was used to search for Runx3-interacting proteins to elucidate the molecular mechanisms of gastric carcinogenesis. Runx3 bound with myc and flag tags (MEF tags) is expressed in HEK293T cells, and the protein complex formed with Runx3 was purified and identified by mass spectrometry. Ku70 and Ku80, members of the DNA repair protein complex, were identified as Runx3-interacting proteins. Runx3, Ku70, and Ku80 associate in vivo, and in vitro interaction between Runx3 and Ku70 was confirmed via His-tag pull-down assay. The amino acids 241-322 of Runx3, which correspond to the transcriptional activation domain, and the amino acids 1-116 of Ku70 were necessary for binding with each other, and immunocytochemistry under confocal laser microscopy demonstrated that Runx3 and Ku70 localized throughout the nucleus excluding the nucleoli. Furthermore, Runx3 highly activated the transcription of p21, the target gene of Runx3, in Ku70 knockdown cells. These results suggest a possible link between a tumor suppressor function and DNA repair.

Inflammatory reactive oxygen species-mediated hemopoietic suppression in Fancc-deficient mice

Patients with the genomic instability syndrome Fanconi anemia (FA) commonly develop progressive bone marrow (BM) failure and have a high risk of cancer. Certain manifestations of the disease suggest that the FA immune system is dysfunctional and may contribute to the pathogenesis of both BM failure and malignancies. In this study, we have investigated inflammation and innate immunity in FA hemopoietic cells using mice deficient in Fanconi complementation group C gene (Fancc). We demonstrate that Fancc-deficient mice exhibit enhanced inflammatory response and are hypersensitive to LPS-induced septic shock as a result of hemopoietic suppression. This exacerbated inflammatory phenotype is intrinsic to the hemopoietic system and can be corrected by the re-expression of a wild-type FANCC gene, suggesting a potential role of the FANCC protein in innate immunity. LPS-mediated hemopoietic suppression requires two major inflammatory agents, TNF-alpha and reactive oxygen species. In addition, LPS-induced excessive accumulation of reactive oxygen species in Fancc(-/-) BM cells overactivates the stress kinase p38 and requires prolonged activation of the JNK. Our data implicate a role of inflammation in pathogenesis of FA and BM failure diseases in general.

Inflammatory ROS promote and cooperate with the Fanconi anemia mutation for hematopoietic senescence

The proinflammatory cytokine tumor necrosis factor alpha (TNFalpha) inhibits hematopoietic stem cell (HSC) expansion, interferes with HSC self-renewal and compromises the ability of HSC to reconstitute hematopoiesis. We have investigated mechanisms by which TNFalpha suppresses hematopoiesis using the genomic instability syndrome Fanconi anemia mouse model deficient for the complementation-group-C gene (Fancc). Examination of senescence makers, such as senescence-associated beta-galactosidase, HP1-gamma, p53 and p16(INK4A) shows that TNFalpha induces premature senescence in bone marrow HSCs and progenitor cells as well as other tissues of Fancc-/- mice. TNFalpha-induced senescence correlates with the accumulation of reactive oxygen species (ROS) and oxidative DNA damage. Neutralization of TNFalpha or deletion of the TNF receptor in Fancc-/- mice (Fancc-/-;Tnfr1-/-) prevents excessive ROS production and hematopoietic senescence. Pretreatment of TNFalpha-injected Fancc-/- mice with a ROS scavenger significantly reduces oxidative base damage, DNA strand breaks and senescence. Furthermore, HSCs and progenitor cells from TNFalpha-treated Fancc-/- mice show increased chromosomal aberrations and have an impaired oxidative DNA-damage repair. These results indicate an intimate link between inflammatory reactive oxygen species and DNA-damage-induced premature senescence in HSCs and progenitor cells, which may play an important role in aging and anemia.

Defective mitochondrial peroxiredoxin-3 results in sensitivity to oxidative stress in Fanconi anemia

Cells from patients with Fanconi anemia (FA), an inherited disorder that includes bone marrow failure and cancer predisposition, have increased sensitivity to oxidative stress through an unknown mechanism. We demonstrate that the FA group G (FANCG) protein is found in mitochondria. Wild-type but not G546R mutant FANCG physically interacts with the mitochondrial peroxidase peroxiredoxin-3 (PRDX3). PRDX3 is deregulated in FA cells, including cleavage by a calpainlike cysteine protease and mislocalization. FA-G cells demonstrate distorted mitochondrial structures, and mitochondrial extracts have a sevenfold decrease in thioredoxin-dependent peroxidase activity. Transient overexpression of PRDX3 suppresses the sensitivity of FA-G cells to H₂O₂, and decreased PRDX3 expression increases sensitivity to mitomycin C. Cells from the FA-A and -C subtypes also have PRDX3 cleavage and decreased peroxidase activity. This study demonstrates a role for the FA proteins in mitochondria witsh sensitivity to oxidative stress resulting from diminished peroxidase activity. These defects may lead to apoptosis and the accumulation of oxidative DNA damage in bone marrow precursors.

Nucleophosmin regulates cell cycle progression and stress response in hematopoietic stem/progenitor cells

Nucleophosmin (NPM) is a multifunctional protein frequently overexpressed in actively proliferating cells. Strong evidence indicates that NPM is required for embryonic development and genomic stability. Here we report that NPM enhances the proliferative potential of hematopoietic stem cells (HSCs) and increases their survival upon stress challenge. Both short term liquid culture and clonogenic progenitor cell assays show a selective expansion of NPM-overexpressing HSCs. Interestingly, HSCs infected with NPM retrovirus show significantly reduced commitment to myeloid differentiation compared with vector-transduced cells, and majority of the NPM-overexpressing cells remains primitive during a 5-day culture. Bone marrow transplantation experiments demonstrate that NPM promotes the self-renewal of long term repopulating HSCs while attenuated their commitment to myeloid differentiation. NPM overexpression induces rapid entry of HSCs into the cell cycle and suppresses the expression of several negative cell cycle regulators that are associated with G(1)-to-S transition. NPM knockdown elevates expression of these negative regulators and exacerbates stress-induced cell cycle arrest. Finally, overexpression of NPM promotes the survival and recovery of HSCs and progenitors after exposure to DNA damage, oxidative stress, and hematopoietic injury both in vivo and in vitro. DNA repair kinetics study suggests that NPM has a role in reducing the susceptibility of chromosomal DNA to damage rather than promoting DNA damage repair. Together, these results indicate that NPM plays an important role in hematopoiesis via mechanisms involving modulation of HSC/progenitor cell cycle progression and stress response.

Oxidative stress as a multiple effector in Fanconi anaemia clinical phenotype

Fanconi anaemia (FA) is a genetic disease characterised by bone marrow failure with excess risk of myelogenous leukaemia and solid tumours. A widely accepted notion in FA research invokes a deficiency of response to DNA damage as the fundamental basis of the 'crosslinker sensitivity' observed in this disorder. However, such an isolated defect cannot readily account for the full cellular and clinical phenotype, which includes a number of other abnormalities, such as malformations, endocrinopathies, and typical skin spots. An extensive body of evidence pointing toward an involvement of oxidative stress in the FA phenotype includes the following: (i) In vitro and ex vivo abnormalities in a number of redox status endpoints; (ii) the functions of several FA proteins in protecting cells from oxidative stress; (iii) redox-related toxicity mechanisms of the xenobiotics evoking excess toxicity in FA cells. The clinical features in FA and the in vivo abnormalities of redox parameters are here reconsidered in view of the pleiotropic clinical phenotype and known biochemical and molecular links to an in vivo prooxidant state, which causes oxidative damage to biomolecules, resulting in an excessive number of acquired abnormalities that may overwhelm the cellular repair capacity rather than a primary deficiency in DNA repair. FA may thus represent a unique model disease in testing the integration between the acquisition of macromolecular damage as a result of oxidative stress and the ability of the mammalian cell to respond effectively to such damage.

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.

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.

Telomere dynamics in Fancg-deficient mouse and human cells

A number of DNA repair proteins also play roles in telomere metabolism. To investigate whether the accelerated telomere shortening reported in Fanconi anemia (FA) hematopoietic cells relates to a direct role of the FA pathway in telomere maintenance, we have analyzed telomere dynamics in Fancg-deficient mouse and human cells. We show here that both hematopoietic (stem and differentiated bone marrow cells, B and T lymphocytes) and nonhematopoietic (germ cells, mouse embryonic fibroblasts [MEFs]) Fancg(-/-) mouse cells display normal telomere length, normal telomerase activity, and normal chromosome end-capping, even in the presence of extensive clastogen-induced cytogenetic instability (mitomycin C [MMC], gamma-radiation). In addition, telomerase-deficient MEFs with humanlike telomere length and decreased Fancg expression (G5 Terc(-/-)/Fancg shRNA3 MEFs) display normal telomere maintenance. Finally, early-passage primary fibroblasts from patients with FA of complementation group G as well as primary human cells with reduced FANCG expression (FANCG shRNA IMR90 cells) show no signs of telomere dysfunction. Our observations indicate that accelerated telomere shortening in patients with FA is not due to a role of FANCG at telomeres but instead may be secondary to the disease. These findings suggest that telomerase-based therapies could be useful prophylactic agents in FA aplastic anemia by preserving their telomere reserve in the context of the disease.