Selective pressure as an essential force in molecular evolution of myeloid leukemic clones: a view from the window of Fanconi anemia

Pediatric Germline Predisposition to Myeloid Neoplasms

PURPOSE OF REVIEW

Advances in the understanding of germline predisposition to pediatric cancers, particularly myeloid neoplasms, have increased rapidly over the last 20 years. Here, we highlight the most up-to-date knowledge regarding known pathogenic germline variants that contribute to the development of myeloid neoplasms in children.

RECENT FINDINGS

This discussion enumerates the most notable myeloid neoplasm-causing germline mutations. These mutations may be organized based on their molecular underpinnings-transcriptional control, splicing and signal transduction control, and a group of heterogeneous bone marrow failure syndromes. We review recent findings related to the biochemical mechanisms that predispose to malignant transformation in each condition. Key genetic discoveries such as novel mutations, degrees of penetrance, principles of the two-hit hypothesis, and co-occurrence of multiple mutations are shared. Clinical pearls, such as information regarding epidemiology, natural history, or prognosis, are also discussed. Germline mutations predisposing to pediatric myeloid neoplasms are frequent, but underrecognized. They hold major clinical implications regarding prognosis, treatment strategies, and screening for other malignancies. Further research is warranted to better characterize each of these conditions, as well as identify additional novel germline pathogenic variants of interest.

Molecular Pathogenesis in Myeloid Neoplasms with Germline Predisposition

Myeloid neoplasms with germline predisposition have recently been added as distinct provisional entities in the 2017 revision of the World Health Organization’s classification of tumors of hematopoietic and lymphatic tissue. Individuals with germline predisposition have increased risk of developing myeloid neoplasms—mainly acute myeloid leukemia and myelodysplastic syndrome. Although the incidence of myeloid neoplasms with germline predisposition remains poorly defined, these cases provide unique and important insights into the biology and molecular mechanisms of myeloid neoplasms. Knowledge of the regulation of the germline genes and their interactions with other genes, proteins, and the environment, the penetrance and clinical presentation of inherited mutations, and the longitudinal dynamics during the process of disease progression offer models and tools that can further our understanding of myeloid neoplasms. This knowledge will eventually translate to improved disease sub-classification, risk assessment, and development of more effective therapy. In this review, we will use examples of these disorders to illustrate the key molecular pathways of myeloid neoplasms.

iPSC modeling of stage-specific leukemogenesis reveals BAALC as a key oncogene in severe congenital neutropenia

Severe congenital neutropenia (CN) is a pre-leukemic bone marrow failure syndrome that can evolve to acute myeloid leukemia (AML). Mutations in CSF3R and RUNX1 are frequently observed in CN patients, although how they drive the transition from CN to AML (CN/AML) is unclear. Here we establish a model of stepwise leukemogenesis in CN/AML using CRISPR-Cas9 gene editing of CN patient-derived iPSCs. We identified BAALC upregulation and resultant phosphorylation of MK2a as a key leukemogenic event. BAALC deletion or treatment with CMPD1, a selective inhibitor of MK2a phosphorylation, blocked proliferation and induced differentiation of primary CN/AML blasts and CN/AML iPSC-derived hematopoietic stem and progenitor cells (HSPCs) without affecting healthy donor or CN iPSC-derived HSPCs. Beyond detailing a useful method for future investigation of stepwise leukemogenesis, this study suggests that targeting BAALC and/or MK2a phosphorylation may prevent leukemogenic transformation or eliminate AML blasts in CN/AML and RUNX1 mutant BAALC(hi) de novo AML.

FANCD2 and HES1 suppress inflammation-induced PPARɣ to prevent haematopoietic stem cell exhaustion

The Fanconi anaemia protein FANCD2 suppresses PPARƔ to maintain haematopoietic stem cell's (HSC) function; however, the underlying mechanism is not known. Here we show that FANCD2 acts in concert with the Notch target HES1 to suppress inflammation-induced PPARƔ in HSC maintenance. Loss of HES1 exacerbates FANCD2-KO HSC defects. However, deletion of HES1 does not cause more severe inflammation-mediated HSC defects in FANCD2-KO mice, indicating that both FANCD2 and HES1 are required for limiting detrimental effects of inflammation on HSCs. Further analysis shows that both FANCD2 and HES1 are required for transcriptional repression of inflammation-activated PPARg promoter. Inflammation orchestrates an overlapping transcriptional programme in HSPCs deficient for FANCD2 and HES1, featuring upregulation of genes in fatty acid oxidation (FAO) and oxidative phosphorylation. Loss of FANCD2 or HES1 augments both basal and inflammation-primed FAO. Targeted inhibition of PPARƔ or the mitochondrial carnitine palmitoyltransferase-1 (CPT1) reduces FAO and ameliorates HSC defects in inflammation-primed HSPCs deleted for FANCD2 or HES1 or both. Finally, depletion of PPARg or CPT1 restores quiescence in these mutant HSCs under inflammatory stress. Our results suggest that this novel FANCD2/HES1/PPARƔ axis may constitute a key component of immunometabolic regulation, connecting inflammation, cellular metabolism and HSC function.

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.

Hematopoietic development: a gap in our understanding of inherited bone marrow failure

Inherited bone marrow failure syndromes (IBMFS) represent a heterogeneous group of multisystem disorders that typically present with cytopenia in early childhood. Efforts to understand the underlying hematopoietic stem cell (HSC) losses have generally focused on postnatal hematopoiesis. However, reflecting the role of many of the involved genes in core cellular functions and the diverse nonhematologic abnormalities seen in patients at birth, studies have begun to explore IBMFS manifestations during fetal development. Here, I consider the current evidence for fetal deficits in the HSC pool and highlight emerging concepts regarding the origins and unique pathophysiology of hematopoietic failure in IBMFS.

The immune receptor Trem1 cooperates with diminished DNA damage response to induce preleukemic stem cell expansion

Fanconi anemia (FA) is an inherited bone marrow failure syndrome with extremely high risk of leukemic transformation. Here we investigate the relationship between DNA damage response (DDR) and leukemogenesis using the Fanca knockout mouse model. We found that chronic exposure of the Fanca^-/- hematopoietic stem cells to DNA crosslinking agent mitomycin C in vivo leads to diminished DDR, and the emergence/expansion of pre-leukemia stem cells (pre-LSCs). Surprisingly, although genetic correction of Fanca deficiency in the pre-LSCs restores DDR and reduces genomic instability, but fails to prevent pre-LSC expansion or delay leukemia development in irradiated recipients. Furthermore, we identified transcription program underlying dysregulated DDR and cell migration, myeloid proliferation, and immune response in the Fanca^-/- pre-LSCs. Forced expression of the downregulated DNA repair genes, Rad51c or Trp53i13, in the Fanca^-/- pre-LSCs partially rescues DDR but has no effect on leukemia, whereas shRNA knockdown of the upregulated immune receptor genes Trem1 or Pilrb improves leukemia-related survival, but not DDR or genomic instability. Furthermore, Trem1 cooperates with diminished DDR in vivo to promote Fanca^-/- pre-LSC expansion and leukemia development. Our study implicates diminishing DDR as a root cause of FA leukemogenesis, which subsequently collaborates with other signaling pathways for leukemogenic transformation.

How I treat MDS and AML in Fanconi anemia

Fanconi anemia (FA) is the most frequent inherited cause of bone marrow failure (BMF). Most FA patients experience hematopoietic stem cell attrition and cytopenia during childhood, which along with intrinsic chromosomal instability, favor clonal evolution and the frequent emergence in their teens or young adulthood of myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). To early identify and further predict bone marrow (BM) clonal progression and enable timely treatment, the follow-up of FA patients includes regular BM morphological and cytogenetic examinations. Allogeneic hematopoietic stem cell transplantation (HSCT) remains the only curative treatment of FA patients with MDS or AML. Although questions remain concerning HSCT itself (including the need for pretransplant chemotherapy, the best conditioning regimen, and the optimal long-term follow-up of such patients especially regarding secondary malignancies), clonal evolution in the absence of significant BM dysplasia and blast cells can be difficult to address in FA patients, for whom the concept of preemptive HSCT is discussed. Illustrated by 3 representative clinical vignettes showing specific features of MDS and AML in FA patients, this paper summarizes our practical approach from diagnosis through treatment in this particular situation.

FANCA and FANCC modulate TLR and p38 MAPK-dependent expression of IL-1β in macrophages

Hematopoietic stem and progenitor cells with inactivated Fanconi anemia (FA) genes, FANCA and FANCC, are hypersensitive to inflammatory cytokines. One of these, tumor necrosis factor α (TNF-α), is also overproduced by FA mononuclear phagocytes in response to certain Toll-like receptor (TLR) agonists, creating an autoinhibitory loop that may contribute to the pathogenesis of progressive bone marrow (BM) failure and selection of TNF-α-resistant leukemic stem cell clones. In macrophages, the TNF-α overproduction phenotype depends on p38 mitogen-activated protein kinase (MAPK), an enzyme also known to induce expression of other inflammatory cytokines, including interleukin 1β (IL-1β). Reasoning that IL-1β might be involved in a like autoinhibitory loop, we determined that (1) TLR activation of FANCA- and FANCC-deficient macrophages induced overproduction of both TNF-α and IL-1β in a p38-dependent manner; (2) exposure of Fancc-deficient BM progenitors to IL-1β potently suppressed the expansion of multipotent progenitor cells in vitro; and (3) although TNF-α overexpression in FA cells is controlled posttranscriptionally by the p38 substrate MAPKAPK-2, p38-dependent overproduction of IL-1β is controlled transcriptionally. We suggest that multiple inflammatory cytokines overproduced by FANCA- and FANCC-deficient mononuclear phagocytes may contribute to the progressive BM failure that characterizes FA, and that to achieve suppression of this proinflammatory state, p38 is a more promising molecular therapeutic target than either IL-1β or TNF-α alone.

TNF-α signaling in Fanconi anemia

Tumor necrosis factor-alpha (TNF-α) is a major pro-inflammatory cytokine involved in systemic inflammation and the acute phase reaction. Dysregulation of TNF production has been implicated in a variety of human diseases including Fanconi anemia (FA). FA is a genomic instability syndrome characterized by progressive bone marrow failure and cancer susceptibility. The patients with FA are often found overproducing TNF-α, which may directly affect hematopoietic stem cell (HSC) function by impairing HSC survival, homing and proliferation, or indirectly change the bone marrow microenvironment critical for HSC homeostasis and function, therefore contributing to disease progression in FA. In this brief review, we discuss the link between TNF-α signaling and FA pathway with emphasis on the implication of inflammation in the pathophysiology and abnormal hematopoiesis in FA.

The Fanconi anemia pathway: repairing the link between DNA damage and squamous cell carcinoma

Fanconi anemia (FA) is a rare inherited recessive disease caused by mutations in one of fifteen genes known to encode FA pathway components. In response to DNA damage, nuclear FA proteins associate into high molecular weight complexes through a cascade of post-translational modifications and physical interactions, followed by the repair of damaged DNA. Hematopoietic cells are particularly sensitive to the loss of these interactions, and bone marrow failure occurs almost universally in FA patients. FA as a disease is further characterized by cancer susceptibility, which highlights the importance of the FA pathway in tumor suppression, and will be the focus of this review. Acute myeloid leukemia is the most common cancer type, often subsequent to bone marrow failure. However, FA patients are also at an extreme risk of squamous cell carcinoma (SCC) of the head and neck and gynecological tract, with an even greater incidence in those individuals who have received a bone marrow transplant and recovered from hematopoietic disease. FA tumor suppression in hematopoietic versus epithelial compartments could be mechanistically similar or distinct. Definition of compartment specific FA activities is now critical to assess the effects of today's bone marrow failure treatments on tomorrow's solid tumor development. It is our hope that current therapies can then be optimized to decrease the risk of malignant transformation in both hematopoietic and epithelial cells. Here we review our current understanding of the mechanisms of action of the Fanconi anemia pathway as it contributes to stress responses, DNA repair and squamous cell carcinoma susceptibility.

p38 MAPK inhibition suppresses the TLR-hypersensitive phenotype in FANCC- and FANCA-deficient mononuclear phagocytes

Fanconi anemia, complementation group C (FANCC)-deficient hematopoietic stem and progenitor cells are hypersensitive to a variety of inhibitory cytokines, one of which, TNFα, can induce BM failure and clonal evolution in Fancc-deficient mice. FANCC-deficient macrophages are also hypersensitive to TLR activation and produce TNFα in an unrestrained fashion. Reasoning that suppression of inhibitory cytokine production might enhance hematopoiesis, we screened small molecules using TLR agonist-stimulated FANCC- and Fanconi anemia, complementation group A (FANCA)-deficient macrophages containing an NF-κB/AP-1-responsive reporter gene (SEAP). Of the 75 small molecules screened, the p38 MAPK inhibitor BIRB 796 and dasatinib potently suppressed TLR8-dependent expression of the reporter gene. Fanconi anemia (FA) macrophages were hypersensitive to the TLR7/8 activator R848, overproducing SEAP and TNFα in response to all doses of the agonist. Low doses (50nM) of both agents inhibited p38 MAPK-dependent activation of MAPKAPK2 (MK2) and suppressed MK2-dependent TNFα production without substantially influencing TNFα gene transcription. Overproduction of TNFα by primary FA cells was likewise suppressed by these agents and involved inhibition of MK2 activation. Because MK2 is also known to influence production and/or sensitivity to 2 other suppressive factors (MIP-1α and IFNγ) to which FA hematopoietic progenitor cells are uniquely vulnerable, targeting of p38 MAPK in FA hematopoietic cells is a rational objective for preclinical evaluation.

Fanconi Anemia

Fanconi anemia (FA) is the most frequent inherited cause of BM failure (BMF). Fifteen FANC genes have been identified to date, the most prevalent being FANCA, FANCC, FANCG, and FANCD2. In addition to classical presentations with progressive BMF during childhood and a positive chromosome breakage test in the blood, atypical clinical and/or biological situations can be seen in which a FA diagnosis has to be confirmed or eliminated. For this, a range of biological tools have been developed, including analysis of skin fibroblasts. FA patients experience a strong selective pressure in the BM that predisposes to clonal evolution and to the emergence in their teens or young adulthood of myelodysplasia syndrome (MDS) and/or acute myeloid leukemia (AML) with a specific pattern of somatic chromosomal lesions. The cellular mechanisms underlying (1) the hematopoietic defect which leads to progressive BMF and (2) somatic clonal evolutions in this background, are still largely elusive. Elucidation of these mechanisms at the molecular and cellular levels should be useful to understand the physiopathology of the disease and to adapt the follow-up and treatment of FA patients. This may also ultimately benefit older, non-FA patients with aplastic anemia, MDS/AML for whom FA represents a model genetic condition.

TNFα facilitates clonal expansion of JAK2V617F positive cells in myeloproliferative neoplasms

Proinflammatory cytokines such as TNFα are elevated in patients with myeloproliferative neoplasms (MPN), but their contribution to disease pathogenesis is unknown. Here we reveal a central role for TNFα in promoting clonal dominance of JAK2(V617F) expressing cells in MPN. We show that JAK2(V617F) kinase regulates TNFα expression in cell lines and primary MPN cells and TNFα expression is correlated with JAK2(V617F) allele burden. In clonogenic assays, normal controls show reduced colony formation in the presence of TNFα while colony formation by JAK2(V617F)-positive progenitor cells is resistant or stimulated by exposure to TNFα. Ectopic JAK2(V617F) expression confers TNFα resistance to normal murine progenitor cells and overcomes inherent TNFα hypersensitivity of Fanconi anemia complementation group C deficient progenitors. Lastly, absence of TNFα limits clonal expansion and attenuates disease in a murine model of JAK2(V617F)-positive MPN. Altogether our data are consistent with a model where JAK2(V617F) promotes clonal selection by conferring TNFα resistance to a preneoplastic TNFα sensitive cell, while simultaneously generating a TNFα-rich environment. Mutations that confer resistance to environmental stem cell stressors are a recognized mechanism of clonal selection and leukemogenesis in bone marrow failure syndromes and our data suggest that this mechanism is also critical to clonal selection in MPN.

Overexpression of IL-3Rα on CD34+CD38- stem cells defines leukemia-initiating cells in Fanconi anemia AML

Patients with Fanconi anemia (FA) have a high risk of developing acute myeloid leukemia (AML). In this study, we attempted to identify cell-surface markers for leukemia-initiating cells in FA-AML patients. We found that the IL-3 receptor-α (IL-3Rα) is a promising candidate as an leukemia-initiating cell-specific antigen for FA-AML. Whereas IL-3Rα expression is undetectable on normal CD34(+)CD38(-) HSCs, it is overexpressed on CD34(+)CD38(-) cells from FA patients with AML. We examined the leukemia-initiating cell activity of IL-3Rα-positive FA-AML cells in a "humanized" FA xenotransplant model in which we separated AML cells into IL-3Rα-positive and IL-3Rα-negative CD34 fractions and transplanted them into irradiated recipient mice. In all 3 FA-AML samples, only IL-3Rα-positive cells showed significant levels of engraftment and developed leukemia in the recipient mice. The FA CD34(+)IL-3Rα(+) blasts isolated from leukemic mice exhibited hypersensitivity to IL-3 deprivation and JAK2-STAT5 overactivation after IL-3 treatment. Finally, treatment of FA CD34(+)IL-3Rα(+) blasts with an IL-3Rα-neutralizing antibody inhibited IL-3-mediated proliferation and STAT5 activation. These results demonstrate that IL-3Rα is a cell-surface marker present on FA-AML leukemia-initiating cells and may be a valuable therapeutic target.

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

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

Knockdown of Fanconi anemia genes in human embryonic stem cells reveals early developmental defects in the hematopoietic lineage

Fanconi anemia (FA) is a genetically heterogeneous, autosomal recessive disorder characterized by pediatric bone marrow failure and congenital anomalies. The effect of FA gene deficiency on hematopoietic development in utero remains poorly described as mouse models of FA do not develop hematopoietic failure and such studies cannot be performed on patients. We have created a human-specific in vitro system to study early hematopoietic development in FA using a lentiviral RNA interference (RNAi) strategy in human embryonic stem cells (hESCs). We show that knockdown of FANCA and FANCD2 in hESCs leads to a reduction in hematopoietic fates and progenitor numbers that can be rescued by FA gene complementation. Our data indicate that hematopoiesis is impaired in FA from the earliest stages of development, suggesting that deficiencies in embryonic hematopoiesis may underlie the progression to bone marrow failure in FA. This work illustrates how hESCs can provide unique insights into human development and further our understanding of genetic disease.

Tumor heterogeneity: causes and consequences

With rare exceptions, spontaneous tumors originate from a single cell. Yet, at the time of clinical diagnosis, the majority of human tumors display startling heterogeneity in many morphological and physiological features, such as expression of cell surface receptors, proliferative and angiogenic potential. To a substantial extent, this heterogeneity might be attributed to morphological and epigenetic plasticity, but there is also strong evidence for the co-existence of genetically divergent tumor cell clones within tumors. In this perspective, we summarize the sources of intra-tumor phenotypic heterogeneity with emphasis on genetic heterogeneity. We review experimental evidence for the existence of both intra-tumor clonal heterogeneity as well as frequent evolutionary divergence between primary tumors and metastatic outgrowths. Furthermore, we discuss potential biological and clinical implications of intra-tumor clonal heterogeneity.

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.

Diamond-Blackfan anemia: diagnosis, treatment, and molecular pathogenesis

Diamond-Blackfan anemia (DBA) is a genetically and clinically heterogeneous disorder characterized by erythroid failure, congenital anomalies, and a predisposition to cancer. Faulty ribosome biogenesis, resulting in proapoptotic erythropoiesis leading to erythroid failure, is hypothesized to be the underlying defect. The genes identified to date that are mutated in DBA all encode ribosomal proteins associated with either the small or large subunit and in these cases haploinsufficiency gives rise to the disease. Extraordinarily robust laboratory and clinical investigations have recently led to demonstrable improvements in clinical care for patients with DBA.

Myelodysplasia and acute leukemia as late complications of marrow failure: future prospects for leukemia prevention

Patients who have acquired and inherited bone marrow failure syndromes are at risk for the development of clonal neoplasms including acute myeloid leukemia, myelodysplastic syndrome, and paroxysmal nocturnal hemoglobinuria. This article reviews the evidence supporting a model of clonal selection, a paradigm that provides a reasonable expectation that these often fatal complications might be prevented in the future.

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.

Diagnosing and treating Diamond Blackfan anaemia: results of an international clinical consensus conference

Diamond Blackfan anaemia (DBA) is a rare, genetically and clinically heterogeneous, inherited red cell aplasia. Classical DBA affects about seven per million live births and presents during the first year of life. However, as mutated genes have been discovered in DBA, non-classical cases with less distinct phenotypes are being described in adults as well as children. In caring for these patients it is often difficult to have a clear understanding of the treatment options and their outcomes because of the lack of complete information on the natural history of the disease. The purpose of this document is to review the criteria for diagnosis, evaluate the available treatment options, including corticosteroid and transfusion therapies and stem cell transplantation, and propose a plan for optimizing patient care. Congenital anomalies, mode of inheritance, cancer predisposition, and pregnancy in DBA are also reviewed. Evidence-based conclusions will be made when possible; however, as in many rare diseases, the data are often anecdotal and the recommendations are based upon the best judgment of experienced clinicians. The recommendations regarding the diagnosis and management described in this report are the result of deliberations and discussions at an international consensus conference.

Cigarette smoke induces genetic instability in airway epithelial cells by suppressing FANCD2 expression

Chromosomal abnormalities are commonly found in bronchogenic carcinoma cells, but the molecular causes of chromosomal instability (CIN) and their relationship to cigarette smoke has not been defined. Because the Fanconi anaemia (FA)/BRCA pathway is essential for maintenance of chromosomal stability, we tested the hypothesis that cigarette smoke suppresses that activity of this pathway. Here, we show that cigarette smoke condensate (CSC) inhibited translation of FANCD2 mRNA (but not FANCC or FANCG) in normal airway epithelial cells and that this suppression of FANCD2 expression was sufficient to induce both genetic instability and programmed cell death in the exposed cell population. Cigarette smoke condensate also suppressed FANCD2 function and induced CIN in bronchogenic carcinoma cells, but these cells were resistant to CSC-induced apoptosis relative to normal airway epithelial cells. We, therefore, suggest that CSC exerts pressure on airway epithelial cells that results in selection and emergence of genetically unstable somatic mutant clones that may have lost the capacity to effectively execute an apoptotic programme. Carcinogen-mediated suppression of FANCD2 gene expression provides a plausible molecular mechanism for CIN in bronchogenic carcinogenesis.

Discovering early molecular determinants of leukemogenesis

Truncating mutations of the G-CSF receptor are found during disease course in nearly half of all patients with severe congenital neutropenia. In this issue of the JCI, Liu et al. demonstrate that these mutations confer a competitive clonal advantage upon HSCs in mice and that the advantage is conditional because it is observed only in the presence of the ligand G-CSF (see the related article beginning on page 946). Once activated, the mutant receptor requires the function of Stat5 in order to effect clonal expansion of this stem cell population. The results support the notion that early molecular steps in this and other neoplastic processes represent adaptations in which, through somatic mutations, "unfit" stem cells gain a measure of fitness by altering their relationships with their microenvironment.

TNF-alpha induces leukemic clonal evolution ex vivo in Fanconi anemia group C murine stem cells

The molecular pathogenesis of the myeloid leukemias that frequently occur in patients with Fanconi anemia (FA) is not well defined. Hematopoietic stem cells bearing inactivating mutations of FA complementation group C (FANCC) are genetically unstable and hypersensitive to apoptotic cytokine cues including IFN-gamma and TNF-alpha, but neoplastic stem cell clones that arise frequently in vivo are resistant to these cytokines. Reasoning that the combination of genetic instability and cytokine hypersensitivity might create an environment supporting the emergence of leukemic stem cells, we tested the leukemia-promoting effects of TNF-alpha in murine stem cells. TNF-alpha exposure initially profoundly inhibited the growth of Fancc-/- stem cells. However, longer-term exposure of these cells promoted the outgrowth of cytogenetically abnormal clones that, upon transplantation into congenic WT mice, led to acute myelogenous leukemia. TNF-alpha induced ROS-dependent genetic instability in Fancc-/- but not in WT cells. The leukemic clones were TNF-alpha resistant but retained their characteristic hypersensitivity to mitomycin C and exhibited high levels of chromosomal instability. Expression of FANCC cDNA in Fancc-/- stem cells protected them from TNF-alpha-induced clonal evolution. We conclude that TNF-alpha exposure creates an environment in which somatically mutated preleukemic stem cell clones are selected and from which unaltered TNF-alpha-hypersensitive Fancc-/- stem cells are purged.

Diamond Blackfan anemia: a disorder of red blood cell development

Diamond Blackfan anemia (DBA) is an inherited hypoplastic anemia that typically presents in the first year of life. The genes identified to date that are mutated in DBA encode ribosomal proteins, and in these cases ribosomal protein haploinsufficiency gives rise to the disease. The developmental timing of DBA presentation suggests that the changes in red blood cell production that occur around the time of birth trigger a pathophysiological mechanism, likely linked to defective ribosome synthesis, which precipitates the hematopoietic phenotype. Variable presentation of other clinical phenotypes in DBA patients indicates that other developmental pathways may also be affected by ribosomal protein haploinsufficiency and that the involvement of these pathways is influenced by modifier genes. Understanding the molecular basis for the developmental timing of DBA presentation promises to shed light on a number of baffling features of this disease. This chapter also attempts to demonstrate how the marriage of laboratory and clinical science may enhance each and permit insights into human disease that neither alone can accomplish.

Hematopoietic changes during progression from Fanconi anemia into acute myeloid leukemia: case report and brief review of the literature

Fanconi anemia (FA) is a rare autosomal recessive disorder characterized by bone marrow (BM) failure and a wide array of physical abnormalities. Around 9% of FA patients develop acute myeloid leukemia (AML), which makes FA a good genetic model to study leukemogenesis. To date, however, no information exists on the functional integrity of the hematopoietic system of FA patients during the period in which they develop AML. Herein, we report on the characterization of hematopoietic progenitor cells from a pediatric FA patient that developed AML. Our results show that significant changes occurred in the hematopoietic system of the patient from the time he presented with FA to the time he developed AML. Such changes included marrow cellularity, frequency of CD34(+) cells and CFC, as well as proliferation potential of progenitor cells in liquid cultures supplemented with recombinant cytokines. Interestingly, no significant changes in the karyotype of marrow cells were observed, indicating that progression from FA into AML may proceed without major chromosomal alterations (i.e. translocations and/or deletions). This study represents one of the first steps towards the cellular characterization of the hematopoietic system in FA patients that develop AML.

Amplification and translocation of 3q26 with overexpression of EVI1 in Fanconi anemia-derived childhood acute myeloid leukemia with biallelic FANCD1/BRCA2 disruption

Fanconi anemia (FA) is an inherited disease with congenital abnormalities and an extreme risk of acute myeloid leukemia (AML). Genetic events occurring during malignant transformation in FA and the biology of FA-associated AML are poorly understood, but are often preceded by the development of chromosomal aberrations involving 3q26-29 in bone marrow of FA patients. We report here the molecular cytogenetic characterization of FA-derived AML cell lines SB1685CB and SB1690CB by conventional and array comparative genomic hybridization, fluorescence in situ hybridization, and SKY. We identified gains of a 3.7 MB chromosomal region on 3q26.2-26.31, which preceded transformation to overt leukemia. This region harbors the oncogenic transcription factor EVI1. A third FA-derived cell line, FA-AML1, carried a translocation with ectopic localization of 3q26 including EVI1. Rearrangements of 3q, which are rare in childhood AML, commonly result in overexpression of EVI1, which determines specific gene expression patterns and confers poor prognosis. We detected overexpression of EVI1 in all three FA-derived AML. Our results suggest a link between the FA defect, chromosomal aberrations involving 3q and overexpression of EVI1. We hypothesize that constitutional or acquired FA defects might be a common factor for the development of 3q abnormalities in AML. In addition, cryptic imbalances as detected here might account for overexpression of EVI1 in AML without overt 3q26 rearrangements.

Bone marrow failure as a risk factor for clonal evolution: prospects for leukemia prevention

Patients with bone marrow failure syndromes are at risk for the development of clonal neoplasms, including paroxysmal nocturnal hemoglobinuria (PNH), myelodysplasia (MDS), and acute myelogenous leukemia (AML). Approximately 10% to 20% of those who survive acquired aplastic anemia will develop a clonal disease within the decade following their diagnosis. The relative risk of clonal neoplasms is very significantly increased in children and adults with inherited bone marrow failure syndromes as well. Until recently, the mechanisms underlying clonal evolution have been opaque, but a sufficient amount of evidence has now accumulated to support a model in which cells resistant to extracellular apoptotic cues are selected from the stem cell pool. Indeed, in the past two years this paradigm has been validated in preclinical models that are robust enough to reconsider new therapeutic objectives in aplastic states and to support the planning and development of rationally designed leukemia prevention trials.

Cytogenetic instability in ovarian epithelial cells from women at risk of ovarian cancer

Fanconi anemia is an inherited cancer predisposition disease characterized by cytogenetic and cellular hypersensitivity to cross-linking agents. Seeking evidence of Fanconi anemia protein dysfunction in women at risk of ovarian cancer, we screened ovarian surface epithelial cells from 25 primary cultures established from 22 patients using cross-linker hypersensitivity assays. Samples were obtained from (a) women at high risk for ovarian cancer with histologically normal ovaries, (b) ovarian cancer patients, and (c) a control group with no family history of breast or ovarian cancer. In chromosomal breakage assays, all control cells were mitomycin C (MMC) resistant, but eight samples (five of the six high-risk and three of the eight ovarian cancer) were hypersensitive. Lymphocytes from all eight patients were MMC resistant. Only one of the eight patients had a BRCA1 germ-line mutation and none had BRCA2 mutations, but FANCD2 was reduced in five of the eight. Ectopic expression of normal FANCD2 cDNA increased FANCD2 protein and induced MMC resistance in both hypersensitive lines tested. No FANCD2 coding region or promoter mutations were found, and there was no genomic loss or promoter methylation in any Fanconi anemia genes. Therefore, in high-risk women with no BRCA1 or BRCA2 mutations, tissue-restricted hypersensitivity to cross-linking agents is a frequent finding, and chromosomal breakage responses to MMC may be a sensitive screening strategy because cytogenetic instability identified in this way antedates the onset of carcinoma. Inherited mutations that result in tissue-specific FANCD2 gene suppression may represent a cause of familial ovarian cancer.

Diamond blackfan anemia: New paradigms for a "not so pure" inherited red cell aplasia

Diamond Blackfan anemia (DBA) is a genetically and clinically heterogeneous disorder characterized by erythroid failure, congenital anomalies, and a predisposition to cancer. Faulty ribosome biogenesis is hypothesized to be the underlying defect, leading to erythroid failure due to accelerated apoptosis in affected erythroid progenitors/precursors. Since first observed in DBA, pro-apoptotic hematopoiesis has been recognized as a common mechanism for hematopoietic failure in virtually all of the inherited bone marrow failure syndromes. Inherited as an autosomal dominant trait, one of what appears to be multiple DBA genes, coding for ribosomal protein RPS19, has been cloned. The discovery of additional genes will no doubt clarify the molecular pathophysiology of this disorder. Even within families, individuals may vary dramatically as to the degree of anemia, treatment response, and the presence of congenital anomalies. The study of DBA has been facilitated by the creation of international patient registries that provide more reliable information regarding clinical presentation, genetics, and outcome, as well as descriptions of congenital malformations and cancer predisposition, than can be culled from the literature. Analysis of registry data has led to improvements in clinical care and provides patients and research specimens for clinical and laboratory investigations.

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.

Inherited Bone Marrow Failure Syndromes: Molecular Features

Recent advances resulting from the identification of the genes responsible for four inherited marrow failure syndromes, Fanconi anemia, dyskeratosis congenita, Diamond-Blackfan anemia, and Shwachman-Diamond syndrome, are reviewed. The interpretation of genetic testing should be guided by an understanding of the limitations of such testing for each disorder. The possibility of an inherited basis for marrow failure must be considered for adults as well as children with aplastic anemia. Shared molecular themes are emerging from functional studies of the genes underlying the different inherited disorders. Genomic instability may result from impaired DNA repair in Fanconi anemia or telomere dysregulation in dyskeratosis congenita. Mutations affecting ribosome assembly or function are associated with Diamond-Blackfan anemia, dyskeratosis congenita, and Shwachman-Diamond syndrome. These findings raise new questions about the molecular mechanisms regulating hematopoiesis and leukemogenesis. Clinical implications arising from these molecular studies are explored.

Scientific and clinical opportunities for modeling blood disorders with embryonic stem cells

Our considerable wealth of data concerning hematologic processes has come despite difficulties working with stem and progenitor cells in vitro and their propensity to differentiate. Key methodologies that have sought to overcome such limitations include transgenic/knock-out animals and in vitro studies using murine embryonic stem cells, because both permit investigation of the formation of hematopoietic tissue from nonhematopoietic precursors. Although there have been many successful studies in model animals for understanding hematopoietic-cell development, differences between lower vertebrates and humans have left gaps in our understanding. Clearly, human-specific strategies to study the onset of hematopoiesis, particularly the earliest events leading to the specification of both normal and abnormal hematopoietic tissue, could bring an investigational renaissance. The recent availability of human embryonic stem (hES) cells suggests that such a system is now at hand. This review highlights the potential of hES cells to model human hematologic processes in vitro with an emphasis on disease targets.

Ex vivo culture of Fancc-/- stem/progenitor cells predisposes cells to undergo apoptosis, and surviving stem/progenitor cells display cytogenetic abnormalities and an increased risk of malignancy

Current strategies for genetic therapy using Moloney retroviruses require ex vivo manipulation of hematopoietic cells to facilitate stable integration of the transgene. While many studies have evaluated the impact of ex vivo culture on normal murine and human stem/progenitor cells, the cellular consequences of ex vivo manipulation of stem cells with intrinsic defects in genome stability are incompletely understood. Here we show that ex vivo culture of Fancc(-/-) bone marrow cells results in a time-dependent increase in apoptosis of primitive Fancc(-/-) progenitor cells in conditions that promote the proliferation of wild-type stem/progenitor cells. Further, recipients reconstituted with the surviving Fancc(-/-) cells have a high incidence of cytogenetic abnormalities and myeloid malignancies that are associated with an acquired resistance to tumor necrosis factor alpha (TNF-alpha). Collectively, these data indicate that the intrinsic defects in the genomic stability of Fancc(-/-) stem/progenitor cells provide a selective pressure for cells that are resistant to apoptosis and have a propensity for the evolution to clonal hematopoiesis and malignancy. These studies could have implications for the design of genetic therapies for treatment of Fanconi anemia and potentially other genetic diseases with intrinsic defects in genome stability.

Fanconi anaemia and leukaemia - clinical and molecular aspects

Fanconi anaemia (FA) is an autosomal recessive chromosomal instability disorder, which is characterized by congenital abnormalities, defective haemopoiesis and a high risk of developing acute myeloid leukaemia and certain solid tumours. It can be caused by mutations in at least eight different genes. Molecular studies have established that a common pathway exists, both between the FA proteins and other proteins involved in DNA damage repair such as NBS1, ATM, BRCA1 and BRCA2. This review summarizes the general clinical and specific haematological features and the current management of FA. Recent molecular advances will also be discussed in the context of the cellular and clinical FA phenotype, with particular emphasis on the haematological aspects of the condition.

Deletion and reduced expression of the Fanconi anemia FANCA gene in sporadic acute myeloid leukemia

Fanconi anemia (FA) is an autosomal recessive chromosomal instability disorder caused by mutations in one of seven known genes (FANCA,C,D2,E,F,G and BRCA2). Mutations in the FANCA gene are the most prevalent, accounting for two-thirds of FA cases. Affected individuals have greatly increased risks of acute myeloid leukemia (AML). This raises the question as to whether inherited or acquired mutations in FA genes might be involved in the development of sporadic AML. Quantitative fluorescent PCR was used to screen archival DNA from sporadic AML cases for FANCA deletions, which account for 40% of FANCA mutations in FA homozygotes. Four heterozygous deletions were found in 101 samples screened, which is 35-fold higher than the expected population frequency for germline FANCA deletions (P<0.0001). Sequencing FANCA in the AML samples with FANCA deletions did not detect mutations in the second allele and there was no evidence of epigenetic silencing by hypermethylation. However, real-time quantitative PCR analysis in these samples showed reduced expression of FANCA compared to nondeleted AML samples and to controls. These findings suggest that gene deletions and reduced expression of FANCA may be involved in the promotion of genetic instability in a subset of cases of sporadic AML.

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.

Oxidative stress and the myelodysplastic syndromes

The evolution of higher organisms from anaerobic to aerobic living has promoted an elaborate mechanism of defense against potentially toxic oxidants. Many environmental toxicants implicated in the pathogenesis of myelodysplastic syndromes (MDS), including benzene and ionizing radiation, exert toxicity via pro-oxidant mechanisms. The emerging data suggest a probable genetic susceptibility to environmental carcinogenesis through functional polymorphic variants in enzymes that metabolize toxicants and/or protect against oxidative stress. The most studied enzyme is NAD(P)H:quinone oxidoreductase (NQO1). CD34+ cells from individuals homozygous for the NQO1 C609T nonfunctional allelic variant are incapable of enzyme induction following exposure to benzene, thus potentially increasing the hematotoxicity of benzene metabolites. Serologic and molecular markers of oxidative stress are present in many patients with MDS and include an increased concentration of the lipid peroxidation product malondialdehyde and the presence of oxidized bases in CD34+ cells. Potential mechanisms of oxidative stress include mitochondrial dysfunction via iron overload and mitochondrial DNA mutation, systemic inflammation, and bone marrow stromal defects. The biological activity of the antioxidant aminothiol amifostine in vivo suggests that these pathways may be meaningful targets for future therapy in MDS patients.

Retroviral-mediated expression of recombinant Fancc enhances the repopulating ability of Fancc-/- hematopoietic stem cells and decreases the risk of clonal evolution

Fanconi anemia (FA) is a chromosomal instability disorder characterized by a progressive bone marrow (BM) failure and an increased incidence of myeloid leukemias. Children with FA are currently being enrolled in clinical trials to evaluate the safety of retroviral-mediated gene transfer. Previously, we used Fancc(-/-) mice to show that Fancc(-/-) hematopoietic stem cells (HSCs) have a profound defect in repopulating ability. Here, we examined whether retroviral-mediated gene transfer of recombinant Fancc (rFancc) would restore the repopulating ability of Fancc(-/-) HSC to wild-type levels. Fancc(-/-) HSCs transduced with a retrovirus encoding rFancc exhibited a repopulating ability that approached wild-type levels. Interestingly, approximately 30% of primary recipients (7 of 22) transplanted with uncorrected Fancc(-/-) cells developed a range of hematopoietic abnormalities including pancytopenia and BM hypoplasia similar to individuals with FA. Hematopoietic abnormalities were detected in only 1 of 22 mice transplanted with Fancc(-/-) cells transduced with a retrovirus encoding rFancc. Moreover, several mice with hematopoietic defects had progenitors that displayed a marked resistance to IFN-gamma, TNF-alpha, and MIP-1alpha compared to both Fancc(-/-) progenitors, which are uniquely hypersensitive to these cytokines, and wild-type progenitors. These data are analogous to studies using progenitors from patients with myelodysplasia and provide functional support for clonal evolution in these mice. Collectively, these data show that gene transfer can enhance HSC repopulating ability and suppresses the tendency for clonal evolution. These studies also reveal potential detrimental effects of ex vivo manipulation for untransduced Fancc(-/-) HSCs.

Cancer incidence in persons with Fanconi anemia

Fanconi anemia (FA) is an autosomal recessive condition associated with congenital abnormalities, progressive pancytopenia, and a predisposition to leukemia and solid tumors. We studied a retrospective cohort of North American patients with FA. We calculated relative risks of cancer compared to the general population and cause-specific hazards of the first major adverse outcomes of FA: bone marrow transplantation (BMT) for marrow complications, acute myeloid leukemia (AML), solid tumors, or death from bone marrow failure. We also estimated the cumulative incidence of each adverse event in the presence of the competing risks. Among 145 patients with FA, 9 developed leukemia and 14 developed a total of 18 solid tumors. The ratio of observed to expected cancers (O/E ratio) was 50 for all cancers, 48 for all solid tumors, and 785 for leukemia; these increased risks were statistically significant. The highest solid tumor O/E ratios were 4317 for vulvar cancer, 2362 for esophageal cancer, and 706 for head and neck cancer. Cause-specific hazards of both death and AML peaked at 1%/y in teenage years; the hazard of BMT peaked at 4%/y at age 7. In contrast, the hazard of a solid tumor approached 8%/y by age 40 years. The cumulative incidence to age 48 was 10% for leukemia, 11% for death from marrow failure, 29% for a solid tumor, and 43% for BMT. The risk of a solid tumor may become even higher as death from aplastic anemia is reduced and as patients survive longer after BMT.

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.

Fanconi anaemia

Fanconi anaemia (FA) is an autosomal recessive disease characterised by congenital abnormalities, defective haemopoiesis, and a high risk of developing acute myeloid leukaemia and certain solid tumours. Chromosomal instability, especially on exposure to alkylating agents, may be shown in affected subjects and is the basis for a diagnostic test. FA can be caused by mutations in at least seven different genes. Interaction pathways have been established, both between the FA proteins and other proteins involved in DNA damage repair, such as ATM, BRCA1 and BRCA2, thereby providing a link with other disorders in which defective DNA damage repair is a feature. This review summarises the clinical features of FA and the natural history of the disease, discusses diagnosis and management, and puts the recent molecular advances into the context of the cellular and clinical FA phenotype.