Human Fanconi A cells are susceptible to TRAIL-induced apoptosis

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.

CD34+ gene expression profiling of individual children with very severe aplastic anemia indicates a pathogenic role of integrin receptors and the proapoptotic death ligand TRAIL

BACKGROUND Very severe aplastic anemia is characterized by a hypoplastic bone marrow due to destruction of CD34(+) stem cells by autoreactive T cells. Investigation of the pathomechanism by patient-specific gene expression analysis of the attacked stem cells has previously been impractical because of the scarcity of these cells at diagnosis.

DESIGN AND METHODS

Employing unbiased RNA amplification, patient-specific gene expression profiling was carried out for CD34(+) cells from patients newly diagnosed with very severe aplastic anemia (n=13), refractory anemia (n=8) and healthy controls (n=10). These data were compared to profiles of myelodysplastic disease (n=55), including refractory anemia (n=18). To identify possible targets of autoimmune attack, presence of autoreactive antibodies was tested in pre-therapeutic sera of patients with very severe aplastic anemia (n=19).

RESULTS

CD34(+) gene expression profiling distinguished between healthy controls, children with aplastic or refractory anemia and clonal disease. Interferon stimulated genes such as the apoptosis inducing death ligand TRAIL were strongly up-regulated in CD34(+) cells of patients with aplastic anemia, in particular in patients responding to immunosuppressive treatment. In contrast, mRNA expression of integrin GPVI and the integrin complexes GPIa/IIa, GPIIb/IIIa, GPIB/GPIX/GPV was significantly down-regulated and corresponding antibodies were detected in 7 of 11 profiled patients and in 11 of 19 aplastic anemia patients. CONCLUSIONS As a potential diagnostic tool, patient-specific gene expression profiling of CD34(+) stem cells made it possible to make the difficult differential diagnosis of most patients with aplastic and refractory anemia. Profiling indicated a prognostic correlation of TRAIL expression and patient benefit from immunosuppressive therapy. Downregulation of integrin expression and concurrent presence of autoreactive anti-integrin-antibodies suggested a previously unrecognized pathological role of integrins in aplastic anemia.

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

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

Following TRAIL's path in the immune system

The members of the tumour necrosis factor (TNF) superfamily of cytokines play important roles in the regulation of various immune-cell functions. Likewise, induction of cell death by apoptosis is indispensable for the normal functioning of the immune system. There are two major pathways of apoptosis induction. The intrinsic, or mitochondrial, pathway is regulated by the activation and interaction of members of the Bcl-2 family. The extrinsic, or death receptor, pathway is triggered by certain TNF family members when they engage their respective cognate receptors on the surface of the target cell. Hence, cell-to-cell-mediated death signals are induced by activation of these death receptor-ligand systems. Besides TNF itself and the CD95 (Fas/APO-1) ligand (FasL/Apo1L), the TNF-related apoptosis-inducing ligand (TRAIL/Apo2L) belongs to the subfamily of ligands that is responsible for extrinsic induction of cell death. Depending on their status of stimulation, TRAIL can be expressed by various cells of the immune system, amongst them natural killer (NK) cells, T cells, natural killer T cells (NKT cells), dendritic cells and macrophages. TRAIL has been implicated in immunosuppressive, immunoregulatory and immune-effector functions. With respect to pathological challenges, TRAIL and its receptors have been shown to play important roles in the immune response to viral infections and in immune surveillance of tumours and metastases. In this review we summarize the current knowledge on the role of TRAIL and its receptors in the immune system and, based on this, we discuss future directions of research into the diverse functions of this fascinating receptor-ligand system.

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.

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.

Tumor-necrosis-factor-related apoptosis-inducing ligand and the regulation of hematopoiesis

PURPOSE OF REVIEW

This review will focus on the emerging role of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)/TRAIL-receptors in the pathophysiology of hematopoiesis and on the potential therapeutic applications of either recombinant TRAIL or anti-TRAIL-R1/-R2 agonistic antibodies for the treatment of hematological malignancies.

RECENT FINDINGS

While CD34 stem/progenitor cells do not express TRAIL-receptors and are protected from TRAIL-induced apoptosis, accumulating evidence points to a role for elevated expression/release of TRAIL at the bone marrow level in the pathophysiology of aplastic anemia, Fanconi anemia, and myelodysplastic syndromes. In-vitro data show promising synergistic effects of recombinant TRAIL in association with proteasome or histone deacetylase inhibitors, natural compounds or small molecules in the therapy of myeloid and lymphoid malignancies. Moreover, although both recombinant TRAIL and anti-TRAIL-R1/-R2 antibodies are well tolerated in vivo, anti-TRAIL-R1/-R2 agonistic antibodies show the potential advantage of avoiding the neutralizing activity of the soluble receptor osteoprotegerin.

SUMMARY

While a chronic pathological elevation of TRAIL at the bone marrow level might contribute to the impairment of normal hematopoiesis, the use of recombinant TRAIL and anti-TRAIL-R1/-R2 agonistic antibodies appears particularly promising for the treatment of hematological malignancies in particular, of multiple myeloma, especially if used in association with innovative therapeutic compounds.

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.