Nf1 haploinsufficiency and Icsbp deficiency synergize in the development of leukemias

Clonal evolution patterns in acute myeloid leukemia with NPM1 mutation

Mutations in the nucleophosmin 1 (NPM1) gene are considered founder mutations in the pathogenesis of acute myeloid leukemia (AML). To characterize the genetic composition of NPM1 mutated (NPM1^mut) AML, we assess mutation status of five recurrently mutated oncogenes in 129 paired NPM1^mut samples obtained at diagnosis and relapse. We find a substantial shift in the genetic pattern from diagnosis to relapse including NPM1^mut loss (n = 11). To better understand these NPM1^mut loss cases, we perform whole exome sequencing (WES) and RNA-Seq. At the time of relapse, NPM1^mut loss patients (pts) feature distinct mutational patterns that share almost no somatic mutation with the corresponding diagnosis sample and impact different signaling pathways. In contrast, profiles of pts with persistent NPM1^mut are reflected by a high overlap of mutations between diagnosis and relapse. Our findings confirm that relapse often originates from persistent leukemic clones, though NPM1^mut loss cases suggest a second "de novo" or treatment-associated AML (tAML) as alternative cause of relapse.

The Granulocyte Progenitor Stage Is a Key Target of IRF8-Mediated Regulation of Myeloid-Derived Suppressor Cell Production

Alterations in myelopoiesis are common across various tumor types, resulting in immature populations termed myeloid-derived suppressor cells (MDSCs). MDSC burden correlates with poorer clinical outcomes, credited to their ability to suppress antitumor immunity. MDSCs consist of two major subsets, monocytic and polymorphonuclear (PMN). Intriguingly, the latter subset predominates in many patients and tumor models, although the mechanisms favoring PMN-MDSC responses remain poorly understood. Ordinarily, lineage-restricted transcription factors regulate myelopoiesis that collectively dictate cell fate. One integral player is IFN regulatory factor (IRF)-8, which promotes monocyte/dendritic cell differentiation while limiting granulocyte development. We recently showed that IRF8 inversely controls MDSC burden in tumor models, particularly the PMN-MDSC subset. However, where IRF8 acts in the pathway of myeloid differentiation to influence PMN-MDSC production has remained unknown. In this study, we showed that: 1) tumor growth was associated with a selective expansion of newly defined IRF8^lo granulocyte progenitors (GPs); 2) tumor-derived GPs had an increased ability to form PMN-MDSCs; 3) tumor-derived GPs shared gene expression patterns with IRF8^-/- GPs, suggesting that IRF8 loss underlies GP expansion; and 4) enforced IRF8 overexpression in vivo selectively constrained tumor-induced GP expansion. These findings support the hypothesis that PMN-MDSCs result from selective expansion of IRF8^lo GPs, and that strategies targeting IRF8 expression may limit their load to improve immunotherapy efficacy.

Regulation of the interferon regulatory factor-8 (IRF-8) tumor suppressor gene by the signal transducer and activator of transcription 5 (STAT5) transcription factor in chronic myeloid leukemia

Tyrosine kinase inhibitors such as imatinib can effectively target the BCR-ABL oncoprotein in a majority of patients with chronic myeloid leukemia (CML). Unfortunately, some patients are resistant primarily to imatinib and others develop drug resistance, prompting interest in the discovery of new drug targets. Although much of this resistance can be explained by the presence of mutations within the tyrosine kinase domain of BCR-ABL, such mutations are not universally identified. Interferon regulatory factor-8 (IRF-8) is a transcription factor that is essential for myelopoiesis. Depressed IRF-8 levels are observed in a majority of CML patients and Irf-8(-/-) mice exhibit a CML-like disease. The underlying mechanisms of IRF-8 loss in CML are unknown. We hypothesized that BCR-ABL suppresses transcription of IRF-8 through STAT5, a proximal BCR-ABL target. Treatment of primary cells from newly diagnosed CML patients in chronic phase as well as BCR-ABL(+) cell lines with imatinib increased IRF-8 transcription. Furthermore, IRF-8 expression in cell line models was necessary for imatinib-induced antitumor responses. We have demonstrated that IRF-8 is a direct target of STAT5 and that silencing of STAT5 induced IRF-8 expression. Conversely, activating STAT5 suppressed IRF-8 transcription. Finally, we showed that STAT5 blockade using a recently discovered antagonist increased IRF-8 expression in patient samples. These data reveal a previously unrecognized BCR-ABL-STAT5-IRF-8 network, which widens the repertoire of potentially new anti-CML targets.

Molecular control of monocyte development

Monocyte development is a tightly regulated and multi-staged process, occurring through several defined progenitor cell intermediates. The key transcription factors, including PU.1, IRF8 and KLF4, growth factors, such as M-CSF and IL-34 and cytokines that drive monocyte development from hematopoietic progenitor cells are well defined. However, the molecular controls that direct differentiation into the Ly6C(hi) inflammatory and Ly6C(lo) monocyte subsets are yet to be completely elucidated. This review will provide a summary of the transcriptional regulation of monocyte development. We will also discuss how these molecular controls are also critical for microglial development despite their distinct haematopoetic origins. Furthermore, we will examine recent breakthroughs in defining mechanisms that promote differentiation of specific monocyte subpopulations.

Interferon regulatory factor-8 is important for histone deacetylase inhibitor-mediated antitumor activity

The notion that epigenetic alterations in neoplasia are reversible has provided the rationale to identify epigenetic modifiers for their ability to induce or enhance tumor cell death. Histone deacetylase inhibitors (HDACi) represent one such class of anti-neoplastic agents. Despite great interest for clinical use, little is known regarding the molecular targets important for response to HDACi-based cancer therapy. We had previously shown that interferon regulatory factor (IRF)-8, originally discovered as a leukemia suppressor gene by regulating apoptosis, also regulates Fas-mediated killing in non-hematologic tumor models. Furthermore, we and others have shown that epigenetic mechanisms are involved in repression of IRF-8 in tumors. Therefore, in our preclinical tumor model, we tested the hypothesis that IRF-8 expression is important for response to HDACi-based antitumor activity. In the majority of experiments, we selected the pan-HDACi, Trichostatin A (TSA), because it was previously shown to restore Fas sensitivity to tumor cells. Overall, we found that: 1) TSA alone and more so in combination with IFN-γ enhanced both IRF-8 expression and Fas-mediated death of tumor cells in vitro; 2) TSA treatment enhanced IRF-8 promoter activity via a STAT1-dependent pathway; and 3) IRF-8 was required for this death response, as tumor cells rendered IRF-8 incompetent were significantly less susceptible to Fas-mediated killing in vitro and to HDACi-mediated antitumor activity in vivo. Thus, IRF-8 status may underlie a novel molecular basis for response to HDACi-based antitumor treatment.

Adverse Prognostic Impact of Abnormal Lesions Detected by Genome-Wide Single Nucleotide Polymorphism Array–Based Karyotyping Analysis in Acute Myeloid Leukemia With Normal Karyotype

Purpose

This study attempted to analyze the prognostic role of single nucleotide polymorphism array (SNP-A) –based karyotying in 133 patients with acute myeloid leukemia with normal karyotype (AML-NK), which presents with diverse clinical outcomes, thus requiring further stratification of patient subgroups according to their prognoses.

Patients and Methods

A total of 133 patients with AML-NK confirmed by metaphase cytogenetics (MC) and fluorescent in situ hybridization analysis were included in this study. Analysis by Genome-Wide Human SNP 6.0 Array was performed by using DNAs derived from marrow samples at diagnosis.

Results

Forty-three patients (32.3%) had at least one abnormal SNP lesion that was not detected by MC. One hundred thirteen abnormal SNP lesions included 55 losses, 23 gains, and 35 copy-neutral losses of heterozygosity. Multivariate analyses showed that detection of abnormal SNP lesions by SNP-A karyotyping results in an unfavorable prognostic value for overall survival (hazard ratio [HR], 2.69; 95% CI, 1.50 to 4.82; P = .001); other significant prognostic factors included secondary AML (HR, 5.55; 95% CI, 1.80 to 17.14; P = .003), presence of the FLT3 mutation (HR, 3.17; 95% CI, 1.71 to 5.87; P < .001), and age (HR, 1.03; 95% CI, 1.01 to 1.05; P = .020).

Conclusion

Our data demonstrated that abnormal SNP lesions detected by SNP-A karyotyping might indicate an adverse prognosis in patients with AML-NK, thus requiring a more sophisticated treatment strategy for improvement of treatment outcomes.

Cutting edge: IRF8 regulates Bax transcription in vivo in primary myeloid cells

A prominent phenotype of IRF8 knockout (KO) mice is the uncontrolled expansion of immature myeloid cells. The molecular mechanism underlying this myeloproliferative syndrome is still elusive. In this study, we observed that Bax expression level is low in bone marrow preginitor cells and increases dramatically in primary myeloid cells in wt mice. In contrast, Bax expression level remained at a low level in primarymyeloid cells in IRF8 KO mice. However, in vitro IRF8 KO bone marrow-differentiated myeloid cells expressed Bax at a level as high as that in wild type myeloid cells. Furthermore, we demonstrated that IRF8 specifically binds to the Bax promoter region in primary myeloid cells. Functional analysis indicated that IRF8 deficiency results in increased resistance of the primary myeloid cells to Fas-mediated apoptosis. Our findings show that IRF8 directly regulates Bax transcription in vivo, but not in vitro during myeloid cell lineage differentiation.

Therapy-related myeloid neoplasms: pathobiology and clinical characteristics

Therapy-related myeloid neoplasms (t-MNs) are serious long-term consequences of cytotoxic treatments for an antecedent disorder. t-MNs are observed after ionizing radiation as well as conventional chemotherapy including alkylating agents, topoisomerase-II-inhibitors and antimetabolites. In addition, adjuvant use of recombinant human granulocyte-colony stimulating factor may also increase the risk of t-MNs. There is clinical and biological overlap between t-MNs and high-risk de novo myelodysplastic syndromes and acute myeloid leukaemia suggesting similar mechanisms of leukaemogenesis. Human studies and animal models point to a prominent role of genetic susceptibilty in the pathogenesis of t-MNs. Common genetic variants have been identified that modulate t-MN risk, and t-MNs have been observed in some cancer predisposition syndromes. In either case, establishing a leukaemic phenotype requires acquisition of somatic mutations - most likely induced by the cytotoxic treatment. Knowledge of the specific nature of the initiating exposure has allowed the identification of crucial pathogenetic mechanisms and for these to be modelled in vitro and in vivo. Prognosis of patients with t-MNs is dismal and at present, the only curative approach for the majority of these individuals is haematopoietic stem cell transplantation, which is characterized by high transplant-related mortality rates. Novel transplantation strategies using reduced intensity conditioning regimens as well as novel drugs - demethylating agents and targeted therapies - await clinical testing and may improve outcome. Ultimately, individual assessment of genetic risk factors may translate into tailored therapies and establish a strategy for reducing t-MN incidences without jeopardizing therapeutic success rates for the primary disorders.

IRF8 regulates acid ceramidase expression to mediate apoptosis and suppresses myelogeneous leukemia

IFN regulatory factor 8 (IRF8) is a key transcription factor for myeloid cell differentiation and its expression is frequently lost in hematopoietic cells of human myeloid leukemia patients. IRF8-deficient mice exhibit uncontrolled clonal expansion of undifferentiated myeloid cells that can progress to a fatal blast crisis, thereby resembling human chronic myelogeneous leukemia (CML). Therefore, IRF8 is a myeloid leukemia suppressor. Whereas the understanding of IRF8 function in CML has recently improved, the molecular mechanisms underlying IRF8 function in CML are still largely unknown. In this study, we identified acid ceramidase (A-CDase) as a general transcription target of IRF8. We demonstrated that IRF8 expression is regulated by IRF8 promoter DNA methylation in myeloid leukemia cells. Restoration of IRF8 expression repressed A-CDase expression, resulting in C16 ceramide accumulation and increased sensitivity of CML cells to FasL-induced apoptosis. In myeloid cells derived from IRF8-deficient mice, A-CDase protein level was dramatically increased. Furthermore, we demonstrated that IRF8 directly binds to the A-CDase promoter. At the functional level, inhibition of A-CDase activity, silencing A-CDase expression, or application of exogenous C16 ceramide sensitized CML cells to FasL-induced apoptosis, whereas overexpression of A-CDase decreased CML cells' sensitivity to FasL-induced apoptosis. Consequently, restoration of IRF8 expression suppressed CML development in vivo at least partially through a Fas-dependent mechanism. In summary, our findings determine the mechanism of IRF8 downregulation in CML cells and they determine a primary pathway of resistance to Fas-mediated apoptosis and disease progression.

Interferon regulatory factor-8-driven myeloid differentiation is regulated by 12/15-lipoxygenase-mediated redox signaling

OBJECTIVE

Several transcription factors determine the cell fate decision between granulocytes and monocytes, but the upstream signal transduction pathways that govern myelopoiesis are largely unknown. Based on our observation of aberrant myeloid cell representation in hematopoietic tissues of 12/15-lipoxygenase (12/15-LOX)-deficient (Alox15) mice, we tested the hypothesis that polyunsaturated fatty acid metabolism regulates myelopoiesis.

MATERIALS AND METHODS

Multicolor flow cytometric analysis and methylcellulose assays were used to compare myelopoiesis and the differentiative capacity of progenitors from Alox15 and wild-type mice. Furthermore, we elucidated the mechanism by which 12/15-LOX is involved in regulation of myelopoiesis.

RESULTS

Granulopoiesis in Alox15 mice is increased while monopoiesis is reduced. Moreover, there is an accumulation of granulocyte-macrophage progenitors that exhibit defective differentiation. Mechanistically, we demonstrate that transcriptional activity of interferon regulatory factor-8 (Irf8), which regulates myelopoiesis, is impaired in Alox15 progenitors and bone marrow-derived macrophages due to loss of 12/15-LOX-mediated redox regulation of Irf8 nuclear accumulation. Restoration of redox signaling in Alox15 bone marrow cells and granulocyte-macrophage progenitors reversed the defect in myeloid differentiation.

CONCLUSIONS

These data establish 12/15-LOX-mediated redox signaling as a novel regulator of myelopoiesis and Irf8.

Cooperation between deficiencies of IRF-4 and IRF-8 promotes both myeloid and lymphoid tumorigenesis

Interferon regulatory factor 4 (IRF-4) plays important functions in B- and T-cell development and immune response regulation and was originally identified as the product of a proto-oncogene involved in chromosomal translocations in multiple myeloma. Although IRF-4 is expressed in myeloid cells, its function in that lineage is not known. The closely related family member IRF-8 is a critical regulator of myelopoiesis, which when deleted in mice results in a syndrome highly similar to human chronic myelogenous leukemia. In early lymphoid development, we have shown previously that IRF-4 and IRF-8 can function redundantly. We therefore investigated the effects of a combined loss of IRF-4 and IRF-8 on hematologic tumorigenesis. We found that mice deficient in both IRF-4 and IRF-8 develop from a very early age a more aggressive chronic myelogenous leukemia-like disease than mice deficient in IRF-8 alone, correlating with a greater expansion of granulocyte-monocyte progenitors. Although these results demonstrate, for the first time, that IRF-4 can function as tumor suppressor in myeloid cells, interestingly, all mice deficient in both IRF-4 and IRF-8 eventually develop and die of a B-lymphoblastic leukemia/lymphoma. Combined losses of IRF-4 and IRF-8 therefore can cooperate in the development of both myeloid and lymphoid tumors.

Wilms' tumor gene 1 protein represses the expression of the tumor suppressor interferon regulatory factor 8 in human hematopoietic progenitors and in leukemic cells

Wilms' tumor gene 1 (WT1) is a transcription factor involved in developmental processes. In adult hematopoiesis, only a small portion of early progenitor cells express WT1, whereas most leukemias show persistently high levels, suggesting an oncogenic role. We have previously characterized oncogenic BCR/ABL1 tyrosine kinase signaling pathways for increased WT1 expression. In this study, we show that overexpression of BCR/ABL1 in CD34+ progenitor cells leads to reduced expression of interferon regulatory factor 8 (IRF8), in addition to increased WT1 expression. Interestingly, IRF8 is known as a tumor suppressor in some leukemias and we investigated whether WT1 might repress IRF8 expression. When analyzed in four leukemia mRNA expression data sets, WT1 and IRF8 were anticorrelated. Upon overexpression in CD34+ progenitors, as well as in U937 cells, WT1 strongly downregulated IRF8 expression. All four major WT1 splice variants induced repression, but not the zinc-finger-deleted WT1 mutant, indicating dependence on DNA binding. A reporter construct with the IRF8 promoter was repressed by WT1, dependent on a putative WT1-response element. Binding of WT1 to the IRF8 promoter was demonstrated by chromatin immunoprecipitation. Our results identify IRF8 as a direct target gene for WT1 and provide a possible mechanism for oncogenic effects of WT1 in leukemia.