The effects of Bcr-Abl on C/EBP transcription-factor regulation and neutrophilic differentiation are reversed by the Abl kinase inhibitor imatinib mesylate

TIF1β activates leukemic transcriptional program in HSCs and promotes BCR::ABL1-induced myeloid leukemia

TIF1β/KAP1/TRIM28, a chromatin modulator, both represses and activates the transcription of genes in normal and malignant cells. Analyses of datasets on leukemia patients revealed that the expression level of TIF1β was increased in patients with chronic myeloid leukemia at the blast crisis and acute myeloid leukemia. We generated a BCR::ABL1 conditional knock-in (KI) mouse model, which developed aggressive myeloid leukemia, and demonstrated that the deletion of the Tif1β gene inhibited the progression of myeloid leukemia and showed longer survival than that in BCR::ABL1 KI mice, suggesting that Tif1β drove the progression of BCR::ABL1-induced leukemia. In addition, the deletion of Tif1β sensitized BCR::ABL1 KI leukemic cells to dasatinib. The deletion of Tif1β decreased the expression levels of TIF1β-target genes and chromatin accessibility peaks enriched with the Fosl1-binding motif in BCR::ABL1 KI stem cells. TIF1β directly bound to the promoters of proliferation genes, such as FOSL1, in human BCR::ABL1 cells, in which TIF1β and FOSL1 bound to adjacent regions of chromatin. Since the expression of Fosl1 was critical for the enhanced growth of BCR::ABL1 KI cells, Tif1β and Fosl1 interacted to activate the leukemic transcriptional program in and cellular function of BCR::ABL1 KI stem cells and drove the progression of myeloid leukemia.

Histone chaperone ASF1A accelerates chronic myeloid leukemia blast crisis by activating Notch signaling

The blast crisis (BC) is the final deadly phase of chronic myeloid leukemia (CML), which remains a major challenge in clinical management. However, the underlying molecular mechanism driving blastic transformation remains unclear. Here, we show that ASF1A, an essential activator, enhanced the transformation to CML-BC by mediating cell differentiation arrest. ASF1A expression was aberrantly increased in bone marrow samples from CML-BC patients compared with newly diagnosed CML-chronic phase (CP) patients. ASF1A inhibited cell differentiation and promoted CML development in vivo. Mechanistically, we identified ASF1A as a coactivator of the Notch transcriptional complex that induces H3K56ac modification in the promoter regions of Notch target genes, and subsequently enhanced RBPJ binding to these promoter regions, thereby enhancing Notch signaling activation to mediate differentiation arrest in CML cells. Thus, our work suggests that targeting ASF1A might represent a promising therapeutic approach and a biomarker to detect disease progression in CML patients.

PI3K/mTOR dual-inhibition with VS-5584 enhances anti-leukemic efficacy of ponatinib in blasts and Ph-negative LSCs of chronic myeloid leukemia

Ponatinib is used for advanced treatment of chronic myeloid leukemia (CML), although low doses to prevent side effects do not suppress survival pathways and eradicate leukemia stem cells (LSCs). We evaluated the potential of ponatinib and PI3K/mTOR dual-inhibitor VS-5584 combination (PoVS) therapy to increase the anti-leukemic effects of ponatinib and investigated the underlying mechanisms at the molecular level. We measured the cytotoxicities of ponatinib, VS-5584, and PoVS (CCK-8 assay), and used the median-effect equation for combination analyses. We investigated the effects of inhibitory concentrations on apoptosis, cell viability and cell-cycle regulation (flow cytometry), protein levels (ELISA, Western blot), transcriptional activities (dual-luciferase reporter assay), gene expressions (qRT-PCR). VS-5584 exerted selective cytotoxic effects against CML and LSC cell lines. VS-5584 inhibited the PI3K/Akt/mTOR pathway, resulting in reduced cell viability, slightly induced caspase-independent apoptosis, prominent G₀/G₁ cell-cycle blockade that is not a consequence of quiescence. Normal hematopoietic stem cell line was the least affected. Moreover, ponatinib and VS-5584 mediated synergistic anti-leukemic effects on leukemic cells. VS-5584 reduced the ponatinib dose required to target leukemic cells. PoVS treatment inhibited PI3K/Akt/mTOR pathway more consistently than either of the two agents alone through reducing p-Akt, p-mTOR, p-S6K, p-PRAS40, p-S6. The subsequent downstream effects were an increase in C/EBP transcriptional activity and decreases in activities of E2F/DP1, Myc/Max, CREB, STAT3, NFκB, AP-1, Elk-1/SRF. Transcriptional regulation resulted in alterations in the expression levels of target mRNAs. Our results highlight PoVS can be a promising treatment strategy for eliminating CML cells and LSCs selectively, with the reduced ponatinib doses.

SIRT7: an influence factor in healthy aging and the development of age-dependent myeloid stem-cell disorders

Molecular alterations within the hematopoietic system influence cellular longevity and development of age-related myeloid stem-cell disorders like acute myeloid leukemia (AML) and chronic myeloid leukemia (CML). A reduced SIRT7-expression in aged murine hematopoietic stem cells (HSC) resulted in reduced longevity and increased proliferation. In this study we investigated age-related changes of SIRT7-expression in healthy humans and relevant pathomechanisms in AML and CML. SIRT7-expression in leukocytes of healthy people decreased in an age-dependent manner. Low SIRT7 mRNA levels were also detected in AML and CML patients. With positive treatment response, SIRT7-expression increased, but showed reduction when patients progressed or relapsed. Pharmacologic inhibition of driver mutations in AML (FLT3-ITD) or CML (BCR-ABL) also restored SIRT7 levels in cell lines and patient samples. Furthermore, SIRT7-expression increased with time during PMA-mediated monocyte differentiation of THP-1 cells. SIRT7-overexpression in THP-1 cells resulted in increased expression of differentiation markers. BCR-ABL, FLT3-ITD, and differentiation-associated SIRT7-expression in general were positively regulated by C/EBPα, -β, and -ε binding to two different C/EBP-binding sites within the SIRT7 promoter. SIRT7 is important in human hematopoietic cell aging and longevity. It might act as tumor suppressor and could potentially serve as general biomarker for monitoring treatment response in myeloid stem-cell disorders.

A Small Molecule Inhibitor, OGP46, Is Effective against Imatinib-Resistant BCR-ABL Mutations via the BCR-ABL/JAK-STAT Pathway

Chronic myeloid leukemia (CML) is caused by the Philadelphia (Ph⁺) chromosome carrying the BCR-ABL oncogene, a constitutively active tyrosine kinase. The discovery of imatinib represents a major success story in the treatment against CML. However, mutations in the BCR-ABL kinase domain are a major cause of resistance to imatinib, demonstrating that BCR-ABL remains a critical drug target. Here, we investigate a novel small molecule inhibitor, OGP46, for its inhibitory activity against K562, a panel of murine BaF3 cell lines stably expressing either wild-type BCR-ABL or its mutant forms, including T315I. OGP46 exhibits potent activity against imatinib-resistant BCR-ABL mutations, including T315I. OGP46 induced cell differentiation accompanied by G0/G1 cell-cycle arrest and suppressed the colony formation capacity of cells. Treatment with OGP46 significantly decreased the mRNA and protein expression of BCR-ABL in K562 and BaF3-p210-T315I cells. Mechanistically, the anti-cancer activity of OGP46 induced by cell differentiation is likely through the BCR-ABL/JAK-STAT pathway in native BCR-ABL and mutant BCR-ABL, including T315I, of CML cells. Our findings highlight that OGP46 is active against not only native BCR-ABL but also 11 clinically relevant BCR-ABL mutations, including T315I mutation, which are resistant to imatinib. Thus, OGP46 may be a novel strategy for overcoming imatinib-resistance BCR-ABL mutations, including T315I.

Differential gene expression changes and their implication on the disease progression in patients with Chronic Myeloid Leukemia

The molecular mechanisms responsible for disease progression of CML are not conclusive. The main functional changes associated with disease evolution in CML was high proliferation rate, decreased apoptosis, blockade of differentiation, and strong resistance to chemotherapeutic agents. The current study analyzed the relative expressional profiles of genes related with proliferation, apoptosis, differentiation, and resistance to chemotherapeutic agents such as c-MYC, BAD, BCL-2, C/EBPα/-β and ABCB1 respectively in different clinical stages of CML by SYBR Green I quantitative real-time (qRT) PCR. We selected a total of 183 CML patients and 30 healthy control samples. The study populations were classified into four groups, including de novo CML-CP (50/183), CML-AP (32/183), CML-BC (51/183) and Imatinib Mesylate or IM resistant CML-CP (50/183) groups. qRT PCR analysis revealed that significant overexpression of c-MYC, ABCB1 and BCL-2 was observed in advanced phases and IM resistant CP of CML compared to healthy controls. Likewise, the mean expression level of BAD, C/EBPα/-β genes were found to be significantly down regulated. Present study concluded that the complex interplay of several candidate genes like overexpression of c-MYC, ABCB1, BCL-2 and down regulation of BAD, C/EBPα/-β played a significant role in the disease evolution and development of drug resistant in CML.

De novo UBE2A mutations are recurrently acquired during chronic myeloid leukemia progression and interfere with myeloid differentiation pathways

Despite the advent of tyrosine kinase inhibitors, a proportion of chronic myeloid leukemia patients in chronic phase fail to respond to imatinib or to second-generation inhibitors and progress to blast crisis. Until now, improvements in the understanding of the molecular mechanisms responsible for chronic myeloid leukemia transformation from chronic phase to the aggressive blast crisis remain limited. Here we present a large parallel sequencing analysis of 10 blast crisis samples and of the corresponding autologous chronic phase controls that reveals, for the first time, recurrent mutations affecting the ubiquitin-conjugating enzyme E2A gene (UBE2A, formerly RAD6A). Additional analyses on a cohort of 24 blast crisis, 41 chronic phase as well as 40 acute myeloid leukemia and 38 atypical chronic myeloid leukemia patients at onset confirmed that UBE2A mutations are specifically acquired during chronic myeloid leukemia progression, with a frequency of 16.7% in advanced phases. In vitro studies show that the mutations here described cause a decrease in UBE2A activity, leading to an impairment of myeloid differentiation in chronic myeloid leukemia cells.

Paradoxical counteraction by imatinib against cell death in myeloid progenitor 32D cells expressing p210BCR-ABL

Chronic myeloid leukemia (CML) is believed to be caused by the tyrosine kinase p210BCR-ABL, which exhibits growth-promoting and anti-apoptotic activities. However, mechanisms that allow cell differentiation in CML still remain elusive. Here we established tetracycline (Tet)-regulatable p210BCR-ABL-expressing murine 32D myeloid progenitor (32D/TetOff-p210) cells to explore p210BCR-ABL-induced cell death and differentiation. Tet-regulatable overexpression of p210BCR-ABL induced cell death due to the activation of both caspase-1 and caspase-3, coincident with the differentiation from myeloid progenitors into CD11b⁺Ly6C⁺Ly6G⁺ cells with segmented nuclei, exemplified as granulocytic myeloid-derived suppressor cells (G-MDSC), and the ability to secrete IL-1β, TNF-α, and S100A8/A9 into the culture supernatant. Treatment with imatinib almost completely abrogated all these phenotypes. Moreover, overexpression of a sensor of activated caspase-1 based on fluorescence resonance energy transfer (FRET) probe enabled us to detect activation of caspase-1 in a human CML cell line, K562. Furthermore, increased numbers of splenic G-MDSC associated with enhancement of S100A8/A9 production were observed in transgenic mice expressing p210BCR-ABL compared with that in wild-type mice. We also propose the novel mode of cell death in this 32D/TetOff-p210 system termed as myeloptosis.

Proliferation and Differentiation of Murine Myeloid Precursor 32D/G-CSF-R Cells

Understanding of the hematopoietic stem and progenitor cell biology has important implications for regenerative medicine and the treatment of hematological pathologies. Despite the most relevant data that can be acquired using in vivo models or primary cultures, the low abundance of hematopoietic stem and progenitor cells considerably restricts the pool of suitable techniques for their investigation. Therefore, the use of cell lines allows sufficient production of biological material for the performance of screenings or assays that require large cell numbers. Here we present a detailed description, readout, and interpretation of proliferation and differentiation assays which are used for the investigation of processes involved in myelopoiesis and neutrophilic differentiation. These experiments employ the 32D/G-CSF-R cytokine dependent murine myeloid cell line, which possesses the ability to proliferate in the presence of IL-3 and differentiate in G-CSF. We provide optimized protocols for handling 32D/G-CSF-R cells and discuss major pitfalls and drawbacks that might compromise the described assays and expected results. Additionally, this article contains protocols for lentiviral and retroviral production, titration, and transduction of 32D/G-CSF-R cells. We demonstrate that genetic manipulation of these cells can be employed to successfully perform functional and molecular studies, which can complement results obtained with primary hematopoietic stem and progenitor cells or in vivo models.

Analysis of the interplay between all-trans retinoic acid and histone deacetylase inhibitors in leukemic cells

The treatment of acute promyelocytic leukemia (APL) with all-trans retinoic acid (ATRA) induces granulocytic differentiation. This process renders APL cells resistant to cytotoxic chemotherapies. Epigenetic regulators of the histone deacetylases (HDACs) family, which comprise four classes (I-IV), critically control the development and progression of APL. We set out to clarify the parameters that determine the interaction between ATRA and histone deacetylase inhibitors (HDACi). Our assays included drugs against class I HDACs (MS-275, VPA, and FK228), pan-HDACi (LBH589, SAHA), and the novel HDAC6-selective compound Marbostat-100. We demonstrate that ATRA protects APL cells from cytotoxic effects of SAHA, MS-275, and Marbostat-100. However, LBH589 and FK228, which have a superior substrate-inhibitor dissociation constant (Ki) for the class I deacetylases HDAC1, 2, 3, are resistant against ATRA-dependent cytoprotective effects. We further show that HDACi evoke DNA damage, measured as induction of phosphorylated histone H2AX and by the comet assay. The ability of ATRA to protect APL cells from the induction of p-H2AX by HDACi is a readout for the cytoprotective effects of ATRA. Moreover, ATRA increases the fraction of cells in the G1 phase, together with an accumulation of the cyclin-dependent kinase inhibitor p21 and a reduced expression of thymidylate synthase (TdS). In contrast, the ATRA-dependent activation of the transcription factors STAT1, NF-κB, and C/EBP hardly influences the responses of APL cells to HDACi. We conclude that the affinity of HDACi for class I HDACs determines whether such drugs can kill naïve and maturated APL cells.

MEF2C and CEBPA: Possible co-regulators in chronic myeloid leukemia disease progression

Chronic myelogenous leukemia (CML), a hematopoietic malignancy, characterized initially by a chronic phase (CP) progresses into blast crisis (BC) with the accumulation of secondary abnormalities. We have reported earlier that MEF2C, a target of miR-223, was significantly up regulated in CML and also showed a negative correlation with miR-223. In this study, gene expression arrays were used to identify the genes regulated by MEF2C during myelopoiesis. Statistical tools were used to understand the correlation between MEF2C and the targets in different phases of CML. Different CML cell lines and CML patient samples were treated with imatinib to study the effect of MEF2C on the target genes. We observed that MEF2C targets a set of myeloid genes including the myeloid transcription factor CEBPA. MEF2C and CEBPA expression patterns are negatively correlated in CML patient samples. We further show that the expression of MEF2C and CEBPA along with CSF3R is sufficient to molecularly classify different stages of CML. Imatinib, the drug of choice for CML, abrogates MEF2C expression and reverses CEBPA repression both in the cell line and the primary cells. We report the existence of a MEF2C and CEBPA correlation in CML disease progression.

The Philadelphia chromosome in leukemogenesis

The truncated chromosome 22 that results from the reciprocal translocation t(9;22)(q34;q11) is known as the Philadelphia chromosome (Ph) and is a hallmark of chronic myeloid leukemia (CML). In leukemia cells, Ph not only impairs the physiological signaling pathways but also disrupts genomic stability. This aberrant fusion gene encodes the breakpoint cluster region-proto-oncogene tyrosine-protein kinase (BCR-ABL1) oncogenic protein with persistently enhanced tyrosine kinase activity. The kinase activity is responsible for maintaining proliferation, inhibiting differentiation, and conferring resistance to cell death. During the progression of CML from the chronic phase to the accelerated phase and then to the blast phase, the expression patterns of different BCR-ABL1 transcripts vary. Each BCR-ABL1 transcript is present in a distinct leukemia phenotype, which predicts both response to therapy and clinical outcome. Besides CML, the Ph is found in acute lymphoblastic leukemia, acute myeloid leukemia, and mixed-phenotype acute leukemia. Here, we provide an overview of the clinical presentation and cellular biology of different phenotypes of Ph-positive leukemia and highlight key findings regarding leukemogenesis.

ETV6-LPXN fusion transcript generated by t(11;12)(q12.1;p13) in a patient with relapsing acute myeloid leukemia with NUP98-HOXA9

ETV6, which encodes an ETS family transcription factor, is frequently rearranged in human leukemias. We show here that a patient with acute myeloid leukemia with t(7;11)(p15;p15) gained, at the time of relapse, t(11;12)(q12.1;p13) with a split ETV6 FISH signal. Using 3'-RACE PCR analysis, we found that ETV6 was fused to LPXN at 11q12.1, which encodes leupaxin. ETV6-LPXN, an in-frame fusion between exon 4 of ETV6 and exon 2 of LPXN, did not transform the interleukin-3-dependent 32D myeloid cell line to cytokine independence; however, an enhanced proliferative response was observed when these cells were treated with G-CSF without inhibition of granulocytic differentiation. The 32D and human leukemia cell lines each transduced with ETV6-LPXN showed enhanced migration towards the chemokine CXCL12. We show here for the first time that LPXN is a fusion partner of ETV6 and present evidence indicating that ETV6-LPXN plays a crucial role in leukemia progression through enhancing the response to G-CSF and CXCL12.

Spred2 modulates the erythroid differentiation induced by imatinib in chronic myeloid leukemia cells

Differentiation induction is currently considered as an alternative strategy for treating chronic myelogenous leukemia (CML). Our previous work has demonstrated that Sprouty-related EVH1 domainprotein2 (Spred2) was involved in imatinib mediated cytotoxicity in CML cells. However, its roles in growth and lineage differentiation of CML cells remain unknown. In this study, we found that CML CD34⁺ cells expressed lower level of Spred2 compared with normal hematopoietic progenitor cells, and adenovirus mediated restoration of Spred2 promoted the erythroid differentiation of CML cells. Imatinib could induce Spred2 expression and enhance erythroid differentiation in K562 cells. However, the imatinib induced erythroid differentiation could be blocked by Spred2 silence using lentiviral vector PLKO.1-shSpred2. Spred2 interference activated phosphorylated-ERK (p-ERK) and inhibited erythroid differentiation, while ERK inhibitor, PD98059, could restore the erythroid differentiation, suggesting Spred2 regulated the erythroid differentiation partly through ERK signaling. Furthermore, Spred2 interference partly restored p-ERK level leading to inhibition of erythroid differentiation in imatinib treated K562 cells. In conclusion, Spred2 was involved in erythroid differentiation of CML cells and participated in imatinib induced erythroid differentiation partly through ERK signaling.

miR-17 deregulates a core RUNX1-miRNA mechanism of CBF acute myeloid leukemia

Background

Core Binding Factor acute myeloid leukemia (CBF-AML) with t(8;21) RUNX1-MTG8 or inv(16) CBFB-MYH11 fusion proteins often show upregulation of wild type or mutated KIT receptor. However, also non-CBF-AML frequently displays upregulated KIT expression. In the first part of this study we show that KIT expression can be also upregulated by miR-17, a regulator of RUNX1, the gene encoding a CBF subunit. Interestingly, both CBF leukemia fusion proteins and miR-17, which targets RUNX1-3′UTR, negatively affect a common core RUNX1-miRNA mechanism that forces myeloid cells into an undifferentiated, KIT-induced, proliferating state. In the second part of this study we took advantage of the conservation of the core RUNX1-miRNA mechanism in mouse and human, to mechanistically demonstrate in a mouse myeloid cell model that increased KIT-induced proliferation is per se a mechanism sufficient to delay myeloid differentiation.

Methods

Human (U937) or mouse (32D) myeloid clonal lines were used, respectively, to test: 1) the effect of RUNX1-MTG8 and CBFB-MYH11 fusion proteins, or upregulation of miR-17, on KIT-induced proliferation and myeloid differentiation, and 2) the effect of upregulation of KIT-induced proliferation per se on myeloid cell differentiation.

Results

In the first part of this study we found that stable miR-17 upregulation affects, like the CBF-AML fusion proteins (RUNX1-MTG8 or CBFB-MYH11), a core RUNX1-miRNA mechanism leading to KIT-induced proliferation of differentiation-arrested U937 myeloid cells. In the second part of the study we harnessed the conservation of this core mechanism in human and mouse to demonstrate that the extent of KIT upregulation in 32D mouse myeloid cells with wild type RUNX1 can per se delay G-CSF-induced differentiation. The integrated information gathered from the two myeloid cell models shows that RUNX1 regulates myeloid differentiation not only by direct transcriptional regulation of coding and non-coding myeloid differentiation functions (e.g. miR-223), but also by modulating KIT-induced proliferation via non-coding miRNAs (e.g. miR-221).

Conclusions

The novelty of this study is dual. On the one hand, miRNAs (e.g. miR-17) can mimic the effects of CBF-AML fusion proteins by affecting a core RUNX1-miRNA mechanism of KIT-induced proliferation of undifferentiated myeloid cells. On the other hand, the extent of KIT-induced proliferation itself can modulate myeloid differentiation of cells with wild type RUNX1 function.

Electronic supplementary material

The online version of this article (doi:10.1186/s12943-014-0283-z) contains supplementary material, which is available to authorized users.

E3 ubiquitin ligase Fbw7 negatively regulates granulocytic differentiation by targeting G-CSFR for degradation

Tight control between activation and attenuation of granulocyte colony stimulating factor receptor (G-CSFR) signaling is essential to regulate survival, proliferation and differentiation of myeloid progenitor cells. Previous studies demonstrated negative regulation of G-CSFR through endosomal-lysosomal routing and ubiquitin-proteasome mediated degradation. However, very few E3 ubiquitin ligases are known to target G-CSFR for ubiquitin-proteasome pathway. Here we identified F-box and WD repeat domain-containing 7 (Fbw7), a substrate recognizing component of Skp-Cullin-F box (SCF) E3 ubiquitin Ligase physically associates with G-CSFR and promotes its ubiquitin-mediated proteasomal degradation. Our data shows that Fbw7 also interacts with and degrades G-CSFR-T718 (a truncated mutant of G-CSFR found in severe congenital neutropenia/acute myeloid leukemia (SCN/AML patients)) though at a quite slower rate compared to G-CSFR. We further show that glycogen synthase kinase 3 beta (GSK3β), like Fbw7 also targets G-CSFR and G-CSFR-T718 for degradation; however, Fbw7 and GSK3β are interdependent in targeting G-CSFR/G-CSFR-T718 for degradation because they are unable to degrade G-CSFR individually when either of them is knocked down. We further show that Fbw7 mediated downregulation of G-CSFR inhibits signal transducer and activator of transcription 3 (STAT3) phosphorylation which is required for G-CSF dependent granulocytic differentiation. In addition, our data also shows that inhibition of Fbw7 restores G-CSFR signaling leading to enhanced STAT3 activity resulting in massive granulocytic differentiation. These data indicate that Fbw7 together with GSK3β negatively regulates G-CSFR expression and its downstream signaling.

Na+/H+ exchanger 1 inhibition contributes to K562 leukaemic cell differentiation

The effect of hypoxia on the differentiation of chronic myeloid leukaemic K562 cells were studied, as was the role of the NHE1 (Na+/H+ exchanger 1). Hypoxia induced differentiation of K562 cells as seen by modifications in their morphological features, up-regulation of C/EBPα (CCAAT/enhancer-binding protein α), and marked IL-8 (interleukin-8) release. Inhibition of NHE1 under hypoxia additionally enhanced the level of C/EBPα and further promoted leukaemic cells differentiation. Pharmacological inhibition of p38 MAPK (mitogen-activated protein kinase) also significantly suppressed C/EBPα expression under hypoxia conditions after NHE1 inhibition. These results indicate the enhancement of hypoxia-induced K562 differentiation by NHE1 inhibition, which may be due to up-regulation of C/EBPα via p38 MAPK signalling pathway, which suggests a possible therapeutic target of NHE1 under hypoxia microenvironment in the treatment of leukaemic diseases.

C/EBPβ promotes BCR-ABL-mediated myeloid expansion and leukemic stem cell exhaustion

The BCR-ABL fusion oncoprotein accelerates differentiation and proliferation of myeloid cells during the chronic phase of chronic myeloid leukemia (CP-CML). Here, the role of CCAAT/enhancer binding protein β (C/EBPβ), a regulator for 'emergency granulopoiesis,' in the pathogenesis of CP-CML was examined. C/EBPβ expression was upregulated in Lineage(-) CD34(+) CD38(-) hematopoietic stem cells (HSCs) and myeloid progenitors isolated from bone marrow of patients with CP-CML. In EML cells, a mouse HSC line, BCR-ABL upregulated C/EBPβ, at least in part, through the activation of STAT5. Myeloid differentiation and proliferation induced by BCR-ABL was significantly impaired in C/EBPβ-deficient bone marrow cells in vitro. Mice that were transplanted with BCR-ABL-transduced C/EBPβ knockout bone marrow cells survived longer than mice that received BCR-ABL-transduced wild-type (WT) bone marrow cells. Significantly higher levels of leukemic stem cells were maintained in BCR-ABL-transduced C/EBPβ-deficient cells than in BCR-ABL-transduced WT cells. These results suggest that C/EBPβ is involved in BCR-ABL-mediated myeloid expansion. Further elucidation of the molecular mechanisms underlying the C/EBPβ-mediated stem cell loss might reveal a novel therapeutic strategy for eradication of CML stem cells.

Chronic myeloid leukemia: mechanisms of blastic transformation

The BCR-ABL1 oncoprotein transforms pluripotent HSCs and initiates chronic myeloid leukemia (CML). Patients with early phase (also known as chronic phase [CP]) disease usually respond to treatment with ABL tyrosine kinase inhibitors (TKIs), although some patients who respond initially later become resistant. In most patients, TKIs reduce the leukemia cell load substantially, but the cells from which the leukemia cells are derived during CP (so-called leukemia stem cells [LSCs]) are intrinsically insensitive to TKIs and survive long term. LSCs or their progeny can acquire additional genetic and/or epigenetic changes that cause the leukemia to transform from CP to a more advanced phase, which has been subclassified as either accelerated phase or blastic phase disease. The latter responds poorly to treatment and is usually fatal. Here, we discuss what is known about the molecular mechanisms leading to blastic transformation of CML and propose some novel therapeutic approaches.

BCR-ABL promotes neutrophil differentiation in the chronic phase of chronic myeloid leukemia by downregulating c-Jun expression

The mechanism that is responsible for mature neutrophil overproduction in the chronic phase (CP) of chronic myeloid leukemia (CML), a neoplastic disease of hematopoietic stem cells carrying a constitutively active tyrosine kinase BCR-ABL, remains obscure. In this study, microarray analysis revealed that c-Jun, a monopoiesis-promoting transcription factor, was downregulated in CML neutrophils. BCR-ABL directly inhibited c-Jun expression, as c-Jun downregulation in primary CML neutrophils and in the CML blast cell lines, KCL22 and K562, was reversed by the tyrosine kinase inhibitor imatinib. We established a myeloid differentiation model in KCL22 cells using zinc-inducible CCAAT/enhancer-binding protein (C/EBP)alpha (KCL22/alpha). Myeloid differentiation was observed in C/EBP-induced KCL22/alpha cells. Imatinib-induced c-Jun upregulation promoted the monocytic differentiation of KCL22/alpha cells. c-Jun knockdown in KCL22/alpha cells by a short interfering RNA redirected their differentiation from the monocytic to the neutrophilic lineage, even after imatinib treatment. A blockade of PI3K-Akt signaling with an Akt inhibitor upregulated c-Jun and induced the monocytic differentiation of KCL22, K562, and C/EBP-induced KCL22/alpha cells. Thus, BCR-ABL downregulates c-Jun expression by activating the PI3K-Akt pathway during CML-CP, thereby allowing C/EBPs to promote neutrophil differentiation.

Data-driven modeling of cellular stimulation, signaling and output response in RAW 264.7 cells

Background

Understanding the relative importance of signaling pathway components which regulate a specific cellular response is a major focus of current efforts in biology. This interest, along with the inherit complexity of these systems, is driving the development of approaches capable of providing both quantitative predictions as well as guiding the design of future experiments. Of particular interest is the establishment of methods for the analysis of cellular-level input-output signaling relationships that have been characterized over time.

Results

Work by the Alliance for Cellular Signaling (AfCS) has provided an extensive profile of ligand-induced changes in protein phosphorylation state and cytokine output response in macrophage-like RAW 264.7 cells. Using model averaging with partial least squares (PLS) or principal components regression (PCR), we compared multivariate models quantitatively predicting cytokine release and identifying key regulatory components of the underlying signaling pathways. We paid particular attention to the effect of metrics extracted from the experimentally derived signaling time courses so as to determine whether the inclusion of such temporal information improved model predictions. Results indicate that we were able to determine the key biological predictors responsible for generating a specific cytokine response, with model R² values ranging from 0.48 to 0.93. Furthermore, for this data set, the use of time metrics was found to be of mixed value, with increased and/or more appropriate sampling likely being required to improve predictive performance.

Conclusion

The use of multivariate approaches and model averaging provides a valuable predictive framework for quantitative studies of these complex biological processes. Results of this work also point to several issues for consideration in the design of similar large-scale interrogations.

Role of p21(WAF1/CIP1) as an attenuator of both proliferative and drug-induced apoptotic signals in BCR-ABL-transformed hematopoietic cells

The constitutive tyrosine kinase activity of the BCR-ABL fusion protein plays a crucial role in the pathogenesis of chronic myeloid leukemia and promotes growth factor-independent survival of hematopoietic cells. In 32D cells, expression levels of retrovirally transduced BCR-ABL were positively correlated with the levels of the cell cycle regulator protein p21, and this upregulation of p21 expression depended on the kinase activity of BCR-ABL. To assess the role of p21 on BCR-ABL-positive hematopoietic cells, we compared proliferation and drug-induced apoptosis in bone marrow (BM) cells from wild-type and p21 knockout mice after retroviral transfer of the BCR-ABL fusion gene. As compared with wild-type cells, p21 knockout cells showed increased proliferation, suggesting that p21 acted as an attenuator of BCR-ABL-mediated cell proliferation. In marked contrast, deletion of p21 promoted apoptosis induction by imatinib and taxol in BCR-ABL-transformed BM cells. These findings demonstrate that p21 has a dual function in BCR-ABL-transformed murine BM cells: It attenuates the effects of two apparently opposed phenomena such as BCR-ABL-mediated cell proliferation and drug-induced apoptosis. This dual function of p21 calls for a cautious evaluation of the suitability of p21 as a secondary target in anticancer therapy.

High levels of the BCR/ABL oncoprotein are required for the MAPK-hnRNP-E2 dependent suppression of C/EBPalpha-driven myeloid differentiation

The inability of myeloid chronic myelogenous leukemia blast crisis (CML-BC) progenitors to undergo neutrophil differentiation depends on suppression of C/EBPalpha expression through the translation inhibitory activity of the RNA-binding protein hnRNP-E2. Here we show that "oncogene dosage" is a determinant factor for suppression of differentiation in CML-BC. In fact, high levels of p210-BCR/ABL are required for enhanced hnRNP-E2 expression, which depends on phosphorylation of hnRNP-E2 serines 173, 189, and 272 and threonine 213 by the BCR/ABL-activated MAPK(ERK1/2). Serine/threonine to alanine substitution abolishes hnRNP-E2 phosphorylation and markedly decreases its stability in BCR/ABL-expressing myeloid precursors. Similarly, pharmacologic inhibition of MAPK(ERK1/2) activity decreases hnRNP-E2 binding to the 5'UTR of C/EBPalpha mRNA by impairing hnRNP-E2 phosphorylation and stability. This, in turn, restores in vitro and/or in vivo C/EBPalpha expression and G-CSF-driven neutrophilic maturation of differentiation-arrested BCR/ABL(+) cell lines, primary CML-BC(CD34+) patient cells and lineage-negative mouse bone marrow cells expressing high levels of p210-BCR/ABL. Thus, increased BCR/ABL oncogenic tyrosine kinase activity is essential for suppression of myeloid differentiation of CML-BC progenitors as it is required for sustained activation of the MAPK(ERK1/2)-hnRNP-E2-C/EBPalpha differentiation-inhibitory pathway. Furthermore, these findings suggest the inclusion of clinically relevant MAPK inhibitors in the therapy of CML-BC.

p210(BCR-ABL) reprograms transformed and normal human megakaryocytic progenitor cells into erythroid cells and suppresses FLI-1 transcription

The BCR-ABL oncoprotein exhibits deregulated protein tyrosine kinase activity and is implicated in the pathogenesis of Philadelphia chromosome (Ph)-positive human leukemias. Here, we report that ectopic expression of p210(BCR-ABL) in the megakaryoblastic Mo7e cell line and in primary human CD34(+) progenitors trigger erythroid differentiation at the expense of megakaryocyte (MK) differentiation. Clonal culture of purified CD41(+)CD42(-) cells, a population highly enriched in MK progenitors, combined with the conditional expression of p210(BCR-ABL) tyrosine kinase activity by imatinib identified a true lineage reprogramming. In both Mo7e or CD41(+)CD42(-) cells transduced with p210(BCR-ABL), lineage switching was associated with a downregulation of the friend leukemia Integration 1 (FLI-1) transcription factor. Re-expression of FLI-1 in p210(BCR-ABL)-transduced Mo7e cells rescued the megakaryoblastic phenotype. Altogether, these results demonstrate that alteration of signal transduction via p210(BCR-ABL) reprograms MK cells into erythroid cells by a downregulation of FLI-1. In addition, our findings underscore the role of kinases in lineage choice and infidelity in pathology and suggest that downregulation of FLI-1 may have important implications in CML pathogenesis.

Expression of CCL9/MIP-1gamma is repressed by BCR/ABL and its restoration suppresses in vivo leukemogenesis of 32D-BCR/ABL cells

Transformation of hematopoietic cells by the BCR/ABL oncogene is caused by perturbation of signal transduction pathways leading to altered patterns of gene expression and activity. By oligonucleotide microarray hybridization of polysomal RNA of untreated and STI571-treated 32D-BCR/ABL cells, we identified the beta-chemokine CCL9 as a gene regulated by BCR/ABL in a tyrosine kinase-dependent manner. BCR/ABL repressed CCL9 expression at the transcriptional level by mechanisms involving suppression of p38 MAP kinase, and modulation of the activity of CDP/cut and C/EBPalpha, two transcription regulators of myeloid differentiation. However, repression of C/EBP-dependent transcription did not prevent the induction of CCL9 expression by STI571, suggesting that C/EBPalpha is involved in maintaining rather than in inducing CCL9 expression. Restoration of CCL9 expression in 32D-BCR/ABL cells had no effect on the in vitro proliferation of these cells, but reduced their leukemogenic potential in vivo, possibly by recruitment of CD3-positive immune cells. Together, these findings suggest that downregulation of chemokine expression may be involved in BCR/ABL-dependent leukemogenesis by altering the relationship between transformed cells and the microenvironment.

Leukemogenesis induced by wild-type and STI571-resistant BCR/ABL is potently suppressed by C/EBPalpha

Chronic phase-to-blast crisis transition in chronic myelogenous leukemia (CML) is associated with differentiation arrest and down-regulation of C/EBPalpha, a transcription factor essential for granulocyte differentiation. Patients with CML in blast crisis (CML-BC) became rapidly resistant to therapy with the breakpoint cluster region-Abelson murine leukemia (BCR/ABL) kinase inhibitor imatinib (STI571) because of mutations in the kinase domain that interfere with drug binding. We show here that the restoration of C/EBPalpha activity in STI571-sensitive or -resistant 32D-BCR/ABL cells induced granulocyte differentiation, inhibited proliferation in vitro and in mice, and suppressed leukemogenesis. Moreover, activation of C/EBPalpha eradicated leukemia in 4 of 10 and in 6 of 7 mice injected with STI571-sensitive or -resistant 32D-BCR/ABL cells, respectively. Differentiation induction and proliferation inhibition were required for optimal suppression of leukemogenesis, as indicated by the effects of p42 C/EBPalpha, which were more potent than those of K298E C/EBPalpha, a mutant defective in DNA binding and transcription activation that failed to induce granulocyte differentiation. Activation of C/EBPalpha in blast cells from 4 patients with CML-BC, including one resistant to STI571 and BMS-354825 and carrying the T315I Abl kinase domain mutation, also induced granulocyte differentiation. Thus, these data indicate that C/EBPalpha has potent antileukemia effects even in cells resistant to ATP-binding competitive tyrosine kinase inhibitors, and they portend the development of anti-leukemia therapies that rely on C/EBPalpha activation.

Inducible activation of CEBPB, a gene negatively regulated by BCR/ABL, inhibits proliferation and promotes differentiation of BCR/ABL-expressing cells

Translational regulation by oncogenic proteins may be a rapid and efficient mechanism to modulate gene expression. We report here the identification of the CEBPB gene as a target of translational regulation in myeloid precursor cells transformed by the BCR/ABL oncogene. Expression of CEBPB was repressed in 32D-BCR/ABL cells and reinduced by imatinib (STI571) via a mechanism that appears to depend on expression of the CUG-repeat RNA-binding protein CUGBP1 and the integrity of the CUG-rich intercistronic region of c/ebpbeta mRNA. Constitutive expression or conditional activation of wild-type CEBPB induced differentiation and inhibited proliferation of 32D-BCR/ABL cells in vitro and in mice, but a DNA binding-deficient CEBPB mutant had no effect. The proliferation-inhibitory effect of CEBPB was, in part, mediated by the CEBPB-induced GADD45A gene. Because expression of CEBPB (and CEBPA) is low in the blast crisis (BC) stage of chronic myelogenous leukemia (CML) and is inversely correlated with BCR/ABL tyrosine kinase levels, these findings point to the therapeutic potential of restoring C/EBP activity in CML-BC and, perhaps, other types of acute leukemia.

C/EBPalpha and the pathophysiology of acute myeloid leukemia

PURPOSE OF REVIEW

The transcription factor C/EBPalpha controls differentiation and proliferation in normal granulopoiesis in a stage-specific manner. Loss of C/EBPalpha function in myeloid cells in vitro and in vivo leads to a block to myeloid differentiation similar to that which is observed in malignant cells from patients with acute myeloid leukemia. The finding of C/EBPalpha alterations in subgroups of acute myeloid leukemia patients suggests a direct link between critically decreased C/EBPalpha function and the development of the disorder.

RECENT FINDINGS

Conditional mouse models provide direct evidence that loss of C/EBPalpha function leads to the accumulation of myeloid blasts in the bone marrow. Targeted disruption of the wild type C/EBPalpha protein, while conserving the dominant-negative 30 kDa isoform of C/EBPalpha, induces an AML-like disease in mice. In hematopoietic stem cells C/EBPalpha serves to limit cell self-renewal. Finally, C/EBPalpha function is disrupted at different levels in specific subgroups of acute myeloid leukemia patients.

SUMMARY

There is evidence that impaired C/EBPalpha function contributes directly to the development of acute myeloid leukemia. Normal myeloid development and acute myeloid leukemia are now thought to reflect opposite sides of the same hematopoietic coin. Restoring C/EBPalpha function represents a promising target for novel therapeutic strategies in acute myeloid leukemia.

A stochastic model of oncogene expression and the relevance of this model to cancer therapy

Background

Ablation of an oncogene or of the activity of the protein it encodes can result in apoptosis and/or inhibit tumor cell proliferation. Therefore, if the oncogene or set of oncogenes contributing maximally to a tumor cell's survival can be identified, such oncogene(s) are the most appropriate target(s) for maximizing tumor cell kill.

Methods and results

A mathematical model is presented that describes cellular phenotypic entropy as a function of cellular proliferation and/or survival, and states of transformation and differentiation. Oncogenes become part of the cellular machinery, block apoptosis and differentiation or promote proliferation and give rise to new states of cellular transformation. Our model gives a quantitative assessment of the amount of cellular death or growth inhibition that result from the ablation of an oncogene's protein product. We review data from studies of chronic myelogenous leukemia and K562 cells to illustrate these principles.

Conclusion

The model discussed in this paper has implications for oncogene-directed therapies and their use in combination with other therapeutic modalities.

Osteopontin is upregulated by BCR-ABL

Chronic myelogenous leukemia (CML) is characterized by its hallmark oncogene BCR-ABL and the progression from a chronic phase toward an acute leukemia, with a differentiation arrest of the leukemic clone. In the present study, we conducted a microarray analysis using an inducible model of BCR-ABL expression based on the TET-OFF system, and we found that osteopontin (OPN), a component of stem cell niche, is overexpressed in BCR-ABL-expressing cells. Studies using mutant forms of BCR-ABL demonstrated that the BCR-ABL-induced OPN overexpression was a tyrosine kinase-dependent event. Furthermore, OPN concentration was significantly increased in the serum of leukemic mice generated by transplantation of BCR-ABL-expressing bone marrow cells. Most importantly, a significant increase of OPN concentration was observed in the serum of CML patients as compared to controls. Overall these results show that OPN is deregulated by BCR-ABL oncogene and suggest that OPN could be involved in CML stem cell biology.

p210BCR-ABL inhibits SDF-1 chemotactic response via alteration of CXCR4 signaling and down-regulation of CXCR4 expression

It has been shown that p210(BCR-ABL) significantly impairs CXCR4 signaling. We report here that the migratory response to SDF-1 was profoundly altered in blast crisis, whereas chronic-phase CD34(+) cells migrated normally to this chemokine. This migratory defect was associated with a low CXCR4 membrane expression. In vitro STI-571 treatment of CD34(+) cells from patients in blast crisis markedly increased the CXCR4 transcript and CXCR4 membrane expression. Because p210(BCR-ABL) frequently increases with disease progression, we determined the effects of high and low p210(BCR-ABL) expression on CXCR4 protein in the granulocyte macrophage colony-stimulating factor-dependent human cell line MO7e. p210(BCR-ABL) expression distinctly alters CXCR4 protein through two different mechanisms depending on its expression level. At low expression, a signaling defect was detected with no modification of CXCR4 expression. However, higher p210(BCR-ABL) expression induced a marked down-regulation of CXCR4 that is related to its decreased transcription. The effect of p210(BCR-ABL) required its tyrosine kinase activity. Collectively, these data indicate that p210(BCR-ABL) could affect CXCR4 by more than one mechanism and suggest that down-regulation of CXCR4 may have important implications in chronic myelogenous leukemia pathogenesis.

C/EBPdelta expression in a BCR-ABL-positive cell line induces growth arrest and myeloid differentiation

CCAAT/enhancer-binding proteins (C/EBPs) are a family of highly conserved transcription factors that have important roles in normal myelopoiesis as well as associated with myeloid disorders. The chronic myelogenous leukemia (CML) cell lines, KCL22 and K562, express exceptionally low levels of endogenous C/EBPs and provide a good model to test the effects of C/EBPs on myeloid differentiation. To explore the possibility that C/EBPdelta can promote differentiation in BCR-ABL-positive cells, we generated stable KCL22 and K562 clones that expressed an inducible C/EBPdelta gene. C/EBPdelta expression resulted in G0/G1 proliferative arrest and a moderate increase in apoptosis of the KCL22 and the K562 cells. Within 4 days of inducing expression of C/EBPdelta, myeloid differentiation of the CML blast cells occurred as shown by morphologic changes and induction of secondary granule-specific genes. We also showed that during granulocytic differentiation of KCL22 cells, the C/EBPdelta protein was detected in immunocomplexes with both Rb and E2F1. Furthermore, expression of C/EBPdelta was associated with downregulation of c-Myc and cyclin E and upregulation of the cyclin-dependent kinase inhibitor p27(Kip1) in both the KCL22 and K562 cell lines. These results show that expression of C/EBPdelta in BCR-ABL-positive leukemic cells in blast crisis is sufficient for neutrophil differentiation and point to the therapeutic potential of ectopic induction of C/EBPdelta in the acute phase of CML.

The BCR-ABL story: bench to bedside and back

The twenty-first century is beginning with a sharp turn in the field of cancer therapy. Molecular targeted therapies against specific oncogenic events are now possible. The BCR-ABL story represents a notable example of how research from the fields of cytogenetics, retroviral oncology, protein phosphorylation, and small molecule chemical inhibitors can lead to the development of a successful molecular targeted therapy. Imatinib mesylate (Gleevec, STI571, or CP57148B) is a direct inhibitor of ABL (ABL1), ARG (ABL2), KIT, and PDGFR tyrosine kinases. This drug has had a major impact on the treatment of chronic myelogenous leukemia (CML) as well as other blood neoplasias and solid tumors with etiologies based on activation of these tyrosine kinases. Analysis of CML patients resistant to BCR-ABL suppression by Imatinib mesylate coupled with the crystallographic structure of ABL complexed to this inhibitor have shown how structural mutations in ABL can circumvent an otherwise potent anticancer drug. The successes and limitations of Imatinib mesylate hold general lessons for the development of alternative molecular targeted therapies in oncology.

C/EBPepsilon interacts with retinoblastoma and E2F1 during granulopoiesis

CCAAT enhancer binding protein epsilon (C/EBPepsilon) is a myeloid specific transcription factor that is essential for terminal granulocytic differentiation. Retinoblastoma (Rb) and E2F1 are critical cell cycle regulators that also have been implicated in several differentiation systems. Here, we demonstrate that C/EBPepsilon interacts with Rb and E2F1 during granulocytic differentiation in NB4 and U937 human myeloid cells and in 32Dcl3 murine myeloid precursor cells. The interaction between C/EBPepsilon and Rb enhances C/EBPepsilon-mediated transcription of myeloid specific genes both in reporter assays and endogenously. The C/EBPepsilon-E2F1 interaction results in repression of E2F1-mediated transcriptional activity. Finally, overexpression of C/EBPepsilon in human myeloid cells leads to down-regulation of c-Myc. We propose that the interactions between C/EBPepsilon, a tissue-specific transcription factor, and the broad-spectrum proteins, Rb and E2F1, are important in C/EBPepsilon-induced terminal granulocytic differentiation.

Clonal dominance of chronic myelogenous leukemia is associated with diminished sensitivity to the antiproliferative effects of neutrophil elastase

Clinical observations suggest that in chronic myelogenous leukemia (CML), the Philadelphia chromosome (Ph+) clone has a growth advantage over normal hematopoiesis. Patients with CML have high levels of neutrophil elastase, which has recently been shown to antagonize the action of granulocyte-colony-stimulating factor (G-CSF) and other growth factors. We therefore compared the effect of elastase on the growth of normal and CML progenitor cells. In 10-day suspension cultures of normal or CML CD34+ cells supplemented with G-CSF, stem cell factor (SCF), and granulocyte macrophage-colony-stimulating factor (GM-CSF), CML cells had diminished sensitivity to the growth inhibitory effect of elastase. When equal numbers of CML and normal CD34+ cells were cocultured for 10 days, there was no change in the relative proportions of normal and leukemic cells (measured by fluorescence in situ hybridization [FISH] or flow cytometry). However, when elastase was added, CML cells predominated at the end of the culture period (78% vs 22% with 1 microg/mL and 80% vs 20% with 5 microg/mL elastase). CML neutrophils substituted effectively for elastase in suppressing the proliferation of normal CD34+ cells, but this effect was abrogated by serine protease inhibitors. These results suggest that elastase overproduction by the leukemic clone can change the growth environment by digesting growth factors, thereby giving advantage to Ph+ hematopoiesis.

Elevated c-Jun expression in acute myeloid leukemias inhibits C/EBPalpha DNA binding via leucine zipper domain interaction

Transcription factor C/EBPalpha induces normal myeloid differentiation, inactivation of C/EBPalpha leads to a differentiation block in acute myeloid leukemias (AML), and overexpression of C/EBPalpha results in AML growth arrest and differentiation. Recent reports suggest that C/EBPalpha is activated or inactivated via protein-protein interactions. We previously reported that C/EBPalpha needs to inactivate the proto-oncogene c-Jun via leucine zipper domain interaction in order to induce granulocytic differentiation. We, therefore, hypothesized that c-Jun expression might be elevated in AML and subsequently inactivate C/EBPalpha. In fact, compared to normal bone marrow mononuclear cells, c-Jun expression is increased in AML patient samples (Affymetrix expression microarray analysis, n=166). c-Jun binds to C/EBPalpha via the leucine zipper domains and prevents C/EBPalpha from DNA binding. Inactivation of C/EBPalpha by c-Jun is necessary for c-Jun to induce proliferation because c-Jun-induced proliferation can be prevented by ectopic overexpression of C/EBPalpha. The dominant-negative 30-kDa C/EBPalpha protein, found in AML, fails to downregulate c-Jun mRNA expression in AML patient samples. Thus, our data suggest a model for AML in which c-Jun promotes proliferation and prevents differentiation by inhibiting C/EBPalpha DNA binding via leucine zipper domain interaction. It might depend on the expression levels of C/EBPalpha and c-Jun, if inhibition of C/EBPalpha by c-Jun or if inhibition of c-Jun by C/EBPalpha is more predominant: proliferation versus differentiation; AML versus normal myeloid development.

Disruption of differentiation in human cancer: AML shows the way

Although much is understood about the ways in which transcription factors regulate various differentiation systems, and one of the hallmarks of many human cancers is a lack of cellular differentiation, relatively few reports have linked these two processes. Recent studies of acute myeloid leukaemia (AML), however, have indicated how disruption of transcription-factor function can disrupt normal cellular differentiation and lead to cancer. This model involves lineage-specific transcription factors, which are involved in normal haematopoietic differentiation. These factors are often targeted in AML--either by direct mutation or by interference from translocation proteins. Uncovering these underlying pathways will improve the diagnosis and treatment of AML, and provide a working model for other types of human cancer, including solid tumours.