Are PU.1 mutations frequent genetic events in acute myeloid leukemia (AML)?

Genome-scale definition of the transcriptional programme associated with compromised PU.1 activity in acute myeloid leukaemia

Transcriptional dysregulation is associated with haematological malignancy. Although mutations of the key haematopoietic transcription factor PU.1 are rare in human acute myeloid leukaemia (AML), they are common in murine models of radiation-induced AML, and PU.1 downregulation and/or dysfunction has been described in human AML patients carrying the fusion oncogenes RUNX1-ETO and PML-RARA. To study the transcriptional programmes associated with compromised PU.1 activity, we adapted a Pu.1-mutated murine AML cell line with an inducible wild-type PU.1. PU.1 induction caused transition from leukaemia phenotype to monocytic differentiation. Global binding maps for PU.1, CEBPA and the histone mark H3K27Ac with and without PU.1 induction showed that mutant PU.1 retains DNA-binding ability, but the induction of wild-type protein dramatically increases both the number and the height of PU.1-binding peaks. Correlating chromatin immunoprecipitation (ChIP) Seq with gene expression data, we found that PU.1 recruitment coupled with increased histone acetylation induces gene expression and activates a monocyte/macrophage transcriptional programme. PU.1 induction also caused the reorganisation of a subgroup of CEBPA binding peaks. Finally, we show that the PU.1 target gene set defined in our model allows the stratification of primary human AML samples, shedding light on both known and novel AML subtypes that may be driven by PU.1 dysfunction.

The transcriptomic landscape and directed chemical interrogation of MLL-rearranged acute myeloid leukemias

Using next-generation sequencing of primary acute myeloid leukemia (AML) specimens, we identified to our knowledge the first unifying genetic network common to the two subgroups of KMT2A (MLL)-rearranged leukemia, namely having MLL fusions or partial tandem duplications. Within this network, we experimentally confirmed upregulation of the gene with the most subtype-specific increase in expression, LOC100289656, and identified cryptic MLL fusions, including a new MLL-ENAH fusion. We also identified a subset of MLL fusion specimens carrying mutations in SPI1 accompanied by inactivation of its transcriptional network, as well as frequent RAS pathway mutations, which sensitized the leukemias to synthetic lethal interactions between MEK and receptor tyrosine kinase inhibitors. This transcriptomics-based characterization and chemical interrogation of human MLL-rearranged AML was a valuable approach for identifying complementary features that define this disease.

Frequency of acute myeloid leukaemia-associated mouse chromosome 2 deletions in X-ray exposed immature haematopoietic progenitors and stem cells

Exposure to ionising radiation can lead to an increased risk of cancer, particularly leukaemia. In radiation-induced acute myeloid leukaemia (rAML), a partial hemizygous deletion of mouse chromosome 2 is a common feature in several susceptible strains. The deletion is an early event detectable 24h after exposure in bone marrow cells using cytogenetic techniques. Expanding clones of bone marrow cells with chromosome 2 deletions can be detected less than a year after exposure to ionising radiation in around half of the irradiated mice. Ultimately, 15-25% of exposed animals develop AML. It is generally assumed that leukaemia originates in an early progenitor cell or haematopoietic stem cell, but it is unknown whether the original chromosome damage occurs at a similar frequency in committed progenitors and stem cells. In this study, we monitored the frequency of chromosome 2 deletions in immature bone marrow cells (Lin(-)) and haematopoietic stem cells/multipotent progenitor cells (LSK) by several techniques, fluorescent in situ hybridisation (FISH) and through use of a reporter gene model, flow cytometry and colony forming units in spleen (CFU-S) following ex vivo or in vivo exposure. We showed that partial chromosome 2 deletions are present in the LSK subpopulation, but cannot be detected in Lin(-) cells and CFU-S12 cells. Furthermore, we transplanted irradiated Lin(-) or LSK cells into host animals to determine whether specific irradiated cell populations acquire an increased proliferative advantage compared to unirradiated cells. Interestingly, the irradiated LSK subpopulation containing cells carrying chromosome 2 deletions does not appear to repopulate as well as the unirradiated population, suggesting that the chromosomal deletion does not provide an advantage for growth and in vivo repopulation, at least at early stages following occurrence.

Epigenetic aberrations in myeloid malignancies (Review)

The development of novel technologies, such as massively parallel DNA sequencing, has led to the identification of several novel recurrent gene mutations, such as DNA methyltransferase (Dnmt)3a, ten-eleven-translocation oncogene family member 2 (TET2), isocitrate dehydrogenase (IDH)1/2, additional sex comb-like 1 (ASXL1), enhancer of zeste homolog 2 (EZH2) and ubiquitously transcribed tetratricopeptide repeat X chromosome (UTX) mutations in acute myeloid leukemia (AML) and other myeloid malignancies. These findings strongly suggest a link between recurrent genetic alterations and aberrant epigenetic regulations, resulting from an abnormal DNA methylation and histone modification status. This review focuses on the current findings of aberrant epigenetic signatures by these newly described genetic alterations. Moreover, epigenetic aberrations resulting from transcription factor aberrations, such as mixed lineage leukemia (MLL) rearrangement, ecotropic viral integration site 1 (Evi1) overexpression, chromosomal translocations and the downregulation of PU.1 are also described.

5-azacitidine in aggressive myelodysplastic syndromes regulates chromatin structure at PU.1 gene and cell differentiation capacity

Epigenetic 5-azacitidine (AZA) therapy of high-risk myelodysplastic syndromes (MDS) and acute myelogenous leukemia (AML) represents a promising, albeit not fully understood, approach. Hematopoietic transcription factor PU.1 is dynamically regulated by upstream regulatory element (URE), whose deletion causes downregulation of PU.1 leading to AML in mouse. In this study a significant group of the high-risk MDS patients, as well as MDS cell lines, displayed downregulation of PU.1 expression within CD34+ cells, which was associated with DNA methylation of the URE. AZA treatment in vitro significantly demethylated URE, leading to upregulation of PU.1 followed by derepression of its transcriptional targets and onset of myeloid differentiation. Addition of colony-stimulating factors (CSFs; granulocyte-CSF, granulocyte-macrophage-CSF and macrophage-CSF) modulated AZA-mediated effects on reprogramming of histone modifications at the URE and cell differentiation outcome. Our data collectively support the importance of modifying the URE chromatin structure as a regulatory mechanism of AZA-mediated activation of PU.1 and induction of the myeloid program in MDS.

RUNX1 regulates corepressor interactions of PU.1

The transcription factor (TF) RUNX1 cooperates with lineage-specifying TFs (eg, PU.1/SPI1) to activate myeloid differentiation genes, such as macrophage and granulocyte macrophage colony-stimulating factor receptors (MCSFR and GMCSFR). Disruption of cooperative gene activation could contribute to aberrant repression of differentiation genes and leukemogenesis initiated by mutations and translocations of RUNX1. To investigate the mechanisms underlying cooperative gene activation, the effects of Runx1 deficiency were examined in an in vitro model of Pu.1-driven macrophage differentiation and in primary cells. Runx1 deficiency decreased Pu.1-mediated activation of Mcsfr and Gmcsfr, accompanied by decreased histone acetylation at the Mcsfr and Gmcsfr promoters, and increased endogenous corepressor (Eto2, Sin3A, and Hdac2) coimmunoprecipitation with Pu.1. In cotransfection experiments, corepressors were excluded from a multiprotein complex containing full-length RUNX1 and PU.1. However, corepressors interacted with PU.1 if wild-type RUNX1 was replaced with truncated variants associated with leukemia. Histone deacetylase (HDAC) enzyme activity is a major component of corepressor function. HDAC inhibition using suberoylanilide hydroxamic acid or MS-275 significantly increased MCSFR and GMCSFR expression in leukemia cell lines that express PU.1 and mutated or translocated RUNX1. RUNX1 deficiency is associated with persistent corepressor interaction with PU.1. Thus, inhibiting HDAC can partly compensate for the functional consequences of RUNX1 deficiency.

Heterozygous deletion of the PU.1 locus in human AML

The transcription factor PU.1 is essential for myeloid development. Targeted disruption of an upstream regulatory element (URE) decreases PU.1 expression by 80% and leads to acute myeloid leukemia (AML) in mice. Here, we sequenced the URE sequences of PU.1 in 120 AML patients. Four polymorphisms (single nucleotide polymorphisms [SNPs]) in the URE were observed, with homozygosity in all SNPs in 37 patients. Among them, we compared samples at diagnosis and remission, and one patient with cytogenetically normal acute myeloid leukemia M2 was identified with heterozygosity in 3 of the SNPs in the URE at remission. Loss of heterozygosity was further found in this patient at 2 polymorphic sites in the 5′ promoter region and in 2 intronic sites flanking exon 4, thus suggesting loss of heterozygosity covering at least 40 kb of the PU.1 locus. Consistently, PU.1 expression in this patient was markedly reduced. Our study suggests that heterozygous deletion of the PU.1 locus can be associated with human AML.

Lineage-specific transcription factor aberrations in AML

Transcription factors play a key role in the commitment of hematopoietic stem cells to differentiate into specific lineages [78]. This is particularly important in that a block in terminal differentiation is the key contributing factor in acute leukemias. This general theme of the role of transcription factors in differentiation may also extend to other tissues, both in terms of normal development and cancer. Consistent with the role of transcription factors in hematopoietic lineage commitment is the frequent finding of aberrations in transcription factors in AML patients. Here, we intend to review recent findings on aberrations in lineage-restricted transcription factors as observed in patients with acute myeloid leukemia (AML).

Identification of a monopartite sequence in PU.1 essential for nuclear import, DNA-binding and transcription of myeloid-specific genes

The Ets transcription factor PU.1 is an essential regulator of normal hematopoiesis, especially within the myeloid lineage. As such, endogenous PU.1 predominantly localizes to the nucleus of mammalian cells to facilitate gene regulation. However, to date, little is known regarding the mechanisms of PU.1 nuclear transport. We found, using HeLa and RAW 264.7 macrophage cells, that PU.1 enters the nucleus via passive diffusion and active transport. The latter can be facilitated by: (i) the classical pathway requiring importin alpha and beta; (ii) the non-classical pathway requiring only importin beta; or (iii) direct interaction with nucleoporins. A group of six positively charged lysine or arginine residues within the Ets DNA-binding domain was determined to be crucial in active nuclear import. These residues directly interact with importin beta to facilitate a predominantly non-classical import pathway. Furthermore, luciferase reporter assays demonstrated that these same six amino acids are crucial for PU.1-mediated transcriptional activation of myeloid-specific genes. Indeed, these residues may represent a consensus sequence vital for nuclear import, DNA-binding and transcriptional activity of Ets family members. By identifying and characterizing the mechanisms of PU.1 nuclear import and the specific amino acids involved, this report may provide insights into the molecular basis of diseases.

Cooperating gene mutations in acute myeloid leukemia: a review of the literature

Acute myeloid leukemia (AML) is a heterogeneous group of neoplastic disorders with great variability in clinical course and response to therapy, as well as in the genetic and molecular basis of the pathology. Major advances in the understanding of leukemogenesis have been made by the characterization and the study of acquired cytogenetic abnormalities, particularly reciprocal translocations observed in AML. Besides these major cytogenetic abnormalities, gene mutations also constitute key events in AML pathogenesis. In this review, we describe the contribution of known gene mutations to the understanding of AML pathogenesis and their clinical significance. To gain more insight in this understanding, we clustered these alterations in three groups: (1) mutations affecting genes that contribute to cell proliferation (FLT3, c-KIT, RAS, protein tyrosine standard phosphatase nonreceptor 11); (2) mutations affecting genes involved in myeloid differentiation (AML1 and CEBPA) and (3) mutations affecting genes implicated in cell cycle regulation or apoptosis (P53, NPM1). This nonexhaustive review aims to show how gene mutations interact with each other, how they contribute to refine prognosis and how they can be useful for risk-adapted therapeutic management of AML patients.

Radiation leukemogenesis: a proteomic approach

OBJECTIVE

The objectives of this study were to identify protein biomarkers of radiation-induced acute myeloid leukemia (rAML) in CBA/CaJ mice, and to examine the similarities or differences in the patterns of protein-expression profiles among AMLs induced by low linear energy transfer (LET) radiation (e.g., gamma- or x-rays), and high LET radiation (i.e., neutrons).

MATERIALS AND METHODS

We used two-dimensional electrophoresis gel in combination with mass spectrometry (MS), i.e., matrix-assisted laser desorption ionization/time-of-flight MS and electrospray ionization-liquid chromatography/tandem mass spectrometry, to identify protein signatures in blood-plasma samples collected from control and rAML mice. There were nine cases of rAML (three cases induced by high LET radiation; six induced by low LET radiation) and eight control mice at similar ages.

RESULTS

The results showed differences in the patterns of protein profiles from blood-plasma samples collected from rAML vs control mice. Moreover, our data demonstrated, both qualitatively and quantitatively, differences between the plasma protein profiles obtained from mice with AML induced by low vs high LET radiation. Most of the proteins that were present at greater levels in normal samples than in rAML samples were associated with normal metabolism and growth. Several acute-phase proteins were upregulated in rAML samples.

CONCLUSION

The data present, for the first time, evidence for increased expression of clusterin and a loss of gelsolin expression in blood plasma as potential biomarkers of rAML in the CBA/CaJ mouse. Results also indicate that two-dimensional electrophoresis, in combination with MS, is a highly sensitive technique for identification of blood-based biomarkers of rAML.

Lentiviral PU.1 overexpression restores differentiation in myeloid leukemic blasts

PU.1, a transcription factor of the ETS family, plays a pivotal role in normal hematopoiesis, and particularly in myeloid differentiation. Altered PU.1 function is possibly implicated in leukemogenesis, as PU.1 gene mutations were identified in some patients with acute myeloid leukemia (AML) and as several oncogenic products (AML1-ETO, promyelocytic leukemia-retinoic acid receptor alpha, FMS-like receptor tyrosine kinase 3 internal tandem duplication) are associated with PU.1 downregulation. To demonstrate directly a role of PU.1 in the blocked differentiation of leukemic blasts, we transduced cells from myeloid cell lines and primary blasts from AML patients with a lentivector encoding PU.1. In NB4 cells we obtained increases in PU.1 mRNA and protein, comparable to increases obtained with all-trans retinoic acid-stimulation. Transduced cells showed increased myelomonocytic surface antigen expression, decreased proliferation rates and increased apoptosis. Similar results were obtained in primary AML blasts from 12 patients. These phenotypic changes are characteristic of restored blast differentiation. PU.1 should therefore constitute an interesting target for therapeutic intervention in AML.

Transcription factors in myeloid development: balancing differentiation with transformation

In recent years, great progress has been made in elucidating the progenitor-cell hierarchy of the myeloid lineage. Transcription factors have been shown to be key determinants in the orchestration of myeloid identity and differentiation fates. Most transcription factors show cell-lineage-restricted and stage-restricted expression patterns, indicating the requirement for tight regulation of their activities. Moreover, if dysregulated or mutated, these transcription factors cause the differentiation block observed in many myeloid leukaemias. Consequently, therapies designed to restore defective transcription factor functions are an attractive option in the treatment of myeloid and other human cancers.

Is PU.1 a dosage-sensitive regulator of haemopoietic lineage commitment and leukaemogenesis?

The transcription factor PU.1 is an essential regulator of haemopoiesis and a suppressor of myeloid leukaemia. PU.1 displays a complex expression pattern characterized by high expression in myeloid cells and low amounts in lymphoid cells. Based on this transcriptional profile, and the analysis of cell lines and mice expressing altered levels of PU.1, a model has been proposed where the concentration of PU.1 determines cell fate, whereas the graded reduction, but not absence, of PU.1 facilitates leukaemogenesis. The recent reports of mouse strains that enable the accurate determination of PU.1 expression and the conditional inactivation of PU.1 in adult haemopoiesis have led us to re-examine our understanding of the complex functions of PU.1. Here, we will discuss the data that, we believe, argue against the dosage-sensitive model of PU.1-mediated lineage commitment and leukaemogenesis.

Pharmacogenetics of alkylator-associated acute myeloid leukemia

Therapy-related acute myeloid leukemia (t-AML) is a lethal late complication of alkylator chemotherapy. The genetic basis of susceptibility to t-AML is poorly understood. Both t-AML and de novo AML are complex genetic diseases, requiring cooperating mutations in interacting pathways for disease initiation and progression. Germline variants of these ‘leukemia pathway’ genes may cooperate with somatic mutations to induce both de novo and therapy-related AML. Several cancer susceptibility syndromes have been identified that cause an inherited predisposition to de novo and t-AML. The genes responsible for these syndromes are also somatically mutated in sporadic AML. We reason that germline polymorphism in any gene somatically mutated in AML could contribute to t-AML risk in the general population. Identification of these susceptibility alleles should help clinicians develop tailored therapies that reduce the relative risk of t-AML.

Inactivation of PU.1 in adult mice leads to the development of myeloid leukemia

Genetically primed adult C57BL mice were deleted of exon 5 of the gene encoding the transcription factor PU.1 by IFN activation of Cre recombinase. After a 13-week delay, conditionally deleted (PU.1(-/-)) mice began dying of myeloid leukemia, and 95% of the mice surviving from early postinduction death developed transplantable myeloid leukemia whose cells were deleted of PU.1 and uniformly Gr-1 positive. The leukemic cells formed autonomous colonies in semisolid culture with varying clonal efficiency, but colony formation was enhanced by IL-3 and sometimes by granulocyte-macrophage colony-stimulating factor. Nine of 13 tumors analyzed had developed a capacity for autocrine IL-3 or granulocyte-macrophage colony-stimulating factor production, and there was evidence of rearrangement of the IL-3 gene. Acquisition of autocrine growth-factor production and autonomous growth appeared to be major events in the transformation of conditionally deleted PU.1(-/-) cells to fully developed myeloid leukemic populations.

Cooperation of activating Ras/rtk signal transduction pathway mutations and inactivating myeloid differentiation gene mutations in M0 AML: a study of 45 patients

According to a two hit model of leukaemogenesis, the association between acute myeloid leukaemia (AML)1 mutations and FLT3 gene alterations has been recently described in M0 AML. To further document this model in M0 AML, we screened a cohort of 45 patients to find an association between genes implicated in myeloid differentiation (AML1, Pu1) and genes contributing to cell proliferation: (FLT3, N-RAS, K-RAS, c-KIT, PTPN11). No mutation of the Pu1 gene was observed, whereas mutation in the Runt domain of AML1 gene was observed in 12 of 45 patients (27%). No point mutation or insertion-deletion in the c-kit gene was found. Three point mutations (7%) and 11 internal tandem duplications (22%) were seen in FLT3 gene. Two N-Ras and one PTPN11 mutations were found. No significant correlation between AML1 mutation and FLT3 alteration was found. On the other hand, abnormal cytogenetic findings, especially unfavourable ones, were significantly more frequent in patients without detectable molecular abnormality. These findings suggest at least two different pathogenetic pathways in M0 AML: one associated with AML1 mutation, sometimes in combination with the activating lesion of the tyrosine kinase pathway and generally with normal karyotype, and the other with unfavourable cytogenetic findings.

ATRA resolves the differentiation block in t(15;17) acute myeloid leukemia by restoring PU.1 expression

Tightly regulated expression of the transcription factor PU.1 is crucial for normal hematopoiesis. PU.1 knockdown mice develop acute myeloid leukemia (AML), and PU.1 mutations have been observed in some populations of patients with AML. Here we found that conditional expression of promyelocytic leukemia-retinoic acid receptor alpha (PML-RARA), the protein encoded by the t(15;17) translocation found in acute promyelocytic leukemia (APL), suppressed PU.1 expression, while treatment of APL cell lines and primary cells with all-trans retinoic acid (ATRA) restored PU.1 expression and induced neutrophil differentiation. ATRA-induced activation was mediated by a region in the PU.1 promoter to which CEBPB and OCT-1 binding were induced. Finally, conditional expression of PU.1 in human APL cells was sufficient to trigger neutrophil differentiation, whereas reduction of PU.1 by small interfering RNA (siRNA) blocked ATRA-induced neutrophil differentiation. This is the first report to show that PU.1 is suppressed in acute promyelocytic leukemia, and that ATRA restores PU.1 expression in cells harboring t(15;17).

Inverse correlation between Flt3 and PU.1 expression in acute myeloblastic leukemias

Over-expression of the Flt3 is prevalent in acute myeloblastic leukemia (AML), playing a role in leukemogenesis while decreased expression of PU.1 induces AML in mice model. Therefore, we speculated that there is an inverse relationship between these two factors. To clarify this, we measured the expression level of Flt3 and PU.1 in 24 primary AML specimens. As a result, there is a significant negative correlation between Flt3 and PU.1 (r=-0.43, p<0.05). Furthermore, we revealed that flt3 gene promoter is suppressed by the over-expression of PU.1, suggesting that PU.1 is a potential suppressor of flt3 gene promoter.

Role of transcription factors C/EBPalpha and PU.1 in normal hematopoiesis and leukemia

Differentiation of hematopoietic stem and progenitor cells is under strict control of a regulatory network orchestrated by lineage-specific transcription factors. A block in normal differentiation is a major contributing factor in the development of solid tumors and leukemias. Cells from patients with acute myeloid leukemia (AML) frequently harbor mutated or dysregulated transcription factor genes, suggesting their involvement in leukemogenesis. As a consequence, these alterations diminish the pool of available molecules of a small number of critical transcription factors, such as CCAAT enhancer binding proteins, PU.1, GATA-1, and AML-1. In this review, we focus on the mechanisms of how this functional pool of transcription factors is maintained during normal and malignant hematopoiesis, including direct protein-protein interactions, competition for DNA binding, and the control of transcription factor genes by proximal and distal regulatory elements. Results of recent studies of mice carrying hypomorphic PU.1 alleles have indicated that reduction in the expression of a single transcription factor is capable of predisposing mice to AML. The implications of these findings for the study of hematopoiesis in the future as well as novel approaches to more disease-specific therapies are discussed.

Gatekeeper pathways and cellular background in the pathogenesis and therapy of AML

Acute myelogenous leukemia (AML) is characterized by the accumulation of immature cells due to disturbed differentiation and proliferation of the myeloid lineage. Genetic alterations affecting transcription factors and receptor tyrosine kinases have been identified in AML and causally linked to the disease. The goal of this review is to address the role of the different genetic alterations in self-renewal and proliferation and to discuss the cellular background in which these events occur during the pathogenesis of AML. Data from AML samples, clinical studies and mouse models for AML will be used to support the different theories regarding the leukemogenesis of AML. Finally, this review wants to highlight the implication of these findings for the therapy of AML.

Mutations of the PU.1 Ets domain are specifically associated with murine radiation-induced, but not human therapy-related, acute myeloid leukaemia

Murine radiation-induced acute myeloid leukaemia (AML) is characterized by loss of one copy of chromosome 2. Previously, we positioned the critical haematopoietic-specific transcription factor PU.1 within a minimally deleted region. We now report a high frequency (>65%) of missense mutation at codon 235 in the DNA-binding Ets domain of PU.1 in murine AML. Earlier studies, outside the context of malignancy, determined that conversion of arginine 235 (R235) to any other amino-acid residue leads to ablation of DNA-binding function and loss of expression of downstream targets. We show that mutation of R235 does not lead to protein loss, and occurs specifically in those AMLs showing loss of one copy of PU.1 (P=0.001, Fisher's exact test). PU.1 mutations were not found in the coding region, UTRs or promoter of human therapy-related AMLs. Potentially regulatory elements upstream of PU.1 were located but no mutations found. In conclusion, we have identified the cause of murine radiation-induced AML and have shown that loss of one copy of PU.1, as a consequence of flanking radiation-sensitive fragile domains on chromosome 2, and subsequent R235 conversion are highly specific to this mouse model. Such a mechanism does not operate, or is extremely rare, in human AML.

Effect of transcription-factor concentrations on leukemic stem cells

Increasing evidence suggests that leukemias are sustained by leukemic stem cells. However, the molecular pathways underlying the transformation of normal cells into leukemic stem cells are still poorly understood. The involvement of a small group of key transcription factors into this process was suggested by their frequent mutation or down-regulation in patients with acute myeloid leukemia (AML). Recent findings in mice with hypomorphic transcription-factor genes demonstrated that leukemic stem-cell formation in AML could directly be caused by reduced transcription-factor activity beyond a critical threshold. Most interestingly, those experimental models and the paucity of biallelic null mutations or deletions in transcription-factor genes in patients suggest that AML is generally associated with graded down-regulation rather than complete disruption of transcription factors. Here, we discuss the effects of transcription-factor concentrations on hematopoiesis and leukemia, with a focus on the regulation of transcription-factor gene expression as a major mechanism that alters critical threshold levels during blood development and cancer.

Development of a human acute myeloid leukaemia screening panel and consequent identification of novel gene mutation in FLT3 and CCND3

A study was undertaken to develop an acute myeloid leukaemia (AML) screening panel to uncover novel recurring gene mutations. Analysis was performed on six genes known to be mutated in AML (RUNX1, FLT3, KIT, CEBPA, PTPN11 and NRAS) and an additional two candidate genes (CCND3 and FES) in a panel of 175 primary human AML samples that included all French-American-British types except M3, and all cytogenetic risk groups. One hundred and fifteen mutations were identified in 97 (55%) patients comprising 81 patients (46%) with one mutation, 14 patients (8%) with two mutations and two patients (1%) with three mutations. Fifty-five of 88 (63%) patients with normal karyotype AML had at least one mutation. Correlation was observed between KIT mutation and 'favourable risk' cytogenetics (P <0.001), CEBPA mutation and 'intermediate risk' cytogenetics (P=0.045), and PTPN11 mutation and 'poor risk' disease (P <0.001). The frequency of individual gene mutation was in accordance with previously published studies. Three novel mutations of FLT3 were detected (Y589D, D839G, Y842H) that would have been overlooked by conventional gel electrophoresis. A 51-bp deletion was detected in CCND3 in a patient with normal karyotype AML. This validated panel now provides an important tool to evaluate other candidate genes in the genesis of myeloid malignancy.

Carrier-independent nuclear import of the transcription factor PU.1 via RanGTP-stimulated binding to Nup153

PU.1 is a transcription factor of the Ets family with important functions in hematopoietic cell differentiation. Using green fluorescent protein-PU.1 fusions, we show that the Ets DNA binding domain of PU.1 is necessary and sufficient for its nuclear localization. Fluorescence and ultrastructural nuclear import assays showed that PU.1 nuclear import requires energy but not soluble carriers. PU.1 interacted directly with two nucleoporins, Nup62 and Nup153. The binding of PU.1 to Nup153, but not to Nup62, increased dramatically in the presence of RanGMPPNP, indicating the formation of a PU.1.RanGTP.Nup153 complex. The Ets domain accounted for the bulk of the interaction of PU.1 with Nup153 and RanGMPPNP. Because Nup62 is located close to the midplane of the nuclear pore complex whereas Nup153 is at its nuclear side, these findings suggest a model whereby RanGTP propels PU.1 toward the nuclear side of the nuclear pore complex by increasing its affinity for Nup153. This notion was confirmed by ultrastructural studies using gold-labeled PU.1 in permeabilized cells.

A pilot study of high-throughput, sequence-based mutational profiling of primary human acute myeloid leukemia cell genomes

In this pilot study, we used primary human acute myeloid leukemia (AML) cell genomes as templates for exonic PCR amplification, followed by high-throughput resequencing, analyzing approximately 7 million base pairs of DNA from 140 AML samples and 48 controls. We identified six previously described, and seven previously undescribed sequence changes that may be relevant for AML pathogenesis. Because the sequencing templates were generated from primary AML cells, the technique favors the detection of mutations from the most dominant clones within the tumor cell mixture. This strategy represents a viable approach for the detection of potentially relevant, nonrandom mutations in primary human cancer cell genomes.