Clinical implications of molecular genetic aberrations in acute myeloid leukemia

Epigenetic dysregulation in cancers by isocitrate dehydrogenase 2 (IDH2)

Recent advances in genome-wide studies have revealed numerous epigenetic regulations brought about by genes involved in cellular metabolism. Isocitrate dehydrogenase (IDH), an essential enzyme, that converts isocitrate into -ketoglutarate (KG) predominantly in the tricarboxylic acid (TCA) cycle, has gained particular importance due to its cardinal role in the metabolic pathway in cells. IDH1, IDH2, and IDH3 are the three isomeric IDH enzymes that have been shown to regulate cellular metabolism. Of particular importance, IDH2 genes are associated with several cancers, including gliomas, oligodendroglioma, and astrocytomas. These mutations lead to the production of oncometabolite D-2-hydroxyglutarate (D-2-HG), which accumulates in cells promoting tumor growth. The enhanced levels of D-2-HG competitively inhibit α-KG dependent enzymes, inhibiting cell TCA cycle, upregulating the cell growth and survival relevant HIF-1α pathway, promoting DNA hypermethylation related epigenetic activity, all of which synergistically contribute to carcinogenesis. The present review discusses epigenetic mechanisms inIDH2 regulation in cells and further its clinical implications.

Phe354Leu polymorphism of the liver kinase B1 gene as a prognostic factor in adult egyptian patients with acute myeloid leukemia

Background

The human liver kinase B1 (LKB1) gene is a significant tumor suppressor widely expressed in all fetal and adult tissues. Despite its established role in solid tumors, the biological and clinical implications of LKB1 gene alterations in hematological malignancies have not been sufficiently recognized.

Aim

This study aimed to determine the frequency of the LKB1 Phe354Leu polymorphism in adult Egyptian patients with cytogenetically normal AML (CN-AML), evaluate its clinical prognostic significance, and investigate its effect on the therapeutic outcome and patient survival.

Methods

Direct sequencing of amplified exon eight of the LKB1 gene was performed to detect the Phe354Leu polymorphism in 72 adult de novo CN-AML patients.

Results

The LKB1 Phe354Leu polymorphism was detected in 16.7% of patients and associated with younger age and lower hemoglobin levels (p < 0.001). Patients in the mutated group had significantly higher total leukocytic count and bone marrow blasts (p = 0.001 and p < 0.001, respectively). The most common FAB subtypes in mutated patients were M4 and M2. The relapse rate was significantly higher in the mutated group (p = 0.004). There was a significant association between the FLT3-ITD polymorphism and LKB1 F354L (p < 0.001). The mutated group had shorter overall survival (p = 0.003). In multivariate analysis, the Phe354Leu polymorphism was a significant independent prognostic variable for the overall and disease-free survival of the studied patients (p = 0.049).

Conclusion

The LKB1 Phe354Leu polymorphism was diagnosed at younger ages in Egyptian CN-AML patients and represented a poor independent prognostic factor in CN-AML. Patients who carried this polymorphism had shorter overall survival and more frequent relapses. Our findings may provide insight into the design of therapeutic targets, and molecular testing of the LKB1 gene is recommended for proper risk stratification of CN-AML patients.

Identification of Survival-Related Genes in Acute Myeloid Leukemia (AML) Based on Cytogenetically Normal AML Samples Using Weighted Gene Coexpression Network Analysis

The prognosis of acute myeloid leukemia (AML) remains a challenge. In this study, we applied the weighted gene coexpression network analysis (WGCNA) to find survival-specific genes in AML based on 42 adult CN-AML samples from The Cancer Genome Atlas (TCGA) database. Eighteen hub genes (ABCA13, ANXA3, ARG1, BTNL8, C11orf42, CEACAM1, CEACAM3, CHI3L1, CRISP2, CYP4F3, GPR84, HP, LTF, MMP8, OLR1, PADI2, RGL4, and RILPL1) were found to be related to AML patient survival time. We then compared the hub gene expression levels between AML peripheral blood (PB) samples ( $n=162$ ) and control healthy whole blood samples ( $n=337$ ). Seventeen of the hub genes showed lower expression levels in AML PB samples. The gene expression analysis was also done among AML BM (bone marrow) samples of different stages: diagnosis ( $n=142$ ), posttreatment ( $n=42$ ), and recurrent ( $n=12$ ) stages. The results showed a significant increase of ANXA3, CEACM1, RGL4, RILPL1, and HP in posttreatment samples compared to diagnosis and/or recurrent samples. Transcription factor (TF) prediction of the hub genes suggested LTF as the top hit, overlapping 10 hub genes, while LTF itself is just one of the hub genes. Also, 3671 correlation links were shown between 128 mRNAs and 209 lncRNAs found in survival time-related modules. Generally, we identified candidate mRNA biomarkers based on CN-AML data which can be extensively used in AML prognosis. In addition, we mapped their potential regulatory mechanisms with correlated lncRNAs, providing new insights into potential targets for therapies in AML.

Prevalence and Prognostic Value of IDH1 R132 Mutation in Newly Diagnosed AML Egyptian Patients with Normal Karyotype

Mutation in IDH1 gene was suggested to be associated with bad prognosis in cytogenetically normal AML (CN-AML). However, there are conflicting data about its prognostic impact. Besides, its prevalence and prognostic significance in Egyptian patients still not fully stated. We aimed to assess the prevalence of IDH1^R132 mutation in Egyptian CN-AML patients, its correlation with FAB subtypes, and clinical outcome of those patients. Sequencing of amplified IDH1 gene exon four from 50 patients was performed to detect codon R132 point mutation. High prevalence of IDH1 mutation was detected in our patients (9/50, 18 %). Mutated IDH1^R132 was associated with older age and higher platelets count (p = 0.04 and 0.01 respectively). The most common FAB subtype associated with mutated IDH1^R132 was AML-M2 followed by M4. In multivariate analysis, IDH1^R132 mutation was found as independent prognostic variable. It was significantly associated with lower CR and shorter OS (p = 0.06 and 0.009 respectively).

Regain control of p53: Targeting leukemia stem cells by isoform-specific HDAC inhibition

Leukemia stem cells (LSCs) are self-renewable, leukemia-initiating populations that are often resistant to traditional chemotherapy and tyrosine kinase inhibitors currently used for treatment of acute or chronic myeloid leukemia. The persistence and continued acquisition of mutations in resistant LSCs represent a major cause of refractory disease and/or relapse after remission. Understanding the mechanisms regulating LSC growth and survival is critical in devising effective therapies that will improve treatment response and outcome. Several recent studies indicate that the p53 tumor suppressor pathway is often inactivated in de novo myeloid leukemia through oncogenic-specific mechanisms, which converge on aberrant p53 protein deacetylation. Here, we summarize our current understanding of the various mechanisms underlying deregulation of histone deacetylases (HDACs), which could be exploited to restore p53 activity and enhance targeting of LSCs in molecularly defined patient subsets.

Prognostic significance of the BAALC gene expression in adult patients with acute myeloid leukemia: A meta-analysis

Numerous studies have investigated the prognostic role of brain and acute leukemia, cytoplasmic (BAALC) gene expression in adult patients with acute myeloid leukemia (AML); however, the results are inconclusive. A meta-analysis was conducted to provide a comprehensive evaluation of the prognostic role of BAALC gene expression in AML. Eligible studies were searched through PubMed, Embase, the Cochrane Library, the China National Knowledge Infrastructure and the China Biology Medicine Disc. Correlations between the BAALC gene expression and clinicopathological features and prognosis were analyzed. A total of 15 studies were examined. The pooled results suggest that high BAALC expression had an unfavorable outcome in AML. The combined hazard ratio (HR) for overall survival (OS) was 1.53 and the summary HR for the disease-free survival rate was 1.64. In addition, subgroup analyses considering cytogenetic and survival analysis were also conducted. High BAALC gene expression appeared to be an adverse prognostic indicator in patients with cytogenetically normal AML (HR for OS, 1.43) and in subgroups of survival analysis with multivariate analysis (HR for OS, 2.35). These results indicate that high BAALC gene expression served as an independent poor prognostic indicator in adult patients with AML.

The Role of miRNA in Haematological Malignancy

Currently, there are over 1,800 annotated human miRNAs, many of which have tissue-specific expression. Numerous studies have highlighted their role in haematopoietic differentiation and proliferation, acting as master regulators of haematopoietic stem cell function. Aberrant expression of miRNAs has been observed in haematological cancers, exhibiting unique expression signatures in comparison to normal counterparts. Functional and target analyses as well as animal models have attempted to annotate how different miRNA may contribute to the pathophysiology of these malignancies from modulating cancer associated genes, functioning directly as oncogenes or tumour suppressor genes or acting as bystanders or regulators of the epigenetic mechanisms in cancer. miRNAs have also been shown to play a role in modulating drug resistance and determining prognosis between the various subtypes of blood cancers. This review discusses the important role that miRNAs play in haematological malignancies by exploring associations that exist between the two and trying to examine evidence of causality to support the tantalising possibility that miRNAs might serve as therapeutic targets in blood cancers.

WT1 overexpression at diagnosis may predict favorable outcome in patients with de novo non-M3 acute myeloid leukemia

We reviewed the frequency and prognostic significance of FLT3 (fms-like tyrosine kinase receptor-3) and NPM (nucleophosmin) gene mutations and WT1 (Wilms' tumor) and BAALC (brain and acute leukemia, cytoplasmic) gene expression in 100 consecutive patients with intermediate and poor cytogenetic risk de novo acute myeloid leukemia (AML) receiving conventional anthracycline-AraC based therapy. We observed a strict relationship between unfavorable karyotype and BAALC >1000 (p = 0.0001). Multivariate analysis of 81 patients with intermediate karyotype revealed that younger age (p = 0.00009), NPM gene mutation (p = 0.002), and WT1 >75th percentile (>2365) (p = 0.003) were independent, positive factors for complete remission (CR). WT1 expression above 2365 was correlated also to longer event-free survival (EFS) and overall survival (OS) in the same subset of patients (p = 0.003 and p = 0.02, respectively); the same finding occurred in younger patients with AML with intermediate karyotype (p = 0.008 and p = 0.01, respectively). In patients with intermediate karyotype, FLT3 internal tandem duplication (ITD) negatively affected EFS (EFS at 30 months: 30% vs. 6% in FLT3-ITD negative and FLT3 positive patients, respectively; p = 0.01) and OS (OS at 30 months: 38% vs. 20%, p = 0.03). The positive prognostic value of high WT1 expression does not have a clear explanation; it may be implicated either with WT1 anti-oncogenic function, or with the stimulating effect of WT1 oncogene on the leukemic cellular cycle, possibly associated with an enhanced response to chemotherapy.

CREB and leukemogenesis

Acute myeloid leukemia (AML) is one of the most common leukemias with a 20% 5-year event-free survival in adults and 50% overall survival in children, despite aggressive chemotherapy treatment and bone marrow transplantation. The incidence and mortality rates for acute leukemia have only slightly decreased over the last 20 years, and therefore greater understanding of the molecular mechanisms associated with leukemic progression is needed. To this end, a number of transcription factors that appear to play a central role in leukemogenesis are being investigated; among them is the cAMP response element binding protein (CREB). CREB is a transcription factor that can regulate downstream targets involving in various cellular functions including cell proliferation, survival, and differentiation. In several studies, the majority of bone marrow samples from patients with acute lymphoid and myeloid leukemia demonstrate CREB overexpression. Moreover, CREB overexpression is associated with a poor outcome in AML patients. This review summarizes the role of CREB in leukemogenesis.

Novel mutations in CEBPA in Korean Patients with acute myeloid leukemia with a normal karyotype

Mutations in the transcription factor CCAAT/enhancer binding protein α gene (CEBPA) are found in 5-14% of the patients with AML and have been associated with a favorable clinical outcome. In this study, we aimed to assess the frequencies and characteristics of mutations in CEBPA. Between 2006 and 2009, CEBPA mutations were assessed using archival DNA samples obtained from 30 consecutive adult patients diagnosed with AML with a normal karyotype at our institution. CEBPA mutations were detected using direct sequencing analyses. These mutations were detected and described with reference to GenBank Accession No. NM_004364.3. In our series, CEBPA mutations were detected in 4 patients (13.3%). These mutations occurred as double mutations in all 4 patients. Among the 8 mutant alleles, 5 were novel (c.179_180dupCG, c.50_53delGCCA, c.178_182delACGTinsTTT, c.243_244insGTCG, and c.923_924insCTC). The frequency of occurrence of CEBPA mutations in Korean patients with AML is comparable to that in previous reports. Long-term follow-up data from a larger series of patients with comprehensive molecular profiling are needed to delineate the prognostic implications.

PCR for monitoring of minimal residual disease in hematologic malignancy

Monitoring minimal residual disease (MRD) is useful to evaluate therapeutic response and risk of relapse in patients with hematologic malignancy. Currently available quantitative MRD assays are fluorescence in situ hybridization of chromosomal aberrations; multiparameter flow cytometry of leukemia-associated immunophenotypes; and quantitative polymerase chain reaction (qPCR) analysis of fusion genes, immunoglobulin/T-cell receptor gene rearrangements, genetic alterations, or over-expressed genes. Among the PCR-based markers, genetic alterations are found in acute myelogenous leukemia patients with cytogenetically normal karyotype and can be considered as applicable targets for monitoring of MRD. Screening, confirmation and quantification procedures are important to develop the patient- or tumor-specific MRD assays using the PCR-based markers. Wild-type blocking PCR or coamplification at lower denaturing temperature-PCR is suited for screening of low-abundant genetic alterations, and allele-specific qPCR using primers including mismatched base and locked nucleic acids can quantify not only insertion and duplication of several nucleotides but also single nucleotide mutation in the presence of an excess amount of wild-type nucleotides. In addition to the well-established MRD markers, such as immunoglobulin/T-cell receptor gene rearrangements and fusion genes, utilizing potential MRD markers such as genetic alterations may expand the spectrum of patients in whom MRD can be monitored.

Low expression of MN1 associates with better treatment response in older patients with de novo cytogenetically normal acute myeloid leukemia

Low MN1 expression bestows favorable prognosis in younger adults with cytogenetically normal acute myeloid leukemia (CN-AML), but its prognostic significance in older patients is unknown. We analyzed pretherapy MN1 expression in 140 older (≥ 60 years) de novo CN-AML patients treated on cytarabine/daunorubicin-based protocols. Low MN1 expressers had higher complete remission (CR) rates (P = .001), and longer overall survival (P = .03) and event-free survival (EFS; P = .004). In multivariable models, low MN1 expression was associated with better CR rates and EFS. The impact of MN1 expression on overall survival and EFS was predominantly in patients 70 years of age or older, with low MN1 expressers with mutated NPM1 having the best outcome. The impact of MN1 expression was also observed in the Intermediate-I, but not the Favorable group of the European LeukemiaNet classification, where low MN1 expressers had CR rates and EFS similar to those of Favorable group patients. MN1 expresser-status-associated gene- and microRNA-expression signatures revealed underexpression of drug resistance and adverse outcome predictors, and overexpression of HOX genes and HOX-gene-embedded microRNAs in low MN1 expressers. We conclude that low MN1 expression confers better prognosis in older CN-AML patients and may refine the European LeukemiaNet classification. Biologic features associated with MN1 expression may help identify new treatment targets.

Emerging FMS-like tyrosine kinase 3 inhibitors for the treatment of acute myelogenous leukemia

INTRODUCTION

The FMS-like tyrosine kinase 3 (FLT3) is highly expressed in acute leukemias. Mutations involving FLT3 are among the most common molecular abnormalities in acute myelogenous leukemia (AML). Available evidence suggests that these molecular lesions confer a shorter disease-free survival and overall survival in patients with intermediate-risk cytogenetics. Therefore, substantial interest in FLT3 as a therapeutic target has led to the development of several promising inhibitors that target this tyrosine kinase.

AREAS COVERED

This review covers the molecular pathways associated with FLT3 activation in patients with AML, the biological rationale for inhibiting FLT3 and recent clinical progress with FLT3 inhibitors for the treatment of AML. Six FLT3 inhibitors undergoing clinical evaluation are discussed. A review of selected published manuscripts on the subject of FLT3 inhibition in AML and a search of the English language manuscripts in PubMed using the index words FLT3 and AML were conducted and articles of interest selected.

EXPERT OPINION

Mutated forms of FLT3, specifically FLT3-internal tandem duplication, have a significant impact on the prognosis of AML patients, particularly those with a normal karyotype. Inhibiting FLT3 may lead to clinical benefit for patients with AML. Newly developed FLT3 inhibitors have shown encouraging activity as monotherapy and in combination with other therapeutic agents.

Towards individualized follow-up in adult acute myeloid leukemia in remission

An increasing body of data has demonstrated that the traditional concept of morphologic complete remission in acute myeloid leukemia, in which less than 5% myeloblasts is regarded as a sufficient response criterion, is not biologically sound. Fortunately, the quantitative reverse-transcribed polymerase chain reaction (RT-PCR) method seems to be a promising alternative because of its high degree of preclinical standardization and extreme sensitivity on the background of an accurate day-to-day estimate of sample quality. Widespread implementation of this has, however, to some extent been hampered by the lack of knowledge of how and when to measure minimal residual disease levels and, even more importantly, how to react preemptively on a molecular relapse defined by a PCR reversal. Thus, only few prospective studies have been published to date to clinically validate this assay. Here, we discuss outstanding issues in the clinical implementation of RT-PCR for fusion transcripts, mutated and overexpressed genes in acute myeloid leukemia patients in complete remission, and propose a set of guidelines, which can be used when designing prospective trials aimed at validating the use of RT-PCR as well as for following these patients based on mathematical models for disease recurrence recently developed in our laboratory.

Broad copy neutral-loss of heterozygosity regions and rare recurring copy number abnormalities in normal karyotype-acute myeloid leukemia genomes

We analyzed, by the latest high-resolution SNP arrays, 19 Normal Karyotype (NK)-AML patients at diagnosis (Dx) and remission (R) phases, to determine the number of tumor-associated copy number abnormalities (CNAs) and copy neutral-loss of heterozygosity (CN-LOH) regions per patient and to identify possible recurring genomic abnormalities. The number of tumor-associated CNAs was determined after comparison of matched Dx/R samples using stringent conditions able to reduce the number of false positive CNAs. With the exception of a single outlier case, a low number of CNAs per patient was detected (median value of 1 somatic loss or gain per patient). However, a high prevalence of CNAs (60-70% of the patients showed at least one tumor-associated gain or loss) and few recurring CNAs were observed, thus providing new hints towards identification of cooperating mutations. An extensive search of all tumor-associated CN-LOH regions >1 Mb revealed only three broad regions (terminal 12Mb of 22q, terminal 27Mb of 1p and the whole chromosome 21) in three patients out of 19 (16%). CN-LOH of the whole chromosome 21 was responsible for homozygosity of a missense mutation (R80C) of RUNX1/AML1. Our study suggests that a relative submicroscopic copy number stability NK-AML genomes is associated with low recurrence of specific CNAs and CN-LOH in NK-AML patient population. Sequencing of candidate genes in the identified CNAs and CN-LOH regions should be considered a priority in the search of novel driver mutations of AML.

Biology, risk stratification, and therapy of pediatric acute leukemias: an update

PURPOSE

We review recent advances in the biologic understanding and treatment of childhood acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML), identify therapeutically challenging subgroups, and suggest future directions of research.

METHODS

A review of English literature on childhood acute leukemias from the past 5 years was performed.

RESULTS

Contemporary treatments have resulted in 5-year event-free survival rates of approximately 80% for childhood ALL and almost 60% for pediatric AML. The advent of high-resolution genome-wide analyses has provided new insights into leukemogenesis and identified many novel subtypes of leukemia. Virtually all ALL and the vast majority of AML cases can be classified according to specific genetic abnormalities. Cooperative mutations involved in cell differentiation, cell cycle regulation, tumor suppression, drug responsiveness, and apoptosis have also been identified in many cases. The development of new formulations of existing drugs, molecularly targeted therapy, and immunotherapies promises to further advance the cure rates and improve quality of life of patients.

CONCLUSION

The application of new high-throughput sequencing techniques to define the complete DNA sequence of leukemia and host normal cells and the development of new agents targeted to leukemogenic pathways promise to further improve outcome in the coming decade.

Acute myeloid leukemia with IDH1 or IDH2 mutation: frequency and clinicopathologic features

Mutations in the isocitrate dehydrogenase 1 (IDH1) and IDH2 genes are reported in acute myeloid leukemia (AML). We studied the frequency and the clinicopathologic features of IDH1 and IDH2 mutations in AML. Mutations in IDH1 (IDH1(R)¹³²) and IDH2 (IDH2(R)¹⁷²) were assessed by Sanger sequencing in 199 AML cases. Point mutations in IDH1(R)¹³² were detected in 12 (6.0%) of 199 cases and in IDH2(R)¹⁷² in 4 (2.0%) of 196 cases. Of the 16 mutated cases, 15 (94%) were cytogenetically normal, for an overall frequency in this group of 11.8%. IDH1(R)¹³² and IDH2(R)¹⁷² mutations were mutually exclusive. Concurrent mutations in NPM1, FLT3, CEBPA, and NRAS were detected only in AML with the IDH1(R)¹³² mutation. The clinical and laboratory variables of patients with AML with IDH mutations showed no significant differences compared with patients with wild-type IDH. We conclude that IDH1(R)¹³² and IDH2(R)¹⁷² mutations occur most often in cytogenetically normal AML cases with an overall frequency of approximately 11.8%.

Prognostic value of FLT3 mutations among different cytogenetic subgroups in acute myeloid leukemia

BACKGROUND

The impact of FMS-like tyrosine kinase 3 (FLT3) mutations and mutation burden among cytogenetic subgroups of patients with acute myeloid leukemia (AML) other than normal karyotype (NK) AML is unclear.

METHODS

Patients with newly diagnosed AML were divided among 3 cytogenetic subgroups: core binding factor (CBF) AML, NK-AML, and poor-risk AML.

RESULTS

In total, 481 patients were included: 13% had, CBF-AML, 57% had NK-AML, and 30% had poor risk AML, and the frequency of any FLT3 mutations was 20%, 32%, and 7.6% in the respective cytogenetic subgroups. FLT3 mutation did not have an impact on event-free survival (EFS) in patients with CBF-AML (P = .84) and poor-risk AML (P = .37). In patients with NK-AML, EFS was worse in the FLT3-internal tandem duplication (ITD) group (20 weeks vs 41 weeks; P < .00,001) but not in the FLT3-tyrosine kinase domain (TKD) point mutation group (61 weeks vs 41 weeks; P = .15). Worse EFS and overall survival (OS) were observed among patients with NK-AML and higher FLT3-ITD burden but not among patients with FLT3-TKD mutation. In multivariate analysis, FLT3-ITD mutation was prognostic of EFS in patients with NK-AML (hazard ratio, 3.1; P = .03).

CONCLUSIONS

FLT3 mutations did not have a prognostic impact in patients with AML who had good-risk and poor-risk karyotypes. In patients with NK-AML, FLT3-ITD mutations led to worse survival, which was even worse among patients who had high mutation burden.

Acute erythroid leukemia as defined in the World Health Organization classification is a rare and pathogenetically heterogeneous disease

The diagnostic criteria for acute erythroid leukemia have been controversial since this disease was initially described. Using the current World Health Organization classification criteria, we retrospectively reviewed cases of acute myeloid leukemia or myelodysplastic syndrome in which erythroid precursors were >or=50% of the bone marrow nucleated cell population and the diagnosis of erythroleukemia was considered using older classification schemes. We collected 90 cases and separated them into four diagnostic groups: acute erythroid leukemia, erythroleukemia or erythroid/myeloid type (n=20); acute myeloid leukemia with myelodysplasia-related changes (n=22); therapy-related acute myeloid leukemia (n=32); and refractory anemia with excess blasts and preceding or concurrent history of erythropoietin therapy for anemia (n=16). Patients with acute erythroid leukemia were the youngest patient group and had the best overall survival. There was a statistically significant difference in overall survival between patients with acute erythroid leukemia versus acute myeloid leukemia with myelodysplasia-related changes (P=0.003) and between patients with acute erythroid leukemia versus therapy-related acute myeloid leukemia (P<0.0001). The presence of complex cytogenetic abnormalities (>3) was the only statistically significant independent variable that adversely affected survival in the acute erythroid leukemia group. Monosomy 5/del(5q) and monosomy 7/del(7q) were overrepresented in the context of complex chromosomal abnormalities. Our data suggest that acute erythroid leukemia, as currently defined in the World Health Organization classification, has become a rare disease. A majority of the cases reported previously as erythroleukemia are now classified as other entities. In addition, our data suggest that the current definition of acute erythroid leukemia, erythroleukemia type remains heterogeneous. One subset of acute erythroid leukemia patients has relatively low blast counts and are diploid. The prognosis of this patient subset is relatively good. The other subset has cytogenetic abnormalities similar to those in myelodysplastic syndromes and a poor prognosis.

MicroRNAs in normal and malignant myelopoiesis

MicroRNAs (miRNAs) are a class of non-coding protein, single-stranded RNA of 18-22 nucleotides, that exert their actions at post-transcriptional level, mostly through base pairing with the 3'-untranslated region of the target mRNA, thus leading to its translational repression and/or degradation. Recent studies have shown that miRNAs play a crucial role in normal hematopoiesis through the control of the expression of key regulators of hematopoiesis (i.e., transcription factors, growth factor receptors, chemokine receptors), involving regulatory loops that selectively operate in the various hematopoietic lineages. Extensive miRNA deregulation has been observed in leukemia and functional studies support a role for miRNAs in the pathogenesis of these disorders.