Acute myeloid leukemias (AMLs) result from multiple genetic alterations in hematopoietic stem cells. We describe a novel t(12;18)(p13;q12) involving ETV6 in a patient with AML. The translocation resulted in overexpression of SETBP1 (18q12), located close to the breakpoint. Overexpression of SETBP1 through retroviral insertion has been reported to confer growth advantage in hematopoietic progenitor cells. We show that SETBP1 overexpression protects SET from protease cleavage, increasing the amount of full-length SET protein and leading to the formation of a SETBP1-SET-PP2A complex that results in PP2A inhibition, promoting proliferation of the leukemic cells. The prevalence of SETBP1 overexpression in AML at diagnosis (n ‫؍‬ 192) was 27.6% and was associated with unfavorable cytogenetic prognostic group, monosomy 7, and EVI1 overexpression (P < .01). Patients with SETBP1 overexpression had a significantly shorter overall survival, and the prognosis impact was remarkably poor in patients older than 60 years in both overall survival (P ‫؍‬ .015) and event-free survival (P ‫؍‬ .015). In summary, our data show a novel leukemogenic mechanism through SETBP1 overexpression; moreover, multivariate analysis confirms the negative prognostic impact of SETBP1 overexpression in AML, especially in elderly patients, where it could be used as a predictive factor in any future clinical trials with PP2A activators. (Blood. 2010;115: 615-625)

Protein phosphatase 2A (PP2A) is a human tumor suppressor that inhibits cellular transformation by regulating the activity of several signaling proteins critical for malignant cell behavior. PP2A has been described as a potential therapeutic target in chronic myeloid leukemia, Philadelphia chromosome-positive acute lymphoblastic leukemia and B-cell chronic lymphocytic leukemia. Here, we show that PP2A inactivation is a recurrent event in acute myeloid leukemia (AML), and that restoration of PP2A phosphatase activity by treatment with forskolin in AML cells blocks proliferation, induces caspase-dependent apoptosis and affects AKT and ERK1/2 activity. Moreover, treatment with forskolin had an additive effect with Idarubicin and Ara-c, drugs used in standard induction therapy in AML patients. Analysis at protein level of the PP2A activation status in a series of patients with AML at diagnosis showed PP2A hyperphosphorylation in 78% of cases (29/37). In addition, we found that either deregulated expression of the endogenous PP2A inhibitors SET or CIP2A, overexpression of SETBP1, or downregulation of some PP2A subunits, might be contributing to PP2A inhibition in AML. In conclusion, our results show that PP2A inhibition is a common event in AML cells and that PP2A activators, such as forskolin or FTY720, could represent potential novel therapeutic targets in AML.

The online version of this article has a Supplementary Appendix. BackgroundProtein phosphatase 2A is a novel potential therapeutic target in several types of chronic and acute leukemia, and its inhibition is a common event in acute myeloid leukemia. Upregulation of SET is essential to inhibit protein phosphatase 2A in chronic myeloid leukemia, but its importance in acute myeloid leukemia has not yet been explored. Design and MethodsWe quantified SET expression by real time reverse transcriptase polymerase chain reaction in 214 acute myeloid leukemia patients at diagnosis. Western blot was performed in acute myeloid leukemia cell lines and in 16 patients' samples. We studied the effect of SET using cell viability assays. Bioinformatics analysis of the SET promoter, chromatin immunoprecipitation, and luciferase assays were performed to evaluate the transcriptional regulation of SET. ResultsSET overexpression was found in 60/214 patients, for a prevalence of 28%. Patients with SET overexpression had worse overall survival (P<0.01) and event-free survival (P<0.01). Deregulation of SET was confirmed by western blot in both cell lines and patients' samples. Functional analysis showed that SET promotes proliferation, and restores cell viability after protein phosphatase 2A overexpression. We identified EVI1 overexpression as a mechanism involved in SET deregulation in acute myeloid leukemia cells. ConclusionsThese findings suggest that SET overexpression is a key mechanism in the inhibition of PP2A in acute myeloid leukemia, and that EVI1 overexpression contributes to the deregulation of SET. Furthermore, SET over-expression is associated with a poor outcome in acute myeloid leukemia, and it can be used to identify a subgroup of patients who could benefit from future treatments based on PP2A activators.

BackgroundDeregulated miRNA expression plays a crucial role in carcinogenesis. Recent studies show different mechanisms leading to miRNA deregulation in cancer; however, alterations affecting miRNAs by DNA copy number variations (CNV) remain poorly studied.ResultsOur integrative analysis including data from high resolution SNPs arrays, mRNA expression arrays, and miRNAs expression profiles in 16 myeloid cell lines highlights that CNV are alternative mechanisms to deregulate the expression of miRNAs in acute myeloid leukemia (AML), and represent a novel approach to identify novel candidate genes involved in AML. We found association between the expression levels of 19 miRNAs and CNVs affecting their loci. Functional analysis showed that NF1 is a direct target of miR-370, and that overexpression of miR-370 has similar effects that NF1 inactivation, increasing proliferation and colony formation in AML cells. Moreover, real time RT-PCR showed that NF1 downregulation is a recurrent event in AML (30.8%), and western blot analysis confirmed this result. MiR-370 overexpression and deletions affecting the NF1 locus were identified as alternative mechanisms to downregulate NF1.Conclusions NF1 downregulation is a common event in AML, and both deletions in the NF1 locus and overexpression of miR-370 are alternative mechanisms to downregulate NF1 in this disease. Our results suggest a leukemogenic role of miR-370 through NF1 downregulation in AML cells. Since NF1 deficiency leads to RAS activation, patients with AML and overexpression of miR-370 may potentially benefit from additional treatment with either RAS or mTOR inhibitors.

Bakground:The EVI1(ecotropic virus integration site 1) gene codes for a zinc-finger transcription factor, whose transcriptional activation leads to a particularly aggressive form of acute myeloid leukaemia (AML). Although, EVI1 interactions with key proteins in hematopoiesis have been previously described, the precise role of this transcription factor in promoting leukaemic transformation is not completely understood. Recent works have identified specific microRNA (miRNA) signatures in different AML subgroups. However, there is no analysis of miRNAs profiles associated with EVI1 overexpression in humans.Methods:We performed QT-RT–PCR to assess the expression of 250 miRNAs in cell lines with or without EVI1 overexpression and in patient samples. We used ChIP assays to evaluated the possible binding of EVI1 binding to the putative miRNA promoter. Proliferation of the different cell lines transfected with the anti- or pre-miRs was quantified by MTT.Results:Our data showed that EVI1 expression was significantly correlated with the expression of miR-1-2 and miR-133-a-1 in established cell lines and in patient samples. ChIP assays confirmed that EVI1 binds directly to the promoter of these two miRNAs. However, only miR-1-2 was involved in abnormal proliferation in EVI1 expressing cell lines.Conclusions:Our data showed that EVI1 controls proliferation in AML through modulation of miR-1-2. This study contributes to further understand the transcriptional networks involving transcription factors and miRNAs in AML.