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Acute myeloid leukemia (AML) arises through clonal expansion of transformed myeloid progenitor cells. The SKI proto-oncogene is highly upregulated in different solid tumors and leukemic cells, but little is known about its transcriptional regulation during leukemogenesis. MYB is an important hematopoietic transcription factor involved in proliferation as well as differentiation and upregulated in most human acute leukemias. Here, we find that MYB protein binds within the regulatory region of the SKI gene in AML cells. Reporter gene assays using MYB binding sites present in the SKI gene locus show MYB-dependent transcriptional activation. SiRNA-mediated depletion of MYB in leukemic cell lines reveals that MYB is crucial for SKI gene expression. Consistently, we observed a positive correlation of MYB and SKI expression in leukemic cell lines and in samples of AML patients. Moreover, MYB and SKI both were downregulated by treatment with histone deacetylase inhibitors. Strikingly, differentiation of AML cells induced by depletion of MYB is attenuated by overexpression of SKI . Our findings identify SKI as a novel MYB target gene, relevant for the MYB-induced differentiation block in leukemic cells., Competing Interests: CONFLICTS OF INTEREST The authors disclose no potential conflicts of interest.

RAS mutations are frequently found among acute myeloid leukemia patients (AML), generating a constitutively active signaling protein changing cellular proliferation, differentiation and apoptosis. We have previously shown that treatment of AML patients with high-dose cytarabine is preferentially beneficial for those harboring oncogenic RAS. On the basis of a murine AML cell culture model, we ascribed this effect to a RAS-driven, p53-dependent induction of differentiation. Hence, in this study we sought to confirm the correlation between RAS status and differentiation of primary blasts obtained from AML patients. The gene expression signature of AML blasts with oncogenic NRAS indeed corresponded to a more mature profile compared to blasts with wildtype RAS, as demonstrated by gene set enrichment analysis (GSEA) and real-time PCR analysis of myeloid ecotropic viral integration site 1 homolog (MEIS1) in a unique cohort of AML patients. In addition, in vitro cell culture experiments with established cell lines and a second set of primary AML cells showed that oncogenic NRAS mutations predisposed cells to cytarabine (AraC) driven differentiation. Taken together, our findings show that AML with inv(16) and NRAS mutation have a differentiation gene signature, supporting the notion that NRAS mutation may predispose leukemic cells to AraC induced differentiation. We therefore suggest that promotion of differentiation pathways by specific genetic alterations could explain the superior treatment outcome after therapy in some AML patient subgroups. Whether a differentiation gene expression status may generally predict for a superior treatment outcome in AML needs to be addressed in future studies.

MicroRNAs (miRNAs) are small, noncoding RNA molecules that regulate growth and differentiation. miRNAs are frequently located at cancer-specific fragile sites in the human genome, such as chromosome 7q. The nuclear oncogene SKI is up-regulated in acute myeloid leukemia (AML) with -7/del7q. Here we asked whether loss of miRNAs on chromosome 7q may explain this up-regulation. miR-29a expression was found to be down-regulated in AML with -7/del7q. Forced expression of miR-29a down-regulated Ski and its target gene, Nr-CAM, whereas miR-29a inhibition induced Ski expression. Luciferase assays validated a functional binding site for miR-29a in the 3' untranslated region of SKI. Finally, in samples of AML patients, we observed an inverse correlation of Ski and miR-29a expression, respectively. In conclusion, up-regulation of Ski in AML with -7/del7q is caused by loss of miR-29a. miR-29a may therefore function as an important tumor suppressor in AML by restraining expression of the SKI oncogene.

Acute myeloid leukemia (AML) patients with internal tandem duplication (ITD) mutations in the Fms-like tyrosine-3 (FLT3) gene have a dismal prognosis. Here we report compassionate-use results with the multikinase and FLT3-ITD inhibitor sorafenib for the treatment of relapsed or refractory FLT3-ITD-positive AML. Sorafenib induced clinically meaningful and very rapid responses in all 6 patients treated either before (n = 2), after (n = 3), or both before and after (n = 1) allogeneic stem cell transplantation (allo-SCT). Sorafenib-induced remissions facilitated allo-SCT in 2 of the 3 refractory patients. Two of the 4 patients who were treated after allo-SCT survived 216 and 221 days, respectively, whereas the other 2 remain in ongoing complete molecular remission. Sorafenib response was associated with an inhibition of the antiapoptotic FLT3-ITD target Stat-5 in vivo. Together, sorafenib monotherapy before or after allo-SCT has remarkable clinical activity in poor risk FLT3-ITD-positive AML and deserves further evaluation in prospective clinical trials.