Kurzfassung

Acute myeloid leukemia (AML) is a clonal disease of the hematopoietic progenitor cell and is characterized by uncontrolled proliferation and a block in differentiation. During malignant transformation the sequential acquisition of somatic mutations within the malignant clone causes aberrant signal transduction and epigenetic dysregulation, finally resulting in altered gene expression. Constitutive activation of signal transducer and activator of transcription 5 (STAT5) proteins, in most cases...Acute myeloid leukemia (AML) is a clonal disease of the hematopoietic progenitor cell and is characterized by uncontrolled proliferation and a block in differentiation. During malignant transformation the sequential acquisition of somatic mutations within the malignant clone causes aberrant signal transduction and epigenetic dysregulation, finally resulting in altered gene expression. Constitutive activation of signal transducer and activator of transcription 5 (STAT5) proteins, in most cases caused by mutations in upstream receptor tyrosine kinases, represents one of the most common rewired signaling pathways in AML and plays an important role in leukemia initiation and maintenance. Beside phosphorylation, the sole presence of unphosphorylated STAT5 proteins in AML stem cells has been shown to play a fundamental role in leukemia development. In previous work our group demonstrated that unphosphorylated Stat5A/B blocks differentiation and shifts the balance toward the maintenance of self-renewal activity whereas genetic depletion of Stat5A/B induced differentiation into mature hematopoietic cells and caused an exhaustion phenotype in vivo. Work in Drosophila melanogaster suggested the existence of a non-canonical STAT signaling pathway in which unphosphorylated STAT proteins translocate into the nucleus and co-localize to heterochromatin whereas loss of STAT was followed by heterochromatin destabilization and re-expression of previously silenced genes. To investigate this unexpected function of unphosphorylated STAT5 in mammalian AML cells, we established several in vitro and in vivo models. Functionally, depletion of STAT5 was followed by reduced cell growth, induction of apoptotic cell death, differentiation and prolonged survival in a transgenic AML mouse model. These observations raise the hypothesis that unphosphorylated STAT5, as in the D. melanogaster model, might be involved in epigenetic silencing of tumor suppressor genes or genes involved in differentiation. To gain insight into the fundamental mechanisms of our findings, the aim of the proposed study is i) to analyze gene expression; and ii) to explore mechanisms of epigenetic regulation of gene expression mediated by unphosphorylated STAT5 in AML blasts. To achieve these goals, we plan to perform RNA sequencing analysis and chromatin immunoprecipitation (ChIP) sequencing analysis upon STAT5A and STAT5B knockdown using our already established cell line models. These data will be a prerequisite for future functional analyses.
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