The role of STAT3 and STAT5 Target Gene Networks in Leukemia

Accumulation of mutations and chromatin conformation changes in hematopoietic stem cells (HSCs) lead to disease development such as Myelodysplastic Syndrome (MDS) and Acute Myeloid Leukemia (AML). MDS and AML are age-associated clonal disorders characterized by hematopoietic dysfunctions and impaired HSCs differentiation. MDS commonly progresses to AML, which is an aggressive type of leukemia. STAT3 and STAT5 are important cellular regulators and their constitutive activation has been linked to various solid and hematologic malignancies, including MDS and AML, through dysregulated transcription. To elucidate the role of STAT3 and STAT5 in leukemic transformation, MDS and AML have been utilized to model pre-leukemia and leukemia respectively. Transcriptome changes have been identified by STAT3, STAT5A or STAT5B knockdowns (KDs) in MDS and AML cells followed by mRNA-sequencing. Genome-wide STAT3, STAT5A and STAT5B DNA binding has been explored through CUT&Tag. Differential gene expression and binding profiles in MDS and AML determined the direct target gene networks of each factor in both conditions, revealing a distinct function including pathways involved in the cell cycle, apoptosis and differentiation. STAT3 and STAT5A differentially regulated genes between MDS and AML have been identified, providing novel targets for biomarker development and therapeutic intervention. Also, a possible interplay between STAT3 and STAT5 activation in AML has been uncovered through increased phosphorylation of STAT5 and STAT3 in STAT3 and STAT5 KDs respectively, and thus via the regulation of common downstream targets. These findings describe a functional overlap between STAT3 and STAT5 in AML for the regulation of shared targets essential for important cellular functions in AML, such as leukemic metabolism. ATAC-sequencing has provided insights into the chromatin conformation states and detected higher accessibility in MDS compared to AML. This finding has unveiled potential changes in accessibility for STAT3 and STAT5 binding sites between MDS and AML, indicating plausible leukemia-promoting changes in AML. Furthermore, STAT5A and STAT5B mediated control of the IKZF2 gene has been determined, through the differential regulation of IKZF2 between MDS and AML after STAT5A and STAT5B KDs. IKZF2 mRNA and protein levels have been reduced after STAT5A and STAT5B KD in AML through an MSI2-independent mechanism, opposing the previously reported MSI2-dependent regulation of IKZF2 expression. Our findings identify a novel STAT5/IKZF2 axis providing an alternative regulatory pathway in leukemia linked to AML maintenance and supporting the concept of regulatory mechanisms between STATs and Ikaros family members in AML. The results are currently validated in primary hematopoietic cells of MDS and AML patients. This Thesis delineates the role of STAT3, STAT5A, and STAT5B and their target gene networks in leukemic transformation, thus providing potential novel biomarkers and therapeutic targets for optimized patient stratification protocols and more efficient treatments, respectively, for MDS and AML.