Aberrant STAT Function in CEBPA-mutant Acute Myeloid Leukemia

ABSTRACT Acute Myeloid Leukemia (AML) is a hematologic malignancy characterized by the accumulation of immature myeloblasts in the bone marrow and blood. Classically, AML is thought to arise due to a combination of signaling mutations that increase proliferation and transcription factor mutations that impair differentiation. A prototypical

example of signaling activation in AML results from mutations in the growth factor receptor, Colony Stimulating Factor 3 Receptor (CSF3R). Mutations in CSF3R, such as CSF3RT618I, cause constitutive activation of the receptor in the absence of ligand and robust signaling through the JAK/STAT pathway. This causes activation

of downstream transcription factors—STAT3 and STAT5, to promote the proliferation of progenitor cells and their differentiation down the myeloid lineage. In patients with AML, mutations in CSF3R are associated with high rates of relapse and poor overall survival, but the advent of effective therapies for these patients has remained

elusive. CSF3R mutations alone are insufficient to cause AML and require a second mutation that impairs myeloid differentiation. The most common co-occurring mutations are loss-of-function mutations in critical myeloid transcription factors, such as CCAAT binding protein alpha (CEBPA). In AML, CEBPA mutations block

CSF3R-driven differentiation, leading to the accumulation of immature myeloid blasts. This differentiation arrest is a fundamental feature of the aggressive phenotype associated with CSF3R-driven AML. The objective of this project is to understand the mechanism by which CEBPA mutations impair CSF3R-driven differentiation. The

aims of this project are to (1) determine the effect of CEBPA mutations on localization of STAT3 to regulatory elements of differentiation-associated genes and (2) to understand the effect of mutant CEBPA on cellular signaling pathways. A novel mouse cell line model of myeloid differentiation with biallelic Cebpa mutations

analogous to those found in patients with AML will facilitate the successful completion of these aims. A combination of transcription factor profiling, gene expression, chromatin architecture and phospho-proteomics will be used to nominate mechanistic targets. Follow up studies will perturb these targets to determine their role

in mutant-CEBPA driven differentiation block. AML is a devastating disease with poor outcomes despite chemotherapeutic treatments. Understanding how mutant CEBPA impairs differentiation will help create a mechanistic foundation for the development of more efficacious therapies to treat patients this disease.