CEPBA in normal blood cell development and in myeloid malignancies

Abstract

Expression levels of the myeloid transcription factor _CEBPA_ in the bone marrow are critical to maintain the balance between normal hematopoiesis and the onset of malignancy. What maintains constant levels of _CEBPA_ in normal hematopoiesis and how these levels are perturbed to trigger leukemogensis are less understood. In this thesis, a critical myeloid-specific regulatory element controls Cebpa expression levels in both mice and humans, was discovered. In the murine hematopoietic system, this regulatory element engages with Cebpa to prime neutrophilic differentiation. Genetic knockout of this regulatory element using CRISPR technology in mouse zygotes resulted in loss of mature neutrophils in the peripheral blood and also perturbed the myeloid progenitor system in the bone marrow. In a subtype of acute myeloid leukemia (AML), _CEBPA_ expression levels are low when relatively compared to other AMLs. What is the fundamental cause of low _CEBPA_ expression levels in AML? This question was approached in two ways: either by potential DNA mutations occuring in the regulatory regions of _CEBPA_ or epigenetic deregulation caused by AML-related oncoproteins. The CEBPA locus was screened for mutations in 300 AML patients. Only one patient harboured a biallelic deletion of the whole _CEBPA_ locus (220kb). However, it is more likely that _CEBPA_ expression levels are deregulated by oncoproteins that bind the identified regulatory element, reverses its active chromatin state and disturbs the chromatin loop between the enhancer and _CEBPA_. The thesis reports the identification of the regulatory element that primes the entry of myelopoiesis in the bone marrow and identifies a mechanism by which oncoproteins block myeloid differentiation in AML by disturbing the transcriptional regulatory mechanism of _CEBPA_ in the bone marrow