Dependence on Myb expression is attenuated in myeloid leukaemia with N-terminal CEBPA mutations

Mutations at the N- or C-terminus of C/EBP alpha are frequent in acute myeloid leukaemia (AML) with normal karyotype. Here, we investigate the role of the transcription factor Myb in AMLs driven by different combinations of CEBPA mutations. Using knockdown of Myb inmurine cell linesmodelling the spectrum of CEBPAmutations, we show that the effect of reduced Myb depends on the mutational statusof the twoCebpa alleles. Importantly, Myb knockdown fails to override the block in myeloid differentiation in cells with biallelic N-terminal C/EBP alpha mutations, demonstrating for the first time that the dependency on Myb is much lower in AML with this mutational profile. By comparing gene expression following Myb knockdown and chromatin immunoprecipitation sequencing data for the binding of C/EBP alpha isoforms, we provide evidence for a functional cooperation between C/EBP alpha and Myb in the maintenance of AML. This co-dependency breaks down when both alleles of CEBPA harbour N-terminal mutations, as a subset of C/EBP alpha-regulated genes only bind the short p30 C/EBP alpha isoform and, unlike other C/EBP alpha-regulated genes, do so without a requirement for Myb.