The transcription factor RUNX1 is a master regulator of hematopoiesis and is frequently mutated in myeloid malignancies. Mutations in its runt homology domain (RHD) frequently disrupt DNA binding and result in loss of RUNX1 function. However, it is not clearly understood how other RUNX1 mutations contribute to disease development. Here, we characterize RUNX1 mutations outside of the RHD. Our analysis of patient datasets revealed that mutations within the C-terminus frequently occur in hematopoietic disorders. Remarkably, most of these mutations were nonsense or frameshift and predicted to be exempt from nonsense mediated mRNA decay. Therefore, this class of mutation is projected to produce DNA-binding proteins that contribute to pathogenesis in a distinct manner. To model this, we introduced the RUNX1R320* mutation into the endogenous gene locus and demonstrated the production of RUNX1R320* protein. Expression of RUNX1R320* resulted in the disruption of RUNX1 regulated processes such as megakaryocytic differentiation through a transcriptional signature different from RUNX1 depletion. To understand the underlying mechanisms, we utilized Global RNA Interactions with DNA by deep sequencing (GRID-seq) to examine enhancer-promoter connections. We identified wide-spread alteration of enhancer-promoter networks within RUNX1 mutant cells. Additionally, we uncovered enrichment of RUNX1R320* and FOXK2 binding at the MYC super enhancer locus, significantly upregulating MYC transcription and signaling pathways. Together, our study demonstrates that most RUNX1 mutations outside the DNA binding domain are not subject to nonsense mediated decay, producing protein products that act in concert with additional cofactors to dysregulate hematopoiesis through mechanisms distinct from that induced by RUNX1 depletion.
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