CRISPR/Cas9 screening of RNA binding proteins (RBPs) that regulate RUNX1 isoform production

The proper balance of hematopoietic stem cell (HSC) self-renewal and differentiation is critical for normal hematopoiesis and disrupted in hematologic malignancy. Among regulators of HSC fate, transcription factors have a well-defined, central role and mutations promote malignant transformation. More recently, studies have illuminated the importance of post-transcriptional regulation by RNA-binding proteins (RBPs) in hematopoiesis and leukemia development. However, the RBPs involved and breadth of regulation are only beginning to be elucidated. Furthermore, the intersection between post-transcriptional regulation and hematopoietic transcription factor function is poorly understood. Here, we studied the post-transcriptional regulation of RUNX1, a key hematopoietic transcription factor. Alternative polyadenylation (APA) of RUNX1 produces functionally antagonistic protein isoforms (RUNX1a versus RUNX1b/c) that mediate HSC self-renewal versus differentiation, an RNA-processing event that is dysregulated in malignancy. Consequently, RBPs that regulate this event directly contribute to healthy and aberrant hematopoiesis. We modeled RUNX1 APA using a split GFP minigene reporter and confirmed the sensitivity of our model to detecting changes in RNA-processing. We utilized this reporter in a CRISPR screen consisting of single guide-RNAs exclusively targeting RBPs and uncovered HNRNPA1 and KHDRBS1 as antagonistic regulators of RUNX1a isoform generation. Overall, our study provides mechanistic insight into the post-transcriptional regulation of a key hematopoietic transcription factor and identifies RBPs which may have a widespread, important function in hematopoiesis.