Mis-splicing of mitotic regulators sensitizes SF3B1-mutated human HSCs to CHK1 inhibition [RNA-seq]

Splicing factor SF3B1 mutations are frequent somatic lesions in myeloid neoplasms that transform hematopoietic stem cells (HSCs) by inducing mis-splicing of target genes. However, the molecular and functional consequences of SF3B1 mutations in human HSCs remain unclear. Here, we identify the mis-splicing program in human HSCs as a targetable vulnerability by precise gene editing of SF3B1 K700E mutations in primary CD34+ cells. Mutant SF3B1 induced pervasive mis-splicing and reduced expression of genes regulating mitosis in single cell transcriptomics, critically BUBR1 and CDC27, leading to altered differentiation and delayed G2/M progression. Mutant SF3B1 mis-splicing or reduced expression of BUBR1 and CDC27 was sufficient to delay G2/M transit, leading to activation of CHK1, sensitizing cells to CHK1 inhibition. Clinical CHK1 inhibitor prexasertib selectively targeted SF3B1-mutant HSCs and abrogated engraftment in vivo. These findings identify mis-splicing of mitotic regulators in SF3B1-mutant HSCs as a targetable vulnerability engaged by pharmacological CHK1 inhibition. RNA-seq was performed on FACS-sorted BFP+CD34+ gene edited HSPCs and clonally derived K562 cells. For each cell type, transcriptome (gene expression and splicing) of SF3B1 mutant cells was compared to wild-type gene edited controls.