Synthetic Introns Identify GPATCH8 as Required for Mis-splicing by SF3B1 Mutations [RNA-seq]
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ABSTRACT: Mutations in the RNA splicing factor SF3B1 are common across hematologic and solid cancers and result in widespread alterations in splicing but therapeutic means to correct this mis-splicing do not exist. Here we utilize synthetic introns uniquely responsive to mutant SF3B1 to identify trans factors required for aberrant mutant SF3B1 splicing activity. This revealed the G-patch domain containing protein GPATCH8 as required for mutant SF3B1-induced splicing alterations and impaired hematopoiesis. GPATCH8 is involved in quality control of branchpoint selection, interacts with the RNA helicase DHX15, and functionally opposes SUGP1, a G-patch protein recently implicated in SF3B1 mutant diseases. Silencing of GPATCH8 corrected one-third of mutant SF3B1 splicing defects and was sufficient to improve hematopoiesis in SF3B1 mutant mouse and human cells. These data identify GPATCH8 as a novel splicing factor required for mis-splicing by mutant SF3B1 and the therapeutic impact of correcting aberrant splicing in SF3B1 mutant cancers.
Project description:Mutations in the RNA splicing factor gene SF3B1 are common across hematologic and solid cancers and result in widespread alterations in splicing, but therapeutic means to correct this mis-splicing do not exist. Here, we utilize synthetic introns uniquely responsive to mutant SF3B1 to identify trans factors required for aberrant mutant SF3B1 splicing activity. This revealed the G-patch domain-containing protein GPATCH8 as required for mutant SF3B1-induced splicing alterations and impaired hematopoiesis. GPATCH8 is involved in quality control of branchpoint selection, interacts with the RNA helicase DHX15, and functionally opposes SUGP1, a G-patch protein recently implicated in SF3B1-mutant diseases. Silencing of GPATCH8 corrected one-third of mutant SF3B1-dependent splicing defects and was sufficient to improve dysfunctional hematopoiesis in SF3B1-mutant mouse and primary human progenitors. These data identify GPATCH8 as a novel splicing factor required for mis-splicing by mutant SF3B1 and highlight the therapeutic impact of correcting aberrant splicing in SF3B1-mutant cancers.
Project description:Mutations in the RNA splicing factor SF3B1 are common across hematologic and solid cancers and result in widespread alterations in splicing but therapeutic means to correct this mis-splicing do not exist. Here we utilize synthetic introns uniquely responsive to mutant SF3B1 to identify trans factors required for aberrant mutant SF3B1 splicing activity. This revealed the G-patch domain containing protein GPATCH8 as required for mutant SF3B1-induced splicing alterations and impaired hematopoiesis. GPATCH8 is involved in quality control of branchpoint selection, interacts with the RNA helicase DHX15, and functionally opposes SUGP1, a G-patch protein recently implicated in SF3B1 mutant diseases. Silencing of GPATCH8 corrected one-third of mutant SF3B1 splicing defects and was sufficient to improve hematopoiesis in SF3B1 mutant mouse and human cells. These data identify GPATCH8 as a novel splicing factor required for mis-splicing by mutant SF3B1 and the therapeutic impact of correcting aberrant splicing in SF3B1 mutant cancers.
Project description:Mutations in the RNA splicing factor SF3B1 are common across hematologic and solid cancers and result in widespread alterations in splicing but therapeutic means to correct this mis-splicing do not exist. Here we utilize synthetic introns uniquely responsive to mutant SF3B1 to identify trans factors required for aberrant mutant SF3B1 splicing activity. This revealed the G-patch domain containing protein GPATCH8 as required for mutant SF3B1-induced splicing alterations and impaired hematopoiesis. GPATCH8 is involved in quality control of branchpoint selection, interacts with the RNA helicase DHX15, and functionally opposes SUGP1, a G-patch protein recently implicated in SF3B1 mutant diseases. Silencing of GPATCH8 corrected one-third of mutant SF3B1 splicing defects and was sufficient to improve hematopoiesis in SF3B1 mutant mouse and human cells. These data identify GPATCH8 as a novel splicing factor required for mis-splicing by mutant SF3B1 and the therapeutic impact of correcting aberrant splicing in SF3B1 mutant cancers.
Project description:SF3B1, which encodes an essential spliceosomal protein, is frequently mutated in myelodysplastic syndromes (MDS) and many cancers. However, the defect of mutant SF3B1 is unknown. Here, we analyzed RNA-sequencing data from MDS patients and confirmed that SF3B1 mutants use aberrant 3' splice sites. To elucidate the underlying mechanism, we purified complexes containing either wild-type or the hotspot K700E mutant SF3B1, and found that levels of a poorly studied spliceosomal protein, SUGP1, were reduced in mutant spliceosomes. Strikingly, SUGP1 knockdown completely recapitulated the splicing errors, whereas SUGP1 overexpression drove the protein, which our data suggests plays an important role in branchsite recognition, into the mutant spliceosome and partially rescued splicing. Other hotspot SF3B1 mutants showed similar altered splicing and diminished interaction with SUGP1. Our study demonstrates that SUGP1 loss is the sole defect of mutant SF3B1 spliceosomes and, since this defect can be rescued, suggests possibilities for therapeutic intervention.
Project description: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.
Project description: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.
Project description:SF3B1 is the most commonly mutated RNA splicing factor in cancer, but the mechanisms by which SF3B1 mutations promote malignancy are poorly understood. Here, we integrated pan-cancer RNA sequencing to identify mutant SF3B1-dependent aberrant splicing with a positive enrichment CRISPR screen to prioritize splicing alterations that functionally promote tumorigenesis. We identify that diverse, recurrent SF3B1 mutations converge on repression of BRD9, a core component of the recently described non-canonical BAF (ncBAF) complex. Mutant SF3B1 recognizes an aberrant deep intronic branchpoint within BRD9, thereby inducing inclusion of an endogenous retrovirus-derived poison exon and BRD9 mRNA degradation. BRD9 depletion causes loss of ncBAF at CTCF-bound loci and promotes melanomagenesis. We demonstrate that BRD9 is a potent tumor suppressor in uveal melanoma, such that correcting BRD9 mis-splicing in SF3B1-mutant cell lines and patient-derived melanoma xenografts with antisense oligonucleotides (ASOs) or by directly targeting its poison exon with CRISPR-directed mutagenesis profoundly suppresses tumor growth. Our results implicate disruption of ncBAF in the diverse malignancies characterized by SF3B1 mutations, identify a single aberrant splicing event which functionally contributes to the pathogenesis of SF3B1-mutant cancers, and suggest a mechanism-based therapeutic for these malignancies.
Project description:Purpose: Mutant SF3B1 is near universally associated with MDS-RS but the exact mechanism of how mutant-SF3B1 induces ring sideroblast formation is unclear. Methods: iPSC samples were differentiated from 5F-HPC derived from MDS-RS patient iPSCs. CD34+ progenitors were isolated using the CD34 enrichment kit (Miltenyi). CD34-CD71+GlyA- pro-erythroblasts were isolated by flow sorting on day 4. CD71+GlyA+ erythroblasts were isolated by flow sorting on day 9. Libraries with DNA fragments of ~300bp were selected using AMPure XP bead purification. Purified libraries were sequenced on an Illumina HiSeq 2000 using paired-end, 50 bp reads. Mis-splicing analysis was performed as described in Inoue et al. 2019. Results: Mis-splicing analysis reveals ~100 genes that are strongly mis-spliced (>40%) by mutant SF3B1 during erythroid differentiation. We identify that mis-splicing of TMEM14C and ABCB7 contribute to RS formation in mutant SF3B1 cells. Conclusion: Our iPS derived mutant SF3B1MDS-RS in vitro model is the first demonstration of a cell line that recapitulates mis-splicing, erythroid differentiation, and ring sideroblast formation.
Project description:Recurrent mutations in RNA splicing factors SF3B1, U2AF1, and SRSF2 have been reported in hematologic cancers including myelodysplastic syndromes (MDS) and chronic lymphocytic leukemia (CLL). However, SF3B1 is the only splicing associated gene to be found mutated in CLL and has been shown to induce aberrant splicing. To investigate if any other genomic aberration caused similar transcriptome changes, we clustered RNASeq samples based on an alternative 3’ splice site (ss) pattern previously identified in SF3B1-mutant CLL patients. Out of 215 samples, we identified 37 (17%) with alternative 3’ ss usage, the majority of which harbored known SF3B1 hotspot mutations. Interestingly, 3 patient samples carried previously unreported in-frame deletions in SF3B1 around K700, the most frequent mutation hotspot. To study the functional effects of these deletions, we used various minigenes demonstrating that recognition of canonical 3’ ss and alternative branchsite are required for aberrant splicing, as observed for SF3B1 p.K700E. The common mechanism of action of these deletions and substitutions result in similar sensitivity of primary cells towards splicing inhibitor E7107. Altogether, these data demonstrate that novel SF3B1 in-frame deletion events identified in CLL result in aberrant splicing, a common biomarker in spliceosome-mutant cancers.
Project description:Recurrent mutations in the spliceosome are observed in several human cancers but their functional and therapeutic significance remain elusive. SF3B1, the most frequently mutated component of the spliceosome in cancer, is involved in the recognition of the branch point sequence (BPS) during selection of the 3’ splice site (ss) in RNA splicing. Here, we report that common and tumor-specific splicing aberrations are induced by SF3B1 mutations and establish aberrant 3’ ss selection as the most frequent splicing defect. Strikingly, mutant SF3B1 utilizes a BPS that differs from that used by wild-type SF3B1 and requires the canonical 3’ ss to enable aberrant splicing during the second step. Approximately 50% of the aberrantly spliced mRNAs are subjected to nonsense-mediated decay resulting in downregulation of gene and protein expression. These findings ascribe functional significance to the consequences of SF3B1 mutations in cancer. 72 samples, including two sets of patient data and cell lines with two additional technical replicates each