Data from: Splicing factor SF3B1K700E mutant dysregulates erythroid differentiation via aberrant alternative splicing of transcription factor TAL1

More than 60% of myeloid dysplasia syndrome (MDS) contains mutations in genes encoding for splicing factors such as SF3B1, U2AF, SRSF2 and ZRSR2. Mutations in SF3B1 are associated with 80% cases of refractory anemia with ring sideroblast (RARS), a subtype of MDS. SF3B1K700E is the most frequently mutated site among mutations on SF3B1. Yet the molecular mechanisms on how mutations of splicing factors lead to defective erythropoiesis are not clear. SF3B1K700E mutant binds to an RNA binding protein, RBM15, stronger than the wild type SF3B1 protein in co-immunoprecipitation assays. In addition, K700E mutant alters the RNA splicing of transcription factors TAL1 and GATA1. Via alternative RNA splicing, a novel short TAL1 transcript variant (TAL1s) is generated. Enhanced interaction between SF3B1 and RBM15 promotes the production of full-length TAL1 (TAL1fl) mRNA, while reduction of RBM15 protein level via PRMT1-mediated degradation pathway changes TAL1s/TAL1fl ratio in favor of TAL1s. TAL1s contains the helix-loop-helix DNA binding domain but not the N terminal region upstream of the DNA binding domain. The TAL1s protein loses its interaction with ETO2, which represses early erythropoiesis. In this vein, overexpression of TAL1s stimulates the transcription of β-hemoglobin in human leukemia K562 cells and promotes erythroid differentiation of human cord blood CD34+ cells cultured in erythropoietin-containing medium. Therefore, mutations of SF3B1 may block erythropoiesis via dysregulation of alternative RNA splicing of transcription factor TAL1, and targeting PRMT1 may alleviate the anemic symptoms in MDS patients.

骨髓增生异常综合征（myeloid dysplasia syndrome, MDS）中超过60%的病例存在编码剪接因子（splicing factors）的基因突变，涉及SF3B1、U2AF、SRSF2及ZRSR2等基因。SF3B1突变与80%的环形铁粒幼红细胞性难治性贫血（refractory anemia with ring sideroblast, RARS）病例相关，而RARS是MDS的一个亚型。SF3B1的突变位点中，K700E是最为常见的突变位点。但目前剪接因子突变引发红细胞生成缺陷的分子机制尚未阐明。在免疫共沉淀（co-immunoprecipitation）实验中，SF3B1K700E突变体与RNA结合蛋白（RNA binding protein）RBM15的结合强度高于野生型SF3B1蛋白。此外，K700E突变会改变转录因子TAL1与GATA1的RNA剪接模式：通过可变RNA剪接（alternative RNA splicing），可产生一种新型短链TAL1转录变体（TAL1s）。SF3B1与RBM15的相互作用增强会促进全长TAL1（TAL1fl）mRNA的生成；而通过PRMT1介导的降解途径降低RBM15蛋白水平，则会改变TAL1s与TAL1fl的比例，使TAL1s的占比提升。TAL1s包含螺旋-环-螺旋DNA结合域（helix-loop-helix DNA binding domain），但缺失该结合域上游的N端区域。TAL1s蛋白会丧失与ETO2的相互作用，而ETO2可抑制早期红细胞生成。据此，过表达TAL1s可刺激人白血病K562细胞中β-珠蛋白（β-hemoglobin）的转录，并促进在含促红细胞生成素（erythropoietin）的培养基中培养的人脐带血CD34+细胞的红系分化。综上，SF3B1突变可能通过干扰转录因子TAL1的可变RNA剪接调控阻断红细胞生成，而靶向PRMT1或可缓解MDS患者的贫血症状。