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.

超过60%的骨髓增生异常综合征（myeloid dysplasia syndrome, MDS）患者存在剪接因子编码基因的突变，涉及的剪接因子包括SF3B1、U2AF、SRSF2及ZRSR2。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是骨髓增生异常综合征的一个亚型。SF3B1K700E是SF3B1突变中最为频发的突变位点。然而，剪接因子突变引发红细胞生成缺陷的分子机制目前仍未阐明。免疫共沉淀实验（co-immunoprecipitation assays）结果显示，相较于野生型SF3B1蛋白，SF3B1K700E突变体与RNA结合蛋白RBM15的结合能力更强。此外，K700E突变会改变转录因子TAL1与GATA1的RNA剪接模式。经由可变RNA剪接，可生成一种新型的短TAL1转录本变体（TAL1s）。SF3B1与RBM15之间的相互作用增强，会促进全长型TAL1（TAL1fl）mRNA的生成；而通过PRMT1介导的降解通路降低RBM15蛋白水平，则会改变TAL1s与TAL1fl的表达比例，使该比例向有利于TAL1s的方向偏移。TAL1s仅包含螺旋-环-螺旋DNA结合结构域，并不具备该结构域上游的N端区域。TAL1s蛋白丧失了与ETO2的相互作用能力，而ETO2可抑制早期红细胞生成过程。据此，在人白血病K562细胞中过表达TAL1s，能够刺激β-血红蛋白的转录；在含促红细胞生成素的培养基中培养时，还可促进人脐带血CD34+细胞的红系分化。综上，SF3B1突变可能通过紊乱转录因子TAL1的可变RNA剪接过程阻断红细胞生成，而靶向PRMT1或可缓解骨髓增生异常综合征患者的贫血症状。