Expression Of Mutant Spliceosomal Protein SF3B1 Results In Dysregulated Hematopoietic Maturation

Whole genome sequencing has recently revealed the prevalence of mutations in proteins directing splicing of RNA in up to half of the patients with Myelodysplastic Syndrome (MDS). Mutations in the protein SF3B1 are particularly common in MDS patients with the phenotypic abnormality termed ring sideroblasts (dysplastic erythroid precursors with perinculear rings formed by iron-laden mitochondria). The most common SF3B1 mutation in MDS patients results in a change from lysine to glutamic acid at amino acid position 700 (K700E). Given that splicing of RNA is a ubiquitous phenomenon, it is unclear how these mutations result in clonal proliferation and dysplastic hematopoiesis; two hallmark features of MDS. Furthermore, direct experimental evidence demonstrating a causative role for SF3B1 mutations in MDS-related phenotypes is lacking. To better understand how mutations of spliceosomal proteins contribute to MDS pathogenesis, we sought to define how expression of mutant SF3B1 changes erythroid maturation in vitro and in vivo. Native SF3B1 cDNA constructs are not amenable to bacterial propagation due to toxicity of its HEAT-domain repeats. We overcame this problem by codon optimization (changing the DNA sequence while preserving the native peptide sequence). Human cord blood derived CD34+ cells were transduced with retroviral vectors to express either the wild-type or K700E mutant of SF3B1. After a week of expansion in cytokines (IL-3, SCF and IL6), cells were induced to erythroid differentiation by addition of erythropoietin (EPO) and analyzed for surface markers of erythroid differentiation (CD 71, CD117, CD105, CD45 and CD235A) at regular intervals. K700E mutant expressing cells were found to have significantly reduced expression of CD105 when compared to wild-type SF3B1-expressing cells (average 50% recuction, n =8). CD105 or endoglin is a TGF-beta receptor accessory receptor expressed at high levels during intermediate stages of erythroid maturation. A more modest reduction of CD71 expression was also noted in K700E-SF3B1 cells. MDS bone marrow is known to express low levels of both CD105 and CD71 making our results clinically relevant. To further characterize how mutant SF3B1 may cause dysplastic hematopoiesis, we studied transduced and transplanted murine progenitor cells in vivo and in colony forming assays. Murine data demonstrate significantly reduced K700E-transduced hematopoietic progenitors (as defined by flow-cytometry) in vivo and impaired erythroid colony formation in vitro. Together, our results suggest that enforced expression of K700E-SF3B1 induces aberrant erythroid maturation and impairs homeostasis of hematopoietic precursor cells. Thus, we provide direct evidence that MDS-associated SF3B1 mutations perturb normal hematopoiesis and offer rationale for using our complementary experimental approach as a platform for elucidating the molecular mechanisms through which mutations in RNA splicing factors promote hematologic disease.