Alternatively Spliced CSF3R in Human Health and Disease

Elevated alternative colony stimulating factor 3 receptor (CSF3R) isoforms are observed in myelodysplastic syndromes (MDS) and other myeloid neoplasms, but their role in driving disease progression has not been fully explained. In this study we report on the role of an MDS-associated splicing factor SRSF2 and its effects on the production of Class III and Class IV CSF3R isoforms and granulopoiesis. Class IV add-back in Csf3r-null mice showed impaired neutrophil differentiation with increased number of CFU-G, which largely consisted of metamyelocytes, while Class III showed greater dysgranulopoiesis accompanied by dysmorphic neutrophils. Alternative CSF3R isoforms were elevated in patients with myeloid neoplasms harboring SRSF2 mutations. Using in vitro splicing assays using a CSF3R minigene in K562 cells, we confirmed increased levels of Class III and Class IV CSF3R isoforms when SRSF2 P95H, P95L, and P95R mutations were co-expressed. Since SRSF2 regulates splicing in part by recognizing exonic splicing enhancer (ESE) sequences on pre-mRNA, deletion of two ESE motifs (5’-GGACCCUG-3’ and 5’-GGCCACUG-3’) within CSF3R exon 17 resulted in decreased Class IV transcript levels. CD34+ cells expressing SRSF2 P95H showed impaired neutrophil differentiation in response to G-CSF treatment, which was accompanied by increased levels of the differentiation-impairing Class IV isoform. Our findings suggest that SRSF2 P95H promotes Class IV splicing by preferentially binding to key ESE sequences in CSF3R exon 17, and that this splicing factor, when mutated as it is recurrently in myelodysplastic syndromes, contributes to dysgranulopoiesis