Mutant SRSF2-associated impaired erythropoiesis is defined by increased mTORC1 signaling due to FYN missplicing

Somatic mutations in RNA splicing regulators, including the serine/arginine-rich protein SRSF2, are frequently observed in myeloid malignancies. Using mouse models and primary human samples, we investigated the impact of SRSF2 mutations on erythropoiesis. We found reduced erythropoiesis in Srsf2^P95H versus wild-type mice upon stress-induced erythropoiesis and identified that SRSF2 mutations correlate with reduced hemoglobin in JAK2-mutant patients with myeloproliferative neoplasms (MPN). Consistent with this, Jak2^V617F-Srsf2^P95H versus Jak2^V617F mice displayed reduced red blood cell counts and erythroid precursor frequencies. RNA-sequencing on erythroid precursors showed reduced expression of heme metabolism and mitotic spindle-related genes, and increased expression of mTORC1 signaling in Srsf2^P95H versus wild-type cells. RNA splicing analyses on the same cells and on human patient samples identified aberrant FYN splicing in SRSF2^mut cells, with increased aberrant FYNB over normal FYNT transcripts. FYNB, but not FYNT, expression resulted in reduced erythroid differentiation and increased phosphorylation of mTORC1 downstream target S6. Additionally, increased S6 phosphorylation was confirmed in primary Srsf2^P95H erythroid cells. mTORC1 pathway inhibition using rapamycin normalized FYNB- and Srsf2^P95H-induced impaired erythropoiesis and significantly increased erythroid colony formation of SRSF2-mutant myelodysplastic neoplasm (MDS) bone marrow cells. Our data reveal targetable molecular mechanisms of impaired erythropoiesis in SRSF2-mutant cells.