LAUSR

SRSF2 P95 mutations are prevalent in myelodysplastic (MDS) and myelodysplastic/myeloproliferative (MDS/MPN) syndromes. They occur early in the course of the disease, are positively selected during disease progression and are associated with poor outcomes. Understanding how founder mutations such as SRSF2 modifies haematopoiesis and ultimately contributes to the development of myeloid bias and MDS/MPN will be critical to understanding the initiation and maintenance of these cancers. We have generated a conditional murine Srsf2P95H mutation model, where the temporal and cell type specific expression of the P95H mutation can occur. As in humans, the Srsf2 P95H mutation is expressed heterozygously from its endogenous locus. Using multiple Cre lines, we find that Srsf2P95H must be expressed in the haematopoietic stem cell (HSC) containing populations to promote a myelo-monocytic bias and expansion during native haematopoiesis. These early phenotypic alterations correspond with transcriptional and RNA splicing changes in lymphoid, myeloid and stem cell related genes. Upon ageing, Srsf2P95H animals develop a progressive, transplantable disease characterised by monocytosis, dysplastic neutrophils and macrocytosis with reduced red blood cells and loss of bone marrow (BM) B lymphopoiesis and erythropoiesis, all features of human MDS/MPN. Long-term (LT) HSC numbers tend to be lower and the repopulation potential of mutant cells decreased unless transplanted with age and microenvironment matched wild type cells. Srsf2P95H cells then reveal a competitiveness consistent with the presence of SRSF2 mutations in age-related clonal haematopoiesis. Diseased mice do not develop leukaemia, but exome sequencing of the bone marrow shows the acquisition of additional mutations currently found in humans with SRSF2 mutation. These results demonstrate a valid and reproducible Srsf2P95H/+ mouse knock-in model that will help elucidate the effect of human SRSF2 mutation on initiation and maintenance of MDS/MPN.