Impaired hematopoiesis and leukemia development in mice with a conditional knock-in allele of a mutant splicing factor gene U2af1

Mutations affecting the spliceosomal protein U2AF1 are commonly found in myelodysplastic syndromes (MDS) and secondary acute myeloid leukemia (sAML). We have generated mice that carry Credependent knock-in alleles of U2af/(S34F), the murine version of the most common mutant allele of U2AF1 encountered in human cancers. Cre-mediated recombination in murine hematopoietic lineages caused changes in RNA splicing, as well as multilineage cytopenia, macrocytic anemia, decreased hematopoietic stem and progenitor cells, lowgrade dysplasias, and impaired transplantability, but without lifespan shortening or leukemia development. In an attempt to identify U2af1/(S34F)-cooperating changes that promote leukemogenesis, we combined U2af1/(S34F) with Runx1 deficiency in mice and further treated the mice with a mutagen, N-ethyl-N-nitrosourea (ENU). Overall, 3 of 16 ENU-treated compound transgenic mice developed AML. However, AML did not arise in mice with other genotypes or without ENU treatment. Sequencing DNA from the three AML5 revealed somatic mutations homologous to those considered to be drivers of human AML, including predicted loss- or gain-of-function mutations in Tet2, Gata2, Idh1, and Ikzf1. However, the engineered U2af1/(534F) missense mutation reverted to WT in two of the three AML cases, implying that U2af1/(534F) is dispensable, or even selected against, once leukemia is established.