Critical Requirement of U2AF1 in the Maintenance and Function of Hematopoietic Stem Cells

Mutations in the RNA spliceosome genes have been frequently found in myelodysplastic syndromes (MDS). U2AF1 is involved in the recognition of the 3' splice site required for the recruitment of the U2 snRNP during pre-mRNA splicing. U2AF1 mutations have been identified in ~11% cases of MDS and are associated with poor prognosis. However, the role of wild type U2AF1 in normal hematopoiesis has remained unknown. To determine the role of U2AF1 in hematopoietic stem/progenitor cell (HSPC) function and normal hematopoiesis, we have generated a conditional U2AF1 knockout (floxed) mouse. We crossed the U2AF1 floxed mouse with Mx1Cre mouse and the expression of Cre recombinase was induced with pI-pC injection at 4 weeks after birth. All induced Mx1Cre;U2AF1fl/fl (U2AF1-deleted) mice became moribund or died between 11-12 days after pI-pC induction. U2AF1-deleted mice exhibited marked decrease in bone marrow (BM) cellularity and significantly reduced numbers of WBC, neutrophil, RBC and platelet counts in their peripheral blood compared with control animals. Flow cytometric analyses revealed a dramatic decrease in myeloid, erythroid and megakaryocytic precursor cells in U2AF1-deficient mice compared with control animals. Hematopoietic progenitor colony assays showed a marked decrease in myeloid (CFU-GM), erythroid (BFU-E), and megakaryocytic (CFU-Mk) colonies in the BM of U2AF1-deficient mice. Histopathologic analysis revealed severe BM aplasia in U2AF1-deficient mice. Together, these data suggest that deletion of U2AF1 results in profound defects in hematopoietic development.

The fatal BM failure in U2AF1-deficient mice prompted us to examine the HSPC compartments in the BM of these animals. We observed a marked decrease in Lin^-Sca-1⁺c-kit⁺(LSK) and long-term hematopoietic stem cells (LT-HSC), short-term HSC (ST-HSC), and multipotential progenitors (MPP) as well as early progenitors including common myeloid progenitors (CMP), granulocyte-macrophage progenitors (GMP), and megakaryocyte-erythroid progenitors (MEP) in the BM of U2AF1-deficient mice, indicating a defect at the earliest stage of adult hematopoietic development.

To determine whether the loss of HSCs in U2AF1-deficient animals is cell autonomous, BM cells from uninduced control (U2AF1fl/fl; no cre) and Mx1Cre;U2AF1fl/fl mice were transplanted into lethally irradiated WT C57BL/6 mice. Six weeks after transplantation, recipients were injected with pI-pC to induce the deletion of U2AF1. All the recipients of U2AF1-deficient BM became moribund within 14 days after pI-pC induction. Deletion of U2AF1 in the recipient animals resulted in pancytopenia and marked decrease in HSC/progenitors, myeloid, erythroid and megakaryocytic cells similar to that observed in the primary U2AF1-deficient mice, suggesting that the hematopoietic defects in U2AF1-deficient HSCs is cell intrinsic.

We performed competitive repopulation assays to further evaluate the function of U2AF1-deficient HSCs. BM cells from uninduced control (U2AF1fl/fl; no cre) and Mx1Cre;U2AF1fl/fl mice (CD45.2⁺) were mixed with CD45.1⁺competitor BM cells at a ratio of 1:1 and then transplanted into lethally irradiated congenic recipient animals (CD45.1⁺). Chimerism analysis in the transplanted animals revealed that U2AF1-deficient mice BM cells were completely unable to compete with WT BM cells. The percentages of U2AF1-deficient CD45.2⁺(donor-derived) LSK, myeloid, B and T cells were markedly reduced in the recipient animals compared with wild type U2AF1 BM donor at 16 weeks after transplantation, indicating that U2AF1-deficiency impairs the repopulation capacity of the HSCs.

To gain insights into the mechanism by which U2AF1controls HSC maintenance and function,we performed RNA-sequencing on purified LSK cells from control and U2AF1-deleted mice. Analysis of RNA-sequencing data revealed significant down regulation of genes related to HSC maintenance, cell cycle and JAK-STAT pathway in U2AF1-deficient LSK cells compared with control LSK. RNA sequencing also identified significantly altered splicing events in several important genes in U2AF1-deficient LSK cells. The most commonly altered splicing events were exon skipping/inclusion. We also observed increased phospho-H2AX and DNA damage in U2AF1-deficient BM cells. Overall, our results suggest an essential role for U2AF1 in the maintenance and function of hematopoietic stem cells.