Survivin, the smallest member of the Inhibitor of Apoptosis (IAP) family of proteins, is expressed in proliferating cells, including fetal tissues, T-lymphocytes and hematopoietic progenitors. Survivin is also highly overexpressed in a wide spectrum of human cancers. Numerous studies have implicated survivin as a key mediator of the spindle assembly checkpoint and cytokinesis, while others have suggested an additional role as an inhibitor of apoptosis. In order to precisely examine the role of survivin in hematopoiesis in vivo, we induced acute deletion of the survivin gene in mice harboring either heterozygous or homozygous floxed alleles of survivin and the MX1-Cre transgene with polyinosinic-polycytidylic acid (pI-pC). Complete loss of survivin in homozygous floxed, MX1-Cre animals resulted in rapid death, which was associated with near total ablation of the bone marrow and marked hypocellularity of the spleen. The peripheral blood of these mutants displayed pancytopenia, including reticulocytopenia, and the presence of abnormal erythrocytes. Hematopoietic progenitors were markedly decreased, including c-kit+ cells, as well as CD71hiTer119hi erythroid precursors. Analysis of the cell cycle parameters revealed that a significant proportion of erythroid progenitors accumulated in G1, while others became polyploid. These defects could not be rescued by overexpression of the anti-apoptotic protein Bcl-2, consistent with a primary function of survivin in cell cycle progression. Bone marrow transplantation studies further confirmed that there exists a cell autonomous requirement for survivin in hematopoietic progenitors. Surprisingly, acute heterozygous deletion of survivin led to defects in erythropoiesis in approximately 20% of the animals. Upon pI-pC treatment, affected survivin heterozygous floxed mice with the MX1-Cre transgene exhibited a striking decrease in CD71Ter119hi cells at the terminal stages of maturation. Morphological analysis of erythroid cells from neonatal spleens of these affected mice revealed a dramatic reduction in enucleated erythrocytes in the CD71Ter119hi population compared to control animals. In addition, the majority of these erythroid cells were megaloblastic, with less condensed nuclear chromatin and nuclear-cytoplasmic dysynchrony. These observations suggest that there is a dose dependence for survivin. We predict that haploinsufficiency of survivin specifically affects “differentiation divisions” that occur concomitant with terminal maturation of red blood cells. Taken together, we show that homozygous deletion of survivin results in a dramatic loss of hematopoietic progenitors and rapid death, while heterozygous deletion leads to a selective defect in red blood cell development.

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