Thrombocytopenia in Mice Lacking the Carboxy-Terminal Regulatory Domain of the Ets Transcription Factor Fli1

Abstract

Targeted disruption of the Fli1 gene results in embryonic lethality. To dissect the roles of functional domains in Fli1, we recently generated mutant Fli1 mice that express a truncated Fli1 protein (Fli1^ΔCTA) that lacks the carboxy-terminal regulatory (CTA) domain. Heterozygous Fli1^ΔCTA mice are viable, while homozygous mice have reduced viability. Early postnatal lethality accounts for 30% survival of homozygotes to adulthood. The peripheral blood of these viable Fli1^ΔCTA/Fli1^ΔCTA homozygous mice has reduced platelet numbers. Platelet aggregation and activation were also impaired and bleeding times significantly prolonged in these mutant mice. Analysis of mRNA from total bone marrow and purified megakaryocytes from Fli1^ΔCTA/Fli1^ΔCTA mice revealed downregulation of genes associated with megakaroyctic development, including c-mpl, gpIIb, gpIV, gpIX, PF4, NF-E2, MafG, and Rab27B. While Fli1 and GATA-1 synergistically regulate the expression of multiple megakaryocytic genes, the level of GATA-1 present on a subset of these promoters is reduced in vivo in the Fli1^ΔCTA/Fli1^ΔCTA mice, providing a possible mechanism for the impared transcription observed. Collectively, these data showed for the first time a hemostatic defect associated with the loss of a specific functional domain of the transcription factor Fli1 and suggest previously unknown in vivo roles in megakaryocytic cell differentiation.

This work was supported by National Institutes of Health grants PO1-CA78582 (to D.K.W.), RO1 HL69123 (to M.O.), and K01 AR051385 (to X.K.Z.) and the Office of Research and Development, Medical Research Services, Department of Veterans Affairs (to M.O., A.C.L., X.K.Z., and G.G.). The Flow Cytometry & Cell Sorting Shared Resource of the Hollings Cancer Center is supported in part by a Cancer Center Support Grant (P30 CA 138313).

We thank Perry V. Halushka for discussion of platelet aggregation and function. We also thank Elizabeth Kruse (the Walter and Eliza Hall Institute of Medical Research, Australia) for providing advice on platelet half-life studies. We acknowledge technical assistance from Haiqun Zeng and James Klein. We thank John Lazarchick and Joan Mueller for providing expertise and the Chrono-log luminometer. We also acknowledge support from the MUSC Sequencing facility and the Flow Cytometry & Cell Sorting Shared Resource of the Hollings Cancer Center. We also thank Clayton Polite and other members of the DLAR staff.