Thymomegaly, Microsplenia, and Defective Homeostatic Proliferation of Peripheral Lymphocytes in p51-Ets1 Isoform-Specific Null Mice

Ets1 is a member of the Ets transcription factor family. Alternative splicing of exon VII results in two naturally occurring protein isoforms: full-length Ets1 (p51-Ets1) and Ets1VII (p42-Ets1). These isoforms bear key distinctions regarding protein-protein interactions, DNA binding kinetics, and transcriptional target specificity. Disruption of both Ets1 isoforms in mice results in the loss of detectable NK and NKT cell activity and defects in B and T lymphocytes. We generated mice that express only the Ets1VII isoform. Ets1VII homozygous mice express no p51-Ets1 and elevated levels of the p42-Ets1 protein relative to the wild type and display increased perinatal lethality, thymomegaly, and peripheral lymphopenia. Proliferation was increased in both the thymus and the spleen, while apoptosis was decreased in the thymus and increased in the spleen of homozygotes. Significant elevations of CD8 and CD8CD4 thymocytes were observed. Lymphoid cell (CD19, CD4, and CD8) reductions were predominantly responsible for diminished spleen cellularity, with fewer memory cells and a failure of homeostatic proliferation to maintain peripheral lymphocytes. Collectively, the Ets1VII mutants demonstrate lymphocyte maturation defects associated with misregulation of p16Ink4a, p27Kip1, and CD44. Thus, a balance in the differential regulation of Ets1 isoforms represents a potential mechanism in the control of lymphoid maturation and homeostasis