TARGETED EXPRESSION OF A CONDITIONAL ONCOGENE IN HEMATOPOIETIC-CELLS OF TRANSGENIC MICE

We have produced two lines of transgenic mice in which the expression of temperature-sensitive SV-40 large T antigen is targeted to bone mar row megakaryocytes via the platelet factor 4 (PF4) tissue-specific pro moter. The progeny of these transgenic mice were observed for about 3 mo, and no malignancies were detected over this period of time. The of fspring of these transgenic mice, 6- to 12-wk of age, served as a sour ce of bone marrow cells, which upon in vitro cultivation at the permis sive temperature yielded immortalized cell lines (MegT). At the permis sive temperature, MegT cells exhibit the characteristics of early 2N a nd 4N megakaryocytes which include the presence of specific gene produ cts such as PF4, glycoprotein IIb, acetylcholinesterase, and CD45 as w ell as absence of molecular markers of other cell lineages such as the macrophage marker Mac-1, the helper cell marker CD4, the mast cell ma rker IgE, the cell marker CD2 or the erythroid cell marker alpha-globi n. rhe inactivation of the oncogene by a shift of temperature from 34- degrees to 39.5-degrees-C produces a reduction in the frequency of the 2N cells, in conjunction with the appearance of 8N and 16N cells, con sisting of 27 and 3% of total cells, respectively. Thus, we have gener ated hematopoietic cell lines that are trapped in the early stages of megakaryocyte commitment, but able to undergo part of the normal progr am of terminal differentiation.