A 2.7-KB PORTION OF THE 5'-FLANKING REGION OF THE MURINE GLYCOPROTEINALPHA(IIB) GENE IS TRANSCRIPTIONALLY ACTIVE IN PRIMITIVE HEMATOPOIETIC PROGENITOR CELLS

The continuous generation of mature blood cells from primitive multipo tent progenitor cells requires a highly complex series of cellular eve nts that are still largely unknown. To examine the molecular events as sociated with the commitment of these hematopoietic progenitor cells t o the megakaryocytic lineage, the alpha subunit of the platelet integr in alpha(IIb)beta(3) was used as marker. Despite an abundance of infor mation regarding the role of this integrin in platelet adhesion and ag gregation, the mechanisms that control the expression of the genes tha t code for these proteins are poorly understood and the earliest hemat opoietic cell capable of expressing them has not been clearly identifi ed. Thus, a strategy was developed to eradicate, using a conditional t oxigene, all the hematopoietic cells capable of expressing the alpha(I Ib) gene in mice. This was achieved by targeting the expression of the gene encoding the herpes simplex virus thymidine kinase (tk), specifi cally to these cell types, using a 2.7-kb fragment of the 5'-flanking region of the murine alpha(IIb) gene. Three transgenic lines having 1, 3, and 4 copies of the transgene, respectively were produced and anal yzed. Administration of ganciclovir (GCV) to these mice induced a seve re thrombocytopenia, which was due to the depletion of the entire mega karyocytic lineage, as shown by bone marrow (BM) culture and electron microscopy analysis. The time required to attain a severe thrombocytop enia was dependent on the level of the expression of the transgene and varied from 7 to 11 days. This condition was completely reversed when GCV treatment was discontinued. Progenitor cell assays showed that th e alpha(IIb) promoter was active in primitive hematopoietic progenitor cells possessing myeloid, erythroid, and megakaryocytic potential and that the transcriptional activity of the promoter decreased progressi vely as differentiation proceeded towards the erythroid and myeloid li neages. (C) 1997 by The American Society of Hematology.