An ex vivo model to study v-Myb-induced leukemogenicity

The v-myb(AMV) oncogene transforms myelomonocytic cells in vitro and induce s acute monoblastic leukemia in chickens. We analyzed the activity of the e volutionarily conserved PEST-like domain (P1 domain) for biochemical and bi ological activities of v-Myb in ex vivo cultures and in vivo. Deletion of t he P1 domain did not affect v-Myb transcriptional. activity, intracellular stability, or subcellular localization. However, it resulted in subtle yet important changes in biological, activities. Although the mutant Delta P1 v -Myb protein blocked the terminal differentiation of the monocyte/macrophag e lineage as efficiently as the wild type (wt) in ex vivo cultures, it fail ed to induce the acute phase of monoblastic leukemia, with its fatal conseq uences, in vivo. Interestingly, in Delta P1 v-myb-infected animals large nu mbers of monoblasts, comparable to those induced by wt v-myb, were present in the bone marrow but very few were found in the peripheral blood. The com parison of ex vivo wt- and DeltaP v-Myb bone marrow cells revealed several important feat-tires of v-Myb transformation: (i) the proliferation of tran sformed monoblasts is not an apparent consequence of the differentiation bl ock with these processes being at least in part independent; (ii) the P1 do main is required for proliferation of v-Myb-mediated transformed monoblasts ; (iii) the mechanism which renders transformed cells growth factor indepen dent does not involve activation of an autocrine growth factor loop; and (i v) deletion of the P1 domain affects self-adhesion properties of v-myb-tran sformed monoblasts as well as their interaction with bone marrow stromal ce lls. These data indicate that the Delta P1 v-myb mutant and ex vivo bone ma rrow cell cultures represent a valuable tool for studies on the mechanisms of leukemia formation. (C) 2001 Academic Press.