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 (PI 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 s tability, or subcellular localization. However, it resulted in subtle yet i mportant changes in biological activities. Although time mutant Delta P1 v- Myb protein blocked the terminal differentiation of the monocyte/macrophage lineage as efficiently as the wild type (wt) in ex vivo cultures, it faile d to induce the acute phase of monoblastic leukemia, with its fatal consequ ences, in vivo. Interestingly, in Delta P1 v-myb-infected animals large num bers of monoblasts, comparable to those induced by wt v-myb, were present i n the bone marrow but very few were found in the peripheral blood. The comp arison of ex vivo wt- and DeltaP v-Myb bone marrow cells revealed several i mportant features of v-Myb transformation: (i) the proliferation of transfo rmed monoblasts is not an apparent consequence of the differentiation block with these processes being at least in part independent; (ii) the P1 domai n is required for proliferation of v-Myb-mediated transformed monoblasts; ( iii) the mechanism which renders transformed cells growth factor independen t does not involve activation of an autocrine growth factor loop; and (iv) deletion of the P1 domain affects self-adhesion properties of v-myb-transfo rmed monoblasts as well as their interaction with bone marrow stromal cells . These data indicate that the Delta P1 v-myb mutant and ex vivo bone marro w cell cultures represent a valuable tool for studies on the mechanisms of leukemia formation. (C) 2001 Academic Press.