V-myc in a simple, single gene retroviral vector causes rapid induction of leukemia and concomitant apoptosis following bone marrow transplantation

Abstract

We have previously developed an in vivo model of leukemogenesis utilizing mice reconstituted with genetically modified bone marrow cells. Based on those studies, a new single gene retroviral vector has been engineered which efficiently transfers vmyc into immature murine bone marrow cells. All reconstituted mice developed leukemia with a short latency period (5-11 weeks). In addition to hyperproliferation associated with elevated levels of PCNA, extensive apoptosis was also observed in all leukemic animals with p53 accumulating in the apoptotic cells. Whereas bax encoded protein, an effector of p53 apoptotic activity was detected in apoptotic cells, p21 Waf1 protein, a potential mediator of p53 growth suppression was not detected in these cells suggesting that v-myc-induced apoptosis was independent of the ability of p53 to induce p21 Waf1 . These results indicate that apoptosis, a part of the cellular response to v-myc expression, does not prevent leukemia development and that hyperproliferation rather than abrogation of oncogeneinduced apoptosis appears to be a critical event in v-mycinduced leukemia.

The v-myc oncogene

“…However, during the transformation process by v-Myc, the proliferative function out-balances the apoptotic function. This phenomenon also occurs in vivo in mice reconstituted with immature bone marrow cells infected with retroviruses expressing v-myc (Dolnikov et al, 1998). All mice develop leukemia with a short latency period (5 ± 11 weeks).…”

Induced p21WAF1 expression acts to reverse myc myelomonocytic cell transformation

“…3 Recently, we have also shown that expression of v-myc in primary murine bone marrow cells induces a similar dual effect: hyperproliferation and apoptosis were observed both in vitro and in vivo, the latter when v -myc ± expressing cells were used to reconstitute the hematopoietic system of lethally irradiated mice. 4 Increased expression of the tumor-suppressor gene p53 was observed in v-myc ± transformed cells in this reconstitution model. 4 Induction of p21 WAF1 , known to be a downstream effector of p53 and responsible for p53-mediated growth -suppressing activity, 5 was not observed in that reconstitution model.…”

“…4 Increased expression of the tumor-suppressor gene p53 was observed in v-myc ± transformed cells in this reconstitution model. 4 Induction of p21 WAF1 , known to be a downstream effector of p53 and responsible for p53-mediated growth -suppressing activity, 5 was not observed in that reconstitution model. 4 It was unclear why p53 accumulation induced by v -myc resulted in apoptosis rather than in p21 WAF1 induction and consequent growth arrest.…”

“…1 The tEMmyc4 clonal cell line expressing a v -myc ± targeted ribozyme (clone Rz1040 ) has been described elsewhere. 3 FDC -P1 and FDLV cells 4 were maintained in RPMI ( Gibco ) supplemented with 10% fetal bovine serum, antibiotics, and 10% WEHI -conditioned medium (WCM) as a source of IL -3. 16 The ÉCre cells transfected with either pLwtp53 or pLp53Ac5 DNA 15 were grown in DMEM supplemented with 10% fetal bovine serum and antibiotics.…”

“…4 Induction of p21 WAF1 , known to be a downstream effector of p53 and responsible for p53-mediated growth -suppressing activity, 5 was not observed in that reconstitution model. 4 It was unclear why p53 accumulation induced by v -myc resulted in apoptosis rather than in p21 WAF1 induction and consequent growth arrest. p21 WAF1 was the first cyclin -dependent kinase ( cdk) inhibitor to be identified and shown to be a downstream effector of p53 in (i) DNA damage-induced growth arrest, ( ii ) cell senescence, and (iii ) direct cdk regulation.…”