Bicistronic retroviral vector reveals capacity of v-erbA to induce erythroleukemia and to co-operate with v-myb.

Previous studies have shown that v-erbA and v-myb can induce the proliferation of avian erythroid cells in culture. To study the combined effects of v-erbA and v-myb, the two oncogenes were engineered into a retrovirus bicistronic vector with an internal ribosomal entry site (IRES) or into a vector with a splice acceptor (SPL). This allowed coexpression of the two proteins and a comparison with the same vector containing either v-erbA or v-myb only. Both the erbA IRES and the erbA/myb IRES virus constructs transformed erythroid cells after infection of bone marrow or blastoderm cultures. The erbA/myb IRES virus exhibited a 5-10-fold higher transformed colony forming efficiency than the erbA IRES virus in the blastoderm assay. Surprisingly, when injected into chicken embryos in the presence of helper virus, both viruses induced an erythroleukemia in about half of the animals. In contrast, no leukemia was observed with a myb IRES virus, with spliced vectors containing v-erbA alone or v-erbA in combination with v-myb, nor with erbA IRES and erbA/myb IRES viruses produced in the absence of helper virus. The average latency of leukemia induction was shorter for the erbA/myb IRES virus (3.5 weeks) than for the erbA IRES virus (5 weeks). Nevertheless, for both viruses the leukemic blasts retained full factor dependence for growth. These results show that v-erbA is capable of inducing an erythroleukemia when expressed by a high titer bicistronic retrovirus under conditions of virus spreading and that its in vitro and in vivo transforming potential can be enhanced by v-myb.