E (XSR) element contributes to the oncogenicity of Avian leukosis virus (subgroup J)

Among the six subgroups of Avian leukosis virus (ALV) that infect chickens, subgroup J (ALV-J) was isolated from meat-type chickens where it predominantly induces myeloid leukosis (ML) and erythroblastosis (EB). The sequence of HPRS-103, the ALV-J prototype virus, shows several distinct features, one of which is the presence of a distinct hairpin stem-loop structure called the E (also called XSR) element in the 3' untranslated region. In order to determine the role of the E element in ALV-induced pathogenicity, a comparison was made of the oncogenicity of viruses derived from the provirus clones of parental and E element-deleted HPRS-103 viruses in two genetically distinct lines of birds. In line 15I birds, deletion of the E element had profound effects on virus replication in vivo, as only 55 % of birds showed evidence of infection, compared with 100 % infection by the parental virus. Furthermore, none of the line 15I birds infected with this virus developed tumours, indicating that the E element does contribute to the oncogenicity of the virus. On the other hand, deletion of the E element had only a marginal effect on the incidence of tumours in line 0 birds. These results indicate that, although the E element per se is not absolutely essential for tumour induction by this subgroup of viruses, it does contribute to oncogenicity in certain genetic lines of chicken.