A model system has been developed to study retroviral oncogenesis, and the genetic factors regulating susceptibility to tumor induction. Avian leukosis virus (ALV) induces bursal lymphoma in chickens, after integration of viral long terminal repeat (LTR) sequences next to the c-myc proto- oncogene. Labile or short-lived proteins appear to regulate c-myc hyperexpression, because LTR-enhanced transcription is specifically decreased after inhibition of protein synthesis. This lability is observed only in pre-B cell types, suggesting that it may be important in the B cell specificity of ALV tumor induction. Moreover, lymphoma-resistant chicken strains exhibit stable LTR-enhanced transcription (unaffected by inhibition of protein synthesis), suggesting that labile regulation of LTR-enhanced c- myc transcription may be important in lymphoma susceptibility. Five LTR binding proteins from bursal lymphoma cells have been characterized using the gel shift assay. Three proteins are specifically labile in pre-B cells (decreased binding after protein synthesis inhibition) and thus may be essential for LTR enhancement. An in vivo footprinting technique will be used to determine if labile protein binding also occurs in vivo. The contribution of each protein binding sequence to LTR enhancement will be analyzed by transfection of wild type or mutant LTR-reporter gene constructs into bursal lymphoma cells. The genes encoding the major labile proteins are being cloned and sequenced, to obtain DNA and antibody probes for analysis of the role of these proteins in lymphomagenesis. Labile protein expression will be compared in different cell types, to determine why it is specifically labile in pre-B cells. Western and Northern blot analyses will determine if this lability is regulated by changes in protein half life or by post- translational modifications such as phosphorylation. Binding protein expression will be analyzed during B cell development, by Western blotting and immunofluorescent staining, to determine if the proteins are labile in ALV target pre-B cells, and stable in mature B cells no longer susceptible to lymphoma. Protein expression will be compared in lymphoma-susceptible and -resistant strains, to determine how stable expression could be involved in resistance to lymphoma. These studies will give insight to the regulation of c-myc hyperexpression during tumor induction, and the role of the labile LTR binding proteins in tumor susceptibility. An understanding of the mechanism of ALV lymphoma susceptibility can be applied to studies of oncogene activation and lymphomagenesis in humans.
