Avian leukosis virus (ALV) induces B cell lymphoma in chickens, after integration of proviral long terminal repeat (LTR) sequences next to the c-myc proto-oncogene. Labile or short-lived transcription factors appear to regulate c-myc hyperexpression, as LTR-enhanced c-myc and viral gene transcription is specifically decreased after inhibition of protein synthesis. This lability is observed only in immature B cells of lymphoma- susceptible strains, while lymphoma-resistant chicken strains show stable LTR-enhanced transcription (unaffected by inhibition o protein synthesis), suggesting that labile regulation of LTR-enhanced c-myc transcription is important for tumor susceptibility. A model has been proposed in which the labile LTR binding proteins transiently down-regulate c-myc hyperexpression in the target immature B cells of lymphoma-susceptible birds, to promote target cell survival and tumor progression. In resistant birds, stable high level c-myc expression would prevent tumor induction. This hypothesis will be examined using genomic DNA polymerase chain reaction (PCR) amplification to monitor the appearance and survival of bursal follicles with proviral c-myc integrations at various stages after ALV infection of lymphoma-susceptible and -resistant chicken embryos. The stage when transformed bursal follicles with clonal proviral c-myc integrations appear will be examined by in situ PCR analysis. Comparison o susceptible and resistant birds will give insight to the developmental age when tumor induction is blocked in resistant birds. The expression of c-myc in these follicles will be directly examined by immunohistochemical staining, to determine if differences in the program of c-myc hyperexpression could regulate target cell survival or tumor progression. A panel of monoclonal antibodies will be used to identify these critical stages when the normal program of B cell development is blocked. Analysis of apoptosis using an assay to detect chromosome breakdown will determine if c-myc hyperexpression-induced cell death is involved in tumor resistance. The effects of c-myc hyperexpression on potential downstream genes will also be examined. Cyclin D1 gene expression will be compared by immunohistochemical analysis of transformed and normal bursal follicles, to determine if this important cell cycle regulatory gene could mediate the proliferative effects of c-myc during B cell tumor induction in vivo. The effects of c-myc on the translational machinery will also be examined by in situ hybridization analysis of ribosomal RNA expression in transformed and normal bursal follicles. These studies will provide insight to the regulation of B cell tumor induction by the c-myc proto- oncogene, and the mechanism controlling susceptibility to lymphomagenesis. An understanding of the mechanism of ALV lymphomagenesis can be applied to studies of c-myc oncogene activation in many types of human cancers including B lymphoma, lung cancer, and breast cancer.
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