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Viral genetic variation, AIDS, and the multistep nature of carcinogenesis: the feline leukemia virus model.

Feline leukemia virus (FeLV) infection in cats serves as a valuable animal model system for understanding the mechanisms of human diseases such as cancer and immunodeficiency. We have used experimental infection with molecularly cloned viruses to isolate and characterize novel FeLV variants that evolved in vivo and that were associated with the development of thymic lymphoma. One variant, FeLV-81T, contained a mutated envelope gene that conferred cytopathicity, enhanced replication rate, and syncytium induction in feline T cells, and is reminiscent of immunodeficiency-inducing strains of FeLV. Another variant transduced a portion of the feline Notch2 gene, which was expressed as a novel truncated protein in the cell nucleus and which we believe functioned as an oncogene in the development of T cell malignancy. Understanding how FeLV variants that either stimulate or destroy lymphocytes evolve and interrelate during disease progression will help elucidate the mechanisms of retroviral pathogenicity.