The role of the long terminal repeat in murine retrovirus-induced thymic lymphoma

Abstract

The main genetic determinant of viral leukemogenicity by replication competent murine leukemia viruses (MuLVs) is the enhancer within the long terminal repeat (LTR). For each virus, the tissue specificity of the LTR enhancer correlates with the tissue type of disease. Proviruses within tumors are found at high frequency adjacent to deregulated proto-oncogene alleles. The enhancer insertion hypothesis states that the LTR enhancer of proviruses adjacent to proto-oncogenes positively activates tissue-specific proto-oncogene expression. The LTR enhancers of T-lymphomagenic SL3-3 virus and nonleukemogenic Akv virus differ in the sequence and arrangement of the comprised elements. Each LTR contains a sequence related to the consensus element, the enhancer core, that differ at a single base pair. Activity of the SL3-3 LTR in transient expression assays is high relative to the Akv LTR in T-cells and the SL3-3 core is necessary for the higher activity. To test the importance of the SL3-3 core element in viral leukemogenicity the Akv core was substituted into the SL3-3 genome. The recombinant virus, termed SAA, was inoculated into newborn, susceptible mice. SAA inoculated mice developed lymphomas more slowly than SL3-3-inoculated mice and in one strain had a substantially lower incidence of disease than SL3-3-inoculated mice. To determine whether the SAA virus reverted to the wild type sequence during leukemogenesis, the LTR enhancer sequences of proviruses within the DNAs of tumor and nontumor tissue were examined by PCR amplification and DNA sequencing. In the majority of SAA-inoculated mice that developed lymphomas, reverted proviruses were detected. In none of the non-diseased, SAA-inoculated mice examined were reverted proviruses detected. Thus, the SL3-3 core is virtually necessary for disease. Proviruses adjacent to c-myc and pim-1 were detected by Southern blot in some of the tumor DNAs. In all such proviruses examined the SL3-3 core or a potential core suppressor mutant is present and in none is only the Akv core present. Thus, the nature of the enhancer in proviruses adjacent to proto-oncogenes is important for viral leukemogenicity. ftn*All degree requirements completed in 1991, but degree will be granted in 1993.