Identification of Genetic Determinants That Regulate Tumorigenicity of Friend Murine Leukemia Virus in Rats

Abstract

Friend murine leukemia virus (FrMLV) clone A8 causes a progressive neurodegenerative disease when the virus is inoculated into newborn rats in addition to the related clones PVC211 and FB29, and of a variant, NT40, derived from FB29 (5,16,24). It has been reported that PVC211, FB29 and NT40 do not induce leukemia in rats, as they do in mice (16,17,23,25). However, clone A8 has high leukemogenic potential against both mice and rats (25). When the A8 virus was inoculated into newborn mice, all of the mice exhibited splenomegaly at 2 3 months post-infection to the same extent as FrMLV clone 57. In the case of inoculation into newborn rats, the A8 virus induced thymoma and leukemic cell infiltration to organs at 7 8 weeks postinfection, but the onset of the disease by 57-virus infection was observed at 20 weeks post-infection (25). The previous studies of chimeric viruses between A8 and 57 revealed that the ClaI-AatII fragment containing the LTR and the 5' half of the 5' leader sequence of A8 is crucial for the induction of aggressive leukemia in rats (25). The LTR of MLV contains enhancer motifs, which are important determinants of leukemogenicity, tissue tropism, and disease specificity (3, 7, 10 13). The 5' leader sequence of MLV, which extends from the 3' end of a primer-binding site to the initiation codon for gag (4, 9), also plays an important role in virus replication. This region contains a donor site for the generation of spliced subgenomic mRNA (4, 22), the packaging signal for the incorporation of genome RNA into virions (1,8), and the internal ribosomal entry mechanism that promotes the translation of gag polyprotein precursors (2). In this study, we restricted the genetic determinants responsible for the induction of thymoma and leukemic infiltration to organs with rapid progression caused by A8 virus in rats. Materials and MethodsViruses and cells. Chimeric viruses combining the high-tumorigenic A8 and the low-tumorigenic 57 (18) were prepared as described previously (24) (Fig. 2). Briefly, to construct R7a, the AatII-HindIII fragment Abstract: A neuropathogenic variant of Friend murine leukemia virus (FrMLV), clone A8, has been shown to cause thymoma and infiltration of leukemic cells to organs at 7 8 weeks post-infection in rats with a more rapid progression than clone 57. We have previously reported that the determinant for induction of aggressive leukemia in rats is located in the ClaI-AatII fragment containing the long terminal repeat (LTR) and the 5' half of the 5' leader sequence of A8 virus. Further studies of chimeric viruses restricted the determinant for the induction of thymoma to only the 0.6-kb ClaI-KpnI fragment of A8. This fragment contains a 0.1 kb region of the 3' terminus of the env gene, the intergenic region, the U3, and the 5' half of the R region in the LTR. Major differences in the fragment between A8 and 57 viruses were found in the U3 region, especially in the enhancer motifs. These results indicate that the enhancer region of A8-LTR contributes to the manifestation of thymom...

Increased Expression of c‐myc Is Associated with Thymoma in Rats Infected with Murine Leukemia Virus A8

“…It is assumed that leukemias and lymphomas induced by retroviruses can activate c-myc by use of virally specified proteins that increase c-myc transcription (3). The viral proteins of A8 may activate c-myc, because during the early phase of infection, viral titers in the thymus of rats infected with A8 were higher than those of rats infected with R7c, Rec5, and R7f (14,15).…”

“…R7c, Rec5, and R7f, which are chimeric viruses between A8 and 57 (8), are low tumorigenicity viruses. The structures of these viruses are described in previous reports (14,15).…”

“…The ability of viruses to cause disease was assessed using newborn Lewis rats purchased from a commercial breeder. The titers of inoculated virus were described in previous reports (14,15). At 7-8 weeks post-infection, exsanguination of the infected animals was performed and the thymuses stored at Ϫ80 C until extraction of DNA and RNA.…”