A Novel Truncated env Gene Isolated from a Feline Leukemia Virus-Induced Thymic Lymphosarcoma

Abstract

We PCR amplified the exogenous feline leukemia virus (FeLV)-related env gene species from lymphosarcomas induced by intradermally administered plasmid DNA of either the prototype FeLV, subgroup A molecular clone, F6A, or a new molecular clone, FeLV-A, Rickard strain (FRA). Of the nine tumors examined, six showed the presence of deleted env species of variable sizes in the tumor DNA. One env mutant, which was detected in a FRA-induced thymic lymphosarcoma, had a large internal deletion beginning from almost the N-terminal surface glycoprotein (SU) up to the middle region of the transmembrane (TM) protein of the env gene. The deduced polypeptide of this truncated env (tenv) retained the complete signal peptide and seven amino acids of the N-terminal mature SU of FRA env gene, followed by eight amino acids from the frameshift in the TM region. To study the biological function of tenv, we used a murine retrovirus vector to produce amphotropic virions. Infection of feline fibroblasts (H927), human fibrosarcoma cells (HT1080), or human B-lymphoma cells (Raji) led to pronounced cytotoxicity, while the tenv virus did not induce significant cytotoxicity to feline T-lymphoma cells (3201B) or human T-lymphoma cells (CEM). Together, these results convincingly demonstrated that the genetic events that led to truncation in the env gene occurred de novo in FeLV lymphomagenesis and that such a product, tenv could induce cytotoxicity to fibroblastic and B-lymphoid cells but not to T-lymphoid tumor cells. This type of selective toxicity might be potentially important in the development of the neoplastic disease.Retroviruses are causative agents in the induction of lymphoid malignancies (leukemia-lymphoma complex) in mammals, including humans. In the domestic cat, an outbred species, which has the highest incidence of lymphoid malignancies of any animal, the disease is naturally associated with chronic feline leukemia virus (FeLV) infection (8, 18). There is solid evidence to indicate that interactions between infectious FeLV and non-infectious inherited endogenous FeLV elements generate recombinant viral quasispecies which represent a variety of chimeric envelope glycoproteins depending on the extent of amino terminal portion replaced by the endogenous env sequences (9,13,24). The viral species with specific adaptive amino acid mutations and with certain sites of recombination are rapidly selected for replication efficiency and are overrepresented at later time points after infection (2,3,14). FeLVs with recombinant env genes are detected with high frequency in naturally as well as experimentally induced feline lymphosarcomas (2,3,10,14,23,25). Evidence also exists to suggest that some defective env genes detected in FeLV-induced lymphosarcomas may be additional factors in the disease process (16,23).Although a previous study addressed the issue of in vivo derivation of defective env genes from an FeLV, subgroup A (FeLV-A) molecular clone (16), administration of an inoculum prepared by propagating the virus in feline cell culture...

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