Tumorigenic Potential of a Recombinant Retrovirus Containing Sequences from Moloney Murine Leukemia Virus and Feline Leukemia Virus

A recombinant retrovirus, termed MoFe2-MuLV, was constructed in which the U3 region of T-lymphoma- genic Moloney murine leukemia virus (Mo-MuLV) was replaced by that of FeLV-945, a provirus of unique long terminal repeat (LTR) structure identified only in non-T-cell, non-B-cell lymphomas of the domestic cat. The LTR of FeLV-945 is unusual in that it contains only a single copy of the transcriptional enhancer followed 25 bp downstream by a 21-bp sequence in triplicate in tandem. Infectivity of MoFe2-MuLV was demonstrated in vitro in SC-1 cells and in vivo in neonatal NIH-Swiss mice. Tumors occurred in MoFe2-MuLV-infected animals following a latency period of 4 to 10 months (average, 6 months). The results of Southern blot analysis of the T-cell receptor beta locus demonstrated that all tumors were lymphomas of T-cell origin. MoFe2-MuLV LTRs were amplified by PCR from tumor DNA and were characterized by nucleotide sequence analysis. LTRs from the tumors that occurred with relatively shorter latency predominantly retained the original MoFe2-MuLV sequence intact and unaltered. Tumors that occurred with relatively longer latency contained LTRs that also retained the 21-bp sequence triplication characteristic of the original virus but had acquired various dupli- cations of enhancer sequences. The repeated identification of enhancer duplications in late-appearing tumors suggests that the duplication affords a selective advantage, although apparently not in the efficient induction of T-cell lymphoma. Proto-oncogenes known to be targets of insertional mutagenesis in the majority of Mo-MuLV-induced tumors or in feline non-T-cell, non-B-cell lymphomas were shown not to be rearranged in any tumor examined. Mink cell focus-inducing (MCF) proviral DNA was readily detectable in some, but not all, tumors. The presence or absence of MCF did not correlate with the kinetics of tumor induction. These studies indicate that the single-enhancer, triplication-containing FeLV LTR, typical of non-T-cell, non-B-cell lymphomas in cats, is competent in the induction of T-cell lymphoma in mice. The findings suggest that the mechanism of MoFe2-MuLV-mediated lymphomagenesis may differ from that of Mo-MuLV-mediated disease, considering the possible involvement of novel oncogenes and the variable presence of MCF recombinants. The replication-competent murine and feline leukemia vi- ruses lack an oncogene within the genome and induce malig- nant disease in a tissue-specific manner. For example, Moloney murine leukemia virus (Mo-MuLV), a replication-competent nonacute retrovirus, induces a T-lymphoblastic lymphoma in virtually 100% of susceptible neonatal mice. Mo-MuLV in- duces T-cell lymphoma with a latency period of 3 to 4 months, an indicator that multiple steps are involved in disease devel- opment. Events characteristically associated with Mo-MuLV- induced lymphoma include (i) virus replication in target cells of the bone marrow, spleen, and thymus, (ii) the appearance of mink cell focus-inducing (MCF) viruses that arise by env gene recombination between Mo-MuLV and endogenous MuLV- related sequences, and (iii) insertional mutagenesis of proto- oncogenes via adjacent proviral integration, thus activating the malignant potential of the proto-oncogene in the target cell for transformation (reviewed in reference 19). The major Mo-MuLV-encoded determinant of both tumor- igenic potency and T-cell disease specificity resides in the long terminal repeat (LTR), particularly within the tandemly dupli-