ESCAPE FROM IN-VIVO RESTRICTION OF MOLONEY MINK CELL FOCUS-INDUCING VIRUSES DRIVEN BY THE MO+PYF101 LONG TERMINAL REPEAT (LTR) BY LTR ALTERATIONS

Mo+PyF101 M-MuLV is a variant Moloney murine leukemia virus containing polyomavirus F101 enhancers inserted just downstream from the M-MuLV enhancers in the long terminal repeat (LTR). The protein coding sequen ces for this virus are identical to those of M-MuLV. Mo+PyF101 M-MuLV induces T-cell disease with a much lower incidence and longer latency than wild-type M-MuLV. We have previously shown that Mo+PyF101 M-MuLV is defective in preleukemic events induced by wild-type M-MuLV, includ ing splenic hematopoietic hyperplasia, bone marrow depletion, and gene ration of recombinant mink cell focus-inducing viruses (MCFs). We also showed that an M-MCF virus driven by the Mo+PyF101 LTR is infectious in vitro but does not propagate in mice. However, in these experiments , when a pseudotypic mixture of Mo+PyF101 M-MuLV and Mo+PyF101 MCF was inoculated into newborn NIH Swiss mice, they died of T-cell leukemia at times almost equivalent to those induced by wild-type M-MuLV. Tumor DNAs from Mo+PyF101 M-MuLV-Mo+PyF101 MCF-inoculated mice were examine d by Southern blot analysis. The predominant forms of Mo+PyF101 MCF pr oviruses in these tumors contained added sequences in the U3 region of the LTR. The U3 regions of representative tumor-derived variant Mo+Py F101 MCFs were cloned by polymerase chain reaction amplification, and sequencing indicated that they had acquired an additional copy of the M-MuLV 75-bp tandem repeat in the enhancer region. NIH 3T3 cell lines infected with altered viruses were obtained from representative Mo+PyF 101 M-MuLV-Mo+PyF101 MCF-induced tumors, and mice were inoculated with the recovered viruses. Leukemogenicity was approximately equivalent t o that in the original Mo+PyF101 M-MuLV-Mo+PyF101 MCF viral stock. Sou thern blot analysis on the resulting tumors now predominantly revealed loss of the polyomavirus sequences. These results suggest that the su ppressive effects of the PyF101 sequences on M-MuLV-induced disease an d potentially on MCF propagation were overcome in two ways: by triplic ation of the M-MuLV direct repeats and by loss of the polyomavirus seq uences.