No abstract

A new type of murine leukemia virus has been detected in thymuses of eukemic and late preleukemic AKR mice, in lymphomas developing in NIH Swiss mice carrying the AKR ecotropic virus-inducing loci Akv-1 or Akv-2, and in

Cells of embryos of the high leukemic mouse strain AKR can be grown in culture as virus-negative cell lines. However, these lines and clonal sublines uniformly have the capacity to initiate synthesis of murine leukemia virus. Exposure of the cells to 5-iododeoxyuridine or 5-bromodeoxyuridine induced synthesis of virus in as high as 0.1 to 0.5 percent of the cells; many of the cells were producing virus as soon as 3 days after initiation of treatment. Induction of virus by these drugs is several orders of magnitude greater than that obtained with any other treatment tested. These studies indicate that the full genome of murine leukemia virus is present in an unexpressed form in all AKR cells and provide a potentially powerful technique for activating leukemia virus genomes in other cell systems.

Recombinant mink cell focus-inducing (MCF) murine leukemic viruses, as well as ecotropic and xenotropic viruses, were tested for ability to accelerate or cause development of lymphoma in AKR and other strains of mice. Of the three classes of virus isolated from AKR, only the MCF viruses were able to accelerate development of AKR lymphoma. This fully supports the idea that the MCF viruses are the proximal cause of spontaneous AKR lymphoma. MCF lymphomagenicity was strain specific, however, in that AKR MCF viruses did not induce lymphomas in many murine strains; they were moderately lymphomagenic in C3H/Bi mice and in National Institutes of Health Swiss partially congenic for Akv-1 or Akv-2. In contrast, MCF viruses from nonthymic hematopoietic neoplasms of C3H/Fg, BALB/c, or mice partially congenic for ecotropic virus loci (Akv-1, Akv-2, Fgv-1, C58v-1, and C58v-2) were not able to accelerate or cause lymphomia in AKR or any other mouse strain tested, including some of the strains of origin. MCF lymphomagenicity correlated with thymic origin in the virus and with ability to replicate in the thymus.

Transcriptional enhancers of replication-competent mouse C-type retroviruses are potent determinants of the distinct disease-inducing phenotypes of different viral isolates and can also strongly influence the incidence and latent period of disease induction. To study the contribution of individual protein-binding sites to viral pathogenicity, we introduced mutations into each of the known nuclear factor-binding sites in the enhancer region of the Moloney murine leukemia virus and injected viruses with these mutations into newborn NFS mice. All viruses induced disease. Viruses with mutations in both copies of the leukemia virus factor a (LVa) site, leukemia virus factor c (LVc) site, or in just the promoter proximal copy of the glucocorticoid response element (GRE) had a latent period of disease onset and disease specificity indistinguishable from that of the wild-type Moloney virus. Viruses with mutations in two or three of the GREs, in both copies of the leukemia virus factor b (LVb) site, in two of the four nuclear factor 1 (NF1) consensus motifs, or in both copies of the conserved viral core element showed a significant delay in latent period of disease induction. Strikingly, viruses with mutations in the core element induced primarily erythroleukemias, and mutations in the LVb site also resulted in a significant incidence of erythroleukemias. These and other genetic and biochemical studies suggest models for how subtle alterations in the highly conserved structure of mouse C-type retrovirus enhancers can produce a dramatic effect on disease specificity.