C-type RNA tumor virus in wild house mice (Mus musculus)

Selective sweeps versus introgression - population genetic dynamics of the murine leukemia virus receptor Xpr1 in wild populations of the house mouse (Mus musculus)

Background

The interaction between viruses and their receptors in the host can be expected to lead to an evolutionary arms race resulting in cycles of rapid adaptations. We focus here on the receptor gene Xpr1 (xenotropic and polytropic retrovirus receptor 1) for murine leukemia viruses (MLVs). In a previous screen for selective sweeps in mouse populations we discovered that a population from Germany was almost monomorphic for Xpr1 haplotypes, while a population from France was polymorphic.

Results

Here we analyze Xpr1 sequences and haplotypes from a broad sample of wild mouse populations of two subspecies, M. m. domesticus and M. m. musculus, to trace the origins of this distinctive polymorphism pattern. We show that the high polymorphism in the population in France is caused by a relatively recent invasion of a haplotype from a population in Iran, rather than a selective sweep in Germany. The invading haplotype codes for a novel receptor variant, which has itself undergone a recent selective sweep in the Iranian population.

Conclusions

Our data support a scenario in which Xpr1 is frequently subject to positive selection, possibly as a response to resistance development against recurrently emerging infectious viruses. During such an infection cycle, receptor variants that may convey viral resistance can be captured from another population and quickly introgress into populations actively dealing with the infectious virus.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0528-5) contains supplementary material, which is available to authorized users.

Generation of fibrosarcomas in vivo by a retrovirus that expresses the normal B chain of platelet-derived growth factor and mimics the alternative splice pattern of the v-sis oncogene

A retrovirus containing the entire human platelet-derived growth factor B-chain (PDGF-B) gene was constructed in order to investigate the in vivo biological activity of its encoded growth factor. When this virus was introduced into newborn mice, it reproducibly generated fibrosarcomas at the site of inoculation. Proviruses in each fibrosarcoma analyzed had lost 149 nucleotides downstream of the PDGF-B coding region. This deletion originated from an alternative or aberrant splice event that occurred within exon 7 of the PDGF-B gene and mimicked the v-sis oncogene. Thus, deletion of this region may be necessary for efficient retrovirus replication or for more potent transforming function. Evidence that the normal growth factor coding sequence was unaltered derived from RNase protection studies and immunoprecipitation analysis. Tumors were generally polyclonal but demonstrated clonal subpopulations. Moreover, tumor-derived cell lines became monoclonal within a few tissue culture passages and rapidly formed tumors in vivo. These findings argue that overexpression of the normal human PDGF-B gene product under retrovirus control can induce the fully malignant phenotype.

Characterization of genome structure of amphotropic and ecotropic wild mouse retroviruses

We studied the RNA genomes of several wild mouse type C retroviruses by using RNase T1-oligonucleotide fingerprinting. The amphotropic and ecotropic viruses of field strain 1504 produced very similar oligonucleotide fingerprints, but each also had several unique oligonucleotides. All of these unique oligonucleotides were located in the env gene region and were probably responsible for the host range differences between these viruses, as well as the lymphomagenic and paralytogenic properties of the viruses. We obtained similar results with the amphotropic and ecotropic viruses of another field strain (4070), which was isolated from a mouse from a different trapping area. The amphotropic viruses of several field strains (strains 1504, 4070, and 1313) were more closely related than the ecotropic viruses of different strains (strains 1504, 4070, and 4996). These findings suggested that the genetic sequences of the amphotropic viruses are more conserved than those of ecotropic viruses isolated from the same wild mice.

Molecular cloning of integrated simian sarcoma virus: genome organization of infectious DNA clones

The integrated form of simian sarcoma virus (SSV) was molecularly cloned in the Charon 16A strain of bacteriophage lambda. In transfection analysis, the recombinant viral DNAs demonstrated the ability to transform cells in tissue culture at high efficiency. Such transformants possessed typical SSV morphology, expressed simian sarcoma associated virus (SSAV) gag gene products in the absence of virus release, and released SSV after superinfection with a type C helper virus. A physical map of the 5.8-kilobase-pair (kbp) recombinant viral DNA clone, deduced from restriction endonuclease analysis, revealed a 5.1-kbp SSV genome containing 0.55-kbp-long terminal repeats flanked by 0.45 and 0.25 kbp of contiguous host cell sequences. By R-loop analysis, the viral DNA molecule contained two regions of homology to SSAV, separated by a 1.0-kbp nonhomologous region. This SSV-specific sequence was shown to be uniquely represented within the normal cellular DNA of diverse mammalian species, including human. Our results demonstrate that this primate transforming retrovirus arose in nature by recombination of a type C helper virus and a host cellular gene.