Mus dunni endogenous virus (MDEV)
Retroviral vectors are being increasingly used to deliver genes to human cells which are then returned to a patient' s body, both for medical research and direct therapeutic goals. One requirement in such ex vivo gene transfer protocols is that the transduced human cells be screened for contaminating replication-competent retroviruses (RCR). RCR was detected using a vector-rescue assay employing Mus dunni cells. We determined that the RCR was being activated from the dunni cells themselves by hydrocortisone present in the patient cell culture medium, and therefore named the novel virus Mus dunni endogenous virus (MDEV). Retroviral interference experiments indicate that MDEV uses a different receptor for cell entry than those used by previously-known mouse retroviruses. The MDEV receptor is widely expressed, as judged by the ability of MDEV to infect many cell types from many species. We molecularly cloned MDEV to further study its envelope and receptor usage. Southern blot analyses demonstrate that MDEV is endogenous to Mus dunni wild Asian mice rather than being a contaminant of the dunni cells, but is not present in other species tested, including other Asian wild mice. The sequence of the entire genome revealed novel features in addition to a distinct envelope. First, MDEV is a chimeric virus, with interior sequences derived from a virus similar to gibbon ape leukemia virus (GALV) and long terminal repeats (LTRs) derived from a virus-like 30 (VL30) retrotransposon(s). MDEV is the first example of a replication-competent retrovirus with VL30 sequences. Second, the enhancer region of the LTR contains more than six 80 bp repeats. We provide evidence that the native MDEV provirus has only one repeat, and that these repeats are multiplied upon activation or passage of the virus through a process we call LTR expansion. This repeat number varies tremendously within an MDEV population, with viruses containing 3.15 to 11.15 repeats. We show that an expanded LTR has a stronger promoter, which likely offers MDEV a replicative advantage. Finally, we have constructed and evaluated packaging cells expressing the MDEV envelope (PD223 cells) to take advantage of the broad distribution of the MDEV receptor.
- Pathology