Acutely Transforming Retrovirus Expressing Nras Generated from HT-1080 Fibrosarcoma Cells Infected with the Human Retrovirus XMRV

Virus from HT-1080 fibrosarcoma cells infected with the human retrovirus XMRV (xenotropic murine leukemia virus-related virus) can induce rare foci of transformation in rat 208F fibroblasts. Characterization of three such foci revealed that one produced an acutely transforming virus at a high titer. The virus consists of a mutant Nras cDNA from the HT-1080 cells inserted into a retroviral vector (added to the HT-1080 cells as a marker for infection) in place of internal vector sequences. These results show that XMRV can generate acutely transforming viruses at a low rate, as is typical of other replication-competent retroviruses, and reveal the potential for transforming virus contamination of retroviral vectors made from transformed cell lines.XMRV (xenotropic murine leukemia virus-related virus) has been associated with prostate cancer (19, 20) and chronic fatigue syndrome (12), although some researchers fail to detect XMRV in other populations with these diseases (4, 8). XMRV is found integrated into human genomic DNA from prostate cancer samples, indicating that it is indeed a human retrovirus and not a laboratory contaminant (3, 9). Because of the potential role of XMRV in prostate cancer, we previously tested XMRV for transforming activity in fibroblast and epithelial cell lines. Although XMRV is a simple retrovirus that does not carry a host-derived oncogene, there is precedence for transformation by retroviral Env genes (21, 22). However, transfection of XMRV proviral DNA or viral envelope expression vectors into 208F rat fibroblasts did not result in transformation, and infection of most cell types tested with XMRV did not induce transformation (13). In contrast, infection of 208F cells with XMRV did result in rare transformed foci suggestive of oncogene activation by XMRV. Characterization of cells from three transformed foci produced by infection of 208F cells with virus from HTX cells (a pseudodiploid subclone of HT-1080 fibrosarcoma cells 18]) infected with XMRV and the LAPSN retroviral vector (included as a marker for infection) revealed that all produced XMRV and that one produced a highly active transforming virus (13).