Direct transformation of rodent fibroblasts by jaagsiekte sheep retrovirusDNA

Jaagsiekte sheep retrovirus (ISRV) is the causative agent of ovine pulmonar y carcinoma, a unique animal model for human bronchioalveolar carcinoma. We previously isolated a JSRV proviral clone and showed that it was both infe ctious and oncogenic. Thus JSRV is necessary and sufficient for the develop ment of ovine pulmonary carcinoma, but no data are available on the mechani sms of transformation. Inspection of the JSRV genome reveals standard retro viral genes, but no evidence for a viral oncogene. However, an alternate OR F in pol (orf;x) might be a candidate for a transforming gene. We tested wh ether the JSRV genome might encode a transforming gene by transfecting an e xpression plasmid for ISRV [pCMVJS21, driven by the cytomegalovirus (CMV) i mmediate early promoter] into mouse NIH 3T3 cells. Foci of transformed cell s appeared in the transfected cultures 2-3 weeks posttransfection; cloned t ransformants showed anchorage independence for growth, and they expressed J SRV RNA, These results indicate that the JRSV genome contains information w ith direct transforming potential for NIH 3T3 cells, Transfection of a muta ted version of pCMVJS21 in which the orf-x protein was terminated by two st op codons also gave transformed foci, Thus, orf-x was eliminated as the can didate transforming gene. In addition, another derivative of pCMVJS21 (pCMV JS21 Delta GP) in which the gag, pol(and orf-x) coding sequences were delet ed also gave transformed foci. These results indicate that the envelope gen e carries the transforming potential. This is an unusual example of a nativ e retroviral structural protein with transformation potential.