Analysis of a Type D Retroviral Capsid Gene Expressed in Ovine Pulmonary Carcinoma and Present in Both Affected and Unaffected Sheep Genomes

Ovine pulmonary carcinoma is a contagious neoplastic disease of sheep which can be transmitted experimentally with cell-free filtrate. Products homologous to structural proteins of type D and type B retroviruses are expressed in the tumors, but an infectious retrovirus has yet to be isolated. Retroviral RNA was isolated from an affected Peruvian sheep and used as a substrate for reverse transcription. The DNA obtained was subjected to polymerase chain reaction amplification using degenerate primers designed to amino acid sequences shared by the capsid peptides of the viruses that cross-react with the proteins found in the tumor tissues. A retroviral capsid region of type D lineage was isolated that proved to be highly homologous to the jaagsiekte sheep retrovirus isolated from affected sheep in South Africa. This capsid sequence was used to detect the virus in the genomes of sheep with and without ovine pulmonary carcinoma. More than 20 endogenous sequences that hybridized with the probe were found in the sheep examined, and assortment of these could be followed in sheep families. Initial experiments did not show any obvious differences in viral presence in the tumors compared with the background of endogenous viral sequences in unaffected lungs.