RNA-dependent DNA polymerase activity in canine lymphosarcoma

Abstract

An assay for RNA-dependent DNA polymerase (RDDP) was used as a screening test for retrovirus expression in canine lymphosarcoma tissue. Enzyme activity of borderline significance was detected with a poly (rC): oligo (dG) template in the supernatants of 3 out of 14 short term cultures of lymphosarcoma cells. Induction with halogenated pyrimidines was not obligatory to detect this activity. In crude preparations of tumour tissue RDDP activity was detected in 2 of 11 cases. The inability to detect enzyme activity with a poly (dA): oligo (dT) template in the culture supernatants or in the tumour preparations, excluded the possibility that there was significant contamination with terminal transferase or cellular DNA polymerases. Confirmatory evidence that particles with retroviral properties were present in the two positive tumour preparations was provided by positive culture supernatant RDDP activity or by the presence of RNA containing particles of oncoviral density. However no oncoviruses were isolated by cocultivation with canine kidney cells (MDCK). In the household of one of the RDDP positive cases a second case of lymphosarcoma had recently occurred in an unrelated dog.