Changes in DNA copy numbers of bovine papillomavirus type 1 after termination of retinoic acid treatment.

The number of bovine papillomavirus type 1 (BPV) DNA copies [plasmid pdBPV-1 (142-6)] was examined in transformed C127 cells of an RIII mouse during exposure to all-trans-retinoic acid (RA) and after its withdrawal. RA treatment of a transformed cell line reduced the number from approximately 60 copies to an average of less than one copy per cell within 5 weeks. The composition of the RA-treated cell population was heterogeneous with respect to BPV DNA copies: 89.7% of the cells had no detectable copies, 8.6% had one copy, 1.7% had fewer than five copies, and one in 13,000 cells carried more than 10 copies. The low number of BPV DNA copies in the RA-treated cell population did not increase when the cells were subcultured before reaching confluence. RA-treated cell populations that contained less than one BPV DNA copy lost the transformed phenotype. However, a small fraction of cells (1 in 13,000) with greater than or equal to 10 BPV DNA copies retained the capacity to develop into transformed colonies. The relevance of these results to the regression of papillomavirus, DNA-carrying human lesions after exposure to retinoids and the redevelopment of these lesions after withdrawal of retinoids is discussed.