Integration and transcription of human papillomavirus type 6 recombinant DNA in mouse cells

Human papillomavirus (HPV) DNA is found in nature mostly in an episomal form. However, in permanent cell lines established from cervical carcinomas in which HPV sequences are present, they are usually integrated in the host genome. In vitro studies of HPV have been hampered because of the difficulty of stably maintaining HPV sequences in transfected cells. We have cloned the entire HPV6 genome into three vectors: pML2, a derivative of pBR322 lacking the "poison sequences"; p142-6, a plasmid containing the complete bovine papillomavirus type 1 (BPV-1) genome which usually remains extrachromosomal in transfected mouse cells; and p302-3, a plasmid containing the bacterial gene conferring resistance to neomycin. Each of the three constructs has been transfected onto C127 mouse cells. In the case of the recombinant molecules cloned into pML2 and into p42-6, the transfection had been done together with the p302-3 plasmid. G418 resistant colonies were selected and analyzed for their DNA content. In all cases (5/5) the transfected molecules were present and integrated in the cellular genome. No free episomal DNA was detected, even in the cells transfected with the BPV-1 containing plasmid. One junction with the flanking cellular sequences is located in the late open reading frames region of the HPV6 genome and there is a disruption of the early region, as has been described for the HPV18 genome integrated into the cervical carcinoma cell line HeLa. Transcription analysis revealed transcripts of heterogeneous sizes, spanning the distal early region (E2S4E5) of the HPV6 genome, as happens in vivo in Condylomata acuminata, although in these genital lesions the HPV6 genome is maintained as a non-integrated episome.