[Biological significance of HPV sequences integrated into cancer cells]

In cervical cancer with integrated human papillomavirus (HPV) type 16 or 18 DNA, E6 and E7 regions of HPV 16 or 18 were always present, suggesting that these regions play an important role in development of cervical cancer. Structure and biological activity of cDNAs representing two types of HPV 18 transcripts in HeLa cells were analyzed. Three open reading frames (E6, E6* and E7) were identified. These open reading frames were introduced into the expression vectors. Among them, E7 alone provided NIH3T3 cells with the ability of anchorage-independent growth. Furthermore, introduction of E7 into mouse primary keratinocytes lead to immortalization of the cells. We also isolated cDNA clones of HPV 16 transcripts from the NIH3T3 transformant which had been obtained by transfection of genomic DNA from a cervical cancer tissue containing HPV 16 DNA. The structure of some of these cDNA clones were analyzed. The clones consisted of joined viral and human sequences and had entire E7 open reading frame together with spliced or truncated E6 open reading frame. These clones were subcloned into the expression vectors. NIH3T3 cells transfected separately by these constructs grew in soft agar but were not tumorigenic in nude mice.