Differential effects of the splice acceptor at nucleotide 3295 of human papillomavirus type 31 on stable and transient viral replication

In human papillomavirus type 31 (HPV-31), the E1-E4 and E5 open reading frames are expressed from polycistronic mRNAs. The major polycistronic mRNAs which encode E1-E4 and E5 are spliced messages which utilize o splice acceptor at nucleotide (nt) 3295 (SPA³²⁹⁵). Our laboratory recently developed a recombinant system for the synthesis of HPVs following immortalization of primary keratinocytes with cloned HPV-31 genomes (M. G. Frattini et al., Proc. Natl. Acad. Sci. USA 93:3062-3067, 1996). These immortalized cell lines are capable of maintaining HPV-31 DNA as episomes and induce the synthesis of virions in organotypic raft culture. In this study, we used these methods to begin an analysis of the roles of E1-E4 and E5 in HPV pathogenesis by mutating the major splice at nt 3295. Mutation of SPA³²⁹⁵ did not significantly alter the ability of HPV-31 genomes to replicate transiently in keratinocytes, nor did the mutation affect the immortalization potential of HPV-31. However, genomes carrying the SPA³²⁹⁵ mutation were not stably maintained as viral episomes, and the resulting immortalized keratinocyte cell line contained multiple, integrated copies of the mutated HPV-31 DNA. Northern analysis indicated that cell lines immortalized with the mutant HPV-31 expressed transcripts which were similar in size and abundance to wild-type messages, including those transcripts which rely on utilization of SPA³²⁹⁵. RNase protection and reverse transcription-PCR revealed that mutation of SPA³²⁹⁵ resulted in the utilization of a cryptic splice acceptor at nt 3298. These data suggest that the requirements for stable maintenance of HPV genomes are more stringent than those for transient replication and that factors which define these requirement rely on the major splice acceptor at nt 3295.