Methods for the construction of human papillomavirus vectors

A number of vector systems have been developed for the delivery of therapeutic genes into cells (1). Many viral vectors suffer from the disadvantage of random integration into the chromosome, making the expression of the cloned genes dependent on the chromosomal context of the inserted DNA. Papillomaviruses (PVs) are potentially important vector systems because of their extrachromosomal replication in target cells. The PVs are small DNA viruses that infect humans and a wide range of animals. Human papillomaviruses (HPVs) induce benign proliferative squamous eplthelial and fibro-epithelial lesions (warts and papillomas) in their natural hosts (2). Some HPVs are also involved in the pathogenesis of anogenital cancer and, in particular, cancer of the cervix (2). Papillomaviruses contain circular, double-stranded DNA of approx 8 kb, and usually replicate extrachromosomally at a copy number estimated to be between 10 and 100 (3). The potential advantages of PV vectors include expression of cloned genes from an extrachromosomal state that may be more amenable to uniform expression and possible elimination of problems associated with integration of DNA into transcriptionally inactive regions of the cellular chromosomes. Since PV DNA is not encapsidated, it may be possible to insert larger DNA sequences into such vectors, provided the DNA can still replicate in a stable manner. Bovine papillomavirus type 1 (BPV-1) vectors have been used to produce stable cell lines expressing foreign proteins (for reviews, see refs. 4-7). The BPV-1 vectors used in these studies contained both replication and transforming genes, and in most cases, extrachromosomal replication of these vectors was accompanied by transformation of the target cells (8-12). Recent advances in our understanding of the replication and transforming genes of PVs have resulted in renewed efforts to develop vectors that can be established as stable extrachromosomal plasmids and express foreign proteins without oncogenic transformation of the host cell. The current article deals mostly with the potential of HPVs as vectors for gene therapy.