In HPV–related cancers, the “high-risk” human papillomaviruses (HPVs) are frequently found integrated into the cellular genome. The integrated subgenomic HPV fragments express viral oncoproteins and carry an origin of DNA replication that is capable of initiating bidirectional DNA re-replication in the presence of HPV replication proteins E1 and E2, which ultimately leads to rearrangements within the locus of the integrated viral DNA. The current study indicates that the E1- and E2-dependent DNA replication from the integrated HPV origin follows the “onion skin”–type replication mode and generates a heterogeneous population of replication intermediates. These include linear, branched, open circular, and supercoiled plasmids, as identified by two-dimensional neutral-neutral gel-electrophoresis. We used immunofluorescence analysis to show that the DNA repair/recombination centers are assembled at the sites of the integrated HPV replication. These centers recruit viral and cellular replication proteins, the MRE complex, Ku70/80, ATM, Chk2, and, to some extent, ATRIP and Chk1 (S317). In addition, the synthesis of histone γH2AX, which is a hallmark of DNA double strand breaks, is induced, and Chk2 is activated by phosphorylation in the HPV–replicating cells. These changes suggest that the integrated HPV replication intermediates are processed by the activated cellular DNA repair/recombination machinery, which results in cross-chromosomal translocations as detected by metaphase FISH. We also confirmed that the replicating HPV episomes that expressed the physiological levels of viral replication proteins could induce genomic instability in the cells with integrated HPV. We conclude that the HPV replication origin within the host chromosome is one of the key factors that triggers the development of HPV–associated cancers. It could be used as a starting point for the “onion skin”–type of DNA replication whenever the HPV plasmid exists in the same cell, which endangers the host genomic integrity during the initial integration and after the de novo infection.

We found that recircularized high-risk (type 16 and 18) and low-risk mucosal (type 6b and 11) and cutaneous (type 5 and 8) human papillomavirus (HPV) genomes replicate readily when delivered into U2OS cells by electroporation. The replication efficiency is dependent on the amount of input HPV DNA and can be followed for more than 3 weeks in proliferating cell culture without selection. Cotransfection of recircularized HPV genomes with a linear G418 resistance marker plasmid has allowed subcloning of cell lines, which, in a majority of cases, carry multicopy episomal HPV DNA. Analysis of the HPV DNA status in these established cell lines showed that HPV genomes exist in these cells as stable extrachromosomal oligomers. When the cell lines were cultivated as confluent cultures, a 3-to 10-fold amplification of the HPV genomes per cell was induced. Two-dimensional (2D) agarose gel electrophoresis confirmed amplification of mono-and oligomeric HPV genomes in these confluent cell cultures. Amplification occurred as a result of the initiation of semiconservative two-dimensional replication from one active origin in the HPV oligomer. Our data suggest that the system described here might be a valuable, cost-effective, and efficient tool for use in HPV DNA replication studies, as well as for the design of cell-based assays to identify potential inhibitors of all stages of HPV genome replication.Due to their association with distinctive human cancers, human papillomaviruses (HPVs) are widely studied. Papillomaviruses have been phylogenetically grouped into genera, species, types, subtypes, and variants (11), and more than 100 HPV types have been identified thus far. The best-characterized ␣-genus HPVs are associated with infections of the mucosal epithelia that lead to the induction of benign tumors. These viruses are divided into high-risk types (e.g., high-risk HPV type 16 [HR-HPV-16] and -18), which have the capability of inducing anogenital malignancies, and low-risk types (e.g., LR-HPV-6 and -11), which induce hyperproliferative mucosal lesions and are rarely associated with malignancy. Mucosal HPV infections tend to clear on their own, but in some cases, latent infection could be established and may persist for years. Cutaneous ␤-genus HPV infections are highly prevalent in the general population and tend to persist (14). Vaccines based on virus-like particles made up of the capsid protein L1 have been developed against and Gardasil/Silgard [Merck Research Laboratories]). Because these virus types are responsible for only a portion of all HPV-induced malignant and benign tumors, a clear need exists for vaccines or antivirals against a broader spectrum of pathogenic HPV types.Despite the differences in viral pathogenesis, progeny virion production invariably depends on cell differentiation and occurs only in terminally differentiated keratinocytes. HPVs require the host's replication machinery to reproduce their genomes, and these viruses have developed a unique replication strategy that is adapted to keratinocyte di...

We describe the extensive and progressive oligomerization of human papillomavirus (HPV) genomes after transfection into the U2OS cell line. The HPV genomic oligomers are extrachromosomal concatemeric molecules containing the viral genome in a head-to-tail orientation. The process of oligomerization does not depend on the topology of the input DNA, and it does not require any other viral factors besides replication proteins E1 and E2. We provide evidence that oligomerization of the HPV18 and HPV11 genomes involves homologous recombination. We also demonstrate oligomerization of the HPV18 and HPV11 genomes in SiHa, HeLa, and C-33 A cell lines and provide examples of oligomeric HPV genomes in clinical samples obtained from HPVinfected patients. Human papillomaviruses (HPV) are important pathogens that cause different epithelial hyperplastic lesions, most commonly manifesting as benign warts or papillomas. Over 100 HPV types have been identified to date (1). These epitheliotropic viruses can be categorized based on their ability to infect mucosal or cutaneous keratinocytes. The mucosal viruses can be further subdivided into low-and high-risk HPVs. A potential for malignant progression is characteristic of high-risk HPV types, such as HPV18, HPV16, HPV31, and HPV45, whereas types such as HPV6 and HPV11 do not show similar associations and are considered low risk (2). Essentially all cervical carcinomas (3) and a quarter of reported head and neck cancers (4) are associated with HPV infections.HPV is a small DNA virus with an approximately 8-kbp genome. During infection of stratified cutaneous or mucosal epithelia, the viral genomes replicate as multicopy extrachromosomal genetic elements in the nuclei of host cells. HPV genomes undergo a three-phase replication cycle linked to the host cell differentiation program (5). The first stage of HPV DNA replication occurs in undifferentiated basal keratinocytes after infection and is referred to as transient amplificational replication. During the first phase, the viral replication factors are produced and the HPV genome is amplified up to 100 of copies per cell during the S phase of the cell cycle. After initial amplification, the expression of viral replication proteins is downregulated to a level sufficient for the stable maintenance phase of episomal genomes in HPV-infected basal cells. Upon differentiation of the infected cells, the regulated expression of viral proteins initiates a second amplification of the viral genome, the production of capsid proteins, and the assembly of viral particles in the uppermost layers of terminally differentiated epithelium. The mechanisms regulating the switch from the initial HPV genome replication to HPV genome maintenance and, subsequently, to vegetative amplification are not entirely understood.Replication of the HPV genome is carried out by the cellular replication machinery, which is directed to the viral origin by the viral replication proteins E1 and E2. The mechanism underlying the initiation of DNA replication is well described...

BackgroundAlthough prophylactic vaccines have been developed against HPV6, HPV11, HPV16 and HPV18 there is the clear unmet medical need in order to justify the development of drugs targeting human papillomavirus replication. The native host cells of HPVs are human primary keratinocytes which can be cultivated in raft cultures. However, this method is difficult to use in high-throughput screening assays and the need for a cost-effective cellular system for screening potential anti-HPV drug candidates during all stages of HPV genome replication remains.MethodsU2OS cells were transfected with HPV11 wt or E8- minicircle genomes and their gene expression was studied via 3′ RACE, 5′ RACE or via real time PCR methods. The DNA replication of these genomes was detected by Southern blot methods.ResultsThe analysis of HPV11 transcripts in U2OS cells showed that the patterns of promoter use, splice sites and polyadenylation cleavage sites are identical to those previously characterized in human HPV-related lesions, human squamous carcinoma cell lines (e.g., SSC-4) and laryngeal papillomas. Transcriptional initiation from the three previously described HPV11 promoters in the E6 and E7 ORFs (P90, P264, and P674-714) were functional, and these promoters were used together with two promoter regions in the E1 ORF (P1092 and P1372). Mutating the E8 ORF ATG start codon to ACG eliminated the translation of fusion proteins from the E8 ORF coupled to E1 and E2 proteins C-terminal sequences, leading to the de-repression of gene expression (particularly from the P1092 promoter) and to the activation of genome replication. These data suggested that the expression of the functional E8^E2 protein is used to control viral gene expression and copy number of the HPV11 genome. The analysis of HPV11 E1 expression plasmids showed that the E6/E7 region, together with the E1 coding region, is crucial for the production of functionally active E1 protein.ConclusionsThe data presented in this paper suggest that in human osteosarcoma cell line U2OS the gene expression pattern of the HPV11 truly reflect the expression profile of the replicating HPV genome and therefore this cellular system is suitable for drug development program targeting HPV replication.

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