Human papillomaviruses (HPVs) comprise an extended family of medically important human pathogens. These small DNA viruses establish persistent infections of squamous epithelia, typically causing benign hyperproliferative lesions. Infections by higher risk genotypes can progress to dysplasias and carcinomas, notably cervical and penile cancers. The double-stranded, circular genome replicates as extrachromosomal plasmids at low copy number in cycling basal keratinocytes. Productive amplification takes place only in differentiating spinous cells. We and others developed transient and cell-free systems to study the mechanisms of HPV DNA replication and showed that replication requires an origin of replication (ori), the HPV ori recognition protein E2, the viral replicative helicase E1, the cellular DNA replication machinery and cyclin/cdk complexes and that the E1 activity is regulated by multiple kinases. Our preliminary results reveal that a number of cellular proteins known to control chromosomal DNA replication also regulate HPV replication in the cell-free replication system, including Cdt1, geminin, TopBP1, and p53. Geminin inhibits Cdt1, which is necessary for loading the cellular replicative helicase MCM complex onto the cellular chromatin. Our results implicate MCM in controlling HPV replication when E1 concentration is low. TopBP1 is involved in both cellular DNA replication and repair. It interacts with DNA polymerase epsilon, a polymerase with critical but yet-to-be elucidated functions. TopBP1 has been reported to interact with HPV16 E2 and to stimulate transient HPV replication. We now show that TopBP1 enhances HPV cell-free replication. p53 is targeted by HPV E6 protein for inactivation and is known to bind E2 and inhibit viral transient amplification replication. We demonstrate that p53 inhibits HPV replication in the presence or absence of E2, indicative of additional mechanisms. We suggest that, in each case, the cell-free system provides an excellent opportunity to elucidate the mechanisms of regulation for both viral and cellular DNA replication. We propose four specific aims. 1. To test a new model for HPV DNA replication and regulation. This model would account for the maintenance mode and the amplification mode of viral DNA replication in infected tissues. 2. To determine the roles of TopBP1 and DNA polymerase epsilon in HPV DNA replication. 3. To test our hypothesis that p53 inhibition of HPV DNA replication is mediated at least in part by interfering with the activities of recombination proteins that have been hypothesized to be involved in chromosomal DNA replication. 4. To elucidate how E1 phosphorylation modulates its replication functions, a continuation of present research.
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