Different Modes of Human Papillomavirus DNA Replication during Maintenance

Abstract

Human papillomavirus (HPV) begins its life cycle by infecting the basal cells of the epithelium. Within these proliferating cells, the viral genomes are replicated, maintained, and passed on to the daughter cells. Using HPV episome-containing cell lines that were derived from naturally infected cervical tissues, we investigated the mode by which the viral DNAs replicate in these cells. We observed that, whereas HPV16 DNA replicated in an ordered once-per-S-phase manner in W12 cells, HPV31 DNA replicated via a random-choice mechanism in CIN612 cells. However, when HPV16 and HPV31 DNAs were separately introduced into an alternate keratinocyte cell line NIKS, they both replicated randomly. This indicates that HPV DNA is inherently capable of replicating by either random-choice or once-per-S-phase mechanisms and that the mode of HPV DNA replication is dependent on the cells that harbor the viral episome. High expression of the viral replication protein E1 in W12 cells converted HPV16 DNA replication to random-choice replication and, as such, it appears that the mode of HPV DNA replication in proliferating cells is dependent on the presence or the increased level of this protein in the host cell. The implications of these observations on maintenance, latency, and persistence are discussed.The host tissue of human papillomaviruses is the stratified epithelium. This tissue is complex in that it is composed of layered sheets of nondividing cells in various stages of terminal differentiation, with the uppermost layer being the most differentiated. Only cells of the bottom-most layer of this tissue, the basal cells, proliferate. Although the HPV life cycle begins with the infection of a basal cell, it only comes to completion when the infected cell reaches the upper layers of the epithelium. As a consequence, the HPV DNA initially finds itself in the nucleus of a proliferating cell, but later in that of a differentiating (nonproliferating) one. This sequential mixture of cell states that the virus has to contend with has undoubtedly shaped the way by which HPV replicates its DNA throughout the varying milieux during its life cycle.It is proposed that immediately after infection, the papillomavirus DNA copy number is amplified to a certain level (50 to 400) per cell (9). This first amplification replication is believed to be rapid and transient, after which the viral DNA is stably maintained at this level in subsequent divisions of the basal cell. This is thought to be achieved by maintenance replication, where the viral episomes approximately double their copy number during the S phase of the host cell cycle and segregate to the resulting two daughter cells. After differentiation of the host cell, the viral DNA undergoes another amplification step, the second amplification replication, which increases the HPV DNA copy number to several hundreds or thousands per cell. This is followed by packaging of the viral DNA into virus particles. Although this triphasic model has not been proven in its entirety, it is supported by va...

The E1 Protein of Human Papillomavirus Type 16 Is Dispensable for Maintenance Replication of the Viral Genome

“…rounds of the viral genome replication per S phase in CIN612-9E cells, and that forced expression of E1 in W12 cells converted HPV16 DNA replication to random-choice replication (12). Interestingly, when HPV16 or HPV31 DNAs are separately introduced into NIKS cells, they both replicate randomly (12). Thus, it is likely that the difference between W12 and CIN612-9E cells depends on expression levels of E1.…”

“…Expression of reintroduced exogenous E1 controlled by doxycycline was detected by anti-HA antibody (lanes 7 and 8). rounds of the viral genome replication per S phase in CIN612-9E cells, and that forced expression of E1 in W12 cells converted HPV16 DNA replication to random-choice replication (12). Interestingly, when HPV16 or HPV31 DNAs are separately introduced into NIKS cells, they both replicate randomly (12).…”

The E8 ∧ E2 Gene Product of Human Papillomavirus Type 16 Represses Early Transcription and Replication but Is Dispensable for Viral Plasmid Persistence in Keratinocytes

“…In contrast, the sequence-related HPV-31 is significantly less prevalent than HPV-16 in cervical carcinomas and is rarely detected in HNC. Mechanisms determining HPV type-specific variations in viral persistence and malignant progression, however, are poorly understood.Papillomaviruses (PVs) replicate as extrachromosomal double-stranded DNA plasmids after infection of the basal keratinocyte host, stably persisting in low copy numbers (10,19,25). Early PV gene expression and plasmid amplification in the initial stages of HPV infection appear to be tightly regulated (18,21,30) through both transcriptional and posttranscriptional mechanisms (reviewed in references 43 and 60).…”

The Viral Etiology of AIDS‐Associated Malignancies

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