Human Papillomavirus Genome Copy Number Is Maintained by S Phase Amplification, Leakage to the Cytosol during Mitosis and Lysosomal Degradation in G1 Phase

Abstract

The current model for human papillomavirus (HPV) replication is comprised of three modes of replication. Following infectious delivery, the viral genome is amplified during the establishment phase to reach up to some hundred copies per cell. HPV genome copy number remains constant during the maintenance stage. Differentiation of infected cells induces HPV genome amplification. Using highly sensitive in situ hybridization (DNAscope) and freshly HPV16-infected as well as established HPV16-positive cell lines, we observed that viral genome is amplified in each S phase of undifferentiated keratinocytes cultured as monolayers. Nuclear viral genome copy number is reset to pre-S phase levels during mitosis. The majority of viral genome fails to tether to host chromosomes and is lost to the cytosol. Cytosolic viral genome gradually decreases during cell cycle progression. Loss of cytosolic genome is blocked in presence of NH4Cl or other drugs interfering with lysosomal acidification, suggesting the involvement of autophagy in viral genome degradation. These observations were also made with HPV31 cell lines obtained from patient samples. Cytosolic viral genome was not detected in UMSCC47 cells carrying integrated HPV16 DNA. Analyses of organotypic raft cultures derived from keratinocytes harboring episomal HPV16 revealed the presence of cytosolic viral genome as well. We conclude that HPV maintains viral genome copy number by balancing viral genome amplification during S phase with loss of viral genome that is lost to the cytosol during mitosis. It seems plausible that restrictions to viral genome tethering to mitotic chromosomes resets genome copy number in each cell cycle. IMPORTANCEHPV genome maintenance is currently being thought to be achieved by regulating expression and activity of viral replication factors E1 and E2. In addition, the E8^E2 repressor has been shown to be important for restricting genome copy number by competing with E1 and E2 for binding to the viral origin of replication and by recruitment of repressor complexes. Herein, we demonstrate that HPV viral genome is amplified in each S phase. Nuclear genome copy number is reset during mitosis by a failure of the majority of genomes to tether to mitotic chromosomes. Rather, they accumulate in the cytoplasm of freshly divided cells. Cytosolic viral DNA is quickly degraded in G1 in a lysosome dependent manner contributing to the genome copy reset. Our data imply that the mode of replication during establishment and maintenance is the same and further suggest that restrictions to genome tethering significantly contributes to viral genome maintenance.