The Role of the Viral E1 and E2 Proteins in the Stable Replication of Human Papillomavirus Type 5 Genome

Abstract

Beta human papillomavirus (HPV) infections are associated with cutaneous squamous cell carcinomas in immunocompromised individuals. There is currently no treatment or vaccines targeting beta HPVs. The oncoprogression is dependent on persistent infections, and despite their clinical relevance, the mechanisms of the persistent beta-HPV replication remain poorly understood. In this study, we aimed to investigate the role of viral replication proteins E1 and E2 in the establishment and long-term maintenance of the episomal HPV5 genome that represents the best-studied beta HPV type. In a transient system, E1 and E2 RNA interference led to a time-dependent decrease in viral genome replication. Cell cycle and viability were unaffected by the E1 and E2 silencing. These results confirmed that E1 and E2 are essential for establishing viral genome replication. Further, we created and characterized a stable cell line bearing the HPV5-E1HA-Nluc-E2Flag genome (H5-Nluc+ cells), as well as compared it to a previously described cell line bearing the HPV5 WT genome (H5+ cells). Southern blot analysis confirmed the episomal nature of viral genomes in both cell lines. H5-Nluc+ cells carried approximately 16-fold more viral genome copies than H5+ cells. During investigation of the physical state of the viral genomes, we observed a previously described dominant oligomeric replicon in H5+ cells that replicates in an E1 and E2-independent manner, while no such replicon was present in the H5-Nluc+ cells. These findings suggest that there are two modes of HPV5 replication, one dependent on the E1 and E2 expression and another supported by host cell machinery, with the latter developing after long-term maintenance of the HPV5 genome. However, whether the E1/E2-independent mode of replication occurs during natural HPV infection remains to be elucidated.