Tormi Reinson will defend his doktoral thesis “Studies on the Genome Replication of Human Papillomaviruses” on 21 January 2016 at 14.15 in the Institute of Technology (Nooruse 1-121) of University of Tartu.
Supervisor: Professor Mart Ustav, University of Tartu
Opponent: Jacques Archambault, PhD, Professor, Institut de Recherches Cliniques de Montréal (IRCM) (Canada)
Human papillomavirus (HPV) is one of the most important cancer-causing agents. In spite of great advancements in understanding of HPV molecular biology in recent decades, it is not fully understood how HPV infection leads to the development of cancer. One of the processes that needs to be better characterized is the replication of viral DNA genome. We developed a new cellular assay system for studying HPV genome replication. The human osteosarcoma cell line U2OS supports the highly efficient replication of high risk, low risk and cutaneous HPV genomes. This system was used to describe involvement of DNA damage response pathways in viral DNA replication. The HPV replication protein E1 was shown to cause DNA double-strand breaks into host genome and thereby activate the DNA damage response. The DNA damaging activity of the E1 protein was not necessary for efficient viral genome replication, and it was suppressed in the context of transiently replicating viral genomes. However, even low levels of E1-induced DNA damage could play a role in the development of HPV-related cancers. We next showed that the ATR pathway is engaged at the HPV genome replication centers. This ATR engagement suggests that DNA replication stress accompanies viral genome duplication. The response to DNA replication stress might play a major role in the integration of the viral genome into a host chromosome and thereby viral-induced carcinogenesis. We also characterized HPV DNA replication timing in the cell cycle and showed that the timing between different viral life cycle phases is different. During HPV stable replication, viral DNA replication takes place only in S-phase, but it starts in S-phase and is extended to G2-phase during the initial amplification of HPV genomes. The timing of viral replication was dependent on the expression levels of the viral replication proteins E1 and E2.