Microscopy image of liver cells (black) in cell culture that are infected with HBV (red).
Individuals who are chronically infected with hepatitis B virus (HBV) are prone to developing liver cancer. Now, Hui-Ling Ko and Ee-Chee Ren at the A*STAR Singapore Immunology Network have demonstrated that the inhibition of a DNA repair enzyme called poly ADP-ribose polymerase 1 (PARP1) by a particular region of the HBV genome can enhance DNA damage within cells, which are then predisposed to the formation of tumors.
High HBV viral DNA levels in the blood have previously been shown to dramatically increase the risk of developing liver cancer in chronically infected individuals. For this reason, Ko and Ren set out to identify the host factors in humans that could be responsible for regulating the levels of HBV DNA.
Replication of HBV is initiated at the HBV core promoter (HBVCP) region of the HBV genome. Using a system in which proteins extracted from a human liver cancer cell line were tested for interaction with HBVCP, the researchers identified PARP1 as a strong binding partner for the HBVCP. When they decreased PARP1 expression in the human liver cancer cells, they observed a drop in HBV replication. PARP1 activity therefore plays a role in regulating HBV levels.
Ko and Ren then identified an 8-nucleotide-long stretch of the HBVCP that is responsible for PARP1 binding, and they called this DNA fragment the ‘PARP1 motif’ of the HBVCP. When they introduced the PARP1 motif onto extracts of liver cancer cells which contained PARP1 protein, they showed that this could inhibit PARP1 activity in a highly specific manner.
By exposing human liver cancer cells to drugs that break down DNA, the researchers showed that DNA damage was enhanced when they introduced the PARP1 motif into the cells. The reduction in PARP1 activity caused by the presence of the PARP1 motif in the HBV genome therefore enhanced the chance for DNA breakage. This could eventually lead to the snapping and swapping of chromosomes — or genomic instability — characteristic of some tumors. “Our work provides insights into how HBV DNA derails host genome stability by interfering with DNA repair,” explains Ren.
The findings suggest that PARP1 is a key factor in the host that regulates HBV replication in infected cells. High levels of HBV DNA then inhibit PARP1, leading to a deficit in DNA repair. DNA damage that is unable to be fixed by PARP1 accumulates and may then predispose HBV-infected cells to develop into cancer.
The A*STAR-affiliated researchers contributing to this research are from the Singapore Immunology Network.
