Hepatitis B virus DNA integration: Implications for diagnostics, therapy, and outcome.

Hepatitis B virus (HBV) DNA integration - originally recognised as a non-functional byproduct of the HBV life cycle - has now been accepted as a significant contributor to HBV pathogenesis and hepatitis D virus (HDV) persistence. Integrated HBV DNA is derived from linear genomic DNA present in viral particles or produced from aberrantly processed relaxed circular genomic DNA following an infection, and can drive expression of hepatitis B surface antigen (HBsAg) and HBx. DNA integration events accumulate over the course of viral infection, ranging from a few percent during early phases to nearly 100 percent of infected cells after prolonged chronic infections. HBV DNA integration events have primarily been investigated in the context of hepatocellular carcinoma development as they can activate known oncogenes and other growth promoting genes, cause chromosomal instability and, presumably, induce epigenetic alterations, promoting tumour growth. More recent evidence suggests that HBsAg expression from integrated DNA might contribute to HBV pathogenesis by attenuating the immune response. Integrated DNA provides a source for envelope proteins required for HDV replication and hence represents a means for HDV persistence. Because integrated DNA is responsible for persistence of HBsAg in the absence of viral replication it impacts established criteria for the resolution of HBV infection, which rely on HBsAg as a diagnostic marker. Integrated HBV DNA has been useful in assessing the turnover of infected hepatocytes which occurs during all phases of chronic hepatitis B including the initial phase of infection historically termed immune tolerant. HBV DNA integration has also been shown to impact the development of novel therapies targeting viral RNAs.