GENE ORGANIZATION AND GENETIC EXPRESSION OF HEPATITIS B VIRUS: RNA AND DNA STUDIES IN A HUMAN HEPATOMA CELL LINE AND IN LONG-TERM CHIMPANZEE HBV CARRIERS
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The studies presented in this thesis are concerned with gene organization and function of Hepatitis B virus DNA sequences in a human hepatoma cell line (PLC/PRF/5) which expresses hepatitis B surface antigen and in the liver of chimpanzee HBV carriers. Our major findings can be summarized as follows: (1) In PLC/PRF/5 cells, all HBV DNA sequences are integrated into the host genome and at least six different integration sites were identified. (2) PLC/PRF/5 cells express two cytoplasmic poly A('+) RNAs (17S and 19S) containing HBsAg gene sequences. RNA mapping studies suggest that a viral RNA initiation site (promoter) is utilized for transcription of both RNAs and that a splicing event is probably involved in the nuclear maturation of viral RNA. These studies indicate further that the 19S poly A('+) RNA might represent a cotranscript of viral and cellular sequences. (3) An active RNA initiation site (promoter) for transcription was identified in cloned HBV DMA by in vitro transcription of HBV DNA sequences in a HeLa cellular extract. An RNA molecule of 1700 nucleotides was transcribed by an RNA polymerase II dependent reaction. This RNA contains hepatitis B virus core antigen gene sequences, as determined by hybridization to specific restriction fragments of cloned HBV DNA. Using a "truncated template assay", the RNA initiation site (promoter) was located (TURN)300 bp upstream from the aminoterminus of the HBcAg gene coding sequence on the HBV genome. (4) In chimpanzee hepatitis B virus (HBV) carriers, the mechanism of viral persistence has been examined by analyzing the properties of viral DNA molecules in liver and serum. These studies have shown that persistence of HBV in the chimpanzee does not require integration of viral DNA sequences into the host genome and led to the identification of supercoiled HBV DNA molecules in infected chimpanzee liver and in concentrated preparations of chimpanzee and human serum Dane particles. It is proposed that most Dane particles may represent interfering noninfectious viral forms containing partially double stranded DNA circles and that particles containing supercoiled HBV DNA may represent the infectious form of hepatitis B virus. The interfering, noninfectious viral forms (D.I. particles) may play an important role in the establishment and maintenance of the persistent HBV infection.