Hepadnaviruses replicate by reverse transcription, which takes place in the cytoplasm of the infected hepatocyte. Viral RNAs, including the pregenome, are transcribed from a covalently closed circular (ccc) viral DNA that is found in the nucleus. Inhibitors of the viral reverse transcriptase can block new DNA synthesis but have no direct effect on the up to 50 or more copies of cccDNA that maintain the infected state. Thus, during antiviral therapy, the rates of loss of cccDNA, infected hepatocytes (1 or more molecules of cccDNA), and replicating DNAs may be quite different. In the present study, we asked how these losses compared when woodchucks chronically infected with woodchuck hepatitis virus were treated with L-FMAU [1-(2-fluoro-5-methyl-␤-L-arabinofuranosyl) uracil], an inhibitor of viral DNA synthesis. Viremia was suppressed for at least 8 months, after which drug-resistant virus began replicating to high titers. In addition, replicating viral DNAs were virtually absent from the liver after 6 weeks of treatment. In contrast, cccDNA declined more slowly, consistent with a half-life of ϳ33 to 50 days. The loss of cccDNA was comparable to that expected from the estimated death rate of hepatocytes in these woodchucks, suggesting that death of infected cells was one of the major routes for elimination of cccDNA. However, the decline in the actual number of infected hepatocytes lagged behind the decline in cccDNA, so that the average cccDNA copy number in infected cells dropped during the early phase of therapy. This observation was consistent with the possibility that some fraction of cccDNA was distributed to daughter cells in those infected hepatocytes that passed through mitosis.

Hepatitis B viruses establish a chronic, productive, and noncytopathic infection of hepatocytes. Viral products are produced by transcription from multiple copies (5-50) of covalently closed circular (ccc) viral DNA. This cccDNA does not replicate, but can be replaced by DNA precursors that are synthesized in the cytoplasm. The present study was carried out to determine if long-term treatment with an inhibitor of viral DNA synthesis would lead to loss of virus products, including cccDNA, from the liver of woodchucks chronically infected with woodchuck hepatitis virus. Viral DNA synthesis was inhibited with the nucleoside analog, lamivudine (2'-deoxy-3'-thiacytidine). Lamivudine treatment produced a slow but progressive decline in viral titers in serum, to about 0.3% or less of the initial level. However, even after maintenance of drug therapy for 3-12 months, > 95% of the hepatocytes in most animals were still infected. Significant declines in the percentage of infected hepatocytes and of intrahepatic cccDNA levels were observed in only three woodchucks, two in the group receiving lamivudine and one in the placebo control group. Moreover, virus titers eventually rose in woodchucks receiving lamivudine, suggesting that drug-resistant viruses began to spread through the liver starting at least as early as 9-12 months of treatment. Three types of mutation that may be associated with drug resistance were found at this time, in a region upstream of the YMDD motif in the active site of the viral reverse transcriptase. The YMDD motif itself remained unchanged. Not unexpectedly, the lamivudine therapy did not have a impact on development of liver cancer.

Many vertebrates, including fishes and marine mammals, rely on acoustic signals for communication. In aquatic environments, where sound propagates five times faster than in air, acoustic signals are particularly important for communication over long distances or in low-light environments. Research on a variety of fishes, however, demonstrates sensitive hearing capabilities even in nonvocalizing fishes, leading to the hypothesis that hearing in fishes evolved for analysis of the complete auditory scene, including both biotic (e.g.