Molecular anatomy and pathophysiologic implications of drug resistance in hepatitis B virus infection.

Synthesis of the hepatitis B virus (HBV) DNA genome occurs within the viral nucleocapsid in a mechanistically ordered fashion. The nucleocapsid contains small pores that permit influx of nucleotide triphosphates and metabolites of nucleoside analogues such as lamivudine for DNA synthesis. Lamivudine is a potent inhibitor of HBV and human immunodeficiency virus (HIV) reverse transcriptases, but substitutions of isoleucine or valine for methionine within the tyrosine-methionine-aspartate-aspartate (YMDD) motif are associated with virologic and clinical resistance to lamivudine therapy. Under lamivudine selection pressure, the high viral production rate and the low fidelity viral polymerase contribute to frequent development of the YMDD mutants. However, the pattern and dynamics of emergence of the mutant viruses over the wild-type virus are determined by multiple factors including replication efficiency, host immune response, and availability of replication space. Structural modeling of HIV reverse transcriptase has permitted key insights into the molecular basis of lamivudine resistance of HBV based on evolutionary relatedness of HIV and HBV. The side groups of isoleucine and valine of the YMDD mutants sterically prevent lamivudine from appropriately configuring into the nucleotide binding site of the reverse transcriptase. Aminotransferase flares are associated with lamivudine therapy and may signify clinical resistance with emergence of YMDD mutants. They may also herald the recovery phase with seroconversion and viral clearance. Reconstitution of the endogenous anti-HBV immune response may be equally important in the control of viral replication by lamivudine and other nucleoside analogues.