Abstract
Hepatitis B virus (HBV) infection is a dynamic process during which molecular variants are selected continuously to adapt to changes. In addition to drug resistant mutations, sequential antiviral therapy may also lead to the selection of deleted mutants. To investigate this process, the following samples were collected from a patient who failed lamivudine therapy and then was switched to adefovir dipivoxil. HBV DNA was sequenced at two separate regions; a 1โkb region of reverse transcriptase (RT) and a 1.5โkb region encompassing the C gene and part of the preS gene. Sequence analysis of the RT region showed that the prevailing lamivudine resistant mutations were reduced after switching to adefovir dipivoxil, and ultimately the mutations were undetectable. Quasispecies distribution and deletion patterns in the C and preS regions were also different between the two antiviral therapies. In lamivudineโtreated samples, wildโtype strains (57.7%) were dominant and deletions in the preS region were observed. However, in the subsequent therapy involving adefovir dipivoxil, a virus population harboring 81 and 96โbp deletions (86%) in the C gene prevailed. Both major deletions encompassed Tโ and Bโcell epitopes. Meanwhile, the frequencies of the preS deletions decreased significantly, except for the 129โbp deletion. Notably, the presence of 81, 96, and 129โbp deletions was always accompanied with some nucleotide substitutions. In conclusion, the prevalence of deletions at the C gene epitopes accompanied with the gradual disappearance of lamivudine resistance mutations may contribute to the survival of HBV under sequential antiviral therapy. J. Med. Virol. 81:1551โ1559, 2009. ยฉ 2009 WileyโLiss, Inc.