Nucleotide sequence of a cloned human HBV mutant (pDKHBV) in duck hepatoma of Qidong County.

This is the first report to describe the presence of an HBV-like DNA sequence in two hepatoma and their tumor surrounding liver tissues, one precancerous and one non-malignant liver tissue of ducks collected from Qidong County of China. The HBV-like sequences were either in an episomal form of 3.2 kb or in an integrated form of various sizes, while the DHBV DNA sequences (3.0 kb) were either present or absent in these tissues and in different size pattern. Furthermore, there was no evidence of cross-hybridization between HBV-like DNA sequences in duck and DHBV DNA. A 3.2 kb HBV-like DNA sequence has been cloned from one duck hepatoma (40 K), designated as pDKHBV. The 3218 bp full-length nucleotide sequence of this clone has been determined, which had no apparent homology with Duck Hepatitis B Virus (DHBV) genome, but was highly homologous to human HBV adw2 subtype (99.0%). The sequence was composed of four open reading frames for HBV gene Pre-S/S, X, C and P respectively. In addition to multiple sites of point mutation, one nt and two nt deletion were detected downstream the initiation codon of Pre-S1 gene and at the 3' end of C gene respectively, thereby suggesting a frameshift mutation in Pre-S gene and C gene. Based on these results, it was implicated that an unusual HBV variant might exist in Qidong County, which might have transmitted into duck through some yet unknown mechanisms. The possibility from exogenous contamination could be thus excluded.