Characteristics of hepatitis B virus isolates of genotype G and their phylogenetic differences from the other six genotypes (A through F)

Abstract

Eight hepatitis B virus (HBV) isolates of genotype G were recovered from patients and sequenced over the entire genome. Six of them had a genomic length of 3,248 bp and two had genomic lengths of 3,239 bp (USG15) and 3,113 bp (USG18) due to deletions. The 10 HBV/G isolates, including the 8 sequenced isolates as well as the original isolate (AF160501) and another isolate (B1-89), had a close sequence homology of 99.3 to 99.8% among themselves (excluding USG18 with a long deletion) but of <88.7% to any of the 68 HBV isolates of the other six genotypes with the full-length sequence known. The eight HBV/G isolates possessed an insertion of 36 bp in the core gene and two stop codons in the precore region, as did the AF160501 and B1-89 isolates. The 10 HBV/G isolates clustered on a branch separate from those bearing the other six genotypes (A through F [A-F]) in the phylogenetic tree constructed from full-length sequences of 78 HBV isolates as well as in those constructed from the core, polymerase, X, and envelope genes. Despite two stop codons in the precore region that prohibited the translation of the HBV e antigen (HBeAg), all of the eight patients with HBV/G infection possessed the HBeAg in serum. By restriction fragment length polymorphism of the surface gene, all of the eight patients were found to be coinfected with HBV of genotype A (HBV/A), which would be responsible for the expression of HBeAg in them. It is worthy of examination to determine how coinfection occurs and whether HBV/G needs HBV/A for replication.

FIG. 1.

Phylogenetic trees constructed for HBV isolates of seven genotypes on the full genome (a) and on the C (b), P (c), X (d), large S (e), and S (f) genes. Confidence values calculated by bootstrap analysis are indicated after each genotype.

FIG. 1.

Phylogenetic trees constructed for HBV isolates of seven genotypes on the full genome (a) and on the C (b), P (c), X (d), large S (e), and S (f) genes. Confidence values calculated by bootstrap analysis are indicated after each genotype.

FIG. 2.

(a) Nucleotide sequences of the eight HBV/G isolates within the pre-C region and a 5′-terminal part of the C gene. Sequences of the original HBV/G strain (AF160501 [25]) and an isolate of retrospective genotype G (adw2 variant [27]) are shown for comparison. The 36-bp insertion in the C gene and two stop codons in the pre-C region are shaded. (b) Amino acid sequence of the C gene product in the eight HBV/G isolates. Sequences of the other six genotypes (A-F) in representative strains and the original HBV/G strain (AF160501 [26]) as well as an isolate with retrospective genotype G (adw2 variant [27]) are shown for comparison.