Typing hepatitis B virus by homology in nucleotide sequence: comparison of surface antigen subtypes

The complete nucleotide sequences of the DNA of three hepatitis B virus (HBV) genomes of subtype adw, cloned from plasma samples of asymptomatic carriers living in the mainland and Okinawa Prefecture of Japan and Indonesia were determined. All three comprised 3215 bp and differed in sequence by only 3.9 to 5.6%. When these isolates were compared with the reported sequences of two HBV genomes of the same subtype derived from American carriers, however, the differences were greater (8.3 to 9.3% to an extent comparable with the nucleotide divergence between an HBV genome of subtype adw and that of a heterotypic subtype, such as adr, ayw or ayr. A total of 18 HBV genomes of various subtypes, including the three described here, 10 reported previously and five unpublished ones, were classified into four groups based on an inter-group divergence in nucleotide sequence of 8% or greater: group A (two adw genomes), group B (four adw), group C (three adw, four adr and one ayr) and group D (four ayw). Thus, the nine genomes of HBV subtype adw were distributed into three groups with considerably different sequences. These results indicate that the four major antigenically defined subtypes of envelope polypeptide do not reflect true genotypic variation of HBV. The fact that d to y, as well as w to r, subtypic change can be induced by an A----G point mutation at nucleotides 365 and 479 in the S gene, respectively, supports this view.