Genetic heterogeneity of hepatitis B and D viruses in chronic infection

Abstract: During the prolonged course of chronic viral infection there is a continuous reciprocal interaction between the host immune defence system and the viral immunogens, and viral genetic mutations capable of circumventing host immune defence could be selected for. Our observation of increasing prevalence of heterogeneity in the DNA sequence coding for the ‘a1 determinant of HBsAg in patients with presumed increasing length of chronic HBV infection is in agreement with the assumption of accumulation of presumed escape mutants. In a large series of Chinese patients with chronic liver disease, we found ‘a1 determinant mutations in only one of 13 patients positive for HBsAg and HBeAg, but in eight (47%) of 17 patients positive for HBsAg and anti-HBe. However, most significant of all was the detection of ‘a1 determinant mutations in as many as 63% (15/24) of serum samples yielding serological evidence of healed HBV infection, but still with HBV DNA presence. The HBV strains found in these patients could therefore be regarded as escape mutants, an interpretation verified by cloning the entire HBV S gene from a number of the mutant strains and expressing the S protein in a mammalian cell culture system. Thus, the protein expressed by a number of mutant S genes was non-reactive in conventional HBsAg assays. cDNA sequencing and phylogenetic analysis showed that at least two independent HDV genotype IA strains had been involved in the introduction of hepatitis D virus among the drug addicts of southern Sweden in the early 1970s. The annual HDV RNA evolution rate was determined to be 1.2x10-3 substitutions/nucleotide, which is of the same magnitude as the rates found for other RNA viruses. Phylogenetic analysis of HDV strains from different parts of the world confirmed the usefulness of the genotype classification. Within genotype I two subgroups, A and B, have previously been described. We observed the occurrence of the HDV genotype I subgroups to manifest geographical restrictions, genotype IA being identified as a western branch found in HDV isolates from USA and West and South Europe, and genotype IB an eastern branch occurring in HDV strains from East European countries and from China, Taiwan and Oceanea. Moreover, we identified a new type I subgroup, tentatively designated IC, occurring as the sole subgroup in several patients from Ethiopia and in a single HDV isolate from Somalia. The Somalian strain and one of the Ethiopian HDV strains were fully sequenced. One HDV isolate from Jordan and one previously described from Lebanon were also found to belong to the IC subgroup.