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Abstract
Background ＆ Aims: Recent studies have demonstrated that the mutants of viruses could emerge and become dominant during persistent virus infection. The mutation of viruses may result in drug resistance, increased viral virulence, or immune escape. Several types of S and pre-S mutants have been described during chronic HBV infection. The significance of the emergence of HBsAg mutants, however, remains to be investigated. This study was therefore designed to investigate whether the emergence of HBV mutants will prevail during the evolution of chronic HBV infection. The virologic implications of the emergence of these HBV mutants were explored. Materials and Methods: Three parts of studies were included: Part (Ⅰ): horizontal study: The prevalence of HBV mutations over the pre-S and S regions were detected in 155 sera samples: 37 with high titer of HBeAg (+), 45 with low titer of HBeAg (+), 48 HBeAg (-), and 25 hepatoma patients. Polymerase chain reaction (PCR) amplification using primers specific for pre-S and S regions was performed, and the PCR products were directly sequenced and analyzed. Part (Ⅱ): longitudinal study: Serial serum samples were performed on 10 chronic HBV patients (total, 50; mean, 5). Part (Ⅲ): functional analysis: HBsAg plasmids of various mutants were constructed to investigate their replication capacity. Results: (1) Five types of pre-S deletion mutants were demonstrated. The frequency of pre-S mutants was 2.63% at HBeAg high titer samples, 13.7% at HBeAg low titer samples, 17.9% at HBeAg(-) samples, and 61.5% at hepatoma samples. Although the pre-S region only rarely mutated at early replicative stage, the S gene was frequently (25%) mutated at the T cell epitope and “a “ determinant. (2) In longitudinal assay, the HBV strains in 5 patients retained the pre-S wildtype throughout, while one of them had point mutation on S gene; in 1 patient, retained the S gene mutation at T cell epitope and “a “ determinant; in 1 patient, the mutant strains on both pre-S and S regions emerged during the clinical course; in 3 patients, the pre-S mutant strains coexisted with wildtype and the pre-S mutant strains disappeared at later clinical course in 2 patients. (3) In functional analysis, the plasmid constructs of pre-S1 (△2948-3079 ) and pre-S2 (△3156-55 ) were constructed. Conclusion: The emergence of pre-S and S mutants increased in serum during the evolution of chronic HBV infection and in patients with advanced disease. The evolution of HBV mutation may evolve from the point mutation of S gene to the emergence of pre-S1 mutant and pre-S2 mutant. The mutation may result in immune escape or elicitation of a new CTL response leading to repeated bouts of clinical diseases. Furthermore, the pre-S mutants may result in the accumulation of HBsAg in cytoplasm and may have effect on replication capacity, infectivity, or biologic significance.
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