RESUMO
Objective To investigate the molecular characteristics and epidemiological signification of patients with low-level HBsAg. Methods PCR and gene sequencing were used to detect HBV DNA and Tyr-Met-Asp-Asp(YMDD) mutant in 136 serum samples with low-level HBsAg and 44 sernm samples with high-level HBsAg. Genotyping was performed in 47 cases with HBV DNA 10~5 copies/L by concentration method and 37 cases with high-level HBsAg. S gene sequences and serotypes were analyzed in 14 cases with HBV DNA 105 copies/L and 29 cases with high-level HBsAg. S gene sequences were compared with the consensus sequence of Chinese strain by BioEdit software. Results The HBV DNA-positive rate, YMDD mutation rate and HBV DNA load (logarithm) in low-level and high-level HBsAg group were 34.6% (47/136), 0% (0/136), 6.5±1.4 and 84.1% (37/44), 9.1% (4/44), 8.9±1.8, respectively. There was statistically significant differences between two groups (for concentration method,χ~2 = 30.8, P < 0.05; for direct method, χ~2 = 53.5, P < 0.05; for YMDD mutation ratio, P = 0.003, For HBV DNA (log), t = 6.5, P < 0.05). The genotypes in low-level HBsAg group included type B (16/47), type C (5/47) and non-classified ones(26/47). There were significant differences between two groups (χ~2=21.8, P <0.01). The serotypss included adw (7/14), ayw (4/14), adr (2/14) and ayr (1/14). There were significant differences in genotypes (χ~2 = 13.5, P < 0.05) but not in serotypes between two groups (χ~2 = 4.7, P >0.05). S gene sequencing results showed no S gnne variation was detected, but there were 6 single nucleotide polymorphisms in 16 cases, which would not result in the alternation of amino acid. Conclusions Low-replication phenomenon of HBV DNA was present in patients with low-level HBsAg. The major genotyps and serotype was type B and adw/ayw, respectively. Polymorphic variants have been found in the S gene. The existence of low-level HBsAg might be related with its own molecular characteristics resulting in low expression of HBsAg or immune tolerance induced by low-level HBsAg after HBV infection.