A novel hepatitis B virus mutant with A-to-G at nt551 in the surface antigen gene

AIM: Hepatitis B surface antigen (HBsAg) mutant of hepatitis B virus (HBV) is one of the important factors that result in immune escape and cause failure of immunization. In this study we reported and characterized a novel HBV mutant with A-to-G at nt551 and intended to provide theoretical data for prevention of HBV infection in China. METHODS: A methodology comprising polymerase chain reaction (PCR), amplifying, M13 bacteriophage cloning and nucleotide sequencing was used to analyze the sera of the pediatric patient who was hepatitis B (HB) immune failure. Expression plasmids containing the mutant S gene and a wild-type (adr) S gene were constructed respectively and the recombinant HBsAg were expressed in COS-7 cells under the regulation of SV40 early promoter. The recombinant proteins were investigated for their immunological reactivity with different monoclonal antibodies (mAb) against 'a' determinant and vaccine-raised human neutralizing antibodies. RESULTS: It was found that there was a new point mutation at nt551 of the HBV (adr) genome from A to G, leading to a substitution of methionine (Met) to valine (Val) at position 133 in the 'a' determinant of HBsAg. Compared to the wildtype HBsAg, the binding activity of the muant HBsAg to mAbs (A6, A11 and S17) and to vaccine-raised human antihepatitis B surface antibody (anti-HBs) decreased significantly. CONCLUSION: According to the facts that the patient has been immunized with HB vaccine and that the serum is antiHBs positive and HBsAg negative, and based on the nucleotide sequence analysis of the mutant HBV S gene and its alteration of antigenicity the HBV is considered to be a new vaccine-induced immune escape mutant different from the known ones.