Effect of frameshift mutation in the pre-C region of hepatitis B virus on the X and C genes.

We have previously cloned a mutant hepatitis B virus (HBV) genome which had one thymidine addition in the pre-C region resulting in a frameshift mutation in the pre-C region and fusion of the X and C genes. We constructed plasmids containing serially deleted and/or back-mutated (authentic) pre-C regions to study the effect of the frameshift mutation. COS cells transfected with plasmids containing the frameshifted pre-C region produced a 21K C protein (P21c) but not a 22K partially processed pre-C protein (P22). On the other hand, COS cells transfected with plasmids containing the back-mutated pre-C region produced P22. This result was also observed in HepG2-K8 cells producing the mutant HBV particles. Therefore, the pre-C region of HBV is likely to be non-essential for virus replication. COS cells transfected with the plasmid containing a fused X-C open reading frame (ORF) produced a 40K X-C fusion protein. This X-C fusion protein exerted transcriptional trans-activation. These results suggest that the mutant HBV has a C gene with a defective pre-C region and a fused X-C ORF, and hence cannot synthesize 16K HBeAg (P16e).