Properties of hepatitis B virus pre-S1 deletion mutants

Deletion mutants of hepatitis B virus (HBV) pre-S proteins were detected in serum in 2 of 10 (20%) individuals with chronic hepatitis B infection following the initiation of interferon treatment. The size of these deletions was up to one-half of the entire pre-S1 region. In vivo, all HBV deletion mutants were found to coexist with a full-length "wild-type" viral genome. The functional properties of a HBV-deleted mutant were studied in detail and revealed a stop codon in the pre-S2 open reading frame in all 20 of the clones sequenced. Several of the deleted mutants produced low-level HBsAg in culture supernatants compared to wild-type virus due to a putative loss of transcription factor binding sites. Transfection experiments in human hepatoma cells (HuH-7) demonstrated that the polymerase gene function was not affected by the large pre-S1 deletions and mutant viral genomes were capable of replication. However, secretion of incapsidated mutant viral genomes was blocked in HuH-7 cells. Cotransfection studies with a plasmid expressing only the HBV pre-S1, pre-S2, and S proteins resulted in complete restoration of viral particle secretion. Our findings suggest that an in vivo trans-complementation phenomenon would have had to occur to permit secretion from the liver into serum of the nucleocapsids containing these deleted viral genomes.