EFFECTS OF A FREQUENT DOUBLE-NUCLEOTIDE BASAL CORE PROMOTER MUTATION AND ITS PUTATIVE SINGLE-NUCLEOTIDE PRECURSOR MUTATIONS ON HEPATITIS-B VIRUS GENE-EXPRESSION AND REPLICATION

The basal core promoter (BCP) of hepatitis B virus (HBV) directs the t ranscription of both precore RNA and core RNA which code for e antigen (HBeAg) and core antigen, respectively, A double mutation in the BCP which converts nucleotide (nt) 1762 from A to T and nt 1764 from G to A is frequently observed in patients with chronic hepatitis B, We rece ntly demonstrated that this double mutation prevented the binding of a liver-enriched factor (LEF) to the BCP, suppressed only precore RNA t ranscription (and hence HBeAg expression), and enhanced progeny virus production, In order to understand the mechanism for the selection of this frequent double mutation, we have extended our previous studies t o further characterize LEF and to compare the effects of this double-n ucleotide mutation (M1) with each single-nucleotide mutation at nt 176 2 (M2) and nt 1764 (M3), Our results indicate that LEF is likely compo sed of a heterodimer formed between the transcription factor chicken o valbumin upstream promoter-transcription factor (COUP-TF) and an unide ntified liver-enriched factor, Further studies reveal that both M1 and M2 prevent the binding of LEF to the BCP, suppress only precore RNA t ranscription, and increase the efficiency of progeny virus synthesis, In contrast, M3 retains some LEF binding activity, does not suppress H BV RNA transcription, and reduces slightly the efficiency of virus pro geny synthesis, The reduced ability of Mg to replicate indicates that it has no selection advantage in itself at the level of the infected h epatocyte, In spite of its enhanced replication rate, M2 is rarely det ected in HBV patients, This indicates the involvement of factors other than intracellular replication rates in the selection of these virus variants in the infected individual.