NUCLEIC-ACID SEQUENCE-ANALYSIS OF THE PRECORE REGION OF HEPATITIS-B VIRUS FROM SERA OF SOUTHERN AFRICAN BLACK ADULT CARRIERS OF THE VIRUS

Our purpose was to ascertain if mutations of the precore region of the hepatitis B virus genome, in particular the 1896 stop codon mutation, are responsible for the 95% hepatitis B e antigen (HBeAg)-negativity rate in southern African black adult carriers. Hepatitis B virus (HBV) DNA was extracted from the serum of 57 asymptomatic carriers (42 HBeA g-negative; 15 HBeAg-positive), the precore region was amplified using the polymerase chain reaction (PCR), and sequenced. Six carriers (14. 6%) had mutations known to prevent HBeAg synthesis: 4 involved the pre core initiation codon (1814), and one created a stop codon at 1874. Th e 1896 mutation occurred alone in one carrier only (2.4%). The frequen cy of the 1896 mutation can be explained by the high prevalence (70%) of the adw subtype in the carriers studied. Inter alia, adw differs fr om ayw in that codon 15 is comprised of CCC instead of CCT. The presen ce of C instead of T in position 1858 precludes the G-to-A mutation at 1896 because the coexistence of these two mutations would destabilize the stem-loop structure of the RNA encapsidation signal, a finding co nfirmed by our observation that the CCC polymorphism and the 1896 muta tion were mutually exclusive. Ten HBeAg-negative carriers (24%) had a missense mutation at position 1862 in the bulge of the RNA encapsidati on signal, which may possibly affect HBeAg expression by interfering w ith either priming of reverse transcription or signal peptide cleavage . We conclude that the 1896 stop codon mutation accounts for a minorit y only of HBeAg-negative black carriers. A missense mutation in the bu lge of the encapsidation signal may contribute to HBeAg negativity.