Genotypic succession of mutations of the hepatitis B virus polymerase associated with lamivudine resistance

Background/Aims: Hepatitis B mutant strains of virus emerging during treatm ent with the nucleoside analog lamivudine are being increasingly recognized . In the majority of lamivudine-resistant isolates the mutations have been reported to occur within the YMDD motif of the viral polymerase, either as a single mutation M552I or as M552V concomitant with L528M. We analyzed the time course and genetic succession pattern during the emergence of lamivud ine resistance. Methods: Seven patients with breakthrough viremia in the setting of chronic hepatitis (n=5) or recurrent HBV after liver transplantation (n=2) were in vestigated, Pre- and post-breakthrough serum samples were evaluated by sing le- or second-round PCR amplification and sequencing analysis, Results: Genotypic succession of the virus populations was observed to occu r from M552I to M552I/L528M (n=2) and from L528M to M552V/L528M (n=1). The double mutations M552I/L528M (n=4) or M552V/L528M (n=2) were found in six o ut of seven patients, and represented the stable virus populations througho ut the follow-up period. Breakthrough viremia was not associated with the s ingle L528M mutation. The mean duration of uninterrupted treatment with lam ivudine until breakthrough was 422 days (range 182-642) and was longer in t he setting of chronic hepatitis B than in recurrent hepatitis B after liver transplantation. HBV DNA levels after breakthrough were lower than pretrea tment levels in the majority of patients with chronic hepatitis but higher after liver transplantation. Conclusion: Our observations show that the virus populations conferring res istance to lamivudine can evolve from single to double mutations at amino a cid 552 and 528 of the HBV polymerase, and that M552I/L528M or M552V/L528M seem to be the predominant mutations arising during long-term antiviral the rapy with lamivudine.