INITIAL APPEARANCE OF THE 184ILE VARIANT IN LAMIVUDINE-TREATED PATIENTS IS CAUSED BY THE MUTATIONAL BIAS OF HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 REVERSE-TRANSCRIPTASE

Treatment of human immunodeficiency virus type 1-infected patients wit h lamivudine (3TC) results in the appearance of drug-resistant virus, variants with a mutation at the 184Met codon (ATG) of the reverse tran scriptase (RT) gene, The 184Ile (ATA) variant appears first, but subse quently the 184Val (GTG) variant outcompetes the 184Ile variant, We de monstrated previously that the 184Val enzyme and the corresponding vir us are more fit than 184Ile, thereby explaining eventual outgrowth of 184Val. In this study, we set out to determine why 184Ile is usually o bserved first after initiation of 3TC therapy. With a limiting dilutio n approach during in vitro selection with 3TC, we measured a significa ntly higher frequency of the G-A substitution toward the ATA codon (18 4Ile; 56%) than the A-->G substitution toward GTG (184Val; 12.5%). Thi s result indicates that the initial appearance of the 184Ile variant i n patients is a consequence of the mutation bias of the RT enzyme, Int erestingly, a novel 3TC-resistant variant which was generated by T-->C substitution (184Thr; 28%) was also observed. The RT enzyme of the 18 4Thr variant was less than 10% active compared with the wild-type enzy me. and the replication capacity of this variant was severely reduced, Selection of the 184Thr variant illustrates that the limiting dilutio n approach allows the selection of drug-resistant variants with subopt imal fitness.