Nucleotide substitution patterns can predict the requirements for drug-resistance of HIV-1 proteins

The enzyme reverse transcriptase (RT) plays a fundamental role in the replication of the human immunodeficiency virus type 1 (HIV-1) and several antiviral agents that target this key enzyme have been developed. Unfortunately, treatment of patients with RT inhibitors results in the appearance of drug-resistant variants with specific mutations in the RT protein. We hypothesized that if 'difficult' resistance mutations (e.g. transversions/double-hits) are consistently observed at certain positions, it is likely that 'easier' nucleotide substitutions (transitions/single-hits) at that codon do not result in a drug-resistant and/or active RT enzyme. In this study,we examined codon changes involved in RT drug resistance against nucleoside and non-nucleoside inhibitors and listed all easier substitutions, which apparently were not selected, either due to reduced enzyme RT activity or lack of drug resistance. These predictions on the requirements for resistance were confirmed by published mutational data on RT variants. We also propose that differences in mutation type can explain the order of appearance of substitutions in case multiple amino acid changes are required for optimal fitness. Differences in mutation pattern have been reported for drug-resistant HIV-1 variants selected in tissue culture compared with variants found in treated patients. In contrast to the in vivo situation, a relatively small population size is handled in in vitro tissue culture systems and this may limit the chances of creating a resistance mutation. Indeed, inspection of the codon changes indicates that the in vitro culture system is more strongly biased towards the relatively easy nucleotide substitutions. These results suggest that the nucleotide substitution pattern can provide important information on RT drug resistance.