Background:  It is generally established that drug-resistance mutations reduce viral fitness and that individual mutations confer high (e.g., M184V) or moderate/low fitness cost. However, it is not known how the fitness cost of a resistance mutation is affected by the presence of other resistance mutations, and how fitness changes relate to persistence of transmitted resistance in the absence of drug. To better understand the consequences of mutation interactions on fitness cost and persistence, we evaluated a large panel of HIV-1 mutants carrying resistance mutations.

Methods:  Fitness cost of drug-resistance mutations was evaluated in 19 HXB2-derived mutants carrying RT mutations alone or in combinations of clinical relevance, and in 4 transmitted drug-resistant isolates. Replicative fitness was measured in the absence of drug using a growth competition assay. Fitness differences were calculated by monitoring the changes in the relative proportion of the less- and the more-fit virus over time.

Results:  Fitness differences between HXB2 wild type and single mutants ranged from 0.4- to 26-fold, with the lowest fitness cost seen in mutants with 70R, 210W, 181C, or 41L (0.4-, 0.9-, 1.3-, and 4-fold, respectively), and the highest seen in viruses with 65R, 184V, or 215Y (26-, 14-, and 11.5-fold, respectively). Interestingly, the fitness cost of mutations was found to vary with the presence of additional RT mutations. The low fitness cost of the 70R mutation was found to increase in HXB2 viruses with 67N/219Q (4.6-fold) and in viruses with only 67N (6-fold). Similarly, the high fitness cost of 184V was reduced in HXB2 viruses carrying the 67N/70R/219Q or 41L/210W/215Y genotypes (2.3- and 8.9-fold, respectively) but remained high in viruses with 41L/210W/215Y/103N or 67N/70R/219Q/215F (23- and 16.1-fold, respectively). A similar wide range of fitness cost of 184V (from 2- to 20-fold) was seen in transmitted isolates carrying 184V alone or in association with 70R, 103N, or 41L/215Y.

Conclusions:  We confirm by using controlled competition assays that resistance mutations confer a wide range of fitness cost which predicts different persistence times. We also demonstrate that the fitness cost of individual resistance mutations is relative and can be decreased or enhanced by other resistance mutations. Our results suggest that modulation of fitness cost of mutations will play a role in the rate of reversion and persistence of transmitted resistance mutations.