Within-host rates of insertion and deletion in the HIV-1 surface envelope glycoprotein
Under selection by neutralizing antibodies, the HIV-1 envelope glycoprotein gp120 undergoes rapid evolution within hosts, particularly in regions encoding the five variable loops (V1-V5). Indel polymorphisms are abundant in these loops, where they can facilitate immune escape by modifying the length, composition and glycosylation profile of these structures. Here, we present a comparative analysis of within-host indel rates and characteristics within the variable regions of gp120. We analyzed a total of 3,437 HIV-1 gp120 sequences sampled longitudinally from 29 different individuals using coalescent models in BEAST. Next, we used Historian to reconstruct ancestral sequences from the resulting tree samples, and fit a Poisson generalized linear model to the distribution of indel events to estimate their rates in the five variable loops. Overall, the mean insertion and deletion rates were 1.6 x 10-3 and 2.5 x 10-3/ nt / year, respectively, with significant variation among loops. Insertions and deletions also followed similar length distributions, except for significantly longer indels in V1 and V4 and shorter indels in V5. Insertions in V1, V2, and V4 tended to create new N-linked glycosylation sites significantly more often than expected by chance, which is consistent with positive selection to alter glycosylation patterns.