Evolutionary and epidemic potential of dengue viruses

Abstract

The epidemiology of infectious diseases caused by RNA virus is largely determined by the evolutionary potential of this kind of agents, characterized by high mutation rates and occasional exchange of genomic segments between strains. Dengue virus (DENV), one of such rapidly evolving viruses, has emerged as the most extended vector-borne viral pathogen in the world in the last decades. In this study, the forces shaping DENV evolution were investigated in the laboratory and in a natural population. Phylogenetic analyses were performed in the prM and E genes of a collection of DENV isolates from Mexico and in selected sequences of viruses isolated around the world. The results showed a continuous appearance of substitutions mostly in synonymous sites, along with the frequent introduction, expansion and extinction of old and new viral strains coming from different continents. The introductions of some of these strains were temporally and geographically related to changes in the incidence and severity of DENV infection. DENV serotype-2 (DENV-2) exhibited a larger number of variants circulating in close temporal relationship and seems to have had a major role in the observed changes in the clinical outcome of the disease. Recombination between different strains of the same serotype was investigated in vitro and in the same collection of viruses. While no clear evidence of this phenomenon was obtained in cell culture, two probable episodes of recombination were detected in the natural population. All DENV-1 isolates from Mexico and the Americas seem to be derived from a strain originated in a remote recombination event between strains of the Eastern Hemisphere. The sequence of a single Mexican DENV-2 isolated in 1983 is better explained as a mosaic between the two major lineages of this serotype circulating in the Americas at that time. Finally, the evolutionary consequences of alternation between arthropod and vertebrate hosts were explored in vitro by serial passages of four DENV-2 strains in a simplified cell culture model. It was found that different, mostly non-synonymous substitutions accumulated depending on the cell-type used to serially propagate the viruses. Some of these amino acid replacements were consistent across strains and were predicted to generate major changes in the structure of the envelope protein of the virus. They include substitutions that disrupt a glycosylation site or that may alter the conformational change occurring during the fusion between viral and cell membranes. Cell culture-adapted viruses reduced their infectivity for mosquitoes in a cell-type dependent manner but retained their infectiousness for the bypassed cell culture system. Thus, different forces seem to govern DENV evolution in vivo and in vitro. While in cell culture host-dependent directional selection is evident, in natural populations purifying selection and gene flow are the dominant forces.