Abstract
Dengue virus serotype 3 (DENV-3) was associated with severe dengue epidemics in Thailand during 1973–1999. We studied Thai DENV-3 viruses isolated from hospitalized children in Bangkok with differing disease severity during that period. Viruses were sequenced at their 5′ and 3′ untranslated regions (UTRs), which are regions that play a pivotal role in viral replication. Our results indicated that the primary sequences as well as the secondary structures at both ends of Thai DENV-3 viruses were highly conserved over almost three decades. We found nucleotide insertions and deletions at the variable region (VR) that is located just downstream of the nonstructural protein 5 (NS5) stop codon among these viruses. The phylogenetic tree derived from the size heterogeneity of VR in the 3′ UTR divided DENV-3 into four genotypes, and Thai DENV-3 viruses in this study belonged to genotype II. The replication efficiency of the candidate viruses with different lengths at the VR were assessed in the mosquito (C6/36) and human (HepG2) cell lines. Our results show that the viruses with nucleotide insertions at VR replicated better than the virus that contained deletions. Our findings indicate that Thai DENV-3 demonstrated a remarkable conservation of nucleotides over 28 years. Correlation with disease severity suggests that both primary sequences and secondary structures of the 3′ UTR do not appear correlated with disease severity in humans.