Complete genome sequencing and evolutionary analysis of dengue virus serotype 1 isolates from an outbreak in Kerala, South India.

In this study, dengue virus (DENV) isolates from a localized, small-scale, non-seasonal dengue outbreak were genetically characterized. The outbreak occurred during the pre-monsoon months (April-May) in a medical college campus in Kerala, South India in 2009 affecting 76 people. Analysis of 39 viral RNA positive serum samples by a serotype specific reverse-transcription polymerase chain reaction identified dengue virus serotype 1 (DENV1) as the causative strain. Formation of a distinct genetic clade was revealed in the initial phylogenetic analysis using nucleotide sequences of a partial (303 bp) Capsid-Pre-membrane protein (C-PrM) coding region of 37 outbreak strains. The sequences of these strains clustered with that of the Genotype III DENV-1 strains from India, and 32 among them formed a single major sub-clade. Whole-genome sequencing (10,693 bp) of two strains (RGCB585/2009 and RGCB592/2009) selected from this major sub-clade, and subsequent phylogenetic analysis using the full-length coding region sequence showed that the sequences grouped with that of the isolates from Thailand (1980), Comoros (1993), Singapore (1993), and Brunei (2005) among the Indo-Pacific isolates. The sequences of the two strains had a nucleotide identity of 97-98 % and an amino acid identity of 98-99 % with these closely related strains. Maximum amino acid similarity was shown with the Singapore 8114/93 isolate (99.6 %). Four mutations-L46M in the capsid, D278N in the NS1, L123I, and L879S in the NS5 protein coding regions-were seen as signature substitutions uniformly in RGCB585/2009 and RGCB592/2009; in another isolate from Kerala (RGCB419/2008) and in the Brunei isolate (DS06-210505). These four isolates also had in common a 21-nucleotide deletion in the hyper-variable region of the 3'-non-translated region. This first report on the complete genome characterization of DENV-1 isolates from India reveals a dengue outbreak caused by a genetically different viral strain. The results point to the possibility of exotic introduction of these circulating viral strains in the region.