NEW YORK – A team led by investigators at Duke-NUS Medical School has unearthed a dengue virus (DENV) mutation that appears to have diminished the disease burden in infected individuals from Tonga during a South Pacific outbreak in the 1970s.
"Our findings point to the importance of understanding DENV genetic factors that may underpin dengue epidemiology," senior and corresponding author Eng Eong Ooi, an emerging infectious diseases researcher at Duke-NUS Medical School, and his colleagues wrote in Science Translational Medicine on Wednesday.
The team focused on a type 2 dengue virus (DENV-2) linked to an outbreak in the South Pacific in the 1970s that had dramatic effects in places such as Fiji, Tahiti, New Caledonia, Niue, and American Samoa but notoriously fizzled out after arriving in Tonga in 1973.
"The absence of dengue outbreak in Tonga, which contrasted with large outbreaks of severe dengue elsewhere in the South Pacific in the 1970s, has been an intriguing but unexplained event," Ooi said in an email, adding that the current study supports the notion that "research investment on the genetics of DENVs is important for understanding dengue epidemiology."
Starting with viral sequence and phylogenetic data that placed the Tonga strain in a distinct cluster compared to DENV-2 clades found in other parts of the South Pacific, the researchers relied on reverse genetics to explore the consequences of three amino acid substitutions that characterized the strain.
The team found that they could restore DENV-2 infectiousness by reversing the H86R substitution in a premembrane (prM) protein-coding gene, suggesting the prM H86R alteration has a role in attenuating the infection capabilities of the Tonga strain.
The investigators then used a series of follow-up experiments, including RT-qPCR-based in vitro viral replication experiments in mammalian cell lines, mouse blood serum, and mosquito samples, to dig into the mechanism behind this apparent attenuation in human cells.
Along with dialed down dengue virus replication in mammalian cell lines, their results suggested that the single amino acid change led to enhanced DENV-2 replication in the gut of the Aedes aegypti mosquito, a vector for the virus.
Together, Ooi explained, the team's findings suggested that the prM alteration "greatly enhanced" DENV-2 virus infection in the midgut of the A. aegypti mosquito, "even while dampening down viral replication in mammalian cells."
"Our findings suggest that infection with Tonga DENV-2 produced low viremia and hence asymptomatic infection in the Tongan population, but virus transmission was sustained through increased infectiousness in the mosquito vector," he and his colleagues reported, though they cautioned that the current study did not look directly at viremia data from infected individuals in Tonga.
In addition to providing insights into the genetic changes behind the unexpected drop in disease cases in Tonga, Ooi explained, the findings may also hold clues for better controlling the virus elsewhere, particularly when it comes to developing safe and effective dengue vaccines.
"[U]nderstanding the viral genetic basis of outbreaks or lack of outbreaks could also contribute to a rational design approach for new live attenuated dengue vaccines," he wrote.