By Nicole Lim
Assistant Director, Communications & External Relations
Despite ongoing efforts to wipe out the Aedes mosquito responsible for spreading dengue, cases continue to rise. Even now, Singapore is in the midst of an unseasonable spike, with more than 600 dengue cases reported a week during the latter half of January.
Population growth and increasing urban density have recently been shown to be the main driver in the number dengue cases, accounting for 86 per cent of the increase in dengue cases between 1974 and 2011.
LKCMedicine Professor of Infectious Diseases Annelies Wilder-Smith said, "We have very good vector control here. So you start to think, why is it that the number of cases is increasing despite our ability to reduce the size of mosquito populations? Urbanisation creates new habitats for the Aedes mosquitoes to breed. Closer proximity between humans also means the mosquitoes get more blood meals in the same distance."
It is not just complete vector control that is proving elusive. Even with several decades of extensive investment in vaccine development, the best candidate, which is being launched this year, offers only partial protection. As for a treatment, research into antivirals is relatively new and, despite some promising candidates, no natural or synthetic compound has shown any efficacy in clinical studies to date.
But efforts to find an effective treatment have just received a boost from a recent breakthrough, in which the scientists from Singapore have for the first time mapped the three-dimensional structure of the largest viral protein of the dengue virus.
The nonstructural protein 5 (NS5) plays two key functions in the virus's survival – it facilitates viral protein translation and at the same time hides it from the host immune system. LKCMedicine Nanyang Assistant Professor of Infectious Diseases Luo Dahai, who led the team, said, "Its function has been studied for more than a decade. It's just that nobody has ever seen how it works."
Nanyang Assistant Professor of Infectious Diseases Luo Dahai led the team of researchers that worked on this breakthrough
By producing the first crystal structure of the full-length NS5 protein, the team of scientists from LKCMedicine, Duke-NUS, NUS and NTU's School of Biological Sciences offers vital information in the race to develop an effective dengue treatment. What makes NS so interesting is that it is not only conserved in all four strains of dengue, it is also found in other Flaviviruses, such as Japanese encephalitis, Zika, and Yellow Fever, and as such its potential is tremendous. Duke-NUS Medical School Professor of Emerging Infectious Diseases Subhash Vasudevan said, "NS5 contains multiple enzyme activities that are regulated by intramolecular protein interactions that were captured in the structures that were solved together with NUS PhD student Zhao Yong Qian. These protein interaction sites may prove to be valuable antiviral targets."
Having captured a key snapshot at atomic resolution, scientists can now infer and further study how ribonucleic acid (RNA) comes in, how it is catalysed and what other factors are necessary for this process.
But knowing what the virus looks like is not enough. To discover an effective treatment, scientists have to be sure that there is no equivalent in the human body that could be affected by a potential drug. "If you don't really know the difference between host and virus, you are likely to develop something that also targets the host, and then you get cytotoxic side effects, which have to be avoided in drug development," said Asst Prof Luo.
And for dengue, that is where the crux lies. When the scientists compared the NS5 structure of dengue with its equivalent in other organisms, they found that it most closely resembles the human one, with 14 per cent of the protein sequence being identical.
While Asst Prof Luo hopes to continue his work on elucidating the full replication process, he and his team are at the same time working with Singapore-MIT Alliance for Research and Technology scientists to develop an assay to test possible compounds. Using mass spectrometry technology, the team hopes to design an assay that is sensitive enough to identify molecules that bind to the unique spots identified on NS5. Once the assay is ready, the team will work with collaborators such as A*STAR's Drug Discovery & Development unit to perform high resolution screens to identify compounds. "We start with basic knowledge and together with others who have specific expertise, we hope to translate this into a new solution with clinical applications," said Asst Prof Luo.
Added Prof Vasudevan, "Finding a 'druggable' site that will work on the four serotypes is the goal. So we will look for small molecules that can inhibit virus replication in the test tube, animal models and ultimately in human clinical trials, that is the long-term goal for us."
Already the discovery has brought the lab to the attention of other groups around the world working on potential dengue treatments, many of whom have started sending their most promising compounds to the Luo lab for screening. Depending on the background research carried out, each screen can take upwards of two months, so Asst Prof Luo is very selective. But he hopes that one of them will provide the next breakthrough.
Prof Wilder-Smith described the discovery as significant. She said, "Every scientific discovery is one step closer to a therapeutic drug for dengue virus infections."