Development of an improved RT-qPCR Assay for detection of Japanese encephalitis virus (JEV) RNA including a systematic review and comprehensive comparison with published methods

Abstract

Background: Japanese encephalitis virus (JEV) is a major cause of encephalitis in Asia, and the commonest cause of mosquito-borne encephalitis worldwide. Detection of JEV RNA remains challenging due to the characteristic brief and low viraemia, with 0-25% of patients positive, and the mainstay of diagnosis remains detection of anti-JEV IgM antibody.

Methods: We performed a systematic review of published RT-PCR protocols, and evaluated them in silico and in vitro alongside new primers and probes designed using a multiple genome alignment of all JEV strains >9,000nt from GenBank, downloaded from the NCBI website (November 2016). The new assays included pan-genotype and genotype specific assays targeting genotypes 1 and 3.

Results: Ten RT-qPCR assays were compared, a pre-existing in-house assay, three published assays and six newly designed assays, using serial RNA dilutions. We selected three assays, one published and two novel assays, with the lowest limit of detection (LOD) for further optimisation and validation. One of the novel assays, detecting NS2A, showed the best results, with LOD approximately 4 copies/ reaction, and no cross-reaction on testing closely related viruses in the JEV serocomplex, West Nile Virus and St. Louis Virus. The optimised assays were validated in consecutive patients with central nervous system infections admitted to hospitals in Laos, testing paired CSF and serum samples.

Conclusions: We succeeded in developing a JEV specific RT-qPCR assay with at least 1 log10 improved sensitivity as compared to existing assays. Further evaluation is required, field-testing the assay in a larger group of patients.

Fig 1

Standard curves of the 1) Pyke, 2) NS2A assays and 3) NS3 assays with G3-RP9-190 on (A) Day 1 and repeated on (B) Day 2. Result of the RT-qPCR run, ‘Cq’, is plotted against the ‘log starting copies number’, at the RNA dilutions detected: Pyke assay 1:10 serial dilutions of G1-769 in triplicate at 10⁻³ to 10⁻⁶; NS2A assay 1:10 serial dilutions of G1-769 in triplicate at 10⁻³ to 10⁻⁷; and NS3 with G3-RP-190 10⁻⁴ to 10⁻⁷. Efficiency = 10^−1/slope-1. R² = Correlation Coefficient. RT-qPCR performed with Fastvirus kit (TaqMan® Fast Virus 1-Step) with a reaction volume of 50μL, sample volume of 30μl, and primer and probe concentrations of 600nM and 300nM respectively. Thermocycling conditions were 50°C for 5 minutes, 95°C for 20 seconds and 45 x (95°C for 15 seconds + x°C for 60 seconds). The optimal annealing temperature ‘x°C’ was different for each assay: 62°C, 60°C and 56°C for the Pyke, NS2A and NS3 assays respectively.