Mutations resulting in amino-acid substitutions of the SARS-CoV-2 spike protein receptor-binding domain (RBD) have been associated with enhanced transmissibility and immune escape of the respective variants, namely Alpha, Beta, Gamma or Delta. Rapid identification of the aforementioned variants of concern and their discrimination of other variants is thus of importance for public health interventions. For this reason, a one-step real-time RT-PCR assay employing four locked nucleic acid (LNA) modified TaqMan probes was developed, to target signature mutations associated with amino-acid substitutions at positions 478, 484 and 501 present in the receptor-binding motif (RBM) of the spike protein RBD. This region contains most contacting residues of SARS-CoV-2 that bind to ACE2. A novel strategy employing the use of non-extendable LNA oligonucleotide blockers that can reduce non-specific hybridization of probes increased the number of different mutated sites examined in a multiplex PCR. The combinatory analysis of the different fluorescence signals obtained enabled the preliminary differentiation of SARS-CoV-2 variants of concern. The assay is sensitive with a LOD of 263 copies/reaction for the Delta variant, 170 copies/reaction for the Beta variant, amplification efficiencies > 91% and a linear range of >5 log10 copies/reaction against all targets. Validation of the assay using known SARS-CoV-2-positive and negative samples from humans and animals revealed its ability to correctly identify the targeted mutations and preliminary characterize the SARS-CoV-2 variants. The novel approach for mutation typing using LNA oligonucleotide blockers can be modified to target signature mutations at four different sites in the RBM and further expand the range of variants detected.
Keywords: LNA oligonucleotide blockers; SARS-CoV-2; mutation; real-time RT-PCR; receptor-binding domain; spike protein; typing; variant of concern.