An expanded high throughput RT-PCR assay to rapidly identify all known SARS-CoV-2 variants of concern using melting temperature coding.

BackgroundThe rapid emergence of new vaccine-resistant SARS-CoV-2 variants of concern (VOC) requires an equally rapid deployment of diagnostic tests to specifically identify each VOC as soon as it arises. Here, we report an expanded version of our previously described sloppy molecular beacon (SMB) Alpha/Beta/Gamma RT-PCR melting temperature (Tm) signature-based assay, which now includes modifications that allow specific detection of Delta (B.1.617.2) and Omicron (B.1.529) VOCs. MethodsWe developed a dual SMB assay (SMB-452), which targeted the T22917G (L452R) mutation in the SARS-CoV-2 spike protein to specifically detect the Delta VOC. We also identified a Tm profile in our existing SMB-501 and SMB-484 assays (which detect mutations in codons 501 and 484 of the SARS-CoV-2 spike protein, respectively) that differentiate the Omicron-specific N501Y (A23063T) and E484A (A23013C) mutations from both wild type (WT) and other VOCs. The entire six SMB three-codon assay was tested using reference SARS-CoV-2 RNAs. The assay was then validated using clinical samples from COVID-19 patients tested with a LightCycler 480 (LC480) (74 samples), Bio-Rad CFX96 (34 samples), Rotor-Gene Q (Qiagen) (34 samples) and an ABI-7500 (34 samples) RT-PCR instruments. Six SMB Tm results were then inputted into an Excel Analysis tool to generate specific VOC identifications. ResultsThe limit of detection (LOD) for the new SMB-452 assay, which specifically identified the Delta variant was 1 genomic equivalent (GE) per reaction. The LODs of the SMB-501 and SMB-484 assays, which detect Omicron were 100 and 103 GE respectively. Clinical validation of the 3-codon assay in the LC480 instrument showed the assay detected 94% of the samples as WT or VOCs in clinical samples and 6% of the tests producing indeterminate results. None of the samples were incorrectly identified as WT or as a different VOC. Thus, excluding samples with indeterminant results, the assay was 100% sensitive and 100% specific compared to sequencing. There was also 100% concordance between the LC480, BioRad, ABI and Qiagen results, excluding negative or indeterminate results; however, the Qiagen assay had significantly more indeterminates than the other assays. ConclusionThis new assay can serve as a robust diagnostic tool for selecting appropriate monoclonal antibody therapy and rapid VOC surveillance.