ABSTRACT
Diagnosis of SARS-CoV-2 infection through rapid, accurate, and sensitive testing is the most important and fundamental step in coping with the COVID-19 epidemic. We have developed a sensitive fluorometric assay to detect SARS-CoV-2 viral RNA without thermal cycling. This assay system, based on tandem isothermal gene amplification (TIGA), is composed of ternary rolling circle amplification (t-RCA) and subsequent strand displacement amplification (SDA) coupled with G-quadruplex-generating RCA (SDA/GQ-RCA). Without the need to convert viral RNA into cDNA, viral RNA forms a ternary complex composed of hairpin primer (HP) and dumbbell padlock DNA during the t-RCA process. t-RCA generates a long chain of single-stranded DNA (ssDNA) with tandemly repeated hairpin structures that are subjected to SDA. SDA produces multiple short ssDNAs from t-RCA products, which then serve as primers for the second RCA reaction. A long ssDNA harboring repeated copies of the G-quadruplex is produced in the second round of RCA. Emission of enhanced fluorescence by thioflavin T, which intercalates into the G-quadruplex, allows fluorometric detection of amplified viral genes. This fluorometric analysis sensitively detected SARS-CoV-2 RNA as low as 5.9 aM, with a linear range between 0.2â¯fM and 200â¯fM within 1â¯h. Hence, this isothermal gene amplification method without reverse transcription of viral RNA can be applied to diagnose COVID-19 with high sensitivity and accuracy as an alternative to current PCR-based diagnosis.