Development and evaluation of a one-pot RPA-Cas12a assay based on a primer-driven reverse screening strategy for preliminary screening of megalocytivirus-related viruses.
A primer-driven reverse-screening strategy was used to identify an RPA-Cas12a target suitable for the rapid preliminary screening of megalocytivirus-related viruses. The ISKNV reference genome NC_003494.1 was used as the initial template, and candidate amplification units were designed according to RPA primer-design requirements, primer physicochemical properties, and the availability of Cas12a protospacer-adjacent motif (PAM) sites and crRNA target sequences. Following preliminary amplification assessment, the retained candidate primers were aligned individually against 75 complete genome sequences of megalocytivirus-related viruses. Of these, 67 sequences met the predefined criteria for target-region integrity, primer-binding-site compatibility, and Cas12a recognition. Retrospective mapping to the reference genome located the candidate amplification region within ORF057L. Based on the resulting candidate detection unit, a one-pot RPA-Cas12a assay incorporating a commercially available lyophilized RPA amplification module was developed. Optimization showed that 400 nM reporter and 80 nM crRNA-1 provided relatively stable fluorescence output. A cut-off value of 1281.6 relative fluorescence units (RFU) was established as the mean plus three standard deviations of the endpoint fluorescence values obtained from 20 qPCR-negative samples. In analytical sensitivity testing, the assay generated fluorescence signals above the negative control at low plasmid copy numbers. However, because only a limited number of replicates were tested at these low template concentrations, these findings were not used to define a formal limit of detection. ISKNV, RSIV, and TRBIV samples tested positive, whereas the MRV sample produced an endpoint fluorescence value below the cut-off. Repeatability analysis of the same sample in six independent reactions yielded a coefficient of variation of 8.03%. Among the 39 samples examined, no discordant qualitative results were observed between the RPA-Cas12a assay and qPCR. These findings support the use of the ORF057L-targeted one-pot RPA-Cas12a assay as a rapid preliminary screening tool for megalocytivirus-related viruses. Nevertheless, its formal limit of detection, inter-batch stability, cross-reactivity with additional non-target pathogens, and clinical diagnostic performance require further evaluation.