Clinical samples.
Thirty nasopharyngeal aspirates from individuals with symptoms of respiratory tract infections were tested positive for HRV-A (5 clinical samples), HRV-B (20 clinical samples), or HRV-C-positive (5 clinical samples) according to the method previously described [25], and used in this study. Ten other specimens tested positive for metapneumovirus (HmPV), adenovirus (ADV), bocavirus (HBOV), or respiratory syncytial virus (RSV) were used as the negative control. These samples were collected by Ningbo Municipal Center for Disease Control and Prevention (NCDC) between January, 2018 and December, 2020.
Reagents.
ssDNA FQ reporters labeled with 6-FAM and BHQ1 at their 5′ and 3′ ends were synthesized by GENEWIZ Inc. (Suzhou, China). Oligonucleotides used for RT-RPA primers and the in vitro synthesis of crRNA were manufactured by Sangon Biotech (Shanghai, China), and shown in Table 1. RT-RPA kit was purchased from Jiangsu lesun biotechnology Co., Ltd (Wuxi, China). Cas12a and its reaction buffer NEB buffer 2.1 were bought from New England Biolabs (MA, USA).
Table 1
Oligonucleotides employed for RT-RPA and crRNA in RT-RPA-Cas12a-mediated assay for rhinovirus species B detection.
Assay | Name | Oligonucleotide sequences (5’-3’) |
RT-RPA | HRV-B-F1 | AYTAGTYTGGTCGATGAGGCT |
HRV-B-F2 | AGCCTGCGTGGCGGCCARCCCAGC |
HRV-B-F3 | ATGTGCTTGRTTGWGANTCCTCCGGCCCCTGAATG |
HRV-B-R1 | ATATGCTGTGACCATAAGAMAA |
HRV-B-R2 | CGGACACCCAAAGTAGTCGGTCCCRTCCCRGAATT |
HRV-B-R3 | GAAACACGGACACCCAAAGTAGTCGGTCCCRTCCC |
HRV-B-R4 | GTWGANACYTGHGCDCCCATGRTCACAGTATAT |
RT-RPA-Cas12-based detection | HRV-B-crRNA1-F | GAAATTAATACGACTCACTATAGGGTAATTTCTACTAAGTGTAGATTCCTCCGGCCCCTGAATGCG |
HRV-B-crRNA1-R | CGCATTCAGGGGCCGGAGGAATCTACACTTAGTAGAAATTACCCTATAGTGAGTCGTATTAATTTC |
HRV-B-crRNA2-F | GAAATTAATACGACTCACTATAGGGTAATTTCTACTAAGTGTAGATACCGACTACTTTGGGTGTCC |
HRV-B-crRNA2-R | GGACACCCAAAGTAGTCGGTATCTACACTTAGTAGAAATTACCCTATAGTGAGTCGTATTAATTTC |
HRV-B-crRNA2r-F | GAAATTAATACGACTCACTATAGGGTAATTTCTACTAAGTGTAGATGGACACCCAAAGTAGTCGGT |
HRV-B-crRNA2r-R | ACCGACTACTTTGGGTGTCCATCTACACTTAGTAGAAATTACCCTATAGTGAGTCGTATTAATTTC |
HRV-B-crRNA3-F | GAAATTAATACGACTCACTATAGGGTAATTTCTACTAAGTGTAGATACTTTGGGTGTCCGTGTTTC |
HRV-B-crRNA3-R | GAAACACGGACACCCAAAGTATCTACACTTAGTAGAAATTACCCTATAGTGAGTCGTATTAATTTC |
HRV-B-crRNA3r-F | GAAATTAATACGACTCACTATAGGGTAATTTCTACTAAGTGTAGATGAAACACGGACACCCAAAGT |
HRV-B-crRNA3r-R | ACTTTGGGTGTCCGTGTTTCATCTACACTTAGTAGAAATTACCCTATAGTGAGTCGTATTAATTTC |
ssDNA-reporter-FQ | 6-FAM-TTATTATT-BHQ1 |
Production of standard RNA of the target region for HRV-B.
Twenty whole genome sequences of HRV-B, including JX193795.1, JN798573.1, JF285331.1, KF958309.1, JF285329.1, OK649426.1, OK649410.1, OK649413.1, OK254848.1, OL133766.1, OK649379.1, OK181492.1, MZ835615.1, K02121.1, MZ629108.1, MW969533.1, MN369041.1, KY369901.1, MT512399.1, and LC495296.1, were retrieved from the National Center for Biotechnology Information, and then aligned using the MEGA X. Comparative genomic analysis reveals that the segment of VP4 gene was highly conserved, and determined as the target region (Fig. 1A). Subsequently, the conserved fragment of VP4 gene composed of approximately 412 nucleotides was synthesized by Shanghai Sangon biotech (Shanghai, China), cloned into the pBluescript II SK (+) to manufacture pBluescript-VP4, and the recombinant plasmid was kept at -80°C for further use.
For in vitro synthesis of standard RNA of the conserved target region for HRV-B, the Sac I-linearized pBluscript-VP4 was used as the template to biosynthesize standard RNA using the in vitro transcription (IVT) T7 Kit (TaKaRa, Dalian, China). Briefly, the IVT reaction mixture, including 5 µL of 10 × transcription buffer, 5 µL of each NTP solution, 1.25 µL of RNase inhibitor, 5 µL of T7 RNA polymerase, 8.75 µL of RNase-free water and 10 µL of linear pBluscript-VP4 plasmid, was conducted at 42 ℃ for 2 h. Then, the number copies of standard RNA of the target region for HRV-B were determined using the method previously reported [22].
Design of RT-RPA primer and crRNA.
Based on the consensus sequences for HRV-B, both four pairs of RT-RPA primers and five pairs of oligonucleotides for preparation of crRNA were designed (Fig. 1B), followed by the synthesis by Suzhou GENEWIZ biotech, and shown in Table 1. The primer specificity was checked by nucleotide BLAST (blastn), and the secondary structure and dimer analysis were performed using Oligo Analyzer 3.1. Preparation of crRNA was mainly conducted using the following two-step. Four pairs of oligonucleotides were annealed to generate double strand DNAs (dsDNA), respectively, and then the resulting dsDNA products were transcribed by in vitro transcription using IVT T7 Kit [22]. The in vitro transcription reaction was performed by incubating the mixture, including 5 µL of 10 × transcription buffer, 5 µL of each NTP solution, 1.25 µL of RNase inhibitor, 5 µL of T7 RNA polymerase, 16.25 µL of RNase-free water and 2.5 µL of annealed dsDNA, at 42 ℃ for 2 h. The resulting crRNA products were subjected to phenol/chloroform extraction, and the amount of crRNA was measured using the spectrophotometer (Metash Instruments, Shanghai, China).
Screening of optimal crRNA candidate for RT-RPA-Cas12a-mediated assay.
To screen the optimal crRNA, five crRNAs (crRNA1, crRNA2, crRNA2r, crRNA3 and crRNA3r) were designed, and their trans-cleavage efficiencies mediated by Cas12a were assessed by the intensity of fluorescent signal, respectively. Meanwhile, to determine the optimal concentration of crRNA for the trans-cleavage activities, final concentrations ranging from 0 nM to 120 nM were applied and assessed in the RT-RPA-Cas12a-mediated assay.
RT-RPA-Cas12a-mediated assay.
The RT-RPA-Cas12a-mediated assay is mainly divided into two steps [22]. The first step of this assay involves the exponential amplification of the target sequence by RT-RPA. The 50 µL RT-RPA reaction mixture was performed in the preheated Axxin T8 isothermal instrument for 20 min at 39 ℃, including 25 µL of reaction buffer V, 1 µL of each primer (10 µM), 15 µL of ddH2O, 0.5 µL RNase inhibitor (40 U), 5 µL of standard RNA and 2.5 µL of 280 mM magnesium acetate. For the negative control, RNase-free water was served as the template in the same volume. At the second stage, the Cas12a-mediated trans-cleavage reaction was carried out in a 50 µL reaction mixture, including 5 µL of 10 × NEB Buffer 2.1, 2 µL of ssDNA FQ (1 µM), 1 µL of 2.5 µM Cas12a, 2 µL of 1 µM crRNA, 0.5 µL RNase inhibitor (40 U), 29.5 µL of deionized water, and 10 µL of RT-RPA products or deionized water (the negative control). The Cas12a-mediated reaction was performed at 37°C for 30 min. The detection results of the assay were obtained with fluorescent signals collected every 20 second with ssDNA FQ as the substrates.
Specificity and sensitivity of RT-RPA-Cas12a-mediated fluorescent assay.
The specificity of RT-RPA-Cas12a-mediated fluorescent assay was performed by using RNA or DNA from several viral samples as templates, including human rhinoviruses B species, the mixture of human rhinoviruses A and C species (HRV-AC) in equal volume, metapneumovirus (HmPV), adenovirus (ADV), bocavirus (HBOV), and respiratory syncytial virus (RSV), and the amount of template for per reaction was the final concentration of 200 copies/µL. 10 different samples tested positive for each test virus, and 10 HRV-AC samples were applied as the negative control.
To assess the sensitivity of RT-RPA-Cas12a-mediated fluorescent assay, a concentration gradient of standard RNA of the target region for HRV-B (1000, 100, 10, 1, 0.5, 0.1, 0.05 and 0 copies/µL), was used as the template. Each reaction was replicated three times and the results were examined using real-time fluorescence readout.
RT-qPCR detection.
The RT-qPCR assay for detection of HRV nucleic acids was performed using HRV kit (bioPerfectus technologies, Jiangsu, China) in an ABI 7500 (Applied Biosystems). Briefly, the total RNAs of 20 HRV-B samples in this study were extracted using automatic nucleic acid extraction instrument (bioPerfectus technologies, Jiangsu, China), respectively. The reaction was carried out in a 20 µL reaction mixture, including 2 × RT-qPCR buffer 10 µL, Enzyme Mix 0.8 µL, 0.4 µL of each primer (10 µM), ROX Dye Ⅱ 0.4 µL, Total RNA 5 µL ,RNase-free ddH2O 3.4 µL.The reactions were performed with first step of reverse transcription at 42°C for 5 min, followed by 95°C for 1 min, 40 cycles of 95°C for 30 s, 60°C for 30 s.
2.9. Statistics.
Data are representative of each sample with at least three independent biological replicates. Statistical analysis of fluorescence values was measured using the GraphPad Prism 8 (GraphPad Software, version 8.0.1), and statistical difference was calculated using the Students' t-test.