Portable, low-cost pathogen detection and strain identification platform
Methods for detecting the presence of a pathogen infection are described. In particular, this document provides a method of detecting target nucleic acids, such as pathogen-specific RNA, in a biological sample obtained from a subject, where the method comprises using one or more toehold switch sensors and an isothermal amplification step to detect the target nucleic acid. Methods specific for detecting and identify the presence of a virus such as Zika virus are also provided.
1. A method of detecting a target nucleic acid in a sample, the method comprising the steps of:
(a) obtaining nucleic acid from a biological sample obtained from a subject;
(b) amplifying the nucleic acid using isothermal amplification;
(c) contacting the amplified nucleic acid to a toehold switch, wherein the toehold switch encodes a reporter protein and comprises one or more single-stranded toehold sequence domains that are complementary to a target nucleic acid or the reverse complement thereof, wherein the contacting occurs under conditions that allow translation of the coding domain in the presence of the target nucleic acid but not in the absence of the target nucleic acid, and detecting the reporter protein as an indicator that the target nucleic acid is present in the amplified nucleic acid of the subject; and
(d) identifying the target nucleic acid as containing a target protospacer adjacent motif (PAM), wherein identifying comprises:
(i) amplifying nucleic acid obtained from the biological sample using a reverse primer designed to append the trigger sequence of one or more toehold switch sequence domains;
(ii) contacting the amplified nucleic acid of (i) to CRISPR/Cas under conditions that allow for sequence-specific cleavage of the contacted nucleic acid by CRISPR/Cas when the target PAM is present in the amplified nucleic acid; and
(iii) detecting activation of the toehold switch, wherein activation does not occur in the event of CRISPR/Cas-mediated sequence-specific cleavage, thereby indicating the presence of the target PAM.
2. The method of claim 1 , wherein the toehold switch comprises one or more single-stranded toehold sequence domains, a fully or partially double-stranded stem domain comprising an initiation codon, a loop domain comprising a ribosome binding site, and a coding domain.
3. The method of claim 2 , wherein the toehold and stem domains are complementary in sequence to a naturally occurring RNA.
4. The method of claim 2 , wherein the loop domain is complementary in sequence to a non-naturally occurring RNA.
5. The method of claim 1 , wherein the target nucleic acid is an RNA specific to a pathogen.
6. The method of claim 1 , wherein the pathogen is selected from the group consisting of a virus, bacterium, fungus, and parasite.
7. The method of claim 1 , wherein the isothermal amplification is selected from the group consisting of NASBA (nucleic acid sequence-based amplification), loop-mediated isothermal amplification (LAMP), recombinase polymerase amplification (RPA), and helicase-dependent amplification (HAD).
8. The method of claim 1 , wherein the biological sample is selected from the group consisting of blood, serum, urine, saliva, tissue, cell, and organ, or a fraction or portion thereof.
9. A method of detecting a target nucleic acid in a sample, the method comprising the steps of:
(a) obtaining RNA from a biological sample obtained from a subject;
(b) amplifying the RNA using isothermal amplification;
(c) contacting the amplified RNA to a toehold switch, wherein the toehold switch encodes a reporter protein and comprises one or more single-stranded toehold sequence domains that are complementary to a target RNA or the reverse complement thereof, wherein the contacting occurs under conditions that allow translation of the coding domain in the presence of the target RNA but not in the absence of the target RNA, and detecting the reporter protein as an indicator that the target RNA is present in the amplified RNA of the subject; and
(d) identifying the target RNA as containing a target protospacer adjacent motif (PAM), wherein identifying comprises:
(i) amplifying RNA obtained from the biological sample using a reverse primer designed to append the trigger sequence of one or more toehold switch sequence domains;
(ii) contacting the amplified RNA of (i) to CRISPR/Cas under conditions that allow for sequence-specific cleavage of the contacted RNA by CRISPR/Cas when the target PAM is present in the amplified RNA; and
(iii) detecting activation of the toehold switch, wherein activation does not occur in the event of CRISPR/Cas-mediated sequence-specific cleavage, thereby indicating the presence of the target RNA.
10. The method of claim 9 , wherein the toehold switch comprises one or more single-stranded toehold sequence domains, a fully or partially double-stranded stem domain comprising an initiation codon, a loop domain comprising a ribosome binding site, and a coding domain.
11. The method of claim 10 , wherein the toehold and stem domains are complementary in sequence to a naturally occurring RNA.
12. The method of claim 10 , wherein the loop domain is complementary in sequence to a non-naturally occurring RNA.
13. The method of claim 9 , wherein the target nucleic acid is an RNA specific to a pathogen.