Method of detecting target nucleic acid molecules
The present application provides methods for detecting a nucleic acid molecule involving the use of a signal code sequence which corresponds to said nucleic acid molecule and a plurality of labelled detection probes which yield signals which make up the signal code sequence. In particular, the invention provides a sequential barcoding and decoding scheme which utilises a sequencing-by-hybridisation (SBH) strategy to sequence and decode a nucleotide barcode sequence, and to differentiate the nucleotide barcode sequence from other nucleotide barcode sequences. In an extension of the method, the application also provides a new coding scheme for providing a target nucleic acid with a detectable “colour” (or similar signal)-based code.
1. A method of detecting a target nucleic acid molecule in a sample, comprising:
(a) contacting the sample with a padlock probe specific for the target molecule, wherein the padlock probe is circularized after hybridization to a target sequence in the target nucleic acid molecule, wherein the padlock probe comprises a nucleotide barcode sequence to identify the padlock probe, and wherein the circularized padlock probe is amplified by rolling circle amplification (RCA) to produce a rolling circle product (RCP), the RCP containing multiple complementary copies of the nucleotide barcode sequence;
(b) assigning a signal code sequence specific to the target nucleic acid molecule,
wherein the signal code sequence may be derived by interrogating the complementary copies of the nucleotide barcode sequence with sequential detection probes each yielding a signal and the signals together make up the signal code sequence, and wherein a complementary copy of the nucleotide barcode sequence comprises a first domain and a second domain, wherein a portion of the first domain overlaps with some but not all of the second domain;
(c) contacting the RCP with a first detection probe to hybridize the first detection probe to the RCP, wherein the first detection probe comprises:
(i) a recognition sequence complementary to the first domain and
(ii) a first reporter;
(d) detecting a signal from the first reporter, thereby obtaining a first signal code of the signal code sequence;
(e) contacting the RCP with a second detection probe to hybridize the second detection probe to the RCP, wherein the second detection probe comprises:
(i) a recognition sequence complementary to the second domain and
(ii) a second reporter,
wherein the second detection probe initiates a strand displacement reaction displacing the first detection probe hybridized to the first domain;
(f) detecting a signal from the second reporter, thereby obtaining a second signal code of the signal code sequence; and
(g) contacting the RCP with additional detection probes to identify additional signal codes of the signal code sequence until sufficient signal codes have been identified to detect the nucleic acid molecule, wherein the additional detection probes have recognition sequences complementary to the first or second domain such that additional detection probes successively hybridize to the first or second domain and displace preceding detection probes hybridized thereto.
2. The method of claim 1 , wherein the first domain is 5′ to the second domain in the complementary copy of the nucleotide barcode sequence.
3. The method of claim 1 , wherein the first domain is 3′ to the second domain in the complementary copy of the nucleotide barcode sequence.
4. The method of claim 1 , wherein the target sequence is in a native genomic DNA or in a naturally occurring RNA molecule, or in a cDNA, or in an amplification product generated from any of the foregoing nucleic acid molecules.
5. The method of claim 1 , wherein the first domain and the second domain each comprises a first subunit and a second subunit, and wherein the second subunit from the first domain fully overlaps with the first subunit of the second domain.
6. The method of claim 5 , wherein the complementary copy of the nucleotide barcode sequence comprises: (i) a first common region adjacent to the first subunit of the first domain, wherein the recognition sequence of the first detection probe comprises a sequence which is complementary to the first common region; and (ii) a second common region adjacent to the second subunit of the second domain, wherein the recognition sequence of the second detection probe comprises a sequence which is complementary to the second common region.
7. The method of claim 6 , wherein the first detection probe, the second detection probe, and the additional detection probes each comprise a displacer toehold overhang region.
8. The method of claim 7 , wherein all of the detection probes comprising a recognition sequence that is complementary to the first domain comprise a first displacer toehold overhang sequence, and all of the detection probes comprising a recognition sequence that is complementary to the second domain comprise a second displacer toehold overhang sequence.
9. The method of claim 8 , wherein (g) further comprises contacting the RCP with either a first displacer probe which comprises a displacer toehold binding region complementary to the first displacer toehold overhang and a sequence complementary to the first common region; or a second displacer probe which comprises a displacer toehold binding region complementary to the second displacer toehold overhang and a sequence complementary to the second common region.
10. The method of claim 1 , wherein the target sequence is a nucleotide barcode sequence provided within the target nucleic acid molecule.
11. The method of claim 1 , wherein the target nucleic acid molecule is linked to an antibody.
12. The method of claim 1 , wherein the first domain and the second domain each comprises a unique subunit.
13. The method of claim 1 , wherein the reporter of each detection probe is a binding site for a reporter probe comprising a detectable label, said binding site being contained in an overhang region of the detection probe which does not hybridize to the RCP, and wherein the signal is detected from said label.
14. The method of claim 13 , wherein the detectable label is a fluorophore.
15. The method of claim 14 , wherein the signal code signal code sequence is a unique fluorophore sequence.
16. The method of claim 1 , for detecting multiple different nucleic acid molecules present in the sample, wherein each different nucleic acid molecule is assigned a different signal code sequence and is detected using a specific padlock probe with a different nucleotide barcode sequence.
17. The method of claim 1 , wherein the sample is a cell or a tissue sample on a solid substrate.