Nucleic acid detection method

1. A method for detecting the presence of a target nucleic acid of defined sequence in a sample which method comprises the steps:

a) contacting said sample with:

i. a first oligonucleotide probe; and

ii. a nicking agent;

wherein the first oligonucleotide probe comprises a first complementarity region capable of sequence specific hybridisation to the target nucleic acid and a cleavage site for the nicking agent; and wherein the nicking agent specifically recognises double-stranded nucleic acid formed when the first oligonucleotide probe hybridises to the target nucleic acid in said sample and cleaves said first oligonucleotide probe to produce a first probe fragment;

b) contacting said first probe fragment with:

iii. a second oligonucleotide probe; and

iv. a nicking agent;

wherein the second oligonucleotide probe comprises a second complementarity region capable of sequence specific hybridisation to the first probe fragment and a cleavage site for the nicking agent; and wherein the nicking agent specifically recognises double-stranded nucleic acid formed when the second oligonucleotide probe hybridises to the first probe fragment and cleaves said second oligonucleotide probe to produce a second probe fragment;

c) optionally contacting said second probe fragment with:

v. a third oligonucleotide probe; and

vi. a nicking agent;

wherein the third oligonucleotide probe comprises a third complementarity region capable of sequence specific hybridisation to the second probe fragment and a cleavage site for the nicking agent; and wherein the nicking agent specifically recognises double-stranded nucleic acid formed when the third oligonucleotide probe hybridises to the second probe fragment and cleaves said third oligonucleotide probe to produce a third probe fragment;

d) optionally repeating step c) n times, wherein n is a positive integer, using fourth and subsequent sequential (3+n)th oligonucleotide probe(s) to produce fourth and subsequent sequential (3+n)th probe fragment(s); and

e) detecting the presence of the probe fragment produced at the end of step (b) and/or step (c) and/or step (d), wherein one or more of said detected probe fragment(s) is not capable of sequence specific hybridisation to the complementarity region of any of the preceding oligonucleotide probes to form a site that is specifically recognised and cleaved by any of the said nicking agent(s) and/or none of the probe fragments produced is capable of sequence specific hybridisation to the first complementarity region of the first oligonucleotide probe, and wherein the presence of said detected probe fragment(s) indicates the presence of the target nucleic acid in said sample.

2. A method according to claim 1 , wherein none of the probe fragments produced in steps (a), (b), and optional steps (c) and (d) are capable of sequence specific hybridisation to the complementarity region of any one of the preceding oligonucleotide probes.

3. A method according to claim 1 , wherein at least one of the oligonucleotide probes is attached to a solid material.

4. A method according to claim 1 , wherein two or more of the nicking agents in different steps (a), (b), and optional steps (c) and (d) are the same.

5. A method according to claim 1 , wherein one or more of the probe fragments produced in step (b), and optional steps (c) and (d) is attached to a moiety that permits its detection, wherein the moiety is selected from a colorimetric or fluorometric dye, and a moiety that is capable of attachment to a colorimetric dye.

6. A method according to claim 5 , wherein said moiety is an enzyme that yields a colorimetric or fluorometric signal following contact with a substrate.

7. A method according to claim 1 , wherein the presence of the probe fragment(s) in step (e) is detected by nucleic acid lateral flow.

8. A method according to claim 7 , wherein the nucleic acid lateral flow utilises one or more nucleic acid(s) capable of sequence specific hybridisation to one or more of the probe fragments in step (e).

9. A method according to claim 7 , wherein a colorimetric signal is produced with a colorimetric dye selected from the group consisting of carbon and gold.

10. A method according to claim 1 , wherein the level of target nucleic acid in said sample is quantified in step (e).

11. A method according to claim 1 , wherein one or more of the nicking agents in steps (a), (b), and optional steps (c) and (d) is selected from the group consisting of a nicking restriction endonuclease, an engineered enzyme, and a programmable nicking enzyme.

12. A method according to claim 1 , wherein one or more of the nicking agents in steps (a), (b), and optional steps (c) and (d) is a double-strand cleaving agent which functions as a nicking agent due to i) strand preference, or ii) only one of the two strands within the double-stranded nucleic acid that is specifically recognised by said double-stranded cleaving agent being capable of cleavage.

13. A method according to claim 1 , wherein one or more of the oligonucleotide probes comprises one or more modifications that render it resistant to nuclease cleavage.

14. A method according to claim 1 , wherein said target nucleic acid is selected from the group consisting of single-stranded RNA, including single-stranded RNA derived from double-stranded RNA and single-stranded RNA derived from double-stranded DNA, or single-stranded DNA, including single-stranded DNA derived from double-stranded DNA, single-stranded DNA derived from double-stranded DNA by use of a nuclease, the product of reverse transcriptase, an RNA polymerase or a DNA polymerase and a probe fragment produced following cleavage of an oligonucleotide probe by a double-strand cleaving agent.

15. A method according claim 1 , wherein the presence of two or more different target nucleic acids of defined sequence are detected in the same sample.

16. A method according to claim 15 , wherein a separate series of steps (a), (b), optional steps (c), (d), and (e) using different first, second and optionally third, fourth and subsequent sequential (3+n)th oligonucleotide probes for each of the two or more target nucleic acids is performed, which separate series of steps may be conducted simultaneously, or wherein step (a) uses a different first oligonucleotide probe for each of the two or more different target nucleic acids but the probe fragment produced in one or more of steps (a), (b), and optional steps (c) and (d) for two or more target nucleic acids is capable of hybridising to the same oligonucleotide probe in a subsequent step.

17. A method according to claim 1 , wherein said sample is selected from the group consisting of a human sample, a forensic sample, an agricultural sample, a veterinary sample, an environmental sample and a biodefence sample, including a biological sample selected from the group consisting of a nasal or nasopharyngeal swab or aspirate, blood or a sample derived from blood, and urine.

18. A method according to claim 1 , wherein said target nucleic acid is viral or derived from viral nucleic acid material, is bacterial or derived from bacterial nucleic acid material, is circulating, cell-free DNA released from cancer cells or foetal cells, or is micro RNA or derived from micro RNA.

19. A method according to claim 1 , wherein the detection of said target nucleic acid is used for the diagnosis, prognosis or monitoring of a disease or a diseased state, selected from the group consisting of an infectious disease, including but not limited to HIV, influenza, RSV, Rhinovirus, norovirus, tuberculosis, HPV, meningitis, hepatitis, MRSA, Ebola, Clostridium difficile , Epstein-Barr virus, malaria, plague, polio, Chlamydia , herpes, gonorrhoea, measles, mumps, rubella, cholera or smallpox, and a cancer, including but not limited to colorectal cancer, lung cancer, breast cancer, pancreatic cancer, prostate cancer, liver cancer, bladder cancer, leukaemia, esophageal cancer, ovarian cancer, kidney cancer, stomach cancer or melanoma.

20. A method for detecting the presence of a target nucleic acid of defined sequence in a sample which method comprises the steps:

a) contacting said sample with:

i. a first oligonucleotide probe attached to a solid material; and

ii. a nicking agent;

wherein the first oligonucleotide probe comprises a first complementarity region capable of sequence specific hybridisation to the target nucleic acid and a cleavage site for the nicking agent; and wherein the nicking agent specifically recognises double-stranded nucleic acid formed when the first oligonucleotide probe hybridises to the target nucleic acid in said sample and cleaves said first oligonucleotide probe to release a first probe fragment from said solid material;

b) contacting said first probe fragment with:

iii. a second oligonucleotide probe attached to a solid material; and

iv. a nicking agent;

wherein the second oligonucleotide probe comprises a second complementarity region capable of sequence specific hybridisation to the first probe fragment and a cleavage site for the nicking agent; and wherein the nicking agent specifically recognises double-stranded nucleic acid formed when the second oligonucleotide probe hybridises to the first probe fragment and cleaves said second oligonucleotide probe to release a second probe fragment from said solid material;

c) contacting said second probe fragment with:

v. a third oligonucleotide probe attached to a solid material and attached to a colorimetric dye or a moiety capable of attachment to a colorimetric dye; and

vi. a nicking agent;

wherein the third oligonucleotide probe comprises a third complementarity region capable of sequence specific hybridisation to the second probe fragment and a cleavage site for the nicking agent; and wherein the nicking agent specifically recognises double-stranded nucleic acid formed when the third oligonucleotide probe hybridises to the second probe fragment and cleaves said third oligonucleotide probe to release a third probe fragment attached to said colorimetric dye or moiety capable of attachment to a colorimetric dye from said solid material, wherein said third probe fragments is not capable of sequence specific hybridisation to the complementarity region of any one of the preceding oligonucleotide probes to form a site that is specifically recognised and cleaved by any of the said nicking agents; and

d) detecting the presence of said third probe fragment produced at the end of step c) by colorimetric detection wherein the presence of a colorimetric signal indicates the presence of the target nucleic acid in said sample.