Abstract:
A method of detecting a target nucleic acid sequence comprising providing a stem-and-loop structured nucleic acid for measurement wherein the nucleic acid comprises complementary sequence portions located at both terminals and a target sequence portion therebetween as well as a double-stranded portion formed by hybridization of the complementary sequence portions located at both terminals and a remaining looped single-stranded portion, providing a probe nucleic acid having a sequence complementary to the target sequence portion wherein one end of the probe nucleic acid being immobilized to a solid substrate surface, reacting the nucleic acid for measurement with the probe nucleic acid to specifically hybridize the target sequence portion of the nucleic acid for measurement to the probe nucleic acid, and detecting presence or absence of the nucleic acid for measurement hybridized to the probe nucleic acid.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. application Ser. No. 11/378,453, filed Mar. 20, 2006, which is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a technique for detecting a target nucleic acid, more particularly to a method for detecting a target nucleic acid sequence present in a LAMP amplified product and a LAMP amplified product for use in detection of a target nucleic acid. 
     2. Description of the Related Art 
     To select correct therapeutic agents, it is important to identify bacteria causing a given infection. Currently, culture method and nucleic acid amplification method are used for microorganism tests. 
     Recently, improvement in culture method has been made in an effort to improve detection sensitivity and to reduce days of culture. Immunochromatography utilizing antigen-antibody reactions rapidly becomes popular as a simple identification method (see, for example, Japanese Patent Publication No. 2001-103981 (paragraph 0011-0012) and Japanese Patent Publication No. 2002-125695 (paragraph 0002)). However, since this method still takes several weeks for bacterial growth, it hardly meets the needs sought in clinical settings. Consequently, patients are often given a wrong treatment until identification results are obtained. 
     In contrast, the nucleic acid amplification method uses specific primers to identify bacterial species or resistant bacteria by investigating presence or absence of amplification. This method, including a sample preparation step, can provide test results within approximately 6-7 hours, making it a very useful rapid test. Furthermore, under the recent genomic analysis competition, entire genetic information of many organisms are being analyzed, and as a result, this method is expected to become available for identification of a wide range of bacterial species. 
     A method well-known as a nucleic acid amplification technique is PCR method (Polymerase Chain Reaction method, Roche). PCR method is widely used as a tool for genetic analyses such as gene cloning and structural determination. However, PCR method has a disadvantage of requiring a complex temperature control device like Thermal Cycler and reaction time over two hours. In PCR method, If synthesis of wrong complementary chains occurs by any chance, the resultant products work as templates for amplification, thereby leading to a wrong identification. In fact, it is difficult to regulate specific amplification based on a difference of only one base in a primer terminal. 
     In addition, due to the fact that double-stranded products are generated in general when target gene products amplified by PCR method are detected by a DNA chip, complementary strands work as competitors for probes upon a hybridization reaction with the probes, reducing hybridization efficiency and detection sensitivity. To address this problem, such methods as digesting or separating complementary strands have been employed to turn a target into a single-stranded sequence. However, this method has still several problems including a need to use enzymes, expensiveness due to the use of magnetic beads, and handling complexity. 
     LAMP method (Loop-mediated isothermal amplification method) has been developed as a gene amplification method to fix this problem. LAMP method completes gene amplification within one hour under an isothermal condition. Also, 100-1,000 times amount of final products are obtained compared to that of PCR products. LAMP method also has an advantage of higher specificity as compared to PCR method because six primer regions are set in LAMP method (see, for example, U.S. Pat. No. 3,313,358). LAMP method is expected to be a promising technique to rapidly detect bacteria, viruses and gene mutations (see, for example, Japanese Patent Publication No. 2003-159100) with high sensitivity. 
     Among tests using LAMP method, a method is now commercialized which detects presence or absence of gene amplification by measuring white turbidity of magnesium pyrophosphate, a by-product which is produced in the course of amplification (see, for example, Japanese Patent Publication No. 2003-174900). In this method where white turbidity of a by-product (magnesium pyrophosphate) is measured, there is no way of confirming whether amplification of an unintended product occurs or not. LAMP method has also a problem that it cannot detect multiple target genes simultaneously. 
     In addition, methods using intercalators or optical properties have been known as procedures for detecting LAMP amplified products (see, for example, Japanese Patent Publication No. 2002-186481). Other known methods are those measuring degree of fluorescence polarization of reaction solution by fluorescently labeling a probe which hybridizes to a single-stranded loop portion present in a LAMP product (see, for example, Japanese Patent Publication 2002-272475) as well as those immobilizing an insoluble carrier to a 5′ terminal side of a primer which hybridizes to a single-stranded loop portion and observing an aggregation reaction associated with amplification reaction (see, for example, Japanese Patent Publication 2002-345499). 
     BRIEF SUMMARY OF THE INVENTION 
     According to embodiments of the present invention, there is provided a method of detecting a target nucleic acid sequence comprising: 
     providing a stem-and-loop structured nucleic acid for measurement wherein the nucleic acid comprises complementary sequence portions located at both terminals and a target sequence portion therebetween as well as a double-stranded portion formed by hybridization of the complementary sequence portions located at both terminals and a remaining looped single-stranded portion; 
     providing a probe nucleic acid having a sequence complementary to the target sequence portion wherein one end of the probe nucleic acid being immobilized to a solid substrate surface; 
     reacting the nucleic acid for measurement with the probe nucleic acid to specifically hybridize the target sequence portion of the nucleic acid for measurement to the probe nucleic acid; and 
     detecting presence or absence of the nucleic acid for measurement hybridized to the probe nucleic acid. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a schematic diagram illustrating an amplification method using a conventional LAMP method. 
         FIG. 2  is a schematic diagram illustrating an amplification product obtained by a conventional LAMP method. 
         FIG. 3  is a schematic diagram illustrating an amplification method used for production of the nucleic acid for measurement according to an embodiment of the present invention. 
         FIG. 4  is a schematic diagram illustrating the nucleic acid for measurement according to an embodiment of the present invention. 
         FIG. 5  schematically illustrates hybridization reaction between nucleic acid probe and a LAMP product in which a target sequence is located in its looped single stranded portion and a nucleic acid probe. In  FIGS. 5A and 5B , 5′ to 3′ sequence orientation of both the nucleic acid probe and the target sequence portion are arranged in a manner where the double-stranded portion in the LAMP product extends away from the substrate plate. In contrast, in  FIG. 5C , the 5′ to 3′ sequence orientation is arranged in a manner where the double stranded portion extends toward the substrate plate. 
         FIG. 6  schematically illustrates hybridization reaction between nucleic acid probe and a LAMP product in which a target sequence is located in its looped single stranded portion. While, in  FIG. 6A , the double-stranded portion in the LAMP product extends away form the substrate plate, in  FIG. 6B , the double-stranded portion extends toward the substrate plate. 
         FIG. 7  shows a primer which is designed to have a target nucleic acid in a double-stranded region (stem region) as well as the position of a target sequence in NAT2 gene. 
         FIG. 8  shows a primer which is designed to have a target nucleic acid in a single-stranded region (loop region) as well as the position of a target sequence in NAT2 gene. 
         FIG. 9  is an electrophoresis of LAMP amplification products designed to have a target nucleic acid in a double-stranded region (stem region) or a single-stranded region (loop region) and an electrophoresis of products treated by restriction enzymes. 
         FIG. 10  shows an arrangement of electrodes used in the Examples to detect LAMP products. 
         FIG. 11  is graphs showing measurement results of electrical detection of LAMP products performed in the Example.  FIG. 11A  is a graph showing measurement results obtained from LAMP products having a target nucleic acid in its double-stranded portion, and  FIG. 11B  is a graph showing measurement results obtained from LAMP products having a target nucleic acid in a looped single-stranded portion. 
         FIG. 12  illustrates an arrangement of electrodes used in the Example 2 to detect SNPs in LAMP products. 
         FIG. 13  is graphs showing measurement results of electrical detection of SNPs in LAMP products performed in the Example 2.  FIG. 13A  is a graph showing measurement results obtained from substrate plates washed quickly with ultrapure water after hydridization.  FIG. 13B  is a graph showing measurement results obtained from substrate plates immersed in washing buffer at 35□ for 40 minutes after hybridization.  FIG. 13C  is a graph showing measurement results obtained from substrate plates immersed in washing buffer at 40□ for 40 minutes after hybridization.  FIG. 13D  is a graph showing measurement results obtained from substrate plates immersed in washing buffer at 45□ for 40 minutes after hybridization. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention will now be described in more detail. 
     &lt;Nucleic Acids for Measurement&gt; 
     Now, primer designing in LAMP method and intermediate products serving as origins of amplification will be described with reference to  FIG. 1  and  FIG. 2 . In LAMP method, six primer regions will be set and four primers are used for gene amplification.  FIG. 1  illustrates double-stranded DNA to be detected. Three regions, F3 region, F2 region and F1 region, in order of proximity to 5′ terminal of the double strand, will be determined and three regions, B3c region, B2c region and B1c region, in order of proximity to 3′ terminal, will be determined. In addition, F3 region, F2 region and B1 region in complementary strand thereof are called F3c region, F2c region and F1c region, and B3c region, B2c region and B1c regions in complementary strand thereof are called B3 region, B2 region and B1 region, respectively. The six regions, F3 region, F2 region, F1 region, B3c region, B2c region, B1c region and complementary strands regions thereto are hereinafter designated as a primer designing region. Primers constituting four basic primers are FIP primer that has a same sequence with the F2 region at its 3′ terminal and a sequence complementary to the F1 region at its 5′ terminal, F3 primer comprised of a same sequence with the F3 region, BIP primer having a sequence complementary to B2c region at its 3′ terminal and a same sequence with the B1c region at its 5′ terminal, and B3 primer comprised of a sequence complementary to the B3c region. LAMP amplification using the four primers described above will cause formation of a dumbbell-shaped intermediate product having a stem-and-loop structure as shown in  FIG. 2 . Having sequences complementary to each other in the same nucleic acid strand, both terminal portions of the intermediate product will self-anneal and form a single-stranded loop. Intermediate product generation and subsequent amplification process have been well known and are described in detail in Japanese Patent 3,313,358 and Japanese Patent Publication 2002-186481. Traditionally, in LAMP amplification products, target sequences are interposed between B1 and F1c or between F1 and B1c. 
     In contrast to traditional target sequences shown in  FIG. 1  and  FIG. 2 , primers used in this invention are designed to have target sequences located in the single-stranded loop portion. Accordingly, in this invention, target sequence regions are designed to be located either between primer regions F1c and F2c, primer regions F2 and F1, primers B1c and B2c or primer regions B2 and B1 as shown in  FIG. 3 . These regions are designated as FPc region, FP region, BPc region and BP region, respectively. Location of target sequence designs are shown in  FIG. 3 . Four kinds of primers will be designed based on the six primer regions designed in this way. LAMP amplification using these primers will provide portions of LAMP amplification products as shown in  FIG. 4 . In these LAMP amplification products, target sequence regions; FPc region, FP region, BP region and BPc region will be located in a single-stranded loop of amplified products. Since F1 region and F1c region, and B1 region and B1c region have complementary sequences from the beginning, they will self-hybridize with each other to give double strands. In this case, F2c region, F2 region, B1c region and B1 region will be located in loop regions of single strands as shown in  FIG. 4 . These regions, F2c region, F2 region, B1c region and B1c region, may overlap, in part, with target sequence regions FPc region, FP region, BPc region and BP region. Since some target sequences included in amplification products are single-stranded as shown in  FIG. 4 , they can be detected, without any denaturation procedure, by specific hybridization with probe nucleic acids FP, FPc, BP and BPc that are complement to each target sequences as shown in  FIG. 4 . As used herein, specific hybridization means that it can detect minor differences when Single Nucleotide Polymorphisms (SNPs) or mutations occur. 
     &lt;Probe Nucleic Acids&gt; 
     As explained in  FIG. 4  according to the present invention, target sequences included in single-stranded loop structures of stem-and-loop structured nucleic acids for measurement are detected by means of probe nucleic acids having sequences complementary thereto. These probe nucleic acids are immobilized onto a solid substrate surface. Probes are used typically comprising a part of a DNA chip. 
     A DNA chip is a several-centimeter-square of glass or silicon substrate onto which several tens to several hundred thousand kinds probes with different sequences are immobilized. DNA chips allow for simultaneous investigation of information on multiple sequences. This enables analyses of gene expression patterns and SNPs to be completed in several days, which required several weeks previously. Currently, DNA chips are mainly used in search for new genes, elucidation of functions and techniques for supporting researches, but recently, they are becoming common techniques for disease diagnosis. 
     As an exemplary DNA chip, a technique of AFFYMETRIX are well known (see, for example, Proc. Natl. Acad. Sci. USA 91, 1994, p 5022-5026). In this technique, fluorescently-labeled sample genes are reacted with probes on a chip and detected by means of a high-sensitive fluorescent analysis apparatus. Another type of a detection method developed is a current detection type DNA chip. In this method, intercalators specifically reactive with double-stranded DNA are added and electrochemical signals generated by the intercalators are measured. Electrical DNA chips are expected to be a promising second-generation DNA chip, because they need no labeling and expensive detection apparatuses (see, for example, Japanese Patent Publication H05-199898). 
     &lt;Hybridization Reaction Between Nucleic Acids for Measurement and Probes&gt; 
     The nucleic acids for measurement are bound to the solid surface via specific hybridization reaction between single-stranded target sequence located therein and the probe nucleic acids. In this invention, it is an additional characteristics that 5′ to 3′ sequence orientations of both the probe nucleic acid and the target sequence portion are arranged so that the double-stranded portion of the nucleic acid for measurement extends away from the solid surface when the probe nucleic acid and the target sequence portion are hybridized (see  FIG. 5A ). Such characteristics have been found based on a finding that if 5′ to 3′ sequence orientations of both the probe nucleic acid and the target sequence portion are arranged as shown in  FIG. 5C , complex conformations characteristics to LAMP products cause steric hindrance together with a probe-bound substrate (see  FIG. 6B ) and this causes reduction in hybridization efficiency. Steric hindrance produced by LAMP products and a solid substrate on which probe nucleic acids are bound can be avoided (see  FIG. 6A ) by arranging 5′ to 3′ sequence orientations of both the probe nucleic acid and the target sequence portion so that the double-stranded portion of the nucleic acid for measurement extends away from the solid surface when the probe nucleic acid and the target sequence portion are hybridized (see  FIG. 5B ), thereby achieving improvement in hybridization efficiency. 
     &lt;Detection of Nucleic Acids for Measurement Hybridized to Probe Nucleic Acids&gt; 
     In this invention, when detecting presence or absence of the nucleic acids for measurement hybridized to probe nucleic acids, detection means is not limited to particular means in any way. For example, detection can be made based on fluorescent labels or by means of electrical detection using a double-strand specific intercalator generating electrical potential 
     EXAMPLE 1 
     A method for nucleic acid detection according to an embodiment of the present invention will now be described more specifically by way of the Examples. 
     In this example, LAMP amplification products are produced as a sample nucleic acid and target nucleic acid(s) present in the LAMP amplification products were detected in an electrical current system after hybridization reaction. For LAMP reaction, two sets of the following primers were used and a part of N-Acetyltransferase 2 (NAT2) gene were amplified 
     (1) Synthetic Oligonucleotide 
     Primer 1 
     Target nucleic acid sequences were designed to be located in a double-stranded region (stem region) of the LAMP amplification products. Peripheral genomic sequences are shown in  FIG. 7 . 
     
       
         
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
           
               
                 NAT F3 Primer: 
                   
               
             
          
           
               
                 (SEQ ID NO: 1) 
                   
               
             
          
           
               
                 5′ACAGAAGAGAGAGGAATCTGGT 3′ 
                   
               
               
                   
               
               
                 NAT FIP Primer: 
               
             
          
           
               
                 (SEQ ID NO: 2) 
                   
               
             
          
           
               
                 5′TGTTTCTTCTTTGGCAGGAGATGAGAAGGACCAAATCAGGAGAGA 
                   
               
               
                   
               
               
                 GCA 3′ 
               
               
                   
               
               
                 NAT B3 Primer: 
               
             
          
           
               
                 (SEQ ID NO: 3) 
                   
               
             
          
           
               
                 5′GATGAAGCCCACCAAACAGTA 3′ 
                   
               
               
                   
               
               
                 NAT BIP Primer: 
               
             
          
           
               
                 (SEQ ID NO: 4) 
                   
               
             
          
           
               
                 5′ATGAATACATACAGACGTCTCC-CTGGGGTCTGCAAGGAAC 3′ 
                   
               
               
                   
               
             
          
         
       
     
     Primer 2 
     Target nucleic acid sequences were designed to be located in a single-stranded region (loop region) of the LAMP amplification products. Peripheral genomic sequences are shown in  FIG. 8 . 
     
       
         
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
           
               
                 NAT F3 Primer: 
                   
               
             
          
           
               
                 (SEQ ID NO: 5) 
                   
               
             
          
           
               
                 5′ACAAACAAAGAATTTCTTAATTCTCAT 3′ 
                   
               
               
                   
               
               
                 NAT FIP Primer: 
               
             
          
           
               
                 (SEQ ID NO: 6) 
                   
               
             
          
           
               
                 5′CGTCTGCAGGTATGTATCATAGACTCAAAAAATATACTTATTTACGC 
                   
               
               
                   
               
               
                 TTGAACC 3′ 
               
               
                   
               
               
                 NAT B3 Primer: 
               
             
          
           
               
                 (SEQ ID NO: 7) 
                   
               
             
          
           
               
                 5′CGACCAGATCTGTATTGTCTT 3′ 
                   
               
               
                   
               
               
                 NAT BIP Primer: 
               
             
          
           
               
                 (SEQ ID NO: 8) 
                   
               
             
          
           
               
                 5′ATAACCACATCATTTTGTTCCTTGCATGAATTTTCTATAGGTGA 
                   
               
               
                   
               
               
                 GGATGA 3′ 
               
             
          
         
       
     
     (2) LAMP Reaction Solution 
     The composition of the LAMP reaction solution was as follows: 
     
       
         
               
               
               
               
             
               
             
               
               
               
               
             
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Sterile ultrapure water 
                 1.15 
                 μL 
               
               
                   
                 Bst DNA polymerase 
                 1 
                 μL 
               
               
                   
                 buffer 
                 12.5 
                 μL 
               
             
          
           
               
                 buffer components 
               
             
          
           
               
                   
                 Tris HCl (pH 8.0) 
                 40 
                 mM 
               
               
                   
                 KCl 
                 20 
                 mM 
               
               
                   
                 MgSO 4   
                 16 
                 mM 
               
               
                   
                 (NH 4 ) 2 SO 4   
                 20 
                 mM 
               
             
          
           
               
                   
                 Tween 20 
                 0.2% 
               
               
                   
                 Betaine 
                 1.6M 
               
             
          
           
               
                   
                 dNTP 
                 2.8 
                 mM 
               
               
                   
                 F3-primer (10 μM) 
                 0.5 
                 μL 
               
               
                   
                 B3-primer (10 μM) 
                 0.5 
                 μL 
               
               
                   
                 FIP-primer (10 μM) 
                 4 
                 μL 
               
               
                   
                 BIP-primer (10 μM) 
                 4 
                 μL 
               
               
                   
                 Template (purified human genome) 
                 1 
                 μL 
               
               
                   
                 Total 
                 25 
                 μL 
               
               
                   
                   
               
             
          
         
       
     
     (3) Nucleic Acid Amplification by LAMP Method 
     Nucleic acid amplification was performed at 58° C. for one hour. For negative control, sterile water was added instead of templates. 
     (4) Confirmation of Nucleic Acid Amplification 
     LAMP products amplified by the methods described above were confirmed by agarose-gel electrophoresis as shown in  FIG. 9 . The presence or absence of products of interest were confirmed by restriction enzyme cleavage. In  FIG. 9 , lanes 1-7 correspond to the following samples 1-7, respectively. Electrophoretic results are shown below. 
     1. 100 bp ladder (TAKARA) 
     2. Negative control: a target nucleic acid is located in a double-stranded region 
     3. Positive control: a target nucleic acid is located in a double-stranded region 
     4. Negative control: a target nucleic acid is located in a single-stranded region 
     5. Positive control: a target nucleic acid is located in a single-stranded region 
     6. Pst I restriction enzyme treated: a target nucleic acid is located in a double-stranded region 
     7. Pst I restriction enzyme treated: a target nucleic acid is located in a single-stranded region 
     Digestion with a restriction enzyme gave fragments as expected by theory. This indicates these LAMP products are specific amplification products. 
     (5) Preparation of Nucleic Acid Probe Immobilized Electrodes 
     Nucleotide sequences of nucleic acid probes are shown below. 
     
       
         
               
               
               
               
             
               
               
               
             
               
               
               
               
             
               
               
               
             
               
               
               
               
             
           
               
                   
                 Positive-probe FP: 
                   
                   
               
               
                   
                 AACCTCGAACAATTG, 
                 (SEQ ID NO: 9) 
               
             
          
           
               
                   
                 3′SH, 5′SH probes total of two 
                   
               
               
                   
                   
               
               
                   
                 Positive-probe FPc: 
               
             
          
           
               
                   
                 CAATTGTTCGAGGTT 
                 (SEQ ID NO: 10) 
                   
               
             
          
           
               
                   
                 3′SH, 5′SH probes total of two 
                   
               
               
                   
                   
               
             
          
           
               
                   
                 N-probe NP: 
                   
                   
               
               
                   
                 CTGGACGAAGACTGA 
                 (SEQ ID NO: 11) 
               
             
          
         
       
     
     FP probe and FPc probe are complementary to each other. A total of four 3′ SH and 5′ SH modified probes were tested for FP probe and FPc probe. In contrast, N-probe served as negative control and had sequences unrelated to four probes described above. 
     Probe solution containing the labeled probes was spotted onto gold electrodes and, after standing one hour, the electrodes were immersed in mercaptohexanol solution and washed 0.2×SSC solution. The electrode were then washed with ultrapure water, air-dried and used as probe-immobilized electrodes. Arrangement of electrodes is as shown in  FIG. 10 . 
     (6) Hybridization of LAMP Products to Nucleic Acid Probes 
     LAMP products amplified in the step (3) above were used as sample nucleic acids. The surface prepared in step (4) on which nucleic acid probes were immobilized was immersed in LAMP products added by 2×SSC salt, hybridization reaction was performed by standing at 35° C. The probe-immobilized surface was quickly washed with ultra pure water. The electrodes were immersed for 15 minutes in phosphate buffer containing 50 μM Hoechst 33258 solution (an intercalator) and oxidation current response of Hoechst 33258 molecule was measured. 
     (7) Results 
     In  FIG. 11 , the results of electrical current measurements were shown as an increment of current values generated in electrodes on which each probes were immobilized. In LAMP products in which target sequences are located in a double-stranded portion (stem portion), no increment in current values derived from hybridization was found for NP, 3′ SH FP, 3′ SH FPc, 5′ SH FP and 5′ SH FPc ( FIG. 11A ). In contrast, in LAMP products in which target nucleic acid is located in a single-stranded loop portion, no increment in current values derived from hybridization was found for NP, 3′ SH FP and 5′ SH FP, but increment in current value probably derived from hybridization was found for 3′ SH FPc and 5′ SH FP ( FIG. 11B ). 
     For 3′ SH FP and 5′ SH FPc, which gave no current signal values increase when used with products having target nucleic acid located in a single-stranded loop portion, the double-stranded portion in LAMP products, upon hybridization with the single-stranded loop portion of LAMP products, extended toward the substrate thereby causing steric hindrance (see  FIG. 5C  and  FIG. 6B ). In contrast, for 3′ SH FPc and 5′ SH FP, which gave current signal value increase, the double-stranded portion in LAMP products extends away from the substrate thereby streic hindrance is avoided (see  FIG. 5B  and  FIG. 6A ). 
     It was found, from these results, that it was essential to locate target nucleic acids in looped single-stranded portions to detect LAMP products with DNA chips. It was also found that 5′ to 3′ sequence orientations of both the probe nucleic acid and the target sequence portion must be arranged so that the double-stranded portion of LAMP products extended away from the solid surface in order to avoid steric hindrance occurred between LAMP products and substrates upon hybridization reaction. 
     EXAMPLE 2 
     By way of an application of a nucleic acid detection method according to this invention, single nucleotide polymorphisms (SNPs) in target nucleic acid sequences were detected. 
     In Example 2, LAMP products were prepared for sample nucleic acids as described in Example 1. After hybridization, SNPs in the target nucleic acid sequences in the LAMP products were detected in a electrical current detection system. 
     (1) LAMP Products Used for Detection 
     LAMP products used were same as those amplified by Primer 2 described in Example 1, (1) Synthetic oligonucleotide. 
     (2) Probes Used for Detection 
     Nucleotide sequences of the nucleic acid probes are shown below: 
     
       
         
               
               
               
             
           
               
                 Positive-probe FPc: 
                   
                   
               
               
                 CAATTGTTCGAGGTT 3′SH 
                 (SEQ ID NO: 10) 
               
               
                 (a probe used in Example 1) 
               
               
                   
               
               
                 Positive-probe FPc SNP: 
               
               
                 CAATTGTTGGAGGTT 3′SH 
                 (SEQ ID NO: 12) 
               
               
                   
               
               
                 Positive-probe FPc 25mer: 
               
               
                 ATCTTCAATTGTTCGAGGTTCAAGC 3′SH 
                 (SEQ ID NO: 14) 
               
               
                   
               
               
                 Positive-probe FPc 25mer SNP: 
               
               
                 ATCTTCAATTGTTGGAGGTTCAAGC 3′SH 
                 (SEQ ID NO: 13) 
               
               
                   
               
               
                 N-probe NP: 
               
               
                 CTGGACGAAGACTGA 3′SH 
                 (SEQ ID NO: 11) 
               
             
          
         
       
     
     FPc and NP were same as those used in Example 1. For FPc SNP, G near the center of FPc sequence was changed into C. FPc 25mer was a target nucleic acid sequence having five base extensions in both upstream and downstream of FPc. For FPc 25mer SNP, G near the center of FPc 25mer sequence was changed into C. All five probes above were 3′SH modified. 
     Probes were immobilized in the same manner as described in the Example 1. Arrangement of electrodes is shown in  FIG. 12 . 
     (3) Hybridization of LAMP Products to Nucleic Acids 
     LAMP products amplified in the above step (1) were used as sample nucleic acids. The surface prepared in step (2) on which nucleic acid probes were immobilized was immersed in LAMP products added by 2×SSC salt, and by standing for 60 minutes at 35° C., hybridization reaction was performed. The substrates washed under four different conditions were made: immersed in 0.2×SSC buffer at 35° C., 40° C. or 45° C. for 40 minutes followed by quick wash with ultra pure water; or just quickly washed with ultrapure water after hybridization. The electrodes were immersed for 15 minutes in phosphate buffer containing 50 μM Hoechst 33258 solution (an intercalator) and oxidation current response of Hoechst 33258 molecule was measured. 
     (4) Results 
     In  FIG. 13 , the results of electrical current measurements were shown as an increment of electrical current generated in electrodes on which each probes had been immobilized. Regarding the FPc and FPcSNP, (A) there was no increase in electrical current values for FPcSNP when used with a substrate quickly washed with ultrapure water after hybridization, on the contrary, significant increase in electrical current values was observed for FPc. In addition, (B) increase in electrical current values was disappeared for both FPc and FPc SNP when used with substrates that had been washed with 0.2×SSC solution at 35° C. for 40 minutes after hybridization. This indicates FPc and FPc SNP can identify SNPs under the washing condition (A). 
     Regarding extended target sequences, FPc 25mer and FPc 25mer SNP, (C) increase in electrical current values was found for both FPc 25mer and FPc 25mer SNPs when used with substrates that had been washed with 0.2×SSC solution at 40□ for 40 minutes after hybridization. In contrast, (D) when substrates had been washed with 0.2×SSC solution at 45° C. for 40 minutes after hybridization, no increase in electrical current values was found for FPc 25mer SNP, but significant increase was found for FPc 25mer. This indicates FPc 25mer and FPc 25mer SNP can identify SNPs under the washing condition (D). 
     These results show that SNPs can be detected by selecting appropriate hybridization or washing conditions depending on different probe sequences, or by selecting optimal probes for defined hybridization and washing condition. 
     This invention shall never be limited to the embodiments specifically described above and, in practicing this invention, constituent elements thereof can be modified to give other embodiments without departing from the spirit or scope of the invention. In addition, various inventions will be given by combining multiple constituent elements disclosed in the embodiment stated above. For example, among all constituent elements mentioned in the embodiments, some constituent elements could be omitted. Furthermore, constituents elements of different embodiments could appropriately combined. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.