Patent Publication Number: US-2020299782-A1

Title: Method for detecting rp2-arhgap6 gene

Description:
This application is a continuation of U.S. patent application Ser. No. 15/753,994, filed Mar. 1, 2018, which is a national stage filing under 35 U.S.C. § 371 of PCT/JP2016/074442, filed Aug. 23, 2016, which claims priority to U.S. Provisional Patent Application No. 62/209,130, filed Aug. 24, 2015, and Japanese Patent Application No. 248292/2015, filed Dec. 21, 2015, the contents of which are each incorporated herein by reference in their entireties. 
     This application incorporates-by-reference the sequence listing contained in the text file named 127206_05803_Sequence_Listing.txt, which was created Mar. 11, 2020 and is 20,360 bytes in size. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a method for detecting a novel fusion gene. 
     BACKGROUND ART 
     The retinitis pigmentosa 2 (X-linked recessive) (RP2) gene exists on the short arm of the human chromosome X, and it is known as a causal gene of retinitis pigmentosa (Nat Genet. 1998; 19(4): 327-332). RP2 is known to have a function to bind to ARL3 (ADP ribosylation factor like GTPase 3), which is one of the small GTPase protein family members, to enhance GTP hydrolase activity of ARL3, i.e. it is a GTPase activating protein (Nat Struct Mol Biol. 2008; 15(4): 373-380). Its association with cancer is currently unknown. 
     Rho GTPase activating protein 6 (ARHGAP6) gene, which has GTPase activating function, exists on the short arm of human chromosome X, same as RP2, and the protein encoded by this gene is a GTPase activating protein possessing a Rho-GAP domain at the center. The ARHGAP6 gene is known to have a function of enhancing the GTP hydrolase activity of the small GTPase protein family, particularly RhoA (Hum Mol Genet. 2000; 9(4): 477-488). With regards to cancer, it is reported that the gene has decreased its expression in the African American population, which has a high incidence rate and mortality rate of colon cancer (PLoS One. 2012; 7(1): e30168), and also that a fusion gene thereof with a claudin 18 (CLDN18) gene was found in patients suffered from diffuse type gastric cancer (Nature 2014; 513(7517): 202-209). 
     There are no reports so far of a fusion gene composed of RP2 and ARHGAP6. 
     SUMMARY OF INVENTION 
     Problem to be Solved by Invention 
     The object of the present invention is to elucidate a new causative gene of cancer, polynucleotide, and thereby provide a method for detecting a polynucleotide or a polypeptide that is encoded by the polynucleotide, as well as a primer set or a detection kit for such detection. 
     Means for Solving the Problems 
     The present inventors isolated and identified from stomach cancer cell line, a novel fusion gene in which a part of the ARHGAP6 gene and a part of the RP2 gene are fused together (Example 1), and found that this fusion gene was the causative gene of cancer, since the survival ratio of the stomach cancer cell line declined with the suppression of expression of fusion genes in stomach cancer cell lines that endogenously express such fusion genes (Example 2). The present inventors constructed a detection method of a fusion gene based on these findings, and provided primer sets for such purpose, thereby using the detection of such fusion gene made it possible to select cancer patients (particularly, stomach cancer patients) that test positive for a fusion gene composed of an RP2 gene and an ARHGAP6 gene (Example 3). 
     In other words, the present invention relates to [1] to [24] shown below. 
     [1] A method for detecting a fusion gene composed of a retinitis pigmentosa 2 (X-linked recessive) (RP2) gene and a Rho GTPase activating protein 6 (ARHGAP6) gene, the method comprising a step of detecting whether a polynucleotide that encodes a polypeptide described by either (1) or (2) shown below exists in a sample obtained from a subject: 
     (1) a polypeptide that comprises an amino acid sequence having no less than 90% identity with an amino acid sequence represented by SEQ ID NO: 2; 
     (2) a polypeptide that comprises an amino acid sequence represented by SEQ ID NO: 2, or a polypeptide that comprises an amino acid sequence represented by SEQ ID NO: 2, in which 1 to 10 amino acids are deleted, substituted, inserted and/or added. 
     [2] The method according to [1], wherein the polypeptide comprises an amino acid sequence having no less than 90% identity with an amino acid sequence represented by SEQ ID NO: 2, and has an ability to develop tumor 
     [3] The method according to [1], wherein the polypeptide comprises an amino acid sequence represented by SEQ ID NO: 2 and has an ability to develop tumor, or the polypeptide comprises an amino acid sequence represented by SEQ ID NO: 2, in which 1 to 10 amino acids are deleted, substituted, inserted and/or added, and has an ability to develop tumor. 
     [4] The method according to [1], wherein the polypeptide consists of an amino acid sequence represented by SEQ ID NO: 2. 
     [5] A method for detecting a fusion gene composed of an RP2 gene and an ARHGAP6 gene comprising a step of detecting whether a polynucleotide that encodes a polypeptide consisting of an amino acid sequence represented by SEQ ID NO: 2 exists in a sample obtained from a subject. 
     [6] The method according to any one of [1] to [5] further comprising a step in which it is judged when a polynucleotide targeted in detection is detected, that a fusion gene composed of an RP2 gene and an ARHGAP6 gene exists. 
     [7] The method according to any one of [1] to [6], further comprising a step of amplifying a nucleic acid existing in a sample obtained from a subject, or a step of hybridizing a probe with a nucleic acid existing in a sample obtained from a subject to detect a polynucleotide targeted in detection. 
     [8] The method according to [7] comprising a step of amplifying the nucleic acid existing in a sample obtained from a subject using a primer set shown below: 
     a primer set for detecting a fusion gene composed of an RP2 gene and an ARHGAP6 gene, the primer set comprising a sense primer designed from a section encoding RP2 and an antisense primer designed from a section encoding ARHGAP6, wherein the antisense primer consists of an oligonucleotide that hybridizes under a stringent condition with a polynucleotide targeted in detection, and the sense primer consists of an oligonucleotide that hybridizes under a stringent condition with a complementary strand of a polynucleotide targeted in detection. 
     [9] The method according to [8], wherein the sense primer consists of an oligonucleotide that hybridizes under a stringent condition with a complementary strand of a polynucleotide consisting of base no. 1 to 102 of SEQ ID NO: 1, and the antisense primer consists of an oligonucleotide that hybridizes under a stringent condition with a polynucleotide consisting of base no. 103 to 2439 of SEQ ID NO: 1. 
     [10] The method according to any one of [7] to [9] comprising a step of amplifying the nucleic acid existing in a sample obtained from a subject using a primer set shown below: 
     a primer set for detecting a fusion gene composed of an RP2 gene and an ARHGAP6 gene, wherein a sense primer consists of an oligonucleotide of at least 16 random consecutive bases between base no. 1 to 102 of SEQ ID NO: 1, and an antisense primer consists of an oligonucleotide complementary to an oligonucleotide of at least 16 random consecutive bases between base no. 103 to 2439 of SEQ ID NO: 1. 
     [11] The method according to any one of [7] to [10] further comprising a step of detecting whether an amplified nucleic acid fragment of a target size was obtained. 
     [12] The method according to [11] further comprising a step in which it is judged when an amplified nucleic acid fragment of a target size is obtained, that a fusion gene composed of an RP2 gene and an ARHGAP6 gene exists. 
     [13] The method according to any one of [7] to [10] further comprising a step of determining a base sequence of an amplified nucleic acid fragment. 
     [14] The method according to [13] further comprising a step in which it is judged when an amplified nucleic acid fragment includes a base sequence of a section encoding RP2 and a base sequence of a section encoding ARHGAP6 in a same fragment, that a fusion gene composed of an RP2 gene and an ARHGAP6 gene exists. 
     [15] The method according to [7] comprising a step of hybridizing a probe with the nucleic acid existing in a sample obtained from a subject, wherein the probe comprises an oligonucleotide that hybridizes with the polynucleotide under a stringent condition. 
     [16] The method according to [15] comprising a step of performing in situ hybridization using a sample obtained from a subject, a probe designed from a section encoding RP2 of the polynucleotide, and a probe designed from a section encoding ARHGAP6 of the polynucleotide. 
     [17] The method according to [16] using multiple types of probes designed from a section encoding RP2, and multiple types of probes designed from a section encoding ARHGAP6. 
     [18] The method according to [7], [16] or [17] using multiple types of adjacent probe pairs comprising an oligonucleotide that is complementary to an oligonucleotide of at least 16 random consecutive bases between base no. 1 to 102 of SEQ ID NO: 1, and multiple types of adjacent probe pairs comprising an oligonucleotide that is complementary to an oligonucleotide of at least 16 random consecutive bases between base no. 103 to 2439 of SEQ ID NO: 1, in a step of hybridizing a probe with the nucleic acid existing in a sample obtained from a subject. 
     [19] The method according to any one of [16] to [18] further comprising a step of amplifying hybridization signals. 
     [20] The method according to any one of [16] to [19] further comprising a step of detecting a signal overlap of a signal from a probe designed from a section encoding RP2 and a signal from a probe designed from a section encoding ARHGAP6. 
     [21] The method according to [20] further comprising a step in which it is judged when two signals are detected at a same area, that a fusion gene composed of an RP2 gene and an ARHGAP6 gene exists. 
     [22] The method according to any one of [1] to [21] comprising a step of obtaining a sample from a subject. 
     [23] The method according to any one of [1] to [22], wherein the subject is a cancer patient. 
     [24] The method according to [23], wherein cancer is stomach cancer. 
     Further, the present invention relates to [25] to [27] shown below. 
     [25] A method for detecting whether cancer exists in a subject comprising the step according to any one of [1] to [21]. 
     [26] The method according to [25] comprising a step of obtaining a sample from a subject. 
     [27] The method according to [25] or [26], wherein cancer is stomach cancer. 
     Further, the present invention relates to [28] to [32] shown below. 
     [28] The method for diagnosing cancer in a subject comprising a step according to any one of [1] to [21]. 
     [29] The method according to [28] comprising a step of obtaining a sample from a subject. 
     [30] The method according to [28] or [29] further comprising a step in which it is judged when a fusion gene composed of an RP2 gene and an ARHGAP6 gene is detected in a sample obtained from a subject, that there is a high possibility of the subject having cancer. 
     [31] The method according to [28] or [29], wherein cancer is stomach cancer. 
     [32] The method according to [29] further comprising a step in which it is judged when a fusion gene composed of an RP2 gene and an ARHGAP6 gene is detected in a sample obtained from a subject, that there is a high possibility of a subject having stomach cancer. 
     Further, the present invention relates to [33] to [36] shown below. 
     [33] A method for identifying a subject that is a candidate for receiving a treatment by an ARHGAP6 function inhibitor and/or a pharmaceutical agent for blocking an abnormal signal induced by a fusion gene composed of an RP2 gene and an ARHGAP6 gene, comprising a step according to any one of [1] to [21], wherein the subject is a cancer patient. 
     [34] The method according to [33] comprising a step of obtaining a sample from a subject. 
     [35] The method according to [33] or [34] further comprising a step in which it is judged when a fusion gene composed of an RP2 gene and an ARHGAP6 gene is detected in a sample obtained from a subject, that the subject is a candidate for receiving a treatment by an ARHGAP6 inhibitor and/or a pharmaceutical agent for blocking an abnormal signal induced by a fusion gene composed of an RP2 gene and an ARHGAP6 gene. 
     [36] The method according to any one of [33] or [35], wherein cancer is stomach cancer. 
     Further, the present invention relates to [37] to [42] shown below. 
     [37] A primer set for detecting a fusion gene composed of an RP2 gene and an ARHGAP6 gene existing in a sample obtained from a subject, the primer set comprising a sense primer designed from a section encoding RP2 and an antisense primer designed from a section encoding ARHGAP6, wherein the antisense primer consists of an oligonucleotide that hybridizes under a stringent condition with the polynucleotide according to any one of [1] to [5], and the sense primer consists of an oligonucleotide that hybridizes under a stringent condition with a complementary strand of the polynucleotide. 
     [38] The primer set according to [37], wherein the sense primer consists of an oligonucleotide that hybridizes under a stringent condition with a complementary strand of a polynucleotide consisting of base no. 1 to 102 of SEQ ID NO: 1, and the antisense primer consists of an oligonucleotide that hybridizes under a stringent condition with a polynucleotide consisting of base no. 103 to 2439 of SEQ ID NO: 1. 
     [39] A primer set for detecting a fusion gene composed of an RP2 gene and an ARHGAP6 gene existing in a sample obtained from a subject, the primer set comprising a sense primer designed from a section encoding RP2 or an antisense primer designed from a section encoding ARHGAP6 of the polynucleotide according to any one of [1] to [5]. 
     [40] The primer set according to any one of [37] to [39], wherein the sense primer consists of an oligonucleotide of at least 16 random consecutive bases between base no. 1 to 102 of SEQ ID NO: 1, and the antisense primer consists of an oligonucleotide that is complementary to an oligonucleotide of at least 16 random consecutive bases between base no. 103 to 2439 of SEQ ID NO: 1. 
     [41] The primer set according to any one of [37] to [40], wherein the subject is a cancer patient. 
     [42] The primer set according to [41], wherein cancer is stomach cancer. 
     Further, the present invention relates to [43] to [48] shown below. 
     [43] A probe for detecting a fusion gene composed of an RP2 gene and an ARHGAP6 gene existing in a sample obtained from a subject, the probe comprising an oligonucleotide that hybridizes under a stringent condition with the polynucleotide according to any one of [1] to [5]. 
     [44] The probe set comprising multiple probes according to [43], the probe set comprising a probe designed from a section encoding RP2 and a probe designed from a section encoding ARHGAP6 of the polynucleotide according to any one of [1] to [5]. 
     [45] The probe set according to [44] comprising multiple types of probes designed from a section encoding RP2 and multiple types of probes designed from a section encoding ARHGAP6. 
     [46] The probe set according to [44] or [45] comprising multiple types of adjacent probe pairs comprising an oligonucleotide that is complementary to an oligonucleotide of at least 16 random consecutive bases between base no. 1 to 102 of SEQ ID NO: 1 and multiple types of adjacent probe pairs comprising an oligonucleotide that is complementary to an oligonucleotide of at least 16 random consecutive bases between base no. 103 to 2439 of SEQ ID NO: 1. 
     [47] The probe or a probe set according to any one of [43] to [46], wherein the subject is a cancer patient. 
     [48] The method according to [47], wherein cancer is stomach cancer. 
     Further, the present invention relates to [49] to [53] shown below. 
     [49] A detection kit for detecting a fusion gene composed of an RP2 gene and an ARHGAP6 gene existing in a sample obtained from the subject, the detection set comprising a primer set according to any one of [37] to [40]. 
     [50] A detection kit for detecting a fusion gene composed of an RP2 gene and an ARHGAP6 gene existing in a sample obtained from a subject, the detection kit comprising a probe or a probe set according to any one of [43] to [46]. 
     [51] The detection kit according to [50] further comprising a reagent for amplifying a signal of hybridization. 
     [52] The detection kit according to any one of [49] to [51], wherein the subject is a cancer patient. 
     [53] The detection kit according to [52], wherein cancer is stomach cancer. 
     Further, the present invention relates to [54] to [63] shown below. 
     [54] A detection method of a fusion protein of RP2 and ARHGAP6 comprising a step of detecting whether a polypeptide according to either (1) or (2) exists in a sample obtained from a subject: 
     (1) a polypeptide that comprises an amino acid sequence having no less than 90% identity with an amino acid sequence represented by SEQ ID NO: 2; 
     (2) a polypeptide that comprises an amino acid sequence represented by SEQ ID NO: 2, or a polypeptide that comprises an amino acid sequence represented by SEQ ID NO: 2, in which 1 to 10 amino acids are deleted, substituted, inserted and/or added. 
     [55] The method according to [54], wherein the polypeptide comprises an amino acid sequence having no less than 90% identity with an amino acid sequence represented by SEQ ID NO: 2, and has an ability to develop tumor. 
     [56] The method according to [54], wherein the polypeptide comprises an amino acid sequence represented by SEQ ID NO: 2 and has an ability to develop tumor, or the polypeptide comprises an amino acid sequence represented by SEQ ID NO: 2, in which 1 to 10 amino acids are deleted, substituted, inserted and/or added, and has an ability to develop tumor. 
     [57] The method according to [54], wherein the polypeptide consists of an amino acid sequence represented by SEQ ID NO: 2. 
     [58] A detection method of a fusion protein of RP2 and ARHGAP6 comprising a step of detecting whether a polypeptide consisting of an amino acid sequence represented by SEQ ID NO: 2 exists in a sample obtained from a subject. 
     [59] The method according to any one of [54] to [58], wherein the step for detecting whether the polypeptide exists comprises a step of bringing an antibody (primary antibody) that recognizes a section derived from an RP2 gene in the polypeptide and an antibody (primary antibody) that recognizes a section derived from an ARHGAP6 gene in the polypeptide in contact with a sample obtained from a subject. 
     [60] The method according to [59] further comprising steps of i) to v) described below: 
     i) a step of adding secondary antibodies that are connected to oligonucleotides and that respectively bind to primary antibodies; ii) a step of adding a ligation solution that contains two types of oligonucleotides that are partially complementary to oligonucleotides connected to the secondary antibodies and a ligase that can ligate the two types of oligonucleotides to form a circular structure when the oligonucleotides approach each other, thereby inducing a ligation reaction; iii) a step of elongating a nucleic acid along a circular structure that is formed; and iv) a step of hybridizing a labeled oligonucleotide probe that can hybridize with an elongated nucleic acid, and v) a step of detecting a labeled signal. 
     [61] The method according to any one of [54] to [60] comprising a step of obtaining a sample from a subject. 
     [62] The method according to any one of [54] to [61], wherein the subject is a cancer patient. 
     [63] The method according to [62], wherein cancer is stomach cancer. 
     Further, the present invention relates to [64] to [66] shown below. 
     [64] A method for detecting whether cancer exists in a subject comprising the step according to any one of [54] to [60]. 
     [65] The method according to [64] comprising a step of obtaining a sample from a subject. 
     [66] The method according to [64] or [65], wherein cancer is stomach cancer. 
     Further, the present invention relates to [67] to [71] shown below. 
     [67] A method for diagnosing cancer in a subject comprising a step according to any one of [54] to [60]. 
     [68] The method according to [67] comprising a step of obtaining a sample from a subject. 
     [69] The method according to [67] or [68] further comprising a step in which it is judged when a fusion protein of RP2 and ARHGAP6 is detected in a sample obtained from a subject, that there is a high possibility of the subject having cancer. 
     [70] The method according to [67] or [68], wherein cancer is stomach cancer. 
     [71] The method according to [68] further comprising a step in which it is judged when a fusion protein of RP2 and ARHGAP6 is detected in a sample obtained from a subject, that there is a high possibility of the subject having stomach cancer. 
     Further, the present invention relates to [72] to [75] shown below. 
     [72] A method for identifying a subject that is a candidate for receiving a treatment by an ARHGAP6 function inhibitor and/or a pharmaceutical agent for blocking an abnormal signal induced by a fusion gene composed of an RP2 gene and an ARHGAP6 gene, the method comprising a step according to any one of [54] to [60], wherein the subject is a cancer patient. 
     [73] The method according to [72] comprising a step of obtaining a sample from a subject. 
     [74] The method according to [72] or [73] further comprising a step in which it is judged when a fusion protein of RP2 and ARHGAP6 is detected in a sample obtained from a subject, that the subject is a candidate for receiving a treatment by an ARHGAP6 inhibitor and/or a pharmaceutical agent for blocking an abnormal signal induced by a fusion gene composed of an RP2 gene and an ARHGAP6 gene. 
     [75] The method according to [72] or [74], wherein cancer is stomach cancer. 
     Further, the present invention relates to [76] to [79] shown below. 
     [76] A detection kit for detecting a fusion protein of RP2 and ARHGAP6 existing in a sample obtained from a subject, the detection kit comprising an antibody (primary antibody) that recognizes a section derived from an RP2 gene in the polypeptide according to any one of [54] to [58], and an antibody (primary antibody) that recognizes a section derived from an ARHGAP6 gene in said polypeptide. 
     [77] The detection kit according to [76] comprising secondary antibodies that are connected to oligonucleotides and that respectively bind to primary antibodies, two types of oligonucleotides that are partially complementary to the oligonucleotides connected to the secondary antibodies, a ligase that can ligate the two types of oligonucleotides to form a circular structure when the oligonucleotides approach each other, and a labeled oligonucleotide probe. 
     [78] The detection kit according to [76] or [77], wherein the subject is a cancer patient. 
     [79] The detection kit according to [78], wherein cancer is stomach cancer. 
     Further, the present invention relates to [80] to [81] shown below. 
     [80] A polypeptide according to any one of (1) to (3) shown below or a polynucleotide encoding said polypeptide: 
     (1) a polypeptide that comprises an amino acid sequence having no less than 90% identity with an amino acid sequence represented by SEQ ID NO: 2; 
     (2) a polypeptide that comprises an amino acid sequence represented by SEQ ID NO: 2, in which 1 to 10 amino acids are deleted, substituted, inserted and/or added; 
     (3) a polypeptide consisting of an amino acid sequence represented by SEQ ID NO: 2. 
     [81] The polypeptide or a polynucleotide encoding said polypeptide according to [80] that has an ability to develop tumor. 
     Advantageous Effect of Invention 
     The detection method of the present invention may be used as a method for detecting cancer (particularly, stomach cancer) that tests positive for a fusion gene composed of an RP2 gene and an ARHGAP6 gene (hereinafter referred to as RP2-ARHGAP6 fusion gene). The primer set, probe, probe set and detection kit of the present invention may be used in a detection method of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows the result of the Western blot. It shows the change in the amount of protein expression of the RP2-ARHGAP6 fusion protein by the ARHGAP6 siRNA treatment. 
         FIG. 2  shows the change in the number of viable cells in the stomach cancer cell line caused by the ARHGAP6 siRNA treatment.  FIG. 2  compares the number of viable cells cultured in a 0.5% bovine serum containing RPMI-1640 medium after introduction of siRNA with that of the control. 
         FIG. 3  shows a result of amplification by PCR of a region containing a fusion point of a RP2-ARHGAP6 fusion gene. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     «Detection Method of the Present Invention» 
     The detection method of the present invention includes a method for detecting a fusion gene, and a method for detecting a fusion protein encoded in the fusion gene. The method for detecting a fusion gene of the present invention or the method for detecting a fusion protein of the present invention includes a step of detecting whether a specific polynucleotide or polypeptide exists in a sample obtained from a subject. 
     Items collected from the subject (samples separated from a living body) are used as the sample obtained from the subject, specifically, any cells, tissues, or body fluids that were collected (blood, oral mucus, circulating tumor cells, exosome, etc.), biopsied samples (samples from the primary focus, cancer cells in the peritoneal lavage solution, cancer cells in ascites, etc.), of which the biopsied samples are preferred. It is possible to use genome DNAs extracted from the collected samples or to use transcription products thereof (products that are obtained by transcription and translation of a genome; e.g. RNA, protein) or cDNA prepared from RNA. It is particularly preferable to use RNA or cDNA that had been formulated. It is also possible to use a stabilized sample fixed in formalin and embedded in paraffin (Formalin-Fixed Paraffin-Embedded sample; FFPE Sample). A FFPE sample sliced into a thin FFPE slice may also be used. A use of a FFPE slice enables a direct detection of a polynucleotide existing in the slice. 
     The method for detecting a fusion gene in the present invention is a method for detecting “a fusion gene composed of an RP2 gene and an ARHGAP6 gene,” wherein the fusion gene is a fusion gene comprising a part of an RP2 gene and a part of an ARHGAP6 gene. An exemplary fusion gene composed of an RP2 gene and an ARHGAP6 gene includes a polynucleotide consisting of a base sequence represented by SEQ ID NO: 1. The polynucleotide consisting of a base sequence represented by SEQ ID NO: 1 is a polynucleotide with a base sequence of base no. 190 (corresponding to the 5′ terminal of the coding sequence (hereinafter referred to as CDS)) to 291 of an RP2 gene (GenBank registration no: NM_006915.2) and base no. 1462 to 3798 (corresponding to the 3′ terminal of CDS) of an ARHGAP6 gene (GenBank registration no: NM_013427.2). Of the base sequence represented by SEQ ID NO: 1, the sequence from base no. 1 to 102 is derived from an RP2 gene, and the sequence from base no. 103 to 2439 is derived from an ARHGAP6 gene. The polynucleotide consisting of a base sequence represented by SEQ ID NO: 1 is also referred to as a “fusion polynucleotide.” The amino acid sequence encoded in base no. 1 to 2439 of SEQ ID NO: 1 is shown in SEQ ID NO: 2. 
     In the “step of detecting whether a polynucleotide exists” in the detection method of a fusion gene of the present invention, the polynucleotide that is the target of detection (referred to in the present specification as the “polynucleotide targeted in detection”) includes, for example, a polynucleotide encoding a polypeptide described in (1) or (2) shown below: 
     (1) a polypeptide that comprises an amino acid sequence having no less than 90% identity with an amino acid sequence represented by SEQ ID NO: 2; 
     (2) a polypeptide that comprises an amino acid sequence having no less than 90% identity with an amino acid sequence represented by SEQ ID NO: 2, and has an ability to develop tumor. 
     In the aforementioned polypeptide, the “identity with an amino acid sequence represented by SEQ ID NO: 2” is preferably 95% or higher, and more preferably 98% or higher. 
     Note that the “identity” as used in the present specification is a value of “Identity” obtained by using a parameter prepared by default by the NEEDLE program (J Mol Biol 1970; 48: 443-453) search. The aforementioned parameter is shown below. 
     Gap penalty=10 
     Extend penalty=0.5 
     Matrix=EBLOSUM62 
     Whether a polypeptide “has an ability to develop tumor” or not may be confirmed by a method shown below in Example 2. One specific method is to introduce siRNA that suppresses the expression of a polynucleotide encoding the polypeptide to a cell expressing the polypeptide (NSC-10C), and to verify that the viability of the cell decreases. 
     In one embodiment of the present invention, the polynucleotide targeted in detection is a polynucleotide encoding a polypeptide according to any one of (1) to (4) shown below: 
     (1) a polypeptide that comprises an amino acid sequence represented by SEQ ID NO: 2, in which 1 to 10 amino acids are deleted, substituted, inserted and/or added; 
     (2) a polypeptide that comprises an amino acid sequence represented by SEQ ID NO: 2, in which 1 to 10 amino acids are deleted, substituted, inserted and/or added, and has an ability to develop tumor; 
     (3) a polypeptide that comprises an amino acid sequence represented by SEQ ID NO: 2 and has an ability to develop tumor; and 
     (4) a polypeptide that consists of an amino acid sequence represented by SEQ ID NO: 2. 
     In the polypeptide of (1) and (2), the number of amino acids that had been deleted, substituted, inserted and/or added in the amino acid sequence represented by SEQ ID NO: 2 is preferably one to a few, more preferably 1 to 7, and even more preferably 1 to 5. 
     An example of a polynucleotide that encodes “a polypeptide that consists of an amino acid sequence represented by SEQ ID NO: 2” includes “a polynucleotide that consists of a base sequence represented by SEQ ID NO: 1.” 
     The method for detecting a fusion gene of the present invention may comprise a step in which it is judged whether the polynucleotide targeted in detection exists by whether said polynucleotide was detected. 
     The method for detecting a fusion gene of the present invention may further comprise a step in which it is judged when a polynucleotide targeted in detection is detected, that a fusion gene composed of an RP2 gene and an ARHGAP6 gene exists. 
     The method for detecting a fusion gene of the present invention may comprise a step of amplifying the nucleic acid existing in the sample obtained from a subject or a step of hybridizing a probe with the nucleic acid existing in the sample obtained from a subject to detect the polynucleotide targeted in detection. 
     The nucleic acid to be used may be a genome DNA, RNA or a cDNA prepared from RNA. The methods of extracting DNA, extracting RNA or preparing cDNA from RNA is commonly known in the field, and it may be performed easily by using a commercially available DNA extraction kit, RNA extraction kit or a cDNA synthesis kit. 
     The step of amplifying a nucleic acid in the sample obtained from a subject may be performed by a commonly known method of amplifying a nucleic acid. Such method includes PCR (Polymerase chain reaction, e.g. realtime PCR), LCR (Ligase chain reaction), SDA (Strand displacement amplification), NASBA (Nucleic acid sequence-based amplification), ICAN (Isothermal and chimeric primer-initiated amplification of nucleic acids), LAMP (Loop-mediated isothermal amplification), TMA (Transcription-mediated amplification, e.g. Gen-Probe&#39;s TMA system), and a preferable method is PCR. 
     Specifically, the nucleic acid (e.g. genome DNA, RNA, or cDNA prepared from RNA, etc.) in the sample obtained from a subject is subjected to a nucleic acid amplification reaction using a primer set designed to specifically amplify a polynucleotide targeted in detection. The primer set to be used is not particularly limited as long as it can specifically amplify the polynucleotide targeted in detection. For example, a use of a primer design software (e.g. Primer Express; Applied Biosystems) allows a person skilled in the art to easily design the primer set based on the base sequence of the polynucleotide targeted in detection. More specifically, a primer set includes a sense primer (5′-primer) designed from a section that encodes the RP2 of a polynucleotide targeted in detection (e.g. any section in an RP2 gene region of the fusion polynucleotide (particularly, cDNA)) and an antisense primer (3′-primer) designed from a section encoding ARHGAP6 of a polynucleotide targeted in detection (e.g. any section in an ARHGAP6 gene region of the fusion polynucleotide (particularly, cDNA)), and the antisense primer consists of an oligonucleotide that hybridizes with a polynucleotide targeted in detection under a stringent condition (preferably, under a highly stringent condition), and the sense primer consists of an oligonucleotide that hybridizes with a complementary strand of a polynucleotide targeted in detection under a stringent condition (preferably, under a highly stringent condition). Otherwise, either the sense primer or the antisense primer may be designed so that it corresponds to the region comprising the fusion point of the polynucleotide targeted in detection. 
     The “stringent condition” in the present specification refers to a hybridization condition of “5×SSPE, 5×Denhardt&#39;s solution, 0.5% SDS, 50% formaldehyde, 200 μg/mL salmon sperm DNA, at 42° C. overnight” and a washing condition of “0.5×SSC, 0.1% SDS, 42° C.” “A highly stringent condition” refers to a hybridization condition of “5×SSPE, 5×Denhardt&#39;s solution, 0.5% SDS, 50% formaldehyde, 200 μg/mL salmon sperm DNA, at 42° C. overnight” and a washing condition of “0.2×SSC, 0.1% SDS, 65° C.” 
     The “fusion point” of a polynucleotide targeted in detection in the present specification is a point in which a section derived from an RP2 gene and a section derived from an ARHGAP6 gene in the polynucleotide targeted in detection are fused together, and the “region comprising the fusion point” in the polynucleotide targeted in detection is, for example, the region comprising bases of base no. 102 and 103 when the polynucleotide targeted in detection is a polynucleotide consisting of a base sequence represented by SEQ ID NO: 1. 
     In an embodiment of the present invention, the sense primer consists of an oligonucleotide hybridizing with a complementary strand of a polynucleotide that consists of base no. 1 to 102 of SEQ ID NO: 1 under a stringent condition, and the antisense primer consists of an oligonucleotide hybridizing with a polynucleotide that consists of base no. 103 to 2439 of SEQ ID NO: 1 under a stringent condition. 
     In an embodiment of the present invention, the sense primer consists of at least 16 consecutive bases of an oligonucleotide between base no. 1 to 102 of SEQ ID NO: 1, and the antisense primer consists of an oligonucleotide that is complementary with at least 16 consecutive bases of an oligonucleotide that consists of base no. 103 to 2439 of SEQ ID NO: 1. 
     In a step to amplify nucleic acid, the sense primer and the antisense primer should preferably be set so that the fragment size of the nucleic acid to be amplified is 1 kb or lower, since a large fragment size of the nucleic acid to be amplified leads to poor amplification efficiency. The primers to be used generally have a chain length of at least 15 bases, preferably at least 16 bases, more preferably at least 18 bases, even more preferably at least 20 bases. In one embodiment of the present invention, the primer has 15 to 40 bases, preferably 16 to 24 bases, more preferably 18 to 24 bases, even more preferably 20 to 24 bases. 
     The primer may be produced by chemical synthesis without being particularly limited thereby. 
     In a preferable embodiment, the detection method of a fusion gene of the present invention further encompasses a step of detecting whether an amplified nucleic acid fragment of a desired size was obtained in addition to a step of amplifying nucleic acid in the sample obtained from a subject. The step of detecting whether an amplified nucleic acid fragment of a desired size was obtained may be performed using electrophoresis, for example. By using electrophoresis, the nucleic acid fragment may be analyzed by agarose gel electrophoresis to confirm whether amplified nucleic acid fragments were produced in the desired size by using ethidium bromide dye, etc. 
     Further, by performing a PCR amplification monitor in the amplification process of the gene (real time PCR) (Genome Res. 1996; 6(10): 986-994), it is possible to perform a quantified analysis of amplified nucleic acid fragments. A possible candidate to be used in the PCR amplification monitoring method is ABI PRISM7900 (Applied Biosystems). 
     When an amplified nucleic acid fragment of the desired size is obtained, that means that a polynucleotide targeted in detection existed in the sample obtained from a subject. The detection method of a fusion gene of the present invention may further include a step in which it is judged when an amplified nucleic acid fragment of the desired size is obtained, that a fusion gene composed of an RP2 gene and an ARHGAP6 gene exists. 
     In a separate preferable embodiment, the detection method of the fusion gene of the present invention further encompasses a step of determining the base sequence of the amplified nucleic acid in addition to a step of amplifying the nucleic acid of the sample obtained from a subject. The step of determining the base sequence of the nucleic acid fragment may use sequencing methods commonly known in the field of art including next generation sequencing methods (Nature Biotechnology 2008; 26: 1135-1145) (e.g. HiSeq2500 (Illumina)), such as the Sanger sequencing (e.g. ABI PRISM3100 (Applied Biosystems) may be used), or sequencing by synthesis, etc. 
     The step of determining the base sequence of the nucleic acid fragment includes not just a step of sequencing the full length of a nucleic acid fragment, but a step of sequencing partial sequences corresponding to both ends of the nucleic acid fragment. 
     When the sequenced nucleic acid fragment includes a base sequence of a section encoding RP2 and a base sequence of a section encoding ARHGAP6 of the polynucleotide targeted in detection in the same fragment, that means that the polynucleotide targeted in detection existed in the sample obtained from a subject. The detection method of the fusion gene of the present invention may further include a step in which it is judged when the amplified nucleic acid fragment includes a base sequence of a section encoding RP2 and a base sequence of a section encoding ARHGAP6 of the polynucleotide targeted in detection in the same fragment, that a fusion gene composed of an RP2 gene and an ARHGAP6 gene exists. 
     The step of hybridizing a probe with a nucleic acid in the sample obtained from a subject may be performed using a probe including an oligonucleotide that hybridizes under a stringent condition (preferably, under a highly stringent condition) with a polynucleotide targeted in detection, and using a commonly known hybridization method. Such methods include, for example, Northern hybridization, dot blot method, DNA micro array method, RNA protection method, in situ hybridization, etc. A preferable method is the in situ hybridization. Detection using the in situ hybridization may be performed by a commonly known fluorescent in situ hybridization (FISH), chromogenic in situ hybridization (CISH), or silver in situ hybridization (SISH). The chain length of the probe used in hybridization may be selected as necessary by a person skilled in the art according to the hybridization method to be used, but the probe preferably has a chain length of at least 16 bases. 
     In one embodiment of the present invention, the probe used in hybridization is an oligonucleotide that hybridizes under a stringent condition (preferably, under a highly stringent condition) with a polynucleotide targeted in detection, or a complementary strand thereof, and it includes an oligonucleotide of at least 16 bases upstream and at least 16 bases downstream of the fusion point on the polynucleotide targeted in detection (a specific example being a sequence of base no. 87 to 118 in SEQ ID NO: 1) or an oligonucleotide that is complementary to said oligonucleotide. 
     In one embodiment of the present invention, the step of hybridizing a probe with a nucleic acid existing in a sample obtained from a subject may be performed according to the commonly known RNA FISH method (J. Mol. Diagn. 2012; 14(1): 22-29). More specifically, in situ hybridization is performed using a sample obtained from a subject (e.g. FFPE fragment), a probe designed from a section encoding RP2 of the polynucleotide targeted in detection (e.g. any section in an RP2 gene region of the fusion polynucleotide), and a probe designed from a section encoding ARHGAP6 of the polynucleotide targeted in detection (e.g. any section in an ARHGAP6 gene region of the fusion polynucleotide). The probes include oligonucleotides that hybridize under a stringent condition (preferably, under a highly stringent condition) with the polynucleotide targeted in detection. 
     In one embodiment of the present invention, the in situ hybridization is performed using multiple detection probes designed from a section encoding RP2 and multiple detection probes designed from a section encoding ARHGAP6. 
     In one embodiment of the present invention, the in situ hybridization is performed using the following probes: 
     multiple types of adjacent probe pairs including oligonucleotides that are complementary to at least 16 random consecutive oligonucleotides in base no. 1 to 102 of SEQ ID NO: 1 (preferably 10 to 25 types, more preferably 18 to 22 types, even more preferably 20 types of probe pairs), and multiple types of adjacent probe pairs including oligonucleotides that are complementary to at least 16 random consecutive oligonucleotides in base no. 103 to 2439 of SEQ ID NO: 1 (preferably 10 to 25 types, more preferably 18 to 22 types, even more preferably 20 types of probe pairs). 
     In a further embodiment, the probe pair to be used in the in situ hybridization may include an adjacent probe pair including an oligonucleotide that is complementary to an oligonucleotide of at least 16 random consecutive bases in the nontranslating region on the 5′ side of the RP2 gene (base no. 1 to 189 of GenBank registration no: NM_006915.2), and/or a probe pair including an oligonucleotide that is complementary to an oligonucleotide of at least 16 random consecutive bases in the nontranslating region on the 3′ side of the ARHGAP6 gene (base no. 3799 to 5118 of GenBank registration no: NM_013427.2). 
     The “adjacent probe pairs” in the present specification consist of two types of probes that are arranged next to each other when they hybridize with the polynucleotide targeted in detection. The probes include an oligonucleotide that is complementary to the polynucleotide targeted in detection, and the length of the oligonucleotide is generally at least 16 bases, preferably at least 18 bases. In one embodiment of the present invention, the length of the oligonucleotide is 16 to 30 bases, preferably 18 to 25 bases. 
     In a preferable embodiment of the present invention, the detection method of the fusion gene of the present invention further encompasses a step of amplifying a hybridization signal in addition to a step of performing in situ hybridization. To perform a step of amplifying a hybridization signal, a reagent that amplifies a hybridization signal may be hybridized with a probe that hybridizes with a nucleic acid contained in the sample. 
     Reagents that amplify a hybridization signal used in in situ hybridization include PreAmplifier Mix QT, Amplifier Mix QT, Label Probe Mix, and Label Probe Diluent QF, which may be obtained from Affymetrix. 
     In a more preferable embodiment, the detection method of the fusion gene of the present invention further encompasses a step of detecting a signal overlap between a signal from a probe designed from a section encoding RP2 and a signal from a probe designed from a section encoding ARHGAP6. By separating the fluorescent reagent or the color reagent that detects a probe designed from a section encoding RP2 and a probe designed from a section encoding ARHGAP6, it is possible to observe whether the signals from the two different probes are in the same area (inside the same molecule). When it is observed that the signals from the two different probes are in the same area (inside the same molecule), that would mean that the polynucleotide targeted in detection existed in the sample obtained from a subject. The detection method of the fusion gene of the present invention may further include a step in which it is judged when the two signals are in the same area (inside the same molecule), that a fusion gene composed of an RP2 gene and an ARHGAP6 gene exists. 
     The probes are not particularly limited, but they may be produced by a chemical synthesis method. 
     The detection method of the fusion protein of the present invention is a method for detecting “a fusion protein of RP2 and ARHGAP6” and the fusion protein is a fusion protein encoded by the fusion gene of the RP2 gene and the ARHGAP6 gene. 
     In the “step of detecting whether polypeptide exists” in the detection method of the fusion protein of the present invention, the polypeptide targeted in detection includes a polypeptide that is encoded by a polynucleotide targeted in detection. 
     The detection method of the fusion protein of the present invention may encompass a step in which it is judged whether a polynucleotide exists by whether the polypeptide targeted in detection is detected. 
     The detection method of the fusion protein of the present invention may further encompass a step in which it is judged when the polypeptide targeted in detection is detected, that a fusion protein of RP2 and ARHGAP6 exists. 
     The step of detecting whether a polypeptide exists may be performed by preparing a lysate derived from a sample obtained from a subject (e.g. cancer tissue or cell obtained from a subject) and measuring the polypeptide targeted in detection, contained in the sample by an immunological measurement method or an enzyme active measurement method, which combine antibodies against proteins that constitute the fusion protein, or a detection method that combines these methods, or by mass spectrometry. Further, this step may be performed by a detection method using an immunological tissue staining technology performed by combining the polypeptide targeted in detection, included in the sample (e.g. FFPE fragment) obtained from a subject, that had undergone appropriate pretreatment (such as, removal of paraffin), with the antibodies against proteins constituting the fusion protein. Otherwise, this step may be performed by exchanging the antibodies against proteins constituting the fusion protein to antibodies that recognize the fusion section of the fusion protein. Exemplary approaches to these methods include the following methods using monoclonal antibodies and polyclonal antibodies specific to the polypeptide targeted in detection: enzyme immunizing measurement, double antibody sandwich ELISA method, fluorescent immunological measurement method, radioimmunological measurement method, Western blot, immunohistologic staining, a detection method combining immune precipitation and mass spectrometry, etc. 
     The “fusion section” of the fusion protein of the present specification refers to a section in the polypeptide targeted in detection, in which the section derived from an RP2 gene and a section derived from an ARHGAP6 gene are fused. 
     The detection using an immunohistologic staining technology may be performed according to Proximity Ligation Assay (Nat. Methods. 2006; 3(12): 995-1000). More specifically, whether the polypeptide targeted in detection exists or not may be detected by using an antibody that recognizes a section derived from the RP2 gene of the polypeptide targeted in detection, and an antibody that recognizes a section derived from an ARHGAP6 gene of a polypeptide targeted in detection, and by detecting that the two antibodies recognize the same molecule by the aforementioned technologies. More specifically, the detection may be performed by i) a step of bringing an antibody (primary antibody) that recognizes a section derived from an RP2 gene of the polypeptide targeted in detection, and the antibody (primary antibody) that recognizes a section derived from an ARHGAP6 gene of the polypeptide targeted in detection, in contact with the sample obtained from the subject; ii) a step of adding secondary antibodies that are connected to oligonucleotides, and binds to the respective primary antibodies, iii) a step of inducing ligation by adding two types of oligonucleotides that are partly complementary to the oligonucleotides connected to the secondary antibodies, and a ligation solution containing ligase that can form a circular structure by ligation of the two types of oligonucleotides when they approach each other; iv) a step of elongating a nucleic acid along the circular structure that was formed, v) a step of hybridizing a labeled oligonucleotide probe that can hybridize with the elongated nucleic acid; and vi) a step of detecting the labeling signal. Such detection may be performed using a PLA probe and reagents included in the Duolink II reagent kit or the Duolink II Bright field reagent kit (Olink). 
     In one embodiment of the present invention, the detection method of the present invention encompasses a step of obtaining a sample from the subject. 
     In one embodiment of the present invention, the subject of the detection method of the present invention is a cancer patient, and in a more specific embodiment, the cancer is stomach cancer. The type of stomach cancer is not particularly limited, but it may be a diffuse type, an intestinal type, or a mix type in the Lauren classification. Further, without being limited thereby, the stomach cancer may be any of papillary adenocarcinoma, tubular adenocarcinoma, poorly differentiated adenocarcinoma, signet ring cell carcinoma, or carcinoma mucoides, etc. 
     In the detection method of the present invention, it is possible to judge when the polynucleotide targeted in detection, or the polypeptide targeted in detection is detected in the sample obtained from the subject, that the subject has cancer (particularly, stomach cancer). 
     The detection step in the detection method of the present invention may be used as a method for detecting whether cancer (particularly, stomach cancer) exists in a subject or a method for diagnosing cancer (particularly, stomach cancer) in the subject. The diagnosis method of the present invention may include, in addition to the aforementioned detection step, a step in which it is judged when the polynucleotide targeted in detection, or the polypeptide targeted in detection is detected in the sample obtained from the subject, that there is a high possibility that the subject has cancer (particularly, stomach cancer). Further, the detection step may be used in a method for identifying a subject (cancer patients of stomach cancer, etc.) that is a candidate for receiving a treatment by an ARHGAP6 function inhibitor and/or a pharmaceutical agent that blocks abnormality signal induced by a fusion gene composed of an RP2 gene and an ARHGAP6 gene. The identification method of the present invention may include, in addition to the detection step, a step in which it is judged when a polynucleotide is detected in a sample obtained from the subject, that the subject is a candidate for receiving a treatment by an ARHGAP6 function inhibitor and/or a pharmaceutical agent that blocks abnormality signal induced by a fusion gene composed of an RP2 gene and an ARHGAP6 gene. 
     «The Primer Set, Probe, Probe Set and Detection Kit of the Present Invention» 
     The present invention encompasses a primer set, probe, probe set and a detection kit used in the detection method of the present invention. 
     The primer set of the present invention includes a sense primer designed from a section encoding RP2 and an antisense primer designed from a section encoding ARHGAP6, and the antisense primer consists of an oligonucleotide that hybridizes with the polynucleotide targeted in detection under a stringent condition (preferably, under a highly stringent condition), and the sense primer consists of an oligonucleotide that hybridizes with a complementary strand of a polynucleotide targeted in detection under a stringent condition (preferably, under a highly stringent condition). 
     In the primer set of the present invention, either the sense primer or the antisense primer may be designed so that it corresponds to a region in a polynucleotide targeted in detection that comprises a fusion point. 
     A specific embodiment of the primer set of the present invention includes the following primer set: 
     a primer set consisting of a sense primer consisting of an oligonucleotide that hybridizes under a stringent condition with a complementary strand of a polynucleotide consisting of base no. 1 to 102 of SEQ ID NO: 1 and an antisense primer consisting of an oligonucleotide that hybridizes under a stringent condition with a polynucleotide consisting of base no. 103 to 2439 of SEQ ID NO: 1. 
     A more specific embodiment of the primer set of the present invention includes the following primer set: 
     a primer set consisting of a sense primer consisting of an oligonucleotide of at least 16 random consecutive bases between base no. 1 to 102 of SEQ ID NO: 1 and an antisense primer consisting of an oligonucleotide that is complementary with at least 16 random consecutive bases between base no. 103 to 2439 of SEQ ID NO: 1. 
     It is preferable for the primer set to have a space of 1 kb or lower between the selected positions of the sense primer and the antisense primer, or a nucleic acid fragment amplified by the sense primer and the antisense primer with a size of 1 kb or lower. Further, the primer of the present invention normally has a chain length of at least 15 bases, preferably at least 16 bases, more preferably at least 18 bases, even more preferably at least 20 bases. In one embodiment of the present invention, the primer has a chain length of 15 to 40 bases, preferably 16 to 24 bases, more preferably 18 to 24 bases, and even more preferably 20 to 24 bases. 
     The primers included in the primer set of the present invention, without being particularly limited, may be produced by a chemical synthesis method. 
     The probes included in the probe of the present invention and the probe set of the present invention includes an oligonucleotide that hybridizes with the polynucleotide targeted in detection under a stringent condition (preferably, under a highly stringent condition). The chain length of the probes included in the probe of the present invention or the probe set of the present invention may be selected as necessary by a person skilled in the art according to the applied hybridization method, but the probe preferably has a chain length of at least 16 bases. 
     In one embodiment of the present invention, the probe of the present invention includes an oligonucleotide of at least 16 bases upstream and at least 16 bases downstream of the fusion point in the polynucleotide targeted in detection (specifically, the sequence between base no. 87 to 118 of SEQ ID NO: 1), or an oligonucleotide that is complementary thereto. 
     In one embodiment of the present invention, the probe set of the present invention includes a probe designed from a section encoding RP2 (e.g. any section in the RP2 gene region of the fusion polynucleotide) and a probe designed from a section encoding ARHGAP6 (e.g. any section in the ARHGAP6 gene region of the fusion polynucleotide). 
     In one embodiment of the present invention, the probe set of the present invention includes multiple types of probes designed from a section encoding RP2 and multiple types of probes designed from a section encoding ARHGAP6. 
     In one embodiment of the present invention, the probe set of the present invention includes the following: 
     multiple types of adjacent probe pairs including an oligonucleotide that is complementary to an oligonucleotide of at least 16 random consecutive bases between base no. 1 to 102 of SEQ ID NO: 1 (preferably 10 to 25 types, more preferably 18 to 22 types, even more preferably 20 types of probe pairs), and multiple types of adjacent probe pairs including an oligonucleotide that is complementary with an oligonucleotide of at least 16 random consecutive bases between base no. 103 to 2439 of SEQ ID NO: 1 (preferably 10 to 25 types, more preferably 18 to 22 types, even more preferably 20 types of probe pairs). 
     The probes of the probe pair include an oligonucleotide that is complementary with the polynucleotide targeted in detection, and the length of the oligonucleotide is normally at least 16 bases, preferably at least 18 bases. In one embodiment of the present invention, the length of the oligonucleotide is 16 to 30 bases, preferably 18 to 25 bases. 
     The probe of the present invention and the probe included in the probe set of the present invention, without being limited thereby, may be produced by chemical synthesis. 
     The present invention encompasses a detection kit including a primer set of the present invention, a probe of the present invention or the probe set of the present invention. The detection kit of the present invention may include in addition to the primer set of the present invention, the probe of the present invention or the probe set of the present invention, components that may be used together with the primer set, the probe or the probe set for the detection of a polynucleotide targeted in detection such as reagents to amplify the signal of hybridization. 
     The present invention also encompasses a detection kit for detecting a polypeptide targeted in detection. Preferably, the detection kit includes an antibody (primary antibody) that recognizes a section derived from an RP2 gene of polypeptide targeted in detection, and an antibody (primary antibody) that recognizes a section derived from an ARHGAP6 gene of polypeptide targeted in detection. More preferably, the present invention may include secondary antibodies connected with oligonucleotides, that are respectively bound to primary antibodies, two types of oligonucleotides that are partially complementary to the oligonucleotides connected to the secondary antibodies, ligase that forms a circular structure by ligation of the two types of oligonucleotides when they approach each other, and labeled oligonucleotide probes. 
     The primer set, probe, probe set, and detection kit of the present invention may be used for the detection method, diagnosis method, identification method of a patient, and identification method of a subject of the present invention. In one embodiment of the present invention, with respect to the primer set, probe, probe set and detection kit of the present invention, the subject is a cancer patient and more specifically, the cancer is stomach cancer. The stomach cancer is not particularly limited, but it may be a diffuse type, an intestinal type, or a mix type in the Lauren classification. Further, without being limited thereby, the stomach cancer may be any of papillary adenocarcinoma, tubular adenocarcinoma, poorly differentiated adenocarcinoma, signet ring cell carcinoma, or carcinoma mucoid. 
     EXAMPLES 
     The Examples may be performed by commonly known methods unless otherwise indicated. When using commercially available reagents or kits, the Examples may be performed according to the manuals of the commercial products. 
     Example 1 Isolation of RP2-ARHGAP6 Fusion Gene 
     Total RNA was prepared from stomach cancer cell line NSC-10C established at the Division of Translational Oncology, National Cancer Center Research Institute, and reverse-transcribed into cDNA with reverse transcriptase (SuperScriptIII; Life Technologies) and Oligo(dT) Primer (Oligo(dT)20 Primer; Life Technologies) according to the standard protocol of the reagent. 
     Next, primers of RP2_full fwd01 represented by SEQ ID NO: 3 and ARHGAP6_full rev01 represented by SEQ ID NO: 4 were used to perform PCR (10 sec. at 98° C., 15 sec. at 55° C., and 3 min at 68° C., 30 cycles, followed by 5 min at 68° C.) using DNA polymerase (PrimeSTAR GXL; TAKARA BIO INC.) with cDNA obtained in the above step as a template. Then, using the aforementioned PCR product diluted by 10-fold as a template, primers of RP2_full fwd02 represented by SEQ ID NO: 5 and ARHGAP6_full rev02 represented by SEQ ID NO: 6 were subjected to PCR (10 sec. at 98° C., 15 sec. at 55° C., and 3 min at 68° C., 30 cycles, followed by 5 min at 68° C.) using the same DNA polymerase. Electrophoresis was performed after the PCR to obtain PCR product of about 2.5 kbp. After adding A to the 3′-end of the PCR product using Takara Taq (TAKARA BIO INC.), it was cloned into a cloning vector (TOPO XL PCR Cloning Kit; Life Technologies), and sequenced by dideoxy sequencing method (BigDye Terminator v3.1 Cycle Sequencing Kit; Life Technologies). Consequently, the PCR product about 2.5 kbp derived from the NSC-10C cell line was found to be a transcription product (SEQ ID NO: 1) in which nucleotide sequence of base no. 190 (corresponding to the 5′ terminal of CDS) to 291 of RP2 (NM_006915.2) registered in NCBI is fused to a nucleotide sequence of base no. 1462 to 3798 of ARHGAP6 (NM_013427.2) (corresponding to the 3′ terminal of CDS). The amino acid sequence of a polypeptide encoded in SEQ ID NO: 1 is represented by SEQ ID NO: 2. 
     Example 2 Evaluation of Ability to Suppress Expression of RP2-ARHGAP6 Fusion Protein in Stomach Cancer Cell Lines Expressing RP2-ARHGAP6 Fusion Gene Using ARHGAP6 siRNA, and Evaluation of Viability of Those Cell Lines Under the Same Condition 
     After culturing the cell line NSC-10C that expresses RP2-ARHGAP6 fusion gene, as shown in Example 1, and two substrains, NSC-10X1aA and NSC-10X1aF, established at the Division of Translational Oncology, National Cancer Center Research Institute, in RPMI-1640 medium (Wako Pure Chemical Industries, Ltd.) containing 10% bovine serum (Gibco), siRNA was introduced into the cells according to the standard protocol of the transfection reagent DharmaFECT1 (GE Healthcare). Specifically, the above cancer cells were seeded at 2×10 5  cells per well to a 6 well plate (140675, Nunc), 75 pmol of siRNA that targets ARHGAP6 (288604, Life Technologies) and control siRNA (AM4611, Life Technologies) were added to the cells (final concentration 75 nM), and the cells were cultured at 37° C. under an environment of 5% CO 2  for 120 h. (hereinafter, the group in which control siRNA was transfected is referred to as the Control siRNA group, and the group in which siRNA that targets ARHGAP6 is transfected is referred to as the ARHGAP6 siRNA group). 
     The suppressive effect of RP2-ARHGAP6 fusion protein by siRNA treatment was evaluated by the Western blot analysis. Specifically, the cultured cells were dissolved in 350 mM dithiothreitol (Fermentas)-containing Laemmli Sample Buffer (Bio-Rad) to extract protein. Protein concentration was measured by Protein Quantification Assay (MACHEREY-NAGEL GmbH &amp; Co. KG). 5 ug or 20 ug of the protein extract was loaded onto a 8% or 12% Poly-Acrylamide gel (Serva) containing SDS (Wako Pure Chemical Industries, Ltd.) and gel electrophoresis was performed for 1 h. under a condition of 40 mA. After 80 min of transfer to a PVDF membrane (Millipore Corporation) under a 60 mA condition using TRANS-BLOT SD SEMI-DRY TRANSFER CELL (Bio-Rad), blocking was performed for 2 h. at room temperature using PBS containing 5% Membrane Blocking Agent (GE Healthcare) (hereinafter referred to as the blocking buffer). The membrane was shaken in a primary antibody solution of anti-ARHGAP6 antibody (HPA036779, Sigma-Aldrich) diluted with a blocking buffer to a rate of 1:250 and anti-β-Actin antibody (4967, Cell Signaling Technology) diluted with a blocking buffer to a rate of 1:3000, and incubated overnight at 4° C. After washing with PBS containing 0.05% Tween 20 (Wako Pure Chemical Industries, Ltd.) (hereinafter referred to as the washing buffer), the membrane was shaken in a secondary antibody solution of HRP labeled anti-rabbit antibody (P0399, Daco) diluted with a blocking buffer to a rate of 1:3000, and incubated for 1 h. at room temperature. After washing with a washing buffer, Pierce Western blot Substrate Plus (Thermo Fisher Scientific Inc.) was added onto the membrane, and the chemiluminescence on a membrane was detected using LAS-4000R (Fuji Film). Western blotting analysis confirmed that the expression of RP2-ARHGAP6 fusion protein was suppressed by the siRNA for ARHGAP6 in all cell lines that endogenously expressed RP2-ARHGAP6 fusion gene ( FIG. 1 ). 
     In order to evaluate the effect of the RP2-ARHGAP6 fusion gene on the viability of the cancer cells, siRNA that targets ARHGAP6 and the control siRNA were transfected into the NSC-10C and the substrain thereof under the same conditions as shown above. After 24 h., the medium was changed to RPMI-1640 medium containing 0.5% bovine serum, and the cells were seeded at 1×10 3  cells per well to a 96 well plate (167008, Nunc), at 100 μL each, so that cells of each group were seeded to 6 wells, and cultured for additional 96 h. at 37° C. under a 5% CO 2  environment. Wells containing only RPMI-1640 medium containing 10% bovine serum without cells was prepared as a control (hereinafter referred to as the medium group). The number of living cells was measured according to the standard protocol of Cell Counting Kit-8 (DOJINDO LABORATORIES). Specifically, 10 μL of the reagent was added per well and the cells were cultured for 4 h. at 37° C. under a 5% CO 2  environment, then, the number of living cells was determined by measuring an absorbance of 450 nm by a micro plate reader (BioTek). Total 4 wells excluding the maximum and the minimum absorbance values of each group were adopted for the analysis. Viability of the Control siRNA group and the ARHGAP6 siRNA group was determined by subtracting the absorbance of the medium group from the absorbance of each group (hereinafter referred to as the correction value), and setting the correction value of the Control siRNA group as 100%. Student&#39;s t-test was used for the significance test between Control siRNA group and ARHGAP6 siRNA group. 
     Consequently, the viability of cells decreased ( FIG. 2 ) when the expression level of RP2-ARHGAP6 fusion protein was suppressed by transfecting siRNA that targets ARHGAP6 into all cell lines that endogenously express RP2-ARHGAP6 fusion gene. Since the p value is less than 0.05 even performing the correction by the multiple test method (by Bonferroni method), it was judged that a significant difference exists. It was thus found that suppressing the expression of the fusion gene in the cancer cells that endogenously express the RP2-ARHGAP6 fusion gene has an effect to suppress the growth of cancer cells and/or to decrease the survival of those cells. 
     Thus, it was found that the RP2-ARHGAP6 defined the tumor advancing capacity of cancer cells. 
     Example 3 Detection of RP2-ARHGAP6 Fusion Gene 
     Total RNA prepared from the substrains of NSC-10C, i.e. NSC-10X1A, NSC-10X1aA, NSC-10X1F, NSC-10X1aF, NSC-10X1aX1 and NSC-10X1aX1a, the stomach cancer cell lines KATO-III (JCRB0611, JCRB cell bank) and HSC-39 (provided from National Cancer Center Japan, Animal Experiment Section) and stomach cancer cell lines NSC-9C, NSC-6C and NSC-16C that were established at the Division of Translational Oncology, National Cancer Center Research Institute, in addition to the stomach cancer cell line NSC-10C that expresses an RP2-ARHGAP6 fusion gene shown in Example 1, were reverse-transcribed into cDNA using reverse transcriptase (SuperScriptIII; Life Technologies) and oligo(dT) primer (oligo(dT) 20 primer; Life Technologies). 
     Next, primers of RP2_CDS_F23 represented by SEQ ID NO: 7 and ARHGAP6_GCF-R1 represented by SEQ ID NO: 8 were used to perform PCR (2 min at 94° C., followed by 15 sec. at 94° C., 15 sec. at 55° C., and 1 min at 68° C., 30 cycles) using DNA polymerase (AccuPrime Taq DNA Polymerase; Life Technologies), and cDNA obtained above as a template (200 ng when converted to total RNA). Likewise, to confirm the equal amount of cDNA template in the reactions, primers of ACTB_F2 represented by SEQ ID NO: 9 and ACTB_R2 represented by SEQ ID NO: 10 were used to perform PCR (2 min at 94° C., followed by 15 sec. at 94° C., 15 sec. at 55° C., and 1 min at 68° C., 25 cycles) using the same DNA polymerase as above. Electrophoresis was performed with 2% agarose gel (Lonza) after PCR reaction, and about 200 bp PCR product was amplified only at NSC-10C and substrains thereof which were already confirmed the expression of RP2-ARHGAP6 fusion gene ( FIG. 3 ). The PCR product amplified by the aforementioned primer set was 214 bp according to the nucleotide sequence of the fusion gene identified in Example 1. Therefore, it was showed that the fusion gene that expresses the cancer cells is detectable with PCR method. 
     INDUSTRIAL APPLICABILITY 
     The detection method of the present invention is a method for detecting a fusion gene composed of the RP2 gene and the ARHGAP6 gene, and it is useful as a method for detecting and diagnosing cancer in a subject. Further, the primer set and the detection kit of the present invention may be used in a method of the present invention.