Patent Publication Number: US-2022226505-A1

Title: Dna, expression vector, transcript, cell, pharmaceutical composition, and medicine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a DNA, an expression vector, a transcript, a cell, a pharmaceutical composition, and a medicine. 
     2. Description of the Related Art 
     It has been reported that as a cause of tumorigenesis, in myelodysplastic syndrome (MDS), myeloid leukemia, lymphocytic leukemia, and uveal melanoma, mutations are often observed in specific amino acids such as SF3B1, SRSF2, U2AF35, and ZRSR2, which are spliceosome constituents (Kenichi Yoshida et al., Nature 2011). Accordingly, a splice donor sequence or a splice acceptor sequence that does not function in normal cells is used for abnormal splicing. This abnormal splicing induces abnormal protein expression or inhibition of normal protein expression, resulting in oncogenic transformation of cells (H Dolatshad et al., Leukemia, 2016; Daichi Inoue et al., Nature, 2019; and Lili Wang et al., Cancer Cell, 2016). 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a novel DNA, expression vector, transcript, cell, pharmaceutical composition, and medicine. 
     An aspect of the invention relates to a DNA encoding a suicide gene having at least one intron sequence. The intron sequence has a donor sequence or an acceptor sequence to be used in abnormal splicing that occurs in a tumor cell and does not occur in a normal cell. In a transcript of the DNA, the suicide gene is expressed when the intron is abnormally spliced, and the suicide gene is not expressed when the intron is not abnormally spliced. The intron sequence may include a sequence selected from introns of genes encoding at least one protein selected from the group consisting of DYNLL1, HINT2, MAP3K7, TMEM14C, and BRD9. The suicide gene may encode a cell death-inducing protein, a cell division-inhibiting protein, or a protein that converts a precursor of a cell death-inducing compound or a cell division-inhibiting compound into the cell death-inducing compound or the cell division-inhibiting compound. The suicide gene may be a gene encoding a protein selected from the group consisting of a herpes simplex virus type 1 thymidine kinase (HSV-TK), a cytosine deaminase (CD), an inducible caspase 9 (iCasp9), a streptolysin O, and a diphtheria toxin. The suicide gene may not be expressed due to a frame shift when normal splicing occurs. The intron sequence may have a branch point. 
     Another aspect of the invention relates to an expression vector containing any of the above-mentioned DNAs. The expression vector may contain a promoter that specifically transcribes the DNA in the tumor cell, a promoter that transcribes the DNA in a eukaryotic cell, or a promoter that specifically transcribes the DNA in a hematopoietic stem cell or a melanocyte. 
     A still another aspect of the invention relates to a transcript of any of the above-mentioned DNAs. 
     A yet another aspect of the invention relates to a cell containing any of the above-mentioned DNAs, any of the above-mentioned expression vectors, or any of the above-mentioned transcripts. 
     A yet another aspect of the invention relates to a pharmaceutical composition containing, as an active ingredient, any of the above-mentioned DNAs, any of the above-mentioned expression vectors, any of the above-mentioned transcripts, or any of the above-mentioned cells. 
     A yet another aspect of the invention relates to a medicine containing any of the above-mentioned pharmaceutical compositions. This medicine may be an antitumor agent. In that case, a treatment target disease may be myelodysplastic syndrome (MDS), myeloid leukemia, lymphocytic leukemia, or uveal melanoma. 
     The invention has made it possible to provide a novel DNA, expression vector, transcript, cell, pharmaceutical composition, and medicine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a recombinant gene having an intron sequence directly following an ATG of a suicide gene according to an embodiment of the invention, the intron sequence having an abnormal acceptor sequence 10- to 20-base upstream of a normal acceptor sequence and to be spliced in a cell with abnormal splicing. When normal splicing occurs, the entire intron sequence is removed, and when the abnormal splicing occurs, 10 to 20 bases on a 3′ side of the intron sequence remain. A guanine is inserted right after an intron. Therefore, when the normal splicing occurs, a frame shift occurs and the suicide gene is not expressed, but when the abnormal splicing occurs, the full-length suicide gene is expressed as the functional protein because C-terminal half of the suicide gene is fused in-frame. 
         FIG. 2  is a schematic diagram of a recombinant gene having an intron sequence, in which splicing occurs only in the cell with abnormal splicing, inserted into the suicide gene according to the embodiment of the invention. When the splicing does not occur, translational arrest in the intron prevents the suicide gene expression. However, when the abnormal splicing occurs, the full-length suicide gene is expressed. 
         FIG. 3A  shows a vector map of an HSV-TK expression vector and  FIG. 3B  shows a diagram of an intron insertion position in a sequence of HSV-TK gene in Examples of the invention.  FIG. 3A : the HSV-TK expression is induced by an SFFV promoter together with a puromycin resistance gene linked by a ribosomal skipping site P2A. An insertion position of an intron of each gene shown in  FIG. 3B  is indicated by a base length from 5′ end of the HSV-TK gene. 
         FIG. 4  is a diagram showing an experiment time schedule in Examples of the invention. 
         FIG. 5  shows graphs showing results of measuring GCV-concentration-dependent cell death in Examples of the invention. A change in number of cells based on the number of cells to which GCV is not added is shown as a viability (%). ▪ indicates an SF3B1 K700E mutant expression group, and ● indicates an wild-type SF3B1 overexpression group as a control. 
         FIG. 6  shows a DNA sequencing result of an HEK293FT cell in which an SF3B1 K700E mutation is introduced into one allele by genome editing in Examples of the invention. It can be seen that C and T overlap each other at the 111th position. 
         FIG. 7  is a graph showing a result of measuring the GCV-concentration-dependent cell death in Examples of the invention. The change in number of cells based on the number of cells to which GCV is not added is shown as the viability (%). ▪ indicates a result of an SF3B1 K700E mutant cell, and ● indicates a result of an SF3B1 normal cell as a control. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an embodiment of the invention will be described in detail with reference to the drawings, and those skilled in the art can easily reproduce the invention from the present specification since objects, features, advantages, and ideas of the invention are apparent to those skilled in the art from the present specification. The embodiment and specific Examples of the invention described below are preferred embodiments of the invention, are presented for illustration or description, and are not intended to limit the invention thereto. It is apparent to those skilled in the art that various changes and modifications can be made based on the description in the present specification within the spirit and scope of the invention disclosed in the present specification. 
     (1) Recombinant Gene 
     A recombinant gene in an embodiment disclosed in the present specification is a DNA encoding a suicide gene having at least one intron sequence. The intron sequence has a donor sequence or an acceptor sequence to be used in a tumor cell with abnormal splicing a, not in a normal cell. In a transcript of the DNA, the suicide gene is expressed when the abnormal splicing occurs, and the suicide gene is not expressed when the abnormal splicing does not occur. In the present specification, “normal” means an event that occurs when a wild-type protein is generated from a wild-type gene, and “abnormal” means an event that occurs when a protein different from the wild-type protein is generated as a result of splicing different from splicing that occurs in a normal process. Here, the wild type may include a mutation other than a mutation caused by the abnormal splicing. For example, the normal cell refers to a cell in which splicing of a target gene occurs normally and the wild-type protein is generated, and an abnormal acceptor sequence refers to an acceptor sequence used in the splicing different from the splicing that occurs in a normal process. 
     The suicide gene is not particularly limited as long as the suicide gene is a gene that inhibits cell proliferation when a protein encoded by the suicide gene is expressed, and examples thereof include a gene encoding a cell death-inducing protein or a cell division-inhibiting protein. Alternatively, the suicide gene may be a gene encoding a protein that converts a precursor (for example, a prodrug) of a cell death-inducing compound or a cell division-inhibiting compound into the cell death-inducing compound or the cell division-inhibiting compound. Specifically, the suicide gene includes, but is not limited to, a gene encoding a herpes simplex virus type 1 thymidine kinase (HSV-TK), a cytosine deaminase (CD), an inducible caspase 9 (iCasp9), a streptolysin O, or a diphtheria toxin. Of these, those with drug-induced cytotoxicity are preferred. For example, since the HSV-TK converts a non-toxic ganciclovir (GCV) into a toxic phosphorylated metabolite, it is necessary to administer the GCV together in order to inhibit the cell proliferation. Further, since the CD converts non-toxic 5-fluorocytosine (5-FC) into toxic 5-fluorouracil (5-FU), it is necessary to administer the 5-FC together in order to inhibit the cell proliferation. Alternatively, the iCasp9, which is a fusion protein of a human caspase 9 and a recombinant human FK-binding protein, causes apoptotic cell death only when the iCasp9 is a dimer, and can therefore exhibit toxicity only when AP1903 or AP20187 that causes dimerization is administered (Karin C et al., Blood, 2005). 
     The tumor cell is not particularly limited as long as the tumor cell causes the abnormal splicing, and examples thereof include myelodysplastic syndrome (MDS), myeloid leukemia, lymphocytic leukemia, and uveal melanoma. A mechanism of the abnormal splicing is also not particularly limited, and the abnormal splicing is preferably caused by mutations in spliceosome constituent factors such as SF3B1, SRSF2, U2AF35, and ZRSR2. 
     The intron sequence is not particularly limited as long as the intron sequence is a sequence having a donor sequence or an acceptor sequence to be used in the abnormal splicing that occurs in the tumor cell and does not occur in the normal cell, and may be an intron sequence derived from a natural gene or an artificial sequence. Even being an artificial sequence, the intron sequence preferably has an identity of 60% or more, more preferably has an identity of 80% or more, still more preferably has an identity of 90% or more, even more preferably has an identity of 95% or more, and yet still more preferably has an identity of 99% or more with an intron sequence of a natural gene in a portion other than the donor sequence or acceptor sequence to be used in the abnormal splicing. Here, the identity can be evaluated by a known program. 
     It is known that splicing occurs in two stages, and a specific base called a branch point is involved in splicing in a first stage. Therefore, the intron sequence used in the recombinant gene of the disclosure preferably contains a branch point. 
     Specifically, examples of the intron sequence include a sequence selected from an intron of a gene encoding at least one protein selected from the group consisting of HINT2, DYNLL1, MAP3K7, BRD9, and TMEM14C. For example, the intron sequence may be sequences (SEQ ID NOs: 1 to 3) in which 30 to 70 base pairs from a 5′ terminal and 30 to 90 base pairs from a 3′ terminal are bound to each other in an intron between exons 4 and 5 of the HINT2 (NM_032593.3), an intron between exons 1 and 2 of the DYNLL1 (NM_003746.3), or an intron between exons 4 and 5 of the MAP3K7 (NM_003188.4). The intron sequence has one base (G in this case) inserted right after the intron sequence such that when the intron sequence has a normal acceptor sequence and normal splicing occurs in the normal cell, a frame shift occurs in the suicide gene and a normal protein is not expressed ( FIG. 1 ). Therefore, a recombinant gene having the intron sequence does not influence the proliferation of the normal cell. In contrast, when an abnormal acceptor sequence in the intron is used and the splicing occurs in the tumor cell, C-terminal half of the suicide gene is fused in-frame, and the full-length suicide gene is expressed as the functional protein, so that the recombinant gene having the intron sequence inhibits the proliferation of the tumor cell ( FIG. 1 ). 
     Another example of the intron sequence is a sequence (SEQ ID NO: 5) in which 30 to 70 bases from the 5′ terminal of the intron between exons 1 and 2 of the TMEM14C (NM_016462.4) are bound to 20 to 50 bases from an abnormal acceptor sequence located about 13 to 20 bases upstream from the 3′ terminal. Since the intron sequence does not have a normal acceptor sequence, in the normal cell, the inserted intron is not removed by splicing, translational arrest or the frame shift occurs within the intron, and the suicide gene is not expressed ( FIG. 2 ). Therefore, a recombinant gene having the intron sequence does not influence the proliferation of the normal cell. In contrast, in the tumor cell, the abnormal acceptor sequence is used, the abnormal splicing occurs, and the intron is removed, so that the normal protein encoded by the suicide gene is expressed, and the recombinant gene having the intron sequence inhibits the proliferation of the tumor cell ( FIG. 2 ). 
     Alternatively, the suicide gene sequence may have, as an intron sequence, a sequence (SEQ ID NO: 4) in which 39 bases from a 5′ terminal and 82 bases from a 3′ terminal of an intron between exons 14 and 15 of the BRD9 (XM_024446194.1) are bound to each other. In this case, in the normal cell, the intron is also not removed by splicing, and therefore, translational arrest occurs and the suicide gene is not expressed ( FIG. 2 ). In contrast, in the tumor cell, the intron is removed by splicing and the suicide gene is expressed ( FIG. 2 ). 
     For the sequences 1 to 4, the following sequences are particularly important, and in the following, the sequence preferably has an identity of 60% or more, more preferably has an identity of 80% or more, still more preferably has an identity of 90% or more, even more preferably has an identity of 95% or more, and yet still more preferably has an identity of 99% or more. 
     SEQ ID NO: 6: sequence from a branch point of an intron to a normal acceptor site of the HINT2 (including a base of the branch point and an acceptor sequence, and the same applies hereinafter) 
     SEQ ID NO: 7: sequence from a branch point of an intron to a normal acceptor site of the DYNLL1 
     SEQ ID NO: 8: sequence from a branch point of an intron to a normal acceptor site of the MAP3K7 
     SEQ ID NO: 9: sequence from a branch point of an intron to an abnormal acceptor site of the BRD9 containing no normal acceptor sequence 
     SEQ ID NO: 10: sequence from a branch point of an intron to an abnormal acceptor site of the TMEM14C containing no normal acceptor sequence 
     An expression vector disclosed in the present specification can express the above-mentioned recombinant suicide gene intracellularly. Examples of the expression vector include a plasmid vector and a viral vector (for example, vectors derived from an adenovirus, a vaccinia virus, a lentivirus, a herpes simplex virus, a baculovirus, a retrovirus, an adeno-associated virus (AAV), rhinovirus, a human immunodeficiency virus (HIV), a Sendai virus, and a filovirus). A vector derived from an AAV, an adenovirus, or a Sendai virus, which has high introduction efficiency and is not incorporated into a genome, is particularly preferred. 
     These expression vectors have a promoter for expressing the recombinant gene. 
     (2) Medicine 
     A pharmaceutical composition disclosed in the present specification may contain, as an active ingredient, the above-mentioned recombinant gene, an expression vector containing the recombinant gene, or a transcript (mRNA and precursor) of the recombinant gene. In addition to these, the pharmaceutical composition may include a delivery agent for administering the DNA to a patient. Known delivery agents can be used, and examples thereof include exosomes, virosomes, lipid nanoparticles, and liposomes. 
     Further, the pharmaceutical composition disclosed in the present specification may contain, as an active ingredient, a cell containing the above-mentioned recombinant gene, an expression vector containing the recombinant gene, or a transcript (mRNA and precursor) of the recombinant gene. The cell can be used, for example, as follows. In administration of a large dose of an anticancer drug or autologous hematopoietic stem cell transplantation after systemic radiation therapy, the above-mentioned DNA vector is introduced into a pre-collected hematopoietic stem cell ex vivo to kill the tumor cell and to allow only the normal cell to survive, which makes it possible to obtain only a normal hematopoietic stem cell, and then the obtained normal hematopoietic stem cell is re-transplanted into the patient. 
     The pharmaceutical composition of the disclosure can be used as a medicine and a target disease is not limited as long as it is a disease caused by the abnormal splicing. It is particularly preferable to use the pharmaceutical composition as an antitumor agent. The tumor cell is not particularly limited as long as the abnormal splicing occurs, and examples thereof include myelodysplastic syndrome (MDS), myeloid leukemia, lymphocytic leukemia, and uveal melanoma. The mechanism of the abnormal splicing is also not particularly limited, and the abnormal splicing is preferably caused by mutations in spliceosome constituent factors (SF3B1, SRSF2, U2AF35, ZRSR2, and the like). 
     Example 1 
     In this example, in order to verify toxicity of a suicide gene in a cell expressing an SF3B1 K700E mutant, pHR-SFFVp-SF3B1 K700E (SEQ ID NO: 18), pHR-SFFVp-HSVTK-puro containing each intron ( FIGS. 3A and 3B ; SEQ ID NOs: 11 to 15), and pHR-SFFVp-HSVTK-puro as an intron-free control (SEQ ID NO: 16) were cotransfected into HEK293FT cells (Thermo Fisher Scientific Inc.), and the number of cells killed by addition of GCV was quantified using an MTT assay method. A detailed procedure will be shown below. A time schedule is shown in  FIG. 4 . Further, the base sequences used will be described below. 
     SEQ ID NO: 11: sequence in which an SFFV promoter, an HSV-TK inserted with an intron of BRD9 containing no normal acceptor sequence, and a puromycin resistance gene linked to the HSV-TK by a P2A sequence are linked to one another 
     SEQ ID NO: 12: sequence in which an SFFV promoter, an HSV-TK inserted with an intron of TMEM14C containing no normal acceptor sequence, and a puromycin resistance gene linked to the HSV-TK by a P2A sequence are linked to one another 
     SEQ ID NO: 13: sequence in which an SFFV promoter, an HSV-TK inserted with an intron of HINT2 containing a normal acceptor sequence (SEQ ID NO: 1), and a puromycin resistance gene linked to the HSV-TK by a P2A sequence are linked to one another 
     SEQ ID NO: 14: sequence in which an SFFV promoter, an HSV-TK inserted with an intron of DYNLL1 containing a normal acceptor sequence (SEQ ID NO: 2), and a puromycin resistance gene linked to the HSV-TK by a P2A sequence are linked to one another 
     SEQ ID NO: 15: sequence in which an SFFV promoter, an HSV-TK inserted with an intron of MAP3K71 containing a normal acceptor sequence (SEQ ID NO: 3) at a position 5, and a puromycin resistance gene linked to the HSV-TK by a P2A sequence are linked to one another 
     SEQ ID NO: 16: sequence in which an SFFV promoter, an HSV-TK, and a puromycin resistance gene linked to the HSV-TK by a P2A sequence are linked to one another 
     A specific experimental method will be described in detail below. 
     First, HEK293FT cells of 1×10 6  cells/mL, which were suspended in a DMEM medium (cDMEM medium) containing 10% bovine serum (Corning Inc.) and penicillin-streptomycin (Nacalai Tesque Inc.), were seeded in a 96-well plate at 50 μL/well and cultured in a cell culture device adjusted to 37° C. and a CO 2  concentration of 5%. 
     After 24 hours, 50 ng of pHR-SFFVp-SF3B1 WT (a wild-type SF3B1 with a FLAG tag added to an N terminal and the amino acid sequence thereof was shown in SEQ ID NO: 19) (SEQ ID NO: 17) or pHR-SFFVp-SF3B1 K700E (SF3B1 K700E with a FLAG tag added to an N-terminal and the amino acid sequence thereof was shown in SEQ ID NO: 20) (SEQ ID NO: 18) and 50 ng of each pHR-SFFVp-HSVTK-puro (SEQ ID NOs: 11 to 16) were cotransfected into the HEK293FT cells by using Lipofectamine 3000, and then cultured in the cell culture device adjusted to 37° C. and a CO 2  concentration of 5%. 
     After 24 hours, a culture supernatant was completely removed and the medium was replaced with cDMEM media each containing 100 μL of GCV (GCV final concentration: 500 μg/mL, 100 μg/mL, 20 μg/mL, 0 μg/mL). Then, the HEK293FT cells were cultured in the cell culture device adjusted to 37° C. and a CO 2  concentration of 5%. 
     After 48 hours, the HEK293FT cells were suspended in the cDMEM medium and a cell concentration was adjusted to 20×10 5  cells/mL, 10×10 5  cells/mL, 5×10 5  cells/mL, 2.5×10 5  cells/mL, 1.25×10 5  cells/mL, and 0×10 5  cells/mL, and then the HEK293FT cells were seeded in a 96-well plate at 100 μL/well to obtain a standard for measuring the number of cells. 
     An MTT solution of an MTT cell number measuring kit (Nacalai Tesque Inc.) was added into the HEK293FT cells at 10 μL/well, and the mixture was cultured in the cell culture device adjusted to 37° C. and a CO 2  concentration of 5% for 4 hours. 
     A solubilized solution of the MTT cell number measuring kit (Nacalai Tesque Inc.) was added into the HEK293FT cells at 100 μL/well, and the mixture was incubated for 20 hours at 37° C. in a moist environment. 
     An absorbance at 570 nm (reference wavelength: 655 nm) was measured with a microplate reader (SH-9000Lab, manufactured by Hitachi High-Tech Corporation). 
     The number of viable cells was calculated based on a standard curve using the above-mentioned serially diluted HEK293FT cells, and a viability was shown as a ratio when the number of cells in a GCV-free group was 100%. Further, specific cytotoxicity in an abnormal splicing cell was evaluated as efficacy by comparison with a control group expressing SF3B1WT (SEQ ID NO: 17).  FIG. 5  shows results under each condition. 
     As shown in  FIG. 5 , under each condition, due to the suicide gene containing each intron, the viability in the cell having an abnormal splicing mechanism was lower than in a normal cell having no such a mechanism. 
     Example 2 
     In this example, in order to verify the toxicity of a suicide gene in a cell having an SF3B1 K700E mutation in one allele in a genome, as in actual cancer cells, HEK293FT cells were genome-edited and an SF3B1 K700E mutant cell strain was established. 
     Meanwhile, an HSV-TK expression lentiviral vector containing the BRD9 intron was prepared using the pHR vector shown in SEQ ID NO: 11 ( FIGS. 3A and 3B ). The lentiviral vector was introduced into the SF3B1 K700E mutant strain and an SF3B1 normal strain as a control, and the number of cells killed by the addition of GCV was quantified using the MTT assay method. A detailed procedure will be shown below. 
     First, HEK293FT cells of 1×10 6  cells/mL suspended in a cDMEM medium were seeded in a 6-well plate at 1.5 mL/well and cultured in a cell culture device adjusted to 37° C. and a CO 2  concentration of 5%. 
     After 24 hours, 6.25 μg of TrueCut Cas9 Protein v2 (Thermo Fisher Scientific Inc.), 1.2 μg of a guide RNA (AACUUUCUGCUCAUCCA) (SEQ ID NO: 21) for SF3B1, and 100 pmol of ssODN (single-stranded DNA: SEQ ID NO: 22) for the SF3B1 K700E mutation were cotransfected into the HEK293FT cells by using Lipofectamine CRISPRMAX (Thermo Fisher Scientific Inc.) and then cultured in the cell culture device adjusted to 37° C. and a CO 2  concentration of 5%. 
     After 24 hours, a culture supernatant was removed, and the HEK293FT cells were separated into single cells with a trypsin-EDTA solution (Nacalai Tesque Inc.) and diluted to 2 cells per 10 wells, and then seeded in a 96-well plate. Then, the SF3B1 K700E mutant strain was screened out by direct sequencing around SF3B1 K700E in wells where cell colonies were obtained, and the obtained mutant strain was propagated. 
     In the direct sequencing, a genome of cells extracted with an alkaline heat extraction method was amplified by a PCR reaction using KOD One (TOYOBO CO., LTD.) and primers (GTTGATATATTGAGAGAATCTGGATG (SEQ ID NO: 23) and AAATCAAAAGGTAATTGGTGGA (SEQ ID NO: 24)), the obtained amplification product was purified and a base sequence was determined by using the amplification product as a template. The base sequence obtained at that time is shown in  FIG. 6 . In this way, it can be confirmed that the SF3B1 K700E mutation is actually introduced into the one allele. 
     Next, in order to prepare a lentivirus, HEK293FT cells of 1.2×10 6  cells/mL suspended in a cDMEM medium were seeded in a T25 flask at 4 mL/well and then cultured in a cell culture device adjusted to 37° C. and a CO 2  concentration of 5%. 
     After 24 hours, an HSV-TK expression vector (pHR-SFFVp-HSVTK-puro containing the BRD9 intron: SEQ ID NO: 11) and a packaging vector (pCMVR8.74 and pMD2.G, pAdVAntage) were cotransfected into the HEK293FT cells using 1 mg/mL of a Polyethylenimine solution (Polysciences Inc.), and then cultured in the cell culture device adjusted to 37° C. and a CO 2  concentration of 5%. After 24 hours, the medium was replaced with a fresh cDMEM medium and the culture supernatant was collected after 48 hours and 72 hours. 
     The collected culture supernatants were added to an SF3B1 K700E mutant cell derived from the HEK293FT cells and a SF3B1 normal cell to infect the prepared lentivirus. 
     After 48 hours, puromycin was added so as to have a final concentration of 4 μg/mL, and the cells were further cultured for 48 hours to select only transduced cells. 
     The obtained SF3B1 K700E mutant cell derived from the HEK293FT cells expressing an HSV-TK containing the BRD9 intron and the obtained SF3B1 normal cell expressing the HSV-TK containing the BRD9 intron were suspended in a cDMEM medium with the cell concentration adjusted to 4×10 5  cells/mL, and then seeded in a 96-well plate at 50 μL/well. 
     50 μL of cDMEM media each containing GCV (GCV final concentration: 100 μg/mL, 20 μg/mL, 4 μg/mL, 0.8 μg/mL, 0.16 μg/mL, 0 μg/mL) were added, and then the mixture was cultured in the cell culture device adjusted to 37° C. and a CO 2  concentration of 5%. 
     After 48 hours, the HEK293FT cells were suspended in the cDMEM medium and the cell concentration was adjusted to 40×10 5  cells/mL, 20×10 5  cells/mL, 10×10 5  cells/mL, 5×10 5  cells/mL, 2.5×10 5  cells/mL, 1.25×10 5  cells/mL, 0.625×10 5  cells/mL, and 0×10 5  cells/mL, and then seeded in a 96-well plate at 100 μL/well and used as a standard for measuring the number of cells. 
     The MTT solution of the MTT cell number measuring kit (Nacalai Tesque Inc.) was added into the HEK293FT cells at 10 μL/well, and the mixture was cultured in the cell culture device adjusted to 37° C. and a CO 2  concentration of 5% for 4 hours. 
     The solubilized solution of the MTT cell number measuring kit (Nacalai Tesque Inc.) was added into the HEK293FT cells at 100 μL/well, and the mixture was incubated at 37° C. After 20 hours, the absorbance at 570 nm (reference wavelength: 655 nm) was measured with the microplate reader. 
     The number of viable cells was calculated based on a standard curve using the above-mentioned serially diluted HEK293FT cells, and the viability was shown as a ratio when the number of cells in the GCV-free group was 100%. Further, the efficacy was evaluated by comparing the cytotoxicity due to GCV addition in the SF3B1 normal cell with the cytotoxicity of the SF3B1 K700E mutant cell.  FIG. 7  is a graph showing the viability of the SF3B1 K700E mutant cell and the viability of the normal cell. 
     As shown in  FIG. 7 , the SF3B1 K700E mutant cell mutated due to the suicide gene containing the BRD9 intron had a viability lower than that of the SF3B1 normal cell.