Patent Publication Number: US-2021171591-A1

Title: Codon optimized nucleic acid encoding a retinitis pigmentosa gtpase regulator (rpgr)

Description:
RELATED APPLICATIONS 
     This application is a Continuation Application of U.S. patent application Ser. No. 15/360,362, filed on Nov. 23, 2016, which is a Continuation Application of U.S. patent application Ser. No. 14/687,227, filed on Apr. 15, 2015 and issued as U.S. Pat. No. 9,534,225 on Jan. 3, 2017, which claims the benefit of U.S. Provisional Application No. 61/979,633, filed on Apr. 15, 2014, the entire contents of which are expressly incorporated herein by reference. 
    
    
     SUBMISSION OF SEQUENCE LISTING 
     The Sequence Listing associated with this application is filed in electronic format via EFS-Web and hereby incorporated by reference into the specification in its entirety. The name of the text file containing the Sequence Listing is “Sequence Listing” and is 16 kb in size. 
     FIELD OF THE INVENTION 
     This invention relates generally to codon optimized nucleic acid sequences encoding a human retinitis pigmentosa GTPase regulator (RPGR). 
     BACKGROUND OF THE INVENTION 
     Retinitis pigmentosa (RP) is an inherited degenerative disease of the retina that affects approximately one in 3,500 individuals, with an estimated 1.5 million patients worldwide. See Churchill et al., 2013, Invest. Ophthalmol. Vis. Sci. 54(2): 1411-1416. RP is caused by progressive loss of rod and cone photoreceptors, resulting in night blindness followed by loss of visual fields. The disease may result in legal or even complete blindness. Mutations in the retinitis pigmentosa GTPase regulator (RPGR) gene account for greater than 70% of the cases of human X-linked retinitis pigmentosa (XLRP), the most severe subtype of RP. See Beltran et al., 2012, PNAS 109(6): 2132-2137 and Bader et al., 2003, Invest. Ophthalmol. Vis. Sci. (44)4: 1458-1463. 
     Alternative splicing of the RPGR gene results in expression of multiple isoforms of the RPGR protein. The mRNA for isoform A contains all 19 exons of the gene, while the mRNA for isoform C contains exons 1 to 15 and a large part of intron 15. Intron 15 is a purine-rich region that contains highly repetitive sequences that code for glutamate and glycine repeats (EEEGEGEGE in human and EEGEGE in mouse), see Vervoort et al., Mutational hot spot within a new RPGR exon in X-linked retinitis pigmentosa. Nat Genet 2000; 25:462-6. Isoform A is constitutively expressed in all tissues while isoform C, which is also referred to as “ORF15”, is the predominant form expressed in the connecting cilium of photoreceptor, see Hong et al., Invest Ophthalmol Vis Sci 2002; 43:3373-82, and Hong et al., Invest Ophthalmol Vis Sci 2003; 44:2413-21. 
     A total of 55% of RPGR-related XLRP is caused by mutations in ORF15, all of which result from deletions that lead to truncated proteins. Most of the other cases are caused by mutations in exons 1-13, which can be either missense or nonsense mutations, with a small number caused by mutations in introns or large deletions. No cases have been identified due to mutations in exons 16 to 19. 
     Recent studies have demonstrated the potential of gene therapy approaches to treating XLRP caused by mutations in the RPGR gene. For example, Beltran et al. have shown that subretinal injections of adeno-associated virus (AAV) vectors expressing human RPGR increased rod and cone photoreceptor function in a canine model of XLRP. 
     However one of the challenges in large-scale production of AAV vectors for clinical use is that nucleic acid sequences encoding a protein of interest such as RPGR may be unstable, resulting in the accumulation of several mutations and deletions. For example, the RPGR gene contains a region of 1.2 kb called ORF15 near the 3′ end of the cDNA that is highly repetitive and GA rich. This region is a mutation “hot spot” in population. This repetitive region is very unstable during cloning and vector preparation and clones obtained generally contain mutations and deletions. These mutations can potentially alter or eliminate RPGR protein function, limiting the use of this protein in gene therapy applications. Therefore a need exists to identify methods of stabilizing RPGR cDNAs during large-scale production of AAV vectors. 
     SUMMARY OF THE INVENTION 
     It has been surprisingly found that the nucleic acid sequence of SEQ ID NO: 1 encoding the human RPGR protein is stable in large scale production of AAV plasmid pTR-IRBP-RPGRsyn. This nucleic acid sequence was developed through codon optimization of the wild type RPGR cDNA. In one aspect, the present invention provides a polynucleotide comprising the nucleic acid sequence of SEQ ID NO: 1 encoding a human RPGR protein. 
     In one aspect, the invention features a polynucleotide comprising the nucleic acid sequence of SEQ ID NO: 1 encoding a human retinitis pigmentosa GTPase regulator (RPGR) protein. 
     In one embodiments, the invention features an expression cassette comprising the polynucleotide of the above aspect, and an expression control sequence operably linked and heterologous to the nucleic acid sequence. 
     In another embodiment, the invention features a vector comprising the polynucleotide of claim  1 . In a further embodiment, the vector is a recombinant adeno-associated (rAAV) expression vector. 
     In another embodiment, the invention features a recombinant herpes simplex virus (rHSV) comprising the polynucleotide of any one of the above aspects. 
     In another embodiment, the invention features a host cell comprising the polynucleotide of any one of the above aspects. In a related embodiment, the host cell is a mammalian cell. In a further related embodiment, the host cell is a HeLa cell, a BHK21 cell or a Vero cell. In another further embodiment, the host cell is a V27 cell. 
     In another embodiment, the expression control sequence is a human interphotoreceptor retinoid-binding protein (IRBP) promoter. In a further related embodiment, the human IRBP promoter comprises a nucleic acid sequence having at least 95% sequence identity to the nucleic acid sequence of SEQ ID NO: 8 and directs preferential expression in rods and cones. In another further embodiment, the human IRBP promoter comprises the nucleic acid sequence of SEQ ID NO: 8. 
     In one embodiment, the polynucleotide comprises the nucleic acid sequence of SEQ ID NO: 7. 
     The invention also features in another embodiment a method of producing the rAAV expression vector of the above aspect, comprising (a) infecting a host cell with a recombinant herpes simplex virus (rHSV) comprising the nucleic acid sequence of SEQ ID NO: 1; (b) incubating the host cell; and (c) following incubation, collecting rAAV from the host cell of step (b). 
     In one embodiment, the host cell is a HeLa cell, a BHK21 cell or a Vero cell. 
     In another embodiment, the rHSV further comprises a human IRBP promoter operably linked to the nucleic acid sequence of SEQ ID NO: 1. In a further embodiment, the human IRBP promoter comprises a nucleic acid sequence having at least 95% sequence identity to the nucleic acid sequence of SEQ ID NO: 8 and directs preferential expression in rods and cones. In a further related embodiment, the human IRBP promoter comprises the nucleic acid sequence of SEQ ID NO: 8. 
     In another embodiment, the rHSV comprises the nucleic acid sequence of SEQ ID NO: 7. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1B  show a sequence alignment of codon optimized RPGR cDNA (RPGRsyn; SEQ ID NO: 1) and the wildtype RPGR cDNA (Genbank Accession No. NM_001034853; SEQ ID NO: 5). 
         FIG. 2  shows a map of plasmid pUC57-RPGRsyn. 
         FIG. 3  shows pUC57-RPGRsyn plasmid DNA clones N5 and N6 prepared by mini-prep and larger scale midi-prep (Midi) and digested with restriction enzymes NotI and PciI. Plasmid DNA from mini-preps was retransformed into SURE2 cells before larger scale production by midi-prep. 
         FIG. 4  shows pUC57-RPGRsyn plasmid DNA from mini-preps (mini_2 and mini_3) digested with restriction enzymes NotI and PciI. Plasmid DNA was not detectable in larger scale midi preps (midi_2 and midi_3). Seeding culture was stored at 4° C. overnight and used as the inoculant for larger scale plasmid production. 
         FIG. 5  shows a map of AAV proviral plasmid pTR-IRBP-RPGRsyn 
         FIG. 6  shows the restriction maps of pTR-IRBP-RPGRsyn plasmid DNA isolated from transformed bacteria after 4 rounds of serial overnight propagation, along with a control plasmid of pTR-IRBP-CNGB3co. Bacteria transformed with pTR-IRBP-RPGRsyn or pTR-IRBP/GNAT2-hCNGB3co plasmids were grown in medium at 37° C., overnight. In the next morning, plasmid DNA was purified from 1.5 mL of overnight culture, and the remaining culture was left at room temperature until late afternoon and then used to inoculate 2 mLs of culture medium (1:1000 dilution) for the 2 nd  round propagation. Same procedures were followed for the 3 rd  and 4 th  round of propagation. Plasmid DNA purified from each round were then analyzed by restriction digestion with Sma I to confirm the integrity of the ITR sequence of the plasmid. Restriction maps kept same for both pTR-IRBP-RPGRsyn and the control plasmid pTR-IRBP-CNGB3co, through the 3 rounds of propagation in bacteria. However, the yield was significantly decreased after 3 rd  round propagation and almost no plasmid restriction fragments were detected after 4 th  round propagation in bacteria. 
         FIG. 7  shows the sequence alignment of the consensus sequence of contigs obtained from pTR-IRBP-RPGRsyn plasmid DNA to the reference pTR-IRBP-RPGRsyn sequence. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention provides a polynucleotide comprising the nucleic acid sequence of SEQ ID NO: 1 encoding a human retinitis pigmentosa GTPase regulator (RPGR) protein. The nucleic acid sequence has been codon optimized for enhanced stability during vector replication, and may be used, for example, for production of adeno-associated virus (AAV) vectors for gene therapy applications. 
     Nucleic acid sequences may be codon optimized to improve stability or heterologous expression in host cells without changing the encoded amino acid sequence. For example, codon optimization may be used to remove sequences that negatively impact gene expression, transcript stability, protein expression or protein stability, such as transcription splice sites, DNA instability motifs, polyadenylation sites, secondary structure, AU-rich RNA elements, secondary ORFs, codon tandem repeats, or long range repeats. Codon optimization may also be used to adjust the G/C content of a sequence of interest. 
     A codon consists of a set of three nucleotides and encodes a specific amino acid or results in the termination of translation (i.e. stop codons). The genetic code is redundant in that multiple codons specify the same amino acid, i.e., there are a total of 61 codons encoding 20 amino acids. Codon optimization replaces codons present in a DNA sequence with preferred codons encoding the same amino acid, for example, codons preferred for mammalian expression. Thus, the amino acid sequence is not altered during the process. Codon optimization can be performed using gene optimization software. The codon optimized nucleotide sequence is translated and aligned to the original protein sequence to ensure that no changes were made to the amino acid sequence. For example, the nucleotide sequence of SEQ ID NO: 1 encoding human RPGR is a codon optimized version of the wild type human RPGR nucleotide sequence (Genbank Accession No. NM_001034853, SEQ ID NO: 5). Both SEQ ID NO: 1 and SEQ ID NO: 5 encode the same RPGR protein (SEQ ID NO: 6). 
     Methods of codon optimization are known in the art and are described, for example, in U.S. Application Publication No. 2008/0194511 and U.S. Pat. No. 6,114,148. 
     The nucleic acid sequences of the present invention can be made as synthetic sequences. Techniques for constructing synthetic nucleic acid sequences are known in the art, and synthetic gene sequences may be purchased from several companies, including DNA 2.0 (Menlo Park, Calif.) and GenScript USA Inc. (Piscataway, N.J.). Alternatively, codon changes can be introduced by standard molecular biology techniques such as site-specific in vitro mutagenesis, PCR, or any other genetic engineering methods known in art which are suitable for specifically changing a nucleic acid sequence. In vitro mutagenesis protocols are described, for example, in In Vitro Mutagenesis Protocols, Braman, ed., 2002, Humana Press, and in Sankaranarayanan, Protocols in Mutagenesis, 2001, Elsevier Science Ltd. 
     The human RPGR gene is located in chromosomal region Xp21.1 and spans 172 kb. Shu et al., 2012, Invest. Ophthalmol. Vis. Sci. 53(7): 3951-3958. There are multiple alternatively spliced transcripts, all of which encode an amino (N)-terminal RCC1-like (RCCL) domain. The RCCL domain is structurally similar to the RCC1 protein, a guanine nucleotide exchange factor for the small guanosine triphosphate-binding protein, Ran. The RPGR gene contains 19 exons (RPGRex1-19), encoding a predicted 90 kDa protein. Exons 2 to 11 encode the RCCL domain, whereas exons 12 to 19 encode a carboxyl (C)-terminal domain rich in acidic residues and ending in an isoprenylation anchorage signal. Mutations found in RPGRex1-19 account for 15% to 20% of XLRP patients, and subsequent studies revealed many more disease-causing mutations within one or more transcripts containing an alternatively spliced C-terminal exon called ORF15 (RPGRORF15). A high frequency of microdeletions, frameshift, and premature stop mutations are found within the ORF15. 
     In one embodiment, the RPGR cDNA used for codon optimization is the full-length human RPGRORF15 clone, variant C, Genbank Accession No. NM_001034853 (SEQ ID NO: 5). See Vervoort et al., 2000, Nat Genet 25: 462-466. This clone contains exons 1-ORF15 and was generated using three-way ligation by step-wise amplifying exons 1-part of 15b (nucleotides 169-1990) from human lymphocytes and 1991-3627 from human genomic DNA. See Beltran et al., 2012, PNAS 109(6): 2132-2137. 
     RPGR is widely expressed and shows a complex expression pattern. See Shu et al., cited above. RPGR transcripts are detected in different tissues, including brain, eye, kidney, lung, and testis in several different species. RPGR protein is detected in retina, trachea, brain, and testis. In human, mouse, and bovine retina, RPGR mainly localizes to photoreceptor connecting cilia, but expression has also been reported in outer segments in some species. RPGR is expressed in the transitional zone of motile cilia and within human and monkey cochlea. 
     The invention also provides an expression cassette comprising the nucleic acid sequence of SEQ ID NO: 1 and an expression control sequence operably linked and heterologous to the nucleic acid sequence. The term “expression control sequence” refers to any genetic element (e.g., polynucleotide sequence) that can exert a regulatory effect on the replication or expression (transcription or translation) of the nucleic acid sequence. Common expression control sequences include promoters, polyadenylation signals, transcription termination sequences, upstream regulatory domains, origins of replication, internal ribosome entry sites (IRES), and enhancers. 
     An expression control sequence is operably linked with a nucleic acid sequence when the expression control sequence is placed in a functional relationship with the second nucleic acid sequence. For example, a promoter is operably linked to a coding sequence if the promoter affects the expression of the coding sequence. The term operably linked encompasses, for example, an arrangement of an expression control sequence with the nucleic acid sequence to be expressed and optionally further expression control sequences, such as a terminator or enhancer, such that each of the expression control sequences can allow, modify, facilitate or otherwise influence expression of the nucleic acid sequence. 
     The term “heterologous” refers to nucleic acid or amino acid sequences that are obtained or derived from different source organisms or from different genes or proteins within the same source organism. For example, an expression control sequence that is not a native expression control sequence of the human RPGR gene is considered to be heterologous to the human RPGR gene. In certain embodiments, the expression control sequence is a promoter that is heterologous to the RPGR gene. 
     In a preferred embodiment, the expression control sequence is a human interphotoreceptor retinoid-binding protein (IRBP) promoter. IRBP is a large glycoprotein that is expressed only in the photoreceptor cells of the retina and to a much lesser extent in pinealocytes in the pineal gland in the brain. See Al-Ubaidi et al., 1992, J Cell Biology, 119(6) 1681-1687. The IRBP promoter region is well characterized. For example, Albini et al. (1990, Nucleic Acids Research 18(17): 5181-5187) describe a nucleotide sequence of the human IRBP promoter region (Genbank Accession No. X53044) containing 2818 bp of the 5′ untranscribed region (SEQ ID NO: 2). Beltran et al. (cited above) demonstrated that a 235 bp fragment of the human IRBP promoter directed GFP expression in both rods and cones of normal canine retina in a dose- and time-dependent manner. A 1.3 kb fragment of the 5′ untranslated region of the human IRBP gene (SEQ ID NO: 3) directed expression of a bacterial reporter gene (chloramphenicol acetyltransferase, CAT) specifically to photoreceptor cells in transgenic mice. See Al Ubaidi et al. 1992, J Cell Biology 119: 1681-1687. Nested deletion analysis of a 1783 bp fragment of the mouse IRBP 5′ flanking region indicated that high promoter activity was maintained with a fragment consisting of 70 bp 5′ to the transcription start site (SEQ ID NO: 4), but that elements upstream of this 70 bp fragment are required for complete tissue-specific regulation. See Boatright, et al., 1997, Molecular Vision 3: 15. 
     In a preferred embodiment, the human IRBP promoter comprises a nucleic acid sequence having at least 95%, 96%, 97%, 98% or 99% sequence identity to SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 4, or SEQ ID NO: 8. In a further preferred embodiment, the human IRBP promoter comprises SEQ ID NO: 8. 
     As used herein, the term “vector” refers to a nucleic acid molecule capable of transporting another nucleic acid to which it has been linked, e.g., a plasmid. Preferred vectors are those capable of autonomous replication and/or expression of nucleic acids to which they are linked. An “rAAV vector” is a recombinant vector that includes nucleic acid sequences derived from adeno-associated virus (AAV). Recombinant AAV is produced in vitro by introduction of gene constructs into cells known as producer cells. Recombinant AAV has been studied extensively as a vehicle for gene therapy and for its potential applicability as a treatment for human diseases based on genetic defects. At the clinical level, the rAAV vector has been used in human clinical trials to deliver the cftr gene to cystic fibrosis patients and the Factor IX gene to hemophilia patients (Flotte, et al., 1998, Methods Enzymol 292:717-732; and Wagner et al., 1998, Lancet 351:1702-1703). Systems for production of rAAV employ three elements: 1) a gene cassette containing the gene of interest, 2) a gene cassette containing AAV rep and cap genes and 3) a source of “helper” virus proteins. Methods of producing rAAV are known in the art and are described, for example, in U.S. Pat. No. 7,091,029. 
     Production of rAAV vectors for gene therapy is carried out in vitro, using suitable producer cell lines. A preferred cell line is 293, but production of rAAV can be achieved using other cell lines, including but not limited to human or monkey cell lines such as Vero, WI 38 and HeLa, and rodent cells, such as BHK cells, e.g. BHK21. 
     In particular embodiments, the rAAV comprises the nucleic acid sequence of SEQ ID NO: 1 encoding the human RPGR protein. The rAAV may further comprise one or more expression control sequences operably linked to the nucleic acid sequence of SEQ ID NO: 1. In a preferred embodiment, the expression control sequence is a human IRBP promoter. In a further preferred embodiment, the human IRBP promoter comprises a nucleic acid sequence having at least 95%, 96%, 97%, 98% or 99% sequence identity to the nucleic acid sequence of SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 4, or SEQ ID NO: 8 and directs preferential expression in rods and cones. In a particularly preferred embodiment, the human IRBP promoter comprises SEQ ID NO: 8. 
     In certain embodiments, the rAAV further comprises an SV40 poly A tail, an SV40 splice donor/splice acceptor (SD/SA) sequence, and a Kozak sequence, each operably linked to the nucleic acid sequence of SEQ ID NO: 1. In a preferred embodiment, the rAAV comprises the nucleic acid sequence of SEQ ID NO: 7. 
     One strategy for delivering all of the required elements for rAAV production to the producer cell line involves transfecting the cells with plasmids containing gene cassettes encoding the necessary gene products, as well as infection of the cells with the helper virus Ad to provide the helper functions. This system employs plasmids with two different gene cassettes. The first is a proviral plasmid encoding the recombinant DNA to be packaged as rAAV. The second is a plasmid encoding the rep and cap genes. Other DNA viruses, such as Herpes simplex virus type 1 (HSV-1) can be used instead of Ad to provide helper virus gene products needed for rAAV production (Conway et al., 1999, Gene Ther. 6:973-985). 
     Another strategy for rAAV production is based on the use of two or more recombinant rHSV-1 viruses to simultaneously co-infect producer cells with all of the components necessary for producing rAAV. This strategy employs at least two different forms of rHSV, each containing a different gene cassette. In addition to supplying the necessary helper functions, each of these rHSV viruses is engineered to deliver different AAV (and other) genes to the producer cells upon infection. The two rHSV forms are referred to as the “rHSV/rc virus” and the “rHSV expression virus.” The rHSV/rc virus contains a gene cassette in which the rep and cap genes from AAV are inserted into the HSV genome. The rep genes are responsible for replication and packaging of the rAAV genome in host cells infected with AAV. The cap genes encode proteins that comprise the capsid of the rAAV produced by the infected cells. 
     The second recombinant HSV is an “rHSV expression virus.” A usual element of an rAAV production system is an expression cassette containing transgene DNA sequences encoding a gene(s) of interest, such as the RPGR gene, along with promoter elements necessary for expression of the gene. In particular embodiments, the rHSV comprises the nucleic acid sequence of SEQ ID NO: 1 encoding the human RPGR protein. Expression vectors engineered for rAAV production are generally constructed with the gene of interest inserted between two AAV-2 inverted terminal repeats (ITRs). The ITRs are responsible for the ability of native AAV to insert its DNA into the genome of host cells upon infection or otherwise persist in the infected cells. The expression cassette is incorporated into the rHSV expression virus described above. This second rHSV virus is used for simultaneous co-infection of the cells along with the rHSV-1/rc virus. 
     The terms “recombinant HSV,” “rHSV,” “rHSV vector,” and “rHSV expression vector” refer to isolated, genetically modified forms of herpes simplex virus (HSV) containing heterologous genes incorporated into the viral genome. Methods for production of rHSV are known in the art and are described, for example, by Conway et al. (1999, Gene Ther. 6:973-985); Conway et al. (1997, J Virol 71: 8780-8789) and U.S. Pat. No. 7,037,723. 
     In particular embodiments, the rHSV comprises the nucleic acid sequence of SEQ ID NO: 1 encoding the human RPGR protein. The rHSV may further comprise one or more expression control sequences for regulating expression of the nucleic acid sequence of SEQ ID NO: 1, wherein the expression control sequence is operably linked to the nucleic acid sequence of SEQ ID NO: 1. In a preferred embodiment, the expression control sequence is a human IRBP promoter that is operably linked to the nucleic acid sequence of SEQ ID NO: 1. In a further preferred embodiment, the human IRBP promoter comprises a nucleic acid sequence having at least 95%, 96%, 97%, 98% or 99% sequence identity to the nucleic acid sequence of SEQ ID SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 4, or SEQ ID NO: 8 and directs preferential expression in rods and cones. In a particularly preferred embodiment, the human IRBP promoter comprises SEQ ID NO: 8. 
     In certain embodiments of the aforementioned methods, the rHSV further comprises an SV40 poly A tail, an SV40 splice donor/splice acceptor (SD/SA) sequence, and a Kozak sequence, each operably linked to the nucleic acid sequence of SEQ ID NO: 1. In a preferred embodiment, the rHSV comprises the nucleic acid sequence of SEQ ID NO: 7. 
     The invention also provides a method of producing an rAAV expression vector comprising a polynucleotide comprising the nucleic acid sequence of SEQ ID NO: 1 encoding a human RPGR protein. In one embodiment, the method comprises (a) infecting a host cell with a recombinant herpes simplex virus (rHSV) comprising the nucleic acid sequence of SEQ ID NO: 1; (b) incubating the host cell; and (c) following incubation, collecting rAAV from the host cell of step (b). 
     Methods of producing rAAV expression vectors by infecting a host cell with an rHSV are known in the art and are described for example in U.S. Pat. No. 7,091,029. For example, in one embodiment, the host cells are infected with rHSV by diluting the virus in growth medium such as DMEM and adding the virus to flasks containing the host cells. The host cells may be incubated with the virus for various intervals, for example, 22, 26, 30, 34, or 46 hours. Following the incubation interval, the virus-infected cells may be harvested by pelleting, followed by resuspension in DMEM. Cell-associated rAAV may be collected from the host cells by lysis of the cells using standard techniques involving three rounds of freezing and thawing (See Conway et al., 1999, cited above). 
     In particular embodiments, the host cell used for producing an rAAV expression vector in the aforementioned methods is a HeLa cell, a BHK21 cell or a Vero cell. 
     The rHSV used in the aforementioned method may further comprise one or more expression control sequences for regulating expression of the nucleic acid sequence of SEQ ID NO: 1 that is operably linked to the nucleic acid sequence of SEQ ID NO: 1. In a preferred embodiment, the expression control sequence is a human IRBP promoter that is operably linked to the nucleic acid sequence of SEQ ID NO: 1. In a further preferred embodiment, the human IRBP promoter comprises a nucleic acid sequence having at least 95%, 96%, 97%, 98% or 99% sequence identity to the nucleic acid sequence of SEQ ID NO: 2, SEQ ID NO: 3, SEQ ID NO: 4, or SEQ ID NO: 8 and directs preferential expression in rods and cones. In a particularly preferred embodiment, the human IRBP promoter comprises SEQ ID NO: 8. 
     In certain embodiments of the aforementioned methods, the rHSV further comprises an SV40 poly A tail, an SV40 splice donor/splice acceptor (SD/SA) sequence, and a Kozak sequence, each operably linked to the nucleic acid sequence of SEQ ID NO: 1. In a preferred embodiment, the rHSV comprises the nucleic acid sequence of SEQ ID NO: 7. 
     Description of Sequences 
       
     
       
         
           
               
               
             
               
                   
               
               
                 SEQ ID 
                   
               
               
                 NO: 
                 Description 
               
               
                   
               
             
            
               
                 1 
                 Codon modified RPGR cDNA 
               
               
                 2 
                 Human IRBP promoter, 2818 bp. Albini et al., 1990, Nuc 
               
               
                   
                 Acid Res 18: 5181-5187). SEQ ID NO: 2 comprises SEQ 
               
               
                   
                 ID NO: 3 and 4. 
               
               
                 3 
                 Human IRBP promoter, 1326 bp. Al Ubaidi et al. 1992, 
               
               
                   
                 J Cell Biology 119: 1681-1687 
               
               
                 4 
                 Mouse IRBP core promoter region, 70 bp. Boatright, 
               
               
                   
                 et al., 1997, Molecular Vision 3: 15. 
               
               
                 5 
                 Wildtype RPGR cDNA, Genbank Accession No. 
               
               
                   
                 NM_001034853 
               
               
                 6 
                 Wildtype RPGR amino acid sequence 
               
               
                 7 
                 3871 bp synthesized sequence comprising SEQ ID NO: 1, 
               
               
                   
                 an SV40 poly A tail, the SV40 SD/SA sequence, Kozak 
               
               
                   
                 sequence, and restriction sites 
               
               
                 8 
                 Human IRBP promoter, 234 bp fragment used in the 
               
               
                   
                 RPGRsyn expression cassette 
               
               
                   
               
            
           
         
       
     
     The following examples serve to illustrate certain embodiments and aspects of the present invention and are not to be construed as limiting the scope of the invention. 
     EXAMPLES 
     Example 1 
     Codon Optimization of the RPGR Gene and Evaluation of Plasmid Stability 
     A wildtype RPGR cDNA in an AAV plasmid used for AAV manufacturing was found to contain several mutations and deletions in the region from nt 2461 to nt 3057. There were a total of 42 bp accumulated deletions or substitutions across this region. The plasmid clone was found to be stable during plasmid propagation in bacteria, and no sequence changes were found in the AAV vector. 
     A 3459 bp coding sequence of the RPGR gene, variant C (SEQ ID NO: 5) was codon-optimized at Genscript, Inc. for mammalian expression. Codon optimization was used both to select codons of high frequency in mammals and to alter GC content to enhance stability, and to reduce the repetitive nature of the gene. The codon optimized version of the RPGR coding sequence (RPGRsyn; SEQ ID NO: 1) shares 72.1% sequence identity with the original gene (SEQ ID NO: 5). See  FIG. 1 . The codon optimized gene encodes the same polypeptide as the original gene, i.e. the polypeptide of SEQ ID NO: 6. The RPGRsyn gene was synthesized at GenScript along with an SV40 poly A tail, the SV40 SD/SA sequence, a Kozak sequence, and restriction sites for cloning purposes. The entire 3871 bp synthesized sequence is provided as SEQ ID NO: 7. 
     A map of the plasmid containing RPGRsyn (pUC57-RPGRsyn) is shown in  FIG. 2 . This plasmid was able to propagate stably in bacteria in small scale plasmid production. This plasmid also maintained its stability in larger scale production after being retransformed into SURE2 cells, a bacteria strain used for cloning of the AAV plasmid. See  FIG. 3 . Clone N5 of the pUC57-RPGRsyn plasmid DNA produced in large scale production was confirmed to be identical to the original plasmid by DNA sequencing. The plasmid yield could range from very low yield to none at all if the seeding culture was stored at 4° C. overnight and used as the inoculant for large scale plasmid production. See  FIG. 4 . The RPGRsyn cDNA was then released from pUC57-RPGRsyn plasmid and inserted into a pTR containing plasmid to generate the AAV proviral plasmid pTR-IRBP-RPGRsyn ( FIG. 5 ). pTR-IRBP-RPGRsyn contains inverted terminal repeats (ITR) of AAV2 and IRBP promoter. Large scale production of the plasmid confirmed to be 100% correct upon DNA sequencing ( FIG. 6 ). To further confirm the stability of pTR-IRBP-RPGRsyn, bacteria transformed with pTR-IRBP-RPGRsyn or pTR-IRBP/GNAT2-hCNGB3co plasmids were grown in medium at 37° C., overnight. In the next morning, plasmid DNA was purified from 1.5 mL of overnight culture, and the remaining culture was left at room temperature until late afternoon and then used to inoculate 2 mLs of culture medium (1:1000 dilution) for the 2 nd  round propagation. Same procedures were followed for the 3 rd  and 4 th  round propagation. Plasmid DNA purified from each round was then analyzed by restriction digestion with Sma I to confirm the integrity of the ITR sequence of the plasmid. As shown in  FIG. 6 , the yield of pTR-IRBP-RPGRsyn declined during the serial passages; however, the same pattern is observed for pTR-IRBP-CNGB3co, a plasmid that contains the stable hCNGB3 cDNA. Therefore, the decline of plasmid yield is related to bacteria itself or other features such as TR, but not to the RPGRsyn. Also noted in  FIG. 7 , the 4.2 kb band containing RPGRsyn has been stable over the passages (it will become loose or smear if unstable). 
     Example 2 
     Construction of AAV Plasmids and Evaluation in Bacteria 
     An AAV plasmid (pTR-IRBP-RPGRsyn) comprising an RPGRsyn expressing cassette comprising the IRBP promoter (234 bp), the RPGRsyn cDNA (SEQ ID NO: 1), and an SV40 polyA signal sequence is constructed. This IRBP fragment is contained within the 235 bp fragment used by Beltran et al. in the canine model (See Beltran et al., 2012, PNAS 109(6): 2132-2137). After construction of pTR-IRBP-RPGRsyn, the plasmid is tested for stability in bacteria using the methods described in Example 1. 
     Once the stability of pTR-IRBP-RPGRsyn is confirmed, an HSV recombination plasmid comprising the IRBP-RPGRsyn expression cassette (pHSV106-IRBP-RPGRsyn) is constructed. pHSV106-IRBP-RPGRsyn is used for construction of HSV-IRBP-RPGRsyn helper vector for large scale production of the AAV vector AAV-IRBP-RPGRsyn. The rHSV helper viruses are propagated in mammalian cells (V27, an ICP27-complementing Vero cell line). RPGRsyn cDNA is more stable in mammalian cells than in bacteria. This increased stability will eliminate the need for large-scale production of an AAV proviral plasmid containing the RPGRsyn cDNA, which is a reagent required for rAAV production by plasmid transfection methods.