Guanine analogues enhance antisense oligonucleotide-induced exon skipping in dystrophin gene in vitro and in vivo

Abstract

Exon skipping has demonstrated great potential for treating Duchenne muscular dystrophy (DMD) and other diseases. We have developed a drug-screening system using C2C12 myoblasts expressing a reporter green fluorescent phosphate (GFP), with its reading frame disrupted by the insertion of a targeted dystrophin exon. A library of 2,000 compounds (Spectrum collection; Microsource Discovery System) was screened to identify drugs capable of skipping targeted dystrophin exons or enhancing the exon-skipping effect by specific antisense oligomers. The 6-thioguanine (6TG) was effective for inducing skipping of both human dystrophin exon 50 (hDysE50) and mouse dystrophin exon 23 (mDysE23) in the cell culture systems and increased exon skipping efficiency (more than threefolds) when used in combination with phosphorodiamidate morpholino oligomers (PMO) in both myoblasts and myotubes. Guanine and its analogues were unable to induce detectable skipping of exon 23 when used alone but enhanced PMO-induced exon skipping significantly (approximately two times) in the muscles of dystrophic mdx mouse in vivo. Our results demonstrate that small-molecule compounds could enhance specific exon skipping synergistically with antisense oligomers for experimental therapy to human diseases.

Figure 1

Graphic illustration of the GFP-reporter vectors for dystrophin exon skipping. (a) Linear vector structure shows that the GFP sequence is split into two regions, 5′-GFP and 3′-GFP, by the mouse dystrophin intron (In31) that contains the 213-bp mouse exon (E23), 3′ side of intron (In22), and 5′ side of intron (In23). (b) Expression of GFP is driven by MCK promoters. Without AON intervention, the vector expresses an out-of-frame GFP/exon 23 chimeric transcript (normal transcript). (c) Skipping exon 23 with AONs restores the GFP-reading frame and expression of GFP protein. (d) Linear vector structure shows that the GFP sequence is split into two regions, 5′-GFP and 3′-GFP, by the human β-globin intron sequence (β-G), which contains the 109-bp human dystrophin exon 50 (E50), flanked by intron 49 (In49) on the 5′ side and intron 50 (In50) on the 3′ side. (e) Expression of GFP is driven by a β-actin promoter. Without AON intervention, the vector expresses the out-of-frame GFP/exon 50 chimeric transcripts (normal transcripts). (f) Skipping exon 50 with AON restores the GFP-reading frame and expression of the protein. Other structures of the vectors are not shown here. AON, antisense oligonucleotide; GFP, green fluorescent phosphate; MCK, muscle creatine kinase.

Figure 2

GFP expression in C2C12E50 cells after 6TG and PMOE50 treatment. (a) Fluorescence microscope images and (b) FACS analysis of the C2C12 and C2C12E50 cells treated with DMSO, 30, 60, and 90 µmol/l 6TG only. (c) Fluorescence microscope images and (d) FACS analysis of the C2C12 and C2C12E50 cells treated with DMSO, 30, 60, and 90 µmol/l 6TG in the presence of 0.3 µmol/l PMOE50. M2 is the population of cells gated for the expression of GFP in b and d. (e) RT-PCR for exon 50 skipping in the C2C12E50 cells. The 318-bp bands are the chimeric transcripts containing exon 50; the 109-bp bands are the transcripts with exon 50 skipped that are clearly detected only in the cells treated with PMOE50, together with 60- and 90-µmol/l 6TG. AON, antisense oligonucleotide; DMSO, dimethyl sulfoxide; FACS, fluorescence-activated cell sorting; GFP, green fluorescent phosphate; MK, size marker; NC, C2C12 cells as negative controls; RT, reverse transcription; 6TG, 6-thioguanine.

Figure 3

GFP expression in C2C12E23 cells treated with 6TG in differentiation media. The cells were cultured in differentiation medium for 3 days, and myotube formation was observed. (a) Fluorescence detection for GFP expression. The cells in the left column were treated with 0, 0.2, 0.4, and 0.6 µmol/l PMOE23 only. The cells in the middle and right columns were treated with 60 and 90 µmol/l 6TG, together with the concentration of PMOE23 marked on the left side of the panel. Strongest GFP expression was observed in the cells treated with both 0.6-µmol/l PMOE23 and 90 µmol/l 6TG (bottom-right column). (b) RT-PCR for exon 23 skipping. Three different doses of 6TG were used in combination with 0, 0.2, 0.4, and 0.6 µmol/l of PMOE23. The 424 bp bands are the transcripts containing exon 23; the 213 bp bands are the transcripts with exon 23 skipped. The numbers listed under the gel image are relative percentage of the exon 23 skipped mRNA (213 bp) to the total levels of unskipped mRNA (424 bp) based on the density measurement by NIH ImageJ. GFP, green fluorescent phosphate; RT, reverse transcription; 6TG, 6-thioguanine.

Figure 4

Synergistic effect of guanine analogues and PMOE50 on inducing exon 50 skipping. C2C12 (as control cells) and C2C12E50 were treated with (a) 2,4,6-triaminoquinazoline and (b) 2,6-dithiopurine at three different doses. All samples were treated with 0.5 µmol/l PMOE50.

Figure 5

Synergistic effect of guanine analogues and PMOE23 on dystrophin expression in TA muscles of mdx mice. (a) Immunohistochemistry for dystrophin expression. Control, untreated TA muscle; PMOE23 only, muscle treated with 2 µg PMOE23 only; The rest of the muscles were treated with both 2-µg PMOE23 and following compounds at 1- and 10-µg concentration. (b) Number of dystrophin-positive fibers after treatment with 2-µg PMOE23 with and without guanine analogues. The numbers of dystrophin-positive fibers were counted in a single cross-section. * and **, significant (P < 0.05) and very significant (P < 0.01) by Student's t-tests (n = 4) when compared to the PMOE23 treatment only. (c) RT-PCR demonstrates the skipping of mDysE23. E22–E23–E24 indicates normal dystrophin transcripts, and E22–E24 indicates transcripts with mDysE23 skipped. mdx control, saline-treated TA muscle of the mdx mouse. (d) Western blots demonstrate the expression of dystrophin protein. Dys, dystrophin detected with monoclonal antibody Dys 1. α-actin was used as a loading control. 2,6-dith, 2,6-dithiopurine; guanine, guanine hydrochloride; RT, reverse transcription; 2,4,6-tri, 2,4,6-triaminoquinazoline; 6TG, 6-thioguanine.