Long-term restoration of cardiac dystrophin expression in golden retriever muscular dystrophy following rAAV6-mediated exon skipping

Abstract

Although restoration of dystrophin expression via exon skipping in both cardiac and skeletal muscle has been successfully demonstrated in the mdx mouse, restoration of cardiac dystrophin expression in large animal models of Duchenne muscular dystrophy (DMD) has proven to be a challenge. In large animals, investigators have focused on using intravenous injection of antisense oligonucleotides (AO) to mediate exon skipping. In this study, we sought to optimize restoration of cardiac dystrophin expression in the golden retriever muscular dystrophy (GRMD) model using percutaneous transendocardial delivery of recombinant AAV6 (rAAV6) to deliver a modified U7 small nuclear RNA (snRNA) carrying antisense sequence to target the exon splicing enhancers of exons 6 and 8 and correct the disrupted reading frame. We demonstrate restoration of cardiac dystrophin expression at 13 months confirmed by reverse transcription-PCR (RT-PCR) and immunoblot as well as membrane localization by immunohistochemistry. This was accompanied by improved cardiac function as assessed by cardiac magnetic resonance imaging (MRI). Percutaneous transendocardial delivery of rAAV6 expressing a modified U7 exon skipping construct is a safe, effective method for restoration of dystrophin expression and improvement of cardiac function in the GRMD canine and may be easily translatable to human DMD patients.

Figure 1

Taqman analysis of vector genome distribution in the heart. (a) For analysis, biopsies were collected from the LVFW and IVS from base to apex as depicted. (b) Biodistribution of vector genomes throughout the heart in individual high- and low- dose GRMD canines. Error bars represent standard deviations. Ant, anterior; gc, genome copies; GRMD, golden retriever muscular dystrophy; IVS, interventricular septum; Lat, lateral; LVFW, left ventricular free wall.

Figure 2

Description of exon skipping strategy for restoration of dystrophin expression in the GRMD canine. (a) The dystrophin gene is large, containing 79 exons. The primary RNA transcript is in normal canines is depicted schematically. For simplicity, we have focused on the region affected by the GRMD mutation. (b) Splicing of the normal primary transcript yields normal mRNA that is translated into normal dystrophin protein. (c) GRMD is caused by a point mutation in dystrophin in the acceptor splice site of intron 6. (d) Splicing of this mutant primary transcript yields an mRNA with exon 6 fused directly to exon 8. The end result is a nonsense mutation with a premature STOP in exon 8. No functional dystrophin is produced from this out-of-frame transcript. (e) Exon skipping is a strategy that can be used to restore the open reading frame, and thus dystrophin expression, in GRMD. In this study, we used rAAV6 vector to express antisense oligonucleotides (AO) to target the exon splicing enhancers (ESE) in exons 6 and 8. By inducing skipping of exons 6 and 8 in this manner, we were able to restore the open reading frame with production of a slightly truncated but functional dystrophin. It should be noted that the major product of this exon skipping strategy is an mRNA with exon 5 fused directly to exon 10 (also in-frame), as cryptic splicing of exon 9 occurs with high frequency. The resultant protein contains 75/79 of the original exons. AAV, adeno-associated virus; GRMD, golden retriever muscular dystrophy; mRNA, messenger RNA; rAAV6, recombinant AAV6.

Figure 3

RT-PCR analysis of exon skipping in GRMD canines treated with rAAV6-U7-ESE6/8. The GRMD mutation causes loss of exon 7, producing an out-of-frame transcript. Treatment with ESE6/8 will cause splicing of exon 5 to exon 9 or exon 10 (as cryptic skipping of exon 9 is a well-described phenomenon), both of which are in-frame transcripts. For analysis, biopsies were collected from the LVFW and IVS from base to apex as depicted in Figure 1a. Representative gel images are shown from GRMD canines treated with (a,b) 1.8 × 10¹² gc/kg of rAAV6-U7-ESE6/8 and euthanized 13 months later (n = 3), and (c,d) 1.4 × 10¹³ gc/kg of rAAV6-U7-ESE6/8 and euthanized 13 months later (n = 2). Samples were considered positive for exon skipping if they contained the 490 bp band indicative of in-frame splicing of exon 5 to exon 10, as depicted in the diagram adjacent to each gel image. Samples were considered negative for exon skipping if they contained only the 953 bp band representing loss of exon 7 secondary to the GRMD mutation. Note that a greater number of samples were positive for exon skipping following treatment with high-dose rAAV6 compared to lower doses of rAAV6. (e) Sequencing of the 490 bp band confirmed splicing of exon 5 to exon 10, which produces an in-frame transcript. (f) Quantitative analysis confirmed that the ratio of in-frame skipped product to unskipped product was higher in canines treated with high-versus low-dose rAAV6. AAV, adeno-associated virus; GRMD, golden retriever muscular dystrophy; IVS, interventricular septum; LVFW, left ventricular free wall; rAAV76, recombinant AAV6; RT-PCR, reverse transcription-PCR.

Figure 4

Western blot analysis of dystrophin expression in GRMD canines treated with rAAV6-U7-ESE6/8. For analysis, biopsies were collected from the LVFW and IVS from base to apex as depicted in Figure 1a. Normal, untreated unaffected canine (positive control); untreated, untreated affected GRMD canine (negative control). (a) Representative blots are shown from GRMD canines treated with either 1.8 × 10¹² gc/kg of rAAV6-U7-ESE6/8 (GRMD 1, GRMD 2) or 1.4 × 10¹³ gc/kg of rAAV6-U7-ESE6/8 (GRMD 3, GRMD 4) and euthanized 13 months later. Note that a greater number of samples were positive for dystrophin following treatment with high-dose rAAV6 compared to low-dose rAAV6. (b) Quantitative, densitometric analysis of each western blot was performed. Dystrophin expression was higher in GRMD canines treated with high-dose rAAV6 (15–20% of normal) versus low-dose rAAV6 (3–5% of normal). AAV, adeno-associated virus; Dys, dystrophin; GAPDH, glyceraldehyde 3-phosphate dehydrogenase; GRMD, golden retriever muscular dystrophy; IVS, interventricular septum; LVFW, left ventricular free wall; rAAV6, recombinant AAV6; Untr, untreated GRMD canine.

Figure 5

Immunohistochemical (IHC) staining for dystrophin in GRMD canines treated with rAAV6-U7-ESE6/8. For analysis, sections were prepared from the LVFW and IVS from base to apex as depicted in Figure 1a. (a) Images from normal (untreated, unaffected) (positive control) and untreated (untreated, affected GRMD) (negative control) are shown for reference. Representative images are shown from GRMD canines treated with (b) 5 × 10¹¹ gc/kg of rAAV6-U7-ESE6/8 and euthanized 2 months later (n = 1), (c) 1.8 × 10¹² gc/kg of rAAV6-U7-ESE6/8 and euthanized 13 months later (n = 3), and (d) 1.4 × 10¹³ gc/kg of rAAV6-U7-ESE6/8 and euthanized 13 months later (n = 2). (e) The inset demonstrates membrane localization of dystrophin in an image taken at high power (×40 objective). Bar, 200 µm. Note that a greater area of myocardium is positive for dystrophin expression following treatment with high-dose rAAV6 compared to the lower doses of rAAV6. AAV, adeno-associated virus; Ant, anterior; GRMD, golden retriever muscular dystrophy; IVS, interventricular septum; Lat, lateral; LVFW, left ventricular free wall; rAAV6, recombinant AAV6.

Figure 6

Analysis of fibrosis in GRMD canines treated with rAAV6-U7-ESE6/8. Myocardium was co-stained for dystrophin and collagen I. Representative slides are shown from GRMD canines treated with 1.4 × 10¹³ gc/kg of rAAV6-U7-ESE6/8 and euthanized 13 months later. An untreated GRMD is shown as control. Note that collagen content is high when dystrophin staining is low or absent. The white arrow depicts collagen staining in a blood vessel wall. Bar, 200 µm. AAV, adeno-associated virus; GRMD, golden retriever muscular dystrophy; rAAV6, recombinant AAV6.

Figure 7

Cardiac MRI assessment of function. (a) Left ventricular myocardium in the short-axis view was divided into six equal segments with the first segment placed in the anterior region. (b) The slice analyzed was located in the mid-papillary region and (c) the average peak circumferential strain (Ecc) was measured in the treated and untreated GRMD canines. (d) Note that both segment 5 and average Ecc are significant if the animal with the lowest dystrophin expression is removed from analysis. *P < 0.05. Error bars represent standard deviations. GRMD, golden retriever muscular dystrophy; MRI, magnetic resonance imaging.