Prevention of dystrophic pathology in severely affected dystrophin/utrophin-deficient mice by morpholino-oligomer-mediated exon-skipping

Abstract

Duchenne muscular dystrophy (DMD) is a severe neuromuscular disorder caused by mutations in the dystrophin gene that result in the absence of functional protein. Antisense-mediated exon-skipping is one of the most promising approaches for the treatment of DMD because of its capacity to correct the reading frame and restore dystrophin expression, which has been demonstrated in vitro and in vivo. In particular, peptide-conjugated phosphorodiamidate morpholino oligomers (PPMOs) have recently been shown to induce widespread high levels of dystrophin expression in the mdx mouse model. Here, we report the efficiency of the PPMO-mediated exon-skipping approach in the utrophin/dystrophin double-knockout mouse (dKO) mouse, which is a much more severe and progressive mouse model of DMD. Repeated intraperitoneal (i.p.) injections of a PPMO targeted to exon 23 of dystrophin pre-mRNA in dKO mice induce a near-normal level of dystrophin expression in all muscles examined, except for the cardiac muscle, resulting in a considerable improvement of their muscle function and dystrophic pathology. These findings suggest great potential for PPMOs in systemic treatment of the DMD phenotype.

Figure 1

Dystrophin expression in tissues from dKO mice following six once-weekly intraperitoneal injections of PPMO at 25 mg/kg/week. (a) Reverse transcriptase-PCR analysis to detect exon 23 skipping efficiency at the RNA level. The 901-bp product represents the full-length transcript, and the products of 688 and 542 bp represent transcripts that exclude exon 23 and exons 22 and 23, respectively. (b) Western blot to detect dystrophin expression in tissues from PPMO-treated dKO mice, compared with untreated dKO and C57BL6 control mice (top gel). Equal loading of 50-µg protein is shown for each sample with α-actinin expression detected as loading control (bottom gel). (c) Immunostaining of muscle tissue cross-sections to detect dystrophin expression and localization in C57BL6 normal control mice (left panel), untreated dKO mice (middle panel), and PPMO-treated mice (right panel; N = 5). Muscle tissues analyzed were from tibialis anterior (TA), gastrocnemius, quadriceps, biceps, diaphragm, and heart muscles. Bar = 100 µm. dKO, double-knockout; PPMO, peptide-conjugated phosphorodiamidate morpholino oligomer.

Figure 2

Expression of dystrophin restores the dystrophin-associated protein complex (DAPC) to the sarcolemma in the PPMO-treated dKO mice. DAPC components α- and β-sarcoglycan, β-dystroglycan, and nNOS were detected by immunostaining in tissue cross-section of tibialis anterior (TA) muscles from PPMO-treated dKO mice (right panel), compared with C57BL6 normal mice (left panel) and untreated dKO mice (middle panel). Bar = 100 µm. dKO, double-knockout; nNOS, neuronal nitric oxide synthase; PPMO, peptide-conjugated phosphorodiamidate morpholino oligomer.

Figure 3

PPMO treatment improves muscle histopathology in dKO mice. (a) The upper panel shows immunofluorescent staining for dystrophin (green) and nuclei (blue) of tibialis anterior cross-sections from C57BL6 normal control mice (left), untreated dKO mice (middle) and PPMO-treated mice (right). The lower panel shows hematoxylin and eosin staining of the same muscles. (b) Percentage of myofibers with centronucleation (N ≈ 1,000 myofibers/cohort). (c) Box plots showing variance of the muscle fiber cross-sectional area (N ≈ 1,000/cohort). Boxes represent the middle quartiles from the 25th to the 75th percentiles, and the bar demonstrates the high and low values. ***P < 0.001 when compared to untreated dKO mice. CN, centronucleation; dKO, double-knockout; PPMO, peptide-conjugated phosphorodiamidate morpholino oligomer.

Figure 4

PPMO treatment averts the onset of dystrophic pathology in the dKO mice. (a) Photograph of an untreated dKO mouse at 10 weeks of age, displaying a strong kyphosis and joint contractures compared with (b) a PPMO-treated mouse at 10 weeks of age, looking healthy. (c) Measurement of serum creatine kinase (CK) levels as an index of ongoing muscle membrane instability in PPMO-treated dKO mice (N = 5) compared with untreated dKO mice (N = 9; ***P < 0.005) and C57BL6 normal control mice (N = 6). dKO, double-knockout; PPMO, peptide-conjugated phosphorodiamidate morpholino oligomer.

Figure 5

PPMO treatment improves muscle function in dKO mice. (a) Forelimb muscle function was assessed using a functional grip strength test to determine the physical improvement of PPMO-treated dKO mice compared with untreated dKO mice and C57BL6 normal control mice (N = 6/cohort). ***P < 0.005 compared to untreated dKO mice and no significant difference to C57BL6 control mice. (b) Extensor digitorum longus (EDL) muscles of PPMO-treated dKO mice were analyzed for their maximal force (peak force) producing capacity compared to untreated dKO mice and C57BL6 control mice. (c) Maximal force was also normalized for cross-sectional area to assess specific force. **P < 0.01 compared to untreated dKO mice and no significant difference to C57BL6. (d) The percentage of Force drop is assessed by measuring the force deficit following a series of five eccentric contractions. *P < 0.05 compared to untreated dKO mice and no significant difference to C57BL6 control mice. Error bars are shown as mean ± SEM (N = 5/cohort). dKO, double-knockout; PPMO, peptide-conjugated phosphorodiamidate morpholino oligomer.