Long-term administration of antisense oligonucleotides into the paraspinal muscles of mdx mice reduces kyphosis

The mdx mouse model of muscular dystrophy has a premature stop codon preventing production of dystrophin. This results in a progressive phenotype causing centronucleation of skeletal muscle fibers, muscle weakness and fibrosis and kyphosis. Antisense oligonucleotides alter RNA splicing to exclude the nonsense mutation, while still maintaining the open reading frame to produce a shorter, but partially functional dystrophin protein that should ameliorate the extent of pathology. The present study investigated the benefits of chronic treatment of mdx mice by once-monthly deep intramuscular injections of antisense oligonucleotides into paraspinal muscles. After 8 months of treatment, mdx mice had reduced development of kyphosis relative to untreated mdx mice, a benefit that was retained until completion of the study at 18 months of age (16 months of treatment). This was accompanied by reduced centronucleation in the latissimus dorsi and intercostals muscles and reduced fibrosis in the diaphragm and latissimus dorsi. These benefits were accompanied by a significant increase in dystrophin production. In conclusion, chronic antisense oligonucleotide treatment provides clear and ongoing benefits to paralumbar skeletal muscle, with associated marked reduction in kyphosis

Alterations in dihydropyridine receptors in dystrophin-deficient cardiac muscle

The deficiency of dystrophin, a critical membrane stabilising protein, in the mdx mouse causes an elevation in intracellular calcium in myocytes. One mechanism that could elicit increases in intracellular calcium is enhanced influx via the L-type calcium channels. This study investigated the effects of the dihydropyridines BayK 8644 and nifedipine and alterations in dihydropyridine receptors in dystrophin-deficient mdx hearts. A lower force of contraction and a reduced potency of extracellular calcium (P<0.05) was evident in mdx left atria. The dihydropyridine agonist Bay K 8644 and antagonist nifedipine had 2.7 and 1.9 fold lower potencies in contracting left atria (P<0.05). This corresponded with a 2.0 fold reduction in dihydropyridine receptor affinity evident from radioligand binding studies of mdx ventricular homogenates (P<0.05). Increased ventricular dihydropyridine receptor protein was evident from both radioligand binding studies and Western blots and was accompanied by increased mRNA levels (P<0.05). Patch clamp studies in isolated ventricular myocytes showed no change in L-type calcium current density, but revealed delayed channel inactivation (P<0.05). This study indicates that a deficiency of dystrophin leads to changes in dihydropyridine receptors and L-type calcium channel properties that may contribute to enhanced calcium influx. Increased influx is a potential mechanism for the calcium overload observed in dystrophin deficient cardiac muscle