Skipping improves muscles

Small antisense molecules offer hope for Duchenne muscular dystrophy gene therapy | By Cathy Holding

The delivery of therapeutic sequences for the treatment of inherited genetic diseases such as Duchenne muscular dystrophy (DMD) has encountered problems such as failure to distribute beyond sites of injection and undesirable immune responses. The use of small molecules that actively affect transcription of endogenous genomic sequences has been investigated as an alternative approach to virus-delivered gene therapies and the introduction of exogenous proteins, as it offers the possibility of systemic delivery.

Antisense oligonucleotides to the boundary sequences of exon and intron 23 of the dystrophin gene in the mouse model of DMD (mdx) have previously been shown to result in successful splicing-out of the mutant exon and production of an in-frame smaller dystrophin protein in cell lines, but previous attempts to use these oligos in vivo were thwarted by apparent low levels of shortened dystrophin production. In the July 7 advanced online publication of Nature Medicine, Qi Long Lu and colleagues at the Medical Research Council Clinical Sciences Centre included the widely used drug delivery agent F127 and showed the successful production of the shorter dystrophin protein at normal levels together with improved muscle function throughout the tibialis anterior muscle in mdx mice following a single injection into the muscle (Nature Medicine, DOI:10.1038/nm897, July 7, 2003).

Following injection, Lu et al. immunostained and examined muscle fibers and observed that large numbers contained normal levels of dystrophin. They then used exon-specific antibodies to demonstrate that the molecule contained all exons except 23. The level of the shortened dystrophin molecule peaked 2 to 4 weeks after injection and persisted for over 2 months. Following a second injection after 3 months, additional muscle fibers were observed to express the dystrophin protein: the administrations did not induce an immune response or a diminution of efficiency with age of mouse.

"As a practical therapy, antisense oligonucleotides have the problem of limited duration of effect and would thus have to be administered at regular intervals… so clinical trials should await further improvements in efficiency that would permit systemic vascular delivery," the authors point out. Nevertheless, "our data establishes the realistic practicality of an approach that is applicable, in principle, to a majority of cases of severe dystrophinopathy," they conclude.