Much like the cardiologist who does bypass surgery to redirect blood flow around damaged coronary arteries, researchers are developing a gene therapy for Duchenne’s muscular dystrophy that bypasses the genetic mutations that cause the disease. The treatment relies on antisense technology, the delivery of small pieces of synthetic DNA or RNA that block the action of mutated gene segments.

The goal of antisense therapy in Duchenne’s muscular dystrophy is to bypass the mutation responsible for the inadequate dystrophin expression that eventually leads to muscle wasting and replacement with adipose and connective tissue. In fact, this has already been achieved in dystrophic mice with intramuscular injections of a 2-O-methylated phosphorothioated antisense oligoribonucleotide (2OMeAO).

Antisense therapy restored normal dystrophin expression in about 20% of the mice’s muscle fibers and increased their muscle strength to about 70% of normal, effects that appeared quickly and persisted for up to two months. “In principle you could cure a large proportion of patients with Duchenne’s muscular dystrophy just by skipping one exon,” Terence A. Partridge, PhD, an investigator and Head of the Muscle Cell Biology Group in the MRC Clinical Sciences Centre at the Imperial College School of Medicine in London told NEUROLOGY REVIEWS. His research team’s study appeared in the August Nature Medicine.

A MOUSE MODEL

Dr. Partridge and colleagues tested 2OMeAO on mice that were divided into three age-groups—2 weeks, about 4 weeks, and 6 months. All of the mice carried a mutation in the dystrophin gene that causes Duchenne’s muscular dystrophy in boys.

The investigators could only achieve normal dystrophin expression in the mice by administering 2OMeAO with the nonionic block copolymer F127, an inactive excipient widely used for drug delivery. “We injected this mixture into each tibialis anterior muscle … using the sense oligonucleotide or saline as controls in contralateral muscles,” Dr. Partridge and colleagues noted.

The dystrophin that the mice subsequently produced presumably lacked exon 23, since this was found to be absent from the mRNA by reverse-transcription polymerase chain reaction and the dystrophin protein contained all of the other exons for which antibody was available. This dystrophin also accumulated all of the accessory proteins that it usually interacts with in the muscle. “So, the dystrophin seemed functional and as complete as we could ascertain,” Dr. Partridge surmised.

EFFICACY, SAFETY, ADMINISTRATION

The number of dystrophin-positive muscle fibers was similar in the three age-groups. However, dystrophin expression in fibers around the injection site was more homogeneous in 2-week-old mice than in older mice.

While physiologic tests of the mice’s muscles showed large gains in absolute and relative strength, antisense therapy failed to make the muscles more resistant to damaging high-intensity exercise, as the investigators had hoped. Inducing a long series of maximal muscle contractions simulated such exercise, explained Dr. Partridge.

“The antisense oligonucleotide seems to be very safe stuff,” he emphasized, pointing out that there was no evidence of an immune response to the treatment and that the F127 administered with it produced no toxicity. “In fact, F127 seems not to be toxic at much higher concentrations than we used,” he related.

An advantage of 2OMeAO compared to most other gene therapies for Duchenne’s muscular dystrophy is that it is quite small, making it a good candidate for systemic delivery. “That is important,” Dr. Partridge said, “because we would need to administer it regularly and it is impossible to inject all of the difficult-to-get-at muscles in Duchenne’s muscular dystrophy patients. If systemic delivery works and has no severe toxic effect in animals, we could begin to look at some initial safety and efficacy trials in humans.”

NR

—Timothy Begany

Suggested Reading
Brun C, Suter D, Pauli C, et al. U7 snRNAs induce correction of mutated dystrophin pre-mRNA by exon skipping. Cell Mol Life Sci. 2003;60:557-566.
Dunant P, Larochelle N, Thirion C, et al. Expression of dystrophin driven by the 1.35-kb MCK promoter ameliorates muscular dystrophy in fast, but not in slow muscles of transgenic mdx mice. Mol Ther. 2003;8:80-89.
Gilbert R, Dudley RW, Liu AB, et al. Prolonged dystrophin expression and functional correction of mdx mouse muscle following gene transfer with a helper-dependent (gutted) adenovirus-encoding murine dystrophin. Hum Mol Genet. 2003;12:1287-1299.
Lu QL, Mann CJ, Lou F, et al. Functional amounts of dystrophin produced by skipping the mutated exon in the mdx dystrophic mouse. Nat Med. 2003;Jul 6 [Epub ahead of print].

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