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potentially be rescued by multiple exon-skipping, which employs multiple DNA-
like molecules as DNA band-aids to skip over the parts of the mutated gene that
block the effective creation of proteins. Synthetic DNA analogs show outstanding
stability and sequence specifi city yet little or no binding to modulator proteins. An
antisense RNA delivered by a retroviral vector in animal models of DMD not only
is capable of inhibiting mutant myotonic dystrophy protein kinase transcripts but
also can ameliorate dystrophic muscle pathology at the cellular levels. Systemic
delivery of the AAV construct results in effective body-wide colonization, signifi -
cant recovery of the functional properties in vivo, and lower creatine kinase serum
levels, suggesting an overall decrease in muscle wasting. The transduced muscles
rescue dystrophin expression and display a signifi cant recovery of function toward
the normal values at single muscle fi ber level. This approach provides rationale for
systemic use of AAV-mediated antisense-U1 small nuclear RNA expression for the
treatment of DMD.
Development of antisense oligonucleotides with higher stability and lower toxic-
ity, such as morpholinos, has made it possible to restore dystrophin effi ciently in
dystrophic mice in vivo with no obvious side effects. Effi cacy and toxicity of intra-
venous antisense oligonucleotide (morpholino)-induced exon skipping has been
tested in the DMD dog model (Yokota et al. 2009 ). Weekly or biweekly systemic
intravenous injections with a three-morpholino cocktail over the course of 5–22
weeks induced therapeutic levels of dystrophin expression throughout the body,
with an average of about 26 % normal levels. This was accompanied by reduced
infl ammatory signals examined by MRI and histology, improved or stabilized timed
running tests, and clinical symptoms. Blood tests indicated no evidence of toxicity.
This study provides a proof of concept for systemic multiexon-skipping therapy. By
skipping more than a single exon, this so-called DNA band-aid becomes applicable
to between 80 % and 90 % DMD patients, including the mutation found in dogs.
This study makes exon-skipping as a systemic treatment for DMD in humans a real
possibility in the near term. Signifi cant challenges still remain. Successful systemic
treatment with morpholinos requires large doses of the antisense molecules and the
technology is costly and diffi cult to obtain. Additionally, treatment in this study
showed diminished success at curbing muscle deterioration of the heart, meaning
that a more effective and specifi c delivery system is needed to rescue the organ’s
delicate tissue in DMD patients.
Exon skipping is not inextricable bound up with splicing regulatory sequences as
the binding of an antisense oligoribonucleotide to sequences within the exon is suf-
fi cient to induce exon skipping. This implies that probably most exons in the genome
are skippable and that exon skipping could be applicable to the majority of muta-
tions, including deletions, duplications, or nonsense mutations in in-frame exons.
Antisense-mediated exon skipping therapy is being developed as personalized
therapy for DMD. Drisapersen (Prosensa Therapeutics BV) and eteplirsen, two
chemically distinct drug candidates, are currently in clinical development. Their
specifi c physicochemical characteristics each have their advantages and disadvan-
tages with regard to safety and pharmacokinetics. Both candidates demonstrated
specifi c exon 51 skipping to increase muscle dystrophin expression in a mutational
subgroup of patients with DMD, and both showed promising effects on the
16 Personalized Management of Genetic Disorders