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Braun. J Transl Genet Genom. 2025;9:35-47 https://dx.doi.org/10.20517/jtgg.2024.79 Page 37

invasion of fibrotic and fatty tissues (by transdifferentiation of muscle cells, activation of fibro-adipogenic
[9]
progenitors, or chemoattraction of non-muscular cells) , which all create physical and physiological
barriers that impede the distribution and efficacy of vector particles. To maximize therapeutic outcomes,
intervention must be timed carefully-early enough to prevent irreversible remodeling, yet not so early as to
encounter the dilutive effects linked to both regenerative cycles and the rapid muscle growth of children,
where muscle mass can increase up to 20-fold after birth. Additionally, the transduction efficiency may be
influenced by patient age, as vector biodistribution in skeletal muscles seems to be more favorable at earlier
[10]
ages .
DMD is a chronic inflammatory disease, which is an important limiting factor. Mononucleated cell
infiltrates (activated cells that can phagocytose foreign particles) can hinder the ability of vector particles to
reach muscle fibers. These infiltrates also pose a risk for inflammatory/immunological overreactions to the
administration of vector particles, which are mainly viral. The risk of a robust innate immune response is
increased due to the high doses to be administered, not only against the vector but also against previously
unrecognized dystrophin epitopes. These immune responses may vary across patients . In particular,
[11]
patients with deletions in the most immunogenic areas of dystrophin, who are “naïve” to this protein, would
be at risk of immunological rejection . This concern is amplified by the fact that dystrophin is expressed
[12]
throughout the body, especially in vital tissues such as the respiratory muscles, heart, and blood vessels.
While a humoral response may not be problematic - since dystrophin is an intracellular protein and thus
inaccessible to antibodies - a cellular immune response must be avoided at all costs.

GENE THERAPIES OF DMD
Gene therapy for DMD is complicated by the complex genotype-phenotype relationships associated with
the disease. Thousands of different mutations in DMD have been found in DMD patients, including those
with Becker muscular dystrophy (BMD), a milder form of the disease. The majority of these mutations
(60%-70%) are deletions, 5%-15% are duplications (20% in BMD), and 20% are point mutations (10% in
BMD). Deletions and duplications predominantly occur in hotspot regions between exons 45-55 and 3-9,
affecting 47% and 7% of patients, respectively [2,13,14] . While some trends in genotype-phenotype correlations
have been observed, other factors - such as genetic modifiers, inflammation, and fibrosis invasion - may also
influence disease progression, making therapy evaluation particularly challenging. The introduction of
molecular therapies would further add to this variability .
[15]

Several therapeutic technologies have been approved or are under development to target specific dystrophin
mutations at the DNA or (pre)-messenger RNA levels. A first targeted therapy conditionally approved by
the European Medical Agency (EMA) for DMD was Ataluren, based on pharmacological readthrough of
premature stop codons during protein translation. Although it has not yet been approved by the US Food
and Drug Administration (FDA), the EMA recommended in June 2024 that the conditional authorization
granted to Ataluren in 2014 not be renewed due to insufficient evidence supporting the medicine’s
effectiveness .
[16]
RNA-based therapies
At the pre-messenger RNA level, the conversion of out-of-frame (leading to DMD) to in-frame deletions
(associated with milder forms) can be achieved by exon skipping (with oligonucleotides or U7snRNPs
expressing an antisense sequence) or by exon deletions (with genome editing) leading to partially functional
BMD-like truncated dystrophins. This treatment strategy is mutation-dependent and each individual
exon-skipping strategy is only applicable to a small subset of the DMD patients. Nevertheless, collectively,
with multiple medicinal products, the exon skipping strategy could potentially treat ~60% to up to 90% of