DMD Exon Skipping: A Treatment for Muscular Dystrophy

Exon skipping is a molecular strategy that tricks the cellular machinery into ignoring a faulty segment of the dystrophin gene, allowing boys with Duchenne muscular dystrophy (DMD) to produce a shorter but partially functional version of the dystrophin protein. Three exon-skipping drugs have received accelerated approval from the FDA, each targeting a different mutation, and together they are amenable to roughly a third of all DMD patients. The approach does not cure the disease, but it converts the severe, progressive muscle wasting of Duchenne into something closer to the milder Becker form of muscular dystrophy, where patients retain mobility and cardiac function far longer.

Why a Missing Piece of Dystrophin Destroys Muscle

Dystrophin is a long, rod-shaped protein that acts like a shock absorber inside muscle fibers, connecting the internal skeleton of each cell to the membrane that surrounds it. The gene encoding dystrophin is enormous, spanning 79 exons, and that size makes it unusually vulnerable to deletions and other mutations. When a deletion knocks the gene’s reading frame out of alignment, the cell cannot produce any usable dystrophin at all. Without it, muscle fibers tear themselves apart with every contraction, triggering rounds of inflammation and replacement by scar tissue and fat. That is Duchenne.

Becker muscular dystrophy results from mutations in the same gene, but the reading frame stays intact. The protein that gets made is shorter than normal and missing an internal stretch, yet it still connects to the membrane and absorbs mechanical stress. The “reading frame hypothesis” predicts that Becker patients produce this semifunctional, internally deleted dystrophin, while Duchenne patients produce a severely truncated protein that falls apart before it can do anything useful.¹ The clinical difference is stark: boys with Duchenne typically lose the ability to walk by about age twelve, need a ventilator in their late teens, and face life-threatening cardiac or respiratory failure in their twenties or thirties. Many Becker patients remain ambulatory into adulthood and live substantially longer.²

How Exon Skipping Restores the Reading Frame

If the core problem in Duchenne is a broken reading frame, the therapeutic logic is straightforward: skip over the troublesome exon during the splicing step so the reading frame clicks back into place. Before a gene’s instructions get turned into protein, the cell copies the DNA into a preliminary RNA transcript, then splices out the non-coding stretches (introns) and stitches the coding stretches (exons) together. Short synthetic molecules called antisense oligonucleotides can bind to a specific exon in that preliminary transcript, hiding it from the splicing machinery so it gets left out along with the introns.

The earliest proof of this idea came from experiments in the late 1990s using antisense molecules aimed at the mdx mouse, a standard lab model for Duchenne. Researchers directed antisense molecules at the splice sites flanking exon 23 of the mouse dystrophin gene. Within 24 hours of treatment, essentially all dystrophin transcripts in the treated muscle cells were missing exon 23, and because that deletion does not disrupt the reading frame, the cells could make a shorter but presumably functional Becker-like dystrophin.³ This was the spark that launched clinical development.

The jump from mice to humans required choosing which human exons to target. Exon 51 was the first priority because skipping it would restore the reading frame for the largest single group of DMD deletions. Roughly 13% of Duchenne patients carry mutations amenable to exon 51 skipping.⁴ Researchers screened multiple antisense sequences and advanced the best candidates toward clinical trials.⁵

The Three Approved Drugs and What They Target

Three exon-skipping drugs are now approved in the United States, each built on the same phosphorodiamidate morpholino oligomer (PMO) backbone, a synthetic chemistry that resists degradation and avoids activating the immune system the way some older chemistries did. All three require weekly intravenous infusions.

Eteplirsen (exon 51): The first to market, approved under the FDA’s accelerated pathway. In the PROMOVI trial, patients treated for 96 weeks showed roughly a sevenfold increase in dystrophin protein over baseline. Compared with mutation-matched natural history controls, the decline on the six-minute walk test was about half as steep, and the annual decline in lung function was also significantly slower.⁶ Long-term data from nonambulatory patients confirmed that dystrophin expression was significantly higher than in untreated controls across all measurement methods.⁷
Viltolarsen (exon 53): Targets mutations amenable to exon 53 skipping, a different subset of patients. A retrospective study with over three years of follow-up found that three of four patients receiving weekly viltolarsen maintained motor function, while the majority of untreated patients deteriorated.⁸
Casimersen (exon 45): The newest of the three, targeting yet another subset of deletions. Early-phase trial data showed it was well tolerated, with most adverse events mild and unrelated to the drug, and drug levels in the blood scaled proportionally with dose without accumulating.⁹

Even with all three drugs combined, only patients whose specific mutation can be corrected by skipping exon 51, 53, or 45 are eligible. An updated analysis of a large DMD mutation database found that single or double exon-skipping strategies could theoretically apply to about 90% of all DMD mutations, but the vast majority of those would require drugs targeting exons for which no approved therapy yet exists.¹⁰ That gap between theoretical applicability and approved options is one of the biggest frustrations for families navigating treatment.

What Becker Patients Tell Us About Expected Outcomes

Because exon skipping essentially manufactures a Becker-type dystrophin, the clinical experience of Becker patients with matching deletions serves as a natural preview. Researchers have studied Becker patients whose internally deleted dystrophin is identical to what a Duchenne patient would produce after successful exon 51 skipping. Those Becker patients tend to have a mild disease course, which is encouraging for the therapy’s long-term potential.⁴ A broader clinical study confirmed that Becker patients with specific in-frame deletions corresponding to various exon-skipping targets generally have a more favorable prognosis, reinforcing the idea that restoring even an imperfect dystrophin can make a meaningful clinical difference.²

This modeling is not perfect, though. A Becker patient has produced that shortened dystrophin since birth, meaning their muscles never went through the severe degeneration-regeneration cycles that Duchenne muscles endure before treatment begins. A DMD patient who starts exon skipping at age five already carries years of accumulated muscle damage. The therapy can slow further decline and partially restore dystrophin, but it cannot reverse scarring and fat replacement that have already occurred. Timing matters enormously.

The Heart Problem

Cardiac failure is the leading cause of death in both Duchenne and Becker patients, so reaching the heart muscle is not optional. Unfortunately, the current generation of PMO-based drugs does a poor job of getting into cardiac tissue after intravenous infusion. They are taken up reasonably well by skeletal muscle, especially the limbs, but the heart sees much lower drug concentrations.

Animal research has shown that attaching short cell-penetrating peptides to the antisense molecule dramatically improves cardiac uptake. In dystrophic mice, peptide-conjugated antisense oligonucleotides achieved significantly higher exon-skipping levels in heart and diaphragm compared to unconjugated versions.¹¹ A different research group demonstrated that cyclic peptide conjugates doubled exon-skipping levels in both skeletal and cardiac muscle.¹² Peptide-conjugated PMOs (PPMOs) have shown effective exon skipping in target muscles and prolonged dystrophin restoration in animals.¹³ Several PPMOs are now in clinical development, and this next generation of drugs may be the first to offer meaningful cardiac protection.

Kidney Safety and Monitoring

All three approved exon-skipping drugs share a class-level safety signal around the kidneys. A systematic analysis of adverse event reports found that proteinuria, the leaking of protein into urine as a sign of kidney stress, was reported for viltolarsen, eteplirsen, and casimersen. Viltolarsen showed the strongest signal, and most adverse events appeared more than 60 days after starting treatment.¹⁴ The likely mechanism involves drug accumulation in the kidney’s tubular cells during normal filtration.

The renal concern is not limited to approved drugs. Drisapersen, an earlier exon 51 candidate built on a different chemistry (2′-O-methyl phosphorothioate), was ultimately abandoned in part because of a poor benefit-risk profile that included notable kidney toxicity. A more recent candidate, vesleteplirsen, has been associated with severe electrolyte imbalances linked to kidney dysfunction.¹⁵ The practical upshot for families: regular kidney monitoring through urine and blood tests is a non-negotiable part of treatment. Corticosteroid use, which is standard background therapy for DMD, can compound kidney stress and may need dose adjustments.

Tracking Treatment Response Without a Biopsy

Muscle biopsies are the gold standard for measuring dystrophin production, but repeatedly taking tissue samples from a child with already-damaged muscles is hard to justify clinically. Researchers have been pursuing two less invasive alternatives: blood-based biomarkers and quantitative MRI.

A group of small RNA molecules called dystromirs, specifically miR-1, miR-133, and miR-206, are released into the bloodstream when muscle fibers break down. In DMD patients these circulate at elevated levels, and in mouse models “cured” through exon skipping, the levels return toward normal.¹⁶ Animal studies of peptide-conjugated PMOs confirmed that these biomarkers track closely with both the level of exon skipping achieved and the amount of dystrophin restored, doing so in a dose-dependent manner.¹⁷ Translating this into human clinical use has been slower. An early human study of eteplirsen-treated patients did find elevated dystromirs that changed after treatment, but the small number of patients and variable individual responses meant the correlation between blood miRNA levels and actual dystrophin restoration was not conclusive.¹⁸

MRI offers a complementary window. Fat-water imaging can quantify how much of a muscle has been replaced by fat, and researchers have used it to compare treated and untreated DMD patients over time.¹⁹ In the upper limbs, fat quantification by MRI was sensitive enough to detect significant progression within six months in a small group of patients, suggesting it could shorten and shrink clinical trials that today rely on functional tests like walking speed.²⁰ For families, these tools may eventually reduce the need for invasive biopsies while giving clinicians a clearer picture of whether the drug is actually working.

Vectorized Exon Skipping With Gene Therapy

The biggest drawback of current PMO drugs is that they wash out of the body within days, requiring lifelong weekly infusions. One way around this is to deliver the antisense sequence not as a naked molecule but as a gene packed inside an adeno-associated virus (AAV) vector. The virus carries a small gene encoding a modified U7 small nuclear RNA linked to the antisense sequence. Once the vector infects a muscle fiber, the cell produces the antisense RNA on its own, continuously, without further drug administration.

This was first demonstrated in mice over two decades ago. A single injection of an AAV vector expressing an antisense U7 RNA achieved persistent exon skipping, sustained dystrophin production at near-normal levels across entire muscle groups, and correction of the muscular dystrophy phenotype.²¹ A follow-up study used the AAV9 capsid, which has a strong affinity for both skeletal and cardiac muscle, and showed rescue in a severely affected mouse model lacking both dystrophin and a compensatory protein called utrophin.²²

There is a catch. Dystrophic muscle fibers are inherently unstable, and the viral genomes they carry can be lost when fibers degenerate and are replaced. Research showed that AAV genomes encoding a therapeutic antisense RNA were maintained better than non-therapeutic controls, because restoring dystrophin stabilized the fibers. But when the dose was suboptimal and dystrophin restoration was incomplete, viral genomes were lost at a high rate. Over 12 months in severely dystrophic mice, dystrophin levels in skeletal muscle declined significantly as viral genomes disappeared, though heart muscle was spared because it cycles less aggressively.²³ The implication is clear: for gene-therapy-based exon skipping to work long-term, the initial dose has to be high enough to restore enough dystrophin to protect the fibers that carry the vector.

CRISPR as a Permanent Fix

Where antisense drugs and AAV-U7 vectors work at the RNA level, leaving the DNA untouched, CRISPR-Cas9 can edit the gene itself. By cutting the DNA near a problematic exon, the cell’s own repair processes can permanently remove that exon from the gene. In principle, a single treatment could correct the mutation in every transduced cell for life, with no ongoing RNA-level intervention required.

A recent study delivered CRISPR-Cas9 intravenously via AAV to mice carrying a deletion of exons 52 through 54, a model that develops early-onset heart disease. A single-cut approach designed to skip exon 55 outperformed a dual-guide strategy that tried to excise the exon entirely, and it completely prevented the onset of cardiac dysfunction in treated mice through twelve weeks of observation.²⁴ That cardiac protection is especially noteworthy given how poorly the current PMO drugs reach the heart.

CRISPR-based exon skipping is still preclinical. Open questions include off-target edits, immune responses to the bacterial Cas9 protein, and whether enough muscle cells can be edited in a full-sized human body. But the concept bridges two worlds: it borrows the exon-skipping logic that already has clinical proof of concept and pairs it with the permanence of a DNA edit.

How Exon Skipping Compares to Micro-Dystrophin Gene Therapy

Micro-dystrophin gene therapy takes a different approach entirely. Instead of fixing the patient’s own gene, it delivers a miniaturized but rationally designed dystrophin gene via AAV. The advantage is that every treated patient receives the same optimized protein, and the design can be tweaked to maximize function rather than being dictated by whichever exons happen to surround the patient’s deletion.²⁵ Several micro-dystrophin programs are in advanced clinical trials.

The trade-off is that AAV gene therapy is a one-shot deal, at least with current technology, because the immune system mounts a strong response against the viral capsid after the first infusion. If the dose is insufficient or the effect wanes, you cannot simply re-dose. Exon-skipping drugs, by contrast, are given continuously and can be dose-adjusted. They also avoid the immune complications of AAV, though they come with their own renal safety concerns. Many researchers expect the future of DMD treatment to involve combinations: perhaps gene therapy for initial dystrophin restoration, with exon skipping or other strategies available as a backup or complement.

Practical Realities for Families

Genetic testing is the gateway to exon-skipping therapy. A confirmed genetic diagnosis specifying the exact deletion or mutation is required before a child can be matched to one of the three approved drugs. Families sometimes receive a clinical diagnosis of DMD based on symptoms and elevated creatine kinase without detailed genotyping, and they should push for full sequencing as early as possible. Time lost before starting treatment is muscle lost permanently.

The treatment itself demands a significant commitment. Weekly intravenous infusions typically take about an hour, plus travel time to an infusion center, and the drugs carry list prices well above a hundred thousand dollars per year. Insurance coverage varies, and the accelerated-approval status of these drugs has sparked ongoing debate about whether the clinical evidence of benefit is strong enough to justify the cost. Some payers require documentation of dystrophin production via muscle biopsy before authorizing continued treatment.

Corticosteroids remain the backbone of DMD management regardless of exon-skipping treatment. Deflazacort or prednisone slows muscle degeneration through anti-inflammatory effects and is typically started around age four or five. Exon skipping is layered on top of, not substituted for, steroid therapy. Physical therapy, cardiac monitoring, and respiratory support round out a comprehensive care plan. Exon skipping adds an important tool to that arsenal, but it has not replaced any of the existing ones.