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© STEVE GRAEPEL

THE GENETICS OF DUCHENNE

Duchenne muscular dystrophy results from mutations in theDMD

gene that encodes the dystrophin protein. There are many types

of mutations that can cause the disease; each disrupts the reading

frame such that translation terminates prematurely, producing no

functional dystrophin protein. DMD mutations are particularly com-

mon in “hotspot” areas of the gene (exons 45–55 and 2–10).

fied in Duchenne patients worldwide, so one of the huge chal-

lenges has been to devise a strategy that would allow you to con-

solidate large numbers of patients with different mutations and

correct them with a common method,” he says. “We believe we

can, in principle, correct somewhere between 60 and 80 per-

cent of Duchenne mutations using this single-cut gene-edit-

ing strategy.”

From pipette to patient

If all goes well, researchers will soon test Duchenne gene edit-

ing in nonhuman primates, and then in human patients. Gers-

bach is collaborating with Sarepta Therapeutics to develop

CRISPR gene editing for boys with the condition, while Olson’s

work has led to the creation of Exonics Therapeutics, a biotech

focused on designing gene editing therapies for Duchenne and

other neuromuscular diseases.

Scaling up to accommodate the size of human muscles will

be difficult. Skeletal muscle accounts for about 40 percent of

human male body weight (closer to 30 percent in females).

Treating such a large amount of tissue requires a big dose of the

nuclease and delivery vector, which presents both manufactur-

ing and safety issues. High doses can exacerbate the potential for

Because the genetic underpinnings of Duchenne are known,

researchers can devise gene-editing fi xes to the problem. Several

potential treatments are now being tested in preclinical and clinical

studies. In some cases, they aim to correct the mutations in the

DMD gene; other times, the goal is simply to restore a shorter

but still partially functional dystrophin protein.

Example: A mutation in exon 44 disrupts the reading frame and causes

translation to stop prematurely, leading to a dysfunctional protein.

Exon deletion: Making cuts on either side of the mutated exon to remove it can

restore the gene’s reading frame.

STOP

STOP

1-43 44 45-79

STOP

STOP

STOP

Strategies for fi xing the problem

Exon reframing: Creating small deletions to repair mutated exon can also restore

the reading frame.

Exon skipping: The mutated exon can be skipped during mRNA processing

by editing the splicing site preceding the exon.

43 44 45 43 45

43 44 45 43 44 45

43 44 45 43 44 45

43 45 46 43 44 45 46

44

Wildtype DMD gene

Full protein

translated

Point mutation or small deletion or duplication

Duchenne-causing mutations

STOP

STOP

STOP

1-34 35 36-79

1-43 45-79

Deletion of one or more exons

Duplication of one or more exons

1-2 2 3-79

Splice-site mutation (in the genomic region preceding an exon)

Exon 43 not included in mRNA

STOP

1-42 44-79

1-42 43 44-79

1-79

Exon knock-in: If an exon is missing entirely, it can be knocked back in to restore

the complete gene sequence.