09.2018 | THE SCIENTIST 41immune reactivity against the bacterial Cas9 protein or the viral
vector, for example. Preexisting immunity can exclude certain
people from the treatment, while patients who develop antibod-
ies after treatment may only have a single shot at the therapy.
The timing of the Duchenne treatment will also be crucial.
“The earlier the intervention, the more effective this therapy can
be,” says Olson. “If one waits till very late in the disease when
there’s a loss of muscle tissue and there’s nothing left to fix, then
that’s going to be a tougher clinical challenge.”
But the biggest challenge is likely to be getting the therapy
into patients, says Long. Treating Duchenne requires correcting
both skeletal muscle—to restore mobility—and cardiac muscle,
to prevent heart failure. This will require systemic delivery of
any therapy through the blood. That’s worked so far in mice, but
it remains to be seen how well it will work in humans. “Delivery
into patients is a huge, huge challenge,” Long says.
Despite the long road ahead, these cutting-edge Duchenne
therapies have advanced at a blistering pace over the past few
years, and the future looks bright. “ There’s no question that
there will be challenges coming, but I’ve never been more opti-
mistic about something,” says Olson. “I think it’s not unreason-
able to imagine that we could get into patients in a few years.”
And Duchenne could be just the beginning. There are hun-
dreds of incurable muscle and heart disorders that are caused
by mutations in a single gene. “Duchenne is the obvious test
bed,” says Gersbach, “but the hope is that technologies devel-
oped for Duchenne can be applied for other myopathies.” g
Sandeep Ravindran is a freelance science writer living in New
York City.
References
- C. Long et al., “Prevention of muscular dystrophy in mice by CRISPR/Cas9–
mediated editing of germline D N A ,” Science, 345:1184–88, 2014. - C. Long et al., “Postnatal genome editing partially restores dystrophin expression
in a mouse model of muscular dystrophy,” Science, 351:400–403, 2016. - C.E. Nelson et al., “In vivo genome editing improves muscle function in a
mouse model of Duchenne muscular dystrophy,” Science, 351:403–407, 2016. - M. Tabebordbar et al., “In vivo gene editing in dystrophic mouse muscle and
muscle stem cells,” Science, 351:407–11, 2016. - L. Amoasii et al., “Single-cut genome editing restores dystrophin expression in
a new mouse model of muscular dystrophy,” Sci Transl Med, 9:eaan8081, 2017. - C. Long et al., “Correction of diverse muscular dystrophy mutations in
human engineered heart muscle by single-site genome editing,” Sci Adv,
4:eaap9004, 2018.
BEYOND DUCHENNE
In 2015, Ronald Cohn used CRISPR to remove duplicated exons,
restoring the full-length dystrophin protein in cells from a Duchenne
patient. Now, the chief pediatrician at the Hospital for Sick Children
in Toronto is testing whether CRISPR can treat another myopathy,
congenital muscular dystrophy type 1A (MDC1A).
MDC1A affects one in 150,000 people worldwide and is caused
by a mutation in a gene called laminin alpha 2. It can lead to severe
muscle wasting, paralysis, and death by the age of 30. Unlike the
gene for dystrophin, laminin alpha 2 lacks redundant regions, so
researchers can’t just excise or skip mutated exons or create a
truncated but functional form of the protein.
In a mouse model of MDC1A, Cohn and his team used AAV9-
delivered CRISPR to make two strategic cuts to repair a mutation
in a splice site, thus restoring a skipped exon and the full-length
protein. The treatment significantly improved the mice’s muscle
strength and function (Nat Med, 23:984–89, 2017). Cohn hopes to
test this treatment in MDC1A patients in the next few years, and
sees even more potential in the future.
“It’s in theory applicable to any disease with a splice-site
mutation, and there are about 50,000 splice-site mutations
associated with diseases,” he says. Cohn is currently testing the
technique on tissues from Duchenne patients with rare splice site
mutations, with promising results.
Other myopathies may also benefit from modern technologies.
Molecular biologist Jocelyn Laporte at the Institute of Genetics
and Molecular and Cellular Biology in France has been working
for decades on rare and severe muscle disorders known as
centronuclear myopathies, which can lead to death in the first
year or two of life. Laporte and his team noticed increased levels
of dynamin 2 in mice lacking the disease-associated gene Mtm1,
and in muscle biopsies from centronuclear myopathy patients.
The researchers used genetic crosses to produce Mtm1 knockout
mice with only one Dnm2 allele (the double knockout is lethal),
reducing dynamin 2 by 50 percent and successfully improving both
the muscle function and longevity of diseased mice (J Clin Invest,
124:1350–63, 2014).
Injecting mice early in life with antisense oligonucleotides or
small hairpin RNAs to reduce dynamin 2 expression prevented
disease development (Nat Commun, 8:15661, 2017). Injecting
them later reversed some symptoms, including decreased muscle
strength and mass (Mol Ther, 26:1082–92, 2018). “Reducing
dynamin 2 really seems to have therapeutic potential,” says Belinda
Cowling, a former postdoc with Laporte and currently head of
research at biotech company Dynacure, launched in 2016 to
translate the work to the clinic.
Laporte and Cowling hope to start clinical trials in 2019
using antisense oligonucleotides to reduce human dynamin
2 expression. “It’s quite an exciting moment to be working on
centronuclear myopathies,” says Cowling. “And given its role
in membrane remodeling, you could imagine that dynamin 2
may play a role in other diseases, so targeting it could also have
broader therapeutic potential.”The hope is that technologies
devel oped for Duchenne can be
applied for other myopathies.
—Charles Gersbach, Duke University