TORONTO -- Researchers at SickKids hospital in Toronto have corrected a specific mutation through gene editing for the very first time in a live animal — a success that puts scientists one step closer to developing life-changing treatments for numerous rare diseases.

The research, which was published Tuesday in the scientific journal EMBO Molecular Medicine, describes how researchers copied the exact mutation of a patient with Duchenne muscular dystrophy (DMD) into a live mouse, and then were able to correct that mutation, essentially curing the mouse.

Dr. Ronald Cohn, principal investigator of the study and president and CEO of SickKids, told in a video interview that it was “really breathtaking and exciting.

“I want […] everybody to take a step back and think how unbelievably fascinating it is that we can even begin to think about how to fix the genetic mutation,” Cohn said.

“Because I can tell you, 10 years ago, if you would have told me there will be one day that technology just is going to fix the genetic mutation, I would have told you, ‘no, I don't think so.’ So we are in a completely new, exciting, different world.”

Researchers were able to correct a duplication mutation, which occurs when a section of DNA appears twice. It’s a particularly difficult mutation to recreate in order to do research, according to a press release, because scientists have to add DNA instead of merely taking away or copying genes.

One of the disorders that can be caused by duplication mutation is DMD, a progressive muscle disorder in which muscle mass is lost over time. It usually only affects boys, but in rare cases can affect girls. Children with this disorder generally lose the ability to walk at around 12 years old, Cohn said.

“It is a life-threatening disorder,” he said. “The biggest skeletal muscle you have is your diaphragm, that helps you breathe. So when that gets weak, you are compromised in your ability to breathe effectively. And it also affects your heart.”

Treatment has improved over the years so that those with this disorder can live into adulthood instead of dying by age 20, Cohn said, but there still is no cure for the disorder.

This new gene editing research could reach even beyond DMD though.

“If you look across the spectrum of our genetic disorders, about 10 per cent are caused by what we call the duplication mutation,” Cohn explained. “And the attractive aspect of our technology is not that it just applies to Duchenne muscular dystrophy, but the concept and the methodology behind it is really applicable to, theoretically, any duplication mutation.”


Cohn has been studying duplication mutations for years.

Around eight years ago, when scientists started to realize that the newly discovered CRISPR methodology could be applied to genome editing, Cohn’s laboratory shifted its focus completely to focus on the possibilities of this technology.

“CRISPR is a very precise methodology that pretty much allows us in a surgical way to perform cuts anywhere in the genome,” Cohn explained. “Cutting out some faulty information and then thereby trying to fix what has been faulty within the genome.”

He and his team quickly realized that CRISPR would be ideal for working with duplication mutations.

“It made sense to think about, if I have these genetic scissors, let's cut out the duplicated material.”

Around six years ago, they successfully used this technology to remove the duplication in patient cells “in a Petri dish situation,” Cohn explained.

The next step was to see if they could successfully apply this technique to a living creature and essentially fix their mutation. But in order to apply this method to a mouse model, researchers would have to first duplicate a genetic disorder within the mouse itself.

That’s where Gavriel came in.

The 19-year-old has been living with a DMD diagnosis since he was four and a half, and his family has been close friends with Cohn for more than a decade.

Gavriel’s mother, Kerry Rosenfeld, told in a video interview that after their son was diagnosed, she and her husband threw themselves into fundraising, researching the disorder and doctors who might have a chance of helping.

The family, who were living in the U.K. at the time, were referred multiple times to numerous experts. Along the way, they were introduced to Cohn, a meeting that sparked a deep friendship — and influenced Cohn’s research.

It was Gavriel who supplied the patient sample that allowed Cohn to replicate DMD in a mouse model.

“You can imagine that in addition to being a scientist, doing this work on the son of one of my closest friends is yet another level,” Cohn said.

After receiving this sample, Cohn and his team used CRISPR to recreate Gavriel’s mutation within an ordinary mouse.

Once the mouse had duplication mutations that were affecting its muscle, researchers injected the mouse again, this time with a “molecule of CRISPR” that was meant to cut out the mutation.

“[It] went into the heart and skeletal muscle of these mice [and] removed the duplications,” Cohn said. “And with that being successful, we didn't see all the damage in the muscle that the sick mouse had before. And the mouse became more stronger because the muscles were happy.”

Images of the mouse’s muscle cells while it was affected by DMD and after it had received the CRISPR treatment show a stark difference, with no sign of the weakened regions that existed before.

The gene’s normal function had been restored by the treatment.


When Gavriel was born, he seemed like any other child, his mother said.

“He could walk, he could run, he could play like everybody else. And when he became nine, 10, 11, he started to slow down, trip a lot, fall a lot, not keep up with his peers,” she said. “It was very, very traumatic. And then at the age of 11, 12, he lost the ability to walk altogether.”

She explained that when a person has DMD, their muscles don’t recover from use.

“You and I, when we walk, when we run, when we exercise, our muscles are damaged, but they're replenished. But every movement Gavriel makes damages the muscles and it's not replenished.”

Around sixper 100,000 people have DMD, according to the Muscular Dystrophy Association, meaning there could be up to 460,000 people affected by this worldwide.

But Gavriel’s specific mutation is even rarer.

A duplication mutation is just one of several mutations that can cause DMD, which occurs when a mutation affects the dystrophin gene, which is the longest gene in human DNA.

“Any problem on that gene causes a lack of dystrophin, which is the protein that stabilizes the muscles and the muscles’ function,” Rosenfeld explained.

Deletion mutations, where a portion of the gene is missing, are the most common type of mutation that causes DMD, Rosenfeld said, with only eight to10 per cent of the DMD community affected by duplication mutations.

This has meant that the majority of research into this field has not focused on duplication mutations.

The family set up the Duchenne Research Fund, a charity aimed at fundraising to support research into numerous scientific avenues that could aid those with DMD. Many years later, they also helped to fund Solid BioSciences, which focuses on developing gene therapies for DMD.

Rosenfeld said she felt like she became something close to an expert on DMD out of necessity, just trying to help her son. 

“[Gavriel] finds himself now, as a 19-year-old man, wheelchair bound for the last six or seven years, losing the ability to use his arms,” she said. “He finds it hard to brush his teeth. He finds it hard to hug us.”

In a virtual event run by the Duchenne Research Fund this week, Gavriel described having DMD as “like four extra subjects [at school].

“It’s mentally, physically draining, you don’t get any time off, there’s constantly assignments coming in,” he said. “No rest days, it comes with you into the shower, it comes with you into bed, it comes with you on holiday. There’s no respite.”


This research may have provided strong evidence that CRISPR can be used safely and effectively to correct duplication mutations in living creatures, but there’s still a long road ahead.

“It's hard to estimate this, but I think we are now at a stage where we are looking for partnership with an industry partner in order to take our research to the next level, develop this technology as a drug so that we can start to think about a clinical trial in humans,” Cohn said.

“Whether that is a process of two, three, four, or five years, I don't know, but in that timeframe, I would hope we would be able to launch some clinical trials.”

He acknowledged that success in an animal trial doesn’t guarantee success when research moves forward to humans.

“But we have done the necessary work to take the next step,” he said.

The research team will also be expanding their research in the meantime by trying this same technique with older mice, he said, “to see whether we can either stabilize the disease or whether there's even a possibility to reverse some of the [damage],” a question which could be crucial for those who are at a more advanced stage in the disease.

“I really do believe […] that in general, this technology, the genome editing technology, is going to change the way we would practice medicine in 10, 15 years. Before I retire, I hope,” Cohn said.

It’s bittersweet for Cohn and the Rosenfelds, knowing this new breakthrough will mean more for future generations than current ones.

“They've managed to make incredible advances, [but] we still have to be extremely cautiously optimistic about the impact of this,” Rosenfeld said. “I believe that it will be life-changing for the next generation of Duchenne patients, but I'm very clear that it's not going to be life changing for my son.”

She said her son was proud to have contributed in some way to Cohn’s research, which has the potential to help so many in the future.

“The legacy that will be left because of the bravery that [Gavriel] has showed will mean that [the experience of] patients and families that are diagnosed with Duchenne, maybe even just 10 years down the line, will be a different story,” she said.