Armed with base editors, scientists from the laboratory of David Liu, PhD, professor and director of the Merkin Institute of Transformative Technologies in Healthcare at the Broad Institute, and their collaborators elsewhere, successfully made changes to the defective gene responsible for Huntington’s disease and Friedreich’s ataxia. Their results are described in a Nature Genetics paper published this week titled “Base editing of trinucleotide repeats that cause Huntington’s disease and Friedreich’s ataxia reduces somatic repeat expansions in patient cells and in mice.”
Huntington’s disease and Friedreich’s ataxia are part of a group of neurological disorders dubbed trinucleotide repeat disorders because they are caused by repeating three-letter stretches of DNA. Past a certain length, these sequences lengthen uncontrollably leading to brain cell death and the breakdown of nerve fibers. In the paper, the scientists explained that they introduced single base changes in the middle of the repeated stretches of DNA. Their results showed that edited sequences in both patient cells and mouse models stayed the same length or became shorter over time.
More work will be needed to catalog the potential side effects of installing these edits in the genome, but the researchers believe that their approach could be a valuable tool. “A lot more studies would be needed before we can know if disrupting these repeats with a base editor could be a viable therapeutic strategy to treat patients,” said Liu. “But being able to illuminate the biological consequences of interrupted repeats is a really useful and important milestone.”
Roughly one in 3,000 people has a disease caused by three-letter repeats in DNA. Individuals with the same disease can inherit different numbers of DNA repeats, and people with more repeats generally experience symptoms sooner and their disease progresses more quickly. Some patients have naturally occurring single-letter ‘interruptions’ within these repeats, and experience milder symptoms that may develop later. These individuals are also less likely to pass their disease along to their children than people with uninterrupted repeats.
This insight gave the scientists an idea. If they could use a base editor to install an interruption that mimics a naturally occurring mutation observed in some patients, they might be able to prevent the repeating DNA sequence from expanding and slow the progression of the disease. To that end, they devised a system using cytosine and adenine base editors that changed some GC pairs to AT pairs within a CAG repeat tract for Huntington’s disease. Another edited several AT pairs to GC pairs within the GAA repeats of Friedreich’s ataxia. To deliver the base editors, the researchers packaged them into dual AAV9 vectors.
“Not only does this study show for the first time that inducing interruptions has a profound stabilizing effect on repeats, but that the base-editing approach we’ve used can also be applied to study any of over a dozen repeat disorders,” said Mandana Arbab, PhD, who was a postdoctoral researcher in Liu’s lab at the time of the study and is now an assistant professor at Boston Children’s Hospital. “There’s still a lot of work to be done, but we’re hopeful that this approach could really accelerate therapeutic development for a lot of these diseases.”
One challenge is that because these repeat sequences also occur elsewhere in the genome, it’s possible that base editors can make edits in those parts of the genome, raising the possibility of unwanted side effects. So far, Liu said his team has found that most off-target editing occurred between genes or in parts of the genome that do not encode proteins. The researchers plan to study these effects in cell populations and animal models that more faithfully mirror human disease. Separately, Liu and his colleagues are working on using prime editors to replace disease-causing repeat tracts with a shorter, stable number of repeats.
The post Base Editors Curb Repeating DNA in Huntington’s and Ataxia Models appeared first on GEN - Genetic Engineering and Biotechnology News.
Huntington’s disease and Friedreich’s ataxia are part of a group of neurological disorders dubbed trinucleotide repeat disorders because they are caused by repeating three-letter stretches of DNA. Past a certain length, these sequences lengthen uncontrollably leading to brain cell death and the breakdown of nerve fibers. In the paper, the scientists explained that they introduced single base changes in the middle of the repeated stretches of DNA. Their results showed that edited sequences in both patient cells and mouse models stayed the same length or became shorter over time.
More work will be needed to catalog the potential side effects of installing these edits in the genome, but the researchers believe that their approach could be a valuable tool. “A lot more studies would be needed before we can know if disrupting these repeats with a base editor could be a viable therapeutic strategy to treat patients,” said Liu. “But being able to illuminate the biological consequences of interrupted repeats is a really useful and important milestone.”
Roughly one in 3,000 people has a disease caused by three-letter repeats in DNA. Individuals with the same disease can inherit different numbers of DNA repeats, and people with more repeats generally experience symptoms sooner and their disease progresses more quickly. Some patients have naturally occurring single-letter ‘interruptions’ within these repeats, and experience milder symptoms that may develop later. These individuals are also less likely to pass their disease along to their children than people with uninterrupted repeats.
This insight gave the scientists an idea. If they could use a base editor to install an interruption that mimics a naturally occurring mutation observed in some patients, they might be able to prevent the repeating DNA sequence from expanding and slow the progression of the disease. To that end, they devised a system using cytosine and adenine base editors that changed some GC pairs to AT pairs within a CAG repeat tract for Huntington’s disease. Another edited several AT pairs to GC pairs within the GAA repeats of Friedreich’s ataxia. To deliver the base editors, the researchers packaged them into dual AAV9 vectors.
“Not only does this study show for the first time that inducing interruptions has a profound stabilizing effect on repeats, but that the base-editing approach we’ve used can also be applied to study any of over a dozen repeat disorders,” said Mandana Arbab, PhD, who was a postdoctoral researcher in Liu’s lab at the time of the study and is now an assistant professor at Boston Children’s Hospital. “There’s still a lot of work to be done, but we’re hopeful that this approach could really accelerate therapeutic development for a lot of these diseases.”
One challenge is that because these repeat sequences also occur elsewhere in the genome, it’s possible that base editors can make edits in those parts of the genome, raising the possibility of unwanted side effects. So far, Liu said his team has found that most off-target editing occurred between genes or in parts of the genome that do not encode proteins. The researchers plan to study these effects in cell populations and animal models that more faithfully mirror human disease. Separately, Liu and his colleagues are working on using prime editors to replace disease-causing repeat tracts with a shorter, stable number of repeats.
The post Base Editors Curb Repeating DNA in Huntington’s and Ataxia Models appeared first on GEN - Genetic Engineering and Biotechnology News.