Chinese researchers have made a major advance in the science of gene editing by developing a new way to treat diseases that cannot be cured through existing methods.
Their new gene editing tools, known as mitochondrial DNA base editors, can fix errors in the DNA inside a cell鈥檚 mitochondria. The scientists used these editors to create rats with diseases caused by these mitochondrial DNA errors and succeeded in making the first direct corrections of these flaws in animals.
These mitochondrial DNA base editors could lead to gene therapies for serious conditions that currently lack curative therapies, according to the team led by researchers from East China Normal University and Lingang Laboratory.
鈥淓fficient generation and correction of mutations in mitochondrial DNA (mtDNA) is challenging,鈥 the researchers said in the first of two papers published in the peer-reviewed journal Nature Biotechnology on June 3.
Mitochondrial DNA mutations have been associated with several genetic diseases in humans. These include Leber hereditary optic neuropathy, which causes vision loss; Leigh syndrome, a neurological disorder; and mitochondrial encephalomyopathy, which can cause stroke-like episodes and muscle weakness.
There is currently no cure for mtDNA diseases, which affect around one in 5,000 people worldwide and can lead to serious symptoms and premature death.
CRISPR-based therapies have become a viable option to treat diseases caused by mutations in the DNA inside a cell鈥檚 nucleus. However, the structure of the mitochondria makes it difficult to apply this technology to manipulate mitochondrial DNA.
The main type of mutations that lead to mtDNA diseases are called specific point mutations. This refers to a change, deletion or addition of a single nucleotide base, or the blocks that make up DNA 鈥 adenine, thymine, cytosine and guanine.
The clinical manifestation of mtDNA diseases often requires more than 50 per cent of a person鈥檚 mtDNA variants to have the mutation. This is not only a challenge for disease treatment but also for creating animal models to mimic these diseases as it requires editing a lot of mtDNA.
While human cells only have two copies of the nuclear genome, with one set of chromosomes inherited from each parent, some human cells can have 1,000 to 2,000 mitochondria, each with their own copies of DNA.
Existing adenine-to-guanine mtDNA base editors 鈥 which make edits to single bases 鈥 are limited by low efficiency, so the team searched for variants with increased targeting compatibility and activity.
In their first paper, which also involved researchers from ShanghaiTech University, BRL Medicine, and Peking University, the team introduced their engineered mtDNA editors, which showed 87 per cent editing efficiency in human cells while reducing off-target mutations.
Using their editor, the team created rats with a type of hearing loss linked to mtDNA mutations, indicating that the editing could reach the mutation threshold necessary to induce disease symptoms in cells.
Animal models of mtDNA-linked diseases such as sensorineural hearing loss 鈥渃an prove invaluable as a resource for both pathological and therapeutic studies鈥, according to the team.
In an accompanying study published the same day, the researchers used their editors to generate a rat model of Leigh syndrome by injecting the mtDNA adenine base editors into rat embryos.
Leigh syndrome is an inherited neurological disorder that manifests within the first year of a child鈥檚 life, causing progressive loss of movement and mental abilities. It typically results in death within a few years.
The resulting rats had heart defects and muscle function impairments, reflecting human disease symptoms.
鈥淲e demonstrated that evolved hyperactive [mtDNA editor] exhibited high specificity in installing pathogenic mutations in rat embryos and achieved a very high point mutation rate to model Leigh syndrome,鈥 the team said.
To correct the mutation, a mtDNA cytosine-to-thymine base editor was injected into Leigh syndrome rat embryos. The result was that, on average, 53 per cent of the faulty mtDNA was converted into healthy, unmutated mtDNA.
This led to improved muscle and heart function in the resulting rats, which matched that of the control rats.
鈥淭o our knowledge, no study has been reported on the direct correction of mtDNA mutations in animal models,鈥 the team said.
The researchers said their findings could offer a 鈥減owerful tool to investigate mtDNA-related diseases鈥 and encourage the development of new treatments through the correction of point mutations.
鈥淎lthough mtDNA base editing is still in its infancy and there will be a long journey to improve it before clinical applications can be realised, our study demonstrates a method for potentially treating mtDNA diseases.鈥