The newest issue of the journal Nature Medicine features two animal studies that show progress is being made toward achieving the holy grail of gene editing: the ability to prevent or treat diseases that are caused by gene mutations. In both cases, the researchers used modified versions of CRISPR-Cas9, the most commonly used gene-editing system.
The first study, from the University of Pennsylvania, reports results from a trial of CRISPR in treating the rare liver disease hereditary tyrosinemia type 1 (HT1) in mouse models. People with the disease can be treated with a drug called nitisinone and a strict diet, but if they progress, they can develop liver failure or cancer.
The Penn team used prenatal gene editing in mouse models of HT1 to improve liver function—in essence to alleviate the effects of the lethal mutation. They reported that the treated mice exhibited improved liver function and were healthier than mice that received nitisinone.
Rather than using CRISPR-Cas9, which fully slices DNA, they employed a system called base editor 3 (BE3), which carried an enzyme to a specific location in the DNA of liver cells. They wanted to show that BE3 can sidestep a common problem with CRISPR-Cas9, which is that after the DNA is cut, the repair process can cause unintended genetic errors.
The enzyme modified a genetic sequence to produce the therapeutic effects in fetal mice. After birth, the mice carried stable amounts of BE3-edited liver cells for three months, the researchers reported.
"We also plan to use the same base-editing technique not just to disrupt a mutation's effects, but to directly correct the mutation," said co-author Kiran Musunuru, M.D., Ph.D., associate professor of medicine at Penn, in a statement.
In a separate study, researchers at ETH Zurich in Switzerland targeted the disease phenylketonuria, which is a metabolic disorder that requires a special diet to control, similar to HT1. The disease is caused by an inherited mutation in the gene responsible for producing the liver enzyme phenylalanine hydroxylase. Without the enzyme, the body cannot metabolize phenylalanine, resulting in a variety of symptoms, including atopic dermatitis, seizures and cognitive delays.
The team took CRISPR-Cas9 and added one enzyme to it called cytidine deaminase. The enzyme converted the DNA base pair that causes phenylketonuria into a healthy base pair, according to a statement. Up to 60% of all copies of the mutated gene were corrected in the livers of adult mice. As a result, levels of phenylalanine fell to normal, the scientists reported. The animals showed no symptoms of the disease, they added.
"The use of a base editor was the key to our success," said the primary author of the study, Lukas Villiger, a doctoral candidate at ETH Zurich, in the statement. The original technique for base editing was developed at the Massachusetts Institute of Technology, he explained, but his team had to “tinker around” with it to get it to work in phenylketonuria, he said in the statement.
While the two studies provide proof of concept for using modified CRISPR to correct inherited disease-causing mutations, both research teams acknowledge more work will have to be done before their gene-editing techniques can be tried in people.
The Penn team used adenovirus vectors to deliver CRISPR and BE3, but the scientists are currently looking into alternative delivery methods, such as lipid nanoparticles, which they hope will lower the risk of unintended immune responses. The ETH team is planning a followup animal study to confirm that their base-editing system doesn’t raise the risk of introducing off-target mutations that could cause inadvertent effects like cancer. The Swiss researchers are also planning studies in larger animal models, such as pigs, which have livers that are more similar in structure to human livers than those of mice.