For the first time, researchers have used gene-editing tools in adult monkeys to disable a gene throughout much of the liver. The approach lowered blood cholesterol levels, suggesting a treatment for heart disease. The study could also pave the way for treating certain genetic diseases caused by a defective, havoc-causing protein.
“It’s very nice work, one of the first demonstrations of gene-editing tools used with high efficiency in nonhuman primates,” says cardiologist and geneticist Kiran Musunuru of the University of Pennsylvania (UPenn), who was not involved in the study.
Gene edited primates are nothing new. China has used the famous CRISPR DNA scissors, which snip DNA at a specific location, in monkey embryos to produce animals with modified genomes for studying diseases. More controversially, researchers there have repaired a disease-causing gene in early human embryos with CRISPR, though the embryos were not allowed to develop. And Sangamo Therapeutics, a company in Richmond, California, has employed an older gene-editing tool called zinc finger nucleases to knock out a gene in some of the cells of HIV patients to help them resist the virus. That treatment is known as “ex vivo” because it involves editing a patient’s blood cells in a dish, then putting the cells back into the patient.
Physicians and researchers, however, dream of delivering CRISPR and other genome editors directly into patients to correct mutated genes or treat diseases in other ways. Sangamo, for example, has launched a small clinical trial testing in vivo—into the body—delivery of its nucleases to guide a new gene to a specific location in a small fraction of a patient’s liver cells so they will crank out a needed protein.
But reducing levels of a problem protein made in the liver requires editing a large fraction of the organ’s cells. Longtime gene therapy researcher James Wilson and colleagues at UPenn wanted to target PCSK9, a gene whose protein hinders the removal of harmful low-density lipoprotein (LDL) cholesterol from the blood; high levels of LDL cholesterol can raise a person’s risk of a heart disease or stroke. A number of companies have developed drugs that inhibit the protein and lower LDL cholesterol.
Wilson’s lab is using seemingly harmless viruses known as adeno-associated viruses (AAV) that are widely used in gene therapy to deliver gene editing tools to cells, where they snip the genome in a specific spot that the cell fails to repair correctly, disabling the gene there. When his team used an injection of AAV to deliver a type of CRISPR designed to cut the PCSK9 gene, the approach succeeded in mice but not in rhesus macaques. So the researchers then used AAV to deliver a different gene-editing tool called a meganuclease supplied by the company Precision BioSciences in Durham, North Carolina.
“It worked incredibly well,” Wilson says: After 4 months, up to 64% of liver cells carried the knocked-out PCSK9 gene in the six treated macaques. At the highest dose, the animal’s blood levels of PCSK9 protein fell by 84% and LDL cholesterol declined 60%, the team reports today in Nature Biotechnology. For unclear reasons, the edited cells stopped making the meganuclease. That’s a good thing because lingering enzyme could cause unwanted edits.
The meganuclease treatment did cause liver enzymes to rise, indicating an immune response, which wasn’t unexpected because this molecule came from algae. Mammals would sense a foreign protein, Wilson says. It also made cuts at sites other than the PCSK9 gene, which could potentially cause cancer.
Still, Wilson thinks that with improvements, such a PCSK9 gene-editing treatment could be offered to heart disease patients with high cholesterol who can’t tolerate drugs that block the PCSK9 protein. It is also promising for metabolic diseases such as amyloidosis, in which defective proteins made by genes in the liver build up and damage the body.
“What’s nice is that the effect on cholesterol was rock steady over the course of a year,” says Musunuru, who while at Harvard University disabled the PCSK9 gene in mice using a more potent, but riskier virus to deliver CRISPR.
However, Musunuru and others note that Wilson’s approach has competition: Intellia Therapeutics, a company in Cambridge, Massachusetts working with CRISPR, recently announced that it has reduced by up to 80% levels of transthyretin, a protein that causes a type of amyloidosis, in monkeys. The company used an injection of nanoparticles containing RNA for CRISPR to get the editing tool into liver cells. Nanoparticles could offer many safety advantages over AAV, notes molecular geneticist Daniel Anderson of the Massachusetts Institute of Technology in Cambridge, who has developed them to deliver CRISPR in mice. While he calls Wilson’s study “exciting,” Anderson says, “It’s too early to tell which of those approaches will translate” into people.