Recent advances in editing DNA have the potential to treat a wider range of human diseases than ever before. But scientists still need to solve the problem of making those changes in all the cells of the body that need them.
Now, a group of researchers from the Broad Institute at Harvard University and the Massachusetts Institute of Technology (MIT) have identified a smaller enzyme that will make it easier to deliver the gene editing machinery directly to cells inside the body.
This cutting edge genome-editing system — known as CRISPR — is already being used to make precise changes to the DNA of laboratory animals.
Researchers hope to eventually target human diseases with the method. By disabling or altering genes in human cells, scientists might one day be able to treat diseases ranging from cystic fibrosis to heart disease and diabetes.
In the case of some illnesses, scientists could extract stem cells from the blood and alter them using CRISPR. They would then return the altered cells to the patient’s body.
For other diseases, though, scientists need to use a disabled virus to deliver the entire CRISPR system to cells. This package must include a bacterial enzyme — known as Cas9 — that makes cuts in the DNA and a piece of RNA that guides the enzyme to the right location.
Scientists Solve a Delivery Problem
One of the most promising delivery vehicles, or vectors, for delivering CRISPR in people is the adeno-associated virus (AAV). This vector is not known to cause human disease and has already been approved in Europe for use in clinical trials.
However, AAV has a limited cargo capacity. This makes it difficult to package all of the pieces necessary for editing the genome.
One solution would be to find a vector that can carry more. But AAV already has a proven track record. Instead, researchers from the Broad Institute set their sights on finding a smaller Cas9 enzyme, one that would fit more easily inside AAV.
This involved sifting through 600 or so Cas9 enzymes from different strains of bacteria. Researchers narrowed this list down to six potential candidates.
“Luckily, one of these smaller Cas9 proteins turned out to be suitable for the development of the methodology described in this paper,” said Eugene Koonin, a senior investigator with the National Center for Biotechnology Information and a contributing author on the study, in a press release.
The Cas9 enzyme presented in the paper, published today in Nature, is from the bacteria Staphylococcus aureus, which can cause staph infections in humans. It is 25 percent smaller than the one currently used with CRISPR, which is from Streptococcus pyogenes.
Smaller Enzyme Effective in Gene Editing
With the packaging problem solved, the researchers set out to test whether the smaller Cas9 enzyme worked as well as the current version.
They looked at the number of unintended cuts, or mistakes, made by Cas9 to other areas of DNA. In this regard, the smaller Cas9 was just as accurate as the enzyme from S. pyogenes.
Next, the researchers put the smaller Cas9 to work on a potential treatment for heart disease. Researchers injected the AAV delivery system — with the smaller Cas9 in tow — into the livers of mice.
The target for Cas9 was a gene called PCSK9 that is associated with high cholesterol and heart disease. Once delivered, Cas9 made cuts to that gene, effectively disabling it.
A week after treatment, cholesterol levels in the mice dropped. These effects lasted for up to a month.
This technology is a long way from treating disease in humans. Like other promising gene editing techniques, CRISPR is likely to experience setbacks along the way.
But the researchers’ success adds to the tools available for editing the genes of people.
“Our long-term goal is to develop CRISPR as a therapeutic platform,” said the team’s lead researcher Feng Zhang, a member of the Broad Institute and an investigator at the McGovern Institute for Brain Research at MIT. “This new Cas9 provides a scaffold to expand our Cas9 repertoire, and help us create better models of disease, identify mechanisms, and develop new treatments.”
DNA Editing Faces Ethical Hurdles
CRISPR also faces other challenges before it can be widely used to treat human diseases.
One is its safety. CRISPR is faster and easier to use than other gene-editing techniques. But that doesn’t mean it’s more accurate. Off-target cuts to DNA can occur when the sequence is similar but not identical to the guide RNA. This could have unintended — and potentially deadly — health consequences.
The precise nature of the gene-editing technique has also raised ethical questions. The technique could be used to cure disease, but it could also be used to enhance qualities like intelligence or physical appearance in so-called “designer babies.”
Some of these changes could be made to the human germline — sperm, eggs, and embryos — so they would be passed down to future generations.
In response to this threat, a group of biologists — including the inventor of the CRISPR approach — has called for a worldwide ban on the use of this technique in humans in any way that could be passed down to offspring.
The moratorium would provide scientists, ethicists, and the public time to study the potential impact of this method.
“We worry about people making changes without the knowledge of what those changes mean in terms of the overall genome,” Dr. David Baltimore, a member of the group, told the New York Times. “I personally think we are just not smart enough — and won’t be for a very long time — to feel comfortable about the consequences of changing heredity, even in a single individual.”