A new family of adeno-associated viral vectors has been shown to be more efficient in delivering genetic cargo

Gene therapies can treat and even cure some genetic diseases, but it is difficult to deliver the treatments to the parts of the body where they are needed. The researchers engineered viruses called adeno-associated viruses (AAV) to deliver the cargo -; as a functional copy of a gene -; to specific cells and organs, but they do not always arrive at the desired destination.

Researchers from the Broad Institute of MIT and Harvard have now developed a family of AAVs capable of reaching a particularly difficult target tissue; the brain. The team shows, in a study published in Withthat their AAVs are more than three times better at delivering their cargo into primate brains than the current primary AAV delivery vehicle, AAV9.

The new AAVs can cross the blood-brain barrier, which prevents many drugs from entering the brain. They also accumulate much less in the liver than AAV9, potentially reducing the risk of hepatic side effects that have been seen in other AAV9-based gene therapies. This family of AAVs, called the PAL family, could be a safer and more efficient way to deliver gene therapies to the brain.

The AAVs were designed in the lab of Pardis Sabeti, who is a fellow at the Broad Institute, a professor at Harvard University and the Harvard TH Chan School of Public Health, and a researcher at the Howard Hughes Medical Institute.

We generated a massive pool of randomly generated AAV capsids and from there we limited ourselves to those capable of entering the brains of mice and macaques, delivering genetic cargo and transcribing it into mRNA.

Allie Stanton, lead study author, Harvard Medical School graduate student in the Sabeti lab

A protective cover

Gene therapies consist of DNA, RNA, or other molecules that are transported around the body by delivery vehicles or vectors. AAVs are promising vectors because, as viruses, they are efficient in delivering their content into cells. Scientists replace natural AAV payloads with therapeutic DNA, gene-editing machinery, or other genetic information that they want to introduce into cells to treat disease.

“AAVs are a very good vector for gene therapy because you can put whatever you want inside its shell, which will protect it and get it into a wide variety of cell types,” Stanton said.

However, the majority of an injected dose of AAV typically ends up in the liver, meaning that high doses of AAV are required to obtain even a fraction in a different target tissue, such as the brain. In some cases, these high doses have resulted in liver damage and even death in clinical trials.

Engineering vectors to effectively target specific cells or organs could help reduce these unwanted side effects. Gene therapy researchers are working to make AAVs safer and more effective by altering the amino acid composition of the virus’s envelope, or capsid. Because there are billions of possible synthetic capsids of AAV, scientists can modify thousands to millions of viruses at a time to find those that fit a specific purpose -; like crossing the blood-brain barrier.

Building Better Vectors

To develop a delivery system that could one day be used for hard-to-treat neurological diseases, Stanton and his colleagues focused on locating AAVs that cross the blood-brain barrier. They turned to a method developed in the Sabeti lab called DELIVER, in which scientists generate millions of capsids and search for AAVs that successfully deliver their payload to certain target cells. Using DELIVER, the team developed the PAL family of AAVs that cross the blood-brain barrier more efficiently than AAV9-; the only viral vector approved by the FDA for use in the nervous system.

They found that PAL AAVs were three times more effective at producing therapeutic mRNA in the macaque brain than AAV9.

The team also discovered that the modified viruses had a unique attraction to the brain. PAL-treated macaques had a quarter of the viral material in their livers like AAV9-treated primates, suggesting that the novel capsids may help limit the liver toxicity of other gene therapies.

The authors say that PAL AAVs could potentially work in humans given the similarity of macaques to humans, but added that AAVs did not work well in mice, making it difficult to test these vectors in mouse models of disease. In the future, the team hopes their work will provide a starting point for even more efficient viral vectors.

“We are encouraged by the early results of PAL-family AAVS, and can see several promising avenues of research using directed evolution and engineering to further increase their efficiency,” Sabeti said.