Scientists uncover viral vectors that cross the blood-brain barrier

By Nick Paul Taylor, The Science Advisory Board contributing writer

November 23, 2022 -- Using a "directed-evolution" strategy in mice and macaques, scientists at the Broad Institute of Massachusetts Institute of Technology (MIT) and Harvard have identified adeno-associated viruses (AAVs) that cross the blood-brain barrier, advancing efforts to develop neurological disease gene therapies.

Studies have revealed the genetic causes of some neurological diseases. However, delivery problems have stymied efforts to translate that knowledge into gene therapies that fix the source of the diseases. Specifically, researchers have struggled to get delivery vehicles and their genetic payloads into the brain, because viral particles are both sequestered by the liver and blocked by the neurological barrier.

In an article published November 22 in the journal Med, scientists at the Broad Institute of MIT and Harvard outlined how they used directed evolution to try to overcome both of the barriers to the efficient delivery of gene therapies to the brain.

"We generated a massive pool of randomly generated AAV capsids and from there narrowed down to ones able to get into the brain of both mice and macaques, deliver genetic cargo, and actually transcribe it into mRNA," study lead author Allie Stanton, a Harvard Medical School graduate student in the lab of Pardis Sabeti, said in a statement.

Having seen earlier efforts to improve capsids in mice fail to work in primate models, Stanton and her collaborators adopted a directed-evolution strategy covering both groups of animals. The collaborators used an mRNA-based directed-evolution strategy in multiple strains of mice, plus a de novo selection in cynomolgus macaques. The goal was to identify families of engineered vectors with increased potency in the brain and decreased tropism for the liver.

The work zeroed in on the PAL family of variants identified in macaques. The family lacked potency in mice but showed significantly enhanced ability to reach the brain, as well as reduced off-target effects in the liver, in young macaques. Tests comparing the variants to AAV9, the current gold standard for vectors that cross the blood-brain barrier, suggest the PAL family could help to realize the full potential of gene therapies in neurological diseases.

Stanton and her colleagues showed PAL AAVs were three times more effective at producing therapeutic mRNA in the macaque brain than AAV9. In addition, macaques treated with the PAL AAVs had one-fourth of the viral material in their livers as AAV9-treated primates. The finding suggests the new capsids could help to minimize liver toxicity, a common side effect of gene therapies in humans. The team sees scope to further improve the PAL AAVs.

While the PAL AAVs didn't work well in mice, making it difficult to test these vectors in mouse models of disease, the researchers contend they could potentially be effective in humans given how similar macaques are to humans. Going forward, they see their research as a starting point for even more effective viral vectors.

"We are encouraged by the early results of the PAL family AAVs and can see several promising lines of investigation using directed evolution and engineering to further increase their efficiency," Sabeti said.

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