Researchers at UNC School of Medicine described crucial structural features that enable gene therapy vectors to reach the brain. The discovery is key for neurological diseases with ongoing gene therapy trials, such as Friedreich’s ataxia (FA).
The study, “Mapping the Structural Determinants Required for AAVrh.10 Transport across the Blood-Brain Barrier,” appeared in the journal Molecular Therapy.
Adeno-associated viruses (AAV) are among the most used vectors in gene therapy. These modified, noninfectious types of viral vectors are designed to target and deliver DNA to specific cells, and have been at the center of clinical-stage treatment strategies.
AAVs are only able to cross the blood-brain barrier at high doses. Besides their high cost, these doses may also cause significant liver toxicity.
Therefore, the research team studied which specific structural characteristics give AAVs the ability to reach the brain, so their specificity and efficiency could be improved.
Researchers combined two known AAVs — AAV1, which does not cross the blood-brain barrier, and AAVrh.10, which does. The investigators performed multiple swaps of different DNA stretches from one AAV to the other and then characterized the new constructs in mice.
Results showed that a key cluster of just eight amino acids (the building blocks of proteins) on the viral capsid of AAVrh.10 enables efficient crossing of the blood-brain barrier. With this key amino acid sequence, AAV1 was now also able to reach the brain.
Improved efficiency may allow for lower vector doses, which is important for avoiding toxicity.
“Functional mapping of this capsid surface footprint provides a roadmap for engineering synthetic AAV capsids for efficient [central nervous system] gene transfer with an improved safety profile,” the researchers wrote.
The results suggest that existing AAVs used in gene therapy to treat central nervous system disorders might be improved by adding the same or a similar set of amino acids.
Besides the reduced risk for adverse effects of a potentially smaller dose, the findings showed that the AAV variants containing the crucial set of amino acids were also less likely to get into non-brain cells, including liver cells.
“AAV variants containing this key amino acid footprint preferentially get into neurons rather than other brain cell types,” Aravind Asokan, PhD, the study’s senior author and a professor of genetics, said in a press release. “Decreased vector uptake in peripheral organs, particularly the liver, could reduce the potential threat of hepatotoxicity.”
With their ongoing work at UNC, researchers aim to determine the precise molecular mechanism through which the amino acids enable the viruses to reach the brain. The investigators are also studying the potential differences between species in these structures.