Gene therapies use genes to treat, prevent or cure diseases and disorders. Small viruses called adeno-associated viruses (AAVs) is a primary way of delivering gene therapies throughout the body, including the brain. AAVs have enormous potential to expand gene therapies by safely delivering genetic material to cells and tissues to treat disorders at their genetic cause.
However, a difficulty in developing AAV therapies has been the need to deliver genes to specific cells and organs. Otherwise, they may cause unwanted effects in other parts of the body. While researchers have identified the genes behind many brain disorders, a strict watchdog known as blood-brain barrier has presented an obstacle to effective treatment. The barrier protects us from toxins and other harmful things by filtering everything that enters and leaves the human brain. However, this barrier can sometimes work very well, keeping out gene therapies that could help treat or cure diseases.
Unfortunately, many existing AAVs cannot efficiently cross the blood-brain barrier. Although some AAVs have successfully crossed the barrier in animal studies, few have shown success when tested in humans. These challenges have hindered the development of treatments for many disorders, prompting researchers to seek more effective ways to deliver gene therapies.
A team of researchers at MIT’s Broad Institute and Harvard University, led by Benjamin Deverman, Ph.D. set out to engineer an AAV that could efficiently cross the blood-brain barrier to deliver genes into the human brain. The study was funded through Brain Research through Advancing Innovative Neurotechnologies® Initiative, or The BRAIN Initiative® and the NIH Common Fund Somatic cell genome editor .
What did the researchers do in the study?
The researchers started by looking for an AAV with a high chance of reaching the human brain. In previous work, Deverman’s lab already had developed an efficient way to make new AAVs looking for those that bind to specific proteins on the surface of target cells or organs.
They used this method to create and screen a library of different AAVs to see if any would bind to a protein called the human transferrin receptor (TfR1), which brings iron to the brain. TfR1 is highly expressed in the human blood-brain barrier and has been shown to transport large molecules across this barrier to reach the brain in human studies.
This step narrowed down the candidate AAVs to one, called BI-hTFR1, that could attach to the TfR1 protein. Having identified a promising AAV, the researchers then tested the AAV in human cells and in mice modified to express the human form of the TfR1 protein. They also tested whether the new AAV could carry genetic material into the brain by comparing it to one of the few existing AAVs that can carry genes into the human central nervous system.
What were the results of the study?
First, the researchers found that TfR1-bound AAV successfully crossed the blood-brain barrier in the human cell model. Furthermore, in a cross-comparison with existing AAV used in gene therapies of the nervous system, significantly more of the new AAV actively traveled through the brain cell barrier.
This finding was further supported when the researchers injected AAV into the bloodstream of mice expressing the human form of TfR1. The results showed that the new AAV successfully entered the brain and spinal cord of mice and did so at much higher levels than the existing AAV, showing 6-12 times greater amounts in the brain. Importantly, this effect was not found for other organs, demonstrating the enhanced entry of the new AAV into the central nervous system. AAV also affected brain cells with critical functions, up to 92%. astrocytes and 71% of neurons that play an important role in how cells develop and communicate.
Finally, the researchers tested the ability of AAV to deliver the human gene GBA1 in the mouse brain. Mutations in GBA1 gene are linked to several neurodegenerative disorders, including Gaucher disease and Parkinson’s disease. The new AAV delivered GBA1 gene throughout the brain. Once again, the results highlighted the enhanced efficacy of the new AAV, which delivered 30 times more copies GBA1 compared to the existing AAV. Together, the results showed that AAV can efficiently enter the brain on a large scale and carry healthy copies of genetic material with it.
What do the results mean?
These findings confirm that AAVs can be targeted to specific proteins to create potent, minimally invasive gene delivery vehicles. The researchers in this study engineered an AAV that, by binding directly to a human receptor protein, crossed the blood-brain barrier to reach critical cells and deliver a disease-related gene throughout the brain.
Importantly, the new AAV was more effective and efficient than the main AAV currently used for gene therapies of the nervous system. Another critical advantage was its human-specific engagement. Because AAV binds to a well-studied human protein found in the blood-brain barrier, researchers believe it has strong potential to work for human gene therapies. The use of mice expressing the human form of the Tfr1 receptor also provides strong evidence that this treatment could prove successful in humans.
Although exciting, the results require further testing in human studies. Researchers must also address common challenges facing any AAV gene therapy, including limits on the size of the gene it can deliver, potential off-target effects on other genes or gene pathways, and the risk of an overactive immune response. Despite these obstacles, with further testing and development, the gene delivery method could open new therapeutic avenues and revolutionize treatment for a range of neurological and mental disorders.
