New vector
Genetic diseases of muscle tissue lead to progressive skeletal muscle depletion and death, and treatment options are either limited or completely absent. One of the methods – gene therapy, which uses a weakened virus to deliver a healthy copy of the affected gene to muscle cells – demonstrates good results in clinical studies for the treatment of muscular dystrophy. However, it is not without drawbacks. So, in order to deliver the payload to each muscle of the body, high doses of the vector virus are needed, and the viruses used in these studies enter the liver more often than into the muscles. This leads to a high accumulation of the virus in the liver and is associated with the development of serious side effects and even death of some study participants.
A group of researchers from the Broad Institute of MIT and Harvard and Harvard University have created a new family of adeno-associated viruses (AAV) for gene therapy, which will help target treatment to muscle tissue, making it safer and more effective for patients with genetic muscle diseases. This is a group of MyoAAV viral vectors, they are more than 10 times more effective at affecting muscles than currently used vectors, and largely ignore the liver. The group showed that due to this increased efficiency, MyoAAV can be used for effective gene delivery at doses approximately 100-250 times lower than with other viral vectors, potentially reducing the risk of liver damage and other serious side effects.
The researchers modified the capsid – the outer protein envelope of the virus – to create MyoAAV, using the method they developed for the directed evolution of capsids through the expression of transgenic RNA in vivo (Directed Evolution of AAV capsids Leveraging In Vivo Expression of transgene RNA, DELIVER).
In experiments on mouse models of Duchenne muscular dystrophy, the most common form of genetic muscle disease, and the rarer X-linked myotubular myopathy, researchers delivered therapeutic genes or the CRISPR-Cas9 genome editing system to muscle cells using MyoAAV. This led to an improvement in muscle function. The researchers also found that MyoAAV is able to effectively deliver gene therapy to the skeletal muscles of non-human primates and to human muscle cells.
Aiming at the muscles
To improve AAV, researchers have created many different capsids by adding random chains of amino acids to the part of the viral envelope that binds to cells. They injected capsids into mice and non-human primates, as well as into samples of human muscle tissue taken from different parts of the body, in order to find such capsids that deliver their genetic cargo more successfully than others. The researchers discovered a family of MyoAAV capsids with a unique surface structure that targets muscle cells and used them to treat genetic muscle diseases in animal models.
Gene repair in mice
In mouse models of Duchenne muscular dystrophy, MyoAAV carrying CRISPR-Cas9 led to a more massive restoration of the dysfunctional dystrophin protein gene in muscle tissue compared to conventional AAV9 carrying the same load. The animals that received MyoAAV demonstrated great muscle strength and performance.
In collaboration with Boston Children's Hospital, the research team has shown that MyoAAV is also effective in treating X-linked myotubular myopathy in mice, a disease that in these animals leads to death after about 10 weeks. All six mice that received MyoAAV in an amount 100 times less than the amount of the virus that is currently used in clinical studies lived as long as healthy mice, while rodents that received AAV9 lived only to 21 weeks of age.
MyoAAV, adapted for non-human primates, delivered genes to the muscles of these animals much more efficiently than the natural capsids currently used in clinical research. MyoAAV has also successfully implanted genes into human myocytes in vitro.
MyoAAV vectors have shown efficacy in different models of muscular dystrophy, which demonstrates the stability of this AAV family. The research team believes that the DELIVER tool has the potential to create viral capsids specific to any type of tissue, providing the basis for the development of therapy for genetic diseases affecting various organs.
Article by M.Tabebordbar et al. Directed evolution of a family of AAV capsid variants enabling potential muscle-directed gene delivery across species is published in the journal Cell.
Aminat Adzhieva, portal "Eternal Youth" http://vechnayamolodost.ru according to the Broad Institute: A new gene-delivery vehicle could make gene therapy for muscle diseases safer and more effective.