life science
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Targeting Issues in AAV Vector Gene Therapy for Hemophilia
emophilia is a bleeding disorder affecting more than 1,125,000 people worldwide: three times higher than the previous estimate of 400,000.1 The disorder is caused by mutations of factor genes in hepatocyte cells of the liver, resulting in partial or total deficiencies of blood-clotting proteins. Physical symptoms range from severe bleeding after a major injury to spontaneous bleeding in muscles and blood build-up in joints, all of which limit the lifestyle and shorten the lifespan of those affected. Hemophilia currently has no cure. Treatments such as injecting clotting factor concentrates exist; however, such therapies require thousands of injections over a lifetime and cost an average of more than $270,000 annually. These factors only make it harder for hemophilia patients living in less industrialized nations or more rural areas to access adequate treatment. Researchers around the world are currently exploring alternative treatment methods, namely gene therapies using non-viral naked DNA or viral vectors. Such experimental techniques use protective molecules or human-modified viruses to transport and introduce genes into target cells. The end result is the creation of necessary proteins that correct mutations and alleviate disease symptoms. Of the two methods, viral vectors are more popular for liver gene therapy. At the University of North Carolina Gene Therapy Center, Dr. Chengwen Li and his team focus on the adeno-associated virus (AAV) for application in viral vector gene therapy. AAV has shown promising results in clinical trials for hemophilia, with patients’ blood-clotting factors increasing to therapeutic Illustration by Heidi Cao
By Kelly Fan levels for long periods of time. Furthermore, in theory, AAV gene therapy has the potential to cure hemophilia with one injection, since the correct genetic code would stay in the human body for autonomous protein creation. This improves upon current treatment options, which involve multiple expensive injections that only alleviate hemophilia symptoms. Many of the AAV’s characteristics render it an attractive gene therapy vector. For example, the virus allows for long-term protein production in the cell, has the ability to infect a broad spectrum of cell types, invokes a relatively low but still present immune response, and lacks the ability to cause illness. 2 Despite its beneficial features, researchers like Dr. Li must continue to search for ways to modify the AAV genome in order to make it more suitable for effective disease treatment. Dr. Li and his lab mainly investigate how to overcome the immune response against AAV vectors, developing new strategies to enhance AAV vector transduction and specifying AAV vector gene therapy for rare diseases like hemophilia. Their most recent research addresses all three necessary improve-
Dr. Chengwen Li
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Figure 1. AAV vector transduction pathway, starting with the vector entering the cell and ending with the therapeutic protein, like clotting factor protein for hemophilia.
ments to AAV gene therapy, with a focus on neutralizing antibodies called Nabs. The fact that “over 90% of humans have already gotten an AAV infection” presents a significant problem for hemophilia-centered AAV gene therapy. 3 Around 50% of previously-infected people generate Nabs, which recognize and block AAV capsids (the outer coating surrounding the genome) from interacting with host cells. As a result, transduction cannot occur and AAV gene therapy would not show many, if any, beneficial effects for hemophilia patients in that 50%. However, many varieties, or serotypes, of AAV exist. The various serotypes have different genetic material but similar protein structures with nuances impacting their interactions.
