A recent article in the journal of Molecular Therapy, representing work from the University of Florida, indicated a possible gene therapy treatment for Pompe disease. Pompe disease results from a mutation in the gene that encodes the enzyme, acid alpha-glucosidase (GAA), which releases glucose from glycogen. Glycogen consists of repeating monomers of glucose and is thus used by the cell for glucose storage and availability. When glucose is released by GAA, it can be used to generate the high energy compound, adenosine triphosphate (ATP), by glycolysis, and to generate pyruvate for the Krebs cycle, also, ultimately increasing the amount of ATP in the cell. Because ATP is so readily hydrolyzed to adenosine monophophate, it drives chemical reactions when these chemical reactions are coupled to the hydrolysis of ATP. Thus, the energy of ATP drives the synthesis of many important cellular molecules.
In Pompe disease, the GAA defficiency leads to a build up and enlargement of the intracellular storage vesicles for glycogen, and it is the abnormal enlargement of these vesicles that interferes with the normal muscle fiber structure, leading to muscle ineffectiveness, particularly ineffectiveness of the diaphragm muscle.
The replacement of a defective gene with a wild-type gene is referred to as gene therapy, and it has been heralded for two decades as an approach with many applications in medicine. Unfortunately, gene therapy approaches to cures to the disease have been few and far between, with a few notable exceptions.
There are many technical problems limiting gene therapy success. For example, in the disease osteogenesis imperfecta (OI), a collagen protein is not made properly. Scientists have known for years which gene is defective, but replacing the gene at just the right time in development and in the proper tissues, has been impossible.
One of the opportunities with a gene therapy approach to Pompe disease is the fact that restoration of the diaphragm function could restore normal lung function. However, this still leaves the difficulty of delivering the normal gene for GAA into enough diaphragm cells to have an effect on the diaphragm function. In general delivery modes for normal genes have been highly inefficient.
Nevertheless, there are some conditions where an inefficient delivery system is not an impediment, or at least not an impediment that cannot be readily overcome. For example, in some cases, it is desirable to deliver a gene for a viral protein, to establish immunity. In this case, very few cells need to express that gene and thereby generate the viral protein. This type of gene therapy can be a desirable approach to vaccination for several reasons. For example, gene vaccines would be extremely inexpensive to produce compared to a conventional vaccine. Once a gene vaccine, or what is more commonly referred to as a DNA vaccine, is established and tested, enough gene vaccine could be produced to vaccinate everyone on the planet for a few thousand dollars. Also, a DNA vaccine could be transported easily and safely, and stored conveniently, forever, anywhere on the globe.
However, the general inefficiency of gene therapy means that diseases can persist due to an insufficient number of cells being able to receive and properly express the normal gene. This is particularly difficult with solid tissues in the body, as opposed to blood cells. Solid tissues have layers and layers of cells with all sides of the cells in contact with other cells, making each individual cell difficult to access.
The authors of the Molecular Therapy report took advantage of mice that have the gene for GAA artificially destroyed, or “knocked out”. While often mice, with gene defects that cause diseases in humans, have different or no health problems, in this case the GAA knockout mice have an incapacitated diaphragm and decreased lung function, making it reasonable to study the therapeutic replacement of the GAA in these mice to improve lung function.
Thus, the authors from the University of Florida reported the success of modifying a gene therapy approach that represented the highly efficient transfer of the normal gene for GAA into diaphragm muscle cells in mice. Their approach is a modification of the use of what are termed, viral vectors. Viral vectors are virus capsids containing a normal, therapeutic gene in place of the normal viral proteins. These capsids can be produced in a laboratory in specialized cells engineered to supply all the proteins necessary to generate the viral capsid in a way that the viral capsid encapsulates the normal GAA gene rather, than the virus genetic material. Because the pathological components of the virus genetic material are not present, a viral vector does not cause a viral infection or the spread of virus internally. The viral vector used in the University of Florida study is termed, AAV for adeno-associated virus, derived from a harmless virus. This viral vector is highly efficient at entering cells.
Nevertheless, the AAV viral vector is not able to deliver the normal GAA gene to enough of the diaphragm cells to improve the function of the diaphragm without a gel first described by the University of Florida team in a 2004 article, also in Molecular Therapy. The gel is derived from a relatively ordinary chemical called glycerol, and the original motivation for testing the gel was the tiny size of the mouse diaphragm, making it less amenable to other modes of applying the virus. By suspending the virus in the glycerol gel and spreading the gel over the diaphragm muscle, in mice that have been anesthetized and where the muscle has been surgically exposed, the authors demonstrated a highly efficient transfer of the normal GAA gene throughout the diaphragm. In the more recent article, the authors demonstrate that this extensive transfer of the gene leads to increased diaphragm muscle contractile strength and increased lung function. However, the use of the glycerol gel does not lead to muscle and lung function maintained over certain, relatively long periods.
What are the benefits of this research? The primary benefit is that the authors have shown that improving the efficiency of transferring the normal gene into muscle cells has a positive effect, even if the disease has progressed prior to treatment. The disease progression factor is important, because in the human cases, there will be disease progression prior to an opportunities for treatment, particularly in cases of experimental treatments. Whether the gel used by the authors will be necessary or useful in humans remains to be seen. It is possible that more conventional approaches will be sufficient in humans. In fact, the same University of Florida group is in the process of starting a human clinical trial with the GAA gene in an AAV viral vector, but in this trial, the AAV viral vector will be used without the gel.
2010 article describing the GAA gene transfer into mice
GAA clinical trial information
