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Scripps scientists unravel mystery of genetic mutation in CMT disease
LA JOLLA, Calif.— Scientists at the Scripps Research Institute have made an important discovery regarding the mutation of a gene associated with Charcot-Marie-Tooth (CMT) disease, and they hope their findings will lead to the development of new therapies for the genetic nerve disease as well as other conditions.
CMT disease, also known as hereditary motor and sensory neuropathy, was named after three physicians who first described the disease in 1886: Jean-Martin Charcot, Pierre Marie and Howard Henry Tooth. The disease is characterized by loss of muscle tissue and touch sensation in body extremities, predominantly in the feet and legs, but also in the hands and arms. Incurable—but not fatal—the disease is one of the most common inherited neurological disorders, affecting one in 2,500 people.
CMT disease is caused by mutations that cause defects in neuronal proteins. Nerve signals are conducted by an axon with a myelin sheath wrapped around it. Most mutations in CMT disease affect the myelin sheath. Some affect the axon. To date, researchers have identified mutations in 39 genes, and have classified CMT disease into two subtypes according to these mutations: Type 1 primarily affects the myelin sheath, and is either dominant, recessive or X-linked. Type 2 primarily affects the axon, and is either dominant or recessive. The two types can also be mixed.
In a further classified CMT disease type 2D, which is caused by mutations in the GARS gene, a person inherits only one faulty copy of the GARS gene from one parent. The GARS gene holds the instructions for producing an enzyme called glycyl-tRNA synthase (GlyRS), which is vital to the process by which amino acids are attached to one another during protein synthesis. Although previous research has resulted in the identification of 11 different kinds of mutations in the GARS gene that cause type 2D, scientists have not understood why some of the mutations affect the protein-building function of the GlyRS enzyme, but others don't—until now.
Publishing their study of this puzzle this month in an online edition of Proceedings of the National Academies of the Sciences (PNAS), the Scripps team suggests that in fact, a change in enzyme activity is not what causes CMT disease at all. Rather, GARS mutations cause a structural opening in the resulting mutant protein which then gain a new function that is toxic to nerve cells, says Scripps Research Associate Prof. Xiang-Lei Yang, the senior author of the study.
"Does the mutation result in a gain of function, or a loss of function? That question is at the core of our study," Yang says.
Thus, scientists have been trying to find a common feature shared by all GlyRS mutants that might explain the disease mechanism. This common feature shared by all GlyRS mutants that might explain the disease mechanism, Yang adds.
To find this common feature, the Scripps team in 2007 examined the three-dimensional structure of GlyRS and published the X-ray crystal structure of the wild-type protein and one of the mutants. However, finding no dramatic conformational change between the structures, the researchers hypothesized that the differences may actually be found in the crystal packing.
"We thought maybe because the molecules in the crystal are so tightly packed against each other, this packing somehow suppressed the conformation of change," Yang explains. "That is why we turned to high-resolution X-ray defraction."
In collaboration with Hui-Min Zhang, a postdoctoral fellow in the laboratory at the National High Magnetic Field Laboratory at Florida State University, Yang and a graduate student, Weiwei He, used hydrogen–deuterium exchange to obtain the structures of five different GlyRS mutants and the wild-type protein in solution. This method gave information about which parts of a protein are in touch with a solution. In collaboration with Min Guo, assistant professor at Scripps Florida, small-angle X-ray scattering was used to measure changes in the overall shape of the protein structure in solution.
Significantly, the scientists observed that each of five spatially dispersed mutations induced the same conformational opening of a consensus area that is mostly buried in the wild-type protein. By becoming more open to potential new partners, the GlyRS mutants may gain a new function that is toxic to nerve cells. That would explain why type 2D is inherited in a dominant fashion, Yang says.
The identified neomorphic surface is thus a candidate for making CMT-associated pathological interactions, and a target for disease correction, the researchers say.
"This was very exciting for us," she says. "While our lab is focused on understanding the multifunctionality of the GARS gene, at the same time, we are trying to understand the disease connection of these mutations. The intent is that we can develop drugs to fit into the opening, blocking its access to other proteins. We would like to be able to come up with a monoclonal antibody that can block the surface and see in mice if that can reverse the CMT phenotype. This would be a very nice proof of concept."
Scripps already has a commercial partner in mind to take these findings to the next level, and the end result could also have implications for other mutation- induced human diseases, such as amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease.
"Some proteins may be relatively unstable and can be easily triggered into another conformation by different types of mutations," explains Yang. "This example deals with the GlyRS protein, but the general idea can be applied to many other mutation-induced human diseases."