Gregg Homanics, PhD
University of Pittsburgh
Gene therapy offers hope of a cure for MSUD. To study the possibilities of this approach it is necessary to have an animal model on which to experiment. Dr. Homanics, Dr. Paul and their colleagues have created such a model with mice genetically engineered to mimic the pathology of the classic and intermediate forms of MSUD.
Prior to the development of the MSUD mouse, the only MSUD animal model was a cow. The mouse model offers numerous advantages over the cow including an amino acid metabolism which is similar to that of humans. Mice are a much more practical model, as they are inexpensive to raise and reproduction is rapid.
Dr. Homanic’s group first created a classic model of MSUD by inactivating the gene responsible for the E2 subunit of the branched-chain keto acid dehydrogenase enzyme complex using gene targeting and embryonic stem cell technologies. Embryonic stem cells were isolated from blastocysts and grown in culture. These cells were injected back into the embryo and implanted into a foster mother.
This process took several years. The resultant animals had a number of limitations. Mice were sick, difficult to treat, and survived only for a short period of time. For these reasons, the researchers felt that development of an intermediate model would allow for more treatment opportunities. The new model was created, with a survival rate of about 50%.
Preliminary results using gene therapy with a single injection into the peritoneal cavity (belly) of the mice have been exciting, showing an increase in liver enzyme activity from about 9% to 30. The group plans further attempts using multiple injections, intravenous administration, and direct administration of the enzyme into the liver. Once treatment modalities are developed using the intermediate model, the group hopes to apply these modalities to the classic model.
Dr. Homanics discussed the value of stem cell therapy. He explained that, unlike viral vectors which address a specific enzyme defect, all subunits are present in stem cells. The result is that individuals with MSUD will be helped with this technology regardless of the specific nature of their defect.
The team is grateful for the support it receives from the NIH, Scott Foster Metabolic Research Fund, and the MSUD Family Support Group.
Curing MSUD: Update on the Mouse Project