Limb-girdle muscular dystrophy type 2I (LGMD2I) is a form of muscular dystrophy that leads to progressive muscle weakness and wasting, primarily in the hips, shoulders, and proximal arms and legs. An autosomal recessive disorder, LGMD2I is caused by mutations in the FKRP (“Fukatin-related protein”) gene. The disease is one of the dystroglycanopathies, a group of muscular dystrophies due to impaired glycosylation of alpha-dystroglycan (α-DG). The resulting insufficient glycoprotein complex on the surface of muscle cells leads to membrane instability and degeneration of the muscle cells, thus creating weakness and debility. Currently, there is no specific therapy for LGMD2I. Dr. Anthony Blaeser aims to fill that void.
Dr. Tony Blaeser is a Research Assistant Professor at the Wake Forest University School of Medicine and is the Associate Director of the McColl-Lockwood Muscular Dystrophy Laboratory in Charlotte, NC. The laboratory gained its name from the McColl and Lockwood families, who endowed the laboratory to search for a treatment for LGMD2I after a member of their families was diagnosed with the disorder.
Dr. Tony Blaeser evaluates the results of molecular studies for his research on gene therapy for limb girdle muscular dystrophy type 2I.
A critical first step toward the goal of a specific treatment was the creation of an animal model. Dr. Blaeser and his co-workers utilized neomycin cassettes to insert point mutations into the FKRP gene of mice. They found that the mutant mice had progressive muscle wasting that closely resembled the human disease. They characterized the pathology and course of the disease, and they identified clinically relevant end points and targets that they could subsequently use to assess the efficacy of potential treatments.
Dr. Blaeser and his co-workers are pursuing several different strategies to find specific treatments, and they have made substantial strides (Figure 1). One strategy is the use of viral gene therapy vectors. Utilizing AAV vectors, Dr. Blaeser has delivered copies of the FKRP gene into muscle cells of the mutant mice and found that the gene can be expressed, the protein can enhance glycosylation, and the effect can mitigate pathology and improve function.
Figure 1. McColl-Lockwood Muscular Dystrophy Lab research target and therapeutic approaches. A. Primary disease of interest is Limb-Girdle muscular dystrophy 2i/R9 (LGMD2i/R9) caused by mutation in Fukutin-related protein (FKRP). Mutation results in loss of glycosylation (matriglycan chain) which can be recovered with gene and drug therapies. B. FKRP critical for final matriglycan elongation needed to produce fully functional glycosylated alpha-dystroglycan (αDG). C. Restoration of αDG glycosylation following gene (AAV delivery of fully functional FKRP) or drug therapy (delivery of supraphysiological levels of ribitol to increase FKRP substrate). FKRP: Fukutin-related protein; αDG: alpha-dystroglycan; R: Ribitol; R-CDP: CDP-ribitol (FKRP substrate); Matriglycan: terminal xylose and glucuronic acid repeats in glycosylation of αDG that binds to extracellular matrix proteins. B and C adapted from Cataldi et al4.
A second strategy takes advantage of the fact that FKRP is an enzyme. In particular, FKRP is ribitol-5-phosphate glycosyltranferase, whose enzymatic action adds ribitol-5-phosphate onto α-DG, as an important step in production of the glycoprotein complex. Because the enzyme uses ribitol as a substrate, Dr. Blaeser and his co-workers reasoned that administration of supplemental ribitol may allow the defective FKRP to glycosylate α-DG to a greater extent, thus mitigating the impact of the mutation. Indeed, they found that administration of ribitol in the drinking water increased glycosylation of α-DG and improved muscle integrity and function in the mutant mice. Both the gene therapy approach (AAV-FKRP) and the small molecule approach (ribitol supplementation) have been so successful and promising that both therapies are now undergoing phase III clinical trials in humans with LGMD2I.
Some people with LGMD2I have a more severe phenotype that involves not just muscle weakness, but also brain malformation and dysfunction. The brain involvement reflects the fact that α-DG is expressed in the developing brain, where it plays a critical role in guiding neuronal migration. Several of the FKRP mutations in mice produce neuronal migration disturbances that closely mimic those seen in humans. Thus, Dr. Blaeser and his co-workers are testing whether the viral gene therapy vectors and ribitol can protect the developing brain against the adverse effects of FKRP mutations, as they do in muscle.
Over the course of the past decade, tremendous advancements have been made in the treatment of one form of muscular dystrophy, Duchenne Muscular Dystrophy. Considering the impact and success of Dr. Blaeser’s research, it is possible that similarly revolutionary treatments will soon emerge for another form of muscular dystrophy, LGMD2I.