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Development of a strategy to treat limb-girdle muscular dystrophy (LGMD2A) using combined cell and gene therapy strategies

Final Report Summary - LGMD2A (Development of a strategy to treat limb-girdle muscular dystrophy (LGMD2A) using combined cell and gene therapy strategies)

Development of a strategy to treat limb-girdle muscular dystrophy using combined cell and gene therapy strategies
This project involves a highly relevant topic for the public health: the development of new therapy strategies against LGMD2A muscular dystrophy, one of the most frequent LGMD form that is especially prevailing in the population of the Basque region. Muscular dystrophies are a group of diseases characterized by the primary wasting of skeletal muscle, which compromises patient mobility and in the most severe cases originate a complete paralysis and premature death. These diseases are among the most difficult to treat, since current pharmacological approaches are implicated in modifying pathways such as calcium fluxes and protease activities that result in systemic toxicity.

In the present project, we propose to achieve the following aims:
Aim 1. To develop a therapeutic approach for LGMD2A patients.
Aim 2. To isolate and optimize the in vitro culture and genetically correct cells from patients with this mutation.
Aim 3. To analyze the efficacy of this combined therapeutic approach in murine models in vivo

To accomplish aims 1 and 2 we have developed and optimized a cellular model with myogenic potential, to test the different therapeutic approaches ex-vivo. We are using human skin fibroblasts from donors and dystrophic patients to reprogram them into myotubes via MyoD infection (14-21days in differentiation medium). These cultures have been characterized in regards to expression of myogenic markers (Figure 1). We have also characterized and validated this model with regards to their intracelullar calcium responses to caffein and KCl depolarization, by comparison with primary myotubes, obtained through the MTCC bank.

We have found that human myotubes are less mature than rodent myotube cultures, since they do not show contraction or striation pattern. Therefore, we are now optimizing a new culture system in which human fibroblast-derived myotubes are innervated with embryonic rat spinal cord explants to record physiological calcium sparks and contraction. This is a great cellular system to assess for differences in calcium homeostasis and other physiological events between control and dystrophic fibers.

We have also tried to isolate and culture CD133+ cells from control subjects. Unfortunately, we have found out that this technique is not very cost-effective, so we have decided not to use this cell model, and continue using the model derived from fibroblasts.

We have found that innervated myotubes derived from human fibroblasts show spontaneous contraction and a well-developed cross-striated pattern (Figure 2). We also found spontaneous intracellular calcium sparks that preceded contractions. We are currently collaborating with Dr. Aizpurua group, in the testing of novel ryanodine receptor (RyR1) stabilizer compounds in our cellular system. In this regard, we have found that addition of S107 (a RyR1 modulator) to this system resulted in significant decreases in the frequency and amplitude of calcium sparks. Normal frequency was immediately restored after washing S107 away, however, calcium spark amplitude remained diminished long after the drug was withdrawn.

The present study is hoped to help to the development of a combined treatment for LGMD2A, and overall it will let us move forward in the knowledge of this disease. It also involves the first reported analysis of calcium homeostasis on human LGMD2A myotubes, and the testing of novel RyR modulators on human samples, and therefore is will be of great relevance to the Healthcare system.
In regards to its bibliometric impact, results obtained from this project would be publishable in high impact journals, such as Journal of Clinical Investigation, Hum Gene Therapy or J Mol Medicine, always depending on the quality of the final results. Novel RyR modulators that are currently being tested have a great potential as therapeutical agents in the treatment of muscular dystrophies. The compounds with greater clinical potential will be patented in due time. We expect that this project will have a great impact in the pharmacological treatment of several muscular diseases, and also could be useful for the treatment of cardiac diseases.

Figure 1. Characterization of primary and fibroblast-derived cultures. Derived cultures show all the myogenic markers found in primary cultures: Dysferlin, myogenin, myosin heavy chain and RyR1. Forced expression of MyoD is detected in the derived myoblasts and myotubes, as expected. FH, human fibroblasts; MbD, Derived Myoblasts; MtD, derived myotubes (14 dpd); MbP, primary myoblasts; MtP, primary myotubes (7 dpd).

Figure 2. A) Myotubes derived from infected fibroblasts and stained with MyoD and DAPI to show infection efficiency. B) Innervation of myotubes with embryonic rat explants. Green shows neurite outgrowth from rat explants after several days of coculturing with human myotubes. Red shows Factin cross-striated pattern in innervated myotubes C) Spontaneous calcium sparks in control and dystrophic contracting myotubes are diminished in the presence of 10 uM S107. Normal frequency is immediately restored after washing S107 away, however, calcium spark amplitude remains diminished long after the drug is withdrawn. a.u= arbitrary units.

Ainara Vallejo Illaramendi
Marie Curie Fellow, Instituto Biodonostia.