September 15 to 17, 2017
INTERNATIONAL WORKSHOP - HOLLAND
Defects in the calpain 3 (CAPN3) gene are the cause of the autosomal recessive type 2A form of cingulate muscular dystrophy (LGMD2A; OMIM 253600), also identified as calpainopathy.
It was originally identified in the population of Reunion Island in the Indian Ocean and was later shown to have a prevalence of between 10 and 70 per million.
CAPN3 deficiency causes a slowly progressive muscle disorder with onset between the first and second decade of life, and loss of autonomic walking after an average of 15 years from onset.
Muscle weakness is symmetrical and prevalent in the axial musculature of the trunk and proximal of the lower limbs. A large percentage of patients present with a reduction in forced vital capacity but, although respiratory complications are possible, these are not the prominent feature of calpainopathies.
No cardiac involvement is present and when reported, it is incidental.
Calpain 3 is a member of the calpain family, non-lysosomal proteases.
Although some forms have been identified in the lens and brain, the full chain form of CAPN3 is primarily expressed in muscle. Expression at the cardiac level is reported in both embryonic and adult murine and human tissue in amounts 100-fold lower than in skeletal muscle. In muscle, the enzyme is present as an inactive enzyme likely through interaction with one of its partners, the giant protein titin. Calpain 3 has been shown to be self-activating via autolysis. The specific role of calpain 3 and the mechanism by which it causes LGMD2A is not fully understood although much evidence points to a role in cytoskeleton remodeling.
To date, there is no cure for the disease. In an attempt to develop a therapeutic gene transfer strategy for LGMD2A, Isabelle Richard's (Paris) team carried out a series of experiments using adeno viral-associated vectors (rAAVs), which are currently a standard means of gene transfer into skeletal muscle. After local injection of CAPN3-expressing rAAV vectors into the skeletal muscle of a calpain 3-deficient mouse model, he demonstrated efficient expression of the CAPN3 transgene in skeletal muscle with correct localization in the sarcomere. This expression is associated with the resumption of proteolytic activity of calpain 3 and the recovery of contractile activity . Upon verification of a systemic distribution of these vectors, a cardiac toxicity related to the ectopic localization of the vector at this level was evidenced.
A second generation of vectors was designed using new promoters and introducing a cardiac-specific microRNA (miR-208a) target sequence. These modifications suppressed cardiac toxicity while preserving the therapeutic effect in skeletal muscle. In light of this, the team conducted a preclinical study in macaca fascicularis to evaluate the biodistribution and safety of the AAV9-calpaine 3 vector with a cardiac-targeted assay. In conclusion, with these studies, I. Richard and his collaborators have demonstrated the safety of the calpain 3 vector and defined the therapeutic dose, supporting the possible use of calpain-3 AAV-mediated transfer in clinical trials in LGMD2A patients.
