Limb-girdle muscular dystrophies (LGMD) are a group of phenotypically and genotypically heterogeneous diseases, characterised by progressive weakness and atrophy of the muscles of the pelvic and shoulder girdle. Mutations of the CAPN3 gene have been associated with limb-girdle muscular dystrophy type 2A (LGMD2A). Patients with LGMD2A have symmetrical and selective involvement of proximal limb-girdle muscles. The disease shows wide intrafamilial and interfamilial clinical variability. The age at onset ranges from 2 to 40 years, but the disease usually first appears in the second or third decade of life, with the development of proximal weakness in the lower limbs. Mutations in CAPN3 result in a cascade of events leading eventually to muscular dystrophy, but the precise underlying mechanisms have yet to be elucidated. However, a defect of calpain 3, the protein encoded by CAPN3, proteolytic activity is largely recognised as the main pathogenic cause of LGMD2A.
The CAPN3 gene (24 exons) spans ~53 kb of genomic DNA and is located on chromosome 15q15.1, ~40 Mb from the p-telomere. The gene is predominantly expressed in skeletal muscle where it is present in the cytosol as well as in the nucleus. The protein encoded by this gene (calpain 3) belongs to the superfamily of calcium-activated neutral proteases, which are non-lysosomal intracellular cysteine proteases. Calpains respond to Ca2+ signals by cleaving specific proteins, frequently components of signalling cascades, thereby irreversibly modifying their function(s).
The P176-C3 probemix contains probes for each of the 24 CAPN3 exons. Two probes are present for exons 1 and 4. In addition, ten reference probes are included, detecting several different autosomal chromosomal locations.
This SALSA® MLPA® probemix is designed to detect deletions/duplications of one or more sequences in the aforementioned gene in a DNA sample. Heterozygous deletions of recognition sequences should give a 35 50% reduced relative peak height of the amplification product of that probe. Note that a mutation or polymorphism in the sequence detected by a probe can also cause a reduction in relative peak height, even when not located exactly on the ligation site! In addition, some probe signals are more sensitive to sample purity and small changes in experimental conditions. Therefore, deletions and duplications detected by MLPA should always be confirmed by other methods. Not all deletions and duplications detected by MLPA will be pathogenic; users should always verify the latest scientific literature when interpreting their findings. We have no information on what percentage of defects in these genes is caused by deletions/duplications of complete exons. Finally, note that most defects in these genes are expected to be small (point) mutations, most of which will not be detected by this SALSA® MLPA® test.