Myotonia Congenita (Thomsen Disease and Becker Type)
Myotonia Congenita (MC) is a genetic neuromuscular channelopathy affecting skeletal muscle fibers (muscles used for movement). MC is caused by mutations in the CLCN1 gene, which codes for voltage-gated chloride (CIC-1) channels within the cell membrane of skeletal muscle fiber cells. Abnormal CIC-1 channels cause inappropriate hyperexcitability of skeletal muscle fibers resulting in prolonged muscle contraction (myotonia).
There are different patterns of inheritance for the two types of MC. Thomsen disease type is autosomal dominant, which means one copy of the mutant gene in each cell is enough to cause the disease. In most cases, an affected person has one parent with the condition. Becker disease type is inherited in an autosomal recessive pattern, which means an individual will need two copies of the mutant gene to cause the disease. Most often, parents of an individual with the autosomal recessive form of MC carry one copy each of the mutant gene, but do not show signs and symptoms of the disorder.