The genetic cause of Bethlem myopathy
Mutations can occur in any of the three collagen type 6-encoding genes: COL6A1, COL6A2, or COL6A3. Collagen 6 is a protein that is part of the extracellular matrix of muscle cells. The extracellular matrix is the space between adjacent cells within a tissue. Collagen 6 has a supportive function within the extracellular matrix, and mutations in any of the collagen 6-encoding genes can result in reduced amounts or abnormal forms of the protein. The impaired supportive function causes progressive loss of muscle strength, joint contractures, and other symptoms.
All three collagen 6 genes are located on an autosomal (non-sex) chromosome. COL6A1 and COL6A2 are located on chromosome 21. COL6A3 is located on chromosome 2. Mutations in an autosomal gene affect men and women equally.
How are the genetic mutations inherited?
Bethlem myopathy is usually inherited in an autosomal dominant pattern. Everyone has two copies of each gene, one from the mother and one from the father. In an autosomal dominant disease, one mutated copy is sufficient to cause the disease. If one parent has Bethlem myopathy, there is a 50 percent chance that the mutated gene is passed on to the next generation. If both parents have Bethlem myopathy, there is a 75 percent chance that a child will inherit at least one mutated gene copy. The disease can sometimes also occur in individuals with no family history of Bethlem myopathy because mutations can arise during the formation of the egg or sperm cell. These are called de novo mutations.
There are rare cases of autosomal recessive Bethlem myopathy, in which both copies of a collagen 6-encoding gene are mutated. These individuals have parents who both carry one copy of the mutated gene, but do not usually show any symptoms. These individuals pass a mutated copy of the gene to all their children, but only those who have two mutated copies, like their parents, develop the disease.
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