Genetic Causes
The genetic abnormality associated with the X-linked form of myotubular myopathy (XLMTM) was first localized in 1990 to the X chromosome at site Xq28. MTM1 codes for the myotubularin protein, a highly conserved lipid phosphatase involved in cellular transport, trafficking and signalling. Approximately 80% of males with myotubular myopathy diagnosed by muscle biopsy have mutations in MTM1, and about 7% of these mutations are genetic deletions.
Centronuclear myopathies where the genetic abnormality is NOT sex-linked (e.g., not located on the X chromosome) are considered autosomal. Autosomal abnormalities can either be dominant or recessive, and are often referred to as AD for "autosomal dominant" or AR for "autosomal recessive").
Many researchers use the term "myotubular myopathy" (MTM) only for cases when the genetic test has come back positive for abnormalities (genetic mutations) at the MTM1 gene on the X chromosome. Cases with a centronuclear (nucleus in the center) appearance on muscle biopsy but a normal genetic test for MTM1 would be referred to as centronuclear myopathy until such time as a specific genetic site is identified to give a more detailed sub-classification.
The possible combinations of inheritance of myotubular myopathy are as follows:
| Inheritance | OMIM | Gene(s) | Description |
|---|---|---|---|
| X-linked recessive | 310400 | MTM1 (X-linked myotubular myopathy) | The X-linked form of MTM/CNM is the most commonly diagnosed type. Almost all cases of X-linked MTM occurs in males. |
| Autosomal recessive | 255200 | BIN1 | A "recessive" abnormality will only cause disease if both copies of the gene are abnormal. |
| Autosomal dominant | 160150 | DNM2 (MYF6 and MTMR14 less common) | A "dominant" abnormality will exert its abnormal influence (e.g., causing a disease or medical condition) regardless of whether the other copy of the gene is normal or not. Within centronuclear myopathies, researchers have identified an autosomal dominant form at a gene called dynamin 2 (DNM2) on chromosome 19, and this particular condition is now referred to as dynamin 2 centronuclear myopathy (DNM2-CNM). |
Sporadic cases have also been reported where there is no previous family history (these cases are presumably due to a new mutation that was not present in either parent).
Read more about this topic: Centronuclear Myopathy
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