What are the genetic changes related to tubular aggregate myopathy ?
Tubular aggregate myopathy can be caused by mutations in the STIM1 gene. The protein produced from this gene is involved in controlling the entry of positively charged calcium atoms (calcium ions) into cells. The STIM1 protein recognizes when calcium ion levels are low and stimulates the flow of ions into the cell through special channels in the cell membrane called calcium-release activated calcium (CRAC) channels. In muscle cells, the activation of CRAC channels by STIM1 is thought to help replenish calcium stores in a structure called the sarcoplasmic reticulum. STIM1 may also be involved in the release of calcium ions from the sarcoplasmic reticulum. This release of ions stimulates muscle tensing (contraction).  The STIM1 gene mutations involved in tubular aggregate myopathy lead to production of a STIM1 protein that is constantly turned on (constitutively active), which means it continually stimulates calcium ion entry through CRAC channels regardless of ion levels. It is unknown how constitutively active STIM1 leads to the muscle weakness characteristic of tubular aggregate myopathy. Evidence suggests that the tubular aggregates are composed of proteins that are normally part of the sarcoplasmic reticulum. Although the mechanism is unknown, some researchers speculate that the aggregates are the result of uncontrolled calcium levels in muscle cells, possibly due to abnormal STIM1 activity.  Mutations in other genes, some of which have not been identified, are also thought to cause some cases of tubular aggregate myopathy.