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What is the cause ?
#6 - 3--jerry--What is the cause ?--2006-03-17 22:37:09
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The changes in muscle and other body systems in myotonic dystrophy are now known to result from a specific genetic change (mutation) which in most cases involves a gene on chromosome number 19. The same change occurs in patients world-wide, but it is variable in extent, even in a single family, because it is unstable. The length of a particular ‘triple repeat sequence’ (CTG) is expanded in patients and this may vary from a slight expansion in mildly affected individuals to a very large one in severely affected children.
Trinucleotide repeats
In the DMPK gene, there is a section of the genetic code where the three letters CTG are repeated a certain number of times. In people who have DM1, this word is repeated too many times—more than the normal number of 37 times—and thus this section of the gene is too big. This enlarged section of the gene is called a trinucleotide repeat expansion.
People who have repeat numbers in the normal range will not develop DM1 and cannot pass it to their children. Having more than 50 repeats causes DM1. People who have 38–49 repeats have a premutation and will not develop DM1, but can pass DM1 onto their children. Having repeats numbers greater than 1,000 causes congenital myotonic dystrophy.
Until recently it has not been clear how genetic change causes the condition: the most likely mechanism is now thought to be that the expanded repeat is converted normally into the next stage (RNA), but then is unable to leave the cell nucleus. As a result of this trapping, a range of other types of RNA are affected, as are the protein they produce, which helps explain how a single genetic change can affect different body processes.
- Until recently it has not been clear how genetic change causes the condition: the most likely mechanism is now thought to be that the expanded repeat is converted normally into the next stage (RNA), but then is unable to leave the cell nucleus. As a result of this trapping, a range of other types of RNA are affected, as are the protein they produce, which helps explain how a single genetic change can affect different body processes.