What DM1 is and how it is inherited
Myotonic dystrophy type 1 (DM1) is caused by a stretch of the DMPK gene in which three DNA letters, CTG, repeat many times. Most people carry 5 to 34 repeats. In DM1 the stretch is longer, from about 50 to several thousand repeats.
The repeat is unstable, and in many tissues it keeps lengthening over a person’s lifetime, which is one reason symptoms differ so much between people and change with age.
A parent with DM1 has a 50 percent chance of passing the expanded gene to each child. The repeat often lengthens when it is passed on, so symptoms can begin earlier in the next generation. Congenital DM1, the form present at birth, is almost always inherited from the mother.
Repeat numbers and forms of DM1
| CTG repeats | Usually means |
|---|---|
| 5 to 34 | The usual range. No DM1. |
| 35 to 49 | Premutation. Usually no symptoms, but the repeat can lengthen when it is passed to a child. |
| 50 to about 150 | Mild DM1. |
| About 100 to 1,000 | Classic DM1. |
| More than 1,000 | Congenital DM1, present at birth. |
The ranges overlap, and the length measured in blood increases with age, so one number does not predict how DM1 will affect a person.
Diagnosis and support
DM1 often goes undiagnosed for years, because its symptoms (muscle weakness and stiffness, cataracts, heart rhythm problems, fatigue and digestive problems) overlap with more common conditions. A genetic test confirms the diagnosis. Testing of other family members is usually arranged through a genetic counselor; the National Society of Genetic Counselors lists counselors by location at findageneticcounselor.nsgc.org.
The Myotonic Dystrophy Foundation publishes information for families, care recommendations for clinicians and support programs at myotonic.org.
3X Genetics and DM1
3X Genetics was founded by a family affected by DM1. Our laboratory measures the DMPK repeat one DNA molecule at a time, including how the repeat length varies from cell to cell. Our DM1 test is in analytical validation and is not yet available to order.
Questions
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