How is Myotonic Dystrophy Type 1 Inherited?

If you or your partner have myotonic dystrophy type 1 (DM1), one of the most pressing questions is whether you will pass the condition to your children, and if so, how it will affect them.

Myotonic dystrophy type 1 is an autosomal dominant genetic condition ^[1]. This means that a person only needs one copy of the altered gene to develop the disease. If one parent has DM1, there is a 50% chance with each pregnancy that their child will inherit the gene and develop the condition, and a 50% chance they will not ^[1]. These odds are the same for every pregnancy, regardless of whether the child is a boy or a girl.

Genetic Anticipation: Why Symptoms Often Worsen in Future Generations

A common pattern in families with DM1 is that the disease tends to start at an earlier age and cause more severe symptoms in each successive generation ^[2][3]. In the medical world, this phenomenon is called genetic anticipation ^[2].

To understand why this happens, it helps to look at the specific gene involved. DM1 is caused by an alteration in the DMPK gene ^[4]. Within this gene, there is a sequence of DNA “letters” (C, T, and G) that repeats over and over ^[4][1].

• In people without the DM1 gene, this CTG sequence repeats fewer than 35 times.
• Intermediate/Premutation (35-49 repeats): People with this number of repeats typically do not develop DM1 symptoms themselves, but there is a risk that the sequence could expand into the disease-causing range when passed to their children ^[4][5].
• In people with DM1, this sequence repeats anywhere from 50 times to several thousand times ^[6][7].

These repeats are unstable. When the gene is passed from parent to child, the number of CTG repeats often increases, or “expands” ^[2][3]. Because there is a direct link between the length of the repeat expansion and the severity of the disease, a child who inherits a longer repeat sequence is likely to experience earlier and more severe symptoms than their parent ^[6][8].

The Maternal Bias and Congenital DM1

The risk of the repeat expanding heavily depends on which parent passes down the gene.

When a father passes the DM1 gene to his child, the repeat sequence usually still expands—leading to some worsening of symptoms—but the increase is generally smaller, and very rarely causes the most severe congenital forms ^[2][3]. On rare occasions, the repeat length may even shrink ^[3].

However, when a mother passes the DM1 gene to her child, there is a much higher risk of the repeat sequence expanding dramatically during the formation of her egg cells ^[2][9]. This significant jump in repeat size, often called a maternal bias, means that mothers with DM1 are much more likely to have a child with the most severe form of the disease, known as congenital myotonic dystrophy (CDM1) ^[10][2][11].

Congenital DM1 is present at birth and can cause severe muscle weakness, breathing difficulties, and developmental delays ^[12][2]. Even mothers who only have mild symptoms of DM1 can have a child with congenital DM1 if the repeat expansion increases drastically ^[8].

Family Planning and Genetic Counseling

Because the severity of DM1 can change so unpredictably from generation to generation, reproductive decision-making can be deeply stressful ^[13][14].

If you have DM1 and are considering having children, it is highly recommended to meet with a genetic counselor ^[15][14]. They can help you understand your specific risks and discuss your family planning options. Advanced technologies, such as preimplantation genetic testing (PGT), can be used during in vitro fertilization (IVF) to test embryos for the DM1 mutation before pregnancy ^[13][14].

For those who are already pregnant, prenatal testing is an option to determine if the baby has inherited the expanded gene. These include chorionic villus sampling (CVS) and amniocentesis, which test cells from the placenta or amniotic fluid ^[4][16].

Important Note on Maternal Health: Beyond the genetic risks to the child, pregnancy itself can pose physical risks to a mother with DM1. The physiological changes of pregnancy can increase the chance of complications such as cardiac issues or excessive amniotic fluid, making it vital to assemble a care team that includes a high-risk obstetrician ^[17][18].