For many people living with myotonic dystrophy type 1 (DM1), the most dangerous complications are not the visible muscle wasting, but the invisible electrical failures of the heart. Cardiac conduction abnormalities appear in up to 75% of adult cases, and life-threatening arrhythmias are responsible for 25% of deaths in this population, making heart failure the second leading cause of mortality for those with the disorder.
A new study from Baylor College of Medicine, published in JCI Insight, provides a critical piece of the puzzle regarding why heart disease in DM1 worsens over time and, more importantly, whether that damage can be undone. The researchers discovered that the heart can continue to decline even if the underlying genetic mutation remains stable, suggesting that the sheer duration of exposure to toxic RNA is a primary driver of organ failure.
The genetic trigger and the protein trap
DM1 is caused by a mutation in the DMPK gene. In a healthy person, this gene contains between 5 and 37 CTG repeats—slight building blocks of DNA. In those with DM1, this number jumps to anywhere from 50 to more than 4,000 repeats.
This mutation produces faulty RNA molecules that act like traps, sequestering proteins called muscleblind-like (MBNL). These MBNL proteins are essential for “splicing”—the process of cutting and joining RNA to ensure genes function correctly. When MBNL is trapped and unavailable, the splicing process fails, leading to the systemic dysfunction characteristic of the disease.
For years, the prevailing theory was that DM1 worsens because these CTG repeats physically expand over a patient’s lifetime—meaning a person born with 300 repeats might eventually have thousands in certain tissues, increasing the toxicity of the RNA.
Even as the study focuses on the heart, DM1 is a multisystemic disorder. Beyond muscle weakness and cardiac issues, it commonly affects the brain and the gastrointestinal tract, where misregulation of RNA splicing in smooth muscle can lead to chronic dysfunction of the esophagus, stomach, and intestines.
Beyond expansion: The cost of prolonged exposure
To test if repeat expansion was the only cause of progression, researchers used an animal model where the toxic RNA was expressed long-term, but the number of repeats remained constant. If expansion were the only driver, the disease should have plateaued.
Instead, the researchers observed a steady, progressive decline. Early on, the animals developed enlarged hearts and electrical abnormalities. Over 14 months, the condition spiraled into weaker heart muscles, life-threatening rhythms, and fibrosis—the development of permanent scar tissue. Eventually, the heart chambers stretched and dilated, and the animals experienced shorter lifespans.
Crucially, the abnormal RNA splicing appeared early and did not obtain worse over time. This indicates that the progressive heart failure was not caused by a growing loss of MBNL function, but rather by the cumulative damage caused by the long-term presence of toxic RNA.
The window for recovery
The study also explored whether “turning off” the toxic RNA could reverse the damage, and they found that timing is the deciding factor.
When the toxic RNA was deactivated after a short period of exposure, the heart’s size, structure, and electrical function largely returned to normal. However, when the RNA was turned off after several months, the recovery was incomplete. While the molecular splicing errors were corrected, the physical damage—thickened heart walls and fibrotic scar tissue—remained.
Fibrosis is particularly concerning because scar tissue disrupts the heart’s electrical signaling, which increases the risk of the same deadly arrhythmias that drive DM1 mortality.
The researchers also noted a stark difference based on biological sex. Male mice developed more severe heart disease, suffered worse rhythm disturbances, and showed less recovery after treatment than female mice, mirroring patterns seen in human patients.
These findings suggest that for DM1 patients, the window for effective intervention may be narrower than previously thought. Because structural damage like fibrosis is harder to undo than molecular errors, early monitoring and treatment of cardiac symptoms are essential to prevent permanent remodeling of the heart.
Common questions about DM1 cardiac research
- Does this imply DM1 heart disease is inevitable? The study shows that cardiac manifestations affect most DM1 patients, but the severity and progression vary, particularly by sex.
- Can current treatments reverse fibrosis? The study indicates that while RNA splicing can be corrected, physical scarring (fibrosis) and thickened heart walls are often not fully reversed if treatment is delayed.
- Why does sex matter in DM1? Male mice in this study showed more severe heart disease and poorer recovery, suggesting biological sex influences both the risk and the response to treatment.
Given that early intervention is key to avoiding permanent heart scarring, how can clinicians better identify the earliest signs of cardiac decline in adult-onset muscular dystrophy?