Pompe disease is caused by mutations in a single gene called GAA, located on chromosome 17. These mutations prevent the body from producing enough of an enzyme needed to break down glycogen, a stored form of sugar. Without this enzyme, glycogen accumulates inside cells and progressively destroys muscle tissue, including the heart and the muscles used for breathing.
The Gene Behind Pompe Disease
The GAA gene provides instructions for making an enzyme called acid alpha-glucosidase (sometimes called acid maltase). This enzyme works inside lysosomes, tiny compartments within cells that function as recycling centers. Lysosomes use digestive enzymes to break complex molecules into simpler ones the cell can reuse. Acid alpha-glucosidase specifically breaks down glycogen into glucose, the simple sugar your body uses for energy.
When mutations in the GAA gene reduce or eliminate the enzyme’s activity, glycogen accumulates in lysosomes throughout the body. This buildup reaches toxic levels, damaging organs and tissues, with muscles bearing the worst of it. More than 600 different mutations in the GAA gene have been identified, and the specific mutations a person carries largely determine how much enzyme activity remains and how severe the disease becomes.
How Pompe Disease Is Inherited
Pompe disease follows an autosomal recessive inheritance pattern. That means a person must inherit two defective copies of the GAA gene, one from each parent, to develop the disease. Parents who carry one mutated copy and one normal copy are called carriers. They typically produce enough enzyme from their one working gene copy to stay healthy and usually have no symptoms.
When both parents are carriers, each pregnancy carries a 25% chance of producing a child with Pompe disease, a 50% chance of producing another carrier, and a 25% chance of producing a child with two normal gene copies. The disease is rare. Newborn screening studies have found that infantile-onset Pompe disease occurs in roughly 1 in 60,000 to 1 in 300,000 births depending on the population, while late-onset forms are more common, appearing in as many as 1 in 17,000 births in some regions.
What Happens Inside Muscle Cells
The damage from Pompe disease unfolds in stages, and it goes well beyond simple glycogen accumulation. Initially, small glycogen-filled lysosomes appear between the intact muscle fibers. As the disease progresses, lysosomes swell in both size and number, and glycogen starts leaking into the surrounding cell fluid. Muscle fibers begin to fragment.
In advanced stages, swollen lysosomes pack tightly together, their membranes rupture, and very few intact muscle fiber fragments remain. Eventually the contractile elements of muscle cells, the parts that generate force, are completely destroyed. The cells bloat as water rushes in. For a long time, researchers thought this lysosomal rupture alone explained the muscle destruction. The picture turns out to be more complex.
A critical part of the damage involves a breakdown in autophagy, the process cells use to clean up their own damaged components. In Pompe disease, the final step of autophagy fails. Cellular debris that would normally be digested accumulates into massive clumps inside muscle fibers. These clumps act as dead weight: they cannot contract, so they reduce the force a muscle can produce per unit of mass. The buildup also traps undegradable waste called lipofuscin, a mix of oxidized proteins, fats, and metals (particularly iron). Lipofuscin further clogs lysosomes, which means even more damaged components go unrecycled. This creates a self-reinforcing cycle where failing lysosomes produce more waste, which further overwhelms the lysosomes, accelerating muscle destruction.
Why Enzyme Levels Determine Severity
Not all Pompe disease looks the same. The amount of residual enzyme activity a person retains shapes when symptoms appear and how quickly they progress. This creates two broad clinical categories.
In classic infantile-onset Pompe disease, infants retain less than 1% of normal enzyme activity. Symptoms appear within the first few months of life and include severe muscle weakness, an enlarged heart, and difficulty feeding. Without treatment, this form is fatal, usually from heart failure, within the first year or two.
In late-onset Pompe disease, residual enzyme activity is higher but still below 30% of normal. Symptoms can appear anywhere from early childhood to well into adulthood. The heart is generally spared in this form. Instead, the disease primarily targets skeletal muscles, particularly those in the trunk, hips, and legs, along with the diaphragm. Progression is slower but still relentless.
Why Breathing Is Especially Vulnerable
The diaphragm, the dome-shaped muscle beneath your lungs that drives breathing, is frequently involved in Pompe disease. Because it works constantly and relies on sustained muscle contraction, the diaphragm is particularly sensitive to the progressive fiber destruction the disease causes.
As the diaphragm weakens, breathing becomes less effective, especially when lying down. In an upright position, gravity helps pull the diaphragm into its natural dome shape, but lying flat removes that assist. A drop of 15% or more in lung capacity between sitting and lying positions signals diaphragm weakness. A drop of 50% or more indicates bilateral weakness and predicts problems with nighttime breathing. Many adults with late-onset Pompe disease first notice something is wrong when they develop unexplained shortness of breath during sleep or wake up feeling unrested, sometimes years before limb weakness becomes obvious.
How Pompe Disease Is Identified
Pompe disease is on the Recommended Uniform Screening Panel in the United States, meaning newborns are screened for it alongside dozens of other conditions. Screening uses a dried blood spot to measure acid alpha-glucosidase activity. If levels come back low, follow-up testing confirms or rules out the diagnosis.
The diagnostic process involves two steps: confirming that enzyme activity is deficient, then identifying two disease-causing mutations in the GAA gene through genetic testing. Enzyme activity can be measured from blood spots, white blood cells, or skin cell cultures, with skin cells providing the most reliable results when initial tests are borderline. One important wrinkle: some people carry gene variants called pseudodeficiencies that lower enzyme activity on lab tests without actually causing disease. Genetic testing distinguishes these harmless variants from true disease-causing mutations.
For adults who were not screened at birth, diagnosis often comes after years of unexplained muscle weakness or breathing difficulties. Late-onset Pompe disease is frequently misdiagnosed as other neuromuscular conditions, and the average delay between symptom onset and correct diagnosis can stretch for years.