Danon disease: symptoms, genetics and outlook

Danon disease is an inherited condition in which the cells of the heart and skeletal muscles do not recycle their own waste material properly. It was historically grouped with the glycogen storage diseases because of a similar appearance under the microscope, and AGSD-UK notes that it has sometimes been called GSD2b. Despite sharing features with Pompe disease (GSD2), it is caused by a fault in a different gene.

The gene involved is LAMP2. MedlinePlus Genetics explains that it makes a protein found in the membrane of lysosomes, the compartments in a cell that digest and recycle material. Without enough working LAMP-2 protein, cellular debris accumulates in structures called autophagic vacuoles. This build-up is thought to make heart muscle cells expand and eventually die, and to cause breakdown of skeletal muscle cells.

The condition is rare enough that many doctors will never see a case. That has two consequences for patients: diagnosis can take time, and care is usually coordinated by specialist cardiology, neuromuscular or genetics teams rather than by local services alone.

MedlinePlus Genetics says cardiomyopathy is the most common feature and occurs in all males and in most females. Most affected males develop hypertrophic cardiomyopathy, a thickening of the heart muscle that can make it harder for the heart to pump. Others have dilated cardiomyopathy, in which the heart is weakened and enlarged. Among females, about half have hypertrophic and half have dilated cardiomyopathy. Either type can lead to heart failure.

Other heart-related features described by MedlinePlus Genetics include palpitations, an abnormal heartbeat (arrhythmia), chest pain and abnormalities of the heart's electrical conduction. A conduction pattern known as pre-excitation, often of the Wolff-Parkinson-White type, is frequently seen.

Skeletal muscle weakness occurs in most males and in some females, typically in the muscles of the shoulders, neck and upper thighs. Many males have raised creatine kinase in the blood, which is a marker of muscle disease. Most males have mild intellectual disability, which is much less common in females. AGSD-UK adds that the eyes can also be affected, and lists fatigue, mobility problems and breathlessness among the secondary symptoms.

• Weakened or thickened heart muscle (cardiomyopathy).

• Palpitations, irregular heartbeat, chest pain.

• Weakness in shoulder, neck and thigh muscles.

• Mild learning difficulties, more common in males.

• Sometimes visual abnormalities, breathing or gut problems.

Because Danon disease affects the heart, anyone known to have it, or with a family history, should be aware of heart warning signs. The NHS advises calling 999 for chest pain that feels tight or heavy or that spreads to the arms, back, neck or jaw, particularly with shortness of breath, as these can be signs of a heart attack. Fainting, a racing or very irregular heartbeat that does not settle, or sudden worsening breathlessness should also be treated as reasons to seek urgent medical help.

For new palpitations or tiredness that is out of character, contact the GP or specialist team that already follows you, or call NHS 111 if you are unsure where to go. People with an established diagnosis often have a care plan from their cardiology team that says who to contact; follow that plan where one exists.

MedlinePlus Genetics explains that the LAMP2 gene is on the X chromosome. Males have one X chromosome, so a single altered copy is enough to cause the condition. Females have two X chromosomes and one altered copy can still cause the condition, although features may be less severe, and some females with one altered copy have no signs or symptoms at all.

A characteristic of X-linked inheritance is that fathers cannot pass an X-linked trait to their sons. In practice, a diagnosis in one person usually leads to an offer of genetic counselling and testing for relatives, so that those who carry the same variant can be offered heart checks. Genetic services in the NHS are the usual route for this discussion.

The sources read for this guide do not set out a UK diagnostic pathway, so it is best described in general terms. Diagnosis usually begins when cardiomyopathy, an unusual ECG pattern, muscle weakness or a high creatine kinase result is found. Specialists then look at the whole picture: heart imaging, assessment of muscle and development, and the family history.

Genetic testing for variants in LAMP2 is what confirms the diagnosis. Because the condition is rare, a referral to a clinical genetics service or an inherited cardiac conditions team is the usual way for a GP to arrange this.

AGSD-UK states that there is no treatment directed at the condition itself, and that the enzyme replacement therapy used for Pompe disease would not help because the same enzyme is not deficient. Research is under way, including a gene therapy trial that AGSD-UK reported had opened in the USA. Care is therefore aimed at the heart and muscle problems as they arise, and AGSD-UK says that heart transplant can be considered if cardiac symptoms worsen.

MedlinePlus Genetics says that, without treatment, males typically live into early adulthood while females typically survive into mid-adulthood, and that cardiomyopathy can lead to heart failure and premature death. Those figures describe the natural history across published cases, and they are a reminder that this is a serious condition. Individual outlook depends on age at presentation, sex, the type of cardiomyopathy and the care available.

For a condition this rare, NHS care is usually the main route, delivered through specialist cardiology, inherited cardiac conditions and genetics services. Private consultations are sometimes used for second opinions or faster access to a specialist, but the same small number of expert centres tend to be involved either way. AGSD-UK says it offers support and can help people make connections, which is a useful complement for a very isolating condition.