Introduction

Danon disease is a lysosomal storage disorder (LSD). It affects the heart, muscles, retina and brain. The body cells contain organelles called lysosomes, which break down and recycle damaged cell components with the help of enzymes. In the case of Danon disease, the worn-out cell components are not degraded. This leads to the buildup of multiple harmful products, which are stored in specific areas of the body. As lysosomes contain a variety of enzymes, a deficiency of these enzymes or related proteins can cause different diseases, known as lysosomal storage disorders. Danone disease increases the risk of heart problems such as heart failure, increased stress on the heart, fatigue, difficulty with walking, heart rhythm disturbance and anxiety. It varies in different individuals, from mild to severe. Sometimes the symptoms may not be noticed. Danon disease is characterised by an X-linked dominant inheritance pattern. Hence, males are more severely affected than females.¹ The condition is inherited. It typically passes from parents to children, and male children are more affected than female children. Symptoms and key features include:^{1, 2}

• Heart disease: chest pain or tightness, fatigue and a sensation of fluttering in the chest
• Muscle weakness: difficulty sitting or walking, weakness in the back, neck, shoulders and  upper arms or upper thighs
• Eye problems:  blurred vision, flashes or eye floaters
• Intellectual disabilities:  behavioural problems, speech or language delays and psychiatric issues (rare). These are more common in males

Why are women affected differently?

Danon disease is a genetic condition caused due to mutations in the LAMP2 gene located on the X chromosome. Two specific chromosomes (X & Y) determine whether an individual is male or female.  Males have XY and females have XX. A male carrying one X chromosome that carries the DNA change or mutation will have the disorder because only one X chromosome is present. However, females have two X chromosomes. If a female carries an X chromosome having the DNA change, some symptoms of the disorder may be present, although it may not be severe because other X chromosomes can still make the required lysosome enzymes effectively. In males, symptoms appear in childhood or adolescence and progress rapidly to severe. In females, there may be no symptoms or they may appear in late adolescence or young adulthood.^{2, 6}

Danon disease is inherited from a parent, mostly a mother as she is more likely to remain healthy and reach reproductive age compared to an affected male (father). Affected mothers have a 50% chance to pass on the gene variant (mutation) to each of their children, both sons and daughters. Very few males may be healthy to reach the age of becoming a father (even after performing a heart transplant). However, fathers who are healthy and have children will pass on the variant to all of their daughters and none of their sons. This pattern of inheritance is consistent with X-linked genetic conditions. As females possess two X chromosomes, they are protected from the effects of gene mutation that cause Danon disease, to a certain degree.^{3, 4}

Stages in female carriers

At a young age, females may not have any symptoms. Their muscle strength and intellect may appear normal. However, as they age, heart disease symptoms can develop. Adult women experience arrhythmias. They may also develop progressive heart failure for which cardiac transplantation may be considered. Some girls and women report muscle symptoms. Visual complaints reported by women can be an early sign of the disease. Skeletal muscles, such as muscles of the back, shoulder, upper legs and the neck muscles can be affected. This can lead to back pain, difficulty in getting out of a chair or walking up steps and difficulty raising one’s arms over the head. The diseased heart muscle, also known as cardiomyopathy, can cause a thickened, stiff heart or an enlarged heart. The severity of cardiomyopathy is the major prognostic factor.⁵

Clinical features in females:

• Variable presentation: female carriers possess a wide range of symptoms, from completely asymptomatic to severe and progressive heart failure
• Late Onset: symptoms often begin later in life as compared to males
• Cardiac Signs: primary symptoms are cardiac, including hypertrophic and dilated cardiomyopathy, and heart failure
• Less Common Symptoms: Some females may experience visual changes, particularly retinal pigmentary abnormalities

Diagnosis

It is difficult to diagnose Danon disease because it is rare and unfamiliar. A family history compatible with X-linked dominant inheritance, along with symptoms in affected relatives, is considered. When symptoms appear, DNA tests (genetic testing) can be done to confirm the diagnosis of Danon disease and rule out other conditions. Further analysis may involve blood tests, eye exam, ultrasound, CT scan, MRI and biopsy.⁷ The process of diagnosis involves:

• Clinical evaluation: initial symptoms such as muscle weakness, cardiomyopathy and cognitive difficulties are considered. Family history is also analysed because Danon disease has an X-linked dominant inheritance pattern
• Lab tests: creatine kinase (CPK) level in the blood may be elevated due to ongoing muscle damage. It is usually elevated in males and found to be normal in females
• Genetic testing: The LAMP2 gene test is conducted to confirm Danon disease
• Muscle biopsy: A muscle biopsy may be performed to examine cells for LAMP2 protein deficiency
• Imaging: Electrocardiography (ECG) can be used to assess the heart and identify characteristic features of Danon disease

Treatment and care

At present, there is no cure for Danon Disease. However, treatment is provided to manage symptoms. Heart transplantation is performed to improve symptoms and extend life. Implantable cardiac defibrillators are used to manage heart arrhythmias. Other treatment options include medication, occupational or speech and language therapies for intellectual disabilities, and physical therapy to strengthen muscles. Medications for heart disease should be given when indicated by clinical signs and symptoms. Individuals with Danon Disease are at high risk of complications during surgery. Care involves a comprehensive team of specialists, including cardiologists, neurologists, genetic counsellors, ophthalmologists, and physiotherapists. Genetic counselling is crucial to understand the condition's genetic basis, inheritance, and reproductive risks. Extensive counselling and mental health support are highly recommended for individuals and families.^2,7

Emotional and family impact

Danon disease significantly impacts individuals and their families by creating challenges with emotional well-being and social functioning, in addition to the physical and cognitive symptoms. The emotional toll includes mood disorders, anxiety, and even severe psychiatric issues like depression and psychosis, while behavioural problems such as attention deficits and social issues can affect social interactions. As Danon disease is inherited, it creates a feeling of uncertainty for individuals who carry this condition, as they may worry about passing it to their children. For families, the disease requires extensive caregiving, management of a complex healthcare system, and extensive coordination with schools, leading to a substantial physical, emotional, and logistical burden. Danone disease can lead to significant emotional challenges because individuals experiencing heart problems have to cope with the risk of heart failure or sudden cardiac events, which in turn can cause fear, anxiety, or feelings of isolation. Genetic counselling provides an important source of guidance, helping families understand their risks and explore options such as genetic testing for relatives. Support groups and mental health care can also play a key role, offering reassurance to families.²

Summary

Danon disease is a rare genetic condition caused by changes in the LAMP2 gene, which affects how cells clear waste. It is passed down in an X-linked pattern, meaning men often show severe disease, but women who carry the gene can also be affected. In female carriers, symptoms can range widely, from no signs at all to serious heart problems. In some women, symptoms do not appear and are only identified through family genetic testing. Others may develop mild symptoms such as fatigue, palpitations, or early heart changes observed by scans. In more severe cases, women may experience cardiomyopathy, a disease of the heart muscle which leads to thickened heart walls and heart failure. Diagnosis usually involves a family history, genetic testing, and regular heart checks with ECGs and imaging. Treatment depends on the severity of the disease and may include medications, implantable devices to control the heart rhythm, or, in advanced cases, a heart transplant. Genetic counselling is also important to support family planning. Overall, early recognition and ongoing care are crucial to protecting health in female carriers.

Frequently asked questions (FAQs)

What is danon disease?

Danon disease is a rare genetic condition caused by a mutation in the LAMP2 gene. It affects how cells clean out waste, leading mainly to heart and muscle problems.

Can women get Danon disease?

Yes. Even though the disease is often more severe in men, women who carry the gene can also develop heart problems, ranging from very mild to very serious.

What symptoms should female carriers look out for?

The symptoms include tiredness, shortness of breath, palpitations, fatigue and chest pain. In more advanced cases, symptoms of heart failure may appear.

How is Danone's disease diagnosed?

This can be done by genetic testing and heart checks such as ECG, echocardiogram, or cardiac MRI.

What treatments are available?

Depending on severity, medications, implantable devices (like pacemakers/ICDs), or, in some cases, heart transplants are available as treatment.