From OMIM
Danon disease is an X-linked dominant disorder predominantly affecting cardiac muscle. Skeletal muscle involvement and mental retardation are variable features. The accumulation of glycogen in muscle and lysosomes originally led to the classification of Danon disease as a variant of glycogen storage disease II (Pompe disease; 232300) with 'normal acid maltase' or alpha-glucosidase (GAA; 606800) (Danon et al., 1981). However, Nishino et al. (2000) stated that Danon disease is not a glycogen storage disease because glycogen is not always increased. Sugie et al. (2005) classified Danon disease as a form of autophagic vacuolar myopathy, characterized by intracytoplasmic autophagic vacuoles with sarcolemmal features. The characteristic vacuole is believed to be an autolysosome surrounded by secondarily-generated membranes containing sarcolemmal proteins, basal lamina, and acetylcholinesterase activity. X-linked myopathy with excessive autophagy (XMEA; 310440) is a distinct disorder with similar pathologic features.  http://www.omim.org/entry/300257

From MedlinePlus Genetics
Danon disease is a condition characterized by weakening of the heart muscle (cardiomyopathy); weakening of the muscles used for movement, called skeletal muscles (myopathy); and intellectual disabilities. People with Danon disease may develop the condition at different ages. Signs and symptoms of this condition appear about 15 years earlier in individuals with a Y chromosome (typical for males) than in individuals with two X chromosomes (typical for females). People with a Y chromosome first experience health problems in childhood or adolescence; without treatment, these individuals typically live into early adulthood. People with two X chromosomes start experiencing health problems in early adulthood and typically survive into mid-adulthood without treatment.

Cardiomyopathy is the most common symptom of Danon disease, and it occurs in all people with a Y chromosome and in most people with two X chromosomes. Beginning in childhood, most affected individuals with a Y chromosome develop hypertrophic cardiomyopathy, which is a thickening of the heart muscle that may make it harder for the heart to pump blood. Others with Danon disease may have dilated cardiomyopathy, which is a condition that weakens and enlarges the heart, preventing it from pumping blood efficiently. About half of people with Danon disease who have two X chromosomes have hypertrophic cardiomyopathy, and the other half have dilated cardiomyopathy. Rarely, individuals with hypertrophic cardiomyopathy later develop dilated cardiomyopathy. Either type of cardiomyopathy can lead to heart failure and premature death.

Individuals with Danon disease can have other heart-related signs and symptoms, including a sensation of fluttering or pounding in the chest (palpitations), an abnormal heartbeat (arrhythmia), or chest pain. Many affected individuals have abnormalities of the electrical signals that control the heartbeat (conduction abnormalities). Affected individuals often have a specific conduction abnormality known as cardiac preexcitation. The type of cardiac preexcitation most often seen in people with Danon disease is called the Wolff-Parkinson-White syndrome pattern.

Skeletal myopathy occurs in most people with Danon disease who have one Y chromosome and in some affected individuals with two X chromosomes. The weakness typically occurs in the muscles of the shoulders, neck, and upper thighs. Many individuals with Danon disease who have one Y chromosome have elevated levels of an enzyme called creatine kinase in their blood, which often indicates muscle disease.

Most people with Danon disease who have one Y chromosome have mild intellectual disabilities, but this is much less common in affected individuals with two X chromosomes.

There can be other signs and symptoms of the condition in addition to the three characteristic features. Several affected individuals have had gastrointestinal disease, breathing problems, or visual abnormalities.  https://medlineplus.gov/genetics/condition/danon-disease