Background Danon Disease (DD) is a rare X-linked autophagic vacuolar myopathy, characterized by high penetrance and severe cardiomyopathy; cognitive, skeletal muscle and vision impairment may occur as well. Due to its rarity, clinical presentation and outcomes are still uncertain. Purpose To describe clinical features and outcomes of DD in female and male patients Methods Individuals and families from United Kingdom, Australia, and United States were recruited through via advertisements on Facebook groups related to DD. Participants completed a survey about symptoms and medical history and provided their medical records to the research team. Results A total of 44 patients (54.5% female) with positive genetic testing for DD were included. De novo mutations occurred in one out of four patients. Cardiomyopathy occurred in 86.3% of patients (18/24 females, 20/20 males) at a mean age of 7.3 years for males and 19.4 years for females (p=0.001). Females presented with either hypertrophic cardiomyopathy (HCM, 66.7%) or dilated cardiomyopathy (DCM, 8.3%) whereas males presented with HCM 90% of the time. 34.2% of patients were diagnosed with Wolff-Parkinson-White syndrome. Twelve patients (7 females, 5 males) underwent cardiac magnetic resonance (CMR) Out of the 9 cases, 8 (88.9%) exhibited extensive patchy late gadolinium-enhancement (LGE) in multiple segments of the left ventricle; 3 cases also had right ventricular LGE. Median cardiac mass index was 155 g/m2 (Q1-Q3: 70–237; v.n. 31–79 g/m2). Overall, 17 (38.6%) patients died or required or heart transplant (HTx). Median age at the time of death or HTx was 17 years and 42 years in males and females, respectively (p=0.025 by the log-rank test) Cognitive impairment, mainly described as learning disabilities, was diagnosed in 90.0% of males (18/20) and 79.2% (19/24) of females; intelligence quotient (IQ) measurement was reported in 8 patients (3 females, 5 males) and 7 of them showed IQ below the average. Symptomatic skeletal myopathy was present in 28 (63.3%) of patients, with a higher prevalence in males (85% vs. 45.8%; p<0.01). Retinopathy was reported in 14 (31.2%) patients and occurred equally in both genders (p=0.34). Conclusions DD causes significant cardiac morbidity with the need for transplant at a young age; in 25% of cases DD is due to a de novo mutation. While in males DD is more frequently multisystemic with a more rapid clinical deterioration, in females the clinical presentation is variable. However, the presence of severe cases in females warrant the clinicians to screen for DD in both sexes with clinical manifestations or positive family history Acknowledgement/Funding Rocket Pharmaceuticals

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