This is a multicenter, international, non-interventional, natural history study designed to collect longitudinal retrospective clinical information on patients with Danon disease (DD). This study is composed of 2 parts:

• Feasibility study: to identify participating sites, assess site and team capabilities, confirm the site and investigator qualification for taking part in the study,
• Retrospective chart review: Data will be collected retrospectively by means of a chart review of living and/or deceased DD patients with a confirmed lysosome associated membrane protein-2 gene (LAMP2) mutation, and,
• For living patients (who have not undergone heart transplantation or placement of a cardiac assist device), availability of at least 6-month cardiology follow-up data,
• For living patients who underwent heart transplantation or placement of a cardiac assist device, and for deceased patients, at least 1 MRI or echo assessment prior to heart transplantation/cardiac assist device placement or death.

Danon disease (DD) is a rare X-linked dominant genetic disorder characterized by cardiomyopathy, skeletal myopathy, and neurocognitive deficits. The disease is caused by a mutation in the lysosome associated membrane protein-2 gene (LAMP2). LAMP2 functions as a lysosomal membrane receptor in autophagy. Most men and many women with LAMP2 gene mutations will develop cardiac disease that includes hypertrophic cardiomyopathy and/or dilated cardiomyopathy, cardiac pre-excitation syndrome, and a propensity for arrhythmias. The prognosis is directly related to the severity of the cardiac disease, and many patients will die from sudden cardiac death. Males are typically more severely affected than females.

There is currently no therapy available to cure, or even prevent disease progression. DD treatment is thus primarily supportive therapy, and survival beyond 25 years without a cardiac transplantation is improbable for afflicted male patients.

Rocket Pharmaceuticals, a clinical-stage company advancing an integrated and sustainable pipeline of genetic therapies for rare pediatric disorders, is developing Rocket Pharmaceuticals Adeno-associated- vector-501 (RP-A501) (AAV9.LAMP2B), an investigational gene therapy product for DD and the first potential gene therapy for monogenic heart failure.

This retrospective, multicenter, international, longitudinal, non- interventional study of DD patients with confirmed LAMP2 mutation aims at characterizing the Natural History of disease in males and females. The Real-world data collected from patients in this natural history study will help support the overall development and protocol design of the RP-A501 gene therapy pivotal trial to be submitted to the health authorities.

Additionally, this study will allow identification of the core centers managing DD patients in several countries as well as the description of the disease characteristics, diagnosis pathway and disease progression. This will inform the development of patient profiles for future studies, including a potential prospective observational study to investigate the burden of disease among DD patients and an international patient registry for Danon disease.

The overarching goal of this retrospective, multicenter, international study is to gain a better understanding of the Natural History of Danon disease, by collecting de-identified information from patients with this condition treated.

Primary objective: Determine how specific cardiac manifestations in patients diagnosed with Danon disease change over time. Essential echocardiography and/or MRI-based parameters include: thickness of the Left Ventricular Posterior Wall at end-diastole (LVPWd); thickness of the Interventricular Septum at end-diastole (IVSd); Left Ventricular mass (LVmass); Left Ventricular Ejection Fraction (LVEF).

Secondary objectives: Describe the progression of other cardiac and extracardiac manifestations in patients diagnosed with Danon disease, including, but not limited to: serologic markers of heart failure, serologic markers of muscle injury, risk and frequency of arrhythmias, comprehensive echocardiogram and MRI-based assessments of cardiomyopathy, overall survival, and event-free survival (as defined subsequently).

Exploratory objectives:

1. Describe the diagnostic pathway of Danon disease, therapeutic management and local centers' approaches to genetic testing for LAMP2 mutations.
2. Identify key Natural History trends to inform a prospective European Union (EU) natural history study and plan for a potential gene therapy trial for DD patients in the EU.

Inclusion Criteria:

1. Patients older than 6 years old and any sex.

2. Patients with a confirmed genetic mutation in LAMP2. 3a. Living patients (who have not undergone heart transplantation or placement of a cardiac assist device):

   • A minimum of 6 months of longitudinal data, including at least 2 echocardiograms or MRIs at least 6 months apart, prior to most recent follow-up visit or enrollment in Sponsor's gene therapy clinical trial.

     3b. Deceased patients or living patients who underwent heart transplantation or cardiac assist device placement:

   • At least 1 echocardiogram or MRI assessment prior to heart transplantation/ cardiac assist device placement or death

4. Informed consent/assent by the patient and/or their legally authorized representative (if the patient is living at the time of enrollment).

Exclusion Criteria:

1. Diagnosis of Danon disease without genetic testing confirmation.

Female and male patients with confirmed diagnosis of Danon disease based on a genetic test positive for a mutation in LAMP2