Gene Therapy for Male Patients With Danon Disease (DD) Using RP-A501; AAV9.LAMP2B

Main Criteria for Inclusion: The study will enroll adult and pediatric males with a confirmed diagnosis of DD. Patients may be of any race or ethnicity. Patients and/or competent custodial parents must provide informed written consent and meet all of the enrollment criteria as detailed subsequently to be eligible to participate. 1. DD diagnosis with any confirmed LAMP2 mutation(s). 2. Cardiac involvement as documented by at least one abnormal finding on electrocardiogram (ECG), echocardiogram, gadolinium-enhanced cardiac magnetic resonance imaging (MRI), or electrophysiology study. 3. Age ≥15 years for cohorts 1 and 2; 8-14 years for cohorts 1A. 4. Male gender. 5. New York Heart Association (NYHA) Class II or III. 6. Adequate hematologic function as defined by hemoglobin, absolute neutrophil count (ANC), and platelet count ≥ lower limit of normal (LLN). 7. Adequate hepatic function as defined by: 1. AST and ALT ≤10.0×ULN or GGT ≤2.0×ULN (transaminase elevations in DD are considered extensively to result from muscle injury; hence the relatively high upper limit for transaminases and consideration of GGT level, and the presence of additional hepatic eligibility markers of bilirubin and PT/INR). 2. Serum bilirubin ≤1.2×ULN (i.e., Grade ≤1 bilirubin increase). 3. PT/INR ≤1.2×ULN (in the absence of anticoagulation). 4. Absence of cirrhosis or other signs of inflammation on liver ultrasound 8. Adequate renal function as defined by creatinine ≤ULN. 9. Ability to provide informed consent (for adult patients and parents/legal guardians of pediatric patients) and assent (for patients age 15-17). 10. Ability to comply with study procedures including investigational therapy and follow-up evaluations. 11. Able to walk \>150 meters unassisted during the 6MWT. 12. Patient has received meningococcal vaccination recommended by Centers for Disease Control as appropriate for age and health condition (vaccination must be performed at least 6 weeks prior to IP administration). Main Criteria of Exclusion: Patients meeting any of the following criteria are not eligible for study participation: 1. I.V. therapy with positive inotropes, vasodilators, or diuretics within the 30 days prior to enrollment (i.e., patient must be stable on oral medical therapy). 2. Prior cardiac transplantation or prior transplant of other organ (lung, liver, other). 3. Prior cardiac surgery and/or percutaneous cardiac intervention for arteriothrombotic complications, or valvuloplasty. 4. Presence or requirement of a Left Ventricular Assisted Device (LVAD). 5. History of intracardiac thrombosis or arteriothromboembolic events including stroke or transient ischemic attack (TIA). 6. Left ventricular ejection fraction (LVEF) \<40% at baseline. 7. History of the following prior arteriothromboembolic complications: myocardial infarction or unstable angina. 8. Significant (greater than moderate) valvular stenosis or regurgitation on echocardiogram. 9. Requires mechanical ventilation. 10. Anti-AAV9 neutralizing antibody titer \>1:40. 11. Concurrent enrollment in any other clinical investigation involving use of an investigational agent for the treatment of CHF or cardiomyopathy. 12. Active hepatitis B or C infection (including patients with positive hepatitis B surface antigen (HBsAg), hepatitis B e antigen (HBeAg), hepatitis B core antibody (HBcAb), or detectable hepatitis B virus (HBV) or hepatitis C virus (HCV) viral load). Patients with previous, adequately resolved HBV or HCV are eligible. 13. Significant medical conditions including documented human immunodeficiency virus (HIV) infection, active viral or other hepatitis, poorly-controlled hypertension or diabetes, poorly controlled cardiac arrhythmia, or uncontrolled viral, bacterial, or fungal infection. 14. Any concomitant medical or psychiatric condition that in the opinion of the Investigator renders the patient unfit for study participation or at higher than acceptable risk for study participation. 15. Active hematologic or solid organ malignancy, not including non-melanoma skin cancer or other carcinoma in situ. Patients with previously resected solid organ malignancies or definitively treated hematologic malignancies may be eligible if there has been no evidence of active malignancy during the prior 3 years. 16. Any contraindication to use of sirolimus which includes hypersensitivity to sirolimus or HC-60 (polyoxyl 60 hydrogenated castor oil). 17. Active or latent tuberculosis.