Danon disease results from pathogenic loss-of-function variants in the LAMP2 gene on the X chromosome. Loss or dysfunction of LAMP-2 protein disrupts lysosomal membrane integrity and impairs autophagy, causing autophagosome accumulation in cardiac and skeletal muscle—the defining pathological finding. ClinGen has classified the LAMP2–Danon disease relationship as DEFINITIVE. The condition is inherited in an X-linked manner: hemizygous males typically manifest the full phenotype, while heterozygous females experience a later-onset, more variable course. De novo pathogenic variants account for an estimated 40% of simplex cases. Germline mosaicism has been described, though its frequency is unknown. No confirmed genotype-phenotype correlations for loss-of-function variants have been identified, though limited data suggest possible milder phenotypes with certain missense variants or exon 9B-confined variants.

Formal diagnostic criteria for Danon disease have not been established. Diagnostic consideration is prompted in males presenting with rapidly progressive hypertrophic cardiomyopathy, skeletal muscle weakness, retinopathy, and intellectual disability. Supportive laboratory findings include elevated creatine kinase (approximately fivefold), elevated aspartate and alanine aminotransferases with preserved hepatic synthetic function. Electrocardiographic pre-excitation, particularly Wolff-Parkinson-White pattern, is highly supportive, and cardiac MRI demonstrating late gadolinium enhancement reflects myocardial fibrosis. Muscle biopsy may show vacuolar myopathy with autophagosome accumulation on electron microscopy; LAMP-2 protein staining can confirm the diagnosis but is not widely available clinically. In females, diagnostic consideration arises with either dilated or hypertrophic cardiomyopathy and retinal changes. Definitive diagnosis requires identification of a hemizygous pathogenic LAMP2 variant in a male proband, or a heterozygous variant in a female proband, on molecular genetic testing.

No FDA-approved therapy targeting the underlying molecular mechanism of Danon disease currently exists. Management of cardiac manifestations is based on expert opinion, as consensus guidelines have not been published. Hypertrophic cardiomyopathy with preserved ejection fraction is managed with approaches adapted from hypertrophic cardiomyopathy guidelines. Heart failure is addressed with standard management approaches including careful fluid volume monitoring. Cardiac transplantation is documented for males with progressive disease; the reported average age of transplantation is 33.7 years. Implantable cardiac defibrillators are employed for arrhythmia risk management. Electrophysiologic studies and ablation therapy have been applied for pre-excitation and arrhythmia, though literature notes potentially reduced ablation efficacy in this condition. Physical therapy addresses skeletal muscle weakness. Low vision aids are documented in the management of retinal disease. Two FDA orphan drug designations for Danon disease-specific candidates are on record: autologous CD34+ hematopoietic stem and progenitor cells (Papillon Therapeutics) and an adeno-associated virus serotype 9 vector delivering the LAMP2B isoform (Rocket Pharmaceuticals).

Prognosis in Danon disease is substantially determined by the severity and progression of cardiac involvement. In males, penetrance for cardiomyopathy approaches 100% by the second decade, with typical progression to severe hypertrophy, heart failure, and a high likelihood of cardiac transplantation in the second or third decade. Published data indicate average ages of cardiac transplantation and death in males of 33.7 and 34.5 years, respectively. Females experience later onset and generally slower progression of cardiac and musculoskeletal disease, though cardiac disease and retinopathy are documented as clinically significant in this population. The penetrance for cardiac disease in females is estimated as high, with onset later in life than in males. Longitudinal natural history data from large cohorts remain limited due to disease rarity, and the ongoing natural history study (NCT06214507) is anticipated to generate more comprehensive prognostic data.

The published literature on Danon disease encompasses 63 classified research articles, led by case reports and case series (29 publications) and review articles (12). Active investigation focuses on gene therapy and natural history characterization. A Phase 2 gene therapy trial (NCT06092034, Rocket Pharmaceuticals) is recruiting males with Danon disease to receive RP-A501, an AAV9 vector delivering the LAMP2B isoform; enrollment began September 2023 with projected completion April 2032. A dedicated natural history study (NCT06214507, Rocket Pharmaceuticals) is actively recruiting through March 2030. A broader glycogen storage disease natural history study at Duke University (NCT06795152) includes Danon disease and is enrolling through December 2034. A cardiac MRI study of rare cardiomyopathies at Chinese Academy of Medical Sciences, Fuwai Hospital (NCT07336394), is concurrently active. Two FDA orphan designations for Danon-specific therapeutic candidates reflect ongoing pipeline development.