Danon Disease

Monogenic (Mendelian): 100%, LAMP2 mutation^[2]

Inheritance: X-linked dominant (LAMP2 gene on Xq24)

LAMP2 protein: Lysosomal-associated membrane protein 2, integral to cellular autophagy; absence/reduced expression leads to intracytoplasmic vacuoles containing autophagic material

Variant types (Brambatti et al 2019^[2]):

• Stop codon or frameshift variants: 63.8% (most common)
• Splice-site variants: 24.6%
• Missense variants: 6.9%
• Most frequent mutation locations: exon 8 (34%), exon 6 (18%)
• Exon 9b mutations, notably associated with severe skeletal myopathy but absent or mild cardiac involvement in males; females carrying same mutations may die suddenly

Males: Severe, early-onset disease (teenagers to 20s); full absence of LAMP2 protein; high early mortality

Females: Variable severity due to X-inactivation mosaicism; later onset (30s–40s); can present with isolated cardiac disease in 73% of cases

Rare, exact population prevalence unknown; may account for up to 5% of paediatric hypertrophic cardiomyopathy (HCM)^[2]

Systematic review of 146 confirmed cases (Brambatti et al 2019^[2]): 92.5% had cardiac abnormalities; 61.6% male, 38.4% female

More severe and earlier-onset in males (hemizygous, complete LAMP2 protein absence); females show variable severity (X-inactivation mosaicism)

Classic clinical triad (present in 42% of males, rarely in females^[2]):

• Cardiomyopathy (HCM phenotype predominant)
• Skeletal myopathy, proximal muscle weakness, elevated CK (present in 50.7%; mainly males)
• Cognitive impairment, mild to moderate in 55.6% of patients; predominantly males (80%)

Cardiac features (Brambatti et al 2019^[2]):

• Males: HCM in 96.2%; DCM at presentation 3.8%
• Females: HCM in 70.3%; DCM at presentation 29.3%, significantly higher than males (p<0.001)
• LVOT obstruction present in 26.2% of HCM patients
• End-stage cardiomyopathy (refractory to medical management) in 43.8% of HCM patients
• WPW pattern in 41.8%: accessory pathway pre-excitation with no clear gender difference
• Cardiac conduction abnormalities in 57.5%
• Late gadolinium enhancement (LGE) on CMR: present in 92.3% of males and 69.2% of females with available data

Non-cardiac features:

• Vision impairment (retinopathy, myopia) in 20.2%, onset median age 15 years
• Stroke/TIA (5 patients); fatal in 2/5 cases
• Elevated liver enzymes (AST, ALT), CK, and LDH in 61.9% of patients with available data

ECG: WPW (delta wave, short PR, broad QRS) in 41.8%; LVH voltage criteria; conduction abnormalities; ventricular tachycardia (10% males, 3.6% females)

Echocardiography/CMR: Severe concentric LVH (often massively beyond typical HCM); assess LVEF, LVOT gradient, MR; CMR with LGE shows diffuse patchy pattern in the majority

Serum CK: Elevated (skeletal myopathy), also check LDH, AST, ALT

Genetic testing: LAMP2 full gene sequencing and deletion/duplication analysis; ClinVar pathogenicity assessment

Muscle biopsy: Vacuolar myopathy with autophagic vacuoles on electron microscopy (pathognomonic but less often required if genetic diagnosis confirmed)

Ophthalmology referral: For retinopathy/myopia assessment (20% affected)

No disease-modifying therapy currently available: management is supportive and symptomatic:

Heart failure: Standard GDMT (beta-blockers, ACE-I, MRA) but disease is often refractory to medical management; progression to end-stage HF occurs in 43.8%^[2]

Arrhythmias/ICD:

• 45% of patients received an ICD (76.5% for primary prevention)^[2]
• Consider ICD early, even with preserved LVEF, given high SCD risk in both sexes
• WPW with accessory pathway, electrophysiological study and ablation consideration

Cardiac transplantation:

• Required in 19.9% (29/146 patients); males, in teens-to-20s; females, in 30s-40s
• Skeletal myopathy can progress post-transplant in males (profound muscle weakness reported in 4 patients post-HTx)
• Two male patients required re-transplantation due to allograft rejection

Emerging therapies: Gene therapy and substrate reduction approaches under investigation

• Rapidly progressive cardiomyopathy: with massive hypertrophy in males, leading to heart failure and early death, often before age 30, so transplant is frequently the only effective option^[2]
• Ventricular pre-excitation (WPW) and ventricular arrhythmia: with sudden death
• Multisystem involvement: skeletal myopathy, cognitive impairment and retinopathy

Composite outcome (death, heart transplant, or LVAD), Brambatti et al 2019^[2]:

• Overall: 34.9% of patients reached composite outcome
• Males: Composite outcome at median age 21 years (range: teens to 30s)
• Females: Composite outcome at median age 38 years (p<0.001 vs males)
• Cause of death (where reported): heart failure 45%, sudden cardiac death 40%, stroke 10%

High SCD risk: ICD recommended even with preserved EF if high-risk features present

Based on ESC 2023 Cardiomyopathy guidelines and Danon registry data^[1][2].

Advanced / complicated = any LVEF decline or symptomatic cardiomyopathy, note rapid progression, especially in males, warranting a low threshold for transplant assessment.

Genotype-positive / phenotype-negative (G+/P−) = a confirmed pathogenic-variant carrier with no overt disease expression yet.

	Genotype+ / Phenotype−	Uncomplicated / Stable	Advanced / Complicated
Frequency	Every 6–12 months (rapid conversion, esp. males)	Every 6 months	Every 3 months
Clinical review	Symptoms, neuromuscular & cognitive review	Symptoms, NYHA, neuromuscular & cognitive status	As above + transplant assessment
ECG	Each visit (pre-excitation, LVH)	Each visit (pre-excitation, conduction, LVH)	Each visit
Echocardiography	6–12 monthly	6-monthly	3-monthly
Holter / ambulatory	Periodic	6-monthly	3-monthly
CMR	Baseline + periodic	Baseline + periodic (LGE burden)	As clinically indicated
Transplant pathway	–	Early referral / listing discussion	Active evaluation

Disclaimer: This table is general guidance based on published guidelines and does not replace clinical judgement. The responsible clinician is accountable for determining the appropriate, individualised follow-up plan for each patient.

• Suspect in young males with severe LVH + WPW pattern + elevated CK, the classic triad is pathognomonic^[1]
• Females can present differently, up to 29.3% with DCM (not HCM); LAMP2 testing warranted in women with unexplained cardiomyopathy and family history^[2]
• Do NOT confuse with sarcomeric HCM, management and prognosis differ substantially
• Rapidly progressive, early and proactive cardiac transplant evaluation is essential in males^[1]
• Screen all first-degree female relatives, can be affected (milder, later onset)
• Multidisciplinary team involvement: cardiology, neurology/myology, ophthalmology, clinical genetics