Introduction

Danon disease is a rare and fatal genetic disorder that mainly involves the heart and skeletal muscles and, in a minority of individuals, the brain. Although the disease was first described decades ago, recent advances in molecular biology have established that the complex clinical presentation of the disease arises due to mutations in the LAMP2 gene. Notably, the presentation of Danon disease in specific tissues mainly relies on the various isoforms of the LAMP2 protein.¹ Elucidation of how the isoforms work and their restricted expression in the body clarifies why the disease mainly attacks specific tissues and how such insights may lead to future therapies.

Danon disease: clinical and genetic overview

Danon disease presents as a lysosomal storage disease inherited through the X chromosome. It presents most frequently in people assigned male at birth (AMAB), who develop the condition in adolescence or early adulthood. People assigned female at birth (AFAB) are also affected but less seriously due to the X inactivation process.

The usual clinical presentation entails:

• Hypertrophic cardiomyopathy (HCM): Thickened heart muscle causing heart failure, arrhythmias, and sudden death
• Skeletal myopathy: Muscular fatigue and weakness
• Variable cognitive impairment: Emerges in a minority of patients

The cause is mutations in the LAMP2 gene, which produces a protein associated with the lysosome.² LAMP2 has a central role in the function of lysosomes, autophagy, and cell constituent degradation and recycling.³

The LAMP2 gene and LAMP2 isoforms

The LAMP2 gene encodes three isoforms—LAMP2A, LAMP2B, and LAMP2C—by alternative splicing of the terminal exon. Each isoform contains a distinct cytoplasmic tail that regulates its intracellular localisation and activity.

LAMP2A: chaperone-mediated autophagy specialist

LAMP2A functions mainly in an autophagy mediated by the proteins called chaperones. This process is important for protein quality control and plays a central role during stress conditions. LAMP2A forms the lysosomal membrane receptor that transports the substrate outside the lysosome.⁴

LAMP2B: Macroautophagy in heart and muscle

LAMP2B contributes to the bulk degradation at the cytoplasmic level of material, including damaged organelles, a process called macroautophagy. It is highly expressed in skeletal and cardiac muscle tissue and holds the elevated degradation flux that must be obtained as a component of tissue health and energy balance. Disabling LAMP2B inhibits the process and causes accumulation of autophagic vacuoles and cellular.⁴

LAMP2C: The latest nucleic acid autophagy player

LAMP2C is the least-characterised isoform and has been implicated in autophagic clearance of nucleic acids (DNA and RNA). The tissue-specific expression and precise physiological function of the isoform continue to be the subject of active research, with some evidence suggesting potential functions in the maintenance of neurons and the immune system.⁴

Tissue-specific expression patterns for LAMP2 isoforms

The three isoforms of LAMP2 are differentially expressed in tissues and account for most selective vulnerability in Danon disease.

Skeletal and cardiac muscle

These tissues essentially express LAMP2B. Owing to the key function of macroautophagy in muscle energy and function metabolism, the absence of LAMP2B leads to the debilitating myopathy and heart muscle disease of the Danon disease.

Kidney and liver

These tissues depend upon LAMP2A for chaperone-mediated autophagy. Even when Danon disease is most connected with muscle symptoms, the tissues may show delicate impairments when it comes to dysfunctional degradation pathways in proteins.

Brain

The three isoforms, including LAMP2C, are present in the central nervous system. This heterogenously directed expression pattern may be the cause of the variable cognitive manifestations in Danon patients, with LAMP2C implicated in nucleic acid autophagy in the neurons.

Pathogenic mechanisms: LAMP2 isoform deficiency as the driver of disease

Inactivation of the autophagy by mutations in the LAMP2 isoforms develops Danon disease, but the clinical effect varies by isoform and tissue.

LAMP2B and cardiac/skeletal muscle dysfunction

The most severe forms arise from loss-of-function mutations in heart and skeletal muscle LAMP2B. This protein plays a critical role in the clearance by autophagy of dysfunctional mitochondrial subunits and aberrant proteins. If macroautophagy does not work efficiently, cells become loaded with autophagic cell space and dysfunctional cell waste, and eventually undergo cell death and tissue fibrosis.⁵

Hypertrophic cardiomyopathy ensues when cardiomyocytes, the muscle cells of the heart, try to compensate for the dysfunction, but ultimately, heart failure follows. It goes the same way with muscle weakness in the skeleton resulting from inappropriate muscle fibre turnover and energy deficits.

Potential role of LAMP2A in other organs

While LAMP2A deficiency is less related to Danon disease classical symptomatology, it can potentially be the root of weak hepatic or renal function capacity through the impaired selective proteolysis of damaged proteins and the resulting cell stress.⁵

Emerging evidence for LAMP2C in neurological symptoms

The function of LAMP2C in nucleic acid autophagy became evident only very recently. Failure in the clearance of RNA and DNA in neurons would predispose to neurodegeneration or cognitive development defect in Danon patients, although additional studies are needed to account for such a correlation.

Clarifying the clinical heterogeneity of Danon disease

The cellular diversity inside a tissue and the variability in clinical symptoms in Danon patients are the result of the following factors:

• Isoform-specific expression: The predominance of expression of LAMP2B in skeletal muscle and heart accounts for maximum severity in the above-mentioned tissues
• X-linked inheritance: People AMABMales harbour a single X chromosome, expressing the mutant gene and causing the disorder to run in severe form.⁶ People AFAB are generally symptomless or only mildly affected because they carry one out of two copies of the mutant gene, and its expression depends on the pattern of inactivation of the X chromosome
• Type and location of mutation: Individual LAMP2 gene mutations may disrupt specific isoforms or overall protein stability and influence the severity of the disease⁷

Therapeutic considerations: targeting LAMP2 isoforms

Current treatment for Danon disease is generally palliative and aimed at heart failure, arrhythmias, and muscle symptoms. The identification of the function of the LAMP2 isoforms offers hope for new therapies.

Gene therapy

Gene therapies for the correction of LAMP2B expression in cardiac tissue are underway for the treatment of the disease-causing origin of cardiomyopathy. Preclinical studies hold promise but are plagued by challenges such as selective delivery and duration of safety.⁸

Isoform-specific therapy

Therapies in the future can potentially be customised to adjust the function of single isoforms or to replace them in selective tissues and reduce muscle and neurological disease.⁹

Challenges

1. Impart tissue-specificity to gene or protein therapies
2. Balancing isoform expression to prevent off-targeting
3. Managing the multisystem nature of Danon disease

FAQs

What is the overall cause of Danon disease?

Danon disease is caused by mutations in the LAMP2 gene, resulting in impaired lysosomal autophagy.

Why are the skeletal muscles and heart mainly affected?

These tissues preferentially express the LAMP2B isoform, which is important for the degradation of cellular organelles (macroautophagy). The deficiency of LAMP2B causes the disease symptoms by interfering with this process.

Can people AFAB get Danon disease?

Yes, but usually with diminished symptoms by virtue of random inactivation of the X chromosome.

Do corrective treatments exist for the underlying cause? 

Treatment is presently supportive, but gene therapy for LAMP2B is a hopeful field of research.

Summary

Danon disease is a multi-organ lysosomal storage disease whose tissue-specific pathogenesis depends on the differential expression and function of LAMP2 gene isoforms. The tissue-specific abundance of LAMP2B in heart and skeletal muscle explains why the most severely affected tissue types develop disease pathology and crippling myopathy and cardiomyopathy. LAMP2A and LAMP2C must therefore be the explanation for disease manifestation in other tissue types, including the brain.

In-depth knowledge about LAMP2 isoforms not only elucidates the pathogenesis of Danon disease but also opens the way to isoform-specific therapies, most notably gene therapies aimed at the cardiac muscle-specific LAMP2B isoform. Other studies on the biological activities of the isoforms will continue to enrich diagnosis, prognosis, and treatment for the patients of this rare disease.