Generalized lysosome-associated membrane protein-2 defect explains multisystem clinical involvement and allows leukocyte diagnostic screening in Danon disease.
Danon disease, an X-linked dominant disorder, results from mutations in the lysosome-associated membrane protein-2 (LAMP2) gene and presents with hypertrophic cardiomyopathy, skeletal myopathy, and mental retardation. To investigate the effects of LAMP2 gene mutations on protein expression in different tissues, we screened LAMP2 gene mutations and LAMP-2 protein deficiency in the skeletal muscle of nine unrelated patients with hypertrophic cardiomyopathy and vacuolar myopathy. We identified three novel families (including one affected mother) with unreported LAMP2 gene null mutations and LAMP-2 protein deficiency in skeletal and myocardial muscle, leukocytes, and fibroblasts. LAMP-2 protein deficiency was detectable in various tissues, including leukocytes, explaining the multisystem clinical involvement. Skeletal muscle immunopathology showed that mutant protein was not localized in the Golgi complex, vacuolar membranes expressed sarcolemmal-specific proteins, and the degree of muscle fiber vacuolization correlated with clinical muscle involvement. In our female patient, muscle histopathology and LAMP-2 protein analysis was inconclusive, indicating that diagnosis in females requires mutation identification. The random X-chromosome inactivation found in muscle and leukocytes excluded the possibility that selective involvement of some tissues in females is due to skewed X-chromosome inactivation. Therefore, biochemical analysis of leukocytes might be used for screening in male patients, but genetic screening is required in females.