A Mild Version of Danon Disease Caused by a Newly Recognized Mutation in the Lysosome-associated Membrane Protein-2 Gene

Update Review about Metabolic Myopathies

The aim of this review is to summarize and discuss recent findings and new insights in the etiology and phenotype of metabolic myopathies. The review relies on a systematic literature review of recent publications. Metabolic myopathies are a heterogeneous group of disorders characterized by mostly inherited defects of enzymatic pathways involved in muscle cell metabolism. Metabolic myopathies present with either permanent (fixed) or episodic abnormalities, such as weakness, wasting, exercise-intolerance, myalgia, or an increase of muscle breakdown products (creatine-kinase, myoglobin) during exercise. Though limb and respiratory muscles are most frequently affected, facial, extra-ocular, and axial muscles may be occasionally also involved. Age at onset and prognosis vary considerably. There are multiple disease mechanisms and the pathophysiology is complex. Genes most recently related to metabolic myopathy include PGM1, GYG1, RBCK1, VMA21, MTO1, KARS, and ISCA2. The number of metabolic myopathies is steadily increasing. There is limited evidence from the literature that could guide diagnosis and treatment of metabolic myopathies. Treatment is limited to mainly non-invasive or invasive symptomatic measures. In conclusion, the field of metabolic myopathies is evolving with the more widespread availability and application of next generation sequencing technologies worldwide. This will broaden the knowledge about pathophysiology and putative therapeutic strategies for this group of neuromuscular disorders.

A novel LAMP2 p.G93R mutation associated with mild Danon disease presenting with familial hypertrophic cardiomyopathy

BACKGROUND

Danon disease (DD) is an X-linked dominant multisystem disorder that is associated with cardiomyopathy, skeletal myopathy, and varying degrees of intellectual disability. It results from mutations in the lysosome-associated membrane protein 2 (LAMP2) gene.

METHODS

Herein, a proband with a mild DD case presenting with a familial hypertrophic cardiomyopathy (HCM) phenotype and additional family members were evaluated. Exome sequencing and Sanger sequencing were performed to explore the genetic basis of DD in the proband. Segregation, in silico, and functional analyses were carried out to explore potential pathogenicity in the candidate mutation.

RESULTS

Exome sequencing and Sanger sequencing identified one novel missense mutation (p.G93R) in the LAMP2 gene in the proband, and this mutation was also identified in three other family members. In silico analysis of LAMP2 predicted that the mutation causes a conformational change and subsequent protein destabilization. Furthermore, functional examination showed that mutation carriers have a significant reduction in LAMP2 expression, which supports that the mutation is pathogenic. Moreover, skewed X chromosome inactivation (XCI) was identified in one female mutation carrier, thus suggesting that skewed XCI may be the reason why this individual escaped the pathogenic influence of the mutation.

CONCLUSION

These findings will aid in diagnosing DD patients carrying this LAMP2 mutation that presents with a HCM phenotype. Furthermore, this study illustrates the importance of utilizing a molecular diagnostic approach in HCM patients and is the first study to report a LAMP2 p.G93R mutation associated with mild DD and identify that XCI serves a protective role in DD etiology.