Metabolic Maturation Exaggerates Abnormal Calcium Handling in a Lamp2 Knockout Human Pluripotent Stem Cell-Derived Cardiomyocyte Model of Danon Disease

Danon disease (DD) is caused by mutations of the gene encoding lysosomal-associated membrane protein type 2 (LAMP2), which lead to impaired autophagy, glycogen accumulation, and cardiac hypertrophy. However, it is not well understood why a large portion of DD patients develop arrhythmia and sudden cardiac death. In the current study, we generated LAMP2 knockout (KO) human iPSC-derived cardiomyocytes (CM), which mimic the LAMP2 dysfunction in DD heart. Morphologic analysis demonstrated the sarcomere disarrangement in LAMP2 KO CMs. In functional studies, LAMP2 KO CMs showed near-normal calcium handling at base level. However, treatment of pro-maturation medium (MM) exaggerated the disease phenotype in the KO cells as they exhibited impaired calcium recycling and increased irregular beating events, which recapitulates the pro-arrhythmia phenotypes of DD patients. Further mechanistic study confirmed that MM treatment significantly enhanced the autophagic stress in the LAMP2 KO CMs, which was accompanied by an increase of both cellular and mitochondrial reactive oxygen species (ROS) levels. Excess ROS accumulation in LAMP2 KO CMs resulted in the over-activation of calcium/calmodulin dependent protein kinase IIδ (CaMKIIδ) and arrhythmogenesis, which was partially rescued by the treatment of ROS scavenger. In summary, our study has revealed ROS induced CaMKIIδ overactivation as a key mechanism that promotes cardiac arrhythmia in DD patients.

Application of Patient-Specific iPSCs for Modelling and Treatment of X-Linked Cardiomyopathies

Inherited cardiomyopathies are among the major causes of heart failure and associated with significant mortality and morbidity. Currently, over 70 genes have been linked to the etiology of various forms of cardiomyopathy, some of which are X-linked. Due to the lack of appropriate cell and animal models, it has been difficult to model these X-linked cardiomyopathies. With the advancement of induced pluripotent stem cell (iPSC) technology, the ability to generate iPSC lines from patients with X-linked cardiomyopathy has facilitated in vitro modelling and drug testing for the condition. Nonetheless, due to the mosaicism of the X-chromosome inactivation, disease phenotypes of X-linked cardiomyopathy in heterozygous females are also usually more heterogeneous, with a broad spectrum of presentation. Recent advancements in iPSC procedures have enabled the isolation of cells with different lyonisation to generate isogenic disease and control cell lines. In this review, we will summarise the current strategies and examples of using an iPSC-based model to study different types of X-linked cardiomyopathy. The potential application of isogenic iPSC lines derived from a female patient with heterozygous Danon disease and drug screening will be demonstrated by our preliminary data. The limitations of an iPSC-derived cardiomyocyte-based platform will also be addressed.

Phenotyping an adult zebrafish lamp2 cardiomyopathy model identifies mTOR inhibition as a candidate therapy

Adult zebrafish is an emerging vertebrate model for studying genetic basis of cardiomyopathies; but whether the simple fish heart can model essential features of hypertrophic cardiomyopathy (HCM) remained unknown. Here, we report a comprehensive phenotyping of a lamp2 knockout (KO) mutant. LAMP2 encodes a lysosomal protein and is a causative gene of Danon disease that is characterized by HCM and massive autophagic vacuoles accumulation in the tissues. There is no effective therapy yet to treat this most lethal cardiomyopathy in the young. First, we did find the autophagic vacuoles accumulation in cardiac tissues from lamp2 KO. Next, through employing a set of emerging phenotyping tools, we revealed heart failure phenotypes in the lamp2 KO mutants, including decreased ventricular ejection fraction, reduced physical exercise capacity, blunted β-adrenergic contractile response, and enlarged atrium. We also noted changes of the following indices suggesting cardiac hypertrophic remodeling in lamp2 KO: a rounded heart shape, increased end-systolic ventricular volume and density of ventricular myocardium, elevated actomyosin activation kinetics together with increased maximal isometric tension at the level of cardiac myofibrils. Lastly, we assessed the function of lysosomal-localized mTOR on the lamp2-associated Danon disease. We found that haploinsufficiency of mtor was able to normalize some characteristics of the lamp2 KO, including ejection fraction, β-adrenergic response, and the actomyosin activation kinetics. In summary, we demonstrate the feasibility of modeling the inherited HCM in the adult zebrafish, which can be used to develop potential therapies.

Early onset cardiomyopathy in females with Danon disease

Danon disease is caused by mutations in the lysosome-associated membrane protein-2 gene, LAMP2, located on the X chromosome. Female carriers with LAMP2 mutations most often present with late onset cardiomyopathy and slow disease progress; however, there are unusual cases that emerge early and show a more severe disease course. We investigated the explanted heart and skeletal muscle biopsies in two girls, aged ten and thirteen years, who underwent cardiac transplantation because of hypertrophic cardiomyopathy secondary to LAMP2 mutations and a 41-year old female with late-onset familial LAMP2 cardiomyopathy with more typical clinical phenotype. The two girls in contrast had clinical features that mimicked severe primary hypertrophic cardiomyopathy caused by mutations in genes encoding sarcomeric proteins. Immunohistochemistry in cardiac muscles showed a remarkable pattern with lack of LAMP2 protein in large regions including thousands of cardiomyocytes that also showed myocyte hypertrophy, lysosomal enlargement and disarray. In other equally large regions there were preserved LAMP2 expression and nearly normal histology. The skeletal muscle biopsy revealed no pathological changes. An uneven distribution of LAMP2 protein may cause deleterious effects depending on which regions of the myocardium are lacking LAMP2 protein in spite of an overall moderate reduction of LAMP2 protein. This may be a more common mechanism behind early aggressive disease in females than an overall skewed X-chromosome inactivation in the tissue.

Exercise training does not correct abnormal cardiac glycogen accumulation in the db/db mouse model of type 2 diabetes

Substrate imbalance is a well-recognized feature of diabetic cardiomyopathy. Insulin resistance effectively limits carbohydrate oxidation, resulting in abnormal cardiac glycogen accumulation. Aims of the present study were to 1) characterize the role of glycogen-associated proteins involved in excessive glycogen accumulation in type 2 diabetic hearts and 2) determine if exercise training can attenuate abnormal cardiac glycogen accumulation. Control (db(+)) and genetically diabetic (db/db) C57BL/KsJ-lepr(db)/lepr(db) mice were subjected to sedentary or treadmill exercise regimens. Exercise training consisted of high-intensity/short-duration (10 days) and low-intensity/long-duration (6 wk) protocols. Glycogen levels were elevated by 35-50% in db/db hearts. Exercise training further increased (2- to 3-fold) glycogen levels in db/db hearts. Analysis of soluble and insoluble glycogen pools revealed no differential accumulation of one glycogen subspecies. Phosphorylation (Ser(640)) of glycogen synthase, an indicator of enzymatic fractional activity, was greater in db/db mice subjected to sedentary and exercise regimens. Elevated glycogen levels were accompanied by decreased phosphorylation (Thr(172)) of 5'-AMP-activated kinase and phosphorylation (Ser(79)) of its downstream substrate acetyl-CoA carboxylase. Glycogen concentration was not associated with increases in other glycogen-associated proteins, including malin and laforin. Novel observations show that exercise training does not correct diabetes-induced elevations in cardiac glycogen but, rather, precipitates further accumulation.

Electron microscopic findings in skin biopsies from patients with Danon disease

Danon disease is a rare lysosomal disorder. It is due to deficiency of lysosomal-associated protein-2. In human LAMP-2 gene is located at chromosome region Xq24. Danon disease is characterized by hypertrophic cardiomyopathy, skeletal myopathy, mental retardation and retinopathy. To date, the morphological characterization of Danon disease has been limited to endomyocardial and skeletal muscle biopsies. In the current study we demonstrated that electron microscopy of a more accessible tissue, skin biopsies, is a useful method in the diagnosis of Danon disease.

Danon disease: case report and detection of new mutation

Danon disease is an X-linked disorder resulting from mutations in the lysosome-associated membrane protein-2 (LAMP2) gene. We report a male patient with skeletal myopathy, mental retardation, and massive hypertrophic obstructive cardiomyopathy necessitating heart transplantation. Immunohistochemistry of skeletal muscle and leukocytes, western blot analysis of leukocytes and cardiac muscle, flow cytometry, and DNA sequencing were performed. Muscle biopsy revealed autophagic vacuolar myopathy and lack of immunohistochemically detectable LAMP-2. Diagnosis of Danon disease was confirmed by western blot analysis of myocardial tissue and peripheral blood sample of the patient showing deficiency of LAMP-2 in myocardium and leukocytes. Moreover, absence of LAMP-2 in lymphocytes, monocytes and granulocytes was shown by flow cytometric analysis. Genetic analysis of the LAMP2 gene revealed a novel 1-bp deletion at position 179 (c.179delC) at the 3' end of exon 2, resulting in a frameshift with a premature stop codon.

Danon disease: a novel LAMP2 mutation affecting the pre-mRNA splicing and causing aberrant transcripts and partial protein expression

LAMP2, the causative gene of Danon disease, located on chromosome Xq24, encodes the lysosome-associated membrane protein-2 (LAMP-2). We describe clinical features and molecular data in an Italian patient with Danon disease. The patient had hyperCKemia, hypertrophic cardiomyopathy, no muscle weakness and slight mental impairment. Muscle biopsy revealed autophagic vacuoles with sarcolemmal features and glycogen storage. Immunohistochemistry and immunoblot revealed traces of LAMP-2 protein in skeletal muscle. Molecular analysis of the LAMP2 gene revealed a novel hemizygous mutation affecting the invariant +1 position of the splice site of intron 8, resulting in aberrant transcripts with skipping of exon 8 in all three LAMP-2 isoforms, skipping of exons 7 and 8 in LAMP-2A and 2C, and a 15 bp deletion in exon 8 of LAMP-2B. Low levels of normal LAMP-2B transcript were also present. Danon disease is an under-recognized and frequently fatal condition, treatable by heart transplantation. Investigation of the primary molecular defect is important for cardiac surveillance and genetic counseling.

Lysosomal myopathies: an excessive build-up in autophagosomes is too much to handle

Lysosomes are membrane-bound acidic organelles that contain hydrolases used for intracellular digestion of various macromolecules in a process generally referred to as autophagy. In normal skeletal and cardiac muscles, lysosomes usually appear morphologically unremarkable and thus are not readily visible on light microscopy. In distinct neuromuscular disorders, however, lysosomes have been shown to be structurally abnormal and functionally impaired, leading to the accumulation of autophagic vacuoles in myofibers. More specifically, there are myopathies in which buildup of these autophagic vacuoles seem to predominate the pathological picture. In such conditions, autophagy is considered not merely a secondary event, but a phenomenon that actually contributes to disease pathomechanism and/or progression. At present, there are two disorders in the muscle which are associated with primary defect in lysosomal proteins, namely Danon disease and Pompe disease. Other myopathies which have prominent autophagy in the skeletal muscle include X-linked myopathy with excessive autophagy (XMEA). In this review, these disorders are briefly characterized, and the role of autophagy in the context of the pathomechanism of these disorders is highlighted.

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.

Phenotypic heterogeneity in two unrelated Danon patients associated with the same LAMP-2 gene mutation

Danon disease, an X-linked cardioskeletal myopathy caused by primary deficiency of lysosome-associated membrane protein-2 (LAMP-2), is clinically characterized by cardiomyopathy, myopathy, and variable mental retardation. The pathological hallmark of the disease is the absence of LAMP-2 immunohistochemical staining in muscle. The LAMP-2 gene mutations reported thus far are generally private mutations. We describe two cases of Danon disease with different clinical presentation, in whom we identified the same exon skipping mutation c.928G>A in the LAMP-2 gene. The first patient was affected by an early onset myopathy and hypertrophic cardiomyopathy (HCM) that partially improved with drug treatment. A first muscle biopsy at age 4 months showed markedly increased glycogen, and acid maltase deficiency was ruled out biochemically. A second muscle biopsy, performed at age 3(1/2) years, showed very mild abnormalities. The second child at age 15 years had mild, diffuse muscle weakness and wasting, moderate mental deficiency, and HCM. Two serial biopsies performed at age 8 and 15 years showed similar findings of multiple esterase-positive vacuoles in type I myofibers. In both patients the immunohistochemical study demonstrated the absence of LAMP-2 in skeletal muscle.