Cardiac MRI and Clinical Outcomes in TMEM43 Arrhythmogenic Cardiomyopathy

Arrhythmogenic cardiomyopathy is an inherited cardiomyopathy that can involve both ventricles. Several genes have been identified as pathogenic in arrhythmogenic cardiomyopathy, including TMEM43. However, there are limited data on cardiac MRI findings in patients with TMEM43 variants to date. In this case series, cardiac MRI findings and clinical outcomes are described in 14 patients with TMEM43 variants, including eight (57%) with the pathogenic p.Ser358Leu variant (six female patients; mean age, 33 years ± 15 [SD]) and six (43%) with a TMEM43 variant of unknown significance (three female patients; mean age, 38 years ± 11). MRI findings demonstrated left ventricular systolic dysfunction in eight (57%) patients and right ventricular dysfunction in four (29%) patients. Among the nine patients with late gadolinium enhancement imaging, left ventricular late gadolinium enhancement was present in seven (78%; all subepicardial) patients. In summary, TMEM43 variants are associated with high prevalence of subepicardial late gadolinium enhancement and left ventricular dysfunction. Keywords: Arrhythmogenic Cardiomyopathy, Arrhythmogenic Right Ventricular Cardiomyopathy, TMEM43, Cardiac MRI, Genetic Variants Supplemental material is available for this article.

Adipogenic Signaling Promotes Arrhythmia Substrates before Structural Abnormalities in TMEM43 ARVC

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a genetic disorder of desmosomal and structural proteins that is characterized by fibro-fatty infiltrate in the ventricles and fatal arrhythmia that can occur early before significant structural abnormalities. Most ARVC mutations interfere with β-catenin-dependent transcription that enhances adipogenesis; however, the mechanistic pathway to arrhythmogenesis is not clear. We hypothesized that adipogenic conditions play an important role in the formation of arrhythmia substrates in ARVC. Cardiac myocyte monolayers co-cultured for 2-4 days with mesenchymal stem cells (MSC) were derived from human-induced pluripotent stem cells with the ARVC5 TMEM43 p.Ser358Leu mutation. The TMEM43 mutation in myocyte co-cultures alone had no significant effect on impulse conduction velocity (CV) or APD. In contrast, when co-cultures were exposed to pro-adipogenic factors for 2-4 days, CV and APD were significantly reduced compared to controls by 49% and 31%, respectively without evidence of adipogenesis. Additionally, these arrhythmia substrates coincided with a significant reduction in IGF-1 expression in MSCs and were mitigated by IGF-1 treatment. These findings suggest that the onset of enhanced adipogenic signaling may be a mechanism of early arrhythmogenesis, which could lead to personalized treatment for arrhythmias associated with TMEM43 and other ARVC mutations.

Altered Expression of TMEM43 Causes Abnormal Cardiac Structure and Function in Zebrafish

Arrhythmogenic cardiomyopathy (ACM) is an inherited heart muscle disease caused by heterozygous missense mutations within the gene encoding for the nuclear envelope protein transmembrane protein 43 (TMEM43). The disease is characterized by myocyte loss and fibro-fatty replacement, leading to life-threatening ventricular arrhythmias and sudden cardiac death. However, the role of TMEM43 in the pathogenesis of ACM remains poorly understood. In this study, we generated cardiomyocyte-restricted transgenic zebrafish lines that overexpress eGFP-linked full-length human wild-type (WT) TMEM43 and two genetic variants (c.1073C>T, p.S358L; c.332C>T, p.P111L) using the Tol2-system. Overexpression of WT and p.P111L-mutant TMEM43 was associated with transcriptional activation of the mTOR pathway and ribosome biogenesis, and resulted in enlarged hearts with cardiomyocyte hypertrophy. Intriguingly, mutant p.S358L TMEM43 was found to be unstable and partially redistributed into the cytoplasm in embryonic and adult hearts. Moreover, both TMEM43 variants displayed cardiac morphological defects at juvenile stages and ultrastructural changes within the myocardium, accompanied by dysregulated gene expression profiles in adulthood. Finally, CRISPR/Cas9 mutants demonstrated an age-dependent cardiac phenotype characterized by heart enlargement in adulthood. In conclusion, our findings suggest ultrastructural remodeling and transcriptomic alterations underlying the development of structural and functional cardiac defects in TMEM43-associated cardiomyopathy.

TMEM43 promotes pancreatic cancer progression by stabilizing PRPF3 and regulating RAP2B/ERK axis

BACKGROUND

Transmembrane protein 43 (TMEM43), a member of the transmembrane protein subfamily, plays a critical role in the initiation and development of cancers. However, little is known concerning the biological function and molecular mechanisms of TMEM43 in pancreatic cancer.

METHODS

In this study, TMEM43 expression levels were analyzed in pancreatic cancer samples compared with control samples. The relationship of TMEM43 expression and disease-free survival (DFS) and overall survival (OS) were assessed in pancreatic cancer patients. In vitro and in vivo assays were performed to explore the function and role of TMEM43 in pancreatic cancer. Coimmunoprecipitation (co-IP) followed by protein mass spectrometry was applied to analyze the molecular mechanisms of TMEM43 in pancreatic cancer.

RESULTS

We demonstrated that TMEM43 expression level is elevated in pancreatic cancer samples compared with control group, and is correlated with poor DFS and OS in pancreatic cancer patients. Knockdown of TMEM43 inhibited pancreatic cancer progression in vitro, decreased the percentage of S phase, and inhibited the tumorigenicity of pancreatic cancer in vivo. Moreover, we demonstrated that TMEM43 promoted pancreatic cancer progression by stabilizing PRPF3 and regulating the RAP2B/ERK axis.

CONCLUSIONS

The present study suggests that TMEM43 contributes to pancreatic cancer progression through the PRPF3/RAP2B/ERK axis, and might be a novel therapeutic target for pancreatic cancer.

A Deafness Associated Protein TMEM43 Interacts with KCNK3 (TASK-1) Two-pore Domain K+ (K2P) Channel in the Cochlea

The TMEM43 has been studied in human diseases such as arrhythmogenic right ventricular cardiomyopathy type 5 (ARVC5) and auditory neuropathy spectrum disorder (ANSD). In the heart, the p.(Ser358Leu) mutation has been shown to alter intercalated disc protein function and disturb beating rhythms. In the cochlea, the p.(Arg372Ter) mutation has been shown to disrupt connexin-linked function in glia-like supporting cells (GLSs), which maintain inner ear homeostasis for hearing. The TMEM43-p.(Arg372Ter) mutant knock-in mice displayed a significantly reduced passive conductance current in the cochlear GLSs, raising a possibility that TMEM43 is essential for mediating the passive conductance current in GLSs. In the brain, the two-pore-domain potassium (K2P) channels are generally known as the “leak channels” to mediate background conductance current, raising another possibility that K2P channels might contribute to the passive conductance current in GLSs. However, the possible association between TMEM43 and K2P channels has not been investigated yet. In this study, we examined whether TMEM43 physically interacts with one of the K2P channels in the cochlea, KCNK3 (TASK-1). Utilizing co-immunoprecipitation (IP) assay and Duolink proximity ligation assay (PLA), we revealed that TMEM43 and TASK-1 proteins could directly interact. Genetic modifications further delineated that the intracellular loop domain of TMEM43 is responsible for TASK-1 binding. In the end, gene-silencing of Task-1 resulted in significantly reduced passive conductance current in GLSs. Together, our findings demonstrate that TMEM43 and TASK-1 form a protein-protein interaction in the cochlea and provide the possibility that TASK-1 is a potential contributor to the passive conductance current in GLSs.

Dilated cardiomyopathy in the era of precision medicine: latest concepts and developments

Dilated cardiomyopathy (DCM) is an umbrella term entailing a wide variety of genetic and non-genetic etiologies, leading to left ventricular systolic dysfunction and dilatation, not explained by abnormal loading conditions or coronary artery disease. The clinical presentation can vary from asymptomatic to heart failure symptoms or sudden cardiac death (SCD) even in previously asymptomatic individuals. In the last 2 decades, there has been striking progress in the understanding of the complex genetic basis of DCM, with the discovery of additional genes and genotype-phenotype correlation studies. Rigorous clinical work-up of DCM patients, meticulous family screening, and the implementation of advanced imaging techniques pave the way for a more efficient and earlier diagnosis as well as more precise indications for implantable cardioverter defibrillator implantation and prevention of SCD. In the era of precision medicine, genotype-directed therapies have started to emerge. In this review, we focus on updates of the genetic background of DCM, characteristic phenotypes caused by recently described pathogenic variants, specific indications for prevention of SCD in those individuals and genotype-directed treatments under development. Finally, the latest developments in distinguishing athletic heart syndrome from subclinical DCM are described.

Exercise and arrhythmic risk in TMEM43 p.S358L arrhythmogenic right ventricular cardiomyopathy

BACKGROUND

High-level exercise has been associated with a malignant phenotype in desmosomal and genotype-negative forms of arrhythmogenic right ventricular cardiomyopathy (ARVC). This is the first study to examine this issue with ARVC secondary to the TMEM43 p.S358L mutation.

OBJECTIVE

The purpose of this study was to evaluate the impact of exercise on arrhythmic risk and cardiac death in TMEM43 p.S358L ARVC.

METHODS

Individuals with the TMEM43 p.S358L mutation enrolled in a prospective registry who had received a primary prevention implantable cardioverter-defibrillator (ICD) were invited to complete the modified Paffenbarger Physical Activity Questionnaire to assess their physical activity in the year before their ICD implantation. Time-to-event analyses using unadjusted and adjusted Cox proportional hazards models evaluated associations between physical activity and first appropriate ICD discharge secondary to malignant ventricular arrhythmia or cardiac death.

RESULTS

In 80 subjects with the TMEM43 p.S358L mutation, exercise ≥9.0 metabolic equivalent of task (MET)-hours/day (high level) in the year before ICD implantation was associated with an adjusted 9.1-fold increased hazard of first appropriate ICD discharge (there were no deaths) relative to physical activity <9.0 MET-hours/day (moderate level) (95% confidence interval [CI] 3.3-24.6 MET-hours/day; P < .001). The median age from birth to first appropriate ICD discharge was 58.5 years (95% CI 56.5-60.5 years) vs 35.8 years (95% CI 28.2-43.4 years) (P < .001) in subjects in moderate- and high-level exercise groups, respectively.

CONCLUSION

Exercise ≥9.0 MET-hours/day is associated with an increased risk of malignant ventricular arrhythmias in the TMEM43 p.S358L subtype of ARVC. Extrapolating these data, we suggest molecular testing be offered in early childhood to inform exercise choices reflective of the genotype.

Clinical characteristics and determinants of the phenotype in TMEM43 arrhythmogenic right ventricular cardiomyopathy type 5

BACKGROUND

Arrhythmogenic right ventricular cardiomyopathy type V (ARVC-5) is the most aggressive heterozygous form of ARVC. It is predominantly caused by a fully penetrant mutation (p.S358L) in the nondesmosomal gene TMEM43-endemic to Newfoundland, Canada. To date, all familial cases reported worldwide share a common ancestral haplotype. It is unknown whether the p.S358L mutation by itself causes ARVC-5 or whether the disease is influenced by genetic or environmental factors.

OBJECTIVE

The purpose of this study was to examine the phenotype, clinical course, and the impact of exercise on patients with p.S358L ARVC-5 without the Newfoundland genetic background.

METHODS

We studied 62 affected individuals and 73 noncarriers from 3 TMEM43-p.S358L Spanish families. The impact of physical activity on the phenotype was also evaluated.

RESULTS

Haplotype analysis revealed that the 3 Spanish families were unrelated to patients with ARVC-5 with the Newfoundland genetic background. Two families shared 10 microsatellite markers in a 4.9 cM region surrounding TMEM43; the third family had a distinct haplotype. The affected individuals showed a 38.7% incidence of sudden cardiac death, which was higher in men. Left ventricular involvement was common, with 40% of mutation carriers showing a left ventricular ejection fraction of <50%. Compared with noncarriers, the R-wave voltage in lead V₃ was lower (3.2 ± 2.8 mV vs 7.5 ± 3.6 mV; P < .001) and QRS complex in right precordial leads wider (104.7 ± 24.0 ms vs 88.2 ± 7.7 ms; P = .001). A history of vigorous exercise showed a trend toward more ventricular arrhythmias only in women (P = .053).

CONCLUSION

ARVC-5 is associated with a high risk of sudden cardiac death and characteristic clinical and electrocardiographic features irrespective of geographical origin and genetic background. Our data suggest that, as in desmosomal ARVC, vigorous physical activity could aggravate the phenotype of TMEM43 mutation carriers.

Severe Cardiac Dysfunction and Death Caused by Arrhythmogenic Right Ventricular Cardiomyopathy Type 5 Are Improved by Inhibition of Glycogen Synthase Kinase-3β

BACKGROUND

Arrhythmogenic cardiomyopathy/arrhythmogenic right ventricular cardiomyopathy (ARVC) is an inherited cardiac disease characterized by fibrofatty replacement of the myocardium, resulting in heart failure and sudden cardiac death. The most aggressive arrhythmogenic cardiomyopathy/ARVC subtype is ARVC type 5 (ARVC5), caused by a p.S358L mutation in TMEM43 (transmembrane protein 43). The function and localization of TMEM43 are unknown, as is the mechanism by which the p.S358L mutation causes the disease. Here, we report the characterization of the first transgenic mouse model of ARVC5.

METHODS

We generated transgenic mice overexpressing TMEM43 in either its wild-type or p.S358L mutant (TMEM43-S358L) form in postnatal cardiomyocytes under the control of the α-myosin heavy chain promoter.

RESULTS

We found that mice expressing TMEM43-S358L recapitulate the human disease and die at a young age. Mutant TMEM43 causes cardiomyocyte death and severe fibrofatty replacement. We also demonstrate that TMEM43 localizes at the nuclear membrane and interacts with emerin and β-actin. TMEM43-S358L shows partial delocalization to the cytoplasm, reduced interaction with emerin and β-actin, and activation of glycogen synthase kinase-3β (GSK3β). Furthermore, we show that targeting cardiac fibrosis has no beneficial effect, whereas overexpression of the calcineurin splice variant calcineurin Aβ1 results in GSK3β inhibition and improved cardiac function and survival. Similarly, treatment of TMEM43 mutant mice with a GSK3β inhibitor improves cardiac function. Finally, human induced pluripotent stem cells bearing the p.S358L mutation also showed contractile dysfunction that was partially restored after GSK3β inhibition.

CONCLUSIONS

Our data provide evidence that TMEM43-S358L leads to sustained cardiomyocyte death and fibrofatty replacement. Overexpression of calcineurin Aβ1 in TMEM43 mutant mice or chemical GSK3β inhibition improves cardiac function and increases mice life span. Our results pave the way toward new therapeutic approaches for ARVC5.

TMEM43-S358L mutation enhances NF-κB-TGFβ signal cascade in arrhythmogenic right ventricular dysplasia/cardiomyopathy

Arrhythmogenic right ventricular dysplasia/cardiomyopathy (ARVD/C) is a genetic cardiac muscle disease that accounts for approximately 30% sudden cardiac death in young adults. The Ser358Leu mutation of transmembrane protein 43 (TMEM43) was commonly identified in the patients of highly lethal and fully penetrant ARVD subtype, ARVD5. Here, we generated TMEM43 S358L mouse to explore the underlying mechanism. This mouse strain showed the classic pathologies of ARVD patients, including structural abnormalities and cardiac fibrofatty. TMEM43 S358L mutation led to hyper-activated nuclear factor κB (NF-κB) activation in heart tissues and primary cardiomyocyte cells. Importantly, this hyper activation of NF-κB directly drove the expression of pro-fibrotic gene, transforming growth factor beta (TGFβ1), and enhanced downstream signal, indicating that TMEM43 S358L mutation up-regulates NF-κB-TGFβ signal cascade during ARVD cardiac fibrosis. Our study partially reveals the regulatory mechanism of ARVD development.

Fetal arrhythmogenic right ventricular cardiomyopathy with double mutations in TMEM43

We herein describe a fetal case of arrhythmogenic right ventricular cardiomyopathy (ARVC) with double mutations in transmembrane protein 43 (TMEM43). RV aneurysm and ventricular arrhythmia were detected during the fetal period. After birth, electrocardiogram showed frequent premature ventricular contractions (PVC) of left bundle branch block morphology and epsilon waves in the right-sided chest leads. Echocardiography also indicated RV aneurysm with regionally decreased systolic function. PVC disappeared after treatment with amiodarone and mexiletin. Mutations in TMEM43, which was recently identified as the causative gene of ARVC type 5, were also confirmed in the present patient and in the patient's mother, and they were therefore diagnosed with ARVC. The present case confirms that symptoms of ARVC can emerge during the fetal period. Pediatricians need to keep in mind the possibility of ARVC when they encounter patients with RV aneurysm and arrhythmia.