In-Silico Modeling of the Functional Role of Reduced Sialylation in Sodium and Potassium Channel Gating of Mouse Ventricular Myocytes

Simulation Modeling of Reduced Glycosylation Effects on Potassium Channels of Mouse Cardiomyocytes

Dilated cardiomyopathy (DCM) is the third most common cause of heart failure and the primary reason for heart transplantation; upward of 70% of DCM cases are considered idiopathic. Our in-vitro experiments showed that reduced hybrid/complex N-glycosylation in mouse cardiomyocytes is linked with DCM. Further, we observed direct effects of reduced N-glycosylation on Kv gating. However, it is difficult to rigorously determine the effects of glycosylation on Kv activity, because there are multiple Kv isoforms in cardiomyocytes contributing to the cardiac excitation. Due to complex functions of Kv isoforms, only the sum of K+ currents (IKsum) can be recorded experimentally and decomposed later using exponential fitting to estimate component currents, such as IKto, IKslow, and IKss. However, such estimation cannot adequately describe glycosylation effects and Kv mechanisms. Here, we propose a framework of simulation modeling of Kv kinetics in mouse ventricular myocytes and model calibration using the in-vitro data under normal and reduced glycosylation conditions through ablation of the Mgat1 gene (i.e., Mgat1KO). Calibrated models facilitate the prediction of Kv characteristics at different voltages that are not directly observed in the in-vitro experiments. A model calibration procedure is developed based on the genetic algorithm. Experimental results show that, in the Mgat1KO group, both IKto and IKslow densities are shown to be significantly reduced and the rate of IKslow inactivation is much slower. The proposed approach has strong potential to couple simulation models with experimental data for gaining a better understanding of glycosylation effects on Kv kinetics.

The congenital disorder of glycosylation in PGM1 (PGM1-CDG) can cause severe cardiomyopathy and unexpected sudden cardiac death in childhood

INTRODUCTION

Sudden cardiac death (SCD) in the young is rare and should always lead to suspicion of a genetic cardiac disorder. We describe a family, in which the proband was a girl deceased by sudden cardiac death in the playground at thirteen years of age. The index-patient had short stature, cleft palate but no previous cardiac symptoms. We found an uncommon cause of cardiomyopathy, due to a congenital disorder of glycosylation (CDG), previously described to cause a variable range of usually mild symptoms, and not previously found to cause SCD as the first symptom of the condition.

METHODS

The index patient underwent postmortem genetic testing/molecular autopsy for genes known to cause SCD, without a detection of causative agent, why two siblings of similar phenotype as the deceased sister underwent clinical-exome genetic sequencing (next generation sequencing). All first-degree relatives underwent clinical examination including cardiac ultrasound, Holter-ECG, exercise stress test and biochemistry panel.

RESULTS

A genetic variant in the gene for phosphoglucomutase 1 (PGM1) was identified in the index patient and her two brothers, all were found to be homozygous for the genetic variant (G230E) NM_002633.2:c.689 G > A in PGM1. This variant has been linked to a congenital disorder of glycosylation (PGM1-CDG), explaining the clinical picture of short stature, cleft palate, liver engagement and cardiomyopathy. During follow-up one of the brothers died unexpectedly after physical exertion during daily life at the age of twelve years. The other brother fainted during similar circumstances at the age of thirteen years. Both parents and three other siblings were found to be heterozygous gene carriers without risk for the disease.

CONCLUSION

Our findings suggest that there is a need of multidisciplinary discussion and genetic testing after unexpected cardiac death in the young. We have to be more flexible in our evaluation of diseases and to consider even uncommon diseases including rare recessive inherited disorders. Our findings also suggest that the autosomal recessive PGM1-CDG might be highly associated with life-threatening cardiomyopathy with arrhythmia or sudden cardiac death as the first symptom presenting from childhood and adolescence.

Reduced hybrid/complex N-glycosylation disrupts cardiac electrical signaling and calcium handling in a model of dilated cardiomyopathy

Dilated cardiomyopathy (DCM) is the third most common cause of heart failure, with ~70% of DCM cases considered idiopathic. We showed recently, through genetic ablation of the MGAT1 gene, which encodes an essential glycosyltransferase (GlcNAcT1), that prevention of cardiomyocyte hybrid/complex N-glycosylation was sufficient to cause DCM that led to heart failure and early death. Our findings are consistent with increasing evidence suggesting a link between aberrant glycosylation and heart diseases of acquired and congenital etiologies. However, the mechanisms by which changes in glycosylation contribute to disease onset and progression remain largely unknown. Activity and gating of voltage-gated Na⁺ and K⁺ channels (Na_v and K_v respectively) play pivotal roles in the initiation, shaping and conduction of cardiomyocyte action potentials (APs) and aberrant channel activity was shown to contribute to cardiac disease. We and others showed that glycosylation can impact Na_v and K_v function; therefore, here, we investigated the effects of reduced cardiomyocyte hybrid/complex N-glycosylation on channel activity to investigate whether chronic aberrant channel function can contribute to DCM. Ventricular cardiomyocytes from MGAT1 deficient (MGAT1KO) mice display prolonged APs and pacing-induced aberrant early re-activation that can be attributed to, at least in part, a significant reduction in K_v expression and activity that worsens over time suggesting heart disease-related remodeling. MGAT1KO Na_v demonstrate no change in expression or maximal conductance but show depolarizing shifts in voltage-dependent gating. Together, the changes in MGAT1KO Na_v and K_v function likely contribute to observed anomalous electrocardiograms and Ca²⁺ handling. These findings provide insight into mechanisms by which altered glycosylation contributes to DCM through changes in Na_v and K_v activity that impact conduction, Ca²⁺ handling and contraction. The MGAT1KO can also serve as a useful model to study the effects of aberrant electrical signaling on cardiac function and the remodeling events that can occur with heart disease progression.

Cardiac complications of congenital disorders of glycosylation (CDG): a systematic review of the literature

Congenital disorders of glycosylation (CDG) are inborn errors of metabolism due to protein and lipid hypoglycosylation. This rapidly growing family of genetic diseases comprises 103 CDG types, with a broad phenotypic diversity ranging from mild to severe poly-organ -system dysfunction. This literature review summarizes cardiac involvement, reported in 20% of CDG. CDG with cardiac involvement were divided according to the associated type of glycosylation: N-glycosylation, O-glycosylation, dolichol synthesis, glycosylphosphatidylinositol (GPI)-anchor biosynthesis, COG complex, V-ATPase complex, and other glycosylation pathways. The aim of this review was to document and interpret the incidence of heart disease in CDG patients. Heart disorders were grouped into cardiomyopathies, structural defects, and arrhythmogenic disorders. This work may contribute to improved early management of cardiac complications in CDG.

Limitations of galactose therapy in phosphoglucomutase 1 deficiency

Introduction

Phosphoglucomutase 1 deficiency (PGM1 deficiency) has been identified as both, glycogenosis and congenital disorder of glycosylation (CDG). The phenotype includes hepatopathy, myopathy, oropharyngeal malformations, heart disease and growth retardation. Oral galactose supplementation at a dosage of 1 g per kg body weight per day is regarded as the therapy of choice.

Results

We report on a patient with a novel disease causing mutation, who was treated for 1.5 years with oral galactose supplementation. Initially, elevated transaminases were reduced and protein glycosylation of serum transferrin improved rapidly. Long-term surveillance however indicated limitations of galactose supplementation at the standard dose: 1 g per kg body weight per day did not achieve permanent correction of protein glycosylation. Even increased doses of up to 2.5 g per kg body weight did not result in complete normalization.

Furthermore, we described for the first time heart rhythm abnormalities, i.e. long QT Syndrome associated with a glycosylation disorder.

Mass spectrometry of IGFBP3, which was assumed to play a major role in growth retardation associated with PGM1 deficiency, revealed no glycosylation abnormalities. Growth rate did not improve under galactose supplementation.

Conclusions

The results of our study indicate that the current standard dose of galactose might be too low to achieve normal glycosylation in all patients. In addition, growth retardation in PGM1 deficiency is complex and multifactorial. Furthermore, heart rhythm abnormalities must be considered when treating patients with PGM1 deficiency.