Gender-based differences in cardiac diseases

It has been observed that the incidence of heart failure and Brugada syndrome are higher in men,while women are more likely to have QT interval prolongation and develop torsades de pointes (TdP).Over the past decade,new studies have improved our understanding of the mechanisms of abnormal repolarization and the relationship between gender differences in cardiac repolarization and presentation of clinical syndromes.Nevertheless,the causes of gender-based differences in cardiac disease are still not completely clear.This review paper briefly summarized what is currently known about gender differences in heart failure,Brugada syndrome and long QT syndrome from molecular mechanisms to clinical presentations.     

Molecular genetic basis of sudden cardiac death

In this review, the up-to-date understanding of the molecular basis of disorders causing sudden death will be described. Two arrhythmic disorders causing sudden death have recently been well described at the molecular level, the long QT syndromes (LQTS) and Brugada syndrome, and in this article we will review the current scientific knowledge of each disease. A third disorder, hypertrophic cardiomyopathy (HCM), a myocardial disorder causing sudden death, has also been well studied. Finally, a disorder in which both myocardial abnormalities and rhythm abnormalities coexist, arrhythmogenic right ventricular dysplasia (ARVD) will also be described. The role of the pathologist in these studies will be highlighted.     

Pharmacological separation of early afterdepolarizations from arrhythmogenic substrate in DeltaKPQ Scn5a murine hearts modelling human long QT 3 syndrome

Aim: To perform an empirical, pharmacological, separation of early afterdepolarizations (EADs) and transmural gradients of repolarization in arrhythmogenesis in a genetically modified mouse heart modelling human long QT syndrome (LQT) 3. Methods: Left ventricular endocardial and epicardial monophasic action potentials and arrhythmogenic tendency were compared in isolated wild type (WT) and Scn5a+/Δ hearts perfused with 0.1 and 1 μm propranolol and paced from the right ventricular epicardium. Results: All spontaneously beating bradycardic Scn5a+/Δ hearts displayed EADs, triggered beats and ventricular tachycardia (VT; n = 7), events never seen in WT hearts (n = 5). Perfusion with 0.1 and 1 μm propranolol suppressed all EADs, triggered beats and episodes of VT. In contrast, triggering of VT persisted following programmed electrical stimulation in 6 of 12 (50%), one of eight (12.5%), but six of eight (75%) Scn5a+/Δ hearts perfused with 0, 0.1 and 1 μm propranolol respectively in parallel with corresponding alterations in repolarization gradients, reflected in action potential duration (ΔAPD₉₀) values. Thus 0.1 μm propranolol reduced epicardial but not endocardial APD₉₀ from 54.7 ± 1.6 to 44.0 ± 2.0 ms, restoring ΔAPD₉₀ from ?3.8 ± 1.6 to 3.5 ± 2.5 ms (all n = 5), close to WT values. However, 1 μm propranolol increased epicardial APD₉₀ to 72.5 ± 1.2 ms and decreased endocardial APD₉₀ from 50.9 ± 1.0 to 24.5 ± 0.3 ms, increasing ΔAPD₉₀ to ?48.0 ± 1.2 ms. Conclusion: These findings empirically implicate EADs in potentially initiating spontaneous arrhythmogenic phenomena and transmural repolarization gradients in the re‐entrant substrate that would sustain such activity when provoked by extrasystolic activity in murine hearts modelling human LQT3 syndrome.     

The role of free fatty acids, pancreatic lipase and Ca+ signalling in injury of isolated acinar cells and pancreatitis model in lipoprotein lipase-deficient mice

Aim: The multifunctional signal molecule calmodulin kinase II (CaMKII) has been associated with cardiac arrhythmogenesis under conditions where its activity is chronically elevated. Recent studies report that its activity is also acutely elevated during acidosis. We test a hypothesis implicating CaMKII in the arrhythmogenesis accompanying metabolic acidification. Methods: We obtained monophasic action potential recordings from Langendorff‐perfused whole heart preparations and single cell action potentials (AP) using whole‐cell patch‐clamped ventricular myocytes. Spontaneous sarcoplasmic reticular (SR) Ca²⁺release events during metabolic acidification were investigated using confocal microscope imaging of Fluo‐4‐loaded ventricular myocytes. Results: In Langendorff‐perfused murine hearts, introduction of lactic acid into the Krebs‐Henseleit perfusate resulted in abnormal electrical activity and ventricular tachycardia. The CaMKII inhibitor, KN‐93 (2 μm ), reversibly suppressed this spontaneous arrhythmogenesis during intrinsic rhythm and regular 8 Hz pacing. However, it failed to suppress arrhythmia evoked by programmed electrical stimulation. These findings paralleled a CaMKII‐independent reduction in the transmural repolarization gradients during acidosis, which previously has been associated with the re‐entrant substrate under other conditions. Similar acidification produced spontaneous AP firing and membrane potential oscillations in patch‐clamped isolated ventricular myocytes when pipette solutions permitted cytosolic Ca²⁺ to increase following acidification. However, these were abolished by both KN‐93 and use of pipette solutions that held cytosolic Ca²⁺ constant during acidosis. Acidosis also induced spontaneous Ca²⁺ waves in isolated intact Fluo‐4‐loaded myocytes studied using confocal microscopy that were abolished by KN‐93. Conclusion: These findings together implicate CaMKII‐dependent SR Ca²⁺ waves in spontaneous arrhythmic events during metabolic acidification.     

Effective long-term control of cardiac events with beta-blockers in a family with a common LQT1 mutation

The congenital long QT syndrome (LQTS) is characterized by a prolonged QT interval on the surface electrocardiogram and an increased risk of recurrent syncope and sudden cardiac death. Mutations in seven genes have been identified as the molecular basis of LQTS. beta-blockers are the treatment of choice to reduce cardiac symptoms. However, long-term follow-up of genotyped families with LQTS has been rarely reported. We have clinically followed a four-generation family with LQTS being treated with beta-blocker therapy over a period of 23 years. Seven family members were carriers of two amino acid alterations in cis (V254M-V417M) in the cardiac potassium channel gene KCNQ1. Voltage-clamp recordings of mutant KCNQ1 protein in Xenopus oocytes showed that only the V254M mutation reduced the IKs current and that the effect of the V417M variant was negligible. The family exhibited the complete clinical spectrum of the disease, from asymptomatic patients to victims of sudden death before beta-blocker therapy. There was no significant reduction in QTc (556 +/- 40 ms(1/2) before therapy, 494 +/- 20 ms(1/2) during 17 years of treatment; n = 5 individuals). Of nine family members, one female died suddenly before treatment, three females of the second generation were asymptomatic, and four individuals of the third and fourth generation were symptomatic. All mutation carriers were treated with beta-blockers and remained asymptomatic for a follow-up up to 23 years. Long-term follow-up of a LQT1 family with a common mutation (V254M) being on beta-blocker therapy was effective and safe. This study underscores the importance of long-term follow-up in families with specific LQT mutations to provide valuable information for clinicians for an appropriate antiarrhythmic treatment.     

Differences in electrophysiological and contractile properties of mammalian cardiac tissues bathed in bicarbonate – and HEPES‐buffered solutions

Aim: The aim of this study was to compare the action potential configuration, contractility, intracellular Ca²⁺ and H⁺ concentrations in mammalian cardiac tissues bathed with Krebs and Tyrode solutions at 37 °C. Results: In Langendorff‐perfused guinea‐pig hearts, loaded with the fluorescent Ca²⁺‐indicator Fura‐2, or H⁺‐sensitive dye carboxy‐SNARF, shifts from Krebs to Tyrode solution caused intra‐cellular acidification, increased diastolic pressure and [Ca²⁺]_i, decreased systolic pressure and [Ca²⁺]_i, leading to a reduction in the amplitude of [Ca²⁺]_i transients and pulse pressure. Contractility was also depressed in canine ventricular trabeculae when transferred from Krebs to Tyrode solution. Shifts from Krebs to Tyrode solution increased the duration of action potentials in multicellular cardiac preparations excised from canine and rabbit hearts but not in isolated cardiomyocytes. All these changes in action potential morphology, contractility, [Ca²⁺]_i and [H⁺]_i were readily reversible by addition of 26 mmol L^?1 bicarbonate to Tyrode solution. Effects of dofetilide and CsCl, both blockers of the delayed rectifier K current, on action potential duration were compared in Krebs and Tyrode solutions. Dofetilide lengthened rabbit ventricular action potentials in a significantly greater extent in Tyrode than in Krebs solution. Exposure of canine Purkinje fibres to CsCl evoked early after depolarizations within 40 min in all preparations incubated with Tyrode solution, but not in those bathed with Krebs solution. Conclusion: It is concluded that the marked differences in action potential morphology, [Ca²⁺]_i, [H⁺]_i and contractility observed between preparations bathed with Krebs and Tyrode solutions are more likely attributable to differences in the intracellular buffering capacities of the two media.     

Mouse models of human arrhythmia syndromes

Sudden cardiac death stemming from ventricular arrhythmogenesis is one of the major causes of mortality in the developed world. Congenital and acquired forms of long QT syndrome (LQTS) are in turn associated with life threatening arrhythmias. Over the past decade our understanding of arrhythmogenic mechanisms in the setting of these diseases has increased greatly due to the creation of a number of animal models. Of these, the genetically amenable mouse has proved to be a particularly powerful tool. This review summarizes the congenital and acquired LQTS and describes the various mouse models that have been created to further probe arrhythmogenic mechanisms.     

一个先天性长QT综合征家系KCNH2基因新突变的确定及其编码蛋白的二级结构预测

目的 确定一个先天性长QT综合征家系的基因突变位点,并对突变所引起的编码蛋白的结构改变进行预测.方法 应用聚合酶链反应和直接测序分析先证者,找到突变位点后合成位点特异性引物,应用多重聚合酶链反应对长QT家系成员进行筛查;利用网络分析软件对突变所引起的编码蛋白结构进行预测.结果 发现了1个KCNH2基因新错义突变,即跨膜片段S2的F463L突变(GenBank接受序列号EU218526);突变没有引起预测跨膜区的改变,但编码蛋白的疏水性及二级结构,突变基因最小自由能二级结构都发生了变化.结论 作者所发现的突变点丰富了长QT综合征离子通道突变的基因库资料,用软件分析基因突变可能引起编码蛋白二级结构的改变有利于理解引起长QT综合征的结构基础.     

KCNQ1 mutations in patients with a family history of lethal cardiac arrhythmias and sudden death

Long QT syndrome (LQTS) is the prototype of the cardiac ion channelopathies which cause syncope and sudden death. LQT1, due to mutations of KCNQ1 (KVLQT1), is the most common form. This study describes the genotype-phenotype characteristics in 10 families with mutations of KCNQ1, including 5 novel mutations. One hundred and two families with a history of lethal cardiac events, 55 LQTS, 9 Brugada syndrome, 18 idiopathic ventricular fibrillation (IVF), and 20 acquired LQTS, were studied by single-strand conformational polymorphism (SSCP) and DNA sequence analyzes. Families found to have KCNQ1 mutations were phenotyped using ECG parameters and cardiac event history, and genotype-phenotype correlation was performed. No mutations were found in Brugada syndrome, IVF, or acquired LQTS families. Ten out of 55 LQTS families had KCNQ1 mutations and 62 carriers were identified. Mutations included G269S in domain S5; W305X, G314C, Y315C, and D317N in the pore region; A341E and Q357R in domain S6; and 1338insC, G568A and T587M mutations in the C-terminus. W305X, G314C, Q357R, 1338insC, and G568A, appeared to be novel mutations. Gene carriers were 26 +/- 19 years (32 females). Baseline QTc was 0.47 +/- 0.03 s (range 0.40-0.57 s) and 40% had normal to borderline QTc (< or = 0.46 s). Typical LQT1 T wave patterns were present in at least one affected member of each family, and in 73% of all affected members. A history of cardiac events was present in 19/62 (31%), 18 with syncope, 2 with aborted cardiac arrest (ACA) and six with sudden death (SD). Two out of 6 SDs (33%) occurred as the first symptom. No difference in phenotype was evident in pore vs. non-pore mutations. KCNQ1 mutations were limited to LQTS families. All five novel mutations produced a typical LQT1 phenotype. Findings emphasize (1) reduced penetrance of QTc and symptoms, resulting in diagnostic challenges, (2) the problem of sudden death as the first symptom (33% of those who died), and (3) genetic testing is important for identification of gene carriers with reduced penetrance, in order to provide treatment and to prevent lethal cardiac arrhythmias and sudden death.     

Spectrum of pathogenic mutations and associated polymorphisms in a cohort of 44 unrelated patients with long QT syndrome

Long QT syndrome (LQTS) is a rare and clinically heterogeneous inherited disorder characterized by a long QT interval on the electrocardiogram, increased risk of syncope and sudden death caused by arrhythmias. This syndrome is mostly caused by mutations in genes encoding various cardiac ion channels. The clinical heterogeneity is usually attributed to variable penetrance. One of the reasons for this variability in expression could be the coexistence of common single nucleotide polymorphisms (SNPs) on LQTS-causing genes and/or unknown genes. Some synonymous and nonsynonymous exonic SNPs identified in LQTS-causing genes may have an effect on the cardiac repolarization process and modulate the clinical expression of a latent LQTS pathogenic mutation. We report the molecular pattern of 44 unrelated patients with LQTS using denaturing high-performance liquid chromatography analysis of the KCNQ1, KCNH2, SCN5A, KCNE1 and KCNE2 genes. Forty-five disease-causing mutations (including 24 novel ones) were identified in this cohort. Most of our patients (84%) showed complex molecular pattern with one mutation (and even two for four patients) associated with several SNPs located in several LQTS genes.     

Effect of exogenous nitric oxide on electrogenesis in myelinated and unmyelinated nerve fibers

Extracellular recording was used to study the effect of sodium nitroprusside, a donor of NO, on parameters of action potential and ionic currents in single sciatic nerve fibers and unmyelinated nerve terminals in the sternal muscle in frogs. Sodium nitroprusside significantly decreased the duration of action potential in Ranvier node and the amplitude of afterdepolarization. In motor nerve terminals bathed in low Ca²⁺ saline, sodium nitroprusside increased phase III amplitude of the nerve terminal response corresponding to outward potas-sium currents. Blockade of voltage-dependent potassium channels with 4-aminopyridine abolished the effects of NO. These data indicate that exogenous NO reduced the duration of action potential and afterdepolarization through enhancement of voltage-dependent potassium currents.     

Sodium channel abnormalities are infrequent in patients with long QT Syndrome: Identification of two novel SCN5A mutations

Long QT syndrome (LQTS) is a heterogeneous disorder caused by mutations of at least five different loci. Three of these, LQT1, LQT2, and LQT5, encode potassium channel subunits. LQT3 encodes the cardiac-specific sodium channel, SCN5A. Previously reported LQTS-associated mutations of SCN5A include a recurring three amino acid deletion (ΔKPQ1505–1507) in four different families, and four different missense mutations. We have examined the SCN5A gene in 88 index cases with LQTS, including four with Jervell and Lange-Nielsen syndrome and the remainder with Romano-Ward syndrome. Screening portions of DIII–DIV, where mutations have previously been found, showed that none of these patients has the three amino acid deletion, ΔKPQ1505–1507, or the other four known mutations. We identified a novel missense mutation, T1645M, in the DIV; S4 voltage sensor immediately adjacent to the previously reported mutation R1644H. We also examined all of the additional pore-forming regions and voltage-sensing regions and discovered another novel mutation, T1304M, at the voltage-sensing region DIII; S4. Neither T1645M nor T1304M were seen in a panel of unaffected control individuals. Five of six T1304M gene carriers were symptomatic. In contrast to previous studies, QT_onset-c was not a sensitive indicator of SCN5A-associated LQTS, at least in this family. These data suggest that mutations of SCN5A are responsible for only a small proportion of LQTS cases. Am. J. Med. Genet. 86:470–476, 1999. © 1999 Wiley-Liss, Inc.     

Ionic currents contributing to the action potential in single ventricular myocytes of the guinea pig studied with action potential clamp

With the action potential clamp procedure we studied the contribution of various ionic currents to the action potential in single ventricular myocytes. Action potentials were elicited by a current pulse through the suction pipette and recorded by a computer. A representative action potential was then repetitively replayed to the same cell under voltage-clamp conditions. Successive pharmacological blocks of ionic currents allowed for the first time the measurement of the contribution of the L-type calcium current (I _Ca) and the [Ca²⁺]_i-activated currents as well as the potassium current to the action potential. Experiments using caffeine as a tool to increase calcium release from the sarcoplasmic reticulum supported the idea that I _NaCa contributes to the plateau during the second half of the action potential and even lasts into diastole, whereas strong elevation of the intracellular [Ca]_i during the action potential additionally activated the non-specific cation channel.     

Bradycardia‐induced long QT syndrome caused by a de novo missense mutation in the S2‐S3 inner loop of HERG

Long QT syndrome is a congenital disorder that presents with a defective cardiac ion channel and is either associated with prolonged action potential or, more commonly, known as an acquired form in which “torsades de pointes” type arrhythmias specifically occur after secondary causes. We report a case of a novel HERG mutation (A490T) that caused a bradycardia‐associated form of long QT syndrome. A 27‐year‐old woman exhibited recurrent syncope due to torsades de pointes associated with a disturbance of the cardiac conduction system. By using polymerase chain reaction and single strand conformational polymorphism analyses, we identified a heterozygous single nucleotide substitution of HERG (G to A at nt 1468). This mutational change was not present in 140 Japanese control individuals. Electrophysiological assays for the A490T mutant HERG channel were conducted in the heterologous expression system with COS7 cells. The mutant channel was found to reconstitute functional channel currents, suggesting the homomeric mutant channel was functional. The mutation did not change the properties of the activation gate and inward rectification, however the current density of this mutant channel was small compared with that of wild type HERG. Taken together, this mutant may cause subtle changes in HERG channel functions (I_Kr) in vivo. In this case, genetic background and unexpected bradycardia may have contributed to the development of long QT syndrome. © 2001 Wiley‐Liss, Inc.     

Cellular calcium as a determinant of action potential duration in rabbit myocardium

It has been suggested that the amount of calcium in the myocardial cell determines force production as well as the duration of the action potential. We have examined two factors proposed to determine the action potential duration: (I) the inotropic state of the muscle and (2) the preceding stimulation interval. The experiments were performed on isolated papillary muscles from rabbits. The preparations were stimulated to contract isometrically at a temperature of 37°C and intracellular recordings of action potentials were made. An increased inotropic state produced either by raised external calcium concentration or by increased stimulation frequency was associated with shortened action potentials, confirming factor (1) above. The dependence of the action potential duration on the preceding stimulation interval (factor (2) above) disappeared over a wide range of intervals when the preparation was exposed to 4 μM D600. The action potential duration was, however, still dependent on the preceding stimulation interval when the extracellular calcium concentration had been lowered from 2.0 mM to 1.0 and 0.5 mM respectively. The results are discussed in terms of a model which postulates an inverse relationship between the amount of calcium in a membrane compartment and the action potential duration.