Long QT Syndrome

Differential conditions for early after-depolarizations and triggered activity in cardiomyocytes derived from transgenic LQT1 and LQT2 rabbits

Early after-depolarization (EAD), or abnormal depolarization during the plateau phase of action potentials, is a hallmark of long-QT syndrome (LQTS). More than 13 genes have been identified as responsible for LQTS, and elevated risks for EADs may depend on genotypes, such as exercise in LQT1 vs. sudden arousal in LQT2 patients. We investigated mechanisms underlying different high-risk conditions that trigger EADs using transgenic rabbit models of LQT1 and LQT2, which lack I(Ks) and I(Kr) (slow and fast components of delayed rectifying K(+) current), respectively. Single-cell patch-clamp studies show that prolongation of action potential duration (APD) can be further enhanced by lowering extracellular potassium concentration ([K(+)](o)) from 5.4 to 3.6 mm. However, only LQT2 myocytes developed spontaneous EADs following perfusion with lower [K(+)](o), while there was no EAD formation in littermate control (LMC) or LQT1 myocytes, although APDs were also prolonged in LMC myocytes and LQT1 myocytes. Isoprenaline (ISO) prolonged APDs and triggered EADs in LQT1 myocytes in the presence of lower [K(+)](o). In contrast, continuous ISO perfusion diminished APD prolongation and reduced the incidence of EADs in LQT2 myocytes. These different effects of ISO on LQT1 and LQT2 were verified by optical mapping of the whole heart, suggesting that ISO-induced EADs are genotype specific. Further voltage-clamp studies revealed that ISO increases L-type calcium current (I(Ca)) faster than I(Ks) (time constant 9.2 s for I(Ca) and 43.6 s for I(Ks)), and computer simulation demonstrated a high-risk window of EADs in LQT2 during ISO perfusion owing to mismatch in the time courses of I(Ca) and I(Ks), which may explain why a sympathetic surge rather than high sympathetic tone can be an effective trigger of EADs in LQT2 perfused hearts. In summary, EAD formation is genotype specific, such that EADs can be elicited in LQT2 myocytes simply by lowering [K(+)](o), while LQT1 myocytes require sympathetic stimulation. Slower activation of I(Ks) than of I(Ca) by ISO may explain why different sympathetic modes, i.e. sympathetic surge vs. high sympathetic tone, are associated with polymorphic ventricular tachycardia in LQTS patients.

Syncope during exercise: just another benign vasovagal event?

In general, syncope in children and adolescents is a benign event. Syncope during exercise may identify patients with a potentially fatal condition. Catecholaminergic polymorphic ventricular tachycardia is characterized by life-threatening ventricular arrhythmias, usually polymorphic ventricular tachycardia or ventricular fibrillation, occurring under conditions of exercise or emotional stress. Catecholaminergic polymorphic ventricular tachycardia is a familial condition that presents with exercise-induced syncope or sudden death in children or young adults. Detailed evaluation should be considered for patients who have syncope during exercise, injure themselves during the fall (i.e., unprotected faint with no antecedent warning prodrome), or who have a family history of syncope, early sudden cardiac death, myocardial disease, or arrhythmias.

Long QT syndrome mutation detection by SNaPshot technique

Long QT syndrome (LQTS) is a cardiac disorder with an abnormality of cardiac rhythm associated with sudden death especially in younger, apparently healthy individuals. If there is no clear cause of death detectable during comprehensive coroner's inquest (autopsy-negative cases), you have to consider LQTS and other heritable arrhythmia syndromes. A molecular genetic screening regarding mutations in associated genes can help to ensure the cause of death and to protect affected family members. Genetic testing of LQTS, currently performed mainly by sequencing, is still very expensive and time consuming. With this study we present a rapid and reasonable method for the simultaneously screening of some of the most common mutations associated with LQTS, focused on the KCNQ1 and KCNH2 genes. With the method of SNaPshot minisequencing, a total of 58 mutations were analyzed in four multiplex assays which were successfully established and optimized. The comparison with samples previously analyzed by direct sequencing showed concordance. Furthermore, autopsy-negative cases were tested but no mutations could be observed in any of the specimen. The presented method is well suitable for LQTS mutation screening. An enhancement to further mutations and population-based investigations regarding mutation frequencies should be the aim of prospective studies.

Mutation and gender-specific risk in type 2 long QT syndrome: implications for risk stratification for life-threatening cardiac events in patients with long QT syndrome

BACKGROUND

Men and women with type 2 long QT syndrome (LQT2) exhibit time-dependent differences in the risk for cardiac events. We hypothesized that data regarding the location of the disease-causing mutation in the KCNH2 channel may affect gender-specific risk in LQT2.

OBJECTIVE

This study sought to risk-stratify LQT2 patients for life-threatening cardiac events based on clinical and genetic information.

METHODS

The risk for life-threatening cardiac events from birth through age 40 years (comprising aborted cardiac arrest [ACA] or sudden cardiac death [SCD]) was assessed among 1,166 LQT2 male (n = 490) and female (n = 676) patients by the location of the LQTS-causing mutation in the KCNH2 channel (prespecified in the primary analysis as pore-loop vs. non-pore-loop).

RESULTS

During follow-up, the cumulative probability of life-threatening cardiac events years was significantly higher among LQT2 women (26%) as compared with men (14%; P <.001). Multivariate analysis showed that the risk for life-threatening cardiac events was not significantly different between women with and without pore-loop mutations (hazard ratio 1.20; P =.33). In contrast, men with pore-loop mutations displayed a significant >2-fold higher risk of a first ACA or SCD as compared with those with non-pore-loop mutations (hazard ratio 2.18; P = .01). Consistently, women experienced a high rate of life-threatening events regardless of mutation location (pore-loop: 35%, non-pore-loop: 23%), whereas in men the rate of ACA or SCD was high among those with pore-loop mutations (28%) and relatively low among those with non-pore-loop mutations (8%).

CONCLUSION

Combined assessment of clinical and mutation-specific data can be used for improved risk stratification for life-threatening cardiac events in LQT2.

The role of translational bioinformatics in drug discovery

The application of translational approaches (e.g. from bed to bench and back) is gaining momentum in the pharmaceutical industry. By utilizing the rapidly increasing volume of data at all phases of drug discovery, translational bioinformatics is poised to address some of the key challenges faced by the industry. Indeed, computational analysis of clinical data and patient records has informed decision-making in multiple aspects of drug discovery and development. Here, we review key examples of translational bioinformatics approaches to emphasize its potential to enhance the quality of drug discovery pipelines, reduce attrition rates and, ultimately, lead to more effective treatments.

LQTS gene LOVD database

The Long QT Syndrome (LQTS) is a group of genetically heterogeneous disorders that predisposes young individuals to ventricular arrhythmias and sudden death. LQTS is mainly caused by mutations in genes encoding subunits of cardiac ion channels (KCNQ1, KCNH2,SCN5A, KCNE1, and KCNE2). Many other genes involved in LQTS have been described recently(KCNJ2, AKAP9, ANK2, CACNA1C, SCNA4B, SNTA1, and CAV3). We created an online database(http://www.genomed.org/LOVD/introduction.html) that provides information on variants in LQTS-associated genes. As of February 2010, the database contains 1738 unique variants in 12 genes. A total of 950 variants are considered pathogenic, 265 are possible pathogenic, 131 are unknown/unclassified, and 292 have no known pathogenicity. In addition to these mutations collected from published literature, we also submitted information on gene variants, including one possible novel pathogenic mutation in the KCNH2 splice site found in ten Chinese families with documented arrhythmias. The remote user is able to search the data and is encouraged to submit new mutations into the database. The LQTS database will become a powerful tool for both researchers and clinicians.

Impact of Mendelian inheritance in cardiovascular disease

Cardiovascular disease is a leading cause of mortality worldwide. While the etiology for the majority of cardiovascular disease is presumed to be a combination of genetic and environmental factors, developments in understanding the basic biology of cardiac disorders have been greatly advanced through discoveries made studying heart diseases that exhibit Mendelian forms of inheritance. Most of these diseases primarily affect children and young adults and include cardiomyopathies, arrhythmias, aortic aneurysms, and congenital heart defects. The discovery of the genetic etiologies for these diseases have had significant impact on our understanding of more complex forms of cardiovascular disease and in some cases have led to novel diagnostic and treatment modalities. In this review, we will summarize these seminal genetic discoveries, highlighting a few that have resulted in significant impact on human disease, and discuss the potential utility of studying Mendelian-inherited heart disease with the development of new genetic technologies and our increased understanding of the human genome.

Nicorandil normalizes prolonged repolarisation in the first transgenic rabbit model with Long-QT syndrome 1 both in vitro and in vivo

Transgenic rabbits expressing loss-of-function pore mutants of the human gene KCNQ1 (K(v)LQT1-Y315S) have a Long QT-Syndrome 1 (LQT1) phenotype. We evaluated for the first time the effect of nicorandil, an opener of ATP-sensitive potassium channels, and of isoproterenol on cardiac action potential duration and heart rate dependent dispersion of repolarisation in transgenic LQT1 rabbits. In vivo LQT1 and littermate control were subjected to transvenous electrophysiological studies; in vitro monophasic action potentials were recorded from explanted Langendorff-perfused hearts. In vivo ventricular effective refractory periods (VERP) at the right ventricular base were significantly prolonged in LQT1 as compared to littermate control, resulting in a more pronounced VERP dispersion in LQT1. This difference in VERP dispersion between LQT1 and littermate control disappeared after infusion of nicorandil. In vitro, mean action potential durations (APD(75) and APD(90)) of LQT1 were significantly prolonged compared to littermate control at baseline. Nicorandil decreased APD(75) and APD(90) in LQT1 and littermate control at all stimulated heart rates. After adding nicorandil, the APD(90) at all hearts rates and the APD(75) at high heart rates were no longer different. Dispersion of repolarisation (∆APD(75) and ∆APD(90)) was heart rate dependently decreased after nicorandil at all tested stimulation cycle lengths only in LQT1. We demonstrated phenotypic differences of LQT1 and littermate control in vivo and in vitro. Nicorandil 20μmol/l improved repolarisation abnormalities and heterogeneities in transgenic LQT1 rabbits.

A common variant of NOS1AP is associated with QT interval duration in a Chinese population with Type 2 diabetes

AIMS

Electrocardiographic ventricular repolarization QT parameters are independent risk factors for cardiovascular events and sudden cardiac death in diabetic patients. The aim of the study was to investigate the association of polymorphisms of the nitric oxide synthase 1 adaptor protein (NOS1AP) gene with QT interval in Chinese subjects with or without Type 2 diabetes.

METHODS

Three single nucleotide polymorphisms (SNPs) (rs10494366, rs12143842 and rs12029454) were genotyped in 1240 Type 2 diabetic patients (631 men and 609 women) and 1196 normal controls (433 men and 763 women). Individuals with overt diseases other than diabetes were excluded. Heart-rate corrected QT interval (QTc) was determined by standard 12-lead ECG and Bazett formula. Sex-pooled analysis and sex-specific analysis for genotype-phenotype association were both conducted.

RESULTS

In the diabetic group, the rs12143842 T allele was associated with a 3.87-ms (P = 0.014, empirical P = 0.039) increase in QTc duration for each additional allele copy, while rs10494366 and rs12029454 exhibited no significant association with QTc. We found no evidence of association for the three SNPs in subjects with normal glucose regulation. No significant SNP-gender and -diabetes affection interaction was observed.

CONCLUSIONS

The genetic variant rs12143842 in NOS1AP is associated with QT interval duration in a Chinese population with Type 2 diabetes. Future studies in different populations are needed to validate this finding and to evaluate the impact of NOS1AP variants on cardiovascular events and sudden cardiac death in diabetic patients.

Traffic-related air pollution and QT interval: modification by diabetes, obesity, and oxidative stress gene polymorphisms in the normative aging study

BACKGROUND

Acute exposure to ambient air pollution has been associated with acute changes in cardiac outcomes, often within hours of exposure.

OBJECTIVES

We examined the effects of air pollutants on heart-rate-corrected QT interval (QTc), an electrocardiographic marker of ventricular repolarization, and whether these associations were modified by participant characteristics and genetic polymorphisms related to oxidative stress.

METHODS

We studied repeated measurements of QTc on 580 men from the Veterans Affairs Normative Aging Study (NAS) using mixed-effects models with random intercepts. We fitted a quadratic constrained distributed lag model to estimate the cumulative effect on QTc of ambient air pollutants including fine particulate matter <or= 2.5 microm in aerodynamic diameter (PM2.5), ozone (O3), black carbon (BC), nitrogen dioxide (NO2), carbon monoxide (CO), and sulfur dioxide (SO2) concentrations during the 10 hr before the visit. We genotyped polymorphisms related to oxidative stress and analyzed pollution-susceptibility score interactions using the genetic susceptibility score (GSS) method.

RESULTS

Ambient traffic pollutant concentrations were related to longer QTc. An interquartile range (IQR) change in BC cumulative during the 10 hr before the visit was associated with increased QTc [1.89 msec change; 95% confidence interval (CI), -0.16 to 3.93]. We found a similar association with QTc for an IQR change in 1-hr BC that occurred 4 hr before the visit (2.54 msec change; 95% CI, 0.28-4.80). We found increased QTc for IQR changes in NO2 and CO, but the change was statistically insignificant. In contrast, we found no association between QTc and PM2.5, SO2, and O3. The association between QTc and BC was stronger among participants who were obese, who had diabetes, who were nonsmokers, or who had higher GSSs.

CONCLUSIONS

Traffic-related pollutants may increase QTc among persons with diabetes, persons who are obese, and nonsmoking elderly individuals; the number of genetic variants related to oxidative stress increases this effect.

Two four-marker haplotypes on 7q36.1 region indicate that the potassium channel gene HERG1 (KCNH2, Kv11.1) is related to schizophrenia: a case control study

Background

The pathobiology of schizophrenia is still unclear. Its current treatment mainly depends on antipsychotic drugs. A leading adverse effect of these medications is the acquired long QT syndrome, which results from the blockade of cardiac HERG1 channels (human ether-a-go-go-related gene potassium channels 1) by antipsychotic agents. The HERG1 channel is encoded by HERG1 (KCNH2, Kv11.1) gene and is most highly expressed in heart and brain. Genetic variations in HERG1 predispose to acquired long QT syndrome. We hypothesized that the blockade of HERG1 channels by antipsychotics might also be significant for their therapeutic mode of action, indicating a novel mechanism in the pathogenesis of schizophrenia.

Methods

We genotyped four single nucleotide polymorphisms (SNPs) in 7q36.1 region (two SNPs, rs1805123 and rs3800779, located on HERG1, and two SNPs, rs885684 and rs956642, at the 3'-downstream intergenic region) and then performed single SNP and haplotype association analyses in 84 patients with schizophrenia and 74 healthy controls after the exclusion of individuals having prolonged or shortened QT interval on electrocardiogram.

Results

Our analyses revealed that both genotype and allele frequencies of rs3800779 (c.307+585G>T) were significantly different between populations (P = 0.023 and P = 0.018, respectively). We also identified that two previously undescribed four-marker haplotypes which are nearly allelic opposite of each other and located in chr7:150225599-150302147bp position encompassing HERG1 were either overrepresented (A-A-A-T, the at-risk haplotype, P = 0.0007) or underrepresented (C-A-C-G, the protective haplotype, P = 0.005) in patients compared to controls.

Conclusions

Our results indicate that the potassium channel gene HERG1 is related to schizophrenia. Our findings may also implicate the whole family of HERG channels (HERG1, HERG2 and HERG3) in the pathogenesis of psychosis and its treatment.

Catecholaminergic polymorphic ventricular tachycardia

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited arrhythmogenic disorder that causes syncopal episodes related with stress or emotion and even sudden cardiac deaths. Signs and symptoms usually begin in childhood. A suspicion of CPVT should be kept in mind when a child or an adolescent suddenly loses consciousness, particularly if this happens upon physical exercise or sudden mental stress. During the past decade, the knowledge of CPVT genetics and physiology has increased. Exercise testing is essential when suspecting arrhythmogenic origin of syncope, and in the case of CPVT, it may be even more sensitive than Holter monitoring. Beta-antiadrenergic medication can substantially decrease the mortality associated with CPVT. Asymptomatic patients with known CPVT gene defects should also be treated because sudden cardiac death may be the first manifestation of the disease. An implantable cardioverter-defibrillator may also be required in the most severe CPVT cases. In this review, we summarise the current knowledge on the clinical characteristics, diagnostic, genetic and prognostic features of CPVT in children. In all, 133 publications covering 60 years were checked, and those written in English and containing ten or more, mainly paediatric CPVT cases, were included. In addition, a CPVT family with three members and delayed diagnoses until late childhood and adulthood is presented.

Genetic basis of malignant channelopathies and ventricular fibrillation in the structurally normal heart

Sudden cardiac death occurs in a minority of patients in the absence of structural or functional abnormalities. In this category, pure electrical heart diseases are responsible for a large number of these unexpected deaths. These conditions include the long QT syndrome, Brugada syndrome, catecholaminergic polymorphic ventricular tachycardia, short QT syndrome (collectively referred to as channelopathies) and idiopathic ventricular fibrillation. This article reviews the current molecular understanding of the electrical diseases of the heart associated with sudden cardiac death, and provides a summary of the causal genes and a flowchart with an overview of the genotype–phenotype correlation of the most common arrhythmia syndromes.

Early identification of risk factors for sudden cardiac death

Sudden cardiac death (SCD) is a global health issue. The unexpected nature of this devastating condition compounds the urgency of discovering methods for early detection of risk, which will lead to more effective prevention. However, the complex and dynamic nature of SCD continues to present a considerable challenge for the early identification of risk factors. Measurement of the left ventricular ejection fraction (LVEF) is currently the only major risk factor used for stratification in clinical practice. Severely decreased LVEF is likely to manifest late in the natural history of SCD, however, and may only affect a small subgroup of patients who will suffer SCD. A growing body of literature describes novel risk markers and predictors of SCD, such as high-risk phenotypes, genetic variants and biomarkers. This Review will discuss the potential utility of these markers as early identifiers of risk, and suggests a framework for the conduct of future studies for the discovery, validation, and deployment of novel SCD risk factors.

The importance of the family history in caring for families with long QT syndrome and dilated cardiomyopathy

In potentially inherited cardiac diseases, the family history is of great importance. We looked at the way cardiologists take a family history in patients with idiopathic dilated cardiomyopathy (DCM) or long QT syndrome (LQTS) and whether this led to screening of relatives or other follow-up. We performed retrospective cross-sectional analyses of adult index patients with DCM or LQTS in a general hospital (GH) or a University Medical Center (UMC). We identified 82 index patients with DCM (34 GH; 48 UMC) and 20 with LQTS (all UMC) between 1996 and 2005. Mean follow-up was 58 months. A family history was recorded in 90% of both LQTS and DCM patients most of the cases restricted to first-degree family members. The genetic aspects, counseling and screening of family members was discussed significantly more often with LQTS than DCM patients (all P < 0.05). Also follow-up (screening of family members, DNA analysis and referral) was performed significantly more often in LQTS than DCM patients. Cardiologists in the UMC referred DCM index patients for genetic counseling more often than those in the GH (25% vs. 6%; P < 0.05). Only a few index patients with DCM were referred to a clinical genetics department. One-third of DCM cases and nearly all LQTS cases are familial. Since early recognition and treatment may reduce morbidity and mortality we recommend cardiologists take a more thorough family history and always consider referring to a clinical genetics department in such index patients.

Neurohormonal regulation of cardiac ion channels in chronic heart failure

Alteration of neurohormonal homeostasis is a hallmark of the pathophysiology of chronic heart failure (CHF). In particular, overactivation of the renin-angiotensin-aldosterone system and the sympathetic catecholaminergic system is consistently observed. Chronic overactivation of these hormonal pathways leads to a detrimental arrhythmogenic remodeling of cardiac tissue due to dysregulation of cardiac ion channels. Sudden cardiac death resulting from ventricular arrhythmias is a major cause of mortality in patients with CHF. All the drug classes known to reduce mortality in patients with CHF are neurohormonal blockers. The aim of this review was to provide an overview of how cardiac ion channels are regulated by hormones known to play a central role in the pathogenesis of CHF.

A new era in clinical genetic testing for hypertrophic cardiomyopathy

Building on seminal studies of the last 20 years, genetic testing for hypertrophic cardiomyopathy (HCM) has become a clinical reality in the form of targeted exonic sequencing of known disease-causing genes. This has been driven primarily by the decreasing cost of sequencing, but the high profile of genome-wide association studies, the launch of direct-to-consumer genetic testing, and new legislative protection have also played important roles. In the clinical management of hypertrophic cardiomyopathy, genetic testing is primarily used for family screening. An increasing role is recognized, however, in diagnostic settings: in the differential diagnosis of HCM; in the differentiation of HCM from hypertensive or athlete's heart; and more rarely in preimplantation genetic diagnosis. Aside from diagnostic clarification and family screening, use of the genetic test for guiding therapy remains controversial, with data currently too limited to derive a reliable mutation risk prediction from within the phenotypic noise of different modifying genomes. Meanwhile, the power of genetic testing derives from the confidence with which a mutation can be called present or absent in a given individual. This confidence contrasts with our more limited ability to judge the significance of mutations for which co-segregation has not been demonstrated. These variants of "unknown" significance represent the greatest challenge to the wider adoption of genetic testing in HCM. Looking forward, next-generation sequencing technologies promise to revolutionize the current approach as whole genome sequencing will soon be available for the cost of today's targeted panel. In summary, our future will be characterized not by lack of genetic information but by our ability to effectively parse it.

The risk of long QT syndrome in the pediatric population

PURPOSE OF REVIEW

Advances in understanding the biophysical underpinnings of long QT syndrome have provided growing insight into the risk of this syndrome in the pediatric population. This review focuses on developments in this area as reflected in the recent literature.

RECENT FINDINGS

QT interval prolongation on the surface ECG is the hallmark of long QT syndrome. This prolongation reflects protracted ventricular repolarization, primarily due to mutations in genes coding for cardiac ion channels. To date, 12 different genes have been implicated, and current genetic testing methods can provide a specific diagnosis in approximately 70% of patients. Clinical indicators, including age, sex, corrected QT duration, and prior syncope are the most powerful predictors of future life-threatening cardiac events. However, diagnosis, risk assessment, and therapeutic strategies are being guided by genetic analysis to an increasing degree.

SUMMARY

Impressive advancements have been made in understanding the genetic and clinical determinants of this heterogeneous syndrome. As genetic testing techniques become more robust, the ability to assess risk in affected individuals and tailor therapy will improve.

Long QT syndrome and associated gene mutation carriers in Japanese children: results from ECG screening examinations

LQTS (long QT syndrome) is caused by mutations in cardiac ion channel genes; however, the prevalence of LQTS in the general population is not well known. In the present study, we prospectively estimated the prevalence of LQTS and analysed the associated mutation carriers in Japanese children. ECGs were recorded from 7961 Japanese school children (4044 males; mean age, 9.9±3.0 years). ECGs were examined again for children who had prolonged QTc (corrected QT) intervals in the initial ECGs, and their QT intervals were measured manually. An LQTS score was determined according to Schwartz's criteria, and ion channel genes were analysed. In vitro characterization of the identified mutants was performed by heterologous expression experiments. Three subjects were assigned to a high probability of LQTS (3.5≤ LQTS score), and eight subjects to an intermediate probability (1.0< LQTS score ≤3.0). Genetic analysis of these II subjects identified three KCNH2 mutations (M124T, 547–553 del GGCGGCG and 2311–2332 del/ins TC). In contrast, no mutations were identified in the 15 subjects with a low probability of LQTS. Electrophysiological studies showed that both the M124T and the 547–553 del GGCGGCG KCNH2 did not suppress the wild-type KCNH2 channel in a dominant-negative manner. These results demonstrate that, in a random sample of healthy Japanese children, the prevalence of a high probability of LQTS is 0.038% (three in 7961), and that LQTS mutation carriers can be identified in at least 0.038% (one in 2653). Furthermore, large-scale genetic studies will be needed to clarify the real prevalence of LQTS by gene-carrier status, as it may have been underestimated in the present study.

Cardiac sodium channel Na(v)1.5 and interacting proteins: Physiology and pathophysiology

The cardiac voltage-gated Na(+) channel Na(v)1.5 generates the cardiac Na(+) current (INa). Mutations in SCN5A, the gene encoding Na(v)1.5, have been linked to many cardiac phenotypes, including the congenital and acquired long QT syndrome, Brugada syndrome, conduction slowing, sick sinus syndrome, atrial fibrillation, and dilated cardiomyopathy. The mutations in SCN5A define a sub-group of Na(v)1.5/SCN5A-related phenotypes among cardiac genetic channelopathies. Several research groups have proposed that Na(v)1.5 may be part of multi-protein complexes composed of Na(v)1.5-interacting proteins which regulate channel expression and function. The genes encoding these regulatory proteins have also been found to be mutated in patients with inherited forms of cardiac arrhythmias. The proteins that associate with Na(v)1.5 may be classified as (1) anchoring/adaptor proteins, (2) enzymes interacting with and modifying the channel, and (3) proteins modulating the biophysical properties of Na(v)1.5 upon binding. The aim of this article is to review these Na(v)1.5 partner proteins and to discuss how they may regulate the channel's biology and function. These recent investigations have revealed that the expression level, cellular localization, and activity of Na(v)1.5 are finely regulated by complex molecular and cellular mechanisms that we are only beginning to understand.

Segmental duplications mediate novel, clinically relevant chromosome rearrangements

Copy number studies have led to an explosion in the discovery of new segmental duplication-mediated deletions and duplications. We have analyzed copy number changes in 2419 patients referred for clinical array comparative genomic hybridization studies. Twenty-three percent of the abnormal copy number changes we found are immediately flanked by segmental duplications > or =10 kb in size and > or =95% identical in direct orientation, consistent with deletions and duplications generated by non-allelic homologous recombination. Here, we describe copy number changes in five previously unreported loci with genomic organization characteristic of NAHR-mediated gains and losses; namely, 2q11.2, 7q36.1, 17q23, 2q13 and 7q11.21. Deletions and duplications of 2q11.2, deletions of 7q36.1 and deletions of 17q23 are interpreted as pathogenic based on their genomic size, gene content, de novo inheritance and absence from control populations. The clinical significance of 2q13 deletions and duplications is still emerging, as these imbalances are also found in phenotypically normal family members and control individuals. Deletion of 7q11.21 is a benign copy number change well represented in control populations and copy number variation databases. Here, we discuss the genetic factors that can modify the phenotypic expression of such gains and losses, which likely play a role in these and other recurrent genomic disorders.