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Biomedical subjects

Jonathan C Makielski

Publications and source records attributed to Jonathan C Makielski.

At least 19 recordsLinked to original sources
PubMed2006

Mutant caveolin-3 induces persistent late sodium current and is associated with long-QT syndrome.

BACKGROUND: Congenital long-QT syndrome (LQTS) is a primary arrhythmogenic syndrome stemming from perturbed cardiac repolarization. LQTS, which affects approximately 1 in 3000 persons, is 1 of the most common causes of autopsy-negative sudden death in the young. Since the sentinel discovery of cardiac channel gene mutations in LQTS in 1995, hundreds of mutations in 8 LQTS susceptibility genes have been identified. All 8 LQTS genotypes represent primary cardiac channel defects (ie, ion channelopathy) except LQT4, which is a functional channelopathy because of mutations in ankyrin-B. Approximately 25% of LQTS remains unexplained pathogenetically. We have pursued a "final common pathway" hypothesis to elicit novel LQTS-susceptibility genes. With the recent observation that the LQT3-associated, SCN5A-encoded cardiac sodium channel localizes in caveolae, which are known membrane microdomains whose major component in the striated muscle is caveolin-3, we hypothesized that mutations in caveolin-3 may represent a novel pathogenetic mechanism for LQTS. METHODS AND RESULTS: Using polymerase chain reaction, denaturing high-performance liquid chromatography, and direct DNA sequencing, we performed open reading frame/splice site mutational analysis on CAV3 in 905 unrelated patients referred for LQTS genetic testing. CAV3 mutations were engineered by site-directed mutagenesis and the molecular phenotype determined by transient heterologous expression into cell lines that stably express the cardiac sodium channel hNa(v)1.5. We identified 4 novel mutations in CAV3-encoded caveolin-3 that were absent in >1000 control alleles. Electrophysiological analysis of sodium current in HEK293 cells stably expressing hNa(v)1.5 and transiently transfected with wild-type and mutant caveolin-3 demonstrated that mutant caveolin-3 results in a 2- to 3-fold increase in late sodium current compared with wild-type caveolin-3. Our observations are similar to the increased late sodium current associated with LQT3-associated SCN5A mutations. CONCLUSIONS: The present study reports the first CAV3 mutations in subjects with LQTS, and we provide functional data demonstrating a gain-of-function increase in late sodium current.

Amino Acid Sequence↗
PubMed2006

The single nucleotide polymorphisms of I(Ks) potassium channel genes and their association with atrial fibrillation in a Chinese population.

Recent studies suggest that genetic mutation of the slow delayed rectifier potassium channel (I(Ks)) may underlie atrial fibrillation (AF). We investigated the association between AF and the single nucleotide polymorphisms (SNPs) of genes KCNQ1, KCNE1 and KCNE4 associated with this channel. Common non-synonymous SNPs in KCNQ1 and KCNE1 known to be frequent in Asian people were selected and direct sequencing of KCNE4 was performed to identify possible SNPs. The AF group consisted of 142 hospitalized patients with AF, the community control group consisted of 120 subjects, and a ward control group consisted of 118 hospitalized patients without AF. Restriction fragment length polymorphism analysis was performed to determine the genotypes. The minor allele frequencies of P448R, R519H, G643S for KCNQ1 and G38S and D85N for KCNE1 in the AF group, the community control group and the ward control group were 9.9, 7.9, 9.3%; 0, 0, -; 4.3, 4.2, 1.7%; 28.4, 31.7, 29.7%; 0.7, 0.4%, -, respectively. There was no significant association between these SNPs and AF phenotype. There were eight SNPs in the whole length of KCNE4 plus 1,000 bases upstream of this gene including the non-synonymous SNP E145D. Logistical regression analysis revealed a difference in the distribution of KCNE4 E145D in the AF and the community control group (minor allele frequency was 34.0 versus 27.1% respectively, OR = 1.66, p = 0.044). We provided the frequencies of non-synonymous SNPs of KCNQ1 and KCNE1 in Chinese population; none of these SNPs was associated with AF. But KCNE4 E145D may be associated with the AF phenotype.

Aged↗
PubMed2006

Partial expression defect for the SCN5A missense mutation G1406R depends on splice variant background Q1077 and rescue by mexiletine.

Mutations in the cardiac Na(+) channel gene SCN5A cause loss of function and underlie arrhythmia syndromes. SCN5A in humans has two splice variants, one lacking a glutamine at position 1077 (Q1077del) and one containing Q1077. We investigated the effect of splice variant background on loss of function and rescue for G1406R, a mutation reported to cause Brugada syndrome. Mutant and wild-type (WT) channels in both backgrounds were transfected into HEK-293 cells and incubated for up to 72 h with and without mexiletine. At 8 h, neither current nor cell surface expression was observed for the mutant in either background, but both were present in WT channels. At 24 h, small (<10% compared with WT) currents were noted and accompanied by cell surface expression. At 48 h, current density was approximately 40% of WT channels for the mutant in the Q1077del variant background but remained at <10% of WT channels in Q1077. Current levels were stable by 72 h. Coexpression with beta(1)- or beta(3)-subunits or insertion of the polymorphism H558R in the background did not significantly affect current expression. Mexiletine restored current density of the mutant channel in both backgrounds to nearly WT levels. The mutant channels also showed a negative shift in inactivation, slower recovery, and enhanced slow inactivation, consistent with a loss of function phenotype. These data show that a trafficking defect may be partial and time dependent and may differ with the splice variant background. Also, expression defects and gating abnormalities may contribute to loss of function for the same mutation.

Alternative Splicing↗
PubMed2006

Genotypic heterogeneity and phenotypic mimicry among unrelated patients referred for catecholaminergic polymorphic ventricular tachycardia genetic testing.

BACKGROUND: Mutations in the RyR2-encoded cardiac ryanodine receptor/calcium release channel and in CASQ2-encoded calsequestrin cause catecholaminergic polymorphic ventricular tachycardia (CPVT1 and CPVT2, respectively). OBJECTIVES: The purpose of this study was to evaluate the extent of genotypic and phenotypic heterogeneity among referrals for CPVT genetic testing. METHODS: Using denaturing high-performance liquid chromatography and DNA sequencing, mutational analysis of 23 RyR2 exons previously implicated in CPVT1, comprehensive analysis of all translated exons in CASQ2 (CPVT2), KCNQ1 (LQT1), KCNH2 (LQT2), SCN5A (LQT3), KCNE1 (LQT5), KCNE2 (LQT6), and KCNJ2 (Andersen-Tawil syndrome [ATS1], also annotated LQT7), and analysis of 10 ANK2 exons implicated in LQT4 were performed on genomic DNA from 11 unrelated patients (8 females) referred to Mayo Clinic's Sudden Death Genomics Laboratory explicitly for CPVT genetic testing. RESULTS: Overall, putative disease causing mutations were identified in 8 patients (72%). Only 4 patients (3 males) hosted CPVT1-associated RyR2 mutations: P164S, V186M, S3938R, and T4196A. Interestingly, 4 females instead possessed either ATS1- or LQT5-associated mutations. Mutations were absent in >400 reference alleles. CONCLUSION: Putative CPVT1-causing mutations in RyR2 were seen in <40% of unrelated patients referred with a diagnosis of CPVT and preferentially in males. Phenotypic mimicry is evident with the identification of ATS1- and LQT5-associated mutations in females displaying a normal QT interval and exercise-induced bidirectional VT, suggesting that observed exercise-induced polymorphic VT in patients may reflect disorders other than CPVT. Clinical consideration for either Andersen-Tawil syndrome or long QT syndrome and appropriate genetic testing may be warranted for individuals with RyR2 mutation-negative CPVT, particularly females.

Adolescent↗
PubMed2006

Spontaneous coronary vasospasm in KATP mutant mice arises from a smooth muscle-extrinsic process.

In the vasculature, ATP-sensitive potassium channels (KATP) channels regulate vascular tone. Mice with targeted gene disruptions of KATP subunits expressed in vascular smooth muscle develop spontaneous coronary vascular spasm and sudden death. From these models, it was hypothesized that the loss of KATP channel activity in arterial vascular smooth muscle was responsible for coronary artery spasm. We now tested this hypothesis using a transgenic strategy where the full-length sulfonylurea receptor containing exon 40 was expressed under the control of a smooth muscle-specific SM22alpha promoter. Two transgenic founder lines were generated and independently bred to sulfonylurea receptor 2 (SUR2) null mice to generate mice that restored expression of KATP channels in vascular smooth muscle. Transgenic expression of the sulfonylurea receptor in vascular smooth muscle cells was confirmed by detecting mRNA and protein from the transgene. Functional restoration was determined by recording pinacidil-based KATP current by whole cell voltage clamping of isolated aortic vascular smooth muscle cells isolated from the transgenic restored mice. Despite successful restoration of KATP channels in vascular smooth muscle, transgene-restored SUR2 null mice continued to display frequent episodes of spontaneous ST segment elevation, identical to the phenotype seen in SUR2 null mice. As in SUR2 null mice, ST segment elevation was frequently followed by atrioventricular heart block. ST segment elevation and coronary perfusion pressure in the restored mice did not differ significantly between transgene-negative and transgene-positive SUR2 null mice. We conclude that spontaneous coronary vasospasm and sudden death in SUR2 null mice arises from a coronary artery vascular smooth muscle-extrinsic process.

ATP-Binding Cassette Transporters↗
PubMed2006

Ranolazine and late cardiac sodium current--a therapeutic target for angina, arrhythmia and more?

Ranolazine is a new antianginal drug approved for clinical use in the United States in January 2006. A study published in this same issue of the British Journal of Pharmacology characterizes ranolazine block of late sodium current caused by the long QT syndrome 3 mutations. This commentary discusses the implications of that study and the background and implications for block of late cardiac sodium current in general.

Acetanilides↗
PubMed2006

Na(+) current in human ventricle: implications for sodium loading and homeostasis.

The Na current (I(Na)) in human ventricle is carried through a specific isoform of the voltage gated Na channel in heart. The pore forming alpha-subunit is encoded by the gene SCN5A. Up to four beta-subunits may be associated, and the larger macromolecular complex may include attachments to cytoskeleton and scaffolding proteins, all of which may affect the gating kinetics of the current. I(Na) underlies initiation and propagation of action potentials in the heart and plays a prominent role in cardiac electrophysiology and arrhythmia. In addition, I(Na) also loads the ventricular cell with Na(+) ions and plays an important role in intracellular Na homeostasis. This review considers the structure and function of the human cardiac Na channel that carries I(Na) with a particular consideration of the implications of alterations in I(Na) in acquired cardiac diseases such as hypertrophy, failure, and ischemia, which affect Na loading.

Action Potentials↗
PubMed2006

SIDS: genetic and environmental influences may cause arrhythmia in this silent killer.

In this issue of the JCI, Bowers et al. show that the common polymorphism of the cardiac voltage-gated sodium channel, type Valpha (SCN5A), designated S1103Y, found in African Americans is associated with an increased risk of sudden infant death syndrome (SIDS). Wild-type and mutant SCN5A channels both functioned typically under normal conditions in vitro, but exposure to acidic intracellular pH levels such as those found in respiratory acidosis--a known risk factor for SIDS--produced abnormal gain-of-function late reopenings of S1103Y channels, behavior that is often associated with cardiac arrhythmias. These pathologic late reopenings were suppressed by low levels of the channel-blocking drug mexiletine. These findings provide an excellent illustration of a causal relationship between the interaction of the environment and genetic background in SIDS and also raise interesting questions about the linkage of a genetic abnormality with a clinical phenotype.

Arrhythmias, Cardiac↗
PubMed2005

Specific serine proteases selectively damage KCNH2 (hERG1) potassium channels and I(Kr).

KCNH2 (hERG1) encodes the alpha-subunit proteins for the rapidly activating delayed rectifier K+ current (I(Kr)), a major K+ current for cardiac myocyte repolarization. In isolated myocytes I(Kr) frequently is small in amplitude or absent, yet KCNH2 channels and I(Kr) are targets for drug block or mutations to cause long QT syndrome. We hypothesized that KCNH2 channels and I(Kr) are uniquely sensitive to enzymatic damage. To test this hypothesis, we studied heterologously expressed K+, Na+, and L-type Ca2+ channels, and in ventricular myocytes I(Kr), slowly activating delayed rectifier K+ current (I(Ks)), and inward rectifier K+ current (I(K1)), by using electrophysiological and biochemical methods. 1) Specific exogenous serine proteases (protease XIV, XXIV, or proteinase K) selectively degraded KCNH2 current (I(KCNH2)) and its mature channel protein without damaging cell integrity and with minimal effects on the other channel currents; 2) immature KCNH2 channel protein remained intact; 3) smaller molecular mass KCNH2 degradation products appeared; 4) protease XXIV selectively abolished I(Kr); and 5) reculturing HEK-293 cells after protease exposure resulted in the gradual recovery of I(KCNH2) and its mature channel protein over several hours. Thus the channel protein for I(KCNH2) and I(Kr) is uniquely sensitive to proteolysis. Analysis of the degradation products suggests selective proteolysis within the S5-pore extracellular linker, which is structurally unique among Kv channels. These data provide 1) a new mechanism to account for low I(Kr) density in some isolated myocytes, 2) evidence that most complexly glycosylated KCNH2 channel protein is in the plasma membrane, and 3) new insight into the rate of biogenesis of KCNH2 channel protein within cells.

Animals↗
PubMed2005

Function and distribution of the SUR isoforms and splice variants.

Alternative splicing allows multiple mRNAs to be generated from a single gene, which in turn can be translated into a group of diverse proteins with different roles and structures. The outcome of alternative splicing leads to the co-existence of multiple splice variants of a gene at different concentrations in different tissues. The pore-forming subunit of the K(ATP) channel (K(IR)6.x) and the regulatory sulfonylurea receptor (SUR(x)) subunits exist in a 4:4 stoichiometry to form hetero-octameric ATP-sensitive potassium channel (K(ATP)) channels, which are widely distributed in various types of tissues at either the plasma membrane (cellK(ATP)) or mitochondrial inner membrane (the mitochondrial form of K(ATP) channel, mitoK(ATP)). They perform important physiological functions in regulating insulin secretion in pancreatic beta-cells, providing ischemic protection in heart and brain, and regulating vascular tone in smooth muscles. Two separate genes, the regulatory subunit protein I (SUR1) and the regulatory subunit protein II (SUR2) encode the high- and low-affinity SUR, respectively. This review summarizes the current studies on the function and distribution of the SUR isoforms and alternative splice variants, and to a lesser extent the K(IR)6.x subunits. The different isoforms and splice variants allow for many K(ATP) channel combinations, and therefore, increases the channel diversity and the possibility of complexity in function.

ATP-Binding Cassette Transporters↗
PubMed2005

Intrinsic mechanism of the enhanced rate-dependent QT shortening in the R1623Q mutant of the LQT3 syndrome.

OBJECTIVE: In the type 3 long QT syndrome (LQT3), arrhythmia events tend to occur at rest or during sleep. One of the mutations, R1623Q, is located in the voltage sensor of the cardiac sodium channel (hH1), and patients with R1623Q mutation have been also reported to show bradycardia-dependent cardiac events. Although the mutant channel has been characterized by inactivation gating defects, the intrinsic mechanism(s) that might explain why arrhythmia attack is most prevalent at slower heart rates has not been investigated. METHODS: cDNA encoding either the wild-type or the R1623Q mutant of hH1 was stably transfected into HEK293 cells. I(Na) was recorded using a whole-cell patch-clamp technique at 23 degrees C. RESULTS: A train of 50 depolarizing pulses from holding potentials (-120 and -80 mV) to -20 mV or a train of 50 action potential waveforms was applied at different frequencies. When using a rectangular waveform voltage clamp protocol, rate-dependent reduction of I(Na) was holding voltage-dependent but was not different between peak I(Na) and late I(Na). However, using the action potential clamp, preferential rate-dependent reduction of the phase 3 I(Na) was obvious as compared with peak I(Na). The discrepancy in the rate-dependent reduction between protocols was attributed to accelerated recovery from inactivation under non-equilibrium condition. CONCLUSION: The rate dependency of phase 3 I(Na) under non-equilibrium gating is a novel mechanism to explain the enhanced rate-dependent QT-shortening in LQT3 patients. Our findings are important for genotype-phenotype correlations in LQT3 mutants as well as for understanding the function of S4 segment of domain IV region in the cardiac Na(+) channel.

Action Potentials↗
PubMed2005

The perplexing complexity of cardiac arrhythmias: beyond electrical remodeling.

Cardiac arrhythmias continue to pose a major medical challenge and significant public health burden. Atrial fibrillation, the most prevalent arrhythmia, affects more than two million Americans annually and is associated with a twofold increase in mortality. In addition, more than 250,000 Americans each year suffer ventricular arrhythmias, often resulting in sudden cardiac death. Despite the high incidence and societal impact of cardiac arrhythmias, presently there are insufficient insights into the molecular mechanisms involved in arrhythmia generation, propagation, and/or maintenance or into the molecular determinants of disease risk, prognosis, and progression. In addition, present therapeutic strategies for arrhythmia abatement often are ineffective or require palliative device therapy after persistent changes in the electrical and conduction characteristics of the heart have occurred, changes that appear to increase the risk for arrhythmia progression. This article reviews our present understanding of the complexity of mechanisms that regulate cardiac membrane excitability and cardiac function and explores the role of derangements in these mechanisms that interact to induce arrhythmogenic substrates. Approaches are recommended for future investigations focused on providing new mechanistic insights and therapeutic interventions.

Arrhythmias, Cardiac↗
PubMed2005

Common human SCN5A polymorphisms have altered electrophysiology when expressed in Q1077 splice variants.

BACKGROUND: Eight common (>0.5%) polymorphisms of SCN5A have been described in the US population. Every human also continuously generates two wild-type (WT) splice variants, one with a glutamine residue at position 1077 (Q1077) and one lacking this glutamine (Q1077del). One polymorphism (H558R) has been studied in both splice variants, five polymorphisms (R34C, R481W, S524Y, P1090L,V1951L) have not been previously studied, and two polymorphisms (S1103Y and R1193Q) have been studied in only one of the two splice variants. OBJECTIVES: The purpose of this study was to examine the electrophysiologic molecular phenotype of the eight common polymorphisms in the two human splice variants of SCN5A. METHODS: Currents from 16 channels (all polymorphisms in both splice variants) were determined by voltage clamp and compared with WT after expression in HEK-293 cells. RESULTS: Six of eight polymorphisms showed a distinct phenotype that depended upon the background splice variant used for expression. Only R34C and V1951L showed no functional differences. S524Y showed a dramatic reduction in current density in the Q1077 background similar to that previously described for H558R. Four other polymorphisms (R481W, P1090L, S1103Y, R1193Q) showed shifts in activation, inactivation, or recovery that depended upon the splice variants. Shifts of a similar magnitude have been reported for arrhythmia syndrome mutations and are thought to be pathogenic. CONCLUSION: The majority of common human SCN5A polymorphisms have a distinct molecular phenotype that depends upon the splice variant background. These findings have implications for the interpretation of previous studies of arrhythmia mutations. The significance of these findings for clinical arrhythmia remains to be elucidated.

Alternative Splicing↗
PubMed2005

Increased late sodium current in myocytes from a canine heart failure model and from failing human heart.

Electrophysiological remodeling of ion channels in heart failure causes action potential prolongation and plays a role in arrhythmia mechanism. The importance of down-regulation of potassium currents is well-known, but a role for Na current (I(Na)) in heart failure is less well established. We studied I(Na) in heart failure ventricular cells from a canine pacing model of heart failure and also from explanted failing human hearts. Peak I(Na) density was significantly decreased by 39% and 57% in the dog model and in human heart failure, respectively. The kinetics of peak I(Na) were not different in heart failure. Late I(Na) was measured 750 ms after the initial depolarization as the saxitoxin (STX)-sensitive current and also as the current remaining after contaminating currents were blocked. Late I(Na) as a percentage of the peak I(Na) was significantly increased in both conditions. In dogs, STX sensitive late I(Na) was 0.5 +/- 0.1% n = 16 cells from eight normal hearts and 3.4 +/- 1.4% n = 12 cells from seven failing hearts; in humans, it was 0.2 +/- 0.1% n = 4 cells from two normal hearts and 2.4 +/- 0.5% n = 10 cells from three human failing hearts (-40 mV). Quantitative measures of mRNA including RNase protection assays and real time quantitative PCR in the dog model showed no differences for different alpha subunit isoforms (NaV1.1, 1.3, 1.5) and for the beta1 and beta2 subunits. This suggests neither alpha subunit isoform switching nor altered beta subunit expression is a mechanism for increased late I(Na). We conclude that a peak I(Na) is decreased, and non-inactivating late I(Na) is increased in heart failure and this may contribute to action potential prolongation and the generation of arrhythmia.

Animals↗
PubMed2005

Blockade of HERG cardiac K+ current by antifungal drug miconazole.

1. Miconazole, an imidazole antifungal agent, is associated with acquired long QT syndrome and ventricular arrhythmias. Miconazole increases the plasma concentration of QT-prolonging drugs by inhibiting the hepatic cytochrome P450 metabolic pathway, but whether it has direct effects on cardiac ion channels has not been elucidated. 2. To determine the mechanism underlying these clinical findings, we investigated the effect of miconazole on human ether-a-go-go-related gene (HERG) K+ channels. 3. HERG channels were heterologously expressed in human embryonic kidney 293 (HEK293) cells and whole-cell currents were recorded using a patch-clamp technique (23 degrees C). 4. Miconazole inhibited HERG peak tail current in a concentration-dependent manner (0.4-40 microM) with an IC50 of 2.1 microM (n=3-5 cells at each concentration, Hill coefficient 1.2). HERG block was not frequency-dependent. It required channel activation, occurred rapidly, and had very slow dissociation properties. 5. The activation curve was shifted in a negative direction (V(1/2): -9.5+/-2.3 mV in controls and -15.3+/-2.4 mV after 4 microM miconazole, P<0.05, n=6). Miconazole did not change other channel kinetics (activation, deactivation, onset of inactivation, recovery from inactivation, steady-state inactivation). 6. The S6 domain mutation, F656C, abolished the inhibitory action of miconazole on HERG current indicating that miconazole preferentially binds to an aromatic amino-acid residue within the pore-S6 region. 7. Our findings indicate that miconazole causes HERG channel block by binding to a common drug receptor, and this involves preferential binding to activated channels. Thus, miconazole prolongs the QT interval by direct inhibition of HERG channels.

Action Potentials↗
PubMed2004

A trafficking defective, Brugada syndrome-causing SCN5A mutation rescued by drugs.

BACKGROUND: The human cardiac SCN5A gene encodes for the alpha subunit of the human cardiac voltage-dependent sodium channel hNav1.5 [Neuron 28 (2) (2000) 365] and carries inward Na current (INa). Mutations in SCN5A cause arrhythmia syndromes including Brugada syndrome (BrS) and congenital long QT syndrome subtype 3 (LQT3). Here, we report a trafficking defective BrS-causing SCN5A mutation that was drug-rescued. METHODS AND RESULTS: A 14-year-old Caucasian male was diagnosed with BrS with typical ECG pattern for BrS and ventricular fibrillation was easily induced. He also had significant HV interval delay ( approximately 65 ms) and high (31 J) defibrillation thresholds (DFTs). Genomic analysis revealed the SCN5A mutation (G1743R). We engineered G1743R into the cardiac Na channel and transfected HEK-293 cells for functional studies. The mutant channel yielded nearly undetectable sodium channel currents. Coexpression with the beta1 subunit, or incubation at low temperature did not increase current density. However, mexiletine, a sodium channel blocker, increased current density 93-fold in G1743R, but only twofold in WT. CONCLUSIONS: This study identifies an expression-defective BrS mutation in SCN5A with pharmacological rescue. The profoundly decreased sodium current associated with the G1743R suggests a molecular basis for the delayed His-Purkinje conduction and elevated DFTs observed in the proband. Whether the mutant channel may be rescued in vivo by mexiletine and normalize the patient's electrophysiologic parameters remains to be tested.

Adolescent↗
PubMed2004

Spectrum and prevalence of cardiac sodium channel variants among black, white, Asian, and Hispanic individuals: implications for arrhythmogenic susceptibility and Brugada/long QT syndrome genetic testing.

OBJECTIVES: The purpose of this study was to determine the prevalence and spectrum of nonsynonymous polymorphisms (amino acid variants) in the cardiac sodium channel among healthy subjects. BACKGROUND: Pathogenic mutations in the cardiac sodium channel gene, SCN5A, cause approximately 15 to 20% of Brugada syndrome (BrS1), 5 to 10% of long QT syndrome (LQT3), and 2 to 5% of sudden infant death syndrome. METHODS: Using single-stranded conformation polymorphism, denaturing high-performance liquid chromatography, and/or direct DNA sequencing, mutational analysis of the protein-encoding exons of SCN5A was performed on 829 unrelated, anonymous healthy subjects: 319 black, 295 white, 112 Asian, and 103 Hispanic. RESULTS: In addition to the four known common polymorphisms (R34C, H558R, S1103Y, and R1193Q), four relatively ethnic-specific polymorphisms were identified: R481W, S524Y, P1090L, and V1951L. Overall, 39 distinct missense variants (28 novel) were elucidated. Nineteen variants (49%) were found only in the black cohort. Only seven variants (18%) localized to transmembrane-spanning domains. Four variants (F1293S, R1512W, and V1951L cited previously as BrS1-causing mutations and S1787N previously published as a possible LQT3-causing mutation) were identified in this healthy cohort. CONCLUSIONS: This study provides the first comprehensive determination of the prevalence and spectrum of cardiac sodium channel variants in healthy subjects from four distinct ethnic groups. This compendium of SCN5A variants is critical for proper interpretation of SCN5A genetic testing and provides an essential hit list of targets for future functional studies to determine whether or not any of these variants mediate genetic susceptibility for arrhythmias in the setting of either drugs or disease.

Bundle-Branch Block↗