Homozygous/Compound Heterozygous Triadin Mutations Associated With Autosomal-Recessive Long-QT Syndrome and Pediatric Sudden Cardiac Arrest: Elucidation of the Triadin Knockout Syndrome.

BACKGROUND: Long-QT syndrome (LQTS) may result in syncope, seizures, or sudden cardiac arrest. Although 16 LQTS-susceptibility genes have been discovered, 20% to 25% of LQTS remains genetically elusive. METHODS AND RESULTS: We performed whole-exome sequencing child-parent trio analysis followed by recessive and sporadic inheritance modeling and disease-network candidate analysis gene ranking to identify a novel underlying genetic mechanism for LQTS. Subsequent mutational analysis of the candidate gene was performed with polymerase chain reaction, denaturing high-performance liquid chromatography, and DNA sequencing on a cohort of 33 additional unrelated patients with genetically elusive LQTS. After whole-exome sequencing and variant filtration, a homozygous p.D18fs*13 TRDN-encoded triadin frameshift mutation was discovered in a 10-year-old female patient with LQTS with a QTc of 500 milliseconds who experienced recurrent exertion-induced syncope/cardiac arrest beginning at 1 year of age. Subsequent mutational analysis of TRDN revealed either homozygous or compound heterozygous frameshift mutations in 4 of 33 unrelated cases of LQTS (12%). All 5 TRDN-null patients displayed extensive T-wave inversions in precordial leads V1 through V4, with either persistent or transient QT prolongation and severe disease expression of exercise-induced cardiac arrest in early childhood (≤3 years of age) and required aggressive therapy. The overall yield of TRDN mutations was significantly greater in patients ≤10 years of age (5 of 10, 50%) compared with older patients (0 of 24, 0%; P=0.0009). CONCLUSIONS: We identified TRDN as a novel underlying genetic basis for recessively inherited LQTS. All TRDN-null patients had strikingly similar phenotypes. Given the recurrent nature of potential lethal arrhythmias, patients fitting this phenotypic profile should undergo cardiac TRDN genetic testing.

Long QT syndrome-associated mutations in intrauterine fetal death.

IMPORTANCE: Intrauterine fetal death or stillbirth occurs in approximately 1 out of every 160 pregnancies and accounts for 50% of all perinatal deaths. Postmortem evaluation fails to elucidate an underlying cause in many cases. Long QT syndrome (LQTS) may contribute to this problem. OBJECTIVE: To determine the spectrum and prevalence of mutations in the 3 most common LQTS susceptible genes (KCNQ1, KCNH2, and SCN5A) for a cohort of unexplained cases. DESIGN, SETTING, AND PATIENTS: In this case series, retrospective postmortem genetic testing was conducted on a convenience sample of 91 unexplained intrauterine fetal deaths (mean [SD] estimated gestational age at fetal death, 26.3 [8.7] weeks) that were collected from 2006-2012 by the Mayo Clinic, Rochester, Minnesota, or the Fondazione IRCCS Policlinico San Matteo, Pavia, Italy. More than 1300 ostensibly healthy individuals served as controls. In addition, publicly available exome databases were assessed for the general population frequency of identified genetic variants. MAIN OUTCOMES AND MEASURES: Comprehensive mutational analyses of KCNQ1 (KV7.1, LQTS type 1), KCNH2 (HERG/KV11.1, LQTS type 2), and SCN5A (NaV1.5, LQTS type 3) were performed using denaturing high-performance liquid chromatography and direct DNA sequencing on genomic DNA extracted from decedent tissue. Functional analyses of novel mutations were performed using heterologous expression and patch-clamp recording. RESULTS: The 3 putative LQTS susceptibility missense mutations (KCNQ1, p.A283T; KCNQ1, p.R397W; and KCNH2 [1b], p.R25W), with a heterozygous frequency of less than 0.05% in more than 10 000 publicly available exomes and absent in more than 1000 ethnically similar control patients, were discovered in 3 intrauterine fetal deaths (3.3% [95% CI, 0.68%-9.3%]). Both KV7.1-A283T (16-week male) and KV7.1-R397W (16-week female) mutations were associated with marked KV7.1 loss-of-function consistent with in utero LQTS type 1, whereas the HERG1b-R25W mutation (33.2-week male) exhibited a loss of function consistent with in utero LQTS type 2. In addition, 5 intrauterine fetal deaths hosted SCN5A rare nonsynonymous genetic variants (p.T220I, p.R1193Q, involving 2 cases, and p.P2006A, involving 2 cases) that conferred in vitro electrophysiological characteristics consistent with potentially proarrhythmic phenotypes. CONCLUSIONS AND RELEVANCE: In this molecular genetic evaluation of 91 cases of intrauterine fetal death, missense mutations associated with LQTS susceptibility were discovered in 3 cases (3.3%) and overall, genetic variants leading to dysfunctional LQTS-associated ion channels in vitro were discovered in 8 cases (8.8%). These preliminary findings may provide insights into mechanisms of some cases of stillbirth.

Most LQT2 mutations reduce Kv11.1 (hERG) current by a class 2 (trafficking-deficient) mechanism.

BACKGROUND: The KCNH2 or human ether-a-go-go related gene (hERG) encodes the Kv11.1 alpha-subunit of the rapidly activating delayed rectifier K+ current (IKr) in the heart. Type 2 congenital long-QT syndrome (LQT2) results from KCNH2 mutations that cause loss of Kv11.1 channel function. Several mechanisms have been identified, including disruption of Kv11.1 channel synthesis (class 1), protein trafficking (class 2), gating (class 3), or permeation (class 4). For a few class 2 LQT2-Kv11.1 channels, it is possible to increase surface membrane expression of Kv11.1 current (IKv11.1). We tested the hypotheses that (1) most LQT2 missense mutations generate trafficking-deficient Kv11.1 channels, and (2) their trafficking-deficient phenotype can be corrected. METHODS AND RESULTS: Wild-type (WT)-Kv11.1 channels and 34 missense LQT2-Kv11.1 channels were expressed in HEK293 cells. With Western blot analyses, 28 LQT2-Kv11.1 channels had a trafficking-deficient (class 2) phenotype. For the majority of these mutations, the class 2 phenotype could be corrected when cells were incubated for 24 hours at reduced temperature (27 degrees C) or in the drugs E4031 or thapsigargin. Four of the 6 LQT2-Kv11.1 channels that had a wild-type-like trafficking phenotype did not cause loss of Kv11.1 function, which suggests that these channels are uncommon sequence variants. CONCLUSIONS: This is the first study to identify a dominant mechanism, class 2, for the loss of Kv11.1 channel function in LQT2 and to report that the class 2 phenotype for many of these mutant channels can be corrected. This suggests that if therapeutic strategies to correct protein trafficking abnormalities can be developed, it may offer clinical benefits for LQT2 patients.

Mutation detection in congenital long QT syndrome: cardiac channel gene screen using PCR, dHPLC, and direct DNA sequencing.

Within the field of molecular cardiac electrophysiology, the previous decade of research elucidated the fundamental genetic substrate underlying many arrhythmogenic disorders such as long QT syndrome (LQTS), catecholaminergic polymorphic ventricular tachycardia (CPVT), Andersen-Tawil syndrome, Brugada Syndrome, and Timothy syndrome. In addition, the genetic basis for cardiomyopathic processes vulnerable to sudden arrhythmic death-hypertrophic cardiomyopathy, dilated cardiomyopathy, and arrhythmogenic right ventricular cardiomyopathy-are understood now in greater detail. The majority of congenital LQTS is understood as a primary cardiac channelopathy that often but not always provides evidence of its presence via a prolonged QT interval on the 12-lead surface electrocardiogram. To date, more than 300 mutations have been identified in five genes encoding key ion channel sub units involved in the orchestration of the heart's action potential. LQTS genetic testing has been performed in research laboratories over the past decade, relying on the techniques of PCR, an intermediate mutation analysis platform such as single-stranded conformation polymorphism (SSCP) or denaturing high-performance liquid chromatography (dHPLC), and subsequent direct DNA sequencing to elucidate the genetic underpinnings of this disorder. Presently, LQTS genetic testing is a clinically available molecular diagnostic test that provides comprehensive open reading frame/splice site mutational analysis via high-throughput DNA sequencing. This chapter will focus on LQTS genetic testing employing the techniques of genomic DNA isolation from peripheral blood, exon-specific PCR amplification, dHPLC hetero-duplex analysis, and direct DNA sequencing.

Whole-Exome Molecular Autopsy After Exertion-Related Sudden Unexplained Death in the Young.

BACKGROUND: Targeted postmortem genetic testing of the 4 major channelopathy-susceptibility genes (KCNQ1, KCNH2, SCN5A, and RYR2) have yielded putative pathogenic mutations in ≤30% of autopsy-negative sudden unexplained death in the young (SUDY) cases with highest yields derived from the subset of exertion-related SUDY. Here, we evaluate the role of whole-exome sequencing in exertion-related SUDY cases. METHODS AND RESULTS: From 1998 to 2010, 32 cases of exertion-related SUDY were referred by Medical Examiners for a cardiac channel molecular autopsy. A mutational analysis of the major long-QT syndrome-susceptibility genes (KCNQ1, KCNH2, and SCN5A) and catecholaminergic polymorphic ventricular tachycardia-susceptibility gene (RYR2) identified a putative pathogenic mutation in 11 cases. Whole-exome sequencing was performed on the remaining 21 targeted gene-negative SUDY cases. After whole-exome sequencing, a gene-specific surveillance of all genes (N=100) implicated in sudden death was performed to identify putative pathogenic mutation(s). Three of these 21 decedents had a clinically actionable, pathogenic mutation (CALM2-F90L, CALM2-N98S, and PKP2-N634fs). Of the 18 remaining cases, 7 hosted at least 1 variant of unknown significance with a minor allele frequency <1:20 000. The overall yield of pathogenic mutations was higher among decedents aged 1 to 10 years (10/11, 91%) than those aged 11 to 19 years (4/21, 19%, P=0.0001). CONCLUSIONS: Molecular screening in this clinical scenario is appropriate with a pathogenic mutation detection rate of 44% using direct DNA sequencing followed by whole-exome sequencing. Only 5 of the 100 interrogated sudden death genes hosted actionable pathogenic mutations for more than one third of these exertion-related, autopsy-negative SUDY cases.

Spectrum and frequency of cardiac channel defects in swimming-triggered arrhythmia syndromes.

BACKGROUND: Swimming is a relatively genotype-specific arrhythmogenic trigger for type 1 long-QT syndrome (LQT1). We hypothesize that mimickers of concealed LQT1, namely catecholaminergic polymorphic ventricular tachycardia (CPVT), may also underlie swimming-triggered cardiac events. METHODS AND RESULTS: Between August 1997 and May 2003, 388 consecutive, unrelated patients were referred specifically for LQTS genetic testing. The presence of a personal and/or family history of a near-drowning or drowning was determined by review of the medical records and/or phone interviews and was blinded to genetic test results. Comprehensive mutational analysis of the 5 LQTS-causing channel genes, KCNQ1 (LQT1), KCNH2 (LQT2), SCN5A (LQT3), KCNE1 (LQT5), and KCNE2 (LQT6), along with KCNJ2 (Andersen-Tawil syndrome) and targeted analysis of 18 CPVT1-associated exons in RyR2, was performed with the use of denaturing high-performance liquid chromatography and direct DNA sequencing. Approximately 11% (43 of 388) of the index cases had a positive swimming phenotype. Thirty-three of these 43 index cases had a "Schwartz" score (> or =4) suggesting high clinical probability of LQTS. Among this subset, 28 patients (85%) were LQT1, 2 patients (6%) were LQT2, and 3 were genotype negative. Among the 10 cases with low clinical probability for LQTS, 9 had novel, putative CPVT1-causing RyR2 mutations. CONCLUSIONS: In contrast to previous studies that suggested universal LQT1 specificity, genetic heterogeneity underlies channelopathies that are suspected chiefly because of a near-drowning or drowning. CPVT1 and strategic genotyping of RyR2 should be considered when LQT1 is excluded in the pathogenesis of a swimming-triggered arrhythmia syndrome.

Prevalence and severity of "benign" mutations in the beta-myosin heavy chain, cardiac troponin T, and alpha-tropomyosin genes in hypertrophic cardiomyopathy.

BACKGROUND: Genotype-phenotype correlative studies have implicated 8 particular mutations that cause hypertrophic cardiomyopathy (HCM) as "benign defects," associated with near-normal survival: N232S, G256E, F513C, V606M, R719Q, and L908V of beta-myosin heavy chain (MYH7); S179F of troponin T (TNNT2); and D175N of alpha-tropomyosin (TPM1). Routine genetic screening of HCM patients for specific mutations is anticipated to provide important diagnostic and prognostic information. The frequency and associated phenotype of these mutations in a large, unselected cohort of HCM is unknown. METHODS AND RESULTS: A total of 293 unrelated HCM patients were genotyped for the presence of a benign mutation. DNA was obtained after informed consent; specific MHY7, TNNT2, and TPM1 fragments were amplified by polymerase chain reaction; and the mutations were detected by denaturing high-performance liquid chromatography and automated DNA sequencing. Only 5 (1.7%) of the 293 patients possessed a benign mutation. Moreover, all 5 subjects with an ascribed benign mutation had already manifested clinically severe expression of HCM, with all 5 requiring surgical myectomy, 3 of the 5 having a family history of sudden cardiac death, and 1 adolescent requiring an orthotopic heart transplant. CONCLUSIONS: These findings demonstrate the rarity of specific mutations in HCM and challenge the notion of mutation-specific clinical outcomes. Fewer than 2% of the subjects harbored a benign mutation, and those patients with a benign mutation experienced a very serious clinical course.

Myosin binding protein C mutations and compound heterozygosity in hypertrophic cardiomyopathy.

OBJECTIVES: We sought to determine the frequency and phenotype of mutations in myosin binding protein C (MYBPC3) in a large outpatient cohort of patients with hypertrophic cardiomyopathy (HCM) seen at our tertiary referral center. BACKGROUND: Mutations in MYBPC3 are one of the most frequent genetic causes of HCM and have been associated with variable onset of disease and prognosis. However, the frequency of mutations and associated clinical presentation have not been established in a large, unrelated cohort of patients. METHODS: Using deoxyribonucleic acid from 389 unrelated patients with HCM, each protein coding exon of MYBPC3 was analyzed for mutations by polymerase chain reaction, denaturing high-performance liquid chromatography, and direct deoxyribonucleic acid sequencing. Clinical data were extracted from patient records blinded to patient genotype. RESULTS: Of 389 patients with HCM, 71 (18%) had mutations in MYBPC3. In all, 46 mutations were identified, 33 of which were novel (72%). Patients with MYBPC3 mutations did not differ significantly from patients with thick filament-HCM, thin filament-HCM, or genotype-negative HCM with respect to age at diagnosis, degree of hypertrophy, incidence of myectomy, or family history of HCM or sudden death. Patients with multiple mutations (n = 10, 2.6%) had the most severe disease presentation. CONCLUSIONS: This study defines the frequency and associated phenotype for MYBPC3 and/or multiple mutations in HCM in the largest cohort to date. In this cohort, unrelated patients with MYBPC3-HCM virtually mimicked the phenotype of those with mutations in the beta-myosin heavy chain. Patients with multiple mutations had the most severe phenotype.

Comprehensive analysis of the beta-myosin heavy chain gene in 389 unrelated patients with hypertrophic cardiomyopathy.

OBJECTIVES: We sought to determine the prevalence and phenotype of beta-myosin heavy chain gene MYH7 mutations in a large cohort of unrelated patients with hypertrophic cardiomyopathy (HCM). BACKGROUND: Hypertrophic cardiomyopathy is a heterogeneous cardiac disease. MYH7 mutations are one of the most common genetic causes of HCM and have been associated with severe hypertrophy, young age of diagnosis, and high risk of sudden cardiac death. However, these clinical findings from large, family studies have not been confirmed in a large unrelated cohort. METHODS: Deoxyribonucleic (DNA) samples obtained from 389 HCM outpatients seen at this tertiary referral center were analyzed for mutations, using polymerase chain reaction, denaturing high-performance liquid chromatography, and DNA sequencing for all 38 protein-coding exons of MYH7. Clinical data were extracted from patient records blinded to patient genotype. RESULTS: Fifty-eight patients (15%) harbored 40 different mutations in MYH7. Compared with HCM patients without MYH7 mutations, HCM patients with MYH7 were younger at diagnosis (32.9 vs. 42.7 years, p = 0.0002), had more hypertrophy (left ventricular wall thickness of 24.2 vs. 21.1 mm, p = 0.0009), and more frequently underwent myectomy (60% vs. 38%, p = 0.002). The HCM patients with MYH7 mutations more often had a family history of HCM (43% vs. 29%, p = 0.006), but there was no difference in family history of sudden death (16% vs. 14%, p = NS). CONCLUSIONS: In this setting, HCM patients with MYH7 were diagnosed at a younger age and had more hypertrophy, but they had no greater frequency of sudden death among first-degree relatives. Although these associations may prove useful for targeted gene screening, caution should be exercised in terms of using pathogenic status in risk stratification.

Prevalence and age-dependence of malignant mutations in the beta-myosin heavy chain and troponin T genes in hypertrophic cardiomyopathy: a comprehensive outpatient perspective.

OBJECTIVES: The goal of this study was to determine the prevalence of "malignant" mutations in hypertrophic cardiomyopathy (HCM). BACKGROUND: Previous genotype-phenotype studies have implicated four mutations (R403Q, R453C, G716R and R719W) as highly malignant defects in the beta-myosin heavy chain (MYH7). In the cardiac troponin T gene (TNNT2), a specific mutation (R92W) has been associated with high risk of sudden death. Routine clinical screening for these malignant mutations has been suggested to identify high-risk individuals. METHODS: We screened 293 unrelated individuals with HCM seen at the Mayo Clinic in Rochester, Minnesota, between April 1997 and October 2000. Deoxyribonucleic acid (DNA) was obtained after informed consent; amplification of MYH7 exons 13 (R403Q), 14 (R453C) and 19 (G716R and R719W), and TNNT2 exon 9 (R92W) was performed by polymerase chain reaction. The mutations were detected using denaturing high-performance liquid chromatography and automated DNA sequencing. RESULTS: The mean age at diagnosis was 42 years with 53 patients diagnosed before age 25. The mean maximal left ventricular wall thickness was 21 mm. Nearly one-third of cases were familial and one-fourth had a family history of sudden cardiac death. Only 3 of the 293 patients possessed one of the five "malignant" mutations, and all 3 patients were <25 years of age at presentation (p < 0.006). CONCLUSIONS: This finding underscores the profound genetic heterogeneity in HCM. Only 1% of unrelated individuals seen at a tertiary referral center for HCM possessed one of the five "malignant" mutations that were examined. Routine clinical testing for these specific mutations is of low yield.

Spectrum and prevalence of cardiac ryanodine receptor (RyR2) mutations in a cohort of unrelated patients referred explicitly for long QT syndrome genetic testing.

BACKGROUND: Mutations in the RyR2-encoded cardiac ryanodine receptor/calcium release channel cause type 1 catecholaminergic polymorphic ventricular tachycardia (CPVT1). OBJECTIVES: Because CPVT and concealed long QT syndrome (LQTS) phenotypically mimic one other, we sought to determine the spectrum and prevalence of RyR2 mutations in a cohort of unrelated patients who were referred specifically for LQTS genetic testing. METHODS: Using denaturing high-performance liquid chromatography and direct DNA sequencing, targeted mutational analysis of 23 RyR2 exons previously implicated in CPVT1 was performed on genomic DNA from 269 unrelated patients (180 females, average age at diagnosis 24 +/- 17 years) who were referred to Mayo Clinic's Sudden Death Genomics Laboratory for LQTS genetic testing. Previously, comprehensive mutational analysis of the five LQTS-associated cardiac channel genes proved negative for this entire subset of patients now designated as "genotype-negative" LQTS referrals. RESULTS: Fifteen distinct RyR2 mutations (14 missense, 1 duplication/insertion, 12 novel) were found in 17 (6.3%) of 269 patients. None of these mutations were present in 400 reference alleles. Two mutations localized to the calstabin-2 (FKBP12.6) binding domain. Upon review of the clinical records, the referral diagnosis for all 17 patients was "atypical" or "borderline" LQTS. CONCLUSION: Putative pathogenic CPVT1-causing mutations in RyR2 were detected in 6% of unrelated, genotype-negative LQTS referrals. These findings suggest that CPVT may be underrecognized among physicians referring patients because of a suspected channelopathy. A diagnosis of "atypical LQTS" may warrant consideration of CPVT and analysis of RyR2 if the standard cardiac channel gene screen for LQTS is negative.

Compendium of cardiac channel mutations in 541 consecutive unrelated patients referred for long QT syndrome genetic testing.

OBJECTIVES: The purpose of this study was to determine the spectrum and prevalence of cardiac channel mutations among a large cohort of consecutive, unrelated patients referred for long QT syndrome (LQTS) genetic testing. BACKGROUND: Congenital LQTS is a primary cardiac channelopathy. More than 300 mutations have been identified in five genes encoding key ion channel subunits. Until the recent release of the commercial clinical genetic test, LQTS genetic testing had been performed in research laboratories during the past decade. METHODS: A cardiac channel gene screen for LQTS-causing mutations in KCNQ1 (LQT1), KCNH2 (LQT2), SCN5A (LQT3), KCNE1 (LQT5), and KCNE2 (LQT6) was performed for 541 consecutive, unrelated patients (358 females, average age at diagnosis 24 +/- 16 years, average QTc 482 +/- 57 ms) referred to Mayo Clinic's Sudden Death Genomics Laboratory for LQTS genetic testing between August 1997 and July 2004. A comprehensive open reading frame and splice site analysis of the 60 protein-encoding exons was conducted using polymerase chain reaction, denaturing high-performance liquid chromatography, and DNA sequencing. RESULTS: Overall, 211 putative pathogenic mutations in KCNQ1 (88), KCNH2 (89), SCN5A (32), KCNE1 (1), and KCNE2 (1) were found in 272 unrelated patients (50%). Among the genotype positive patients (N = 272), 243 had single pathogenic mutations (LQT1: n = 120 patients; LQT2: n = 93; LQT3: n = 26; LQT5: n = 3; LQT6: n = 1), and 29 patients (10% of genotype-positive patients and 5% overall) had two LQTS-causing mutations. The majority of mutations were missense mutations (154/210 [73%]), singletons (identified in only a single unrelated patient: 165/210 [79%]), and novel (125/211 [59%]). None of the mutations identified were seen in more than 1,500 reference alleles. Those patients harboring multiple mutations were younger at diagnosis (15 +/- 11 years vs 24 +/- 16 years, P = .003). CONCLUSIONS: In this comprehensive cardiac channel gene screen of the largest cohort of consecutive, unrelated patients referred for LQTS genetic testing, half of the patients had an identifiable mutation. The majority of mutations continue to represent novel singletons that expand the published compendium of LQTS-causing mutations by 35%. These observations should facilitate diagnostic interpretation of the clinical genetic test for LQTS.

Genotype-phenotype Correlations of Hypertrophic Cardiomyopathy When Diagnosed in Children, Adolescents, and Young Adults.

BACKGROUND: Hypertrophic cardiomyopathy (HCM) is the most common heritable cardiovascular disease and a leading cause of identifiable sudden cardiac death (SCD) in the young. Herein, we sought to determine the genotype-phenotype correlations in a cohort of unrelated, genotyped patients diagnosed with HCM at a young age, as well as to characterize the differences between HCM diagnosed in adulthood and HCM diagnosed at a young age. METHODS AND RESULTS: From 1999 to 2011, 1053 unrelated patients diagnosed with HCM were enrolled in research-based genetic testing. The electronic medical record was reviewed to identify those with HCM diagnosed at ≤21 years (N = 137, mean age at diagnosis 13.2 ± 6 years, 64% male). From this cohort of patients recruited from a tertiary care referral center, the genetic test was positive in 71 (52%), which was significantly higher than patients diagnosed >21 years (31%; P < .001). Genotype-positive patients had increased maximum left ventricular wall thickness (24.9 ± 8.0 vs. 21.6 ± 7.4 mm, P = .01) and higher incidence of reverse-curve ventricular septal morphology (71% vs. 40%, P < .001). Unrelated to genotype status, 26/137 patients (19%) experienced significant HCM-related morbidity/mortality including progressive heart failure symptoms in 12, transplantation in 4, and death in 10. CONCLUSIONS: Among patients diagnosed with HCM during the first two decades of life, the yield of genetic testing is significantly higher than when diagnosed at later age. While the phenotype of young HCM patients is worse than patients whose HCM is diagnosed at later age, the phenotypes of genotype-positive and genotype-negative young patients were similar. Independent of genotype, nearly 30% of the patients with follow-up in this study had symptom progression, transplant, or death.

A novel TPM1 mutation in a family with hypertrophic cardiomyopathy and sudden cardiac death in childhood.

We sought to define the pathogenic mutation in a family with hypertrophic cardiomyopathy (HC) and a markedly arrhythmogenic phenotype. The proband was an 8-year-old female with a sentinel event of sudden death. Screening echocardiograms revealed HC in 2 of her 3 siblings and her father. Her youngest male sibling was diagnosed with HC at age 2 years and died suddenly at age 6 years from ventricular fibrillation despite an implanted cardioverter defibrillator. Using DNA extracted from peripheral lymphocytes, linkage exclusion was performed by haplotype analysis of polymorphic markers for the HC genes. Genes not excluded by linkage were analyzed for mutations using denaturing high-performance liquid chromatography (DHPLC) and direct DNA sequencing. Using this strategy, a 610 T>G nucleotide substitution in the alpha-tropomyosin gene (TPM1) was identified resulting in a novel L185R (Leucine [L] to Arginine [R]) missense mutation. This mutation was a spontaneous germ-line mutation originating in the proband's father. L185R-TPM1 cosegregated with family members having clinical evidence of HC, including the proband as confirmed by molecular autopsy. The mutation was not present in 400 reference alleles. Thus, a novel missense mutation in TPM1 was discovered in a family with HC and sudden death in childhood. Unlike previously defined mutations that may disrupt the interactions between alpha-tropomyosin monomers, the L185R mutation may affect troponin-T binding. Defining the pathogenic mutation enabled definitive molecular diagnosis of 2 surviving children.

Ethnic differences in cardiac potassium channel variants: implications for genetic susceptibility to sudden cardiac death and genetic testing for congenital long QT syndrome.

OBJECTIVE: To determine the spectrum, frequency, and ethnic-specificity of channel variants in the potassium channel genes implicated in congenital long QT syndrome (LQTS) among healthy subjects. SUBJECTS AND METHODS: Genomic DNA from 744 apparently healthy individuals-305 black, 187 white, 134 Asian, and 118 Hispanic--was subject to a comprehensive mutational analysis of the 4 LQTS-causing potassium channel genes: KCNQ1 (LQT1), KCNH2 (LQT2), KCNE1 (LQT5), and KCNE2 (LQT6). RESULTS: Overall, 49 distinct amino acid-altering variants (36 novel) were identified: KCNQ1 (n = 16), KCNH2 (n = 25),KCNE1 (n = 5), and KCNE2 (n = 3). More than half of these variants (26/49) were found exclusively in black subjects. The known K897T-HERG and the G38S-min K common polymorphisms were identified in all 4 ethnic groups. Excluding these 2 common polymorphisms, 25% of black subjects had at least 1 nonsynonymous potassium channel variant compared with 14% of white subjects (P < .01). CONCLUSIONS: To our knowledge, this study represents the first comprehensive determination of the frequency and spectrum of cardiac channel variants found among healthy subjects from 4 major ethnic groups. Defining the population burden of genetic variants in these critical cardiac ion channels is crucial for proper interpretation of genetic test results of individuals at risk for LQTS. This compendium provides a resource for epidemiological and functional investigation of variant effects on the repolarization properties of cardiac tissues, including susceptibility to lethal cardiac arrhythmias.

Identification of a common genetic substrate underlying postpartum cardiac events in congenital long QT syndrome.

OBJECTIVES: The aim of this study was to elucidate the genetic basis for long QT syndrome (LQTS) in patients with a personal or family history of postpartum cardiac events. BACKGROUND: The postpartum period is a time of increased arrhythmogenic susceptibility in women with LQTS. METHODS: Between August 1997 and May 2003, 388 unrelated patients (260 females, average age at diagnosis, 23 years, and average QTc, 482 ms) were referred to Mayo Clinic's Sudden Death Genomics Laboratory for LQTS genetic testing. Comprehensive mutational analysis of the 5 LQTS-causing channel genes was performed. The postpartum period was defined as the 20 weeks after delivery. Cardiac events included sudden cardiac death, aborted cardiac arrest, and syncope. The presence of a personal and/or family history of cardiac events during postpartum period was determined by review of the medical records and/or phone interviews and was blinded to the status of genetic testing. RESULTS: Fourteen patients (3.6% of cohort) had personal (n = 4) and/or family history (n = 11) of cardiac events during the defined postpartum period. Thirteen of 14 patients (93%) possessed an LQT2 mutation and 1 had an LQT1 mutation. Postpartum cardiac events were found more commonly in patients with LQT2 (13 of 80, 16%) than in patients with LQT1 (1 of 103, <1%, P = .0001). CONCLUSIONS: There is a relatively gene-specific molecular basis underlying cardiac events during the postpartum period in LQTS. Along with previous gene-specific associations involving swimming and LQT1 as well as auditory triggers and LQT2, this association between postpartum cardiac events and LQT2 can facilitate strategic genotyping.

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.

Characterization of a phenotype-based genetic test prediction score for unrelated patients with hypertrophic cardiomyopathy.

OBJECTIVES: To determine the prevalence and spectrum of mutations and genotype-phenotype relationships in the largest hypertrophic cardiomyopathy (HCM) cohort to date and to provide an easy, clinically applicable phenotype-derived score that provides a pretest probability for a positive HCM genetic test result. PATIENTS AND METHODS: Between April 1, 1997, and February 1, 2007, 1053 unrelated patients with the clinical diagnosis of HCM (60% male; mean ± SD age at diagnosis, 44.4 ± 19 years) had HCM genetic testing for the 9 HCM-associated myofilament genes. Phenotyping was performed by review of electronic medical records. RESULTS: Overall, 359 patients (34%) were genotype positive for a putative HCM-associated mutation in 1 or more HCM-associated genes. Univariate and multivariate analyses identified the echocardiographic reverse curve morphological subtype, an age at diagnosis younger than 45 years, a maximum left ventricular wall thickness of 20 mm or greater, a family history of HCM, and a family history of sudden cardiac death as positive predictors of positive genetic test results, whereas hypertension was a negative predictor. A score, based on the number of predictors of a positive genetic test result, predicted a positive genetic test result ranging from 6% when only hypertension was present to 80% when all 5 positive predictor markers were present. CONCLUSION: In this largest HCM cohort published to date, the overall yield of genetic testing was 34%. Although all the patients were diagnosed clinically as having HCM, the presence or absence of 6 simple clinical/echocardiographic markers predicted the likelihood of mutation-positive HCM. Phenotype-guided genetic testing using the Mayo HCM Genotype Predictor score provides an easy tool for an effective genetic counseling session.

Cardiac channel molecular autopsy: insights from 173 consecutive cases of autopsy-negative sudden unexplained death referred for postmortem genetic testing.

OBJECTIVE: To perform long QT syndrome and catecholaminergic polymorphic ventricular tachycardia cardiac channel postmortem genetic testing (molecular autopsy) for a large cohort of cases of autopsy-negative sudden unexplained death (SUD). METHODS: From September 1, 1998, through October 31, 2010, 173 cases of SUD (106 males; mean ± SD age, 18.4 ± 12.9 years; age range, 1-69 years; 89% white) were referred by medical examiners or coroners for a cardiac channel molecular autopsy. Using polymerase chain reaction, denaturing high-performance liquid chromatography, and DNA sequencing, a comprehensive mutational analysis of the long QT syndrome susceptibility genes (KCNQ1, KCNH2, SCN5A, KCNE1, and KCNE2) and a targeted analysis of the catecholaminergic polymorphic ventricular tachycardia type 1-associated gene (RYR2) were conducted. RESULTS: Overall, 45 putative pathogenic mutations absent in 400 to 700 controls were identified in 45 autopsy-negative SUD cases (26.0%). Females had a higher yield (26/67 [38.8%]) than males (19/106 [17.9%]; P<.005). Among SUD cases with exercise-induced death, the yield trended higher among the 1- to 10-year-olds (8/12 [66.7%]) compared with the 11- to 20-year-olds (4/27 [14.8%]; P=.002). In contrast, for those who died during a period of sleep, the 11- to 20-year-olds had a higher yield (9/25 [36.0%]) than the 1- to 10-year-olds (1/24 [4.2%]; P=.01). CONCLUSION: Cardiac channel molecular autopsy should be considered in the evaluation of autopsy-negative SUD. Several interesting genotype-phenotype observations may provide insight into the expected yields of postmortem genetic testing for SUD and assist in selecting cases with the greatest potential for mutation discovery and directing genetic testing efforts.

Repeat long QT syndrome genetic testing of phenotype-positive cases: prevalence and etiology of detection misses.

BACKGROUND: Approximately 75% of long QT syndrome (LQTS) has been explained genetically through research-based and, more recently, commercial genetic testing. While novel LQTS-susceptibility genes or mutations in unexplored regions of known genes underlie the genetic mechanism for some of the 25% "genotype-negative" remnant, it is likely that some cases represent false-negative test results owing to mutation detection failures. OBJECTIVE: To determine the prevalence and etiology of false negatives that occurred with research-based mutational analysis involving denaturing high-performance liquid chromatography (DHPLC) followed by DNA sequencing (DHPLC-SEQ) in our previously published cohort of unrelated patients referred for LQTS genetic testing. METHODS: Forty-four LQTS cases (29 men, average age 23 ± 15 years, average corrected QT interval 516 ± 56 ms) deemed genotype negative following DHPLC-SEQ were selected for repeat genetic testing using direct DNA sequencing. RESULTS: LQTS-causing mutations were identified in 7 of 44 (16%) phenotype-positive/previously genotype-negative subjects, including 4 mutations in KCNQ1 (S225L, G568R, R591H, and R594Q), 2 in KCNH2 (H70R and G925R), and 1 in SCN5A (V411M). None of these variants were seen in more than 2600 reference alleles. Analysis of the misses revealed (1) normal DHPLC detection profile in 2, (2) allelic dropout in 2, (3) failure to correctly optimize DHPLC conditions in 1, and (4) failure to detect abnormal DHPLC signal in 2. CONCLUSIONS: Repeat genetic testing using direct DNA sequencing may be warranted for LQTS phenotype-positive individuals who were pronounced genotype negative during the decade of research-based mutational analysis that involved intermediate mutation detection methods such as DHPLC.