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Background— The prevalence of genetic arrhythmogenic diseases is unknown. For the long-QT syndrome (LQTS), figures ranging from 1:20 000 to 1:5000 were published, but none was based on actual data. Our objective was to define the prevalence of LQTS. Methods and Results— In 18 maternity hospitals, an ECG was performed in 44 596 infants 15 to 25 days old (43 080 whites). In infants with a corrected QT interval (QTc) >450 ms, the ECG was repeated within 1 to 2 weeks. Genetic analysis, by screening 7 LQTS genes, was performed in 28 of 31 (90%) and in 14 of 28 infants (50%) with, respectively, a QTc >470 ms or between 461 and 470 ms. A QTc of 451 to 460, 461 to 470, and >470 ms was observed in 177 (0.41%), 28 (0.06%), and 31 infants (0.07%). Among genotyped infants, disease-causing mutations were found in 12 of 28 (43%) with a QTc >470 ms and in 4 of 14 (29%) with a QTc of 461 to 470 ms. One genotype-negative infant (QTc 482 ms) was diagnosed as affected by LQTS on clinical grounds. Among family members of genotype-positive infants, 51% were found to carry disease-causing mutations. In total, 17 of 43 080 white infants were affected by LQTS, demonstrating a prevalence of at least 1:2534 apparently healthy live births (95% confidence interval, 1:1583 to 1:4350). Conclusions— This study provides the first data-based estimate of the prevalence of LQTS among whites. On the basis of the nongenotyped infants with QTc between 451 and 470 ms, we advance the hypothesis that this prevalence might be close to 1:2000. ECG-guided molecular screening can identify most infants affected by LQTS and unmask affected relatives, thus allowing effective preventive measures.

Background— In congenital long-QT syndrome (LQTS), a genetically heterogeneous disorder that predisposes to sudden cardiac death, genetic factors other than the primary mutation may modify the probability of life-threatening events. Recent evidence indicates that common variants in NOS1AP are associated with the QT-interval duration in the general population. Methods and Results— We tested the hypothesis that common variants in NOS1AP modify the risk of clinical manifestations and the degree of QT-interval prolongation in a South African LQTS population (500 subjects, 205 mutation carriers) segregating a founder mutation in KCNQ1 (A341V) using a family-based association analysis. NOS1AP variants were significantly associated with the occurrence of symptoms (rs4657139, P =0.019; rs16847548, P =0.003), with clinical severity, as manifested by a greater probability for cardiac arrest and sudden death (rs4657139, P =0.028; rs16847548, P =0.014), and with greater likelihood of having a QT interval in the top 40% of values among all mutation carriers (rs4657139, P =0.03; rs16847548, P =0.03). Conclusions— These findings indicate that NOS1AP , a gene first identified as affecting the QTc interval in a general population, also influences sudden death risk in subjects with LQTS. The association of NOS1AP genetic variants with risk for life-threatening arrhythmias suggests that this gene is a genetic modifier of LQTS, and this knowledge may be clinically useful for risk stratification for patients with this disease, after validation in other LQTS populations.

Background— Clinical heterogeneity among patients with long-QT syndrome (LQTS) sharing the same disease-causing mutation is usually attributed to variable penetrance. One potential explanation for this phenomenon is the coexistence of modifier gene alleles, possibly common single nucleotide polymorphisms, altering arrhythmia susceptibility. We demonstrate this concept in a family segregating a novel, low-penetrant KCNH2 mutation along with a common single nucleotide polymorphism in the same gene. Methods and Results— The proband is a 44-year-old white woman with palpitations associated with presyncope since age 20, who presented with ventricular fibrillation and cardiac arrest. Intermittent QT prolongation was subsequently observed (max QTc, 530 ms), and LQT2 was diagnosed after the identification of a missense KCNH2 mutation (A1116V) altering a conserved residue in the distal carboxyl-terminus of the encoded HERG protein. The proband also carried the common KCNH2 polymorphism K897T on the nonmutant allele. Relatives who carried A1116V without K897T were asymptomatic, but some exhibited transient mild QTc prolongation, suggesting latent disease. Heterologous expression studies performed in cultured mammalian cells and using bicistronic vectors linked to different fluorescent proteins demonstrated that coexpression of A1116V with K897T together resulted in significantly reduced current amplitude as compared with coexpression of either allele with WT-HERG. Thus, the presence of KCNH2 -K897T is predicted to exaggerate the I Kr reduction caused by the A1116V mutation. These data explain why symptomatic LQTS occurred only in the proband carrying both alleles. Conclusions— We have provided evidence that a common KCNH2 polymorphism may modify the clinical expression of a latent LQT2 mutation. A similar mechanism may contribute to the risk for sudden death in more prevalent cardiac diseases.

Background : Stillbirths (intrauterine fetal deaths occurring after the 22 th week of gestation) notably contribute to perinatal mortality. Among them, 25–50% of cases remain unexplained after thorough investigation. We recently demonstrated that 10% of Sudden Infant Death Syndrome (SIDS) cases carry functionally relevant genetic variants in Long QT Syndrome (LQTS) genes. We previously hypothesized that severe forms of LQTS could contribute to mortality not only shortly after birth but also before. Accordingly, we are now screening LQTS genes in unexplained stillbirths. Materials and Methods : We receive DNA from an ongoing Italian multicenter study on rigorously defined ``unexplained stillbirths”. DNA is extracted from placenta or umbilical cord. Through DHPLC and sequence analysis we screen the main LQTS genes: KCNQ1, KCNH2, SCN5A, KCNE1 and KCNE2. Any amino-acid substitution identified in the samples is checked in a control population of 122 Caucasian controls. Here, we report the preliminary data on the first 17 cases (gestational age of death: 23–38 weeks). Results : A total of 3 missense mutations have been identified in 3 of 17 stillbirths (18%). Two were on SCN5A and one on KCNH2 . The two mutations on SCN5A (V1951L; P2006A) have been previously associated to SIDS and shown to increase the late sodium current. The mutation on KCNH2 is a novel genetic variant absent in 244 reference alleles and never described in any control populations. We are currently performing the electrophysiological cellular studies to define its functional effect. Conclusions : The initial data from this ongoing study already indicate that a yet to be defined but potentially significant number of currently unexplained intrauterine fetal deaths might be caused by ion channel diseases such as LQTS. The completion of our study will soon provide a reliable quantification of this contribution. There are potentially important implications for the affected families, ranging from prevention of recurrences to identification of other affected family members and to possible prevention of SIDS or later sudden deaths in subsequent siblings.