A novel rare variant in SCN1Bb linked to Brugada syndrome and SIDS by combined modulation of Na(v)1.5 and K(v)4.3 channel currents.

BACKGROUND Cardiac sodium channel β-subunit mutations have been associated with several inherited cardiac arrhythmia syndromes. OBJECTIVE To identify and characterize variations in SCN1Bb associated with Brugada syndrome (BrS) and sudden infant death syndrome (SIDS). METHODS All known exons and intron borders of the BrS-susceptibility genes were amplified and sequenced in both directions. Wild type (WT) and mutant genes were expressed in TSA201 cells and studied using co-immunoprecipitation and whole-cell patch-clamp techniques. RESULTS Patient 1 was a 44-year-old man with an ajmaline-induced type 1 ST-segment elevation in V1 and V2 supporting the diagnosis of BrS. Patient 2 was a 62-year-old woman displaying a coved-type BrS electrocardiogram who developed cardiac arrest during fever. Patient 3 was a 4-month-old female SIDS case. A R214Q variant was detected in exon 3A of SCN1Bb (Na(v)1B) in all three probands, but not in any other gene previously associated with BrS or SIDS. R214Q was identified in 4 of 807 ethnically-matched healthy controls (0.50%). Co-expression of SCN5A/WT + SCN1Bb/R214Q resulted in peak sodium channel current (I(Na)) 56.5% smaller compared to SCN5A/WT + SCN1Bb/WT (n = 11-12, P<0.05). Co-expression of KCND3/WT + SCN1Bb/R214Q induced a Kv4.3 current (transient outward potassium current, I(to)) 70.6% greater compared with KCND3/WT + SCN1Bb/WT (n = 10-11, P<0.01). Co-immunoprecipitation indicated structural association between Na(v)β1B and Na(v)1.5 and K(v)4.3. CONCLUSION Our results suggest that R214Q variation in SCN1Bb is a functional polymorphism that may serve as a modifier of the substrate responsible for BrS or SIDS phenotypes via a combined loss of function of sodium channel current and gain of function of transient outward potassium current.

Gas embolism following intraosseous medication application proven by post-mortem multislice computed tomography and autopsy

The post-mortem use of modern imaging techniques such as multislice computed tomography (MSCT) is becoming increasingly important as an aid for conventional autopsy. This article presents a case of a 4-month-old boy who died from sudden infant death syndrome (SIDS) with intravascular gas after an intraosseus medication application documented by post-mortem MSCT. It is most likely that the gas entered the body during resuscitation. This case emphasises the advantage of post-mortem imaging as a complementary aid for the autopsy. We conclude that during emergency treatment, the medical staff should be aware of the possibility of causing a gas embolism following intraosseus medication. Resuscitation with an inserted, disconnected intraosseous needle should be avoided.

LQTS-associated mutation A257G in α1-syntrophin interacts with the intragenic variant P74L to modify its biophysical phenotype.

The SNTA1-encoded α1-syntrophin (SNTA1) missense mutation, p.A257G, causes long QT syndrome (LQTS) by pathogenic accentuation of Nav1.5's sodium current (I Na). Subsequently, we found p.A257G in combination with the SNTA1 polymorphism, p.P74L in 4 victims of sudden infant death syndrome (SIDS) as well as in 3 adult controls. We hypothesized that p.P74L-SNTA1 could functionally modify the pathogenic phenotype of p.A257G-SNTA1, thus explaining its occurrence in non-LQTS populations. The SNTA1 variants p.P74L, p.A257G, and the combination variant p.P74L/p.A257G were engineered using PCR-based overlap-extension and were co-expressed heterologously with SCN5A in HEK293 cells. I Na was recorded using the whole-cell method. Compared to wild-type (WT), the significant increase in peak I Na and window current found with p.A257G was reversed by the intragenic variant p.P74L (p.P74L/p.A257G). These results report for the first time the intragenic rescue of an LQT-associated SNTA1 mutation when found in combination with the SNTA1 polymorphism p.P74L, suggesting an ever-increasing picture of complexity in terms of genetic risk stratification for arrhythmia.

Loss-of-function mutations in the KCNJ8-encoded Kir6.1 K(ATP) channel and sudden infant death syndrome.

BACKGROUND Approximately 10% of sudden infant death syndrome (SIDS) may stem from cardiac channelopathies. The KCNJ8-encoded Kir6.1 (K(ATP)) channel critically regulates vascular tone and cardiac adaptive response to systemic metabolic stressors, including sepsis. KCNJ8-deficient mice are prone to premature sudden death, particularly with infection. We determined the spectrum, prevalence, and function of KCNJ8 mutations in a large SIDS cohort. METHODS AND RESULTS Using polymerase chain reaction, denaturing high-performance liquid chromatography, and DNA sequencing, comprehensive open reading frame/splice-site mutational analysis of KCNJ8 was performed on genomic DNA isolated from necropsy tissue on 292 unrelated SIDS cases (178 males, 204 white; age, 2.9±1.9 months). KCNJ8 mutations were coexpressed heterologously with SUR2A in COS-1 cells and characterized using whole-cell patch-clamp. Two novel KCNJ8 mutations were identified. A 5-month-old white male had an in-frame deletion (E332del) and a 2-month-old black female had a missense mutation (V346I). Both mutations localized to Kir6.1's C-terminus, involved conserved residues and were absent in 400 and 200 ethnic-matched reference alleles respectively. Both cases were negative for mutations in established channelopathic genes. Compared with WT, the pinacidil-activated K(ATP) current was decreased 45% to 68% for Kir6.1-E332del and 40% to 57% for V346I between -20 mV and 40 mV. CONCLUSIONS Molecular and functional evidence implicated loss-of-function KCNJ8 mutations as a novel pathogenic mechanism in SIDS, possibly by predisposition of a maladaptive cardiac response to systemic metabolic stressors akin to the mouse models of KCNJ8 deficiency.

Sudden infant death syndrome-associated mutations in the sodium channel beta subunits.

BACKGROUND Approximately 10% of sudden infant death syndrome (SIDS) cases may stem from potentially lethal cardiac channelopathies, with approximately half of channelopathic SIDS involving the Na(V)1.5 cardiac sodium channel. Recently, Na(V) beta subunits have been implicated in various cardiac arrhythmias. Thus, the 4 genes encoding Na(V) beta subunits represent plausible candidate genes for SIDS. OBJECTIVE This study sought to determine the spectrum, prevalence, and functional consequences of sodium channel beta-subunit mutations in a SIDS cohort. METHODS In this institutional review board-approved study, mutational analysis of the 4 beta-subunit genes, SCN1B to 4B, was performed using polymerase chain reaction, denaturing high-performance liquid chromatography, and direct DNA sequencing of DNA derived from 292 SIDS cases. Engineered mutations were coexpressed with SCN5A in HEK 293 cells and were whole-cell patch clamped. One of the putative SIDS-associated mutations was similarly studied in adenovirally transduced adult rat ventricular myocytes. RESULTS Three rare (absent in 200 to 800 reference alleles) missense mutations (beta3-V36M, beta3-V54G, and beta4-S206L) were identified in 3 of 292 SIDS cases. Compared with SCN5A+beta3-WT, beta3-V36M significantly decreased peak I(Na) and increased late I(Na), whereas beta3-V54G resulted in a marked loss of function. beta4-S206L accentuated late I(Na) and positively shifted the midpoint of inactivation compared with SCN5A+beta4-WT. In native cardiomyocytes, beta4-S206L accentuated late I(Na) and increased the ventricular action potential duration compared with beta4-WT. CONCLUSION This study provides the first molecular and functional evidence to implicate the Na(V) beta subunits in SIDS pathogenesis. Altered Na(V)1.5 sodium channel function due to beta-subunit mutations may account for the molecular pathogenic mechanism underlying approximately 1% of SIDS cases.

Alpha1-syntrophin mutations identified in sudden infant death syndrome cause an increase in late cardiac sodium current.

BACKGROUND Sudden infant death syndrome (SIDS) is a leading cause of death during the first 6 months after birth. About 5% to 10% of SIDS may stem from cardiac channelopathies such as long-QT syndrome. We recently implicated mutations in alpha1-syntrophin (SNTA1) as a novel cause of long-QT syndrome, whereby mutant SNTA1 released inhibition of associated neuronal nitric oxide synthase by the plasma membrane Ca-ATPase PMCA4b, causing increased peak and late sodium current (I(Na)) via S-nitrosylation of the cardiac sodium channel. This study determined the prevalence and functional properties of SIDS-associated SNTA1 mutations. METHODS AND RESULTS Using polymerase chain reaction, denaturing high-performance liquid chromatography, and DNA sequencing of SNTA1's open reading frame, 6 rare (absent in 800 reference alleles) missense mutations (G54R, P56S, T262P, S287R, T372M, and G460S) were identified in 8 (approximately 3%) of 292 SIDS cases. These mutations were engineered using polymerase chain reaction-based overlap extension and were coexpressed heterologously with SCN5A, neuronal nitric oxide synthase, and PMCA4b in HEK293 cells. I(Na) was recorded using the whole-cell method. A significant 1.4- to 1.5-fold increase in peak I(Na) and 2.3- to 2.7-fold increase in late I(Na) compared with controls was evident for S287R-, T372M-, and G460S-SNTA1 and was reversed by a neuronal nitric oxide synthase inhibitor. These 3 mutations also caused a significant depolarizing shift in channel inactivation, thereby increasing the overlap of the activation and inactivation curves to increase window current. CONCLUSIONS Abnormal biophysical phenotypes implicate mutations in SNTA1 as a novel pathogenic mechanism for the subset of channelopathic SIDS. Functional studies are essential to distinguish pathogenic perturbations in channel interacting proteins such as alpha1-syntrophin from similarly rare but innocuous ones.