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 (BrS) and sudden infant death syndromes (SIDS). METHODS AND RESULTS Patient 1 was a 44-y/o male with an ajmaline-induced Type-1 ST-segment elevation in V1 and V2 supporting the diagnosis of BrS. Patient 2 was a 62-y/o female displaying a coved-type BrS ECG who developed cardiac arrest during fever. Patient 3 was a 4-m/o female SIDS case. All known exons and intron borders of BrS and SIDS susceptibility genes were amplified and sequenced in both directions. A R214Q variant was detected in exon 3A of SCN1Bb (Navβ1B) 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%). Wild type (WT) and mutant genes were expressed in TSA201 cells and studied using whole-cell patch-clamp and co-immunoprecipitation techniques. Co-expression of SCN5A/WT+SCN1Bb/R214Q resulted in peak sodium channel current (INa) 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 (Ito) 70.6% greater compared with KCND3/WT+SCN1Bb/WT(n=10–11, p<0.01). Co-immunoprecipitation indicated structural association between Navβ1B and Nav1.5 and Kv4.3. CONCLUSION Our results suggest that R214Q variation in SCN1Bb is a functional polymorphism that may serve as a modifier of the substrate responsible for Brugada or SIDS phenotypes via a combined loss of function of INa and gain of function of Ito.

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The standard methods for toxicity testing using rodent models cannot keep pace with the increasing number of chemicals in our environment due to time and resource limitations. Hence, there is an unmet need for fast, sensitive, and costeffective alternate models to reliably predict toxicity. As part of Tox21 Phase III's effort, a 90-compound library was created and made available to researchers to screen for neurotoxicants using novel technology and models. The chemical library was evaluated in zebrafish in a dose-range finding test for embryo-toxicity (ie, mortality or morphological alterations induced by each chemical). In addition, embryos exposed to the lowest effect level and nonobservable effect level were used to measure the internal concentration of the chemicals within the embryos by bioanalysis. Finally, considering the lowest effect level as the highest testing concentration, a functional assay was performed based on locomotor activity alteration in response to light-dark changes. The quality control chemicals included in the library, ie, negative controls and replicated chemicals, indicate that the assays performed were reliable. The use of analytical chemistry pointed out the importance of measuring chemical concentration inside embryos, and in particular, in the case of negative chemicals to avoid false negative classification. Overall, the proposed approach presented a good sensitivity and supports the inclusion of zebrafish assays as a reliable, relevant, and efficient screening tool to identify, prioritize, and evaluate chemical toxicity.

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