A novel gain-of-function sodium channel ß2 subunit mutation in idiopathic small fiber neuropathy.

Small fiber neuropathy (SFN) is a common condition affecting thinly myelinated Aδ and unmyelinated C fibers, often resulting in excruciating pain and dysautonomia. SFN has been associated with several conditions, but a significant number of cases have no discernible cause. Recent genetic studies have identified potentially pathogenic gain-of-function mutations in several pore-forming voltage-gated sodium channel α subunits (NaV) in a subset of patients with SFN, but the auxiliary sodium channel ß subunits have been less implicated in the development of the disease. ß subunits modulate NaV trafficking and gating, and several mutations have been linked to epilepsy and cardiac dysfunction. Recently, we provided the first evidence for the contribution of a mutation in the ß2 subunit to pain in human painful diabetic neuropathy. Here, we provide the first evidence for the involvement of a sodium channel ß subunit mutation in the pathogenesis of SFN with no other known causes. We show, through current-clamp analysis, that the newly identified Y69H variant of the ß2 subunit induces neuronal hyperexcitability in dorsal root ganglion neurons, lowering the threshold for action potential firing and allowing for increased repetitive action potential spiking. Underlying the hyperexcitability induced by the ß2-Y69H variant, we demonstrate an upregulation in tetrodotoxin-sensitive, but not tetrodotoxin-resistant sodium currents. This provides the first evidence for the involvement of ß2 subunits in SFN and strengthens the link between sodium channel ß subunits and the development of neuropathic pain in humans.NEW & NOTEWORTHY Small fiber neuropathy (SFN) often has no discernible cause, although mutations in the voltage-gated sodium channel α subunits have been implicated in some cases. We identify a patient suffering from SFN with a mutation in the auxiliary ß2 subunit and no other discernible causes for SFN. Functional assessment confirms this mutation renders dorsal root ganglion neurons hyperexcitable and upregulates tetrodotoxin-sensitive sodium currents. This study strengthens a newly emerging link between sodium channel ß2 subunit mutations and human pain disorders.

MicroRNA­449a regulates the progression of brain aging by targeting SCN2B in SAMP8 mice.

Our previous study demonstrated that the expression of sodium channel voltage­gated beta 2 (SCN2B) increased with aging in senescence­accelerated mouse prone 8 (SAMP8) mice, and was identified to be associated with a decline in learning and memory, while the underlying mechanism is unclear. In the present study, multiple differentially expressed miRNAs, which may be involved in the process of aging by regulating target genes, were identified in the prefrontal cortex and hippocampus of SAMP8 mice though miRNA microarray analysis. Using bioinformatics prediction, SCN2B was identified to be one of the potential target genes of miR­449a, which was downregulated in the hippocampus. Previous studies demonstrated that miR­449a is involved in the occurrence and progression of aging by regulating a variety of target genes. Therefore, it was hypothesized that miR­449a may be involved in the process of brain aging by targeting SCN2B. To verify this hypothesis, the following experiments were conducted: A reverse transcription­quantitative polymerase chain reaction assay revealed that the expression level of miR­449a was significantly decreased in the prefrontal cortex and hippocampus of 12­month old SAMP8 mice; a dual­luciferase reporter assay verified that miR­449a regulated SCN2B expression by binding to the 3'­UTR 'seed region'; an anti­Ago co­immunoprecipitation combined with Affymetrix microarray analyses demonstrated that the target mRNA highly enriched with Ago­miRNPs was confirmed to be SCN2B. Finally, overexpression of miR­449a or inhibition of SCN2B promoted the extension of hippocampal neurons in vitro. The results of the present study suggested that miR­449a was downregulated in the prefrontal cortex and hippocampus of SAMP8 mice and may regulate the process of brain aging by targeting SCN2B.

Trafficking and Function of the Voltage-Gated Sodium Channel ß2 Subunit.

The voltage-gated sodium channel is vital for cardiomyocyte function, and consists of a protein complex containing a pore-forming α subunit and two associated ß subunits. A fundamental, yet unsolved, question is to define the precise function of ß subunits. While their location in vivo remains unclear, large evidence shows that they regulate localization of α and the biophysical properties of the channel. The current data support that one of these subunits, ß2, promotes cell surface expression of α. The main α isoform in an adult heart is NaV1.5, and mutations in SCN5A, the gene encoding NaV1.5, often lead to hereditary arrhythmias and sudden death. The association of ß2 with cardiac arrhythmias has also been described, which could be due to alterations in trafficking, anchoring, and localization of NaV1.5 at the cardiomyocyte surface. Here, we will discuss research dealing with mechanisms that regulate ß2 trafficking, and how ß2 could be pivotal for the correct localization of NaV1.5, which influences cellular excitability and electrical coupling of the heart. Moreover, ß2 may have yet to be discovered roles on cell adhesion and signaling, implying that diverse defects leading to human disease may arise due to ß2 mutations.

Neuroprotection induced by Navß2­knockdown in APP/PS1 transgenic neurons is associated with NEP regulation.

Voltage­gated sodium channel ß2 (Navß2), as an unconventional substrate of ß­site amyloid precursor protein cleaving enzyme 1, is involved in regulating the neuronal surface expression of sodium channels. A previous study demonstrated that knockdown of Navß2 protected neurons and induced spatial cognition improvement by partially reducing pathological amyloidogenic processing of amyloid precursor protein (APP) in aged APP/presenilin 1 (PS1) transgenic mice. The present study aimed to investigate whether Navß2 knockdown altered APP metabolism via regulation of the Aß­degrading enzyme neprilysin (NEP). APPswe/PS1ΔE9 mice (APP/PS1 transgenic mice with a C57BL/6J genetic background) carrying a Navß2­knockdown mutation (APP/PS1/Navß2­kd) or without Navß2 knockdown (APP/PS1) were used for cell culture and further analysis. The present results demonstrated that in APP/PS1 mouse­derived neurons, Navß2 knockdown partially reversed the reduction in pathological APP cleavage, and the recovery of neurite extension and neuron area. Additionally, Navß2 knockdown increased NEP activity and levels, and the levels of intracellular domain fragment binding to the NEP promoter. The present findings suggested that knockdown of Navß2 reversed the APP/PS1 mutation­induced deficiency in amyloid ß degradation by regulating NEP.

MiR-34a is differentially expressed in dorsal root ganglia in a rat model of chronic neuropathic pain.

INTRODUCTION: Recent evidence shows that numerous microRNAs (miRNAs) regulate pain-related genes in chronic pain. The aim of the present study was to further explore the regulation of miRNAs and their effect on the expression of pain-associated target genes in experimental neuropathic pain. METHODS: Male Wistar rats underwent chronic constriction injury (CCI) of the sciatic nerve or Sham procedure. After assessment of mechanical allodynia, the ipsilateral dorsal root ganglia (DRG) were harvested. MiRNA expression levels were analysed with Agilent microRNA microarrays and real time quantitative PCR. An interaction between miRNAs and pain-relevant genes was confirmed by luciferase assays. Western Blot analysis and ELISA were performed to evaluate protein expression, respectively. RESULTS: Mechanical allodynia developed within 6 days after CCI. MiRNA-arrays revealed the differential expression of 49 miRNAs after 4 h, of 3 miRNAs after 1 d, of 26 miRNAs after 6 d and of 28 miRNAs after 12 d in the CCI group versus Sham. Time-dependent down regulation of miR-34a was verified by qPCR. Bioinformatic prediction revealed an interaction with several pain-relevant targets including voltage-gated sodium channel ß2 subunit (SCN2B) and vesicle-associated membrane protein 2 (VAMP-2), both of which were subsequently confirmed by luciferase assay. VAMP-2 expression was statistically significantly increased 12 d after CCI. A non-significant upregulation of SCN2B in the DRG after CCI was confirmed by ELISA. DISCUSSION: Peripheral mononeuropathic pain in rats was associated with distinct alterations of miRNA expression in the ipsilateral DRG. Notably, miR-34a was time-dependently down regulated. We validated SCN2B and VAMP-2 as new targets of miR-34a. While SCN2B expression was only marginally altered, VAMP-2 expression was increased. The present study underlines that the induction and maintenance of neuropathic pain is accompanied by expression changes of miRNAs in the peripheral nervous system, adding several previously unreported miRNAs, including miR-34a.

Genetic interpretation and clinical translation of minor genes related to Brugada syndrome.

Brugada syndrome (BrS) is an inherited arrhythmogenic disease associated with sudden cardiac death. The main gene is SCN5A. Additional variants in 42 other genes have been reported as deleterious, although these variants have not yet received comprehensive pathogenic analysis. Our aim was to clarify the role of all currently reported variants in minor genes associated with BrS. We performed a comprehensive analysis according to the American College of Medical Genetics and Genomics guidelines of published clinical and basic data on all genes (other than SCN5A) related to BrS. Our results identified 133 rare variants potentially associated with BrS. After applying current recommendations, only six variants (4.51%) show a conclusive pathogenic role. All definitively pathogenic variants were located in four genes encoding sodium channels or related proteins: SLMAP, SEMA3A, SCNN1A, and SCN2B. In total, 33.83% of variants in 19 additional genes were potentially pathogenic. Beyond SCN5A, we conclude definitive pathogenic variants associated with BrS in four minor genes. The current list of genes associated with BrS, therefore, should include SCN5A, SLMAP, SEMA3A, SCNN1A, and SCN2B. Comprehensive genetic interpretation and careful clinical translation should be done for all variants currently classified as potentially deleterious for BrS.

SCN1B and SCN2B gene variants analysis in dravet syndrome patients: Analysis of 22 cases.

Previous research identified SCN1B variants in some cases of Dravet syndrome (DS). We investigated whether SCN1B and SCN2B variants are commonly happened in DS patients without SCN1A variants. A total of 22 DS patients without SCN1A variants and 100 healthy controls were enrolled in this genetic study. DNA from DS patients was sequenced by Sanger method in whole exons of SCN1B and SCN2B genes. We identified two exon variants (c.351C>T, p.G117G and c.467C>T, p.T156M), which were present both in 1000 egenomes database and in healthy controls with a frequency of 0.54% and 4%, 0.06% and 0%, respectively. Additionally, eight intron or 3 prime UTR variants showing benign clinical significance have also been identified. Our results suggest that variants of SCN1B and SCN2B may not be common causes of DS according to our data. Further large sample-size cohort studies are needed to confirm our conclusion.

Trafficking and localisation to the plasma membrane of Nav 1.5 promoted by the ß2 subunit is defective due to a ß2 mutation associated with Brugada syndrome.

BACKGROUND INFORMATION: Cardiac channelopathies arise by mutations in genes encoding ion channel subunits. One example is Brugada Syndrome (BrS), which causes arrhythmias and sudden death. BrS is often associated with mutations in SCN5A, encoding Nav 1.5, the α subunit of the major cardiac voltage-gated sodium channel. This channel forms a protein complex including one or two associated ß subunits as well as other proteins. RESULTS: We analysed regulation of Nav 1.5 localisation and trafficking by ß2, specifically, Nav 1.5 arrival to the cell surface. We used polarised Madin-Darby canine kidney (MDCK) cells and mouse atria-derived HL-1 cells, which retain phenotypic features of adult cardiomyocytes. In both, Nav 1.5 was found essentially intracellular, mainly in the endoplasmic reticulum, whereas ß2 localised to the plasma membrane, and was restricted to the apical surface in MDCK cells. A fraction of ß2 interacted with Nav 1.5, despite their limited overlap. Importantly, ß2 promoted Nav 1.5 localisation to the cell surface. Both ß2 WT and the BrS-associated mutation D211G (substitution of Asp for Gly) effectively reached the plasma membrane. Strikingly, however, ß2 D211G was defective in promoting Nav 1.5 surface localisation. CONCLUSIONS: Our data sustain that ß2 promotes surface localisation of Nav 1.5, which can be affected due to ß2 mutations associated with channelopathies. SIGNIFICANCE: Our findings add to the understanding of ß2 role in Nav 1.5 trafficking and localisation, which must influence cell excitability and electrical coupling in the heart. This study will contribute to knowledge on development of arrhythmias.

Scn2b Deletion in Mice Results in Ventricular and Atrial Arrhythmias.

BACKGROUND: Mutations in SCN2B, encoding voltage-gated sodium channel ß2-subunits, are associated with human cardiac arrhythmias, including atrial fibrillation and Brugada syndrome. Because of this, we propose that ß2-subunits play critical roles in the establishment or maintenance of normal cardiac electric activity in vivo. METHODS AND RESULTS: To understand the pathophysiological roles of ß2 in the heart, we investigated the cardiac phenotype of Scn2b null mice. We observed reduced sodium and potassium current densities in ventricular myocytes, as well as conduction slowing in the right ventricular outflow tract region. Functional reentry, resulting from the interplay between slowed conduction, prolonged repolarization, and increased incidence of premature ventricular complexes, was found to underlie the mechanism of spontaneous polymorphic ventricular tachycardia. Scn5a transcript levels were similar in Scn2b null and wild-type ventricles, as were levels of Nav1.5 protein, suggesting that similar to the previous work in neurons, the major function of ß2-subunits in the ventricle is to chaperone voltage-gated sodium channel α-subunits to the plasma membrane. Interestingly, Scn2b deletion resulted in region-specific effects in the heart. Scn2b null atria had normal levels of sodium current density compared with wild type. Scn2b null hearts were more susceptible to atrial fibrillation, had increased levels of fibrosis, and higher repolarization dispersion than wild-type littermates. CONCLUSIONS: Genetic deletion of Scn2b in mice results in ventricular and atrial arrhythmias, consistent with reported SCN2B mutations in human patients.

MicroRNA-9 induces defective trafficking of Nav1.1 and Nav1.2 by targeting Navß2 protein coding region in rat with chronic brain hypoperfusion.

BACKGROUND: Previous studies have demonstrated that the trafficking defects of Nav1.1/Nav1.2 are involved in the dementia pathophysiology. However, the detailed mechanisms are not fully understood. Moreover, whether the impaired miRNAs regulation linked to dementia is a key player in sodium channel trafficking disturbance remains unclear. The cognitive impairment induced by chronic cerebral ischemia through chronic brain hypoperfusion (CBH) is likely reason to precede dementia. Therefore, our goal in the present study was to examine the role of microRNA-9 (miR-9) in regulating Nav1.1/Nav1.2 trafficking under CBH generated by bilateral common carotid artery occlusion (2VO). RESULTS: The impairment of Nav1.1/Nav1.2 trafficking and decreased expression of Navß2 were found in the hippocampi and cortices of rats following CBH generated by bilateral 2VO. MiR-9 was increased in both the hippocampi and cortices of rats following CBH by qRT-PCR. Intriguingly, miR-9 suppressed, while AMO-miR-9 enhanced, the trafficking of Nav1.1/Nav1.2 from cytoplasm to cell membrane. Further study showed that overexpression of miR-9 inhibited the Navß2 expression by targeting on its coding sequence (CDS) domain by dual luciferase assay. However, binding-site mutation or miR-masks failed to influence Navß2 expression as well as Nav1.1/Nav1.2 trafficking process, indicating that Navß2 is a potential target for miR-9. Lentivirus-mediated miR-9 overexpression also inhibited Navß2 expression and elicited translocation deficits to cell membrane of Nav1.1/Nav1.2 in rats, whereas injection of lentivirus-mediated miR-9 knockdown could reverse the impaired trafficking of Nav1.1/Nav1.2 triggered by 2VO. CONCLUSIONS: We conclude that miR-9 may play a key role in regulating the process of Nav1.1/Nav1.2 trafficking via targeting on Navß2 protein in 2VO rats at post-transcriptional level, and inhibition of miR-9 may be a potentially valuable approach to prevent Nav1.1/Nav1.2 trafficking disturbance induced by CBH.

Comprehensive Genetic Characterization of a Spanish Brugada Syndrome Cohort.

BACKGROUND: Brugada syndrome (BrS) is a rare genetic cardiac arrhythmia that can lead to sudden cardiac death in patients with a structurally normal heart. Genetic variations in SCN5A can be identified in approximately 20-25% of BrS cases. The aim of our work was to determine the spectrum and prevalence of genetic variations in a Spanish cohort diagnosed with BrS. METHODOLOGY/PRINCIPAL FINDINGS: We directly sequenced fourteen genes reported to be associated with BrS in 55 unrelated patients clinically diagnosed. Our genetic screening allowed the identification of 61 genetic variants. Of them, 20 potentially pathogenic variations were found in 18 of the 55 patients (32.7% of the patients, 83.3% males). Nineteen of them were located in SCN5A, and had either been previously reported as pathogenic variations or had a potentially pathogenic effect. Regarding the sequencing of the minority genes, we discovered a potentially pathogenic variation in SCN2B that was described to alter sodium current, and one nonsense variant of unknown significance in RANGRF. In addition, we also identified 40 single nucleotide variations which were either synonymous variants (four of them had not been reported yet) or common genetic variants. We next performed MLPA analysis of SCN5A for the 37 patients without an identified genetic variation, and no major rearrangements were detected. Additionally, we show that being at the 30-50 years range or exhibiting symptoms are factors for an increased potentially pathogenic variation discovery yield. CONCLUSIONS: In summary, the present study is the first comprehensive genetic evaluation of 14 BrS-susceptibility genes and MLPA of SCN5A in a Spanish BrS cohort. The mean pathogenic variation discovery yield is higher than that described for other European BrS cohorts (32.7% vs 20-25%, respectively), and is even higher for patients in the 30-50 years age range.

A missense mutation in the sodium channel ß2 subunit reveals SCN2B as a new candidate gene for Brugada syndrome.

Brugada Syndrome (BrS) is a familial disease associated with sudden cardiac death. A 20%-25% of BrS patients carry genetic defects that cause loss-of-function of the voltage-gated cardiac sodium channel. Thus, 70%-75% of patients remain without a genetic diagnosis. In this work, we identified a novel missense mutation (p.Asp211Gly) in the sodium ß2 subunit encoded by SCN2B, in a woman diagnosed with BrS. We studied the sodium current (INa ) from cells coexpressing Nav 1.5 and wild-type (ß2WT) or mutant (ß2D211G) ß2 subunits. Our electrophysiological analysis showed a 39.4% reduction in INa density when Nav 1.5 was coexpressed with the ß2D211G. Single channel analysis showed that the mutation did not affect the Nav 1.5 unitary channel conductance. Instead, protein membrane detection experiments suggested that ß2D211G decreases Nav 1.5 cell surface expression. The effect of the mutant ß2 subunit on the INa strongly suggests that SCN2B is a new candidate gene associated with BrS.