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BackgroundThe epinephrine infusion test (EIT) typically induces marked QT prolongation in LQT1, but not LQT3, while the efficacy of β‐blocker therapy is established in LQT1, but not LQT3. We encountered an LQT3 family, with an SCN5A V1667I mutation, that exhibited epinephrine‐induced marked QT prolongation.MethodsWild‐type (WT) or V1667I‐SCN5A was transiently expressed into tsA‐201 cells, and whole‐cell sodium currents (INa) were recorded using patch‐clamp techniques. To mimic the effects of epinephrine, INa was recorded after the application of protein kinase A (PKA) activator, 8‐CPT‐cAMP (200 μM), for 10 minutes.ResultsThe peak density of V1667I‐INa was significantly larger than WT‐INa (WT: 469 ± 48 pA/pF, n = 20; V1667I: 690 ± 62 pA/pF, n = 19, P < .01). The steady‐state activation (SSA) and fast inactivation rate of V1667I‐INa were comparable to WT‐INa. V1667I‐INa displayed a significant depolarizing shift in steady‐state inactivation (SSI) in comparison to WT‐INa (V1/2‐WT: −88.1 ± 0.8 mV, n = 17; V1667I: −82.5 ± 1.1 mV, n = 17, P < .01), which increases window currents. Tetrodotoxin (30 μM)‐sensitive persistent V1667I‐INa was comparable to WT‐INa. However, the ramp pulse protocol (RPP) displayed an increased hump in V1667I‐INa in comparison to WT‐INa. Although 8‐CPT‐cAMP shifted SSA to hyperpolarizing potentials in WT‐INa and V1667I‐INa to the same extent, it shifted SSI to hyperpolarizing potentials much less in V1667I‐INa than in WT‐INa (V1/2‐WT: −92.7 ± 1.3 mV, n = 6; V1667I: −85.3 ± 1.6 mV, n = 6, P < .01). Concordantly, the RPP displayed an increased hump in V1667I‐INa, but not in WT‐INa.ConclusionsWe demonstrated an increase of V1667I‐INa by PKA activation, which may provide a rationale for the efficacy of β‐blocker therapy in some cases of LQT3.

The segment 4 (S4) voltage sensor in voltage-gated sodium channels (Na v s) have domain-specific functions, and the S4 segment in domain DIV (DIVS4) plays a key role in the activation and fast inactivation processes through the coupling of arginine residues in DIVS4 with residues of putative gating charge transfer center (pGCTC) in DIVS1-3. In addition, the first four arginine residues (R1-R4) in Na v DIVS4 have position-specific functions in the fast inactivation process, and mutations in these residues are associated with diverse phenotypes of Na v-related diseases (sodium channelopathies). R1 and R2 mutations commonly display a delayed fast inactivation, causing a gain-of-function, whereas R3 and R4 mutations commonly display a delayed recovery from inactivation and profound use-dependent current attenuation, causing a severe loss-offunction. In contrast, mutations of residues of pGCTC in Na v DIVS1-3 can also alter fast inactivation. Such alterations in fast inactivation may be caused by disrupted interactions of DIVS4 with DIVS1-3. Despite fast inactivation of Na v s occurs from both the open-state (open-state inactivation; OSI) and closed state (closed-state inactivation; CSI), changes in CSI have received considerably less attention than those in OSI in the pathophysiology of sodium channelopathies. CSI can be altered by mutations of arginine residues in DIVS4 and residues of pGCTC in Na v s, and altered CSI can be an underlying primary biophysical defect of sodium channelopathies. Therefore, CSI should receive focus in order to clarify the pathophysiology of sodium channelopathies.

Background RYR2, encoding cardiac ryanodine receptor, is the major responsible gene for catecholaminergic polymorphic ventricular tachycardia (CPVT). Meanwhile, KCNJ2, encoding inward‐rectifier potassium channel (IK1), can be the responsible gene for atypical CPVT. We recently encountered a family with CPVT and sought to identify a responsible gene variant.MethodsA targeted panel sequencing (TPS) was employed in the proband. Copy number variation (CNV) in RYR2 was identified by focusing on read numbers in the TPS and long‐range PCR. Cascade screening was conducted by a Sanger method and long‐range PCR. KCNJ2 wild‐type (WT) or an identified variant was expressed in COS‐1 cells, and whole‐cell currents (IK1) were recorded using patch‐clamp techniques.ResultsA 40‐year‐old female experienced cardiopulmonary arrest while cycling. Her ECG showed sinus bradycardia with prominent U‐waves (≥0.2 mV). She had left ventricular hypertrabeculation at apex. Exercise induced frequent polymorphic ventricular arrhythmias. Her sister died suddenly at age 35 while bouldering. Her father and paternal aunt, with prominent U‐waves, received permanent pacemaker due to sinus node dysfunction. The initial TPS and cascade screening identified a KCNJ2 E118D variant in all three symptomatic patients. However, after focusing on read numbers, we identified a novel exon3 deletion of RYR2 (RYR2‐exon3 deletion) in all of them. Functional analysis revealed that KCNJ2 E118D generated IK1 indistinguishable from KCNJ2 WT, even in the presence of catecholaminergic stimulation.ConclusionsFocusing on the read numbers in the TPS enabled us to identify a novel CNV, RYR2‐exon3 deletion, which was associated with phenotypic features of this family.

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