Functional Characterization of a Novel Frameshift Mutation in the C-terminus of the Nav1.5 Channel Underlying a Brugada Syndrome with Variable Expression in a Spanish Family

Abstract

IntroductionWe functionally analyzed a frameshift mutation in the SCN5A gene encoding cardiac Na+ channels (Nav1.5) found in a proband with repeated episodes of ventricular fibrillation who presented bradycardia and paroxysmal atrial fibrillation. Seven relatives also carry the mutation and showed a Brugada syndrome with an incomplete and variable expression. The mutation (p.D1816VfsX7) resulted in a severe truncation (201 residues) of the Nav1.5 C-terminus. Methods and ResultsWild-type (WT) and mutated Nav1.5 channels together with hNavβ1 were expressed in CHO cells and currents were recorded at room temperature using the whole-cell patch-clamp. Expression of p.D1816VfsX7 alone resulted in a marked reduction (≈90%) in peak Na+ current density compared with WT channels. Peak current density generated by p.D1816VfsX7+WT was ≈50% of that generated by WT channels. p.D1816VfsX7 positively shifted activation and inactivation curves, leading to a significant reduction of the window current. The mutation accelerated current activation and reactivation kinetics and increased the fraction of channels developing slow inactivation with prolonged depolarizations. However, late INa was not modified by the mutation. p.D1816VfsX7 produced a marked reduction of channel trafficking toward the membrane that was not restored by decreasing incubation temperature during cell culture or by incubation with 300 μM mexiletine and 5 mM 4-phenylbutirate. ConclusionDespite a severe truncation of the C-terminus, the resulting mutated channels generate currents, albeit with reduced amplitude and altered biophysical properties, confirming the key role of the C-terminal domain in the expression and function of the cardiac Na+ channel.

Diagnostic Approach to Unexplained Cardiac Arrest (from the FIVI-Gen Study)

“…In 1 of these 3 cases, co-segregation with the pathogenic variant p.D1816Vfs in SCN5A was confirmed in 7 carrier relatives ( Figure 3A). This mutation causes severe truncation (201 residues) of the C-terminus of Nav1.5 and the functional analysis revealed a marked reduction in channel trafficking towards the membrane and in sodium current density compared with non-mutated channels (WT) 18 . In another case, despite a negative adrenaline test in the proband, who had severe neurological damage, an upper limit of normal QTc was detected in a firstdegree relative.…”

De Novo Mutation in the SCN5A Gene Associated with Brugada Syndrome

“…We found that the density of Na + currrent was reduced to ∼60% of the WT Na + current density, and the voltage dependence of inactivation was also shifted towards negative voltages compared to the WT channel. Some research showed that when co-expressing trafficking-deficient BrS SCN5A mutations with WT Na v 1.5 channel (0.5:0.5 in ratio) in heterologous expression systems, it is observed that the Na + current was reduced by about 50% compared to the Na + current produced by expressing only WT Na v 1.5 channel [33][34][35]. Such reduction of Na + current is due to the haploinsufficiency of WT Na v 1.5 channel.…”

Genetics and Epigenetics of Atrial Fibrillation

“…However, Olson and colleagues identified the first SCN5A missense mutation that was associated with several cardiac phenotypes including AF [ 76 ]. Subsequently, multiple mutations in SCN5A gene have been identified in patients with AF, alone [ 77 , 78 , 79 , 80 , 81 ] or with combined cardiac conditions [ 76 , 82 , 83 , 84 , 85 , 86 ]. Several variants of SCNA5 have been reported in patients with common polygenic AF as well [ 87 ].…”