A Novel and Lethal De Novo LQT-3 Mutation in a Newborn with Distinct Molecular Pharmacology and Therapeutic Response

Abstract

Background SCN5A encodes the α-subunit (Nav1.5) of the principle Na+ channel in the human heart. Genetic lesions in SCN5A can cause congenital long QT syndrome (LQTS) variant 3 (LQT-3) in adults by disrupting inactivation of the Nav1.5 channel. Pharmacological targeting of mutation-altered Na+ channels has proven promising in developing a gene-specific therapeutic strategy to manage specifically this LQTS variant. SCN5A mutations that cause similar channel dysfunction may also contribute to sudden infant death syndrome (SIDS) and other arrhythmias in newborns, but the prevalence, impact, and therapeutic management of SCN5A mutations may be distinct in infants compared with adults.Methods and ResultsHere, in a multidisciplinary approach, we report a de novo SCN5A mutation (F1473C) discovered in a newborn presenting with extreme QT prolongation and differential responses to the Na+ channel blockers flecainide and mexiletine. Our goal was to determine the Na+ channel phenotype caused by this severe mutation and to determine whether distinct effects of different Na+ channel blockers on mutant channel activity provide a mechanistic understanding of the distinct therapeutic responsiveness of the mutation carrier. Sequence analysis of the proband revealed the novel missense SCN5A mutation (F1473C) and a common variant in KCNH2 (K897T). Patch clamp analysis of HEK 293 cells transiently transfected with wild-type or mutant Na+ channels revealed significant changes in channel biophysics, all contributing to the proband's phenotype as predicted by in silico modeling. Furthermore, subtle differences in drug action were detected in correcting mutant channel activity that, together with both the known genetic background and age of the patient, contribute to the distinct therapeutic responses observed clinically.SignificanceThe results of our study provide further evidence of the grave vulnerability of newborns to Na+ channel defects and suggest that both genetic background and age are particularly important in developing a mutation-specific therapeutic personalized approach to manage disorders in the young.

Induced pluripotent stem cells used to reveal drug actions in a long QT syndrome family with complex genetics

“…He was found to be heterozygous for a de novo SCN5A mutation, F1473C, in the heart Na v 1.5 Na + channel inactivation gate and for a common polymorphism (K897T) in the hERG channel (KCNH2; Fig. 1 A; Bankston et al, 2007b). Neither parent was found to carry the Na + channel mutation, but the mother was homozygous for the hERG polymorphism ( Fig.…”

“…5). We focused on Na + channel biophysical properties principally related to channel inactivation because the F1473C mutation had been found to alter Na + channel inactivation in heterologous expression studies in HEK293 cells (Bankston et al, 2007b). First, comparison of the biophysical properties of all clones studied from either parent indicated remarkable similarity to properties of Na + channels measured in CMs derived from hESCs (hESC-CMs; Satin et al, 2004), validating the use of iPSC-CMs in the measurement of Na + channel activity.…”

“…PCR products were purified after de novo mutation in the principal heart sodium channel (SCN5A) and a common polymorphism in KCNH2 that codes for hERG, a critical heart potassium channel known to contribute to most cases of drug-induced LQTS (Mitcheson et al, 2000). In a first study, we had characterized the biophysical properties of the mutant sodium channels alone expressed in HEK293 cells and found that the mutation resulted in defects in inactivation consistent with the LQT phenotype observed in the proband (Bankston et al, 2007b). However, this study did not explain why the proband was resistant to drug therapy, and it did not investigate whether the hERG polymorphism played a role in the observed phenotype or in the response to drug therapy, nor did it report the function of the mutant sodium channel expressed in a cardiac cellular background genetically identical to that of the mutation carrier.…”

“…Action potentials were triggered at 0.2 Hz by 3 ms of suprathreshold stimuli. Sodium current (I Na ), measured as tetrodotoxin (TTX; 50 µM)-sensitive current, was recorded with a previously described internal recording solution (Bankston et al, 2007b) and the following external solution: 130 mM NaCl, 5 mM CsCl, 2 mM CaCl 2 , 1.2 mM MgCl 2 , 0.05 mM NiSO 4 , 10 mM HEPES, 5 mM glucose, pH 7.4, and 1 µM isradipine. I Na late current (I NaL ) was measured as the TTX (50 µM)-sensitive current measured at 100 ms during depolarization to 10 mV from a 90-mV holding potential.…”

“…As a result, a dual chamber implantable cardioverter defibrillator (ICD)/pacemaker was implanted epicardially to help reduce the number of arrhythmias experienced by the patient. Heterologous expression of mutant Na + channels in mammalian (HEK293) cells identified a severe defect in inactivation and illustrated the utility of Na + channel blockers to correct the mutation-induced channel dysfunction (Bankston et al, 2007b). The contribution of the KCNH2 polymorphism to the disease phenotype and pro-and anti-arrhythmic drug effects was unknown despite studies suggesting a causal link between this polymorphism and arrhythmia (Bezzina et al, 2003;Paavonen et al, 2003;Anson et al, 2004;Crotti et al, 2005;Rhodes et al, 2008;Sinner et al, 2008).…”