Two novel SCN5A loss-of-function mutations affect patients with severe arrhytmogenic syndromes

SCN5A gene encodes for the α subunit of the cardiac isoform of the voltage gated sodium channel. Mutations in this gene have been correlated with several arrhytmogenic syndromes among which Long QT Syndrome (LQTS) type 3 and Brugada Syndrome (BrS) are the most studied. Here we describe two novel mutations related to these phenotypes. The c.86_87delinsTG causes the substitution of Ala 29 in the N-terminus of the protein with a Val and was found in a 36 years old man diagnosed with BrS. The mutation c.5089T>C substitute the Phe 1697 with a Ser in the intramembrane pore forming region and was found in a 45 years old woman diagnosed with LQTS. Whole cell patch-clamp studies on HEK293 cells revealed that both the mutations are responsible for a loss-of-function of the protein. In particular the A29V completely abolished the inward current. The F1697S mutation reduced the current density (-183.1±37.1 pA/pF, n=37 in WT vs -107.3±33.7 pA/pF, n=29 in F1697S; p<0.01) and caused a significant negative shift in the half-maximal voltage (V1/2) of steady- state inactivation curve and a positive one in the V1/2 of the activation curve (-8 and +6.4 mV, respectively). These gating changes caused a shift and a reduction in the window current that may markedly modify the action potential duration. Moreover, F1697S substitution slowed down the recovery from inactivation, being 8.1±0.2 ms (n=27) in WT and 12±0.4 ms (n=17) in F1697S the time necessary for the recovery of the 50% of the channels. These findings suggest that both the mutations strongly reduce the sodium inward current anticipating a higher risk of arrhythmogenesis for the patients. Still more has to be done in order to explain the clinical phenotype of LQTS in light of a loss-offunction of the F1697S Nav1.5 channel.