FGF12 is a candidate Brugada syndrome locus

Background Less than 30% of the cases of Brugada syndrome (BrS) have an identified genetic cause. Of the known BrS-susceptibility genes, loss-of-function mutations in SCN5A or CACNA1C and their auxiliary subunits are most common. On the basis of the recent demonstration that fibroblast growth factor (FGF) homologous factors (FHFs; FGF11-FGF14) regulate cardiac Na ⁺ and Ca²⁺ channel currents, we hypothesized that FHFs are candidate BrS loci. Objective The goal of this study was to test whether FGF12 is a candidate BrS locus. Methods We used quantitative polymerase chain reaction to identify the major FHF expressed in the human ventricle and then queried a phenotype-positive, genotype-negative BrS biorepository for FHF mutations associated with BrS. We queried the effects of an identified mutant with biochemical analyses combined with electrophysiological assessment. We designed a novel rat ventricular cardiomyocyte system in which we swapped the endogenous FHF with the identified mutant and defined its effects on multiple ionic currents in their native milieu and on the cardiac action potential. Results We identified FGF12 as the major FHF expressed in the human ventricle. In 102 individuals in the biorepository, we identified a single missense mutation in FGF12-B (Q7R-FGF12). The mutant reduced binding to the Na_V1.5 C terminus, but not to junctophilin-2. In adult rat cardiac myocytes, Q7R-FGF12, but not wild-type FGF12, reduced Na⁺ channel current density and availability without affecting Ca²⁺ channel function. Furthermore, the mutant, but not wild-type FGF12, reduced action potential amplitude, which is consistent with a mutant-induced loss of Na⁺ channel function. Conclusions These multilevel investigations strongly suggest that Q7R-FGF12 is a disease-associated BrS mutation. Moreover, these data suggest for the first time that FHF effects on Na⁺ and Ca²⁺ channels are separable. Most significantly, this study establishes a new method to analyze effects of human arrhythmogenic mutations on cardiac ionic currents.