Background:
SCN5A encodes the voltage-gated sodium channel alpha subunit Nav1.5, importantly contributing to cardiac depolarisation. Loss-of-function variants in SCN5A are associated with high phenotypic variability and multiple cardiac diseases including Brugada syndrome (BrS). GPD1L, encoding the glycerol-3-phosphate dehydrogenase-1-like protein, is linked to BrS, as well, but disease mechanisms are less established. In this study a German family with individuals affected by BrS of different severity was investigated, aiming at the identification of the genetic and electrophysiological mechanisms underlying the disease.

Methods and Results:
Next-generation sequencing, including all known BrS-associated genes, was used to identify possible genetic variants in the family. The index patient, who was severely affected by BrS with recurrent ventricular arrhythmias, carried previously uncharacterized variants of the voltage-gated sodium channel Nav1.5 (SCN5A-G1661R) and glycerol-3-phosphate dehydrogenase-1-like protein (GPD1L-A306del) in a double heterozygous conformation. The brother and father of the index patient exclusively carrying the SCN5A variant showed asymptomatic Brugada ECG patterns, while the mother solely carrying GPD1L-A306del lacked any clinical phenotype.

To evaluate functional changes caused by the variants, Nav1.5 channels were transiently expressed in HEK-293T cells in the presence and absence of GPD1L, and the sodium current was measured by whole-cell patch‑clamp recordings. Homozygous expression of SCN5A-G1661R failed to produce any detectable current. The heterozygous constellation, simulated by expression of equal amounts of wildtype and mutant Nav1.5, showed a reduction of current amplitude by ~50% without changing the gating parameters, suggesting haploinsufficiency. With respect to the underlying pathomechanisms, immunostaining of Nav1.5 revealed a lack of mutant channels at the cell membrane, suggesting defective trafficking.      
The presence of wildtype GPD1L resulted in a trend towards increased sodium current amplitudes and a hyperpolarizing shift in steady-state activation and -inactivation compared to sole SCN5A expression. Application of the GPD1L-A306del variant shifted steady-state activation to more hyperpolarized and inactivation to more depolarized potentials. Finally, immunostaining indicated increased Nav1.5 membrane expression in the presence of wildtype GPD1L, and a reduction in the presence of mutant GPD1L.                      

Conclusion: