Available clinical data and identification of the SCN5A mutations
Patient #1 was an infant with a QTc of 700 ms and a 2:1 atrioventricular block (AVB) at birth (Fig 1A) . The 2:1 AVB was detected in utero at a gestational age of 31 WA. This infant received 2 mg/kg of propranolol, and a pacemaker was implanted a few days after birth. He experienced salvos of torsades de pointes. Flecainide and mexiletine were unsuccessful in preventing torsades. The patient died at 3 months of age after experiencing ventricular fibrillation.
Patient #2 had a QTc of 600 ms and a 2:1 AVB block at birth. The 2:1 AVB was detected in utero at a gestational age of 25 WA. A pacemaker was implanted a few days after birth, and 2 mg/kg of propranolol was started. The infant experienced several episodes or torsades de pointes and died at 1 month of age after experiencing ventricular fibrillation.
The genomic DNAs of the two infants were screened using specific primers for variant changes in several genes associated with arrhythmia syndrome (see Supplemental Table 1), in particular in all 28 SCN5A exons. Molecular screening by automatic sequencing analysis of the genomic DNAs revealed two novel mutations in the SCN5A gene (G1481V and Q1491H), namely, c. 4473 G>T, which resulted in a Glu1491His substitution, and c. 4442 G>T, which resulted in a Gly1481Val substitution (Fig 1B) . No mutations were detected in other genes. Applying the American College of Medical Genetics and Genomics (ACMG) guidelines15, revealed unclear initial results. This is illustrated and confirmed by Varsome (https://varsome.com) and CardioClassifier (https://www.cardioclassifier.org) which respectively returned both variations as likely pathogenic (class 4) or uncertain significance (class 3). This initial variant classification consequently precluded their use in future presymptomatic diagnosis. This warranted further detailed biophysical characterization (see below) The two substitutions are highly conserved in the Na+ channels of many species and are located in the III-IV linker, which plays a role in channel inactivation in all voltage-gated Na+ channels. It was hypothesized that these two mutations were responsible for the sudden death events. Family screening documented normal phenotypes in both parents of the two infants.