Abstract 1504: Scn4b Is A Genetic Modifier Of Cardiac Conduction Disease In Mice
Background: Conduction slowing of the cardiac electrical impulse may evoke cardiac arrhythmias that result in sudden cardiac death. We have previously generated a mouse model of cardiac Na+ channelopathy carrying the Scn5a-1798insD/+ mutation, the mouse homolog of SCN5A-1795insD associated with conduction disease in humans. We here employed this model to identify novel modifier genes for cardiac conduction disease.
Methods and Results: The Scn5a1798insD± mutation was generated in the 129P2 strain and transferred into the FVB/N strain. Conduction disease as assessed by ECG, epicardial mapping in isolated hearts and patch-clamp studies in isolated myocytes, was more severe in 129P2-Scn5a1798insD± (mut129P2) mice compared to FVB/N-Scn5a1798insD± (mutFVB/N) mice. Genome-wide mRNA expression profiling of ventricular tissue identified multiple transcripts differentially expressed between the 2 strains. Notably, Scn4b encoding the Na+ channel β4 subunit, was 11-fold lower in 129P2. Accordingly, Western blot analysis demonstrated robust β4 protein expression in FVB/N ventricles while it was consistently undetectable in 129P2. DNA sequencing of the Scn4b promoter uncovered differences between the 2 strains likely underlying this effect. Systematic surface ECG analysis of 230 mutant F2 (mutF2) mice, resulting from the mutFVB/N X mut129P2 cross, yielded a distribution of conduction indices indicative of an efficient segregation of genetic modifier loci. Association studies between the identified Scn4b promoter variants and conduction indices in mutF2 mice uncovered an association with heart rate (P=0.01) and QRS interval (P=0.02). Moreover, qRT-PCR analysis in mutF2 mice showed an association between Scn4b genotype and ventricular Scn4b mRNA levels. Finally, in HEK cells heterologously expressing Nav1.5, co-expression of β4 increased peak Na+ current.
We have demonstrated that Scn4b is a genetic modifier of conduction disease in mice.
Differences in Scn4b mRNA and protein expression underlie this effect.
Differences in Scn4b mRNA expression are caused by variants within the Scn4b locus itself (ciseQTL).
Differences in Scn4b expression affect cardiac conduction by modulating Na+ current amplitude.