Clinical and functional characterization of an SCN5A mutation associated with dilated cardiomyopathy

Genes involved in calcium and potassium regulation in cardiac myocytes are implicated in the etiology of idiopathic Dilated Cardiomyopathy (DCM) and the dilation-related phenotype Arrhythmogenic Right Ventricular Dysplasia (ARVD). Dysregulation of sodium ions in the myocyte, due to mutations in the cardiac sodium channel gene (SCN5A), lead to arrhythmogenic disorders and are implicated in DCM etiology. Linkage-analysis of the Familial Dilated Cardiomyopathy (FDC) locus CMD1E indicates the causative mutation for this phenotype co-localizes with SCN5A in the genome, and CMD1E family members carry a subset of arrhythmic disorders overlapping arrhythmias caused by SCN5A mutations. CMD1E proband sequencing identified a putative disease-causing D1275N missense mutation in SCN5A that segregates with the instance of disease in this family. Substantiating a link between sodium ion dysregulation and dilation etiology, SCN5A screens of DCM and ARVD registrants identified additional putative disease-causing mutations including de-novo missense changes E446K, V12761, and F1520L. Comparative functional studies involving transiently expressed D1275N and wild type SCN5A variants in a Chinese Hamster Ovary (CHO) model cell system indicate alteration significantly disrupts normal channel function. Altering the amino at this position to a positively charged lysine (D1275K) or an arginine (D1275R) eliminates sodium current. Co-expressing a combination of the SCN5A alpha subunit-modulating proteins beta1 (SCN1B) and beta 2 (SCN2B) with the wild type and D1275N variants elicits additional perturbations in basic biophysical function. Investigating the role of this conserved amino among analogous positions within homologous domains of SCN5A, I generated and evaluated variants D197N and the double mutant D1275N/D197N. I also evaluated the position-analogous D1595H variant recently linked to DCM for changes that may reveal common underlying biophysical elements involved in the dilation phenotype. In summary, my thesis work broadens the putative genetic and functional link between sodium channel dysregulation and the manifestation of cardiac dilation. It shows that modulation by beta subunits is an important determinant of cardiac sodium channel function, and characterizes the role of a highly conserved amino acid at analogous critical positions in the protein. The altered functional properties of the sodium channel due to these single mutations must underlie the development of cardiac arrhythmia and dilation.