Sodium channel mutations and susceptibility to heart failure and atrial fibrillation

Abstract

Context: Dilated cardiomyopathy (DCM), a genetically heterogeneous disorder, causes heart failure and rhythm disturbances. The majority of identified DCM genes encode structural proteins of the contractile apparatus and cytoskeleton. Recently, genetic defects in calcium and potassium regulation have been discovered in patients with DCM, implicating an alternative disease mechanism. The full spectrum of genetic defects in DCM, however, has not been established.

Objectives: To identify a novel gene for DCM at a previously mapped locus, define the spectrum of mutations in this gene within a DCM cohort, and determine the frequency of DCM among relatives inheriting a mutation in this gene.

Design, setting, and participants: Refined mapping of a DCM locus on chromosome 3p in a multigenerational family and mutation scanning in 156 unrelated probands with DCM, prospectively identified at the Mayo Clinic between 1987 and 2004. Relatives underwent screening echocardiography and electrocardiography and DNA sample procurement.

Main outcome measure: Correlation of identified mutations with cardiac phenotype.

Results: Refined locus mapping revealed SCN5A, encoding the cardiac sodium channel, as a candidate gene. Mutation scans identified a missense mutation (D1275N) that cosegregated with an age-dependent, variably expressed phenotype of DCM, atrial fibrillation, impaired automaticity, and conduction delay. In the DCM cohort, additional missense (T220I, R814W, D1595H) and truncation (2550-2551insTG) SCN5A mutations, segregating with cardiac disease or arising de novo, were discovered in unrelated probands. Among individuals with an SCN5A mutation 27% had early features of DCM (mean age at diagnosis, 20.3 years), 38% had DCM (mean age at diagnosis, 47.9 years), and 43% had atrial fibrillation (mean age at diagnosis, 27.8 years).

Conclusions: Heritable SCN5A defects are associated with susceptibility to early-onset DCM and atrial fibrillation. Similar or even identical mutations may lead to heart failure, arrhythmia, or both.

Figures

Figure 1

Pedigree Struture and Clinica Features of Index Family With SCN5A Mutation Phenotypic traits are variably expressed, designated by shaded quadrants within pedigree symbols. Genotypes for closely spaced DNA markers are shown as numbers, representing different lengths of short tandem repeat marker alleles that distinguish the paternally and maternally inherited chromosomal region. Markers are located within the previously reported disease gene locus on chromosome 3p22-p25. The distance of each marker from the p-arm telomere is indicated by Mb (megabase). The 4 central markers (5-4-5-2) within the shaded chromosome segment define a haplotype, a group of alleles inherited as a unit, common to all family members with cardiac disease. Recombination events indicate that the disease-causing gene resides between markers D3S3727 and D3S3559. SCN5A, located at 38.6-Mb, was investigated as a candidate gene. A point mutation in SCN5A that alters a single amino acid, D1275N, was identified in all affected family members. Mutation-carrier status is indicated by letter symbols for the amino acid at position 1275: aspartic acid (D) in the normal protein and asparagine (N) in the mutant protein. Amino acids shown in parentheses were inferred, together with their corresponding haplotypes. SVT indicates supraventricular tachycardia.

Figure 2

Identification of SCN5A Mutation and Location of Cardiac Sodium Channel Defects A, Heteroduplex mutation scans of exons comprising the entire coding region of SCN5A were performed by denaturing high-performance liquid chromatography (DHPLC). Heterozygous variation in DNA sequence was detected in exons 6, 16, 17, 21, and 27 for the 5 family probands in Figure 1 and Figure 3. In contrast to normal exons generating single peaks on chromatographic profiles, exons harboring mutations had anomalous profiles characterized by 2 peaks. B, To determine if detected variations were benign or pathogenic, genomic DNA sequencing was performed. In 4 of the exons (exons 6, 16, 21, and 27), mutations were discovered in 1 copy of the gene, resulting in amino acid substitutions. In the remaining exon (exon 17), insertion of 2 bases disrupts the coding sequence (only the mutant gene is shown). C, Regions in the cardiac sodium channel protein altered by mutations were aligned with protein sequences of other human and nonhuman sodium channels. The 4 missense mutations alter highly conserved amino acids as indicated by vertical boxes wherein identical amino acids are designated by dots. The frameshift mutation creates a series of 18 anomalous amino acids, shown by the horizontal box in the fs851 panel, and an early stop codon, designated by an asterisk. D, Two-dimensional schematic of SCN5A, demonstrating the predicted transmembrane topology. SCN5A is a monomeric channel composed of 4 repeat domains (DI-DIV), each with 5 homologous hydrophobic transmembrane segments (S1-S3, S5, and S6) and 1 positively charged voltage-sensing segment (S4). Mutations map to all 4 domains, altering residues within membrane-spanning segments; 2 mutations occur within S4 voltage-sensing segments. The insertion or frameshift mutation severely truncates the protein by removing 13 of 24 transmembrane segments.

Figure 3

Pedigree Structures and Clinical Features of Additional Families With SCN5A Mutations Haplotypes at the chromosome 3p locus where SCN5A is located are shown. Each shaded haplotype defines a chromosomal segment that harbors a mutant SCN5A gene. In 2 families, point mutations caused amino acid substitutions: D1595H in DC-30 and T220I in DC-31. In DC-26, the insertion of 2 bases in the mutant gene results in a truncated protein that terminates in a string of 18 anomalous amino acids (fs851 [frameshift at amino acid 851]; Figure 2C). In DC-96, neither of the proband’s parents and none of her 7 siblings had cardiac disease. The haplotypes she inherited from her father (dark blue) and mother (light purple) are also inherited by other siblings, yet she is the only family member with a mutation in SCN5A. These findings indicate that the point mutation, R814W, arose as a spontaneous, or de novo, event on either the paternal or maternal chromosome. SVT indicates supraventricular tachycardia.