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Inherited disorders of heart rhythm and cardiac conduction have received tremendous attention during the past 15 years. The discovery of mutations in several genes, mostly encoding subunits of ion channels, has revealed the molecular basis for various familial arrhythmia susceptibility syndromes and enabled deployment of clinical genetic testing to identify at-risk persons. Proper risk stratification in these settings is crucial for selecting the most appropriate treatments for individual patients, but knowledge of the primary mutation doesn't always help predict risk of serious cardiac events.
With more widespread use of genetic testing in disorders such as the congenital long QT syndrome (LQTS) and Brugada syndrome (BrS) has come the recognition that not all mutations carriers are equally affected.1-4 When asymptomatic mutation carriers belong to a family that segregate a Mendelian phenotype (i.e., caused by a single gene defect) such as LQTS we say that the mutation exhibits reduced or incomplete penetrance. Similarly, certain mutations may confer variable risk of disease expression or variation in the signs and symptoms evident in related individuals (variable expressivity). Both incomplete penetrance and variable expressivity are likely due to genetic, environmental or developmental factors that condition the host or help trigger disease onset. Genetic factors distinct from the primary disease-causing mutation are referred to as modifiers. Identification of genetic modifiers of Mendelian disorders may improve the precision of molecular diagnostics and provide additional clues about disease mechanism. But, the search for genetic modifiers can be quite challenging.
In this issue of Circulation Research, Remme and colleagues report on a search for candidate genetic modifiers of a cardiac conduction disorder caused by mutation of the cardiac sodium channel gene, SCN5A.5 A specific mutation, in-frame insertion of an aspartic acid residue at position 1795 in the human protein (designated 1795insD), was previously identified by this group in a large Dutch family segregating a mixed phenotype of LQTS and BrS with additional features of bradycardia and impaired cardiac conduction.6 When the corresponding mutation is transgenically engineered in murine Scn5a (corresponding to 1798insD), the resulting heterozygous mice exhibit features of the human disorder including bradycardia, prolonged QT interval and impaired cardiac conduction most evident in the right ventricle.7 Importantly, the investigators recognized that the severity of the cardiac conduction defect exhibited significant strain-dependence when the mutation was transmitted to different inbred mouse lines. In particular, they observed that the conduction defect was less severe when the mutation was present in FVB/N mice than when the same allele was present on a 129P2 genomic background. Further, flecainide aggravated bradycardia and QRS widening more in 129P2 than in FVB/N mutant mice. Because inbred mouse strains have been extremely well characterized genetically, the recognition of a strain-dependent phenotype provides an outstanding opportunity to map genes that contribute to variable phenotype.
To provide clues about the potential genetic basis for these phenotype disparities, Remme and colleagues examined differences in gene expression between cardiac tissues obtained from the two inbred strains. Using microarray technology, the investigators discovered 76 genes that exhibited significantly different mRNA expression levels between the two strains. About half of the genes were expressed at a higher level in the 129P2 mice as compared to FVB/N, while the other genes detected were expressed at a lower level. Inspecting this list of differentially expressed genes, the group discovered that Scn4b encoding a sodium channel accessory subunit (β4) was expressed at a substantially lower level in 129P2 mice. They further demonstrated that right ventricular tissue from 129P2 mice had very low levels of β4 protein. Logically, they considered the possibility that differential expression of β4 contributed to the strain-dependent conduction defect observed in Scn5a-1798insD mutant mice.
Differences in functional behavior of cardiac sodium current between the two mouse strains were also observed. Specifically, sodium currents recorded from 129P2 myocytes exhibited significant differences in the voltage-dependence of activation, a depolarized shift in the conductance-voltage relationship, compared with FVB/N myocytes. This finding suggested a potential biophysical mechanism for impaired conduction velocity (i.e., insufficient sodium current activation), an idea corroborated using a computational approach. These findings provide indirect support for their hypothesis that Scn4b is a genetic modifier of impaired cardiac conduction.
Further proof of this hypothesis will require additional mouse genetic studies. If Scn4b is a dominant modifier of the murine phenotype, then genetic mapping studies should complement the finding of differential expression between strains by showing linkage to this locus. Alternatively, transgenic overexpression of Scn4b in 129P2 mice carrying Scn5a-1798insD may provide corroborative evidence if the strain differences in phenotype are eliminated. Importantly, there were 75 other differentially expressed genes between the two mouse strains. Although Scn4b is an attractive candidate modifier because of its known association with voltage-gated sodium channels, further studies should address the contribution of other differentially expressed genes on modulating the degree of impaired cardiac conduction. Perhaps these studies will reveal other previously unrecognized molecular pathways regulating cardiac conduction. Sodium channel β subunit proteins come in four ‘flavors’ (β1 through β4) encoded by distinct genes (SCN1B through SCN4B).8 Mutations in some β subunit genes have been linked to familial epilepsy and inherited disorders of heart rhythm thus emphasizing their likely physiological importance.9-11 The four β subunit genes are expressed widely in neuronal tissue, muscle and heart but there are no systematic biochemical data to indicate with which specific pore forming α-subunits they partner in various tissues. Expression of all four β subunit proteins has been demonstrated in mouse heart with two patterns of localization. The β1 and β3 subunits localize along the z-lines suggesting expression in the t-tubules whereas β2 and β4 have been shown to locate in punctate clusters in a subgroup of intercalated disks.12 How these distinct patterns of localization mediate the physiological actions of different β subunits in heart is not known.
In neuronal tissue, several functional activities have been ascribed to β subunits including regulation of excitability, cell adhesion, neural development and possibly control of transcription.8 Co-expression of β subunits with recombinant brain and heart sodium channel α-subunits in heterologous cells causes subtle but significant alterations of functional properties.13,14 Relevant to the study by Remme and colleagues, co-expression of β4 with cardiac sodium channels in vitro results in virtually no functional change in sodium current.15 However, these in vitro findings do not necessarily rule out important physiological activities for sodium channel β subunits in heart, a point emphasized by the discovery of a missense mutation in SCN4B associated with congenital LQTS.11
Although these reported mouse studies provide important new and interesting findings, an important next step for this research will be to determine if SCN4B is a genetic modifier of human diseases associated with SCN5A mutations including the original Dutch family segregating the 1795insD allele, and perhaps other inherited arrhythmias such as BrS. Ultimately, such studies will help shape new risk assessment strategies that consider the primary mutation as well as genetic modifiers.
Sources of Funding: A.L.G. is funded by NIH grants HL068880 and HL083374.
Disclosures: There are no relevant disclosures.