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(Circulation. 2004;110:2163-2167.)
© 2004 American Heart Association, Inc.
Heart Failure |
From the University of Colorado Cardiovascular Institute (W.P.M., L.K., M.R.G.T., P.R.F., D.D., E.W., L.M.), Human Medical Genetics Program (W.P.M., M.R.G.T., P.R.F., L.M.), and Division of Cardiology (E.W., L.M.), University of Colorado Health Sciences Center, Denver, Colo.
Reprint requests to Dr Luisa Mestroni, University of Colorado Cardiovascular Institute, Bioscience Park Center, 12635 E Montview Blvd, Suite 150, Aurora, CO 80010-7116. E-mail Luisa.Mestroni{at}UCHSC.edu
Received February 20, 2003; revision received May 4, 2004; accepted May 18, 2004.
| Abstract |
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Methods and Results Family members were studied, and the positional candidate gene SCN5A was screened for mutations. We identified, by direct sequencing, a heterozygous G-to-A mutation at position 3823 that changed an aspartic acid to asparagine (D1275N) in a highly conserved residue of exon 21. This mutation was present in all affected family members, was absent in 300 control chromosomes, and predicted a change of charge within the S3 segment of domain III.
Conclusions Our findings expand the clinical spectrum of disorders of the cardiac sodium channel to include cardiac dilation and dysfunction and support the hypothesis that genes encoding ion channels can be implicated in dilated cardiomyopathies.
Key Words: genetics cardiomyopathy conduction arrhythmia heart block
| Introduction |
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-subunit of the cardiac sodium channel (SCN5A) gene.2 SCN5A mutations have been associated with progressive cardiac conduction defect (Lenègre syndrome), isolated cardiac conduction disease, AV conduction block, sick sinus syndrome, sudden infant death syndrome, long-QT syndrome, and Brugada syndrome.39 This chromosomal region also contains a locus for right ventricular cardiomyopathy (ARVD5).10 We hypothesized that SCN5A mutations might be responsible for causing the conduction-related phenotype within this family (pedigree DN-ADFDC3; Figure 1A) and report the identification of an SCN5A mutation associated with the disease phenotype.
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| Methods |
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Criteria for affected status were based on the scoring system proposed by Olson and Keating.2 In brief, points were assigned as follows: asymptomatic sinus bradycardia (heart rate
50 bpm), 1 point; incomplete bundle-branch block age
30 years (QRS interval 0.10 to 0.12 second), 1 point; symptomatic supraventricular tachycardia at
30 years of age, 2 points; first-degree AV block
30 years of age (PR interval >0.22 second), 2 points; clinical heart failure
55 years of age, 2 points; echocardiographic atrial dilation, 2 points; echocardiographic ventricular dilation, 2 points; ventricular dilation (cross-sectional dimension >95% for body surface area), 2 points; left ventricular systolic dysfunction (shortening fraction <28%), 3 points; complete bundle-branch block at
30 years of age (QRS interval >0.12 second), 3 points; symptomatic second-degree AV block at
30 years of age, 4 points; symptomatic sinus node dysfunction (sinus pause or arrest), 4 points; and stroke at
40 years of age, 5 points. Patient status was considered affected with a score of 5 or greater, uncertain with 1 to 4 points, and unaffected with 0 points.2
Molecular Genetic Studies
Genotyping of family members was conducted with genomic DNA extracted from either peripheral blood leukocytes or buccal swab sampling (Epitope, Inc) according to standard protocols. Using publicly available genetic maps (NCBI, http://www.ncbi.nlm.nih.gov/), we localized positional candidate genes to the 3p22-p25 genomic region shared by affected individuals. Current contig maps place SCN5A
7.5 megabases centromeric to the "affected" haplotype marker D3S1211, in a region with a multipoint lod score >3.2 We hypothesized that SCN5A was a likely candidate because of the conduction-related component of this familys phenotype. Primers for all of the protein-coding exons of the SCN5A gene were designed from a prior report12 or optimized with Primer 3 Input software (http://www-genome.wi.mit.edu/cgi-bin/primer/primer3_www.cgi) based on the NCBI and Celera (http://publication.celera.com/humanpub/index.jsp) human sequence databases. The probands genomic DNA was amplified by polymerase chain reaction (PCR) followed by bidirectional sequencing with the ABI 377 DNA Sequencer (Applied Biosystems). Sequencing results were compared with wild-type sequences published in NCBI by the BLAST comparative search algorithms (www.ncbi.nlm.nih.gov/BLAST/). As controls, 300 chromosomes from healthy, unrelated individuals were screened with denaturing high-performance liquid chromatography protocols as described previously.13
We sequenced the promoter region and first exon of Connexin40 (Cx40) for polymorphisms that were previously reported as being in a causative relationship with the D1275N SCN5A mutation for a phenotype of atrial standstill.14 Primers for the relevant regions of the Cx40 gene were used as designed and reported by Groenewegen et al.14 Bidirectional sequencing was performed, and results were compared with the wild-type sequences published in NCBI.
| Results |
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A heterozygous mutation (G3823A) was found in exon 21 of SCN5A, which led to the substitution of an aspartic acid (acidic) by an asparagine (polar, hydrophilic; D1275N) in the first third of the S3 transmembrane region of domain III (Figure 1B).12 This mutation cosegregates within the family with the affected phenotype and with the haplotype characterized by Olson and Keating.2 No recombination event was observed, and the mutation was absent in more than 300 control chromosomes. This amino acid residue is highly conserved among voltage-gated sodium channels and in calcium channels, potassium channels, and across species.15
The reported 44 (G
A) and +71 (A
G) allelic variations in the promoter region and first exon of the Cx40 gene14 were screened in 11 members of the pedigree, as shown in the Table. Three affected individuals, including the proband (III-4, III-10, and IV-2), were heterozygous, carrying the wild-type and the Cx40 polymorphic alleles. Five affected individuals were homozygous for the wild-type sequence at these positions. None of the tested pedigree members proved to be homozygous for the polymorphic allele.
| Discussion |
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Electrophysiological studies performed by Groenewegen et al14 in xenopus oocytes found that the D1275N mutation shifted the activation curve of the sodium channel conductance by 3.8 mV.14 The shift disappeared when the ß-1 subunit was coexpressed. They surmise that this shift in the activation curve toward more positive voltages may result in reduced excitability of myocytes expressing the mutation. Studies in mammalian cells should also be performed owing to significant discrepancies among the electrophysiological profiles of the particular cellular backgrounds on which wild-type channels are expressed.
DCM due to SCN5A mutations has been reported in LQT316 and congenital conduction disease.17 DCM has also been shown to associate with mutations in other ion channel proteins; a patient with a homozygous HERG mutation was found to have DCM,18 and recently, Bienengraeber et al19 reported that mutations in the ABCC9 gene encoding the SUR2A subunit of the cardiac potassium channel conferred susceptibility to DCM. Furthermore, mutations in the ryanodine receptor (ion channel) have been shown to cause a phenotype related to DCM, arrhythmogenic right ventricular dysplasia type 2 (ARVD2).20 The association of an ion channel mutation with dilated cardiomyopathy seen in the DN-ADFDC3 pedigree, however, is particularly remarkable. There is a strong correlation between the penetrance of a conduction disorder and the manifestation of dilatation in this pedigree. This correlation is 100% for affected individuals beyond the youngest generation. Overall, these data suggest that DCM can result not only from structural changes in the myocytes but also from altered ion homeostasis.
The findings of the present study expand the clinical spectrum of disorders of the cardiac sodium channel to include cardiac dilation and dysfunction and support the hypothesis that genes encoding ion channels can be strongly implicated in DCMs.
| Acknowledgments |
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| References |
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14. Groenewegen WA, Firouzi M, Bezzina CR, et al. A cardiac sodium channel mutation cosegregates with a rare connexin40 genotype in familial atrial standstill. Circ Res. 2003; 92: 1422.
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17. Chen S, Chung MK, Martin D, et al. SNP S1103Y in the cardiac sodium channel gene SCN5A is associated with cardiac arrhythmias and sudden death in a white family. J Med Genet. 2002; 39: 913915.
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20. Tiso N, Stephan DA, Nava A, et al. Identification of mutations in the cardiac ryanodine receptor gene in families affected with arrhythmogenic right ventricular cardiomyopathy type 2 (ARVD2). Hum Mol Genet. 2001; 10: 189194.
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