**8. Polymorphisms in the genes coding ion channels**

#### **8.1. KCNN3 rs13376333 gene polymorphism**

Differences in populations due to myocardial membrane stability, conduction routes or genetic polymorphisms are important factors in predisposing to AF development. In recent association studies, gene polymorphisms found on chromosomes 4q25, 16q22 and 1q21 have been identified as genetic risk factors for AF development. Moreover, these genetic variants are more important in the development of early onset AF. The relationship between these gene polymorphisms and the risk of developing AF has been explored in different populations. From these populations, in one Chinese and European origin considerable differences have been found in terms of these polymorphisms. rs2200733 on the 4q25 chromosome and rs106261 gene polymorphisms on the 16q22 chromosome have been observed quite often in the Chinese population in particular. However, the relationship between the rs7193343 gene polymorphism on the 16q22 chromosome and the risk of developing AF was not significant in the Chinese Han population. In a study conducted by Ellinor et al. with the European population, KCNN3 single nucleotide gene polymorphism, which is associated with AF in the new genetic locus, has been discovered in the potassium medium/small conductance calciumactivating channel. The KCNN3 gene encodes voltage-independent calcium and activated potassium channels. The KCNN3 rs13376333 gene polymorphism is located between the first and second exons of the KCNN3 gene. There are three subtypes of potassium channels as SK1, SK2 and SK3. Atrial myocytes are formed by the subunits of these channels to form heteromultimeric complexes. The expression of SK3 channels is similar to the expressions of SK1 and SK2 channels. There are studies showing that the relationship between these SK channels and AF is significant. However, more studies are needed to determine the role of SK3 channels in AF development. Studies were also conducted in the Asian population to investigate AF associations with KCNN3 gene polymorphisms, which are the ionic channel gene identified in AF GWAS. However, a large number of replication studies are needed to determine this relationship. In a study conducted by Chang et al., KCNN3 rs13376333 gene polymorphism in the Taiwanese population was found to be an important risk factor for the development of AF. Also Ellinor et al. showed that there is a significant association between AF and KCNN3 rs13376333 gene polymorphism. In a study conducted with Chinese Han population, KCNN3 rs13376333 gene polymorphism has not been identified as a genetic risk factor in the development of AF. In Taiwan and China populations, the T allele of KCNN3 rs13376333 gene polymorphism was observed at a significantly lower frequency [17].

#### **8.2. SCN10A gene polymorphism**

**7. G-protein β3 subunit C825T gene polymorphism**

G-protein β3 subunit C825T gene polymorphism plays an important role in the change of electrophysiological properties of human atrium. This polymorphism occurs in 10th exon of gene, which encodes the G-protein β3 subunit. It has been determined by Siffert et al. that this polymorphism is a genetic risk factor in the development of hypertension. Increased human atrial internal rectifier regulatory potentials have been associated with the TT genotype of G-protein β3 subunit C825T gene polymorphism. There is also a significant relationship between the TT genotype of the G-protein β3 subunit C825T gene polymorphism and the increased internal rectifier flow and reduced acetylcholine stimulating potassium flux in the human atrium. In the European white population, 825 T allele of G-protein β3 subunit C825T gene polymorphism was found to be significantly associated with various cardiovascular disorders such as increased obesity, hypertension, left ventricular hypertrophy and coronary artery disease. In a study performed by Schreieck et al., heterozygote T and homozygote T allele carriage were found to be low risk factors for AF development. G-protein β3 subunit the TT and CT genotypes of the C825T gene polymorphism play an important role in atrial cellular electrophysiological changes. In a study conducted by Dobrev et al., it was determined that TT genotype of this polymorphism correlates with the downregulation of acetylcholine mRNA transcripts in human atrial myocytes. Although there is no relationship between G-protein β3 subunit C825T gene polymorphism and any arrhythmia, in some studies this polymorphism has been associated with the risk of developing AF. In conclusion, gene polymorphisms encoding ion channels are very important in AF pathogenesis. Identification of these polymorphisms will elucidate the multigenic mechanism of AF predisposition [16]. It is presented primer sequences

that used to determine G-protein β3 subunit C825T gene polymorphism in **Table 7**.

Forward 5′-TTCTCCCACGAGAGCATCATCT-3′ Reverse 5′-GTCGTCGTAGCCAGCGAATAGTA-3′ Allele 825C probe 5′-CATCACGTCCGTGGCCTTCTCC-3′ Allele 825T probe 5′-CATCACGTCTGTGGCCTTCTCCCT-3′

Differences in populations due to myocardial membrane stability, conduction routes or genetic polymorphisms are important factors in predisposing to AF development. In recent association studies, gene polymorphisms found on chromosomes 4q25, 16q22 and 1q21 have been identified as genetic risk factors for AF development. Moreover, these genetic variants

**8. Polymorphisms in the genes coding ion channels**

**Table 7.** Sequence of primers for G-protein β3 subunit C825T gene polymorphism.

**Genes Primer Primer sequences**

G-protein β3 subunit C825T

PCR, polymerase chain reaction.

gene

12 Cardiac Arrhythmias

**8.1. KCNN3 rs13376333 gene polymorphism**

Voltage-gated sodium channels play an important role in impulse generation and conduction during the rising phase of action potential in excitable cells. There are sodium channel isoforms in the heart. These channels include voltage-gated sodium 1.1, voltage-gated sodium 1.3, voltage-gated sodium 1.5 (Nav 1.5), voltage-gated sodium 1.6 and voltage-gated sodium 1.8 channels. The Nav 1.5 encoded by SCN5A is responsible for the regulation of cardiac conduction. The Nav1.5 channel plays a very important role in cardiac impulse spread. As a result of the activation of sodium channels, the cardiac action potential is rapidly increasing. Each sodium channel consists of an α subunit and modulating β subunits. The α subunit of the NaV1.5 channel is encoded by the SCN5A gene. Each of the Nav1.5 α subunit consists of 4 homologous domains (DI-DIV) with 6 transmembrane alpha helices (S1-S6). The S1-S4 domains are repeatable and these domains constitute the voltage sensing areas of the channel. The functional pore and selectivity filter of the sodium channel consists of S5, S6 and S5-S6 loops. More than 300 mutations have been identified in the SCN5A gene. SCN5A mutations determined to be associated with Brugada Syndrome (BrS) lead to variable reductions in the sodium flow inward with channel transit changes. These channel passing changes delayed activation, increased inactivation, slow recovery from inactivation, or impaired exchange of channel. As a result, decrease in expression occurs in the cell membrane. Consequently, these mechanisms cause loss of function in the cardiac sodium channel. The most common genotype found among BrS patients stems from mutations in the SCN5A gene. As a result of these mutations occurring in the gene, there is a loss of function in the cardiac sodium channel through different mechanisms. Depolarization or repolarization of cardiac action potential may be affected by due to reduced sodium current. Nevertheless, the underlying pathophysiological mechanism of the BrS phenotype is still being discussed [18]. BrS is defined as a disease characterized by sudden cardiac death characterized by a right bundle branch with an ST segment elevation in leads V1 and V2 in 1992. This syndrome was found to be associated with sudden cardiac death, especially in young men [19]. BrS, determined to be genetic, is a cardiac electrical disorder. BrS, an arrhythmogenic and autosomal dominant inherited cardiac syndrome, is characterized by typical electrocardiographic changes. In a study conducted in the Chinese population, localized in the domain II S4 segment of NaV1.5 α subunit protein, a new mutation, L812Q mutation, has been described. In this study, it was shown that this mutation improved the sodium channel inactivation process and disrupted the membrane expression of the canal in BrS patients [18]. In a Dutch population study, it was determined that SCN5A gene mutations, which cause loss of function in BrS patients, are associated with dilation and deterioration in contractile function of both ventricles [20]. In another study, SCN5A showed a high penetrance for BrS in a large family with the E1784K mutation. In addition, in the same study, overexpressing phenotypes of BrS were shown in E1784K and H558R carriers after the fourth decades of their lifes [21]. There is an effect in the cardiac electrophysiological properties of sodium-gated voltage channel 1.8 via the effect intrinsic on cardiac ganglion neurons. In the isolated ventricular myocardium, it is known that the sodium-gated voltage 1.8 channel is not expressed. In isolated intrinsic cardiac ganglia, there are immunoelectrochemical studies indicating that significant amounts of sodium-gated voltage 1.8 channels are expressed. Facer et al. have shown that sodium-gated voltage 1.8 channel immunoreactive sensory nerves are present in human atrial myocardium. Voltage-gated sodium 1.8 channel is encoded by SCN10A and is a tetradoxin (TTX)-resistant sodium channel. This channel is expressed in dorsal root ganglia, cranial sensory ganglion sensory neurons. The SCN10A gene, which contains 27 exons, is localized on chromosome 3q22.2. The SCN10A gene has been shown to be associated with cardiac transmission. Because the SCN10A gene plays a role in increasing the PR interval and QRS duration in the electrocardiogram. Therefore, it was found that there is a relation between SCN10A and AF development. The SCN10A sodium-gated voltage 1.8 channel plays an important role in modulating the induction of AF. Verkerk et al. have demonstrated that the SCN10A sodium-gated voltage 1.8 channel is present in intrinsic cardiac neurons. In a study conducted by Chambers et al., a significant relationship was found between SCN10A rs6795970 gene polymorphism and PR interval. SCN10A rs6795970 (G>A) gene polymorphism is a missense mutation and causes an A1073V amino acid substitution in the sodium-gated voltage channel 1.8 IDII/III intracellular cycle. In a study conducted by Ritchie et al., the G allele of SCN10A rs6795970 gene polymorphism was found to be a genetic risk factor for the development of AF. In another study performed by Sabbari et al., G allele of SCN10A rs6795970 gene polymorphism was associated with increased risk of AF. A significant association was found between the SCN10A rs6800541 gene polymorphism and AF development in the study conducted by Pfeufer et al. [22, 23].

**8.3. KCNE1 G38S gene polymorphism**

KCNN3 rs13376333

SCN10A-1 rs6795970

SCN10A-2 rs6795970

KCNE1 rs1805127

KCNE1 rs1892593

KCNE1 widely known as a potassium ion channel encoding gene for humans and it is localized on chromosome 21q22.1–21q22.2 encoding the subunit of the potassium ion channel (IKs). KCNE1 plays an important role in atrial and ventricular repolarization. The KCNE1 gene was discovered by Murai et al. in 1989. Studies have shown that KV7.1, the α subunit of the IKS current, plays an important role in AF pathogenesis. The regulatory β subunits of the IKS current also bind to the KCNE1 gene. Biophysical properties of these β subunits of KV.71 can be altered by expression together. The β subunits of IKS contain 130 amino acids, which is called the Mink protein. Several single nucleotide gene polymorphisms have been identified in the KCNE1 gene. The most common of these polymorphisms is the KCNE1 G38S (rs1805127 G>A, G38S) polymorphism. The KCNE1 gene polymorphism is characterized by a glycine or serine amino acid substitution in the 38th position of the gene. As a result, stronger IKs flows occur. Various studies have been carried out to demonstrate that the KCNE1 gene and polymorphisms are highly effective in AF pathogenesis. In a study conducted by Lai et al., a significant association was found between the risk of developing AF in the Taiwanese population and the KCNE1 G38S gene polymorphism. Despite this conclusion in the Taiwanese population, it has been determined that this polymorphism is not a genetic risk factor in the development of AF in the Chinese population. Studies conducted with European and Uighur populations have also found that KCNE1 G38S polymorphism is a risk factor associated with AF. A total of 14 studies were conducted to investigate the relationship between KCNE1 G38S gene polymorphism and the risk of developing AF. In eight of these studies, a significant relationship was found between the risk of developing KCNE1 G38S gene polymorphism and AF. However, no significant relationship was determined in other six studies. In a meta-analysis study conducted by Jiang et al., to evaluate the relationship between KCNE1 G38S polymorphism and AF, it is concluded the KCNE1 G38S gene polymorphism increased AF risk. In a study carried out by Yadav et al., in the North Indian population, KCNE1 G38S gene polymorphism was found to be not a risk factor for postoperative AF development. In a study by Chen et al., it was found that the arrhythmia matrix is important

Gene Polymorphisms Associated with Atrial Fibrillation http://dx.doi.org/10.5772/intechopen.76920 15

**SNP rs Forward primer (5′–3′) Reverse primer (5′–3′)**

PCR, polymerase chain reaction; SNP, single nucleotide polymorphism.

**Table 8.** Primer sequences used in PCR for polymorphisms in the genes coding ion channels.

TGAGAGCACCTGCAGACATC GCAGCAAGAAGTGGGTCAAT

ATGACCCGAACTGACCTTCC TGACGCTAAAATCCAGCCAGT

TGACAGAGGAGCAGAAGAAATACTACA GTTGAGGCAGATGAGGACCA

GTGACGCCCTTTCTGACCAA CCAGATGGTTTTCAACGACA

TGGGCTCTATTTTCAG CCATTGGTCATTTTCC

#### **8.3. KCNE1 G38S gene polymorphism**

activation, increased inactivation, slow recovery from inactivation, or impaired exchange of channel. As a result, decrease in expression occurs in the cell membrane. Consequently, these mechanisms cause loss of function in the cardiac sodium channel. The most common genotype found among BrS patients stems from mutations in the SCN5A gene. As a result of these mutations occurring in the gene, there is a loss of function in the cardiac sodium channel through different mechanisms. Depolarization or repolarization of cardiac action potential may be affected by due to reduced sodium current. Nevertheless, the underlying pathophysiological mechanism of the BrS phenotype is still being discussed [18]. BrS is defined as a disease characterized by sudden cardiac death characterized by a right bundle branch with an ST segment elevation in leads V1 and V2 in 1992. This syndrome was found to be associated with sudden cardiac death, especially in young men [19]. BrS, determined to be genetic, is a cardiac electrical disorder. BrS, an arrhythmogenic and autosomal dominant inherited cardiac syndrome, is characterized by typical electrocardiographic changes. In a study conducted in the Chinese population, localized in the domain II S4 segment of NaV1.5 α subunit protein, a new mutation, L812Q mutation, has been described. In this study, it was shown that this mutation improved the sodium channel inactivation process and disrupted the membrane expression of the canal in BrS patients [18]. In a Dutch population study, it was determined that SCN5A gene mutations, which cause loss of function in BrS patients, are associated with dilation and deterioration in contractile function of both ventricles [20]. In another study, SCN5A showed a high penetrance for BrS in a large family with the E1784K mutation. In addition, in the same study, overexpressing phenotypes of BrS were shown in E1784K and H558R carriers after the fourth decades of their lifes [21]. There is an effect in the cardiac electrophysiological properties of sodium-gated voltage channel 1.8 via the effect intrinsic on cardiac ganglion neurons. In the isolated ventricular myocardium, it is known that the sodium-gated voltage 1.8 channel is not expressed. In isolated intrinsic cardiac ganglia, there are immunoelectrochemical studies indicating that significant amounts of sodium-gated voltage 1.8 channels are expressed. Facer et al. have shown that sodium-gated voltage 1.8 channel immunoreactive sensory nerves are present in human atrial myocardium. Voltage-gated sodium 1.8 channel is encoded by SCN10A and is a tetradoxin (TTX)-resistant sodium channel. This channel is expressed in dorsal root ganglia, cranial sensory ganglion sensory neurons. The SCN10A gene, which contains 27 exons, is localized on chromosome 3q22.2. The SCN10A gene has been shown to be associated with cardiac transmission. Because the SCN10A gene plays a role in increasing the PR interval and QRS duration in the electrocardiogram. Therefore, it was found that there is a relation between SCN10A and AF development. The SCN10A sodium-gated voltage 1.8 channel plays an important role in modulating the induction of AF. Verkerk et al. have demonstrated that the SCN10A sodium-gated voltage 1.8 channel is present in intrinsic cardiac neurons. In a study conducted by Chambers et al., a significant relationship was found between SCN10A rs6795970 gene polymorphism and PR interval. SCN10A rs6795970 (G>A) gene polymorphism is a missense mutation and causes an A1073V amino acid substitution in the sodium-gated voltage channel 1.8 IDII/III intracellular cycle. In a study conducted by Ritchie et al., the G allele of SCN10A rs6795970 gene polymorphism was found to be a genetic risk factor for the development of AF. In another study performed by Sabbari et al., G allele of SCN10A rs6795970 gene polymorphism was associated with increased risk of AF. A significant association was found between the SCN10A rs6800541 gene polymorphism and

14 Cardiac Arrhythmias

AF development in the study conducted by Pfeufer et al. [22, 23].

KCNE1 widely known as a potassium ion channel encoding gene for humans and it is localized on chromosome 21q22.1–21q22.2 encoding the subunit of the potassium ion channel (IKs). KCNE1 plays an important role in atrial and ventricular repolarization. The KCNE1 gene was discovered by Murai et al. in 1989. Studies have shown that KV7.1, the α subunit of the IKS current, plays an important role in AF pathogenesis. The regulatory β subunits of the IKS current also bind to the KCNE1 gene. Biophysical properties of these β subunits of KV.71 can be altered by expression together. The β subunits of IKS contain 130 amino acids, which is called the Mink protein. Several single nucleotide gene polymorphisms have been identified in the KCNE1 gene. The most common of these polymorphisms is the KCNE1 G38S (rs1805127 G>A, G38S) polymorphism. The KCNE1 gene polymorphism is characterized by a glycine or serine amino acid substitution in the 38th position of the gene. As a result, stronger IKs flows occur. Various studies have been carried out to demonstrate that the KCNE1 gene and polymorphisms are highly effective in AF pathogenesis. In a study conducted by Lai et al., a significant association was found between the risk of developing AF in the Taiwanese population and the KCNE1 G38S gene polymorphism. Despite this conclusion in the Taiwanese population, it has been determined that this polymorphism is not a genetic risk factor in the development of AF in the Chinese population. Studies conducted with European and Uighur populations have also found that KCNE1 G38S polymorphism is a risk factor associated with AF. A total of 14 studies were conducted to investigate the relationship between KCNE1 G38S gene polymorphism and the risk of developing AF. In eight of these studies, a significant relationship was found between the risk of developing KCNE1 G38S gene polymorphism and AF. However, no significant relationship was determined in other six studies. In a meta-analysis study conducted by Jiang et al., to evaluate the relationship between KCNE1 G38S polymorphism and AF, it is concluded the KCNE1 G38S gene polymorphism increased AF risk. In a study carried out by Yadav et al., in the North Indian population, KCNE1 G38S gene polymorphism was found to be not a risk factor for postoperative AF development. In a study by Chen et al., it was found that the arrhythmia matrix is important


PCR, polymerase chain reaction; SNP, single nucleotide polymorphism.

**Table 8.** Primer sequences used in PCR for polymorphisms in the genes coding ion channels.

at the onset or maintenance of AF. The arrhythmia matrix is formed by the interaction of proteins encoded by KCNE1 with other proteins. Therefore, the KCNE1 gene plays a very important role in regulating cardiac rhythm. Studies involving subgroup analyzes also found that the risk of developing AF in white populations with risk alleles was higher than in the Chinese population. The pathogenesis of AF is unknown. However, as a result of mutations in the genes encoding the ion channel, AF can develop due to a decrease in IKs. Environmental factors and genetic factors play a role in the pathogenesis of AF. It has been determined that different polymorphisms in genes encoding ion channels other than the KCNE1 G38S gene polymorphism may also be important risk factors for AF development [24, 25]. It is presented primer sequences that used to determine polymorphisms in the genes coding ion channels in **Table 8**.

role in the onset and maintenance of AF. Myocarditis, pericardiotomy and C-reactive protein (CRP) levels were associated with AF, a dysrhythmia, in studies conducted. However, in some other studies, it has been determined that there is a relationship between AF and the induction of inflammatory response. Previous studies have suggested that AF may be due to inflammatory processes and there is a significant relationship between the CRP levels and the risk of developing AF in these studies. A study performed by Lo et al. found a significant relationship between high basal CRP levels and increased postoperative AF risk. Non-Willebrand factor expression, which is effective in tissue factor, fibrinogen, factor VIII and prothrombic state, is induced by the IL-6 gene, which plays an important role in inflammation. Another study by Gaudino et al. found that −174 G/C polymorphism, a polymorphism in the promoter region of the interleukin-6 (IL-6) gene, was a significant effect on the inflammatory response and was associated with the risk of postoperative AF development. Also Marcus et al., in their study showed a significant relationship between increased IL-6 levels and the risk of developing AF. There is also a study showing that patients with high CRP levels have higher AF risk than patients with normal CRP levels [27].

Gene Polymorphisms Associated with Atrial Fibrillation http://dx.doi.org/10.5772/intechopen.76920 17

There are four unique nucleotide polymorphisms on the 4q25 chromosomal region, rs2200733, rs2220427, rs2634073 and rs10033464, and in studies conducted in European and Chinese populations, a significant relationship was found between these polymorphisms and the risk of developing AF. There are no known biological roles of these single nucleotide polymorphisms. These polymorphisms near to the homedomain transcription factor 2 (PITX2) gene and potentially alter the function of this factor. PITX2 is involved in the cardiac pathogenesis of ischemic and pulmonary venous access pathways. rs2200733 and rs13143308 that among the polymorphisms found on the 4q25 chromosome have also been identified as genetic risk factors for AF development. There are also several epidemiological cohorts recently showing a significant association between rs2200733, rs10033464 single nucleotide polymorphisms located in the 4q25 chromosome and AF development. In a recent study, rs2200733 polymorphism was

found to be a genetic risk factor for AF development, proliferation and recurrence [28].

PRRX1 (paired-related HomeBox 1) is a gene encoding homedomain transcription factor that is expressed high in the developing heart. As a result of GWAS, the molecular mechanisms related to AF have been tried to be elucidated. In a recent meta-GWAS, significant correlations were found between the risk of developing rs3903239 polymorphism and AF on the 1q24 chromosome of the PRRX1 gene. In another study conducted with the Greek population, the role of the genetic interaction between PRRX1 rs3903239 and PITX2 rs2200733 gene polymorphisms in the development of AF was investigated and no significant interaction could be detected between these polymorphisms in AF patients. In addition, there was no significant difference in terms of PRRX1 rs3903239 allele frequencies and genotypes between AF patients and healthy controls in the same study. In another study conducted with the Chinese population, PRRX1 rs3903239 gene polymorphism was not detected as a significant genetic risk factor for AF [29].

**11. Gene polymorphisms on chromosome 4q25**

**12. PRRX1 rs3903239 gene polymorphism**
