The chromosome 21q21 locus had not previously been linked to arrhythmia and the mechanism by which common genetic variation at this locus confers an increased risk to VF is unknown. Common genetic variation is expected to impact on disease either by changing the amino acid sequence of a protein (and thereby its function) or by impacting on the level of gene expression and consequently protein level. The SNPs at chromosome 21q21 impacting on risk for VF are not located within a gene. Inspection of a 1 megabase (Mb) interval spanning both upstream and downstream of these SNPs identified three genes all located within a region of 0.5 Mb downstream. One is
BTG3, located at a distance of 179 kb from the association signal. This gene encodes B cell translocation gene 3, a member of the
PC3,
BTG and
TOB family of growth inhibitory genes [
13]. Its mRNA expression level is relatively high in the testis, lung and ovary but is low in heart. Another gene in the region is
C21orf91, located at 374 kb from the association signal; it encodes the human orthologue of the chicken
C21orf91 (also known as
EURL, encoding the early undifferentiated retina and lens protein), which is known to be expressed in the developing chick retina and lens and has been suggested to play a role in the development of these structures [
14].
The most plausible candidate gene in the region for the observed effect at this locus is, however, the
CXADR gene, located 98 kb downstream of the association signal. This gene encodes the Coxsackievirus and adenovirus receptor (CAR) protein, which is mostly expressed in the heart, brain and pancreas [
15]. CAR has a long-recognised role as viral receptor in the pathogenesis of viral myocarditis and its sequela of dilated cardiomyopathy [
16,
17]. Upon infection, the enteroviral (as is the Coxsackie B virus) protease 2A cleaves dystrophin, resulting in disruption of the dystrophin–glycoprotein complex, a component of the cytoskeleton of cardiac myocytes [
18]. Interestingly, frequency of active Coxsackie B virus infection has been reported to be high in a group of MI patients who died suddenly [
19]. The physiological role of CAR was initially less clear. It is expressed at the intercalated disc between cardiomyocytes, suggesting a role in conduction of the cardiac electrical impulse and cell–cell communication [
20,
21]. Indeed, two studies have reported a physiological role for CAR in localisation of connexin 45 at the intercalated disks of atrioventricular node cardiomyocytes as well as its role in conduction of the cardiac impulse within this cardiac compartment [
22,
23]. In addition, protein levels of connexin 43, which is expressed in atrial and ventricular myocytes and in distal parts of the conduction system, were reduced in CAR-deficient mice, suggesting that loss of CAR could impact on ventricular conduction [
23]. Thus,
CXADR is a candidate gene for the association reported here.
However, providing support for a role of
CXADR (or any other gene) in mediating the observed effect is a daunting task. The SNPs mediating the association signal on chromosome 21q21 are not linked to amino acid changing variants within the CAR coding region (or any other gene in the region). This precludes an effect through functional variation of a gene product, implying that the role of these SNPs in modulation of arrhythmia risk is to be sought in effects on gene expression. Effects of common genetic variation on gene expression is considered to be an important mechanism impacting on inter-individual variation in disease susceptibility, and many GWAS studies have in fact identified many signals within non-coding (intronic and intergenic) regions [
24]. This will require studying gene expression as a function of genotype at this locus in the human heart.