Reduced connexin40 protein expression in the right atrial appendage of patients bearing the minor connexin40 allele (-44 G --> A)

Association between polymorphisms in connexin 40 gene (Cx40) and risk of atrial fibrillation: a meta-analysis based on 3,452 subjects

INTRODUCTION

Atrial fibrillation (AF) is a common cardiac arrhythmia that is associated with heart failure and stroke, leading sometimes to death. But the pathogenesis of AF remains unclear. Numerous studies have investigated whether the connexin 40 (Cx40) polymorphisms influences the risk of AF, but the results are controversial.

METHODS

We searched English and Chinese databases and calculated the odds ratio (OR) and 95% confidence interval (CI) to examine the existence of genetic associations between the Cx40 polymorphisms and the risk of AF. All relevant studies were screened and meta-analyzed using Review Manager 5.0.

RESULTS

A total of 12 studies, including 10 studies for -44 polymorphism (rs35594137) and 4 studies for -26 polymorphism (rs10465885), were identified for the meta-analysis. For -44 polymorphism, the results showed a significantly increased risk of AF in the five genetic models in the overall analysis. Furthermore, in subgroup analysis, increased AF risks were also observed in Asian and non-Asian populations. For -26 polymorphism, the overall OR revealed an increased risk of AF in dominant model. In subgroup analysis, increased AF risk was only found in recessive genetic model of the Asian population.

CONCLUSIONS

The Cx40 polymorphisms were positively associated with AF in both populations, especially on -44 polymorphism.

The intercalated disc: a unique organelle for electromechanical synchrony in cardiomyocytes

The intercalated disc (ID) is a highly specialized structure that connects cardiomyocytes via mechanical and electrical junctions. Although described in some detail by light microscopy in the 19th century, it was in 1966 that electron microscopy images showed that the ID represented apposing cell borders and provided detailed insight into the complex ID nanostructure. Since then, much has been learned about the ID and its molecular composition, and it has become evident that a large number of proteins, not all of them involved in direct cell-to-cell coupling via mechanical or gap junctions, reside at the ID. Furthermore, an increasing number of functional interactions between ID components are emerging, leading to the concept that the ID is not the sum of isolated molecular silos but an interacting molecular complex, an "organelle" where components work in concert to bring about electrical and mechanical synchrony. The aim of the present review is to give a short historical account of the ID's discovery and an updated overview of its composition and organization, followed by a discussion of the physiological implications of the ID architecture and the local intermolecular interactions. The latter will focus on both the importance of normal conduction of cardiac action potentials as well as the impact on the pathophysiology of arrhythmias.

Two polymorphisms in the Cx40 promoter are associated with hypertension and left ventricular hypertrophy preferentially in men

BACKGROUND

Lack of connexin40, a gap junction protein expressed in endothelial and renin-producing cells, results in hypertension and cardiac hypertrophy in mice due to unleashed renin production caused by disruption of the pressure-induced feedback inhibition. We analysed human GJA5 consisting of two exons (exon1A or 1B and exon2) in a selected cohort identified by a single nucleotide polymorphism (SNP) in the GJA5 intron for polymorphisms and putative association with hypertension and left ventricular hypertrophy (LVH).

METHODS

Individuals carrying a SNP in the intron of GJA5 (rs791295) were selected from the MONICA/KORA cohort (n = 1677) and searched for GJA5 polymorphisms. We accessed DNA of 178 probands, of which 26 suffered from LVH, 112 were hypertensive and 29 normotensive (unknown: 11).

RESULTS

Sequencing of the GJA5 coding region did not reveal alterations suggesting the expression of functional connexin40 in all probands. Sequencing of the upstream region of transcript 1A including exon1A revealed two previously described linked SNPs (rs35594137 -44G>A; rs11552588 + 71A>G) at an increased frequency. Moreover, the rare genotype was significantly associated with hypertension and LVH with a preponderance in men. Functional analysis in a reporter gene assay verified promoter activity, however, it was unchanged by the identified SNPs after expressing respective reporter constructs in HeLa and human endothelial cells.

CONCLUSION

We suggest to consider the -44G>A SNP upstream of the connexin40 transcript 1A indeed as a risk factor for hypertension in men. However, the underlying mechanisms remain unclear but animal data suggest that renin-producing cells may be involved and contribute to hypertension.

Connexin40 abnormalities and atrial fibrillation in the human heart

Normal atrial conduction requires similar abundances and homogeneous/overlapping distributions of two connexins (Cx40 and Cx43). The remodeling of myocyte connections and altered electrical conduction associated with atrial fibrillation (AF) likely involves perturbations of these connexins. We conducted a comprehensive series of experiments to examine the abundances and distributions of Cx40 and Cx43 in the atria of AF patients. Atrial appendage tissues were obtained from patients with lone AF (paroxysmal or chronic) or normal controls. Connexins were localized by double label immunofluorescence confocal microscopy, and their overlap was quantified. Connexin proteins and mRNAs were quantified by immunoblotting and qRT-PCR. PCR amplified genomic DNA was sequenced to screen for connexin gene mutations or polymorphisms. Immunoblotting showed reductions of Cx40 protein (to 77% or 49% of control values in samples from patients with paroxysmal and chronic AF, respectively), but no significant changes of Cx43 protein levels in samples from AF patients. The extent of Cx43 immunostaining and its distribution relative to N-cadherin were preserved in the AF patient samples. Although there was variability of Cx40 staining among paroxysmal AF patients, all had some fields with substantial Cx40 heterogeneity and reduced overlap with Cx43. Cx40 immunostaining was severely reduced in all chronic AF patients. qRT-PCR showed no change in Cx43 mRNA levels, but reductions in total Cx40 mRNA (to <50%) and Cx40 transcripts A (to ~50%) and B (to <25%) as compared to controls. No Cx40 coding region mutations were identified. The frequency of promoter polymorphisms did not differ between AF patient samples and controls. Our data suggest that reduced Cx40 levels and heterogeneity of its distribution (relative to Cx43) are common in AF. Multiple mechanisms likely lead to reductions of functional Cx40 in atrial gap junctions and contribute to the pathogenesis of AF in different patients.

Prevalence and spectrum of GJA5 mutations associated with lone atrial fibrillation

Atrial fibrillation (AF) is the most common form of cardiac arrhythmia observed in clinical practice and a major contributor to cardiovascular morbidity and mortality. Accumulating evidence indicates a substantial genetic basis for AF. However, AF is genetically heterogeneous and the hereditary components responsible for AF remain to be identified in the majority of patients. The cardiac gap junction protein α 5 (GJA5) is specifically expressed in atrial myocytes and is associated with the coordinated electrical activation of the atria, providing a rationale to screen GJA5 as a logical candidate gene for AF. A cohort of 310 unrelated patients with lone AF and their available relatives were included in this study. A group of 200 unrelated healthy individuals matched for age, gender and race were also included as controls. The entire coding region and splice sites of the GJA5 gene were initially sequenced in 310 unrelated AF patients. The relatives of mutation carriers and 200 controls were subsequently genotyped for the presence of identified mutations. As a result, 4 novel heterozygous GJA5 mutations, p.K107R, p.L223M, p.Q236H and p.I257L, were identified in 4 of 310 unrelated AF patients, respectively, with a prevalence of ~1.29%. Genetic analysis of the carriers' families showed that in each family the missense mutation was present in all the affected family members. Absent in the 400 reference alleles, these mutations altered the amino acids highly conserved among various species, with the exception of p.I257L. In conclusion, this study expands the spectrum of GJA5 mutations associated with AF and provides novel insights into the molecular basis of AF, suggesting potential implications for the improved, gene-specific rhythm control strategies.