A novel PITX2c loss‑of‑function mutation underlies lone atrial fibrillation

In Silico Characterization of Pathogenic Homeodomain Missense Mutations in the PITX2 Gene

Paired homologous domain transcription factor 2 (PITX2) is critically involved in ocular and cardiac development. Mutations in PITX2 are consistently reported in association with Axenfeld-Rieger syndrome, an autosomal dominant genetic disorder and atrial fibrillation, a common cardiac arrhythmia. In this study, we have mined missense mutations in PITX2 gene from NCBI-dbSNP and Ensembl databases, evaluated the pathogenicity of the missense variants in the homeodomain and C-terminal region using five in silico prediction tools SIFT, PolyPhen2, GERP, Mutation Assessor and CADD. Fifteen homeodomain mutations G42V, G42R, R45W, S49Y, R53W, E53D, E55V, R62H, P65S, R69H, G75R, R84G, R86K, R87W, R91P were found to be highly pathogenic by both SIFT, PolyPhen2 were further functionally characterized using I-Mutant 2.0, Consurf, MutPred and Project Hope. The findings of the study can be used for prioritizing mutations in the context of genetic studies.

Genetic Atrial Cardiomyopathies: Common Features, Specific Differences, and Broader Relevance to Understanding Atrial Cardiomyopathy

Atrial cardiomyopathy is a condition that causes electrical and contractile dysfunction of the atria, often along with structural and functional changes. Atrial cardiomyopathy most commonly occurs in conjunction with ventricular dysfunction, in which case it is difficult to discern the atrial features that are secondary to ventricular dysfunction from those that arise as a result of primary atrial abnormalities. Isolated atrial cardiomyopathy (atrial-selective cardiomyopathy [ASCM], with minimal or no ventricular function disturbance) is relatively uncommon and has most frequently been reported in association with deleterious rare genetic variants. The genes involved can affect proteins responsible for various biological functions, not necessarily limited to the heart but also involving extracardiac tissues. Atrial enlargement and atrial fibrillation are common complications of ASCM and are often the predominant clinical features. Despite progress in identifying disease-causing rare variants, an overarching understanding and approach to the molecular pathogenesis, phenotypic spectrum, and treatment of genetic ASCM is still lacking. In this review, we aim to analyze the literature relevant to genetic ASCM to understand the key features of this rather rare condition, as well as to identify distinct characteristics of ASCM and its arrhythmic complications that are related to specific genotypes. We outline the insights that have been gained using basic research models of genetic ASCM in vitro and in vivo and correlate these with patient outcomes. Finally, we provide suggestions for the future investigation of patients with genetic ASCM and improvements to basic scientific models and systems. Overall, a better understanding of the genetic underpinnings of ASCM will not only provide a better understanding of this condition but also promises to clarify our appreciation of the more commonly occurring forms of atrial cardiomyopathy associated with ventricular dysfunction.

Atrial fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia despite substantial efforts to understand the pathophysiology of the condition and develop improved treatments. Identifying the underlying causative mechanisms of AF in individual patients is difficult and the efficacy of current therapies is suboptimal. Consequently, the incidence of AF is steadily rising and there is a pressing need for novel therapies. Research has revealed that defects in specific molecular pathways underlie AF pathogenesis, resulting in electrical conduction disorders that drive AF. The severity of this so-called electropathology correlates with the stage of AF disease progression and determines the response to AF treatment. Therefore, unravelling the molecular mechanisms underlying electropathology is expected to fuel the development of innovative personalized diagnostic tools and mechanism-based therapies. Moreover, the co-creation of AF studies with patients to implement novel diagnostic tools and therapies is a prerequisite for successful personalized AF management. Currently, various treatment modalities targeting AF-related electropathology, including lifestyle changes, pharmaceutical and nutraceutical therapy, substrate-based ablative therapy, and neuromodulation, are available to maintain sinus rhythm and might offer a novel holistic strategy to treat AF.

Mechanistic Insights into Axenfeld-Rieger Syndrome from Zebrafish foxc1 and pitx2 Mutants

Axenfeld-Rieger syndrome (ARS) encompasses a group of developmental disorders that affect the anterior segment of the eye, as well as systemic developmental defects in some patients. Malformation of the ocular anterior segment often leads to secondary glaucoma, while some patients also present with cardiovascular malformations, craniofacial and dental abnormalities and additional periumbilical skin. Genes that encode two transcription factors, FOXC1 and PITX2, account for almost half of known cases, while the genetic lesions in the remaining cases remain unresolved. Given the genetic similarity between zebrafish and humans, as well as robust antisense inhibition and gene editing technologies available for use in these animals, loss of function zebrafish models for ARS have been created and shed light on the mechanism(s) whereby mutations in these two transcription factors cause such a wide array of developmental phenotypes. This review summarizes the published phenotypes in zebrafish foxc1 and pitx2 loss of function models and discusses possible mechanisms that may be used to target pharmaceutical development and therapeutic interventions.

Novel PITX2 Homeodomain-Contained Mutations from ATRIAL Fibrillation Patients Deteriorate Calcium Homeostasis

Atrial fibrillation (AF) is the most common cardiac arrhythmia in the human population, with an estimated incidence of 1–2% in young adults but increasing to more than 10% in 80+ years patients. Pituitary Homeobox 2, Paired Like Homeodomain 2 (PITX2c) loss-of-function in mice revealed that this homeodomain (HD)-containing transcription factor plays a pivotal role in atrial electrophysiology and calcium homeostasis and point to PITX2 as a candidate gene for AF. To address this issue, we recruited 31 AF patients for genetic analyses of both the known risk alleles and PITX2c open reading frame (ORF) re-sequencing. We found two-point mutations in the homedomain of PITX2 and three other variants in the 5’untranslated region. A 65 years old male patient without 4q25 risk variants but with recurrent AF displayed two distinct HD-mutations, NM_000325.5:c.309G>C (Gln103His) and NM_000325.5:c.370G>A (Glu124Lys), which both resulted in a change within a highly conserved amino acid position. To address the functional impact of the PITX2 HD mutations, we generated plasmid constructs with mutated version of each nucleotide variant (MD4 and MD5, respectively) as well as a dominant negative control construct in which the PITX2 HD was lacking (DN). Functional analyses demonstrated PITX2c MD4 and PITX2c MD5 decreased Nppa-luciferase transactivation by 50% and 40%, respectively, similar to the PITX2c DN (50%), while Shox2 promoter repression was also impaired. Co-transactivation with other cardiac-enriched co-factors, such as Gata4 and Nkx2.5, was similarly impaired, further supporting the pivotal role of these mutations for correct PITX2c function. Furthermore, when expressed in HL1 cardiomyocyte cultures, the PITX2 mutants impaired endogenous expression of calcium regulatory proteins and induced alterations in sarcoplasmic reticulum (SR) calcium accumulation. This favored alternating and irregular calcium transient amplitudes, causing deterioration of the beat-to-beat stability upon elevation of the stimulation frequency. Overall this data demonstrate that these novel PITX2c HD-mutations might be causative of atrial fibrillation in the carrier.

Genetics of atrial fibrillation

PURPOSE OF REVIEW

Atrial fibrillation is the most common sustained cardiac arrhythmia. In addition to traditional risk factors, it is increasingly recognized that a genetic component underlies atrial fibrillation development. This review aims to provide an overview of the genetic cause of atrial fibrillation and clinical applications, with a focus on recent developments.

RECENT FINDINGS

Genome-wide association studies have now identified around 140 genetic loci associated with atrial fibrillation. Studies into the effects of several loci and their tentative gene targets have identified novel pathways associated with atrial fibrillation development. However, further validations of causality are still needed for many implicated genes. Genetic variants at identified loci also help predict individual atrial fibrillation risk and response to different therapies.

SUMMARY

Continued advances in the field of genetics and molecular biology have led to significant insight into the genetic underpinnings of atrial fibrillation. Potential clinical applications of these studies include the identification of new therapeutic targets and development of genetic risk scores to optimize management of this common cardiac arrhythmia.

Genetics of Atrial Fibrillation

Background

Atrial fibrillation (AF) is a common arrhythmia seen in clinical practice. Occasionally, no common risk factors are present in patients with this arrhythmia. This suggests the potential underlying role of genetic factors associated with predisposition to developing AF.

Methods and Results

We conducted a comprehensive review of the literature through large online libraries, including PubMed. Many different potassium and sodium channel mutations have been discussed in their relation to AF. There have also been non–ion channel mutations that have been linked to AF. Genome‐wide association studies have helped in identifying potential links between single‐nucleotide polymorphisms and AF. Ancestry studies have also highlighted a role of genetics in AF. Blacks with a higher percentage of European ancestry are at higher risk of developing AF. The emerging field of ablatogenomics involves the use of genetic profiles in their relation to recurrence of AF after catheter ablation.

Conclusions

The evidence for the underlying role of genetics in AF continues to expand. Ultimately, the role of genetics in risk stratification of AF and its recurrence is of significant interest. No established risk scores that are useful in clinical practice are present to date.

Proarrhythmia in the p.Met207Val PITX2c-Linked Familial Atrial Fibrillation-Insights From Modeling

Functional analysis has shown that the p.Met207Val mutation was linked to atrial fibrillation and caused an increase in transactivation activity of PITX2c, which caused changes in mRNA synthesis related to ionic channels and intercellular electrical coupling. We assumed that these changes were quantitatively translated to the functional level. This study aimed to investigate the potential impact of the PITX2c p.Met207Val mutation on atrial electrical activity through multiscale computational models. The well-known Courtemanche-Ramirez-Nattel (CRN) model of human atrial cell action potentials (APs) was modified to incorporate experimental data on the expected p.Met207Val mutation-induced changes in ionic channel currents (I_NaL, I_Ks, and I_Kr) and intercellular electrical coupling. The cell models for wild-type (WT), heterozygous (Mutant/Wild type, MT/WT), and homozygous (Mutant, MT) PITX2c cases were incorporated into homogeneous multicellular 1D and 2D tissue models. Effects of this mutation-induced remodeling were quantified as changes in AP profile, AP duration (APD) restitution, conduction velocity (CV) restitution and wavelength (WL). Temporal and spatial vulnerabilities of atrial tissue to the genesis of reentry were computed. Dynamic behaviors of re-entrant excitation waves (Life span, tip trajectory and dominant frequency) in a homogeneous 2D tissue model were characterized. Our results suggest that the PITX2c p.Met207Val mutation abbreviated atrial APD and flattened APD restitution curves. It reduced atrial CV and WL that facilitated the conduction of high rate atrial excitation waves. It increased the tissue's temporal vulnerability by increasing the vulnerable window for initiating reentry and increased the tissue spatial vulnerability by reducing the substrate size necessary to sustain reentry. In the 2D models, the mutation also stabilized and accelerated re-entrant excitation waves, leading to rapid and sustained reentry. In conclusion, electrical and structural remodeling arising from the PITX2c p.Met207Val mutation may increase atrial susceptibility to arrhythmia due to shortened APD, reduced CV and increased tissue vulnerability, which, in combination, facilitate initiation and maintenance of re-entrant excitation waves.

PITX2 and NEURL1 SNP polymorphisms in Hungarian atrial fibrillation patients determined by quantitative real-time PCR and melting curve analysis

Atrial fibrillation (AF) is the most common cardiac arrhythmia affecting 1-2% of the general population. Some common variants located in or next to PITX2 and NEURL1 genes are proved to play role in the occurrence of AF. The aim of our study was to investigate whether rs2595104 in the 4q25 chromosome region and rs6584555 SNP in the NEURL1 gene on chromosome 10 is associated with AF in a Caucasian population. We genotyped DNA samples of 76 AF patients and 77 healthy controls using quantitative real-time PCR followed by melting curve analysis. The minor A allele frequency of rs2595104 in PITX2 was 0.38 and 0.44 in the control group and in AF patients, respectively. There was no significant difference in allele and genotype distribution between the two groups (p = 0.52). The allele frequency based log additive odds ratio is 1.22 (C.I. = 0.76-1.94; p = 0.42). The frequency of minor rs6584555 C allele in NEURL1 was 0.22 in the control group and 0.23 in AF patients. Again there were no significant differences in allele and genotype frequencies between AF patients and controls (p = 0.92). The log additive odds ratio is 1,15 (C.I. = 0.66-2.01; p = 0,63). The heterozygous genotype of rs2595104 had the highest frequency compared to the other genotypes in both groups. In case of the rs6584555 SNP the homozygous genotype of the major allele (TT) had the highest frequency in both groups (0.59). The frequency of homozygous genotype for risk allele had the lowest frequency for both SNPs [rs2595104 (AA): 0.19 in patients, 0.12 in controls; rs6584555 (CC): 0.05 in patients, 0.03 in controls]. We did not find significant association between SNP rs2595104 and rs6584555 andAF. We performed a protein-protein network analysis to assess functional connection among the protein products. The proteins coded by PITX2 and NEURL1 are connected indirectly via CTNNB1 and either JAG1 or DLL4 proteins. These interactive proteins are components of two major channels of cell communication pathways, the Wnt and Notch signaling pathways.

A Novel PITX2c Gain-of-Function Mutation, p.Met207Val, in Patients With Familial Atrial Fibrillation

Genome-wide studies have associated several genetic variants upstream of PITX2 on chromosome 4q25 with atrial fibrillation (AF), suggesting a potential role of PITX2 in AF. Our study aimed at identifying rare coding variants in PITX2 predisposing to AF. The Polymerase chain reaction sequencing of PITX2c was performed in 60 unrelated patients with idiopathic AF. The p.Met207Val variant was identified in 1 of 60 French patients with early onset AF and in none of 389 French referents. This variant, located in the inhibitory domain 1 distal to the homeodomain, was evaluated by the software MutationTaster as a disease-causing mutation with a probability of 0.999. Reporter gene assays demonstrated that p.Met207Val caused a 3.1-fold increase in transactivation activity of PITX2c in HeLa cells in comparison with its wild-type counterpart. When the variant was coexpressed with wild-type PITX2c in the HL-1 immortalized mouse atrial cell line, this gain-of-function caused an increase in the mRNA level of KCNH2 (2.6-fold), SCN1B (1.9-fold), GJA5 (3.1-fold), GJA1 (2.1-fold), and KCNQ1 in the homozygous form (1.8-fold). These genes encode for the IKr channel α subunit, the β-1 Na+ channel subunit, connexin 40, connexin 43 and the IKs channel α subunit, respectively. These conditions may contribute to the propensity to AF found in patients carrying the p.Met207Val variant. In conclusion, the present report is the first to associate a gain-of-function mutation of PITX2c with increased vulnerability to AF, therefore, restoration of normal PITX2c function may be a potential therapeutic target in AF patients.

Genetics of Atrial Fibrilation: In Search of Novel Therapeutic Targets

Atrial fibrillation (AF) is the most frequent arrhythmogenic disease in humans, ranging from 2% in the general population and rising up to 10-12% in 80+ years. Genetic analyses of AF familiar cases have identified a series of point mutations in distinct ion channels, supporting a causative link. However, these genetic defects only explain a minority of AF patients. Genomewide association studies identified single nucleotide polymorphisms (SNPs), close to PITX2 on 4q25 chromosome, that are highly associated to AF. Subsequent GWAS studies have identified several new loci, involving additional transcription and growth factors. Furthermore, these risk 4q25 SNPs serve as surrogate biomarkers to identify AF recurrence in distinct surgical and pharmacological interventions. Experimental studies have demonstrated an intricate signalling pathway supporting a key role of the homeobox transcription factor PITX2 as a transcriptional regulator. Furthermore, cardiovascular risk factors such as hyperthyroidism, hypertension and redox homeostasis have been identified to modulate PITX2 driven gene regulatory networks. We provide herein a state-of-the-art review of the genetic bases of atrial fibrillation, our current understanding of the genetic regulatory networks involved in AF and its plausible usage for searching novel therapeutic targets.

Generation and cardiac subtype-specific differentiation of PITX2-deficient human iPS cell lines for exploring familial atrial fibrillation

Loss-of-function mutations in the PITX2 transcription factor gene have been shown to cause familial atrial fibrillation (AF). To potentially model aspects of AF and unravel PITX2-regulated downstream genes for drug target discovery, we here report the generation of integration-free PITX2-deficient hiPS cell lines. We also show that both PITX2 knockout hiPS cells and isogenic wild-type controls can selectively be differentiated into human atrial cardiomyocytes, to potentially uncover differentially expressed gene sets between these groups.

Genetic basis of atrial fibrillation

Atrial fibrillation is the most common sustained arrhythmia and remains as one of main challenges in current clinical practice. The disease may be induced secondary to other diseases such as hypertension, valvular heart disease, and heart failure, conferring an increased risk of stroke and sudden death. Epidemiological studies have provided evidence that genetic factors play an important role and up to 30% of clinically diagnosed patients may have a family history of atrial fibrillation. To date, several rare variants have been identified in a wide range of genes associated with ionic channels, calcium handling protein, fibrosis, conduction and inflammation. Important advances in clinical, genetic and molecular basis have been performed over the last decade, improving diagnosis and treatment. However, the genetics of atrial fibrillation is complex and pathophysiological data remains still unraveling. A better understanding of the genetic basis will induce accurate risk stratification and personalized clinical treatment. In this review, we have focused on current genetics basis of atrial fibrillation.

Nkx2.5/CARP signaling pathway contributes to the regulation of ion channel remodeling induced by rapid pacing in rat atrial myocytes

Remodeling of atrial electrophysiology and structure is the primary feature of atrial fibrillation (AF). Evidence suggests that abnormalities in the expression levels of embryological cardiovascular development‑associated transcription factors, including Nkx2.5, are crucial for the development of AF. Rat atrial myocardial cells (AMCs) in culture dishes were placed in an electric field and stimulated. Transmission electron microscopy was used to observe the ultrastucture prior to and following rapid pacing. The action potential durations and effective refractory periods were measured. RT‑PCR and western blotting were performed to investigate the effect of rapid pacing on the expression levels of ion channel and nuclear proteins in AMCs. Nkx2.5 short interfering RNA (siRNA) transfection was performed. Through this in vitro rat AMC culture and rapid pacing model, it was demonstrated that rapid pacing shortened the action potential and downregulated the expression levels of L‑type calcium and potassium channels. Expression levels of Nkx2.5 and cardiac ankyrin repeat protein (CARP) were significantly upregulated by rapid pacing at the mRNA and protein levels. siRNA‑mediated Nkx2.5 silencing attenuated the rapid pacing‑induced downreglation of ion channel levels. These results suggest that the Nkx2.5/CARP signaling pathway contributes to the early electrical remodeling process of AF.

Prevalence and Spectrum of TBX5 Mutation in Patients with Lone Atrial Fibrillation

Atrial fibrillation (AF), the most common type of cardiac rhythm disturbance encountered in clinical practice, is associated with substantially increased morbidity and mortality. Aggregating evidence demonstrates that abnormal cardiovascular development is involved in the pathogenesis of AF. A recent study has revealed that the TBX5 gene, which encodes a T-box transcription factor key to cardiovascular development, was associated with AF and atypical Holt-Oram syndrome. However, the prevalence and spectrum of TBX5 mutation in patients with lone AF remain unclear. In this study, the coding regions and splicing junction sites of TBX5 were sequenced in 192 unrelated patients with lone AF and 300 unrelated ethnically-matched healthy individuals used as controls. The causative potential of the identified TBX5 variation was evaluated by MutationTaster and PolyPhen-2. The functional effect of the mutant TBX5 was assayed by using a dual-luciferase reporter assay system. As a result, a novel heterozygous TBX5 mutation, p.H170D, was identified in a patient, with a mutational prevalence of approximately 0.52%. This mutation, which was absent in the 300 control individuals, altered the amino acid completely conserved evolutionarily across species, and was predicted to be disease-causing. Functional deciphers showed that the mutant TBX5 was associated with significantly reduced transcriptional activity when compared with its wild-type counterpart. Furthermore, the mutation significantly decreased the synergistic activation between TBX5 and NKX2-5 or GATA4. The findings expand the mutational spectrum of TBX5 linked to AF and provide new evidence that dysfunctional TBX5 may contribute to lone AF.

PITX2 Loss-of-Function Mutation Contributes to Congenital Endocardial Cushion Defect and Axenfeld-Rieger Syndrome

Congenital heart disease (CHD), the most common type of birth defect, is still the leading non-infectious cause of infant morbidity and mortality in humans. Aggregating evidence demonstrates that genetic defects are involved in the pathogenesis of CHD. However, CHD is genetically heterogeneous and the genetic components underpinning CHD in an overwhelming majority of patients remain unclear. In the present study, the coding exons and flanking introns of the PITX2 gene, which encodes a paired-like homeodomain transcription factor 2essential for cardiovascular morphogenesis as well as maxillary facial development, was sequenced in 196 unrelated patients with CHD and subsequently in the mutation carrier's family members available. As a result, a novel heterozygous PITX2 mutation, p.Q102X for PITX2a, or p.Q148X for PITX2b, or p.Q155X for PITX2c, was identified in a family with endocardial cushion defect (ECD) and Axenfeld-Rieger syndrome (ARS). Genetic analysis of the pedigree showed that the nonsense mutation co-segregated with ECD and ARS transmitted in an autosomal dominant pattern with complete penetrance. The mutation was absent in 800 control chromosomes from an ethnically matched population. Functional analysis by using a dual-luciferase reporter assay system revealed that the mutant PITX2 had no transcriptional activity and that the mutation eliminated synergistic transcriptional activation between PITX2 and NKX2.5, another transcription factor pivotal for cardiogenesis. To our knowledge, this is the first report on the association of PITX2 loss-of-function mutation with increased susceptibility to ECD and ARS. The findings provide novel insight into the molecular mechanisms underpinning ECD and ARS, suggesting the potential implications for the antenatal prophylaxis and personalized treatment of CHD and ARS.

NKX2-6 mutation predisposes to familial atrial fibrillation

Atrial fibrillation (AF) is the most common form of sustained cardiac arrhythmia and is associated with substantially increased morbidity and mortality rates. Aggregating evidence demonstrates that genetic defects are involved in the pathogenesis of AF and a number of AF-associated genes have been identified. Nevertheless, AF is a genetically heterogeneous disorder and the genetic components underpinning AF in an overwhelming majority of patients remain unclear. In this study, the entire coding exons and splice junction sites of the NK2 homeobox 6 (NKX2-6) gene, which encodes a homeodomain transcription factor important for cardiovascular development, were sequenced in 150 unrelated patients with lone AF, and a novel heterozygous NKX2-6 mutation, p.Q175H, was identified in an index patient. Genetic analysis of the available family members of the mutation carrier revealed that the mutation co-segregated with AF transmitted in an autosomal dominant pattern. The missense mutation was absent in the 200 unrelated ethnically matched healthy individuals used as controls and the altered amino acid was completely conserved evolutionarily among species. Due to unknown transcriptional targets of NKX2-6, the functional characteristics of the mutation as regards transcriptional activity were analyzed using NKX2-5 as a surrogate. Alignment between human NKX2-6 and NKX2-5 proteins displayed that the Q175H-mutant NKX2-6 was equivalent to the Q181H-mutant NKX2-5, and the introduction of Q181H into NKX2-5 significantly decreased its transcriptional activity at the atrial natriuretic factor promoter. The present study firstly associates genetically defective NKX2-6 with enhanced susceptibility to AF, providing novel insight into the molecular mechanisms underlying AF and suggesting potential strategies for the antenatal prophylaxis and personalized treatment of AF.

GATA6 loss-of-function mutations contribute to familial dilated cardiomyopathy

Dilated cardiomyopathy (DCM), the most prevalent form of primary heart muscle disease, is the third most common cause of heart failure and the most frequent reason for cardiac transplantation. Mounting evidence has demonstrated that genetic risk factors are crucial in the pathogenesis of DCM. However, DCM is genetically heterogeneous, and the genetic basis of DCM in a large majority of cases remains unclear. In the current study, the coding exons and flanking introns of the GATA6 gene, which encodes a zinc‑finger transcription factor essential for cardiogenesis, was sequenced in 140 unrelated patients with DCM, and two novel heterozygous mutations, p.C447Y and p.H475R, were identified in two index patients with DCM, respectively. Analysis of the pedigrees showed that in each family the mutation co-segregated with DCM transmitted in an autosomal-dominant pattern, with complete penetrance. The missense mutations were absent in 400 control chromosomes and predicted to be disease-causing by MutationTaster or probably damaging by PolyPhen-2. The alignment of multiple GATA6 proteins across species revealed that the altered amino acids were completely conserved evolutionarily. The functional assays showed that the mutated GATA6 proteins were associated with significantly reduced transcriptional activation in comparison with their wild-type counterpart. To the best of our knowledge, this is the first study on the association of GATA6 loss-of-function mutations with enhanced susceptibility to familial DCM, which provides novel insight into the molecular mechanism of DCM and suggests potential implications for the antenatal prophylaxis and allele-specific treatment of DCM.

Emerging directions in the genetics of atrial fibrillation

Atrial fibrillation (AF) is the most common arrhythmia and is associated with increased morbidity. As the population ages and the prevalence of AF continues to rise, the socioeconomic consequences of AF will become increasingly burdensome. Although there are well-defined clinical risk factors for AF, a significant heritable component is also recognized. To identify the molecular basis for the heritability of AF, investigators have used a combination of classical Mendelian genetics, candidate gene screening, and genome-wide association studies. However, these avenues have, as yet, failed to define the majority of the heritability of AF. The goal of this review is to describe the results from both candidate gene and genome-wide studies, as well as to outline potential future avenues for creating a more complete understanding of AF genetics. Ultimately, a more comprehensive view of the genetic underpinnings for AF will lead to the identification of novel molecular pathways and improved risk prediction of this complex arrhythmia.

Novel PITX2c loss-of-function mutations associated with complex congenital heart disease

Congenital heart disease (CHD) is the most common form of birth defect in humans and is the leading non-infectious cause of infant mortality. Emerging evidence strongly suggests that genetic risk factors play an important role in the pathogenesis of CHD. However, CHD is of pronounced genetic heterogeneity, and the genetic defects responsible for CHD in an overwhelming majority of patients remain unclear. In this study, the entire coding region and splice junction sites of the PITX2c gene, which encodes a paired-like homeodomain transcription factor crucial for proper cardiovascular morphogenesis, was sequenced in 170 unrelated neonates with CHD. The available relatives of the mutation carriers and 200 unrelated ethnically matched healthy individuals were genotyped. The disease-causing potential of the PITX2c sequence variations was predicted by MutationTaster and PolyPhen-2. The functional effect of the mutations was characterized using a luciferase reporter assay system. As a result, 2 novel heterozygous PITX2c mutations, p.R91Q and p.T129S, were identified in 2 unrelated newborns with transposition of the great arteries and ventricular septal defect, respectively. A genetic scan of the pedigrees revealed that each mutation co-segregated with CHD transmitted in an autosomal dominant pattern with complete penetrance. The mutations, which altered the amino acids completely conserved evolutionarily, were absent in 400 normal chromosomes and were predicted to be causative. Functional analysis revealed that the PITX2c mutations were both associated with significantly diminished transcriptional activity compared with their wild-type counterpart. This study demonstrates the association between PITX2c loss-of-function mutations and the transposition of the great arteries and ventricular septal defect in humans, providing further insight into the molecular mechanisms responsible for CHD.

Pitx2c is reactivated in the failing myocardium and stimulates myf5 expression in cultured cardiomyocytes

BACKGROUND

Pitx2 (paired-like homeodomain 2 transcription factor) is crucial for heart development, but its role in heart failure (HF) remains uncertain. The present study lays the groundwork implicating Pitx2 signalling in different modalities of HF.

METHODOLOGY/PRINCIPAL FINDINGS

A variety of molecular, cell-based, biochemical, and immunochemical assays were used to evaluate: (1) Pitx2c expression in the porcine model of diastolic HF (DHF) and in patients with systolic HF (SHF) due to dilated and ischemic cardiomyopathy, and (2) molecular consequences of Pitx2c expression manipulation in cardiomyocytes in vitro. In pigs, the expression of Pitx2c, physiologically downregulated in the postnatal heart, is significantly re-activated in left ventricular (LV) failing myocardium which, in turn, is associated with increased expression of a restrictive set of Pitx2 target genes. Among these, Myf5 was identified as the top upregulated gene. In vitro, forced expression of Pitx2c in cardiomyocytes, but not in skeletal myoblasts, activates Myf5 in dose-dependent manner. In addition, we demonstrate that the level of Pitx2c is upregulated in the LV-myocardium of SHF patients.

CONCLUSIONS/SIGNIFICANCE

The results provide previously unrecognized evidence that Pitx2c is similarly reactivated in postnatal/adult heart at distinct HF phenotypes and suggest that Pitx2c is involved, directly or indirectly, in the regulation of Myf5 expression in cardiomyocytes.

A novel PITX2c loss-of-function mutation associated with familial atrial fibrillation

Atrial fibrillation (AF) represents the most prevalent form of sustained cardiac arrhythmia and contributes substantially to cardiovascular morbidity and mortality. Aggregating evidence demonstrates that genetic risk factors play an important role in the pathogenesis of AF. However, AF is a genetically heterogeneous disease and the genetic defects responsible for AF in an overwhelming majority of patients remain unclear. In the present study, the whole coding region and splice junction sites of the PITX2c gene, which encodes a paired-like homeobox transcription factor essential for normal cardiovascular development, were sequenced in 160 unrelated patients with lone AF, and a novel heterozygous mutation, c.349C > T equivalent to p.P117S, was identified in a patient with positive family history of AF. The missense mutation, which co-segregated with AF in the family with complete penetrance and was absent in 700 unrelated ethnically matched healthy individuals, altered the amino acid completely conserved evolutionarily across species and was predicted to be pathogenic by MutationTaster and PolyPhen-2. Biological assays revealed that the mutant PITX2c protein was associated with significantly decreased transcriptional activity when compared with its wild-type counterpart. The findings implicate PITX2c loss-of-function mutation in familial AF for the first time, providing novel insight into the molecular pathology of AF.

Prevalence and spectrum of PITX2c mutations associated with congenital heart disease

Congenital heart disease (CHD) is the most common form of birth defect and is the leading noninfectious cause of infant death. A growing body of evidence demonstrates that genetic risk factors are involved in the pathogenesis of CHD. However, CHD is a genetically heterogeneous disease and the genetic defects underlying CHD in an overwhelming majority of patients remain unclear. In this study, the whole coding region and splice junction sites of the PITX2c gene, which encodes variant 3 of paired-like homeodomain transcription factor 2 crucial for normal cardiovascular morphogenesis, were sequenced in 382 unrelated patients with CHD, and 2 novel heterozygous mutations, p.W147X and p.N153D, were identified in 2 unrelated patients with CHD, respectively, including a 1-year-old male patient with double outlet right ventricle in combination with ventricular septal defect and a 4-year-old female patient with ventricular septal defect. The mutations were absent in 400 control chromosomes and were both predicted to be disease-causing by MutationTaster. Multiple alignments of PITX2c proteins across species displayed that the altered amino acids were completely conserved evolutionarily. Functional analysis revealed that the mutated PITX2c proteins were associated with a significantly reduced transactivational activity compared with their wild-type counterpart. These findings provide a novel insight into the molecular mechanisms implicated in CHD, suggesting potential implications for the antenatal prophylaxis and allele-specific treatment of CHD.