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From the onset of the pandemic, evidence of cardiac involvement in acute COVID-19 abounded. Cardiac presentations ranged from arrhythmias to ischemia, myopericarditis/myocarditis, ventricular dysfunction to acute heart failure, and even cardiogenic shock. Elevated serum cardiac troponin levels were prevalent among hospitalized patients with COVID-19; the higher the magnitude of troponin elevation, the greater the COVID-19 illness severity and in-hospital death risk. Whether these consequences were due to direct SARS-CoV-2 infection of cardiac cells or secondary to inflammatory responses steered early cardiac autopsy studies. SARS-CoV-2 was reportedly detected in endothelial cells, cardiac myocytes, and within the extracellular space. However, findings were inconsistent and different methodologies had their limitations. Initial autopsy reports suggested that SARS-CoV-2 myocarditis was common, setting off studies to find and phenotype inflammatory infiltrates in the heart. Nonetheless, subsequent studies rarely detected myocarditis. Microthrombi, cardiomyocyte necrosis, and inflammatory infiltrates without cardiomyocyte damage were much more common. In vitro and ex vivo experimental platforms have assessed the cellular tropism of SARS-CoV-2 and elucidated mechanisms of viral entry into and replication within cardiac cells. Data point to pericytes as the primary target of SARS-CoV-2 in the heart. Infection of pericytes can account for the observed pericyte and endothelial cell death, innate immune response, and immunothrombosis commonly observed in COVID-19 hearts. These processes are bidirectional and synergistic, rendering a definitive order of events elusive. Single-cell/nucleus analyses of COVID-19 myocardial tissue and isolated cardiac cells have provided granular data about the cellular composition and cell type–specific transcriptomic signatures of COVID-19 and microthrombi-positive COVID-19 hearts. Still, much remains unknown and more in vivo studies are needed. This review seeks to provide an overview of the current understanding of COVID-19 cardiac pathophysiology. Cell type–specific mechanisms and the studies that provided such insights will be highlighted. Given the unprecedented pace of COVID-19 research, more mechanistic details are sure to emerge since the writing of this review. Importantly, our current knowledge offers significant clues about the cardiac pathophysiology of long COVID-19, the increased postrecovery risk of cardiac events, and thus, the future landscape of cardiovascular disease.

Cardiac allograft vasculopathy (CAV) is the leading cause of late allograft failure and mortality after heart transplantation. As current standards of diagnosis and treatment of CAV have significant limitations, understanding cell-specific responses may prove critical for developing improved detection strategies and novel therapeutics. This study is the first to successfully utilize human endomyocardial biopsy (EMB) samples to isolate large numbers of intact nuclei for single-nuclear transcriptomics. These data also lay the groundwork for ongoing experiments to study serial, routinely-collected EMB specimens after heart transplantation to identify novel biomarkers and pathways through which early CAV pathogenesis can be interrupted, thereby prolonging allograft survival.

Approximately 50% of all heart failure (HF) diagnoses can be classified as HF with preserved ejection fraction (HFpEF). HFpEF is more prevalent in females compared with males, but the underlying mechanisms are unknown. We previously showed that pressure overload (PO) in male felines induces a cardiopulmonary phenotype with essential features of human HFpEF. The goal of this study was to determine if slow progressive PO induces distinct cardiopulmonary phenotypes in females and males in the absence of other pathological stressors. Female and male felines underwent aortic constriction (banding) or sham surgery after baseline echocardiography, pulmonary function testing, and blood sampling. These assessments were repeated at 2 and 4 mo postsurgery to document the effects of slow progressive pressure overload. At 4 mo, invasive hemodynamic studies were also performed. Left ventricle (LV) tissue was collected for histology, myofibril mechanics, extracellular matrix (ECM) mass spectrometry, and single-nucleus RNA sequencing (snRNAseq). The induced pressure overload (PO) was not different between sexes. PO also induced comparable changes in LV wall thickness and myocyte cross-sectional area in both sexes. Both sexes had preserved ejection fraction, but males had a slightly more robust phenotype in hemodynamic and pulmonary parameters. There was no difference in LV fibrosis and ECM composition between banded male and female animals. LV snRNAseq revealed changes in gene programs of individual cell types unique to males and females after PO. Based on these results, both sexes develop cardiopulmonary dysfunction but the phenotype is somewhat less advanced in females.

A pandemic of historic impact, coronavirus disease 2019 (COVID-19) has potential consequences on the cardiovascular health of millions of people who survive infection worldwide. Severe acute respiratory syndrome-coronavirus 2 (SARS-CoV-2), the etiologic agent of COVID-19, can infect the heart, vascular tissues, and circulating cells through ACE2 (angiotensin-converting enzyme 2), the host cell receptor for the viral spike protein. Acute cardiac injury is a common extrapulmonary manifestation of COVID-19 with potential chronic consequences. This update provides a review of the clinical manifestations of cardiovascular involvement, potential direct SARS-CoV-2 and indirect immune response mechanisms impacting the cardiovascular system, and implications for the management of patients after recovery from acute COVID-19 infection.

Atrial fibrillation (AF) affects over 1% of the population and is a leading cause of stroke and heart failure in the elderly. A feared side effect of sodium channel blocker therapy, ventricular pro-arrhythmia, appears to be relatively rare in patients with AF. The biophysical reasons for this relative safety of sodium blockers are not known.\ud \ud Our data demonstrates intrinsic differences between atrial and ventricular cardiac voltage-gated sodium currents (INa), leading to reduced maximum upstroke velocity of action potential and slower conduction, in left atria compared to ventricle. Reduced atrial INa is only detected at physiological membrane potentials and is driven by alterations in sodium channel biophysical properties and not by NaV1.5 protein expression. Flecainide displayed greater inhibition of atrial INa, greater reduction of maximum upstroke velocity of action potential, and slowed conduction in atrial cells and tissue.\ud \ud Our work highlights differences in biophysical properties of sodium channels in left atria and ventricles and their response to flecainide. These differences can explain the relative safety of sodium channel blocker therapy in patients with atrial fibrillation.

Genome-wide association studies have uncovered over a 100 genetic loci associated with atrial fibrillation (AF), the most common arrhythmia. Many of the top AF-associated loci harbor key cardiac transcription factors, including PITX2, TBX5, PRRX1, and ZFHX3. Moreover, the vast majority of the AF-associated variants lie within noncoding regions of the genome where causal variants affect gene expression by altering the activity of transcription factors and the epigenetic state of chromatin. In this review, we discuss a transcriptional regulatory network model for AF defined by effector genes in Genome-wide association studies loci. We describe the current state of the field regarding the identification and function of AF-relevant gene regulatory networks, including variant regulatory elements, dose-sensitive transcription factor functionality, target genes, and epigenetic states. We illustrate how altered transcriptional networks may impact cardiomyocyte function and ionic currents that impact AF risk. Last, we identify the need for improved tools to identify and functionally test transcriptional components to define the links between genetic variation, epigenetic gene regulation, and atrial function.

Introduction: Approximately 50% of all heart failure (HF) diagnoses can be classified as HF with preserved ejection fraction (HFpEF), which has no FDA approved therapies. HFpEF is more prevalent in females compared to males, but the underlying mechanisms for the development as a sex-based disorder are unknown. We previously described how slow progressive pressure overload (PO) in male felines recapitulates the HFpEF phenotype but have not investigated the female phenotype. Hypothesis: Females will develop a less severe HFpEF phenotype compared to males under the same pathological stress. Methods & Results: Male (m) and female (f) domestic short felines (age 2mo) underwent aortic constriction (m: n=11; f: n=10) using a customized pre-shaped band or a sham procedure (m: n=7; f: n=7). Before surgery (baseline), there was no difference in body weight (BW) between groups and lung compliance was not different. Echocardiography revealed no significant difference in the ratio of left atrium to aortic root (LA/Ao), LA ejection fraction (LA EF), left ventricle (LV) ejection fraction, LV wall-thickness, and E/A ratio. By 4mo post-surgery, both males and females had developed cardiac dysfunction and decreased lung compliance. At this time, females had significantly smaller BWs than males. Despite the difference in BW, LV wall thickness and changes in E/A ratio were similar in both sexes in banded vs. shams. Importantly, LV EF did not change in any group. There was a decrease in LA EF and increased LA/Ao in all banded animals. Invasive hemodynamics at 4mo post-surgery showed no differences between sexes for the systolic pressure gradient generated by aortic banding. Banded males had higher LV end-diastolic pressure vs. banded females (m: 15.0±2.7mmHg; f: 8.1±1.9mmHg). However, there was a trend towards prolongation of tau and lower dp/dt min in banded females, suggestive of abnormal relaxation. There were no differences between banded males and females in heart weight/BW or cardiomyocyte cross-sectional area and both developed fibrosis. Conclusions: Exposure of male and female felines to PO resulted in similar cardiac hypertrophy, fibrosis, and decreased lung compliance. Females had lower LVEDP than males suggesting they may be protected from diastolic dysfunction.

Rationale: Genome-wide association studies have identified a large number of common variants (single-nucleotide polymorphisms) associated with atrial fibrillation (AF). These variants are located mainly in noncoding regions of the genome and likely include variants that modulate the function of transcriptional regulatory elements (REs) such as enhancers. However, the actual REs modulated by variants and the target genes of such REs remain to be identified. Thus, the biological mechanisms by which genetic variation promotes AF has thus far remained largely unexplored. Objective: To identify REs in genome-wide association study loci that are influenced by AF-associated variants. Methods and Results: We screened 2.45 Mbp of human genomic DNA containing 12 strongly AF-associated loci for RE activity using self-transcribing active regulatory region sequencing and a recently generated monoclonal line of conditionally immortalized rat atrial myocytes. We identified 444 potential REs, 55 of which contain AF-associated variants ( P −8 ). Subsequently, using an adaptation of the self-transcribing active regulatory region sequencing approach, we identified 24 variant REs with allele-specific regulatory activity. By mining available chromatin conformation data, the possible target genes of these REs were mapped. To define the physiological function and target genes of such REs, we deleted the orthologue of an RE containing noncoding variants in the Hcn4 (potassium/sodium hyperpolarization-activated cyclic nucleotide-gated channel 4) locus of the mouse genome. Mice heterozygous for the RE deletion showed bradycardia, sinus node dysfunction, and selective loss of Hcn4 expression. Conclusions: We have identified REs at multiple genetic loci for AF and found that loss of an RE at the HCN4 locus results in sinus node dysfunction and reduced gene expression. Our approach can be broadly applied to facilitate the identification of human disease-relevant REs and target genes at cardiovascular genome-wide association studies loci.

Background: The human heart requires a complex ensemble of specialized cell types to perform its essential function. A greater knowledge of the intricate cellular milieu of the heart is critical to increase our understanding of cardiac homeostasis and pathology. As recent advances in low-input RNA sequencing have allowed definitions of cellular transcriptomes at single-cell resolution at scale, we have applied these approaches to assess the cellular and transcriptional diversity of the nonfailing human heart. Methods: Microfluidic encapsulation and barcoding was used to perform single nuclear RNA sequencing with samples from 7 human donors, selected for their absence of overt cardiac disease. Individual nuclear transcriptomes were then clustered based on transcriptional profiles of highly variable genes. These clusters were used as the basis for between-chamber and between-sex differential gene expression analyses and intersection with genetic and pharmacologic data. Results: We sequenced the transcriptomes of 287 269 single cardiac nuclei, revealing 9 major cell types and 20 subclusters of cell types within the human heart. Cellular subclasses include 2 distinct groups of resident macrophages, 4 endothelial subtypes, and 2 fibroblast subsets. Comparisons of cellular transcriptomes by cardiac chamber or sex reveal diversity not only in cardiomyocyte transcriptional programs but also in subtypes involved in extracellular matrix remodeling and vascularization. Using genetic association data, we identified strong enrichment for the role of cell subtypes in cardiac traits and diseases. Intersection of our data set with genes on cardiac clinical testing panels and the druggable genome reveals striking patterns of cellular specificity. Conclusions: Using large-scale single nuclei RNA sequencing, we defined the transcriptional and cellular diversity in the normal human heart. Our identification of discrete cell subtypes and differentially expressed genes within the heart will ultimately facilitate the development of new therapeutics for cardiovascular diseases.

Atrial fibrillation (AF) is the most common cardiac arrhythmia with well-established clinical and genetic risk components. Genome-wide association studies (GWAS) have identified 17 independent susceptibility signals for AF at 14 genomic regions, but the mechanisms through which these loci confer risk to AF remain largely undefined. This problem is not unique to AF, as the field of functional genomics, which attempts to bridge this gap from genotype to phenotype, has only uncovered the mechanisms for a handful of GWAS loci. Recent functional genomic studies have made great strides towards translating genetic discoveries to an underlying mechanism, but the large-scale application of these techniques to AF has remain limited. These advances, as well as the continued unresolved challenges for both common variation in AF and the functional genomics field in general, will be the subject of the following review.

Background— Atrial fibrillation (AF) affects over 33 million individuals worldwide. Genome-wide association studies have identified at least 30 AF loci, but the mechanisms through which individual variants lead to altered disease risk have remained unclear for the majority of these loci. At the 1q24 locus, we hypothesized that the transcription factor PRRX1 could be a strong candidate gene as it is expressed in the pulmonary veins, a source of AF in many individuals. We sought to identify the molecular mechanism, whereby variation at 1q24 may lead to AF susceptibility. Methods and Results— We sequenced a ≈158 kb region encompassing PRRX1 in 962 individuals with and without AF. We identified a broad region of association with AF at the 1q24 locus. Using in silico prediction and functional validation, we identified an enhancer that interacts with the promoter of PRRX1 in cells of cardiac lineage. Within this enhancer, we identified a single-nucleotide polymorphism, rs577676, which alters enhancer activity in a mouse atrial cell line and in embryonic zebrafish and differentially regulates PRRX1 expression in human left atria. We found that suppression of PRRX1 in human embryonic stem cell–derived cardiomyocytes and embryonic zebrafish resulted in shortening of the atrial action potential duration, a hallmark of AF. Conclusions— We have identified a functional genetic variant that alters PRRX1 expression, ultimately resulting in electrophysiological alterations in atrial myocytes that may promote AF.

Background— Restrictive cardiomyopathy (RCM) is a rare cardiomyopathy characterized by impaired diastolic ventricular function resulting in a poor clinical prognosis. Rarely, heritable forms of RCM have been reported, and mutations underlying RCM have been identified in genes that govern the contractile function of the cardiomyocytes. Methods and Results— We evaluated 8 family members across 4 generations by history, physical examination, electrocardiography, and echocardiography. Affected individuals presented with a pleitropic syndrome of progressive RCM, atrioventricular septal defects, and a high prevalence of atrial fibrillation. Exome sequencing of 5 affected members identified a single novel missense variant in a highly conserved residue of FLNC (filamin C; p.V2297M). FLNC encodes filamin C—a protein that acts as both a scaffold for the assembly and organization of the central contractile unit of striated muscle and also as a mechanosensitive signaling molecule during cell migration and shear stress. Immunohistochemical analysis of FLNC localization in cardiac tissue from an affected family member revealed a diminished localization at the z disk, whereas traditional localization at the intercalated disk was preserved. Stem cell-derived cardiomyocytes mutated to carry the effect allele had diminished contractile activity when compared with controls. Conclusion— We have identified a novel variant in FLNC as pathogenic variant for familial RCM—a finding that further expands on the genetic basis of this rare and morbid cardiomyopathy.

Atrial fibrillation (AF) is the most common arrhythmic disorder, and currently affects nearly 3 million Americans, 8.8 million Europeans, and an estimated 30 million individuals worldwide. The clinical risk factors for AF are numerous, with age, sex, hypertension, obesity, and ischemic heart disease among the most prevalent. Over the last ten years, a preponderance of evidence also suggests a large genetic contribution to AF. The earliest report of familial AF dates to the early 1940s1. Since then, it has become apparent that AF in referral populations2,3 and in the community is heritable4,5. Indeed, having a family member with AF is associated with a 40% increased risk for the arrhythmia6. Once the heritability was recognized, traditional genetics techniques for the discovery of rare, monogenic causes of AF were used to identify the initial AF genes. These studies in turn, informed candidate gene screening in AF cohorts. To identify additional sources of heritability for AF, large-scale analyses of common variation through genome wide association studies (GWAS) has recently yielded data identifying risk loci in many regions of the genome. In spite of these advances, the combination of these techniques has, as yet, failed to completely identify the heritability of AF in the population. It is the goal of this review to examine the previous studies on rare variants, address the findings of the recent GWAS studies, and describe future avenues towards defining the heritability of AF.

Objective: Nonsyndromic Mitral valve prolapse (MVP) is a common degenerative valvular heart disease with severe health consequences, including arrhythmia, heart failure and sudden death. MVP is cha...

Background A recent genome-wide association study identified a susceptibility locus for atrial fibrillation at the KCNN3 gene. Since the KCNN3 gene encodes for a small conductance calcium-activated potassium channel, we hypothesized that overexpression of the SK3 channel increases susceptibility to cardiac arrhythmias. Methods and results We characterized the cardiac electrophysiological phenotype of a mouse line with overexpression of the SK3 channel. We generated homozygote ( SK3T/T ) and heterozygote ( SK3+/T ) mice with overexpression of the channel and compared them with wild-type (WT) controls. We observed a high incidence of sudden death among SK3T/T mice (7 of 19 SK3T/T mice). Ambulatory monitoring demonstrated that sudden death was due to heart block and bradyarrhythmias. SK3T/T mice displayed normal body weight, temperature, and cardiac function on echocardiography; however, histological analysis demonstrated that these mice have abnormal atrioventricular node morphology. Optical mapping demonstrated that SK3T/T mice have slower ventricular conduction compared with WT controls ( SK3T/T vs. WT; 0.45 ± 0.04 vs. 0.60 ± 0.09 mm/ms, P = 0.001). Programmed stimulation in 1-month-old SK3T/T mice demonstrated inducible atrial arrhythmias (50% of SK3T/T vs. 0% of WT mice) and also a shorter atrioventricular nodal refractory period ( SK3T/T vs. WT; 43 ± 6 vs. 52 ± 9 ms, P = 0.02). Three-month-old SK3T/T mice on the other hand displayed a trend towards a more prolonged atrioventricular nodal refractory period ( SK3T/T vs. WT; 61 ± 1 vs. 52 ± 6 ms, P = 0.06). Conclusion Overexpression of the SK3 channel causes an increased risk of sudden death associated with bradyarrhythmias and heart block, possibly due to atrioventricular nodal dysfunction.

Background The genetic basis of atrial fibrillation (AF) and congenital heart disease remains incompletely understood. Objective We sought to determine the causative mutation in a family with AF, atrial septal defects, and ventricular septal defects. Methods We evaluated a pedigree with 16 family members, 1 with an atrial septal defect, 1 with a ventricular septal defect, and 3 with AF; we performed whole exome sequencing in 3 affected family members. Given that early-onset AF was prominent in the family, we then screened individuals with early-onset AF, defined as an age of onset GATA6 . Variants were functionally characterized using reporter assays in a mammalian cell line. Results Exome sequencing in 3 affected individuals identified a conserved mutation, R585L, in the transcription factor gene GATA6 . In the Massachusetts General Hospital Atrial Fibrillation (MGH AF) Study, the mean age of AF onset was 47.1 ± 10.9 years; 79% of the participants were men; and there was no evidence of structural heart disease. We identified 3 GATA6 variants (P91S, A177T, and A543G). Using wild-type and mutant GATA6 constructs driving atrial natriuretic peptide promoter reporter, we found that 3 of the 4 variants had a marked upregulation of luciferase activity (R585L: 4.1-fold, P P = .0002; A177T; 1.7-fold, P = .03). In addition, when co-overexpressed with GATA4 and MEF2C, GATA6 variants exhibited upregulation of the alpha myosin heavy chain and atrial natriuretic peptide reporter activity. Conclusion Overall, we found gain-of-function mutations in GATA6 in both a family with early-onset AF and atrioventricular septal defects as well as in a family with sporadic, early-onset AF.

Background: To date, 17 loci associated with atrial fibrillation (AF) have been identified, and there are 4 regions at 4q25 around PITX2 gene strongly associated with AF. However, the functional variants at all known AF loci remain elusive. Methods and Results: Among the 4 AF loci at 4q25, we focused on one associated region of ~84 kb extending from Chr4: 111,520,926 to 111,604,727 that includes the PITX2 gene itself (defined as all SNPs with r 2 > 0.6 from the sentinel SNP rs1448818). To identify potential functional variants , we analyzed DNase hypersensitivity, histone modification and mammalian conservation from ENCODE and Roadmap Epigenomics Project. Additionally, cap analysis of gene expression (CAGE) data was also investigated. We identified 13 candidate regions, 5 of which showed enhancer activity in zebrafish skeletal muscle and heart. Using luciferase assays in a mouse atrial cell line (HL-1), we then identified one SNP (rs2595104) in which the T risk allele was associated with 31% decreased activity compared to the G non-risk allele (p = 0.007). rs2595104 lies ~10 kb upstream of PITX2c transcriptional start site and is in high linkage disequilibrium (LD) (r 2 = 0.67) with rs1448818. In silico investigation of rs2595104 using UniPROBE and HaploReg database revealed differential binding of each allele to activating enhancer binding protein 2 alpha (TFAP2a). Electrophoretic mobility shift assays showed that TFAP2a bound robustly to the non-risk G allele at rs2595104 but not to the risk T allele. Chromatin immunopricipitation followed by allele-specific qPCR in human embryonic stem cell-derived cardiomyocytes (hESC-CM) further suggested that TFAP2a was preferentially recruited to the non-risk G allele at rs2595104 in an AF-relevant cell type. hESC-CMs with homozygous deletion of rs2595104-containing enhancer using CRSPR-Cas9 expressed less PITX2c (35 ± 6%, p = 0.036), compared to the wild-type hESC-CMs. Finally, mutating the G allele at rs2595104 to T allele by CRSPR-Cas9 led to 21% reduction in PITX2c level in hESC-CMs (p = 0.027). Conclusion: We have found that the AF-associated SNP rs2595104 alters PITX2c expression via interaction with TFAP2a. Such a pathway may ultimately contribute to AF susceptibility at the PITX2 locus.

Introduction: Genome-wide association studies have identified 14 independent loci for atrial fibrillation (AF). The 4q25 locus upstream of the left-right asymmetry gene PITX2 is, by far, the strongest association signal for AF. However, as with most GWAS loci, the functional variants are noncoding, presumed to be regulatory, and remain unknown. We therefore sought to rapidly identify the functional variants at an AF locus by combining high throughput sequencing and massively parallel reporter assays. Methods and Results: We sequenced a ~750kb region encompassing the PITX2 locus in 462 individuals with early-onset AF from the MGH AF Study and 464 referents from the Framingham Heart Study. The SNP most significantly associated with AF in our sequenced sample was rs2129983, which is 140kb from PITX2 (OR=2.43, P =8.9X10 -16 ). rs2129983 is approximately 1.7kb from the most significantly associated SNP in a prior AF GWAS, rs6817105 (r 2 =0.52). From the targeted sequencing analysis, we identified 262 SNVs with a MAF >0.5% within a genomic region bounded by SNPs with an r2 greater than 0.4 with the top variant. To identify functional variants, we then utilized a massively parallel reporter assay (MPRA) in order to measure enhancer activity at each SNP across the entire AF locus. In both HL-1 and C2C12 myoblasts, MPRA identified many distinct SNP regions with differential enhancer activity. Using AF-association status as a standard, we were able to identify a series of variants that have both differential activity in either cell line tested and also a high level of association (rs17042076, rs4469143). Mechanistically, these functional SNPs are predicted to alter transcription factor binding. Conclusions: We have comprehensively identified the AF-associated variation at 4q25 and determined which of these variants are functional through differential enhancer activity. Here, in addition to identifying the causative variation for AF at 4q25, we provide a generalizable pathway for translating this work to other loci, a method that could expedite the identification of causative genetic variants at other disease loci.

Introduction: Genome-wide association studies have identified 9 genomic loci associated with atrial fibrillation (AF). Hypothesis: We sought to identify the functional variant at the 1q24 locus for AF, located upstream of the paired related homeobox 1 gene ( PRRX1 ). Methods: We used morpholino-mediated knockdown in zebrafish to assess the role of PRRX1 in cardiac function and development. To identify potential enhancers at the PRRX1 locus we analyzed DNase hypersensitivity, histone methylation, and mammalian conservation data from ENCODE. Tissue-specific enhancer activity was evaluated by microinjection of eGFP reporter constructs for each putative enhancer into zebrafish and luciferase assays in a mouse atrial myocyte (HL-1) cell line. To determine physical interaction between the AF-associated enhancer and PRRX1 promoter we analyzed available Hi-C data and performed chromatin conformation capture (3C). The functional SNP was localized using luciferase assays in HL-1 cells. The effect of the functional SNP on gene expression in human left atrial tissue was measured by qPCR. Results: Knockdown of the PRRX1 ortholog in zebrafish resulted in atrial dilation and shortening of atrial action potential duration (APD 80 : 114.8±2.2ms vs 126±1.5ms in controls, p=0.0004). Of the 4 regions tested at the 1q24 locus, 2 adjacent regions exhibited enhancer activity in the zebrafish myocardium. 3C demonstrated an increased interaction frequency between the enhancer and PRRX1 promoter regions in cells of cardiac lineage when compared to controls (103±57%, p=0.038). Screening for functional SNPs within these regions revealed that the AF risk allele (C) at SNP rs577676 associated with ~4 fold increased enhancer activity as compared to the non-risk (T) allele in HL-1 cells. Finally, regional eQTL analysis of human atrial tissue showed that rs577676 correlated with PRRX1 expression. Conclusions: We have implicated PRRX1 in cardiac electrophysiology by demonstrating that knockdown of the gene results in atrial dilation and shortening of atrial action potential duration. Further, we have found that SNP rs577676 modifies an enhancer regulating PRRX1 expression.

Background Atrial fibrillation (AF) is the most common arrhythmia, and a recent genome-wide association study identified the hyperpolarization-activated cyclic nucleotide-gated channel 4 (HCN4) as a novel AF susceptibility locus. HCN4 encodes for the cardiac pacemaker channel, and HCN4 mutations are associated with familial sinus bradycardia and AF. Objective The purpose of this study was to determine whether novel variants in the coding region of HCN4 contribute to the susceptibility for AF. Methods We sequenced the coding region of HCN4 for novel variants from 527 cases with early-onset AF from the Massachusetts General Hospital AF Study and 443 referents from the Framingham Heart Study. We used site-directed mutagenesis, cellular electrophysiology, immunocytochemistry, and confocal microscopy to functionally characterize novel variants. Results We found the frequency of novel coding HCN4 variants was 2-fold greater for individuals with AF (7 variants) compared to the referents (3 variants). We determined that of the 7 novel HCN4 variants in our AF cases, 1 (p.Pro257Ser, located in the amino-terminus adjacent to the first transmembrane spanning domain) did not traffic to cell membrane, whereas the remaining 6 were not functionally different from wild type. In addition, the 3 novel variants in our referents did not alter function compared to wild-type. Coexpression studies showed that the p.Pro257Ser mutant channel failed to colocalize with the wild-type HCN4 channel on the cell membrane. Conclusion Our findings are consistent with HCN4 haploinsufficiency as the likely mechanism for early-onset AF in the p.Pro257Ser carrier.

Background: Recently, the gene responsible for encoding SK3, KCNN3, was implicated in a genome-wide association study as a susceptibility locus for atrial fibrillation. SK3 is one of a family of three small conductance, voltage-independent, calcium-activated potassium channels which is abundantly expressed in cardiac tissues. Therefore, we hypothesized that transcriptional misregulation of the KCNN3 gene could result in arrhythmogenesis. Methods: We characterized a mouse line which had constitutive ubiquitous overexpression of Kcnn3 using ambulatory cardiac rhythm monitoring, optical mapping and in vivo electrophysiology studies. Both homozygote (SK3 T/T ) and heterozygote (SK3 T/+ ) animals were compared to their wild-type (SK3 +/+ ) littermates for assessment of cardiac phenotypes. Results: SK3 T/T mice displayed increased expression of SK3, as assessed by qRT-PCR, in all four chambers of the heart, although highest increase of expression was observed in the ventricles. Examinations of SK3 T/T mice revealed no gross morphological changes in the myocardium, no observable cardiac fibrosis, and normal echocardiograms when compared to SK3 +/+ mice. However, pups from SK3 T/+ crosses did not produce expected Mendelian ratios (SK3 +/+ = 17%), and 7 out of 19 SK3 +/+ mice died suddenly by 3 months of age, whereas all of the SK3 T/+ or SK3 +/+ survived. To address potential mechanisms, we performed ambulatory monitoring on mice beginning at approximately one month of age. Of the 6 SK3 +/+ mice, 4 died suddenly, whereas neither of the wild-type controls expired. The cardiac rhythm recorded at the time of death in three of the four mice was heart block followed by severe bradycardia. Additionally, during the period of ambulatory monitoring, SK3 T/T mice had a lower mean heart rate compared to the SK3 +/+ control mice, and SK3 T/T mice also displayed frequent episodes of atrioventricular dissociation, both at rest and during periods of activity. SK3 T/T mice also displayed more pronounced variability of the heart rate and the PR interval. Optical mapping revealed slower ventricular conduction velocity. Conclusion: These data suggest a mechanism whereby overexpression of SK3 leads to sudden death due to bradyarrhythmias and heart block.

Fibromuscular dysplasia (FMD) is a nonatherosclerotic vascular disease leading to stenosis, dissection and aneurysm affecting mainly the renal and cerebrovascular arteries. FMD has higher prevalence in females (80-90%) and predisposes to hypertension and stroke but its pathophysiology is unclear. We performed a multistage genetic association study (1,154 patients and 3,895 controls) from five case-control cohorts. We investigated the association between rs9349379 in the phosphatase and actin regulator 1 gene (PHACTR1) and carotid traits by echo-tracking in healthy volunteers, the expression of PHACTR1 by genotypes in human fibroblasts, protein staining pattern in human carotids, and vasculature development after Phactr1 knockdown in zebrafish. We show significant and replicated association of rs9349379, a common variant (effect allele frequency 0.60) in PHACTR1 with FMD (OR = 1.39, P = 7.4 × 10-10). rs9349379 associates with greater intima media thickness (P = 3.9 × 10-5) and wall to lumen ratio (P = 0.001), previously reported to be increased in FMD patients. Immunohistochemistry detected PHACTR1 in endothelium and smooth muscle cells of FMD and normal carotid arteries. We also found higher expression of PHACTR1 in FMD risk allele carriers (N = 86, P = 0.003). Phactr1 knockdown in zebrafish resulted in dilated vessels indicating impaired vascular development. We report the first susceptibility locus for FMD in the largest genetic association study conducted so far. Our data reveal a common genetic variant in PHACTR1, previously reported to associate with coronary artery disease, migraine, and cervical artery dissection supporting a complex genetic pattern of inheritance and indices of shared pathophysiology between FMD and other cardiovascular and neuromuscular diseases.

Background Fibromuscular dysplasia (FMD) is a nonatherosclerotic vascular disease leading to arterial stenosis, aneurysm and dissection, mainly in renal and carotid artery. FMD has higher prevalence in females (80-90%) and is associated with hypertension and stroke. The pathophysiology of FMD is unclear and a genetic origin is suspected. Methods We performed a genetic association study in European ancestry individuals. The discovery included 249 cases and 689 controls, in which we analyzed 25,606 common variants (MAF>0.05) using an exome-chip array. Results We followed up 13 loci ( p −4 ) in 393 cases and 2537 controls and replicated a signal on Chr6. Three additional studies (combined n cases = 512, n controls = 669) confirmed this association, with an overall OR of 1.39, ( p = 7.4×10 −10 , n all cases = 1154, n all controls = 3895). The FMD risk variant is intronic to the phosphatase and actin regulator 1 gene (PHACTR1), involved in angiogenesis and cell migration. PHACTR1 is a risk locus for coronary artery disease, migraine, and cervical artery dissection, which may occur in FMD. We found a significant association between the risk allele and higher central pulse pressure ( p =0.0009), increased intima media thickness ( p =0.001) and wall cross-sectional area ( p =0.003) of carotids assessed by echotracking in 3800 population-based individuals. RNA expression of PHACTR1 in primary cultured human fibroblasts is 1.7 fold higher in FMD patients ( n =20, matched to 20 controls) and we showed that FMD risk allele is an eQTL for PHACTR1 in fibroblast of 57 FMD patients ( p =0.02). Finally, Phactr1 knockdown of zebrafish showed significantly dilated vessels ( p =0.003) indicating impaired development of vasculature. Conclusions Here we report the first risk locus for FMD with the largest genetic association study conducted so far. Our data reveal a common genetic variant at PHACTR1 providing indices of shared pathophysiology between FMD and other cardiovascular and neurovascular diseases.

Nonsyndromic mitral valve prolapse (MVP) is a common degenerative valvulopathy of unknown aetiology that predisposes to heart failure and sudden death. Here we investigate functional evidence for candidate genes near genome-wide association study (GWAS) loci to understand the mechanisms underlying the genetic susceptibility to MVP. To prioritize genes, we used literature and databases (e. g ENCODE, eQTLs). We analysed the expression pattern of 3 candidate genes during valve development in mouse embryos by immunohistochemistry (IHC) i) completion of endothelial-to-mesenchymal transformation (E13.5), ii) valve sculpting and elongation (E17.5) and iii) adult form (9 months). Morpholino knockdown (KD) was performed for 8 genes in zebrafish and assayed the function of developing atrioventrocular (AV) canal by scoring the embryos according to the presence of erythrocyte regurgitation. On chr3, the association is in an intron of LMCD1, a repressor of GATA6 previously implicated in cardiac hypertrophy. KD of Lmcd1 in zebrafish resulted in a significant atrioventricular valve defect with regurgitation. On Chr2, the associated variants were upstream to TNS1 encoding a focal adhesion and actin-interacting protein. Tensin1 is expressed during valve morphogenesis in mice and maintained in the adult endothelial and valvular interstitial cells. Hematoxylin and eosin histological staining in 9-month Tns1-/- mice show enlarged posterior mitral leaflets with myxomatous aspects. In addition, zebrafish KD of Tns1 induced AV regurgitation. The functional validation of genes located in or near MVP susceptibility loci identifies new MVP mechanisms and stresses the role of cytoskeleton integrity in valve development. This study reveals LMCD1 and TNS1 at play as early as during valve development and can potentially be targeted during adulthood to improve the natural history of MVP.