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The brown bear (Ursus arctos) is the second largest and most widespread extant terrestrial carnivore on Earth and has recently emerged as a medical model for human metabolic diseases. Here, we report a fully phased chromosome-level assembly of a male North American brown bear built by combining Pacific Biosciences (PacBio) HiFi data and publicly available Hi-C data. The final genome size is 2.47 Gigabases (Gb) with a scaffold and contig N50 length of 70.08 and 43.94 Megabases (Mb), respectively. Benchmarking Universal Single-Copy Ortholog (BUSCO) analysis revealed that 94.5% of single copy orthologs from Mammalia were present in the genome (the highest of any ursid genome to date). Repetitive elements accounted for 44.48% of the genome and a total of 20,480 protein coding genes were identified. Based on whole genome alignment to the polar bear, the brown bear is highly syntenic with the polar bear, and our phylogenetic analysis of 7,246 single-copy orthologs supports the currently proposed species tree for Ursidae. This highly contiguous genome assembly will support future research on both the evolutionary history of the bear family and the physiological mechanisms behind hibernation, the latter of which has broad medical implications.

Extracellular vesicles (EVs) mediate intercellular signaling by transferring their cargo to recipient cells, but the functional consequences of signaling are not fully appreciated. RBC-derived EVs are abundant in circulation and have been implicated in regulating immune responses. Here, we use a transgenic mouse model for fluorescence-based mapping of RBC-EV recipient cells to assess the role of this intercellular signaling mechanism in heart disease. Using fluorescent-based mapping, we detected an increase in RBC-EV-targeted cardiomyocytes in a murine model of ischemic heart failure. Single cell nuclear RNA sequencing of the heart revealed a complex landscape of cardiac cells targeted by RBC-EVs, with enrichment of genes implicated in cell proliferation and stress signaling pathways compared with non-targeted cells. Correspondingly, cardiomyocytes targeted by RBC-EVs more frequently express cellular markers of DNA synthesis, suggesting the functional significance of EV-mediated signaling. In conclusion, our mouse model for mapping of EV-recipient cells reveals a complex cellular network of RBC-EV-mediated intercellular communication in ischemic heart failure and suggests a functional role for this mode of intercellular signaling.

The electrocardiographic PR interval reflects atrioventricular conduction, and is associated with conduction abnormalities, pacemaker implantation, atrial fibrillation (AF), and cardiovascular mortality. Here we report a multi-ancestry (N = 293,051) genome-wide association meta-analysis for the PR interval, discovering 202 loci of which 141 have not previously been reported. Variants at identified loci increase the percentage of heritability explained, from 33.5% to 62.6%. We observe enrichment for cardiac muscle developmental/contractile and cytoskeletal genes, highlighting key regulation processes for atrioventricular conduction. Additionally, 8 loci not previously reported harbor genes underlying inherited arrhythmic syndromes and/or cardiomyopathies suggesting a role for these genes in cardiovascular pathology in the general population. We show that polygenic predisposition to PR interval duration is an endophenotype for cardiovascular disease, including distal conduction disease, AF, and atrioventricular pre-excitation. These findings advance our understanding of the polygenic basis of cardiac conduction, and the genetic relationship between PR interval duration and cardiovascular disease.

The brown bear (Ursus arctos) is the second largest and most widespread extant terrestrial carnivore on Earth and has recently emerged as a medical model for human metabolic diseases. Here, we report a fully phased chromosome-level assembly of a male North American brown bear built by combining Pacific Biosciences (PacBio) HiFi data and publicly available Hi-C data. The final genome size is 2.47 Gigabases (Gb) with a scaffold and contig N50 length of 70.08 and 43.94 Megabases (Mb), respectively. Benchmarking Universal Single-Copy Ortholog (BUSCO) analysis revealed that 94.5% of single copy orthologs from Mammalia were present in the genome (the highest of any ursid genome to date). Repetitive elements accounted for 44.48% of the genome and a total of 20,480 protein coding genes were identified. Based on whole genome alignment to the polar bear, the brown bear is highly syntenic with the polar bear, and our phylogenetic analysis of 7,246 single-copy orthologs supports the currently proposed species tree for Ursidae. This highly contiguous genome assembly will support future research on both the evolutionary history of the bear family and the physiological mechanisms behind hibernation, the latter of which has broad medical implications. [ABSTRACT FROM AUTHOR]

Genome-wide association studies found that increased risk for atrial fibrillation (AF), the most common human heart arrhythmia, is associated with noncoding sequence variants located in proximity to PITX2. Cardiomyocyte-specific epigenomic and comparative genomics uncovered 2 AF-associated enhancers neighboring PITX2 with varying conservation in mice. Chromosome conformation capture experiments in mice revealed that the Pitx2c promoter directly contacted the AF-associated enhancer regions. CRISPR/Cas9- mediated deletion of a 20-kb topologically engaged enhancer led to reduced Pitx2c transcription and AF predisposition. Allele-specific chromatin immunoprecipitation sequencing on hybrid heterozygous enhancer knockout mice revealed that long-range interaction of an AF-associated region with the Pitx2c promoter was required for maintenance of the Pitx2c promoter chromatin state. Long-range looping was mediated by CCCTC-binding factor (CTCF), since genetic disruption of the intronic CTCF-binding site caused reduced Pitx2c expression, AF predisposition, and diminished active chromatin marks on Pitx2. AF risk variants located at 4q25 reside in genomic regions possessing long-range transcriptional regulatory functions directed at PITX2. [ABSTRACT FROM AUTHOR]

Supplemental Digital Content is available in the text. [ABSTRACT FROM AUTHOR]

Importance: Atrial fibrillation (AF) is the most common arrhythmia affecting 1% of the population. Young individuals with AF have a strong genetic association with the disease, but the mechanisms remain incompletely understood.Objective: To perform large-scale whole-genome sequencing to identify genetic variants related to AF.Design, Setting, and Participants: The National Heart, Lung, and Blood Institute's Trans-Omics for Precision Medicine Program includes longitudinal and cohort studies that underwent high-depth whole-genome sequencing between 2014 and 2017 in 18 526 individuals from the United States, Mexico, Puerto Rico, Costa Rica, Barbados, and Samoa. This case-control study included 2781 patients with early-onset AF from 9 studies and identified 4959 controls of European ancestry from the remaining participants. Results were replicated in the UK Biobank (346 546 participants) and the MyCode Study (42 782 participants).Exposures: Loss-of-function (LOF) variants in genes at AF loci and common genetic variation across the whole genome.Main Outcomes and Measures: Early-onset AF (defined as AF onset in persons <66 years of age). Due to multiple testing, the significance threshold for the rare variant analysis was P = 4.55 × 10-3.Results: Among 2781 participants with early-onset AF (the case group), 72.1% were men, and the mean (SD) age of AF onset was 48.7 (10.2) years. Participants underwent whole-genome sequencing at a mean depth of 37.8 fold and mean genome coverage of 99.1%. At least 1 LOF variant in TTN, the gene encoding the sarcomeric protein titin, was present in 2.1% of case participants compared with 1.1% in control participants (odds ratio [OR], 1.76 [95% CI, 1.04-2.97]). The proportion of individuals with early-onset AF who carried a LOF variant in TTN increased with an earlier age of AF onset (P value for trend, 4.92 × 10-4), and 6.5% of individuals with AF onset prior to age 30 carried a TTN LOF variant (OR, 5.94 [95% CI, 2.64-13.35]; P = 1.65 × 10-5). The association between TTN LOF variants and AF was replicated in an independent study of 1582 patients with early-onset AF (cases) and 41 200 control participants (OR, 2.16 [95% CI, 1.19-3.92]; P = .01).Conclusions and Relevance: In a case-control study, there was a statistically significant association between an LOF variant in the TTN gene and early-onset AF, with the variant present in a small percentage of participants with early-onset AF (the case group). Further research is necessary to understand whether this is a causal relationship. [ABSTRACT FROM AUTHOR]

BACKGROUND: Inhibition of PKC-α (protein kinase C-α) enhances contractility and cardioprotection in animal models, but effects in humans are unknown. Genotypes at rs9912468 strongly associate with PRKCA expression in the left ventricle, enabling genetic approaches to measure effects of reduced PKC-α in human populations. METHODS AND RESULTS: We analyzed the cis expression quantitative trait locus for PRKCA marked by rs9912468 using 313 left ventricular specimens from European Ancestry patients. The forward strand minor allele (G) at rs9912468 is associated with reduced PKC-α transcript abundance (1.7-fold reduction in minor allele homozygotes, P=1x10-41). This association was cardiac specific in expression quantitative trait locus data sets that span 16 human tissues. Cardiac epigenomic data revealed a predicted enhancer in complete (R²=1.0) linkage disequilibrium with rs9912468 within intron 2 of PRKCA. We cloned this region and used reporter constructs to verify cardiac-specific enhancer activity in vitro in cardiac and noncardiac cells and in vivo in zebrafish. The PRKCA enhancer contains 2 common genetic variants and 4 haplotypes; the haplotype correlated with the rs9912468 PKC-α-lowering allele (G) showed lowest activity. In contrast to previous reports in animal models, the PKC-α-lowering allele is associated with adverse left ventricular remodeling (higher mass, larger diastolic dimension), reduced fractional shortening, and higher risk of dilated cardiomyopathy in human populations. CONCLUSIONS: These findings support PKC-α as a regulator of the human heart but suggest that PKC-α inhibition may adversely affect the left ventricle depending on timing and duration. Pharmacological studies in human subjects are required to discern potential benefits and harms of PKC-α inhibitors as an approach to treat heart disease. [ABSTRACT FROM AUTHOR]

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 <66 years, for mutations in 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 < .0001; P91S: 2.5-fold, 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. [ABSTRACT FROM AUTHOR]

The most significantly associated genetic locus for atrial fibrillation (AF) is in chromosomal region 4q25, where four independent association signals have been identified. Although model-system studies suggest that altered PITX2c expression might underlie the association, the link between specific variants and the direction of effect on gene expression remains unknown for all four signals. In the present study, we analyzed the AF-associated region most proximal to PITX2 at 4q25. First, we identified candidate regulatory variants that might confer AF risk through a combination of mammalian conservation, DNase hypersensitivity, and histone modification from ENCODE and the Roadmap Epigenomics Project, as well as through in vivo analysis of enhancer activity in embryonic zebrafish. Within candidate regions, we then identified a single associated SNP, rs2595104, which displayed dramatically reduced enhancer activity with the AF risk allele. CRISPR-Cas9-mediated deletion of the rs2595104 region and editing of the rs2595104 risk allele in human stem-cell-derived cardiomyocytes resulted in diminished PITX2c expression in comparison to that of the non-risk allele. This differential activity was mediated by activating enhancer binding protein 2 alpha (TFAP2a), which bound robustly to the non-risk allele at rs2595104, but not to the risk allele, in cardiomyocytes. In sum, we found that the AF-associated SNP rs2595104 altered PITX2c expression via interaction with TFAP2a. Such a pathway could ultimately contribute to AF susceptibility at the PITX2 locus associated with AF. [ABSTRACT FROM AUTHOR]

Background: The genetic basis for dilated cardiomyopathy (DCM) can be difficult to determine, particularly in familial cases with complex phenotypes. Next generation sequencing may be useful in the management of such cases. Methods: We report two large families with pleiotropic inherited cardiomyopathy. In addition to DCM, the phenotypes included atrial and ventricular septal defects, cardiac arrhythmia and sudden death. Probands underwent whole exome sequencing to identify potentially causative variants. Results: Each whole exome sequence yielded over 18,000 variants. We identified distinct mutations affecting a common amino acid in NKX2.5. Segregation analysis of the families support the pathogenic role of these variants. Conclusion: Our study emphasizes the utility of next generation sequencing in identifying causative mutations in complex inherited cardiac disease. We also report a novel pathogenic NKX2.5 mutation. [ABSTRACT FROM AUTHOR]

Fibromuscular dysplasia (FMD) is a nonatherosclerotic vascular disease leading to stenosis, dissection and aneurysm affecting mainly the renal and cerebrovascular arteries. FMD is often an underdiagnosed cause of hypertension and stroke, has higher prevalence in females (~80%) but its pathophysiology is unclear. We analyzed ~26K common variants (MAF>0.05) generated by exome-chip arrays in 249 FMD patients and 689 controls. We replicated 13 loci (P<10-4) in 402 cases and 2,537 controls and confirmed an association between FMD and a variant in the phosphatase and actin regulator 1 gene (PHACTR1). Three additional case control cohorts including 512 cases and 669 replicated this result and overall reached the genomic level of significance (OR = 1.39, P = 7.4×10-10, 1,154 cases and 3,895 controls). The top variant, rs9349379, is intronic to PHACTR1, a risk locus for coronary artery disease, migraine, and cervical artery dissection. The analyses of geometrical parameters of carotids from ~2,500 healthy volunteers indicate higher intima media thickness (P = 1.97×10-4) and wall to lumen ratio (P = 0.002) in rs9349379-A carriers, suggesting indices of carotid hypertrophy previously described in carotids of FMD patients. Immunohistochemistry detected PHACTR1 in endothelium and smooth muscle cells of FMD and normal human carotids. The expression of PHACTR1 by genotypes in primary human fibroblasts showed higher expression in rs9349379-A carriers (N = 86, P = 0.003). Phactr1 knockdown in zebrafish resulted in dilated vessels indicating subtle impaired vascular development. We report the first susceptibility locus for FMD and provide evidence for a complex genetic pattern of inheritance and indices of shared pathophysiology between FMD and other cardiovascular and neurovascular diseases. [ABSTRACT FROM AUTHOR]

Genetic variants identified by genome-wide association studies explain only a modest proportion of heritability, suggesting that meaningful associations lie 'hidden' below current thresholds. Here, we integrate information from association studies with epigenomic maps to demonstrate that enhancers significantly overlap known loci associated with the cardiac QT interval and QRS duration. We apply functional criteria to identify loci associated with QT interval that do not meet genome-wide significance and are missed by existing studies. We demonstrate that these 'sub-threshold' signals represent novel loci, and that epigenomic maps are effective at discriminating true biological signals from noise. We experimentally validate the molecular, gene- regulatory, cellular and organismal phenotypes of these sub-threshold loci, demonstrating that most sub-threshold loci have regulatory consequences and that genetic perturbation of nearby genes causes cardiac phenotypes in mouse. Our work provides a general approach for improving the detection of novel loci associated with complex human traits. [ABSTRACT FROM AUTHOR]

Background--Atrial fibrillation (AF) affects >30 million individuals worldwide and is associated with an increased risk of stroke, heart failure, and death. AF is highly heritable, yet the genetic basis for the arrhythmia remains incompletely understood. Methods and Results--To identify new AF-related genes, we used a multifaceted approach, combining large-scale genotyping in 2 ethnically distinct populations, cis-eQTL (expression quantitative trait loci) mapping, and functional validation. Four novel loci were identified in individuals of European descent near the genes NEURL (rs12415501; relative risk [RR]=1.18; 95% confidence interval [Cl], 1.13-1.23; P=6.5xl0-16), GJA1 (rsl3216675; RR=1.10; 95% CI, 1.06-1.14; P=2.2x10-8), TBX5 (rs10507248; RR=1.12; 95% CI, 1.08-1.16; P=5.7xl0-11), and CAND2 (rs4642101; RR=1.10; 95% Cl, 1.06-1.14; P=9.8x10-9). In Japanese, novel loci were identified near NEURL (rs6584555; RR=1.32; 95% CI, 1.26-1.39; P=2.0x10-25) and CUX2 (rs6490029; RR=1.12; 95% CI, 1.08-1.16; P=3.9x10-9). The top single-nucleotide polymorphisms or their proxies were identified as cis-eQTLs for the genes CAND2 (P=2.6x10-19), GJA1 (P=2.66x10-6), and TBX5 (P=1.36x10-5). Knockdown of the zebrafish orthologs of NEURL and CAND2 resulted in prolongation of the atrial action potential duration (17% and 45%, respectively). Conclusions--We have identified 5 novel loci for AF. Our results expand the diversity of genetic pathways implicated in AF and provide novel molecular targets for future biological and pharmacological investigation. [ABSTRACT FROM AUTHOR]

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. [Copyright &y& Elsevier]

Genetic variants identified by genome-wide association studies explain only a modest proportion of heritability, suggesting that meaningful associations lie 'hidden' below current thresholds. Here, we integrate information from association studies with epigenomic maps to demonstrate that enhancers significantly overlap known loci associated with the cardiac QT interval and QRS duration. We apply functional criteria to identify loci associated with QT interval that do not meet genome-wide significance and are missed by existing studies. We demonstrate that these 'sub-threshold' signals represent novel loci, and that epigenomic maps are effective at discriminating true biological signals from noise. We experimentally validate the molecular, gene-regulatory, cellular and organismal phenotypes of these sub-threshold loci, demonstrating that most sub-threshold loci have regulatory consequences and that genetic perturbation of nearby genes causes cardiac phenotypes in mouse. Our work provides a general approach for improving the detection of novel loci associated with complex human traits. DOI: http://dx.doi.org/10.7554/eLife.10557.001

Ischemia and subsequent reperfusion (IR) produces injury to brain, eye and other tissues, contributing to the progression of important clinical pathologies. The response of cells to IR involves activation of several signaling pathways including those activating hypoxia and heat shock responsive transcription factors. However, specific roles of these responses in limiting cell damage and preventing cell death after IR have not been fully elucidated. Here, we have examined the role of heat shock factor 1 (HSF1) in the response of zebrafish embryos to hypoxia and subsequent return to normoxic conditions (HR) as a model for IR. Heat shock preconditioning elevated heat shock protein expression and protected zebrafish embryo eye and brain tissues against HR-induced apoptosis. These effects were inhibited by translational suppression of HSF1 expression. Reduced expression of HSF1 also increased cell death in brain and eye tissues of embryos subjected to hypoxia and reperfusion without prior heat shock. Surprisingly, reduced expression of HSF1 had only a modest effect on hypoxiainduced expression of Hsp70 and no effect on hypoxia-induced expression of Hsp27. These results establish the zebrafish embryo as a model for the study of ischemic injury in the brain and eye and reveal a critical role for HSF1 in the response of these tissues to HR. Our results also uncouple the role of HSF1 expression from that of Hsp27, a well characterized heat shock protein considered essential for cell survival after hypoxia. Alternative roles for HSF1 are considered. [ABSTRACT FROM AUTHOR]

The article offers information on identification of a functional genetic variant that alters PRRX1 gene expression in atrial myocytes and provides a link with atrial fibrillation (AF). It mentions the molecular mechanism of transcription factor PRRX1 which is expressed in the pulmonary veins. It also states the role of electrophysiological alterations in atrial myocytes in promoting AF.

The ability to track disease without tissue biopsy in patients is a major goal in biology and medicine. Here, we identify and characterize cardiomyocyte-derived extracellular vesicles in circulation (EVs; "cardiovesicles") through comprehensive studies of induced pluripotent stem cell-derived cardiomyocytes, genetic mouse models, and state-of-the-art mass spectrometry and low-input transcriptomics. These studies identified two markers ( POPDC2 , CHRNE ) enriched on cardiovesicles for biotinylated antibody-based immunocapture. Captured cardiovesicles were enriched in canonical cardiomyocyte transcripts/pathways with distinct profiles based on human disease type (heart failure, myocardial infarction). In paired myocardial tissue-plasma from patients, highly expressed genes in cardiovesicles were largely cardiac-enriched (vs. "bulk" EVs, which were more organ non-specific) with high expression in myocardial tissue by single nuclear RNA-seq, largely in cardiomyocytes. These results demonstrate the first "liquid" biopsy discovery platform to interrogate cardiomyocyte states noninvasively in model systems and in human disease, allowing non-invasive characterization of cardiomyocyte biology for discovery and therapeutic applications.

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. NEW & NOTEWORTHY We performed a comprehensive assessment to evaluate the effects of slow progressive pressure overload on cardiopulmonary function in a large animal model of heart failure with preserved ejection fraction (HFpEF) in males and females. Functional and structural assessments were performed at the organ, tissue, cellular, protein, and transcriptional levels. This is the first study to compare snRNAseq and ECM mass spectrometry of HFpEF myocardium from males and females. The results broaden our understanding of the pathophysiological response of both sexes to pressure overload. Both sexes developed a robust cardiopulmonary phenotype, but the phenotype was equal or a bit less robust in females.

Acute cardiac injury is prevalent in critical COVID-19 and associated with increased mortality. Its etiology remains debated, as initially presumed causes - myocarditis and cardiac necrosis - have proved uncommon. To elucidate the pathophysiology of COVID-19-associated cardiac injury, we conducted a prospective study of the first 69 consecutive COVID-19 decedents at CUIMC in New York City. Of 6 acute cardiac histopathologic features, presence of microthrombi was the most commonly detected among our cohort. We tested associations of cardiac microthrombi with biomarkers of inflammation, cardiac injury, and fibrinolysis and with in-hospital antiplatelet therapy, therapeutic anticoagulation, and corticosteroid treatment, while adjusting for multiple clinical factors, including COVID-19 therapies. Higher peak erythrocyte sedimentation rate and C-reactive protein were independently associated with increased odds of microthrombi, supporting an immunothrombotic etiology. Using single-nuclei RNA-sequencing analysis on 3 patients with and 4 patients without cardiac microthrombi, we discovered an enrichment of prothrombotic/antifibrinolytic, extracellular matrix remodeling, and immune-potentiating signaling among cardiac fibroblasts in microthrombi-positive, relative to microthrombi-negative, COVID-19 hearts. Non-COVID-19, nonfailing hearts were used as reference controls. Our study identifies a specific transcriptomic signature in cardiac fibroblasts as a salient feature of microthrombi-positive COVID-19 hearts. Our findings warrant further mechanistic study as cardiac fibroblasts may represent a potential therapeutic target for COVID-19-associated cardiac microthrombi.