GATA6 reporter gene reveals myocardial phenotypic heterogeneity that is related to variations in gap junction coupling

GATA6 mutations: Characterization of two novel patients and a comprehensive overview of the GATA6 genotypic and phenotypic spectrum

The first human mutations in GATA6 were described in a cohort of patients with persistent truncus arteriosus, and the phenotypic spectrum has expanded since then. This study underscores the broad phenotypic spectrum by presenting two patients with de novo GATA6 mutations, both exhibiting complex cardiac defects, pancreatic, and other abnormalities. Furthermore, we provided a detailed overview of all published human genetic variation in/near GATA6 published to date and the associated phenotypes (n = 78). We conclude that the most common phenotypes associated with a mutation in GATA6 were structural cardiac and pancreatic abnormalities, with a penetrance of 87 and 60%, respectively. Other common malformations were gallbladder agenesis, congenital diaphragmatic hernia, and neurocognitive abnormalities, mostly developmental delay. Fifty-eight percent of the mutations were de novo, and these patients more often had an anomaly of intracardiac connections, an anomaly of the great arteries, and hypothyroidism, compared with those with inherited mutations. Functional studies mostly support loss-of-function as the pathophysiological mechanism. In conclusion, GATA6 mutations give a wide range of phenotypic defects, most frequently malformations of the heart and pancreas. This highlights the importance of detailed clinical evaluation of identified carriers to evaluate their full phenotypic spectrum.

GATA6 loss-of-function mutation contributes to congenital bicuspid aortic valve

Congenital bicuspid aortic valve (BAV), the most common form of birth defect in humans, is associated with substantial morbidity and mortality. Increasing evidence demonstrates that genetic risk factors play a key role in the pathogenesis of BAV. However, BAV is a genetically heterogeneous disease and the genetic determinants underpinning BAV in an overwhelming majority of patients remain unknown. In the present study, the coding exons and flanking introns of the GATA6 gene, which encodes a zinc-finger transcription factor essential for the normal development of the aortic valves, were sequenced in 152 unrelated patients with congenital BAV. The available relatives of a proband harboring an identified GATA6 mutation and 200 unrelated, ethnically matched healthy individuals used as controls were also genotyped for GATA6. The functional characteristics of the mutation were analyzed by using a dual-luciferase reporter assay system. As a result, a novel heterozygous GATA6 mutation, p.E386X, was identified in a family with BAV transmitted in an autosomal dominant mode. The nonsense mutation was absent in 400 control chromosomes. Biological assays revealed that the mutant GATA6 protein had no transcriptional activity compared with its wild-type counterpart. Furthermore, the mutation disrupted the synergistic transcriptional activation between GATA6 and GATA4, another transcription factor causally linked to BAV. In conclusion, this study firstly associates GATA6 loss-of-function mutation with enhanced susceptibility to familial BAV, which provides novel insight into the molecular mechanism of BAV, implying potential implications for genetic counseling and personalized management of BAV patients.

SUR1-TRPM4 and AQP4 form a heteromultimeric complex that amplifies ion/water osmotic coupling and drives astrocyte swelling

Astrocyte swelling occurs after central nervous system injury and contributes to brain swelling, which can increase mortality. Mechanisms proffered to explain astrocyte swelling emphasize the importance of either aquaporin-4 (AQP4), an astrocyte water channel, or of Na⁺ -permeable channels, which mediate cellular osmolyte influx. However, the spatio-temporal functional interactions between AQP4 and Na⁺ -permeable channels that drive swelling are poorly understood. We hypothesized that astrocyte swelling after injury is linked to an interaction between AQP4 and Na⁺ -permeable channels that are newly upregulated. Here, using co-immunoprecipitation and Förster resonance energy transfer, we report that AQP4 physically co-assembles with the sulfonylurea receptor 1-transient receptor potential melastatin 4 (SUR1-TRPM4) monovalent cation channel to form a novel heteromultimeric water/ion channel complex. In vitro cell-swelling studies using calcein fluorescence imaging of COS-7 cells expressing various combinations of AQP4, SUR1, and TRPM4 showed that the full tripartite complex, comprised of SUR1-TRPM4-AQP4, was required for fast, high-capacity transmembrane water transport that drives cell swelling, with these findings corroborated in cultured primary astrocytes. In a murine model of brain edema involving cold-injury to the cerebellum, we found that astrocytes newly upregulate SUR1-TRPM4, that AQP4 co-associates with SUR1-TRPM4, and that genetic inactivation of the solute pore of the SUR1-TRPM4-AQP4 complex blocked in vivo astrocyte swelling measured by diolistic labeling, thereby corroborating our in vitro functional studies. Together, these findings demonstrate a novel molecular mechanism involving the SUR1-TRPM4-AQP4 complex to account for bulk water influx during astrocyte swelling. These findings have broad implications for the understanding and treatment of AQP4-mediated pathological conditions.

A loss-of-function and H2B-Venus transcriptional reporter allele for Gata6 in mice

Background

The GATA-binding factor 6 (Gata6) gene encodes a zinc finger transcription factor that often functions as a key regulator of lineage specification during development. It is the earliest known marker of the primitive endoderm lineage in the mammalian blastocyst. During gastrulation, GATA6 is expressed in early cardiac mesoderm and definitive endoderm progenitors, and is necessary for development of specific mesoderm and endoderm-derived organs including the heart, liver, and pancreas. Furthermore, reactivation or silencing of the Gata6 locus has been associated with certain types of cancer affecting endodermal organs.

Results

We have generated a Gata6^H2B-Venus knock-in reporter mouse allele for the purpose of labeling GATA6-expressing cells with a bright nuclear-localized fluorescent marker that is suitable for live imaging at single-cell resolution.

Conclusions

Expression of the Venus reporter was characterized starting from embryonic stem (ES) cells, through mouse embryos and adult animals. The Venus reporter was not expressed in ES cells, but was activated upon endoderm differentiation. Gata6^{H2B-Venus/H2B-Venus} homozygous embryos did not express GATA6 protein and failed to specify the primitive endoderm in the blastocyst. However, null blastocysts continued to express high levels of Venus in the absence of GATA6 protein, suggesting that early Gata6 transcription is independent of GATA6 protein expression. At early post-implantation stages of embryonic development, there was a strong correlation of Venus with endogenous GATA6 protein in endoderm and mesoderm progenitors, then later in the heart, midgut, and hindgut. However, there were discrepancies in reporter versus endogenous protein expression in certain cells, such as the body wall and endocardium. During organogenesis, detection of Venus in specific organs recapitulated known sites of endogenous GATA6 expression, such as in the lung bud epithelium, liver, pancreas, gall bladder, stomach epithelium, and vascular endothelium. In adults, Venus was observed in the lungs, pancreas, liver, gall bladder, ovaries, uterus, bladder, skin, adrenal glands, small intestine and corpus region of the stomach. Overall, Venus fluorescent protein under regulatory control of the Gata6 locus was expressed at levels that were easily visualized directly and could endure live and time-lapse imaging techniques. Venus is co-expressed with endogenous GATA6 throughout development to adulthood, and should provide an invaluable tool for examining the status of the Gata6 locus during development, as well as its silencing or reactivation in cancer or other disease states.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-015-0086-5) contains supplementary material, which is available to authorized users.

Inhibition of G9a Histone Methyltransferase Converts Bone Marrow Mesenchymal Stem Cells to Cardiac Competent Progenitors

The G9a histone methyltransferase inhibitor BIX01294 was examined for its ability to expand the cardiac capacity of bone marrow cells. Inhibition of G9a histone methyltransferase by gene specific knockdown or BIX01294 treatment was sufficient to induce expression of precardiac markers Mesp1 and brachyury in bone marrow cells. BIX01294 treatment also allowed bone marrow mesenchymal stem cells (MSCs) to express the cardiac transcription factors Nkx2.5, GATA4, and myocardin when subsequently exposed to the cardiogenic stimulating factor Wnt11. Incubation of BIX01294-treated MSCs with cardiac conditioned media provoked formation of phase bright cells that exhibited a morphology and molecular profile resembling similar cells that normally form from cultured atrial tissue. Subsequent aggregation and differentiation of BIX01294-induced, MSC-derived phase bright cells provoked their cardiomyogenesis. This latter outcome was indicated by their widespread expression of the primary sarcomeric proteins muscle α-actinin and titin. MSC-derived cultures that were not initially treated with BIX01294 exhibited neither a commensurate burst of phase bright cells nor stimulation of sarcomeric protein expression. Collectively, these data indicate that BIX01294 has utility as a pharmacological agent that could enhance the ability of an abundant and accessible stem cell population to regenerate new myocytes for cardiac repair.

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.

Cryoinjury models of the adult and neonatal mouse heart for studies of scarring and regeneration

A major limitation in studies of the injured heart is animal-to-animal variability in wound size resulting from commonly used techniques such as left anterior descending coronary artery ligation. This variability can make standard errors sufficiently large that mean separation between treatment and control groups can be difficult without replicating numbers (n) of animals in groups by excessive amounts. Here, we describe the materials and protocol necessary for delivering a standardized non-transmural cryoinjury to the left ventricle of an adult mouse heart that may in part obviate the issue of injury variance between animals. As reported previously, this cryoinjury model generates a necrotic wound to the ventricle of consistent size and shape that resolves into a scar of uniform size, shape, and organization. The cryo-model also provides an extended injury border zone that exhibits classic markers of remodeling found in surviving cardiac tissue at the edge of a myocardial infarction, including connexin43 (Cx43) lateralization. In a further extension of the method, we describe how we have adapted the model to deliver a cryoinjury to the apex of the heart of neonatal mice-a modification that may be useful for studies of myocardial regeneration in mammals.

Somatic mutations in the GATA6 gene underlie sporadic tetralogy of Fallot

Tetralogy of Fallot (TOF) is the most common cyanotic congenital heart disease associated with significant morbidity and mortality in humans. However, the molecular etiology underlying TOF in most patients remains largely unknown. In the present study, sequence analysis of the GATA6 gene was performed from fresh-frozen cardiac tissues and matched blood samples of 52 unrelated patients who underwent surgical repair of TOF. The cardiac tissues and matched blood specimens from 46 patients who underwent cardiac valve replacement due to rheumatic heart disease and blood samples from 200 healthy individuals as controls were genotyped. The functional characteristics of the mutations were assessed using a luciferase reporter assay system. Based on the results, two novel heterozygous GATA6 mutations, p.G367X and p.G394C, were identified in the cardiac tissues of 2 TOF patients, respectively. No mutations were found in the cardiac tissues from 46 patients with rheumatic heart disease and in the blood samples from the 298 participants. Functional analysis demonstrated that the GATA6 mutants were consistently associated with significantly reduced transcriptional activation compared with their wild-type counterpart. This is the first report on the link of somatic GATA6 mutation to TOF, providing novel insight into the molecular mechanism involved in TOF.

Novel GATA6 mutations associated with congenital ventricular septal defect or tetralogy of fallot

Congenital heart disease (CHD) is the most common form of developmental malformation and is the leading noninfectious cause of infant mortality. Emerging evidence indicates that genetic defects are involved in the pathogenesis of CHD. Nevertheless, CHD is genetically heterogeneous, and the molecular basis for CHD in a majority of patients remains unknown. In this study, the whole coding region of GATA6, a gene encoding a zinc-finger transcription factor crucial for normal cardiogenesis, was sequenced in 380 unrelated patients with CHD. The relatives of the index patients harboring the identified mutations and 200 unrelated control individuals were subsequently genotyped. The functional effect of the mutations was characterized using a luciferase reporter assay system. As a result, two novel heterozygous GATA6 mutations, p.D404Y and p.E460X, were identified in two families with ventricular septal defect and tetralogy of Fallot, respectively. The mutations co-segregated with CHD in the families with complete penetrance, and were absent in 400 control chromosomes. Functional analysis demonstrated that the mutated GATA6 proteins were associated with significantly decreased transactivational activity in comparison with their wild-type counterpart. These findings provide novel insight into the molecular mechanism implicated in CHD, suggesting potential implications for the early prophylaxis and personalized treatment of CHD.

GATA6 loss-of-function mutation in atrial fibrillation

Atrial fibrillation (AF) is the most common type of sustained cardiac arrhythmia and is associated with substantial morbidity and mortality. Increasing evidence demonstrates that hereditary defects are involved in the pathogenesis of AF. However, AF is of remarkable genetic heterogeneity, and the heritable components responsible for AF in the majority of patients remain unclear. In this study, the entire coding region of the GATA6 gene, which encodes a zinc-finger transcription factor crucial for cardiogenesis, was sequenced in 138 unrelated patients with lone AF, and a novel heterozygous GATA6 mutation, c.704A > C equivalent to p.Y235S, was identified in a patient. The detected substitution, which altered the amino acid highly conserved evolutionarily across species, was absent in 200 unrelated ethnically matched healthy individuals, and was predicted to be disease-causing by MutationTaster. Genetic analysis of the available relatives of the mutation carrier showed that in the family the variation co-segregated with the disease transmitted as an autosomal dominant trait, with complete penetrance. The functional analysis performed using a luciferase reporter assay system revealed that the mutant GATA6 protein resulted in significantly decreased transcriptional activity compared with its wild-type counterpart. These findings provide novel insight into the molecular pathophysiology implicated in AF, suggesting the potential implications in the prophylactic strategy and effective therapy for this common arrhythmia.

Inhibition of heart formation by lithium is an indirect result of the disruption of tissue organization within the embryo

Lithium is a commonly used drug for the treatment of bipolar disorder. At high doses, lithium becomes teratogenic, which is a property that has allowed this agent to serve as a useful tool for dissecting molecular pathways that regulate embryogenesis. This study was designed to examine the impact of lithium on heart formation in the developing frog for insights into the molecular regulation of cardiac specification. Embryos were exposed to lithium at the beginning of gastrulation, which produced severe malformations of the anterior end of the embryo. Although previous reports characterized this deformity as a posteriorized phenotype, histological analysis revealed that the defects were more comprehensive, with disfigurement and disorganization of all interior tissues along the anterior-posterior axis. Emerging tissues were poorly segregated and cavity formation was decreased within the embryo. Lithium exposure also completely ablated formation of the heart and prevented myocardial cell differentiation. Despite the complete absence of cardiac tissue in lithium treated embryos, exposure to lithium did not prevent myocardial differentiation of precardiac dorsal marginal zone explants. Moreover, precardiac tissue freed from the embryo subsequent to lithium treatment at gastrulation gave rise to cardiac tissue, as demonstrated by upregulation of cardiac gene expression, display of sarcomeric proteins, and formation of a contractile phenotype. Together these data indicate that lithium's effect on the developing heart was not due to direct regulation of cardiac differentiation, but an indirect consequence of disrupted tissue organization within the embryo.