Two novel frameshift mutations in NKX2.5 result in novel features including visceral inversus and sinus venosus type ASD

Nkx2.5: a crucial regulator of cardiac development, regeneration and diseases

Cardiomyocytes fail to regenerate after birth and respond to mitotic signals through cellular hypertrophy rather than cellular proliferation. Necrotic cardiomyocytes in the infarcted ventricular tissue are eventually replaced by fibroblasts, generating scar tissue. Cardiomyocyte loss causes localized systolic dysfunction. Therefore, achieving the regeneration of cardiomyocytes is of great significance for cardiac function and development. Heart development is a complex biological process. An integral cardiac developmental network plays a decisive role in the regeneration of cardiomyocytes. During this process, genetic epigenetic factors, transcription factors, signaling pathways and small RNAs are involved in regulating the developmental process of the heart. Cardiomyocyte-specific genes largely promote myocardial regeneration, among which the Nkx2.5 transcription factor is one of the earliest markers of cardiac progenitor cells, and the loss or overexpression of Nkx2.5 affects cardiac development and is a promising candidate factor. Nkx2.5 affects the development and function of the heart through its multiple functional domains. However, until now, the specific mechanism of Nkx2.5 in cardiac development and regeneration is not been fully understood. Therefore, this article will review the molecular structure, function and interaction regulation of Nkx2.5 to provide a new direction for cardiac development and the treatment of heart regeneration.

Whole-exome sequencing reveals a combination of extremely rare single-nucleotide polymorphism of DNAH9 and RSPH1 genes in a Japanese fetus with situs viscerum inversus

Randomization of left-right body asymmetry, situs viscerum inversus (heterotaxy), is commonly associated with primary ciliary dyskinesia (PCD) resulting from an abnormal ciliary structure, with approximately 50% of PCD patients exhibiting organ laterality defects. I herein report an intrauterine fetal death case, in which an autopsy revealed two lobes of the bilateral lungs as well as heterotaxy of abdominal organs (right-sided spleen and inversion of the alimentary and biliary organs). Whole-exome sequencing (WES) identified a heterozygous single-nucleotide change (c.12775T>C) in exon 68 of the DNAH9 gene, which is a rare single-nucleotide polymorphism (SNP) of rs746081639 and results in the amino acid change of p.C4259R. WES also identified a rare SNP of rs763089682 (c.121G>A) in the RSPH1 gene that causes a heterozygous amino acid alteration of p.G41R. The frequencies of both SNPs, C in rs746081639 and A in rs763089682, are 0.00000824, and a polyphen-2 analysis predicted these amino acid changes to be probably damaging, with a score of 1.000. The combination of extremely rare SNPs in DNAH9 and RSPH1 genes might have been the possible mechanism underlying the development of the laterality defect in the present case.

An update on genetic variants of the NKX2-5

NKX2-5 is a homeodomain transcription factor that plays a crucial role in heart development. It is the first gene where a single genetic variant (GV) was found to be associated with congenital heart diseases in humans. In this study, we carried out a comprehensive survey of NKX2-5 GVs to build a unified, curated, and updated compilation of all available GVs. We retrieved a total of 1,380 unique GVs. From these, 970 had information on their frequency in the general population and 143 have been linked to pathogenic phenotypes in humans. In vitro effect was ascertained for 38 GVs. The homeodomain had the biggest cluster of pathogenic variants in the protein: 49 GVs in 60 residues, 23 in its third α-helix, where 11 missense variants may affect protein-DNA interaction or the hydrophobic core. We also pinpointed the likely location of pathogenic GVs in four linear motifs. These analyses allowed us to assign a putative explanation for the effect of 90 GVs. This study pointed to reliable pathogenicity for GVs in helix 3 of the homeodomain and may broaden the scope of functional and structural studies that can be done to better understand the effect of GVs in NKX2-5 function.

The determination factors of left-right asymmetry disorders- a short review

Laterality defects in humans, situs inversus and heterotaxy, are rare disorders, with an incidence of 1:8000 to 1:10 000 in the general population, and a multifactorial etiology. It has been proved that 1.44/10 000 of all cardiac problems are associated with malformations of left-right asymmetry and heterotaxy accounts for 3% of all congenital heart defects. It is considered that defects of situs appear due to genetic and environmental factors. Also, there is evidence that the ciliopathies (defects of structure or function) are involved in development abnormalities. Over 100 genes have been reported to be involved in left-right patterning in model organisms, but only a few are likely to candidate for left-right asymmetry defects in humans. Left-right asymmetry disorders are genetically heterogeneous and have variable manifestations (from asymptomatic to serious clinical problems). The discovery of the right mechanism of left-right development will help explain the clinical complexity and may contribute to a therapy of these disorders.

Genetics of Congenital Heart Defects: The NKX2-5 Gene, a Key Player

Congenital heart defects (CHDs) represent the biggest fraction of morbid congenital anomalies worldwide. Owing to their complex inheritance patterns and multifactorial etiologies, these defects are difficult to identify before complete manifestation. Research over the past two decades has established firmly the role of genetics in the development of these congenital defects. While syndromic CHDs are more straightforward, non-syndromic CHDs are usually characterized by multiple mutations that affect intricate inter-connected developmental pathways. Knock-out and gene expression studies in mice and other genetic models have been performed to elucidate the roles of these implicated genes. Functional analysis has not been able to resolve the complete picture, as increasingly more downstream effects are continuously being assigned to CHD mutant factors. NKX2-5, a cardiac transcription factor, has received much attention for its role in cardiac dysmorphogenesis. Approximately 50 different mutations in this gene have been identified to date, and only a few have been functionally characterized. The mutant NKX2-5 factor can regulate a number of off-targets downstream to facilitate CHD development. This review summarizes the genetic etiology of congenital heart defects and emphasizes the need for NKX2-5 mutation screening.

Familial Atrial Septal Defect and Sudden Cardiac Death: Identification of a Novel NKX2-5 Mutation and a Review of the Literature

OBJECTIVE

Atrial septal defect (ASD) is the second most common congenital heart defect (CHD) and is observed in families as an autosomal dominant trait as well as in nonfamilial CHD. Mutations in the NKX2-5 gene, located on chromosome 5, are associated with ASD, often combined with conduction disturbances, cardiomyopathies, complex CHD, and sudden cardiac death as well. Here, we show that NKX2-5 mutations primarily occur in ASD patients with conduction disturbances and heritable ASD. Furthermore, these families are at increased risk of sudden cardiac death.

RESULTS

We screened 39 probands with familial CHD for mutations in NKX2-5 and discovered a novel mutation in one family (2.5%) with ASD and atrioventricular block. A review of the literature revealed 59 different NKX2-5 mutations in 202 patients. Mutations were significantly more common in familial cases compared to nonfamilial cases (P = 7.1 × 10(-9) ). The majority of patients (74%) had ASD with conduction disturbance. Nineteen patients (15%) of 120 with familial ASD and conduction disturbance died from sudden cardiac death of which nine (8%) were confirmed mutation carriers, and 10 were possible carriers.

CONCLUSIONS

NKX2-5 mutations mainly occur in familial CHD, the signature phenotype is ASD with conduction disturbances and mutation carriers are at increased risk of sudden cardiac death. We suggest that familial ASD patients should be screened for NKX2-5 mutations and, if they are mutation carriers, implantation of an implantable cardioverter-defibrillator should be considered in these patients.

Genetic basis of human left-right asymmetry disorders

Humans and other vertebrates exhibit left-right (LR) asymmetric arrangement of the internal organs, and failure to establish normal LR asymmetry leads to internal laterality disorders, including situs inversus and heterotaxy. Situs inversus is complete mirror-imaged arrangement of the internal organs along LR axis, whereas heterotaxy is abnormal arrangement of the internal thoraco-abdominal organs across LR axis of the body, most of which are associated with complex cardiovascular malformations. Both disorders are genetically heterogeneous with reduced penetrance, presumably because of monogenic, polygenic or multifactorial causes. Research in genetics of LR asymmetry disorders has been extremely prolific over the past 17 years, and a series of loci and disease genes involved in situs inversus and heterotaxy have been described. The review highlights the classification, chromosomal abnormalities, pathogenic genes and the possible mechanism of human LR asymmetry disorders.

Duplication and deletion of CFC1 associated with heterotaxy syndrome

Heterotaxy syndrome, which causes significant morbidity and mortality, is a class of congenital disorders, in which normal left-right asymmetry cannot be properly established. To explore the role of copy number variants (CNVs) in the occurrence of heterotaxy syndrome, we recruited 93 heterotaxy patients and studied 12 of them by the Affymetrix Genome-Wide Human SNP 6.0 Array. The results were confirmed in the remaining 81 patients and 500 healthy children by quantitative real-time polymerase chain reaction (qPCR). The analysis of the SNP6.0 array showed a duplication of chromosome 2q21.1, which was verified by qPCR. The result of qPCR in the other 81 patients showed that 8/81 patients had the CNVs of 2q21.1 and the only overlapping gene in these patients is CFC1. However, in the 500 healthy children, only one carried the duplication of CFC1 (p=3.5×10(-7)). The duplication and deletion of CFC1 may play key roles in the occurrence of heterotaxy syndrome. Moreover, the transposed great arteries, double outlet right ventricle, single atrium, and single ventricle may share a common genetic etiology with the heterotaxy syndrome.

Novel copy-number variants in a population-based investigation of classic heterotaxy

PURPOSE

Heterotaxy is a clinically and genetically heterogeneous disorder. We investigated whether screening cases restricted to a classic phenotype would result in the discovery of novel, potentially causal copy-number variants.

METHODS

We identified 77 cases of classic heterotaxy from all live births in New York State during 1998-2005. DNA extracted from each infant's newborn dried blood spot was genotyped with a microarray containing 2.5 million single-nucleotide polymorphisms. Copy-number variants were identified with PennCNV and cnvPartition software. Candidates were selected for follow-up if they were absent in unaffected controls, contained 10 or more consecutive probes, and had minimal overlap with variants published in the Database of Genomic Variants.

RESULTS

We identified 20 rare copy-number variants including a deletion of BMP2, which has been linked to laterality disorders in mice but not previously reported in humans. We also identified a large, terminal deletion of 10q and a microdeletion at 1q23.1 involving the MNDA gene; both are rare variants suspected to be associated with heterotaxy.

CONCLUSION

Our findings implicate rare copy-number variants in classic heterotaxy and highlight several candidate gene regions for further investigation. We also demonstrate the efficacy of copy-number variant genotyping in blood spots using microarrays.

NKX2.5 mutation identification on exome sequencing in a patient with heterotaxy

Exome sequencing enables us to screen most of the protein coding genes in an unbiased way, this technique represents an ideal tool to identify previously under- or unappreciated phenotypes associated with known disease genes and genetic disorders. Here we present an illustrative case that required exome sequencing to identify a genetic alteration associated with the clinical features. The phenotype of the proband included heterotaxy, double outlet right ventricle, common atrioventricular canal, total anomalous pulmonary venous connection, asplenia, failure to thrive and short stature. Exome sequencing demonstrated a frameshift mutation c.397_400del (p.P133GfsTer 42) in NKX2.5. Although a single previous case of heterotaxy was reported in a large familial case of NKX2.5, heterotaxy is not clinically appreciated to be a part of the phenotypic spectrum associated with NKX2.5 mutations. This case report demonstrates the utility of exome sequencing in expanding a phenotypic spectrum of a known Mendelian disorder. We predict that this type of unexpected identification of mutations in known-disease associated genes in patients with atypical or expanded phenotypes will occur with increasing frequency as the use of exome and genome sequencing become more common tools in diagnosing patients with syndromic and non-syndromic foms of structural birth defects.

A novel mutation of the axonemal dynein heavy chain gene 5 (DNAH5) in a Japanese neonate with asplenia syndrome

Asplenia syndrome (Ivemark syndrome) is a complex disorder composed of asplenia, malpositioning of the visceral organs and congenital heart defects. To elucidate the underlying molecular mechanism of asplenia syndrome, we herein analyzed the fatal case of a male neonate who exhibited three lobes of the left lung, asplenia and complex heart anomalies and died 6 hours after delivery. A whole-exome sequence (WES) analysis followed by Sanger sequence identified a heterozygous single nucleotide change (c.7829A>G) in exon 47 of the axonemal dynein heavy chain gene 5 (DNAH5), which results in the missense mutation of p.Glu2610Gly. This mutation was found only in the neonate, but not in his parents, implying de novo mutation of DNAH5 that codes dynein heavy chain, a component of outer dynein arm. The WES analysis also identified a heterozygous single nucleotide substitution (c.3697C>T) in the axonemal dynein heavy chain gene 7 (DNAH7), resulting in p.Arg1233Cys, and a rare SNP (c.2029G>A, p.Gly677Ser) of the axonemal dynein intermediate chain gene 1 (DNAI1) in the patient and his mother, but not in his father. The mutation of p.Glu2610Gly in DNAH5 is novel and we here present a first Japanese case of asplenia syndrome who exhibited a DNAH5 mutation.

Genetic basis of congenital cardiovascular malformations

Cardiovascular malformations are a singularly important class of birth defects and due to dramatic improvements in medical and surgical care, there are now large numbers of adult survivors. The etiologies are complex, but there is strong evidence that genetic factors play a crucial role. Over the last 15 years there has been enormous progress in the discovery of causative genes for syndromic heart malformations and in rare families with Mendelian forms. The rapid characterization of genomic disorders as major contributors to congenital heart defects is also notable. The genes identified encode many transcription factors, chromatin regulators, growth factors and signal transduction proteins- all unified by their required roles in normal cardiac development. Genome-wide sequencing of the coding regions promises to elucidate genetic causation in several disorders affecting cardiac development. Such comprehensive studies evaluating both common and rare variants would be essential in characterizing gene-gene interactions, as well as in understanding the gene-environment interactions that increase susceptibility to congenital heart defects.

Mutational spectrum of the NKX2-5 gene in patients with lone atrial fibrillation

Atrial fibrillation (AF) is the most common form of sustained cardiac arrhythmia in humans and is responsible for substantial morbidity and mortality worldwide. Emerging evidence indicates that abnormal cardiovascular development is involved in the pathogenesis of AF. In this study, the coding exons and splice sites of the NKX2-5 gene, which encodes a homeodomain-containing transcription factor essential for cardiovascular genesis, were sequenced in 146 unrelated patients with lone AF as well as the available relatives of the mutation carriers. A total of 700 unrelated ethnically matched healthy individuals used as controls were genotyped. The disease-causing potential of the identified NKX2-5 variations was predicted by MutationTaster and PolyPhen-2. The functional characteristics of the mutant NKX2-5 proteins were analyzed using a dual-luciferase reporter assay system. As a result, two heterozygous NKX2-5 mutations, including a previously reported p.E21Q and a novel p.T180A mutation, were identified in two families with AF transmitted in an autosomal dominant pattern. The mutations co-segregated with AF in the families with complete penetrance. The detected substitutions, which altered the amino acids highly conserved evolutionarily across species, were absent in 700 control individuals and were both predicted to be causative. Functional analyses demonstrated that the NKX2-5 mutants were associated with significantly decreased transcriptional activity compared with their wild-type counterpart. The findings expand the spectrum of NKX2-5 mutations linked to AF and provide additional evidence that dysfunctional NKX2-5 may confer vulnerability to AF, suggesting the potential benefit for the early prophylaxis and personalized treatment of AF.

Nkx2-5 Regulates Tdgf1 (Cripto) Early During Cardiac Development

Congenital Heart Disease (CHD) is the most frequent and deadly birth defect. Patients with CHD that survive the neonatal period often progress to develop advanced heart failure requiring specialized treatment including cardiac transplantation. A full understanding of the transcriptional networks that direct cardiac progenitors during heart development will enhance our understanding of both normal cardiac function and pathological states. These findings will also have important applications for emerging therapies and the treatment of congenital heart disease. Furthermore, a number of shared transcriptional pathways or networks have been proposed to regulate the development and regeneration of tissues such as the heart. We have utilized transgenic technology to isolate and characterize cardiac progenitor cells from the developing mouse heart and have begun to define specific transcriptional networks of cardiovascular development. Initial studies identified Tdgf1 as a potential target of Nkx2-5. To mechanistically dissect the regulation of this molecular program, we utilized an array of molecular biological techniques to confirm that Nkx2-5 is an upstream regulator of the Tdgf1 gene in early cardiac development. These studies further define Nkx2-5 mediated transcriptional networks and enhance our understanding of cardiac morphogenesis.

Novel GATA5 loss-of-function mutations underlie familial atrial fibrillation

OBJECTIVE

This study aimed to identify novel GATA5 mutations that underlie familial atrial fibrillation.

METHODS

A total of 110 unrelated patients with familial atrial fibrillation and 200 unrelated, ethnically matched healthy controls were recruited. The entire coding region of the GATA5 gene was sequenced in 110 atrial fibrillation probands. The available relatives of the mutation carriers and 200 controls were subsequently genotyped for the identified mutations. The functional effect of the mutated GATA5 was characterized using a luciferase reporter assay system.

RESULTS

Two novel heterozygous GATA5 mutations (p.Y138F and p.C210G) were identified in two of the 110 unrelated atrial fibrillation families. These missense mutations cosegregated with AF in the families and were absent in the 400 control chromosomes. A cross-species alignment of GATA5 protein sequence showed that the altered amino acids were completely conserved evolutionarily. A functional analysis revealed that the mutant GATA5 proteins were associated with significantly decreased transcriptional activation when compared with their wild-type counterpart.

CONCLUSION

The findings expand the spectrum of GATA5 mutations linked to AF and provide novel insights into the molecular mechanism involved in the pathogenesis of atrial fibrillation, suggesting potential implications for the early prophylaxis and personalized treatment of this common arrhythmia.

A novel GATA5 loss-of-function mutation underlies lone atrial fibrillation

Atrial fibrillation (AF), the most common sustained cardiac arrhythmia, is associated with significantly increased morbidity and mortality. Cumulative evidence highlights the importance of genetic defects in the pathogenesis of AF. However, AF is of remarkable heterogeneity and the genetic determinants of AF in a vast majority of patients remain illusive. In this study, the coding exons and splice junctions of the GATA5 gene, which encodes a zinc-finger transcription factor essential for normal cardiogenesis, were sequenced in 118 unrelated patients with lone AF. The available relatives of the index patient carrying an identified mutation and 200 unrelated ethnically-matched healthy individuals used as controls were genotyped. The functional effect of the mutant GATA5 was characterized in contrast to its wild-type counterpart using a luciferase reporter assay system. As a result, a novel heterozygous GATA5 mutation, p.W200G, was identified in a family with AF inherited as an autosomal dominant trait. The mutation was absent in 200 control individuals and the altered amino acid was completely conserved evolutionarily across species. Functional analysis showed that the mutation of GATA5 was associated with a significantly decreased transcriptional activity. These findings provide novel insight into the molecular mechanism involved in AF, suggesting potential implications for the early prophylaxis and gene-specific therapy of AF.

The role of Shox2 in SAN development and function

Embryonic development is a tightly regulated process, and many families of genes functions to provide a regulatory genetic network to achieve such a program. The homeobox genes are an extensive family that encodes transcription factors with a characteristic 60-amino acid homeodomain. Mutations in these genes or in the encoded proteins might result in structural malformations, physiological defects, and even embryonic death. Mutations in the short-stature homeobox gene (SHOX) is associated with idiopathic short stature in humans, as observed in patients with Turner syndrome and/or Leri-Weill dyschondrosteosis. A closely related human homolog, SHOX2, has not been linked to any syndrome or defect so far. In mice, a SHOX ortholog gene is not present in the genome; however, a true SHOX2 ortholog has been identified. Analyses of Shox2 knockout mouse models have showed crucial functions during embryonic development, including limb skeletogenesis, palatogenesis, temporomandibular joint formation, and cardiovascular development. During embryonic cardiac development, Shox2 is restrictedly expressed in the sinus venosus region, including the sinoatrial node (SAN) and the sinus valves. Shox2 null mutant is embryonically lethal due to cardiovascular defects, including a severely hypoplastic SAN and sinus valves attributed to a significantly decreased level of cell proliferation in addition to an abnormal low heartbeat rate (bradycardia). In addition, it has been demonstrated that Shox2 regulates a genetic network through the repression of Nkx2.5 to maintain the SAN fate and thus plays essential roles in its proper formation and differentiation.

Novel GATA6 loss-of-function mutation responsible for familial atrial fibrillation

Atrial fibrillation (AF) is the most commonly sustained cardiac arrhythmia, and confers a substantially increased risk of morbidity and mortality. Increasing evidence has indicated that hereditary defects are implicated in AF. However, AF is genetically heterogeneous and the genetic etiology of AF in a significant portion of patients remains unclear. In this study, the entire coding sequence and splice junctions of the GATA6 gene, which encodes a zinc-finger transcription factor crucial for cardiogenesis, were sequenced in 140 unrelated patients with lone AF. The available relatives of the index patient carrying an identified mutation and 200 unrelated ethnically-matched healthy individuals used as the controls were genotyped. The functional characteristics of the mutant GATA6 were assessed in contrast to its wild-type counterpart using a luciferase reporter assay system. As a result, a novel heterozygous GATA6 mutation, p.G469V, was identified in a family with AF inherited in an autosomal dominant pattern. The mutation was absent in the 200 control individuals and the altered amino acid was completely conserved across species. Functional analysis demonstrated that the GATA6 mutation was associated with a significantly decreased transcriptional activity. The findings provide novel insight into the molecular mechanism involved in the pathogenesis of AF, as well as insight into potential therapies for the prevention and treatment of AF.

Mutation spectrum of the GATA4 gene in patients with idiopathic atrial fibrillation

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia associated with substantially increased morbidity and mortality. Growing evidence strongly implicates hereditary determinants in the pathogenesis of AF. However, AF is genetically heterogeneous and the genetic defects responsible for AF in the majority of cases remain to be identified. In this study, all the coding exons and splice junctions of GATA4, a gene encoding a zinc-finger transcription factor critical for normal cardiac morphogenesis, were sequenced in a cohort of 150 unrelated patients with idiopathic AF. The available relatives of the mutation carriers and a total of 200 unrelated ethnically matched healthy individuals used as controls were genotyped for the presence of mutations identified in index patients. The functional effect of the mutant GATA4 was characterized using a luciferase reporter assay system. As a result, two novel heterozygous GATA4 mutations (p.Y38D and p.P103A) were identified in 2 unrelated families with AF, respectively. In each family the mutation co-segregated with AF and was absent in the 400 control chromosomes. Functional analysis showed that the mutations of GATA4 were associated with a significantly decreased transcriptional activity. The findings expand the mutation spectrum of GATA4 linked to AF, and further support the notion that compromised GATA4 confers genetic susceptibility to AF.

Mutations in ZIC3 and ACVR2B are a common cause of heterotaxy and associated cardiovascular anomalies

BACKGROUND

Heterotaxy syndrome is caused by left-right asymmetry disturbances and is associated with abnormal lateralisation of the abdominal and thoracic organs. The heart is frequently involved and the severity of the abnormality usually determines the outcome.

METHODS

We performed a direct sequence analysis of the coding sequence of genes including Zinc Finger Protein of the Cerebellum 3, Left-Right Determination Factor 2, Activin A Receptor Type IIB, and Cryptic in 47 patients with laterality defects and congenital cardiac disease.

RESULTS

Of the 47 patients, 31 (66%) had atrioventricular septal defects, 34 (72%) had abnormal systemic venous return, 25 (53%) had transposed or malposed great arteries, and 20 (43%) had pulmonary venous abnormalities. We identified two novel genetic changes in Zinc Finger Protein of the Cerebellum 3, and these variants were not present in 100 ethnically matched control samples. One previously reported missense mutation in Activin A Receptor Type IIB was identified in two unrelated subjects. The genetic changes identified in this study are all located in conserved regions and are predicted to affect protein function in left-right axis formation and cardiovascular development.

CONCLUSIONS

Mutations in Zinc Finger Protein of the Cerebellum 3 and Activin A Receptor Type IIB were identified in 4 of the 47 patients with heterotaxy syndrome for a yield of approximately 8.5%. Our results expand the mutation spectrum of monogenic heterotaxy syndrome with associated cardiac anomalies and suggest that there are other causes of heterotaxy yet to be identified.

Mechanisms of cardiogenesis in cardiovascular progenitor cells

Self-renewing cells of the vertebrate heart have become a major subject of interest in the past decade. However, many researchers had a hard time to argue against the orthodox textbook view that defines the heart as a postmitotic organ. Once the scientific community agreed on the existence of self-renewing cells in the vertebrate heart, their origin was again put on trial when transdifferentiation, dedifferentiation, and reprogramming could no longer be excluded as potential sources of self-renewal in the adult organ. Additionally, the presence of self-renewing pluripotent cells in the peripheral blood challenges the concept of tissue-specific stem and progenitor cells. Leaving these unsolved problems aside, it seems very desirable to learn about the basic biology of this unique cell type. Thus, we shall here paint a picture of cardiovascular progenitor cells including the current knowledge about their origin, basic nature, and the molecular mechanisms guiding proliferation and differentiation into somatic cells of the heart.

GATA4 loss-of-function mutations in familial atrial fibrillation

BACKGROUND

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia and a major source of the substantially increased morbidity and mortality. Growing studies demonstrate that genetic defects play pivotal roles in a subgroup of AF. However, AF is a genetically heterogeneous disorder and the molecular basis of AF in a majority of cases remains unknown.

METHODS

The whole coding region of the GATA4 gene, which encodes a zinc-finger transcription factor essential for cardiogenesis, was analyzed in 130 unrelated probands with AF in contrast to 200 unrelated ethnically matched healthy individuals used as controls. The available family members of the probands harboring the identified mutations were genotyped. The functional effect of the mutant GATA4 was characterized using a luciferase reporter assay system.

RESULTS

Two novel heterozygous GATA4 mutations, p.S70T and p.S160T, were identified in 2 unrelated families with AF inherited as an autosomal dominant trait, respectively, which co-segregated with AF in each family with complete penetrance. Functional analysis showed that the mutations of GATA4 were associated with a significantly decreased transcriptional activity.

CONCLUSION

The findings provide new insight into the molecular mechanism involved in the pathogenesis of AF, suggesting the potential implications in the genetic diagnosis and gene-specific therapy of this common arrhythmia.

Ectopic expression of Nkx2.5 suppresses the formation of the sinoatrial node in mice

The sinoatrial node (SAN), functionally known as the pacemaker, regulates the cardiac rhythm or heartbeat. Several genes are expressed in the developing SAN and form a genetic network regulating the fate of the SAN cells. The short stature homeobox gene Shox2 is an important player in the SAN genetic network by regulating the expression of different cardiac conduction molecular markers including the early cardiac differentiation marker Nkx2.5. Here we report that the expression patterns of Shox2 and Nkx2.5 are mutually exclusive from the earliest stages of the venous pole and the SAN formation. We show that tissue specific ectopic expression of Shox2 in the developing mouse heart downregulates the expression of Nkx2.5 and causes cardiac malformations; however, it is not sufficient to induce a SAN cell fate switch in the working myocardium. On the other hand, tissue specific overexpression of Nkx2.5 in the heart leads to severe hypoplasia of the SAN and the venous valves, dis-regulation of the SAN genetic network, and change of the SAN cell fate into working myocardium, and causes embryonic lethality, recapitulating the phenotypes including bradycardia observed in Shox2(-/-) mutants. These results indicate that Nkx2.5 activity is detrimental to the normal formation of the SAN. Taken together, our results demonstrate that Shox2 downregulation of Nkx2.5 is essential for the proper development of the SAN and that Shox2 functions to shield the SAN from becoming working myocardium by acting upstream of Nkx2.5.

Mutations of the GATA4 and NKX2.5 genes in Chinese pediatric patients with non-familial congenital heart disease

A number of mutations in GATA4 and NKX2.5 have been identified to be causative for a subset of familial congenital heart defects (CHDs) and a small number of sporadic CHDs. In this study, we evaluated common GATA4 and NKX2.5 mutations in 135 Chinese pediatric patients with non-familial congenital heart defects. Two novel mutations in the coding region of GATA4 were identified, namely, 487C >T (Pro163Ser) in exon 1 in a child with tetralogy of Fallot and 1220C >A (Pro407Gln) in exon 6 in a pediatric patient with outlet membranous ventricular septal defect. We also found 848C >A (Pro283Gln) in exon 2 of the NKX2.5 gene in a pediatric patient with ventricular septal defect, patent ductus arteriosus and aortic isthmus stenosis. None of the mutations was detected in healthy control subjects (n = 114). This study suggests that GATA4 and NKX2.5 missense mutations may be associated with congenital heart defects in pediatric Chinese patients. Further clinical studies with large samples are warranted.

Stem cell models of cardiac development and disease

The past few years have witnessed remarkable advances in stem cell biology and human genetics, and we have arrived at an era in which patient-specific cell and tissue models are now practical. The recent identification of cardiovascular progenitor cells, as well as the identification of genetic variants underlying congenital heart disorders and adult disease, opens the door to the development of human models of human cardiovascular disease. We review the current understanding of the contribution of progenitor cells to cardiogenesis and outline how pluripotent stem cells can be applied to the modeling of cardiovascular disorders of genetic origin. A key challenge will be to implement these models in an efficient manner to develop a molecular understanding of how genes lead to disease and to screen for genes and drugs that modify the disease process.

NKX2-5: an update on this hypermutable homeodomain protein and its role in human congenital heart disease (CHD)

Congenital heart disease (CHD) is among the most prevalent and fatal of all birth defects. Deciphering its causes, however, is complicated, as many patients affected by CHD have no family history of the disease. There is also widespread heterogeneity of cardiac malformations within affected individuals. Nonetheless, there have been tremendous efforts toward a better understanding of the molecular and cellular events leading to CHD. Notably, certain cardiac-specific transcription factors have been implicated in mammalian heart development and disruption of their activity has been demonstrated in CHD. The homeodomain transcription factor NKX2-5 is an important member of this group. Indeed, more than 40 heterozygous NKX2-5 germline mutations have been observed in individuals with CHD, and these are spread along the coding region, with many shown to impact protein function. Thus, NKX2-5 appears to be hypermutable, yet the overall detection frequency in sporadic CHD is about 2% and NKX2-5 mutations are one-time detections with single-positives or private to families. Furthermore, there is lack of genotype-phenotype correlation, in which the same cardiac malformations have been exhibited in different NKX2-5 mutations or the same NKX2-5 mutation associated with diverse malformations. Here, we summarize published NKX2-5 germline mutations and explore different avenues in disease pathogenesis to support the notion of a multifactorial cause of CHD where possibly several genes and associated pathways are involved.

Review of genetic factors in intestinal malrotation

Intestinal malrotation is well covered in the surgical literature from the point of view of operative management, but few reviews to date have attempted to provide a comprehensive examination of the topic from the point of view of aetiology, in particular genetic aetiology. Following a brief overview of molecular embryology of midgut rotation, we present in this article instances of and case reports and case series of intestinal malrotation in which a genetic aetiology is likely. Autosomal dominant, autosomal recessive, X-linked and chromosomal forms of the disorder are represented. Most occur in syndromic form, that is to say, in association with other malformations. In many instances, recognition of a specific syndrome is possible, one of several examples discussed being the recently described association of intestinal malrotation with alveolar capillary dysplasia, due to mutations in the forkhead box transcription factor FOXF1. New advances in sequencing technology mean that the identification of the genes mutated in these disorders is more accessible than ever, and paediatric surgeons are encouraged to refer to their colleagues in clinical genetics where a genetic aetiology seems likely.

Genetic factors in non-syndromic congenital heart malformations

The genetic defect in most patients with non-syndromic congenital heart malformations (CHM) is unknown, although more than 40 different genes have already been implicated. Only a minority of CHM seems to be due to monogenetic mutations, and the majority occurs sporadically. The multifactorial inheritance hypothesis of common diseases suggesting that the cumulative effect of multiple genetic and environmental risk factors leads to disease, might also apply for CHM. We review here the monogenic disease genes with high-penetrance mutations, susceptibility genes with reduced-penetrance mutations, and somatic mutations implicated in non-syndromic CHM.

Genetic origins of pediatric heart disease

Pediatric heart disease comprises many forms of cardiovascular disease in the young including cardiovascular malformations (CVM), cardiomyopathies, vasculopathies, e.g., Marfan syndrome, and cardiac arrhythmias. CVM are an important component of pediatric heart disease and constitute a major portion of clinically significant birth defects. In the past decade, the complementary nature of genetic, developmental, and biochemical approaches have contributed to extraordinary advances in understanding the origins of pediatric heart disease. Results of the studies of the cardiac transcription factor, NKX2.5, illustrate these accomplishments and at the same time provide a forecast of the nature of future genetic studies to better understand the origins of pediatric heart disease.

Left-right asymmetry in gut development: what happens next?

The gastrointestinal tract is an asymmetrically patterned organ system. The signals which initiate left-right asymmetry in the developing embryo have been extensively studied, but the downstream steps required to confer asymmetric morphogenesis on the gut organ primordia are less well understood. In this paper we outline key findings on the tissue mechanics underlying gut asymmetry, across a range of species, and use these to synthesise a conserved model for asymmetric gut morphogenesis. We also discuss the importance of correct establishment of left-right asymmetry for gut development and the consequences of perturbations in this process.

Genetic screening of 104 patients with congenitally malformed hearts revealed a fresh mutation of GATA4 in those with atrial septal defects

We analysed the GATA binding protein 4 gene, or GATA4, along with the NK2 transcription factor related, locus 5 gene, or NKX2.5, to determine their genetic contribution to 104 sporadic patients in Indonesia with congenitally malformed hearts, 76 cases having atrial septal defect and 28 tetralogy of Fallot. We found only 1 novel mutation of GATA4 in those with atrial septal defects. Analysis of the genetic background of the parents of the patient showed for the first time that a new mutation of GATA4 can cause sporadic atrial septal defects. We failed to discover any other mutations of either the GATA4 or NKX2-5 genes, supporting the marked genetic heterogeneity of human congenital cardiac defects.

Screening NKX2.5 mutation in a sample of 230 Han Chinese children with congenital heart diseases

Congenital heart disease (CHD) is the most common developmental anomaly, affecting approximately 1% of all newborns. Genetic factors play an important role in CHD's development. Germline mutations in NK2 transcription factor related, locus 5 (NKX2.5) have been identified as the factors responsible for various forms of CHD. In this study, we investigated mutations of the NKX2.5 gene's coding region in 230 nonsyndromic CHD patients belonging to the Chinese Han nationality by PCR, denaturing high-performance liquid chromatography, and sequencing. Pathogenic mutations were not found among the patients. Two known single-nucleotide polymorphisms (rs2277923 and rs3729753) were detected, but the differences in the allele and genotype frequencies were insignificant between CHD and the controls (p > 0.05). The data we gathered suggest that NKX2.5 mutations are highly rare in CHD patients of the Chinese Han nationality. Therefore, NKX2.5 mutation investigation should be limited within a number of familial and special phenotype of CHD in Chinese patients.

Shox2 is essential for the differentiation of cardiac pacemaker cells by repressing Nkx2-5

The pacemaker is composed of specialized cardiomyocytes located within the sinoatrial node (SAN), and is responsible for originating and regulating the heart beat. Recent advances towards understanding the SAN development have been made on the genetic control and gene interaction within this structure. Here we report that the Shox2 homeodomain transcription factor is restrictedly expressed in the sinus venosus region including the SAN and the sinus valves during embryonic heart development. Shox2 null mutation results in embryonic lethality due to cardiovascular defects, including an abnormal low heart beat rate (bradycardia) and severely hypoplastic SAN and sinus valves attributed to a significantly decreased level of cell proliferation. Genetically, the lack of Tbx3 and Hcn4 expression, along with ectopic activation of Nppa, Cx40, and Nkx2-5 in the Shox2(-/-) SAN region, indicates a failure in SAN differentiation. Furthermore, Shox2 overexpression in Xenopus embryos results in extensive repression of Nkx2-5 in the developing heart, leading to a reduced cardiac field and aberrant heart formation. Reporter gene expression assays provide additional evidence for the repression of Nkx2-5 promoter activity by Shox2. Taken together our results demonstrate that Shox2 plays an essential role in the SAN and pacemaker development by controlling a genetic cascade through the repression of Nkx2-5.

Identification and functional characterization of NODAL rare variants in heterotaxy and isolated cardiovascular malformations

NODAL and its signaling pathway are known to play a key role in specification and patterning of vertebrate embryos. Mutations in several genes encoding components of the NODAL signaling pathway have previously been implicated in the pathogenesis of human left-right (LR) patterning defects. Therefore, NODAL, a member of TGF-beta superfamily of developmental regulators, is a strong candidate to be functionally involved in congenital LR axis patterning defects or heterotaxy. Here we have investigated whether variants in NODAL are present in patients with heterotaxy and/or isolated cardiovascular malformations (CVM) thought to be caused by abnormal heart tube looping. Analysis of a large cohort of cases (n = 269) affected with either classic heterotaxy or looping CVM revealed four different missense variants, one in-frame insertion/deletion and two conserved splice site variants in 14 unrelated subjects (14/269, 5.2%). Although similar with regard to other associated defects, individuals with the NODAL mutations had a significantly higher occurrence of pulmonary valve atresia (P = 0.001) compared with cases without a detectable NODAL mutation. Functional analyses demonstrate that the missense variant forms of NODAL exhibit significant impairment of signaling as measured by decreased Cripto (TDGF-1) co-receptor-mediated activation of artificial reporters. Expression of these NODAL proteins also led to reduced induction of Smad2 phosphorylation and impaired Smad2 nuclear import. Taken together, these results support a role for mutations and rare deleterious variants in NODAL as a cause for sporadic human LR patterning defects.

Physiology, pathology and relatedness of human tissues from gene expression meta-analysis

BACKGROUND

Development and maintenance of the identity of tissues is of central importance for multicellular organisms. Based on gene expression profiles, it is possible to divide genes in housekeeping genes and those whose expression is preferential in one or a few tissues and which provide specialized functions that have a strong effect on the physiology of the whole organism.

RESULTS

We have surveyed the gene expression in 78 normal human tissues integrating publicly available microarray gene expression data. A total amount of 1601 genes were identified as selectively expressed in one or more tissues. The tissue-selective genes covered a wide range of cellular and molecular functions, and could be linked to 361 human diseases with Mendelian inheritance. Based on the gene expression profiles, we were able to form a network of tissues reflecting their functional relatedness and, to certain extent, their development. Using co-citation driven gene network technique and promoter analysis, we predicted a transcriptional module where the co-operation of the transcription factors E2F and NF-kappaB can possibly regulate a number of genes involved in the neurogenesis that takes place in the adult hippocampus.

CONCLUSIONS

Here we propose that integration of gene expression data from Affymetrix GeneChip experiments is possible through re-annotation and commonly used pre-processing methods. We suggest that some functional aspects of the tissues can be explained by the co-operation of multiple transcription factors that regulate the expression of selected groups of genes.

Genetic basis for congenital heart defects: current knowledge: a scientific statement from the American Heart Association Congenital Cardiac Defects Committee, Council on Cardiovascular Disease in the Young: endorsed by the American Academy of Pediatrics

The intent of this review is to provide the clinician with a summary of what is currently known about the contribution of genetics to the origin of congenital heart disease. Techniques are discussed to evaluate children with heart disease for genetic alterations. Many of these techniques are now available on a clinical basis. Information on the genetic and clinical evaluation of children with cardiac disease is presented, and several tables have been constructed to aid the clinician in the assessment of children with different types of heart disease. Genetic algorithms for cardiac defects have been constructed and are available in an appendix. It is anticipated that this summary will update a wide range of medical personnel, including pediatric cardiologists and pediatricians, adult cardiologists, internists, obstetricians, nurses, and thoracic surgeons, about the genetic aspects of congenital heart disease and will encourage an interdisciplinary approach to the child and adult with congenital heart disease.

Novel point mutation in the NKX2-5 gene in a Moroccan family with atrioventricular conduction disturbance and an atrial septal defect in the oval fossa

Defects of the oval fossa usually occur as isolated malformations, but can show an autosomal dominant pedigree in familial cases. Several mutations have been described for the transcription factor NKX2-5, and co-segregate with varied cardiac anomalies. We have identified by sequence analysis a novel missense heterozygous mutation in the NKX2-5 gene, specifically a substitution of glutamine for proline at codon 160, in a Moroccan family, the affected members having a deficiency of the floor of the oval fossa and atrioventricular block.

A novel CSX/NKX2-5 mutation causes autosomal-dominant AV block: are atrial fibrillation and syncopes part of the phenotype?

The prevalence of congenital heart defects is approximately 1% of all live births. Identifying the genes responsible for cardiac malformation is the first step to understand pathogenesis. Heterozygous mutations in the CSX/NKX2-5 (NKX2E) gene have been identified to cause atrial septal defect (ASD) and/or atrioventricular (AV) conduction disturbance in some families. However, there is great variability in expressivity of the phenotype between the patients with a CSX/NKX2-5 mutation. We screened four sporadic patients and three index cases of families with ASD and/or conduction defects. In one of them, a CSX/NKX2-5 mutation was identified. This novel mutation (p.Tyr256X) was inherited in a three-generation family causing five individuals to have cardiac anomalies ranging from ASD to arrhythmias. Interestingly, all the observed AV conduction disturbances were at the nodal level, manifesting first as an AV block of the first degree and evolving toward a second-degree block. Atrial fibrillation, previously reported in three individuals with CSX/NKX2-5 mutations, was observed in three patients.

Transcription factors and congenital heart defects

Although there have been important advances in diagnostic modalities and therapeutic strategies for congenital heart defects (CHD), these malformations still lead to significant morbidity and mortality in the human population. Over the past 10 years, characterization of the genetic causes of CHD has begun to elucidate some of the molecular causes of these defects. Linkage analysis and candidate-gene approaches have been used to identify gene mutations that are associated with both familial and sporadic cases of CHD. Complementation of the human studies with developmental studies in mouse models provides information for the roles of these genes in normal development as well as indications for disease pathogenesis. Biochemical analysis of these gene mutations has provided further insight into the molecular effects of these genetic mutations. Here we review genetic, developmental, and biochemical studies of six cardiac transcription factors that have been identified as genetic causes for CHD in humans.

Localization of candidate regions for a novel gene for Kartagener syndrome

Asymmetric positioning of internal organs is a characteristics of vertebrates. The normal left-right anatomic positioning, situs solitus, sometimes does not occur normaly, leading to laterality defects. Studies in animal models have shown that laterality decisions are mediated by a cascade of genes that lead to the asymmetric expression of Nodal, LEFTA, LEFTB and PITX2 in the lateral plate mesoderm. A search for mutations in genes implicated in left-right patterning in animal models allowed genes associated with heterotaxia defects in humans to be identified. However, these genes explain only a small percentage of human situs defects, suggesting that other genes must play a role. In this study, we report a consanguineous family of Turkish origin, composed of two unaffected parents and three children, two of whom presented Kartagener syndrome. On the basis of their family history, we hypothesize autosomal recessive mode of inheritance. A genotype analysis with polymorphic markers did not show linkage with any known genes or loci causing laterality disorders. Array CGH did not detect a duplication or microdeletion greater than 1 Mb as a possible cause. Genome wide screening using 10 K Affymetrix SNP chips was performed, allowing the identification of two regions of autozygosity, one in chromosome 1 and the other on chromosome 7. In the chromosome 1 locus, a strong candidate gene, encoding the kinesin-associated protein 3 (KIF3AP) was not mutated, based on SSCP/heteroduplex analysis and direct sequencing. These data provide a basis for the identification of a novel gene implicated in Kartagener syndrome.

Human laterality disorders

Heterotaxia is a group of congenital disorders characterized by a misplacement of one or more organs according to the left-right axis. Bilateral asymmetry of internal organs is conserved among all vertebrate species. Analyses in animal models such as mouse, chicken, frog and zebrafish allowed for a remarkable progress of knowledge on the embryonic and genetic mechanisms underlying internal left-right asymmetry. In this review we focus on the insights from these model organisms that are useful for a better understanding of the etiology and pathogenesis of human heterotaxia. The known causes of human heterotaxia are reviewed and situated within the conceptual framework that originates from vertebrate model organisms. Furthermore, we attempt to apply the rapidly increasing insights gained from both animal models and human genetics to clinical practice in order to contribute to a more accurate conceptual classification, genetic diagnosis and counseling.

Differentiation of cardiac Purkinje fibers requires precise spatiotemporal regulation of Nkx2-5 expression

Nkx2-5 gene mutations cause cardiac abnormalities, including deficits of function in the atrioventricular conduction system (AVCS). In the chick, Nkx2-5 is elevated in Purkinje fiber AVCS cells relative to working cardiomyocytes. Here, we show that Nkx2-5 expression rises to a peak as Purkinje fibers progressively differentiate. To disrupt this pattern, we overexpressed Nkx2-5 from embryonic day 10, as Purkinje fibers are recruited within developing chick hearts. Overexpression of Nkx2-5 caused inhibition of slow tonic myosin heavy chain protein (sMHC), a late Purkinje fiber marker but did not affect Cx40 levels. Working cardiomyocytes overexpressing Nkx2-5 in these hearts ectopically up-regulated Cx40 but not sMHC. Isolated embryonic cardiomyocytes overexpressing Nkx2-5 also displayed increased Cx40 and suppressed sMHC. By contrast, overexpression of a human NKX2-5 mutant did not effect these markers in vivo or in vitro, suggesting one possible mechanism for clinical phenotypes. We conclude that a prerequisite for normal Purkinje fiber maturation is precise regulation of Nkx2-5 levels.

Medical sequencing of candidate genes for nonsyndromic cleft lip and palate

Nonsyndromic or isolated cleft lip with or without cleft palate (CL/P) occurs in wide geographic distribution with an average birth prevalence of 1/700. We used direct sequencing as an approach to study candidate genes for CL/P. We report here the results of sequencing on 20 candidate genes for clefts in 184 cases with CL/P selected with an emphasis on severity and positive family history. Genes were selected based on expression patterns, animal models, and/or role in known human clefting syndromes. For seven genes with identified coding mutations that are potentially etiologic, we performed linkage disequilibrium studies as well in 501 family triads (affected child/mother/father). The recently reported MSX1 P147Q mutation was also studied in an additional 1,098 cleft cases. Selected missense mutations were screened in 1,064 controls from unrelated individuals on the Centre d'Étude du Polymorphisme Humain (CEPH) diversity cell line panel. Our aggregate data suggest that point mutations in these candidate genes are likely to contribute to 6% of isolated clefts, particularly those with more severe phenotypes (bilateral cleft of the lip with cleft palate). Additional cases, possibly due to microdeletions or isodisomy, were also detected and may contribute to clefts as well. Sequence analysis alone suggests that point mutations in FOXE1, GLI2, JAG2, LHX8, MSX1, MSX2, SATB2, SKI, SPRY2, and TBX10 may be rare causes of isolated cleft lip with or without cleft palate, and the linkage disequilibrium data support a larger, as yet unspecified, role for variants in or near MSX2, JAG2, and SKI. This study also illustrates the need to test large numbers of controls to distinguish rare polymorphic variants and prioritize functional studies for rare point mutations.

Nonsyndromic or isolated cleft lip with or without cleft palate (CL/P) is a birth defect with wide geographic distribution, occurring with an average frequency of 1/700 live births. Treatment can be provided, but it will involve medical, surgical, dental, and psychological personnel. Several different genes have been implicated in different cases. Here the researchers report the results of sequencing 20 different genes in 184 CL/P cases selected with an emphasis on more severe cases and cases with a positive family history for CL/P. Genes were selected based on previous work done by others and by the researchers' group. The authors' results suggest that point mutations in these candidate genes are likely to contribute to about 5% of CL/P, and particularly those with more severe phenotypes (bilateral cleft of the lip with cleft palate). This study also illustrates the need to test large numbers of controls to distinguish rare polymorphic variants and allow investigators to focus functional studies on the rare point mutations that seem to be disease-causing, so that researchers might better understand the mechanisms that play a role in CL/P.

Genetics of human heterotaxias

The past decade has seen remarkable advances in defining the molecular mechanisms underlying formation of the embryonic left right (LR) axis. This information is slowly transforming our understanding of human birth defects that are caused by disturbed LR axis patterning. Reversals, isomerisms, or segmental discordances of thoraco-abdominal organ position, that is, classic heterotaxy, clearly indicate embryonic disruption of normal LR patterning. Other isolated birth defects, particularly cardiovascular malformations, may be caused by deficiencies in the same pathways. Here, we review the distinctive clinical features of human heterotaxias and try to summarize the known connections between them and the corresponding developmental pathways.