TBX5 genetic testing validates strict clinical criteria for Holt-Oram syndrome

TBX5 variants and cardiac phenotype: A systematic review of the literature and a novel variant

T-Box Transcription Factor 5 (TBX5) variants are associated with Holt-Oram syndrome. Holt-Oram syndrome display phenotypic variability, regarding upper limb defects, congenital heart defects, and arrhythmias. To investigate the genotype-phenotype relationship between TBX5 variants and cardiac disease, we performed a systematic review of the literature. Through the systematic review we identified 108 variants in TBX5 associated with a cardiac phenotype in 277 patients. Arrhythmias were more frequent in patients with a missense variant (48% vs 30%, p = 0.009) and upper limb abnormalities were more frequent in patients with protein-truncating variants (85% vs 64%, p = 0.0008). We found clustering of missense variants in the T-box domain. Furthermore, we present a family with atrial septal defects. By whole exome sequencing, we identified a novel missense variant p.Phe232Leu in TBX5. The cardiac phenotype included atrial septal defect, arrhythmias, heart failure, and dilated cardiomyopathy. Clinical examination revealed subtle upper limb abnormalities. Thus, the family corresponds to the diagnostic criteria of Holt-Oram syndrome. We provide an overview of cardiac phenotypes associated with TBX5 variants and show an increased risk of arrhythmias associated to missense variants compared to protein-truncating variants. We report a novel missense variant in TBX5 in a family with an atypical Holt-Oram syndrome phenotype.

Breaking enhancers to gain insights into developmental defects

Despite ground-breaking genetic studies that have identified thousands of risk variants for developmental diseases, how these variants lead to molecular and cellular phenotypes remains a gap in knowledge. Many of these variants are non-coding and occur at enhancers, which orchestrate key regulatory programs during development. The prevailing paradigm is that non-coding variants alter the activity of enhancers, impacting gene expression programs, and ultimately contributing to disease risk. A key obstacle to progress is the systematic functional characterization of non-coding variants at scale, especially since enhancer activity is highly specific to cell type and developmental stage. Here, we review the foundational studies of enhancers in developmental disease and current genomic approaches to functionally characterize developmental enhancers and their variants at scale. In the coming decade, we anticipate systematic enhancer perturbation studies to link non-coding variants to molecular mechanisms, changes in cell state, and disease phenotypes.

TBX5 variant with the novel phenotype of mixed‑type total anomalous pulmonary venous return in Holt‑Oram Syndrome and variable intrafamilial heart defects

Variants in T‑box transcription factor 5 (TBX5) can result in a wide phenotypic spectrum, specifically in the heart and the limbs. TBX5 has been implicated in causing non‑syndromic cardiac defects and Holt‑Oram syndrome (HOS). The present study investigated the underlying molecular etiology of a family with heterogeneous heart defects. The proband had mixed‑type total anomalous pulmonary venous return (mixed‑type TAPVR), whereas her mother had an atrial septal defect. Genetic testing through trio‑based whole‑exome sequencing was used to reveal the molecular etiology. A nonsense variant was identified in TBX5 (c.577G>T; p.Gly193*) initially showing co‑segregation with a presumably non‑syndromic presentation of congenital heart disease. Subsequent genetic investigations and more complete phenotyping led to the correct diagnosis of HOS, documenting the novel association of mixed‑type TAPVR with HOS. Finally, protein modeling of the mutant TBX5 protein that harbored this pathogenic nonsense variant (p.Gly193*) revealed a substantial drop in the quantity of non‑covalent bonds. The decrease in the number of non‑covalent bonds suggested that the resultant mutant dimer was less stable compared with the wild‑type protein, consequently affecting the protein's ability to bind DNA. The present findings extended the phenotypic cardiac defects associated with HOS; to the best of our knowledge, this is the first association of mixed‑type TAPVR with TBX5. Prior to the current analysis, the molecular association of TAPVR with HOS had never been documented; hence, this is the first genetic investigation to report the association between TAPVR and HOS. Furthermore, it was demonstrated that the null‑variants reported in the T‑box domain of TBX5 were associated with a wide range of cardiac and/or skeletal anomalies on both the inter‑and intrafamilial levels. In conclusion, genetic testing was highlighted as a potentially powerful approach in the prognostication of the proper diagnosis.

In vitro relationships of galactic cosmic radiation and epigenetic clocks in human bronchial epithelial cells

Ionizing radiation is a well-appreciated health risk, precipitant of DNA damage, and contributor to DNA methylation variability. Nevertheless, relationships of ionizing radiation with DNA methylation-based markers of biological age (i.e. epigenetic clocks) remain poorly understood. Using existing data from human bronchial epithelial cells, we examined in vitro relationships of three epigenetic clock measures (Horvath DNAmAge, MiAge, and epiTOC2) with galactic cosmic radiation (GCR), which is particularly hazardous due to its high linear energy transfer (LET) heavy-ion components. High-LET 56Fe was significantly associated with accelerations in epiTOC2 (β = 192 cell divisions, 95% CI: 71, 313, p-value = .003). We also observed a significant, positive interaction of 56Fe ions and time-in-culture with epiTOC2 (95% CI: 42, 441, p-value = .019). However, only the direct 56Fe ion association remained statistically significant after adjusting for multiple hypothesis testing. Epigenetic clocks were not significantly associated with high-LET 28Si and low-LET X-rays. Our results demonstrate sensitivities of specific epigenetic clock measures to certain forms of GCR. These findings suggest that epigenetic clocks may have some utility for monitoring and better understanding the health impacts of GCR.

Prenatal diagnosis of Holt-Oram syndrome

Objectives

To detect common congenital disorders in Holt-Oram syndrome.

Case presentation

We present a case of a 32 years old primigravida pregnant woman affected by Holt-Oram syndrome referred to our institution for second trimester routine anatomy scan. The ultrasound reported a bilateral aplasia radii, slightly curved ulna and bilateral twisted hand with four digital rays. A significant enlargement of the right atrium without tricuspid regurgitation was also detected. The patient refused the amniocentesis and the postnatal evaluation confirmed the diagnosis of Holt-Oram syndrome.

Conclusions

Holt-Oram syndrome is an autosomal dominant genetic condition. It is characterized by abnormalities in the bones of the upper limb and congenital heart malformation. The mutation can be inherited, but most cases result from a new mutation in patients without family history of the disorder.

Genomic and physiological analyses of the zebrafish atrioventricular canal reveal molecular building blocks of the secondary pacemaker region

The atrioventricular canal (AVC) is the site where key structures responsible for functional division between heart regions are established, most importantly, the atrioventricular (AV) conduction system and cardiac valves. To elucidate the mechanism underlying AVC development and function, we utilized transgenic zebrafish line sqet31Et expressing EGFP in the AVC to isolate this cell population and profile its transcriptome at 48 and 72 hpf. The zebrafish AVC transcriptome exhibits hallmarks of mammalian AV node, including the expression of genes implicated in its development and those encoding connexins forming low conductance gap junctions. Transcriptome analysis uncovered protein-coding and noncoding transcripts enriched in AVC, which have not been previously associated with this structure, as well as dynamic expression of epithelial-to-mesenchymal transition markers and components of TGF-β, Notch, and Wnt signaling pathways likely reflecting ongoing AVC and valve development. Using transgenic line Tg(myl7:mermaid) encoding voltage-sensitive fluorescent protein, we show that abolishing the pacemaker-containing sinoatrial ring (SAR) through Isl1 loss of function resulted in spontaneous activation in the AVC region, suggesting that it possesses inherent automaticity although insufficient to replace the SAR. The SAR and AVC transcriptomes express partially overlapping species of ion channels and gap junction proteins, reflecting their distinct roles. Besides identifying conserved aspects between zebrafish and mammalian conduction systems, our results established molecular hallmarks of the developing AVC which underlies its role in structural and electrophysiological separation between heart chambers. This data constitutes a valuable resource for studying AVC development and function, and identification of novel candidate genes implicated in these processes.

Genetics of congenital heart disease: a narrative review of recent advances and clinical implications

Congenital heart disease (CHD) is the most common human birth defect and remains a leading cause of mortality in childhood. Although advances in clinical management have improved the survival of children with CHD, adult survivors commonly experience cardiac and non-cardiac comorbidities, which affect quality of life and prognosis. Therefore, the elucidation of genetic etiologies of CHD not only has important clinical implications for genetic counseling of patients and families but may also impact clinical outcomes by identifying at-risk patients. Recent advancements in genetic technologies, including massively parallel sequencing, have allowed for the discovery of new genetic etiologies for CHD. Although variant prioritization and interpretation of pathogenicity remain challenges in the field of CHD genomics, advances in single-cell genomics and functional genomics using cellular and animal models of CHD have the potential to provide novel insights into the underlying mechanisms of CHD and its associated morbidities. In this review, we provide an updated summary of the established genetic contributors to CHD and discuss recent advances in our understanding of the genetic architecture of CHD along with current challenges with the interpretation of genetic variation. Furthermore, we highlight the clinical implications of genetic findings to predict and potentially improve clinical outcomes in patients with CHD.

Functional analysis of two novel TBX5 variants present in individuals with Holt-Oram syndrome with different clinical manifestations

Holt-Oram syndrome (HOS) is a rare disorder characterized by cardiac and upper-limb defects. Pathogenic variants in TBX5-a gene encoding a transcription factor important for heart and skeletal development-are the only known cause of HOS. Here, we present the identification and functional analysis of two novel TBX5 pathogenic variants found in two individuals with HOS presenting distinct phenotypes. The individual with the c.905delA variant has a severe cardiac phenotype but mild skeletal defects, unlike the individual with the c.246_249delGATG variant who has no cardiac problems but severe upper limbs malformations, including phocomelia. Both frameshift variants, c.246_249delGATG and c.905delA, generate mRNAs harbouring premature stop codons which, if not degraded by nonsense mediated decay, will lead to the production of shorter TBX5 proteins, p.Gln302Argfs*92 and p.Met83Phefs*6, respectively. Immunocytochemistry results suggest that both mutated proteins are produced and furthermore, like the wild-type protein, p.Gln302Argfs*92 mutant appears to be mainly localized in the nucleus, in contrast with p.Met83Phefs*6 mutant that displays a higher level of cytoplasmic localization. In addition, luciferase activity analysis revealed that none of the TBX5 mutants are capable of transactivating the NPPA promoter. In conclusion, our results provide evidence that both pathogenic variants cause a severe TBX5 loss-of-function, dramatically reducing its biological activity. The absence of cardiac problems in the individual with the p.Met83Phefs*6 variant supports the existence of other mechanisms/genes underlying the pathogenesis of HOS and/or the existence of an age-related delay in the development of a more serious cardiac phenotype. Further studies are required to understand the differential effects observed in the phenotypes of both individuals.

Decoding Genetics of Congenital Heart Disease Using Patient-Derived Induced Pluripotent Stem Cells (iPSCs)

Congenital heart disease (CHD) is the most common cause of infant death associated with birth defects. Recent next-generation genome sequencing has uncovered novel genetic etiologies of CHD, from inherited and de novo variants to non-coding genetic variants. The next phase of understanding the genetic contributors of CHD will be the functional illustration and validation of this genome sequencing data in cellular and animal model systems. Human induced pluripotent stem cells (iPSCs) have opened up new horizons to investigate genetic mechanisms of CHD using clinically relevant and patient-specific cardiac cells such as cardiomyocytes, endothelial/endocardial cells, cardiac fibroblasts and vascular smooth muscle cells. Using cutting-edge CRISPR/Cas9 genome editing tools, a given genetic variant can be corrected in diseased iPSCs and introduced to healthy iPSCs to define the pathogenicity of the variant and molecular basis of CHD. In this review, we discuss the recent progress in genetics of CHD deciphered by large-scale genome sequencing and explore how genome-edited patient iPSCs are poised to decode the genetic etiologies of CHD by coupling with single-cell genomics and organoid technologies.

TBX5-encoded T-box transcription factor 5 variant T223M is associated with long QT syndrome and pediatric sudden cardiac death

Long QT syndrome (LQTS) is a genetic disease resulting in a prolonged QT interval on a resting electrocardiogram, predisposing affected individuals to polymorphic ventricular tachycardia and sudden death. Although a number of genes have been implicated in this disease, nearly one in four individuals exhibiting the LQTS phenotype are genotype-negative. Whole-exome sequencing identified a missense T223M variant in TBX5 that cosegregates with prolonged QT interval in a family with otherwise genotype-negative LQTS and sudden death. The TBX5-T223M variant was absent among large ostensibly healthy populations (gnomAD) and predicted to be pathogenic by in silico modeling based on Panther, PolyPhen-2, Provean, SIFT, SNAP2, and PredictSNP prediction tools. The variant was located in a highly conserved region of TBX5 predicted to be part of the DNA-binding interface. A luciferase assay identified a 57.5% reduction in the ability of TBX5-T223M to drive expression at the atrial natriuretic factor promotor compared to wildtype TBX5 in vitro. We conclude that the variant is pathogenic in this family, and we put TBX5 forward as a disease susceptibility allele for genotype-negative LQTS. The identification of this familial variant may serve as a basis for the identification of previously unknown mechanisms of LQTS with broader implications for cardiac electrophysiology.

Genetic Basis of Human Congenital Heart Disease

Congenital heart disease (CHD) is the most common major congenital anomaly with an incidence of ∼1% of live births and is a significant cause of birth defect-related mortality. The genetic mechanisms underlying the development of CHD are complex and remain incompletely understood. Known genetic causes include all classes of genetic variation including chromosomal aneuploidies, copy number variants, and rare and common single-nucleotide variants, which can be either de novo or inherited. Among patients with CHD, ∼8%-12% have a chromosomal abnormality or aneuploidy, between 3% and 25% have a copy number variation, and 3%-5% have a single-gene defect in an established CHD gene with higher likelihood of identifying a genetic cause in patients with nonisolated CHD. These genetic variants disrupt or alter genes that play an important role in normal cardiac development and in some cases have pleiotropic effects on other organs. This work reviews some of the most common genetic causes of CHD as well as what is currently known about the underlying mechanisms.

Transcriptional Patterning of the Ventricular Cardiac Conduction System

RATIONALE

The heartbeat is organized by the cardiac conduction system (CCS), a specialized network of cardiomyocytes. Patterning of the CCS into atrial node versus ventricular conduction system (VCS) components with distinct physiology is essential for the normal heartbeat. Distinct node versus VCS physiology has been recognized for more than a century, but the molecular basis of this regional patterning is not well understood.

OBJECTIVE

To study the genetic and genomic mechanisms underlying node versus VCS distinction and investigate rhythm consequences of failed VCS patterning.

METHODS AND RESULTS

Using mouse genetics, we found that the balance between T-box transcriptional activator, Tbx5, and T-box transcriptional repressor, Tbx3, determined the molecular and functional output of VCS myocytes. Adult VCS-specific removal of Tbx5 or overexpression of Tbx3 re-patterned the fast VCS into slow, nodal-like cells based on molecular and functional criteria. In these cases, gene expression profiling showed diminished expression of genes required for VCS-specific fast conduction but maintenance of expression of genes required for nodal slow conduction physiology. Action potentials of Tbx5-deficient VCS myocytes adopted nodal-specific characteristics, including increased action potential duration and cellular automaticity. Removal of Tbx5 in vivo precipitated inappropriate depolarizations in the atrioventricular (His)-bundle associated with lethal ventricular arrhythmias. TBX5 bound and directly activated cis-regulatory elements at fast conduction channel genes required for fast physiological characteristics of the VCS action potential, defining the identity of the adult VCS.

CONCLUSIONS

The CCS is patterned entirely as a slow, nodal ground state, with a T-box dependent, physiologically dominant, fast conduction network driven specifically in the VCS. Disruption of the fast VCS gene regulatory network allowed nodal physiology to emerge, providing a plausible molecular mechanism for some lethal ventricular arrhythmias.

Familial dilated cardiomyopathy associated with pathogenic TBX5 variants: Expanding the cardiac phenotype associated with Holt-Oram syndrome

Holt-Oram syndrome (HOS) is a rare, autosomal dominant disorder caused by heterozygous pathogenic variants in cardiac T-box transcription factor, TBX5. Classically, it is associated with upper limb malformations and variable cardiac abnormalities. Limb manifestations are considered to be invariably present, ranging in severity from limitation in movement, to triphalangeal thumbs, absent thumbs, shortened forearms, or phocomelia. Cardiac involvement is characterized by congenital heart defects, most commonly septal structural malformations, and conduction system disease. Recently, novel TBX5 variants have also been reported in association with dilated cardiomyopathy (DCM). In this context, we report eight individuals from four unrelated families, in whom pathogenic variants in TBX5 segregated with an atypical HOS phenotype. Affected individuals exhibit relatively mild skeletal features of HOS, with a predominant cardiac phenotype, which includes several individuals affected by non-ischaemic DCM. To our knowledge, these represent the first reported cases of DCM in families with skeletal features of HOS, some of whom also harbored variants previously linked to a classical HOS phenotype (p. Arg279* and p.Arg237Gln). This finding supports diverse roles of TBX5 in cardiovascular development and function, and confirms the importance of long-term cardiac surveillance for individuals affected by HOS. Furthermore, these families highlight the wide phenotypic variability of HOS, which may include comparatively mild upper limb findings in respect to cardiac manifestations.

TBX5 R264K acts as a modifier to develop dilated cardiomyopathy in mice independently of T-box pathway

Background

TBX5 is a transcription factor that has an important role in development of heart. TBX5 variants in the region encoding the T-box domain have been shown to cause cardiac defects, such as atrial septal defect or ventricular septal defect, while TBX5 variants have also been identified in a few cardiomyopathy patients and considered causative. We identified a TBX5 variant (c.791G>A, p.Arg264Lys), that is over-represented in cardiomyopathy patients. This variant is located outside of the T-box domain, and its pathogenicity has not been confirmed by functional analyses.

Objective

To investigate whether the TBX5 R264K is deleterious and could contribute to the pathogenesis of cardiomyopathy.

Methods and results

We developed mice expressing Tbx5 R264K. Mice homozygous for this variant displayed compensated dilated cardiomyopathy; mild decreased fractional shortening, dilatation of the left ventricle, left ventricular wall thinning and increased heart weight without major heart structural disorders. There was no difference in activation of the ANF promotor, a transcriptional target of Tbx5, compared to wild-type. However, analysis of RNA isolated from left ventricular samples showed significant increases in the expression of Acta1 in left ventricle with concomitant increases in the protein level of ACTA1.

Conclusions

Mice homozygous for Tbx5 R264K showed compensated dilated cardiomyopathy. Thus, TBX5 R264K may have a significant pathogenic role in some cardiomyopathy patients independently of T-box domain pathway.

Holt-Oram Syndrome: Hands are the Clue to the Diagnosis

Holt–Oram syndrome or heart–hand syndrome consists of phenotypic and genotypic abnormalities. It is characterized by abnormalities of upper limbs and congenital cardiac defects. It is an autosomal dominant disorder due to a mutation in TBX5 gene located on chromosome 12, but sporadic cases have also been reported. We describe a 26-year-old female with a history of shortness of breath for 5 years. She had bilateral hand deformities, and on evaluation, found to have ostium secundum atrial septal defect which is common cardiac defect in Holt–Oram syndrome.

Genetic Basis for Congenital Heart Disease: Revisited: A Scientific Statement From the American Heart Association

This review provides an updated summary of the state of our knowledge of the genetic contributions to the pathogenesis of congenital heart disease. Since 2007, when the initial American Heart Association scientific statement on the genetic basis of congenital heart disease was published, new genomic techniques have become widely available that have dramatically changed our understanding of the causes of congenital heart disease and, clinically, have allowed more accurate definition of the pathogeneses of congenital heart disease in patients of all ages and even prenatally. Information is presented on new molecular testing techniques and their application to congenital heart disease, both isolated and associated with other congenital anomalies or syndromes. Recent advances in the understanding of copy number variants, syndromes, RASopathies, and heterotaxy/ciliopathies are provided. Insights into new research with congenital heart disease models, including genetically manipulated animals such as mice, chicks, and zebrafish, as well as human induced pluripotent stem cell-based approaches are provided to allow an understanding of how future research breakthroughs for congenital heart disease are likely to happen. It is anticipated that this review will provide a large range of health care-related personnel, including pediatric cardiologists, pediatricians, adult cardiologists, thoracic surgeons, obstetricians, geneticists, genetic counselors, and other related clinicians, timely information on the genetic aspects of congenital heart disease. The objective is to provide a comprehensive basis for interdisciplinary care for those with congenital heart disease.

Fetal cardiac abnormalities: Genetic etiologies to be considered

Congenital heart diseases are a common prenatal finding. The prenatal identification of an associated genetic syndrome or a major extracardiac anomaly helps to understand the etiopathogenic diagnosis. Besides, it also assesses the prognosis, management, and familial recurrence risk while strongly influences parental decision to choose termination of pregnancy or postnatal care. This review article describes the most common genetic diagnoses associated with a prenatal finding of a congenital heart disease and a suggested diagnostic process.

Navigating the non-coding genome in heart development and Congenital Heart Disease

Congenital Heart Disease (CHD) is characterised by a wide range of cardiac defects, from mild to life-threatening, which occur in babies worldwide. To date, there is no cure to CHD, however, progress in surgery has reduced its mortality allowing children affected by CHD to reach adulthood. In an effort to understand its genetic basis, several studies involving whole-genome sequencing (WGS) of patients with CHD have been undertaken and generated a great wealth of information. The majority of putative causative mutations identified in WGS studies fall into the non-coding part of the genome. Unfortunately, due to the lack of understanding of the function of these non-coding mutations, it is challenging to establish a causal link between the non-coding mutation and the disease. Thus, here we review the state-of-the-art approaches to interpret non-coding mutations in the context of CHD and address the following questions: What are the non-coding sequences important for cardiac function? Which technologies are used to identify them? Which resources are available to analyse them? What mutations are expected in these non-coding sequences? Learning from developmental process, what is their expected role in CHD?

Holt-Oram syndrome: clinical and molecular description of 78 patients with TBX5 variants

Holt-Oram syndrome (HOS) is an autosomal dominant condition characterised by the association of congenital heart defect (CHD), with or without rhythm disturbances and radial defects, due to TBX5 variants. The diagnosis is challenged by the variability of expression and the large phenotypic overlap with other conditions, like Okihiro syndrome, TAR syndrome or Fanconi disease. We retrospectively reviewed 212 patients referred for suspicion of HOS between 2002 and 2014, who underwent TBX5 screening. A TBX5 variant has been identified in 78 patients, representing the largest molecular series ever described. In the cohort, 61 met the previously described diagnostic criteria and 17 have been considered with an uncertain HOS diagnosis. A CHD was present in 91% of the patients with a TBX5 variant, atrial septal defects being the most common (61.5%). The genotype-phenotype study highlights the importance of some critical features in HOS: the septal characteristic of the CHD, the bilateral and asymmetric characteristics of the radial defect and the presence of shoulder or elbow mobility defect. Besides, 21 patients presented with an overlapping condition. Among them, 13 had a typical HOS presentation. We discuss the strategies that could be adopted to improve the molecular delineation of the remaining typical patients.

A novel de novo TBX5 mutation in a patient with Holt-Oram syndrome

Holt-Oram syndrome (HOS) is an autosomal dominant disorder characterized by congenital cardiac defects and congenital deformities of the upper limbs. Herein, we report the case of a 2-year-old patient presenting with clinical diagnostic criteria of HOS with interatrial and interventricular communication associated with hip dysplasia and upper limb reduction composed of radial ray anomaly. A novel de novo, potentially pathogenic variant in the TBX5 gene at NM_181486.2:c.243-1G>C was identified.

Holt-Oram Syndrome in a Patient with Crohn's Disease: a Rare Case Report and Literature Review

Introduction:

Holt-Oram syndrome (HOS) is an uncommon autosomal dominant disorder defined by congenital cardiac defects, some anatomical deformities in the upper limb and conduction abnormalities. Sequence alteration of TBX5 gene located on chromosome 12 has associated with HOS.

Case report:

We present the case of a 26-year-old female with known upper limb alteration and ventricular septal defect who later in life developed Crohn’s disease.

Conclusion:

To the best of our knowledge association of Holt-Oram syndrome with Crohn’s disease has not been reported in literature before. Therefore, a possible genetic connection between Holt-Oram syndrome and Crohn’s disease remains to be determined.

Holt-Oram Syndrome With Multiple Cardiac Abnormalities

Holt-Oram syndrome (HOS) is a rare monogenic disorder characterized by upper limb abnormalities, congenital heart defects and/or conduction abnormalities. It is determined by mutations of TBX5 gene and is inherited in an autosomal dominant manner. Penetrance is complete, but variable expressivity is present, which gives sometimes diagnostic difficulties. Our case is a young adult with a personal history of preaxial polydactyly operated in infancy, multiple cardiac malformations (atrial septal defect, bicuspid aortic valve, left ventricular non-compaction) and radiologic findings consistent with HOS. Family history is negative for HOS. In conclusion, we present a case of HOS diagnosed in the adult period to highlight the diagnostic problems for the proband and the family and the importance of an early diagnostic.

Electrical disorders in atrial septal defect: genetics and heritability

Atrial septal defect (ASD) is one of the most common types of congenital heart diseases (CHDs). Most ASDs occur sporadically, but some are inherited and associated with cardiac conduction defects such as atrioventricular block (AVB) or bundle branch block. Mutations in genes encoding transcription factor gene TBX5 and NKX2-5, were found in Holt-Oram syndrome (HOS) and ASD with atrioventricular (AV) conduction defects, respectively. HOS is characterized by upper limb anomaly in addition to ASD and AVB (heart-hand syndrome). ASD associated with NKX2-5 is rare but is reported to cause sudden cardiac death (SCD) or cardiomyopathy. We provide a review of these two diseases.

Clinical and epidemiological features of Heart-Hand Syndrome: a hospital-based study in China

Heart–hand syndrome (HHS) is a clinically and genetically heterogeneous disorder characterized by the co-occurrence of a congenital cardiac disease and an upper limb malformation. This study revealed the clinical and epidemiological features of HHS in China. The study was based on patients with congenital upper limb malformation treated in Beijing Ji Shui Tan hospital from October 1^st, 2013 to October 1^st, 2016. We reviewed the patients’ medical records and identified patients with abnormal ultrasonic cardiogram and/or electrocardiogram (ECG). A total of 1462 patients (910 male and 552 female) were identified to be treated for congenital upper limb malformation. Among them, 172 (11.8%) had abnormal ultrasonic cardiogram and/or ECG. Abnormal heart structure were discovered in 121 patients and 51 patients had abnormal ECG. The most common type of abnormal heart structure was tricuspid regurgitation (53/121, 43.8%), while the most common abnormal ECG was wave patterns (22/51, 43.1%). This hospital-based study suggests that the rate of congenital heart disease is high in patients treated for congenital upper extremity malformation in China. Surgeons and anesthetists should be aware of the comorbidity and preoperational examination of congenital heart diseases is highly needed to avoid complications during operation.

KLF13 is a genetic modifier of the Holt-Oram syndrome gene TBX5

TBX5, a member of the T-box family of transcription factors, is a dosage sensitive regulator of heart development. Mutations in TBX5 are responsible for Holt-Oram Syndrome, an autosomal dominant disease with variable and partially penetrant cardiac defects suggestive of the existence of genetic and environmental modifiers. KLF13, a member of the Krüppel-like family of zinc finger proteins is co-expressed with TBX5 in several cardiac cells including atrial cardiomyocytes and cells of the interatrial septum. We report that KLF13 interacts physically and functionally with TBX5 to synergistically activate transcription of cardiac genes. We show that TBX5 contacts KLF13 via its T-domain and find that several disease-causing mutations therein have decreased KLF13 interaction. Whereas Klf13 heterozygote mice have no detectable cardiac defects, loss of a Klf13 allele in Tbx5 heterozygote mice significantly increases the penetrance of TBX5-dependent cardiac abnormalities including atrial, atrial-ventricular and ventricular septal defects. The results reveal for the first time combinatorial interaction between a T-box protein and a KLF family member and its importance for heart and possibly other organ development. The data also suggest that, in human, KLF13 may be a genetic modifier of the Holt-Oram Syndrome gene TBX5.

An interspecies heart-to-heart: Using Xenopus to uncover the genetic basis of congenital heart disease

PURPOSE OF REVIEW

Given the enormous impact congenital heart disease has on child health, it is imperative that we improve our understanding of the disease mechanisms that underlie patient phenotypes and clinical outcomes. This review will outline the merits of using the frog model, Xenopus, as a tool to study human cardiac development and left-right patterning mechanisms associated with congenital heart disease.

RECENT FINDINGS

Patient-driven gene discovery continues to provide new insight into the mechanisms of congenital heart disease, and by extension, patient phenotypes and outcomes. By identifying gene variants in CHD patients, studies in Xenopus have elucidated the molecular mechanisms of how these candidate genes affect cardiac development, both cardiogenesis as well as left-right patterning, which can have a major impact on cardiac morphogenesis. Xenopus has also proved to be a useful screening tool for the biological relevance of identified patient-mutations, and ongoing investigations continue to illuminate disease mechanisms.

SUMMARY

Analyses in model organisms can help to elucidate the disease mechanisms underlying CHD patient phenotypes. Using Xenopus to disentangle the genotype-phenotype relationships of well-known and novel disease genes could enhance the ability of physicians to efficaciously treat patients and predict clinical outcomes, ultimately improving quality of life and survival rates of patients born with congenital heart disease.

TBX5: A Key Regulator of Heart Development

TBX5 is a member of the T-box transcription factor family and is primarily known for its role in cardiac and forelimb development. Human patients with dominant mutations in TBX5 are characterized by Holt-Oram syndrome, and show defects of the cardiac septa, cardiac conduction system, and the anterior forelimb. The range of cardiac defects associated with TBX5 mutations in humans suggests multiple roles for the transcription factor in cardiac development and function. Animal models demonstrate similar defects and have provided a useful platform for investigating the roles of TBX5 during embryonic development. During early cardiac development, TBX5 appears to act primarily as a transcriptional activator of genes associated with cardiomyocyte maturation and upstream of morphological signals for septation. During later cardiac development, TBX5 is required for patterning of the cardiac conduction system and maintenance of mature cardiomyocyte function. A comprehensive understanding of the integral roles of TBX5 throughout cardiac development and adult life will be critical for understanding human cardiac morphology and function.

The Cardiac TBX5 Interactome Reveals a Chromatin Remodeling Network Essential for Cardiac Septation

Human mutations in the cardiac transcription factor gene TBX5 cause congenital heart disease (CHD), although the underlying mechanism is unknown. We report characterization of the endogenous TBX5 cardiac interactome and demonstrate that TBX5, long considered a transcriptional activator, interacts biochemically and genetically with the nucleosome remodeling and deacetylase (NuRD) repressor complex. Incompatible gene programs are repressed by TBX5 in the developing heart. CHD mis-sense mutations that disrupt the TBX5-NuRD interaction cause depression of a subset of repressed genes. Furthermore, the TBX5-NuRD interaction is required for heart development. Phylogenetic analysis showed that the TBX5-NuRD interaction domain evolved during early diversification of vertebrates, simultaneous with the evolution of cardiac septation. Collectively, this work defines a TBX5-NuRD interaction essential to cardiac development and the evolution of the mammalian heart, and when altered may contribute to human CHD.

A novel de novo TBX5 mutation in a patient with Holt-Oram syndrome leading to a dramatically reduced biological function

Background

The Holt–Oram syndrome (HOS) is an autosomal dominant disorder affecting 1/100.000 live births. It is defined by upper limb anomalies and congenital heart defects with variable severity. We describe a dramatic phenotype of a male, 15‐month‐old patient being investigated for strict diagnostic criteria of HOS.

Methods and results

Genetic analysis revealed a so far unpublished TBX5 mutation, which occurs de novo in the patient with healthy parents. TBX5 belongs to the large family of T‐box transcription factors playing major roles in morphogenesis and cell‐type specification. The mutation located in the DNA‐binding domain at position 920 (C→A) leads to an amino acid change at position 85 (proline → threonine). Three‐dimensional analysis of the protein structure predicted a cis to trans change in the respective peptide bond, thereby probably provoking major conformational and functional alterations of the protein. The p.Pro85Thr mutation showed a dramatically reduced activation (97%) of the NPPA promoter in luciferase assays and failed to induce NPPA expression in HEK 293 cells compared to wild‐type TBX5 protein. The mutation did not interfere with the nuclear localization of the protein.

Conclusion

These results suggest that the dramatic functional alteration of the p.Pro85Thr mutation leads to the distinctive phenotype of the patient.

A Matter of the Heart: The African Clawed Frog Xenopus as a Model for Studying Vertebrate Cardiogenesis and Congenital Heart Defects

The African clawed frog, Xenopus, is a valuable non-mammalian model organism to investigate vertebrate heart development and to explore the underlying molecular mechanisms of human congenital heart defects (CHDs). In this review, we outline the similarities between Xenopus and mammalian cardiogenesis, and provide an overview of well-studied cardiac genes in Xenopus, which have been associated with congenital heart conditions. Additionally, we highlight advantages of modeling candidate genes derived from genome wide association studies (GWAS) in Xenopus and discuss commonly used techniques.

Role of Genetic Factors in the Pathogenesis of Radial Deficiencies in Humans

Radial deficiencies (RDs), defined as under/abnormal development or absence of any of the structures of the forearm, radial carpal bones and thumb, occur with a live birth incidence ranging from 1 out of 30,000 to 1 out 6,000 newborns and represent about one third/one fourth of all the congenital upper limb anomalies. About half of radial disorders have a mendelian cause and pattern of inheritance, whereas the remaining half appears sporadic with no known gene involved. In sporadic forms certain anomalies, such as thumb or radial hypoplasia, may occur either alone or in association with systemic conditions, like vertebral abnormalities or renal defects. All the cases with a mendelian inheritance are syndromic forms, which include cardiac defects (in Holt-Oram syndrome), bone marrow failure (in Fanconi anemia), platelet deficiency (in thrombocytopenia-absent-radius syndrome), ocular motility impairment (in Okihiro syndrome). The genetics of radial deficiencies is complex, characterized by genetic heterogeneity and high inter- and intra-familial clinical variability: this review will analyze the etiopathogenesis and the genotype/phenotype correlations of the main radial deficiency disorders in humans.

Malfunction defended

Historical accounts of biological are thought to have, as a point in their favour, their being able to accommodate malfunction. Recently, this has been brought into doubt by Paul Sheldon Davies's argument for the claim that both selected malfunction (that of the selected functions account) and weak etiological malfunction (that of the weak etiological account), are impossible. In this paper I suggest that in light of Davies's objection, historical accounts of biological function need to be adjusted to accommodate malfunction. I propose a historical account which places two conditions on membership of a functional kind. My claim is that it is in virtue of a trait's meeting these conditions that it is a member of a functional kind, and can thus malfunction. I suggest that a version of my proposal can be adopted by both the selected effects and weak etiological theorists, and so conclude that such a proposal meets Davies's objection.

A unique TBX5 microdeletion with microinsertion detected in patient with Holt-Oram syndrome

Holt-Oram syndrome (HOS) is an autosomal dominant condition characterized by upper limb and congenital heart defects and caused by numerous germline mutations of TBX5 producing preterminal stop codons. Here, we report on a novel and unusual heterozygous TBX5 microdeletion with microinsertion (microindel) mutation (c.627delinsGTGACTCAGGAAACGCTTTCCTGA), which is predicted to synthesize a truncated TBX5 protein, detected in a sporadic patient with clinical features of HOS prenatally diagnosed by ultrasonography. This uncommon and relatively large inserted sequence contains sequences derived from nearby but not adjacent templates on both sense and antisense strands, suggesting two possible models, which require no repeat sequences, causing this complex microindel through the bypass of large DNA adducts via an error-prone DNA polymerase-mediated translesion synthesis.

Asymmetric cell convergence-driven zebrafish fin bud initiation and pre-pattern requires Tbx5a control of a mesenchymal Fgf signal

Tbx5 plays a pivotal role in vertebrate forelimb initiation, and loss-of-function experiments result in deformed or absent forelimbs in all taxa studied to date. Combining single-cell fate mapping and three-dimensional cell tracking in the zebrafish, we describe a Tbx5a-dependent cell convergence pattern that is both asymmetric and topological within the fin-field lateral plate mesoderm during early fin bud initiation. We further demonstrate that a mesodermal Fgf24 convergence cue controlled by Tbx5a underlies this asymmetric convergent motility. Partial reduction in Tbx5a or Fgf24 levels disrupts the normal fin-field cell motility gradient and results in anteriorly biased perturbations of fin-field cell convergence and truncations in the pectoral fin skeleton, resembling aspects of the forelimb skeletal defects that define individuals with Holt-Oram syndrome. This study provides a quantitative reference model for fin-field cell motility during vertebrate fin bud initiation and suggests that a pre-pattern of anteroposterior fate specification is already present in the fin-field before or during migration because perturbations to these early cell movements result in the alteration of specific fates.

Clinical and molecular characterisation of Holt-Oram syndrome focusing on cardiac manifestations

BACKGROUND

Holt-Oram syndrome is characterised by CHD and limb anomalies. Mutations in TBX5 gene, encoding the T-box transcription factor, are responsible for the development of Holt-Oram syndrome, but such mutations are variably detected in 30-75% of patients.

METHODS

Clinically diagnosed eight Holt-Oram syndrome patients from six families were evaluated the clinical characteristics, focusing on the cardiac manifestations, in particular, and molecular aetiologies. In addition to the investigation of the mutation of TBX5, SALL4, NKX2.5, and GATA4 genes, which are known to regulate cardiac development by physically and functionally interacting with TBX5, were also analyzed. Multiple ligation-dependent probe amplification analysis was performed to detect exonic deletion and duplication mutations in these genes.

RESULTS

All included patients showed cardiac septal defects and upper-limb anomalies. Of the eight patients, seven underwent cardiac surgery, and four suffered from conduction abnormalities such as severe sinus bradycardia and complete atrioventricular block. Although our patients showed typical clinical findings of Holt-Oram syndrome, only three distinct TBX5 mutations were detected in three families: one nonsense, one splicing, and one missense mutation. No new mutations were identified by testing SALL4, NKX2.5, and GATA4 genes.

CONCLUSIONS

All Holt-Oram syndrome patients in this study showed cardiac septal anomalies. Half of them showed TBX5 gene mutations. To understand the genetic causes for inherited CHD such as Holt-Oram syndrome is helpful to take care of the patients and their families. Further efforts with large-scale genomic research are required to identify genes responsible for cardiac manifestations or genotype-phenotype relation in Holt-Oram syndrome.

Tbx5 and Osr1 interact to regulate posterior second heart field cell cycle progression for cardiac septation

RATIONALE

Mutations of TBX5 cause Holt-Oram syndrome (HOS) in humans, a disease characterized by atrial or occasionally ventricular septal defects in the heart and skeletal abnormalities of the upper extremity. Previous studies have demonstrated that Tbx5 regulates Osr1 expression in the second heart field (SHF) of E9.5 mouse embryos. However, it is unknown whether and how Tbx5 and Osr1 interact in atrial septation.

OBJECTIVE

To determine if and how Tbx5 and Osr1 interact in the posterior SHF for cardiac septation.

METHODS AND RESULTS

In the present study, genetic inducible fate mapping showed that Osr1-expressing cells contribute to atrial septum progenitors between E8.0 and E11.0. Osr1 expression in the pSHF was dependent on the level of Tbx5 at E8.5 and E9.5 but not E10.5, suggesting that the embryo stage before E10.5 is critical for Tbx5 interacting with Osr1 in atrial septation. Significantly more atrioventricular septal defects (AVSDs) were observed in embryos with compound haploinsufficiency for Tbx5 and Osr1. Conditional compound haploinsufficiency for Tbx5 and Osr1 resulted in a significant cell proliferation defect in the SHF, which was associated with fewer cells in the G2 and M phases and a decreased level of Cdk6 expression. Remarkably, genetically targeted disruption of Pten expression in atrial septum progenitors rescued AVSDs caused by Tbx5 and Osr1 compound haploinsufficiency. There was a significant decrease in Smo expression, which is a Hedgehog (Hh) signaling pathway modulator, in the pSHF of Osr1 knockout embryos at E9.5, implying a role for Osr1 in regulating Hh signaling.

CONCLUSIONS

Tbx5 and Osr1 interact to regulate posterior SHF cell cycle progression for cardiac septation.