Variation in severity of cardiac disease in Holt-Oram syndrome

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.

Modeling Human TBX5 Haploinsufficiency Predicts Regulatory Networks for Congenital Heart Disease

Haploinsufficiency of transcriptional regulators causes human congenital heart disease (CHD); however, the underlying CHD gene regulatory network (GRN) imbalances are unknown. Here, we define transcriptional consequences of reduced dosage of the CHD transcription factor, TBX5, in individual cells during cardiomyocyte differentiation from human induced pluripotent stem cells (iPSCs). We discovered highly sensitive dysregulation of TBX5-dependent pathways-including lineage decisions and genes associated with heart development, cardiomyocyte function, and CHD genetics-in discrete subpopulations of cardiomyocytes. Spatial transcriptomic mapping revealed chamber-restricted expression for many TBX5-sensitive transcripts. GRN analysis indicated that cardiac network stability, including vulnerable CHD-linked nodes, is sensitive to TBX5 dosage. A GRN-predicted genetic interaction between Tbx5 and Mef2c, manifesting as ventricular septation defects, was validated in mice. These results demonstrate exquisite and diverse sensitivity to TBX5 dosage in heterogeneous subsets of iPSC-derived cardiomyocytes and predicts candidate GRNs for human CHDs, with implications for quantitative transcriptional regulation in disease.

Anterior lateral plate mesoderm gives rise to multiple tissues and requires tbx5a function in left-right asymmetry, migration dynamics, and cell specification of late-addition cardiac cells

In this study, we elucidate a single cell resolution fate map in the zebrafish in a sub-section of the anterior Lateral Plate Mesoderm (aLPM) at 18 hpf. Our results show that this tissue is not organized into segregated regions but gives rise to intermingled pericardial sac, peritoneum, pharyngeal arch and cardiac precursors. We further report upon asymmetrical contributions of lateral aLPM-derived heart precursors-specifically that twice as many heart precursors arise from the right side versus the left side of the embryo. Cell tracking analyses and large-scale cell labeling of the lateral aLPM corroborate these differences and show that the observed asymmetries are dependent upon Tbx5a expression. Previously, it was shown that cardiac looping was affected in Tbx5a knock-down and knock-out zebrafish (Garrity et al., 2002; Parrie et al., 2013); our present data also implicate tbx5a function in cell specification, establishment and maintenance of cardiac left-right asymmetry.

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.

Growing up with a rare genetic disease: an interpretative phenomenological analysis of living with Holt-Oram syndrome

BACKGROUND

Holt-Oram syndrome (HOS) is a rare genetic disease characterized by variable radial upper limb and cardiac defects. The aim of this research was to shed light on people's subjective perceptions of their diseases, how these perceptions provide meaning, and the consequences the syndrome can have in daily life and across all life stages.

METHODS

Semistructured interviews with ten participants diagnosed with HOS were conducted in France and analyzed using interpretative phenomenological analysis.

RESULTS

Participants' experiences fall under two main themes, namely, "stages of self-construction as different" and "when I am no longer the only one involved", each of which has three subthemes. From childhood onwards, symptoms monopolize the physical and psychological spheres. The feeling of being different is unavoidable until the patient can appropriate his or her condition, and by the end of adolescence, the patient generally feels that he or she has adapted to the syndrome. In adulthood, other concerns arise, such as the fear of rejection, the need to better understand the genetic issues of the condition and the desire for offspring to not experience the same life difficulties.

CONCLUSION

The findings underscore the specific psychological issues associated with the syndrome at different life stages and the need for holistic genetic treatment with dedicated reference centers to improve care and further address these issues.IMPLICATIONS FOR REHABILITATIONHolt-Oram syndrome is a genetic disease characterized by abnormalities of the upper limbs and shoulder girdle and associated with a congenital heart defect.Specific issues arise at different stages of life: the physical consequences of the syndrome arise during childhood, the self-construction of pervasive difference during adolescence, the fear of being rejected as a young adult, and concerns about future parenthood and the transmission of the syndrome and the desire that one's child not be confronted with the same difficulties in adulthood.The complexity and entanglement of medical and existential issues related to HOS requires the development of multidisciplinary consultations that promote holistic care.The rarity of the syndrome and the lack of knowledge about HOS among health professionals and the general public make it necessary both to establish reference centers and to create patient associations to support patients.

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.

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.

Perspective: Is Random Monoallelic Expression a Contributor to Phenotypic Variability of Autosomal Dominant Disorders?

Several factors have been proposed as contributors to interfamilial and intrafamilial phenotypic variability in autosomal dominant disorders, including allelic variation, modifier genes, environmental factors and complex genetic and environmental interactions. However, regardless of the similarity of genetic background and environmental factors, asymmetric limb or trunk anomalies in a single individual and variable manifestation between monozygotic twins have been observed, indicating other mechanisms possibly involved in expressivity of autosomal dominant diseases. One such example is Holt-Oram syndrome (HOS), which is characterized by congenital cardiac defects and forelimb anomalies, mainly attributed to mutations in the TBX5 gene. We hypothesize that monoallelic expression of the TBX5 gene occurs during embryo development, and, in the context of a mutation, random monoallelic expression (RME) can create discrepant functions in a proportion of cells and thus contribute to variable phenotypes. A hybrid mouse model was used to investigate the occurrence of RME with the Tbx5 gene, and single-cell reverse transcription PCR and restriction digestion were performed for limb bud cells from developing embryos (E11.5) of the hybrid mice. RME of Tbx5 was observed in approximately two-thirds of limb bud cells. These results indicate that RME of the Tbx5 gene occurs frequently during embryo development, resulting in a mosaic expression signature (monoallelic, biallelic, or null) that may provide a potential explanation for the widespread phenotypic variability in HOS. This model will further provide novel insights into the variability of autosomal dominant traits and a better understanding of the complex expressivity of disease conditions.

Genetics of Congenital Heart Disease: Past and Present

Congenital heart disease is the most common congenital anomaly, representing an important cause of infant morbidity and mortality. Congenital heart disease represents a group of heart anomalies that include septal defects, valve defects, and outflow tract anomalies. The exact genetic, epigenetic, or environmental basis of congenital heart disease remains poorly understood, although the exact mechanism is likely multifactorial. However, the development of new technologies including copy number variants, single-nucleotide polymorphism, next-generation sequencing are accelerating the detection of genetic causes of heart anomalies. Recent studies suggest a role of small non-coding RNAs, micro RNA, in congenital heart disease. The recently described epigenetic factors have also been found to contribute to cardiac morphogenesis. In this review, we present past and recent genetic discoveries in congenital heart disease.

T-Box Genes in Human Development and Disease

T-box genes are important development regulators in vertebrates with specific patterns of expression and precise roles during embryogenesis. They encode transcription factors that regulate gene transcription, often in the early stages of development. The hallmark of this family of proteins is the presence of a conserved DNA binding motif, the "T-domain." Mutations in T-box genes can cause developmental disorders in humans, mostly due to functional deficiency of the relevant proteins. Recent studies have also highlighted the role of some T-box genes in cancer and in cardiomyopathy, extending their role in human disease. In this review, we focus on ten T-box genes with a special emphasis on their roles in human disease.

A novel mutated sequence in the T-box transcription factor-5 (TBX-5) gene (c.241A>T) in Holt-Oram syndrome

We report a case of a 31-year-old pregnant woman who was admitted to our perinatology outpatient clinic because of a fetal ventricular septal defect and limb reduction in the upper extremities of fetus revealed by ultrasonographic investigation diagnosed in the 16(th) week of gestation. First child of the family was diagnosed with Holt-Oram syndrome who had atrial septal defect and upper limb anomalies, whereas the father was documented to have arrhythmia and shortening of upper limbs. The pregnancy was terminated in the 16(th) week of gestation with the consent of the family. We performed mutation analysis in T-box transcription factor-5 (TBX5) gene coding exons, including exon/intron boundaries from peripheral blood or skin fibroblasts. The sequence analysis revealed c.241 adenine (A)>thymine (T) [p. arginine (Arg) 81 Tryptophan (Trp)] alteration in exon-3 of the TBX5 gene in affected family members and fetus. This is a novel mutation causing Holt-Oram syndrome.

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.

Holt-Oram Syndrome Associated with Aortic Atresia: A Rare Association

Holt-Oram syndrome (HOS) is a rare autosomal dominant disorder that causes abnormalities of the upper limbs and heart. It is seen in 1:1, 00, 000 live births. It is linked to a single-gene TBX5 “protein-producing” mutation with gene map locus 12q24. Most commonly it is characterized by the cardiac septation defects and pre-axial radial ray abnormalities. We are reporting a case of HOS with aortic atresia which is a rare association.

Holt Oram syndrome: a registry-based study in Europe

BACKGROUND

Holt-Oram syndrome (HOS) is an autosomal dominant disorder characterised by upper limb anomalies and congenital heart defects. We present epidemiological and clinical aspects of HOS patients using data from EUROCAT (European Surveillance of Congenital Anomalies) registries.

METHODS

The study was based on data collected during 1990-2011 by 34 registries. The registries are population-based and use multiple sources of information to collect data on all types of birth using standardized definitions, methodology and coding. Diagnostic criteria for inclusion in the study were the presence of radial ray abnormalities and congenital heart disease (CHD), or the presence of either radial ray anomaly or CHD, with family history of HOS.

RESULTS

A total of 73 cases of HOS were identified, including 11 (15.1%) TOPFA and 62 (84.9%) LB. Out of 73 HOS cases, 30.8% (20/65) were suspected prenatally, 55.4% (36/65) at birth, 10.7% (7/65) in the first week of life, and 3.1% (2/65) in the first year of life. The prenatal detection rate was 39.2% (20/51), with no significant change over the study period. In 55% (11/20) of prenatally detected cases, parents decided to terminate pregnancy. Thumb anomalies were reported in all cases. Agenesis/hypoplasia of radius was present in 49.2% (30/61), ulnar aplasia/hypoplasia in 24.6% (15/61) and humerus hypoplasia/phocomelia in 42.6% (26/61) of patients. Congenital heart defects (CHD) were recorded in 78.7% (48/61) of patients. Isolated septal defects were present in 54.2 (26/48), while 25% (12/48) of patients had complex/severe CHD. The mean prevalence of HOS diagnosed prenatally or in the early years of life in European registries was 0.7 per 100,000 births or 1:135,615 births.

CONCLUSIONS

HOS is a rare genetic condition showing regional variation in its prevalence. It is often missed prenatally, in spite of the existence of major structural anomalies. When discovered, parents in 45% (9/20) of cases opt for the continuation of pregnancy. Although a quarter of patients have severe CHD, the overall first week survival is very good, which is important information for counselling purposes.

Holt-Oram syndrome with aortopulmonary window--a rare association

A 4-month-old male infant presented with recurrent cough for 2 months. He had a shortened right upper limb with absent right thumb and continuous murmur in the left parasternal area. The X-ray showed an absent radius and the first metacarpal and phalyngeal bones on the right side. Echocardiogram revealed aortopulmonary window and small secundum atrial septal defect. Aortopulmonary window was successfully treated by device closure. Holt-Oram syndrome with aortopulmonary window is an extremely rare association.

Holt-oram syndrome in adult presenting with heart failure: a rare presentation

Holt-Oram syndrome is a rare inherited disorder involving the hands, arms, and the heart. The defects involve carpal bones of the wrist and the thumb and the associated cardiac anomalies like atrial or ventricular septal defects. Congenital cardiac and upper-limb malformations frequently occur together and are classified as heart-hand syndromes. The most common amongst the heart-hand disorders is the Holt-Oram syndrome, which is characterized by septal defects of the heart and preaxial radial ray abnormalities. Its incidence is one in 100,000 live births. Approximately three out of four patients have some cardiac abnormality with common associations being either an atrial septal defect or ventricular septal defect. Herein, we report a rare sporadic case of Holt-Oram syndrome with atrial septal defect with symptoms of heart failure in a forty-five-year-old lady who underwent emergency cardiac surgery for the symptoms.

Diversity of congenital cardiac defects and skeletal deformities associated with the Holt-Oram syndrome

INTRODUCTION

The Holt–Oram syndrome is a rare congenital disorder involving the skeletal and cardiovascular systems. It is characterized by upper limb deformities and cardiac malformations, atrial septal defects in particular.

PRESENTATION OF CASE

Four consecutive patients 1–15 years old with the Holt–Oram syndrome presented over a 10 year span for surgical treatment of their cardiac maladies. The spectrum of the heart defects and skeletal deformities encountered in these patients are described and discussed.

DISCUSSION

The Holt–Oram syndrome is an autosomal dominant condition; however absence of the morphological features of the trait in close family members is not rare. Although patients are known to predominately present with atrial septal defects, other cardiovascular anomalies, including rhythm abnormalities, are not uncommon. Skeletal disorders vary as well.

CONCLUSION

Cardiovascular disorders, skeletal malformations and familial expression of the Holt–Oram syndrome, vary widely.

Unexplained right atrial enlargement may be a sign of Holt-Oram syndrome in the fetus

Two cases of ultrasound diagnosis of Holt-Oram syndrome are described. Both were characterized by significant right atrial enlargement that was not due to concurrent tricuspid regurgitation or other cardiac anomalies. In both cases the associated skeletal anomaly was subtle and barely visible using ultrasound. Interestingly, despite the fact that Holt-Oram syndrome is also called atriodigital dysplasia, unexplained right atrial enlargement has not been described in this context in the fetus before. When such a finding is detected, we believe a thorough search for upper limb abnormalities should be carried out and genetic testing for Holt-Oram syndrome should be discussed with the parents.

Holt-oram syndrome associated with double outlet right ventricle: A rare association

Holt-Oram syndrome is a rare inherited disorder that causes abnormalities of the hands, arms, and the heart. Most commonly, there are defects in the carpal bones of the wrist and in the bones of the thumb along with cardiac defects such as atrial or ventricular septal defects. We report a case of Holt-Oram syndrome with a rare association of double outlet right ventricle.

The pathogenesis of atrial and atrioventricular septal defects with special emphasis on the role of the dorsal mesenchymal protrusion

Partitioning of the four-chambered heart requires the proper formation, interaction and fusion of several mesenchymal tissues derived from different precursor populations that together form the atrioventricular mesenchymal complex. This includes the major endocardial cushions and the mesenchymal cap of the septum primum, which are of endocardial origin, and the dorsal mesenchymal protrusion (DMP), which is derived from the Second Heart Field. Failure of these structures to develop and/or fully mature results in atrial septal defects (ASDs) and atrioventricular septal defects (AVSD). AVSDs are congenital malformations in which the atria are permitted to communicate due to defective septation between the inferior margin of the septum primum and the atrial surface of the common atrioventricular valve. The clinical presentation of AVSDs is variable and depends on both the size and/or type of defect; less severe defects may be asymptomatic while the most severe defect, if untreated, results in infantile heart failure. For many years, maldevelopment of the endocardial cushions was thought to be the sole etiology of AVSDs. More recent work, however, has demonstrated that perturbation of DMP development also results in AVSD. Here, we discuss in detail the formation of the DMP, its contribution to cardiac septation and describe the morphological features as well as potential etiologies of ASDs and AVSDs.

Tetralogy of Fallot with Holt-Oram syndrome

Holt-Oram syndrome (HOS) is characterised by mild to severe congenital cardiac defects and skeletal abnormalities of the upper limb. This syndrome is also referred to as Hand-Heart syndrome. The most common cardiac disorder is an ostium secundum detected an atrial septal defect (ASD), followed by ventricular septal defect (VSD) and ostium primum ASD. We report a case of HOS with tetralogy of Fallot (TOF). This association is very rare and is hardly reported in the literature.

TBX5 intragenic duplication: a family with an atypical Holt-Oram syndrome phenotype

Holt-Oram syndrome (HOS) is a rare autosomal dominant heart-hand syndrome due to mutations in the TBX5 transcription factor. Affected individuals can have structural cardiac defects and/or conduction abnormalities, and exclusively upper limb defects (typically bilateral, asymmetrical radial ray defects). TBX5 mutations reported include nonsense, missense, splicing mutations and exon deletions. Most result in a null allele and haploinsufficiency, but some impair nuclear localisation of TBX5 protein or disrupt its interaction with co-factors and downstream targets. We present a five generation family of nine affected individuals with an atypical HOS phenotype, consisting of ulnar ray defects (ulnar hypoplasia, short fifth fingers with clinodactyly) and very mild radial ray defects (short thumbs, bowing of the radius and dislocation of the radial head). The cardiac defects seen are those more rarely reported in HOS (atrioventricular septal defect, hypoplastic left heart syndrome, mitral valve disease and pulmonary stenosis). Conduction abnormalities include atrial fibrillation, atrial flutter and sick sinus syndrome. TBX5 mutation screening (exons 3-10) identified no mutations. Array comparative genomic hybridisation (CGH) revealed a 48 kb duplication at 12q24.21, encompassing exons 2-9 of the TBX5 gene, with breakpoints within introns 1-2 and 9-10. The duplication segregates with the phenotype in the family, and is likely to be pathogenic. This is the first known report of an intragenic duplication of TBX5 and its clinical effects; an atypical HOS phenotype. Further functional studies are needed to establish the effects of the duplication and pathogenic mechanism. All typical/atypical HOS cases should be screened for TBX5 exon duplications.

Xenopus: An emerging model for studying congenital heart disease

Congenital heart defects affect nearly 1% of all newborns and are a significant cause of infant death. Clinical studies have identified a number of congenital heart syndromes associated with mutations in genes that are involved in the complex process of cardiogenesis. The African clawed frog, Xenopus, has been instrumental in studies of vertebrate heart development and provides a valuable tool to investigate the molecular mechanisms underlying human congenital heart diseases. In this review, we discuss the methodologies that make Xenopus an ideal model system to investigate heart development and disease. We also outline congenital heart conditions linked to cardiac genes that have been well studied in Xenopus and describe some emerging technologies that will further aid in the study of these complex syndromes.

Separation of the PROX1 gene from upstream conserved elements in a complex inversion/translocation patient with hypoplastic left heart

Hypoplastic left heart (HLH) occurs in at least 1 in 10 000 live births but may be more common in utero. Its causes are poorly understood but a number of affected cases are associated with chromosomal abnormalities. We set out to localize the breakpoints in a patient with sporadic HLH and a de novo translocation. Initial studies showed that the apparently simple 1q41;3q27.1 translocation was actually combined with a 4-Mb inversion, also de novo, of material within 1q41. We therefore localized all four breakpoints and found that no known transcription units were disrupted. However we present a case, based on functional considerations, synteny and position of highly conserved non-coding sequence elements, and the heterozygous Prox1(+/-) mouse phenotype (ventricular hypoplasia), for the involvement of dysregulation of the PROX1 gene in the aetiology of HLH in this case. Accordingly, we show that the spatial expression pattern of PROX1 in the developing human heart is consistent with a role in cardiac development. We suggest that dysregulation of PROX1 gene expression due to separation from its conserved upstream elements is likely to have caused the heart defects observed in this patient, and that PROX1 should be considered as a potential candidate gene for other cases of HLH. The relevance of another breakpoint separating the cardiac gene ESRRG from a conserved downstream element is also discussed.

Holt-Oram syndrome associated with anomalies of the feet

Holt-Oram syndrome (HOS) (OMIM 142900) is characterized by upper-extremity malformations involving the radial, thenar, or carpal bones and a personal and/or family history of congenital heart defects (CHDs). It is inherited in an autosomal dominant manner. The TBX5 gene located on chromosome 12 (12q24.1) is the only gene currently known to be associated with HOS and is associated with variable phenotypes. We report on the clinical and molecular characterization of a HOS family with three affected individuals and a novel mutation (Lys88ter). We discuss genotype-phenotype correlations, the presence of foot anomalies in one affected individual, and the role of atypical features in HOS differential diagnosis.

[Holt-Oram syndrome: genetic counseling and diagnosis with prenatal ultrasonography]

Authors report the cases of the autosomal dominant Holt-Oram syndrome encountered during genetic counseling in the authors' institution between 1976 and 2005. A male patient with Holt-Oram syndrome considered the 50% risk of transmission of the disease too high, thus he decided against fathering children. Prenatal fetal ultrasonography and echocardiography were performed in two pregnant women affected with Holt-Oram syndrome. In one case the possibility of the syndrome was excluded, in the other Holt-Oram syndrome was diagnosed. In two further cases Holt-Oram syndrome was prenatally diagnosed on the basis of ultrasonography in two midtrimester fetuses from unaffected parents. In one case the cardiac and skeletal malformations proved to be so serious that the couple requested termination of the pregnancy. The pathologic examination of the fetus confirmed the prenatal diagnosis.

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.

Genetics of congenital heart diseases in syndromic and non-syndromic patients: new advances and clinical implications

Congenital heart defects (CHDs) are the most common birth defects in humans and over the last 20 years significant progress has been made in the understanding of the molecular and genetic determinants of an increasing number of CHDs. Fundamental to this progress has been the contribution of five fields of research: the epidemiological results of the Baltimore-Washington Infant Study (BWIS); the pathogenetic classification introduced by Clark; the Human Genome Project; genotype-phenotype correlation and familial recurrence studies; and transgenic animals. The recently advanced cytogenetic techniques can now detect subtle rearrangements in chromosomes, which may be overlooked by standard methods and, more recently, molecular instruments such as linkage analysis and positional cloning are being used to identify genes causing Mendelian monogenic syndromes with CHDs, such as Holt-Oram, Ellis-van Creveld and Noonan/LEOPARD syndromes. Finally, useful information is yet available with regard to genes causing isolated CHDs in individuals who do not have a genetic syndrome (an example is the mutation of NKX2.5 and GATA4 genes causing atrial septal defect). The future perspectives for the genetics of CHDs will involve three fields of interest: diagnosis; therapy; and prognosis.

Perinatal/neonatal case presentation: unexpected severe respiratory insufficiency in a newborn with Holt-Oram Syndrome

Holt-Oram syndrome is an autosomal dominant condition characterized by skeletal and cardiac defects. Pulmonary malformation is not reported to belong to the spectrum of this condition. We report a second case of a newborn with Holt-Oram syndrome who developed severe respiratory insufficiency shortly after birth. We discuss possible genetic links between abnormal pulmonary morphogenesis and Holt-Oram syndrome.

Autosomal dominant inheritance of left ventricular outflow tract obstruction

Most nonsyndromic congenital heart malformations (CHMs) in humans are multifactorial in origin, although an increasing number of monogenic cases have been reported recently. We describe here four new families with presumed autosomal dominant inheritance of left ventricular outflow tract obstruction (LVOTO), consisting of hypoplastic left heart (HLHS) or left ventricle (HLV), aortic valve stenosis (AS) and bicuspid aortic valve (BAV), hypoplastic aortic arch (HAA), and coarctation of the aorta (CoA). LVOTO in these families shows a wide clinical spectrum with some family members having severe anomalies such as hypoplastic left heart, and others only minor anomalies such as mild aortic valve stenosis. This supports the suggestion that all anomalies of the LVOTO spectrum are developmentally related and can be caused by a single gene defect.

Identification of the TBX5 transactivating domain and the nuclear localization signal

TBX5 is a member of the T-box gene family and encodes a transcription factor involved in cardiac and limb development. Mutations of TBX5 cause Holt-Oram syndrome (HOS), an autosomal-dominant condition with congenital cardiac defects and forelimb anomalies. Here, we used a GAL4-TBX5 fusion protein in a modified yeast-one hybrid system to elucidate the TBX5 transactivating domain. Using a series of deletion mutations of TBX5, we narrowed down its functional domain to amino acids 339-379 of its C-terminal half; point mutagenesis analysis then showed that the loss of amino acids 349-351 abolished transactivation. This result was confirmed in mammalian cells. Furthermore, wild-type TBX5, but not TBX5 with mutations at the amino acids 349-351, has ability to inhibit NCI-H1299 cell growth also suggesting that these amino acids are crucial for the TBX5 function in mammalian cells. In addition, to identify the nuclear localization signal of TBX5, we searched for cluster of basic amino acids. We found that the deletion of the KRK sequence at amino acids 325-327 mislocalizes TBX5 to cytoplasm, suggesting that these amino acids serve as a nuclear localization signal. These studies enhance our understanding of the structure-function relationship of TBX5 and suggest that truncation mutations of TBX5 could cause HOS through the loss of its transactivating domain and/or the nuclear localization signal.

Fetal cardiac anomalies and genetic syndromes

Cardiac anomalies may occur in isolation or can be part of a genetic syndrome. In this article, we describe some of the genetic syndromes commonly associated with cardiac anomalies where there are other sonographic features that may aid accurate prenatal diagnosis.

TBX5, a gene mutated in Holt-Oram syndrome, is regulated through a GC box and T-box binding elements (TBEs)

TBX5 is a member of the T-box gene family and encodes a transcription factor that regulates the expression of other gene(s) in the developing heart and limbs. Mutations of TBX5 cause Holt-Oram syndrome (HOS), an autosomal dominant condition characterized by congenital heart defects and limb anomalies. How TBX5 gene expression is regulated is still largely unknown. In order to identify transcription factors regulating TBX5 expression, we examined the 5'-flanking region of the human TBX5 gene. We determined that up to 300 bp of the 5'-flanking region of the TBX5 gene was necessary for promoter activity in mouse cardiomyocyte ECL2 cells. One GC box, three potential T-box-like binding elements (TBE-A, -B, and -C), and one NKX2.5 binding site were identified. Site-directed mutagenesis of the potential binding sites revealed that the GC box, TBE-B, TBE-C, and NKX2.5 are functionally positive for the expression of TBX5. DNA footprint analysis showed that these binding regions are resistant to DNaseI digestion. Electrophoretic mobility shift assays (EMSAs) further demonstrated the protein-DNA interactions at the GC box and the potential TBE-B, TBE-C, and NKX2.5 sites in a sequence-specific manner. The ability of TBX5 to regulate its own promoter was demonstrated by the ability of ectopically expressed human TBX5 to increase reporter expression. We conclude that the GC box, T-box-like binding elements, and NKX2.5 binding site play important roles in the regulation of TBX5 expression, and that TBX5 is likely to be autoregulated as part of the mechanism of its transcription.

Expressivity of Holt-Oram syndrome is not predicted by TBX5 genotype

Mutations in TBX5, a T-box-containing transcription factor, cause cardiac and limb malformations in individuals with Holt-Oram syndrome (HOS). Mutations that result in haploinsufficiency of TBX5 are purported to cause cardiac and limb defects of similar severity, whereas missense mutations, depending on their location in the T box, are thought to cause either more severe heart or more severe limb abnormalities. These inferences are, however, based on the analysis of a relatively small number of independent cases of HOS. To better understand the relationship between mutations in TBX5 and the variable expressivity of HOS, we screened the coding and noncoding regions of TBX5 and SALL4 for mutations in 55 probands with HOS. Seventeen mutations, including six missense mutations in TBX5 and two mutations in SALL4, were found in 19 kindreds with HOS. Fewer than 50% of individuals with nonsense or frameshift mutations in TBX5 had heart and limb defects of similar severity, and only 2 of 20 individuals had heart or limb malformations of the severity predicted by the location of their mutations in the T box. These results suggest that neither the type of mutation in TBX5 nor the location of a mutation in the T box is predictive of the expressivity of malformations in individuals with HOS.

A dominantly inherited malformation syndrome with short stature, upper limb anomaly, minor craniofacial anomalies, and absence of TBX5 mutations: report of a Thai family

We report on a Thai family with dominantly inherited malformation syndrome with upper limb anomalies, short stature, quadricuspid aortic valve, and minor craniofacial anomalies. The affected individuals comprised a mildly affected mother, a moderately affected daughter, and a most severely affected son. The daughter and son had short stature. The craniofacial abnormalities comprised frontal bossing, hypoplastic nasal bones, depressed nasal bridge, and broad nasal alae. The upper limb defects varies among the patients, ranging from radial ray defects in the mother through radial and ulnar ray defects with unilateral humeral hypoplasia in the daughter to radial ray defects with severe oligodactyly and bilateral humeral hypoplasia in the son. All patients in this family had hypoplasia of the shoulder girdle and resembled what is observed in many families with Holt-Oram syndrome. Moreover, the son showed quadricuspid aortic valve with mild aortic regurgitation. However, the present family did not show any mutation of the TBX5 gene, a disease-causing gene of Holt-Oram syndrome. The present family deserves further investigation on other genes that play a role in the development of the upper limbs, particularly of radial rays.

Congenital heart disease and genetic syndromes: specific correlation between cardiac phenotype and genotype

The increasing role of genetic factors in the etiology of congenital heart defects is shown by the high frequency of genetic syndromes and extracardiac malformations in these patients. The accurate study of cardiac anatomy disclosed that peculiar morphologic subtypes of heart defects are related to specific genetic conditions. The correlation between anatomic cardiac patterns and some genetic anomalies (trisomy, deletion, mutation) suggests that specific morphogenetic mechanisms put in motion by gene(s) can result in a specific cardiac phenotype. In this review we analyze the cardiac morphology and the frequent genetic syndromes in five groups of congenital heart diseases: right-sided obstructions, left-sided obstructions, atrioventricular canal defects, ventricular septal defects, and conotruncal defects. Progress in this field is due not only to new research in molecular biology, but also to the attention of clinicians to a detailed cardiac diagnosis and to specific correlations between genotype and phenotype.

Chamber-specific cardiac expression of Tbx5 and heart defects in Holt-Oram syndrome

To further define the role of a T-box transcription factor, Tbx5, in cardiac development, we have examined its expression in the developing mouse and chick heart and correlated this pattern with cardiac defects caused by human TBX5 mutations in Holt-Oram syndrome. Early in the developing heart, Tbx5 is uniformly expressed throughout the entire cardiac crescent. Upon formation of the linear heart tube, Tbx5 is expressed in a graded fashion, stronger near the posterior end and weaker at the anterior end. As the heart tube loops, asymmetric Tbx5 expression continues; Tbx5 is expressed in the presumptive left ventricle, but not the right ventricle or outflow tract. This pattern of expression is maintained in more mature hearts. Expression in the ventricular septum is restricted to the left side and is contiguous with left ventricular free wall expression. Trabeculae, vena cavae (inferior and superior), and the atrial aspect of the atrioventricular valves also express high levels of Tbx5. These patterns of Tbx5 expression provide an embryologic basis for the prevalence of atrial septal defects (ostium primum and secundum), ventricular muscular septal defects, and left-sided malformations (endocardial cushion defects, hypoplastic left heart, and aberrant trabeculation) observed in patients with Holt-Oram syndrome.