Novel mutations in the TBX5 gene in patients with Holt-Oram Syndrome

The Holt-Oram syndrome (HOS) is an autosomal dominant condition characterized by upper limb and cardiac malformations. Mutations in the TBX5 gene cause HOS and have also been associated with isolated heart and arm defects. Interactions between the TBX5, GATA4 and NKX2.5 proteins have been reported in humans. We screened the TBX5, GATA4, and NKX2.5 genes for mutations, by direct sequencing, in 32 unrelated patients presenting classical (8) or atypical HOS (1), isolated congenital heart defects (16) or isolated upper-limb malformations (7). Pathogenic mutations in the TBX5 gene were found in four HOS patients, including two new mutations (c.374delG; c.678G > T) in typical patients, and the hotspot mutation c.835C > T in two patients, one of them with an atypical HOS phenotype involving lower-limb malformations. Two new mutations in the GATA4 gene were found in association with isolated upper-limb malformations, but their clinical significance remains to be established. A previously described possibly pathogenic mutation in the NKX2.5 gene (c.73C > 7) was detected in a patient with isolated heart malformations and also in his clinically normal father.     

Knowing left from right: the molecular basis of laterality defects

The apparent symmetry of the vertebrate body conceals profound asymmetries in the development and placement of internal organs. Asymmetric organ development is controlled in part by genes expressed asymmetrically in the early embryo, and alterations in the activities of these genes can result in severe defects during organogenesis. Recently, data from different vertebrates have allowed researchers to put forward a model of genetic interactions that explains how asymmetric patterns of gene expression in the early embryo are translated into spatial patterns of asymmetric organ development. This model helps us to understand the molecular basis of a number of congenital malformations in humans.     

Ccm1 is required for arterial morphogenesis: implications for the etiology of human cavernous malformations

Hemorrhagic stroke is a significant cause of morbidity and mortality in children, and is frequently associated with intracranial vascular malformations. One prevalent form of these vascular malformations, cerebral cavernous malformation, is characterized by thin-walled vascular cavities that hemorrhage and has been linked to loss-of-function mutations in CCM1. The neural and epithelial expression of CCM1 in adulthood suggests that cavernous malformations may be the result of primary neural defects. In this study, we generated mice lacking Ccm1 and demonstrate that Ccm1 is ubiquitously expressed early in embryogenesis and is essential for vascular development. Homozygous mutant embryos die in mid-gestation and the first detectable defects are exclusively vascular in nature. The precursor vessels of the brain become dilated starting at E8.5, reminiscent of the intracranial vascular defects observed in the human disease. In addition, there is marked enlargement and increased endothelial proliferation of the caudal dorsal aorta, as well as variable narrowing of the branchial arch arteries and proximal dorsal aorta. These vascular defects are not secondary to primary neural defects, as neural morphology and marker expression are normal even subsequent to the onset of vascular pathology. The defects in the vascular structure of embryos lacking Ccm1 are associated with early downregulation of artery-specific markers, including the Efnb2- and Notch-related genes. Finally, consistent with the murine data, we found that there is an analogous reduction in Notch gene expression in arterioles from humans with mutations in CCM1. Our studies suggest that cavernous malformations result from primary vascular rather than neural defects.     

Reduced NODAL signaling strength via mutation of several pathway members including FOXH1 is linked to human heart defects and holoprosencephaly

Abnormalities of embryonic patterning are hypothesized to underlie many common congenital malformations in humans including congenital heart defects (CHDs), left-right disturbances (L-R) or laterality, and holoprosencephaly (HPE). Studies in model organisms suggest that Nodal-like factors provide instructions for key aspects of body axis and germ layer patterning; however, the complex genetics of pathogenic gene variant(s) in humans are poorly understood. Here we report our studies of FOXH1, CFC1, and SMAD2 and summarize our mutational analysis of three additional components in the human NODAL-signaling pathway: NODAL, GDF1, and TDGF1. We identify functionally abnormal gene products throughout the pathway that are clearly associated with CHD, laterality, and HPE. Abnormal gene products are most commonly detected in patients within a narrow spectrum of isolated conotruncal heart defects (minimum 5%-10% of subjects), and far less commonly in isolated laterality or HPE patients (approximately 1% for each). The difference in the mutation incidence between these groups is highly significant. We show that apparent gene dosage discrepancies between humans and model organisms can be reconciled by considering a broader combination of sequence variants. Our studies confirm that (1) the genetic vulnerabilities inferred from model organisms with defects in Nodal signaling are indeed analogous to humans; (2) the molecular analysis of an entire signaling pathway is more complete and robust than that of individual genes and presages future studies by whole-genome analysis; and (3) a functional genomics approach is essential to fully appreciate the complex genetic interactions necessary to produce these effects in humans.     

Denys-Drash syndrome: a role for the WT1 tumour suppressor gene in urogenital development

The study of inherited disease provides unique insight into basic developmental and biochemical processes. By linking the pathogenesis of complex malformations to mutations of specific genes, the function of those genes in normal developmental biology can be inferred. One such disorder is the Denys-Drash syndrome, where identification of genetic lesions within the WT1 tumour suppressor gene has provided astonishing insight into events regulating development of the urogenital system.     

Mouse Zic5 deficiency results in neural tube defects and hypoplasia of cephalic neural crest derivatives

Zic family genes encode zinc finger proteins, which are homologues of the Drosophila pair-rule gene odd-paired. In the present study, we characterized the fifth member of the mouse Zic family gene, mouse Zic5. Zic5 is located near Zic2, which is responsible for human brain malformation syndrome (holoprosencephaly, or HPE). In embryonic stages, Zic5 was expressed in dorsal part of neural tissues and limbs. Expression of Zic5 overlapped with those of other Zic genes, most closely with Zic2, but was not identical. Targeted disruption of Zic5 resulted in insufficient neural tube closure at the rostral end, similar to that seen in Zic2 mutant mice. In addition, the Zic5-deficient mice exhibited malformation of neural-crest-derived facial bones, especially the mandible, which had not been observed in other Zic family mutants. During the embryonic stages, there were delays in the development of the first branchial arch and extension of the trigeminal and facial nerves. Neural crest marker staining revealed fewer neural crest cells in the dorsal cephalic region of the mutant embryos without significant changes in their migration. When mouse Zic5 was overexpressed in Xenopus embryos, expression of a neural crest marker was enhanced. These findings suggested that Zic5 is involved in the generation of neural crest tissue in mouse development. ZIC5 is also located close to ZIC2 in humans, and deletions of 13q32, where ZIC2 is located, lead to congenital brain and digit malformations known as the "13q32 deletion syndrome". Based on both their similar expression pattern in mouse embryos and the malformations observed in Zic5-deficient mutant mice, human ZIC5 might be involved in the deletion syndrome.     

Investigating Different Duplication Pattern of Essential Genes in Mouse and Human

Gene duplication is one of the major driving forces shaping genome and organism evolution and thought to be itself regulated by some intrinsic properties of the gene. Comparing the essential genes among mouse and human, we observed that the essential genes avoid duplication in mouse while prefer to remain duplicated in humans. In this study, we wanted to explore the reasons behind such differences in gene essentiality by cross-species comparison of human and mouse. Moreover, we examined essential genes that are duplicated in humans are functionally more redundant than that in mouse. The proportion of paralog pseudogenization of essential genes is higher in mouse than that of humans. These duplicates of essential genes are under stringent dosage regulation in human than in mouse. We also observed slower evolutionary rate in the paralogs of human essential genes than the mouse counterpart. Together, these results clearly indicate that human essential genes are retained as duplicates to serve as backed up copies that may shield themselves from harmful mutations.     

A locus for hereditary capillary malformations mapped on chromosome 5q

Capillary malformations (port-wine stains) are the most common vascular malformations occurring in 0.3% of live births. Most capillary malformations occur sporadically and present as a solitary lesion. Capillary malformations can also occur as a component of well-described syndromes. Familial occurrence of multiple capillary malformations has been described in the literature, suggesting autosomal dominant inheritance with variable expression in this subgroup. A hereditary basis underlying the development of solitary capillary malformations has not been found, but may well be possible. We have mapped a locus for an autosomal dominant disorder in a three-generation family that manifested itself with multiple cutaneous capillary malformations to chromosome 5q13-22. This locus spans 48 cM between the markers D5S647 and D5S659 and harbours several candidate genes. By defining the gene(s) responsible for capillary malformations, we will gain more insight in the pathogenesis of this disorder. It is likely that genes implicated in these familial cases may be involved in the more sporadic cases.     

Making the mouse embryo transparent: identifying developmental malformations using magnetic resonance imaging

Developmental malformations are a major cause of childhood mortality and are typically characterized by lesions that allow survival of the embryo through gestation. The genetics of developmental malformations are powerfully studied by using high-throughput, phenotype-driven screens (e.g., following zebrafish or mouse mutagenesis) or by genotype-driven studies using transgenic or knockout mice. With regard to either approach, the mouse is anatomically and phylogenetically closer to humans than any other genetically tractable model organism. This is particularly important in the cardiovascular and respiratory systems, which have unique mammalian features. The identification of murine models of developmental malformations is, however, hindered by the opacity of the late gestational mouse embryo. In this review, we describe recent advances in magnetic resonance imaging that make it possible to rapidly identify malformations in the developing mouse embryo with high efficiency.     

Induction of congenital malformation in mice by parental irradiation: transmission to later generations

In order to investigate the genetic basis of the increased incidence of congenital malformations in the offspring of irradiated mice, the frequency of malformations among the offspring of individual F1 sons of irradiated females was studied in detail. Among 90 fully tested F1 sons of females which had been mated 15-21 days after receiving 360 cGy X-rays 4 were definite or probable carriers of dominant genes giving a low penetrance of malformations. This confirms that the malformations seen in the first generation are of genetic origin and can be transmitted to later generations. However, the incidence and penetrance of the mutant genes detected were too low to account for all the anomalies found in the first generation. It was concluded that the genetic basis of the original anomalies was heterogeneous, with some due to genetic changes of high penetrance and rapidly eliminated, and others due to genes of low penetrance like those found in this work. Other malformations, in both the irradiated and control series, were probably of non-genetic origin.     

Human and chimpanzee gene expression differences replicated in mice fed different diets

Although the human diet is markedly different from the diets of closely related primate species, the influence of diet on phenotypic and genetic differences between humans and other primates is unknown. In this study, we analyzed gene expression in laboratory mice fed diets typical of humans and of chimpanzees. The effects of human diets were found to be significantly different from that of a chimpanzee diet in the mouse liver, but not in the brain. Importantly, 10% of the genes that differ in their expression between humans and chimpanzee livers differed also between the livers of mice fed the human and chimpanzee diets. Furthermore, both the promoter sequences and the amino acid sequences of these diet-related genes carry more differences between humans and chimpanzees than random genes. Our results suggest that the mouse can be used to study at least some aspects of human-specific traits.     

Thoracic skeletal defects and cardiac malformations: A common epigenetic link?

Congenital heart defects (CHDs) are the most common birth defects in humans. In addition, cardiac malformations represent the most frequently identified anomaly in teratogenicity experiments with laboratory animals. To explore the mechanisms of these drug-induced defects, we developed a model in which pregnant rats are treated with dimethadione, resulting in a high incidence of heart malformations. Interestingly, these heart defects were accompanied by thoracic skeletal malformations (cleft sternum, fused ribs, extra or missing ribs, and/or wavy ribs), which are characteristic of anterior-posterior (A/P) homeotic transformations and/or disruptions at one or more stages in somite development. A review of other teratogenicity studies suggests that the co-occurrence of these two disparate malformations is not unique to dimethadione, rather it may be a more general phenomenon caused by various structurally unrelated agents. The coexistence of cardiac and thoracic skeletal malformations has also presented clinically, suggesting a mechanistic link between cardiogenesis and skeletal development. Evidence from genetically modified mice reveals that several genes are common to heart development and to formation of the axial skeleton. Some of these genes are important in regulating chromatin architecture, while others are tightly controlled by chromatin-modifying proteins. This review focuses on the role of these epigenetic factors in development of the heart and axial skeleton, and examines the hypothesis that posttranslational modifications of core histones may be altered by some developmental toxicants.     

Interactions of polymorphisms in different clock genes associated with circadian phenotypes in humans

Several studies have shown that mutations and polymorphisms in clock genes are associated with abnormal circadian parameters in humans and also with more subtle non-pathological phenotypes like chronotypes. However, there have been conflicting results, and none of these studies analyzed the combined effects of more than one clock gene. Up to date, association studies in humans have focused on the analysis of only one clock gene per study. Since these genes encode proteins that physically interact with each other, combinations of polymorphisms in different clock genes could have a synergistic or an inhibitory effect upon circadian phenotypes. In the present study, we analyzed the combined effects of four polymorphisms in four clock genes (Per2, Per3, Clock and Bmal1) in people with extreme diurnal preferences (morning or evening). We found that a specific combination of polymorphisms in these genes is more frequent in people who have a morning preference for activity and there is a different combination in individuals with an evening preference for activity. Taken together, these results show that it is possible to detect clock gene interactions associated with human circadian phenotypes and bring an innovative idea of building a clock gene variation map that may be applied to human circadian biology.     

Genetics, chance, and morphogenesis.

We posit that chance plays a major role in the occurrence of many common malformations that cluster in families but recur less frequently than expected for simple Mendelian traits. Once the role of random effects is accepted, the segregation of such malformations may be explained on the basis of Mendelian transmission of a single abnormal gene that predisposes to, but does not always result in, the abnormal phenotype. We apply a stochastic (probabilistic) single-gene model to the occurrence of malformations in mouse and man. The stochastic single-gene model suggests the feasibility of isolating individual genes that determine morphogenesis and sets limits on the precision with which the recurrence of malformations can be predicted.