Tbx6 regulates left/right patterning in mouse embryos through effects on nodal cilia and perinodal signaling

TBX6 as a cause of a combined skeletal-kidney dysplasia syndrome

TBX6 encodes transcription-factor box 6, a transcription factor critical to paraxial mesoderm segmentation and somitogenesis during embryonic development. TBX6 haploinsufficiency is believed to drive the skeletal and kidney phenotypes associated with the 16p11.2 deletion syndrome. Heterozygous and biallelic variants in TBX6 are associated with vertebral and rib malformations (TBX6-associated congenital scoliosis) and spondylocostal dysostosis, and heterozygous TBX6 variants are associated with increased risk of genitourinary tract malformations. Combined skeletal and kidney phenotypes in individuals harboring heterozygous or biallelic TBX6 variants are rare. Here, we present seven individuals with vertebral and rib malformations and structural kidney differences associated with heterozygous TBX6 gene deletion in trans with a hypomorphic TBX6 allele or biallelic TBX6 variants. Our case series highlights the association between TBX6 and both skeletal and kidney disease.

Resolution of the curse of dimensionality in single-cell RNA sequencing data analysis

Single-cell RNA sequencing (scRNA-seq) can determine gene expression in numerous individual cells simultaneously, promoting progress in the biomedical sciences. However, scRNA-seq data are high-dimensional with substantial technical noise, including dropouts. During analysis of scRNA-seq data, such noise engenders a statistical problem known as the curse of dimensionality (COD). Based on high-dimensional statistics, we herein formulate a noise reduction method, RECODE (resolution of the curse of dimensionality), for high-dimensional data with random sampling noise. We show that RECODE consistently resolves COD in relevant scRNA-seq data with unique molecular identifiers. RECODE does not involve dimension reduction and recovers expression values for all genes, including lowly expressed genes, realizing precise delineation of cell fate transitions and identification of rare cells with all gene information. Compared with representative imputation methods, RECODE employs different principles and exhibits superior overall performance in cell-clustering, expression value recovery, and single-cell-level analysis. The RECODE algorithm is parameter-free, data-driven, deterministic, and high-speed, and its applicability can be predicted based on the variance normalization performance. We propose RECODE as a powerful strategy for preprocessing noisy high-dimensional data.

Differential MicroRNA Expression in Porcine Endometrium Related to Spontaneous Embryo Loss during Early Pregnancy

Litter size is an important indicator to measure the production capacity of commercial pigs. Spontaneous embryo loss is an essential factor in determining sow litter size. In early pregnancy, spontaneous embryo loss in porcine is as high as 20–30% during embryo implantation. However, the specific molecular mechanism underlying spontaneous embryo loss at the end of embryo implantation remains unknown. Therefore, we comprehensively used small RNA sequencing technology, bioinformatics analysis, and molecular experiments to determine the microRNA (miRNA) expression profile in the healthy and arresting embryo implantation site of porcine endometrium on day of gestation (DG) 28. A total of 464 miRNAs were identified in arresting endometrium (AE) and healthy endometrium (HE), and 139 differentially expressed miRNAs (DEMs) were screened. We combined the mRNA sequencing dataset from the SRA database to predict the target genes of these miRNAs. A quantitative real-time PCR assay identified the expression levels of miRNAs and mRNAs. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyses were performed on differentially expressed target genes of DEMs, mainly enriched in epithelial development and amino acids metabolism-related pathways. We performed fluorescence in situ hybridization (FISH) and the dual-luciferase report gene assay to confirm miRNA and predicted target gene binding. miR-205 may inhibit its expression by combining 3′-untranslated regions (3′ UTR) of tubulointerstitial nephritis antigen-like 1 (TINAGL1). The resulting inhibition of angiogenesis in the maternal endometrium ultimately leads to the formation of arresting embryos during the implantation period. This study provides a reference for the effect of miRNA on the successful implantation of pig embryos in early gestation.

Role of Primary Cilia in Skeletal Disorders

Primary cilia are highly conserved microtubule-based organelles that project from the cell surface into the extracellular environment and play important roles in mechanosensation, mechanotransduction, polarity maintenance, and cell behaviors during organ development and pathological changes. Intraflagellar transport (IFT) proteins are essential for cilium formation and function. The skeletal system consists of bones and connective tissue, including cartilage, tendons, and ligaments, providing support, stability, and movement to the body. Great progress has been achieved in primary cilia and skeletal disorders in recent decades. Increasing evidence suggests that cells with cilium defects in the skeletal system can cause numerous human diseases. Moreover, specific deletion of ciliary proteins in skeletal tissues with different Cre mice resulted in diverse malformations, suggesting that primary cilia are involved in the development of skeletal diseases. In addition, the intact of primary cilium is essential to osteogenic/chondrogenic induction of mesenchymal stem cells, regarded as a promising target for clinical intervention for skeletal disorders. In this review, we summarized the role of primary cilia and ciliary proteins in the pathogenesis of skeletal diseases, including osteoporosis, bone/cartilage tumor, osteoarthritis, intervertebral disc degeneration, spine scoliosis, and other cilium-related skeletal diseases, and highlighted their promising treatment methods, including using mesenchymal stem cells. Our review tries to present evidence for primary cilium as a promising target for clinical intervention for skeletal diseases.

dmrt2 and myf5 Link Early Somitogenesis to Left-Right Axis Determination in Xenopus laevis

The vertebrate left-right axis is specified during neurulation by events occurring in a transient ciliated epithelium termed left-right organizer (LRO), which is made up of two distinct cell types. In the axial midline, central LRO (cLRO) cells project motile monocilia and generate a leftward fluid flow, which represents the mechanism of symmetry breakage. This directional fluid flow is perceived by laterally positioned sensory LRO (sLRO) cells, which harbor non-motile cilia. In sLRO cells on the left side, flow-induced signaling triggers post-transcriptional repression of the multi-pathway antagonist dand5. Subsequently, the co-expressed Tgf-β growth factor Nodal1 is released from Dand5-mediated repression to induce left-sided gene expression. Interestingly, Xenopus sLRO cells have somitic fate, suggesting a connection between LR determination and somitogenesis. Here, we show that doublesex and mab3-related transcription factor 2 (Dmrt2), known to be involved in vertebrate somitogenesis, is required for LRO ciliogenesis and sLRO specification. In dmrt2 morphants, misexpression of the myogenic transcription factors tbx6 and myf5 at early gastrula stages preceded the misspecification of sLRO cells at neurula stages. myf5 morphant tadpoles also showed LR defects due to a failure of sLRO development. The gain of myf5 function reintroduced sLRO cells in dmrt2 morphants, demonstrating that paraxial patterning and somitogenesis are functionally linked to LR axis formation in Xenopus.

Identification of Two Novel DNAAF2 Variants in Two Consanguineous Families with Primary Ciliary Dyskinesia

Background

Dynein axonemal assembly factor 2 (DNAAF2) is involved in the early preassembly of dynein in the cytoplasm, which is essential for motile cilia function. Primary ciliary dyskinesia (PCD) associated with DNAAF2 variants has rarely been reported in females with infertility. Moreover, there is no report linking DNAAF2 to scoliosis in human.

Materials and Methods

We recruited patients from two consanguineous families with a clinical diagnosis of PCD and collected their clinical history, laboratory tests, and radiographic data. Sequencing and bioinformatics analysis were then performed. Immunofluorescence and high-speed microscope analysis were used to support the pathogenicity of the variant.

Results

Proband 1, a 26-year-old female from family I, exhibited scoliosis, bronchiectasis, sinusitis, situs inversus, and infertility. We found a novel homozygous missense variant in DNAAF2, c.491T>C, p.(Leu164Pro) in this patient. Subsequent immunofluorescence indicated the absence of outer dynein arm and inner dynein arm of cilia, and high-speed microscopy analysis showed that the most of the cilia are static, which support the pathogenicity of this variant. Proband 2, a 53-year-old female, presented with bronchiectasis, sinusitis, and infertility. In this patient, a new homozygous frameshift variant DNAAF2, c.822del, p.(Ala275Profs*10) was identified. The disease-causing variants mentioned above are not included in the current authorized genetic databases.

Conclusion

Our findings expand the spectrum of DNAAF2 variants and link DNAAF2 to female infertility and likely scoliosis in patients with PCD.

Role of Ca2+ transients at the node of the mouse embryo in breaking of left-right symmetry

Immotile cilia sense extracellular signals such as fluid flow, but whether Ca²⁺ plays a role in flow sensing has been unclear. Here, we examined the role of ciliary Ca²⁺ in the flow sensing that initiates the breaking of left-right (L-R) symmetry in the mouse embryo. Intraciliary and cytoplasmic Ca²⁺ transients were detected in the crown cells at the node. These Ca²⁺ transients showed L-R asymmetry, which was lost in the absence of fluid flow or the PKD2 channel. Further characterization allowed classification of the Ca²⁺ transients into two types: cilium-derived, L-R-asymmetric transients (type 1) and cilium-independent transients without an L-R bias (type 2). Type 1 intraciliary transients occurred preferentially at the left posterior region of the node, where L-R symmetry breaking takes place. Suppression of intraciliary Ca²⁺ transients delayed L-R symmetry breaking. Our results implicate cilium-derived Ca²⁺ transients in crown cells in initiation of L-R symmetry breaking in the mouse embryo.

Vertebrate cranial mesoderm: developmental trajectory and evolutionary origin

Vertebrate cranial mesoderm is a discrete developmental unit compared to the mesoderm below the developing neck. An extraordinary feature of the cranial mesoderm is that it includes a common progenitor pool contributing to the chambered heart and the craniofacial skeletal muscles. This striking developmental potential and the excitement it generated led to advances in our understanding of cranial mesoderm developmental mechanism. Remarkably, recent findings have begun to unravel the origin of its distinct developmental characteristics. Here, we take a detailed view of the ontogenetic trajectory of cranial mesoderm and its regulatory network. Based on the emerging evidence, we propose that cranial and posterior mesoderm diverge at the earliest step of the process that patterns the mesoderm germ layer along the anterior-posterior body axis. Further, we discuss the latest evidence and their impact on our current understanding of the evolutionary origin of cranial mesoderm. Overall, the review highlights the findings from contemporary research, which lays the foundation to probe the molecular basis of unique developmental potential and evolutionary origin of cranial mesoderm.

Whole-Exome Sequencing Identified a TBX6 Loss of Function Mutation in a Patient with Distal Vaginal Atresia

STUDY OBJECTIVE

The purpose of this study was to determine if there are any genetic changes with whole-exome sequencing associated with distal vaginal atresia.

DESIGN

This was a retrospective genetics study of 5 patients who presented with distal vaginal atresia who were recruited between 2017 and 2018. Whole-exome sequencing was performed in each subject with distal vaginal atresia. Sanger sequencing was used to confirm the potential causative genetic mutation.

SETTING

Beijing Obstetrics and Gynecology Hospital, Capital Medical University, Beijing, China.

PARTICIPANTS AND MAIN OUTCOME MEASURES

The main outcome measure was the rare mutations potentially associated with distal vaginal atresia in 5 patients.

RESULTS

A truncating mutation c.266delC (p.P89Rfs*5) in the T-box transcription factor 6 (TBX6) gene, which is highly expressed in the human vagina, was identified in 1 patient using whole-exome sequencing. The deletion of the 16p11.2 region containing the TBX6 locus has also been reported previously to have the clinical feature of Müllerian agenesis. This mutation was paternally inherited by the patient. This truncating mutation was absent from all of the databases we checked, suggesting that the variant is rare and pathogenic.

CONCLUSION

We showed, to our knowledge, for the first time, that the mutation in TBX6 might be associated with human distal vaginal atresia.

The copy number variation landscape of congenital anomalies of the kidney and urinary tract

Congenital anomalies of the kidney and urinary tract (CAKUT) are a major cause of pediatric kidney failure. We performed a genome-wide analysis of copy number variants (CNVs) in 2,824 cases and 21,498 controls. Affected individuals carried a significant burden of rare exonic (i.e. affecting coding regions) CNVs and were enriched for known genomic disorders (GD). Kidney anomaly (KA) cases were most enriched for exonic CNVs, encompassing GD-CNVs and novel deletions; obstructive uropathy (OU) had a lower CNV burden and an intermediate prevalence of GD-CNVs; vesicoureteral reflux (VUR) had the fewest GD-CNVs but was enriched for novel exonic CNVs, particularly duplications. Six loci (1q21, 4p16.1-p16.3, 16p11.2, 16p13.11, 17q12, and 22q11.2) accounted for 65% of patients with GD-CNVs. Deletions at 17q12, 4p16.1-p16.3, and 22q11.2 were specific for KA; the 16p11.2 locus showed extensive pleiotropy. Using a multidisciplinary approach, we identified TBX6 as a driver for the CAKUT subphenotypes in the 16p11.2 microdeletion syndrome.

The Roles of Primary Cilia in Cardiovascular Diseases

Primary cilia are microtubule-based organelles found in most mammalian cell types. Cilia act as sensory organelles that transmit extracellular clues into intracellular signals for molecular and cellular responses. Biochemical and molecular defects in primary cilia are associated with a wide range of diseases, termed ciliopathies, with phenotypes ranging from polycystic kidney disease, liver disorders, mental retardation, and obesity to cardiovascular diseases. Primary cilia in vascular endothelia protrude into the lumen of blood vessels and function as molecular switches for calcium (Ca²⁺) and nitric oxide (NO) signaling. As mechanosensory organelles, endothelial cilia are involved in blood flow sensing. Dysfunction in endothelial cilia contributes to aberrant fluid-sensing and thus results in vascular disorders, including hypertension, aneurysm, and atherosclerosis. This review focuses on the most recent findings on the roles of endothelial primary cilia within vascular biology and alludes to the possibility of primary cilium as a therapeutic target for cardiovascular disorders.

A Rare Case of Human Diphallia Associated with Hypospadias

Diphallia or penile duplication is a rare congenital variant with an estimated frequency of 1 per 5 to 6 million live births. The extent of duplication varies widely and typically occurs with other malformations including urogenital, gastrointestinal, and musculoskeletal anomalies. Here we present a case of human diphallia that was detected during routine dissection of an 84-year-old cadaver. Upon thorough examination, this case was characterized as a complete bifid penis which was accompanied by hypospadias with no other anatomical abnormalities detected. To gain insights into the etiology of this case, we analyzed DNA procured from the body for putative genetic variants using Next Generation Sequencing (NGS) technology. Our results support clinical observations consistent with human diphallia being a polygenic syndrome and identify new genetic variants that might underlie its etiology.

Tbx6 controls left-right asymmetry through regulation of Gdf1

The Tbx6 transcription factor plays multiple roles during gastrulation, somite formation and body axis determination. One of the notable features of the Tbx6 homozygous mutant phenotype is randomization of left/right axis determination. Cilia of the node are morphologically abnormal, leading to the hypothesis that disrupted nodal flow is the cause of the laterality defect. However, Tbx6 is expressed around but not in the node, leading to uncertainty as to the mechanism of this effect. In this study, we have examined the molecular characteristics of the node and the genetic cascade determining left/right axis determination. We found evidence that a leftward nodal flow is generated in Tbx6 homozygous mutants despite the cilia defect, establishing the initial asymmetric gene expression in Dand5 around the node, but that the transduction of the signal from the node to the left lateral plate mesoderm is disrupted due to lack of expression of the Nodal coligand Gdf1 around the node. Gdf1 was shown to be a downstream target of Tbx6 and a Gdf1 transgene partially rescues the laterality defect.

Summary: Tbx6 affects morphology of the cilia of the node, but a leftward nodal flow is still generated. Downstream of nodal flow, Tbx6 regulates the Nodal coligand Gdf1 leading to disruption of left/right axis determination.

Growth and Morphogenesis during Early Heart Development in Amniotes

In this review, we will focus on the growth and morphogenesis of the developing heart, an aspect of cardiovascular development to which Antoon Moorman and colleagues have extensively contributed. Over the last decades, genetic studies and characterization of regionally regulated gene programs have provided abundant novel insights into heart development essential to understand the basis of congenital heart disease. Heart morphogenesis, however, is inherently a complex and dynamic three-dimensional process and we are far from understanding its cellular basis. Here, we discuss recent advances in studying heart morphogenesis and regionalization under the light of the pioneering work of Moorman and colleagues, which allowed the reinterpretation of regional gene expression patterns under a new morphogenetic framework. Two aspects of early heart formation will be discussed in particular: (1) the initial formation of the heart tube and (2) the formation of the cardiac chambers by the ballooning process. Finally, we emphasize that in addition to analyses based on fixed samples, new approaches including clonal analysis, single-cell sequencing, live-imaging and quantitative analysis of the data generated will likely lead to novel insights in understanding early heart tube regionalization and morphogenesis in the near future.

Multiple Roles of Pitx2 in Cardiac Development and Disease

Cardiac development is a complex morphogenetic process initiated as bilateral cardiogenic mesoderm is specified at both sides of the gastrulating embryo. Soon thereafter, these cardiogenic cells fuse at the embryonic midline configuring a symmetrical linear cardiac tube. Left/right bilateral asymmetry is first detected in the forming heart as the cardiac tube bends to the right, and subsequently, atrial and ventricular chambers develop. Molecular signals emanating from the node confer distinct left/right signalling pathways that ultimately lead to activation of the homeobox transcription factor Pitx2 in the left side of distinct embryonic organ anlagen, including the developing heart. Asymmetric expression of Pitx2 has therefore been reported during different cardiac developmental stages, and genetic deletion of Pitx2 provided evidence of key regulatory roles of this transcription factor during cardiogenesis and thus congenital heart diseases. More recently, impaired Pitx2 function has also been linked to arrhythmogenic processes, providing novel roles in the adult heart. In this manuscript, we provide a state-of-the-art review of the fundamental roles of Pitx2 during cardiogenesis, arrhythmogenesis and its contribution to congenital heart diseases.

Cell lineage of timed cohorts of Tbx6-expressing cells in wild-type and Tbx6 mutant embryos

Tbx6 is a T-box transcription factor with multiple roles in embryonic development as evidenced by dramatic effects on mesoderm cell fate determination, left/right axis determination, and somite segmentation in mutant mice. The expression of Tbx6 is restricted to the primitive streak and presomitic mesoderm, but some of the phenotypic features of mutants are not easily explained by this expression pattern. We have used genetically-inducible fate mapping to trace the fate of Tbx6-expressing cells in wild-type and mutant embryos to explain some of the puzzling features of the mutant phenotype. We created an inducible Tbx6-creERT2 transgenic mouse in which cre expression closely recapitulates endogenous Tbx6 expression both temporally and spatially. Using a lacZ-based Cre reporter and timed tamoxifen injections, we followed temporally overlapping cohorts of cells that had expressed Tbx6 and found contributions to virtually all mesodermally-derived embryonic structures as well as the extraembryonic allantois. Contribution to the endothelium of major blood vessels may account for the embryonic death of homozygous mutant embryos. In mutant embryos, Tbx6-creERT2-traced cells contributed to the abnormally segmented anterior somites and formed the characteristic ectopic neural tubes. Retention of cells in the mutant tail bud indicates a deficiency in migratory behavior of the mutant cells and the presence of Tbx6-creERT2-traced cells in the notochord, a node derivative provides a possible explanation for the heterotaxia seen in mutant embryos.

Summary: Embryonic cells that transiently express the transcription factor, Tbx6, during the process of gastrulation have been tracked in later development in wild-type and Tbx6 homozygous mutant embryos, where they give rise to the ectopic neural tubes characteristic of the mutant phenotype.

Comparison of two related lines of tauGFP transgenic mice designed for lineage tracing

Background

The tauGFP reporter fusion protein is produced nearly ubiquitously by the TgTP6.3 transgene in TP6.3 mice and its localisation to microtubules offers some advantages over soluble GFP as a lineage marker. However, TgTP6.3^Tg/Tg homozygotes are not viable and TgTP6.3^Tg/− hemizygotes are smaller than wild-type. TP6.4 mice carry the TgTP6.4 transgene, which was produced with the same construct used to generate TgTP6.3, so we investigated whether TgTP6.4 had any advantages over TgTP6.3.

Results

Although TgTP6.4^Tg/Tg homozygotes died before weaning, TgTP6.4^Tg/− hemizygotes were viable and fertile and only males were significantly lighter than wild-type. The TgTP6.4 transgene produced the tauGFP fusion protein by the 2-cell stage and it was widely expressed in adults but tauGFP fluorescence was weak or absent in several tissues, including some neural tissues. The TgTP6.4 transgene expression pattern changed over several years of breeding and mosaic transgene expression became increasingly common in all expressing tissues. This mosaicism was used to visualise clonal lineages in the adrenal cortex of TgTP6.4^Tg/− hemizygotes and these were qualitatively and quantitatively comparable to lineages reported previously for other mosaic transgenic mice, X-inactivation mosaics and chimaeras. Mosaicism occurred less frequently in TP6.3 than TP6.4 mice and was only observed in the corneal epithelium and adrenal cortex.

Conclusions

Mosaic expression makes the TgTP6.4 transgene unsuitable for use as a conventional cell lineage marker but such mosaicism provides a useful system for visualising clonal lineages that arise during development or maintenance of adult tissues. Differences in the occurrence of mosaicism between related transgenic lines, such as that described for lines TP6.3 and TP6.4, might provide a useful system for investigating the mechanism of transgene silencing.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-017-0149-x) contains supplementary material, which is available to authorized users.

Co-expression of Tbx6 and Sox2 identifies a novel transient neuromesoderm progenitor cell state

The elongation of body axis during development is a key aspect of body plan. Bi-potential neuromesoderm progenitors (NMPs) ensure the axial growth of embryos by contributing both to the spinal cord and mesoderm. The current model for the mechanism controlling NMP deployment invokes Tbx6, a T-box factor, to drive mesoderm differentiation of NMPs. Here, we identify a new population of Tbx6+ cells in a subdomain of NMP niche in mouse embryos. Based on co-expression of a progenitor marker Sox2, we identify this population to represent a transient cell state in the mesoderm-fated NMP lineage. Genetic lineage tracing confirms the presence of Tbx6+ NMP cell state. Furthermore, we report a novel aspect of documented Tbx6 mutant phenotype, i.e., an increase from two to four ectopic neural tubes, corresponding to the switch in NMP niche, highlighting the importance of Tbx6 function in NMP fate decision. This study emphasizes the function of Tbx6 as the bi-stable switch turning mesoderm fate “on” and progenitor state “off”, and thus, has implications for the molecular mechanism driving NMP fate choice.

Copy-number variant analysis of classic heterotaxy highlights the importance of body patterning pathways

Classic heterotaxy consists of congenital heart defects with abnormally positioned thoracic and abdominal organs. We aimed to uncover novel, genomic copy-number variants (CNVs) in classic heterotaxy cases. A microarray containing 2.5 million single-nucleotide polymorphisms (SNPs) was used to genotype 69 infants (cases) with classic heterotaxy identified from California live births from 1998 to 2009. CNVs were identified using the PennCNV software. We identified 56 rare CNVs encompassing genes in the NODAL (NIPBL, TBX6), BMP (PPP4C), and WNT (FZD3) signaling pathways, not previously linked to classic heterotaxy. We also identified a CNV involving FGF12, a gene previously noted in a classic heterotaxy case. CNVs involving RBFOX1 and near MIR302F were detected in multiple cases. Our findings illustrate the importance of body patterning pathways for cardiac development and left/right axes determination. FGF12, RBFOX1, and MIR302F could be important in human heterotaxy, because they were noted in multiple cases. Further investigation into genes involved in the NODAL, BMP, and WNT body patterning pathways and into the dosage effects of FGF12, RBFOX1, and MIR302F is warranted.

Etiologies of uterine malformations

Ranging from aplastic uterus (including Mayer-Rokitansky-Kuster-Hauser syndrome) to incomplete septate uterus, uterine malformations as a group are relatively frequent in the general population. Specific causes remain largely unknown. Although most occurrences ostensibly seem sporadic, familial recurrences have been observed, which strongly implicate genetic factors. Through the study of animal models, human syndromes, and structural chromosomal variation, several candidate genes have been proposed and subsequently tested with targeted methods in series of individuals with isolated, non-isolated, or syndromic uterine malformations. To date, a few genes have garnered strong evidence of causality, mainly in syndromic presentations (HNF1B, WNT4, WNT7A, HOXA13). Sequencing of candidate genes in series of individuals with isolated uterine abnormalities has been able to suggest an association for several genes, but confirmation of a strong causative effect is still lacking for the majority of them. We review the current state of knowledge about the developmental origins of uterine malformations, with a focus on the genetic variants that have been implicated or associated with these conditions in humans, and we discuss potential reasons for the high rate of negative results. The evidence for various environmental and epigenetic factors is also reviewed. © 2016 Wiley Periodicals, Inc.

Rare variants in the notch signaling pathway describe a novel type of autosomal recessive Klippel-Feil syndrome

Klippel-Feil syndrome is a rare disorder represented by a subgroup of segmentation defects of the vertebrae and characterized by fusion of the cervical vertebrae, low posterior hairline, and short neck with limited motion. Both autosomal dominant and recessive inheritance patterns were reported in families with Klippel-Feil. Mutated genes for both dominant (GDF6 and GDF3) and recessive (MEOX1) forms of Klippel-Feil syndrome have been shown to be involved in somite development via transcription regulation and signaling pathways. Heterotaxy arises from defects in proteins that function in the development of left-right asymmetry of the developing embryo. We describe a consanguineous family with a male proband who presents with classical Klippel-Feil syndrome together with heterotaxy (situs inversus totalis). The present patient also had Sprengel's deformity, deformity of the sternum, and a solitary kidney. Using exome sequencing, we identified a homozygous frameshift mutation (c.299delT; p.L100fs) in RIPPLY2, a gene shown to play a crucial role in somitogenesis and participate in the Notch signaling pathway via negatively regulating Tbx6. Our data confirm RIPPLY2 as a novel gene for autosomal recessive Klippel-Feil syndrome, and in addition-from a mechanistic standpoint-suggest the possibility that mutations in RIPPLY2 could also lead to heterotaxy. © 2015 Wiley Periodicals, Inc.

Microscale imaging of cilia-driven fluid flow

Cilia-driven fluid flow is important for multiple processes in the body, including respiratory mucus clearance, gamete transport in the oviduct, right-left patterning in the embryonic node, and cerebrospinal fluid circulation. Multiple imaging techniques have been applied toward quantifying ciliary flow. Here, we review common velocimetry methods of quantifying fluid flow. We then discuss four important optical modalities, including light microscopy, epifluorescence, confocal microscopy, and optical coherence tomography, that have been used to investigate cilia-driven flow.

Intraciliary calcium oscillations initiate vertebrate left-right asymmetry

Background

Bilateral symmetry during vertebrate development is broken at the left-right organizer (LRO) by ciliary motility and the resultant directional flow of extracellular fluid. However, how ciliary motility is perceived and transduced into asymmetrical intracellular signaling at the LRO remains controversial. Previous work has indicated that sensory cilia and polycystin-2 (Pkd2), a cation channel, are required for sensing ciliary motility, yet their function and the molecular mechanism linking both to left-right signaling cascades is unknown.

Results

Here, we report novel intraciliary calcium oscillations (ICOs) at the LRO that connect ciliary sensation of ciliary motility to downstream left-right signaling. Utilizing cilia-targeted genetically-encoded calcium indicators in live zebrafish embryos, we show that ICOs depend on Pkd2 and are left-biased at the LRO in response to ciliary motility. Asymmetric ICOs occur with onset of LRO ciliary motility, thus representing the earliest known LR asymmetric molecular signal. Suppression of ICOs using a cilia-targeted calcium sink demonstrates that they are essential for LR development.

Conclusions

These findings demonstrate that intraciliary calcium initiates LR development and identify cilia as a functional ion signaling compartment connecting ciliary motility and flow to molecular LR signaling.

The transcription factor Pitx2 positions the embryonic axis and regulates twinning

Embryonic polarity of invertebrates, amphibians and fish is specified largely by maternal determinants, which fixes cell fates early in development. In contrast, amniote embryos remain plastic and can form multiple individuals until gastrulation. How is their polarity determined? In the chick embryo, the earliest known factor is cVg1 (homologous to mammalian growth differentiation factor 1, GDF1), a transforming growth factor beta (TGFβ) signal expressed posteriorly before gastrulation. A molecular screen to find upstream regulators of cVg1 in normal embryos and in embryos manipulated to form twins now uncovers the transcription factor Pitx2 as a candidate. We show that Pitx2 is essential for axis formation, and that it acts as a direct regulator of cVg1 expression by binding to enhancers within neighbouring genes. Pitx2, Vg1/GDF1 and Nodal are also key actors in left-right asymmetry, suggesting that the same ancient polarity determination mechanism has been co-opted to different functions during evolution.

The T-box gene family: emerging roles in development, stem cells and cancer

The T-box family of transcription factors exhibits widespread involvement throughout development in all metazoans. T-box proteins are characterized by a DNA-binding motif known as the T-domain that binds DNA in a sequence-specific manner. In humans, mutations in many of the genes within the T-box family result in developmental syndromes, and there is increasing evidence to support a role for these factors in certain cancers. In addition, although early studies focused on the role of T-box factors in early embryogenesis, recent studies in mice have uncovered additional roles in unsuspected places, for example in adult stem cell populations. Here, I provide an overview of the key features of T-box transcription factors and highlight their roles and mechanisms of action during various stages of development and in stem/progenitor cell populations.

Nature and extent of left/right axis defects in T(Wis) /T(Wis) mutant mouse embryos

BACKGROUND

Mutations in the T-box gene Brachyury have well known effects on invagination of the endomesodermal layer during gastrulation, but the gene also plays a role in the determination of left/right axis determination that is less well studied. Previous work has implicated node morphology in this effect. We use the T(Wis) allele of Brachyury to investigate the molecular and morphological effects of the T locus on axis determination in the mouse.

RESULTS

Similar to embryos mutant for the T allele, T(Wis) /T(Wis) embryos have a high incidence of ventral and/or reversed heart looping. In addition, heterotaxia between the direction of heart looping and the direction of embryo turning is common. Scanning electron microscopy reveals defects in node morphology including irregularity, smaller size, and a decreased number of cilia, although the cilia appear morphologically normal. Molecular analysis shows a loss of perinodal expression of genes involved in Nodal signaling, namely Cer2, Gdf1, and Nodal itself. There is also loss of Dll1 expression, a key component of the Notch signaling pathway, in the presomitic mesoderm.

CONCLUSIONS

Morphological abnormalities of the node as well as disruptions of the molecular cascade of left/right axis determination characterize T(Wis) /T(Wis) mutants. Decreased Notch signaling may account for both the morphological defects and the absence of expression of genes in the Nodal signaling pathway.

Follow your gut: relaying information from the site of left-right symmetry breaking in the mouse

A central unresolved question in the molecular cascade that drives establishment of left-right (LR) asymmetry in vertebrates are the mechanisms deployed to relay information between the midline site of symmetry-breaking and the tissues which will execute a program of asymmetric morphogenesis. The cells located between these two distant locations must provide the medium for signal relay. Of these, the gut endoderm is an attractive candidate tissue for signal transmission since it comprises the epithelium that lies between the node, where asymmetry originates, and the lateral plate, where asymmetry can first be detected. Here, focusing on the mouse as a model, we review our current understanding and entertain open questions concerning the relay of LR information from its origin.

Charting Brachyury-mediated developmental pathways during early mouse embryogenesis

To gain insights into coordinated lineage-specification and morphogenetic processes during early embryogenesis, here we report a systematic identification of transcriptional programs mediated by a key developmental regulator--Brachyury. High-resolution chromosomal localization mapping of Brachyury by ChIP sequencing and ChIP-exonuclease revealed distinct sequence signatures enriched in Brachyury-bound enhancers. A combination of genome-wide in vitro and in vivo perturbation analysis and cross-species evolutionary comparison unveiled a detailed Brachyury-dependent gene-regulatory network that directly links the function of Brachyury to diverse developmental pathways and cellular housekeeping programs. We also show that Brachyury functions primarily as a transcriptional activator genome-wide and that an unexpected gene-regulatory feedback loop consisting of Brachyury, Foxa2, and Sox17 directs proper stem-cell lineage commitment during streak formation. Target gene and mRNA-sequencing correlation analysis of the T(c) mouse model supports a crucial role of Brachyury in up-regulating multiple key hematopoietic and muscle-fate regulators. Our results thus chart a comprehensive map of the Brachyury-mediated gene-regulatory network and how it influences in vivo developmental homeostasis and coordination.

Cilia and coordination of signaling networks during heart development

Primary cilia are unique sensory organelles that coordinate a wide variety of different signaling pathways to control cellular processes during development and in tissue homeostasis. Defects in function or assembly of these antenna-like structures are therefore associated with a broad range of developmental disorders and diseases called ciliopathies. Recent studies have indicated a major role of different populations of cilia, including nodal and cardiac primary cilia, in coordinating heart development, and defects in these cilia are associated with congenital heart disease. Here, we present an overview of the role of nodal and cardiac primary cilia in heart development.

Regulation of organogenesis and stem cell properties by T-box transcription factors

T-box transcription factors containing the common DNA-binding domain T-box contribute to the organization of multiple tissues in vertebrates and invertebrates. In mammals, 17 T-box genes are divided into five subfamilies depending on their amino acid homology. The proper distribution and expression of individual T-box transcription factors in different tissues enable regulation of the proliferation and differentiation of tissue-specific stem cells and progenitor cells in a suitable time schedule for tissue organization. Consequently, uncontrollable expressions of T-box genes induce abnormal tissue organization, and eventually cause various diseases with malformation and malfunction of tissues and organs. Furthermore, some T-box transcription factors are essential for maintaining embryonic stem cell pluripotency, improving the quality of induced pluripotent stem cells, and inducing cell-lineage conversion of differentiated cells. These lines of evidence indicate fundamental roles of T-box transcription factors in tissue organization and stem cell properties, and suggest that these transcription factors will be useful for developing therapeutic approaches in regenerative medicine.

TBX3 Directs Cell-Fate Decision toward Mesendoderm

Cell-fate decisions and pluripotency are dependent on networks of key transcriptional regulators. Recent reports demonstrated additional functions of pluripotency-associated factors during early lineage commitment. The T-box transcription factor TBX3 has been implicated in regulating embryonic stem cell self-renewal and cardiogenesis. Here, we show that TBX3 is dynamically expressed during specification of the mesendoderm lineages in differentiating embryonic stem cells (ESCs) in vitro and in developing mouse and Xenopus embryos in vivo. Forced TBX3 expression in ESCs promotes mesendoderm specification by directly activating key lineage specification factors and indirectly by enhancing paracrine Nodal/Smad2 signaling. TBX3 loss-of-function analyses in the Xenopus underline its requirement for mesendoderm lineage commitment. Moreover, we uncovered a functional redundancy between TBX3 and Tbx2 during Xenopus gastrulation. Taken together, we define further facets of TBX3 actions and map TBX3 as an upstream regulator of the mesendoderm transcriptional program during gastrulation.

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T-box transcription factor TBX3 is involved in early embryonic events

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Key transcription factor promoters of mesendoderm formation are occupied by TBX3

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TBX3 promotes mesendodermal fate of mESCs

TBX6, LHX1 and copy number variations in the complex genetics of Müllerian aplasia

Background

Müllerian aplasia (MA) is a congenital disorder of the female reproductive tract with absence of uterus and vagina with paramount impact on a woman’s life. Despite intense research, no major genes have been found to explain the complex genetic etiology.

Methods and Results

We have used several genetic methods to study 112 patients with MA. aCGH identified CNVs in 8/50 patients (16%), including 16p11.2 and 17q12 deletions previously associated with MA. Subsequently, another four patients were shown to carry the ~0.53 Mb deletion in 16p11.2. More importantly, sequencing of TBX6, residing within 16p11.2, revealed two patients carrying a splice site mutation. Two previously reported TBX6 variants in exon 4 and 6 were shown to have a significantly higher frequency in patients (8% and 5%, respectively) than in controls (2% each). We also sequenced LHX1 and found three apparently pathogenic missense variants in 5/112 patients. Altogether, we identified either CNVs or variations in TBX6 or LHX1 in 30/112 (26.8%) MA patients. CNVs were found in 12/112 (10.7%), patients, novel variants in TBX6 or LHX1 in 7/112 (6.3%), and rare variants in TBX6 in 15/112 (13.4%) patients. Furthermore, four of our patients (4/112, 3.6%) were shown to carry variants in both TBX6 and LHX1 or a CNV in combination with TBX6 variants lending support to the complex genetic etiology of MA.

Conclusions

We have identified TBX6 as a new gene associated with MA. Our results also support the relevance of LHX1 and CNVs in the development of this congenital malformation.

A genome-wide association study identifies two risk loci for congenital heart malformations in Han Chinese populations

Congenital heart malformation (CHM) is the most common form of congenital human birth anomaly and is the leading cause of infant mortality. Although some causative genes have been identified, little progress has been made in identifying genes in which low-penetrance susceptibility variants occur in the majority of sporadic CHM cases. To identify common genetic variants associated with sporadic non-syndromic CHM in Han Chinese populations, we performed a multistage genome-wide association study (GWAS) in a total of 4,225 CHM cases and 5,112 non-CHM controls. The GWAS stage included 945 cases and 1,246 controls and was followed by 2-stage validation with 2,160 cases and 3,866 controls. The combined analyses identified significant associations (P < 5.0 × 10⁻⁸) at 1p12 (rs2474937 near TBX15; odds ratio (OR) = 1.40; P = 8.44 × 10⁻¹⁰) and 4q31.1 (rs1531070 in MAML3; OR = 1.40; P = 4.99 × 10⁻¹²). These results extend current knowledge of genetic contributions to CHM in Han Chinese populations.

Reporter mouse lines for fluorescence imaging

The use of live imaging approaches to examine and understand the dynamic processes that take place during mouse development has become widespread. Several groups have reported their success in generating different reporter mouse lines that express a variety of fluorescent markers for imaging. However, there is currently no established database of the reporter mouse lines available for live imaging, such as the Cre transgenic lines (Cre-X-Mice). Researchers therefore often have difficulties in determining which reporter mouse line meets their research purposes. In this review, we summarize some of the reporter mouse lines that have been generated for live imaging studies, and discuss their characteristics.

Asymmetric distribution of dynamic calcium signals in the node of mouse embryo during left-right axis formation

In the node of mouse embryo, rotational movements of cilia generate an external liquid flow known as nodal flow, which determines left-right asymmetric gene expression. How nodal flow is converted into asymmetric gene expression is still controversial, but the increase of Ca(2+) levels in endodermal cells to the left of the node has been proposed to play a role. However, Ca(2+) signals inside the node itself have not yet been described. By our optimized Ca(2+) imaging method, we were able to observe dynamic Ca(2+) signals in the node in live mouse embryos. Pharmacological disruption of Ca(2+) signals did not affect ciliary movements or nodal flow, but did alter the expression patterns of the Nodal and Cerl-2 genes. Quantitative analyses of Ca(2+) signal frequencies and distributions showed that during left-right axis establishment, formerly symmetric Ca(2+) signals became biased to the left side. In iv/iv mutant embryos that showed randomized laterality due to ciliary immotility, Ca(2+) signals were found to be variously left-sided, right-sided, or bilateral, and thus symmetric on average. In Pkd2 mutant embryos, which lacked polycystin-2, a Ca(2+)-permeable cation channel necessary for left-right axis formation, the Ca(2+) signal frequency was lower than in wild-type embryos. Our data support a model in which dynamic Ca(2+) signals in the node are involved in left-right patterning.

T-box factors: insights into the evolutionary emergence of the complex heart

The heart as a functional organ first appeared in bilaterians as a single peristaltic pump and evolved through arthropods, fish, amphibians, and finally mammals into a four-chambered engine controlling blood-flow within the body. The acquisition of cardiac complexity in the evolving heart was a product of gene duplication events and the co-option of novel signaling pathways to an ancestral cardiac-specific gene network. T-box factors belong to an evolutionary conserved family of transcriptional regulators with diverse roles in development. Their regulatory functions are integral in the initiation and potentiation of heart development, and mutations in these genes are associated with congenital heart defects. In this review we will discuss the evolutionary conserved cardiac regulatory functions of this family as well as their implication in disease in an aim to facilitate future gene-targeted and regenerative therapeutic remedies.

Spatially coordinated kinase signaling regulates local axon degeneration

In addition to being a hallmark of neurodegenerative disease, axon degeneration is used during development of the nervous system to prune unwanted connections. In development, axon degeneration is tightly regulated both temporally and spatially. Here, we provide evidence that degeneration cues are transduced through various kinase pathways functioning in spatially distinct compartments to regulate axon degeneration. Intriguingly, glycogen synthase kinase-3 (GSK3) acts centrally, likely modulating gene expression in the cell body to regulate distally restricted axon degeneration. Through a combination of genetic and pharmacological manipulations, including the generation of an analog-sensitive kinase allele mutant mouse for GSK3β, we show that the β isoform of GSK3, not the α isoform, is essential for developmental axon pruning in vitro and in vivo. Additionally, we identify the dleu2/mir15a/16-1 cluster, previously characterized as a regulator of B-cell proliferation, and the transcription factor tbx6, as likely downstream effectors of GSK3β in axon degeneration.

Evolution of the tbx6/16 subfamily genes in vertebrates: insights from zebrafish

In any comparative studies striving to understand the similarities and differences of the living organisms at the molecular genetic level, the crucial first step is to establish the homology (orthology and paralogy) of genes between different organisms. Determination of the homology of genes becomes complicated when the genes have undergone a rapid divergence in sequence or when the involved genes are members of a gene family that has experienced a differential gain or loss of its constituents in different taxonomic groups. Organisms with duplicated genomes such as teleost fishes might have been especially prone to these problems because the functional redundancies provided by the duplicate copies of genes would have allowed a rapid divergence or loss of genes during evolution. In this study, we will demonstrate that much of the ambiguities in the determination of the homology between fish and tetrapod genes resulting from the problems like these can be eliminated by complementing the sequence-based phylogenies with nonsequence information, such as the exon-intron structure of a gene or the composition of a gene's genomic neighbors. We will use the Tbx6/16 subfamily genes of zebrafish (tbx6, tbx16, tbx24, and mga genes), which have been well known for the ambiguity of their evolutionary relationships to the Tbx6/16 subfamily genes of tetrapods, as an illustrative example. We will show that, despite the similarity of sequence and expression to the tetrapod Tbx6 genes, zebrafish tbx6 gene is actually a novel T-box gene more closely related to the tetrapod Tbx16 genes, whereas the zebrafish tbx24 gene, hitherto considered to be a novel gene due to the high level of sequence divergence, is actually an ortholog of tetrapod Tbx6 genes. We will also show that, after their initial appearance by the multiplication of a common ancestral gene at the beginning of vertebrate evolution, the Tbx6/16 subfamily of vertebrate T-box genes might have experienced differential losses of member genes in different vertebrate groups and gradual pooling of member gene's functions in surviving members, which might have prevented the revelation of the true identity of member genes by way of the comparison of sequence and function.

Fss/Tbx6 is required for central dermomyotome cell fate in zebrafish

The dermomyotome is a pool of progenitor cells on the surface of the myotome. In zebrafish, dermomyotome precursors (anterior border cells, ABCs) can be first identified in the anterior portion of recently formed somites. They must be prevented from undergoing terminal differentiation during segmentation, even while mesodermal cells around them respond to signaling cues and differentiate.

T-box containing transcription factors regulate many aspects of mesoderm fate including segmentation and somite patterning. The fused somites (fss) gene is the zebrafish ortholog of tbx6. We demonstrate that in addition to its requirement for segmentation, fss/tbx6 is also required for the specification of ABCs and subsequently the central dermomyotome. The absence of Tbx6-dependent central dermomyotome cells in fss/tbx6 mutants is spatially coincident with a patterning defect in the myotome.

Using transgenic fish with a heat-shock inducible tbx6 gene in the fss/tbx6 mutant background, we further demonstrate that ubiquitous fss/tbx6 expression has spatially distinct effects on recovery of the dermomyotome and segment boundaries, suggesting that the mechanism of Fss/Tbx6 action is distinct with respect to dermomyotome development and segmentation.

We propose that Fss/Tbx6 is required for preventing myogenic differentiation of central dermomyotome precursors before and after segmentation and that central dermomyotome cells represent a genetically and functionally distinct subpopulation within the zebrafish dermomyotome.

Tbx6 is a determinant of cardiac and neural cell fate decisions in multipotent P19CL6 cells

Multipotent P19CL6 cells differentiate into cardiac myocytes or neural lineages when stimulated with dimethyl sulfoxide (DMSO) or retinoic acid (RA), respectively. Expression of the transcription factor Tbx6 was found to increase during cardiac myocyte differentiation and to decrease during neural differentiation. Overexpression of Tbx6 was not sufficient to drive P19CL6 cells to a cardiac myocyte fate or to accelerate DMSO-induced differentiation. In contrast, knockdown of Tbx6 dramatically inhibited DMSO-induced differentiation of P19CL6 cells to cardiac myocytes, as evidenced by the loss of striated muscle-specific markers and spontaneous beating. Tbx6 knockdown was also accompanied by almost complete loss of Nkx2.5, a transcription factor involved in the specification of the cardiac myocyte lineage, indicating that Nkx2.5 is downstream of Tbx6. In distinction to its positive role in cardiac myocyte differentiation, Tbx6 knockdown augmented RA-induced differentiation of P19CL6 cells to both neurons and glia, and accelerated the rate of neurite formation. Conversely, Tbx6 overexpression attenuated differentiation to neural lineages. Thus, in the P19CL6 model, Tbx6 is required for cardiac myocyte differentiation and represses neural differentiation. We propose a model in which Tbx6 is a part of a molecular switch that modulates divergent differentiation programs within a single progenitor cell.

Gut endoderm is involved in the transfer of left-right asymmetry from the node to the lateral plate mesoderm in the mouse embryo

In the mouse, the initial signals that establish left-right (LR) asymmetry are determined in the node by nodal flow. These signals are then transferred to the lateral plate mesoderm (LPM) through cellular and molecular mechanisms that are not well characterized. We hypothesized that endoderm might play a role in this process because it is tightly apposed to the node and covers the outer surface of the embryo, and, just after nodal flow is established, higher Ca(2+) flux has been reported on the left side near the node, most likely in the endoderm cells. Here we studied the role of endoderm cells in the transfer of the LR asymmetry signal by analyzing mouse Sox17 null mutant embryos, which possess endoderm-specific defects. Sox17(-/-) embryos showed no expression or significantly reduced expression of LR asymmetric genes in the left LPM. In Sox17 mutant endoderm, the localization of connexin proteins on the cell membrane was greatly reduced, resulting in defective gap junction formation, which appeared to be caused by incomplete development of organized epithelial structures. Our findings suggest an essential role of endoderm cells in the signal transfer step from the node to the LPM, possibly using gap junction communication to establish the LR axis of the mouse.

Interaction of Wnt3a, Msgn1 and Tbx6 in neural versus paraxial mesoderm lineage commitment and paraxial mesoderm differentiation in the mouse embryo

Paraxial mesoderm is the tissue which gives rise to the skeletal muscles and vertebral column of the body. A gene regulatory network operating in the formation of paraxial mesoderm has been described. This network hinges on three key factors, Wnt3a, Msgn1 and Tbx6, each of which is critical for paraxial mesoderm formation, since absence of any one of these factors results in complete absence of posterior somites. In this study we determined and compared the spatial and temporal patterns of expression of Wnt3a, Msgn1 and Tbx6 at a time when paraxial mesoderm is being formed. Then, we performed a comparative characterization of mutants in Wnt3a, Msgn1 and Tbx6. To determine the epistatic relationship between these three genes, and begin to decipher the complex interplay between them, we analyzed double mutant embryos and compared their phenotypes to the single mutants. Through the analysis of molecular markers in mutants, our data support the bipotential nature of the progenitor cells for paraxial mesoderm and establish regulatory relationships between genes involved in the choice between neural and mesoderm fates.

Role of the gut endoderm in relaying left-right patterning in mice

Analysis of Sox17 mutant mice reveals that gap junction coupling across the gut endoderm of the embryo transmits the left-right asymmetric signal from the node to the site of asymmetric organogenesis in mice.

Establishment of left-right (LR) asymmetry occurs after gastrulation commences and utilizes a conserved cascade of events. In the mouse, LR symmetry is broken at a midline structure, the node, and involves signal relay to the lateral plate, where it results in asymmetric organ morphogenesis. How information transmits from the node to the distantly situated lateral plate remains unclear. Noting that embryos lacking Sox17 exhibit defects in both gut endoderm formation and LR patterning, we investigated a potential connection between these two processes. We observed an endoderm-specific absence of the critical gap junction component, Connexin43 (Cx43), in Sox17 mutants. Iontophoretic dye injection experiments revealed planar gap junction coupling across the gut endoderm in wild-type but not Sox17 mutant embryos. They also revealed uncoupling of left and right sides of the gut endoderm in an isolated domain of gap junction intercellular communication at the midline, which in principle could function as a barrier to communication between the left and right sides of the embryo. The role for gap junction communication in LR patterning was confirmed by pharmacological inhibition, which molecularly recapitulated the mutant phenotype. Collectively, our data demonstrate that Cx43-mediated communication across gap junctions within the gut endoderm serves as a mechanism for information relay between node and lateral plate in a process that is critical for the establishment of LR asymmetry in mice.

Superficially, humans, like other vertebrates, are bilaterally symmetrical. Nonetheless, the internal configuration of visceral organs reveals a stereotypical asymmetry. For example, human hearts are generally located on the left and the liver on the right side within the body cavity. How this left-right asymmetry is established is an area of interest, for both intrinsic biological significance and its medical application. In the mouse, the initial event that breaks left-right symmetry occurs at the node, a specialized organ located in the midline of the developing embryo. Somehow this initial asymmetry leads to a cascade of events that results in the activation of a genetic circuit on the left side of the embryo, which then leads to asymmetric organ formation. Here we show that the laterality information that is generated at the node is transferred to the lateral extremity of the embryo across the gut endoderm, which is the precursor tissue of the respiratory and digestive tracts and associated organs such as lungs, liver, and pancreas. Sox17 mutant mouse embryos exhibit defects in gut endoderm formation and fail to establish left-right asymmetry. Analysis of the mutants reveals that gap junction coupling across the gut endoderm is the mechanism of left-right information relay from the midline site of symmetry breaking to the site of asymmetric organogenesis in mice.

Genomic alterations that contribute to the development of isolated and non-isolated congenital diaphragmatic hernia

BACKGROUND

Congenital diaphragmatic hernia (CDH) is a life threatening birth defect. Most of the genetic factors that contribute to the development of CDH remain unidentified.

OBJECTIVE

To identify genomic alterations that contribute to the development of diaphragmatic defects.

METHODS

A cohort of 45 unrelated patients with CDH or diaphragmatic eventrations was screened for genomic alterations by array comparative genomic hybridisation or single nucleotide polymorphism based copy number analysis.

RESULTS

Genomic alterations that were likely to have contributed to the development of CDH were identified in 8 patients. Inherited deletions of ZFPM2 were identified in 2 patients with isolated diaphragmatic defects and a large de novo 8q deletion overlapping the same gene was found in a patient with non-isolated CDH. A de novo microdeletion of chromosome 1q41q42 and two de novo microdeletions on chromosome 16p11.2 were identified in patients with non-isolated CDH. Duplications of distal 11q and proximal 13q were found in a patient with non-isolated CDH and a de novo single gene deletion of FZD2 was identified in a patient with a partial pentalogy of Cantrell phenotype.

CONCLUSIONS

Haploinsufficiency of ZFPM2 can cause dominantly inherited isolated diaphragmatic defects with incomplete penetrance. These data define a new minimal deleted region for CDH on 1q41q42, provide evidence for the existence of CDH related genes on chromosomes 16p11.2, 11q23-24 and 13q12, and suggest a possible role for FZD2 and Wnt signalling in pentalogy of Cantrell phenotypes. These results demonstrate the clinical utility of screening for genomic alterations in individuals with both isolated and non-isolated diaphragmatic defects.

The multiple roles of Notch signaling during left-right patterning

The establishment of left-right (LR) asymmetry is regulated by intricate signaling mechanisms during embryogenesis and this asymmetry is critical for morphogenesis as well as the positioning of internal organs within the organism. Recent progress including elucidation of ion transporters, leftward nodal flow, and regulation of asymmetric gene expression contributes to our understanding of how the breaking of the symmetry is initiated and how this laterality information is subsequently transmitted to the organ primordium. A number of developmental signaling pathways have been implicated in this complex process. In this review, we will focus on the roles of the Notch signaling pathway during development of LR asymmetry. The Notch signaling pathway is a short-range communication system between neighboring cells. While Notch signaling plays essential roles in regulating the morphogenesis of the node and left-specific expression of Nodal in the lateral plate mesoderm, a hallmark gene in LR patterning, Notch signaling also suppresses the expression of Pitx2 that is a direct downstream target of Nodal during later stages of development. This negative activity of Notch signaling towards left-specific activity was recently shown to be inhibited by the B cell lymphoma 6 (BCL6)/BCL6 co-repressor (BcoR) transcriptional repressor complex in a target-specific manner. The complex regulation of Notch-dependent gene expression for LR asymmetry will be highlighted in this review.

Disruption of a ciliary B9 protein complex causes Meckel syndrome

Nearly every ciliated organism possesses three B9 domain-containing proteins: MKS1, B9D1, and B9D2. Mutations in human MKS1 cause Meckel syndrome (MKS), a severe ciliopathy characterized by occipital encephalocele, liver ductal plate malformations, polydactyly, and kidney cysts. Mouse mutations in either Mks1 or B9d2 compromise ciliogenesis and result in phenotypes similar to those of MKS. Given the importance of these two B9 proteins to ciliogenesis, we examined the role of the third B9 protein, B9d1. Mice lacking B9d1 displayed polydactyly, kidney cysts, ductal plate malformations, and abnormal patterning of the neural tube, concomitant with compromised ciliogenesis, ciliary protein localization, and Hedgehog (Hh) signal transduction. These data prompted us to screen MKS patients for mutations in B9D1 and B9D2. We identified a homozygous c.301A>C (p.Ser101Arg) B9D2 mutation that segregates with MKS, affects an evolutionarily conserved residue, and is absent from controls. Unlike wild-type B9D2 mRNA, the p.Ser101Arg mutation failed to rescue zebrafish phenotypes induced by the suppression of b9d2. With coimmunoprecipitation and mass spectrometric analyses, we found that Mks1, B9d1, and B9d2 interact physically, but that the p.Ser101Arg mutation abrogates the ability of B9d2 to interact with Mks1, further suggesting that the mutation compromises B9d2 function. Our data indicate that B9d1 is required for normal Hh signaling, ciliogenesis, and ciliary protein localization and that B9d1 and B9d2 are essential components of a B9 protein complex, disruption of which causes MKS.