The mouse rib-vertebrae mutation disrupts anterior-posterior somite patterning and genetically interacts with a Delta1 null allele

Functionally distinct roles for T and Tbx6 during mouse development

The mouse T-box transcription factors T and Tbx6 are co-expressed in the primitive streak and have unique domains of expression; T is expressed in the notochord, while Tbx6 is expressed in the presomitic mesoderm. T-box factors are related through a shared DNA binding domain, the T-domain, and can therefore bind to similar DNA sequences at least in vitro. We investigated the functional similarities and differences of T and Tbx6 DNA binding and transcriptional activity in vitro and their interaction genetically in vivo. We show that at one target, Dll1, the T-domains of T and Tbx6 have different affinities for the binding sites present in the mesoderm enhancer. We further show using in vitro assays that T and Tbx6 differentially affect transcription with Tbx6 activating expression tenfold higher than T, that T and Tbx6 can compete at target gene enhancers, and that this competition requires a functional DNA binding domain. Next, we addressed whether T and Tbx6 can compete in vivo. First, we generated embryos that express Tbx6 at greater than wild-type levels embryos and show that these embryos have short tails, resembling the T heterozygous phenotype. Next, using the dominant-negative TWis allele, we show that Tbx6+/− TWis/+ embryos share similarities with embryos homozygous for the Tbx6 hypomorphic allele rib-vertebrae, specifically fusions of several ribs and malformation of some vertebrae. Finally, we tested whether Tbx6 can functionally replace T using a knockin approach, which resulted in severe T null-like phenotypes in chimeric embryos generated with ES cells heterozygous for a Tbx6 knockin at the T locus. Altogether, our results of differences in affinity for DNA binding sites and transcriptional activity for T and Tbx6 provide a potential mechanism for the failure of Tbx6 to functionally replace T and possible competition phenotypes in vivo.

Summary: Mouse Tbx6 fails to compensate for heterozygous loss of T; instead ectopic Tbx6 in the T expression-domain in knockin embryos generates T null-like phenotypes suggestive of competition.

Progress and perspective of TBX6 gene in congenital vertebral malformations

Congenital vertebral malformation is a series of significant health problems affecting a large number of populations. It may present as an isolated condition or as a part of an underlying syndromes occurring with other malformations and/or clinical features. Disruption of the genesis of paraxial mesoderm, somites or axial bones can result in spinal deformity. In the course of somitogenesis, the segmentation clock and the wavefront are the leading factors during the entire process in which TBX6 gene plays an important role. TBX6 is a member of the T-box gene family, and its important pathogenicity in spinal deformity has been confirmed. Several TBX6 gene variants and novel pathogenic mechanisms have been recently revealed, and will likely have significant impact in understanding the genetic basis for CVM. In this review, we describe the role which TBX6 plays during human spine development including its interaction with other key elements during the process of somitogenesis. We then systematically review the association between TBX6 gene variants and CVM associated phenotypes, highlighting an important and emerging role for TBX6 and human malformations.

Patterning spinal nerves and vertebral bones

A prominent anatomical feature of the peripheral nervous system is the segmentation of mixed (motor, sensory and autonomic) spinal nerves alongside the spinal cord. During early development their axon growth cones avoid the developing vertebral elements by traversing the anterior/cranial half of each somite-derived sclerotome, so ensuring the separation of spinal nerves from vertebral bones as axons extend towards their peripheral targets. A glycoprotein expressed on the surface of posterior half-sclerotome cells confines growth cones to the anterior half-sclerotomes by contact repulsion. A closely similar glycoprotein is expressed in avian and mammalian grey matter, where we hypothesize it may have evolved to regulate neural plasticity in birds and mammals.

A study of vertebra number in pigs confirms the association of vertnin and reveals additional QTL

Background

Formation of the vertebral column is a critical developmental stage in mammals. The strict control of this process has resulted in little variation in number of vertebrae across mammalian species and no variation within most mammalian species. The pig is quite unique as considerable variation exists in number of thoracic vertebrae as well as number of lumbar vertebrae. At least two genes have been identified that affect number of vertebrae in pigs yet considerable genetic variation still exists. Therefore, a genome-wide association (GWA) analysis was conducted to identify additional genomic regions that affect this trait.

Results

A total of 1883 animals were phenotyped for the number of ribs and thoracolumbar vertebrae as well as successfully genotyped with the Illumina Porcine SNP60 BeadChip. After data editing, 41,148 SNP markers were included in the GWA analysis. These animals were also phenotyped for kyphosis. Fifty-three 1 Mb windows each explained at least 1.0 % of the genomic variation for vertebrae counts while 16 regions were significant for kyphosis. Vertnin genotype significantly affected vertebral counts as well. The region with the largest effect for number of lumbar vertebrae and thoracolumbar vertebrae were located over the Hox B gene cluster and the largest association for thoracic vertebrae number was over the Hox A gene cluster. Genetic markers in significant regions accounted for approximately 50 % of the genomic variation. Less genomic variation for kyphosis was described by QTL regions and no region was associated with kyphosis and vertebra counts.

Conclusions

The importance of the Hox gene families in vertebral development was highlighted as significant associations were detected over the A, B and C families. Further evaluation of these regions and characterization of variants within these genes will expand our knowledge on vertebral development using natural genetic variants segregating in commercial swine.

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-015-0286-9) contains supplementary material, which is available to authorized users.

Building the backbone: the development and evolution of vertebral patterning

The segmented vertebral column comprises a repeat series of vertebrae, each consisting of two key components: the vertebral body (or centrum) and the vertebral arches. Despite being a defining feature of the vertebrates, much remains to be understood about vertebral development and evolution. Particular controversy surrounds whether vertebral component structures are homologous across vertebrates, how somite and vertebral patterning are connected, and the developmental origin of vertebral bone-mineralizing cells. Here, we assemble evidence from ichthyologists, palaeontologists and developmental biologists to consider these issues. Vertebral arch elements were present in early stem vertebrates, whereas centra arose later. We argue that centra are homologous among jawed vertebrates, and review evidence in teleosts that the notochord plays an instructive role in segmental patterning, alongside the somites, and contributes to mineralization. By clarifying the evolutionary relationship between centra and arches, and their varying modes of skeletal mineralization, we can better appreciate the detailed mechanisms that regulate and diversify vertebral patterning.

Novel ENU-Induced Mutation in Tbx6 Causes Dominant Spondylocostal Dysostosis-Like Vertebral Malformations in the Rat

Congenital vertebral malformations caused by embryonic segmentation defects are relatively common in humans and domestic animals. Although reverse genetics approaches in mice have provided information on the molecular mechanisms of embryonic somite segmentation, hypothesis-driven approaches cannot adequately reflect human dysmorphology within the population. In a N-ethyl-N-nitrosourea (ENU) mutagenesis project in Kyoto, the Oune mutant rat strain was isolated due to a short and kinked caudal vertebra phenotype. Skeletal staining of heterozygous rats showed partial loss of the cervical vertebrae as well as hemivertebrae and fused vertebral blocks in lumbar and sacral vertebrae. In homozygous embryos, severe displacement of the whole vertebrae was observed. The Oune locus was genetically mapped to rat chromosome 1 using 202 backcross animals and 50 genome-wide microsatellite markers. Subsequently, a miss-sense mutation in the Tbx6 gene was identified in the critical region. Although the mutation is located within the T-box domain near a predicted dimmer-interface, in vitro experiments revealed that the Tbx6 variant retains normal DNA binding ability and translational efficiency. However, the variant has decreased transcriptional activation potential in response to Notch-mediated signaling. Recently, it was reported that a dominant type of familial spondylocostal dysostosis is caused by a stoploss mutation in TBX6. Thus, we propose that partial dysfunction of Tbx6 leads to similar congenital vertebral malformations in both humans and rats. The Oune strain could be a unique animal model for dominant spondylocostal dysostosis and is useful for molecular dissection of the pathology of congenital vertebral malformations in humans.

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.

Early transcriptional targets of MyoD link myogenesis and somitogenesis

In order to identify early transcriptional targets of MyoD prior to skeletal muscle differentiation, we have undertaken a transcriptomic analysis on gastrula stage Xenopus embryos in which MyoD has been knocked-down. Our validated list of genes transcriptionally regulated by MyoD includes Esr1 and Esr2, which are known targets of Notch signalling, and Tbx6, mesogenin, and FoxC1; these genes are all are known to be essential for normal somitogenesis but are expressed surprisingly early in the mesoderm. In addition we found that MyoD is required for the expression of myf5 in the early mesoderm, in contrast to the reverse relationship of these two regulators in amniote somites. These data highlight a role for MyoD in the early mesoderm in regulating a set of genes that are essential for both myogenesis and somitogenesis.

PMesogenin1 and 2 function directly downstream of Xtbx6 in Xenopus somitogenesis and myogenesis

T-box transcription factor tbx6 and basic-helix-loop-helix transcription factor pMesogenin1 are reported to be involved in paraxial mesodermal differentiation. To clarify the relationship between these genes in Xenopus laevis, we isolated pMesogenin2, which showed high homology with pMesogenin1. Both pMesogenin1 and 2 appeared to be transcriptional activators and were induced by a hormone-inducible version of Xtbx6 without secondary protein synthesis in animal cap assays. The pMesogenin2 promoter contained three potential T-box binding sites with which Xtbx6 protein was shown to interact, and a reporter gene construct containing these sites was activated by Xtbx6. Xtbx6 knockdown reduced pMesogenin1 and 2 expressions, but not vice versa. Xtbx6 and pMesogenin1 and 2 knockdowns caused similar phenotypic abnormalities including somite malformation and ventral body wall muscle hypoplasia, suggesting that Xtbx6 is a direct regulator of pMesogenin1 and 2, which are both involved in somitogenesis and myogenesis including that of body wall muscle in Xenopus laevis.

Molecular basis for skeletal variation: insights from developmental genetic studies in mice

Skeletal variations are common in humans, and potentially are caused by genetic as well as environmental factors. We here review molecular principles in skeletal development to develop a knowledge base of possible alterations that could explain variations in skeletal element number, shape or size. Environmental agents that induce variations, such as teratogens, likely interact with the molecular pathways that regulate skeletal development.

Transcriptional Repression by the T-box Proteins Tbx18 and Tbx15 Depends on Groucho Corepressors

Tbox18 (Tbx18) and Tbox15 (Tbx15) encode a closely related pair of vertebrate-specific T-box (Tbx) transcription factors. Functional analyses in the mouse have proven the requirement of Tbx15 in skin and skeletal development and of Tbx18 in the formation of the vertebral column, the ureter, and the posterior pole of the heart. Despite the accumulation of genetic data concerning the embryological roles of these genes, it is currently unclear how Tbx18 and Tbx15 exert their function on the molecular level. Here, we have initiated a molecular analysis of Tbx18 and Tbx15 proteins and have characterized functional domains for nuclear localization, DNA binding, and transcriptional modulation. We show that both proteins homo- and heterodimerize, bind to various combinations of T half-sites, and repress transcription in a Groucho-dependent manner. Competition with activating T-box proteins may constitute one mode of action as we show that Tbx18 interacts with Gata4 and Nkx2-5 and competes Tbx5-mediated activation of the cardiac Natriuretic peptide precursor type a-promoter and that ectopic expression of Tbx18 down-regulates Tbx6-activated Delta-like 1 expression in the somitic mesoderm in vivo.

The vertebrate segmentation clock and its role in skeletal birth defects

The segmental structure of the vertebrate body plan is most evident in the axial skeleton. The regulated generation of somites, a process called somitogenesis, underlies the vertebrate body plan and is crucial for proper skeletal development. A genetic clock regulates this process, controlling the timing of somite development. Molecular evidence for the existence of the segmentation clock was first described in the expression of Notch signaling pathway members, several of which are expressed in a cyclic fashion in the presomitic mesoderm (PSM). The Wnt and fibroblast growth factor (FGF) pathways have also recently been linked to the segmentation clock, suggesting that a complex, interconnected network of three signaling pathways regulates the timing of somitogenesis. Mutations in genes that have been linked to the clock frequently cause abnormal segmentation in model organisms. Additionally, at least two human disorders, spondylocostal dysostosis (SCDO) and Alagille syndrome (AGS), are caused by mutations in Notch pathway genes and exhibit vertebral column defects, suggesting that mutations that disrupt segmentation clock function in humans can cause congenital skeletal defects. Thus, it is clear that the correct, cyclic function of the Notch pathway within the vertebrate segmentation clock is essential for proper somitogenesis. In the future, with a large number of additional cyclic genes recently identified, the complex interactions between the various signaling pathways making up the segmentation clock will be elucidated and refined.

A genetic screen for modifiers of the delta1-dependent notch signaling function in the mouse

The Notch signaling pathway is an evolutionarily conserved transduction pathway involved in embryonic patterning and regulation of cell fates during development. Recent studies have demonstrated that this pathway is integral to a complex system of interactions, which are also involved in distinct human diseases. Delta1 is one of the known ligands of the Notch receptors. Mice homozygous for a loss-of-function allele of the Delta1 gene Dll1(lacZ/lacZ) die during embryonic development. Here, we present the results of two phenotype-driven modifier screens. Heterozygous Dll1(lacZ) knockout animals were crossed with ENU-mutagenized mice and screened for dysmorphological, clinical chemical, and immunological variants that are dependent on the Delta1 loss-of-function allele. First, we show that mutagenized heterozygous Dll1(lacZ) offspring have reduced body weight and altered specific clinical chemical parameters, including changes in metabolites and electrolytes relevant for kidney function. In our mutagenesis screen we have successfully generated 35 new mutant lines. Of major interest are 7 mutant lines that exhibit a Dll1(lacZ/+)-dependent phenotype. These mutant mouse lines provide excellent in vivo tools for studying the role of Notch signaling in kidney and liver function, cholesterol and iron metabolism, cell-fate decisions, and during maturation of T cells in the immune system.

The long and short of it: somite formation in mice

A fundamental characteristic of the vertebrate body plan is its segmentation along the anterior-posterior axis. This segmental pattern is established during embryogenesis by the formation of somites, the transient epithelial blocks of cells that derive from the unsegmented presomitic mesoderm. Somite formation involves a molecular oscillator, termed the segmentation clock, in combination with gradients of signaling molecules such as fibroblast growth factor 8, WNT3A, and retinoic acid. Disruption of somitogenesis in humans can result in disorders such as spondylocostal dysostosis, which is characterized by vertebral malformations. This review summarizes recent findings concerning the role of Notch signaling in the segmentation clock, the complex regulatory network that governs somitogenesis, the genes that cause inherited spondylocostal dysostosis, and the mechanisms that regulate bilaterally symmetric somite formation.

FGF8, Wnt8 and Myf5 are target genes of Tbx6 during anteroposterior specification in Xenopus embryo

The T-box transcription factor Tbx6 is required for somite formation and loss-of-function or reduced activity of Tbx6 result in absence of posterior paraxial mesoderm or disorganized somites, but how it is involved in a regulatory hierarchy during Xenopus early development is not clear. We show here that Tbx6 is expressed in the lateral and ventral mesoderm of early gastrula, and it is necessary and sufficient to directly and indirectly regulate the expression of a subset of early mesodermal and endodermal genes. Ectopic expression of Tbx6 inhibits early neuroectodermal gene expression by strongly inducing the expression of posterior mesodermal genes, and expands the mesoderm territory at the expense of neuroectoderm. Conversely, overexpression of a dominant negative Tbx6 mutant in the ventral mesoderm inhibits the expression of several mesodermal genes and results in neural induction in a dose-dependent manner. Using a hormone-inducible form of Tbx6, we have identified FGF8, Xwnt8 and XMyf5 as immediate early responsive genes of Tbx6, and the induction of these genes by Tbx6 is independent of Xbra and VegT. These target genes act downstream and mediate the function of Tbx6 in anteroposterior specification. Our results therefore identify a regulatory cascade governed by Tbx6 in the specification of posterior mesoderm during Xenopus early development.

Regulation of Tbx6 expression by Notch signaling

Somites are the first overt sign of segmentation in the vertebrate embryo and form from bilateral strips of paraxial mesoderm. Paraxial mesoderm arises from the primitive streak; it then migrates laterally and comes to lie on both sides of the neural tube. In the mouse, the T-box transcription factor Tbx6 is required for both somite formation and patterning. Tbx6 expression corresponds both temporally and spatially to somite formation, with expression in the primitive streak and presomitic mesoderm. Its expression in the latter could simply be explained by maintenance following its initial activation in the primitive streak. Alternatively, its expression in the presomitic mesoderm may be contributed by separate regulatory elements possibly under the control of different signals. We have begun to investigate how Tbx6 expression is controlled during development using a transgenic approach to identify the cis-acting regulatory regions. We show that it is possible to separate an element required for presomitic mesoderm expression from that driving expression in the primitive streak. Further, we show that a binding site for the Notch transcription factor RBP-Jkappa is necessary for Tbx6 presomitic mesoderm enhancer activity, indicating that Notch signaling is upstream of Tbx6 in the pathway directing somite formation and patterning.

Identification of Dll1 (Delta1) target genes during mouse embryogenesis using differential expression profiling

The Notch signaling pathway has pleiotropic functions during mammalian embryogenesis. It is required for the patterning and differentiation of the presomitic and somitic paraxial mesoderm and of the neural tube. We used DNA-chip expression profiling and 2D-gel electrophoresis combined with peptide mass fingerprinting to identify genes and proteins differentially regulated in E10.5 Dll1 (delta-like 1, Delta1) mutant embryos. The differential expression profiling approach identified 47 regulated transcripts and 40 differentially expressed proteins. The majority of these genes has until now not been associated with Notch signaling. Subsequent whole-mount in situ hybridization confirmed that a subset of the identified transcripts has restricted and distinct patterns of expression in E10.5 mouse embryos. For most genes these expression patterns were affected in the presomitic mesoderm, in differentiating somites of Dll1 mutant embryos and in the neural tube and cells differentiating from it. Similar effects were observed in embryos homozygous for the Headturner (Htu) and pudgy (pu) mutations, which are alleles of the Notch ligands Jag1 and Dll3. The regulated expression of a subset of the proteins was validated by immunoblots. Remarkably six of the proteins down-regulated in Dll1 mutant embryos are proteasome subunits. The large set of regulated genes identified in this differential expression profiling approach is an important resource for further functional studies.

Dll1 is a downstream target of Tbx6 in the paraxial mesoderm

Tbx6 is a member of the T-box family of transcription factors. In the mouse, Tbx6 is expressed in the primitive streak, tail bud, and presomitic mesoderm and is essential for the specification of posterior paraxial mesoderm; in its absence, posterior somites are replaced by ectopic neural tubes. Analysis of embryos expressing reduced levels of Tbx6 also revealed that it is required for the correct patterning of the somites as well as their initial specification. As a first step toward identifying downstream targets of Tbx6, we examined the DNA binding properties of Tbx6 and identified a Tbx6 consensus binding site. Previously, we have shown that expression of Dll1, which encodes a Notch ligand, is lost in the Tbx6 mutant and that Tbx6 and Dll1 genetically interact, indicating that Dll1 may be a direct target of Tbx6 in the paraxial mesoderm. We uncovered four putative Tbx6 binding sites within a Dll1 paraxial mesoderm enhancer and show that Tbx6 can bind two of these sites in vitro. Altogether, these results lend further support for Dll1 being a direct target of Tbx6 in the presomitic mesoderm.

WNT signaling, in synergy with T/TBX6, controls Notch signaling by regulating Dll1 expression in the presomitic mesoderm of mouse embryos

Notch signaling in the presomitic mesoderm (psm) is critical for somite formation and patterning. Here, we show that WNT signals regulate transcription of the Notch ligand Dll1 in the tailbud and psm. LEF/TCF factors cooperate with TBX6 to activate transcription from the Dll1 promoter in vitro. Mutating either T or LEF/TCF sites in the Dll1 promoter abolishes reporter gene expression in vitro as well as in the tail bud and psm of transgenic embryos. Our results indicate that WNT activity, in synergy with TBX6, regulates Dll1 transcription and thereby controls Notch activity, somite formation, and patterning.

New mutant mouse with skeletal deformities caused by mutation in delta like 3 (Dll3) gene

We have established a new mouse strain with vertebral deformities caused by an autosomal single recessive mutation (oma). The mutant mice showed short trunk and short and kinky tail. The skeletal preparations of newborn and prenatal mice showed disorganized vertebrae and numerous vertebral and rib fusions which are thought to be caused by patterning defects at the stage of somitegenesis. Linkage analysis localized the oma locus on the proximal region of mouse chromosome 7 close to Dll3 gene. Dll3 is the gene involved in the Notch signaling pathway and null-mutation of the gene has been reported to cause vertebral deformities. The phenotypic similarity between oma and Dll3 null-mutant mice suggests that the causative gene for the oma mutant is the Dll3 gene. We, therefore, investigated the nucleotide sequence of the Dll3 gene of the oma mouse and found a single nucleotide substitution of G to T which causes missense mutation of glycine to cysteine at codon 409. Since the amino acid substitution is a nonconservative amino acid substitution at the conserved portion of the Dll3 protein, and the substitution is specific to the mutant mice, we concluded that the nucleotide substitution of the Dll3 gene is responsible for the skeletal deformities of the oma mouse.

A Notch feeling of somite segmentation and beyond

Vertebrate segmentation is manifested during embryonic development as serially repeated units termed somites that give rise to vertebrae, ribs, skeletal muscle and dermis. Many theoretical models including the "clock and wavefront" model have been proposed. There is compelling genetic evidence showing that Notch-Delta signaling is indispensable for somitogenesis. Notch receptor and its target genes, Hairy/E(spl) homologues, are known to be crucial for the ticking of the segmentation clock. Through the work done in mouse, chick, Xenopus and zebrafish, an oscillator operated by cyclical transcriptional activation and delayed negative feedback regulation is emerging as the fundamental mechanism underlying the segmentation clock. Ubiquitin-dependent protein degradation and probably other posttranslational regulations are also required. Fgf8 and Wnt3a gradients are important in positioning somite boundaries and, probably, in coordinating tail growth and segmentation. The circadian clock is another biochemical oscillator, which, similar to the segmentation clock, is operated with a negative transcription-regulated feedback mechanism. While the circadian clock uses a more complicated network of pathways to achieve homeostasis, it appears that the segmentation clock exploits the Notch pathway to achieve both signal generation and synchronization. We also discuss mathematical modeling and future directions in the end.

Critical role for Tbx6 in mesoderm specification in the mouse embryo

Tbx6 is a member of the T-box family of transcription factor genes. Two mutant alleles of this gene establish that Tbx6 is involved in both the specification and patterning of the somites along the entire length of the embryo. The null allele, Tbx6(tm1Pa), causes abnormal patterning of the cervical somites and improper specification of more posterior paraxial mesoderm, such that it forms ectopic neural tubes. In this study, we use this allele to further investigate the mechanism of action of the Tbx6 gene and investigate possible genetic interactions. We have tested the developmental and differentiation potential of Tbx6(tm1Pa)/Tbx6(tm1Pa) cells in ectopic sites, in vitro, and in chimeras in vivo. We have also documented cell proliferation and cell death in mutant tail buds in an attempt to explain the mechanism of tail bud enlargement in the Tbx6 mutant embryos. Our results indicate specific developmental restrictions on the differentiation of posterior cells lacking Tbx6, once they have traversed the primitive streak, but no restrictions in differentiation of anterior somites, or of Tbx6 null embryonic stem (ES) cells. We further demonstrate that Tbx6 null ES cells fail to populate posterior somites in chimeric embryos. To discover whether different T-box proteins interact on the same down stream targets in areas of expression overlap, we have explored potential interactions between Tbx6 and T (Brachyury) in genetic crosses. Our results reveal that the T(Wis) mutation is epistatic to the Tbx6(tm1Pa) mutation and that there is no apparent genetic interaction. However, homozygosity for Tbx6(tm1Pa) and heterozygosity for T(Wis) mutation shows a combinatorial interaction at the phenotypic level.

Defective somite patterning in mouse embryos with reduced levels of Tbx6

During vertebrate embryogenesis, paraxial mesoderm gives rise to somites, which subsequently develop into the dermis, skeletal muscle, ribs and vertebrae of the adult. Mutations that disrupt the patterning of individual somites have dramatic effects on these tissues, including fusions of the ribs and vertebrae. The T-box transcription factor, Tbx6, is expressed in the paraxial mesoderm but is downregulated as somites develop. It is essential for the formation of posterior somites, which are replaced with ectopic neural tubes in Tbx6-null mutant embryos. We show that partial restoration of Tbx6 expression in null mutants rescues somite development, but that rostrocaudal patterning within them is defective, ultimately resulting in rib and vertebral fusions, demonstrating that Tbx6 activity in the paraxial mesoderm is required not simply for somite specification but also for their normal patterning. Somite patterning is dependent upon Notch signaling and we show that Tbx6 genetically interacts with the Notch ligand, delta-like 1 (Dll1). Dll1 expression, which is absent in the Tbx6-null mutant, is restored at reduced levels in the partially rescued mutants, suggesting that Dll1 is a target of Tbx6. We also identify the spontaneous mutation rib-vertebrae as a hypomorphic mutation in Tbx6. The similarity in the phenotypes we describe here and that of some human birth defects, such as spondylocostal dysostosis, raises the possibility that mutations in Tbx6 or components of this pathway may be responsible for these defects.

The mouse rib-vertebrae mutation is a hypomorphic Tbx6 allele

Rib-vertebrae (rv) is an autosomal recessive mutation in mouse that affects somite formation, morphology, and patterning. Expression of Notch pathway components is affected in the paraxial mesoderm of rv mutant embryos, and rv and a null allele of the Notch ligand delta1 show non-allelic non-complementation. By fine genetic mapping and complementation testing we have identified Tbx6, a gene essential for paraxial mesoderm formation, as the gene mutated in rv. Compound heterozygotes carrying a Tbx6 null allele and rv show a phenotype that is milder than in homozygous Tbx6 null but more severe than in homozygous rv mutants. Tbx6 expression is down-regulated in rv mutant embryos. An insertion in the promoter region upstream of the transcriptional start is present in the genome of rv mutants but not in different strains of mice wild type for Tbx6. Our results indicate that rv is a regulatory mutation of Tbx6 causing a hypomorphic phenotype.

Efficient generation and mapping of recessive developmental mutations using ENU mutagenesis

Treatment with N-ethyl-N-nitrosourea (ENU) efficiently generates single-nucleotide mutations in mice. Along with the renewed interest in this approach, much attention has been given recently to large screens with broad aims; however, more finely focused studies have proven very productive as well. Here we show how mutagenesis together with genetic mapping can facilitate the rapid characterization of recessive loci required for normal embryonic development. We screened third-generation progeny of mutagenized mice at embryonic day (E) 18.5 for abnormalities of organogenesis. We ascertained 15 monogenic mutations in the 54 families that were comprehensively analyzed. We carried out the experiment as an outcross, which facilitated the genetic mapping of the mutations by haplotype analysis. We mapped seven of the mutations and identified the affected locus in two lines. Using a hierarchical approach, it is possible to maximize the efficiency of this analysis so that it can be carried out easily with modest infrastructure and resources.