Expression and functional analysis of Tgif during mouse midline development

Copy number variation in pituitary stalk interruption syndrome: A large case series of sporadic non-syndromic patients and literature review

Abnormal hypothalamic/posterior pituitary development appears to be a major determinant of pituitary stalk interruption syndrome (PSIS). The observation of familial cases and associated congenital abnormalities suggests a genetic basis. Single-gene mutations explain less than 5% of the cases, and whole exome sequencing has shown heterogeneous results. The present study aimed to assess copy number variation (CNV) using array-based comparative genomic hybridization (aCGH) in patients with non-syndromic PSIS and comprehensively review data from the literature on CNV analysis in congenital hypopituitarism (CH) patients. Twenty-one patients with sporadic CH from our outpatient clinics presented with ectopic posterior pituitary (EPP) and no central nervous system abnormalities on magnetic resonance image (MRI) or any other malformations on physical examination at presentation were enrolled in the study. aCGH using a whole-genome customized 400K oligonucleotide platform was performed in our patients. For the literature review, we searched for case reports of patients with CH and CNV detected by either karyotype or aCGH reported in PubMed up to November 2021. Thirty-five distinct rare CNVs were observed in 18 patients (86%) and two of them (6%) were classified as pathogenic: one deletion of 1.8 Mb in chromosome 17 (17q12) and one deletion of 15 Mb in chromosome 18 (18p11.32p11.21), each one in a distinct patient. In the literature review, 67 pathogenic CNVs were published in 83 patients with CH, including the present study. Most of these patients had EPP (78% out of the 45 evaluated by sellar MRI) and were syndromic (70%). The most frequently affected chromosomes were X, 18, 20 and 1. Our study has found that CNV can be a mechanism of genetic abnormality in non-syndromic patients with CH and EPP. In future studies, one or more genes in those CNVs, both pathogenic and variant of uncertain significance, may be considered as good candidate genes.

Regulating Retinoic Acid Availability during Development and Regeneration: The Role of the CYP26 Enzymes

This review focuses on the role of the Cytochrome p450 subfamily 26 (CYP26) retinoic acid (RA) degrading enzymes during development and regeneration. Cyp26 enzymes, along with retinoic acid synthesising enzymes, are absolutely required for RA homeostasis in these processes by regulating availability of RA for receptor binding and signalling. Cyp26 enzymes are necessary to generate RA gradients and to protect specific tissues from RA signalling. Disruption of RA homeostasis leads to a wide variety of embryonic defects affecting many tissues. Here, the function of CYP26 enzymes is discussed in the context of the RA signalling pathway, enzymatic structure and biochemistry, human genetic disease, and function in development and regeneration as elucidated from animal model studies.

Functions of TGIF homeodomain proteins and their roles in normal brain development and holoprosencephaly

Holoprosencephaly (HPE) is a frequent human forebrain developmental disorder with both genetic and environmental causes. Multiple loci have been associated with HPE in humans, and potential causative genes at 14 of these loci have been identified. Although TGIF1 (originally TGIF, for Thymine Guanine-Interacting Factor) is among the most frequently screened genes in HPE patients, an understanding of how mutations in this gene contribute to the pathogenesis of HPE has remained elusive. However, mouse models based on loss of function of Tgif1, and the related Tgif2 gene, have shed some light on how human TGIF1 variants might cause HPE. Functional analyses of TGIF proteins and of TGIF1 single nucleotide variants from HPE patients, combined with analysis of forebrain development in mouse embryos lacking both Tgif1 and Tgif2, suggest that TGIFs regulate the transforming growth factor ß/Nodal signaling pathway and sonic hedgehog (SHH) signaling independently. Although, some developmental processes that are regulated by TGIFs may be Nodal-dependent, it appears that the forebrain patterning defects and HPE in Tgif mutant mouse embryos is primarily due to altered signaling via the Shh pathway.

Link between the causative genes of holoprosencephaly: Zic2 directly regulates Tgif1 expression

One of the causal genes for holoprosencephaly (HPE) is ZIC2 (HPE5). It belongs to the zinc finger protein of the cerebellum (Zic) family of genes that share a C2H2-type zinc finger domain, similar to the GLI family of genes. In order to clarify the role of Zic2 in gene regulation, we searched for its direct target genes using chromatin immunoprecipitation (ChIP). We identified TGIF1 (HPE4), another holoprosencephaly-causative gene in humans. We identified Zic2-binding sites (ZBS) on the 5′ flanking region of Tgif1 by in vitro DNA binding assays. ZBS were essential for Zic2-dependent transcriptional activation in reporter gene assays. Zic2 showed a higher affinity to ZBS than GLI-binding sequences. Zic2-binding to the cis-regulatory element near the Tgif1 promoter may be involved in the mechanism underlying forebrain development and incidences of HPE.

Tgif1 and Tgif2 Repress Expression of the RabGAP Evi5l

Mouse embryos conditionally lacking Tgif1 and Tgif2 have holoprosencephaly and defects in left-right asymmetry. To identify pathways affected by loss of Tgif function during embryogenesis, we performed transcriptome profiling on whole mouse embryos. Among the genes with altered expression in embryos lacking Tgifs were a number with links to cilium function. One of these, Evi5l, encodes a RabGAP that is known to block the formation of cilia when overexpressed. Evi5l expression is increased in Tgif1; Tgif2-null embryos and in double-null mouse embryo fibroblasts (MEFs). Knockdown of Tgifs in a human retinal pigment epithelial cell line also increased EVI5L expression. We show that TGIF1 binds to a conserved consensus TGIF site 5' of the human and mouse Evi5l genes and represses Evi5l expression. In primary MEFs lacking both Tgifs, the number of cells with primary cilia was significantly decreased, and we observed a reduction in the transcriptional response to Shh pathway activation. Reducing Evi5l expression in MEFs lacking Tgifs resulted in a partial restoration of cilium numbers and in the transcriptional response to activation of the Shh pathway. In summary, this work shows that Tgifs regulate ciliogenesis and suggests that Evi5l mediates at least part of this effect.

Genetic and Molecular Analyses indicate independent effects of TGIFs on Nodal and Gli3 in neural tube patterning

Holoprosencephaly (HPE) is a prevalent craniofacial developmental disorder that has both genetic and environmental causes. The gene encoding TG-interacting factor 1 (TGIF1) is among those that are routinely screened in HPE patients. However, the mechanisms by which TGIF1 variants cause HPE are not fully understood. TGIF1 is a transcriptional repressor that limits the output of the Transforming Growth Factor ß (TGFß)/Nodal signaling pathway, and HPE in patients with TGIF1 variants has been suggested to be due to increased Nodal signaling. Mice lacking both Tgif1 and its paralog, Tgif2, have HPE, and embryos lacking Tgif function do not survive past mid-gestation. Here, we show that in the presence of a Nodal heterozygous mutation, proliferation defects are rescued and a proportion of embryos lacking all Tgif function survive to late gestation. However, these embryos have a classic HPE phenotype, suggesting that this is a Nodal-independent effect of Tgif loss of function. Further, we show that the Gli3 gene is a direct target for repression by Tgifs, independent of TGFß/Nodal signaling, consistent with Tgif mutations causing HPE via Nodal-independent effects on the Sonic Hedgehog (Shh) pathway. Based on this work, we propose a model for distinct functions of Tgifs in the Nodal and Shh/Gli3 pathways during forebrain development.

Tgif1 and Tgif2 Regulate Axial Patterning in Mouse

Tgif1 and Tgif2 are transcriptional repressors that inhibit the transcriptional response to transforming growth factor β signaling, and can repress gene expression by direct binding to DNA. Loss of function mutations in TGIF1 are associated with holoprosencephaly (HPE) in humans. In mice, embryos lacking both Tgif1 and Tgif2 fail to complete gastrulation, and conditional double null embryos that survive past gastrulation have HPE and do not survive past mid-gestation. Here we show that in mice of a relatively pure C57BL/6 strain background, loss of Tgif1 alone results in defective axial patterning and altered expression of Hoxc6. The primary defects in Tgif1 null embryos are the presence of extra ribs on the C7 vertebra, consistent with a posterior transformation phenotype. In addition we observed defective cervical vertebrae, primarily C1-C5, in both adult mice and embryos that lacked Tgif1. The combination of Tgif1 and Tgif2 mutations increases the severity and penetrance of the posterior transformation phenotype, without altering the type of defects seen. Similarly, exposure of Tgif1 mutant embryos to retinoic acid at E8.5 increased the severity and penetrance of the Tgif1 phenotype. This suggests that Tgif1 and Tgif2 regulate axial patterning and that reduced TGIF function sensitizes embryos to the effects of retinoic acid.

Effective population size, extended linkage disequilibrium and signatures of selection in the rare dog breed lundehund

The Lundehund is an old dog breed with remarkable anatomical features including polydactyly in all four limbs and extraordinary flexibility of the spine. We genotyped 28 Lundehund using the canine Illumina high density beadchip to estimate the effective population size (N_e) and inbreeding coefficients as well as to identify potential regions of positive selection. The decay of linkage disequilibrium was slow with r² = 0.95 in 50 kb distance. The last 7-200 generations ago, Ne was at 10-13. An increase of N_e was noted in the very recent generations with a peak value of 19 for N_e at generation 4. The FROH estimated for 50-, 65- and 358-SNP windows were 0.87, 087 and 0.81, respectively. The most likely estimates for F_ROH after removing identical-by-state segments due to linkage disequilibria were at 0.80-0.81. The extreme loss of heterozygosity has been accumulated through continued inbreeding over 200 generations within a probably closed population with a small effective population size. The mean inbreeding coefficient based on pedigree data for the last 11 generations (F_Ped = 0.10) was strongly biased downwards due to the unknown coancestry of the founders in this pedigree data. The long-range haplotype test identified regions with genes involved in processes of immunity, olfaction, woundhealing and neuronal development as potential targets of selection. The genes QSOX2, BMPR1B and PRRX2 as well as MYOM1 are candidates for selection on the Lundehund characteristics small body size, increased number of digits per paw and extraordinary mobility, respectively.

Role of TG-interacting factor (Tgif) in lipid metabolism

TG interacting factors (Tgifs) 1 and 2 are members of the TALE (three-amino-acid loop extension) superfamily of homeodomain proteins. These two proteins bind to the same DNA sequence and share a conserved C-terminal repression domain. Mutations in TGIF1 have been linked to holoprosencephaly, which is a human genetic disease that affects craniofacial development. As these proteins can interact with the ligand binding domain of retinoid X receptor α, a common heterodimeric partner of several nuclear receptors [e.g., liver X receptors (LXRs) and peroxisome proliferator-activated receptors (PPARs)], Tgif1 and Tgif2 might repress other transcriptional pathways activated by lipids. In line with this, Tgif1 interacts with LXRα and Tgif1 null mice have increased expression of the two Lxrα target genes apolipoproteins (Apo) c2 and a4. Also, we have recently identified Tgif1 to function as a transcriptional repressor of the cholesterol esterifying enzyme acyl-coenzyme A:cholesterol acyltransferase 2 (gene name SOAT2). As no studies yet have shown involvement of Tgif2 in the lipid metabolism, this review will focus on the role of Tgif1 in lipid and cholesterol metabolism. This article is part of a Special Issue entitled: Linking transcription to physiology in lipodomics.

Premature senescence and increased TGFβ signaling in the absence of Tgif1

Transforming growth factor β (TGFβ) signaling regulates cell cycle progression in several cell types, primarily by inducing a G1 cell cycle arrest. Tgif1 is a transcriptional corepressor that limits TGFβ responsive gene expression. Here we demonstrate that primary mouse embryo fibroblasts (MEFs) lacking Tgif1 proliferate slowly, accumulate increased levels of DNA damage, and senesce prematurely. We also provide evidence that the effects of loss of Tgif1 on proliferation and senescence are not limited to primary cells. The increased DNA damage in Tgif1 null MEFs can be partially reversed by culturing cells at physiological oxygen levels, and growth in normoxic conditions also partially rescues the proliferation defect, suggesting that in the absence of Tgif1 primary MEFs are less able to cope with elevated levels of oxidative stress. Additionally, we show that Tgif1 null MEFs are more sensitive to TGFβ-mediated growth inhibition, and that treatment with a TGFβ receptor kinase inhibitor increases proliferation of Tgif1 null MEFs. Conversely, persistent treatment of wild type cells with low levels of TGFβ slows proliferation and induces senescence, suggesting that TGFβ signaling also contributes to cellular senescence. We suggest that in the absence of Tgif1, a persistent increase in TGFβ responsive transcription and a reduced ability to deal with hyperoxic stress result in premature senescence in primary MEFs.

Loss of Tgif function causes holoprosencephaly by disrupting the SHH signaling pathway

Holoprosencephaly (HPE) is a severe human genetic disease affecting craniofacial development, with an incidence of up to 1/250 human conceptions and 1.3 per 10,000 live births. Mutations in the Sonic Hedgehog (SHH) gene result in HPE in humans and mice, and the Shh pathway is targeted by other mutations that cause HPE. However, at least 12 loci are associated with HPE in humans, suggesting that defects in other pathways contribute to this disease. Although the TGIF1 (TG-interacting factor) gene maps to the HPE4 locus, and heterozygous loss of function TGIF1 mutations are associated with HPE, mouse models have not yet explained how loss of Tgif1 causes HPE. Using a conditional Tgif1 allele, we show that mouse embryos lacking both Tgif1 and the related Tgif2 have HPE-like phenotypes reminiscent of Shh null embryos. Eye and nasal field separation is defective, and forebrain patterning is disrupted in embryos lacking both Tgifs. Early anterior patterning is relatively normal, but expression of Shh is reduced in the forebrain, and Gli3 expression is up-regulated throughout the neural tube. Gli3 acts primarily as an antagonist of Shh function, and the introduction of a heterozygous Gli3 mutation into embryos lacking both Tgif genes partially rescues Shh signaling, nasal field separation, and HPE. Tgif1 and Tgif2 are transcriptional repressors that limit Transforming Growth Factor β/Nodal signaling, and we show that reducing Nodal signaling in embryos lacking both Tgifs reduces the severity of HPE and partially restores the output of Shh signaling. Together, these results support a model in which Tgif function limits Nodal signaling to maintain the appropriate output of the Shh pathway in the forebrain. These data show for the first time that Tgif1 mutation in mouse contributes to HPE pathogenesis and provide evidence that this is due to disruption of the Shh pathway.

Holoprosencephaly (HPE) is a devastating genetic disease affecting human brain development. HPE affects more than 1/8,000 live births and up to 1/250 conceptions. Several genetic loci are associated with HPE, and the mutated genes have been identified at some. We have analyzed the role of the TGIF1 gene, which is present at one of these loci (the HPE4 locus) and is mutated in a subset of human HPE patients. We show that Tgif1 mutations in mice cause HPE when combined with a mutation in the closely related Tgif2 gene. This provides the first evidence from model organisms that TGIF1 is in fact the gene at the HPE4 locus that causes HPE when mutated. The Sonic Hedgehog signaling pathway is the best understood pathway in the pathogenesis of HPE, and mutation of the Sonic Hedgehog gene in both humans and mice causes HPE. We show that mutations in Tgif1 and Tgif2 in mice cause HPE by disrupting the Sonic Hedgehog signaling pathway, further emphasizing the importance of this pathway for normal brain development. Thus we confirm TGIF1 as an HPE gene and provide genetic evidence that Tgif1 mutations cause HPE by disrupting the interplay of the Nodal and Sonic Hedgehog pathways.

Minimal evidence for a direct involvement of twisted gastrulation homolog 1 (TWSG1) gene in human holoprosencephaly

Holoprosencephaly (HPE) is the most common disorder of human forebrain and facial development. Presently understood etiologies include both genetic and environmental factors, acting either alone, or more likely, in combination. The majority of patients without overt chromosomal abnormalities or recognizable associated syndromes have unidentified etiologies. A potential candidate gene, Twisted Gastrulation Homolog 1 (TWSG1), was previously suggested as a contributor to the complex genetics of human HPE based on (1) cytogenetic studies of patients with 18p deletions, (2) animal studies of TWSG1 deficient mice, and (3) the relationship of TWSG1 to bone morphogenetic protein (BMP) signaling, which modulates the primary pathway implicated in HPE, Sonic Hedgehog (SHH) signaling. Here we present the first analysis of a large cohort of patients with HPE for coding sequence variations in TWSG1. We also performed fine mapping of 18p for a subset of patients with partial 18p deletions. Surprisingly, minimal evidence for alterations of TWSG1 was found, suggesting that sequence alterations of TWSG1 are neither a common direct cause nor a frequent modifying factor for human HPE pathologies.

An update to the list of mouse mutants with neural tube closure defects and advances toward a complete genetic perspective of neural tube closure

The number of mouse mutants and strains with neural tube defects (NTDs) now exceeds 240, including 205 representing specific genes, 30 for unidentified genes, and 9 multifactorial strains. These mutants identify genes needed for embryonic neural tube closure. Reports of 50 new NTD mutants since our 2007 review (Harris and Juriloff, 2007) were considered in relation to the previously reviewed mutants to obtain new insights into mechanisms of NTD etiology. In addition to null mutations, some are hypomorphs or conditional mutants. Some mutations do not cause NTDs on their own, but do so in digenic, trigenic, and oligogenic combinations, an etiology that likely parallels the nature of genetic etiology of human NTDs. Mutants that have only exencephaly are fourfold more frequent than those that have spina bifida aperta with or without exencephaly. Many diverse cellular functions and biochemical pathways are involved; the NTD mutants draw new attention to chromatin modification (epigenetics), the protease-activated receptor cascade, and the ciliopathies. Few mutants directly involve folate metabolism. Prevention of NTDs by maternal folate supplementation has been tested in 13 mutants and reduces NTD frequency in six diverse mutants. Inositol reduces spina bifida aperta frequency in the curly tail mutant, and three new mutants involve inositol metabolism. The many NTD mutants are the foundation for a future complete genetic understanding of the processes of neural fold elevation and fusion along mechanistically distinct cranial-caudal segments of the neural tube, and they point to several candidate processes for study in human NTD etiology.

Aberrant forebrain signaling during early development underlies the generation of holoprosencephaly and coloboma

In this review, we highlight recent literature concerning the signaling mechanisms underlying the development of two neural birth defects, holoprosencephaly and coloboma. Holoprosencephaly, the most common forebrain defect, occurs when the cerebral hemispheres fail to separate and is typically associated with mispatterning of embryonic midline tissue. Coloboma results when the choroid fissure in the eye fails to close. It is clear that Sonic hedgehog (Shh) signaling regulates both forebrain and eye development, with defects in Shh, or components of the Shh signaling cascade leading to the generation of both birth defects. In addition, other intercellular signaling pathways are known factors in the incidence of holoprosencephaly and coloboma. This review will outline recent advances in our understanding of forebrain and eye embryonic pattern formation, with a focus on zebrafish studies of Shh and retinoic acid pathways. Given the clear overlap in the mechanisms that generate both diseases, we propose that holoprosencephaly and coloboma can represent mild and severe aspects of single phenotypic spectrum resulting from aberrant forebrain development. This article is part of a Special Issue entitled Zebrafish Models of Neurological Diseases.

The molecular genetics of holoprosencephaly

Holoprosencephaly (HPE) has captivated the imagination of Man for millennia because its most extreme manifestation, the single-eyed cyclopic newborn infant, brings to mind the fantastical creature Cyclops from Greek mythology. Attempting to understand this common malformation of the forebrain in modern medical terms requires a systematic synthesis of genetic, cytogenetic, and environmental information typical for studies of a complex disorder. However, even with the advances in our understanding of HPE in recent years, there are significant obstacles remaining to fully understand its heterogeneity and extensive variability in phenotype. General lessons learned from HPE will likely be applicable to other malformation syndromes. Here we outline the common, and rare, genetic and environmental influences on this conserved developmental program of forebrain development and illustrate the similarities and differences between these malformations in humans and those of animal models.

Tgif1 and Tgif2 regulate Nodal signaling and are required for gastrulation

Tgif1 and Tgif2 are transcriptional co-repressors that limit the response to TGFbeta signaling and play a role in regulating retinoic-acid-mediated gene expression. Mutations in human TGIF1 are associated with holoprosencephaly, but it is unclear whether this is a result of deregulation of TGFbeta/Nodal signaling, or of effects on other pathways. Surprisingly, mutation of Tgif1 in mice results in only relatively mild developmental phenotypes in most strain backgrounds. Here, we show that loss-of-function mutations in both Tgif1 and Tgif2 result in a failure of gastrulation. By conditionally deleting Tgif1 in the epiblast, we demonstrate that a single wild-type allele of Tgif1 in the extra-embryonic tissue allows the double null embryos to gastrulate and begin organogenesis, suggesting that extra-embryonic Tgif function is required for patterning the epiblast. Genetically reducing the dose of Nodal in embryos lacking all Tgif function results in partial rescue of the gastrulation defects. Conditional double null embryos have defects in left-right asymmetry, which are also alleviated by reducing the dose of Nodal. Together, these data show that Tgif function is required for gastrulation, and provide the first clear evidence that Tgifs limit the transcriptional response to Nodal signaling during early embryogenesis.

REST and CoREST modulate neuronal subtype specification, maturation and maintenance

Background

The repressor element-1 silencing transcription factor/neuron-restrictive silencer factor (REST/NRSF) is a master regulator of neuronal gene expression. REST functions as a modular scaffold for dynamic recruitment of epigenetic regulatory factors including its primary cofactor, the corepressor for element-1-silencing transcription factor (CoREST), to genomic loci that contain the repressor element-1 (RE1) binding motif. While REST was initially believed to silence RE1 containing neuronal genes in neural stem cells (NSCs) and non-neuronal cells, emerging evidence shows an increasingly complex cell type- and developmental stage-specific repertoire of REST target genes and functions that include regulation of neuronal lineage maturation and plasticity.

Methodology/Principal Findings

In this study, we utilized chromatin immunoprecipitation on chip (ChIP-chip) analysis to examine REST and CoREST functions during NSC-mediated specification of cholinergic neurons (CHOLNs), GABAergic neurons (GABANs), glutamatergic neurons (GLUTNs), and medium spiny projection neurons (MSNs). We identified largely distinct but overlapping profiles of REST and CoREST target genes during neuronal subtype specification including a disproportionately high percentage that are exclusive to each neuronal subtype.

Conclusions/Significance

Our findings demonstrate that the differential deployment of REST and CoREST is an important regulatory mechanism that mediates neuronal subtype specification by modulating specific gene networks responsible for inducing and maintaining neuronal subtype identity. Our observations also implicate a broad array of factors in the generation of neuronal diversity including but not limited to those that mediate homeostasis, cell cycle dynamics, cell viability, stress responses and epigenetic regulation.

Maternal Tgif1 regulates nodal gene expression in Xenopus

In Xenopus, the maternal transcription factor VegT is necessary and sufficient to initiate the expression of nodal-related genes, which are central to many aspects of early development. However, little is known about regulation of VegT activity. Using maternal loss-of-function experiments, we show that the maternal homeoprotein, Tgif1, antagonizes VegT and plays a central role in anteroposterior patterning by negatively regulating a subset of nodal-related genes. Depletion of Tgif1 causes the anteriorization of embryos and the up-regulation of nodal paralogues nr5 and nr6. Furthermore, Tgif1 inhibits activation of nr5 by VegT in a manner that requires a C-terminal Sin3 corepressor-interacting domain. Tgif1 has been implicated in the transcriptional corepression of transforming growth factor-beta (TGFbeta) and retinoid signaling. However, we show that Tgif1 does not inhibit these pathways in early development. These results identify an essential role for Tgif1 in the control of nodal expression and provide insight into Tgif1 function and mechanisms controlling VegT activity.

TGIF, a homeodomain transcription factor, regulates retinal progenitor cell differentiation

TG-interacting factor (TGIF) is a TALE homeodomain protein expressed predominantly in the central nervous system and functions as a transcriptional repressor. Several mutations in TGIF have been identified in patients with holoprosencephaly, the most common congenital malformation of the developing human forebrain. However, the precise role of TGIF in neural development is not well understood. We found that TGIF was expressed strongly in the mouse retina during early stages of development, and that its expression gradually decreased as retinal development progressed. In vitro explant cultures of mouse retina mimic the in vivo development of retinal subtypes. Forced expression of TGIF using a retrovirus in explant culture induced the differentiation of amacrine cells from retinal progenitor cells. A TGIF paralog, TGIF2, showed a similar transition in expression during retinal development, and TGIF2 also promoted amacrine cell differentiation in a retinal explant culture system. However, no apparent difference between wild-type and TGIF-knockout mouse retina was observed, suggesting that TGIF and TGIF2 function redundantly in that tissue. Forced expression of TGIF homeodomain (HD)-EnR (repressing) rather than TGIF HD-VP16 (activating) resulted in a phenotype similar to that induced by wild-type TGIF, suggesting that TGIFs may act as transcriptional repressors to induce amacrine genesis.

Maternal Tgif is required for vascularization of the embryonic placenta

The mammalian placenta is the site of exchange of nutrients and waste between mother and embryo. In humans, placental insufficiency can result in intrauterine growth retardation, perinatal death and spontaneous abortion. We show that in C57BL/6J mice a null mutation in the gene encoding the transcriptional corepressor, Tgif, causes placental defects. The major defects are decreased vascularization of the placenta, due to a decrease in the fetal blood vessels, and decreased expression of the gap junction protein Gjb2 (Cx26). These defects result in severe growth retardation in a proportion of Tgif null embryos in Tgif heterozygous mothers, and an overall growth delay in Tgif null animals. Placental defects are much more severe if the mother also completely lacks Tgif function, and placentas from heterozygous Tgif embryos are defective in a Tgif null mother. Embryo transfer experiments show that even the placenta from a wild type embryo is compromised in the absence of maternal Tgif. These results demonstrate that Tgif functions in the normal development of the placenta, and suggest a role for maternal factors in regulating the morphogenesis of embryonically-derived placental tissues.

Murine models of holoprosencephaly

Holoprosencephaly (HPE), the most common developmental defect of the forebrain and midface, is caused by a failure to delineate the midline in these structures. Both genetic and environmental etiologies exist for HPE, and clinical presentation is highly variable. HPE occurs in sporadic and inherited forms, and even HPE in pedigrees is characterized by incomplete penetrance and variable expressivity. Heterozygous mutations in eight different genes have been identified in human HPE, and disruption of Sonic hedgehog expression and/or signaling in the rostroventral region of the embryo is a major common effect of these mutations. An understanding of the mechanisms whereby genetic defects and teratogenic exposures become manifest as developmental anomalies of varying severity requires experimental models that accurately reproduce the spectrum of defects seen in human HPE. The mouse has emerged as such a model, because of its ease of genetic manipulation and similarity to humans in development of the forebrain and face. HPE is generally observed in mice homozygous for mutations in orthologs of human HPE genes though, unlike humans, rarely in mice with heterozygous mutations. Moreover, reverse genetics in the mouse has provided a wealth of new candidate human HPE genes. Construction of hypomorphic alleles, interbreeding to produce double mutants, and analysis of these mutations on different genetic backgrounds has generated multiple models of HPE and begun to provide insight into the conundrum of the HPE spectrum. Here, we review forebrain development with an emphasis on the pathways known to be defective in HPE and describe the strengths and weaknesses of various murine models of HPE.

Zebrafish model of holoprosencephaly demonstrates a key role for TGIF in regulating retinoic acid metabolism

Holoprosencephaly (HPE) is the most common human congenital forebrain defect, affecting specification of forebrain tissue and subsequent division of the cerebral hemispheres. The causes of HPE are multivariate and heterogeneous, and include exposure to teratogens, such as retinoic acid (RA), and mutations in forebrain patterning genes. Many of the defects in HPE patients resemble animal models with aberrant RA levels, which also show severe forebrain abnormalities. RA plays an important role in early neural patterning of the vertebrate embryo: expression of RA-synthesizing enzymes initiates high RA levels in the trunk, which are required for proper anterior-posterior patterning of the hindbrain and spinal cord. In the forebrain and midbrain, RA-degrading enzymes are expressed, protecting these regions from the effects of RA. However, the mechanisms that regulate RA-synthesizing and RA-degrading enzymes are poorly understood. Mutations in the gene TGIF are associated with incidence of HPE. We demonstrate in zebrafish that Tgif plays a key role in regulating RA signaling, and is essential to properly pattern the forebrain. Tgif is necessary for normal initiation of genes that control RA synthesis and degradation, resulting in defects in RA-dependent central nervous system patterning in Tgif-depleted embryos. The loss of the forebrain-specific RA-degrading enzyme cyp26a1 causes a forebrain phenotype that mimics tgif morphants. We propose a model in which Tgif controls forebrain patterning by regulating RA degradation. The consequences of abnormal RA levels for forebrain patterning are profound, and imply that in human patients with TGIF deficiencies, increased forebrain RA levels contribute to the development of HPE.

Holoprosencephaly: new models, new insights

Holoprosencephaly (HPE) is a common congenital malformation that is characterised by a failure to divide the forebrain into left and right hemispheres and is usually accompanied by defects in patterning of the midline of the face. HPE exists in inherited, autosomal dominant (familial) forms and mutation-associated sporadic forms, but environmental factors are also implicated. There are several features of HPE that are not well understood, including the extremely variable clinical presentation, even among obligate carriers of familial mutations, and the restriction of structural anomalies to the ventral anterior midline, despite association with defects in signal transduction pathways that regulate development of many additional body structures. The new animal models described in this review may help unravel these puzzles. Furthermore, these model systems suggest that human HPE arises from a complex interaction between the timing and strength of developmental signalling pathways, genetic variation and exposure to environmental agents.

Functional analysis of mutations in TGIF associated with holoprosencephaly

Holoprosencephaly (HPE) is the most common structural malformation of the forebrain and face in humans. Our current understanding of the pathogenesis of HPE attempts to integrate genetic susceptibility, evidenced by mutations in the known HPE genes, with the epigenetic influence of environmental factors. Mutations or deletions of the human TGIF gene have been associated with HPE in multiple population cohorts. Here we examine the functional effects of all previously reported mutations, and describe four additional variants. Of the eleven sequence variations in TGIF, all but four can be demonstrated to be functionally abnormal. In contrast, no potentially pathogenic sequence alterations were detected in the related gene TGIF2. These results provide further evidence of a role for TGIF in HPE and demonstrate the importance of functional analysis of putative disease-associated alleles.

Intragenic deletion of Tgif causes defectsin brain development

TG-interacting factor (TGIF) is a homeodomain-containing protein and functions as a transcriptional repressor within the TGF-beta and retinoic acid signaling pathways. Heterozygous mutations of TGIF have been found in patients with holoprosencephaly (HPE), which is the most common congenital brain malformation in humans. However, targeted null deletions of the entire Tgif gene in mice surprisingly revealed no apparent brain defects. We report here that deletion of the third exon of Tgif gene resulted in a defined spectrum of brain developmental defects including exencephaly, microcephaly, HPE, and abnormalities in embryonic brain ventricle formation and cleavage. These defects could be detected in mice both heterozygous and homozygous for the targeted Tgif deletion. Moreover, expression of dorsal-ventral patterning genes including Shh, Pax6 and Nkx2.2 was altered. The ventricular neuroepithelium exhibited focalized increase of cell proliferation rate and resultant tissue expansion. The incidence of brain abnormalities within the mutant mice was dependent on its genetic background, suggesting that additional genetic modifiers functionally interact with Tgif during embryonic brain development. The intragenic Tgif deletion mouse, therefore, would serve as a useful model that can be used to unravel the genetic complexity implicated in the pathogenesis of HPE.

Embryonic fibroblasts from mice lacking Tgif were defective in cell cycling

Holoprosencephaly (HPE) is the most common structural anomaly of the human brain, resulting from incomplete cleavage of the developing forebrain during embryogenesis. Haploinsufficient mutations in the TG-interacting factor (TGIF) gene were previously identified in a subset of HPE families and sporadic patients, and this gene is located within a region of chromosome 18 that is associated with nonrandom chromosomal aberrations in HPE patients. TGIF is a three-amino-acid loop extension (TALE) homeodomain-containing transcription factor that functions both as a corepressor of the transforming growth factor beta (TGF-beta) pathway and as a competitor of the retinoic acid pathway. Here we describe mice deficient in Tgif that exhibited laterality defects and growth retardation and developed kinked tails. Cellular analysis of mutant mouse embryonic fibroblasts (MEFs) demonstrated for the first time that Tgif regulates proliferation and progression through the G1 cell cycle phase. Additionally, wild-type human TGIF was able to rescue this proliferative defect in MEFs. In contrast, a subset of human Tgif mutations detected in HPE patients was unable to rescue the proliferative defect. However, an absence of Tgif did not alter the normal inhibition of proliferation caused by treatment with TGF-beta or retinoic acid. Developmental control of proliferation by Tgif may play a role in the pathogenesis of HPE.