Expression of a novel murine homeobox gene in the developing cerebellar external granular layer during its proliferation

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.

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.

A novel combination of homeobox genes is expressed in mesenchymal chorionic stem/stromal cells in first trimester and term pregnancies

Human chorionic mesenchymal stem/stromal cells (CMSCs) derived from the placenta are similar to adult tissue-derived MSCs. The aim of this study was to investigate the role of these cells in normal placental development. Transcription factors, particularly members of the homeobox gene family, play crucial roles in maintaining stem cell proliferation and lineage specification in embryonic tissues. In adult tissues and organs, stem cells proliferate at low levels in their niche until they receive cues from the microenvironment to differentiate. The homeobox genes that are expressed in the CMSC niche in placental tissues have not been identified. We used the novel strategy of laser capture microdissection to isolate the stromal component of first trimester villi and excluded the cytotrophoblast and syncytiotrophoblast layers that comprise the outer layer of the chorionic villi. Microarray analysis was then used to screen for homeobox genes in the microdissected tissue. Candidate homeobox genes were selected for further RNA analysis. Immunohistochemistry of candidate genes in first trimester placental villous stromal tissue revealed homeobox genes Meis1, myeloid ectropic viral integration site 1 homolog 2 (MEIS2), H2.0-like Drosophila (HLX), transforming growth factor β-induced factor (TGIF), and distal-less homeobox 5 (DLX5) were expressed in the vascular niche where CMSCs have been shown to reside. Expression of MEIS2, HLX, TGIF, and DLX5 was also detected in scattered stromal cells. Real-time polymerase chain reaction and immunocytochemistry verified expression of MEIS2, HLX, TGIF, and DLX5 homeobox genes in first trimester and term CMSCs. These data suggest a combination of regulatory homeobox genes is expressed in CMSCs from early placental development to term, which may be required for stem cell proliferation and differentiation.

Sub-circuits of a gene regulatory network control a developmental epithelial-mesenchymal transition

Epithelial-mesenchymal transition (EMT) is a fundamental cell state change that transforms epithelial to mesenchymal cells during embryonic development, adult tissue repair and cancer metastasis. EMT includes a complex series of intermediate cell state changes including remodeling of the basement membrane, apical constriction, epithelial de-adhesion, directed motility, loss of apical-basal polarity, and acquisition of mesenchymal adhesion and polarity. Transcriptional regulatory state changes must ultimately coordinate the timing and execution of these cell biological processes. A well-characterized gene regulatory network (GRN) in the sea urchin embryo was used to identify the transcription factors that control five distinct cell changes during EMT. Single transcription factors were perturbed and the consequences followed with in vivo time-lapse imaging or immunostaining assays. The data show that five different sub-circuits of the GRN control five distinct cell biological activities, each part of the complex EMT process. Thirteen transcription factors (TFs) expressed specifically in pre-EMT cells were required for EMT. Three TFs highest in the GRN specified and activated EMT (alx1, ets1, tbr) and the 10 TFs downstream of those (tel, erg, hex, tgif, snail, twist, foxn2/3, dri, foxb, foxo) were also required for EMT. No single TF functioned in all five sub-circuits, indicating that there is no EMT master regulator. Instead, the resulting sub-circuit topologies suggest EMT requires multiple simultaneous regulatory mechanisms: forward cascades, parallel inputs and positive-feedback lock downs. The interconnected and overlapping nature of the sub-circuits provides one explanation for the seamless orchestration by the embryo of cell state changes leading to successful EMT.

TG-interacting factor (TGIF) downregulates SOX3 gene expression in the NT2/D1 cell line

SOX3 is a member of the Sox gene family implicated in brain formation and cognitive function. It is considered to be one of the earliest neural markers in vertebrates, playing a role in specifying neuronal fate. Recently, we have established the first link between TALE (three-amino-acid loop extension) proteins, PBX1 (pre-B-cell leukemia homeobox 1) and MEIS1 (myeloid ecotropic viral integration site 1 homologue), and the expression of the human SOX3 gene. Here we present the evidence that TGIF (TG-interacting factor) is an additional TALE superfamily member involved in the regulation of human SOX3 gene expression in NT2/D1 cells by direct interaction with the consensus binding site that is conserved in primate orthologue promoters. Functional analysis demonstrated that mutation of the TGIF binding site resulted in the activation of SOX3 promoter. TGIF overexpression downregulates SOX3 promoter activity and decreases endogenous SOX3 protein expression in both uninduced and retinoic acid (RA)-induced NT2/D1 cells. Up to now, this is the first transcription factor identified as a negative regulator of SOX3 gene expression. The obtained results further underscore the significance of TALE proteins as important transcriptional regulators of SOX3 gene expression.

Differential roles of TGIF family genes in mammalian reproduction

Background

TG-interacting factors (TGIFs) belong to a family of TALE-homeodomain proteins including TGIF1, TGIF2 and TGIFLX/Y in human. Both TGIF1 and TGIF2 act as transcription factors repressing TGF-β signalling. Human TGIFLX and its orthologue, Tex1 in the mouse, are X-linked genes that are only expressed in the adult testis. TGIF2 arose from TGIF1 by duplication, whereas TGIFLX arose by retrotransposition to the X-chromosome. These genes have not been characterised in any non-eutherian mammals. We therefore studied the TGIF family in the tammar wallaby (a marsupial mammal) to investigate their roles in reproduction and how and when these genes may have evolved their functions and chromosomal locations.

Results

Both TGIF1 and TGIF2 were present in the tammar genome on autosomes but TGIFLX was absent. Tammar TGIF1 shared a similar expression pattern during embryogenesis, sexual differentiation and in adult tissues to that of TGIF1 in eutherian mammals, suggesting it has been functionally conserved. Tammar TGIF2 was ubiquitously expressed throughout early development as in the human and mouse, but in the adult, it was expressed only in the gonads and spleen, more like the expression pattern of human TGIFLX and mouse Tex1. Tammar TGIF2 mRNA was specifically detected in round and elongated spermatids. There was no mRNA detected in mature spermatozoa. TGIF2 protein was specifically located in the cytoplasm of spermatids, and in the residual body and the mid-piece of the mature sperm tail. These data suggest that tammar TGIF2 may participate in spermiogenesis, like TGIFLX does in eutherians. TGIF2 was detected for the first time in the ovary with mRNA produced in the granulosa and theca cells, suggesting it may also play a role in folliculogenesis.

Conclusions

The restricted and very similar expression of tammar TGIF2 to X-linked paralogues in eutherians suggests that the evolution of TGIF1, TGIF2 and TGIFLX in eutherians was accompanied by a change from ubiquitous to tissue-specific expression. The distribution and localization of TGIF2 in tammar adult gonads suggest that there has been an ultra-conserved function for the TGIF family in fertility and that TGIF2 already functioned in spermatogenesis and potentially folliculogenesis long before its retrotransposition to the X-chromosome of eutherian mammals. These results also provide further evidence that the eutherian X-chromosome has actively recruited sex and reproductive-related genes during mammalian evolution.

Homeodomain subtypes and functional diversity

The homeodomain is a protein domain of about 60 amino acids that is encoded by homeobox genes. The homeodomain is a DNA binding domain, and hence homeodomain proteins are essentially transcription factors (TFs). They have been shown to play major roles in many developmental processes of animals, as well as fungi and plants. A primary function of homeodomain proteins is to regulate the expression of other genes in development and differentiation. Thousands of homeobox genes have been identified, and they can be grouped into many different classes. Often other conserved protein domains are found linked to a homeodomain. Several particular types of homeobox genes are organized into chromosomal clusters. The best-known cluster, the HOX cluster, is found in all bilaterian animals. Tetrapods contain four HOX clusters that arose through duplication in early vertebrate evolution. The genes in these clusters are called Hox genes. Lower chordates, insects and nematodes tend to have only one HOX cluster. Of particular interest is that many of the HOX cluster genes function in the process of pattern formation along the anterior-posterior body axis. Many other types of homeodomain proteins play roles in the determination of cell fates and cell differentiation. Homeobox genes thus perform key roles for all aspects of the development of an organism.

Hedgehog signaling update

In vertebrate hedgehog signaling, hedgehog ligands are processed to become bilipidated and then multimerize, which allows them to leave the signaling cell via Dispatched 1 and become transported via glypicans and megalin to the responding cells. Hedgehog then interacts with a complex of Patched 1 and Cdo/Boc, which activates endocytic Smoothened to the cilium. Patched 1 regulates the activity of Smoothened (1) via Vitamin D3, which inhibits Smoothened in the absence of hedgehog ligand or (2) via oxysterols, which activate Smoothened in the presence of hedgehog ligand. Hedgehog ligands also interact with Hip1, Patched 2, and Gas1, which regulate the range as well as the level of hedgehog signaling. In vertebrates, Smoothened is shortened at its C-terminal end and lacks most of the phosphorylation sites of importance in Drosophila. Cos2, also of importance in Drosophila, plays no role in mammalian transduction, nor do its homologs Kif7 and Kif27. The cilium may provide a function analogous to that of Cos2 by linking Smoothened to the modulation of Gli transcription factors. Disorders associated with the hedgehog signaling network follow, including nevoid basal cell carcinoma syndrome, holoprosencephaly, Smith-Lemli-Opitz syndrome, Greig cephalopolysyndactyly syndrome, Pallister-Hall syndrome, Carpenter syndrome, and Rubinstein-Taybi syndrome.

Novel homeobox genes are differentially expressed in placental microvascular endothelial cells compared with macrovascular cells

Angiogenesis is fundamental to normal placental development and aberrant angiogenesis contributes substantially to placental pathologies. The complex process of angiogenesis is regulated by transcription factors leading to the formation of endothelial cells that line the microvasculature. Homeobox genes are important transcription factors that regulate vascular development in embryonic and adult tissues. We have recently shown that placental homeobox genes HLX, DLX3, DLX4, MSX2 and GAX are expressed in placental endothelial cells. Hence, the novel homeobox genes TLX1, TLX2, TGIF, HEX, PHOX1, MEIS2, HOXB7, and LIM6 were detected that have not been reported in endothelial cells previously. Importantly, these homeobox genes have not been previously reported in placental endothelial cells and, with the exception of HEX, PHOX1 and HOXB7, have not been described in any other endothelial cell type. Reverse transcriptase PCR was performed on cDNA from freshly isolated placental microvascular endothelial cells (PLEC), and the human placental microvascular endothelial cell line HPEC. cDNAs prepared from control term placentae, human microvascular endothelial cells (HMVEC) and human umbilical vein macrovascular endothelial cells (HUVEC) were used as controls. PCR analyses showed that all novel homeobox genes tested were expressed by all endothelial cells types. Furthermore, real-time PCR analyses revealed that homeobox genes TLX1, TLX2 and PHOX1 relative mRNA expression levels were significantly decreased in HUVEC compared with microvascular endothelial cells, while the relative mRNA expression levels of MEIS2 and TGIF were significantly increased in macrovascular cells compared with microvascular endothelial cells. Thus we have identified novel homeobox genes in microvascular endothelial cells and have shown that homeobox genes are differentially expressed between micro- and macrovascular endothelial cells.

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.

Holoprosencephaly: clinical, anatomic, and molecular dimensions

Holoprosencephaly is addressed under the following headings: alobar, semilobar, and lobar holoprosencephaly; arrhinencephaly; agenesis of the corpus callosum; pituitary abnormalities; hindbrain abnormalities; syntelencephaly; aprosencephaly/atelencephaly; neural tube defects; facial anomalies; median cleft lip; minor facial anomalies; single maxillary central incisor; holoprosencephaly-like phenotype; epidemiology; genetic causes of holoprosencephaly; teratogenic causes of holoprosencephaly; SHH mutations; ZIC2 mutations; SIX3 mutations; TGIF mutations; PTCH mutations; GLI2 mutations; FAST1 mutations; TDGF1 mutations; and DHCR7 mutations.

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.

Expression and functional analysis of Tgif during mouse midline development

The Tgif gene encodes a homeodomain protein that functions as a transforming growth factor beta (TGF-beta) repressor by binding to Smad2. Mutations in the TGIF gene are associated with human holoprosencephaly, a common birth defect caused by the failure of anterior ventral midline formation. However, Smad2-mediated TGF-beta signaling in the axial mesendoderm has been demonstrated to be essential for ventral midline formation, and loss of a Smad2 antagonist should in principle promote rather than inhibit ventral midline formation. This suggests a more complex mechanism for the function of TGIF in controlling ventral midline formation. To explore the role of TGIF in ventral forebrain formation and patterning, we investigated Tgif expression and function during mouse development by in situ hybridization and gene targeting. We found that Tgif is highly expressed in the anterior neural plate, consistent with the proposed neural differentiation model in which TGF-beta suppression is required for normal neural differentiation. This result suggests a possible role for Tgif in anterior neural differentiation and patterning. However, targeted disruption of the Tgif gene during mouse development does not cause any detectable defects in development and growth. Both histological examination and gene expression analysis showed that Tgif-/- embryos have a normal ventral specification in the central nervous system, including the forebrain region. One interpretation of these results is that the loss of TGIF function is compensated by other TGF-beta antagonists such as c-Ski and SnoN during vertebrate anterior neural development.

Expression pattern of TG-interacting factor 2 during mouse development

TGIF2 is a close homologue of TGIF that functions as a transcriptional corepressor by interaction with transforming growth factor-beta (TGF-beta)-activated Smads. Mutations of TGIF have been found in holoprosencephaly, a genetic disease affecting forebrain and craniofacial developmental. Here we analyzed the expression pattern of TGIF2 during mouse embryogenesis. Expression of TGIF2 transcripts was detected at high level at E12.5 and E15.5 in the nervous system including neopallial cortex, mesencephalon, metencephalon, medulla oblongata, spinal cord, trigeminal ganglion, vestibulocochlear ganglion and dorsal root ganglion. In addition, TGIF2 transcripts could be detected in other tissues including heart, lung, liver, pancreas, kidney, small intestine and nasal cavity. These data indicated that TGIF2 has a wide but well controlled expression pattern during mouse development.

Targeted disruption of Tgif, the mouse ortholog of a human holoprosencephaly gene, does not result in holoprosencephaly in mice

5'-TG-3'-interacting factor or transforming growth factor beta (TGF-beta)-induced factor (TGIF) belongs to a family of evolutionarily conserved proteins that are characterized by an atypical three-amino-acid loop extension homeodomain. In vitro studies have implicated TGIF as a transcriptional repressor and corepressor in retinoid and TGF-beta signaling pathways that regulate several important biological processes. Heterozygous nonsense and missense mutations of the human TGIF gene have been associated with holoprosencephaly, the most common congenital malformation of the forebrain. In mice, Tgif mRNA is expressed ubiquitously in the ventricular neuroepithelium at embryonic day 10.5 (E10.5) but displays a medial to lateral gradient in the developing cerebral cortex at E12.5. The expression quickly declines by E14.5. The spatiotemporal expression profile of Tgif is consistent with its involvement in midline forebrain development. To better understand the function of Tgif in forebrain patterning and proliferation in vivo, we generated mice lacking Tgif by targeted deletion of exons 2 and 3, which encode 98% of the amino acids. Tgif(-)(/)(-) mice had no detectable Tgif protein by Western blotting. Surprisingly, however, these mice were viable and fertile. In addition, there were no discernible derangements in any of the major organ systems, including the forebrain. Overall our results point to a possible functional redundancy of Tgif, potentially provided by the closely related Tgif2.

Molecular screening of SHH, ZIC2, SIX3, and TGIF genes in patients with features of holoprosencephaly spectrum: Mutation review and genotype-phenotype correlations

Holoprosencephaly (HPE; 1 out of 16,000 live births; 1 out of 250 conceptuses) is a complex brain malformation resulting from incomplete cleavage of the prosencephalon, affecting both the forebrain and the face. Clinical expressivity is variable, ranging from a single cerebral ventricle and cyclopia to clinically unaffected carriers in familial dominant autosomic HPE. The disease is genetically heterogeneous, but additional environmental agents also contribute to the etiology of HPE. In our cohort of 200 patients, 34 heterozygous mutations were identified, 24 of them being novel ones: 13 out of 17 in the Sonic hedgehog gene (SHH); 4 out of 7 in ZIC2; and 7 out of 8 in SIX3. The two mutations identified in TGIF have already been reported. Novel phenotypes associated with a mutation have been described, such as abnormalities of the pituitary gland and corpus callosum, colobomatous microphthalmia, choanal aperture stenosis, and isolated cleft lip. This study confirms the great genetic heterogeneity of the disease, the important phenotypic variability in HPE families, and the difficulty to establish genotype-phenotype correlations.

Drosophila TGIF proteins are transcriptional activators

The information carried by transforming growth factor beta (TGF-beta) signaling molecules induces profound responses in target cells. To restrict this information to appropriate cells, TGF-beta signaling pathways are tightly regulated by dynamic interactions with transcriptional activators and repressors. Numerous cross-species experiments have shown that TGF-beta family members and their signal transduction machinery (receptors and Smad signal transducers) are functionally conserved between vertebrates and invertebrates. TG-interacting factor (TGIF) is a homeodomain-containing transcriptional corepressor of TGF-beta-dependent gene expression in mammals that is associated with holoprosencephaly in humans. Here we report a biochemical analysis of TGIF from zebra fish and Drosophila. Our study reveals an unprecedented role reversal between vertebrate and invertebrate TGIF proteins. Zebra fish TGIF, like its mammalian relative, interacts with general corepressors and represses TGF-beta-responsive gene expression. We identified a tandem duplication of TGIF genes in Drosophila. In contrast to vertebrate TGIFs, both Drosophila TGIFs strongly activate transcription. We also demonstrate that Drosophila TGIF proteins physically interact with both Mad and dSmad2, suggesting a role in Dpp and activin signaling. Thus, dTGIF may be the first transcription factor in the Drosophila activin pathway. Overall, our study suggests that assumptions about the functional equivalence of conserved proteins must be validated experimentally.

Drosophila TGIF is essential for developmentally regulated transcription in spermatogenesis

We have investigated the role of TGIF, a TALE-class homeodomain transcription factor, in Drosophila development. In vertebrates, TGIF has been implicated, by in vitro analysis, in several pathways, most notably as a repressor modulating the response to TGFbeta signalling. Human TGIF has been associated with the developmental disorder holoprosencephaly. Drosophila TGIF is represented by the products of two tandemly repeated highly similar genes, achintya and vismay. We have generated mutations that delete both genes. Homozygous mutant flies are viable and appear morphologically normal, but the males are completely sterile. The defect lies at the primary spermatocyte stage and differentiation is blocked prior to the onset of the meiotic divisions. We show that mutants lacking TGIF function fail to activate transcription of many genes required for sperm manufacture and of some genes required for entry into the meiotic divisions. This groups TGIF together with two other genes producing similar phenotypes, always early and cookie monster, as components of the machinery required for the activation of the spermatogenic programme of transcription. TGIF is the first sequence-specific transcription factor identified in this pathway. By immunolabelling in mouse testes we show that TGIF is expressed in the early stages of spermatogenesis consistent with a conserved role in the activation of the spermatogenesis transcription programme.

TGIF2 interacts with histone deacetylase 1 and represses transcription

TG-interacting factor (TGIF) is a transcriptional repressor, which represses transcription by binding directly to DNA or interacts with transforming growth factor beta (TGF beta)-activated Smads, thereby repressing TGF beta-responsive gene expression. Mutation of TGIF in humans causes holoprosencephaly, a severe genetic disorder affecting craniofacial development. Searching human expressed sequence tag data bases revealed the presence of clones encoding a TGIF-related protein (TGIF2), which contains two regions of high sequence identity with TGIF. Here we show that, like TGIF, TGIF2 recruits histone deacetylase, but in contrast to TGIF, is unable to interact with the corepressor CtBP. TGIF2 and TGIF have very similar DNA-binding homeodomains, and TGIF2 represses transcription when bound to DNA via a TGIF binding site. TGIF2 interacts with TGF beta-activated Smads and represses TGF beta-responsive transcription. TGIF2 appears to be a context-independent transcriptional repressor, which can perform similar functions to TGIF and may play a role in processes, which, when disrupted by mutations in TGIF, cause holoprosencephaly.

Molecular evolution of the homeodomain family of transcription factors

The homeodomain family of transcription factors plays a fundamental role in a diverse set of functions that include body plan specification, pattern formation and cell fate determination during metazoan development. Members of this family are characterized by a helix-turn-helix DNA-binding motif known as the homeodomain. Homeodomain proteins regulate various cellular processes by specifically binding to the transcriptional control region of a target gene. These proteins have been conserved across a diverse range of species, from yeast to human. A number of inherited human disorders are caused by mutations in homeodomain-containing proteins. In this study, we present an evolutionary classification of 129 human homeodomain proteins. Phylogenetic analysis of these proteins, whose sequences were aligned based on the three-dimensional structure of the homeodomain, was performed using a distance matrix approach. The homeodomain proteins segregate into six distinct classes, and this classification is consistent with the known functional and structural characteristics of these proteins. An ancestral sequence signature that accurately describes the unique sequence characteristics of each of these classes has been derived. The phylogenetic analysis, coupled with the chromosomal localization of these genes, provides powerful clues as to how each of these classes arose from the ancestral homeodomain.

Epidermal growth factor signaling via Ras controls the Smad transcriptional co-repressor TGIF

Smad transcription factors mediate the actions of transforming growth factor-beta (TGF-beta) cytokines during development and tissue homeostasis. TGF-beta receptor-activated Smad2 regulates gene expression by associating with transcriptional co-activators or co-repressors. The Smad co-repressor TGIF competes with the co-activator p300 for Smad2 association, such that TGIF abundance helps determine the outcome of a TGF-beta response. Small alterations in the physiological levels of TGIF can have profound effects on human development, as shown by the devastating brain and craniofacial developmental defects in heterozygotes carrying a hypomorphic TGIF mutant allele. Here we show that TGIF levels modulate sensitivity to TGF-beta-mediated growth inhibition, that TGIF is a short-lived protein and that epidermal growth factor (EGF) signaling via the Ras-Mek pathway causes the phosphorylation of TGIF at two Erk MAP kinase sites, leading to TGIF stabilization and favoring the formation of Smad2-TGIF co-repressor complexes in response to TGF-beta. These results identify the first mechanism for regulating TGIF levels and suggest a potential link for Smad and Ras pathway convergence at the transcriptional level.

The interaction of the carboxyl terminus-binding protein with the Smad corepressor TGIF is disrupted by a holoprosencephaly mutation in TGIF

The homeodomain protein TGIF represses transcription in part by recruiting histone deacetylases. TGIF binds directly to DNA to repress transcription or interacts with TGF-beta-activated Smads, thereby repressing genes normally activated by TGF-beta. Loss of function mutations in TGIF result in holoprosencephaly (HPE) in humans. One HPE mutation in TGIF results in a single amino acid substitution in a conserved PLDLS motif within the amino-terminal repression domain. We demonstrate that TGIF interacts with the corepressor carboxyl terminus-binding protein (CtBP) via this motif. CtBP, which was first identified by its ability to bind the adenovirus E1A protein, interacts both with gene-specific transcriptional repressors and with a subset of polycomb proteins. Efficient repression of TGF-beta-activated gene responses by TGIF is dependent on interaction with CtBP, and we show that TGIF is able to recruit CtBP to a TGF-beta-activated Smad complex. Disruption of the PLDLS motif in TGIF abolishes the interaction of CtBP with TGIF and compromises the ability of TGIF to repress transcription. Thus, at least one HPE mutation in TGIF appears to prevent CtBP-dependent transcriptional repression by TGIF, suggesting an important developmental role for the recruitment of CtBP by TGIF.

Mutations in holoprosencephaly

Holoprosencephaly (HPE) is the most common developmental defect of the forebrain and midface in humans. In holoprosencephaly the cerebral hemispheres of the brain fail to separate into distinct left and right hemispheres. This malformation is due to the improper specification and formation of the forebrain during early development. When one considers the great number and kinds of genetic interactions that must occur to properly pattern the developing forebrain, it is not surprising that HPE is extremely heterogeneous. In addition to teratogenic agents, several genes are implicated as the cause of HPE. At least 12 different loci have been associated with HPE and now several distinct human genes for holoprosencephaly have been identified. These genes include Sonic Hedgehog (SHH), ZIC2, SIX3, and TG-interacting factor (TGIF). Here we present an overview of the presently known genes causing human holoprosencephaly. We discuss their functional role in development of the forebrain and summarize the mutations and polymorphisms that have been identified within them. Hum Mutat 16:99-108, 2000. Published 2000 Wiley-Liss, Inc.

Genetic approaches to understanding brain development: holoprosencephaly as a model

Holoprosencephaly (HPE) is the most common major developmental defect of the forebrain in humans. Clinical expression is variable, ranging from a small brain with a single cerebral ventricle and cyclopia to clinically unaffected carriers in familial HPE. Significant etiologic heterogeneity exists in HPE and includes both genetic and environmental causes. Defects in the cell signaling pathway involving the Sonic Hedgehog (SHH) gene, as well as defects in the cholesterol biosynthesis have been shown to cause HPE in humans. More recently, HPE genes from additional signaling pathways have been identified. These discoveries and current genetic approaches serve as a paradigm for studying normal and abnormal brain morphogenesis. MRDD Research Reviews 6:15-21, 2000.

Mutations in TGIF cause holoprosencephaly and link NODAL signalling to human neural axis determination

Holoprosencephaly (HPE) is the most common structural defect of the developing forebrain in humans (1 in 250 conceptuses, 1 in 16,000 live-born infants). HPE is aetiologically heterogeneous, with both environmental and genetic causes. So far, three human HPE genes are known: SHH at chromosome region 7q36 (ref. 6); ZIC2 at 13q32 (ref. 7); and SIX3 at 2p21 (ref. 8). In animal models, genes in the Nodal signalling pathway, such as those mutated in the zebrafish mutants cyclops (refs 9,10), squint (ref. 11) and one-eyed pinhead (oep; ref. 12), cause HPE. Mice heterozygous for null alleles of both Nodal and Smad2 have cyclopia. Here we describe the involvement of the TG-interacting factor (TGIF), a homeodomain protein, in human HPE. We mapped TGIF to the HPE minimal critical region in 18p11.3. Heterozygous mutations in individuals with HPE affect the transcriptional repression domain of TGIF, the DNA-binding domain or the domain that interacts with SMAD2. (The latter is an effector in the signalling pathway of the neural axis developmental factor NODAL, a member of the transforming growth factor-beta (TGF-beta) family.) Several of these mutations cause a loss of TGIF function. Thus, TGIF links the NODAL signalling pathway to the bifurcation of the human forebrain and the establishment of ventral midline structures.

Holoprosencephaly: molecular study of a California population

Holoprosencephaly (HPE) is a common developmental anomaly of the forebrain and midface in which the cerebral hemispheres fail to separate into distinct left and right halves. HPE is extremely heterogeneous. In addition to teratogenic agents, several genes are implicated in the cause of HPE. Using samples from a population-based birth defects registry in California, we performed a mutational analysis of the known HPE genes Sonic Hedgehog (SHH), ZIC2, and SIX3, in addition to two HPE candidate genes, TG-interacting factor (TGIF), and Patched (PTC), on a group of sporadic HPE patients. This is the first molecular study of HPE in a population-based sample of patients. Among these patients, a deletion in the homeodomain of SIX3 and several polymorphisms in SIX3 and TGIF were identified. No sequence changes were detected in SHH, ZIC2, and PTC. Our results suggest that mutations in the currently recognized HPE genes may explain <5% of all sporadic HPE cases.

Multiple modes of repression by the Smad transcriptional corepressor TGIF

TGIF is a DNA-binding homeodomain protein that has been demonstrated to play a role in transforming growth factor beta-regulated transcription and implicated in the control of retinoid-responsive transcription. We investigated the intrinsic transcriptional activity of TGIF fused to a heterologous DNA-binding domain. Our results demonstrate that TGIF is a transcriptional repressor able to repress transcription from several different promoters. Repression by TGIF is insensitive to the distance at which it is bound from the promoter. Moreover, the wild type TGIF effectively represses transcription when bound to its cognate DNA-binding site via its homeodomain. Deletion analysis revealed the presence of at least two separable repression domains within TGIF. Repression by one of these is dependent on the activity of histone deacetylases (HDACs), whereas the other appears not to require HDAC activity. Finally, we demonstrate that TGIF interacts with HDACs via its carboxyl-terminal repression domain. Together, these results suggest that TGIF is a multifunctional transcriptional repressor, which acts in part by recruiting HDAC activity.

The mutational spectrum of the sonic hedgehog gene in holoprosencephaly: SHH mutations cause a significant proportion of autosomal dominant holoprosencephaly

Holoprosencephaly (HPE) is a common developmental anomaly of the human forebrain and midface where the cerebral hemispheres fail to separate into distinct left and right halves. We have previously reported haploinsufficiency for Sonic Hedgehog ( SHH ) as a cause for HPE. We have now performed mutational analysis of the complete coding region and intron-exon junctions of the SHH gene in 344 unrelated affected individuals. Herein, we describe 13 additional unrelated affected individuals with SHH mutations, including nonsense and missense mutations, deletions and an insertion. These mutations occur throughout the extent of the gene. No specific genotype-phenotype association is evident based on the correlation of the type or position of the mutations. In conjunction with our previous studies, we have identified a total of 23 mutations in 344 unrelated cases of HPE. They account for 14 cases of familial HPE and nine cases of sporadic HPE. Mutations in SHH were detected in 10 of 27 (37%) families showing autosomal dominant transmission of the HPE spectrum, based on structural anomalies. Interestingly, three of the patients with an SHH mutation also had abnormalities in another gene that is expressed during forebrain development. We suggest that the interactions of multiple gene products and/or environmental elements may determine the final phenotypic outcome for a given individual and that variations among these factors may cause the wide variability in the clinical features seen in HPE.

Molecular mechanisms of holoprosencephaly

Holoprosencephaly (HPE) is the most common developmental defect of the forebrain in humans. Several distinct human genes for holoprosencephaly have now been identified. They include Sonic hedgehog (SHH), ZIC2, and SIX3. Many additional genes involved in forebrain development are rapidly being cloned and characterized in model vertebrate organisms. These include Patched (Ptc), Smoothened (Smo), cubitus interuptus (ci)/Gli, wingless (wg/Wnt, decapentaplegic (dpp)/BMP, Hedgehog interacting protein (Hip), nodal, Smads, One-eyed pinhead (Oep), and TG-Interacting Factor (TGIF). However, further analysis is needed before their roles in HPE can be established. Here we present an overview of the presently known genes causing human holoprosencephaly and describe candidate genes involved in forebrain development identified in other systems. A model is discussed for how these genes may interact within and between several different signaling pathways to direct the formation of the forebrain.

Meis2, a novel mouse Pbx-related homeobox gene induced by retinoic acid during differentiation of P19 embryonal carcinoma cells

We report the cDNA cloning, partial genomic organization, and expression pattern of Stra10, a novel retinoic acid-inducible gene in P19 embryonal carcinoma cells. Four murine cDNA isoforms have been isolated, which are likely to result from alternative splicing. The predicted protein sequences exhibit approximately 85% identity with the Pbx-related Meis1 homeobox gene products, which are involved in myeloid leukemia in BXH-2 mice, and one of the Stra10 isoforms corresponds to the recently published Meis2 sequence (Nakamura et al. [1996] Oncogene 13:2235-2242). The Meis2 homeodomain is identical to that of Meis1, and is most closely related to those of the Pbx/TGIF homeobox gene products. By in situ hybridization analysis, we show that the Meis2 gene displays spatially restricted expression patterns in the developing nervous system, limbs, face, and in various viscera. In adult mice, Meis2 is mainly expressed in the brain and female genital tract, with a different distribution of the alternative splice forms in these organs.

Expression of Meis2, a Knotted-related murine homeobox gene, indicates a role in the differentiation of the forebrain and the somitic mesoderm

Knotted (Kn) genes are expressed within restricted domains of the plant meristems and play a key role in the control of plant morphogenesis. We have isolated the Kn-related gene Meis2 in mouse, which labels the lateral somitic compartment and its derivatives during early mouse embryogenesis and later becomes a marker for the dorso-ectodermal region overlying cells of the paraxial mesoderm. Meis2 is also highly expressed in specific areas of the developing central nervous system from embryonic day 9 (e9) onward. In later developmental stages, a strong expression is detectable in differentiating nuclei and regions of the forebrain, midbrain, hindbrain, and spinal cord. This temporal and spatial expression pattern suggests that Meis2 may play an important role in the cascade of induction leading to somitic differentiation as well as in brain regionalization and histogenesis.

Cloning and mapping of the MEIS1 gene, the human homolog of a murine leukemogenic gene

The mammalian homeobox genes encode a family of transcription factors that are important in a wide range of cellular processes, including hematopoiesis. Aberrant expression of some homeobox genes is known to be oncogenic. We report the cloning and initial characterization of a human homeobox gene, MEIS1, identified in a survey of homeobox genes expressed in the human fetal liver. The complete cDNA sequence shows that MEIS1 is likely to be the human homolog of Meis1, a mouse gene that is known to be activated in myeloid leukemia by retroviral insertion. We found that the MEIS1 gene is expressed at low levels in normal immunohematopoietic tissues, including the fetal liver. However, consistent with its possible role in myeloid leukemogenesis, MEIS1 was expressed in a subset of myeloid leukemia cell lines, with the highest expression seen in those with a megakaryocytic-erythroid phenotype. The gene is also expressed at high levels in the cerebellum. The gene is located on human chromosome region 2p13-p14 and contains the previously identified anonymous markers D2S134 and NIB1519. Whether this gene, which is leukemogenic in mice, also plays a leukemogenic role in humans will require further study.