Hedgehog signaling: cooking with Gas1

TALPID3/KIAA0586 Regulates Multiple Aspects of Neuromuscular Patterning During Gastrointestinal Development in Animal Models and Human

TALPID3/KIAA0586 is an evolutionary conserved protein, which plays an essential role in protein trafficking. Its role during gastrointestinal (GI) and enteric nervous system (ENS) development has not been studied previously. Here, we analyzed chicken, mouse and human embryonic GI tissues with TALPID3 mutations. The GI tract of TALPID3 chicken embryos was shortened and malformed. Histologically, the gut smooth muscle was mispatterned and enteric neural crest cells were scattered throughout the gut wall. Analysis of the Hedgehog pathway and gut extracellular matrix provided causative reasons for these defects. Interestingly, chicken intra-species grafting experiments and a conditional knockout mouse model showed that ENS formation did not require TALPID3, but was dependent on correct environmental cues. Surprisingly, the lack of TALPID3 in enteric neural crest cells (ENCC) affected smooth muscle and epithelial development in a non-cell-autonomous manner. Analysis of human gut fetal tissues with a KIAA0586 mutation showed strikingly similar findings compared to the animal models demonstrating conservation of TALPID3 and its necessary role in human GI tract development and patterning.

Sonic Hedgehog-Gli1 Signaling and Cellular Retinoic Acid Binding Protein 1 Gene Regulation in Motor Neuron Differentiation and Diseases

Cellular retinoic acid-binding protein 1 (CRABP1) is highly expressed in motor neurons. Degenerated motor neuron-like MN1 cells are engineered by introducing SOD^G93A or AR-65Q to model degenerated amyotrophic lateral sclerosis (ALS) or spinal bulbar muscular atrophy neurons. Retinoic acid (RA)/sonic hedgehog (Shh)-induced embryonic stem cells differentiation into motor neurons are employed to study up-regulation of Crabp1 by Shh. In SOD^G93A or AR-65Q MN1 neurons, CRABP1 level is reduced, revealing a correlation of motor neuron degeneration with Crabp1 down-regulation. Up-regulation of Crabp1 by Shh is mediated by glioma-associated oncogene homolog 1 (Gli1) that binds the Gli target sequence in Crabp1′s neuron-specific regulatory region upstream of minimal promoter. Gli1 binding triggers chromatin juxtaposition with minimal promoter, activating transcription. Motor neuron differentiation and Crabp1 up-regulation are both inhibited by blunting Shh with Gli inhibitor GANT61. Expression data mining of ALS and spinal muscular atrophy (SMA) motor neurons shows reduced CRABP1, coincided with reduction in Shh-Gli1 signaling components. This study reports motor neuron degeneration correlated with down-regulation in Crabp1 and Shh-Gli signaling. Shh-Gli up-regulation of Crabp1 involves specific chromatin remodeling. The physiological and pathological implication of this regulatory pathway in motor neuron degeneration is supported by gene expression data of ALS and SMA patients.

Hedgehog Signaling Components Are Expressed in Choroidal Neovascularization in Laser-induced Retinal Lesion

Choroidal neovascularization is one of the major pathological changes in age-related macular degeneration, which causes devastating blindness in the elderly population. The molecular mechanism of choroidal neovascularization has been under extensive investigation, but is still an open question. We focused on sonic hedgehog signaling, which is implicated in angiogenesis in various organs. Laser-induced injuries to the mouse retina were made to cause choroidal neovascularization. We examined gene expression of sonic hedgehog, its receptors (patched1, smoothened, cell adhesion molecule down-regulated by oncogenes (Cdon) and biregional Cdon-binding protein (Boc)) and downstream transcription factors (Gli1-3) using real-time RT-PCR. At seven days after injury, mRNAs for Patched1 and Gli1 were upregulated in response to injury, but displayed no upregulation in control retinas. Immunohistochemistry revealed that Patched1 and Gli1 proteins were localized to CD31-positive endothelial cells that cluster between the wounded retina and the pigment epithelium layer. Treatment with the hedgehog signaling inhibitor cyclopamine did not significantly decrease the size of the neovascularization areas, but the hedgehog agonist purmorphamine made the areas significantly larger than those in untreated retina. These results suggest that the hedgehog-signaling cascade may be a therapeutic target for age-related macular degeneration.

Sonic hedgehog patterning during cerebellar development

The morphogenic factor sonic hedgehog (Shh) actively orchestrates many aspects of cerebellar development and maturation. During embryogenesis, Shh signaling is active in the ventricular germinal zone (VZ) and represents an essential signal for proliferation of VZ-derived progenitors. Later, Shh secreted by Purkinje cells sustains the amplification of postnatal neurogenic niches: the external granular layer and the prospective white matter, where excitatory granule cells and inhibitory interneurons are produced, respectively. Moreover, Shh signaling affects Bergmann glial differentiation and promotes cerebellar foliation during development. Here we review the most relevant functions of Shh during cerebellar ontogenesis, underlying its role in physiological and pathological conditions.

Design and characterization of a photo-activatable hedgehog probe that mimics the natural lipidated form

We have generated a photoactivatable form of sonic hedgehog protein by modifying the N-terminal cysteine with the heterobifunctional photocrosslinker 4-maleimidobenzophenone (Bzm). The Bzm modification on ShhN imparted a significant increase in activity as assessed in the C3H10T1/2 functional assay with potency comparable to that of the endogenous dual-lipidated form of ShhN (ShhNp). Reversed-phase HPLC analysis indicated that the increase in activity compared to unmodified ShhN may be due in part to the hydrophobic nature of the benzophenone group. In contrast to the fully processed ShhNp, Bzm-ShhN is monomeric as assessed by analytical SEC and does not require detergent to be soluble. Further, we demonstrated that the Bzm-ShhN was able to crosslink in vitro in the presence of a known binding partner, heparin. We suggest that Bzm-ShhN can serve as a relatively facile and preferred source of ShhNp for in vitro assays and as a probe to identify novel Hh protein interactions.

Holoprosencephaly: signaling interactions between the brain and the face, the environment and the genes, and the phenotypic variability in animal models and humans

Holoprosencephaly (HPE) is the most common developmental defect of the forebrain characterized by inadequate or absent midline division of the forebrain into cerebral hemispheres, with concomitant midline facial defects in the majority of cases. Understanding the pathogenesis of HPE requires knowledge of the relationship between the developing brain and the facial structures during embryogenesis. A number of signaling pathways control and coordinate the development of the brain and face, including Sonic hedgehog, Bone morphogenetic protein, Fibroblast growth factor, and Nodal signaling. Mutations in these pathways have been identified in animal models of HPE and human patients. Because of incomplete penetrance and variable expressivity of HPE, patients carrying defined mutations may not manifest the disease at all, or have a spectrum of defects. It is currently unknown what drives manifestation of HPE in genetically at-risk individuals, but it has been speculated that other gene mutations and environmental factors may combine as cumulative insults. HPE can be diagnosed in utero by a high-resolution prenatal ultrasound or a fetal magnetic resonance imaging, sometimes in combination with molecular testing from chorionic villi or amniotic fluid sampling. Currently, there are no effective preventive methods for HPE. Better understanding of the mechanisms of gene-environment interactions in HPE would provide avenues for such interventions.

Gas1 interferes with AβPP trafficking by facilitating the accumulation of immature AβPP in endoplasmic reticulum-associated raft subdomains

The amyloid-β protein precursor (AβPP) is a type I transmembrane protein that undergoes maturation during trafficking in the secretory pathway. Proper maturation and trafficking of AβPP are necessary prerequisites for AβPP processing to generate amyloid-β (Aβ), the core component of Alzheimer's disease senile plaques. Recently, we reported that the glycosylphosphatidylinositol (GPI)-anchored protein growth arrest-specific 1 (Gas1) binds to and interferes with the maturation and processing of AβPP. Gas1 expression led to a trafficking blockade of AβPP between the endoplasmic reticulum (ER) and the Golgi. GPI-anchored proteins can exit the ER by transiting through raft subdomains acting as specialized sorting platforms. Here, we show that Gas1 co-partitioned and formed a complex with AβPP in raft fractions, wherein Gas1 overexpression triggered immature AβPP accumulation. Pharmacological interference of ER to Golgi transport increased immature AβPP accumulation upon Gas1 expression in these raft fractions, which were found to be positive for the COPII protein complex component Sec31A, a specific marker for ER exit sites. Furthermore, a Gas1 mutant lacking the GPI anchor that could not transit through rafts was still able to form a complex with AβPP but did not lead to immature AβPP accumulation in rafts. Together these data show that Gas1 interfered with AβPP trafficking by interacting with AβPP to facilitate its translocation into specialized ER-associated rafts where immature AβPP accumulated.

Neogenin regulates Sonic Hedgehog pathway activity during digit patterning

BACKGROUND

Digit patterning integrates signaling by the Sonic Hedgehog (SHH), fibroblast growth factor (FGF), and bone morphogenetic protein (BMP) pathways. GLI3, a component of the SHH pathway, is a major regulator of digit number and identity. Neogenin (encoded by Neo1) is a cell surface protein that serves to transduce signals from several ligands, including BMPs, in various developmental contexts. Although neogenin is implicated in BMP signaling, it has not been linked to SHH signaling and its role in digit patterning is unknown.

RESULTS

We report that Neo1 mutant mice have preaxial polydactyly with low penetrance. Expression of SHH target genes, but not BMP target genes, is altered in Neo1 mutant limb buds. Analysis of mice carrying mutations in both Neo1 and Gli3 reveals that, although neogenin plays a role in constraint of digit numbers, suppressing polydactyly, it is also required for the severe polydactyly caused by loss of GLI3. Furthermore, embryo fibroblasts from Neo1 mutant mice are sensitized to SHH pathway activation in vitro.

CONCLUSIONS

Our findings indicate that neogenin regulates SHH signaling in the limb bud to achieve proper digit patterning.

Missense substitutions in the GAS1 protein present in holoprosencephaly patients reduce the affinity for its ligand, SHH

Holprosencephaly (HPE) is the most common disorder of the developing forebrain in humans, and is characterized by varying degrees of abnormal union of the cerebral hemispheres. These defects are typically co-associated with midline craniofacial anomalies. The combination of forebrain and craniofacial defects that comprise HPE can present along a broad and variable phenotypic spectrum. Both the SHH and NODAL signaling pathways play important roles in the pathogenesis of this disorder. Disruption of these pathways by chromosomal rearrangements, mutations in pathway-related genes and/or biochemical alterations are proposed to contribute to HPE in a large number of patients. Additional factors that are not yet fully delineated are also very likely to be involved in the pathogenesis and phenotypic heterogeneity of the disorder. Genetic loss of GAS1, a cell membrane receptor and positive regulator of SHH, has been demonstrated to contribute to the HPE phenotypic spectrum in animal models. We have evaluated the coding and flanking sequence of GAS1 in 394 patients who have clinical findings within the HPE phenotypic spectrum, and now report five novel missense sequence variants among five unrelated HPE probands. Finally, we tested the effect of these variants (as well as previously reported GAS1 variants) on the ability of GAS1 to bind to SHH. Here, we demonstrate that sequence variants in GAS1 can impair its physical interaction with SHH, suggesting a decrease in the SHH downstream signaling cascade as a pathogenic mechanism of disease.

Transcriptome analysis of embryonic mammary cells reveals insights into mammary lineage establishment

INTRODUCTION

The mammary primordium forms during embryogenesis as a result of inductive interactions between its constitutive tissues, the mesenchyme and epithelium, and represents the earliest evidence of commitment to the mammary lineage. Previous studies of embryonic mouse mammary epithelium indicated that, by mid-gestation, these cells are determined to a mammary cell fate and that a stem cell population has been delimited. Mammary mesenchyme can induce mammary development from simple epithelium even across species and classes, and can partially restore features of differentiated tissue to mouse mammary tumours in co-culture experiments. Despite these exciting properties, the molecular identity of embryonic mammary cells remains to be fully characterised.

METHODS

Here, we define the transcriptome of the mammary primordium and the two distinct cellular compartments that comprise it, the mammary primordial bud epithelium and mammary mesenchyme. Pathway and network analysis was performed and comparisons of embryonic mammary gene expression profiles to those of both postnatal mouse and human mammary epithelial cell sub-populations and stroma were made.

RESULTS

Several of the genes we have detected in our embryonic mammary cell signatures were previously shown to regulate mammary cell fate and development, but we also identified a large number of novel candidates. Additionally, we determined genes that were expressed by both embryonic and postnatal mammary cells, which represent candidate regulators of mammary cell fate, differentiation and progenitor cell function that could signal from mammary lineage inception during embryogenesis through postnatal development. Comparison of embryonic mammary cell signatures with those of human breast cells identified potential regulators of mammary progenitor cell functions conserved across species.

CONCLUSIONS

These results provide new insights into genetic regulatory mechanisms of mammary development, particularly identification of novel potential regulators of mammary fate and mesenchymal-epithelial cross-talk. Since cancers may represent diseases of mesenchymal-epithelial communications, we anticipate these results will provide foundations for further studies into the fundamental links between developmental, stem cell and breast cancer biology.

Growth arrest-specific 1 binds to and controls the maturation and processing of the amyloid-beta precursor protein

Alzheimer's disease (AD), the most common neurodegenerative disorder, is characterized by cerebral deposition of amyloid-β (Aβ), a series of peptides derived from the processing of the amyloid-β precursor protein (APP). To identify new candidate genes for AD, we recently performed a transcriptome analysis to screen for genes preferentially expressed in the hippocampus and located in AD linkage regions. This strategy identified CALHM1 (calcium homeostasis modulator 1), a gene modulating AD age at onset and Aβ metabolism. Here, we focused our attention on another candidate identified using this screen, growth arrest-specific 1 (Gas1), a gene involved in the central nervous system development. We found that Gas1 formed a complex with APP and controlled APP maturation and processing. Gas1 expression inhibited APP full glycosylation and routing to the cell surface by leading to a trafficking blockade of APP between the endoplasmic reticulum and the Golgi. Gas1 expression also resulted in a robust inhibition of APP transport into multivesicular bodies, further demonstrating that Gas1 negatively regulated APP intracellular trafficking. Consequently, Gas1 overexpression led to a reduction in Aβ production, and conversely, Gas1 silencing in cells expressing endogenously Gas1 increased Aβ levels. These results suggest that Gas1 is a novel APP-interacting protein involved in the control of APP maturation and processing.

Extrinsic and intrinsic control of germ cell proliferation in Caenorhabditis elegans

The germ cells of Caenorhabditis elegans serve as a useful model to study the balance between proliferation and differentiation within the context of development and changing environmental signals experienced by the animal. Germ cells adjacent to a stem cell niche in the distal region of the gonad retain the capacity to divide during adulthood, making them unique from other cells in the organism. We will highlight recent advances in our understanding of mechanisms that control proliferation, as well as the signaling pathways involved in promoting mitosis at the expense of differentiation.

Boc modifies the holoprosencephaly spectrum of Cdo mutant mice

Holoprosencephaly (HPE) is caused by a failure to form the midline of the forebrain and/or midface. It is one of the most common human birth defects, but clinical expression is extremely variable. HPE is associated with mutations in the sonic hedgehog (SHH) pathway. Mice lacking the Shh pathway regulator Cdo (also called Cdon) display HPE with strain-dependent penetrance and expressivity, implicating silent modifier genes as one cause of the variability. However, the identities of potential HPE modifiers of this type are unknown. We report here that whereas mice lacking the Cdo paralog Boc do not have HPE, Cdo;Boc double mutants on a largely Cdo-resistant genetic background have lobar HPE with strong craniofacial anomalies and defects in Shh target gene expression in the developing forebrain. Boc is therefore a silent HPE modifier gene in mice. Furthermore, Cdo and Boc have specific, selective roles in Shh signaling in mammals, because Cdo;Boc double-mutant mice do not display the most severe HPE phenotype seen in Shh-null mice, nor do they have major defects in digit patterning or development of vertebrae, which are also Shh-dependent processes. This is in contrast to reported observations in Drosophila, where genetic removal of the Cdo and Boc orthologs Ihog and Boi results in a complete loss of response to the hedgehog ligand. Therefore, there is evolutionary divergence between mammals and insects in the requirement of the hedgehog pathway for Cdo/Ihog family members, with mammalian development involving additional factors and/or distinct mechanisms at this level of pathway regulation.

The cell-surface proteins Dally-like and Ihog differentially regulate Hedgehog signaling strength and range during development

Hedgehog (Hh) acts as a morphogen in various developmental contexts to specify distinct cell fates in a concentration-dependent manner. Hh signaling is regulated by two conserved cell-surface proteins: Ig/fibronectin superfamily member Interference hedgehog (Ihog) and Dally-like (Dlp), a glypican that comprises a core protein and heparan sulfate glycosaminoglycan (GAG) chains. Here, we show in Drosophila that the Dlp core protein can interact with Hh and is essential for its function in Hh signaling. In wing discs, overexpression of Dlp increases short-range Hh signaling while reducing long-range signaling. By contrast, Ihog has biphasic activity in Hh signaling in cultured cells: low levels of Ihog increase Hh signaling, whereas high levels decrease it. In wing discs, overexpression of Ihog represses high-threshold targets, while extending the range of low-threshold targets, thus showing opposite effects to Dlp. We further show that Ihog and its family member Boi are required to maintain Hh on the cell surface. Finally, Ihog and Dlp have complementary expression patterns in discs. These data led us to propose that Dlp acts as a signaling co-receptor. However, Ihog might not act as a classic co-receptor; rather, it may act as an exchange factor by retaining Hh on the cell surface, but also compete with the receptor for Hh binding.

GLI1 is a central mediator of EWS/FLI1 signaling in Ewing tumors

The Ewing Sarcoma Family Tumors (ESFT) consist of the classical pathologic entities of Ewing Sarcoma and peripheral Primitive Neuroectodermal Tumor. Occurring largely in the childhood through young adult years, these tumors have an unsurpassed propensity for metastasis and have no defined cell of origin. The biology of these aggressive malignancies centers around EWS/FLI1 and related EWS/ETS chimeric transcription factors, which are largely limited to this tumor class. Much progress has been made in the identification of a network of loci whose expression is modulated by EWS/FLI1 and its congeners. To date, little progress has been made in reconstructing the sequence of direct and indirect events that produce this network of modulated loci. The recent identification of GLI1 as an upregulated target of EWS/ETS transcription factors suggests a target which may be a more central mediator in the ESFT signaling network. In this paper, we further define the relationship of EWS/FLI1 expression and GLI1 upregulation in ESFT. This relationship is supported with data from primary tumor specimens. It is consistently observed across multiple ESFT cell lines and with multiple means of EWS/FLI1 inhibition. GLI1 inhibition affects tumor cell line phenotype whether shRNA or endogenous or pharmacologic inhibitors are employed. As is seen in model transformation systems, GLI1 upregulation by EWS/FLI1 appears to be independent of Hedgehog stimulation. Consistent with a more central role in ESFT pathogenesis, several known EWS/FLI1 targets appear to be targeted through GLI1. These findings further establish a central role for GLI1 in the pathogenesis of Ewing Tumors.

FOS proliferating network construction in early colorectal cancer (CRC) based on integrative significant function cluster and inferring analysis

The aim is to setup single distinguished molecular network. We constructed FOS proliferating network from 22 colorectal samples of the same GEO dataset by GRNInfer tool and DAVID based on linear programming and a decomposition procedure with integrated Kappa statistics and fuzzy heuristic clustering. In the control, we found no proliferating subnetwork. In CRC, we identified one FOS proliferating module (SFRP2, ADAMTS1, SYNPO2, VIP, ADAM33 inhibition to FOS and MGP, FOSB activation to FOS. FOS activation to IGFBP5, LGI1, GAS1 and FOS inhibition to VIP). These results may be useful for developing novel prognostic markers and therapeutic targets in CRC.

Structural insights into hedgehog ligand sequestration by the human hedgehog-interacting protein HHIP

Hedgehog (Hh) morphogens have fundamental roles in development, whereas dysregulation of Hh signaling leads to disease. Multiple cell-surface receptors are responsible for transducing and/or regulating Hh signals. Among these, the Hedgehog-interacting protein (Hhip) is a highly conserved, vertebrate-specific inhibitor of Hh signaling. We have solved a series of crystal structures for the human HHIP ectodomain and Desert hedgehog (DHH) in isolation, as well as HHIP in complex with DHH (HHIP-DHH) and Sonic hedgehog (Shh) (HHIP-Shh), with and without Ca2+. The interaction determinants, confirmed by biophysical studies and mutagenesis, reveal previously uncharacterized and distinct functions for the Hh Zn2+ and Ca2+ binding sites--functions that may be common to all vertebrate Hh proteins. Zn2+ makes a key contribution to the Hh-HHIP interface, whereas Ca2+ is likely to prevent electrostatic repulsion between the two proteins, suggesting an important modulatory role. This interplay of several metal binding sites suggests a tuneable mechanism for regulation of Hh signaling.

Trafficking, development and hedgehog

Embryogenesis is mediated by a relatively small number of developmental signaling pathways, and the morphogens, receptors and transcription factors integral to these cascades are considered the master regulators of development. However, superimposed on this is an additional layer of control by complex intracellular trafficking networks. The importance of trafficking in controlling the processes of morphogenesis and development is highlighted by recent data regarding the transport and localisation of the morphogen sonic hedgehog (Shh) and the machinery that leads to its secretion, modification, cellular internalisation and signal transduction. Here we review the regulation of hedgehog signaling by intracellular trafficking, including the role of the primary cilium and lipids in mediating pathway activity.

The Hedgehog protein family

The Hedgehog (Hh) pathway is one of the fundamental signal transduction pathways in animal development and is also involved in stem-cell maintenance and carcinogenesis. The hedgehog (hh) gene was first discovered in Drosophila, and members of the family have since been found in most metazoa. Hh proteins are composed of two domains, an amino-terminal domain HhN, which has the biological signal activity, and a carboxy-terminal autocatalytic domain HhC, which cleaves Hh into two parts in an intramolecular reaction and adds a cholesterol moiety to HhN. HhC has sequence similarity to the self-splicing inteins, and the shared region is termed Hint. New classes of proteins containing the Hint domain have been discovered recently in bacteria and eukaryotes, and the Hog class, of which Hh proteins comprise one family, is widespread throughout eukaryotes. The non-Hh Hog proteins have carboxy-terminal domains (the Hog domain) highly similar to HhC, although they lack the HhN domain, and instead have other amino-terminal domains. Hog proteins are found in many protists, but the Hh family emerged only in early metazoan evolution. HhN is modified by cholesterol at its carboxyl terminus and by palmitate at its amino terminus in both flies and mammals. The modified HhN is released from the cell and travels through the extracellular space. On binding its receptor Patched, it relieves the inhibition that Patched exerts on Smoothened, a G-protein-coupled receptor. The resulting signaling cascade converges on the transcription factor Cubitus interruptus (Ci), or its mammalian counterparts, the Gli proteins, which activate or repress target genes.

The mode of Hedgehog binding to Ihog homologues is not conserved across different phyla

Hedgehog (Hh) proteins specify tissue pattern in metazoan embryos by forming gradients that emanate from discrete sites of expression and elicit concentration-dependent cellular differentiation or proliferation responses. Cellular responses to Hh and the movement of Hh through tissues are both precisely regulated, and abnormal Hh signalling has been implicated in human birth defects and cancer. Hh signalling is mediated by its amino-terminal domain (HhN), which is dually lipidated and secreted as part of a multivalent lipoprotein particle. Reception of the HhN signal is modulated by several cell-surface proteins on responding cells, including Patched (Ptc), Smoothened (Smo), Ihog (known as CDO or CDON in mammals) and the vertebrate-specific proteins Hip (also known as Hhip) and Gas1 (ref. 11). Drosophila Ihog and its vertebrate homologues CDO and BOC contain multiple immunoglobulin and fibronectin type III (FNIII) repeats, and the first FNIII repeat of Ihog binds Drosophila HhN in a heparin-dependent manner. Surprisingly, pull-down experiments suggest that a mammalian Sonic hedgehog N-terminal domain (ShhN) binds a non-orthologous FNIII repeat of CDO. Here we report biochemical, biophysical and X-ray structural studies of a complex between ShhN and the third FNIII repeat of CDO. We show that the ShhN-CDO interaction is completely unlike the HhN-Ihog interaction and requires calcium, which binds at a previously undetected site on ShhN. This site is conserved in nearly all Hh proteins and is a hotspot for mediating interactions between ShhN and CDO, Ptc, Hip and Gas1. Mutations in vertebrate Hh proteins causing holoprosencephaly and brachydactyly type A1 map to this calcium-binding site and disrupt interactions with these partners.

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.