Human Forebrain Research Articles

Creation of Computerized 3D MRI-Integrated Atlases of the Human Basal Ganglia and Thalamus

Functional brain imaging and neurosurgery in subcortical areas often requires visualization of brain nuclei beyond the resolution of current magnetic resonance imaging (MRI) methods. We present techniques used to create: (1) a lower resolution 3D atlas, based on the Schaltenbrand and Wahren print atlas, which was integrated into a stereotactic neurosurgery planning and visualization platform (VIPER); and (2) a higher resolution 3D atlas derived from a single set of manually segmented histological slices containing nuclei of the basal ganglia, thalamus, basal forebrain, and medial temporal lobe. Both atlases were integrated to a canonical MRI (Colin27) from a young male participant by manually identifying homologous landmarks. The lower resolution atlas was then warped to fit the MRI based on the identified landmarks. A pseudo-MRI representation of the high-resolution atlas was created, and a non-linear transformation was calculated in order to match the atlas to the template MRI. The atlas can then be warped to match the anatomy of Parkinson's disease surgical candidates by using 3D automated non-linear deformation methods. By way of functional validation of the atlas, the location of the sensory thalamus was correlated with stereotactic intraoperative physiological data. The position of subthalamic electrode positions in patients with Parkinson's disease was also evaluated in the atlas-integrated MRI space. Finally, probabilistic maps of subthalamic stimulation electrodes were developed, in order to allow group analysis of the location of contacts associated with the best motor outcomes. We have therefore developed, and are continuing to validate, a high-resolution computerized MRI-integrated 3D histological atlas, which is useful in functional neurosurgery, and for functional and anatomical studies of the human basal ganglia, thalamus, and basal forebrain.

NOTCH, a new signaling pathway implicated in holoprosencephaly

Genetics of Holoprosencephaly (HPE), a congenital malformation of the developing human forebrain, is due to multiple genetic defects. Most genes that have been implicated in HPE belong to the sonic hedgehog signaling pathway. Here we describe a new candidate gene isolated from array comparative genomic hybridization redundant 6qter deletions, DELTA Like 1 (DLL1), which is a ligand of NOTCH. We show that DLL1 is co-expressed in the developing chick forebrain with Fgf8. By treating chick embryos with a pharmacological inhibitor, we demonstrate that DLL1 interacts with FGF signaling pathway. Moreover, a mutation analysis of DLL1 in HPE patients revealed a three-nucleotide deletion. These various findings implicate DLL1 in early patterning of the forebrain and identify NOTCH as a new signaling pathway involved in HPE.

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.

Multiple Origins of Human Neocortical Interneurons Are Supported by Distinct Expression of Transcription Factors

Cortical γ-aminobutyric acid (GABA)ergic interneurons in rodents originate mainly in ventrally positioned ganglionic eminences (GEs), but their origin in primates is still debated. We studied human fetal forebrains during the first half of gestation (5-23 gestational weeks, gw) for the expression of ventral transcription factors, Nkx2.1, Dlx1,2, Lhx6, and Mash1, important for development of neocortical interneurons. In embryonic (5-8 gw) human forebrain, these factors were expressed in the GE but also dorsally in the neocortical ventricular/subventricular zones (VZ/SVZ). Furthermore, their expression was retained in cells of all fetal cortical layers up to midgestation (20 gw). Nkx2.1 continued to be expressed not only in the GE but also in a subpopulation of neocortical interneurons. Moreover, proliferation marker Ki67 revealed that calretinin(+), Mash1(+), and Nkx2.1(+) cells proliferate in the neocortical VZ/SVZ at midgestation. At least some of the Mash1(+) progenitors in the neocortical SVZ could be colabeled with GABA, whereas others were oligodendrocyte progenitors, indicating a link between the 2 lineages. Taken together, these results suggest the existence of several categories of dorsal interneuronal progenitors in the human neocortical VZ/SVZ, in addition to ventrally derived cortical interneurons described in rodents. These human-specific developmental events may underlie human brain's higher complexity and capacity to process information.

Long-term Delivery of Nerve Growth Factor by Encapsulated Cell Biodelivery in the Göttingen Minipig Basal Forebrain

Nerve growth factor (NGF) prevents cholinergic degeneration in Alzheimer's disease (AD) and improves memory in AD animal models. In humans, the safe delivery of therapeutic doses of NGF is challenging. For clinical use, we have therefore developed an encapsulated cell (EC) biodelivery device, capable of local delivery of NGF. The clinical device, named NsG0202, houses an NGF-secreting cell line (NGC-0295), which is derived from a human retinal pigment epithelial (RPE) cell line, stably genetically modified to secrete NGF. Bioactivity and correct processing of NGF was confirmed in vitro. NsG0202 devices were implanted in the basal forebrain of Göttingen minipigs and the function and retrievability were evaluated after 7 weeks, 6 and 12 months. All devices were implanted and retrieved without associated complications. They were physically intact and contained a high number of viable and NGF-producing NGC-0295 cells after explantation. Increased NGF levels were detected in tissue surrounding the devices. The implants were well tolerated as determined by histopathological brain tissue analysis, blood analysis, and general health status of the pigs. The NsG0202 device represents a promising approach for treating the cognitive decline in AD patients.

Holoprosencephaly: recommendations for diagnosis and management

This review presents recent advances in our understanding and clinical management of holoprosencephaly (HPE). HPE is the most common developmental disorder of the human forebrain and involves incomplete or failed separation of the cerebral hemispheres. The epidemiology, clinical features, causes, diagnostic approach, management, and outcomes of HPE are discussed. Chromosomal abnormalities account for the most commonly identified cause of HPE. However, there are often unidentifiable causes in patients with nonsyndromic, nonchromosomal forms of HPE. The prevalence of HPE may be underestimated given that patients with mild forms often are not diagnosed until they present with severely affected children. Pregestational maternal diabetes mellitus is the most recognized risk factor for HPE, as supported by recent large-scale epidemiological studies. Genetic studies using microarray-based comparative genomic hybridization technology have resulted in better characterization of important HPE loci. HPE encompasses a wide spectrum of forebrain and midline defects, with an accompanying wide spectrum of clinical manifestations. A coordinated, multidisciplinary care team is required for clinical management of this complex disorder. Further research will enable us to better understand the pathogenesis and causes of HPE, and thus to improve the genetic counseling of patients and their families.

Phasic and sustained fear in humans elicits distinct patterns of brain activity

Aversive events are typically more debilitating when they occur unpredictably than predictably. Studies in humans and animals indicate that predictable and unpredictable aversive events can induce phasic and sustained fear, respectively. Research in rodents suggests that anatomically related but distinct neural circuits may mediate phasic and sustained fear. We explored this issue in humans by examining threat predictability in three virtual reality contexts, one in which electric shocks were predictably signaled by a cue, a second in which shocks occurred unpredictably but never paired with a cue, and a third in which no shocks were delivered. Evidence of threat-induced phasic and sustained fear was presented using fear ratings and skin conductance. Utilizing recent advances in functional magnetic resonance imaging (fMRI), we were able to conduct whole-brain fMRI at relatively high spatial resolution and still have enough sensitivity to detect transient and sustained signal changes in the basal forebrain. We found that both predictable and unpredictable threat evoked transient activity in the dorsal amygdala, but that only unpredictable threat produced sustained activity in a forebrain region corresponding to the bed nucleus of the stria terminalis complex. Consistent with animal models hypothesizing a role for the cortex in generating sustained fear, sustained signal increases to unpredictable threat were also found in anterior insula and a frontoparietal cortical network associated with hypervigilance. In addition, unpredictable threat led to transient activity in the ventral amygdala–hippocampal area and pregenual anterior cingulate cortex, as well as transient activation and subsequent deactivation of subgenual anterior cingulate cortex, limbic structures that have been implicated in the regulation of emotional behavior and stress responses. In line with basic findings in rodents, these results provide evidence that phasic and sustained fear in humans may manifest similar signs of distress, but appear to be associated with different patterns of neural activity in the human basal forebrain.

Prospective Identification, Isolation, and Profiling of a Telomerase-Expressing Subpopulation of Human Neural Stem Cells, using sox2 Enhancer-Directed Fluorescence-Activated Cell Sorting

Sox2 is expressed by neural stem and progenitor cells, and a sox2 enhancer identifies these cells in the forebrains of both fetal and adult transgenic mouse reporters. We found that an adenovirus encoding EGFP placed under the regulatory control of a 0.4 kb sox2 core enhancer selectively identified multipotential and self-renewing neural progenitor cells in dissociates of human fetal forebrain. Upon EGFP-based fluorescence-activated cell sorting (FACS), the E/sox2:EGFP(+) isolates were propagable for up to 1 year in vitro, and remained multilineage competent throughout. E/sox2:EGFP(+) cells expressed more telomerase enzymatic activity than matched E/sox2:EGFP-depleted populations, and maintained their telomeric lengths with successive passage. Gene expression analysis of E/sox2:EGFP-sorted neural progenitor cells, normalized to the unsorted forebrain dissociates from which they derived, revealed marked overexpression of genes within the notch and wnt pathways, and identified multiple elements of each pathway that appear selective to human neural progenitors. Sox2 enhancer-based FACS thus permits the prospective identification and direct isolation of a telomerase-active population of neural stem cells from the human fetal forebrain, and the elucidation of both the transcriptome and dominant signaling pathways of these critically important cells.

Early microglial colonization of the human forebrain and possible involvement in periventricular white‐matter injury of preterm infants

Amoeboid microglial subpopulations visualized by antibodies against ionized calcium-binding adapter molecule 1, CD68, and CD45 enter the forebrain starting at 4.5 postovulatory or gestational weeks (gw). They penetrate the telencephalon and diencephalon via the meninges, choroid plexus, and ventricular zone. Early colonization by amoeboid microglia-macrophages is first restricted to the white matter, where these cells migrate and accumulate in patches at the junctions of white-matter pathways, such as the three junctions that the internal capsule makes with the thalamocortical projection, external capsule and cerebral peduncle, respectively. In the cerebral cortex anlage, migration is mainly radial and tangential towards the immature white matter, subplate layer, and cortical plate, whereas pial cells populate the prospective layer I. A second wave of microglial cells penetrates the brain via the vascular route at about 12-13 gw and remains confined to the white matter. Two main findings deserve emphasis. First, microglia accumulate at 10-12 gw at the cortical plate-subplate junction, where the first synapses are detected. Second, microglia accumulate in restricted laminar bands, most notably around 19-30 gw, at the axonal crossroads in the white matter (semiovale centre) rostrally, extending caudally in the immature white matter to the visual radiations. This accumulation of proliferating microglia is located at the site of white-matter injury in premature neonates. The spatiotemporal organization of microglia in the immature white and grey matter suggests that these cells may play active roles in developmental processes such as axonal guidance, synaptogenesis, and neurodevelopmental apoptosis as well as in injuries to the developing brain, in particular in the periventricular white-matter injury of preterm infants.

Characterization of the Netrin/RGMa receptor neogenin in neurogenic regions of the mouse and human adult forebrain

In the adult rodent forebrain, astrocyte-like neural stem cells reside within the subventricular zone (SVZ) and give rise to progenitors and neuroblasts, which then undergo chain migration along the rostral migratory stream (RMS) to the olfactory bulb, where they mature into fully functional interneurons. Neurogenesis also occurs in the adult human SVZ, where neural precursors similar to the rodent astrocyte-like stem cell and neuroblast have been identified. A migratory pathway equivalent to the rodent RMS has also recently been described for the human forebrain. In the embryo, the guidance receptor neogenin and its ligands netrin-1 and RGMa regulate important neurogenic processes, including differentiation and migration. We show in this study that neogenin is expressed on neural stem cells (B cells), progenitor cells (C cells), and neuroblasts (A cells) in the adult mouse SVZ and RMS. We also show that netrin-1 and RGMa are ideally placed within the neurogenic niche to activate neogenin function. Moreover, we find that neogenin and RGMa are also present in the neurogenic regions of the human adult forebrain. We show that neogenin is localized to cells displaying stem cell (B cell)-like characteristics within the adult human SVZ and RMS and that RGMa is expressed by the same or a closely apposed cell population. This study supports the hypothesis that, as in the embryo, neogenin regulates fundamental signalling pathways important for neurogenesis in the adult mouse and human forebrain.

Spatiotemporal Distribution of PAX6 and MEIS2 Expression and Total Cell Numbers in the Ganglionic Eminence in the Early Developing Human Forebrain

The development of the human neocortex is a complex and highly regulated process involving a time-related expression of many transcription factors including the homeobox genes Pax6 and Meis2. During early development, Pax6 is expressed in nuclei of radial glia cells in the neocortical proliferative zones and controls the differentiation and neurogenetic fate of these cells in the dorsal telencephalon in rodents. Animal studies on the Meis2 gene have revealed expression in the developing telencephalon and Meis2 is known to regulate the expression of Pax6 in the eye and pancreas. Because of this functional relation between Pax6 and Meis2, we studied the spatial and temporal expression of PAX6, and MEIS2 using a developmental series of human fetal brains at 7–19 postconceptional weeks with emphasis on the forebrain to investigate whether the two genes are expressed in the same regions and zones in the same time window. We demonstrate by in situ hybridization and immunohistochemistry that the two homeobox genes are expressed during early fetal brain development in humans. PAX6 mRNA and protein were located in the proliferative zones of the neocortex and in single cells in the cortical preplate at 7 fetal weeks and in the developing cortical plate from 8 or 9 to 19 fetal weeks. The expression of PAX6 expanded into the ganglionic eminence just prior to the stage at which a stereological estimation showed an exponential rise in total cell number in this area. The MEIS2 gene was also present in the proliferative zones of the human fetal neocortex and a higher expression of MEIS2 than PAX6 was observed in these areas at 9 fetal weeks. Further, MEIS2 was expressed at a very high level in the developing ganglionic eminence and at a more moderate level in the cortical plate.

BMP antagonism protects Nodal signaling in the gastrula to promote the tissue interactions underlying mammalian forebrain and craniofacial patterning

Holoprosencephaly (HPE) is the most common forebrain and craniofacial malformation syndrome in humans. The genetics of HPE suggest that it often stems from a synergistic interaction of mutations in independent loci. In mice, several combinations of mutations in Nodal signaling pathway components can give rise to HPE, but it is not clear whether modest deficits of Nodal signaling along with lesions in other pathways might also cause such defects. We find that HPE results from simultaneous reduction of Nodal signaling and an organizer BMP (bone morphogenetic protein) antagonist, either Chordin or Noggin. These defects result from reduced production of tissues that promote forebrain and craniofacial development. Nodal promotes the expression of genes in the anterior primitive streak that are important for the development of these tissues, whereas BMP inhibits their expression. Pharmacological and transgenic manipulation of these signaling pathways suggests that BMP and Nodal antagonize each other prior to intracellular signal transduction. Biochemical experiments in vitro indicate that secreted Bmp2 and Nodal can form extracellular complexes, potentially interfering with receptor activation. Our results reveal that the patterning of forebrain and medial craniofacial elements requires a fine balance between BMP and Nodal signaling during primitive streak development, and provide a potential mechanistic basis for a new multigenic model of HPE.

Expression of the Rap1 Guanine Nucleotide Exchange Factor, MR-GEF, Is Altered in Individuals with Bipolar Disorder

In the rodent forebrain GABAergic neurons are generated from progenitor cells that express the transcription factors Dlx1 and Dlx2. The Rap-1 guanine nucleotide exchange factor, MR-GEF, is turned on by many of these developing GABAergic neurons. Expression of both Dlx1/2 and MR-GEF is retained in both adult mouse and human forebrain where, in human, decreased Dlx1 expression has been associated with psychosis. Using in situ hybridization studies we show that MR-GEF expression is significantly down-regulated in the forebrain of Dlx1/2 double mutant mice suggesting that MR-GEF and Dlx1/2 form part of a common signalling pathway during GABAergic neuronal development. We therefore compared MR-GEF expression by in situ hybridization in individuals with major psychiatric disorders (schizophrenia, bipolar disorder, major depression) and control individuals. We observed a significant positive correlation between layers II and IV of the dorso-lateral prefrontal cortex (DLPFC) in the percentage of MR-GEF expressing neurons in individuals with bipolar disorder, but not in individuals with schizophrenia, major depressive disorder or in controls. Since MR-GEF encodes a Rap1 GEF able to activate G-protein signalling, we suggest that changes in MR-GEF expression could potentially influence neurotransmission.

A Hypomorphic Allele in the FGF8 Gene Contributes to Holoprosencephaly and Is Allelic to Gonadotropin-Releasing Hormone Deficiency in Humans

Holoprosencephaly (HPE), the most common malformation of the human forebrain, may arise due to interacting genetic and environmental factors. To date, at least 12 contributory genes have been identified. Fibroblast growth factor 8 (Fgf8) belongs to the FGF family of genes expressed in several developmental signaling centers, including the anterior neural ridge, which is implicated in midline anomalies in mice. In humans, FGF8 mutations have been previously reported in facial clefting and in hypogonadotropic hypogonadism, but have not been reported in patients with HPE. We screened 360 probands with HPE for sequence variations in FGF8 using High Resolution DNA Melting (HRM) and sequenced all identified variations. Here we describe a total of 8 sequence variations in HPE patients, including a putative loss-of-function mutation in 3 members of a family with variable forms of classic HPE, and relate these findings to the phenotypes seen in other conditions.

The internal state of medium spiny neurons varies in response to different input signals

BackgroundParkinson's disease, schizophrenia, Huntington's chorea and drug addiction are manifestations of malfunctioning neurons within the striatum region at the base of the human forebrain. A key component of these neurons is the protein DARPP-32, which receives and processes various types of dopamine and glutamate inputs and translates them into specific biochemical, cellular, physiological, and behavioral responses. DARPP-32's unique capacity of faithfully converting distinct neurotransmitter signals into appropriate responses is achieved through a complex phosphorylation-dephosphorylation system that evades intuition and predictability.ResultsTo gain deeper insights into the functioning of the DARPP-32 signal transduction system, we developed a dynamic model that is robust and consistent with available clinical, pharmacological, and biological observations. Upon validation, the model was first used to explore how different input signal scenarios are processed by DARPP-32 and translated into distinct static and dynamic responses. Secondly, a comprehensive perturbation analysis identified the specific role of each component on the system's signal transduction ability.ConclusionsOur study investigated the effects of various patterns of neurotransmission on signal integration and interpretation by DARPP-32 and showed that the DARPP-32 system has the capability of discerning surprisingly many neurotransmission scenarios. We also screened out potential mechanisms underlying this capability of the DARPP-32 system. This type of insight deepens our understanding of neuronal signal transduction in normal medium spiny neurons, sheds light on neurological disorders associated with the striatum, and might aid the search for intervention targets in neurological diseases and drug addiction.

The α and β estrogen receptors in the human hypothalamus and basal forebrain in Alzheimer’s disease

Estrogen therapy in early menopause decreases the risk of Alzheimer’s disease. In contrast to the epidemiological and experimental evidence, no positive effects of estrogens on cognition were noted when estrogen therapy was prescribed to Alzheimer’s disease patients. We hypothesize that changes in canonical and alternatively spliced estrogen receptors (ERs) may play a role in these controversial findings. We found that nuclear expression of the ER α increases in the cholinergic structures of the basal forebrain and hypothalamic nuclei involved in cognitive functions and decreases in the hippocampus. Furthermore, certain brain areas in people over 50 years old had ER α splice variants with defective ligandand DNA-binding domains compromised AF1 activation function. The presence of different sets of classical and mutant estrogen receptors in different brain regions may account for the controversial effects of estrogens on cognitive functions in the climacteric period.

In Vitro and in Vivo Enhanced Generation of Human A9 Dopamine Neurons from Neural Stem Cells by Bcl-XL

Human neural stem cells derived from the ventral mesencephalon (VM) are powerful research tools and candidates for cell therapies in Parkinson disease. Previous studies with VM dopaminergic neuron (DAn) precursors indicated poor growth potential and unstable phenotypical properties. Using the model cell line hVM1 (human ventral mesencephalic neural stem cell line 1; a new human fetal VM stem cell line), we have found that Bcl-X(L) enhances the generation of DAn from VM human neural stem cells. Mechanistically, Bcl-X(L) not only exerts the expected antiapoptotic effect but also induces proneural (NGN2 and NEUROD1) and dopamine-related transcription factors, resulting in a high yield of DAn with the correct phenotype of substantia nigra pars compacta (SNpc). The expression of key genes directly involved in VM/SNpc dopaminergic patterning, differentiation, and maturation (EN1, LMX1B, PITX3, NURR1, VMAT2, GIRK2, and dopamine transporter) is thus enhanced by Bcl-X(L). These effects on neurogenesis occur in parallel to a decrease in glia generation. These in vitro Bcl-X(L) effects are paralleled in vivo, after transplantation in hemiparkinsonian rats, where hVM1-Bcl-X(L) cells survive, integrate, and differentiate into DAn, alleviating behavioral motor asymmetry. Bcl-X(L) then allows for human fetal VM stem cells to stably generate mature SNpc DAn both in vitro and in vivo and is thus proposed as a helpful factor for the development of cell therapies for neurodegenerative conditions, Parkinson disease in particular.

Vascular Endothelial Growth Factor and Its High-Affinity Receptor (VEGFR-2) Are Highly Expressed in the Human Forebrain and Cerebellum During Development

Vascular endothelial growth factor (VEGF) is an angiogenic and neurotrophic factor in both adult and neonatal animals, but its expression and role have been incompletely studied in the developing human brain. We analyzed the distribution of VEGF and its high-affinity receptor VEGFR-2 in the human forebrain and cerebellum at developmental stages from 14 weeks' gestation (WG) to the13th postnatal month. Tissue samples free of detectable neuropathologic abnormalities were assessed by immunohistochemistry and confocal microscopy using anti-human VEGF and VEGFR-2 antibodies. The VEGFR-2 was first expressed in the whole cerebral mantle and in migrating cells in the intermediate zone, whereas VEGFwas found in superficial layers of the cortical plate, in radial glia, and in the cerebellar external germinal cell layer. From 23 WG, temporospatial VEGFR-2 expression was superimposable on that ofVEGF in the cortical plate, intermediate zone, basal ganglia, limbicstructures, and external germinal cell layer. The VEGF/VEGFR-2-positive astrocytes were observed during their generation and migration from 23 WG to the first postnatal month. The VEGF-positive mature oligodendrocytes were observed in myelinating structures in the forebrain from birth and in the cerebellum from 24WG. These data suggest that VEGF and VEGFR-2 are likely involved in several aspects of human brain development.

Management of children with holoprosencephaly

Holoprosencephaly (HPE) is the most common malformation of the embryonic forebrain in humans. Although HPE occurs along a continuous spectrum, it has been categorized into four types from most severe to least severe: alobar, semilobar, lobar, and middle interhemispheric (MIH) variant. Facial malformations are often associated with HPE and usually correlate with the severity of brain malformation. With the most severely affected newborns, there is a high mortality rate in the first month of life, however, with milder forms of HPE, the majority survive beyond infancy. The Carter Centers for Brain Research in Holoprosencephaly and Related Malformations have enrolled 182 living children in a prospective research study. Based on previously published reports using this database, reports from other investigators, as well as our experience and personal observations, the range of developmental, neurological, and medical problems found in children with HPE is described in this article. Virtually all children with HPE have some developmental disability and the severity correlates with the severity of the brain malformation on neuroimaging. Common medical problems include hydrocephalus, seizures, motor impairment, oromotor dysfunction with risk of poor nutrition and aspiration, chronic lung disease, gastroesophageal reflux, constipation, hypothalamic dysfunction with disturbed sleep-wake cycles and temperature dysregulation, as well as endocrine dysfunction. Diabetes insipidus in particular is found in about 70% of children with classic HPE. Recommendations for management of these problems are given based on experiences of the authors and familiarity with the literature.

Abnormal sterol metabolism in holoprosencephaly

Holoprosencephaly (HPE) is the most common structural malformation of the developing forebrain in humans. The HPE phenotype is extremely variable and the etiology is heterogeneous. Among a variety of embryological toxins that can induce HPE, inhibitors, and other pertubations of cholesterol biosynthesis have been shown to be important factors, most likely because cholesterol is required in the Sonic hedgehog signaling cascade. Decreased levels of maternal cholesterol during pregnancy increase the risk for preterm delivery, but they are not associated with congenital malformations. However, if the fetus is affected by an inborn error of endogenous cholesterol synthesis, a reduction of maternal cholesterol concentration and cholesterol transport over the placenta aggravates the phenotypic expression. Exposure to lipophilic statins in early pregnancy may be associated with a substantial risk for structural CNS defects.