Human malformations of the midbrain and hindbrain: review and proposed classification scheme

Foxc1 dependent mesenchymal signalling drives embryonic cerebellar growth

Loss of Foxc1 is associated with Dandy-Walker malformation, the most common human cerebellar malformation characterized by cerebellar hypoplasia and an enlarged posterior fossa and fourth ventricle. Although expressed in the mouse posterior fossa mesenchyme, loss of Foxc1 non-autonomously induces a rapid and devastating decrease in embryonic cerebellar ventricular zone radial glial proliferation and concurrent increase in cerebellar neuronal differentiation. Subsequent migration of cerebellar neurons is disrupted, associated with disordered radial glial morphology. In vitro, SDF1α, a direct Foxc1 target also expressed in the head mesenchyme, acts as a cerebellar radial glial mitogen and a chemoattractant for nascent Purkinje cells. Its receptor, Cxcr4, is expressed in cerebellar radial glial cells and conditional Cxcr4 ablation with Nes-Cre mimics the Foxc1-/- cerebellar phenotype. SDF1α also rescues the Foxc1-/- phenotype. Our data emphasizes that the head mesenchyme exerts a considerable influence on early embryonic brain development and its disruption contributes to neurodevelopmental disorders in humans.

Recent advances in the genetic etiology of brain malformations

In the past few years, the increasing accessibility of next-generation sequencing technology has translated to a number of significant advances in our understanding of brain malformations. Genes causing brain malformations, previously intractable due to their complex presentation, rarity, sporadic occurrence, or molecular mechanism, are being identified at an unprecedented rate and are revealing important insights into central nervous system development. Recent discoveries highlight new associations of biological processes with human disease including the PI3K-AKT-mTOR pathway in brain overgrowth syndromes, the trafficking of cellular proteins in microcephaly-capillary malformation syndrome, and the role of the exosome in the etiology of pontocerebellar hypoplasia. Several other gene discoveries expand our understanding of the role of mitosis in the primary microcephaly syndromes and post-translational modification of dystroglycan in lissencephaly. Insights into polymicrogyria and heterotopias show us that these 2 malformations are complex in their etiology, while recent work in holoprosencephaly and Dandy-Walker malformation suggest that, at least in some instances, the development of these malformations requires "multiple-hits" in the sonic hedgehog pathway. The discovery of additional genes for primary microcephaly, pontocerebellar hypoplasia, and spinocerebellar ataxia continue to impress upon us the significant degree of genetic heterogeneity associated with many brain malformations. It is becoming increasingly evident that next-generation sequencing is emerging as a tool to facilitate rapid and cost-effective molecular diagnoses that will be translated into routine clinical care for these rare conditions in the near future.

The fundamentals of fetal MR imaging: Part 1

Congenital malformations detected in any fetal system using ultrasound may be further evaluated with magnetic resonance imaging (MRI) to improve counseling, to plan deliveries appropriately, and sometimes to enable fetal interventions. In this first half of a 2-part review, the history and safety factors regarding fetal MRI, as well as the practical aspects of image acquisition, are discussed. In addition, as central nervous system anomalies are most commonly and best evaluated using fetal MRI, challenging central nervous system anomalies, such as fetal ventriculomegaly, posterior anomalies, and neural tube defects, detected using prenatal ultrasound are also reviewed with a focus on the fundamental implications of these diagnoses.

Pontocerebellar hypoplasia type 1 with a milder phenotype in a two-year-old girl

The rare association of pontocerebellar hypoplasia with anterior horn cell involvement has been classified as pontocerebellar hypoplasia type 1. Its classic phenotype is usually severe. However, the pontocerebellar hypoplasia type 1 may have wider variability in clinical and radiological features. There may be a genetic heterogeneity as well. We described here a young girl with relatively milder clinical phenotype with cerebellar atrophy with absent pontine involvement, further adding to the clinical phenotype.

Covert orienting in three etiologies of congenital hydrocephalus: the effect of midbrain and posterior fossa dysmorphology

Covert orienting is related to the integrity of the midbrain, but the specificity of the relation is unclear. We compared covert orienting in three etiologies of congenital hydrocephalus (aqueductal stenosis [AS], Dandy-Walker malformation [DWM], and spina bifida myelomeningocele [SBM]--with and without tectal beaking) to explore the effects of midbrain and posterior fossa malformations. We hypothesized a stepwise order of group performance reflecting the degree of midbrain tectum dysmorphology. Performance on an exogenously cued covert orienting task was compared using repeated measures analysis of covariance, controlling for age. Individuals with SBM and tectal beaking demonstrated the greatest disengagement cost in the vertical plane, whereas individuals with AS performed as well as a typically developing (TD) group. Individuals with SBM but no tectal beaking and individuals with DWM showed greater disengagement costs in the vertical plane relative to the TD group, but better performance relative to the group with SBM and tectal beaking. Individuals with AS, DWM, and SBM and tectal beaking demonstrated poorer inhibition of return than TD individuals. Impairments in attentional disengagement in SBM are not attributable to the general effects of hydrocephalus, but are instead associated with specific midbrain anomalies that are part of the Chiari II malformation.

Main congenital cerebral anomalies: how prenatal imaging aids counseling

The purpose of this article is to discuss some common cerebral lesions that may be detected during prenatal screening: corpus callosum dysgenesis, absent septum pellucidum, localized parenchymal ischemic-hemorrhagic lesions, megacisterna magna, Blake's pouch cyst, posterior fossa arachnoid cyst and Dandy-Walker malformation. For each cerebral defect, the main imaging findings are reminded, certain differential diagnoses are discussed and prenatal diagnostic accuracy is analyzed with emphasis on uncertainties encountered during analysis of ultrasound or magnetic resonance images. Detecting cerebral lesions in fetuses requires rapid counseling by neuropediatricians. Keeping in mind that the prenatal diagnostic accuracy is not 100%, the neuropediatricians have to answer the parents' questions regarding the outcome of the unborn child as well as the risk of recurrence for future pregnancies. This article is based on the authors' large experience in both prenatal imaging and neurocounseling. The frequently asked questions are set up. Answers are provided, underscoring the importance of an appropriate description of the cerebral defect, and therefore the pivotal role of prenatal imaging. However, prenatal neurocounseling remains challenging and the parents must be aware of uncertainties regarding both diagnostic accuracy and prognostic evaluation.

Long-term developmental outcome of children with a fetal diagnosis of isolated inferior vermian hypoplasia

OBJECTIVES

Isolated inferior vermian hypoplasia (iiVH) is one of the most common fetal cerebellar anomalies presenting for fetal neurological counselling with controversial postnatal neurodevelopmental outcome. In the present study, we characterised the long-term neurodevelopmental outcome of prenatally diagnosed iiVH at school age.

DESIGN AND PATIENTS

We prospectively followed 20 children with fetal MRI diagnosis of iiVH including their postnatal MRI result and developmental outcome at school age (mean 6.1 years±1.9 years SD) using a comprehensive age-appropriate developmental testing battery, which encompassed cognitive, language, social and behavioural domains. Parental stress level and socioeconomic status were also evaluated.

RESULTS

All children with postnatally confirmed iiVH had a normal neurodevelopmental outcome. A subgroup of children (2/20) who demonstrated cognitive delays and behavioural impairments had more extensive cerebellar malformation. Despite a normal developmental outcome, the parents of children with postnatally confirmed iiVH had higher parental stress compared with those parents whose children had normal postnatal MRI.

CONCLUSIONS

Children with postnatally confirmed iiVH show age appropriate functioning at school age. Postnatal MRI is important to confirm the diagnosis of iiVH and to exclude associated anomalies that impact neurodevelopmental outcome. A diagnosis of iiVH is associated with persistent elevated parental stress despite normal developmental outcomes in these children suggesting the need for ongoing parental support.

Both rare and de novo copy number variants are prevalent in agenesis of the corpus callosum but not in cerebellar hypoplasia or polymicrogyria

Agenesis of the corpus callosum (ACC), cerebellar hypoplasia (CBLH), and polymicrogyria (PMG) are severe congenital brain malformations with largely undiscovered causes. We conducted a large-scale chromosomal copy number variation (CNV) discovery effort in 255 ACC, 220 CBLH, and 147 PMG patients, and 2,349 controls. Compared to controls, significantly more ACC, but unexpectedly not CBLH or PMG patients, had rare genic CNVs over one megabase (p = 1.48×10⁻³; odds ratio [OR] = 3.19; 95% confidence interval [CI] = 1.89–5.39). Rare genic CNVs were those that impacted at least one gene in less than 1% of the combined population of patients and controls. Compared to controls, significantly more ACC but not CBLH or PMG patients had rare CNVs impacting over 20 genes (p = 0.01; OR = 2.95; 95% CI = 1.69–5.18). Independent qPCR confirmation showed that 9.4% of ACC patients had de novo CNVs. These, in comparison to inherited CNVs, preferentially overlapped de novo CNVs previously observed in patients with autism spectrum disorders (p = 3.06×10⁻⁴; OR = 7.55; 95% CI = 2.40–23.72). Interestingly, numerous reports have shown a reduced corpus callosum area in autistic patients, and diminished social and executive function in many ACC patients. We also confirmed and refined previously known CNVs, including significantly narrowing the 8p23.1-p11.1 duplication present in 2% of our current ACC cohort. We found six novel CNVs, each in a single patient, that are likely deleterious: deletions of 1p31.3-p31.1, 1q31.2-q31.3, 5q23.1, and 15q11.2-q13.1; and duplications of 2q11.2-q13 and 11p14.3-p14.2. One ACC patient with microcephaly had a paternally inherited deletion of 16p13.11 that included NDE1. Exome sequencing identified a recessive maternally inherited nonsense mutation in the non-deleted allele of NDE1, revealing the complexity of ACC genetics. This is the first systematic study of CNVs in congenital brain malformations, and shows a much higher prevalence of large gene-rich CNVs in ACC than in CBLH and PMG.

Here, we systematically test the genetic etiology of three common developmental brain malformations: agenesis of the corpus callosum (ACC), cerebellar hypoplasia (CBLH), and polymicrogyria (PMG) by copy number variation (CNV) analysis in a large cohort of brain malformation patients and controls. We found significantly more ACC but not CBLH or PMG patients with rare genic CNVs over one megabase and with rare CNVs impacting over 20 genes when compared with controls. De novo CNVs were found in 9.4% of ACC patients, and interestingly many such CNVs overlapped with de novo CNVs observed in autism. Notably, numerous studies have demonstrated a reduction in the corpus callosum area in autistic brains. Our analysis also refined previously known large CNVs that cause these malformations, and identified six novel CNVs that are likely deleterious. One ACC patient had inherited a deletion from the father which, through exome sequencing, was found to uncover a recessive nonsense mutation in NDE1 on the non-deleted allele inherited from the mother. Our study is the first to systematically evaluate the burden of rare genic CNVs in congenital brain malformations and shows that large gene-rich CNVs are more common in ACC than in CBLH and PMG.

Dandy-Walker malformation and Wisconsin syndrome: novel cases add further insight into the genotype-phenotype correlations of 3q23q25 deletions

Background

The Dandy-Walker malformation (DWM) is one of the commonest congenital cerebellar defects, and can be associated with multiple congenital anomalies and chromosomal syndromes. The occurrence of overlapping 3q deletions including the ZIC1 and ZIC4 genes in few patients, along with data from mouse models, have implicated both genes in the pathogenesis of DWM.

Methods and results

Using a SNP-array approach, we recently identified three novel patients carrying heterozygous 3q deletions encompassing ZIC1 and ZIC4. Magnetic resonance imaging showed that only two had a typical DWM, while the third did not present any defect of the DWM spectrum. SNP-array analysis in further eleven children diagnosed with DWM failed to identify deletions of ZIC1-ZIC4. The clinical phenotype of the three 3q deleted patients included multiple congenital anomalies and peculiar facial appearance, related to the localization and extension of each deletion. In particular, phenotypes resulted from the variable combination of three recognizable patterns: DWM (with incomplete penetrance); blepharophimosis, ptosis, and epicanthus inversus syndrome; and Wisconsin syndrome (WS), recently mapped to 3q.

Conclusions

Our data indicate that the 3q deletion is a rare defect associated with DWM, and suggest that the hemizygosity of ZIC1-ZIC4 genes is neither necessary nor sufficient per se to cause this condition. Furthermore, based on a detailed comparison of clinical features and molecular data from 3q deleted patients, we propose clinical diagnostic criteria and refine the critical region for WS.

Midbrain and hindbrain malformations: advances in clinical diagnosis, imaging, and genetics

Historically, the midbrain and hindbrain have been considered of secondary importance to the cerebrum, which has typically been acknowledged as the most important part of the brain. In the past, radiologists and pathologists did not regularly examine these structures-also known as the brainstem and cerebellum-because they are small and difficult to remove without damage. With recent developments in neuroimaging, neuropathology, and neurogenetics, many developmental disorders of the midbrain and hindbrain have emerged as causes of neurodevelopmental dysfunction. These research advances may change the way in which we treat these patients in the future and will enhance the clinical acumen of the practising neurologist and thereby improve the diagnosis and treatment of these patients.

Role of Shh in the development of molecularly characterized tegmental nuclei in mouse rhombomere 1

Hindbrain rhombomeres in general are differentially specified molecularly by unique combinations of Hox genes with other developmental genes. Rhombomere 1 displays special features, including absence of Hox gene expression. It lies within the hindbrain range of the Engrailed genes (En1, En2), controlled by the isthmic organizer via diffusion of FGF8. It is limited rostrally by the isthmus territory, and caudally by rhombomere 2. It is double the normal size of any other rhombomere. Its dorsal part generates the cerebellar hemispheres and its ventral part gives rise to several populations, such as some raphe nuclei, the interpeduncular nucleus, the rhabdoid nucleus, anterior, dorsal, ventral and posterodorsal tegmental nuclei, the cholinergic pedunculopontine and laterodorsal tegmental nuclei, rostral parts of the hindbrain reticular formation, the locus coeruleus, and part of the lateral lemniscal and paralemniscal nuclei, among other formations. Some of these populations migrate tangentially before reaching their final positions. The morphogen Sonic Hedgehog (Shh) is normally released from the local floor plate and underlying notochord. In the present report we explore, first, whether Shh is required in the specification of these r1 populations, and, second, its possible role in the guidance of tangentially migrating neurons that approach the midline. Our results indicate that when Shh function is altered selectively in a conditional mutant mouse strain, most populations normally generated in the medial basal plate of r1 are completely absent. Moreover, the relocation of some neurons that normally originate in the alar plate and migrate tangentially into the medial basal plate is variously altered. In contrast, neurons that migrate radially (or first tangentially and then radially) into the lateral basal plate were not significantly affected.

Stretching morphogenesis of the roof plate and formation of the central canal

Background

Neurulation is driven by apical constriction of actomyosin cytoskeleton resulting in conversion of the primitive lumen into the central canal in a mechanism driven by F-actin constriction, cell overcrowding and buildup of axonal tracts. The roof plate of the neural tube acts as the dorsal morphogenetic center and boundary preventing midline crossing by neural cells and axons.

Methodology/Principal Findings

The roof plate zebrafish transgenics expressing cytosolic GFP were used to study and describe development of this structure in vivo for a first time ever. The conversion of the primitive lumen into the central canal causes significant morphogenetic changes of neuroepithelial cells in the dorsal neural tube. We demonstrated that the roof plate cells stretch along the D–V axis in parallel with conversion of the primitive lumen into central canal and its ventral displacement. Importantly, the stretching of the roof plate is well-coordinated along the whole spinal cord and the roof plate cells extend 3× in length to cover 2/3 of the neural tube diameter. This process involves the visco-elastic extension of the roof place cytoskeleton and depends on activity of Zic6 and the Rho-associated kinase (Rock). In contrast, stretching of the floor plate is much less extensive.

Conclusions/Significance

The extension of the roof plate requires its attachment to the apical complex of proteins at the surface of the central canal, which depends on activity of Zic6 and Rock. The D–V extension of the roof plate may change a range and distribution of morphogens it produces. The resistance of the roof plate cytoskeleton attenuates ventral displacement of the central canal in illustration of the novel mechanical role of the roof plate during development of the body axis.

Diffusion tensor imaging and fiber tractography in brain malformations

Diffusion tensor imaging (DTI) is an advanced MR technique that provides qualitative and quantitative information about the micro-architecture of white matter. DTI and its post-processing tool fiber tractography (FT) have been increasingly used in the last decade to investigate the microstructural neuroarchitecture of brain malformations. This article aims to review the use of DTI and FT in the evaluation of a variety of common, well-described brain malformations, in particular by pointing out the additional information that DTI and FT renders compared with conventional MR sequences. In addition, the relevant existing literature is summarized.

Tumors of the superior medullary velum in infancy and childhood: report of 6 cases

OBJECT

The superior medullary velum (SMV) is a thin lamina of white matter located between the superior cerebellar peduncles horizontally and between the midbrain and cerebellum vertically. The SMV has not previously been described as the primary location of a posterior fossa tumor, although it can be secondarily invaded by a tumor from the cerebellum or quadrigeminal plate. This paper aims to define clinical and radiological features of tumors primarily arising from the SMV during childhood.

METHODS

The authors observed 6 infants and children harboring neoplasms of the SMV who were treated at Ann & Robert Lurie Children's Hospital of Chicago (formerly Children's Memorial Hospital) in Chicago, Illinois. Pathological diagnosis of the neoplasms was an atypical teratoid/rhabdoid tumor (ATRT) in 5 patients, and a juvenile pilocytic astrocytoma (JPA) in the remaining child. The tumors were diagnosed during infancy in all patients, with ages ranging from 3 months to 10 months, except for the patient with a JPA (diagnosed at 5 years old). All patients presented with signs and symptoms of increased intracranial pressure due to obstructive hydrocephalus.

RESULTS

Characteristic MRI features were noted, consistent with a mass in both the fourth ventricle and the cerebellomesencephalic fissure and quadrigeminal cistern, resulting in the circumferential displacement of the neural structures surrounding the SMV. The tumor was removed effectively in gross-total fashion through the occipital transtentorial approach in all patients. This approach offers a wide exposure of the region. However, all infants with ATRT suffered tumor dissemination and died between 4 and 11 months after diagnosis, in spite of radical resection and oncological treatment. The 1 child with JPA is alive and well 30 months after tumor resection.

CONCLUSIONS

To the best of the authors' knowledge, this is the first description in the literature that focuses on tumors originating from the SMV. This entity must be promptly recognized on preoperative radiological studies to carefully plan the subsequent surgical and clinical management.

Dominantly inherited nonprogressive cerebellar hypoplasia identified in utero

Cerebellar hypoplasia is the hallmark of a heterogeneous group of disorders that are caused by genetic and metabolic disorders. Prenatal identification of cerebellar hypoplasia and accurate prediction of outcome are challenging. Autosomal dominant nonprogressive cerebellar ataxia is a rare disorder that typically presents with early hypotonia and delayed motor milestones followed by the onset of mild ataxia and occasionally cognitive impairment. We present a case of a mother and her female fetus. Fetal sonography and magnetic resonance imaging (MRI) showed generalized cerebellar hypoplasia. The mother had mild learning difficulties and clinically showed minor features of cerebellar ataxia. Her MRI also demonstrated extreme cerebellar hypoplasia. The diagnosis of autosomal dominant nonprogressive cerebellar ataxia was suggested. This is the first report of prenatal diagnosis of autosomal dominant nonprogressive cerebellar ataxia. We recommend obtaining a family history, examining the parents, and when appropriate obtaining an MRI before counseling parents of a fetus with a brain malformation.

Cochlear implantation in patient with Dandy-walker syndrome

INTRODUCTION

 Dandy Walker Syndrome is a congenital abnormality in the central nervous system, characterized by a deficiency in the development of middle cerebelar structures, cystic dilatation of the posterior pit communicating with the fourth ventricle and upward shift of the transverse sinuses, tentorium and dyes. Among the clinical signs are occipital protuberances, a progressive increase of the skull, bowing before the fontanels, papilledema, ataxia, gait disturbances, nystagmus, and intellectual impairment.

OBJECTIVES

 To describe a case of female patient, 13 years old with a diagnosis of this syndrome and bilateral hearing loss underwent cochlear implant surgery under local anesthesia and sedation.

CASE REPORT

 CGS, 13 years old female was referred to the Otolaryngological Department of Otolaryngology Institute of Parana with a diagnosis of "Dandy-Walker syndrome" for Otolaryngological evaluation for bilateral hearing loss with no response to the use of hearing aids. Final Comments: The field of cochlear implants is growing rapidly. We believe that the presence of Dandy-Walker syndrome cannot be considered a contraindication to the performance of cochlear implant surgery, and there were no surgical complications due to neurological disorders with very favorable results for the patient who exhibits excellent discrimination. It has less need for lip reading with improvement in speech quality.

Joubert syndrome: brain and spinal cord malformations in genotyped cases and implications for neurodevelopmental functions of primary cilia

Joubert syndrome (JS) is an autosomal recessive ciliopathy characterized by hypotonia, ataxia, abnormal eye movements, and intellectual disability. The brain is malformed, with severe vermian hypoplasia, fourth ventriculomegaly, and "molar tooth" appearance of the cerebral and superior cerebellar peduncles visible as consistent features on neuroimaging. Neuropathological studies, though few, suggest that several other brain and spinal cord structures, such as the dorsal cervicomedullary junction, may also be affected in at least some patients. Genetically, JS is heterogeneous, with mutations in 13 genes accounting for approximately 50% of patients. Here, we compare neuropathologic findings in five subjects with JS, including four with defined mutations in OFD1 (2 siblings), RPGRIP1L, or TCTN2. Characteristic findings in all JS genotypes included vermian hypoplasia, fragmented dentate and spinal trigeminal nuclei, hypoplastic pontine and inferior olivary nuclei, and nondecussation of corticospinal tracts. Other common findings, seen in multiple genotypes but not all subjects, were dorsal cervicomedullary heterotopia, nondecussation of superior cerebellar peduncles, enlarged arcuate nuclei, hypoplastic reticular formation, hypoplastic medial lemnisci, and dorsal spinal cord disorganization. Thus, while JS exhibits significant neuropathologic as well as genetic heterogeneity, no genotype-phenotype correlations are apparent as yet. Our findings suggest that primary cilia are important for neural patterning, progenitor proliferation, cell migration, and axon guidance in the developing human brain and spinal cord.

Basic genetic principles applied to posterior fossa malformations

Recent advances in neuroimaging techniques turned possible for neuroradiologists to be frequently the first one to detect possible brain structural anomalies. However, with all the recent advances in genetics and embryology, understanding posterior fossa malformation's principles is being hardest to be achieved than previously. Studies in vertebrate models provide a developmental framework in which to categorize human hindbrain malformations and serve to inform our thinking regarding candidate genes involved in disrupted developmental processes. The main focus of this review was to survey the basic principles of the rhombomere division, anteroposterior and dorsoventral patterning, alar and basal zone concept, and axonal path finding to integrate the knowledge of human hindbrain malformations for better understanding the genetic basis of hindbrain development.

Pontocerebellar hypoplasia in association with de novo 19p13.11p13.12 microdeletion

The pontocerebellar hypoplasias (PCHs) are a group of clinically variable disorders characterized by abnormally small cerebellum and brainstem, generally inherited in an autosomal recessive pattern. While PCHs have been grouped into six subtypes, clinical diagnosis is equivocal until a genetic diagnosis is established. We report a patient with PCH, intrauterine growth restriction, ventricular septal defect, rib anomalies, midgut malrotation, and facial dysmorphic features. Using SNP analysis, we identified three de novo deletions of: 1.055 Mb at 6q24.3q25.1 (148174730-149229583); 169 kb at 16p13.2 (6565411-6733934); and 2.530 Mb at 19p13.11p13.12 (13857587-16387798), which were confirmed by FISH. 19p13 deletions are rare aberrations. Of patients previously described with overlapping 19p13.12 deletions and multiple anomalies, only one presented with PCH. Deleted in both that patient and the patient reported here, is DDX39, a DEAD-box RNA helicase. DDX39 is part of the homeostatic machinery that regulates the switch of cellular proliferation and differentiation. It is highly expressed in the developing central nervous system and optic cup of Xenopus laevis. The brain abnormalities in the patient reported here were more severe than the previously reported patient, which may be due to additional deletions or undetected point mutations in the nondeleted allele. Notably, the patient reported here also has a partial deletion of RBFOX1 (A2BP1), which lies within the autism susceptibility locus on 16p13.2. Our findings suggest chromosomal microarray analysis may be useful in determining etiology of syndromic PCH.

Bifid tongue, corneal clouding, and Dandy-Walker malformation in a male infant with otopalatodigital syndrome type 2

We report on a male infant with otopalatodigital syndrome type 2 (OPD2) associated with a novel c.514C>G FLNA mutation and unusual clinical features including bifid tongue and congenital corneal clouding. Bifid tongue and congenital corneal clouding have each only been described once previously in a patient with OPD2, and this is the first description of Dandy-Walker malformation (DWM) in OPD2. The presence of these clinical findings in a mutation-confirmed case of OPD2 supports the notion that corneal clouding, bifid tongue, and DWM are part of the constellation of abnormalities caused by mutations in FLNA.

Multiple developmental programs are altered by loss of Zic1 and Zic4 to cause Dandy-Walker malformation cerebellar pathogenesis

Heterozygous deletions encompassing the ZIC1;ZIC4 locus have been identified in a subset of individuals with the common cerebellar birth defect Dandy-Walker malformation (DWM). Deletion of Zic1 and Zic4 in mice produces both cerebellar size and foliation defects similar to human DWM, confirming a requirement for these genes in cerebellar development and providing a model to delineate the developmental basis of this clinically important congenital malformation. Here, we show that reduced cerebellar size in Zic1 and Zic4 mutants results from decreased postnatal granule cell progenitor proliferation. Through genetic and molecular analyses, we show that Zic1 and Zic4 have Shh-dependent function promoting proliferation of granule cell progenitors. Expression of the Shh-downstream genes Ptch1, Gli1 and Mycn was downregulated in Zic1/4 mutants, although Shh production and Purkinje cell gene expression were normal. Reduction of Shh dose on the Zic1(+/-);Zic4(+/-) background also resulted in cerebellar size reductions and gene expression changes comparable with those observed in Zic1(-/-);Zic4(-/-) mice. Zic1 and Zic4 are additionally required to pattern anterior vermis foliation. Zic mutant folial patterning abnormalities correlated with disrupted cerebellar anlage gene expression and Purkinje cell topography during late embryonic stages; however, this phenotype was Shh independent. In Zic1(+/-);Zic4(+/-);Shh(+/-), we observed normal cerebellar anlage patterning and foliation. Furthermore, cerebellar patterning was normal in both Gli2-cko and Smo-cko mutant mice, where all Shh function was removed from the developing cerebellum. Thus, our data demonstrate that Zic1 and Zic4 have both Shh-dependent and -independent roles during cerebellar development and that multiple developmental disruptions underlie Zic1/4-related DWM.

Impairment of the tRNA-splicing endonuclease subunit 54 (tsen54) gene causes neurological abnormalities and larval death in zebrafish models of pontocerebellar hypoplasia

Pontocerebellar hypoplasia (PCH) represents a group (PCH1-6) of neurodegenerative autosomal recessive disorders characterized by hypoplasia and/or atrophy of the cerebellum, hypoplasia of the ventral pons, progressive microcephaly and variable neocortical atrophy. The majority of PCH2 and PCH4 cases are caused by mutations in the TSEN54 gene; one of the four subunits comprising the tRNA-splicing endonuclease (TSEN) complex. We hypothesized that TSEN54 mutations act through a loss of function mechanism. At 8 weeks of gestation, human TSEN54 is expressed ubiquitously in the brain, yet strong expression is seen within the telencephalon and metencephalon. Comparable expression patterns for tsen54 are observed in zebrafish embryos. Morpholino (MO) knockdown of tsen54 in zebrafish embryos results in loss of structural definition in the brain. This phenotype was partially rescued by co-injecting the MO with human TSEN54 mRNA. A developmental patterning defect was not associated with tsen54 knockdown; however, an increase in cell death within the brain was observed, thus bearing resemblance to PCH pathophysiology. Additionally, N-methyl-N-nitrosourea mutant zebrafish homozygous for a tsen54 premature stop-codon mutation die within 9 days post-fertilization. To determine whether a common disease pathway exists between TSEN54 and other PCH-related genes, we also monitored the effects of mitochondrial arginyl-tRNA synthetase (rars2; PCH1 and PCH6) knockdown in zebrafish. Comparable brain phenotypes were observed following the inhibition of both genes. These data strongly support the hypothesis that TSEN54 mutations cause PCH through a loss of function mechanism. Also we suggest that a common disease pathway may exist between TSEN54- and RARS2-related PCH, which may involve a tRNA processing-related mechanism.

Novel approaches to studying the genetic basis of cerebellar development

The list of genes that when mutated cause disruptions in cerebellar development is rapidly increasing. The study of both spontaneous and engineered mouse mutants has been essential to this progress, as it has revealed much of our current understanding of the developmental processes required to construct the mature cerebellum. Improvements in brain imaging, such as magnetic resonance imaging (MRI) and the emergence of better classification schemes for human cerebellar malformations, have recently led to the identification of a number of genes which cause human cerebellar disorders. In this review we argue that synergistic approaches combining classical molecular techniques, genomics, and mouse models of human malformations will be essential to fuel additional discoveries of cerebellar developmental genes and mechanisms.

Dandy-Walker malformation associated with heterozygous ZIC1 and ZIC4 deletion: Report of a new patient

We report on a female patient with Dandy-Walker malformation possibly caused by heterozygous loss of ZIC1 and ZIC4. The patient presented with mental retardation, epilepsy, and multiple congenital malformations including spina bifida, mild dysmorphic facial features including, thick eyebrows, broad nose, full lips, macroglossia, and hypoplasia of the cerebellar vermis with enlargement of the fourth ventricle on brain magnetic resonance imaging, which is consistent with Dandy-Walker malformation. A chromosome analysis showed interstitial deletion of chromosome 3q23-q25.1. Fluorescence in situ hybridization (FISH) and microarray-based genomic analysis revealed the heterozygous deletion of ZIC1 and ZIC4 loci on 3q24. Her facial features were not consistent with those observed in blepharophimosis-ptosis-epicanthus inversus syndrome (BPES) involving FOXL2 abnormality. Other deleted genes at 3q23-25.1 might contribute to the dysmorphic facial appearance. A milder phenotype as the Dandy-Walker malformation in our patient supports the idea that modifying loci/genes can influence the development of cerebellar malformation.

Autopsy case of later-onset pontocerebellar hypoplasia type 1: pontine atrophy and pyramidal tract involvement

The combination of pontocerebellar hypoplasia and anterior horn cell degeneration is classified as pontocerebellar hypoplasia type 1. Although most cases exhibit severe muscle weakness and hypotonia neonatally with short life spans, some cases exhibit a later onset with a longer life span and show cerebellar atrophy without pontine involvement. We present a child who exhibited neurological deterioration and progressive atrophy of the cerebellum and pons, with onset of symptoms at 20 months and death at 15 years of age. The pathological findings disclosed anterior horn cell degeneration and pyramidal tract involvement in addition to pontocerebellar atrophy, leading to the diagnosis of pontocerebellar hypoplasia type 1. The present case suggests that the degenerative pattern of later-onset pontocerebellar hypoplasia type 1 is similar to that of prenatal-onset cases. Further reports of later-onset cases with histopathological examination are required to elucidate the nosology and etiology of the disorder.

Clinical and brain imaging heterogeneity of severe microcephaly

Microcephaly may be present at birth or develop postnatally. Classification according to the genetic cause cannot always predict the severity of the clinical course. The aim of this research was to group a large cohort of patients with primary microcephaly into more discrete subtypes, to optimize assessment of the patients based on their clinical and brain imaging findings. Medical records and brain images were reviewed for 4442 patients with brain malformations diagnosed and treated over 24 years and identified 247 patients classified as having microcephaly with simplified gyri alone or in association with additional brain abnormalities. For each case, clinical records were retrospectively reviewed for consanguinity, positive family history, sex, associated anomalies, and cranial magnetic resonance imaging. A subset (n = 12) of representative patients with the most complete available data was studied in greater detail, to define the most common subtypes and clinical presentations. Overall, four relatively common brain imaging presentations were identified, involving abnormalities in the gyral pattern, extra-axial space, and small size of the brainstem and cerebellum. Classifying patients with microcephaly according to brain imaging findings could enable more accurate counseling of the families with regard to prognosis.

Neuroradiologic features of CASK mutations

Mutations of the CASK gene are associated with X-linked mental retardation with microcephaly and disproportionate brain stem and cerebellar hypoplasia in females. The areas of the cerebrum, corpus callosum, pons, midbrain, and cerebellar vermis and hemisphere and a ratio of cerebrum/corpus callosum areas were measured in 5 female patients with CASK mutations, 67 female controls, and 5 patients with pontine hypoplasia. MR imaging in patients with CASK mutations revealed a normal size of the corpus callosum and a low ratio of the cerebrum/corpus callosum with a reduced area of the cerebrum, pons, midbrain, and cerebellar vermis and hemispheres. The 5 patients with pontine hypoplasia showed thinning of the corpus callosum and a high ratio of the cerebrum/corpus callosum, irrespective of the size of the cerebrum. The normal size of the corpus callosum, which gives an impression of callosal thickening at first glance, may be an imaging clue to detect patients with CASK mutations.

Usher syndrome associated with a variant of Dandy-Walker malformation

Three cases of Usher syndrome associated with a variant of Dandy-Walker malformation in three siblings from consanguineous Turkish parents are described. The siblings had retinitis pigmentosa and hearing loss. Two of the siblings also had mental retardation, which is not a constant finding in Usher syndrome. Dandy-Walker malformation might have contributed to the mental retardation in two of these patients and might be a coincidental finding with Usher syndrome.

Pontine tegmental cap dysplasia with a 2q13 microdeletion involving the NPHP1 gene: insights into malformations of the mid-hindbrain

The case of a young man with multiple brain and somatic anomalies that presented diagnostic difficulties, is discussed in this report. A majority of his features were suggestive of Joubert syndrome--although it was felt that he did not fully meet diagnostic criteria. The subsequent evaluations included a magnetic resonance image of the brain, that was found to be consistent with pontine tegmental cap dysplasia. Chromosomal microarray studies showed a 2q13 deletion. A gene associated with Joubert syndrome, NPHP1, is within this region. This case highlights several important aspects of the diagnosis and nosology of malformations of the mid-hind brain.

Delayed gyration with pontocerebellar hypoplasia type 1

We report two sibling cases of pontocerebellar hypoplasia type 1 (PCH-1), which showed severe generalized hypotonia, psychomotor delay, and visual impairment. Magnetic resonance imaging in the neonatal period revealed delayed gyration compared to the postconceptional ages in both cases. The elder brother died with pneumonia at 10months of age, and the younger brother survived to over 6years of age. Repeated neuroimaging in the younger brother revealed improvement of the delayed gyration and progressive atrophy, not only in the pons and cerebellum, but also in the cerebrum. To the best of our knowledge, this is the first report of delayed gyration in PCH-1.

A developmental and genetic classification for midbrain-hindbrain malformations

Advances in neuroimaging, developmental biology and molecular genetics have increased the understanding of developmental disorders affecting the midbrain and hindbrain, both as isolated anomalies and as part of larger malformation syndromes. However, the understanding of these malformations and their relationships with other malformations, within the central nervous system and in the rest of the body, remains limited. A new classification system is proposed, based wherever possible, upon embryology and genetics. Proposed categories include: (i) malformations secondary to early anteroposterior and dorsoventral patterning defects, or to misspecification of mid-hindbrain germinal zones; (ii) malformations associated with later generalized developmental disorders that significantly affect the brainstem and cerebellum (and have a pathogenesis that is at least partly understood); (iii) localized brain malformations that significantly affect the brain stem and cerebellum (pathogenesis partly or largely understood, includes local proliferation, cell specification, migration and axonal guidance); and (iv) combined hypoplasia and atrophy of putative prenatal onset degenerative disorders. Pertinent embryology is discussed and the classification is justified. This classification will prove useful for both physicians who diagnose and treat patients with these disorders and for clinical scientists who wish to understand better the perturbations of developmental processes that produce them. Importantly, both the classification and its framework remain flexible enough to be easily modified when new embryologic processes are described or new malformations discovered.

Looking at cerebellar malformations through text-mined interactomes of mice and humans

We have generated and made publicly available two very large networks of molecular interactions: 49,493 mouse-specific and 52,518 human-specific interactions. These networks were generated through automated analysis of 368,331 full-text research articles and 8,039,972 article abstracts from the PubMed database, using the GeneWays system. Our networks cover a wide spectrum of molecular interactions, such as bind, phosphorylate, glycosylate, and activate; 207 of these interaction types occur more than 1,000 times in our unfiltered, multi-species data set. Because mouse and human genes are linked through an orthological relationship, human and mouse networks are amenable to straightforward, joint computational analysis. Using our newly generated networks and known associations between mouse genes and cerebellar malformation phenotypes, we predicted a number of new associations between genes and five cerebellar phenotypes (small cerebellum, absent cerebellum, cerebellar degeneration, abnormal foliation, and abnormal vermis). Using a battery of statistical tests, we showed that genes that are associated with cerebellar phenotypes tend to form compact network clusters. Further, we observed that cerebellar malformation phenotypes tend to be associated with highly connected genes. This tendency was stronger for developmental phenotypes and weaker for cerebellar degeneration.

We described and made publicly available the largest existing set of text-mined statements; we also presented its application to an important biological problem. We have extracted and purified two large molecular networks, one for humans and one for mouse. We characterized the data sets, described the methods we used to generate them, and presented a novel biological application of the networks to study the etiology of five cerebellum phenotypes. We demonstrated quantitatively that the development-related malformations differ in their system-level properties from degeneration-related genes. We showed that there is a high degree of overlap among the genes implicated in the developmental malformations, that these genes have a strong tendency to be highly connected within the molecular network, and that they also tend to be clustered together, forming a compact molecular network neighborhood. In contrast, the genes involved in malformations due to degeneration do not have a high degree of connectivity, are not strongly clustered in the network, and do not overlap significantly with the development related genes. In addition, taking into account the above-mentioned system-level properties and the gene-specific network interactions, we made highly confident predictions about novel genes that are likely also involved in the etiology of the analyzed phenotypes.

Development of the human cerebellum and its disorders

The cerebellum arises from two anatomically and molecularly different proliferative compartments: the cerebellar ventricular zone and the rhombic lip. The protracted development makes the cerebellum vulnerable to a broad spectrum of developmental disorders, of which the more frequent (the Dandy-Walker and related malformations and the pontocerebellar hypoplasias) are discussed in this article. Several genes for congenital malformations of the human cerebellum have recently been identified, including genes causing Joubert syndrome, the Dandy-Walker malformation, and pontocerebellar hypoplasias.

Morphological spectrum of prenatal cerebellar disruptions

There is increasing evidence that the cerebellum is susceptible to both prenatal infections and haemorrhages as well as being vulnerable in extremely preterm babies, but not to perinatal and postnatal hypoxic-ischaemic injuries. Starting with the imaging appearance we describe and illustrate a spectrum of prenatal cerebellar disruptions: cerebellar agenesis; unilateral cerebellar hypoplasia; unilateral cerebellar cleft; global cerebellar hypoplasia; vanishing cerebellum in myelomeningocele; and disruption of cerebellar development in preterm infants. We discuss neuroradiological characteristics, possible disruptive events, and clinical findings in the different morphological patterns. Remarkably, the same disruptive agent can cause different neuroradiological patterns, which appear likely to represent a morphological spectrum. The analysis of imaging patterns is crucial in recognising cerebellar disruptions. Recognition of cerebellar disruptions and their differentiation from cerebellar malformations is important in terms of diagnosis, prognosis, and genetic counselling.

Model organisms inform the search for the genes and developmental pathology underlying malformations of the human hindbrain

Congenital malformations of the human hindbrain, including the cerebellum, are poorly understood largely because their recognition is a relatively recent advance for imaging diagnostics. Cerebellar malformations are the most obvious and best characterized hindbrain malformations due to their relative ease of viewing by magnetic resonance imaging and the recent identification of several causative genes (Millen et al. Curr Opin Neurobiol 18:12-19, 2008). Malformations of the pons and medulla have also been described both in isolation and in association with cerebellar malformations (Barkovich et al. Ann Neurol 62:625-639, 2007). Although little is understood regarding the specific developmental pathologies underlying hindbrain malformations in humans, much is known regarding the mechanisms and genes driving hindbrain development in vertebrate model organisms. Thus, studies in vertebrate models provide a developmental framework in which to categorize human hindbrain malformations and serve to provide information regarding disrupted developmental processes and candidate genes. Here, we survey the basic principles of vertebrate hindbrain development and integrate our current knowledge of human hindbrain malformations into this framework.

FOXC1 is required for normal cerebellar development and is a major contributor to chromosome 6p25.3 Dandy-Walker malformation

Dandy-Walker malformation (DWM), the most common human cerebellar malformation, has only one characterized associated locus. Here we characterize a second DWM-linked locus on 6p25.3, showing that deletions or duplications encompassing FOXC1 are associated with cerebellar and posterior fossa malformations including cerebellar vermis hypoplasia (CVH), mega-cisterna magna (MCM) and DWM. Foxc1-null mice have embryonic abnormalities of the rhombic lip due to loss of mesenchyme-secreted signaling molecules with subsequent loss of Atoh1 expression in vermis. Foxc1 homozygous hypomorphs have CVH with medial fusion and foliation defects. Human FOXC1 heterozygous mutations are known to affect eye development, causing a spectrum of glaucoma-associated anomalies (Axenfeld-Rieger syndrome, ARS; MIM no. 601631). We report the first brain imaging data from humans with FOXC1 mutations and show that these individuals also have CVH. We conclude that alteration of FOXC1 function alone causes CVH and contributes to MCM and DWM. Our results highlight a previously unrecognized role for mesenchyme-neuroepithelium interactions in the mid-hindbrain during early embryogenesis.

Neurodevelopmental outcomes in children with cerebellar malformations: a systematic review

Cerebellar malformations are increasingly diagnosed in the fetal period. Consequently, their consideration requires stressful and often critical decisions from both clinicians and families. This has resulted in an emergent need to understand better the impact of these early life lesions on child development. We performed a comprehensive literature search of studies describing neurodevelopmental outcomes of cerebellar malformations between January 1997 and December 2007. Overall, the data suggested that children with isolated inferior vermis hypoplasia (IVH) and mega cisterna magna (MCM) have a good developmental outcome, whereas children with molar tooth sign/Joubert syndrome, vermis hypoplasia, pontocerebellar hypoplasia (PCH) type II, and cerebellar agenesis experience moderate to severe global developmental delays. Reports for Dandy-Walker malformation (DWM) were conflicting; however, the presence of a normally lobulated vermis and the absence of associated brain anomalies were associated with a more favourable outcome. Finally, children with isolated cerebellar hypoplasia experienced fewer impairments. Important methodological limitations highlighted include a lack of standardized outcome measure use in 79% of studies and the predominant use of retrospective study designs (85%), with 40% limited to case reports or case-series. In summary, rigorous outcome studies describing the spectrum of disabilities in survivors are urgently needed to accurately delineate the long-term neurodevelopmental consequences of cerebellar malformations.