Genotypic and phenotypic analysis of 396 individuals with mutations in Sonic Hedgehog

Characterization of subtle brain abnormalities in a mouse model of Hedgehog pathway antagonist-induced cleft lip and palate

Subtle behavioral and cognitive deficits have been documented in patient cohorts with orofacial clefts (OFCs). Recent neuroimaging studies argue that these traits are associated with structural brain abnormalities but have been limited to adolescent and adult populations where brain plasticity during infancy and childhood may be a confounding factor. Here, we employed high resolution magnetic resonance microscopy to examine primary brain morphology in a mouse model of OFCs. Transient in utero exposure to the Hedgehog (Hh) signaling pathway antagonist cyclopamine resulted in a spectrum of facial dysmorphology, including unilateral and bilateral cleft lip and palate, cleft of the secondary palate only, and a non-cleft phenotype marked by midfacial hypoplasia. Relative to controls, cyclopamine-exposed fetuses exhibited volumetric differences in several brain regions, including hypoplasia of the pituitary gland and olfactory bulbs, hyperplasia of the forebrain septal region, and expansion of the third ventricle. However, in affected fetuses the corpus callosum was intact and normal division of the forebrain was observed. This argues that temporally-specific Hh signaling perturbation can result in typical appearing OFCs in the absence of holoprosencephaly--a condition classically associated with Hh pathway inhibition and frequently co-occurring with OFCs. Supporting the premise that some forms of OFCs co-occur with subtle brain malformations, these results provide a possible ontological basis for traits identified in clinical populations. They also argue in favor of future investigations into genetic and/or environmental modulation of the Hh pathway in the etiopathogenesis of orofacial clefting.

Molecular analysis of holoprosencephaly in South America

Holoprosencephaly (HPE) is a spectrum of brain and facial malformations primarily reflecting genetic factors, such as chromosomal abnormalities and gene mutations. Here, we present a clinical and molecular analysis of 195 probands with HPE or microforms; approximately 72% of the patients were derived from the Latin American Collaborative Study of Congenital Malformations (ECLAMC), and 82% of the patients were newborns. Alobar HPE was the predominant brain defect in almost all facial defect categories, except for patients without oral cleft and median or lateral oral clefts. Ethmocephaly, cebocephaly, and premaxillary agenesis were primarily observed among female patients. Premaxillary agenesis occurred in six of the nine diabetic mothers. Recurrence of HPE or microform was approximately 19%. The frequency of microdeletions, detected using Multiplex Ligation-dependant Probe Amplification (MLPA) was 17% in patients with a normal karyotype. Cytogenetics or QF-PCR analyses revealed chromosomal anomalies in 27% of the probands. Mutational analyses in genes SHH, ZIC2, SIX3 and TGIF were performed in 119 patients, revealing eight mutations in SHH, two mutations in SIX3 and two mutations in ZIC2. Thus, a detailed clinical description of new HPE cases with identified genetic anomalies might establish genotypic and phenotypic correlations and contribute to the development of additional strategies for the analysis of new cases.

Pathogenic mutations in GLI2 cause a specific phenotype that is distinct from holoprosencephaly

BACKGROUND

Mutations in GLI2 have been associated with holoprosencephaly (HPE), a neuroanatomic anomaly resulting from incomplete cleavage of the developing forebrain, and an HPE-like phenotype involving pituitary anomalies and polydactyly.

OBJECTIVE

To characterise the genotypic and phenotypic findings in individuals with GLI2 variants and clarify clinical findings in individuals with loss-of-function mutations.

METHODS

Through the National Institutes of Health and collaborating centres, ∼400 individuals with HPE spectrum disorders, endocrine disorders or craniofacial anomalies were screened for GLI2 mutations. Results were combined with all published cases. We compared the clinical and molecular features of individuals with truncating mutations to individuals with variants of unknown significance (defined as not resulting in protein truncation, reported in normal controls and/or deemed unlikely to be pathogenic by functional prediction software).

RESULTS

112 individuals with variants in GLI2 were identified, with 43 having truncating mutations. Individuals with truncating mutations were more likely to have both pituitary anomalies and polydactyly versus those with variants of unknown significance (p<0.0001 by Fisher's exact test); only 1 of 43 had frank HPE. These individuals were more likely to have recognised penetrance (polydactyly or pituitary anomalies or both) than those without truncating mutations (p=0.0036 by Fisher's exact test). A common facial phenotype was seen in individuals (with midface hypoplasia, cleft lip/palate and hypotelorism) with truncating mutations.

CONCLUSIONS

Individuals with truncating mutations in GLI2 typically present with pituitary anomalies, polydactyly and subtle facial features rather than HPE. This will be helpful in screening populations for GLI2 mutations and for counselling affected patients.

TRIAL REGISTRATION

98-HG-0249/04-HG-0093.

The teratogenic effects of prenatal ethanol exposure are exacerbated by Sonic Hedgehog or GLI2 haploinsufficiency in the mouse

Disruption of the Hedgehog signaling pathway has been implicated as an important molecular mechanism in the pathogenesis of fetal alcohol syndrome. In severe cases, the abnormalities of the face and brain that result from prenatal ethanol exposure fall within the spectrum of holoprosencephaly. Single allele mutations in the Hh pathway genes Sonic Hedgehog (SHH) and GLI2 cause holoprosencephaly with extremely variable phenotypic penetrance in humans. Here, we tested whether mutations in these genes alter the frequency or severity of ethanol-induced dysmorphology in a mouse model. Timed pregnancies were established by mating Shh^+/− or Gli2^+/− male mice backcrossed to C57BL/6J strain, with wildtype females. On gestational day 7, dams were treated with two ip doses of 2.9 g/kg ethanol (or vehicle alone), administered four hrs apart. Fetuses were then genotyped and imaged, and the severity of facial dysmorphology was assessed. Following ethanol exposure, mean dysmorphology scores were increased by 3.2- and 6.6-fold in Shh^+/− and Gli2^+/− groups, respectively, relative to their wildtype littermates. Importantly, a cohort of heterozygous fetuses exhibited phenotypes not typically produced in this model but associated with severe holoprosencephaly, including exencephaly, median cleft lip, otocephaly, and proboscis. As expected, a correlation between the severity of facial dysmorphology and medial forebrain deficiency was observed in affected animals. While Shh^+/− and Gli2^+/− mice have been described as phenotypically normal, these results illustrate a functional haploinsufficiency of both genes in combination with ethanol exposure. By demonstrating an interaction between specific genetic and environmental risk factors, this study provides important insights into the multifactorial etiology and complex pathogenesis of fetal alcohol syndrome and holoprosencephaly.

Neural crest cell signaling pathways critical to cranial bone development and pathology

Neural crest cells appear early during embryogenesis and give rise to many structures in the mature adult. In particular, a specific population of neural crest cells migrates to and populates developing cranial tissues. The ensuing differentiation of these cells via individual complex and often intersecting signaling pathways is indispensible to growth and development of the craniofacial complex. Much research has been devoted to this area of development with particular emphasis on cell signaling events required for physiologic development. Understanding such mechanisms will allow researchers to investigate ways in which they can be exploited in order to treat a multitude of diseases affecting the craniofacial complex. Knowing how these multipotent cells are driven towards distinct fates could, in due course, allow patients to receive regenerative therapies for tissues lost to a variety of pathologies. In order to realize this goal, nucleotide sequencing advances allowing snapshots of entire genomes and exomes are being utilized to identify molecular entities associated with disease states. Once identified, these entities can be validated for biological significance with other methods. A crucial next step is the integration of knowledge gleaned from observations in disease states with normal physiology to generate an explanatory model for craniofacial development. This review seeks to provide a current view of the landscape on cell signaling and fate determination of the neural crest and to provide possible avenues of approach for future research.

Fetal alcohol spectrum disorder: pathogenesis and mechanisms

This chapter provides an overview of animal model-based studies that have generated information critical to our understanding of the pathogenesis and mechanisms underlying alcohol-induced birth defects, in particular those involving the brain. Focus is placed on the developing organism itself, rather than the mother, placenta, or other extraembryonic tissues. Components of the cascades of alcohol-induced damage that are considered herein are excessive cell death, changes in the cell cycle and proliferation, cell migration, cell morphogenesis, and gene expression as well as free radical damage and interference with cell signaling. The roles played by one or more of these various factors in the genesis of structural and functional birth defects are dependent upon alcohol exposure patterns and dosage, the involved tissue, and the prenatal stage(s) at the time of exposure. Technologic advances and rapidly increasing knowledge in the fields of genetics, cell, developmental, and neurobiology are critical to accurately piecing together experimental evidence in refining our understanding of the genesis of alcohol-induced birth defects, to the planning and execution of future studies, and to applying the knowledge gained to diminish the severity or occurrence of fetal alcohol spectrum disorder.

Currarino Syndrome and HPE Microform Associated with a 2.7-Mb Deletion in 7q36.3 Excluding SHH Gene

Holoprosencephaly (HPE) is the most common forebrain defect in humans. It results from incomplete midline cleavage of the prosencephalon and can be caused by environmental and genetic factors. HPE is usually described as a continuum of brain malformations from the most severe alobar HPE to the middle interhemispheric fusion variant or syntelencephaly. A microform of HPE is limited to craniofacial features such as congenital nasal pyriform aperture stenosis and single central maxillary incisor, without brain malformation. Among the heterogeneous causes of HPE, point mutations and deletions in the SHH gene at 7q36 have been identified as well as extremely rare chromosomal rearrangements in the long-range enhancers of this gene. Here, we report a boy with an HPE microform associated with a Currarino syndrome. Array CGH detected a de novo 2.7-Mb deletion in the 7q36.3 region including the MNX1 gene, usually responsible for the Currarino triad but excluding SHH, which is just outside the deletion. This new case provides further evidence of the importance of the SHH long-range enhancers in the HPE spectrum.