Previously undescribed nonsense mutation in SHH caused autosomal dominant holoprosencephaly with wide intrafamilial variability

MYCN in human development and diseases

Somatic mutations in MYCN have been identified across various tumors, playing pivotal roles in tumorigenesis, tumor progression, and unfavorable prognoses. Despite its established notoriety as an oncogenic driver, there is a growing interest in exploring the involvement of MYCN in human development. While MYCN variants have traditionally been associated with Feingold syndrome type 1, recent discoveries highlight gain-of-function variants, specifically p.(Thr58Met) and p.(Pro60Leu), as the cause for megalencephaly-polydactyly syndrome. The elucidation of cellular and murine analytical data from both loss-of-function (Feingold syndrome model) and gain-of-function models (megalencephaly-polydactyly syndrome model) is significantly contributing to a comprehensive understanding of the physiological role of MYCN in human development and pathogenesis. This review discusses the MYCN's functional implications for human development by reviewing the clinical characteristics of these distinct syndromes, Feingold syndrome, and megalencephaly-polydactyly syndrome, providing valuable insights into the understanding of pathophysiological backgrounds of other syndromes associated with the MYCN pathway and the overall comprehension of MYCN's role in human development.

Solitary Median Maxillary Central Incisor Syndrome: An Exploration of the Pathogenic Mechanism

This study aimed to identify the genetic cause of one Chinese family with solitary median maxillary central incisor (SMMCI) and explore the relationship between genotype and its phenotype. One Chinese family with clinical diagnosis of SMMCI was collected. Single Nucleotide Polymorphism (SNP) array was performed and identified variation was confirmed by whole-genome sequencing (WGS). The reported chromosomal abnormalities and pathogenic genes in patients with SMMCI in literature were reviewed and summarized. The proband was an 8-year-old boy presenting a typical solitary median maxillary central incisor with a range of other phenotypic anomalies, including ptosis. SNP array revealed a 14.3 Mbp heterozygous deletion at chromosome 18p11.32-p11.21 in the proband but not in the unaffected parents. WGS further confirmed the identified deletion. 194 genes were involved in the chromosome region. Among them, 12 genes had been shown to be associated with diseases, including TGIF1, a reported SMMCI gene. The de novo 18p deletion resulted in SMMCI in the present study. Our results provide new genetic evidence that structural abnormality in chromosome 18p contributes to solitary median maxillary central incisor.

Molecular Bases of Human Malformation Syndromes Involving the SHH Pathway: GLIA/R Balance and Cardinal Phenotypes

Human hereditary malformation syndromes are caused by mutations in the genes of the signal transduction molecules involved in fetal development. Among them, the Sonic hedgehog (SHH) signaling pathway is the most important, and many syndromes result from its disruption. In this review, we summarize the molecular mechanisms and role in embryonic morphogenesis of the SHH pathway, then classify the phenotype of each malformation syndrome associated with mutations of major molecules in the pathway. The output of the SHH pathway is shown as GLI activity, which is generated by SHH in a concentration-dependent manner, i.e., the sum of activating form of GLI (GLIA) and repressive form of GLI (GLIR). Which gene is mutated and whether the mutation is loss-of-function or gain-of-function determine in which concentration range of SHH the imbalance occurs. In human malformation syndromes, too much or too little GLI activity produces symmetric phenotypes affecting brain size, craniofacial (midface) dysmorphism, and orientation of polydactyly with respect to the axis of the limb. The symptoms of each syndrome can be explained by the GLIA/R balance model.

A Deletion in GDF7 is Associated with a Heritable Forebrain Commissural Malformation Concurrent with Ventriculomegaly and Interhemispheric Cysts in Cats

An inherited neurologic syndrome in a family of mixed-breed Oriental cats has been characterized as forebrain commissural malformation, concurrent with ventriculomegaly and interhemispheric cysts. However, the genetic basis for this autosomal recessive syndrome in cats is unknown. Forty-three cats were genotyped on the Illumina Infinium Feline 63K iSelect DNA Array and used for analyses. Genome-wide association studies, including a sib-transmission disequilibrium test and a case-control association analysis, and homozygosity mapping, identified a critical region on cat chromosome A3. Short-read whole genome sequencing was completed for a cat trio segregating with the syndrome. A homozygous 7 bp deletion in growth differentiation factor 7 (GDF7) (c.221_227delGCCGCGC [p.Arg74Profs]) was identified in affected cats, by comparison to the 99 Lives Cat variant dataset, validated using Sanger sequencing and genotyped by fragment analyses. This variant was not identified in 192 unaffected cats in the 99 Lives dataset. The variant segregated concordantly in an extended pedigree. In mice, GDF7 mRNA is expressed within the roof plate when commissural axons initiate ventrally-directed growth. This finding emphasized the importance of GDF7 in the neurodevelopmental process in the mammalian brain. A genetic test can be developed for use by cat breeders to eradicate this variant.

Comparative Analysis of Mutational Profile of Sonic hedgehog Gene in Gallbladder Cancer

BACKGROUND

Gallbladder cancer has high incidence in northeastern India; mortality too is high as the disease is often diagnosed late. Numerous studies have shown the role of sonic hedgehog (shh) in different cancers, an important ligand of the hedgehog signaling pathway.

AIM

This study was carried out to evaluate the shh gene mutations in gallbladder cancer patients.

METHODS

PCR-SSCP was performed for shh gene in 50 samples each of gallbladder cancer, cholelithiasis, and control. The samples showing aberration in banding pattern were sequenced.

RESULTS

Variation in banding pattern was observed in 20% gallbladder cancer cases, 10% in cholelithiasis, and none of the control (χ ² = 11.111; p < 0.05). Sequencing results revealed seven novel point mutations in GBC cases. These novel mutations were found to be associated with histopathology (p < 0.05) and stage (p < 0.05) of gallbladder cancer.

CONCLUSION

This study reveals several novel individual and repetitive mutations of shh gene in GBC and cholelithiasis samples that may be used as diagnostic markers for gallbladder carcinogenesis.

Multisystem Involvement in a Patient with a PTCH1 Mutation: Clinical and Imaging Findings

In this article, we report on a Brazilian female patient born to consanguineous parents and presenting with alobar holoprosencephaly, severe eye involvement, and unusual skin hyperpigmented lesions. She was found to have a mutation (c.2240T > C; p.Val751Gly) in exon 15 of the PTCH1 gene. Mutations in this gene are associated with the nevoid basal cell carcinoma syndrome (NBCCS, OMIM 109400) and, in other instances, with holoprosencephaly (holoprosencephaly-7, OMIM 610828). Severe eye involvement ranging from orbital coloboma to microphthalmia has been seldom reported in patients with NBCCS with PTCH1 mutations. To our knowledge, this is the first report of an individual with central nervous system, skin, and eye manifestations due to a PTCH1 mutation. Mechanisms involved in these multisystem manifestations are discussed.

Solitary median maxillary central incisor syndrome (SMMCI) with congenital nasal puriform aperture stenosis: literature review and case report with comprehensive dental treatment and 14 years follow-up

BACKGROUND

Solitary median maxillary central incisor syndrome [SMMCI] is an extremely rare anomaly, especially when no other abnormalities are present. The defect is often found together with various nasal abnormalities and short stature with or without decreased levels of growth hormone. In more severe cases, SMMCI has been associated with holoprosencephaly, the CHARGE and the VACTERL association. Also, published sporadic cases have been related with rare variants of ectodermal dysplasia, chromosomal abnormalities, precocious puberty, hypothalamic hamartoma, congenital heart defects, physical/mental retardation, genital hypoplasia and ear abnormalities. For these reasons when the initial diagnosis is made by the paediatric dentist, ENT, neurological and paediatric evaluations are essential.

CASE REPORT

A 4-year-old boy with SMMCI was referred for dental treatment. Clinical/radiographic examination revealed a symmetrical primary and permanent SMMCI, a skeletal Class I and a unilateral crossbite. Medical history indicated respiratory distress and surgery soon after birth due to congenital nasal puriform aperture stenosis. Gradual orthodontic treatment started at the age of 4 years and completed at the age of 13 years. Following maxillary expansion, upper lateral segments were moved backwards and anterior space was created for accommodating a second central incisor. Retainers with a supplementary acrylic incisor were provided for aesthetic and functional replacement until the age of 16 years, when a fixed Maryland ceramic bridge was placed.

FOLLOW-UP

Two years recall, at the age of 18 years, revealed a satisfactory and stable aesthetic and functional result.

CONCLUSION

Successful dental management of SMMCI patients is possible, following a detailed long-lasting treatment plan requiring multidisciplinary paediatric dental, orthodontic and prosthetic approach.

Utilizing prospective sequence analysis of SHH, ZIC2, SIX3 and TGIF in holoprosencephaly probands to describe the parameters limiting the observed frequency of mutant gene×gene interactions

Clinical molecular diagnostic centers routinely screen SHH, ZIC2, SIX3 and TGIF for mutations that can help to explain holoprosencephaly and related brain malformations. Here we report a prospective Sanger sequence analysis of 189 unrelated probands referred to our diagnostic lab for genetic testing. We identified 28 novel unique mutations in this group (15%) and no instances of deleterious mutations in two genes in the same subject. Our result extends that of other diagnostic centers and suggests that among the aggregate 475 prospectively sequenced holoprosencephaly probands there is negligible evidence for direct gene-gene interactions among these tested genes. We model the predictions of the observed mutation frequency in the context of the hypothesis that gene×gene interactions are a prerequisite for forebrain malformations, i.e. the "multiple-hit" hypothesis. We conclude that such a direct interaction would be expected to be rare and that more subtle genetic and environmental interactions are a better explanation for the clinically observed inter- and intra-familial variability.

Solitary median maxillary central incisor in association with hemifacial microsomia: A rare case report and review of literature

Solitary median maxillary central incisor (SMMCI) is a rare dental anomaly. It is estimated to occur in 1:50,000 live births. The SMMCI tooth differs from the normal central incisor in that the crown form is symmetric and it develops and erupts precisely in the midline of the maxillary dental arch in both primary and permanent dentitions. Presence of SMMCI with hemifacial microsomia (HFM) is a very rare clinical condition. We report a case of HFM in a male of Indian origin who presented with SMMCI in both primary and permanent dentitions. The association of HFM with SMMCI may be due to defective development of neural crest cells and/or lack of space in maxilla.

Novel sonic hedgehog mutation in a couple with variable expression of holoprosencephaly

Holoprosencephaly (HPE) is the most common developmental defect of the forebrain and midface in humans. sporadic and inherited mutations in the human sonic hedgehog (SHH) gene cause 37% of familial HPE. A couple was referred to our unit with a family history of two spontaneous first trimester miscarriages and a daughter with HPE who presented early neonatal death. The father had a repaired median cleft lip, absence of central incisors, facial medial hypoplasia, and cleft palate. Intelligence and a brain CT scan were normal. Direct paternal sequencing analysis showed a novel nonsense mutation (W127X). Facial characteristics are considered as HPE microforms, and the pedigree suggested autosomal dominant inheritance with a variable expression of the phenotype. This study reinforces the importance of an exhaustive evaluation of couples with a history of miscarriages and neonatal deaths with structural defects.

Hedgehog signaling update

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

Palate morphogenesis: current understanding and future directions

In the past, most scientists conducted their inquiries of nature via inductivism, the patient accumulation of "pieces of information" in the pious hope that the sum of the parts would clarify the whole. Increasingly, modern biology employs the tools of bioinformatics and systems biology in attempts to reveal the "big picture." Most successful laboratories engaged in the pursuit of the secrets of embryonic development, particularly those whose research focus is craniofacial development, pursue a middle road where research efforts embrace, rather than abandon, what some have called the "pedestrian" qualities of inductivism, while increasingly employing modern data mining technologies. The secondary palate has provided an excellent paradigm that has enabled examination of a wide variety of developmental processes. Examination of cellular signal transduction, as it directs embryogenesis, has proven exceptionally revealing with regard to clarification of the "facts" of palatal ontogeny-at least the facts as we currently understand them. Herein, we review the most basic fundamentals of orofacial embryology and discuss how functioning of TGFbeta, BMP, Shh, and Wnt signal transduction pathways contributes to palatal morphogenesis. Our current understanding of palate medial edge epithelial differentiation is also examined. We conclude with a discussion of how the rapidly expanding field of epigenetics, particularly regulation of gene expression by miRNAs and DNA methylation, is critical to control of cell and tissue differentiation, and how examination of these epigenetic processes has already begun to provide a better understanding of, and greater appreciation for, the complexities of palatal morphogenesis.

The unfolding clinical spectrum of holoprosencephaly due to mutations in SHH, ZIC2, SIX3 and TGIF genes

Holoprosencephaly is a severe malformation of the brain characterized by abnormal formation and separation of the developing central nervous system. The prevalence is 1:250 during early embryogenesis, the live-born prevalence is 1:16 000. The etiology of HPE is extremely heterogeneous and can be teratogenic or genetic. We screened four known HPE genes in a Dutch cohort of 86 non-syndromic HPE index cases, including 53 family members. We detected 21 mutations (24.4%), 3 in SHH, 9 in ZIC2 and 9 in SIX3. Eight mutations involved amino-acid substitutions, 7 ins/del mutations, 1 frame-shift, 3 identical poly-alanine tract expansions and 2 gene deletions. Pathogenicity of mutations was presumed based on de novo character, predicted non-functionality of mutated proteins, segregation of mutations with affected family-members or combinations of these features. Two mutations were reported previously. SNP array confirmed detected deletions; one spanning the ZIC2/ZIC5 genes (approx. 100 kb) the other a 1.45 Mb deletion including SIX2/SIX3 genes. The mutation percentage (24%) is comparable with previous reports, but we detected significantly less mutations in SHH: 3.5 vs 10.7% (P=0.043) and significantly more in SIX3: 10.5 vs 4.3% (P=0.018). For TGIF1 and ZIC2 mutation the rate was in conformity with earlier reports. About half of the mutations were de novo, one was a germ line mosaic. The familial mutations displayed extensive heterogeneity in clinical manifestation. Of seven familial index patients only two parental carriers showed minor HPE signs, five were completely asymptomatic. Therefore, each novel mutation should be considered as a risk factor for clinically manifest HPE, with the caveat of reduced clinical penetrance.

Early pathogenesis of holoprosencephaly

Holoprosencephaly (HPE) is one of the most common malformations encountered in early human embryos. It is assumed that more than 90% of HPE embryos die in utero and are eliminated by spontaneous abortion. Embryonic HPE displays some characteristic craniofacial phenotypes, which are not necessarily comparable to those in postnatal HPE cases. In this article, we summarize our studies on HPE in human embryos and discuss the pathogenesis of HPE malformations.

The mutational spectrum of holoprosencephaly-associated changes within the SHH gene in humans predicts loss-of-function through either key structural alterations of the ligand or its altered synthesis

Mutations within either the SHH gene or its related pathway components are the most common, and best understood, pathogenetic changes observed in holoprosencephaly patients; this fact is consistent with the essential functions of this gene during forebrain development and patterning. Here we summarize the nature and types of deleterious sequence alterations among over one hundred distinct mutations in the SHH gene (64 novel mutations) and compare these to over a dozen mutations in disease-related Hedgehog family members IHH and DHH. This combined structural analysis suggests that dysfunction of Hedgehog signaling in human forebrain development can occur through truncations or major structural changes to the signaling domain, SHH-N, as well as due to defects in the processing of the mature ligand from its pre-pro-precursor or defective post-translation bi-lipid modifications with palmitate and cholesterol.

Unique organization of the frontonasal ectodermal zone in birds and mammals

The faces of birds and mammals exhibit remarkable morphologic diversity, but how variation arises is not well-understood. We have previously demonstrated that a region of facial ectoderm, which we named the frontonasal ectodermal zone (FEZ), regulates proximo-distal extension and dorso-ventral polarity of the upper jaw in birds. In this work, we examined the equivalent ectoderm in murine embryos and determined that the FEZ is conserved in mice. However, our results revealed that fundamental differences in the organization and constituents of the FEZ in mice and chicks may underlie the distinct growth characteristics that distinguish mammalian and avian embryos during the earliest stages of development. Finally, current models suggest that neural crest cells regulate size and shape of the upper jaw, and that signaling by Bone morphogenetic proteins (Bmps) within avian neural crest helps direct this process. Here we show that Bmp expression patterns in neural crest cells are regulated in part by signals from the FEZ. The results of our work reconcile how a conserved signaling center that patterns growth of developing face may generate morphologic diversity among different animals. Subtle changes in the organization of gene expression patterns in the FEZ could underlie morphologic variation observed among and within species, and at extremes, variation could produce disease phenotypes.

Fetal ethanol exposure activates protein kinase A and impairs Shh expression in prechordal mesendoderm cells in the pathogenesis of holoprosencephaly

BACKGROUND

In humans, fetal ethanol exposure can cause holoprosencephaly (HPE), one of the most common birth defects that is characterized by brain, facial, and oral abnormalities. However, the pathogenesis of HPE is not clear. In the present study, we investigated the teratogenic mechanism of ethanol-induced brain and facial malformations in mice.

METHODS

Pregnant C57BL/6J mice were administered ethanol on E7 and facial and brain malformations were characterized on E10.5. We examined the effect of fetal ethanol exposure on Shh expression and activation of protein kinase A (PKA) because mutations in the human Shh gene are the most frequent cause of autosomal-dominant inherited HPE and PKA is a potent endogenous antagonist of Shh signaling.

RESULTS

Fetal ethanol exposure on E7 induced severe midline defects characteristic of HPE. Ethanol exposure impaired Shh expression and induced excessive apoptosis only along the anterior edge of the prechordal mesendoderm (PME). In addition, ethanol activated PKA in anterior PME cells. Pretreatment of embryos with antioxidants, such as vitamins C or E, prevented the development of ethanol-induced HPE.

CONCLUSIONS

Shh expression in PME cells is involved in the pathogenesis of ethanol-induced HPE. Ethanol may impair Shh expression indirectly by activating PKA. The inhibition of excessive apoptosis in PME cells by antioxidants implies that oxidative stress may underlie the teratogenic actions of ethanol. Thus, antioxidant treatment may be a simple preventative measure that could reduce the incidence of HPE following fetal ethanol exposure.

Single median maxillary central incisor: new data and mutation review

BACKGROUND

Single median maxillary central incisor (SMMCI) is a rare anomaly that may occur alone or associated with other conditions, frequently as part of the holoprosencephaly (HPE) spectrum. However, it has been suggested that SMMCI alone, or associated with some midline defects, may be considered a different entity from HPE (OMIM: 147250). Families with SMMCI, without HPE cases, are difficult to counsel for the risk of HPE in future generations because the same midline defects described as part of the "SMMCI syndrome" can also be part of the HPE spectrum.

METHODS

We screened five cases of SMMCI for mutations in three HPE genes, SHH, TGIF, and SIX3.

RESULTS

A missense mutation c.686C>T was found in the gene SIX3 of one patient, which did not differ from the accepted 20% of known HPE gene mutations among all HPE cases. Our results and an extensive literature review of gene mutations in patients with SMMCI showed that 27/28 of them were in HPE genes: SHH (n = 21), SIX3 (n = 3), TGIF (n = 1), GLI2 (n = 1), and PTCH (n = 1), and only one in the SALL4 gene.

CONCLUSIONS

The clinical findings in patients with SMMCI without HPE in families with mutations in HPE genes cannot be distinguished from the findings reported in the SMMCI syndrome. Therefore, persons with SMMCI and their relatives should be carefully investigated for related midline disorders, especially of the HPE spectrum, and all known HPE genes screened.

Functional analysis of mutations in TGIF associated with holoprosencephaly

Holoprosencephaly (HPE) is the most common structural malformation of the forebrain and face in humans. Our current understanding of the pathogenesis of HPE attempts to integrate genetic susceptibility, evidenced by mutations in the known HPE genes, with the epigenetic influence of environmental factors. Mutations or deletions of the human TGIF gene have been associated with HPE in multiple population cohorts. Here we examine the functional effects of all previously reported mutations, and describe four additional variants. Of the eleven sequence variations in TGIF, all but four can be demonstrated to be functionally abnormal. In contrast, no potentially pathogenic sequence alterations were detected in the related gene TGIF2. These results provide further evidence of a role for TGIF in HPE and demonstrate the importance of functional analysis of putative disease-associated alleles.

Solitary median maxillary central incisor (SMMCI) syndrome

Solitary median maxillary central incisor syndrome (SMMCI) is a complex disorder consisting of multiple, mainly midline defects of development resulting from unknown factor(s) operating in utero about the 35th-38th day(s) from conception. It is estimated to occur in 1:50,000 live births. Aetiology is uncertain. Missense mutation in the SHH gene (I111F) at 7q36 may be associated with SMMCI. The SMMCI tooth differs from the normal central incisor, in that the crown form is symmetric; it develops and erupts precisely in the midline of the maxillary dental arch in both primary and permanent dentitions. Congenital nasal malformation (choanal atresia, midnasal stenosis or congenital pyriform aperture stenosis) is positively associated with SMMCI. The presence of an SMMCI tooth can predict associated anomalies and in particular the serious anomaly holoprosencephaly. Common congenital anomalies associated with SMMCI are: severe to mild intellectual disability, congenital heart disease, cleft lip and/or palate and less frequently, microcephaly, hypopituitarism, hypotelorism, convergent strabismus, oesophageal and duodenal atresia, cervical hemivertebrae, cervical dermoid, hypothyroidism, scoliosis, absent kidney, micropenis and ambiguous genitalia. Short stature is present in half the children. Diagnosis should be made by eight months of age, but can be made at birth and even prenatally at 18-22 weeks from the routine mid-trimester ultrasound scan. Management depends upon the individual anomalies present. Choanal stenosis requires emergency surgical treatment. Short stature may require growth hormone therapy. SMMCI tooth itself is mainly an aesthetic problem, which is ideally managed by combined orthodontic, prosthodontic and oral surgical treatment; alternatively, it can be left untreated.

Single maxillary central incisor, holoprosencephaly, and holoprosencephaly-like phenotype

Three patients--one with alobar holoprosencephaly and two with a holoprosencephaly-like phenotype--are reported with no identifiable mutations. In each case, one parent had a single maxillary central incisor (SMCI). We briefly review the holoprosencephaly-like phenotype and present a table of 25 conditions with SMCI.

New insights into craniofacial morphogenesis

No region of our anatomy more powerfully conveys our emotions nor elicits more profound reactions when disease or genetic disorders disfigure it than the face. Recent progress has been made towards defining the tissue interactions and molecular mechanisms that control craniofacial morphogenesis. Some insights have come from genetic manipulations and others from tissue recombinations and biochemical approaches, which have revealed the molecular underpinnings of facial morphogenesis. Changes in craniofacial architecture also lie at the heart of evolutionary adaptation, as new studies in fish and fowl attest. Together, these findings reveal much about molecular and tissue interactions behind craniofacial development.

Functional characterization of sonic hedgehog mutations associated with holoprosencephaly

Mutations of the developmental gene Sonic hedgehog (SHH) and alterations of SHH signaling have been associated with holoprosencephaly (HPE), a rare disorder characterized by a large spectrum of brain and craniofacial anomalies. Based on the crystal structure of mouse N-terminal and Drosophila C-terminal hedgehog proteins, we have developed three-dimensional models of the corresponding human proteins (SHH-N, SHH-C) that have allowed us to identify within these two domains crucial regions associated with HPE missense mutations. We have further characterized the functional consequences linked to 11 of these mutations. In transfected HEK293 cells, the production of the active SHH-N fragment was dramatically impaired for eight mutants (W117R, W117G, H140P, T150R, C183F, L271P, I354T, A383T). The supernatants from these cell cultures showed no significant SHH-signaling activity in a reporter cell-based assay. Two mutants (G31R, D222N) were associated with a lower production of SHH-N and signaling activity. Finally, one mutant harboring the A226T mutation displays an activity comparable with the wild-type protein. This work demonstrates that most of the HPE-associated SHH mutations analyzed have a deleterious effect on the availability of SHH-N and its biological activity. However, because of the lack of correlation between genotype and phenotype for SHH-associated mutations, our study suggests that other factors intervene in the development of the spectrum of HPE anomalies.

[Congenital malformations of the brain. 2: Malformations of the corpus callosum and holoprocencephalies]

The corpus callosum is formed between the 7th and the 20th gestational week. If this process is disrupted, partial or complete callosal agenesis may ensue. As large parts of the supra- and infratentorial brain are created during this critical period, associated anomalies need always to be searched for when callosal agenesis is present. Associations with neuro-genetic syndromes also exist. The corpus callosum is generally formed from front to back ("front-to-back rule"). Therefore, a partial callosal agenesis usually involves the posterior portion of the corpus callosum, while a secondary lesion of the corpus callosum does not follow this rule. Holoprosencephalies are a notable exception to this rule, as the frontal part of the corpus callosum is absent in spite of their classification as congenital malformations. They represent a disturbance of the differentiation and cleavage of the prosencephalon with a disruption of the separation of the cerebral hemispheres. Holoprosencephalies can be due to genetic causes, but also to intrauterine infections or other teratogenic causes. The holoprosencephalies are subdivided into alobar, semilobar and lobar holoprosencephalies. This article aims to describe the most important features of callosal agenesis and holoprosencephalies highlighting the respective imaging characteristics.