The Role of Sonic Hedgehog in Human Holoprosencephaly and Short-Rib Polydactyly Syndromes

A novel missense variant located within the zinc finger domain of the GLI3 gene was identified in a Vietnamese pedigree with index finger polydactyly

BACKGROUND

Polydactyly, particularly of the index finger, remains an intriguing anomaly for which no specific gene or locus has been definitively linked to this phenotype. In this study, we conducted an investigation of a three-generation family displaying index finger polydactyly.

METHODS

Exome sequencing was conducted on the patient, with a filtration to identify potential causal variation. Validation of the obtained variant was conducted by Sanger sequencing, encompassing all family members.

RESULTS

Exome analysis uncovered a novel heterozygous missense variant (c.1482A>T; p.Gln494His) at the zinc finger DNA-binding domain of the GLI3 protein within the proband and all affected family members. Remarkably, the variant was absent in unaffected individuals within the pedigree, underscoring its association with the polydactyly phenotype. Computational analyses revealed that GLI3 p.Gln494His impacts a residue that is highly conserved across species.

CONCLUSION

The GLI3 zinc finger DNA-binding region is an essential part of the Sonic hedgehog signaling pathway, orchestrating crucial aspects of embryonic development through the regulation of target gene expression. This novel finding not only contributes valuable insights into the molecular pathways governing polydactyly during embryonic development but also has the potential to enhance diagnostic and screening capabilities for this condition in clinical settings.

Semilobar Holoprosencephaly Caused by a Novel and De Novo ZIC2 Pathogenic Variant

Holoprosencephaly (HPE) is the most common embryonic forebrain developmental anomaly. It involves incomplete or absent division of the prosencephalon into two distinct cerebral hemispheres during the early stages of organogenesis. HPE is etiologically heterogeneous, and its clinical presentation is very variable. We report a case of a 7 month old female infant, diagnosed with non-syndromic semilobar holoprosencephaly, caused by a novel, de novo pathogenic variant in ZIC2 - one of the most commonly mutated genes in non-syndromic HPE coding for the ZIC2 transcription factor. The patient presented with microcephaly, mild facial dysmorphic features, central hypotonia and spasticity on all four extremities. Ultrasound imaging demonstrated the absence of septum pellucidum, semilobar fusion of the hemispheres and mega cisterna magna and brain MRI with confirmed the diagnosis of HPE. Early diagnosis and management are important for the prevention and treatment of complications associated with this condition.

Novel sonic hedgehog gene variant in a patient with hyponatremia, microsomia, and midline defects; phenotype description in association with a variant of unknown significance [c.755_757del p.(Phe252del)] and an approach to salt-wasting in SHH-related adrenal disorders

OBJECTIVE

To contribute a novel sonic hedgehog (SHH) gene variant in association with a novel-meagerly described phenotype and discuss SHH signaling pathway pathology.

CASE PRESENTATION

We present a 5-year-old boy with excessive hyponatremia and natriuresis, microform holoprosencephaly and microsomia, with morphologically intact hypothalamic-pituitary-adrenal (HPA) axis, and hypoaldosteronism, yet without hyperreninemia, hyperkalemia, dehydration episodes, or glucocorticoid insufficiency. Extensive workup excluded common causes of salt-wasting and revealed a novel variant of unknown significance on the sonic hedgehog (SHH) gene; NM_000193.4:c.755_757del (p.Phe252del), in heterozygosity.

CONCLUSIONS

Salt-wasting in children is predominantly caused by central nervous system lesions, renal tubular dysfunction, or adrenal insufficiency. The SHH protein is a signaling molecule, essential in embryogenesis-including HPA axis differentiation. Inactivating SHH variants disrupt the signaling pathway, leading to dysplasia or dysfunction of target organs. What's new: • We analyze the patient's phenotype in the light of this novel variant • Patient's isolated aldosterone deficiency possibly implies a selective signaling defect affecting the development of adrenal zona glomerulosa • Unexplained hyporeninemia and hypokalemia in the context of hypoaldosteronism raise questions on SHH signaling pathophysiology.

Genetic analysis and prenatal diagnosis of short-rib thoracic dysplasia 3 with or without polydactyly caused by compound heterozygous variants of DYNC2H1 gene in four Chinese families

Background: To describe the genetic variation of dynein cytoplasmic 2 heavy chain 1 (DYNC2H1) gene in four Chinese families affected with short-rib thoracic dysplasia 3 with or without polydactyly (SRTD3), and to provide evidence for accurate prenatal diagnosis and genetic counseling.

Methods: The detailed clinical prenatal sonographic features of four fetuses with SRTD3 were carried out. Trio-whole exome sequencing (WES) and proband-WES sequencing was applied to filtrated causative variants in four families. The causative variants of each family were validated in by Sanger sequencing. Bioinformation analysis was applied to predict the harmfulness of these mutations and perform the protein-protein interaction network and Gene Ontology (GO) analysis. A vitro minigene splicing assay was conducted to assess the influence of the splice site variant.

Results: Typical characterization of the four fetuses included short long bones, short ribs, narrow chest, hand and foot posture abnormalities, femur short in diameter and slightly bowing, cardiac abnormalities, and so on. Moreover, eight compound heterozygous variants of DYNC2H1 (NM_001080463.2): c.3842A>C (p.Tyr1281Ser) and c.8833-1G>A, c.8617A>G (p.Met2873Val) and c.7053_7054del (p.Cys2351Ter), c.5984C>T (p.Ala1995Val) and c.10219C>T (p.Arg3407Ter), c.5256del (p.Ala1753GlnfsTer13) and c.9737C>T (p.Thr3246Ile), were identified. Among which, c.10219C>T (p.Arg3407Terp), c.5984C>T (p.Ala1995Val) and c.9737C>T (p.Thr3246Ile) were reported in ClinVar databases, and c.8617A>G (p.Met2873Val), c.10219C>T (p.Arg3407Ter), c.5984C>T (p.Ala1995Val) were found in HGMD databases. Four variants (c.3842A>C (p.Tyr1281Ser), c.8833-1G>A, c.7053_7054del (p.Cys2351Ter) and c.5256del (p.Ala1753GlnfsTer13) were first reported as novel mutations. According to the ACMG guidelines, c.8617A>G (p.Met2873Val), c.7053_7054del (p.Cys2351Ter), c.5984C>T (p.Ala1995Val), c.10219C>T (p.Arg3407Ter) and c.5256del (p.Ala1753GlnfsTer13) were rated as pathogenic or likely pathogenic variants, others variants were predicted to be variants of uncertain significance mutations. The minigene assay results indicated that c.8833-1G>A caused the skipping over exon 56, resulting in exon 56 loss.

Conclusion: In our study, we analyzed the genetic mutations in four fetuses with SRTD3 by whole exome sequencing and identified pathogenic variants causing SRTD3. Our results expand the mutation spectrum of DYNC2H1 in SRTD3, which is helpful for the accurate prenatal diagnosis of SRTD3 fetuses and provide useful strategies for genetic counseling.

The Emerging Role of Hedgehog Signaling in Viral Infections

The hedgehog (HH) signaling pathway is one of the key pathways that is indispensable for many developmental processes and postnatal tissue homeostasis. Dysregulated HH signaling could lead to developmental disorders and tumorigenesis in a variety of tissues via inherited or sporadic mutation, gene overexpression, and crosstalk with other signaling pathways. Recently, accumulating evidence has shown that HH signaling is targeted by viruses to facilitate viral transcription, immune evasion, and uncontrolled growth, leading to effective viral replication and pathogenesis. In this study, we will summarize recent advances in functional interaction between HH signaling and different types of viruses, particularly focusing on the pathological role of HH signaling in viral infections and related diseases.

OFD1: One gene, several disorders

The OFD1 protein is necessary for the formation of primary cilia and left–right asymmetry establishment but additional functions have also been ascribed to this multitask protein. When mutated, this protein results in a variety of phenotypes ranging from multiorgan involvement, such as OFD type I (OFDI) and Joubert syndromes (JBS10), and Primary ciliary dyskinesia (PCD), to the engagement of single tissues such as in the case of retinitis pigmentosa (RP23). The inheritance pattern of these condition differs from X‐linked dominant male‐lethal (OFDI) to X‐linked recessive (JBS10, PCD, and RP23). Distinctive biological peculiarities of the protein, which can contribute to explain the extreme clinical variability and the genetic mechanisms underlying the different disorders are discussed. The extensive spectrum of clinical manifestations observed in OFD1‐mutated patients represents a paradigmatic example of the complexity of genetic diseases. The elucidation of the mechanisms underlying this complexity will expand our comprehension of inherited disorders and will improve the clinical management of patients.

Patient-derived cellular models of primary ciliopathies

Primary ciliopathies are rare inherited disorders caused by structural or functional defects in the primary cilium, a subcellular organelle present on the surface of most cells. Primary ciliopathies show considerable clinical and genetic heterogeneity, with disruption of over 100 genes causing the variable involvement of several organs, including the central nervous system, kidneys, retina, skeleton and liver. Pathogenic variants in one and the same gene may associate with a wide range of ciliopathy phenotypes, supporting the hypothesis that the individual genetic background, with potential additional variants in other ciliary genes, may contribute to a mutational load eventually determining the phenotypic manifestations of each patient. Functional studies in animal models have uncovered some of the pathophysiological mechanisms linking ciliary gene mutations to the observed phenotypes; yet, the lack of reliable human cell models has previously limited preclinical research and the development of new therapeutic strategies for primary ciliopathies. Recent technical advances in the generation of patient-derived two-dimensional (2D) and three-dimensional (3D) cellular models give a new spur to this research, allowing the study of pathomechanisms while maintaining the complexity of the genetic background of each patient, and enabling the development of innovative treatments to target specific pathways. This review provides an overview of available models for primary ciliopathies, from existing in vivo models to more recent patient-derived 2D and 3D in vitro models. We highlight the advantages of each model in understanding the functional basis of primary ciliopathies and facilitating novel regenerative medicine, gene therapy and drug testing strategies for these disorders.

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.