Concepts in Multifactorial Etiology of Developmental Disorders: Gene-Gene and Gene-Environment Interactions in Holoprosencephaly

Prenatally Diagnosed Holoprosencephaly: Review of the Literature and Practical Recommendations for Pediatric Neurologists

Holoprosencephaly (HPE) is one of the most common malformations in embryonic development. HPE represents a continuum spectrum that involves the midline cleavage of forebrain structures. Facial malformations of varying degrees of severity are also observed. It is probable that HPE results from a combination of genetic mutations and environmental influences during the initial weeks of pregnancy. Some patients with HPE experience early death, whereas others go on to experience neurodevelopmental impairment. Accurate fetal imaging can facilitate diagnosis and prenatal counseling, although more subtle brain abnormalities can be difficult to diagnose prenatally. Fetal counseling can be complex, given that the etiopathogenesis remains unclear and variable penetrance is prevalent in inherited genetic mutations. The aim of this narrative review is to examine the literature on HPE and to offer recommendations for pediatric neurologists for fetal counseling and postnatal care.

Examination of piperonyl butoxide developmental toxicity as a Sonic hedgehog pathway inhibitor targeting limb and palate morphogenesis

Piperonyl butoxide (PBO) is a pesticide synergist with widespread use and human exposure that was discovered to inhibit Sonic hedgehog (Shh) signaling, a pathway required for numerous developmental processes. Previous examinations of PBO's potential for developmental toxicity have generated seemingly conflicting results. We investigated the impact of acute PBO exposure targeting Shh pathway activity during palate and limb morphogenesis. Timed-pregnant C57BL/6 J mice were exposed to a single PBO dose (67-1800 mg/kg) at gestational day (GD) 9.75, and litters were collected at GD10.25 and GD10.75 to examine Shh pathway activity or GD17 for phenotypic assessment. PBO exposure induced dose-dependent limb malformations and cleft palate in the highest dose group. Following PBO exposure, reduced expression of the Shh pathway activity markers Gli1 and Ptch1 was observed in the embryonic limb buds and craniofacial processes. These findings provide additional evidence that prenatal PBO exposure targeting Shh pathway activity can result in malformations in mice that parallel common etiologically complex human birth defects.

Alobar Holoprosencephaly in a Newborn: A Case Report of Prenatal Diagnosis and a Review of the Literature

Holoprosencephaly (HPE) is a severe and complex congenital brain malformation caused by a defect in the midline cleavage of the prosencephalon during early embryonic development. It is the most common prosencephalic malformation in humans and is categorized into three classical forms based on the severity of this cleavage defect: alobar, semilobar, and lobar HPE. A milder interhemispheric variant, called syntelencephaly, is also considered a form of HPE. This condition may arise from genetic, environmental, or teratogenic factors. Patients with HPE often present with facial dysmorphia, with severity generally correlating with the extent of brain malformation. HPE is diagnosed prenatally through ultrasound and brain magnetic resonance imaging (MRI). Through this case report and a review of the literature, we discuss the etiopathogenic and diagnostic aspects of HPE, along with the management of this congenital malformation, illustrated by the antenatal diagnosis of a newborn with alobar HPE, confirmed by brain MRI at 32 days of age.

DISP1 deficiency: Monoallelic and biallelic variants cause a spectrum of midline craniofacial malformations

PURPOSE

DISP1 encodes a transmembrane protein that regulates the secretion of the morphogen, Sonic hedgehog, a deficiency of which is a major cause of holoprosencephaly (HPE). This disorder covers a spectrum of brain and midline craniofacial malformations. The objective of the present study was to better delineate the clinical phenotypes associated with division transporter dispatched-1 (DISP1) variants.

METHODS

This study was based on the identification of at least 1 pathogenic variant of the DISP1 gene in individuals for whom detailed clinical data were available.

RESULTS

A total of 23 DISP1 variants were identified in heterozygous, compound heterozygous or homozygous states in 25 individuals with midline craniofacial defects. Most cases were minor forms of HPE, with craniofacial features such as orofacial cleft, solitary median maxillary central incisor, and congenital nasal pyriform aperture stenosis. These individuals had either monoallelic loss-of-function variants or biallelic missense variants in DISP1. In individuals with severe HPE, the DISP1 variants were commonly found associated with a variant in another HPE-linked gene (ie, oligogenic inheritance).

CONCLUSION

The genetic findings we have acquired demonstrate a significant involvement of DISP1 variants in the phenotypic spectrum of midline defects. This underlines its importance as a crucial element in the efficient secretion of Sonic hedgehog. We also demonstrated that the very rare solitary median maxillary central incisor and congenital nasal pyriform aperture stenosis combination is part of the DISP1-related phenotype. The present study highlights the clinical risks to be flagged up during genetic counseling after the discovery of a pathogenic DISP1 variant.

Whole-genome sequencing analysis in fetal structural anomalies: novel phenotype-genotype discoveries

OBJECTIVES

The identification of structural variants and single-nucleotide variants is essential in finding molecular etiologies of monogenic genetic disorders. Whole-genome sequencing (WGS) is becoming more widespread in genetic disease diagnosis. However, data on its clinical utility remain limited in prenatal practice. We aimed to expand our understanding of implementing WGS in the genetic diagnosis of fetal structural anomalies.

METHODS

We employed trio WGS with a minimum coverage of 40× on the MGI DNBSEQ-T7 platform in a cohort of 17 fetuses presenting with aberrations detected by ultrasound, but uninformative findings of standard chromosomal microarray analysis (CMA) and exome sequencing (ES).

RESULTS

Causative genetic variants were identified in two families, with an increased diagnostic yield of 11.8% (2/17). Both were exon-level copy-number variants of small size (3.03 kb and 5.16 kb) and beyond the detection thresholds of CMA and ES. Moreover, to the best of our knowledge, we have described the first prenatal instance of the association of FGF8 with holoprosencephaly and facial deformities.

CONCLUSIONS

Our analysis demonstrates the clinical value of WGS in the diagnosis of the underlying etiology of fetuses with structural abnormalities, when routine genetic tests have failed to provide a diagnosis. Additionally, the novel variants and new fetal manifestations have expanded the mutational and phenotypic spectrums of BBS9 and FGF8. © 2023 International Society of Ultrasound in Obstetrics and Gynecology.

Impact of Sonic Hedgehog-dependent sphenoid bone defect on craniofacial growth

OBJECTIVES

The main objective of this study was to evaluate how an apparently minor anomaly of the sphenoid bone, observed in a haploinsufficient mouse model for Sonic Hedgehog (Shh), affects the growth of the adult craniofacial region. This study aims to provide valuable information to orthodontists when making decisions regarding individuals carrying SHH mutation.

MATERIALS AND METHODS

The skulls of embryonic, juvenile and adult mice of two genotypes (Shh heterozygous and wild type) were examined and measured using landmark-based linear dimensions. Additionally, we analysed the clinical characteristics of a group of patients and their relatives with SHH gene mutations.

RESULTS

In the viable Shh+/ - mouse model, bred on a C57BL/6J background, we noted the presence of a persistent foramen at the midline of the basisphenoid bone. This particular anomaly was attributed to the existence of an ectopic pituitary gland. We discovered that this anomaly led to premature closure of the intrasphenoidal synchondrosis and contributed to craniofacial deformities in adult mice, including a longitudinally shortened skull base. This developmental anomaly is reminiscent of that commonly observed in human holoprosencephaly, a disorder resulting from a deficiency in SHH activity. However, sphenoid morphogenesis is not currently monitored in individuals carrying SHH mutations.

CONCLUSION

Haploinsufficiency of Shh leads to isolated craniofacial skeletal hypoplasia in adult mouse. This finding highlights the importance of radiographic monitoring of the skull base in all individuals with SHH gene mutations.

Impact of prenatal exposure to metallic elements on neural tube defects: Insights from human investigations

Metallic elements play a pivotal role in maternal and fetal health. Metals can cross the placental barrier and be absorbed by fetuses, where they may affect closure of the neural tube during embryonic development. Neural tube defects (NTDs), which result from aberrant closure of the neural tube three to four weeks post-conception, have a multifactorial and complex etiology that combines genetic variants and environmental exposure. Recent advances in population-level association studies have investigated the link between maternal environmental exposure and NTDs, particularly the influence of metals on the incidence of NTDs. Herein, we present a broad and qualitative review of current literature on the association between maternal and prenatal metal exposure via the maternal peripheral blood, amniotic fluid, placenta, umbilical cord, and maternal hair, and the risk of developing NTDs. Specifically, we identify the various aggravating or attenuating effects of metallic exposure on the risk of NTD formation. This review provides novel insights into the association between environmental metals and NTDs and has important applications for NTD prevention and mitigating environmental exposure to metals.

Gene-environment interactions in birth defect etiology: Challenges and opportunities

Birth defects are relatively common congenital outcomes that significantly impact affected individuals, their families, and communities. Effective development and deployment of prevention and therapeutic strategies for these conditions requires sufficient understanding of etiology, including underlying genetic and environmental causes. Tremendous progress has been made in defining the genetic basis of familial and syndromic forms of birth defects. However, the majority of birth defect cases are considered nonsyndromic and thought to result from multifactorial gene-environment interactions. While substantial advances have been made in elucidating the genetic landscape of these etiologically complex conditions, significant biological and technical constraints have stymied progress toward a refined knowledge of environmental risk factors. Defining specific gene-environment interactions in birth defect etiology is even more challenging. However, progress has been made, including demonstration of critical proofs of concept and development of new conceptual and technical approaches for resolving complex gene-environment interactions. In this review, we discuss current views of multifactorial birth defect etiology, comparing them with other diseases that also involve gene-environment interactions, including primary immunodeficiency and cancer. We describe how various model systems have illuminated mechanisms of multifactorial etiology and these models' individual strengths and weaknesses. Finally, suggestions for areas of future emphasis are proposed.

Ecce Homo: Moving past labels to lives

OBJECTIVES

We report a contemporary individual who died with advanced holoprosencephaly (HPE) to encourage recognition of rare diseases (RDs), especially congenital conditions in archaeological samples. We also explore the range of conditions associated with hydrocephalus in support of nuanced interpretations of this disease.

MATERIALS

The skeleton of a 17-year-old male who died with clinically diagnosed HPE, along with an age and sex matched comparative sample of 6 individuals who suffered accidental death and who were normal.

METHODS

We observed and measured all remains using standard osteological methods. The clinical records for Ecce Homo were scrutinized; his family was interviewed, and his skull was X-rayed.

RESULTS

The morphology and morphometry of Ecce Homo's skeleton display irregularities along the cranial midline and the postcranial skeleton consistent with anomalies derived from HPE as well as related congenital disorders and physical anomalies.

CONCLUSIONS

We have reported HPE here and developed a differential diagnosis with closely related conditions. Likewise we relate the information generated in the clinical history and interviews with the family of Ecce Homo to facilitate an understanding of the social context.

SIGNIFICANCE

This case is exceptional in providing information from the life context of a contemporary individual who suffered from a rare disease (HPE), with skeletal remains could be studied extensively. The differential diagnosis is useful in identifying HPE and other closely related conditions.

LIMITATIONS

This is a single example with clinical intervention.

SUGGESTIONS FOR FURTHER RESEARCH

Future osteological research should occur on other cases of HPE; molecular studies may offer further clarity.

Promoting Oligodendrocyte Differentiation from Human Induced Pluripotent Stem Cells by Activating Endocannabinoid Signaling for Treating Spinal Cord Injury

Transplantation of oligodendrocyte progenitor cell (OPC) at the injury site is being developed as a potential therapeutic strategy for promoting remyelination and locomotor function recovery after spinal cord injury (SCI). To this end, the development of expandable and functional human OPCs is crucial for testing their efficacy in SCI. In mice and rats, the endocannabinoid signaling system is crucial for the survival, differentiation, and maturation of OPCs. Similar studies in humans are lacking currently. Endocannabinoids and exogenous cannabinoids exert their effects mainly via cannabinoid receptors (CB1R and CB2R). We demonstrated that these receptors were differentially expressed in iPSC-derived human NSCs and OPCs, and they could be activated by WIN55212-2 (WIN), a potent CB1R/CB2R agonist to upregulate the endocannabinoid signaling during glial induction. WIN primed NSCs generated more OLIG2 + glial progenitors and migratory PDGFRα + OPC in a CB1/CB2 dependent manner compared to unprimed NSCs. Furthermore, WIN-induced OPCs (WIN-OPCs) robustly differentiated into functional oligodendrocytes and myelinate in vitro and in vivo in a mouse spinal cord injury model. RNA-Seq revealed that WIN upregulated the biological process of positive regulation of oligodendrocyte differentiation. Mechanistically, WIN could act as a partial smoothed (SMO) inhibitor or activate CB1/CB2 to form heteromeric complexes with SMO leading to the inhibition of GLI1 in the Sonic hedgehog pathway. The partial and temporal inhibition of GLI1 during glial induction is shown to promote OPCs that differentiate faster than control's. Thus, CB1R/CB2R activation results in more efficient generation of OPCs that can mature and efficiently myelinate.