Somatic and de novo Germline Variants of MEDs in Human Neural Tube Defects

Manifold functions of Mediator complex in neurodevelopmental disorders

Neurodevelopmental disorders (NDDs) encompass a diverse range of impairments affecting brain development and functions, often presenting as deficits in motor skills, cognitive abilities, language development and neuropsychiatric health. The emergence of next-generation sequencing has unveiled numerous genetic variants linked to NDDs, implicating molecular pathways involved in essential neuronal processes such as synaptic plasticity, neuronal architecture and proteostasis. Central to these processes is the Mediator complex, a highly conserved multi-subunit assembly crucial for RNA polymerase II (Pol II)-dependent transcription. The Mediator functions as a key regulator of gene expression, playing a pivotal role in coordinating cellular processes essential for neuronal differentiation and developmental signaling cascades. Increasingly evidence has shown that its dysfunction is highly associated with the pathogenesis of NDDs. This review aims to comprehensively examine the structural and functional characteristics of individual mediator subunits. We will focus on clinical case reports and recent preclinical studies that highlight the connection between genetic abnormalities in the Mediator complex and specific neurodevelopmental phenotypes, ultimately guiding the development of enhanced diagnostic tools and therapeutic interventions. Furthermore, this review will advance our understanding of the general role transcriptional regulation plays in the etiology of NDDs.

Human organoids potentially boost research into environmental factors of neural tube defects

Human neural tube closure occurs during the third to fourth gestational week, often before people realize they are pregnant. Ethical issues limit collection of embryonic human neural tube tissue. However, the development of human neural tube organoids is beginning to empower the study of neural tube closure and neural tube defects. A previous review summarized human neural tube organoid models which are grown on top of or embedded in Matrigel or Hydrogel. Recent advances in human neural tube organoid models through micropatterned or microfluidic methods recapitulate diverse and complex neural tube features. In this review, our goal is to summarize these human iPSC-derived advanced organoid models. Moreover, these organoid models provide the possibility of testing how environmental factors influence the process of neural tube closure. Focusing on folic acid supplementation which can reduce the prevalence of neural tube defects, we review experimental evidence for three molecular mechanisms of folic acid function. Our perspective is to boost research on the impacts of environmental factors on reducing the risk of neural tube defects by utilizing human neural tube organoid models.

Novel insights into the phenotypic spectrum and pathogenesis of Hardikar syndrome

PURPOSE

Hardikar syndrome (HS, MIM #301068) is a female-specific multiple congenital anomaly syndrome characterized by retinopathy, orofacial clefting, aortic coarctation, biliary dysgenesis, genitourinary malformations, and intestinal malrotation. We previously showed that heterozygous nonsense and frameshift variants in MED12 cause HS. The phenotypic spectrum of disease and the mechanism by which MED12 variants cause disease is unknown. We aim to expand the phenotypic and molecular landscape of HS and elucidate the mechanism by which MED12 variants cause disease.

METHODS

We clinically assembled and molecularly characterized a cohort of 11 previously unreported individuals with HS. Additionally, we studied the effect of MED12 deficiency on ciliary biology, hedgehog, and yes-associated protein (YAP) signaling; pathways implicated in diseases with phenotypic overlap with HS.

RESULTS

We report novel phenotypes associated with HS, including cardiomyopathy, arrhythmia, and vascular anomalies, and expand the molecular landscape of HS to include splice site variants. We additionally demonstrate that MED12 deficiency causes decreased cell ciliation, and impairs hedgehog and YAP signaling.

CONCLUSION

Our data support updating HS standard-of-care to include regular cardiac imaging, arrhythmia screening, and vascular imaging. We further propose that dysregulation of ciliogenesis and YAP and hedgehog signaling contributes to the pathogenesis of HS.

Activation of lipophagy ameliorates cadmium-induced neural tube defects via reducing low density lipoprotein cholesterol levels in mouse placentas

Neural tube defects (NTDs) represent a prevalent and severe category of congenital anomalies in humans. Cadmium (Cd) is an environmental teratogen known to cause fetal NTDs. However, its underlying mechanisms remain elusive. This study aims to investigate the therapeutic potential of lipophagy in the treatment of NTDs, providing valuable insights for future strategies targeting lipophagy activation as a means to mitigate NTDs.We successfully modeled NTDs by Cd exposure during pregnancy. RNA sequencing was employed to investigate the transcriptomic alterations and functional enrichment of differentially expressed genes in NTD placental tissues. Subsequently, pharmacological/genetic (Atg5-/- placentas) experiments confirmed that inducing placental lipophagy can alleviate Cd induced-NTDs. We found that Cd exposure caused NTDs. Further analyzed transcriptomic data from the placentas with NTDs which revealed significant downregulation of low-density lipoprotein receptor associated protein 1(Lrp1) gene expression responsible for positive regulation of low-density lipoprotein cholesterol (LDL-C) transport. Correspondingly, there was an increase in maternal serum/placenta/amniotic fluid LDL-C content. Subsequently, we have discovered that Cd exposure activated placental lipophagy. Pharmacological/genetic (Atg5-/- placentas) experiments confirmed that inducing placental lipophagy can alleviate Cd induced-NTDs. Furthermore, our findings demonstrate that activation of placental lipophagy effectively counteracts the Cd-induced elevation in LDL-C levels. Lipophagy serves to mitigate Cd-induced NTDs by reducing LDL-C levels within mouse placentas.

In vivo high-content imaging and regression analysis reveal non-cell autonomous functions of Shroom3 during neural tube closure

During neural tube closure, neural ectoderm cells constrict their apical surfaces to bend and fold the tissue into a tube that will become the central nervous system. Recent data from mice and humans with neural tube defects suggest that key genes required for neural tube closure can exert non-cell autonomous effects on cell behavior, but the nature of these effects remains obscure. Here, we coupled tissue-scale, high-resolution time-lapse imaging of the closing neural tube of Xenopus to multivariate regression modeling, and we show that medial actin accumulation drives apical constriction non-autonomously in neighborhoods of cells, rather than solely in individual cells. To further explore this effect, we examined mosaic crispant embryos and identified both autonomous and non-autonomous effects of the apical constriction protein Shroom3.

Pathogenesis of neural tube defects: The regulation and disruption of cellular processes underlying neural tube closure

Neural tube closure (NTC) is crucial for proper development of the brain and spinal cord and requires precise morphogenesis from a sheet of cells to an intact three-dimensional structure. NTC is dependent on successful regulation of hundreds of genes, a myriad of signaling pathways, concentration gradients, and is influenced by epigenetic and environmental cues. Failure of NTC is termed a neural tube defect (NTD) and is a leading class of congenital defects in the United States and worldwide. Though NTDs are all defined as incomplete closure of the neural tube, the pathogenesis of an NTD determines the type, severity, positioning, and accompanying phenotypes. In this review, we survey pathogenesis of NTDs relating to disruption of cellular processes arising from genetic mutations, altered epigenetic regulation, and environmental influences by micronutrients and maternal condition. This article is categorized under: Congenital Diseases > Genetics/Genomics/Epigenetics Neurological Diseases > Genetics/Genomics/Epigenetics Neurological Diseases > Stem Cells and Development.

Spina Bifida: A Review of the Genetics, Pathophysiology and Emerging Cellular Therapies

Spina bifida is the most common congenital defect of the central nervous system which can portend lifelong disability to those afflicted. While the complete underpinnings of this disease are yet to be fully understood, there have been great advances in the genetic and molecular underpinnings of this disease. Moreover, the treatment for spina bifida has made great advancements, from surgical closure of the defect after birth to the now state-of-the-art intrauterine repair. This review will touch upon the genetics, embryology, and pathophysiology and conclude with a discussion on current therapy, as well as the first FDA-approved clinical trial utilizing stem cells as treatment for spina bifida.

Eph and Ephrin Variants in Malaysian Neural Tube Defect Families

Neural tube defects (NTDs) are common birth defects with a complex genetic etiology. Mouse genetic models have indicated a number of candidate genes, of which functional mutations in some have been found in human NTDs, usually in a heterozygous state. This study focuses on Ephs-ephrins as candidate genes of interest owing to growing evidence of the role of this gene family during neural tube closure in mouse models. Eph-ephrin genes were analyzed in 31 Malaysian individuals comprising seven individuals with sporadic spina bifida, 13 parents, one twin-sibling and 10 unrelated controls. Whole exome sequencing analysis and bioinformatic analysis were performed to identify variants in 22 known Eph-ephrin genes. We reported that three out of seven spina bifida probands and three out of thirteen family members carried a variant in either EPHA2 (rs147977279), EPHB6 (rs780569137) or EFNB1 (rs772228172). Analysis of public databases shows that these variants are rare. In exome datasets of the probands and parents of the probands with Eph-ephrin variants, the genotypes of spina bifida-related genes were compared to investigate the probability of the gene–gene interaction in relation to environmental risk factors. We report the presence of Eph-ephrin gene variants that are prevalent in a small cohort of spina bifida patients in Malaysian families.

MED13L-related intellectual disability due to paternal germinal mosaicism

The MED13L-related intellectual disability or MRFACD syndrome (Mental retardation and distinctive facial features with or without cardiac defects; MIM # 616789) is one of the most common forms of syndromic intellectual disability with about a hundred cases reported so far. Affected individuals share overlapping features comprising intellectual disability, hypotonia, motor delay, remarkable speech delay, and a recognizable facial gestalt. De novo disruption of the MED13L gene by deletions, duplications, or sequence variants has been identified as deleterious. Siblings affected by intragenic deletion transmitted from a mosaic parent have been reported once in the literature. We now present the first case of paternal germinal mosaicism for a missense MED13L variant causing MRFACD syndrome in one of the father's children and being the likely cause of intellectual disability and facial dysmorphism in the other. As part of the Mediator complex, the MED proteins have an essential role in regulating transcription. Thirty-two subunits of the Mediator complex genes have been linked to congenital malformations that are now acknowledged as transcriptomopathies. The MRFACD syndrome has been suggested to represent a recognizable phenotype.

Unraveling the complex genetics of neural tube defects: From biological models to human genomics and back

Neural tube defects (NTDs) are a classic example of preventable birth defects for which there is a proven-effective intervention, folic acid (FA); however, further methods of prevention remain unrealized. In the decades following implementation of FA nutritional fortification programs throughout at least 87 nations, it has become apparent that not all NTDs can be prevented by FA. In the United States, FA fortification only reduced NTD rates by 28-35% (Williams et al., 2015). As such, it is imperative that further work is performed to understand the risk factors associated with NTDs and their underlying mechanisms so that alternative prevention strategies can be developed. However, this is complicated by the sheer number of genes associated with neural tube development, the heterogeneity of observable phenotypes in human cases, the rareness of the disease, and the myriad of environmental factors associated with NTD risk. Given the complex genetic architecture underlying NTD pathology and the way in which that architecture interacts dynamically with environmental factors, further prevention initiatives will undoubtedly require precision medicine strategies that utilize the power of human genomics and modern tools for assessing genetic risk factors. Herein, we review recent advances in genomic strategies for discovering genetic variants associated with these defects, and new ways in which biological models, such as mice and cell culture-derived organoids, are leveraged to assess mechanistic functionality, the way these variants interact with other genetic or environmental factors, and their ultimate contribution to human NTD risk.