Microcephaly, an etiopathogenic vision

SMPD4-mediated sphingolipid metabolism regulates brain and primary cilia development

Genetic variants in multiple sphingolipid biosynthesis genes cause human brain disorders. A recent study looked at people from 12 unrelated families with variants in the gene SMPD4, a neutral sphingomyelinase that metabolizes sphingomyelin into ceramide at an early stage of the biosynthesis pathway. These individuals have severe developmental brain malformations, including microcephaly and cerebellar hypoplasia. The disease mechanism of SMPD4 was not known and so we pursued a new mouse model. We hypothesized that the role of SMPD4 in producing ceramide is important for making primary cilia, a crucial organelle mediating cellular signaling. We found that the mouse model has cerebellar hypoplasia due to failure of Purkinje cell development. Human induced pluripotent stem cells lacking SMPD4 exhibit neural progenitor cell death and have shortened primary cilia, which is rescued by adding exogenous ceramide. SMPD4 production of ceramide is crucial for human brain development.

An Association Between Fetal Subarachnoid Space and Various Pathologies Using MR Imaging

Background/Objectives: This study aimed to explore a relationship between the fetal subarachnoid space (SAS) width and various fetal pathologies, employing fetal brain MRI scans. Methods: A retrospective collection of fetal brain MRI scans of 78 fetuses was performed with sonographic indications of microcephaly, macrocephaly, or fetal growth restriction (FGR), during a 7-year period at a single tertiary center. The SAS width (named the SAS index) was manually measured in millimeters in ten specific anatomical locations (four in the axial plane and six in the coronal plane), and then converted to centiles by comparing it to (previously collected) data of apparently healthy fetuses. We evaluated the median SAS centiles using the Kruskal-Wallis and Mann-Whitney U tests for statistical comparison. Results: Seventy-eight subjects (mean gestational age of MRI scan 34.2 ± 2.2 weeks) were evaluated. The median SAS centiles were consistently higher in the macrocephaly group compared to the microcephaly group in all ten anatomical locations (statistically significant except coronal left inferior temporal gyri). Most pronounced difference was displayed in the insula gyri (axial and coronal). The median SAS centiles were higher in the microcephaly group when compared with FGR across all ten anatomical locations (all were statistically significant except for coronal frontal and insula gyri), and the maximal difference was found in the frontal gyri of both planes. The median SAS indexes (IQR) of the three groups in millimeters: macrocephaly 91.55 (86.35-101.05), microcephaly 59.46 (50.00-66.91), and FGR 53.21 (49.71-59.10), p < 0.001. Conclusions: We found a statistically significant association between the fetal subarachnoid space and various fetal pathologies: macrocephaly, microcephaly, and FGR.

Malformations of Cortical Development: Updated Imaging Review

Malformations of cortical development (MCD) are structural anomalies that disrupt the normal process of cortical development. Patients with these anomalies frequently present with seizures, developmental delay, neurologic deficits, and cognitive impairment, resulting in a wide spectrum of neurologic outcomes. The severity and type of malformation, in addition to the genetic pathways of brain development involved, contribute to the observed variability. While neuroimaging plays a central role in identifying congenital anomalies in vivo, the precise definition and classification of cortical developmental defects have undergone significant transformations in recent years due to advances in molecular and genetic knowledge. The authors provide a concise overview of embryologic brain development, recently standardized nomenclature, and the categorization system for abnormalities in cortical development, offering valuable insights into the interpretation of their neuroradiologic patterns. ©RSNA, 2024 Supplemental material is available for this article. The slide presentation from the RSNA Annual Meeting is available for this article.

HiPSC-derived 3D neural models reveal neurodevelopmental pathomechanisms of the Cockayne Syndrome B

Cockayne Syndrome B (CSB) is a hereditary multiorgan syndrome which-through largely unknown mechanisms-can affect the brain where it clinically presents with microcephaly, intellectual disability and demyelination. Using human induced pluripotent stem cell (hiPSC)-derived neural 3D models generated from CSB patient-derived and isogenic control lines, we here provide explanations for these three major neuropathological phenotypes. In our models, CSB deficiency is associated with (i) impaired cellular migration due to defective autophagy as an explanation for clinical microcephaly; (ii) altered neuronal network functionality and neurotransmitter GABA levels, which is suggestive of a disturbed GABA switch that likely impairs brain circuit formation and ultimately causes intellectual disability; and (iii) impaired oligodendrocyte maturation as a possible cause of the demyelination observed in children with CSB. Of note, the impaired migration and oligodendrocyte maturation could both be partially rescued by pharmacological HDAC inhibition.

Assessment of toxicity of pyriproxyfen, Bacillus thuringiensis, and malathion and their mixtures used for mosquito control on embryo-larval development and behavior of zebrafish

Pyriproxyfen (PPF), Bacillus thuringiensis israelensis (BTI), and malathion (MLT) are widely used worldwide to control the population of mosquitos that transmit arboviruses. The current work aimed to evaluate the toxicity of these single pesticides and their binary mixtures of PPF + BTI, PPF + MLT, and MLT + BTI on the embryo-larval stage of zebrafish (Danio rerio) as an animal model. Epiboly, mortality, apical endpoints, affected animals, heart rate, morphometric, thigmotaxis, touch sensitivity, and optomotor response tests were evaluated. PPF and MLT and all mixtures reduced the epiboly percentage. Mortality increased significantly in all exposed groups, except BTI, with MLT being the most toxic. The observed apical endpoints were pericardial and yolk sac edemas, and tail and spine deformation. Exposure to MLT showed a higher percentage of affected animals. A reduction in heart rate was also observed in MLT- and PPF + MLT-exposed groups. The PPF + MLT mixture decreased head measurements. Behavioral alterations were observed, with a decrease in thigmotaxis and touch sensitivity responses in PPF + MLT and MLT + BTI groups. Finally, optomotor responses were affected in all groups. The above data obtained suggest that the MLT + PFF mixture has the greatest toxicity effects. This mixture affected embryo-larval development and behavior and is close to the reality in several cities that use both pesticides for mosquito control rather than single pesticides, leading to a reevaluation of the strategy for mosquito control.

Developmental toxicity of pyriproxyfen induces changes in the ultrastructure of neural cells and in the process of skull ossification

Some studies relate the use of pyriproxyfen (PPF) in drinking water with damage to embryonic neurodevelopment, including a supposed association with cases of microcephaly. However, the effects on neural cells and skull ossification in embryos remain unclear. This study aims to investigate the effects of PPF on the structure and ultrastructure of brain cells and its influence on the skull ossification process during embryonic development. Chicken embryos, used as an experimental model, were exposed to concentrations of 0.01 and 10 mg/l PPF at E1. The findings demonstrated that PPF led to notable ultrastructural alterations such as reduced cilia and microvilli of ependymal cells and damage to mitochondria, endoplasmic reticulum, Golgi bodies, and cell membranes in neural cells. The frequency of changes and the degree of these cell damage between the forebrain and midbrain were similar. PPF induced a reduction in fox3 transcript levels, specific for differentiation of neurons, and a reduction in the NeuN protein content related to mature neurons and dendritic branches. PPF impacted the ossification process of the skull, as evidenced by the increase in the ossified area and the decrease in inter-bone spacing. In conclusion, this study highlights the ability of PPF to affect neurodevelopmental processes by inducing ultrastructural damage to neural cells, concomitant with a reduction in NeuN and fox3 expression. This detrimental impact coupled with deficiencies in skull ossification can prevent the proper growth and development of the brain.

Prenatal evaluation of genetic variants in fetuses with small head circumference: A single-center retrospective study

OBJECTIVES

To comprehensively evaluate the contributions of numerical chromosomal abnormality, copy number variant (CNV), and sequence variant (SV) to fetuses with small head circumference in a Chinese cohort using chromosome microarray analysis and whole exome sequencing.

METHODS

A total of 157 fetuses with small heads defined as head circumference < - 2 standard deviation (SD) were recruited between October 2014 and March 2023. We used the ultrasonic measurement parameter Z-score to define small head as possible microcephaly (3 < Z ≤ -2), microcephaly (-5 < Z ≤ -3), or pathologic microcephaly (Z ≤ -5). Ultrasound findings and genetic results were analyzed.

RESULTS

The overall diagnostic yield of chromosomal abnormalities by microarray analysis was 13 %. Whole exome sequencing revealed eight novel variants and two interesting candidate genes and provided a 25.4 % incremental yield compared with microarray analysis. Of the detected SVs, 56 % were de novo and the most common inheritance pattern was autosomal dominant inheritance presented in 11/16 fetuses. Compared with isolated small heads, non-isolated small heads had a significantly higher detection rate of chromosomal abnormalities (16 % vs. 3.0 %, P = 0.049) but not SVs (24 % vs. 5.5 %, P = 0.126). Subgroup analysis showed that intracranial anomalies had a similar high detection rate of SVs in fetuses with all small heads subgroups while no chromosomal abnormalities and causative SVs were found in fetuses with isolated possible microcephaly.

CONCLUSIONS

Ultrasound findings of small fetal head circumference < 3 SD below the mean, especially those with intracranial structural abnormalities, indicate the need for genetic counseling. Genetic variants, mainly copy number variants and SV, may be responsible for the substantial proportion of small fetal head circumference, while most are de novo. Whole exome sequencing and microarray analysis are effective diagnostic approaches for this population.

Second report of TEDC1-related microcephaly caused by a novel biallelic mutation in an Iranian consanguineous family

BACKGROUND

Primary autosomal recessive microcephaly (MCPH) is a rare developmental disorder characterized by cognitive impairment, delayed neurodevelopment, and reduced brain size. It is a genetically heterogeneous condition, and several genes have been identified as associated with MCPH.

METHODS AND RESULTS

In this study, we utilized whole-exome sequencing (WES) to identify disease-causing variations in two brothers from an Iranian family affected by MCPH, who had consanguineous parents. In the patients, we detected a novel homozygous missense mutation (c.806A > G, p.Gln269Arg) in the TEDC1 gene in one of the patients. Co-segregation analysis using Sanger sequencing confirmed that this variant was inherited from parents. The identified variant was evaluated for its pathogenicity and novelty using various databases. Additionally, bioinformatics tools were employed to predict the three-dimensional structure of the mutant TEDC1 protein.

CONCLUSIONS

This study presents the second documented report of a mutation in the TEDC1 gene associated with MCPH. The identification of this novel biallelic mutation as a causative factor for MCPH in the proband further underscores the utility of genetic testing techniques, such as WES, as reliable diagnostic tools for individuals with this condition.

SMPD4 mediated sphingolipid metabolism regulates brain and primary cilia development

Genetic variants in multiple sphingolipid biosynthesis genes cause human brain disorders. A recent study collected patients from twelve unrelated families with variants in the gene SMPD4 , a neutral sphingomyelinase which metabolizes sphingomyelin into ceramide at an early stage of the biosynthesis pathway. These patients have severe developmental brain malformations including microcephaly and cerebellar hypoplasia. However, the mechanism of SMPD4 was not known and we pursued a new mouse model. We hypothesized that the role of SMPD4 in producing ceramide is important for making primary cilia, a crucial organelle mediating cellular signaling. We found that the mouse model has cerebellar hypoplasia due to failure of Purkinje cell development. Human induced pluripotent stem cells exhibit neural progenitor cell death and have shortened primary cilia which is rescued by adding exogenous ceramide. SMPD4 production of ceramide is crucial for human brain development.

DNA damage and repair: underlying mechanisms leading to microcephaly

DNA-damaging agents and endogenous DNA damage constantly harm genome integrity. Under genotoxic stress conditions, the DNA damage response (DDR) machinery is crucial in repairing lesions and preventing mutations in the basic structure of the DNA. Different repair pathways are implicated in the resolution of such lesions. For instance, the non-homologous DNA end joining and homologous recombination pathways are central cellular mechanisms by which eukaryotic cells maintain genome integrity. However, defects in these pathways are often associated with neurological disorders, indicating the pivotal role of DDR in normal brain development. Moreover, the brain is the most sensitive organ affected by DNA-damaging agents compared to other tissues during the prenatal period. The accumulation of lesions is believed to induce cell death, reduce proliferation and premature differentiation of neural stem and progenitor cells, and reduce brain size (microcephaly). Microcephaly is mainly caused by genetic mutations, especially genes encoding proteins involved in centrosomes and DNA repair pathways. However, it can also be induced by exposure to ionizing radiation and intrauterine infections such as the Zika virus. This review explains mammalian cortical development and the major DNA repair pathways that may lead to microcephaly when impaired. Next, we discuss the mechanisms and possible exposures leading to DNA damage and p53 hyperactivation culminating in microcephaly.

Microcephaly and Chorioretinopathy Relevance as a Differential Diagnosis

Microcephaly and chorioretinopathy are genetic disorders that are inherited in an autosomal recessive manner. The most frequent ocular manifestation is the presence of lacunar atrophy in the retina and choroid. The diagnosis of this condition can be challenging as several potential causes and related syndromes need to be ruled out. We present two cases of microcephaly and chorioretinopathy in Mexican patients, their clinical characterization, and discuss the differential diagnoses that should be considered. An 8-year-old girl was examined due to a history of decreased vision in both eyes. Fundus examination showed excavated, well-defined, sectorial, bilateral, and symmetrical areas of chorioretinal atrophy. An 18-year-old male had a history of poor vision since childhood. Previous ophthalmological examinations reported bilateral symmetric chorioretinal atrophy with pigment accumulation. Both patients had a prior diagnosis of microcephaly and language delay. Blood tests and a comprehensive systemic evaluation ruled out intrauterine infections. The electroretinogram showed decreased amplitude and increased implicit time in the photopic and scotopic responses. Genetic tests revealed mutations in the TUBGCP4 gene, leading to a diagnosis of microcephaly and chorioretinopathy. As observed in these cases, there was variability in retinal lesions. The presence of chorioretinal lacunae and genetic testing can help to correctly diagnose this disorder.

An integrative systems biology strategy to support the development of adverse outcome pathways (AOPs): a case study on radiation-induced microcephaly

Adverse Outcome Pathways (AOPs) are useful tools for assessing the potential risks associated with exposure to various stressors, including chemicals and environmental contaminants. They provide a framework for understanding the causal relationships between different biological events that can lead to adverse outcomes (AO). However, developing an AOP is a challenging task, particularly in identifying the molecular initiating events (MIEs) and key events (KEs) that constitute it. Here, we propose a systems biology strategy that can assist in the development of AOPs by screening publicly available databases, literature with the text mining tool AOP-helpFinder, and pathway/network analyses. This approach is straightforward to use, requiring only the name of the stressor and adverse outcome to be studied. From this, it quickly identifies potential KEs and literature providing mechanistic information on the links between the KEs. The proposed approach was applied to the recently developed AOP 441 on radiation-induced microcephaly, resulting in the confirmation of the KEs that were already present and identification of new relevant KEs, thereby validating the strategy. In conclusion, our systems biology approach represents a valuable tool to simplify the development and enrichment of Adverse Outcome Pathways (AOPs), thus supporting alternative methods in toxicology.

Imaging of Microcephaly

One of the most common definitions of microcephaly cited is that of an occipitofrontal circumference (OFC) of the head that is less than two standard deviations below the average for age (or gestational age, if identified prenatally) and sex. Similarly, severe microcephaly is defined as an OFC that is less than three standard deviations below the average. Microcephaly is not a diagnosis, but rather, a finding that is secondary to a multitude of etiologies that can be categorized as prenatal versus postnatal, genetic versus environmental, and congenital versus acquired.

Teleost Fish and Organoids: Alternative Windows Into the Development of Healthy and Diseased Brains

The variety in the display of animals’ cognition, emotions, and behaviors, typical of humans, has its roots within the anterior-most part of the brain: the forebrain, giving rise to the neocortex in mammals. Our understanding of cellular and molecular events instructing the development of this domain and its multiple adaptations within the vertebrate lineage has progressed in the last decade. Expanding and detailing the available knowledge on regionalization, progenitors’ behavior and functional sophistication of the forebrain derivatives is also key to generating informative models to improve our characterization of heterogeneous and mechanistically unexplored cortical malformations. Classical and emerging mammalian models are irreplaceable to accurately elucidate mechanisms of stem cells expansion and impairments of cortex development. Nevertheless, alternative systems, allowing a considerable reduction of the burden associated with animal experimentation, are gaining popularity to dissect basic strategies of neural stem cells biology and morphogenesis in health and disease and to speed up preclinical drug testing. Teleost vertebrates such as zebrafish, showing conserved core programs of forebrain development, together with patients-derived in vitro 2D and 3D models, recapitulating more accurately human neurogenesis, are now accepted within translational workflows spanning from genetic analysis to functional investigation. Here, we review the current knowledge of common and divergent mechanisms shaping the forebrain in vertebrates, and causing cortical malformations in humans. We next address the utility, benefits and limitations of whole-brain/organism-based fish models or neuronal ensembles in vitro for translational research to unravel key genes and pathological mechanisms involved in neurodevelopmental diseases.

Development of an Adverse Outcome Pathway for radiation-induced microcephaly via expert consultation and machine learning

BACKGROUND

Brain development during embryogenesis and in early postnatal life is particularly complex and involves the interplay of many cellular processes and molecular mechanisms, making it extremely vulnerable to exogenous insults, including ionizing radiation (IR). Microcephaly is one of the most frequent neurodevelopmental abnormalities that is characterized by small brain size, and is often associated with intellectual deficiency. Decades of research span from epidemiological data on in utero exposure of the A-bomb survivors, to studies on animal and cellular models that allowed deciphering the most prominent molecular mechanisms leading to microcephaly. The Adverse Outcome Pathway (AOP) framework is used to organize, evaluate and portray the scientific knowledge of toxicological effects spanning different biological levels of organizations, from the initial interaction with molecular targets to the occurrence of a disease or adversity. In the present study, the framework was used in an attempt to organize the current scientific knowledge on microcephaly progression in the context of ionizing radiation (IR) exposure. This work was performed by a group of experts formed during a recent workshop organized jointly by the Multidisciplinary European Low Dose Initiative (MELODI) and the European Radioecology Alliance (ALLIANCE) associations to present the AOP approach and tools. Here we report on the development of a putative AOP for congenital microcephaly resulting from IR exposure based on discussions of the working group and we emphasize the use of a novel machine-learning approach to assist in the screening of the available literature to develop AOPs.

CONCLUSION

The expert consultation led to the identification of crucial biological events for the progression of microcephaly upon exposure to IR, and highlighted current knowledge gaps. The machine learning approach was successfully used to screen the existing knowledge and helped to rapidly screen the body of evidence and in particular the epidemiological data. This systematic review approach also ensured that the analysis was sufficiently comprehensive to identify the most relevant data and facilitate rapid and consistent AOP development. We anticipate that as machine learning approaches become more user-friendly through easy-to-use web interface, this would allow AOP development to become more efficient and less time consuming.