Autosomal recessive primary microcephaly (MCPH): a review of clinical, molecular, and evolutionary findings

Exome sequencing reveled a compound heterozygous mutations in RTTN gene causing developmental delay and primary microcephaly

RTTN (Rotatin) (OMIM 614833) is a large centrosomal protein coding gene. RTTN mutations are responsible for syndromic forms of malformation of brain development, leading to polymicrogyria, microcephaly, primordial dwarfism, seizure along with many other malformations. In this study we have identified a compound heterozygous mutation in RTTN gene having NM_173630 c.5225A > G p.His1742Arg in exon 39 and NM_173630 c.6038G > T p.Cys2013Phe in exon 45 of a consanguineous Saudi family leading to brain malformation, seizure, developmental delay, dysmorphic feature and microcephaly. Whole exome sequencing (WES) techniques was used to identify the causative mutation in the affected members of the family. WES data analysis was done and obtained data were further confirmed by using Sanger sequencing analysis. Moreover, the mutation was ruled out in 100 healthy control from normal population. To the best of our knowledge the novel compound heterozygous mutation observed in this study is the first report from Saudi Arabia. The identified compound heterozygous mutation will further explain the role of RTTN gene in development of microcephaly and neurodevelopmental disorders.

Molecular Genetics of Microcephaly Primary Hereditary: An Overview

MicroCephaly Primary Hereditary (MCPH) is a rare congenital neurodevelopmental disorder characterized by a significant reduction of the occipitofrontal head circumference and mild to moderate mental disability. Patients have small brains, though with overall normal architecture; therefore, studying MCPH can reveal not only the pathological mechanisms leading to this condition, but also the mechanisms operating during normal development. MCPH is genetically heterogeneous, with 27 genes listed so far in the Online Mendelian Inheritance in Man (OMIM) database. In this review, we discuss the role of MCPH proteins and delineate the molecular mechanisms and common pathways in which they participate.

A missense variant in NUF2, a component of the kinetochore NDC80 complex, causes impaired chromosome segregation and aneuploidy associated with microcephaly and short stature

Mutations in proteins involved in cell division and chromosome segregation, such as microtubule-regulating, centrosomal and kinetochore proteins, are associated with microcephaly and/or short stature. In particular, the kinetochore plays an essential role in mitosis and cell division by mediating connections between chromosomal DNA and spindle microtubules. To date, only a few genes encoding proteins of the kinetochore complex have been identified as causes of syndromes that include microcephaly. We report a male patient with a rare de novo missense variant in NUF2, after trio whole-exome sequencing analysis. The patient presented with microcephaly and short stature, with additional features, such as bilateral vocal cord paralysis, micrognathia and atrial septal defect. NUF2 encodes a subunit of the NDC80 complex in the outer kinetochore, important for correct microtubule binding and spindle assembly checkpoint. The mutated residue is buried at the calponin homology (CH) domain at the N-terminus of NUF2, which interacts with the N-terminus of NDC80. The variant caused the loss of hydrophobic interactions in the core of the CH domain of NUF2, thereby impairing the stability of NDC80-NUF2. Analysis using a patient-derived lymphoblastoid cell line revealed markedly reduced protein levels of both NUF2 and NDC80, aneuploidy, increased micronuclei formation and spindle abnormality. Our findings suggest that NUF2 may be the first member of the NDC80 complex to be associated with a human disorder.

The N-terminal BRCT domain determines MCPH1 function in brain development and fertility

MCPH1 is a causal gene for the neurodevelopmental disorder, human primary microcephaly (MCPH1, OMIM251200). Most pathogenic mutations are located in the N-terminal region of the gene, which encodes a BRCT domain, suggesting an important function of this domain in brain size determination. To investigate the specific function of the N-terminal BRCT domain in vivo, we generated a mouse model lacking the N’-BRCT domain of MCPH1 (referred as Mcph1-ΔBR1). These mutant mice are viable, but exhibit reduced brain size, with a thinner cortex due to a reduction of neuroprogenitor populations and premature neurogenic differentiation. Mcph1-ΔBR1 mice (both male and female) are infertile; however, almost all female mutants develop ovary tumours. Mcph1-ΔBR1 MEF cells exhibit a defect in DNA damage response and DNA repair, and show the premature chromosome condensation (PCC) phenotype, a hallmark of MCPH1 patient cells and also Mcph1 knockout cells. In comparison with Mcph1 complete knockout mice, Mcph1-ΔBR1 mice faithfully reproduce all phenotypes, indicating an essential role of the N-terminal BRCT domain for the physiological function of MCPH1 in the control of brain size and gonad development as well as in multiple cellular processes.

Genome editing in stem cells for genetic neurodisorders

The recent advent of genome editing techniques and their rapid improvement paved the way in establishing innovative human neurological disease models and in developing new therapeutic opportunities. Human pluripotent (both induced or naive) stem cells and neural stem cells represent versatile tools to be applied to multiple research needs and, together with genomic snip and fix tools, have recently made possible the creation of unique platforms to directly investigate several human neural affections. In this chapter, we will discuss genome engineering tools, and their recent improvements, applied to the stem cell field, focusing on how these two technologies may be pivotal instruments to deeply unravel molecular mechanisms underlying development and function, as well as disorders, of the human brain. We will review how these frontier technologies may be exploited to investigate or treat severe neurodevelopmental disorders, such as microcephaly, autism spectrum disorder, schizophrenia, as well as neurodegenerative conditions, including Parkinson's disease, Huntington's disease, Alzheimer's disease, and spinal muscular atrophy.

Brain organoids: an ensemble of bioassays to investigate human neurodevelopment and disease

Understanding etiology of human neurological and psychiatric diseases is challenging. Genomic changes, protracted development, and histological features unique to human brain development limit the disease aspects that can be investigated using model organisms. Hence, in order to study phenotypes associated with human brain development, function, and disease, it is necessary to use alternative experimental systems that are accessible, ethically justified, and replicate human context. Human pluripotent stem cell (hPSC)-derived brain organoids offer such a system, which recapitulates features of early human neurodevelopment in vitro, including the generation, proliferation, and differentiation of neural progenitors into neurons and glial cells and the complex interactions among the diverse, emergent cell types of the developing brain in three-dimensions (3-D). In recent years, numerous brain organoid protocols and related techniques have been developed to recapitulate aspects of embryonic and fetal brain development in a reproducible and predictable manner. Altogether, these different organoid technologies provide distinct bioassays to unravel novel, disease-associated phenotypes and mechanisms. In this review, we summarize how the diverse brain organoid methods can be utilized to enhance our understanding of brain disorders. FACTS: Brain organoids offer an in vitro approach to study aspects of human brain development and disease. Diverse brain organoid techniques offer bioassays to investigate new phenotypes associated with human brain disorders that are difficult to study in monolayer cultures. Brain organoids have been particularly useful to study phenomena and diseases associated with neural progenitor morphology, survival, proliferation, and differentiation. OPEN QUESTION: Future brain organoid research needs to aim at later stages of neurodevelopment, linked with neuronal activity and connections, to unravel further disease-associated phenotypes. Continued improvement of existing organoid protocols is required to generate standardized methods that recapitulate in vivo-like spatial diversity and complexity.

Great Expectations: Induced pluripotent stem cell technologies in neurodevelopmental impairments

Somatic cells such as skin fibroblasts, umbilical cord blood, peripheral blood, urinary epithelial cells, etc., are transformed into induced pluripotent stem cells (iPSCs) by reprogramming technology, a milestone in the stem-cell research field. IPSCs are similar to embryonic stem cells (ESCs), exhibiting the potential to differentiate into various somatic cells. Still, the former avoid problems of immune rejection and medical ethics in the study of ESCs and clinical trials. Neurodevelopmental disorders are chronic developmental brain dysfunctions that affect cognition, exercise, social adaptability, behavior, etc. Due to various inherited or acquired causes, they seriously affect the physical and psychological health of infants and children. These include generalized stunting / mental disability (GDD/ID), Epilepsy, autism spectrum disease (ASD), and attention deficit hyperactivity disorder (ADHD). Most neurodevelopmental disorders are challenging to cure. Establishing a neurodevelopmental disorder system model is essential for researching and treating neurodevelopmental disorders. At this stage, the scarcity of samples is a bigger problem for studying neurological diseases based on the donor, ethics, etc. Some iPSCs are reprogrammed from somatic cells that carry disease-causing mutations. They differentiate into nerve cells by induction, which has the original characteristics of diseases. Disease-specific iPSCs are used to study the mechanism and pathogenesis of neurodevelopmental disorders. The process provided samples and the impetus for developing drugs and developing treatment plans for neurodevelopmental disorders. Here, this article mainly introduced the development of iPSCs, the currently established iPSCs disease models, and artificial organoids related to neurodevelopmental impairments. This technology will promote our understanding of neurodevelopmental impairments and bring great expectations to children with neurological disorders.

A novel mitosis-specific Cep215 domain interacts with Cep192 and phosphorylated Aurora A for organization of spindle poles

The centrosome, which consists of centrioles and pericentriolar material (PCM), becomes mature and assembles mitotic spindles by increasing the number of microtubules (MTs) emanating from the PCM. Among the molecules involved in centrosome maturation, Cep192 and Aurora A (AurA, also known as AURKA) are primarily responsible for recruitment of γ-tubulin and MT nucleators, whereas pericentrin (PCNT) is required for PCM organization. However, the role of Cep215 (also known as CDK5RAP2) in centrosome maturation remains elusive. Cep215 possesses binding domains for γ-tubulin, PCNT and MT motors that transport acentrosomal MTs towards the centrosome. We identify a mitosis-specific centrosome-targeting domain of Cep215 (215N) that interacts with Cep192 and phosphorylated AurA (pAurA). Cep192 is essential for targeting 215N to centrosomes, and centrosomal localization of 215N and pAurA is mutually dependent. Cep215 has a relatively minor role in γ-tubulin recruitment to the mitotic centrosome. However, it has been shown previously that this protein is important for connecting mitotic centrosomes to spindle poles. Based on the results of rescue experiments using versions of Cep215 with different domain deletions, we conclude that Cep215 plays a role in maintaining the structural integrity of the spindle pole by providing a platform for the molecules involved in centrosome maturation.

Novel neuroclinical findings of autosomal recessive primary microcephaly 15 in a consanguineous Iranian family

Major facilitator superfamily domain-containing 2A (MFSD2A) is required for brain uptake of Docosahexaenoic acid and Lysophosphatidylcholine, both are essential for the normal neural development and function. Mutations in MFSD2A dysregulate the activity of this transporter in brain endothelial cells and can lead to microcephaly. In this study, we describe an 11-year-old male who is affected by autosomal recessive primary microcephaly 15. This patient also shows severe intellectual disability, recurrent respiratory and renal infections, low birth weight, and developmental delay. After doing clinical and neuroimaging evaluations, due to heterogeneity of neurogenetic disorders, no narrow clinical diagnosis was possible, therefore, we utilized targeted-exome sequencing to identify any causative genetic factors. This revealed a homozygous in-frame deletion (NM_001136493.1: c.241_243del; p.(Val81del)) in the MFSD2A gene as the most likely disease-susceptibility variant which was confirmed by Sanger sequencing. Neuroimaging revealed lateral ventricular asymmetry, corpus callosum hypoplasia, type B of cisterna magna, and widening of Sylvian fissures. All of these novel phenotypes are associated with autosomal recessive primary microcephaly-15 (MCPH15). According to the genotype-phenotype data, p.(Val81del) can be considered a likely pathogenic variant leading to non-lethal microcephaly. However, further cumulative data and molecular approaches are required to accurately identify genotype-phenotype correlations in MFSD2A.

Novel compound heterozygous variants in the STIL gene identified in a Chinese family with presentation of foetal microcephaly

Primary microcephaly 7 (MCPH7) is an autosomal recessive human neurodevelopmental disorder characterized by microcephaly, sloping forehead, and prominent midface. The STIL gene encodes a protein that regulates the mitotic spindle checkpoint. STIL is the pathogenic gene of MCPH7. Although more than 25 genes have been reported to cause MCPH, many patients lack a molecular diagnosis. The clinical manifestations and genetic factors of MCPH7 remain to be revealed. This research reported two consecutive microcephalic foetuses from unaffected parents. Prenatal ultrasound examination and pre- and postnatal MRI studies were performed. Whole-genome sequencing (WGS) was performed using blood derived from the umbilical cord, and variants were confirmed by Sanger sequencing on the parents. Ultrasound examination showed that the two foetuses suffered primary microcephaly. Using the WGS approach, novel compound heterozygous variants in STIL (c.2344_2347delTTGC, p. Leu782Thrfs*2 in exon 13; c.3838C > T, p. Arg1280Cys in exon 17) were identified in two foetuses with MCPH7. The MRI results of the two siblings were quite similar. Postnatal MRI confirmed the ultrasound and prenatal examinations. The two foetuses had typical microcephaly. Ultrasound and MRI showed that the two foetuses had a thick skull plate, significantly reduced bilateral frontal lobe, upward rotated cerebellum vermis, and dilated fourth ventricle. Our findings have important implications for prenatal diagnosis and genetic counselling for any patients with MCPH7. We extend both the mutational spectrum in the STIL gene and the clinical spectrum of MCPH7.

Protein phosphatase 1 regulatory subunit 35 is required for ciliogenesis, notochord morphogenesis, and cell-cycle progression during murine development

Protein phosphatases regulate a wide array of proteins through post-translational modification and are required for a plethora of intracellular events in eukaryotes. While some core components of the protein phosphatase complexes are well characterized, many subunits of these large complexes remain unstudied. Here we characterize a loss-of-function allele of the protein phosphatase 1 regulatory subunit 35 (Ppp1r35) gene. Homozygous mouse embryos lacking Ppp1r35 are developmental delayed beginning at embryonic day (E) 7.5 and have obvious morphological defects at later stages. Mutants fail to initiate turning and do not progress beyond the size or staging of normal E8.5 embryos. Consistent with recent in vitro studies linking PPP1R35 with the microcephaly protein Rotatin and with a role in centrosome formation, we show that Ppp1r35 mutant embryos lack primary cilia. Histological and molecular analysis of Ppp1r35 mutants revealed that notochord development is irregular and discontinuous and consistent with a role in primary cilia, that the floor plate of the neural tube is not specified. Similar to other mutant embryos with defects in centriole function, Ppp1r35 mutants displayed increased cell death that is prevalent in the neural tube and an increased number of proliferative cells in prometaphase. We hypothesize that loss of Ppp1r35 function abrogates centriole homeostasis, resulting in a failure to produce functional primary cilia, cell death and cell cycle delay/stalling that leads to developmental failure. Taken together, these results highlight the essential function of Ppp1r35 during early mammalian development and implicate this gene as a candidate for human microcephaly.

In vitro modeling for inherited neurological diseases using induced pluripotent stem cells: from 2D to organoid

Stem cells are characterized by self-renewal and by their ability to differentiate into cells of various organs. With massive progress in 2D and 3D cell culture techniques, in vitro generation of various types of such organoids from patient-derived stem cells is now possible. As in vitro differentiation protocols are usually made to resemble human developmental processes, organogenesis of patient-derived stem cells can provide key information regarding a range of developmental diseases. Human stem cell-based in vitro modeling as opposed to using animal models can particularly benefit the evaluation of neurological diseases because of significant differences in structure and developmental processes between the human and the animal brain. This review focuses on stem cell-based in vitro modeling of neurodevelopmental disorders, more specifically, the fundamentals and technical advancements in monolayer neuron and brain organoid cultures. Furthermore, we discuss the drawbacks of the conventional culture method and explore the advanced, cutting edge 3D organoid models for several neurodevelopmental diseases, including genetic diseases such as Down syndrome, Rett syndrome, and Miller-Dieker syndrome, as well as brain malformations like macrocephaly and microcephaly. Finally, we discuss the limitations of the current organoid techniques and some potential solutions that pave the way for accurate modeling of neurological disorders in a dish.

Brain Organoids: Tiny Mirrors of Human Neurodevelopment and Neurological Disorders

Unravelling the complexity of the human brain is a challenging task. Nowadays, modern neurobiologists have developed 3D model systems called "brain organoids" to overcome the technical challenges in understanding human brain development and the limitations of animal models to study neurological diseases. Certainly like most model systems in neuroscience, brain organoids too have limitations, as these minuscule brains lack the complex neuronal circuitry required to begin the operational tasks of human brain. However, researchers are hopeful that future endeavors with these 3D brain tissues could provide mechanistic insights into the generation of circuit complexity as well as reproducible creation of different regions of the human brain. Herein, we have presented the contemporary state of brain organoids with special emphasis on their mode of generation and their utility in modelling neurological disorders, drug discovery, and clinical trials.

Primary Microcephaly with Novel Variant of MCPH1 Gene in Twins: Both Manifesting in Childhood at the Same Time with Hashimoto's Thyroiditis

This study is a clinical report on twin females affected by primary microcephaly who displayed at molecular analysis of heterozygous novel MCPH1 variant. The twins at the age of 10 years developed, in coincidental time, a diagnosis of autoimmune juvenile thyroiditis. The main clinical features presented by the twins consisted of primary microcephaly with occipitofrontal circumference measuring -2 or -3 standard deviation, facial dysmorphism, typical nonsyndromic microcephaly, and mild intellectual disability. Molecular analysis of the major genes involved in primary microcephaly was performed and the following result was found in the twins: MCPH1 ; chr8.6357416; c.2180 C > T (rs 199861426), p.Pro727. Leu; heterozygous; missense; variant of uncertain significance (class 3). At the age of 10 years, the twins started to have, in coincidental time, marked asthenia and episodes of emotiveness, and laboratory exams disclosed a high level of antithyroid peroxidase leading to the diagnosis of autoimmune juvenile thyroiditis with normal thyroid function. The novel heterozygous MCPH1 variant found in the twins may be directly or indirectly involved in the onset of the primary microcephaly. The thyroid disorder in the twins and its onset, in a coincidental time, confirmed the effect of genetic predisposition on the pathogenesis of the immune thyroiditis.

Unusual context of CENPJ variants and primary microcephaly: compound heterozygosity and nonconsanguinity in an Argentinian patient

Primary microcephaly (MCPH) is a genetically heterogeneous disorder showing an autosomal recessive mode of inheritance. Patients with MCPH present head circumference values two or three standard deviations (SDs) significantly below the mean for age- and sex-matched populations. MCPH is associated with a nonprogressive mild to severe intellectual disability, with normal brain structure in most patients, or with a small brain and gyri without visceral malformations. We present the case of an adult patient born from Argentinian nonconsanguineous healthy parents. He had a head circumference >5 SD below the mean, cerebral neuroimaging showing hypoplasia of the corpus callosum, bilateral migration disorder with heterotopia of the sylvian fissure and colpocephaly. The patient was compound heterozygous for pathogenic variants in the CENPJ gene (c.289dupA inherited from his mother and c.1132 C > T inherited from his father). Our patient represents an uncommon situation for the usual known context of CENPJ and MCPH, including family origin (Argentinian), pedigree (nonconsanguineous), and genotype (a compound heterozygous case with two variants predicting a truncated protein). Next-generation sequencing studies applied in a broader spectrum of clinical presentations of MCPH syndromes may discover additional similar patients and families.

MCPH1 Lack of Function Enhances Mitotic Cell Sensitivity Caused by Catalytic Inhibitors of Topo II

The capacity of Topoisomerase II (Topo II) to remove DNA catenations that arise after replication is essential to ensure faithful chromosome segregation. Topo II activity is monitored during G2 by a specific checkpoint pathway that delays entry into mitosis until the chromosomes are properly decatenated. Recently, we demonstrated that the mitotic defects that are characteristic of cells depleted of MCPH1 function, a protein mutated in primary microcephaly, are not a consequence of a weakened G2 decatenation checkpoint response. However, the mitotic defects could be accounted for by a minor defect in the activity of Topo II during G2/M. To test this hypothesis, we have tracked at live single cell resolution the dynamics of mitosis in MCPH1 depleted HeLa cells upon catalytic inhibition of Topo II. Our analyses demonstrate that neither chromosome alignment nor segregation are more susceptible to minor perturbation in decatenation in MCPH1 deficient cells, as compared with control cells. Interestingly, MCPH1 depleted cells were more prone to mitotic cell death when decatenation was perturbed. Furthermore, when the G2 arrest that was induced by catalytic inhibition of Topo II was abrogated by Chk1 inhibition, the incidence of mitotic cell death was also increased. Taken together, our data suggest that the MCPH1 lack of function increases mitotic cell hypersensitivity to the catalytic inhibition of Topo II.

Centrosomes: The good and the bad for brain development

Centrosomes nucleate and organise the microtubule cytoskeleton in animal cells. These membraneless organelles are key structures for tissue organisation, polarity and growth. Centrosome dysfunction, defined as deviation in centrosome numbers and/or structural integrity, has major impact on brain size and functionality, as compared with other tissues of the organism. In this review, we discuss the contribution of centrosomes to brain growth during development. We discuss in particular the impact of centrosome dysfunction in Drosophila and mammalian neural stem cell division and fitness, which ultimately underlie brain growth defects.

A case report of microcephaly and refractory West syndrome associated with WDR62 mutation

The autosomal recessive form of primary microcephaly (MCPH) is a rare disorder characterized by microcephaly with variable degree of intellectual disability. WDR62 has been reported as the second causative gene of MCPH2. West syndrome is a severe epilepsy syndrome composed of the triad of spasms, hypsarrhythmia, and mental retardation. There are limited clinical reports regarding WDR62 mutation and West syndrome. Here we report a boy who was identified with WDR62 mutation and was followed up from age 3 months to 5 months and 14 days. He had the first seizure as the classic epileptic spasm at the age of 3 months. Psychomotor retardation was noted before the seizure occurred. The head circumference was 38.5 cm (SD 2.6) when he was 4 months old, no dysmorphic facial features were observed. He couldn’t support his head steadily or turn over. He was able to laugh when tricked by the parents, but couldn’t track the sound and light. At the early stage, the electroencephalogram showed multifocal discharges, which evolved into hypsarrhythmia one month later, and brain MRI showed developmental malformation of cerebral gyrus. Two heterozygous mutations were identified in WDR62 by whole exome sequencing c.1535G > A, p.R512Q and c.2618dupT, p.K874Qfs*40. The patient was administrated with oral sodium valproate, nitrazepam, intramuscular adrenocorticotropic hormone for 2 weeks, and followed by prednisone, levetiracetam, topiramate and vigabatrin. However, there was no significant improvement on the seizure control after these treatments. According to the genetic report and clinical manifestation, we speculated that the WDR62 compound heterozygous mutation is responsible for the serious clinical phenotype.

The journey of Zika to the developing brain

Zika virus is a mosquito-borne Flavivirus originally isolated from humans in 1952. Following its re-emergence in Brazil in 2015, an increase in the number of babies born with microcephaly to infected mothers was observed. Microcephaly is a neurodevelopmental disorder, characterised phenotypically by a smaller than average head size, and is usually developed in utero. The 2015 outbreak in the Americas led to the World Health Organisation declaring Zika a Public Health Emergency of International Concern. Since then, much research into the effects of Zika has been carried out. Studies have investigated the structure of the virus, its effects on and evasion of the immune response, cellular entry including target receptors, its transmission from infected mother to foetus and its cellular targets. This review discusses current knowledge and novel research into these areas, in hope of developing a further understanding of how exposure of pregnant women to the Zika virus can lead to impaired brain development of their foetus. Although no longer considered an epidemic in the Americas, the mechanism by which Zika acts is still not comprehensively and wholly understood, and this understanding will be crucial in developing effective vaccines and treatments.

Nearly complete deletion of BubR1 causes microcephaly through shortened mitosis and massive cell death

BUB-related 1 (BubR1) encoded by Budding Uninhibited by Benzimidazole 1B (BUB1B) is a crucial mitotic checkpoint protein ensuring proper segregation of chromosomes during mitosis. Mutations of BUB1B are responsible for mosaic variegated aneuploidy (MVA), a human congenital disorder characterized by extensive abnormalities in chromosome number. Although microcephaly is a prominent feature of MVA carrying the BUB1B mutation, how BubR1 deficiency disturbs neural progenitor proliferation and neuronal output and leads to microcephaly is unknown. Here we show that conditional loss of BubR1 in mouse cerebral cortex recapitulates microcephaly. BubR1-deficient cortex includes a strikingly reduced number of late-born, but not of early-born, neurons, although BubR1 expression is substantially reduced from an early stage. Importantly, absence of BubR1 decreases the proportion of neural progenitors in mitosis, specifically in metaphase, suggesting shortened mitosis owing to premature chromosome segregation. In the BubR1 mutant, massive apoptotic cell death, which is likely due to the compromised genomic integrity that results from aberrant mitosis, depletes progenitors and neurons during neurogenesis. There is no apparent alteration in centrosome number, spindle formation or primary cilia, suggesting that the major effect of BubR1 deficiency on neural progenitors is to impair the mitotic checkpoint. This finding highlights the importance of the mitotic checkpoint in the pathogenesis of microcephaly. Furthermore, the ependymal cell layer does not form in the conditional knockout, revealing an unrecognized role of BubR1 in assuring the integrity of the ventricular system, which may account for the presence of hydrocephalus in some patients.

Normal early development in siblings with novel compound heterozygous variants in ASPM

Autosomal recessive primary microcephaly 5 (MCPH5) is caused by pathogenic variants in ASPM. Using whole-exome sequencing, we diagnosed two siblings with MCPH5. A known pathogenic variant (NM_018136.4: c.9697C > T, p.(Arg3233*)) and a novel pathogenic variant (c.1402_1406del, p.(Asn468Serfs*2)) of ASPM were identified in affected siblings with normal intelligence. Their pathogenic variants were not located in the critical regions of ASPM, but the relationship between the genotypes and their normal intelligence was unclear.

Three-Dimensional Models for Studying Neurodegenerative and Neurodevelopmental Diseases

Human brain possesses a unique anatomy and physiology. For centuries, methodological barriers and ethical challenges in accessing human brain tissues have restricted researchers into using 2-D cell culture systems and model organisms as a tool for investigating the mechanisms underlying neurological disorders in humans. However, our understanding regarding the human brain development and diseases has been recently extended due to the generation of 3D brain organoids, grown from human stem cells or induced pluripotent stem cells (iPSCs). This system evolved into an attractive model of brain diseases as it recapitulates to a great extend the cellular organization and the microenvironment of a human brain. This chapter focuses on the application of brain organoids in modelling several neurodevelopmental and neurodegenerative diseases.

Whole Exome Sequencing Identifies Three Novel Mutations in the ASPM Gene From Saudi Families Leading to Primary Microcephaly

Autosomal recessive primary microcephaly (MCPH) is a neurodevelopmental defect that is characterized by reduced head circumference at birth along with non-progressive intellectual disability. Till date, 25 genes related to MCPH have been reported so far in humans. The ASPM (abnormal spindle-like, microcephaly-associated) gene is among the most frequently mutated MCPH gene. We studied three different families having primary microcephaly from different regions of Saudi Arabia. Whole exome sequencing (WES) and Sanger sequencing were done to identify the genetic defect. Collectively, three novel variants were identified in the ASPM gene from three different primary microcephaly families. Family 1, showed a deletion mutation leading to a frameshift mutation c.1003del. (p.Val335^*) in exon 3 of the ASPM gene and family 2, also showed deletion mutation leading to frameshift mutation c.1047del (p.Gln349Hisfs^*18), while in family 3, we identified a missense mutation c.5623A>G leading to a change in protein (p.Lys1875Glu) in exon 18 of the ASPM gene underlying the disorder. The identified respective mutations were ruled out in 100 healthy control samples. In conclusion, we found three novel mutations in the ASPM gene in Saudi families that will help to establish a disease database for specified mutations in Saudi population and will further help to identify strategies to tackle primary microcephaly in the kingdom.

Microcephaly Modeling of Kinetochore Mutation Reveals a Brain-Specific Phenotype

Most genes mutated in microcephaly patients are expressed ubiquitously, and yet the brain is the only major organ compromised in most patients. Why the phenotype remains brain specific is poorly understood. In this study, we used in vitro differentiation of human embryonic stem cells to monitor the effect of a point mutation in kinetochore null protein 1 (KNL1;CASC5), identified in microcephaly patients, during in vitro brain development. We found that neural progenitors bearing a patient mutation showed reduced KNL1 levels, aneuploidy, and an abrogated spindle assembly checkpoint. By contrast, no reduction of KNL1 levels or abnormalities was observed in fibroblasts and neural crest cells. We established that the KNL1 patient mutation generates an exonic splicing silencer site, which mainly affects neural progenitors because of their higher levels of splicing proteins. Our results provide insight into the brain-specific phenomenon, consistent with microcephaly being the only major phenotype of patients bearing KNL1 mutation.

Using 3D neural spheroids, Javed et al. investigate a mutation in KNL1 that causes microcephaly. Their study shows that, despite ubiquitous mutant KNL1 expression, KNL1 mRNA processing is affected only in neural precursors due to difference in splicing protein levels, offering insights into why the phenotype remains brain specific in patients.

Erythromycin Estolate Inhibits Zika Virus Infection by Blocking Viral Entry as a Viral Inactivator

Recently, Zika virus (ZIKV) has attracted much attention in consideration of its association with severe neurological complications including fetal microcephaly. However, there are currently no prophylactic vaccines or therapeutic drugs approved for clinical treatments of ZIKV infection. To determine the potential anti-ZIKV inhibitors, we screened a library of clinical drugs with good safety profiles. Erythromycin estolate (Ery-Est), one of the macrolide antibiotics, was found to effectively inhibit ZIKV infection in different cell types and significantly protect A129 mice from ZIKV-associated neurological signs and mortality. Through further investigation, Ery-Est was verified to inhibit ZIKV entry by disrupting the integrity of the viral membrane which resulted in the loss of ZIKV infectivity. Furthermore, Ery-Est also showed inhibitory activity against dengue virus (DENV) and yellow fever virus (YFV). Thus, Ery-Est may be a promising drug for patients with ZIKV infection, particularly pregnant women.

A Novel Frameshift Mutation in Abnormal Spindle-Like Microcephaly (ASPM) Gene in an Iranian Patient with Primary Microcephaly: A Case Report

Autosomal recessive primary microcephaly (MCPH) is a rare genetic disorder, leading to the defect of neurogenic brain development. Individuals with MCPH reveal reduced head circumference and intellectual disability. Several MCPH loci have been identified from several populations. Genetic heterogeneity of this disorder represents molecular testing challenge. An 8 yr old female, born from consanguineous parents, was attended to Fardis Central Lab, Alborz, Iran. Based on the reduced circumference and intellectual disability, MCPH was diagnosed. Whole exome sequencing of the patient identified a novel homozygous frameshift mutation (c.2738dupT, p.Cys914fs) in exon 9 Abnormal Spindle-like Microcephaly (ASPM) gene. By Sanger sequencing, segregation analysis showed that both parents were heterozygous carriers for this variant. The novel frameshift mutation likely truncates the protein, resulting in loss of normal function ASPM in homozygous mutation carriers. The study might add a new pathogenic variant in mutations of the ASPM gene as a causative variant in patients with MCPH and might be helpful in genetic counseling of consanguineous families.

Novel compound heterozygous mutations in MCPH1 gene causes primary microcephaly in Saudi family

Objectives:

To identify genetic variation involved in primary microcephaly.

Methods:

In present study we identified 4 generation Saudi family showing primary microcephaly. We performed whole exome sequencing along with Sanger sequencing to find the genetic defect in this family. This study was conducted in King Abdulaziz University started from 2016 and the results presented in this manuscript are from one of the family.

Results:

Two novel missense variants (c.982G>A and c.1273T>A) were identified in heterozygous state in exon 8 of MCPH1 gene. The detected missense variants cause a tyrosine to asparagine substitution of residue 425 and a valine to isoleucine substitution at residue 310. MCPH1 gene encodes a DNA damage response protein. The encoded protein play a role in G2/M DNA damage checkpoint arrest via maintenance of inhibitory phosphorylation of cyclin-dependent kinase 1. The respective mutation was ruled out in 100 control samples.

Conclusion:

We found novel compound heterozygous mutation in Saudi family that will help to build database for genetic mutations in population and pave way to devise strategies to tackle such disorders in future.

Brain Organoids as Tools for Modeling Human Neurodevelopmental Disorders

Brain organoids recapitulate in vitro the specific stages of in vivo human brain development, thus offering an innovative tool by which to model human neurodevelopmental disease. We review here how brain organoids have been used to study neurodevelopmental disease and consider their potential for both technological advancement and therapeutic development.

Evolution of ASPM coding variation in apes and associations with brain structure in chimpanzees

Studying genetic mechanisms underlying primate brain morphology can provide insight into the evolution of human brain structure and cognition. In humans, loss-of-function mutations in the gene coding for ASPM (Abnormal Spindle Microtubule Assembly) have been associated with primary microcephaly, which is defined by a significantly reduced brain volume, intellectual disability and delayed development. However, less is known about the effects of common ASPM variation in humans and other primates. In this study, we characterized the degree of coding variation at ASPM in a large sample of chimpanzees (N = 241), and examined potential associations between genotype and various measures of brain morphology. We identified and genotyped five non-synonymous polymorphisms in exons 3 (V588G), 18 (Q2772K, K2796E, C2811Y) and 27 (I3427V). Using T1-weighted magnetic resonance imaging of brains, we measured total brain volume, cerebral gray and white matter volume, cerebral ventricular volume, and cortical surface area in the same chimpanzees. We found a potential association between ASPM V588G genotype and cerebral ventricular volume but not with the other measures. Additionally, we found that chimpanzee, bonobo, and human lineages each independently show a signature of accelerated ASPM protein evolution. Overall, our results suggest the potential effects of ASPM variation on cerebral cortical development, and emphasize the need for further functional studies. These results are the first evidence suggesting ASPM variation might play a role in shaping natural variation in brain structure in nonhuman primates.

Normal and Disordered Formation of the Cerebral Cortex : Normal Embryology, Related Molecules, Types of Migration, Migration Disorders

The expansion and folding of the cerebral cortex occur during brain development and are critical factors that influence cognitive ability and sensorimotor skills. The disruption of cortical growth and folding may cause neurological disorders, resulting in severe intellectual disability and intractable epilepsy in humans. Therefore, understanding the mechanism that regulates cortical growth and folding will be crucial in deciphering the key steps of brain development and finding new therapeutic targets for the congenital anomalies of the cerebral cortex. This review will start with a brief introduction describing the anatomy of the brain cortex, followed by a description of our understanding of the proliferation, differentiation, and migration of neural progenitors and important genes and molecules that are involved in these processes. Finally, various types of disorders that develop due to malformation of the cerebral cortex will be discussed.

From microcephaly to megalencephaly: determinants of brain size

Expansion of the human brain, and specifically the neocortex, is among the most remarkable evolutionary processes that correlates with cognitive, emotional, and social abilities. Cortical expansion is determined through a tightly orchestrated process of neural stem cell proliferation, migration, and ongoing organization, synaptogenesis, and apoptosis. Perturbations of each of these intricate steps can lead to abnormalities of brain size in humans, whether small (microcephaly) or large (megalencephaly). Abnormalities of brain growth can be clinically isolated or occur as part of complex syndromes associated with other neurodevelopmental problems (eg, epilepsy, autism, intellectual disability), brain malformations, and body growth abnormalities. Thorough review of the genetic literature reveals that human microcephaly and megalencephaly are caused by mutations of a rapidly growing number of genes linked within critical cellular pathways that impact early brain development, with important pathomechanistic links to cancer, body growth, and epilepsy. Given the rapid rate of causal gene identification for microcephaly and megalencephaly understanding the roles and interplay of these important signaling pathways is crucial to further unravel the mechanisms underlying brain growth disorders and, more fundamentally, normal brain growth and development in humans. In this review, we will (a) overview the definitions of microcephaly and megalencephaly, highlighting their classifications in clinical practice; (b) overview the most common genes and pathways underlying microcephaly and megalencephaly based on the fundamental cellular processes that are perturbed during cortical development; and (c) outline general clinical molecular diagnostic workflows for children and adults presenting with microcephaly and megalencephaly.

Novel compound heterozygous mutations in WDR62 gene leading to developmental delay and Primary Microcephaly in Saudi Family

Objective:

Primary microcephaly (MCPH) is a rare autosomal recessive disorder characterized by impaired congenital reduction of brain size along with head circumference and intellectual disability. MCPH is a heterogeneous disorder and more than twenty four genes associated with this disease have been identified so far. The objective of this study was to find out the novel genes or mutations leading to the genetic defect in a Saudi family with primary microcephaly.

Methods:

Whole exome sequencing was carried out to find the novel mutation and the results was further validated using Sanger sequencing analysis. This study was done in the Center of excellence in Genomic Medicine and Research, King Abdulaziz University under KACST project during 2017 and 2018.

Results:

We report a novel compound heterozygous mutations c.797C>T in exon 7 and c.1102G>A in exon 9 of the WD repeat domain 62 (WDR62) (OMIM 604317) gene in two affected siblings in Saudi family with intellectual disability, speech impediments walking difficulty along with primary microcephaly. Two rare, missense variants were detected in heterozygous state in the WDR62 gene in these two affected individuals from the heterozygous parents.

Conclusions:

A compound heterozygous mutations c.797C>T in exon 7 and c.1102G> A in exon 9 of the WDR62 gene was identified. WDR62 gene is very important gene and mutation can lead to neuro developmental defects, brain malformations, reduced brain and head size. These results should be taken into consideration during prognostic discussions and mutation spectrum with affected patients and their families in the Saudi population.

Transgenic rhesus monkeys carrying the human MCPH1 gene copies show human-like neoteny of brain development

Brain size and cognitive skills are the most dramatically changed traits in humans during evolution and yet the genetic mechanisms underlying these human-specific changes remain elusive. Here, we successfully generated 11 transgenic rhesus monkeys (8 first-generation and 3 second-generation) carrying human copies of MCPH1, an important gene for brain development and brain evolution. Brain-image and tissue-section analyses indicated an altered pattern of neural-cell differentiation, resulting in a delayed neuronal maturation and neural-fiber myelination of the transgenic monkeys, similar to the known evolutionary change of developmental delay (neoteny) in humans. Further brain-transcriptome and tissue-section analyses of major developmental stages showed a marked human-like expression delay of neuron differentiation and synaptic-signaling genes, providing a molecular explanation for the observed brain-developmental delay of the transgenic monkeys. More importantly, the transgenic monkeys exhibited better short-term memory and shorter reaction time compared with the wild-type controls in the delayed-matching-to-sample task. The presented data represent the first attempt to experimentally interrogate the genetic basis of human brain origin using a transgenic monkey model and it values the use of non-human primates in understanding unique human traits.

Tumor suppressor MCPH1 regulates gene expression profiles related to malignant conversion and chromosomal assembly

Strong inherited predisposition to breast cancer is estimated to cause about 5–10% of all breast cancer cases. As the known susceptibility genes, such as BRCA1 and BRCA2, explain only a fraction of this, additional predisposing genes and related biological mechanisms are actively being searched for. We have recently identified a recurrent MCPH1 germline mutation, p.Arg304ValfsTer3, as a breast cancer susceptibility allele. MCPH1 encodes a multifunctional protein involved in maintenance of genomic integrity and it is also somatically altered in various cancer types, including breast cancer. Additionally, biallelic MCPH1 mutations are causative for microcephaly and at cellular level premature chromosome condensation. To study the molecular mechanisms leading to cancer predisposition and malignant conversion, here we have modeled the effect of MCPH1 p.Arg304ValfsTer3 mutation using gene‐edited MCF10A breast epithelial cells. As a complementary approach, we also sought for additional potential cancer driver mutations in MCPH1 p.Arg304ValfsTer3 carrier breast tumors. We show that mutated MCPH1 de‐regulates transcriptional programs related to invasion and metastasis and leads to downregulation of histone genes. These global transcriptional changes are mirrored by significantly increased migration and invasion potential of the cells as well as abnormal chromosomal condensation both before and after mitosis. These findings provide novel molecular insights to MCPH1 tumor suppressor functions and establish a role in regulation of transcriptional programs related to malignant conversion and chromosomal assembly. The MCPH1 p.Arg304ValfsTer3 carrier breast tumors showed recurrent tumor suppressor gene TP53 mutations, which were also significantly over‐represented in breast tumors with somatically inactivated MCPH1.

What's new?

Even though several breast cancer susceptibility genes have been identified, additional molecular mechanisms behind predisposition and the promotion of malignant conversion remain obscure. Here, the authors show that a previously‐identified breast cancer predisposing allele in tumor suppressor MCPH1 deregulates transcriptional programs related to invasion and metastasis and leads to down‐regulation of histone genes. These global transcriptional changes are mirrored by increased cell migration and invasion potential and abnormal chromosomal condensation. The findings provide novel molecular insights into MCPH1 tumor suppressor functions and establish a role in the regulation of transcriptional programs related to malignant conversion and chromosomal assembly.

Innate immune response in patients with acute Zika virus infection

Innate immunity receptors (Toll-like receptors/TLRs and RIG-like receptors/RLRs) are important for the initial recognition of Zika virus (ZIKV), modulation of protective immune response, and IFN-α and IFN-β production. Immunological mechanisms involved in protection or pathology during ZIKV infection have not yet been determined. In this study, we evaluated the mRNA expression of innate immune receptors (TLR3, TLR7, TLR8, TLR9, melanoma differentiation-associated protein 5/MDA-5, and retinoic acid inducible gene/RIG-1), its adapter molecules (Myeloid Differentiation Primary Response Gene 88/Myd88, Toll/IL-1 Receptor Domain-Containing Adaptor-Inducing IFN-β/TRIF), and cytokines (IL-6, IL-12, TNF-α, IFN-α, IFN-β, and IFN-γ) in the acute phase of patients infected by ZIKV using real-time PCR in peripheral blood. Patients with acute ZIKV infection had high expression of TLR3, IFN-α, IFN-β, and IFN-γ when compared to healthy controls. In addition, there was a positive correlation between TLR3 expression compared to IFN-α and IFN-β. Moreover, viral load is positively correlated with TLR8, RIG-1, MDA-5, IFN-α, and IFN-β. On the other hand, patients infected by ZIKV showed reduced expression of RIG-1, TLR8, Myd88, and TNF-α molecules, which are also involved in antiviral immunity. Similar expressions of TLR7, TLR9, MDA-5, TRIF, IL-6, and IL-12 were observed between the group of patients infected with ZIKV and control subjects. Our results indicate that acute infection (up to 5 days after the onset of symptoms) by ZIKV in patients induces the high mRNA expression of TLR3 correlated to high expression of IFN-γ, IFN-α, and IFN-β, even though the high viral load is correlated to high expression of TLR8, RIG-1, MDA-5, IFN-α, and IFN-β in ZIKV patients.

Mutations in the microtubule-associated protein MAP11 (C7orf43) cause microcephaly in humans and zebrafish

Microtubule associated protein 11 (MAP11, previously termed C7orf43) encodes a highly conserved protein whose function is unknown. Through genome-wide linkage analysis combined with whole exome sequencing, we demonstrate that human autosomal recessive primary microcephaly is caused by a truncating mutation in MAP11. Moreover, homozygous MAP11-orthologue CRISPR/Cas9 knock-out zebrafish presented with microcephaly and decreased neuronal proliferation, recapitulating the human phenotype. We demonstrate that MAP11 is ubiquitously transcribed with high levels in brain and cerebellum. Immunofluorescence and co-immunoprecipitation studies in SH-SY5Y cells showed that MAP11 associates with mitotic spindles, co-localizing and physically associating with α-tubulin during mitosis. MAP11 expression precedes α-tubulin in gap formation of cell abscission at the midbody and is co-localized with PLK1, a key regulator of cytokinesis, at the edges of microtubule extensions of daughter cells post cytokinesis abscission, implicating a role in mitotic spindle dynamics and in regulation of cell abscission during cytokinesis. Finally, lentiviral-mediated silencing of MAP11 diminished SH-SY5Y cell viability, reducing proliferation rather than affecting apoptosis. Thus, MAP11 encodes a microtubule-associated protein that plays a role in spindle dynamics and cell division, in which mutations cause microcephaly in humans and zebrafish.

Spectrum of clinical heterogeneity of β-tubulin TUBB5 gene mutations

Microcephaly is a rare condition in which the occipitofrontal circumference in a child is more than two standard deviations below the mean of children of the same age and gender. It is mainly caused by genetic abnormalities that interfere with the growth of the cerebral cortex during early months of fetal development. We present a case of a 12 years old patient with microcephaly. To identify a possible genetic origin of the phenotype, we performed array CGH and exome sequencing in the patient. Exome sequencing revealed the presence of a de novo missense mutation in the TUBB5 gene (E401K). Mutations in the TUBB5 are mainly responsible for microcephaly but the clinical spectrum is wide, from patients with severe developmental delay, and the presence of different brain malformations, to patients with only slightly cognitive impairment and normal motor development. Our patient shows a milder phenotype than other patients carrying the same mutation. These differences in the clinical features suggest that other factors, presumably genetic or epigenetic, could be modulating clinical expressivity of TUBB5. It is therefore evident that more functional studies are needed to understand the pathology that underlies the clinical spectrum of tubulin associated disease states.

Brain Organoids-A Bottom-Up Approach for Studying Human Neurodevelopment

Brain organoids have recently emerged as a three-dimensional tissue culture platform to study the principles of neurodevelopment and morphogenesis. Importantly, brain organoids can be derived from human stem cells, and thus offer a model system for early human brain development and human specific disorders. However, there are still major differences between the in vitro systems and in vivo development. This is in part due to the challenge of engineering a suitable culture platform that will support proper development. In this review, we discuss the similarities and differences of human brain organoid systems in comparison to embryonic development. We then describe how organoids are used to model neurodevelopmental diseases. Finally, we describe challenges in organoid systems and how to approach these challenges using complementary bioengineering techniques.

Novel SASS6 compound heterozygous mutations in a Chinese family with primary autosomal recessive microcephaly

Primary autosomal recessive microcephaly (MCPH) is a rare hereditary disease characterized by congenitally small with brain circumference of the head below 3 standard deviations (SD). By far, 18 MCPH genes have been reported to be associated with the disease. SASS6 gene functioned in assembly of centrioles that the majority of MCPH genes present at the centrosome. There was only research reporting a homozygous missense mutation in SASS6 gene detected in a consanguineous Pakistani family. By conducting Whole-exome sequencing (WES) and Sanger sequencing on the family trio, we identified two novel splice site mutations c.127-13A>G and c.1867+2T>A in compound heterozygous hereditary form in the SASS6 gene. The two mutations were confirmed to alter mRNA splicing by RT-PCR assay. Our finding supported the role of SASS6 in the pathogenesis of microcephaly, expanding mutation spectrums and contributing to understanding of molecular mechanisms of MCPH.

Effect of head circumference in combination with facial profile line on ultrasonic diagnosis of microcephaly

Objective: Recently, microcephaly has usually been misdiagnosed only by ultrasound via measurement of head circumference (HC). Therefore, the aim of this study is to find another diagnostic index to supplement the original diagnostic method of microcephaly, to improve the detection rate of fetal microcephaly and to reduce the misdiagnosis rate.Methods: We retrospectively analyzed 123 pregnant women from February 2012 to January 2017 with fetal HC less than two standard deviations (SD). The facial profile line (FPL) was determined by ultrasonography. The first method (M1) was only used HC to determine whether the fetus was microcephaly, the second one (M2) was to combine HC and FPL for the diagnosis of microcephaly. Results were classified into five orderly categories by experienced sonographers. ROC curve was drawn to evaluate the diagnostic effect.Results: Among the pregnant women, 14 cases of fetal head circumference were less than 3SD, 109 were -2SD < HC≤ -3SD. A total of 12 cases were confirmed of microcephaly by magnetic resonance imaging (MRI) or postnatal, 10 cases of HC were less than 3SD, 2 were -2SD < HC≤ -3SD. The area under the ROC curve for M1 and M2 were 0.751 and 0.983 respectively.Conclusion: The HC in combination with FPL can be used to evaluate the fetal HC and forehead development quickly, and to improve the sensitivity and specificity of diagnosing fetal microcephaly.

Research advancements in the neurological presentation of flaviviruses

Owing to the large-scale epidemic of Zika virus disease and its association with microcephaly, properties that allow flaviviruses to cause nervous system diseases are an important area of investigation. At present, although potential pathogenic mechanisms of flaviviruses in the nervous system have been examined, they have not been completely elucidated. In this paper, we review the possible mechanisms of blood-brain barrier penetration, the pathological effects on neurons, and the association between virus mutations and neurotoxicity. A hypothesis on neurotoxicity caused by the Zika virus is presented. Clarifying the mechanisms of virulence of flaviviruses will be helpful in finding better antiviral drugs and optimizing the treatment of symptoms.

Small head circumference at birth: an 8-year retrospective cohort study in China

OBJECTIVE

Head circumference is considered a reliable assessment of the volume of the underlying brain. We sought to identify risk factors (maternal factors or antenatal antecedents) for microcephaly and to assess the effects of microcephaly on neonatal outcomes.

DESIGN

Retrospective cohort study.

SETTING

Data for all births in 2009-2017 were obtained from the Guangzhou Maternal-Fetal Care Database.

PARTICIPANTS

All singleton liveborn infants between 33 and 42 weeks' gestation (n=45 663) were categorised using the Intergrowth-21st standard for microcephaly.

MAIN OUTCOME MEASURES

Prevalence of mild, absolute and relative microcephaly at birth. We estimated associations of (1) maternal characteristics including Cantonese origin, parity, exposure to teratogens, TORCH infections (ie, Toxoplasma gondii, rubella virus, cytomegalovirus, herpes simplex virus), in vitro fertilisation conception, pre-eclampsia and maternal congenital anomalies with risk of each category of microcephaly, and (2) microcephaly with risk of in-hospital mortality and severe morbidity.

RESULTS

A total of 2709 infants had a head circumference z-score >2 SD, resulting in an overall prevalence of microcephaly of 59.3 per 1000 infants, consisting of mild (54.1 per 1000), absolute (2.8 per 1000) and relative microcephaly (2.4 per 1000). In multiple logistic regression, absolute microcephaly was associated with in utero exposure to teratogens (OR 4.2, 95% CI 2.0 to 8.8) and TORCH agents (OR 3.2, 95% CI 1.1 to 9.5). Mild microcephaly was associated with Cantonese descent (OR) 1.5, 95% CI 1.3 to 1.7) and primiparity (OR 1.7, 95% CI 1.5 to 2.0). Absolute microcephaly was associated with a significantly higher odds of neonatal seizure (OR 8.7, 95% CI 1.1 to 69.1). Mild microcephaly was not associated with adverse neonatal outcomes overall.

CONCLUSIONS

Cantonese origin, exposure to teratogens, pre-eclampsia and TORCH infection may be risk factors for microcephaly. The high prevalence of relative microcephaly and associated poor outcomes suggests that high-risk women merit closer clinical management and follow-up to maximise fetal head development during pregnancy.

Two Novel Mutations (c.883-4_890del and c.1684C>G) of WDR62 Gene Associated With Autosomal Recessive Primary Microcephaly: A Case Report

Background: Autosomal recessive primary microcephaly (Microcephaly Primary Hereditary, MCPH) is a rare disorder, affecting 1 in 10,000 children in areas where consanguineous marriages are common. WDR62 gene mutations are the second most common cause of MCPH. Herein, we report a case of primary microcephaly caused by two novel WDR62 mutations, which is, to our knowledge, the first such case report in East Asia. Case presentation: A 6-year-old girl visited our outpatient clinic as a result of microcephaly and delayed development. The patient was born at 36 weeks 4 days through cesarean section. Her birth weight was 1.8 kg (<1st percentile), and she was noted to have microcephaly (head circumference at birth was 28 cm, <-3SD). On examination, delayed speech development and microcephaly with an occipitofrontal head circumference of 43.5 cm (<-3SD) were noted. The patient's gross and fine motor development was normal. Her intelligence quotient was 43 (<0.1 percentile), the same as a 27-month-old child, and her social intelligence quotient was 76.92. Brain imaging revealed simplified gyral patterns of the cerebral cortex; however, laboratory findings, including organic acids, were normal. Multiplex ligation-dependent probe amplification technique for microdeletion syndrome and chromosomal microarray, showed no abnormality. Clinical exome sequencing test revealed two novel heterozygous variants in the WDR62 gene at two different sites: in the boundary of intron 7 and exon 8 (NM_001083961.1: c.883-4_890del) and in exon 13 (NM_001083961.1: c.1684C>G). The patient's parents were identified as heterozygous carriers for each variation. Conclusion: We report on two novel heterozygous mutations in East Asia. Our data expand the understanding of WDR62 mutations.

The conservation landscape of the human ribosomal RNA gene repeats

Ribosomal RNA gene repeats (rDNA) encode ribosomal RNA, a major component of ribosomes. Ribosome biogenesis is central to cellular metabolic regulation, and several diseases are associated with rDNA dysfunction, notably cancer, However, its highly repetitive nature has severely limited characterization of the elements responsible for rDNA function. Here we make use of phylogenetic footprinting to provide a comprehensive list of novel, potentially functional elements in the human rDNA. Complete rDNA sequences for six non-human primate species were constructed using de novo whole genome assemblies. These new sequences were used to determine the conservation profile of the human rDNA, revealing 49 conserved regions in the rDNA intergenic spacer (IGS). To provide insights into the potential roles of these conserved regions, the conservation profile was integrated with functional genomics datasets. We find two major zones that contain conserved elements characterised by enrichment of transcription-associated chromatin factors, and transcription. Conservation of some IGS transcripts in the apes underpins the potential functional significance of these transcripts and the elements controlling their expression. Our results characterize the conservation landscape of the human IGS and suggest that noncoding transcription and chromatin elements are conserved and important features of this unique genomic region.

Primary microcephaly case from the Karachay-Cherkess Republic poses an additional support for microcephaly and Seckel syndrome spectrum disorders

Background

Primary microcephaly represents an example of clinically and genetically heterogeneous condition. Here we describe a case of primary microcephaly from the Karachay-Cherkess Republic, which was initially diagnosed with Seckel syndrome.

Case presentation

Clinical exome sequencing of the proband revealed a novel homozygous single nucleotide deletion in ASPM gene, c.1386delC, resulting in preterm termination codon. Population screening reveals allele frequency to be less than 0.005. Mutations in this gene were not previously associated with Seckel syndrome.

Conclusions

Our case represents an additional support for the clinical continuum between Seckel Syndrome and primary microcephaly.

Identification of a Novel Nonsense ASPM Mutation in a Large Consanguineous Pakistani Family Using Targeted Next-Generation Sequencing

AIMS

To identify the pathogenic mutation underlying microcephaly primary hereditary (MCPH) in a large consanguineous Pakistani family.

METHODS

A five-generation family with an autosomal recessive transmission of MCPH was recruited. Targeted next-generation DNA sequencing was carried out to analyze the genomic DNA sample from the proband with MCPH using a previously designed panel targeting 46 known microcephaly-causing genes. Sanger sequencing was performed to verify all identified variants.

RESULTS

We found a novel homozygous nonsense mutation, c.7543C>T, in the ASPM gene. This mutation led to the substitution of an arginine with a stop codon at amino acid residue 2515 (p.Arg2515Ter). The mutation cosegregated with the MCPH phenotype in all affected and obligate carrier family members, but was not present in public databases (dbSNP147, Exome Variant Server, the 1000 Genomes Project, Exome Aggregation Consortium, Human Gene Mutation Database, and ClinVar) or 200 control individuals. The c.7543C>T mutation in ASPM may activate nonsense-mediated mRNA decay pathways and could underlie the pathogenesis of MCPH through a loss-of-function mechanism.

CONCLUSIONS

The c.7543C>T (p.Arg2515Ter) mutation in ASPM is a novel pathogenic mutation for the typical MCPH phenotype in this family.

A novel mutation of WDR62 gene associated with severe phenotype including infantile spasm, microcephaly, and intellectual disability

The autosomal recessive form of primary microcephaly (MCPH) is a rare disorder characterized by head circumference of at least 3 standard deviation below the mean. The MCPH exhibits genetic heterogeneity with thirteen loci (MCPH1-MCPH13) identified, and associated with variable degree of intellectual disability. It has been reported that WDR62 is the second causative gene of autosomal recessive microcephaly (MCPH2) playing a significant role in spindle formation and the proliferation of neuronal progenitor cells. We report a clinical feature, electroclinical findings, and clinical course of a patient with a severe phenotype of MCPH2 including microcephaly, refractory infantile spasms and intellectual disability. Genetic analysis detected a new homozygous splicing variant c.3335+1G>C in the WD repeat domain 62 (WDR62) gene, inherited from both heterozygous healthy parents, and an additional new heterozygous missense mutation c.1706T>A of G protein-coupled receptor 56 (GPR56) gene inherited from his healthy father. The study seeks to broaden the knowledge of clinical and electroclinical findings of MCPH2 and to contribute to a better characterization of the genotype-phenotype correlation.

Humans in a Dish: The Potential of Organoids in Modeling Immunity and Infectious Diseases

For many decades, human infectious diseases have been studied in immortalized cell lines, isolated primary cells from blood and a range of animal hosts. This research has been of fundamental importance in advancing our understanding of host and pathogen responses but remains limited by the absence of multicellular context and inherent differences in animal immune systems that result in altered immune responses. Recent developments in stem cell biology have led to the in vitro growth of organoids that faithfully recapitulate a variety of human tissues including lung, intestine and brain amongst many others. Organoids are derived from human stem cells and retain the genomic background, cellular organization and functionality of their tissue of origin. Thus they have been widely used to characterize stem cell development, numerous cancers and genetic diseases. We believe organoid technology can be harnessed to study host–pathogen interactions resulting in a more physiologically relevant model that yields more predictive data of human infectious diseases than current systems. Here, we highlight recent work and discuss the potential of human stem cell-derived organoids in studying infectious diseases and immunity.