Autosomal recessive primary microcephaly: an analysis of locus heterogeneity and phenotypic variation

Autosomal recessive primary microcephaly type 2 associated with a novel WDR62 splicing variant that disrupts the expression of the functional transcript

Background

Autosomal recessive primary microcephaly (MCPH) is a rare neurodevelopmental disorder characterized primarily by congenital microcephaly and intellectual disability but without extra-central nervous system malformations. This investigation aimed to elucidate the genetic underpinnings of microcephaly in a patient from a Chinese consanguineous family.

Methods

A comprehensive clinical assessment, including brain magnetic resonance imaging (MRI), electroencephalogram (EEG), and genetic analyses, was conducted to evaluate the patient's condition. Whole-exome sequencing (WES) was employed to identify the causative gene, followed by Sanger sequencing, to confirm the mutation and its segregation within the family. Reverse transcript polymerase chain reaction (RT-PCR) was utilized to detect changes in splicing. Western blot was employed to reveal the difference of protein expression level between the wild-type and mutant WDR62 in vitro.

Results

The patient exhibited classic MCPH symptoms, including microcephaly, recurrent epilepsy, delayed psychomotor development, and intellectual disability. Additionally, asymmetrical limb length was noted as a prominent feature. MRI findings indicated reduced brain volume with cortical malformations, while EEG demonstrated heightened sharp wave activity. A molecular analysis uncovered a novel homozygous variant c.4154-6 C > G in the WDR62 intron, and a functional analysis confirmed the pathogenicity of this mutation, resulting in the formation of an abnormal transcript with premature termination codons.

Conclusion

This study enhances our understanding of the genetic heterogeneity associated with MCPH and highlights the pivotal role of genetic testing in the diagnosing and managing of rare neurodevelopmental disorders. Furthermore, it highlights the potential of emerging genetic therapies in treating conditions such as MCPH2.

Loss of Aspm causes increased apoptosis of developing neural cells during mouse cerebral corticogenesis

Abnormal spindle-like microcephaly associated (ASPM) is a causative gene of primary autosomal recessive microcephaly. Microcephaly is considered to be a consequence of a small brain, but the associated molecular mechanisms are not fully understood. In this study, we generated brain-specific Aspm knockout mice to evaluate the fetal brain phenotype and observed cortical reduction in the late stage of murine cortical development. It has been reported that the total number of neurons is regulated by the number of neural stem and progenitor cells. In the Aspm knockout mice, no apparent change was shown in the neural progenitor cell proliferation and there was no obvious effect on the number of newly generated neurons in the developing cortex. On the other hand, the knockout mice showed a constant increase in apoptosis in the cerebral cortex from the early through the late stages of cortical development. Furthermore, apoptosis occurred in the neural progenitor cells associated with DNA damage. Overall, these results suggest that apoptosis of the neural progenitor cells is involved in the thinning of the mouse cerebral cortex, due to the loss of the Aspm gene in neocortical development.

Screening ANLN and ASPM as bladder urothelial carcinoma-related biomarkers based on weighted gene co-expression network analysis

Introduction: Bladder cancer (BLCA) is one of the most common malignancies in the urinary system with a poor prognosis and high treatment costs. Identifying potential prognostic biomarkers is significant for exploring new therapeutic and predictive targets of BLCA.

Methods: In this study, we screened differentially expressed genes using the GSE37815 dataset. We then performed a weighted gene co‐expression network analysis (WGCNA) to identify the genes correlated with the histologic grade and T stage of BLCA using the GSE32548 dataset. Subsequently, Kaplan Meier survival analysis and Cox regression were used to further identify prognosis‐related hub genes using the datasets GSE13507 and TCGA‐BLCA. Moreover, we detected the expression of the hub genes in 35 paired samples, including BLCA and paracancerous tissue, from the Shantou Central Hospital by qRT‐polymerase chain reaction.

Results: This study showed that Anillin (ANLN) and Abnormal spindle-like microcephaly-associated gene (ASPM) were prognostic biomarkers for BLCA. High expression of ANLN and ASPM was associated with poor overall survival.The qRT‐PCR results revealed that ANLN and ASPM genes were upregulated in BLCA, and there was a correlation between the expression of ANLN and ASPM in cancer tissues and paracancerous tissue. Additionally, the increasing multiples in the ANLN gene was obvious in high-grade BLCA.

Discussion: In summary, this preliminary exploration indicated a correlation between ANLN and ASPM expression. These two genes, serving as the risk factors for BLCA progression, might be promising targets to improve the occurrence and progression of BLCA.

Dual Molecular Diagnoses of Recessive Disorders in a Child from Consanguineous Parents: Case Report and Literature Review

The widespread use of whole exome sequencing (WES) resulted in the discovery of multilocus pathogenic variations (MPV), defined as two or more distinct or overlapping Mendelian disorders occurring in a patient, leading to a blended phenotype. In this study, we report on a child with autosomal recessive primary microcephaly-5 (MCPH5) and nephropathic cystinosis. The proband is the first child of consanguineous parents, presenting a complex phenotype including neurodevelopmental delay, microcephaly, growth restriction, significant delay of bone maturation, lissencephaly, and abnormality of neuronal migration, photophobia, and renal tubular acidosis. WES revealed two pathogenic and homozygous variants: a c.4174C>T variant in the ASPM gene and a c.382C>T variant in the CTNS gene, explaining the complex phenotype. The literature review showed that most of the patients harboring two variants in recessive disease genes are born to consanguineous parents. To the best of our knowledge, the patient herein described is the first one harboring pathogenic variants in both the ASPM and CTNS genes. These findings highlight the importance of searching for MPV in patients with complex phenotypes investigated by genome-wide testing methods, especially for those patients born to consanguineous parents.

MCPH1: A Novel Case Report and a Review of the Literature

Microcephaly primary hereditary (MCPH) is a congenital disease characterized by nonsyndromic reduction in brain size due to impaired neurogenesis, often associated with a variable degree of intellectual disability (ID). The genetic etiology of MCPH is heterogeneous and comprises more than 20 loci, nearly all following a recessive inheritance pattern. The first causative gene identified, MCPH1 or Microcephalin, encodes a centrosomal protein that modulates chromosome condensation and cell cycle progression. It is also involved in DNA damage response and telomere maintenance in the nucleus. Despite numerous studies on MCPH1 function, MCPH1-affected individuals are rare and the available clinical reports are not sufficient to define the natural history of the disease. Here, we present a novel patient with congenital microcephaly, ID, language delay, short stature, and other minor features such as strabismus. magnetic resonance imaging revealed ventriculomegaly, simplified gyral pattern in the frontal lobes, and a neuronal migration defect. Genetic testing detected a homozygous deletion of exons 1–8 of MCPH1. We compare the patients’ characteristics with a list of features from MCPH1 cases described in the literature, in an effort to provide additional clues for a comprehensive definition of disease presentation and evolution.

OUP accepted manuscript

Bloom syndrome (BS) is an autosomal recessive disease clinically characterized by primary microcephaly, growth deficiency, immunodeficiency and predisposition to cancer. It is mainly caused by biallelic loss-of-function mutations in the BLM gene, which encodes the BLM helicase, acting in DNA replication and repair processes. Here, we describe the gene expression profiles of three BS fibroblast cell lines harboring causative, biallelic truncating mutations obtained by single-cell (sc) transcriptome analysis. We compared the scRNA transcription profiles from three BS patient cell lines to two age-matched wild-type controls and observed specific deregulation of gene sets related to the molecular processes characteristically affected in BS, such as mitosis, chromosome segregation, cell cycle regulation and genomic instability. We also found specific upregulation of genes of the Fanconi anemia pathway, in particular FANCM, FANCD2 and FANCI, which encode known interaction partners of BLM. The significant deregulation of genes associated with inherited forms of primary microcephaly observed in our study might explain in part the molecular pathogenesis of microcephaly in BS, one of the main clinical characteristics in patients. Finally, our data provide first evidence of a novel link between BLM dysfunction and transcriptional changes in condensin complex I and II genes. Overall, our study provides novel insights into gene expression profiles in BS on an sc level, linking specific genes and pathways to BLM dysfunction.

Time is of the essence: the molecular mechanisms of primary microcephaly

In this review, Phan et al. discuss the different models that have been proposed to explain how centrosome dysfunction impairs cortical development, and review the evidence supporting a unified model in which centrosome defects reduce cell proliferation in the developing cortex by prolonging mitosis and activating a mitotic surveillance pathway. Last, they also extend their discussion to centrosome-independent microcephaly mutations, such as those involved in DNA replication and repair

Primary microcephaly is a brain growth disorder characterized by a severe reduction of brain size and thinning of the cerebral cortex. Many primary microcephaly mutations occur in genes that encode centrosome proteins, highlighting an important role for centrosomes in cortical development. Centrosomes are microtubule organizing centers that participate in several processes, including controlling polarity, catalyzing spindle assembly in mitosis, and building primary cilia. Understanding which of these processes are altered and how these disruptions contribute to microcephaly pathogenesis is a central unresolved question. In this review, we revisit the different models that have been proposed to explain how centrosome dysfunction impairs cortical development. We review the evidence supporting a unified model in which centrosome defects reduce cell proliferation in the developing cortex by prolonging mitosis and activating a mitotic surveillance pathway. Finally, we also extend our discussion to centrosome-independent microcephaly mutations, such as those involved in DNA replication and repair.

Rare Does Not Mean Worthless: How Rare Diseases Have Shaped Neurodevelopment Research in the NGS Era

The advent of next-generation sequencing (NGS) is heavily changing both the diagnosis of human conditions and basic biological research. It is now possible to dig deep inside the genome of hundreds of thousands or even millions of people and find both common and rare genomic variants and to perform detailed phenotypic characterizations of both physiological organs and experimental models. Recent years have seen the introduction of multiple techniques using NGS to profile transcription, DNA and chromatin modifications, protein binding, etc., that are now allowing us to profile cells in bulk or even at a single-cell level. Although rare and ultra-rare diseases only affect a few people, each of these diseases represent scholarly cases from which a great deal can be learned about the pathological and physiological function of genes, pathways, and mechanisms. Therefore, for rare diseases, state-of-the-art investigations using NGS have double valence: their genomic cause (new variants) and the characterize the underlining the mechanisms associated with them (discovery of gene function) can be found. In a non-exhaustive manner, this review will outline the main usage of NGS-based techniques for the diagnosis and characterization of neurodevelopmental disorders (NDDs), under whose umbrella many rare and ultra-rare diseases fall.

Phenotypes and genotypes in non-consanguineous and consanguineous primary microcephaly: High incidence of epilepsy

BACKGROUND

Primary microcephaly (PM) is defined as a significant reduction in occipitofrontal circumference (OFC) of prenatal onset. Clinical and genetic heterogeneity of PM represents a diagnostic challenge.

METHODS

We performed detailed phenotypic and genomic analyses in a large cohort (n = 169) of patients referred for PM and could establish a molecular diagnosis in 38 patients.

RESULTS

Pathogenic variants in ASPM and WDR62 were the most frequent causes in non-consanguineous patients in our cohort. In consanguineous patients, microarray and targeted gene panel analyses reached a diagnostic yield of 67%, which contrasts with a much lower rate in non-consanguineous patients (9%). Our series includes 11 novel pathogenic variants and we identify novel candidate genes including IGF2BP3 and DNAH2. We confirm the progression of microcephaly over time in affected children. Epilepsy was an important associated feature in our PM cohort, affecting 34% of patients with a molecular confirmation of the PM diagnosis, with various degrees of severity and seizure types.

CONCLUSION

Our findings will help to prioritize genomic investigations, accelerate molecular diagnoses, and improve the management of PM patients.

Novel Pathogenic Mutation Mapping of ASPM Gene in Consanguineous Pakistani Families with Primary Microcephaly

Autosomal recessive primary microcephaly (MCPH) is a neurodevelopmental disorder characterized by a congenitally reduced head circumference (-3 to -5 SD) and non-progressive intellectual disability. The objective of the study was to evaluate pathogenic mutations in the ASPM gene to understand etiology and molecular mechanism of primary microcephaly. Blood samples were collected from various families across different remote areas of Pakistan from February 2017 to May 2019 who were identified to be affected with primary microcephaly. DNA extraction was performed using the salting-out method; the quality and quantity of DNA were evaluated using spectrophotometry and 1% agarose gel electrophoresis, respectively in University of the Punjab. Mutation analysis was performed by whole exome sequencing from the Cologne Center for Genomics, University of Cologne. Sanger sequencing was done in University of the Punjab to confirm the pathogenic nature of mutation. A novel 4-bp deletion mutation c.3877_3880delGAGA was detected in exon 17 of the ASPM gene in two primary microcephaly affected families (A and B), which resulted in a frame shift mutation in the gene followed by truncated protein synthesis (p.Glu1293Lysfs*10), as well as the loss of the calmodulin-binding IQ domain and the Armadillo-like domain in the ASPM protein. Using the in-silico tools Mutation Taster, PROVEAN, and PolyPhen, the pathogenic effect of this novel mutation was tested; it was predicted to be "disease causing," with high pathogenicity scores. One previously reported mutation in exon 24 (c.9730C>T) of the ASPM gene resulting in protein truncation (p.Arg3244*) was also observed in family C. Mutations in the ASPM gene are the most common cause of MCPH in most cases. Therefore, enrolling additional affected families from remote areas of Pakistan would help in identifying or mapping novel mutations in the ASPM gene of primary microcephaly.

Microcephaly, an etiopathogenic vision

Microcephaly is defined by an occipital-frontal head circumference (OFD) 2 standard deviations (SD) smaller than the average expected for age, gender and population. Its incidence has been reported between 1.3 and 150 cases per 100,000 births. Currently, new clinical characteristics, causes and pathophysiological mechanisms related to microcephaly continue to be identified. Its etiology is varied and heterogeneous, with genetic and non-genetic factors that produce alterations in differentiation, proliferation, migration, repair of damage to deoxyribonucleic acid and neuronal apoptosis. It requires a multidisciplinary diagnostic approach that includes a medical history, detailed prenatal and postnatal clinical evaluation, cerebral magnetic resonance imaging, neuropsychological evaluation, and in some cases complementary tests such as metabolic screening, tests to rule out infectious processes and genetic testing. There is no specific treatment or intervention to increase cerebral growth; however, timely intervention strategies and programs can be established to improve motor and neurocognitive development, as well as to provide genetic counseling. The objective of this work is to review the available information and reinforce the proposal to carry out an etiopathogenic approach for microcephaly diagnosis and management.

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.

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.

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.

Evolutionary Changes in Transcriptional Regulation: Insights into Human Behavior and Neurological Conditions

Understanding the biological basis for human-specific cognitive traits presents both immense challenges and unique opportunities. Although the question of what makes us human has been investigated with several different methods, the rise of comparative genomics, epigenomics, and medical genetics has provided tools to help narrow down and functionally assess the regions of the genome that seem evolutionarily relevant along the human lineage. In this review, we focus on how medical genetic cases have provided compelling functional evidence for genes and loci that appear to have interesting evolutionary signatures in humans. Furthermore, we examine a special class of noncoding regions, human accelerated regions (HARs), that have been suggested to show human-lineage-specific divergence, and how the use of clinical and population data has started to provide functional information to examine these regions. Finally, we outline methods that provide new insights into functional noncoding sequences in evolution.

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.

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.

A New Way to Treat Brain Tumors: Targeting Proteins Coded by Microcephaly Genes?: Brain tumors and microcephaly arise from opposing derangements regulating progenitor growth. Drivers of microcephaly could be attractive brain tumor targets

New targets for brain tumor therapies may be identified by mutations that cause hereditary microcephaly. Brain growth depends on the repeated proliferation of stem and progenitor cells. Microcephaly syndromes result from mutations that specifically impair the ability of brain progenitor or stem cells to proliferate, by inducing either premature differentiation or apoptosis. Brain tumors that derive from brain progenitor or stem cells may share many of the specific requirements of their cells of origin. These tumors may therefore be susceptible to disruptions of the protein products of genes that are mutated in microcephaly. The potential for the products of microcephaly genes to be therapeutic targets in brain tumors are highlighted hereby reviewing research on EG5, KIF14, ASPM, CDK6, and ATR. Treatments that disrupt these proteins may open new avenues for brain tumor therapy that have increased efficacy and decreased toxicity.

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.

Fundamental Elements in Autism: From Neurogenesis and Neurite Growth to Synaptic Plasticity

Autism spectrum disorder (ASD) is a set of neurodevelopmental disorders with a high prevalence and impact on society. ASDs are characterized by deficits in both social behavior and cognitive function. There is a strong genetic basis underlying ASDs that is highly heterogeneous; however, multiple studies have highlighted the involvement of key processes, including neurogenesis, neurite growth, synaptogenesis and synaptic plasticity in the pathophysiology of neurodevelopmental disorders. In this review article, we focus on the major genes and signaling pathways implicated in ASD and discuss the cellular, molecular and functional studies that have shed light on common dysregulated pathways using in vitro, in vivo and human evidence.

Highlights

Autism spectrum disorder (ASD) has a prevalence of 1 in 68 children in the United States.

ASDs are highly heterogeneous in their genetic basis.

ASDs share common features at the cellular and molecular levels in the brain.

Most ASD genes are implicated in neurogenesis, structural maturation, synaptogenesis and function.

The E3 ubiquitin ligase APC/CCdh1 degrades MCPH1 after MCPH1-βTrCP2-Cdc25A-mediated mitotic entry to ensure neurogenesis

Mutations of microcephalin (MCPH1) can cause the neurodevelopmental disorder primary microcephaly type 1. We previously showed that MCPH1 deletion in neural stem cells results in early mitotic entry that distracts cell division mode, leading to exhaustion of the progenitor pool. Here, we show that MCPH1 interacts with and promotes the E3 ligase βTrCP2 to degrade Cdc25A independent of DNA damage. Overexpression of βTrCP2 or the knockdown of Cdc25A remedies the high mitotic index and rescues the premature differentiation of Mcph1-deficient neuroprogenitors in vivo MCPH1 itself is degraded by APC/C^Cdh1, but not APC/C^Cdc20, in late mitosis and G1 phase. Forced MCPH1 expression causes cell death, underlining the importance of MCPH1 turnover after mitosis. Ectopic expression of Cdh1 leads to premature differentiation of neuroprogenitors, mimicking differentiation defects of Mcph1-knockout neuroprogenitors. The homeostasis of MCPH1 in association with the ubiquitin-proteasome system ensures mitotic entry independent of cell cycle checkpoint. This study provides a mechanistic understanding of how MCPH1 controls neural stem cell fate and brain development.

The genetics of congenitally small brains

Primary microcephaly (PM) refers to a congenitally small brain, resulting from insufficient prenatal production of neurons, and serves as a model disease for brain volumic development. Known PM genes delineate several cellular pathways, among which the centriole duplication pathway, which provide interesting clues about the cellular mechanisms involved. The general interest of the genetic dissection of PM is illustrated by the convergence of Zika virus infection and PM gene mutations on congenital microcephaly, with CENPJ/CPAP emerging as a key target. Physical (protein-protein) and genetic (digenic inheritance) interactions of Wdr62 and Aspm have been demonstrated in mice, and should now be sought in humans using high throughput parallel sequencing of multiple PM genes in PM patients and control subjects, in order to categorize mutually interacting genes, hence delineating functional pathways in vivo in humans.

Genetic heterogeneity in Pakistani microcephaly families revisited

Autosomal recessive primary microcephaly (MCPH) is a rare and heterogeneous genetic disorder characterized by reduced head circumference, low cognitive prowess and, in general, architecturally normal brains. As many as 14 different loci have already been mapped. We recruited 35 MCPH families in Pakistan and could identify the genetic cause of the disease in 31 of them. Using homozygosity mapping complemented with whole-exome, gene panel or Sanger sequencing, we identified 12 novel mutations in 3 known MCPH-associated genes - 9 in ASPM, 2 in MCPH1 and 1 in CDK5RAP2. The 2 MCPH1 mutations were homozygous microdeletions of 164,250 and 577,594 bp, respectively, for which we were able to map the exact breakpoints. We also identified four known mutations - three in ASPM and one in WDR62. The latter was initially deemed to be a missense mutation but we demonstrate here that it affects splicing. As to ASPM, as many as 17 out of 27 MCPH5 families that we ascertained in our sample were found to carry the previously reported founder mutation p.Trp1326*. This study adds to the mutational spectra of four known MCPH-associated genes and updates our knowledge about the genetic heterogeneity of MCPH in the Pakistani population considering its ethnic diversity.

A novel splice-site mutation in the ASPM gene underlies autosomal recessive primary microcephaly

BACKGROUND

Autosomal recessive primary microcephaly (MCPH) is a clinically and genetically heterogeneous disorder. Patients with MCPH exhibit reduced occipito-frontal head circumference and non-progressive intellectual disability. To date, 17 genes have been known as an underlying cause of MCPH in humans. ASPM (abnormal spindle-like, microcephaly associated) is the most commonly mutated MCPH gene.

OBJECTIVE

Identify the genetic defect underlying MCPH in a Saudi family.

DESIGN

A cross-sectional clinical genetic study of a Saudi family.

SETTING

Madinah Maternity and Children Hospital and Centre for Genetics and Inherited Diseases, Taibah University.

PATIENTS AND METHODS

A molecular analysis was carried out on DNA samples from 10 individuals of a Saudi family segregating MCPH. DNA was isolated from the peripheral blood of 10 individuals, including 2 patients, and whole exome sequencing was performed using the Nextera Rapid Capture kit and NextSeq500 instrument. VariantStudio was used to filter and prioritize variants.

MAIN OUTCOME MEASURE(S)

Detection of mutation in the ASPM gene in a family segregating autoso- mal recessive primary microcephaly.

RESULTS

A novel homozygous splice-site variant (c.3742-1G > C) in the ASPM gene was identified. The variant is predicted to have an effect on splicing. Human Splice Finder, an in silico tool, predicted skipping of exon 16 due to this variant.

CONCLUSION

Skipping of exon 16 may change the order and number of IQ motifs in the ASPM protein leading to typical MCPH phenotype.

LIMITATIONS

Single family study.

The Janus soul of centrosomes: a paradoxical role in disease?

The centrosome is the main microtubule organizing center of animal cells. It contributes to spindle assembly and orientation during mitosis and to ciliogenesis in interphase. Numerical and structural defects in this organelle are known to be associated with developmental disorders such as dwarfism and microcephaly, but only recently, the molecular mechanisms linking centrosome aberrations to altered physiology are being elucidated. Defects in centrosome number or structure have also been described in cancer. These opposite clinical outcomes--arising from reduced proliferation and overproliferation respectively--can be explained in light of the tissue- and developmental-specific requirements for centrosome functions. The pathological outcomes of centrosome deficiencies have become clearer when considering its consequences. Among them, there are genetic instability (mainly aneuploidy, a defect in chromosome number), defects in the symmetry of cell division (important for cell fate specification and tissue architecture) and impaired ciliogenesis. In this review, we discuss the origins and the consequences of centrosome flaws, with particular attention on how they contribute to developmental diseases.

The DNA damage response molecule MCPH1 in brain development and beyond

Microcephalin (MCPH1) is identified as being responsible for the neurodevelopmental disorder primary microcephaly type 1, which is characterized by a smaller-than-normal brain size and mental retardation. MCPH1 has originally been identified as an important regulator of telomere integrity and of cell cycle control. Genetic and cellular studies show that MCPH1 controls neurogenesis by coordinating the cell cycle and the centrosome cycle and thereby regulating the division mode of neuroprogenitors to prevent the exhaustion of the progenitor pool and thereby microcephaly. In addition to its role in neurogenesis, MCPH1 plays a role in gonad development. MCPH1 also functions as a tumor suppressor in several human cancers as well as in mouse models. Here, we review the role of MCPH1 in DNA damage response, cell cycle control, chromosome condensation and chromatin remodeling. We also summarize the studies on the biological functions of MCPH1 in brain size determination and in pathologies, including infertility and cancer.

Genetic and environmental modulation of neurodevelopmental disorders: Translational insights from labs to beds

Neurodevelopmental disorders (NDDs) are a heterogeneous group of prevalent neuropsychiatric illnesses with various degrees of social, cognitive, motor, language and affective deficits. NDDs are caused by aberrant brain development due to genetic and environmental perturbations. Common NDDs include autism spectrum disorder (ASD), intellectual disability, communication/speech disorders, motor/tic disorders and attention deficit hyperactivity disorder. Genetic and epigenetic/environmental factors play a key role in these NDDs with significant societal impact. Given the lack of their efficient therapies, it is important to gain further translational insights into the pathobiology of NDDs. To address these challenges, the International Stress and Behavior Society (ISBS) has established the Strategic Task Force on NDDs. Summarizing the Panel's findings, here we discuss the neurobiological mechanisms of selected common NDDs and a wider NDD+ spectrum of associated neuropsychiatric disorders with developmental trajectories. We also outline the utility of existing preclinical (animal) models for building translational and cross-diagnostic bridges to improve our understanding of various NDDs.

The Impact of rs3762271 and rs930557 Polymorphisms of ASPM and MCPH1 Genes on the Anatomy and Function of the Brain

The ASPM and MCPH1 genes are involved in early neurogenesis and are thus potential candidates for affecting the formation of the anatomical and functional characteristics of the brain. However, the results of studies to date have been conflicting, an issue for which the factor of ethnicity may be responsible. We aimed to examine whether the rs3762271 and rs930557 polymorphisms of these two genes can influence brain anatomy and function. We enrolled 97 Caucasian neonates, with males predominating (53.6%). The anatomy of the brain was examined using ultrasound, while Doppler ultrasound was used to establish the blood flow indices in particular brain blood vessels. Genetic analysis was carried out using the polymerase chain reaction-restriction fragment length polymorphism method. The CC and AA homozygotes of rs3762271 were more common in males. The CC genotype of rs3762271 was significantly associated with birth weight (pRE = .03) and body length (pRE = .02). One mutant allele of rs3762271 was significantly associated with higher values of maximum (Vmax, p = .04), minimum (Vmin, p = .04), and average (Vmean, p = .02) speed in the pericallosal artery in newborns of both genders. Similar relationships were found in females only (Vmax p = .03, Vmean p = .02). The CC genotype of rs930557 was more frequently observed in male infants, but no impact on any anthropometric indices or anatomical and functional parameters of the brain was established. The ASPM gene may play a role in shaping the functional parameters of the brain in Caucasians.

Refining the phenotype associated with CASC5 mutation

Autosomal recessive primary microcephaly is a neurodevelopmental disorder characterized by congenitally reduced head circumference by at least two standard deviations (SD) below the mean for age and gender. It is associated with nonprogressive mental retardation of variable degree, minimal neurological deficit with no evidence of architectural anomalies of the brain. So far, 12 genetic loci (MCPH1-12) and corresponding genes have been identified. Most of these encode centrosomal proteins. CASC5 is one the most recently unravelled genes responsible for MCPH with mutations reported in three consanguineous families of Moroccan origin, all of whom harboured the same CASC5 homozygous mutation (c.6125G>A; p.Met2041Ile). Here, we report the identification, by whole exome sequencing, of the same missense mutation in a consanguineous Algerian family. All patients exhibited a similar clinical phenotype, including congenital microcephaly with head circumferences ranging from -3 to -4 standard deviations (SD) after age 5 years, moderate to severe cognitive impairment, short stature (adult height -3 SD), dysmorphic features included a sloping forehead, thick eyebrows, synophris and a low columella. Severe vermis hypoplasia and a large cyst of the posterior fossa were observed in one patient. Close microsatellite markers showed identical alleles in the Algerian the previously and Moroccan patients. This study confirms the involvement of CASC5 in autosomal recessive microcephaly and supports the hypothesis of a founder effect of the c.6125G>A mutation. In addition, this report refines the phenotype of this newly recognized form of primary microcephaly.

Molekulare Grundlagen der autosomal-rezessiven primären Mikrozephalie

Die primäre autosomal-rezessive Mikrozephalie (MCPH) ist eine genetisch sehr heterogene Erkrankung, die klinisch definiert wird durch das Vorliegen einer kongenitalen, nicht progressiven Mikrozephalie, einer mentalen Retardierung variablen Ausmaßes bei weitgehend normaler Körpergröße und das Fehlen von zusätzlichen Fehlbildungen und weiteren neurologischen Befunden. Bislang konnten Mutationen in 14 verschiedenen Genen identifiziert werden, deren Produkte auf zellulärer Ebene insbesondere bei Vorgängen der Zellteilung, der Zellzyklusregulierung und bei der Aktivierung von DNA-Reparaturmechanismen nach DNA-Schädigungen eine wichtige Rolle spielen. Darüber hinaus sind auch syndromale Formen der Mikrozephalie bekannt, zu denen u. a. das Seckel-Syndrom sowie der mikrozephale osteodysplastische primordiale Kleinwuchs Typ II (MOPD II) zählen.

MCPH1: a window into brain development and evolution

The development of the mammalian cerebral cortex involves a series of mechanisms: from patterning, progenitor cell proliferation and differentiation, to neuronal migration. Many factors influence the development of the cerebral cortex to its normal size and neuronal composition. Of these, the mechanisms that influence the proliferation and differentiation of neural progenitor cells are of particular interest, as they may have the greatest consequence on brain size, not only during development but also in evolution. In this context, causative genes of human autosomal recessive primary microcephaly, such as ASPM and MCPH1, are attractive candidates, as many of them show positive selection during primate evolution. MCPH1 causes microcephaly in mice and humans and is involved in a diverse array of molecular functions beyond brain development, including DNA repair and chromosome condensation. Positive selection of MCPH1 in the primate lineage has led to much insight and discussion of its role in brain size evolution. In this review, we will present an overview of MCPH1 from these multiple angles, and whilst its specific role in brain size regulation during development and evolution remain elusive, the pieces of the puzzle will be discussed with the aim of putting together the full picture of this fascinating gene.

Molecular genetics of human primary microcephaly: an overview

Autosomal recessive primary microcephaly (MCPH) is a neurodevelopmental disorder that is characterised by microcephaly present at birth and non-progressive mental retardation. Microcephaly is the outcome of a smaller but architecturally normal brain; the cerebral cortex exhibits a significant decrease in size. MCPH is a neurogenic mitotic disorder, though affected patients demonstrate normal neuronal migration, neuronal apoptosis and neural function. Twelve MCPH loci (MCPH1-MCPH12) have been mapped to date from various populations around the world and contain the following genes: Microcephalin, WDR62, CDK5RAP2, CASC5, ASPM, CENPJ, STIL, CEP135, CEP152, ZNF335, PHC1 and CDK6. It is predicted that MCPH gene mutations may lead to the disease phenotype due to a disturbed mitotic spindle orientation, premature chromosomal condensation, signalling response as a result of damaged DNA, microtubule dynamics, transcriptional control or a few other hidden centrosomal mechanisms that can regulate the number of neurons produced by neuronal precursor cells. Additional findings have further elucidated the microcephaly aetiology and pathophysiology, which has informed the clinical management of families suffering from MCPH. The provision of molecular diagnosis and genetic counselling may help to decrease the frequency of this disorder.

A novel single base pair duplication in WDR62 causes primary microcephaly

BACKGROUND

Primary microcephaly is a disorder of the brain resulting in a reduced head circumference that can come along with intellectual disability but with hardly any other neurological abnormalities.

CASE PRESENTATION

In this study we report on three Pakistani males from a consanguineous family with 2, 4 and 25 years, diagnosed with autosomal recessive primary microcephaly. By genotyping, Sanger sequencing and using bioinformatical approaches the disease causing mutation was identified and evaluated.

CONCLUSION

By using a 250K SNP array, we were able to detect an 11Mb large autozygous region in the MCPH2 locus on chromosome 19q13.12. Sequencing of the associated gene, WDR62, revealed the frameshift causing single base pair duplication, c.2527dupG. This mutation is predicted to affect the structural features of WDR62 which in turn changes the conformation and function of the protein. Aspartic acid (D) at position 843 was found to be conserved among various ortholog species. The present findings will be helpful in genetic diagnosis of patients and future studies of WDR62.

Simplified gyral pattern in severe developmental microcephalies? New insights from allometric modeling for spatial and spectral analysis of gyrification

The strong positive-allometric relationship between brain size, cortical extension and gyrification complexity, recently highlighted in the general population, could be modified by brain developmental disorders. Indeed, in case of brain growth insufficiency, the pathophysiological relevance of the "simplified gyral pattern" phenotype is strongly disputed since almost no genotype-phenotype correlations have been found in primary microcephalies. Using surface scaling analysis and newly-developed spectral analysis of gyrification (Spangy), we tested whether the gyral simplification in groups of severe microcephalies related to ASPM, PQBP1 or fetal-alcohol-syndrome could be fully explained by brain size reduction according to the allometric scaling law established in typically-developing control groups, or whether an additional disease effect was to be suspected. We found the surface area reductions to be fully explained by scaling effect, leading to predictable folding intensities measured by gyrification indices. As for folding pattern assessed by spectral analysis, scaling effect also accounted for the majority of the variations, but an additional negative or positive disease effect was found in the case of ASPM and PQBP1-linked microcephalies, respectively. Our results point out the necessity of taking allometric scaling into account when studying the gyrification variability in pathological conditions. They also show that the quantitative analysis of gyrification complexity through spectral analysis can enable distinguishing between even (predictable, non-specific) and uneven (unpredictable, maybe disease-specific) gyral simplifications.

The first case of CDK5RAP2-related primary microcephaly in a non-consanguineous patient identified by next generation sequencing

Primary autosomal recessive microcephaly (MCPH) is a genetically heterogeneous condition characterized by congenital microcephaly and intellectual disability. To date, 10 MCPH loci have been identified and due to the genetic heterogeneity of this condition, molecular testing for MCPH can be complicated. Our methods involved employing a next generation sequencing panel of MCPH-related genes allowing for the evaluation of multiple disease loci simultaneously. Next generation sequencing analysis of a 6 year old female with primary microcephaly identified novel compound heterozygous mutations (c.524_528del and c.4005-1G>A) in the CDK5RAP2 gene. A review of the published literature to date reveals that only three mutations have been previously reported in the CDK5RAP2 gene in the homozygous state in three Northern Pakistani and one Somali consanguineous MCPH families. Our patient represents the first non-consanguineous Caucasian individual to have been identified with CDK5RAP2-related MCPH. As only a handful of patients have been reported in the literature with CDK5RAP2-related MCPH, we anticipate the identification of individuals with CDK5RAP2 mutations from all ethnic backgrounds will continue. Our patient contributes to the ethnic and genotypic spectrum of CDK5RAP2-related MCPH and supports the occurrence of this genetic condition beyond that of consanguineous families of certain ethnic populations. Our results also highlight the utility of multi-gene sequencing panels to elucidate the etiology of genetically heterogeneous conditions.

The diverse genetic landscape of neurodevelopmental disorders

Advances in genetic tools and sequencing technology in the past few years have vastly expanded our understanding of the genetics of neurodevelopmental disorders. Recent high-throughput sequencing analyses of structural brain malformations, cognitive and neuropsychiatric disorders, and localized cortical dysplasias have uncovered a diverse genetic landscape beyond classic Mendelian patterns of inheritance. The underlying genetic causes of neurodevelopmental disorders implicate numerous cell biological pathways critical for normal brain development.

Homo floresiensis contextualized: a geometric morphometric comparative analysis of fossil and pathological human samples

The origin of hominins found on the remote Indonesian island of Flores remains highly contentious. These specimens may represent a new hominin species, Homo floresiensis, descended from a local population of Homo erectus or from an earlier (pre-H. erectus) migration of a small-bodied and small-brained hominin out of Africa. Alternatively, some workers suggest that some or all of the specimens recovered from Liang Bua are pathological members of a small-bodied modern human population. Pathological conditions proposed to explain their documented anatomical features include microcephaly, myxoedematous endemic hypothyroidism (“cretinism”) and Laron syndrome (primary growth hormone insensitivity). This study evaluates evolutionary and pathological hypotheses through comparative analysis of cranial morphology. Geometric morphometric analyses of landmark data show that the sole Flores cranium (LB1) is clearly distinct from healthy modern humans and from those exhibiting hypothyroidism and Laron syndrome. Modern human microcephalic specimens converge, to some extent, on crania of extinct species of Homo. However in the features that distinguish these two groups, LB1 consistently groups with fossil hominins and is most similar to H. erectus. Our study provides further support for recognizing the Flores hominins as a distinct species, H. floresiensis, whose affinities lie with archaic Homo.

Analysis of ASPM in an ethnically diverse cohort of 400 patient samples: perspectives of the molecular diagnostic laboratory

Primary Autosomal Recessive Microcephaly (MCPH) is characterized by congenital microcephaly usually without additional clinical findings. The most common gene implicated in MCPH is ASPM and a large percentage of mutations described have been homozygous and in consanguineous families primarily of East Asian and Middle Eastern origin. ASPM sequencing was performed on 400 patients between the years 2009 and 2012. Seventy of the patient samples were also analyzed for copy number changes in the ASPM gene. Forty protein truncating mutations, including 29 novel mutations, were identified in 39 patients with MCPH. Approximately one third of patients were compound heterozygotes, indicative of non-consanguinity in these patients. In addition, 46 non-synonymous variants were identified and interpreted as variants of uncertain significance. No deletion/duplication in ASPM was identified in the patients analyzed. A wide ethnic distribution was observed, including the first reported patients with ASPM-related MCPH of Hispanic descent. Clinical information was collected for 26 of the ASPM-positive patients and 41 of the ASPM-negative patients. As more individuals are identified with MCPH, we anticipate that we will continue to identify ASPM mutation-positive patients from all ethnic origins supporting the occurrence of this genetic condition beyond that of consanguineous families of certain ethnic populations.

A Novel Deletion Mutation in ASPM Gene in an Iranian Family with Autosomal Recessive Primary Microcephaly

OBJECTIVE

Autosomal recessive primary microcephaly (MCPH) is a neurodevelopmental and genetically heterogeneous disorder with decreased head circumference due to the abnormality in fetal brain growth. To date, nine loci and nine genes responsible for the situation have been identified. Mutations in the ASPM gene (MCPH5) is the most common cause of MCPH. The ASPM gene with 28 exons is essential for normal mitotic spindle function in embryonic neuroblasts.

MATERIALS & METHODS

We have ascertained twenty-two consanguineous families with intellectual disability and different ethnic backgrounds from Iran. Ten out of twenty-two families showed primary microcephaly in clinical examination. We investigated MCPH5 locus using homozygosity mapping by microsatellite marker.

RESULT

Sequence analysis of exon 8 revealed a deletion of nucleotide (T) in donor site of splicing site of ASPM in one family. The remaining nine families were not linked to any of the known loci .More investigation will be needed to detect the causative defect in these families.

CONCLUSION

[corrected] We detected a novel mutation in the donor splicing site of exon 8 of the ASPM gene. This deletion mutation can alter the ASPM transcript leading to functional impairment of the gene product.

The microtubule-associated protein ASPM regulates spindle assembly and meiotic progression in mouse oocytes

The microtubule-associated protein ASPM (abnormal spindle-like microcephaly-associated) plays an important role in spindle organization and cell division in mitosis and meiosis in lower animals, but its function in mouse oocyte meiosis has not been investigated. In this study, we characterized the localization and expression dynamics of ASPM during mouse oocyte meiotic maturation and analyzed the effects of the downregulation of ASPM expression on meiotic spindle assembly and meiotic progression. Immunofluorescence analysis showed that ASPM localized to the entire spindle at metaphase I (MI) and metaphase II (MII), colocalizing with the spindle microtubule protein acetylated tubulin (Ac-tubulin). In taxol-treated oocytes, ASPM colocalized with Ac-tubulin on the excessively polymerized microtubule fibers of enlarged spindles and the numerous asters in the cytoplasm. Nocodazole treatment induced the gradual disassembly of microtubule fibers, during which ASPM remained colocalized with the dynamic Ac-tubulin. The downregulation of ASPM expression by a gene-specific morpholino resulted in an abnormal meiotic spindle and inhibited meiotic progression; most of the treated oocytes were blocked in the MI stage with elongated meiotic spindles. Furthermore, coimmunoprecipitation combined with mass spectrometry and western blot analysis revealed that ASPM interacted with calmodulin in MI oocytes and that these proteins colocalized at the spindle. Our results provide strong evidence that ASPM plays a critical role in meiotic spindle assembly and meiotic progression in mouse oocytes.

ASPM regulates Wnt signaling pathway activity in the developing brain

Autosomal recessive primary microcephaly (MCPH) is a neural developmental disorder in which patients display significantly reduced brain size. Mutations in Abnormal Spindle Microcephaly (ASPM) are the most common cause of MCPH. Here, we investigate the underlying functions of Aspm in brain development and find that Aspm expression is critical for proper neurogenesis and neuronal migration. The Wnt signaling pathway is known for its roles in embryogenesis, and genome-wide siRNA screens indicate that ASPM is a positive regulator of Wnt signaling. We demonstrate that knockdown of Aspm results in decreased Wnt-mediated transcription, and that expression of stabilized β-catenin can rescue this deficit. Finally, coexpression of stabilized β-catenin can rescue defects observed upon in vivo knockdown of Aspm. Our findings provide an impetus to further explore Aspm's role in facilitating Wnt-mediated neurogenesis programs, which may contribute to psychiatric illness etiology when perturbed.

A primary microcephaly protein complex forms a ring around parental centrioles

Autosomal recessive primary microcephaly (MCPH) is characterised by a significant reduction in prenatal human brain growth, without alteration of cerebral architecture. The genetic aetiology of MCPH is bi-allelic mutations in genes coding for a subset of centrosomal proteins^1-10. While at least three of these proteins have been implicated in centrosome duplication¹¹, the nature of centrosome dysfunction that underlies the neurodevelopmental defect in MCPH is unclear. Here we report a homozygous MCPH-causing mutation in the human CEP63 gene. CEP63 forms a complex with another MCPH protein, CEP152, a conserved centrosome duplication factor^12-15. Together, they are essential for maintaining normal centrosome numbers in cells. Using super-resolution microscopy we find that CEP63 and CEP152 co-localise in a discrete ring around the proximal end of the parental centriole, a pattern specifically disrupted in CEP63-deficient patient-derived cells. This work suggests that the CEP152-CEP63 ring-like structure ensures normal neurodevelopment and its impairment particularly affects human cerebral cortex growth.