Autosomal Recessive Primary Microcephaly (MCPH): clinical manifestations, genetic heterogeneity and mutation continuum

Genetic Microcephaly in a Saudi Population: Unique Spectrum of Affected Genes Including a Novel One

Background: Genetic microcephaly is linked to an increased risk of developmental disabilities, epilepsy, and motor impairment. The aim of this study is to describe the spectrum of identifiable genetic etiologies, clinical characteristics, and radiologic features of genetic microcephaly in patients referred to a tertiary center in Saudi Arabia. Method: This is a retrospective chart review study of all patients with identifiable genetic microcephaly presenting to a tertiary center in Saudi Arabia. The patients' demographics, clinical, laboratory, radiologic, and molecular findings were collected. Results: Of the total 128 cases referred, 52 cases (40%) had identifiable genetic causes. Monogenic disorders were found in 48 cases (92%), whereas chromosomal disorders were found in only 4 cases (8%). Developmental disability was observed in 40 cases (84%), whereas only 8 cases (16%) had borderline IQ or mild developmental delay. Epilepsy was seen in 29 cases (56%), and motor impairment was seen in 26 cases (50%). Brain magnetic resonance imaging (MRI) revealed abnormalities in 26 (50%) of the cohort. Hereditary neurometabolic disorders were seen in 7 (15%) of the 48 cases with monogenic disorders. The most common gene defect was ASPM, which is responsible for primary microcephaly type 5 and was seen in 10 cases (19%). A novel PLK1 gene pathogenic mutation was seen in 3 cases (6%). Conclusion: Single gene defect is common in this Saudi population, with the ASPM gene being the most common. Hereditary neurometabolic disorders are a common cause of genetic microcephaly. Furthermore, we propose the PKL1 gene mutation as a possible novel cause of genetic microcephaly.

DNA damage and repair: underlying mechanisms leading to microcephaly

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

The neurological and non-neurological roles of the primary microcephaly-associated protein ASPM

Primary microcephaly (MCPH), is a neurological disorder characterized by small brain size that results in numerous developmental problems, including intellectual disability, motor and speech delays, and seizures. Hitherto, over 30 MCPH causing genes (MCPHs) have been identified. Among these MCPHs, MCPH5, which encodes abnormal spindle-like microcephaly-associated protein (ASPM), is the most frequently mutated gene. ASPM regulates mitotic events, cell proliferation, replication stress response, DNA repair, and tumorigenesis. Moreover, using a data mining approach, we have confirmed that high levels of expression of ASPM correlate with poor prognosis in several types of tumors. Here, we summarize the neurological and non-neurological functions of ASPM and provide insight into its implications for the diagnosis and treatment of MCPH and cancer.

YIPF5 (p.W218R) mutation induced primary microcephaly in rabbits

Primary microcephaly (PMCPH) is a rare autosomal recessive neurodevelopmental disorder with a global prevalence of PMCPH ranging from 0.0013% to 0.15%. Recently, a homozygous missense mutation in YIPF5 (p.W218R) was identified as a causative mutation of severe microcephaly. In this study, we constructed a rabbit PMCPH model harboring YIPF5 (p.W218R) mutation using SpRY-ABEmax mediated base substitution, which precisely recapitulated the typical symptoms of human PMCPH. Compared with wild-type controls, the mutant rabbits exhibited stunted growth, reduced head circumference, altered motor ability, and decreased survival rates. Further investigation based on model rabbit elucidated that altered YIPF5 function in cortical neurons could lead to endoplasmic reticulum stress and neurodevelopmental disorders, interference of the generation of apical progenitors (APs), the first generation of progenitors in the developing cortex. Furthermore, these YIPF5-mutant rabbits support a correlation between unfolded protein responses (UPR) induced by endoplasmic reticulum stress (ERS), and the development of PMCPH, thus providing a new perspective on the role of YIPF5 in human brain development and a theoretical basis for the differential diagnosis and clinical treatment of PMCPH. To our knowledge, this is the first gene-edited rabbit model of PMCPH. The model better mimics the clinical features of human microcephaly than the traditional mouse models. Hence, it provides great potential for understanding the pathogenesis and developing novel diagnostic and therapeutic approaches for PMCPH.

Incidence, aetiology and neurodisability associated with severe microcephaly: a national surveillance study

OBJECTIVE

To determine the incidence, causes and neurodevelopmental impact of severe microcephaly (head circumference <-3SD) up to age 2 years.

DESIGN

Binational active paediatric surveillance study undertaken in 2017-2018 to identify and characterise new diagnoses of severe microcephaly.

SETTING

UK and Ireland.

PARTICIPANTS

Infants aged under 12 months at diagnosis.

INTERVENTIONS

Observational study.

MAIN OUTCOME MEASURES

Incidence, aetiology and neurodevelopmental outcomes at age 2 years.

RESULTS

Fifty-nine infants met the case definition, of whom 30 (51%) were girls; 24 (41%) were born preterm (<37 weeks' gestation); and 34 (58%) were of 'white' ethnicity. Eight (14%) children died before 12 months of age. Incidence of severe microcephaly was 5.5 per 100 000 infants (95% CI 4.0 to 7.3). Higher relative risk (RR) was associated with preterm birth (RR 7.7, 95% CI 3.8 to 15.1) and British Asian ethnicity (RR 3.6, 95% CI 1.6 to 7.8). Microcephaly was mainly due to genetic causes (59%), brain ischaemia/hypoxia (10%) and congenital infection (8%), and 19% remained undetermined. Each child was referred on average to eight specialists, and 75% had abnormal brain imaging. By 2 years of age, 55 children experienced neurodevelopmental abnormalities, including feeding problems (68%), motor delay (66%), visual impairment (37%), hearing loss (24%) and epilepsy (41%).

CONCLUSIONS

Although severe microcephaly is uncommon, it is associated with high mortality, complex multimorbidity and neurodisability, thus representing a significant ongoing burden for families and healthcare services. Potentially preventable causes include preterm birth, hypoxic/ischaemic brain injury and congenital infections. Clinical guidelines are essential to standardise aetiological investigation and optimise multidisciplinary management.

Novel and recurrent ASPM mutations of founder effect in Chinese population

PURPOSE

Mutations in ASPM are the most common causes of primary microcephaly (MCPH), which is a rare brain developmental disorder with few studies in Chinese population so far. This study aimed to identify the common pathogenic variants of ASPM and estimated the incidence of MCPH5 in Guangxi population.

METHODS

We ascertained six MCPH cases caused by ASPM mutations in Guangxi Zhuang Autonomous Region, Whole-exome sequencing (WES) was performed to uncover the causal variants. The haplotype analysis was used to estimate the age of the recurrent variation.

RESULTS

Five different pathogenic variants were identified in this batch of MCPH5 cases, including two novel variants p.Ser842fs*9 and p.Lys1340Argfs*29. An rarely reported pathogenic variant, c.1789C>T/p.Arg597* was found to be a founder mutation in local population. We evaluated all ASPM variants detected among 2674 non-microcephalic individuals and estimated the MCPH5 incidence to be 5.03/1,000,000 in Guangxi population.

CONCLUSIONS

We reported the first case series of Chinese MCPH cases with ASPM mutation and revealed a highly recurrent founder mutation in this local population. MCPH5 may be the major type of congenital microcephaly in Chinese population.

Primary Dwarfism, Microcephaly, and Chorioretinopathy due to a PLK4 Mutation in Two Siblings

INTRODUCTION

Primary autosomal recessive microcephalies (MCPHs) are characterized by primary dwarfism with MCPH and may present delayed psychomotor development and visual impairment. Biallelic loss of function variants in the PLK4 gene, which encodes the polo-like kinase 4 protein involved in centriole biogenesis, has been recently identified in several patients with MCPH and various ethnic backgrounds.

CASE PRESENTATION

Here, we describe 2 siblings of different sex from Equatorial Guinea harboring a homozygous frameshift mutation in PLK4 (c.1299_1303del, p.Phe433Leufs*6). A Seckel syndrome spectrum phenotype was present in both siblings, with short stature, severe MCPH, reduced brain volume, and distinctive facial features. They also presented severe intellectual disability, lissencephaly/pachygyria, subependymal heterotopia, and ophthalmological impairment. One of them suffered from deafness, and scoliosis was observed in the other.

DISCUSSION/CONCLUSION

Biallelic variants in PLK4 lead to a syndrome where severe short stature, MCPH, and cognitive impairment are constant features. However, ocular, skeletal, and other neurological manifestations can vary upon the same genetic basis.

Two New Cases of Primary Microcephaly with Neuronal Migration Defect Caused by Truncating Mutations in the ASPM Gene

Autosomal recessive primary microcephaly (MCPH) is a uncommon disorder due to congenital deficiency in the development of the cerebral cortex, characterized by a head circumference below 2 SD. MCPH is a group of diseases with genetic heterogeneity and has been reported by the Online Mendelian Inheritance In Man® (OMIM) database and associated with 25 different genes. It is known that MCPH cases are most frequently associated with abnormal spindle-like, microcephaly-associated (ASPM) gene mutations. The ASPM protein consists of an N-terminal 81 IQ (isoleucine-glutamine) domain, a calponin-homology domain, and a C-terminal domain. It interacts with calmodulin and calmodulin-related proteins via the IQ domain and acts as a part in mitotic spindle function. The basic characteristics of cases with ASPM gene mutations are microcephaly (below -3 SD) present before 1 year of age, intellectual disability, and the absence of other congenital anomalies. Macroscopic organization of the brain is preserved in cases with ASPM mutation, and a decrease in brain volume, particularly gray matter volume loss and a simplified gyral pattern are observed. Cortical migration defects are a very rare finding in patients with ASPM mutations. In the present study, we aimed to discuss the clinical and genetic findings in 2 cases with cortical dysplasia in which truncated variants in the ASPM gene were detected, particularly in terms of genotype-phenotype correlation in comparison with the literature.

Whole exome sequencing identifies a novel mutation in ASPM and ultra-rare mutation in CDK5RAP2 causing Primary microcephaly in consanguineous Pakistani families

Background & Objectives:

Primary Microcephaly (MCPH) is a rare neurogenetic disease, manifesting congenitally reduced head circumference and non-progressive intellectual disability (ID). To date, twenty-eight genes with biallelic mutations have been reported for this disorder. The study aimed for molecular genetic characterization of Pakistani families segregating MCPH.

Methods:

We studied two unrelated consanguineous families (family A and B) presenting >2 patients with diagnostic symptoms of MCPH, born to asymptomatic parents. We employed whole-exome sequencing (WES) of probands to find putative causal mutations. The candidate variants were further confirmed and analyzed for co-segregation by Sanger sequencing of all available members of each family. This study was conducted at Government College University, Faisalabad, Pakistan, and Cologne Center for Genomics (CCG), University of Cologne, Germany; during 2017-2020.

Results:

We identified a novel homozygous variant c.10097_10098delGA, p.(Gly3366Glufs*19) in exon 26 of ASPM gene in family A which presents with moderate intellectual disability, speech impairment, visual abnormalities, seizures, and ptyalism. Family B was found to segregate nonsense, homozygous variant c.448C>T p.(Arg150*) in CDK5RAP2. The patients also exhibited mild to severe seizures without ptyalism that has not been previously reported in patients with mutations in the CDK5RAP2 gene.

Conclusion:

We report a novel mutation in ASPM and ultra-rare mutation in the CDK5RAP2 gene, both causing primary microcephaly. The study expands the mutational spectrum of the ASPM gene to 212, and also adds to the clinical spectrum of CDK5RAP2 mutations. It also demonstrated the utility of WES in the investigation and genetic diagnosis of genetically heterogeneous disorders like MCPH. These findings would aid in diagnostic and preventive strategies including carrier screening, cascade testing, and genetic counselling.

Autosomal Recessive Primary Microcephaly: Not Just a Small Brain

Microcephaly or reduced head circumference results from a multitude of abnormal developmental processes affecting brain growth and/or leading to brain atrophy. Autosomal recessive primary microcephaly (MCPH) is the prototype of isolated primary (congenital) microcephaly, affecting predominantly the cerebral cortex. For MCPH, an accelerating number of mutated genes emerge annually, and they are involved in crucial steps of neurogenesis. In this review article, we provide a deeper look into the microcephalic MCPH brain. We explore cytoarchitecture focusing on the cerebral cortex and discuss diverse processes occurring at the level of neural progenitors, early generated and mature neurons, and glial cells. We aim to thereby give an overview of current knowledge in MCPH phenotype and normal brain growth.

Modeling human neurodevelopmental diseases with brain organoids

Studying the etiology of human neurodevelopmental diseases has long been a challenging task due to the brain's complexity and its limited accessibility. Human pluripotent stem cells (hPSCs)-derived brain organoids are capable of recapitulating various features and functionalities of the human brain, allowing the investigation of intricate pathogenesis of developmental abnormalities. Over the past years, brain organoids have facilitated identifying disease-associated phenotypes and underlying mechanisms for human neurodevelopmental diseases. Integrating with more cutting-edge technologies, particularly gene editing, brain organoids further empower human disease modeling. Here, we review the latest progress in modeling human neurodevelopmental disorders with brain organoids.

Two novel truncating variants of the ASPM gene identified in a nonconsanguineous Chinese family associated with primary microcephaly

Primary autosomal recessive microcephaly 5 (MCPH5) is a rare neurodevelopmental disorder with a relatively high incidence in regions where consanguineous marriage is widely practiced; So far, only a few MCPH5 cases have been reported from China. Here, we report clinical and molecular characteristics of two Chinese MCPH5 patients, a 24-year-old woman proband and her brother, a 19-year-old man, from a nonconsanguineous family. Main manifestations in the proband were small head circumference, premature closure of fontanelles, impaired concentration and moderate intellectual disability. The proband's brother had similar symptoms, but he was hyperactive and had a more severe sloping forehead. Brain imaging revealed global reduction in brain size, especially in the frontal lobes bilaterally and anterior horns of lateral ventricles. Sequencing results revealed that both patients carried a novel nonsense variant p.Tyr2004* (c.6012_6013delTA) and a novel frameshift variant p.Arg2005Serfs*48 (c.6015_6016delGG) in the ASPM gene. These variants were interpreted to be pathogenic in the in-silico analysis. Our findings help to expand the mutation spectrum of ASPM and provide new opportunities for assisting the traditional clinical diagnosis on the cases with atypical characteristics.

Thirty novel sequence variants impacting human intracranial volume

Intracranial volume, measured through magnetic resonance imaging and/or estimated from head circumference, is heritable and correlates with cognitive traits and several neurological disorders. We performed a genome-wide association study meta-analysis of intracranial volume (n = 79 174) and found 64 associating sequence variants explaining 5.0% of its variance. We used coding variation, transcript and protein levels, to uncover 12 genes likely mediating the effect of these variants, including GLI3 and CDK6 that affect cranial synostosis and microcephaly, respectively. Intracranial volume correlates genetically with volumes of cortical and sub-cortical regions, cognition, learning, neonatal and neurological traits. Parkinson's disease cases have greater and attention deficit hyperactivity disorder cases smaller intracranial volume than controls. Our Mendelian randomization studies indicate that intracranial volume associated variants either increase the risk of Parkinson's disease and decrease the risk of attention deficit hyperactivity disorder and neuroticism or correlate closely with a confounder.

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.

Tuning SAS-6 architecture with monobodies impairs distinct steps of centriole assembly

Centrioles are evolutionarily conserved multi-protein organelles essential for forming cilia and centrosomes. Centriole biogenesis begins with self-assembly of SAS-6 proteins into 9-fold symmetrical ring polymers, which then stack into a cartwheel that scaffolds organelle formation. The importance of this architecture has been difficult to decipher notably because of the lack of precise tools to modulate the underlying assembly reaction. Here, we developed monobodies against Chlamydomonas reinhardtii SAS-6, characterizing three in detail with X-ray crystallography, atomic force microscopy and cryo-electron microscopy. This revealed distinct monobody-target interaction modes, as well as specific consequences on ring assembly and stacking. Of particular interest, monobody MB_CRS6-15 induces a conformational change in CrSAS-6, resulting in the formation of a helix instead of a ring. Furthermore, we show that this alteration impairs centriole biogenesis in human cells. Overall, our findings identify monobodies as powerful molecular levers to alter the architecture of multi-protein complexes and tune centriole assembly.

Goldberg-Shprintzen syndrome protein KIF1BP is a CITK interactor implicated in cytokinesis

Goldberg-Shprintzen disease (GOSHS) is a rare microcephaly syndrome accompanied by intellectual disability, dysmorphic facial features, peripheral neuropathy and Hirschsprung disease. It is associated with recessive mutations in the gene encoding kinesin family member 1-binding protein (KIF1BP, also known as KIFBP). The encoded protein regulates axon microtubules dynamics, kinesin attachment and mitochondrial biogenesis, but it is not clear how its loss could lead to microcephaly. We identified KIF1BP in the interactome of citron kinase (CITK, also known as CIT), a protein produced by the primary hereditary microcephaly 17 (MCPH17) gene. KIF1BP and CITK interact under physiological conditions in mitotic cells. Similar to CITK, KIF1BP is enriched at the midbody ring and is required for cytokinesis. The association between KIF1BP and CITK can be influenced by CITK activity, and the two proteins may antagonize each other for their midbody localization. KIF1BP knockdown decreases microtubule stability, increases KIF23 midbody levels and impairs midbody localization of KIF14, as well as of chromosome passenger complex. These data indicate that KIF1BP is a CITK interactor involved in midbody maturation and abscission, and suggest that cytokinesis failure may contribute to the microcephaly phenotype observed in GOSHS.

Further Delineation of Phenotype and Genotype of Primary Microcephaly Syndrome with Cortical Malformations Associated with Mutations in the WDR62 Gene

Type 2 congenital microcephaly (MCPH2) is a brain development disorder characterized by primary microcephaly with or without brain malformations. MCPH2 is caused by mutations in the WDR62 gene. We present three new patients with MCPH2 and compound heterozygous mutations in the WDR62 gene. In all the cases, the parents were healthy and unrelated. All children were clinically diagnosed with congenital microcephaly and retardation of motor and speech development. Sequencing results in the presented patients revealed five new variants in the WDR62 gene (c.4273C>T, c.1711_1712insTA, c.1777_1778delGA, c.1642+2T>G, c.194T>A) and one previously described in the German population (c.2864_2867delACAG). In two of the presented cases, variants in the SMAD4, DKC1, and ATRX genes were also found with unknown effects on the course of the disease. Moreover, in the article we collected and compared the most common clinical symptoms, dysmorphic features, and changes in radiographic examinations of the brain observed in 120 patients with recessive primary microcephaly type 2 caused by mutations in the WDR62 gene.

The microcephaly gene Donson is essential for progenitors of cortical glutamatergic and GABAergic neurons

Biallelic mutations in DONSON, an essential gene encoding for a replication fork protection factor, were linked to skeletal abnormalities and microcephaly. To better understand DONSON function in corticogenesis, we characterized Donson expression and consequences of conditional Donson deletion in the mouse telencephalon. Donson was widely expressed in the proliferation and differentiation zones of the embryonic dorsal and ventral telencephalon, which was followed by a postnatal expression decrease. Emx1-Cre-mediated Donson deletion in progenitors of cortical glutamatergic neurons caused extensive apoptosis in the early dorsomedial neuroepithelium, thus preventing formation of the neocortex and hippocampus. At the place of the missing lateral neocortex, these mutants exhibited a dorsal extension of an early-generated paleocortex. Targeting cortical neurons at the intermediate progenitor stage using Tbr2-Cre evoked no apparent malformations, whereas Nkx2.1-Cre-mediated Donson deletion in subpallial progenitors ablated 75% of Nkx2.1-derived cortical GABAergic neurons. Thus, the early telencephalic neuroepithelium depends critically on Donson function. Our findings help explain why the neocortex is most severely affected in individuals with DONSON mutations and suggest that DONSON-dependent microcephaly might be associated with so far unrecognized defects in cortical GABAergic neurons. Targeting Donson using an appropriate recombinase is proposed as a feasible strategy to ablate proliferating and nascent cells in experimental research.

The cerebral cortex constitutes the largest part of the mammalian brain and is generated prenatally by highly proliferative progenitors. Genes encoding proteins that are essential for chromosomal segregation, mitotic division, DNA repair, and DNA damage response are frequently mutated in individuals diagnosed with microcephaly, a clinical condition characterized by cerebrocortical hypotrophy. Recent findings suggest that biallelic mutations in DONSON, a replication fork stabilization factor, cause microcephaly and skeletal defects, but this has not been formally tested. Here, we find that Cre-mediated Donson deletion in progenitors of cortical glutamatergic and cortical GABAergic neurons causes extensive programmed cell death at early stages of cortical development in mice. Cell death is induced in the proliferation zones and the postmitotic differentiation zones of the targeted progenitors. Mice undergoing Donson ablation in glutamatergic progenitors do not develop the hippocampus and dorsolateral neocortex, which leads to a dorsal shift of the early-generated piriform cortex. Donson deletion in GABAergic progenitors eliminates the vast majority of GABAergic neurons and oligodendrocyte precursors arising in the targeted lineage. We thus establish that Donson is essential for diverse early telencephalic progenitors. Targeting Donson might be used to kill off highly proliferating cells in experimental and probably therapeutic settings.

Centrosome defects cause microcephaly by activating the 53BP1-USP28-TP53 mitotic surveillance pathway

Mutations in centrosome genes deplete neural progenitor cells (NPCs) during brain development, causing microcephaly. While NPC attrition is linked to TP53-mediated cell death in several microcephaly models, how TP53 is activated remains unclear. In cultured cells, mitotic delays resulting from centrosome loss prevent the growth of unfit daughter cells by activating a pathway involving 53BP1, USP28, and TP53, termed the mitotic surveillance pathway. Whether this pathway is active in the developing brain is unknown. Here, we show that the depletion of centrosome proteins in NPCs prolongs mitosis and increases TP53-mediated apoptosis. Cell death after a delayed mitosis was rescued by inactivation of the mitotic surveillance pathway. Moreover, 53BP1 or USP28 deletion restored NPC proliferation and brain size without correcting the upstream centrosome defects or extended mitosis. By contrast, microcephaly caused by the loss of the non-centrosomal protein SMC5 is also TP53-dependent but is not rescued by loss of 53BP1 or USP28. Thus, we propose that mutations in centrosome genes cause microcephaly by delaying mitosis and pathologically activating the mitotic surveillance pathway in the developing brain.

RRP7A links primary microcephaly to dysfunction of ribosome biogenesis, resorption of primary cilia, and neurogenesis

Primary microcephaly (MCPH) is characterized by reduced brain size and intellectual disability. The exact pathophysiological mechanism underlying MCPH remains to be elucidated, but dysfunction of neuronal progenitors in the developing neocortex plays a major role. We identified a homozygous missense mutation (p.W155C) in Ribosomal RNA Processing 7 Homolog A, RRP7A, segregating with MCPH in a consanguineous family with 10 affected individuals. RRP7A is highly expressed in neural stem cells in developing human forebrain, and targeted mutation of Rrp7a leads to defects in neurogenesis and proliferation in a mouse stem cell model. RRP7A localizes to centrosomes, cilia and nucleoli, and patient-derived fibroblasts display defects in ribosomal RNA processing, primary cilia resorption, and cell cycle progression. Analysis of zebrafish embryos supported that the patient mutation in RRP7A causes reduced brain size, impaired neurogenesis and cell proliferation, and defective ribosomal RNA processing. These findings provide novel insight into human brain development and MCPH.

The RRP7A a gene is involved in ribosome biogenesis. Here the authors report a homozygous missense mutation segregating with primary microcephaly, and show that this occurs via functional defects in both nucleoli and primary cilia disrupting cell proliferation and neurogenesis.

The Spindle-Associated Microcephaly Protein, WDR62, Is Required for Neurogenesis and Development of the Hippocampus

Primary microcephaly genes (MCPH) are required for the embryonic expansion of the mammalian cerebral cortex. However, MCPH mutations may spare growth in other regions of the developing forebrain which reinforces context-dependent functions for distinct MCPH genes in neurodevelopment. Mutations in the MCPH2 gene, WD40-repeat protein 62 (WDR62), are causative of primary microcephaly and cortical malformations in humans. WDR62 is a spindle microtubule-associated phosphoprotein that is required for timely and oriented cell divisions. Recent studies in rodent models confirm that WDR62 loss or mutation causes thinning of the neocortex and disrupted proliferation of apical progenitors reinforcing critical requirements in the maintenance of radial glia. However, potential contributions for WDR62 in hippocampal development had not been previously defined. Using CRISPR/Cas9 gene editing, we generated mouse models with patient-derived non-synonymous missense mutations (WDR62^V66M and WDR62^R439H) and a null mutation (herein referred to as WDR62^Stop) for comparison. We find that WDR62 deletion or mutation resulted in a significant reduction in the thickness of the hippocampal ventricular zone and the area of the dentate gyrus (DG). This was associated with the mitotic arrest and depletion of radial glia and intermediate progenitors in the ammonic neuroepithelium. As a consequence, we find that the number of mitotic dentate precursors in the migratory stream and granule neurons in the DG was reduced with WDR62 mutation. These findings reveal that WDR62 is required for neurogenesis and the growth of the hippocampus during embryonic development.

International consensus recommendations on the diagnostic work-up for malformations of cortical development

Malformations of cortical development (MCDs) are neurodevelopmental disorders that result from abnormal development of the cerebral cortex in utero. MCDs place a substantial burden on affected individuals, their families and societies worldwide, as these individuals can experience lifelong drug-resistant epilepsy, cerebral palsy, feeding difficulties, intellectual disability and other neurological and behavioural anomalies. The diagnostic pathway for MCDs is complex owing to wide variations in presentation and aetiology, thereby hampering timely and adequate management. In this article, the international MCD network Neuro-MIG provides consensus recommendations to aid both expert and non-expert clinicians in the diagnostic work-up of MCDs with the aim of improving patient management worldwide. We reviewed the literature on clinical presentation, aetiology and diagnostic approaches for the main MCD subtypes and collected data on current practices and recommendations from clinicians and diagnostic laboratories within Neuro-MIG. We reached consensus by 42 professionals from 20 countries, using expert discussions and a Delphi consensus process. We present a diagnostic workflow that can be applied to any individual with MCD and a comprehensive list of MCD-related genes with their associated phenotypes. The workflow is designed to maximize the diagnostic yield and increase the number of patients receiving personalized care and counselling on prognosis and recurrence risk.

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.

PDCD6IP, encoding a regulator of the ESCRT complex, is mutated in microcephaly

Primary microcephaly (PM) is a highly heterogeneous neurodevelopmental disorder with many contributing risk genes and loci identified to date. We report a consanguineous family with PM, intellectual disability and short stature. Using whole exome sequencing, we identified a homozygous frameshift variant in programmed cell death 6 interacting protein (PDCD6IP, c.154_158dup; p.Val54Profs*18). This gene, PDCD6IP, plays an important role in the endosomal sorting complexes required for transport (ESCRT) pathway in the abscission stage of cytokinesis and apoptosis, and is required for normal brain development in mice. The clinical features observed in our patient were similar to the phenotypes observed in mouse and zebrafish models of PDCD6IP mutations in previous studies. This study provides evidence that clinical manifestations of PDCD6IP mutations as seen in our patients with PM and ID may be a novel cause for neurodevelopmental disorders.

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 Yin and Yang of Autosomal Recessive Primary Microcephaly Genes: Insights from Neurogenesis and Carcinogenesis

The stem cells of neurogenesis and carcinogenesis share many properties, including proliferative rate, an extensive replicative potential, the potential to generate different cell types of a given tissue, and an ability to independently migrate to a damaged area. This is also evidenced by the common molecular principles regulating key processes associated with cell division and apoptosis. Autosomal recessive primary microcephaly (MCPH) is a neurogenic mitotic disorder that is characterized by decreased brain size and mental retardation. Until now, a total of 25 genes have been identified that are known to be associated with MCPH. The inactivation (yin) of most MCPH genes leads to neurogenesis defects, while the upregulation (yang) of some MCPH genes is associated with different kinds of carcinogenesis. Here, we try to summarize the roles of MCPH genes in these two diseases and explore the underlying mechanisms, which will help us to explore new, attractive approaches to targeting tumor cells that are resistant to the current therapies.

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.

Tight junction protein occludin regulates progenitor Self-Renewal and survival in developing cortex

Occludin (OCLN) mutations cause human microcephaly and cortical malformation. A tight junction component thought absent in neuroepithelium after neural tube closure, OCLN isoform-specific expression extends into corticogenesis. Full-length and truncated isoforms localize to neuroprogenitor centrosomes, but full-length OCLN transiently localizes to plasma membranes while only truncated OCLN continues at centrosomes throughout neurogenesis. Mimicking human mutations, full-length OCLN depletion in mouse and in human CRISPR/Cas9-edited organoids produce early neuronal differentiation, reduced progenitor self-renewal and increased apoptosis. Human neural progenitors were more severely affected, especially outer radial glial cells, which mouse embryonic cortex lacks. Rodent and human mutant progenitors displayed reduced proliferation and prolonged M-phase. OCLN interacted with mitotic spindle regulators, NuMA and RAN, while full-length OCLN loss impaired spindle pole morphology, astral and mitotic microtubule integrity. Thus, early corticogenesis requires full-length OCLN to regulate centrosome organization and dynamics, revealing a novel role for this tight junction protein in early brain development.

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.

Update on the Animal Models and Underlying Mechanisms for ZIKV-Induced Microcephaly

The circulation of Zika virus (ZIKV) in nearly 80 countries and territories poses a significant global threat to public health. ZIKV is causally linked to severe developmental defects in the brain, recognized as congenital Zika syndrome (CZS), which includes microcephaly and other serious congenital neurological complications. Since the World Health Organization declared the ZIKV outbreak a public health emergency of international concern, remarkable progress has been made in the generation of different ZIKV infection animal models to gain insight into cellular targets and pathogenesis and to explore the associated underlying mechanisms. Here we focus on summarizing our current understanding of the effects of ZIKV on mammalian brain development in different developmental stages and discuss the potential underlying mechanisms of ZIKV-induced CZS, as well as future perspectives.

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.

10q23.31 microduplication encompassing PTEN decreases mTOR signalling activity and is associated with autosomal dominant primary microcephaly

Background

Hereditary primary microcephaly (MCPH) is mainly characterised by decreased occipitofrontal circumference and variable degree of intellectual disability. MCPH with a dominant pattern of inheritance is a rare condition, so far causally linked to pathogenic variants in the ALFY, DPP6, KIF11 and DYRK1A genes.

Objective

This study aimed at identifying the causative variant of the autosomal dominant form of MCPH in a Brazilian family with three affected members.

Methods

Following clinical evaluation of two sibs and their mother presenting with autosomal dominant MCPH, array comparative genome hybridisation was performed using genomic DNA from peripheral blood of the family members. Gene and protein expression studies were carried out in cultured skin fibroblasts.

Results

A 382 kb microduplication at 10q23.31 was detected, encompassing the entire PTEN, KLLN and ATAD1 genes. PTEN haploinsufficiency has been causally associated with macrocephaly and autism spectrum disorder and, therefore, was considered the most likely candidate gene to be involved in this autosomal dominant form of MCPH. In the patients’ fibroblasts, PTEN mRNA and protein were found to be overexpressed, and the phosphorylation patterns of upstream and downstream components of the mammalian target of rapamycin (mTOR) signalling pathway were dysregulated.

Conclusions

Taken together, our results demonstrate that the identified submicroscopic 10q23.31 duplication in a family with MCPH leads to markedly increased expression of PTEN and reduced activity of the mTOR signalling pathway. These results suggest that the most probable pathomechanism underlying the microcephaly phenotype in this family involves downregulation of the mTOR pathway through overexpression of PTEN.

A novel non sense mutation in WDR62 causes autosomal recessive primary microcephaly: a case report

Background

Autosomal recessive primary microcephaly (MCPH) is a rare genetically heterogeneous disorder of neurogenic brain development characterized by a reduced head circumference at birth with no remarkable anomalies of brain architecture and variable degrees of intellectual impairment. Clinical and genetic heterogeneity in genetic disorders represent a major diagnostic challenge.

Case presentation

Two patients, 11 and 9 years old, born from consanguineous parents, were referred to the department of medical genetics at the National Institute of Health in Rabat. The diagnosis of MCPH was made, based on reduced head circumference without brain architecture abnormalities. The two patients were subject to the whole-exome sequencing, which allowed to diagnose a novel homozygous mutation c.1027C > T; p.Gln343* in exon 8 of WDR62, a gene already known to be related to MCPH. Sanger sequencing confirmed the segregation of the mutation in the family.

Conclusion

Our data expends the spectrum of mutations in WDR62 gene, proves the efficiency and cost-effectiveness of whole exome sequencing for the molecular diagnosis of genetically heterogeneous disorders such MCPH. Exome sequencing led to the rapid and cost-effective identification of a novel homozygous mutation in WDR62 gene, thereby facilitating genetic counseling.

Primary microcephaly caused by novel compound heterozygous mutations in ASPM

Autosomal recessive primary microcephaly (microcephaly primary hereditary, MCPH) is a genetically heterogeneous rare developmental disorder that is characterized by prenatal onset of abnormal brain growth, which leads to intellectual disability of variable severity. We report a 5-year-old male who presented with a severe form of primary microcephaly. Targeted panel sequencing revealed compound heterozygous truncating mutations of the abnormal spindle-like microcephaly-associated (ASPM) gene, which confirmed the MCPH5 diagnosis. A novel NM_018136.4: c.9742_9745del (p.Lys3248Serfs*13) deletion mutation was identified.

Novel mutations c.28G>T (p.Ala10Ser) and c.189G>T (p.Glu63Asp) in WDR62 associated with early onset acanthosis and hyperkeratosis in a patient with autosomal recessive microcephaly type 2

Microcephaly (MCPH) is a developmental disorder characterized by reduced brain size and intellectual disability. A proportion of microcephaly is caused by defects in a single gene. Microcephaly 2 (MCPH2) is one of the most frequent subtypes of MCPH.WD repeat-containing protein 62 gene (WDR62) is the most frequently mutated gene in MCPH2 patients. Phenotypes involving dermatological changes in MCPH2 have not been reported. We have identified and investigated a 5-year-old Chinese girl with markedly reduced brain size (86% of normal size), intellectual disability and psychomotor developmental delay. The patient also exhibited spattered blisters and reduced hair density on her head, anisochromasia with reticular hyperpigmentation and hypopigmentation on the trunk, which she has had since the age of 4 and had been found by her parents. Histological examination of a skin biopsy revealed acanthosis, hyperkeratosis and necrotic keratinocytes. Whole exome and Sanger sequencing identified two novel missense mutations, c.28G>T and c.189G>T, in the WDR62 gene. Both the mutations non-synonymously affect evolutionarily conserved amino acids and are predicted to be disease causing. We report the first case of MCPH2 that also presented with marked dermatological changes. Our findings expand the mutational and phenotypical spectra of MCPH2 and are valuable in the mutation-based pre- and post-natal screening and genetic diagnosis for MCPH2.

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.

The relationship between preoperative fetal head circumference and 2-year cognitive performance after laser surgery for twin-twin transfusion syndrome

OBJECTIVE

To determine the relationship between preoperative fetal head circumference (HC) and cognitive performance among children treated with laser surgery for twin-twin transfusion syndrome (TTTS).

METHODS

Donor and recipient twin HCs were measured preoperatively (16-26 weeks' gestation) and at 2 years corrected age. Multilevel multivariate regression models were used to test pregnancy and child-level risk factors for lower Battelle Developmental Inventory Second Edition (BDI-2) scores. A repeated-measures ANOVA was used to examine HC growth among recipients and donors between preoperative and 2 years.

RESULTS

Ninety-nine children were evaluated. The average BDI-2 score for the cohort was 101.4 (SD = 12.2). After controlling for covariates, larger preoperative HC percentiles were significantly associated with an increase in total BDI-2 scores (β = 0.29; P < 0.001), where a 12.5% increase in preoperative HC percentile was associated with 1-point increase in total BDI-2 score. The mean recipient and donor twin HC percentiles preoperatively and at age 2 years were 51^st percentile vs 20^th percentile (P = .050) and 60^th percentile vs 49^th percentile (P = .676), respectively.

CONCLUSION

Smaller preoperative HC percentiles identified children at risk of lower, but still within normal range, total BDI-2 scores. The discordance in HC percentiles between the donor and recipient twin decreased after laser surgery.

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.

Citron Kinase Deficiency Leads to Chromosomal Instability and TP53-Sensitive Microcephaly

Mutations in citron (CIT), leading to loss or inactivation of the citron kinase protein (CITK), cause primary microcephaly in humans and rodents, associated with cytokinesis failure and apoptosis in neural progenitors. We show that CITK loss induces DNA damage accumulation and chromosomal instability in both mammals and Drosophila. CITK-deficient cells display "spontaneous" DNA damage, increased sensitivity to ionizing radiation, and defective recovery from radiation-induced DNA lesions. In CITK-deficient cells, DNA double-strand breaks increase independently of cytokinesis failure. Recruitment of RAD51 to DNA damage foci is compromised by CITK loss, and CITK physically interacts with RAD51, suggesting an involvement of CITK in homologous recombination. Consistent with this scenario, in doubly CitK and Trp53 mutant mice, neural progenitor cell death is dramatically reduced; moreover, clinical and neuroanatomical phenotypes are remarkably improved. Our results underscore a crucial role of CIT in the maintenance of genomic integrity during brain development.

MCPH1, mutated in primary microcephaly, is required for efficient chromosome alignment during mitosis

MCPH1 gene, mutated in primary microcephaly, regulates cell progression into mitosis. While this role has been extensively investigated in the context of DNA damage, its function during unperturbed cell cycles has been given less attention. Here we have analyzed the dynamics of chromosome condensation and cell cycle progression in MCPH1 deficient cells under undamaging conditions. Our study demonstrates that chromosome condensation is uncoupled from cell cycle progression when MCPH1 function is lacking, resulting in cells that prematurely condense their chromosomes during mid G2-phase and delay decondensation at the completion of mitosis. However, mitosis onset occurs on schedule in MCPH1 deficient cells. We also revealed active Cdk1 to be mandatory for the premature onset of chromosome condensation during G2 and the maintenance of the condensed state thereafter. Interestingly, a novel cellular phenotype was observed while monitoring cell cycle progression in cells lacking MCPH1 function. Specifically, completion of chromosome alignment at the metaphase plate was significantly delayed. This deficiency reveals that MCPH1 is required for efficient chromosome biorientation during mitosis.

Novel STIL Compound Heterozygous Mutations Cause Severe Fetal Microcephaly and Centriolar Lengthening

STIL (SCL/TAL1 interrupting locus) is a core component of the centriole duplication process. STIL mutations have been associated with both autosomal recessive primary microcephaly (MCPH) and holoprosencephaly. In this report, we describe a family with multiple miscarriages and 2 terminations of pregnancy due to marked fetal microcephaly, delayed cortical gyrification, and dysgenesis of the corpus callosum. Whole exome sequencing allowed us to identify novel compound heterozygous mutations in STIL. The mutations lie, respectively, in the CPAP/CENPJ and the hsSAS6 interacting domains of STIL. M-phase synchronized amniocytes from both affected fetuses did not display an aberrant number of centrioles, as shown previously for either STIL-depleted or overexpressing cells. However, we observed an elongation of at least 1 centriole for each duplicated centrosome. These preliminary results may point to a novel mechanism causing MCPH and embryonic lethality in humans.

Genetic basis of brain size evolution in cetaceans: insights from adaptive evolution of seven primary microcephaly (MCPH) genes

BACKGROUND

Cetacean brain size expansion is an enigmatic event in mammalian evolution, yet its genetic basis remains poorly explored. Here, all exons of the seven primary microcephaly (MCPH) genes that play key roles in size regulation during brain development were investigated in representative cetacean lineages.

RESULTS

Sequences of MCPH2-7 genes were intact in cetaceans but frameshift mutations and stop codons was identified in MCPH1. Extensive positive selection was identified in four of six intact MCPH genes: WDR62, CDK5RAP2, CEP152, and ASPM. Specially, positive selection at CDK5RAP2 and ASPM were examined along lineages of odontocetes with increased encephalization quotients (EQ) and mysticetes with reduced EQ but at WDR62 only found along odontocete lineages. Interestingly, a positive association between evolutionary rate (ω) and EQ was identified for CDK5RAP2 and ASPM. Furthermore, we tested the binding affinities between Calmodulin (CaM) and ASPM IQ motif in cetaceans because only CaM combined with IQ, can ASPM perform the function in determining brain size. Preliminary function assay showed binding affinities between CaM and IQ motif of the odontocetes with increased EQ was stronger than for the mysticetes with decreased EQ. In addition, evolution rate of ASPM and CDK5RAP2 were significantly related to mean group size (as one measure of social complexity).

CONCLUSIONS

Our study investigated the genetic basis of cetacean brain size evolution. Significant positive selection was examined along lineages with both increased and decreased EQ at CDK5RAP2 and ASPM, which is well matched with cetacean complex brain size evolution. Evolutionary rate of CDK5RAP2 and ASPM were significantly related to EQ, suggesting that these two genes may have contributed to EQ expansion in cetaceans. This suggestion was further indicated by our preliminary function test that ASPM might be mainly linked to evolutionary increases in EQ. Most strikingly, our results suggested that cetaceans evolved large brains to manage complex social systems, consisting with the 'social brain hypothesis', as evolutionary rate of ASPM and CDK5RAP2 were significantly related to mean group size.

Microcephaly and Zika virus: Neuroradiological aspects, clinical findings and a proposed framework for early evaluation of child development

BACKGROUND AND AIMS

As the recent outbreak of microcephaly cases caused by Zika virus has been declared a global health emergency, providing assessment guidelines for multidisciplinary teams providing early developmental screening and stimulation to infants with microcephaly is much needed. Thus, the aim of this manuscript is to provide an overview on what is known about neuroradiological aspects and clinical findings in infants with microcephaly caused by Zika virus and to propose a framework for early evaluation of child development.

METHODS

The keywords "Zika virus" and "microcephaly" were searched in PubMed database for articles published from incept to May 2017. These texts were reviewed, and the ones addressing neuroradiological and clinical findings in infants were selected. Recommendations for early assessment were made based on the International Classification of Functionality Disability and Health (ICF) model.

OUTCOMES AND RESULTS

The database search yielded 599 publications and 36 were selected. The studies detected microcephaly with diffuse brain malformations and calcifications, ventriculomegaly, optic nerve hypoplasia, macular atrophy, cataracts, impaired visual and hearing function, arthrogryposis, spasticity, hyperreflexia, irritability, tremors, and seizures, but very little is known about early development. Early assessments were described based on the ICF domains (Body Function and Structures, Activities and Participation and Contextual factors).

CONCLUSION AND IMPLICATIONS

Studies published showed abnormal brain, optic, neurologic and orthopedic findings, but very little is known about other aspects of functioning in infants with microcephaly caused by Zika virus. The biopsychosocial model based on the ICF paradigm provides an adequate framework to describe the condition of the infant with microcephaly receiving rehabilitative efforts to minimize disability. Efforts towards early identification of developmental delays should be taken within the first six months of life.

Non-model model organisms

Model organisms are widely used in research as accessible and convenient systems to study a particular area or question in biology. Traditionally only a handful of organisms have been widely studied, but modern research tools are enabling researchers to extend the set of model organisms to include less-studied and more unusual systems. This Forum highlights a range of 'non-model model organisms' as emerging systems for tackling questions across the whole spectrum of biology (and beyond), the opportunities and challenges, and the outlook for the future.

WDR62 Regulates Early Neural and Glial Progenitor Specification of Human Pluripotent Stem Cells

Mutations in WD40-repeat protein 62 (WDR62) are commonly associated with primary microcephaly and other developmental cortical malformations. We used human pluripotent stem cells (hPSC) to examine WDR62 function during human neural differentiation and model early stages of human corticogenesis. Neurospheres lacking WDR62 expression showed decreased expression of intermediate progenitor marker, TBR2, and also glial marker, S100β. In contrast, inhibition of c-Jun N-terminal kinase (JNK) signalling during hPSC neural differentiation induced upregulation of WDR62 with a corresponding increase in neural and glial progenitor markers, PAX6 and EAAT1, respectively. These findings may signify a role of WDR62 in specifying intermediate neural and glial progenitors during human pluripotent stem cell differentiation.

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.