The molecular landscape of ASPM mutations in 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.

Case Report: Compound heterozygous nonsense mutations in TRMT10A are associated with microcephaly, delayed development, and periventricular white matter hyperintensities

Microcephaly is a fairly common feature observed in children with delayed development, defined as head circumference less than 2 standard deviations below the mean for age and gender. It may be the result of an acquired insult to the brain, such prenatal or perinatal brain injury (congenital infection or hypoxic ischemic encephalopathy), or be a part of a genetic syndrome. There are over 1000 conditions listed in OMIM (Online Mendelian Inheritance in Man) where microcephaly is a key finding; many of these are associated with specific somatic features and non-CNS anomalies. The term primary microcephaly is used when microcephaly and delayed development are the primary features, and they are not part of another recognized syndrome.

In this case report, we present the clinical features of siblings (brother and sister) with primary microcephaly and delayed development, and subtle dysmorphic features. Both children had brain MRI studies that showed periventricular and subcortical T2/FLAIR hyperintensities, without signs of white matter volume loss, and no parenchymal calcifications by CT scan. The family was enrolled in a research study for whole exome sequencing of probands and parents. Analysis of variants determined that the children were compound heterozygotes for nonsense mutations, c.277C>T (p.Arg93*) and c.397C>T (p.Arg133*), in the TRMT10A gene. Mutations in this gene have only recently been reported in children with microcephaly and early onset diabetes mellitus.

Our report adds to current knowledge of TRMT10A related neurodevelopmental disorders and demonstrates imaging findings suggestive of delayed or abnormal myelination of the white matter in this disorder. Accurate diagnosis through genomic testing, as in the children described here, allows for early detection and management of medical complications, such as diabetes mellitus.

What next-generation sequencing (NGS) technology has enabled us to learn about primary autosomal recessive microcephaly (MCPH)

The impact that next-generation sequencing technology (NGS) is having on many aspects of molecular and cell biology, is becoming increasingly apparent. One of the most noticeable outcomes of the new technology in human genetics, has been the accelerated rate of identification of disease-causing genes. Especially for rare, heterogeneous disorders, such as autosomal recessive primary microcephaly (MCPH), the handful of genes previously known to harbour disease-causing mutations, has grown at an unprecedented rate within a few years. Knowledge of new genes mutated in MCPH over the last four years has contributed to our understanding of the disorder at both the clinical and cellular levels. The functions of proteins such as WDR62, CASC5, PHC1, CDK6, CENP-E, CENP-F, CEP63, ZNF335, PLK4 and TUBGPC, have been added to the complex network of critical cellular processes known to be involved in brain growth and size. In addition to the importance of mitotic spindle assembly and structure, centrosome and centriole function and DNA repair and damage response, new mechanisms involving kinetochore-associated proteins and chromatin remodelling complexes have been elucidated. Two of the major contributions to our clinical knowledge are the realisation that primary microcephaly caused by mutations in genes at the MCPH loci is seldom an isolated clinical feature and is often accompanied either by additional cortical malformations or primordial dwarfism. Gene-phenotype correlations are being revisited, with a new dimension of locus heterogeneity and phenotypic variability being revealed.

Transcriptional regulator PRDM12 is essential for human pain perception

Pain perception has evolved as a warning mechanism to alert organisms to tissue damage and dangerous environments. In humans, however, undesirable, excessive or chronic pain is a common and major societal burden for which available medical treatments are currently suboptimal. New therapeutic options have recently been derived from studies of individuals with congenital insensitivity to pain (CIP). Here we identified 10 different homozygous mutations in PRDM12 (encoding PRDI-BF1 and RIZ homology domain-containing protein 12) in subjects with CIP from 11 families. Prdm proteins are a family of epigenetic regulators that control neural specification and neurogenesis. We determined that Prdm12 is expressed in nociceptors and their progenitors and participates in the development of sensory neurons in Xenopus embryos. Moreover, CIP-associated mutants abrogate the histone-modifying potential associated with wild-type Prdm12. Prdm12 emerges as a key factor in the orchestration of sensory neurogenesis and may hold promise as a target for new pain therapeutics.

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.

[Concordance between a head circumference growth function and intellectual disability in relation with the cause of microcephaly]

INTRODUCTION

Our aim was to investigate the correlations between patterns of head growth and intellectual disability among distinct aetiological presentations of microcephaly.

PATIENTS AND METHODS

3,269 head circumference (HC) charts of patients from a tertiary neuropediatric unit were reviewed and 136 microcephalic participants selected. Using the Z-scores of registered HC measurements we defined the variables: HC Minimum, HC Drop and HC Catch-up. We classified patients according to the presence or absence of intellectual disability (IQ below 71) and according to the cause of microcephaly (idiopathic, familial, syndromic, symptomatic and mixed).

RESULTS

Using Discriminant Analysis a C-function was defined as C=HC Minimum + HC Drop with a cut-off level of C=-4.32 Z-score. In our sample 95% of patients scoring below this level, severe microcephaly, were classified in the disabled group while the overall concordance was 66%. In the symptomatic-mixed group the concordance between HC function and outcome reached 82% in contrast to only 54% in the idiopathic-syndromic group (P-value=0.0002).

CONCLUSIONS

We defined a HC growth function which discriminates intellectual disability of microcephalic patients better than isolated HC measurements, especially for those with secondary and mixed aetiologies.

Molecular and cellular basis of autosomal recessive primary microcephaly

Autosomal recessive primary microcephaly (MCPH) is a rare hereditary neurodevelopmental disorder characterized by a marked reduction in brain size and intellectual disability. MCPH is genetically heterogeneous and can exhibit additional clinical features that overlap with related disorders including Seckel syndrome, Meier-Gorlin syndrome, and microcephalic osteodysplastic dwarfism. In this review, we discuss the key proteins mutated in MCPH. To date, MCPH-causing mutations have been identified in twelve different genes, many of which encode proteins that are involved in cell cycle regulation or are present at the centrosome, an organelle crucial for mitotic spindle assembly and cell division. We highlight recent findings on MCPH proteins with regard to their role in cell cycle progression, centrosome function, and early brain development.

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.

Microcephaly models in the developing zebrafish retinal neuroepithelium point to an underlying defect in metaphase progression

Autosomal recessive primary microcephaly (MCPH) is a congenital disorder characterized by significantly reduced brain size and mental retardation. Nine genes are currently known to be associated with the condition, all of which encode centrosomal or spindle pole proteins. MCPH is associated with a reduction in proliferation of neural progenitors during fetal development. The cellular mechanisms underlying the proliferation defect, however, are not fully understood. The zebrafish retinal neuroepithelium provides an ideal system to investigate this question. Mutant or morpholino-mediated knockdown of three known MCPH genes (stil, aspm and wdr62) and a fourth centrosomal gene, odf2, which is linked to several MCPH proteins, results in a marked reduction in head and eye size. Imaging studies reveal a dramatic rise in the fraction of proliferating cells in mitosis in all cases, and time-lapse microscopy points to a failure of progression through prometaphase. There was also increased apoptosis in all the MCPH models but this appears to be secondary to the mitotic defect as we frequently saw mitotically arrested cells disappear, and knocking down p53 apoptosis did not rescue the mitotic phenotype, either in whole retinas or clones.

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 single strand that links multiple neuropathologies in human disease

The development of the human central nervous system is a complex process involving highly coordinated periods of neuronal proliferation, migration and differentiation. Disruptions in these neurodevelopmental processes can result in microcephaly, a neuropathological disorder characterized by a reduction in skull circumference and total brain volume, whereas a failure to maintain neuronal health in the adult brain can lead to progressive neurodegeneration. Defects in the cellular pathways that detect and repair DNA damage are a common cause of both these neuropathologies and are associated with a growing number of hereditary human disorders. In particular, defects in the repair of DNA single strand breaks, one of the most commonly occurring types of DNA lesion, have been associated with three neuropathological diseases: ataxia oculomotor apraxia 1, spinocerebellar ataxia with neuronal neuropathy 1 and microcephaly, early-onset, intractable seizures and developmental delay. A striking similarity between these three human diseases is that they are all caused by mutations in DNA end processing factors, suggesting that a particularly crucial stage of DNA single strand break repair is the repair of breaks with 'damaged' termini. Additionally all three disorders lack any extraneurological symptoms, such as immunodeficiency and cancer predisposition, which are typically found in other human diseases associated with defective DNA repair. However despite these similarities, two of these disorders present with progressive cerebellar degeneration, whereas the third presents with severe microcephaly. This review discusses the molecular defects behind these disorders and presents several hypotheses based on current literature on a number of important questions, in particular, how do mutations in different end processing factors within the same DNA repair pathway lead to such different neuropathologies?

Exploring the implications of INDELs in neuropsychiatric genetics: challenges and perspectives

The decade passed after publishing the Human Genome first draft faced an enormous growth at the understanding of the genomic variation among different subjects, populations, and groups of patients. Single nucleotide polymorphisms (SNPs) and insertion or deletions (INDELs) have been increasingly recognized as a major type of genetic variations, with potential impact in protein activities and gene expression changes observed in complex genetic traits, like neuropsychiatric diseases. INDELs represent the second most common class of variations after SNPs, but there is still an important gap between the number of INDELs reported and the actual knowledge about the functional implications of such variations. There are approximately 10 million SNPs already reported, and the human populations are expected to collectively harbor at least 1.6-2.5 million INDELs. One of the major challenges is to find better platforms to screen for INDELs in a high throughput manner. The discordance in between the data from different studies might be explained by the diverse approaches employed to sequence the genomes with variable platforms. Short INDEL variations increased the scope of genetic markers in human genetic diseases, and various studies showed that common microdeletions and smaller INDELs might be highly associated with neuropsychiatric diseases such as schizophrenia, autism, mental retardation, and Alzheimer disease. The rapidly increasing amount of resequencing, genotyping, and personal genome data generated by large-scale genetic human projects require the development of integrated bioinformatics tools able to efficiently manage and analyze these genetic data. Our group is currently dealing with different approaches that might optimize sequencing and bioinformatics analyses of short INDELs to broaden our research capabilities of identifying those intriguing genetic variations. Hopefully, INDELs might become a new trend in association studies in neuropsychiatric genetics since so far the level of significant and positive associations with the standard SNPs reported presents limited predictive application.

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

Autosomal Recessive Primary Microcephaly (MCPH) is a rare disorder of neurogenic mitosis characterized by reduced head circumference at birth with variable degree of mental retardation. In MCPH patients, brain size reduced to almost one-third of its original volume due to reduced number of generated cerebral cortical neurons during embryonic neurogensis. So far, seven genetic loci (MCPH1-7) for this condition have been mapped with seven corresponding genes (MCPH1, WDR62, CDK5RAP2, CEP152, ASPM, CENPJ, and STIL) identified from different world populations. Contribution of ASPM and WDR62 gene mutations in MCPH World wide is more than 50%. By and large, primary microcephaly patients are phenotypically indistinguishable, however, recent studies in patients with mutations in MCPH1, WDR62 and ASPM genes showed a broader clinical and/or cellular phenotype. It has been proposed that mutations in MCPH genes can cause the disease phenotype by disturbing: 1) orientation of mitotic spindles, 2) chromosome condensation mechanism during embryonic neurogenesis, 3) DNA damage-response signaling, 4) transcriptional regulations and microtubule dynamics, 5) certain unknown centrosomal mechanisms that control the number of neurons generated by neural precursor cells. Recent discoveries of mammalian models for MCPH have open up horizons for researchers to add more knowledge regarding the etiology and pathophysiology of MCPH. High incidence of MCPH in Pakistani population reflects the most probable involvement of consanguinity. Genetic counseling and clinical management through carrier detection/prenatal diagnosis in MCPH families can help reducing the incidence of this autosomal recessive disorder.

Angelman syndrome protein UBE3A interacts with primary microcephaly protein ASPM, localizes to centrosomes and regulates chromosome segregation

Many proteins associated with the phenotype microcephaly have been localized to the centrosome or linked to it functionally. All the seven autosomal recessive primary microcephaly (MCPH) proteins localize at the centrosome. Microcephalic osteodysplastic primordial dwarfism type II protein PCNT and Seckel syndrome (also characterized by severe microcephaly) protein ATR are also centrosomal proteins. All of the above findings show the importance of centrosomal proteins as the key players in neurogenesis and brain development. However, the exact mechanism as to how the loss-of-function of these proteins leads to microcephaly remains to be elucidated. To gain insight into the function of the most commonly mutated MCPH gene ASPM, we used the yeast two-hybrid technique to screen a human fetal brain cDNA library with an ASPM bait. The analysis identified Angelman syndrome gene product UBE3A as an ASPM interactor. Like ASPM, UBE3A also localizes to the centrosome. The identification of UBE3A as an ASPM interactor is not surprising as more than 80% of Angelman syndrome patients have microcephaly. However, unlike in MCPH, microcephaly is postnatal in Angelman syndrome patients. Our results show that UBE3A is a cell cycle regulated protein and its level peaks in mitosis. The shRNA knockdown of UBE3A in HEK293 cells led to many mitotic abnormalities including chromosome missegregation, abnormal cytokinesis and apoptosis. Thus our study links Angelman syndrome protein UBE3A to ASPM, centrosome and mitosis for the first time. We suggest that a defective chromosome segregation mechanism is responsible for the development of microcephaly in Angelman syndrome.

The microcephaly gene aspm is involved in brain development in zebrafish

MCPH is a neurodevelopmental disorder characterized by a global reduction in cerebral cortical volume. Homozygous mutation of the MCPH5 gene, also known as ASPM, is the most common cause of the MCPH phenotype. To elucidate the roles of ASPM during embryonic development, the zebrafish aspm was identified, which is specifically expressed in proliferating cells in the CNS. Morpholino-mediated knock-down of aspm resulted in a significant reduction in head size. Furthermore, aspm-deficient embryos exhibited a mitotic arrest during early development. These findings suggest that the reduction in brain size in MCPH might be caused by lack of aspm function in the mitotic cell cycle and demonstrate that the zebrafish can provide a model system for congenital diseases of the human nervous system.

A transcription factor map as revealed by a genome-wide gene expression analysis of whole-blood mRNA transcriptome in multiple sclerosis

Background

Several lines of evidence suggest that transcription factors are involved in the pathogenesis of Multiple Sclerosis (MS) but complete mapping of the whole network has been elusive. One of the reasons is that there are several clinical subtypes of MS and transcription factors that may be involved in one subtype may not be in others. We investigate the possibility that this network could be mapped using microarray technologies and contemporary bioinformatics methods on a dataset derived from whole blood in 99 untreated MS patients (36 Relapse Remitting MS, 43 Primary Progressive MS, and 20 Secondary Progressive MS) and 45 age-matched healthy controls.

Methodology/Principal Findings

We have used two different analytical methodologies: a non-standard differential expression analysis and a differential co-expression analysis, which have converged on a significant number of regulatory motifs that are statistically overrepresented in genes that are either differentially expressed (or differentially co-expressed) in cases and controls (e.g., V$KROX_Q6, p-value <3.31E-6; V$CREBP1_Q2, p-value <9.93E-6, V$YY1_02, p-value <1.65E-5).

Conclusions/Significance

Our analysis uncovered a network of transcription factors that potentially dysregulate several genes in MS or one or more of its disease subtypes. The most significant transcription factor motifs were for the Early Growth Response EGR/KROX family, ATF2, YY1 (Yin and Yang 1), E2F-1/DP-1 and E2F-4/DP-2 heterodimers, SOX5, and CREB and ATF families. These transcription factors are involved in early T-lymphocyte specification and commitment as well as in oligodendrocyte dedifferentiation and development, both pathways that have significant biological plausibility in MS causation.

Human ASPM participates in spindle organisation, spindle orientation and cytokinesis

BACKGROUND

Mutations in the Abnormal Spindle Microcephaly related gene (ASPM) are the commonest cause of autosomal recessive primary microcephaly (MCPH) a disorder characterised by a small brain and associated mental retardation. ASPM encodes a mitotic spindle pole associated protein. It is suggested that the MCPH phenotype arises from proliferation defects in neural progenitor cells (NPC).

RESULTS

We show that ASPM is a microtubule minus end-associated protein that is recruited in a microtubule-dependent manner to the pericentriolar matrix (PCM) at the spindle poles during mitosis. ASPM siRNA reduces ASPM protein at the spindle poles in cultured U2OS cells and severely perturbs a number of aspects of mitosis, including the orientation of the mitotic spindle, the main determinant of developmental asymmetrical cell division. The majority of ASPM depleted mitotic cells fail to complete cytokinesis. In MCPH patient fibroblasts we show that a pathogenic ASPM splice site mutation results in the expression of a novel variant protein lacking a tripeptide motif, a minimal alteration that correlates with a dramatic decrease in ASPM spindle pole localisation. Moreover, expression of dominant-negative ASPM C-terminal fragments cause severe spindle assembly defects and cytokinesis failure in cultured cells.

CONCLUSIONS

These observations indicate that ASPM participates in spindle organisation, spindle positioning and cytokinesis in all dividing cells and that the extreme C-terminus of the protein is required for ASPM localisation and function. Our data supports the hypothesis that the MCPH phenotype caused by ASPM mutation is a consequence of mitotic aberrations during neurogenesis. We propose the effects of ASPM mutation are tolerated in somatic cells but have profound consequences for the symmetrical division of NPCs, due to the unusual morphology of these cells. This antagonises the early expansion of the progenitor pool that underpins cortical neurogenesis, causing the MCPH phenotype.

Allelic diversity in human developmental neurogenetics: insights into biology and disease

One of the biggest challenges in neuroscience is illuminating the architecture of developmental brain disorders, which include structural malformations of the brain and nerves, intellectual disability, epilepsy, and some psychiatric conditions like autism and potentially schizophrenia. Ongoing gene identification reveals a great diversity of genetic causes underlying abnormal brain development, illuminating new biochemical pathways often not suspected based on genetic studies in other organisms. Our greater understanding of genetic disease also shows the complexity of allelic diversity, in which distinct mutations in a given gene can cause a wide range of distinct diseases or other phenotypes. These diverse alleles not only provide a platform for discovery of critical protein-protein interactions in a genetic fashion, but also illuminate the likely genetic architecture of as yet poorly characterized neurological disorders.

Mutations in mouse Aspm (abnormal spindle-like microcephaly associated) cause not only microcephaly but also major defects in the germline

Mutations in ASPM (abnormal spindle-like microcephaly associated) cause primary microcephaly in humans, a disorder characterized by a major reduction in brain size in the apparent absence of nonneurological anomalies. The function of the Aspm protein in neural progenitor cell expansion, as well as its localization to the mitotic spindle and midbody, suggest that it regulates brain development by a cell division-related mechanism. Furthermore, evidence that positive selection affected ASPM during primate evolution has led to suggestions that such a function changed during primate evolution. Here, we report that in Aspm mutant mice, truncated Aspm proteins similar to those causing microcephaly in humans fail to localize to the midbody during M-phase and cause mild microcephaly. A human ASPM transgene rescues this phenotype but, interestingly, does not cause a gain of function. Strikingly, truncated Aspm proteins also cause a massive loss of germ cells, resulting in a severe reduction in testis and ovary size accompanied by reduced fertility. These germline effects, too, are fully rescued by the human ASPM transgene, indicating that ASPM is functionally similar in mice and humans. Our findings broaden the spectrum of phenotypic effects of ASPM mutations and raise the possibility that positive selection of ASPM during primate evolution reflects its function in the germline.

Mutation analysis of the ASPM gene in 18 Pakistani families with autosomal recessive primary microcephaly

Autosomal recessive primary microcephaly (MCPH) is a rare neurological disorder, in which the patients exhibit reduced occipital frontal head circumference (>3 standard deviations) and mild-to-severe mental retardation. Autosomal recessive primary microcephaly is genetically heterogeneous and 7 loci have been reported to date. Mutations in ASPM (abnormal spindle-like, microcephaly associated) gene are the most common cause of autosomal recessive primary microcephaly in the majority of the reported families. In the current investigation, we have located and studied 21 families with autosomal recessive primary microcephaly. Genotyping using polymorphic microsatellite markers linked to 7 autosomal recessive primary microcephaly loci revealed linkage of 18 families to the MCPH5 locus. Sequence analysis of the ASPM gene in 18 linked families detected 2 novel nonsense mutations (c.2101C>T/p.Q701X; c.9492T>G/p.Y3164X) in 2 families and 2 novel deletion mutations (c.6686delGAAA/p.R2229TfsX9; c.77delG/p.G26AfsX41) in 2 other families. Three previously described mutations (c.3978G>A/p.W1326X; c.1260delTCAAGTC/p.S420SfsX32; c.9159delA/p.K3053NfsX4) were also detected in 11 families. These identified mutations extended the body of evidence implicating the ASPM gene in the pathogenesis of human hereditary primary microcephaly.

Many roads lead to primary autosomal recessive microcephaly

Autosomal recessive primary microcephaly (MCPH), historically referred to as Microcephalia vera, is a genetically and clinically heterogeneous disease. Patients with MCPH typically exhibit congenital microcephaly as well as mental retardation, but usually no further neurological findings or malformations. Their microcephaly with grossly preserved macroscopic organization of the brain is a consequence of a reduced brain volume, which is evident particularly within the cerebral cortex and thus results to a large part from a reduction of grey matter. Some patients with MCPH further provide evidence of neuronal heterotopias, polymicrogyria or cortical dysplasia suggesting an associated neuronal migration defect. Genetic causes of MCPH subtypes 1-7 include mutations in genes encoding microcephalin, cyclin-dependent kinase 5 regulatory associated protein 2 (CDK5RAP2), abnormal spindle-like, microcephaly associated protein (ASPM), centromeric protein J (CENPJ), and SCL/TAL1-interrupting locus (STIL) as well as linkage to the two loci 19q13.1-13.2 and 15q15-q21. Here, we provide a timely overview of current knowledge on mechanisms leading to microcephaly in humans with MCPH and abnormalities in cell division/cell survival in corresponding animal models. Understanding the pathomechanisms leading to MCPH is of high importance not only for our understanding of physiologic brain development (particularly of cortex formation), but also for that of trends in mammalian evolution with a massive increase in size of the cerebral cortex in primates, of microcephalies of other etiologies including environmentally induced microcephalies, and of cancer formation.

Primary microcephaly: do all roads lead to Rome?

The relatively large brain and expanded cerebral cortex of humans is unusual in the animal kingdom and is thought to have promoted our adaptability and success as a species. One approach for investigating neurogenesis is the study of autosomal recessive primary microcephaly (MCPH), in which prenatal brain growth is significantly reduced without an effect on brain structure. To date, eight MCPH loci and five genes have been identified. Unexpectedly, all MCPH proteins are ubiquitous and localise to centrosomes for at least part of the cell cycle. Here, we focus on recent functional studies of MCPH proteins that reveal the centrosome as a final integration point for many regulatory pathways affecting prenatal neurogenesis in mammals.

Genetics and biology of microcephaly and lissencephaly

Genetic microcephaly and lissencephaly are 2 of the most common brain malformations. Each of them is a heterogeneous group of disorders caused by mutations of many different genes. They are a significant cause of neurological morbidity in children worldwide, responsible for many cases of mental retardation, cerebral palsy, and epilepsy. Recent advances in molecular genetics have led to the identification of several genes causing these disorders, and thus accurate molecular diagnosis and improved genetic counseling has become available for many patients and their families. More recently identified genes include STIL, causing primary autosomal recessive microcephaly (microcephaly vera), and TUBA1A, causing lissencephaly. Numerous other disease genes are likely still to be identified. Functional studies of genes that cause microcephaly and lissencephaly have provided valuable insight into the molecular mechanisms of human brain development.

Compound heterozygous ASPM mutations associated with microcephaly and simplified cortical gyration in a consanguineous Algerian family

Homozygous mutations in the ASPM gene are a major cause of autosomal recessive primary microcephaly (MCPH). Here we report on a consanguineous Algerian family in which three out of five children presented with severe microcephaly, simplified cortical gyration, mild to severe mental retardation and low to low-normal birth weight. Given the parental consanguinity with the unaffected parents being third cousins once removed, the most probable pattern of inheritance was autosomal recessive. Linkage and mutational analyses identified compound heterozygous truncating mutations within the ASPM gene segregating with MCPH (c.2389C>T [p.Arg797X] and c.7781_7782delAG [p.Gln2594fsX6]). These results highlight some of the pitfalls of genetic analysis in consanguineous families. They also suggest that low birth weight may be a feature of MCPH, a finding that needs confirmation, and confirm that ASPM mutations are associated with simplified cortical gyration.