Primary autosomal recessive microcephaly: homozygosity mapping of MCPH4 to chromosome 15

A novel KNL1 intronic splicing variant likely destabilizes the KMN complex, causing primary microcephaly

Primary microcephaly (MCPH) is an autosomal recessive disorder characterized by head circumference of at least two standard deviations below the mean. Biallelic variants in the kinetochore gene KNL1 is a known cause of MCPH4. KNL1 is the central component of the KNL1-MIS12-NSL1 (KMN) network, which acts as the signaling hub of the kinetochore and is required for correct chromosomal segregation during mitosis. We identified biallelic KNL1 variants in two siblings from a non-consanguineous family with microcephaly and intellectual disability. The two siblings carry a frameshift variant predicted to prematurely truncate the transcript and undergo nonsense mediated decay, and an intronic single nucleotide variant (SNV) predicted to disrupt splicing. An in vitro splicing assay and qPCR from blood-derived RNA confirmed that the intronic variant skips exon 23, significantly reducing levels of the canonical transcript. Protein modeling confirmed that absence of exon 23, an inframe exon, would disrupt a key interaction within the KMN network and likely destabilize the kinetochore signaling hub, disrupting mitosis. Therefore, this splicing variant is pathogenic and, in trans with a frameshift variant, causes the MCPH phenotype associated with KLN1. This finding furthers the association of splicing variants as a common pathogenic variant class for KNL1.

Emerging roles of centrosome cohesion

The centrosome, consisting of centrioles and the associated pericentriolar material, is the main microtubule-organizing centre (MTOC) in animal cells. During most of interphase, the two centrosomes of a cell are joined together by centrosome cohesion into one MTOC. The most dominant element of centrosome cohesion is the centrosome linker, an interdigitating, fibrous network formed by the protein C-Nap1 anchoring a number of coiled-coil proteins including rootletin to the proximal end of centrioles. Alternatively, centrosomes can be kept together by the action of the minus end directed kinesin motor protein KIFC3 that works on interdigitating microtubules organized by both centrosomes and probably by the actin network. Although cells connect the two interphase centrosomes by several mechanisms into one MTOC, the general importance of centrosome cohesion, particularly for an organism, is still largely unclear. In this article, we review the functions of the centrosome linker and discuss how centrosome cohesion defects can lead to diseases.

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.

Emerging Methods in Modeling Brain Development and Disease with Human Pluripotent Stem Cells

The Nobel Prize-winning discovery that human somatic cells can be readily reprogrammed into pluripotent cells has revolutionized our potential to understand the human brain. The rapid technological progression of this field has made it possible to easily obtain human neural cells and even intact tissues, offering invaluable resources to model human brain development. In this chapter, we present a brief history of hPSC-based approaches to study brain development and then, provide new insights into neurological diseases, focusing on those driven by aberrant cell death. Furthermore, we will shed light on the latest technologies and highlight the methods that researchers can use to employ established hPSC approaches in their research. Our intention is to demonstrate that hPSC-based modeling is a technical approach accessible to all researchers who seek a deeper understanding of the human brain.

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

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

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

Inhibiting microcephaly genes as alternative to microtubule targeting agents to treat brain tumors

Medulloblastoma (MB) and gliomas are the most frequent high-grade brain tumors (HGBT) in children and adulthood, respectively. The general treatment for these tumors consists in surgery, followed by radiotherapy and chemotherapy. Despite the improvement in patient survival, these therapies are only partially effective, and many patients still die. In the last decades, microtubules have emerged as interesting molecular targets for HGBT, as various microtubule targeting agents (MTAs) have been developed and tested pre-clinically and clinically with encouraging results. Nevertheless, these treatments produce relevant side effects since they target microtubules in normal as well as in cancerous cells. A possible strategy to overcome this toxicity could be to target proteins that control microtubule dynamics but are required by HGBT cells much more than in normal cell types. The genes mutated in primary hereditary microcephaly (MCPH) are ubiquitously expressed in proliferating cells, but under normal conditions are selectively required during brain development, in neural progenitors. There is evidence that MB and glioma cells share molecular profiles with progenitors of cerebellar granules and of cortical radial glia cells, in which MCPH gene functions are fundamental. Moreover, several studies indicate that MCPH genes are required for HGBT expansion. Among the 25 known MCPH genes, we focus this review on KNL1, ASPM, CENPE, CITK and KIF14, which have been found to control microtubule stability during cell division. We summarize the current knowledge about the molecular basis of their interaction with microtubules. Moreover, we will discuss data that suggest these genes are promising candidates as HGBT-specific targets.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSION

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

Dissecting the Genetic and Etiological Causes of Primary Microcephaly

Autosomal recessive primary microcephaly (MCPH; “small head syndrome”) is a rare, heterogeneous disease arising from the decreased production of neurons during brain development. As of August 2020, the Online Mendelian Inheritance in Man (OMIM) database lists 25 genes (involved in molecular processes such as centriole biogenesis, microtubule dynamics, spindle positioning, DNA repair, transcriptional regulation, Wnt signaling, and cell cycle checkpoints) that are implicated in causing MCPH. Many of these 25 genes were only discovered in the last 10 years following advances in exome and genome sequencing that have improved our ability to identify disease-causing variants. Despite these advances, many patients still lack a genetic diagnosis. This demonstrates a need to understand in greater detail the molecular mechanisms and genetics underlying MCPH. Here, we briefly review the molecular functions of each MCPH gene and how their loss disrupts the neurogenesis program, ultimately demonstrating that microcephaly arises from cell cycle dysregulation. We also explore the current issues in the genetic basis and clinical presentation of MCPH as additional avenues of improving gene/variant prioritization. Ultimately, we illustrate that the detailed exploration of the etiology and inheritance of MCPH improves the predictive power in identifying previously unknown MCPH candidates and diagnosing microcephalic patients.

Loss of NARS1 impairs progenitor proliferation in cortical brain organoids and leads to microcephaly

Asparaginyl-tRNA synthetase1 (NARS1) is a member of the ubiquitously expressed cytoplasmic Class IIa family of tRNA synthetases required for protein translation. Here, we identify biallelic missense and frameshift mutations in NARS1 in seven patients from three unrelated families with microcephaly and neurodevelopmental delay. Patient cells show reduced NARS1 protein, impaired NARS1 activity and impaired global protein synthesis. Cortical brain organoid modeling shows reduced proliferation of radial glial cells (RGCs), leading to smaller organoids characteristic of microcephaly. Single-cell analysis reveals altered constituents of both astrocytic and RGC lineages, suggesting a requirement for NARS1 in RGC proliferation. Our findings demonstrate that NARS1 is required to meet protein synthetic needs and to support RGC proliferation in human brain development.

Asparaginyl-tRNA synthetase1 (NARS1) is required for protein synthesis. Here, the authors identify biallelic NARS1 mutations in individuals with microcephaly and neurodevelopmental delay. Cortical brain organoid modeling recapitulates microcephaly characteristics and scRNA-seq reveals a role for NARS1 in radial glial cell proliferation.

The journey of Zika to the developing brain

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

Effect of KNL1 on the proliferation and apoptosis of colorectal cancer cells

Objective:

To identify the expression of kinetochore scaffold 1 (KNL1) in colorectal tumor tissues and to clarify the role of this gene in the proliferation capability of colorectal cancer cells.

Methods:

A total of 108 paired colorectal tumor and normal tissue samples were collected from patients with colorectal cancer and subjected to quantitative polymerase chain reaction and immunohistochemistry analyses. Expression levels of KNL1 mRNA and protein were compared between tumor and normal tissues, and KNL1 levels were evaluated in relation to the patients’ tumor differentiation, sex, lymph node metastasis, TNM stage, infiltration depth, age, and tumor location. Survival curves were also constructed and compared between patients with tumor samples with and without KLN1 protein expression. KNL1 was under-expressed in colorectal cancer cells in vitro using lentiviral transfection with short hairpin RNA, and its function was evaluated by proliferation, colony-formation, and apoptosis assays. Expression levels of BUB1 protein were also compared between tumor and normal tissues, and the correlation between KNL1 expression and BUB1 expression in colorectal cancer tissues was examined.

Results:

KNL1 mRNA and protein were both highly expressed in colorectal tumor tissues compared with paired normal tissues. KNL1 downregulation significantly inhibited colorectal cancer cell proliferation and colony formation, and promoted apoptosis. KNL1 protein expression was significantly associated with tumor differentiation, but not with sex, lymph node metastasis, TNM stage, infiltration depth, age, or tumor location. KNL1 protein expression was also significantly associated with poorer survival. Moreover, there was a significant correlation between KNL1 and BUB1 in colorectal cancer tissues.

Conclusions:

KNL1 plays an effective role in decreasing apoptosis and promoting the proliferation of colorectal cancer cells, suggesting that its inhibition may represent a promising therapeutic approach in patients with colorectal cancer.

The Mitotic Apparatus and Kinetochores in Microcephaly and Neurodevelopmental Diseases

Regulators of mitotic division, when dysfunctional or expressed in a deregulated manner (over- or underexpressed) in somatic cells, cause chromosome instability, which is a predisposing condition to cancer that is associated with unrestricted proliferation. Genes encoding mitotic regulators are growingly implicated in neurodevelopmental diseases. Here, we briefly summarize existing knowledge on how microcephaly-related mitotic genes operate in the control of chromosome segregation during mitosis in somatic cells, with a special focus on the role of kinetochore factors. Then, we review evidence implicating mitotic apparatus- and kinetochore-resident factors in the origin of congenital microcephaly. We discuss data emerging from these works, which suggest a critical role of correct mitotic division in controlling neuronal cell proliferation and shaping the architecture of the central nervous system.

Microcephaly Modeling of Kinetochore Mutation Reveals a Brain-Specific Phenotype

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

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

A case of severe TBCE-negative hypoparathyroidism-retardation-dysmorphism syndrome: Case report and literature review

Hypoparathyroidism-retardation-dysmorphism syndrome (HRD) is a rare autosomal recessive disorder attributed to the mutations in the tubulin-specific chaperone E (TBCE) gene, which is vital for microtubule function during mitosis, organelle positioning, and neuronal cytokinesis. HRD is a congenital syndromic hypoparathyroidism associated with growth deficiency, microcephaly, intellectual disability, ocular anomalies, and facial dysmorphism. To our knowledge, there is only one published case of mild HRD-like syndrome with no identifiable genetic etiology. We report a case of severe TBCE-negative phenotypic HRD in a 4-year-old female from India presenting with hypocalcemic seizures due to congenital hypoparathyroidism, extreme microcephaly, growth deficiency, ocular anomalies, and facial dysmorphism. SNP microarray and whole exome sequencing (WES) did not detect any abnormalities in TBCE or other genes of interest. WES revealed two variants of unknown clinical significance in CASC5 gene, which codes for a protein in the kinetochore and, interestingly similar to TBCE, is essential for proper microtubule function during mitosis and cell proliferation and has been implicated in primary microcephaly disorders. However, further targeted sequencing in the parents revealed both variants inherited from the unaffected mother. Significant copy number variant noise in the proband and her parents limited further analysis. At this time the role of variants in the CASC5 gene is unclear and cannot explain our patient's phenotype. In conclusion, we report a severe case of phenotypic HRD syndrome, in which extensive genetic evaluation failed to reveal an etiology. Our case demonstrates that the pathogenesis of HRD may be genetically heterogenous, meriting further genetic investigations.

Whole exome sequencing identifies a novel homozygous frameshift mutation in the ASPM gene, which causes microcephaly 5, primary, autosomal recessive

Microcephaly is a genetically heterogeneous disorder and is one of the frequently notable conditions in paediatric neuropathology which exists either as a single entity or in association with other co-morbidities. More than a single gene is implicated in true microcephaly and the list is growing with the recent advancements in sequencing technologies. Using massive parallel sequencing, we identified a novel frame shift insertion in the abnormal spindle-like microcephaly-associated protein gene in a client with true autosomal recessive primary microcephaly.  Exome sequencing in the present case helped in identifying the true cause behind the disease, which helps in the premarital counselling for the sibling to avoid future recurrence of the disorder in the family.

D40/KNL1/CASC5 and autosomal recessive primary microcephaly

Autosomal recessive primary microcephaly (MCPH) is a very rare neuro-developmental disease with brain size reduction. More than a dozen loci encoding proteins of diverse function have been shown to be responsible for MCPH1-13. Mutations in the D40/KNL1/CASC5 gene, which was initially characterized as a gene involved in chromosomal translocation in leukemia and as a member of the cancer/testis gene family, was later found to encode a kinetochore protein essential for mitotic cell division and to cause MCPH4. Although our previous studies showed that this gene is required for cell growth and division in vitro and in animal experiments, the revelation that mutations in this gene caused microcephaly provides in vivo evidence of a critical role in brain growth. In this review, we describe mutated gene targets responsible for MCPH1-13 and summarize clinical studies of, and molecular and biological aspects of the gene and encoded protein responsible for MCPH4.

The genetics of congenitally small brains

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

Adaptive evolution of interleukin-3 (IL3), a gene associated with brain volume variation in general human populations

Greatly expanded brain volume is one of the most characteristic traits that distinguish humans from other primates. Recent studies have revealed genes responsible for the dramatically enlarged human brain size (i.e., the microcephaly genes), and it has been well documented that many microcephaly genes have undergone accelerated evolution along the human lineage. In addition to being far larger than other primates, human brain volume is also highly variable in general populations. However, the genetic basis underlying human brain volume variation remains elusive and it is not known whether genes regulating human brain volume variation also have experienced positive selection. We have previously shown that genetic variants (near the IL3 gene) on 5q33 were significantly associated with brain volume in Chinese population. Here, we provide further evidence that support the significant association of genetic variants on 5q33 with brain volume. Bioinformatic analyses suggested that rs31480 is likely to be the causal variant among the studied SNPs. Molecular evolutionary analyses suggested that IL3 might have undergone positive selection in primates and humans. Neutrality tests further revealed signatures of positive selection of IL3 in Han Chinese and Europeans. Finally, extended haplotype homozygosity (EHH) and relative EHH analyses showed that the C allele of SNP rs31480 might have experienced recent positive selection in Han Chinese. Our results suggest that IL3 is an important genetic regulator for human brain volume variation and implied that IL3 might have experienced weak or modest positive selection in the evolutionary history of humans, which may be due to its contribution to human brain volume.

A novel homozygous splicing mutation of CASC5 causes primary microcephaly in a large Pakistani family

Primary microcephaly is a disorder characterized by a small head and brain associated with impaired cognitive capabilities. Mutations in 13 different genes encoding centrosomal proteins and cell cycle regulators have been reported to cause the disease. CASC5, a gene encoding a protein important for kinetochore formation and proper chromosome segregation during mitosis, has been suggested to be associated with primary microcephaly-4 (MCPH4). This was based on one mutation only and circumstantial functional evidence. By combining homozygosity mapping and whole-exome sequencing in an MCPH family from Pakistan, we identified a second mutation (NM_170589.4;c.6673-19T>A) in CASC5. This mutation induced skipping of exon 25 of CASC5 resulting in a frameshift and the introduction of a premature stop codon (p.Met2225Ilefs*7). The C-terminally truncated protein lacks 118 amino acids that encompass the region responsible for the interaction with the hMIS12 complex, which is essential for proper chromosome alignment and segregation. Furthermore, we showed a down-regulation of CASC5 mRNA and reduction of the amount of CASC5 protein by quantitative RT-PCR and western blot analysis, respectively. As a further sign of functional deficits, we observed dispersed dots of CASC5 immunoreactive material outside the metaphase plate of dividing patient fibroblasts. Normally, CASC5 is a component of the kinetochore of metaphase chromosomes. A higher mitotic index in patient cells indicated a mitotic arrest in the cells carrying the mutation. We also observed lobulated and fragmented nuclei as well as micronuclei in the patient cells. Moreover, we detected an altered DNA damage response with higher levels of γH2AX and 53BP1 in mutant as compared to control fibroblasts. Our findings substantiate the proposed role of CASC5 for primary microcephaly and suggest that it also might be relevant for genome stability.

Targeted Knockdown of the Kinetochore Protein D40/Knl-1 Inhibits Human Cancer in a p53 Status-Independent Manner

The D40 gene encodes a kinetochore protein that plays an essential role in kinetochore formation during mitosis. Short inhibitory RNA against D40, D40 siRNA, has been shown to deplete the D40 protein in the human cancer cell line HeLa, which harbors wild-type p53, and this activity was followed by the significant inhibition of cell growth and induction of apoptotic cell death. The p53-null cancer cell line, PC-3M-luc, is also sensitive to the significant growth inhibition and cell death induced by D40 siRNA. The growth of PC-3M-luc tumors transplanted into nude mice was inhibited by the systemic administration of D40 siRNA and the atelocollagen complex. Furthermore, D40 siRNA significantly inhibited growth and induced apoptotic cell death in a cell line with a gain-of-function (GOF) mutation in p53, MDA-MB231-luc, and also inhibited the growth of tumors transplanted into mice when administered as a D40 siRNA/atelocollagen complex. These results indicated that D40 siRNA induced apoptotic cell death in human cancer cell lines, and inhibited their growth in vitro and in vivo regardless of p53 status. Therefore, D40 siRNA is a potential candidate anti-cancer reagent.

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.

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 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.

Mutations in CENPE define a novel kinetochore-centromeric mechanism for microcephalic primordial dwarfism

Defects in centrosome, centrosomal-associated and spindle-associated proteins are the most frequent cause of primary microcephaly (PM) and microcephalic primordial dwarfism (MPD) syndromes in humans. Mitotic progression and segregation defects, microtubule spindle abnormalities and impaired DNA damage-induced G2-M cell cycle checkpoint proficiency have been documented in cell lines from these patients. This suggests that impaired mitotic entry, progression and exit strongly contribute to PM and MPD. Considering the vast protein networks involved in coordinating this cell cycle stage, the list of potential target genes that could underlie novel developmental disorders is large. One such complex network, with a direct microtubule-mediated physical connection to the centrosome, is the kinetochore. This centromeric-associated structure nucleates microtubule attachments onto mitotic chromosomes. Here, we described novel compound heterozygous variants in CENPE in two siblings who exhibit a profound MPD associated with developmental delay, simplified gyri and other isolated abnormalities. CENPE encodes centromere-associated protein E (CENP-E), a core kinetochore component functioning to mediate chromosome congression initially of misaligned chromosomes and in subsequent spindle microtubule capture during mitosis. Firstly, we present a comprehensive clinical description of these patients. Then, using patient cells we document abnormalities in spindle microtubule organization, mitotic progression and segregation, before modeling the cellular pathogenicity of these variants in an independent cell system. Our cellular analysis shows that a pathogenic defect in CENP-E, a kinetochore-core protein, largely phenocopies PCNT-mutated microcephalic osteodysplastic primordial dwarfism-type II patient cells. PCNT encodes a centrosome-associated protein. These results highlight a common underlying pathomechanism. Our findings provide the first evidence for a kinetochore-based route to MPD in humans.

Autosomal recessive primary microcephalies (MCPH)

Autosomal recessive primary microcephaly (MCPH) is a genetically heterogeneous disease characterized by a pronounced reduction in volume of otherwise architectonical normal brains and intellectual deficit. Here, we summarize the genetic causes of MCPH types 1-12 known to date.

Congenital microcephaly

The underlying etiologies of genetic congenital microcephaly are complex and multifactorial. Recently, with the exponential growth in the identification and characterization of novel genetic causes of congenital microcephaly, there has been a consolidation and emergence of certain themes concerning underlying pathomechanisms. These include abnormal mitotic microtubule spindle structure, numerical and structural abnormalities of the centrosome, altered cilia function, impaired DNA repair, DNA Damage Response signaling and DNA replication, along with attenuated cell cycle checkpoint proficiency. Many of these processes are highly interconnected. Interestingly, a defect in a gene whose encoded protein has a canonical function in one of these processes can often have multiple impacts at the cellular level involving several of these pathways. Here, we overview the key pathomechanistic themes underlying profound congenital microcephaly, and emphasize their interconnected nature.

The diverse genetic landscape of neurodevelopmental disorders

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

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.

CDK6 associates with the centrosome during mitosis and is mutated in a large Pakistani family with primary microcephaly

Autosomal recessive primary microcephaly (MCPH) is characterized by reduced head circumference, reduction in the size of the cerebral cortex with otherwise grossly normal brain structure and variable intellectual disability. MCPH is caused by mutations of 11 different genes which code for proteins implicated in cell division and cell cycle regulation. We studied a consanguineous eight-generation family from Pakistan with ten microcephalic children using homozygosity mapping and found a new MCPH locus at HSA 7q21.11-q21.3. Sanger sequencing of the most relevant candidate genes in this region revealed a homozygous single nucleotide substitution c.589G>A in CDK6, which encodes cyclin-dependent kinase 6. The mutation changes a highly conserved alanine at position 197 into threonine (p.Ala197Thr). Post hoc whole-exome sequencing corroborated this mutation's identification as the causal variant. CDK6 is an important protein for the control of the cell cycle and differentiation of various cell types. We show here for the first time that CDK6 associates with the centrosome during mitosis; however, this was not observed in patient fibroblasts. Moreover, the mutant primary fibroblasts exhibited supernumerary centrosomes, disorganized microtubules and mitotic spindles, an increased centrosome nucleus distance, reduced cell proliferation and impaired cell motility and polarity. Upon ectopic expression of the mutant protein and knockdown of CDK6 through shRNA, we noted similar effects. We propose that the identified CDK6 mutation leads to reduced cell proliferation and impairs the correct functioning of the centrosome in microtubule organization and its positioning near the nucleus which are key determinants during neurogenesis.

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

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

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

OBJECTIVE

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

MATERIALS & METHODS

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

RESULT

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

CONCLUSION

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

Kinetochore KMN network gene CASC5 mutated in primary microcephaly

Several genes expressed at the centrosome or spindle pole have been reported to underlie autosomal recessive primary microcephaly (MCPH), a neurodevelopmental disorder consisting of an important brain size reduction present since birth, associated with mild-to-moderate mental handicap and no other neurological feature nor associated malformation. Here, we report a mutation of CASC5 (aka Blinkin, or KNL1, or hSPC105) in MCPH patients from three consanguineous families, in one of which we initially reported the MCPH4 locus. The combined logarithm of odds score of the three families was >6. All patients shared a very rare homozygous mutation of CASC5. The mutation induced skipping of exon 18 with subsequent frameshift and truncation of the predicted protein. CASC5 is part of the KMN network of the kinetochore and is required for proper microtubule attachment to the chromosome centromere and for spindle-assembly checkpoint (SAC) activation during mitosis. Like MCPH gene ASPM, CASC5 is upregulated in the ventricular zone (VZ) of the human fetal brain. CASC5 binds BUB1, BUBR1, ZWINT-1 and interestingly it binds to MIS12 through a protein domain which is truncated by the mutation. CASC5 localized at the equatorial plate like ZWINT-1 and BUBR1, while ASPM, CEP152 and PCTN localized at the spindle poles in our patients and in controls. Comparison of primate and rodent lineages indicates accelerated evolution of CASC5 in the human lineage. Our data provide strong evidence for CASC5 as a novel MCPH gene, and underscore the role of kinetochore integrity in proper volumetric development of the human brain.

A novel MCPH1 isoform complements the defective chromosome condensation of human MCPH1-deficient cells

Biallelic mutations in MCPH1 cause primary microcephaly (MCPH) with the cellular phenotype of defective chromosome condensation. MCPH1 encodes a multifunctional protein that notably is involved in brain development, regulation of chromosome condensation, and DNA damage response. In the present studies, we detected that MCPH1 encodes several distinct transcripts, including two major forms: full-length MCPH1 (MCPH1-FL) and a second transcript lacking the six 3′ exons (MCPH1Δe9–14). Both variants show comparable tissue-specific expression patterns, demonstrate nuclear localization that is mediated independently via separate NLS motifs, and are more abundant in certain fetal than adult organs. In addition, the expression of either isoform complements the chromosome condensation defect found in genetically MCPH1-deficient or MCPH1 siRNA-depleted cells, demonstrating a redundancy of both MCPH1 isoforms for the regulation of chromosome condensation. Strikingly however, both transcripts are regulated antagonistically during cell-cycle progression and there are functional differences between the isoforms with regard to the DNA damage response; MCPH1-FL localizes to phosphorylated H2AX repair foci following ionizing irradiation, while MCPH1Δe9–14 was evenly distributed in the nucleus. In summary, our results demonstrate here that MCPH1 encodes different isoforms that are differentially regulated at the transcript level and have different functions at the protein level.

Two Missense Mutations in the Primary Autosomal Recessive Microcephaly Gene MCPH1 Disrupt the Function of the Highly Conserved N-Terminal BRCT Domain of Microcephalin

Primary microcephaly MCPH1 is an extremely rare autosomal recessive disorder associated with congenital microcephaly, mental retardation and a distinctive cellular phenotype of misregulated chromosome condensation. The MCPH1 gene encodes an 835-amino acid protein, microcephalin, which contains 1 N-terminal and 2 C-terminal BRCT (BRCA1 C-terminus) domains. BRCT domains are predominantly found in proteins involved in cell cycle control and DNA repair. Here we describe 1 novel and 1 previously reported MCPH1 missense mutation, p.Trp75Arg and p.Ser72Leu, respectively, in the N-terminal BRCT domain of microcephalin associated with severe congenital microcephaly. Both residues are entirely conserved in the MCPH1 orthologs of all vertebrate species and Drosophila. Proliferating lymphocytes of the patients with p.Trp75Arg and p.Ser72Leu show the unique cellular MCPH1 phenotype of misregulated chromosome condensation, indicating that these missense alterations disrupt the function of the N-terminal BRCT domain of the protein. Interestingly, both residues are strictly conserved in BRCT domains of BRCA1. ClustalW alignments show that the residue p.Ser72 of microcephalin corresponds to p.Ser1715 of the N-terminal BRCT domain of BRCA1, while the microcephalin residue p.Trp75 is analogous to p.Trp1718 in the N-terminal BRCT and to p.Trp1837 in C-terminal BRCT domains of BRCA1. Missense alterations for all 3 corresponding BRCA1 residues were described and are predicted to be deleterious resulting in the destabilization of the BRCA1 protein. Our data on the 2 MCPH1 missense alterations provide further evidence for the functional significance of these residues in BRCT domains.

WDR62 missense mutation in a consanguineous family with primary microcephaly

We report on a consanguineous couple with two affected sons who presented with primary microcephaly and moderate to severe intellectual disabilities. A SNP array uncovered two overlapping regions of copy-neutral absence of heterozygosity (AOH) in both sibs. This led to sequencing of WDR62, a gene that codes for a spindle pole protein recently identified as a cause of primary microcephaly. A homozygous missense mutation in WDR62, p.E400K, was found in both boys and segregated with the condition in this family. WDR62 is one of seven genes responsible for autosomal recessive primary microcephaly (MCPH), and appears to be one of the most frequently involved in MCPH following ASPM. Studies of ASPM and WDR62 should perhaps be pursued in all cases of primary microcephaly with or without gross brain malformations.

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.

What's the hype about CDK5RAP2?

Cyclin dependent kinase 5 regulatory subunit-associated protein 2 (CDK5RAP2) has gained attention in the last years following the discovery, in 2005, that recessive mutations cause primary autosomal recessive microcephaly. This disease is seen as an isolated developmental defect of the brain, particularly of the cerebral cortex, and was thus historically also referred to as microcephalia vera. Unraveling the pathomechanisms leading to this human disease is fascinating scientists because it can convey insight into basic mechanisms of physiologic brain development (particularly of cortex formation). It also finds itself in the spotlight because of its implication in trends in mammalian evolution with a massive increase in the size of the cerebral cortex in primates. Here, we provide a timely overview of the current knowledge on the function of CDK5RAP2 and mechanisms that might lead to disease in humans when the function of this protein is disturbed.

Genetic regulation of human brain development: lessons from Mendelian diseases

One of the fundamental goals in human genetics is to link gene function to phenotype, yet the function of the majority of the genes in the human body is still poorly understood. This is especially true for the developing human brain. The study of human phenotypes that result from inherited, mutated alleles is the most direct evidence for the requirement of a gene in human physiology. Thus, the study of Mendelian central nervous system (CNS) diseases can be an extremely powerful approach to elucidate such phenotypic/genotypic links and to increase our understanding of the key components required for development of the human brain. In this review, we highlight examples of how the study of inherited neurodevelopmental disorders contributes to our knowledge of both the "normal" and diseased human brain, as well as elaborate on the future of this type of research. Mendelian disease research has been, and will continue to be, key to understanding the molecular mechanisms that underlie human brain function, and will ultimately form a basis for the design of intelligent, mechanism-specific treatments for nervous system disorders.

Mutations in centrosomal protein CEP152 in primary microcephaly families linked to MCPH4

Primary microcephaly is a rare condition in which brain size is substantially diminished without other syndromic abnormalities. Seven autosomal loci have been genetically mapped, and the underlying causal genes have been identified for MCPH1, MCPH3, MCPH5, MCPH6, and MCPH7 but not for MCPH2 or MCPH4. The known genes play roles in mitosis and cell division. We ascertained three families from an Eastern Canadian subpopulation, each with one microcephalic child. Homozygosity analysis in two families using genome-wide dense SNP genotyping supported linkage to the published MCPH4 locus on chromosome 15q21.1. Sequencing of coding exons of candidate genes in the interval identified a nonconservative amino acid change in a highly conserved residue of the centrosomal protein CEP152. The affected children in these two families were both homozygous for this missense variant. The third affected child was compound heterozygous for the missense mutation plus a second, premature-termination mutation truncating a third of the protein and preventing its localization to centrosomes in transfected cells. CEP152 is the putative mammalian ortholog of Drosphila asterless, mutations in which affect mitosis in the fly. Published data from zebrafish are also consistent with a role of CEP152 in centrosome function. By RT-PCR, CEP152 is expressed in the embryonic mouse brain, similar to other MCPH genes. Like some other MCPH genes, CEP152 shows signatures of positive selection in the human lineage. CEP152 is a strong candidate for the causal gene underlying MCPH4 and may be an important gene in the evolution of human brain size.

Cdk5rap2 interacts with pericentrin to maintain the neural progenitor pool in the developing neocortex

Primary autosomal-recessive microcephaly (MCPH) and Majewski osteodysplastic primordial dwarfism type II (MOPDII) are both genetic diseases that result in decreased brain size at birth. MCPH is thought to arise from alterations in the size of the neural progenitor pool, but the cause of this defect has not been thoroughly explored. We find that one of the genes associated with MCPH, Cdk5rap2, is highly expressed in the neural progenitor pool and that its loss results in a depletion of apical progenitors and increased cell-cycle exit leading to premature neuronal differentiation. We link Cdk5rap2 function to the pericentriolar material protein pericentrin, loss of function of which is associated with MOPDII. Depletion of pericentrin in neural progenitors phenocopies effects of Cdk5rap2 knockdown and results in decreased recruitment of Cdk5rap2 to the centrosome. Our findings uncover a common mechanism, involving aberrations in the neurogenesis program, that may underlie the development of microcephaly in multiple diseases.

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.

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.

A novel oculo-oto-facial dysplasia in a Native Alaskan community with autosomal recessive inheritance

We describe a novel autosomal recessive malformation syndrome in four related individuals from a geographically isolated Native Alaskan community, who have facial defects similar to those of individuals with Treacher Collins (TCS) and Miller syndrome. Distinctive findings include malar and mandibular hypoplasia, lower eyelid coloboma, choanal atresia, orofacial clefting, and external ear malformation with preauricular tags. Intellect is normal and profound mixed hearing loss has been observed in affected adults. Variable extracranial findings include atrioseptal defect, renal dysplasia, and imperforate anus, however, no limb defects have been observed. Cranial imaging studies demonstrate relative prominence of the zygoma, inferior orbital maxillary hypoplasia, and lateral orbital wall defects with an accessory superior bony projection off the zygoma lateral to the orbital rim. We propose that these individuals have inherited a novel autosomal recessive condition we have termed oculo-oto-facial dysplasia (OOFD) with unique radiographic findings.

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.

Mutations in STIL, encoding a pericentriolar and centrosomal protein, cause primary microcephaly

Primary microcephaly (MCPH) is an autosomal-recessive congenital disorder characterized by smaller-than-normal brain size and mental retardation. MCPH is genetically heterogeneous with six known loci: MCPH1-MCPH6. We report mapping of a novel locus, MCPH7, to chromosome 1p32.3-p33 between markers D1S2797 and D1S417, corresponding to a physical distance of 8.39 Mb. Heterogeneity analysis of 24 families previously excluded from linkage to the six known MCPH loci suggested linkage of five families (20.83%) to the MCPH7 locus. In addition, four families were excluded from linkage to the MCPH7 locus as well as all of the six previously known loci, whereas the remaining 15 families could not be conclusively excluded or included. The combined maximum two-point LOD score for the linked families was 5.96 at marker D1S386 at theta = 0.0. The combined multipoint LOD score was 6.97 between markers D1S2797 and D1S417. Previously, mutations in four genes, MCPH1, CDK5RAP2, ASPM, and CENPJ, that code for centrosomal proteins have been shown to cause this disorder. Three different homozygous mutations in STIL, which codes for a pericentriolar and centrosomal protein, were identified in patients from three of the five families linked to the MCPH7 locus; all are predicted to truncate the STIL protein. Further, another recently ascertained family was homozygous for the same mutation as one of the original families. There was no evidence for a common haplotype. These results suggest that the centrosome and its associated structures are important in the control of neurogenesis in the developing human brain.

Previously described sequence variant in CDK5RAP2 gene in a Pakistani family with autosomal recessive primary microcephaly

Background

Autosomal Recessive Primary Microcephaly (MCPH) is a disorder of neurogenic mitosis. MCPH leads to reduced cerebral cortical volume and hence, reduced head circumference associated with mental retardation of variable degree. Genetic heterogeneity is well documented in patients with MCPH with six loci known, while pathogenic sequence variants in four respective genes have been identified so far. Mutations in CDK5RAP2 gene at MCPH3 locus have been least involved in causing MCPH phenotype.

Methods

All coding exons and exon/intron splice junctions of CDK5RAP2 gene were sequenced in affected and normal individuals of Pakistani MCPH family of Kashmiri origin, which showed linkage to MCPH3 locus on chromosome 9q33.2.

Results

A previously described nonsense mutation [243 T>A (S81X)] in exon 4 of CDK5RAP2 gene has been identified in the Pakistani family, presented here, with MCPH Phenotype. Genomic and cDNA sequence comparison revealed that the exact nomenclature for this mutation is 246 T>A (Y82X).

Conclusion

Recurrent observation of Y82X mutation in CDK5RAP2 gene in this Pakistani family may be a sign of confinement of a rare ancestral haplotype carrying this pathogenic variant within Northern Pakistani population, as this has not been reported in any other population.