ASPM and the Evolution of Cerebral Cortical Size in a Community of New World Monkeys

The ASPM (abnormal spindle-like microcephaly associated) gene has been proposed as a major determinant of cerebral cortical size among primates, including humans. Yet the specific functions of ASPM and its connection to human intelligence remain controversial. This debate is limited in part by a taxonomic focus on Old World monkeys and apes. Here we expand the comparative context of ASPM sequence analyses with a study of New World monkeys, a radiation of primates in which enlarged brain size has evolved in parallel in spider monkeys (genus Ateles) and capuchins (genus Cebus). The primate community of Costa Rica is perhaps a model system because it allows for independent pairwise comparisons of smaller- and larger-brained species within two taxonomic families. Accordingly, we analyzed the complete sequence of exon 18 of ASPM in Ateles geoffroyi, Alouatta palliata, Cebus capucinus, and Saimiri oerstedii. As the analysis of multiple species in a genus improves phylogenetic reconstruction, we also analyzed eleven published sequences from other New World monkeys. Our exon-wide, lineage-specific analysis of eleven genera and the ratio of rates of nonsynonymous to synonymous substitutions (dN/dS) on ASPM revealed no detectable evidence for positive selection in the lineages leading toAteles or Cebus, as indicated by dN/dS ratios of ,1.0 (0.6502 and 0.4268, respectively). Our results suggest that a multitude of interacting genes have driven the evolution of larger brains among primates, with different genes involved in this process in different encephalized lineages, or at least with evidence for positive selection not readily apparent for the same genes in all lineages. The primate community of Costa Rica may serve as a model system for future studies that aim to elucidate the molecular mechanisms underlying cognitive capacity and cortical size

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

Molecular Genetic Analysis of Autosomal Recessive Primary Microcephaly in Pakistani Kindreds

Autosomal recessive primary microcephaly (MCPH) is a rare genetic disorder in which the afflicted individuals have head circumference more than 3 SDs below the age- and sex-related mean. The reduced head circumference is due to a small but architecturally normal cerebral cortex. MCPH is characterized by a pronounced heterogeneity with seven loci, designated MCPH1-7, have already been identified. The underlying genetic defects were found in the following seven genes, MCPH1, WDR62, CDK5RAP2, CEP152, ASPM, CENPJ, and STIL/SIL. The incidence of this disorder is highest in Pakistan (Woods et al., 2005). Here, I ascertained thirty families with MCPH from various regions of Pakistan. Homozygosity mapping revealed linkage in 19 families to the MCPH5 locus, in 2 to MCPH2, in 2 to MCPH4, in 1 to MCPH1, in 1 to MCPH6, and in 5 families linkage to all known MCPH loci was excluded. Families linked to the MCPH1, MCPH2, MCPH5, and MCPH6 loci were also subjected to direct genomic sequencing of the corresponding genes, i.e. MCPH1, WDR62, ASPM, and CENPJ, respectively. This revealed one, two, and nine novel mutations in MCPH1, WDR62, and ASPM, respectively. Genome-wide linkage analysis in the 5 families previously excluded to be linked to any of the known loci resulted in 5 different new gene loci, MCPH8-MCPH12, situated on different chromosomes. For two of the five new loci, namely MCPH8 on chromosome 7q21-q22 (LOD score 10.47) and MCPH9 on chromosome 4p14-4q12 (LOD score 2.53), the causative genes could be identified. Positional candidate gene sequencing revealed mutations in CDK6 (c.589G>A, p.A197T) at the MCPH8 locus and in CEP135 (c.970delC, p.Gln324Serfs*2) at the MCPH9 locus as the most likely pathogenic variants. These variants were not found in 768 chromosomes from healthy Pakistani controls. These two novel MCPH proteins cyclin-dependent kinase 6 (CDK6) and a centrosomal protein of 135kDa (CEP135) presented as transient or permanent components of the centrosome. Cdk6 and Cep135 showed a high expression level in the developing neuroepithelium of the mouse cerebral cortex of E11.5 and E15.5 embryos. In human cell lines, the localization of CDK6 at the spindle pole was observed. Primary fibroblasts of the patient with the CDK6 mutation failed to grow normally and showed an aberrant nuclear shape as well as centrosome-nucleus distance. CDK6 suppression by shRNA mimicked the defects in cell proliferation, nuclear shape, and microtubule organization. Likewise, overexpression of mutant CDK6 resulted in the production of multiple centrosomes and disorganised microtubules. Primary fibroblasts of the patient with the CEP135 mutation showed multiple and fragmented centrosomes, a disorganised microtubule system, misshapen and fragmented nuclei, and sometimes a complete loss of centrosomes. Altered levels of wild-type and mutant CEP135 protein by overexpression caused disorganization of microtubules, while overexpression of mutant CEP135 showed also multiple centrosomes observed before in the patient’s primary fibroblasts. Based on the data on CDK6, I propose that mutation p.A197T may lead to a reduced cell proliferation and may also affect the correct functioning of the centrosome in microtubule organisation and its positioning near the nucleus. The abnormal centrosome number associated with mutant CEP135 strengthens its role in centriole biogenesis, whereas a disorganisation of the microtubule network points to its role at the centrosome as a microtubule organising center. The data obtained lend further support to the hypothesis that the exquisite control of the cleavage furrow orientation in mammalian neural precursor cell mitosis, controlled in great part by the centrosomes and spindle poles, is critical in the etiology of MCPH (Fish et al., 2006, Thornton and Woods, 2009)

Ciliary signalling and mechanotransduction in the pathophysiology of craniosynostosis

Craniosynostosis (CS) is the second most prevalent inborn craniofacial malformation; it results from the premature fusion of cranial sutures and leads to dimorphisms of variable severity. CS is clinically heterogeneous, as it can be either a sporadic isolated defect, more frequently, or part of a syndromic phenotype with mendelian inheritance. The genetic basis of CS is also extremely heterogeneous, with nearly a hundred genes associated so far, mostly mutated in syndromic forms. Several genes can be categorised within partially overlapping pathways, including those causing defects of the primary cilium. The primary cilium is a cellular antenna serving as a signalling hub implicated in mechanotransduction, housing key molecular signals expressed on the ciliary membrane and in the cilioplasm. This mechanical property mediated by the primary cilium may also represent a cue to understand the pathophysiology of non‐syndromic CS. In this review, we aimed to highlight the implication of the primary cilium components and active signalling in CS pathophysiology, dissecting their biological functions in craniofacial development and in suture biomechanics. Through an in‐depth revision of the literature and computational annotation of disease‐associated genes we categorised 18 ciliary genes involved in CS aetiology. Interestingly, a prevalent implication of midline sutures is observed in CS ciliopathies, possibly explained by the specific neural crest origin of the frontal bone

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 hydrocephalus inducing gene product, Hydin, positions axonemal central pair microtubules

<p>Abstract</p> <p>Background</p> <p>Impairment of cilia and flagella function underlies a growing number of human genetic diseases. Mutations in <it>hydin </it>in <it>hy3 </it>mice cause lethal communicating hydrocephalus with early onset. Hydin was recently identified as an axonemal protein; however, its function is as yet unknown.</p> <p>Results</p> <p>Here we use RNAi in <it>Trypanosoma brucei </it>to address this issue and demonstrate that loss of Hydin causes slow growth and a loss of cell motility. We show that two separate defects in newly-formed flagellar central pair microtubules underlie the loss of cell motility. At early time-points after RNAi induction, the central pair becomes mispositioned, while at later time points the central pair is lost. While the basal body is unaffected, both defects originate at the basal plate, reflecting a role for TbHydin throughout the length of the central pair.</p> <p>Conclusion</p> <p>Our data provide the first evidence of Hydin's role within the trypanosome axoneme, and reveal central pair anomalies and thus impairment of ependymal ciliary motility as the likely cause of the hydrocephalus observed in the <it>hy3 </it>mouse.</p

Chlamydomonas reinhardtii hydin is a central pair protein required for flagellar motility

Mutations in Hydin cause hydrocephalus in mice, and HYDIN is a strong candidate for causing hydrocephalus in humans. The gene is conserved in ciliated species, including Chlamydomonas reinhardtii. An antibody raised against C. reinhardtii hydin was specific for an ∼540-kD flagellar protein that is missing from axonemes of strains that lack the central pair (CP). The antibody specifically decorated the C2 microtubule of the CP apparatus. An 80% knock down of hydin resulted in short flagella lacking the C2b projection of the C2 microtubule; the flagella were arrested at the switch points between the effective and recovery strokes. Biochemical analyses revealed that hydin interacts with the CP proteins CPC1 and kinesin-like protein 1 (KLP1). In conclusion, C. reinhardtii hydin is a CP protein required for flagellar motility and probably involved in the CP–radial spoke control pathway that regulates dynein arm activity. Hydrocephalus caused by mutations in hydin likely involves the malfunctioning of cilia because of a defect in the CP

A homozygous AKNA frameshift variant is associated with microcephaly in a Pakistani family

Primary microcephaly (MCPH) is a prenatal condition of small brain size with a varying degree of intellectual disability. It is a heterogeneous genetic disorder with 28 associated genes reported so far. Most of these genes encode centrosomal proteins. Recently, AKNA was recognized as a novel centrosomal protein that regulates neurogenesis via microtubule organization, making AKNA a likely candidate gene for MCPH. Using linkage analysis and whole-exome sequencing, we found a frameshift variant in exon 12 of AKNA (NM_030767.4: c.2737delG) that cosegregates with microcephaly, mild intellectual disability and speech impairment in a consanguineous family from Pakistan. This variant is predicted to result in a protein with a truncated C-terminus (p.(Glu913Argfs*42)), which has been shown to be indispensable to AKNA’s localization to the centrosome and a normal brain development. Moreover, the amino acid sequence is altered from the beginning of the second of the two PEST domains, which are rich in proline (P), glutamic acid (E), serine (S), and threonine (T) and common to rapidly degraded proteins. An impaired function of the PEST domains may affect the intracellular half-life of the protein. Our genetic findings compellingly substantiate the predicted candidacy, based on its newly ascribed functional features, of the multifaceted protein AKNA for association with MCPH

Molecular causes of primary microcephaly and related diseases: a report from the UNIA Workshop

The International University of Andalucía (UNIA) Current Trends in Biomedicine Workshop on Molecular Causes of Primary Microcephaly and Related Diseases took place in Baeza, Spain, November 18–20, 2019. This meeting brought together scientists from Europe, the USA and China to discuss recent advances in our molecular and genetic understanding of a group of rare neurodevelopmental diseases characterised by primary microcephaly, a condition in which head circumference is smaller than normal at birth. Microcephaly can be caused by inherited mutations that affect key cellular processes, or environmental exposure to radiation or other toxins. It can also result from viral infection, as exemplified by the recent Zika virus outbreak in South America. Here we summarise a number of the scientific advances presented and topics discussed at the meeting

A review on microcephaly genes

В обзоре обобщены данные последних лет по изучению генов микроцефалии. Обсуждаются результаты молекулярно-генетического исследования этих генов, сделана оценка семи определенных локусов (MCPH1–MCPH7), описаны соответствующие им гены и белковые продукты генов, их вероятная роль в нормальном развитии мозга, а также мутации в этих генах. Ключевые слова: микроцефалия, MCPH локус, MCPH ген, мутация, нейрогенез.В огляді узагальнено дані останніх років з вивчення генів мікроцефалії. Обговорюються результати молекулярно-генетичного дослідження цих генів, зроблено оцінку семи визначених локусів (MCPH1–MCPH7), описано відповідні їм гени і білкові продукти генів, їхня можлива роль у нормальному розвитку мозку, а також мутації у цих генах. Ключові слова: мікроцефалія, локус MCPH, ген MCPH, мутація, нейрогенез.This review aims to summarize the recent findings regarding microcephaly genes. We have discussed the molecular genetics studies of microcephaly genes including a comprehensive appraisal of the seven mapped loci (MCPH1–MCPH7), their corresponding genes and protein products of the genes, their likely role in normal brain development and the details of the mutations reported in these genes. Keywords: microcephaly, MCPH loci, MCPH gene, mutation, neurogenesis