Abnormal spindle-like microcephaly-associated protein, also known as abnormal spindle protein homolog or Asp homolog, is a protein that in humans is encoded by the ASPM gene. [5] ASPM is located on chromosome 1, band q31 (1q31). [6] The ASPM gene contains 28 exons and codes for a 3477 amino-acid-long protein. [6] The ASPM protein is conserved across species including human, mouse, Drosophila, and C. elegans. [6] Defective forms of the ASPM gene are associated with autosomal recessive primary microcephaly. [5] [7]
"ASPM" is an acronym for "Abnormal Spindle-like, Microcephaly-associated", which reflects its being an ortholog to the Drosophila melanogaster "abnormal spindle" (asp) gene. The expressed protein product of the asp gene is essential for normal mitotic spindle function in embryonic neuroblasts and regulation of neurogenesis. [6] [8]
A new allele of ASPM arose sometime in the past 14,000 years (mean estimate 5,800 years), during the Holocene, it seems to have swept through much of the European and Middle-Eastern population. Although the new allele is evidently beneficial, researchers do not know what it does.[ citation needed ]
The mouse gene, Aspm, is expressed in the primary sites of prenatal cerebral cortical neurogenesis. The difference between Aspm and ASPM is a single, large insertion coding for so-called IQ domains. [9] Studies in mice also suggest a role of the expressed Aspm gene product in mitotic spindle regulation. [10] The function is conserved, the C. elegans protein ASPM-1 was shown to be localized to spindle asters, where it regulates spindle organization and rotation by interacting with calmodulin, dynein and NuMA-related LIN-5. [11]
One mouse study looking at medulloblastoma growth in mice to study the Aspm gene, an ortholog to human ASPM, suggests that Aspm expression may drive postnatal cerebellar neurogenesis. [12] This process occurs late in embryogenesis and immediately after birth over a time span of about 2 weeks in mice and 12 months in humans, and is regulated by the expression of the Shh gene. [13] In proliferating cerebellar granule neuron progenitors (CGNPs), Shh expression in mouse models showed four times the amount of Aspm expression than those deprived of Shh expression in-vivo. This induction of Aspm and up-regulation during cerebellar neurogenesis was also seen in real-time PCR, where its expression was relatively high at the peak of neurogenesis and much lower at the end of neurogenesis. Additionally, the study indicates that Aspm is necessary for cerebellar neurogenesis. In the presence of Aspm KO mutations and deletions, experimental mice models show decreased cerebellar volume under MRI, compared to the controls. [14] In addition to mutated Aspm's effects on neurogenesis, these mutations may also play a role in neural differentiation. When looking at adult brains in Aspm KO mice, there was a trend in overall size reduction, and variations in cortical thickness between mutant and wild type models. In the somatosensory cortex, KO mice had a significantly thicker layer I cortex, thinner layer VI cortex, and an overall decrease in cortical thickness in the cortical plate. Certain transcription factors expressions were also abnormal in the KO mice. For example, Tbr1 and Satb2 had an increased presence in the cortical sub-plate, the first of which is important for differentiation and neuronal migration, and the second of which is a regulator of transcription and chromosomal remodeling. [15]
While mouse studies have established the role of Aspm mutations in microcephaly, several have linked this mutation to other significant defects. [16] One study showed nerve fiber impairments in which the shape and form of cortex and white matter tissue was altered. This was shown postnatally comparing KO mice and controls, where both cell number and cortical thickness was decreased in KO mice. Using a cell staining methodology for histological analysis, the study also showed shorter distances between adjacent neurons in KO mice, indicating abnormalities in cell alignment in the absence of normal Aspm. [17]
Another significant impact of mutated Aspm is seen in germline abnormalities within mouse models. Mutations in Aspm were shown to reduce fertility in both female and male mice, indicated by a decrease in the rate of pregnancy and consequently the number of offspring, as well as a decrease in female ovarian size, as well as male sperm count and testicular size. The focus on severe germline mutations (as opposed to only mild microcephaly) in these mouse models raises the question as to whether or not human ASPM selection may be more significantly linked to reproduction than brain size. [18] [19] In addition to mouse models, a study using ferrets reveals more about ASPM and its role in determining cortical size and thickness. The researchers from this study chose ferrets over mouse models due to incongruencies between Aspm effects in mice versus ASPM effects in humans - humans with microcephaly due to this gene mutation tend to have significantly reduced brain sizes (about 50% reduction), whereas the analogous mutation in mice only results in mild brain size reduction. [19] Ferrets also show more similarities to humans in terms of brain structure; ferrets' brains have gyrification in high amounts similar to humans, different from the relatively smooth brains of mice. As a result, there is less cortical surface area in mice compared to that of ferrets and humans. [20] In this 2018 study, researchers targeted Aspm exon 15, where a mutation in humans is linked to severe cases of microcephaly. [21] With a loss of function in Aspm, ferrets with Aspm mutations saw a 40% decrease in overall brain size coupled with no reduction in body size, similar to the effects of loss of ASPM in humans. The study also looked at the neurodevelopmental pathways and mechanisms leading to neurogenesis in the KO ferrets compared to the WT controls, specifically studying three different neuron progenitor cell (NPC) types, all of which express the mitotic marker Ki-67 and undergo radial glial migration to the cortical plate. [22] [23] [24] They found that outer subventricular zone (OSVZ) NPCs were largely displaced, especially frontally and dorsally which mirrors the effects seen in cortical volume reductions due to ASPM KO.
Human primary microcephaly (MCPH) is a distinct subtype that is genetically inherited as an autosomal recessive trait. [25] MCPH is characterized by a smaller cerebral cortex associated with mild to moderate mental retardation and no other neurological deficits. [6] [26] Additionally, MCPH is associated with the absence of environmental causes such as intrauterine infections, exposure to prenatal radiation or drugs, maternal phenylketonuria, and birth asphyxia. [26] MCPH has an incidence rate of 1/30,000 to 1/250,000 in western populations. [27] To date, mutations in six loci and four genes associated with microcephaly have been discovered in humans. [28] ASPM, one of these genes, is found at the MCPH5 locus. [29] The most common cause of MCPH in humans is homozygous genetic mutation of the ASPM gene, orthologous to the Drosophila abnormal spindle gene (asp). [6] In humans, the ASPM gene may play a strong role in the growth of the cerebral cortex. [28] A total of 22 mutations have been discovered in the ASPM gene in individuals from Pakistan, Turkey, Yemen, Saudi Arabia, Jordan, and the Netherlands. [6] [21]
A study completed in Karnataka, South India by Kumar et al. analyzed the genetics of MCPH due to mutations in the ASPM gene. [26] The study included nine families with blood relatives across many familial generations. [26] Kumar et al. performed High-resolution G-banding chromosome analysis and haplotype analysis of individuals and families of those affected by MCPH. [26] Kumar et al. found that the South Indian families affected by mutations in the MCPH5 locus did not share a common disease haplotype; thus the authors proposed that different mutations in the ASPM gene are responsible for MCPH. [26]
A similar genetic study of MCPH in Pakistani families was done by Gul et al. in order to evaluate the relationship between ASPM gene mutations and microcephaly. [28] The study was approved by the Institutional Review Board of Quaid-I-Azam University in Islamabad, Pakistan, and involved extraction of DNA and PCR techniques in order to genetically map the ASPM gene. [28] Genotyping using microsatellite regions in the gene revealed that MCPH5 locus mutations were the most common cause of MCPH. [28] Genotyping further linked mutations in the MCPH2 locus, MCPH4 locus, and the MCPH6 locus to microcephaly. [28] Sequence analysis of ASPM in humans revealed four novel mutations; these four types of mutations are an insertion of four nucleotides (9118insCATT), a nonsense mutation (L3080X), a deletion of seven nucleotides (1260delTCAAGTC), and a missense mutation (Q3180P). [28] Gul et al. found that parents who were heterozygous carriers for ASPM had normal cerebral circumferences and normal intelligence levels. [28] The scientists were unable to identify mutations at the MCPH5 locus in nine families who had members affected by MCPH. [28] They concluded that the mutations could be located in the regulatory sequences of ASPM, or that a gene other than ASPM located in the same region could be mutated. [28]
The types of mutations causing MCPH in humans was expanded by a study done by Pichon et al. on an individual with primary microcephaly, as the study revealed a translocation breakpoint in the ASPM gene. [30] Pichon et al. obtained BAC clones with BamHI digestion fragments of the "RP11-32D17" insert and used Fluorescence in situ Hybridization (FISH) in order to label the clones with fluorescein-12-dUTP. [30] In order to precisely locate the translocation breakpoint, the BamHI digestion fragments of "RP11-32D17" were analyzed. [30] The translocation breakpoint was located to be within intron 17 of the ASPM gene. [30] The translocation resulted in a truncated ASPM protein, which is most likely a non-functioning protein also seen in truncating point mutations reported in MCPH patients. [30]
A new allele (version) of ASPM appeared sometime within the last 14,100 years, with a mean estimate of 5,800 years ago. The new allele has a frequency of about 50% in populations of the Middle East and Europe, it is less frequent in East Asia, and has low frequencies among Sub-Saharan African populations. [31] It is also found with an unusually high percentage among the people of Papua New Guinea, with a 59.4% occurrence. [32]
The mean estimated age of the ASPM allele of 5,800 years ago roughly correlates with the development of written language, spread of agriculture and development of cities. [33] [ better source needed ] Currently, two alleles of this gene exist: the older (pre-5,800 years ago) and the newer (post-5,800 years ago). About 10% of humans have two copies of the new ASPM allele, while about 50% have two copies of the old allele. The other 40% of humans have one copy of each. Of those with an instance of the new allele, 50% of them are an identical copy. [34] [35] The allele affects genotype over a large (62 kbp) region, a so called selective sweep which signals a rapid spread of a mutation (such as the new ASPM) through the population; this indicates that the mutation is somehow advantageous to the individual. [32] [36]
Testing the IQ of those with and without new ASPM allele has shown no difference in average IQ, providing no evidence to support the notion that the gene increases intelligence. [36] [37] [38] Other genes related to brain development appear to have come under selective pressure in different populations. The DAB1 gene, involved in organizing cell layers in the cerebral cortex, shows evidence of a selective sweep in the Chinese. The SV2B gene, which encodes a synaptic vesicle protein, likewise shows evidence of a selective sweep in African-Americans. [39] [40]
Reelin, encoded by the RELN gene, is a large secreted extracellular matrix glycoprotein that helps regulate processes of neuronal migration and positioning in the developing brain by controlling cell–cell interactions. Besides this important role in early development, reelin continues to work in the adult brain. It modulates synaptic plasticity by enhancing the induction and maintenance of long-term potentiation. It also stimulates dendrite and dendritic spine development and regulates the continuing migration of neuroblasts generated in adult neurogenesis sites like the subventricular and subgranular zones. It is found not only in the brain but also in the liver, thyroid gland, adrenal gland, fallopian tube, breast and in comparatively lower levels across a range of anatomical regions.
Forkhead box protein P2 (FOXP2) is a protein that, in humans, is encoded by the FOXP2 gene. FOXP2 is a member of the forkhead box family of transcription factors, proteins that regulate gene expression by binding to DNA. It is expressed in the brain, heart, lungs and digestive system.
Microcephaly is a medical condition involving a smaller-than-normal head. Microcephaly may be present at birth or it may develop in the first few years of life. Brain development is often affected; people with this disorder often have an intellectual disability, poor motor function, poor speech, abnormal facial features, seizures and dwarfism.
Microcephalin (MCPH1) is a gene that is expressed during fetal brain development. Certain mutations in MCPH1, when homozygous, cause primary microcephaly—a severely diminished brain. Hence, it has been assumed that variants have a role in brain development. However, in normal individuals no effect on mental ability or behavior has yet been demonstrated in either this or another similarly studied microcephaly gene, ASPM. However, an association has been established between normal variation in brain structure, as measured with MRI but only in females, and common genetic variants within both the MCPH1 gene and another similarly studied microcephaly gene, CDK5RAP2.
Bilateral frontoparietal polymicrogyria is a genetic disorder with autosomal recessive inheritance that causes a cortical malformation. Our brain has folds in the cortex to increase surface area called gyri and patients with polymicrogyria have an increase number of folds and smaller folds than usual. Polymicrogyria is defined as a cerebral malformation of cortical development in which the normal gyral pattern of the surface of the brain is replaced by an excessive number of small, fused gyri separated by shallow sulci and abnormal cortical lamination. From ongoing research, mutation in GPR56, a member of the adhesion G protein-coupled receptor (GPCR) family, results in BFPP. These mutations are located in different regions of the protein without any evidence of a relationship between the position of the mutation and phenotypic severity. It is also found that GPR56 plays a role in cortical pattering.
Pachygyria is a congenital malformation of the cerebral hemisphere. It results in unusually thick convolutions of the cerebral cortex. Typically, children have developmental delay and seizures, the onset and severity depending on the severity of the cortical malformation. Infantile spasms are common in affected children, as is intractable epilepsy.
Paired box protein Pax-6, also known as aniridia type II protein (AN2) or oculorhombin, is a protein that in humans is encoded by the PAX6 gene.
The Disabled-1 (Dab1) gene encodes a key regulator of Reelin signaling. Reelin is a large glycoprotein secreted by neurons of the developing brain, particularly Cajal-Retzius cells. DAB1 functions downstream of Reln in a signaling pathway that controls cell positioning in the developing brain and during adult neurogenesis. It docks to the intracellular part of the Reelin very low density lipoprotein receptor (VLDLR) and apoE receptor type 2 (ApoER2) and becomes tyrosine-phosphorylated following binding of Reelin to cortical neurons. In mice, mutations of Dab1 and Reelin generate identical phenotypes. In humans, Reelin mutations are associated with brain malformations and mental retardation. In mice, Dab1 mutation results in the scrambler mouse phenotype.
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SCL-interrupting locus protein is a protein that in humans is encoded by the STIL gene. STIL is present in many different cell types and is essential for centriole biogenesis. This gene encodes a cytoplasmic protein implicated in regulation of the mitotic spindle checkpoint, a regulatory pathway that monitors chromosome segregation during cell division to ensure the proper distribution of chromosomes to daughter cells. The protein is phosphorylated in mitosis and in response to activation of the spindle checkpoint, and disappears when cells transition to G1 phase. It interacts with a mitotic regulator, and its expression is required to efficiently activate the spindle checkpoint.
The Abelson helper integration site 1 (AHI1) is a protein coding gene that is known for the critical role it plays in brain development. Proper cerebellar and cortical development in the human brain depends heavily on AHI1. The AHI1 gene is prominently expressed in the embryonic hindbrain and forebrain. AHI1 specifically encodes the Jouberin protein and mutations in the expression of the gene is known to cause specific forms of Joubert syndrome. Joubert syndrome is autosomal recessive and is characterized by the brain malformations and mental retardation that AHI1 mutations have the potential to induce. AHI1 has also been associated with schizophrenia and autism due to the role it plays in brain development. An AHI1 heterozygous knockout mouse model was studied by Bernard Lerer and his group at Hadassah Medical Center in Jerusalem to elucidate the correlation between alterations in AHI1 expression and the pathogenesis of neuropsychiatric disorders. The core temperatures and corticosterone secretions of the heterozygous knockout mice after exposure to environmental and visceral stress exhibited extreme repression of autonomic nervous system and hypothalamic-pituitary-adrenal responses. The knockout mice demonstrated an increased resilience to different types of stress and these results lead to a correlation between emotional regulation and neuropsychiatric disorders.
Centromere protein J is a protein that in humans is encoded by the CENPJ gene. It is also known as centrosomal P4.1-associated protein (CPAP). During cell division, this protein plays a structural role in the maintenance of centrosome integrity and normal spindle morphology, and it is involved in microtubule disassembly at the centrosome. This protein can function as a transcriptional coactivator in the Stat5 signaling pathway and also as a coactivator of NF-kappaB-mediated transcription, likely via its interaction with the coactivator p300/CREB-binding protein.
Sacsin also known as DnaJ homolog subfamily C member 29 (DNAJC29) is a protein that in humans is encoded by the SACS gene. Sacsin is a Hsp70 co-chaperone.
CDK5 regulatory subunit-associated protein 2 is a protein that in humans is encoded by the CDK5RAP2 gene. It has necessary roles in the formation and stability of microtubules from the centrosome and has been found to be linked to human brain size variation in males. Multiple transcript variants exist for this gene, but the full-length nature of only two has been determined.
Beta-1,3-N-acetylglucosaminyltransferase lunatic fringe, also known as Lunatic Fringe, is a protein encoded in humans by the LFNG gene.
WD repeat-containing protein 62 is a protein that in humans is encoded by the WDR62 gene.
Dentatorubral–pallidoluysian atrophy (DRPLA) is an autosomal dominant spinocerebellar degeneration caused by an expansion of a CAG repeat encoding a polyglutamine tract in the atrophin-1 protein. It is also known as Haw River Syndrome and Naito–Oyanagi disease. Although this condition was perhaps first described by Smith et al. in 1958, and several sporadic cases have been reported from Western countries, this disorder seems to be very rare except in Japan.
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Kohlschütter–Tönz syndrome (KTS), also called amelo-cerebro-hypohidrotic syndrome, is a rare inherited syndrome characterized by epilepsy, psychomotor delay or regression, intellectual disability, and yellow teeth caused by amelogenesis imperfecta. It is a type A ectodermal dysplasia.
Microlissencephaly (MLIS) is a rare congenital brain disorder that combines severe microcephaly with lissencephaly. Microlissencephaly is a heterogeneous disorder, i.e. it has many different causes and a variable clinical course. Microlissencephaly is a malformation of cortical development (MCD) that occurs due to failure of neuronal migration between the third and fifth month of gestation as well as stem cell population abnormalities. Numerous genes have been found to be associated with microlissencephaly, however, the pathophysiology is still not completely understood.