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 ASPMgene.[1] ASPM is located on chromosome 1, band q31 (1q31).[2] Defective forms of the ASPM gene are associated with autosomal recessive primary microcephaly.[1][3]
"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.[2][4]
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.
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.[5] Studies in mice also suggest a role of the expressed Aspm gene product in mitotic spindle regulation.[6] 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.[7]
Evolution
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.[8] It is also found with an unusually high percentage among the people of Papua New Guinea, with a 59.4% occurrence.[9]
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.[10] 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.[11] 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.[9][12]
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.[12][13][14] However statistical analysis has shown that the older forms of the gene are found more heavily in populations that speak tonal languages like Chinese or many Sub-Saharan African languages.[15]
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.[16][17]
↑ 2.02.1Bond J, Roberts E, Mochida GH, Hampshire DJ, Scott S, Askham JM, et al. (Oct 2002). "ASPM is a major determinant of cerebral cortical size". Nature Genetics. 32 (2): 316–20. doi:10.1038/ng995. PMID12355089.
↑Kaindl AM, Passemard S, Kumar P, Kraemer N, Issa L, Zwirner A, et al. (2010). "Many roads lead to primary autosomal recessive microcephaly". Progress in Neurobiology. 90 (3): 363–83. doi:10.1016/j.pneurobio.2009.11.002. PMID19931588.
↑Kouprina N, Pavlicek A, Collins NK, Nakano M, Noskov VN, Ohzeki J, et al. (2005). "The microcephaly ASPM gene is expressed in proliferating tissues and encodes for a mitotic spindle protein". Human Molecular Genetics. 14 (15): 2155–65. doi:10.1093/hmg/ddi220. PMID15972725.
↑van der Voet M, Berends CW, Perreault A, Nguyen-Ngoc T, Gönczy P, Vidal M, et al. (Mar 2009). "NuMA-related LIN-5, ASPM-1, calmodulin and dynein promote meiotic spindle rotation independently of cortical LIN-5/GPR/Galpha". Nature Cell Biology. 11 (3): 269–77. doi:10.1038/ncb1834. PMID19219036.
↑Evans PD, Gilbert SL, Mekel-Bobrov N, Vallender EJ, Anderson JR, Vaez-Azizi LM, et al. (Sep 2005). "Microcephalin, a gene regulating brain size, continues to evolve adaptively in humans". Science. 309 (5741): 1717–20. Bibcode:2005Sci...309.1717E. doi:10.1126/science.1113722. PMID16151009. Lay summary – New York Times: Researchers Say Human Brain Is Still Evolving.
↑ 9.09.1Mekel-Bobrov N, Gilbert SL, Evans PD, Vallender EJ, Anderson JR, Hudson RR, et al. (Sep 2005). "Ongoing adaptive evolution of ASPM, a brain size determinant in Homo sapiens". Science. 309 (5741): 1720–2. Bibcode:2005Sci...309.1720M. doi:10.1126/science.1116815. PMID16151010.
↑ 12.012.1Currat M, Excoffier L, Maddison W, Otto SP, Ray N, Whitlock MC, et al. (Jul 2006). "Comment on "Ongoing adaptive evolution of ASPM, a brain size determinant in Homo sapiens" and "Microcephalin, a gene regulating brain size, continues to evolve adaptively in humans"". Science. 313 (5784): 172, author reply 172. doi:10.1126/science.1122712. PMID16840683.
↑Woods RP, Freimer NB, De Young JA, Fears SC, Sicotte NL, Service SK, et al. (Jun 2006). "Normal variants of Microcephalin and ASPM do not account for brain size variability". Human Molecular Genetics. 15 (12): 2025–9. doi:10.1093/hmg/ddl126. PMID16687438.
↑Mekel-Bobrov N, Posthuma D, Gilbert SL, Lind P, Gosso MF, Luciano M, et al. (Mar 2007). "The ongoing adaptive evolution of ASPM and Microcephalin is not explained by increased intelligence". Human Molecular Genetics. 16 (6): 600–8. doi:10.1093/hmg/ddl487. PMID17220170.