Saturday, May 26, 2012

Segmental genome duplication responsible for our brain power

Genome duplication is the trigger for evolution. Complex multicellular organisms have evolved from simple unicellular organisms by genome duplication and other genome rearrangements. Genome duplication can be of two types: whole genome duplication in which a diploid organism becomes a tetraploid and provide whole set of additional genes i.e for every old gene a new copy is provided. This type of change is considered to be responsible for major changes in organisms including speciation. The second type of duplication is called segmental duplication or copy number variation.  Here only a part of the genome gets duplicated and provide additional copies of only those genes in the duplicated regions. This type of  variation is considered to be responsible for differences among closely related organisms for example between monkeys and humans. Immediately after sequencing genomes of chimp and other primates Scientists were expecting to find a segmental duplication of  genomic regions possessing genes associated with  the brain i.e neural system but were not successful.

Recently  two  groups of Scientists led by Evan Eichler and Franck Polleux have found a part of the missing clue.  They identified segmental duplication that led to
 three extra copies of a gene called SRGAP2 (Slit-Robo Rho GTPase-activating protein 2), which is involved in brain development.  This duplication is not found in other animals. This duplicated copies were missing in the original human genome sequence.

The cortical development gene Slit-Robo Rho GTPase-activating protein 2 (SRGAP2) duplicated three times exclusively in humans. The promoter and first nine exons of SRGAP2 duplicated from 1q32.1 (SRGAP2A) to 1q21.1 (SRGAP2B) ∼3.4 million years ago (mya). Two larger duplications later copied SRGAP2B to chromosome 1p12 (SRGAP2C) and to proximal 1q21.1 (SRGAP2D) ∼2.4 and ∼1 mya, respectively. Sequence and expression analyses show that SRGAP2C is the most likely duplicate to encode a functional protein and is among the most fixed human-specific duplicate genes. Our data suggest a mechanism where incomplete duplication created a novel gene function—antagonizing parental SRGAP2 function—immediately “at birth” 2–3 mya, which is a time corresponding to the transition from Australopithecus to Homo and the beginning of neocortex expansion. This is just the beginning to understand the genes which made us human.  SRGAP2 is just one of  the many  genes that have been duplicated specifically in humans. Several of the others are also involved in brain development and are missing from the human reference genome. Identifying these genes and further studies will lead to better understanding of what made us human.


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