Tuesday, 3 April 2012

“What Makes each Brain Unique” (ScientificAmerican, 2012)

Genetic variation has been long known to be a product of the genes inherited from our parents as well as the environmental influences that the individual is exposed to. However, studies have found that even specially cloned mice that are genetically identical and subject to exactly the same care, portray very different behaviours in regard to dealing with stress, fear avoidance and learning ability (Gage & Muotri, 2012). Scientists have made recent advances in our understanding of the workings of a third factor that contributes to this phenomenon; mobile segments of DNA known colloquially as ‘jumping genes’ or ‘transposable genetic elements’ (TE) that are able to randomly migrate to other areas of the genome (Gage & Muotri 2012). It is through this process that TEs are able to alter gene expression and the function of affected cells. (Campbell, 2012, 443). Accounting for around 50% of the human genome, TEs are an abundant feature common to all organisms (Wessler, 2006) and recent evidence suggests that their activity could influence the individual’s behaviour, cognition and disposition to mental illness.

There are two types of transposing elements in eukaryotes; transposons which migrate to another region within the genome via a DNA intermediate and retrotransposons which insert a copy of themselves, produced via an RNA intermediate, into another site, leaving the parent retrotransposon in original site (Campbell, 2012, 443).

Fig 2. “Variation in Indian Corn color due to the presence of genetic elements called transposons by Barbara McClintock” [DR. BRAD MOGEN, VISUALS UNLIMITED /SCIENCE PHOTO LIBRARY]

First evidence of TEs predates the Watson-Crick DNA model. In the 1940s, American Barbara McClintock’s first discovered what we now recognise as transposons, when studying the “variegations in corn kernels” (Fig.1) (Campbell, 2012, 443).  Recent studies led by Fred Gage at Stalk Institute for Biomedical Studies in La Jolla have focussed on one type of retrotransposon, Long Interspersed element (L1)  (Gage & Muotri, 2012). They monitored transposition in genetically engineered mice whose cells were made to fluoresce green upon the insertion of an L1 element. Their data revealed that retrotransposons are particularly active in the brain more than any other tissues of the body. As noted by Gage and Muotri (2012) this challenges the long-perceived view that the genetic code of the adult brain is identical for each individual and is static for the life of the neurons. It is this TE activity that indicates yet another source of genetic variability within populations, giving rise to both beneficial mutations which overtime has enables adaptation and survival of species, as well as harmful mutations which bring about disease such as HIV, whose virus operates in a manner similar to retrotransposons (Gage & Muotri, 2012).
This evidence indicates that no two individuals are genetically alike including “identical” twins. This undermines the very foundations upon which many behavioural genetics twin studies are based with geneticists aiming to control for the effects of genes to investigate the environmental factors believed to play a role in psychiatric and other disorders (Gage & Muotri, 2012).  Moreover, this evidence has directed new research into investigating the potential link between retrotransposons and psychiatric disorders. Circumstantial evidence has suggested a link between L1 activity and autism, Rett syndrome and schizophrenia (Gage & Muotri, 2012). Continued research in this area may revolutionise methods of diagnosis, treatment and prevention of mental illness in the not too distant future.

Gage, F H. & Muotri A R. 2012, ‘What makes each Brain Unique’, Scientific American, Vol. 306, no.3, pp.26-31
Wessler, S. 2006, ‘Transposable elements and the evolution of eukaryotic genomes’, Proceedings of the National Academy of Science , vol. 103, no. 47, viewed 17 March 2012 <http://www.pnas.org/content/103/47/17600.full.pdf+html>

Reece, J B. et al., 2012, Campbell Biology 9th Edition Australian Version, Pearson, Frenchs Forest, NSW.

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