Jumping Genes in the Brain Ensure That Even Identical Twins Are Different

Image: Jean-Francois Podevin

In Brief

• Genes we inherit and environmental factors both influence human behaviors. Scientists have recently discovered other underlying processes at work.
• So-called jumping genes, segments of DNA that can copy and paste them?selves into new places in the genome, can alter the activity of full-length genes. Occasionally they will turn on neighboring genes in these locations. That activity occurs more in the brain than other areas, resulting in different traits and behaviors, even in closely related individuals.
• These mobile genetic elements may also turn out to play a role in people?s disposition to psychiatric disorders.
• Researchers are now beginning to investigate whether jumping genes help us adapt to rapidly changing environmental conditions.

Your brain is special.

So is mine. Differences arise at every level of the organ?s astonishingly intricate architecture; the human brain contains 100 billion neurons, which come in thousands of types and collectively form an estimate of more than 100 trillion interconnections. These differences, in turn, lead to variances in the ways we think, learn and behave and in our propensity for mental illness.

How does diversity in brain wiring and function arise? Variations in the genes we inherit from our parents can play a role. Yet even identical twins raised by the same parents can differ markedly in their mental functioning, behavioral traits, and risk of mental illness or neurodegenerative disease. In fact, mice bred to be genetically identical that are then handled in exactly the same way in the laboratory display differences in learning ability, fear avoidance and responses to stress even when age, gender and care are held constant. Something more has to be going on.

Certainly the experiences we have in life matter as well; they can, for instance, influence the strength of the connections between particular sets of neurons. But researchers are increasingly finding tantalizing indications that other factors are at work?for instance, processes that mutate genes or affect gene behavior early in an embryo?s development or later in life. Such phenomena include alternative splicing, in which a single gene can give rise to two or more different proteins. Proteins carry out most of the operations in cells, and thus which proteins are made in cells will affect the functioning of the tissues those cells compose. Many researchers are also exploring the role of epigenetic changes?DNA modifications that alter gene activity (increasing or decreasing the synthesis of specific proteins) without changing the information in genes.

In the past few years the two of us and our colleagues have come on especially intriguing suspects that seem to operate more in the brain than in other tissues: jumping genes. Such genes, which have been found in virtually all species, including humans, can paste copies of themselves into other parts of the genome (the full set of DNA in the nucleus) and alter the functioning of the affected cell, making it behave differently from an otherwise identical cell right next to it. Many such insertions in many different cells would be expected to yield subtle or not so subtle differences in cognitive abilities, personality traits and susceptibility to neurological problems.

Our early findings of gene jumping in the brain have led us to another question: Given that the brain?s proper functioning is essential to survival, why has evolution allowed a process that tinkers with its genetic programming to persist? Although we still do not have a definite answer, mounting evidence suggests that by inducing variability in brain cells, jumping genes may imbue organisms with the flexibility to adapt quickly to changing circumstances. Therefore, these jumping genes?or mobile elements, as they are called?may have been retained evolutionarily because, from the standpoint of promoting survival of the species, this adaptation benefit outweighs the risks.

Ancient invaders
The idea that mobile elements exist and move about in the genome is not new, but the recent evidence that they are so active in the brain came as a surprise. Gene jumping was first discovered in plants, even before James Watson and Francis Crick spelled out the double-helical structure of DNA in 1953. In the 1940s Barbara McClintock of Cold Spring Harbor Laboratory observed that ?controlling elements? moved from one place to another in the genetic material of corn plants. She discovered that under stress, certain regions in the genome could migrate and turn genes on and off in their new location. The products of McClintock?s experiments were the now famous ears of corn with seeds of varying colors?a demonstration of genetic mosaicism, in which genes in a particular cell may be switched on or off in a pattern that differs from that of neighboring cells that are otherwise identical.

Source: http://rss.sciam.com/click.phdo?i=11f973270cb0881061dfff3f950f853b

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