Random “jumping genes” help make each brain unique, researchers find

<body leftmargin="0" bgcolor="#000060" text="#FFFFFF" link="#FFFF00" vlink="#FF9900"> <p class="MsoNormal"><b><font face="Arial" size="5">Random “jumping genes” give our brains uniqueness, researchers find</font></b></p> <p class="MsoNormal"><font face="Arial"><font size="1"> June 17, 2005<br> Courtesy the Salk Institute<br> and World Science staff<br> </font> <br> </font> <b><font face="Arial" size="2">Do genes or experience shape us most? Scientists and philosophers have debated this question in some form or another for centuries, but the debate usually assumes only two possible ingredients: what we inherit, and what we learn.</font></b> </p> <p class="MsoNormal"> <b><font face="Arial" size="2">But now, scientists are finding that a third ingredient has inserted itself somewhere in between these two: a peculiar form of genetic randomness. And this may explain a longstanding puzzle of why no two brains look the same, not even those of otherwise identical twins, the researchers said.<br> <br> The randomness comes in the form of stretches of genetic code that jump from one place in the genome to another, changing the genetic information in single brain cells, the researchers said. If enough of these jumps occur, they could allow individual brains to develop in distinctly different ways, they added.<br> <br> The bits of DNA, called mobile elements, add “variety and flexibility” to brain cells, said Fred H. Gage, of the Laboratory of Genetics at the Salk Institute for Biological Studies in La Jolla, Calif. Gage is the lead author of a study on the subject published in this week’s issue of the research journal Nature. <br> <br> Gage also said the randomness of the mobile element activity is reminiscent of evolutionary theory, which holds that random mutations lead to gradual changes in life forms, and eventually new species. In a similar way, random mobile element activity creates diversity among brain cells.<br> <br> This process is restricted to the brain and leaves other organs unaffected, he added: “You wouldn’t want that added element of individuality in your heart.”<br> <br> Cells in the developing brain of an embryo, which mature into brain cells, all look and act more or less the same. Yet, these precursors ultimately give rise to a panoply of nerve cells that are enormously diverse and together form the brain. <br> <br> Identifying the mechanisms that lead to this diversification has been a longstanding challenge, Gage said. “People have speculated that there might be a mechanism to create diversity in brain like there is in the immune system.”<br> <br> In their study, the researchers closely tracked a single human mobile genetic element in rat brain precursor cells, which were grown in a laboratory. Then they introduced the code, called an LINE-1 element, into mice, and found that it jumped “all over the brain” in them, said M. Carolina N. Marchetto, a co-author of the study.<br> <br> These elements are part of a class of DNA regions that make up 17 percent of the code in our genome, but very little is known about them. Almost all of them are marooned at a permanent spot by mutations rendering them dysfunctional, but in humans a hundred or so are free to move via a “copy and paste” mechanism. <br> <br> Long dismissed as useless gibberish or “junk” DNA, they were thought to be intracellular parasites or leftovers from our distant evolutionary past. It is known that they are active in testis and ovaries, which explains how they potentially play a role in evolution by passing on new insertions to future generations. “But nobody has ever demonstrated mobility convincingly” in other types of cells, said Gage. <br> <br> The elements appear to “jump” in the brain both during adulthood and development, according to the researchers. But they do so only in neurons, the types of brain cells that conduct nerve impulses, and that therefore are believed to be the ones that directly carry information and produce our thoughts. Other types of brain cells, which carry out supporting functions, are unaffected by the jumping. <br> <br> It seems that cells that are developing into neurons, “open up genes and expose their DNA to insertions,” said Alysson R. Muotri, another of the researchers, also with the Salk Institute. “What we have shown for the first time is that a single insertion can mess up gene expression and influence the function of individual cells,” he added. <br> <br> Future studies might examine It how often the elements move in human neurons, how tightly this process is controlled, or what happens it goes awry, Gage suggested. </font></b> </p> </body>