Scientists identify jumping genes in human brain
Researchers have discovered that the human brain is home to large numbers of so-called 'jumping genes'. The findings have implications for our understanding of brain development and individuality, and could shed new light on the causes of neurological diseases. The work, which is published online by the journal Nature, was partly supported by a Marie Curie researcher mobility grant from the EU's Seventh Framework Programme (FP7). Jumping genes, also known as mobile elements, are small pieces of deoxyribonucleic acid (DNA) that can use a 'copy and paste' mechanism to insert duplicates of themselves into other parts of the genome. 'It is known that these mobile elements are important in lower organisms, such as plants and yeast, but in mammals they are generally considered to be remnants of our past,' commented Professor Fred Gage of the Salk Institute in the US. 'Yet they are extremely abundant. Approximately 50% of the total human genome is made up of remnants of mobile elements. If this were true junk, we would be getting rid of it.' So far, the only human cells known to shuffle their genes around are the cells of the immune system. Immune cells move the genes that code for antibodies so that the cell can produce the variety of antibodies needed to recognise a wide range of antigens. Previously, Professor Gage and his team had found that mobile pieces of DNA called LINE-1 elements (or Long interspersed element 1) randomly jump around the genome in the brain cells of mice. In this latest study, he sought to discover whether the same was going on in human brains. Studies of human brain cells growing in a Petri dish suggested that this might be the case. The team then isolated DNA from brain, liver and heart samples taken from adult humans and compared the levels of LINE-1 activity. As expected, there were many more copies of LINE-1 per cell in the brain cells than in the liver and heart cells of the same individual from whom the sample had been taken. 'This was proof that these elements really are jumping in neurons,' commented the lead author of the paper, Nicole Coufal of the Salk Institute. Further investigations revealed that the genetic switch that activates the LINE-1 element is permanently locked in the 'off' position in most tissues of the body, whereas it is generally 'on' in the brain. 'This is a potential mechanism to create the neural diversity that makes each person unique,' noted Professor Gage. 'The brain has 100 billion neurons with 100 trillion connections, but mobile pieces of DNA could give individual neurons a slightly different capacity from each other.' The researchers also suggest that these mobile elements may play a role in driving evolution, by creating more diversity than is created during normal cell division, when the DNA is copied exactly. 'It's a different way of looking at diversity. The brain lives for 80 years with the environment coming at us unpredictably, and this provides an added element of adaptability,' said Professor Gage. 'It makes sense that there would be this added level of complexity.' The findings could also increase our understanding of neurological disorders, some of which could be caused by unregulated jumping genes. The researchers now plan to look for differences in the activity of jumping genes in individuals with neurological conditions.