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Tuesday, July 13, 2010

Jumping genes really move around

Jumping genes, more formally known as ‘transposons’ (short for transposable elements) are segments of DNA that can replicate and reinsert themselves into new locations on the genome. The most common type are ‘retrotransposons’, which go through an RNA intermediate in order to move around. It turns out that these genetic elements are much more active than was previously thought.

Before I continue, I want to be clear about what’s going on. When a transposon ‘jumps’ to a new location, the original piece of DNA is not removed from its location in the genome. That piece is replicated, either directly or through an RNA intermediate, so that there are now two transposons, one left in the original spot, and the second, new piece that is free to insert itself into a new location. The insertion site is chosen at random. If the new transposon inserts itself into the middle of an essential gene, the results can be devastating. Alternatively, the insertion may be completely benign. In either case, the genome now has one more copy of the transposon than it did before.

It had been thought that this sort of event occurred about once in every 20 births. Not so. Scott Devine, currently at the University of Maryland, and his team from the University of Maryland, and from Emory University, where the work was initiated, have found that LINE-1 (L1) and Alu retrotransposons are so active that virtually every newborn has one in a new location. In addition, transposons are very likely to move to new locations during tumor formation. Six out of 20 lung tumors had transposon insertions in new locations compared to adjacent normal tissues.

Taken together, these data indicate that the movement of retrotransposons may be a major factor in both human evolution and in disease.