GENES MAKE UP ONLY ABOUT 2 PERCENT of the human genome. The rest was for many years ignored as “junk DNA.” But over the past decade biologists have come to understand that this space is an incredibly important part of the genetic code, home to a vast unexamined treasure trove of information that controls how genes behave. A more thorough investigation of junk DNA could upend our understanding of the delicate interplay between genes and the environment and could lead to entirely new strategies in medicine's endless struggle against disease.
New examples of junk DNA's importance seem to emerge every few months. Researchers publishing in the September issue of Nature Medicine reported that the rare nervous system cancer neuroblastoma may in part have junk DNA to blame; a small piece of RNA made from junk DNA disables a cancer-inhibiting gene in people suffering from the disease. Similarly, those afflicted with a rare form of muscular dystrophy have between one and 10 copies of a particular slice of junk DNA on the end of the fourth chromosome. Junk DNA isn't just relevant for rare diseases, either: this past February a paper in Nature linked a region of junk DNA on the ninth chromosome to heart disease risk.
Junk DNA may also help organisms adapt to changing environments. In May 2009 scientists at the University of Leuven in Belgium reported that gene activity on a yeast chromosome is directly controlled by the number of repeats in a section of junk DNA. Because the number of repeats changes more frequently than other stretches of DNA do, this setup allows the organism to evolve more quickly.
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So does junk DNA deserve a new, more respectful name? Scientists disagree. Some junk DNA may be obviously useful, but the potential benefits of the rest “may be much more subtle and hard to trace,” says Kevin Verstrepen, a co-author of the yeast study. In time, though, one biologist's junk may turn out to be another's jewel.
