ARI SHAPIRO, HOST:
Your body has about 40 trillion cells, and they all arose from a single fertilized egg. They begin as faithful copies of that original egg, but as we age, they change and mutate so that those cells are no longer perfect clones of the original. NPR's Richard Harris reports on a study in Science magazine that shows that our bodies' cells make up a mosaic with many subtle variations in their DNA.
RICHARD HARRIS, BYLINE: There's plenty of evidence that ourselves pick up mutations as we age. Some skin cells morph into moles. Blood cells also pick up subtle mutations. And some mutations can lead to cancer.
KEREN YIZHAK: But no one has really characterized this phenomenon across many, many tissues and across great amounts of individuals.
HARRIS: Keren Yizhak took that as a challenge as she started working as a postdoc at the Broad Institute at MIT and Harvard. She and her colleagues tapped into genetic information from about 500 people, cataloguing 29 different tissues. They found populations of mutated cells in just about everyone they studied.
YIZHAK: The skin, the lung and the esophagus were the ones that we found the highest amount of mutations.
HARRIS: That make sense because those tissues are always renewing themselves, and they're always bombarded with sunlight and other potentially damaging agents like smoke. Now, doctors are used to finding mutations in diseased tissue - namely, tumors - but Gad Getz, who heads the lab that did the work, said there's a twist here.
GAD GETZ: These are all normal tissues. They're not cancerous.
HARRIS: These tissues are just the normal you. You aren't simply a clone of the cells you started with, despite what you may have learned in biology class.
GETZ: You're just like a big puzzle with different pieces of different sizes, and all of them are very much similar to your original DNA.
HARRIS: But you are actually a mosaic of cells with small variations. This, in itself, is fascinating, but it also has implications for detecting cancer. It turns out that many of the mutations that cause these cell populations to proliferate are also involved in cancerous growth. So blood tests being developed to detect early signs of cancer could be fooled by these mutations, Yizhak says.
YIZHAK: And the fact that they already appear in normal cells means that we need to be cautious, more cautious about detection of early event of cancer.
GETZ: We need to distinguish between ones that eventually could become cancer and ones that maybe are on a dead end and will never become cancers. And I think we don't know yet what dictates the path that cells would take.
HARRIS: Dr. Jay Shendure, a Howard Hughes investigator at the University of Washington, says this is an interesting observation. It may be a bit unsettling to realize that we're actually made up of a gradually changing mosaic of cells.
JAY SHENDURE: I think we should feel OK about it (laughter).
HARRIS: He thinks of these mutations as a record of the changes that we accumulate as we grow from a single fertilized egg into an adult human being. It turns out that these mutations aren't just scattered across our tissues, but they apparently result in discrete clumps of cells.
SHENDURE: That entire process has been marked by nature in each of us.
HARRIS: And he says, as the technology for reading our genes improves, it's easy to see where all this is heading.
SHENDURE: We're only going to see more and more of this, and we're going to see it kind of at almost every scale imaginable.
HARRIS: From the microscopic to the obvious. It seems we are endlessly complex, endlessly fascinating and still full of mysteries. Richard Harris, NPR News.
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