Searching for Cancer Maps in Free-Floating DNA

Jay Shendure, a geneticist at the University of Washington. Dr. Shendure and his colleagues have taken important steps toward identifying the origins of cell-free DNA, which could have ramifications in finding clues about diseases.

Loose pieces of DNA course through our veins. As cells in our body die, they cast off fragments of genes, some of which end up in the bloodstream, saliva and urine.

Cell-free DNA is like a message in a bottle, delivering secrets about what’s happening inside our bodies. Pregnant women, for example, carry cell-free DNA from their fetuses. A test that analyzes fetal DNA has proved to be more accurate in screening for Down syndrome than standard blood tests.

In 2012, Jay Shendure, a geneticist at the University of Washington, and his colleagues were able to reconstruct the entire genome of a fetus from cell-free DNA in a pregnant woman’s saliva. A team of Stanford University researchers collected DNA fragments from the blood of patients who had received heart transplants and managed to find DNA from their donated hearts. (Tellingly, levels were highest in patients who were rejecting their hearts.)

These days, scientists are especially excited by the prospect of using cell-free DNA to test for cancer. Instead of relying on invasive biopsies, they hope to find blood-borne fragments that carry distinctive cancer mutations.

Unfortunately, the genetic sequence of a piece of cell-free DNA doesn’t tell researchers where in the body it originated — a valuable clue for doctors looking for diseases. “Knowing the origin of circulating DNA is of great importance,” said Alain R. Thierry, director of research at France’sNational Institute of Health and Medical Research.

All the cells in our body typically descend from a single fertilized egg, and they inherit all the same genes. The reason we aren’t uniform sacs of protoplasm is that our cells turn those same genes on and off in distinctive patterns, thereby developing into different tissues. They’re like musicians at a piano recital: They sit at the same keyboard, but they play different songs.

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