Almost all healthy people harbor patches of mutated cells

Normal isn’t always normal. A new study finds that large groups of cells in healthy tissues carry mutations, including ones tied to cancer.

About 95 percent of healthy people had patches of mutated cells in at least one of the 29 tissues examined, including kidney, muscle and liver, researchers report in the June 7 Science. Most of those mutations found in the 488 people in the study are harmless, but some have been linked to various cancers.

About 40 percent of tissues had at least one big patch of mutated cells, and about 5 percent of the studied samples had five or more mutant patches, Keren Yizhak of the Broad Institute of MIT and Harvard and colleagues discovered.

Skin, esophagus and lung tissues had more of these mutant patches than other tissues, the researchers found. Those three tissue types are exposed to more ultraviolet light, pollution, smoke or other environmental factors that may cause mutations than internal organs, which are not directly exposed to these external factors. In people of European ancestry, sun-exposed skin accumulated more mutations than covered skin did. African Americans didn’t have the same increase in mutations in their sun-exposed skin.

Age also affected the number of mutations, with mutations popping up more often after age 45 in tissues that divide to make new cells. Tissues that don’t actively grow didn’t tend to build up age-related mutations, the researchers found.

It isn’t yet possible to tell how close a tissue is to becoming cancerous, but the study is a first step toward answering that question, Cristian Tomasetti, a cancer researcher at Johns Hopkins School of Medicine wrote in a commentary in the same issue of Science.