We know that mutations in DNA occur in all of our cells as they divide over the course of our lives. Some of these mutations cause cancers though most do not. But we don’t really know exactly what distinguishes the former from the latter—what makes some mutations drivers of tumorigenesis while others are tolerated as part of healthy tissue?

Sometimes the difference is the particular gene that is mutated; sometimes it is the tissue or cell type in which the mutation occurs, or the time of life, or the presence of other mutations in the same cell. Part of the reason we can’t tease out these disparate effects is because we don’t have that much information about the accrual of mutations in normal, healthy cells over the course of life. A new analysis in last week's Science looks at precisely that issue, but it only muddies the picture further.

Surveying the mutant landscape

The researchers examined cells lining the esophagus of nine deceased organ donors ranging in age from 20 to 75. Four of them had been smokers, but none of them had any chronic diseases or were on prescription meds. These cells have a high rate of proliferation, as they are constantly being sloughed off and replaced.

The software they used found 6,935 independent mutations in 844 tissue samples. Both the number of mutations and the sizes of the patches of cells with the mutation both increased with age. They estimated that the healthy cells had a few hundred mutations per cell in the 20-year-old donor, but that number rose to more than 2,000 mutations per cell among older donors. In total, 30 to 80 percent of the cells—more in older patients—carried mutations.

There was a high density of cancer-associated mutations among the cells examined—higher in these normal esophagus cells than in skin cells exposed to the Sun. The clones seem to be driven by positive selection; the mutations are conferring a competitive advantage to the cells that carry them, allowing them to grow faster than neighboring cells.

Many of the genes carrying mutations are involved in cellular differentiation, the process by which a cell adopts a mature identity, like lung or muscle. In some ways, this is the flip side of proliferation—by turning these cells away from differentiating and maturing, the mutations are promoting their continued growth.

Most of the mutated genes are considered to be canonical drivers of esophageal squamous cell carcinomas, so they could be forming proto-tumors. But they are definitely not tumors yet. The normal cells still have fewer mutations overall than cancer cells, and the mutations are arising by different mechanisms than they do in cancer cells.

Causal or chance?

One of the genes that they found that was mutated (NOTCH1, named after mutations of the gene in Drosophila that cause a notch in their wings) usually acts as a tumor suppressor in esophageal cells. Mutations that render it nonfunctional relieve this suppression, allowing tumors to flourish. NOTCH1 is mutated in about 10 percent of esophageal squamous cell carcinomas and is thus thought to be a driver of tumorigenesis. But this study, while small, showed that NOTCH1 is mutated in a large percentage (30 to 80, increasing with age) of normal esophageal cells—making the mutations less frequent in cancer cells than normal cells.

“These data have unveiled a hidden world of somatic mutation and clonal competition in normal esophagus. We have detected thousands of mutations per cell, hundreds of positively selected clones per square centimeter, and clones with cancer-associated mutations colonizing most of the esophageal epithelium with age, all without grossly detectable changes in histology,” conclude the authors. In other words, we see lots of mutations that look like they should be dangerous but are present in cells that look perfectly normal.

Mutations like those in NOTCH1 may thus not be driver mutations for cancers—they may just be prevalent in tumor cells because they’re prevalent in the normal cells from which the tumors derive. If the results from this limited sample size hold true across the population at large, they could have important ramifications for how we diagnose and treat tumors.

Science, 2018. DOI: 10.1126/science.aau3879 (About DOIs).