Scientists who scoured the DNA of brain tumors, searching gene by gene for bad actors, were puzzled. The cancers had none of the mutations in growth-causing genes that are typical of other tumors, yet they grew quickly, with no brakes. The question was why — what had altered their genetic instructions to lead to runaway cell division?

The surprising answer, researchers reported Wednesday in the journal Nature, is that the three-dimensional structure and organization of the DNA had been disrupted. As a result, two genetic neighborhoods that are usually separated, as if they are two gated communities, were merged. The effect was to allow a powerful snippet of DNA from one neighborhood into the one next door, where it woke up a near-dormant growth gene. And the cells took off.

This is an entirely new way for cells to become cancerous, researchers said, and is probably not unique to the brain cancers, low- and intermediate-grade gliomas, that were the subject of the new study. Its lead author, Dr. Bradley E. Bernstein, a member of the Broad Institute in Cambridge, Mass., and a pathology professor at Massachusetts General Hospital, says he has already found a similar phenomenon in about a dozen other tumors. And, he says, the discovery suggests a potential treatment with an existing chemotherapy drug that restores the walls separating the DNA sections.

“It’s really exciting,” said Dr. Peter Dirks, a brain cancer expert at the Hospital for Sick Children in Toronto, adding that at least with gliomas, the suggested treatments could be tested in clinical trials very soon.