Sequencing the genomes of tumor cells has revealed thousands of mutations associated with cancer. One way to discover the role of these mutations is to breed a strain of mice that carry the genetic flaw -- but breeding such mice is an expensive, time-consuming process.

Now, MIT researchers have found an alternative: They have shown that a gene-editing system called CRISPR can introduce cancer-causing mutations into the livers of adult mice, enabling scientists to screen these mutations much more quickly.

In a study appearing in the Aug. 6 issue of Nature, the researchers generated liver tumors in adult mice by disrupting the tumor suppressor genes p53 and pten. They are now working on ways to deliver the necessary CRISPR components to other organs, allowing them to investigate mutations found in other types of cancer.

"The sequencing of human tumors has revealed hundreds of oncogenes and tumor suppressor genes in different combinations. The flexibility of this technology, as delivery gets better in the future, will give you a way to pretty rapidly test those combinations," says Institute Professor Phillip Sharp, an author of the paper.

Tyler Jacks, director of MIT's Koch Institute for Integrative Cancer Research and the David H. Koch Professor of Biology, is the paper's senior author. The lead authors are Koch Institute postdocs Wen Xue, Sidi Chen, and Hao Yin.

Gene disruption

CRISPR relies on cellular machinery that bacteria use to defend themselves from viral infection. Researchers have copied this bacterial system to create gene-editing complexes that include a DNA-cutting enzyme called Cas9 bound to a short RNA guide strand that is programmed to bind to a specific genome sequence, telling Cas9 where to make its cut.

In some cases, the researchers simply snip out part of a gene to disrupt its function; in others, they also introduce a DNA template strand that encodes a new sequence to replace the deleted DNA.

To investigate the potential usefulness of CRISPR for creating mouse models of cancer, the researchers first used it to knock out p53 and pten, which protect cells from becoming cancerous by regulating cell growth. Previous studies have shown that genetically engineered mice with mutations in both of those genes will develop cancer within a few months.

Studies of such genetically engineered mice have yielded many important discoveries, but the process, which requires introducing mutations into embryonic stem cells, can take more than a year and costs hundreds of thousands of dollars. "It's a very long process, and the more genes you're working with, the longer and more complicated it becomes," Jacks says.

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Gene-editing technique offers new way to model cancer

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