Researchers at the Massachusetts Institute of Technology (MIT) have developed a way to determine, by sequencing DNA of liver cells, whether those cells have been exposed to a potent carcinogen, thereby paving the way for earlier diagnosis of the deadly disease.

In many parts of the world, exposure to a fungal product called aflatoxin is believed to cause up to 80 per cent of liver cancer cases. This fungus is often found in corn, peanuts, and other crops that are dietary staples in those regions.

The new technique can determine whether liver cells have been exposed to aflatoxin and help predict whether someone has a high risk of developing liver cancer, potentially many years before tumours actually appear. "What we're doing is creating a fingerprint," said John Essigmann, Professor at MIT.  "It's really a measure of prior exposure to something that causes cancer," Essigmann, who is the senior author of a paper describing the findings in the journal Proceedings of the National Academy of Sciences, added.

This approach could also be used to generate profiles for other common carcinogens, Essigmann said. In the new study, the MIT team set out to see if they could identify mutations produced by aflatoxin long before cancer develops.  First, the researchers exposed mice to a single dose of aflatoxin, four days after birth. After this exposure, all of the mice eventually developed liver cancer. 

The researchers sequenced DNA from those tumours and also from liver cells removed only 10 weeks after exposure, before tumours developed. To find mutations at 10 weeks, the researchers used a powerful genome sequencing technique that can identify very rare mutations -- which occur in about one in 10 million to 100 million DNA base pairs.

Unlike most DNA sequencing techniques, the one used in this paper, developed by researchers at the University of Washington, combines data from two complementary strands of DNA. With the new technique, the researchers found that at 10 weeks, a distinctive pattern of mutations that can serve as a "fingerprint" for aflatoxin exposure had already emerged.