Ninety-six percent of a chimpanzee's genome is the same as a human's. It's the other 4 percent, and the vast differences, that pique the interest of Georgia Tech's Soojin Yi. For instance, why do humans have a high risk of cancer, even though chimps rarely develop the disease?

In research published in September's American Journal of Human Genetics, Yi looked at brain samples of each species. She found that differences in certain DNA modifications, called methylation, may contribute to phenotypic changes.

The results also hint that DNA methylation plays an important role for some disease-related phenotypes in humans, including cancer and autism.

"Our study indicates that certain human diseases may have evolutionary epigenetic origins," says Yi, a faculty member in the School of Biology. "Such findings, in the long term, may help to develop better therapeutic targets or means for some human diseases. "

DNA methylation modifies gene expression but doesn't change a cell's genetic information. To understand how it differs between the two species, Yi and her research team generated genome-wide methylation maps of the prefrontal cortex of multiple humans and chimps.

They found hundreds of genes that exhibit significantly lower levels of methylation in the human brain than in the chimpanzee brain. Most of them were promoters involved with protein binding and cellular metabolic processes.

"This list of genes includes disproportionately high numbers of those related to diseases," said Yi.

"They are linked to autism, neural-tube defects and alcohol and other chemical dependencies. This suggests that methylation differences between the species might have significant functional consequences. They also might be linked to the evolution of our vulnerability to certain diseases, including cancer."

Yi, graduate student Jia Zeng and postdoctoral researcher Brendan Hunt worked with a team of researchers from Emory University and UCLA. The Yerkes National Primate Research Center provided the animal samples used in the study. It was also funded by the Georgia Tech Fund for Innovation in Research and Education (GT-FIRE) and National Science Foundation grants (MCB-0950896 and BCS-0751481). The content is solely the responsibility of the principal investigators and does not necessarily represent the official views of the NSF.