Check out the big brain on the genetically modified mouse

The mouse embryo developed a giant brain after being injected with the human version of a DNA sequence called HARE5 (Image: Silver lab, Duke University)

Scientists at Duke University have pinpointed a regulator of gene activity that could lend insight into why we're so different from chimpanzees despite having a near-identical genetic makeup (94 per cent of our DNA is the same). When injected into a mouse embryo, the human version of a particular DNA sequence important for brain development caused the embryo to grow a considerably larger brain than other embryos treated with the chimpanzee version.

Humans and chimps split along the evolutionary ladder around four to six million years ago. Our brains then expanded dramatically as we picked up sophisticated abilities for things like abstract language, math, and reasoning. But it's not clear how that could happen without a larger DNA divergence.

The team at Duke went data mining through databases of human and chimp genomes. They identified 106 DNA enhancers – short bits of DNA that control gene activity – that are significantly different between the two species. Of these, six are believed to be involved in brain development. And one of those six, which the researchers dubbed HARE5, looked particularly promising because it is near a key gene in brain development and disease.

Human HARE5 and chimpanzee HARE5 were then implanted in mouse embryos. The human enhancer triggered earlier in development and proved more active in general than the chimp enhancer, leading to a 12 percent difference in brain volume, principally in the neocortex, which is involved in high-level functions such as language and reasoning. The researchers noticed the difference before even checking to see which was which (they monitored the embryo developments "blind," or without indication of genotype).

"What we found is a piece of the genetic basis for why we have a bigger brain," said co-author Gregory Wray. "It really shows in sharp relief just how complicated those changes must have been. This is probably only one piece – a little piece [of the puzzle]."

The human HARE5 and chimp HARE5 mice will now be studied further as they mature into adulthood. The researchers hope this will teach them more about how HARE5 affects brain structure and behavior. The other five HARE sequences will also be tested in hopes that they will reveal part of the secret behind mysteries like why humans get diseases such as autism and Alzheimer's while chimps do not.

A paper describing the research was published in the journal Current Biology.

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