We have all heard that the sequence of human DNA differs from chimpanzee DNA by only about 1%. Yet humans are capable of building complex civilizations while the chimps are still eating bugs in the forest. If you compare the human brain to the brain of any other primate, it’s easy to see where our sophisticated cognitive abilities come from.
DNA is the blueprint for making proteins, cells and organs, so is there something special hidden in that 1% sequence difference that gives humans bigger brains? In particular, scientists have focused on regions in the human genome that have undergone rapid sequence changes in the human lineage, but not in other primates. Besides looking for differences in genes that make proteins, we can also look for changes in regulatory regions, like enhancers, that control when and where the genes are expressed.
A recent paper in Current Biology by Boyd et al. explores these questions by studying a human-accelerated regulatory enhancer (HARE5), which differs significantly between humans and chimps.
Enhancer activity of HARE5
How do you study enhancers? One way is to use a reporter gene. Enhancers drive expression of nearby genes, so what if you swapped out a nearby gene and replaced it with a gene for a fluorescent protein? Then you can look at your organism and wherever you see the fluorescent protein, the enhancer is active, meaning that the normal “nearby gene” is normally expressed in those cells. Instead of doing these experiments with chimps and humans, which would take forever and be unethical in some cases, the authors put these reporter constructs into mice. The enhancers from the chimps and humans drove expression of the reporter gene in the embryonic mouse brains. The gene adjacent to the human enhancer was expressed earlier in development and more strongly than when placed next to the chimp enhancer (in other words, a lot more protein is being made).
|Reporter gene experiment. The mouse brain images are actual results from Figure 2 in Boyd et al. (2015).|
This tells us that whatever normal gene is near HARE5, it is probably expressed earlier and way more in humans than in chimps. There are just 10 sequence differences in the human HARE5 (i.e. mutations), which is enough to affect the way the enhancer functions and activates expression of genes.
Frizzled expression is regulated by HARE5
So which genes are near the HARE5 sequence? The closest gene is called Frizzled 8 and it is a receptor that responds to signals sent by other cells. Frizzled 8 (FZD8) is a well known component of the Wnt signaling pathway that regulates many aspects of embryonic development, including neurogenesis (formation of new neurons). The authors demonstrate that the mouse HARE5 physically interacts with Fzd8, which is a necessary first step of gene expression, so Fzd8 is likely affected by the HARE5 sequence differences in humans and chimps.
The authors wanted to see what would happen to development of the mouse brain when Fzd8 is expressed in the same pattern as in humans or chimps. They repeated the earlier experiments, but this time instead of using a reporter gene, they put the mouse Fzd8 gene next to the chimp or human HARE5 sequence. They injected these DNA constructs into mice and waited to see what would happen to embryonic brain development. When the chimp-HARE5 was driving expression of Fzd8, not much changed in terms of mouse brain development. However, when the human-HARE5 sequence was activating the mouse Fzd8 gene, the mouse brain grew 12% bigger!!
Let me be clear here– they are not expressing the human Fzd gene in mice. No, they are using the human enhancer to drive expression of the mouse Fzd8 gene, so presumably it is expressed more and earlier in development (like they saw in the reporter gene experiment). The neural progenitor cells (pre-neurons) divided faster than in a normal mouse, leading to formation of more neurons, and a bigger brain!
10 sequence changes in an enhancer may be one reason why I am able to write and you are able to read and understand this blog. Whoa. No news yet about whether these mice with bigger brains are also able to read and write… I’m sure they’re saving that for another paper.
I should say too, that there are probably a number of other similar changes to other enhancers and genes that all led to the rapid development of the big ol’ human brain.