言語遺伝子

Natureから

Scientists discovered FOXP2 in the 1990s by studying a British family known as 'KE' in which three generations suffered from severe speech and language problems1. Those with language problems were found to share an inherited mutation that inactivates one copy of FOXP2.

A few years ago, researchers at the MPI Leipzig engineered mice to make the human FOXP2 protein4. The ‘humanized’ mice were less intrepid explorers and, when separated from their mothers, pups produced altered ultrasonic squeaks compared to pups with the mouse version of FOXP2.

Their brains, compared with those of normal mice, contained neurons with more and longer dendrites — the tendrils that help neurons communicate with each other. Another difference was that cells in a brain region called the basal ganglia were quicker to become unresponsive after repeated electrical stimulation, a trait called ‘long-term depression’ that is implicated in learning and memory.

In humans, she says, the mutation to FOXP2 might have helped our species learn the complex muscle movements needed to form basic sounds and then combine these sounds into words and sentences.

Another MPI team member, Ulrich Bornschein, presented work at the neuroscience meeting showing that the changes to brain circuitry that lead to quicker learning come about with just one of the two amino-acid changes in the human form of FOXP2. The second mutation may do nothing.

“That makes sense,” says Genevieve Konopka, a neuroscientist at the University of Texas Southwestern Medical Center in Dallas, who also studies FOXP2. Carnivores, including dogs and wolves, independently evolved the other human FOXP2 mutation, with no obvious effect on their brains.

KEという家系で３代にわたって言語の不自由が認められFOXP2（ウィングドヘリックスドメインを持つ転写因子）のアミノ酸置換が２か所で認められた。ヒト型のFOXP2を持つトランスジェニックマウスが作られたところ脳の基底核の樹状突起が長く、数が多かった。そのニューロンは記憶と関係するとされる長期抑制が早く生じた。FOXP2のヒト型変異は複雑な音をつくりだす筋肉のコントロールと単語や文章を結びつける学習に役立ったのではないかと考えられる。ヒト型の二つのアミノ酸変異のうち１つはイヌやオオカミでも独立に生じているが言語とは関係していない。
ちょっと単純化しすぎの話じゃないか？という気がするが、転写因子は多数の遺伝子に同時に影響するのでありえなくはないかな。