16 November 2009
ByAppeared in BioNews 534
Scientists believe that they have found a gene that helps explain the fact that humans are the only animal that has developed speech. Subtle variations in the human version of the gene, known as FOXP2, appear to underpin the human development of language, according to recent research carried out by scientists at the University of California, Los Angeles (UCLA), US, and published in the journal Nature. When comparing the human and chimp version of FOXP2 the researchers discovered that they had different effects on genes in the brains of the two species, and these differences may affect the way in which the brain develops. The researchers believe this could help explain why humans alone are capable of language, and hope that the findings will lead to treatments for speech and language disorders in the future, as well as other conditions, such as autism, that are characterised by communication deficits.
All animals have a FOXP2 gene, and it is almost identical in most mammals and birds. The gene encodes a transcription factor (also called FOXP2) that binds to DNA and acts as a 'dimmer-switch', governing the extent that other genes are expressed as proteins. However, the gene only rose to prominence when it was found to be the cause of a rare inherited speech and language disorder in a large family in London, UK, in 2001. Half the members of the family had severe problems in understanding and articulating speech and researchers from Oxford University found that those affected all had genetic mutations that disrupted the function of FOXP2
That discovery prompted further research and in 2002 a German team identified that the human version of FOXP2 differed from the chimp version by only two amino acids out of 740, hinting at the gene's crucial role in the human development of speech and language. Then, earlier this year, scientists from Leipzig, Germany, spliced the human version of FOXP2 into mice (whose FOXP2 gene differs from the human one by just three amino acids) and, whilst the mice did not start 'speaking', it was noticed that they squeaked a little differently from normal mice and the shape and activity of neurons changed in the brain area that is affected in humans that have the FOXP2 related language disorders. Since humans and mice last shared a common ancestor 70 million years ago it may be unsurprising that the human FOXP2 gene did not have a greater transformative effect on the mice brains. However, the human evolutionary lineage split from that of chimps only seven million years ago, meaning that our brains and genes are very similar to a chimps, and any difference in the effects of FOXP2 should be easier to notice.
To determine if the human version of FOXP2 does something differently from the chimp version a team led by Daniel Geschwind, from the UCLA, grew human brain cells lacking FOXP2 in petri dishes then inserted the human version of FOXP2 in some cells, and the chimp version in others. They then recorded how many genes were affected. The human version caused 61 genes to be significantly up-regulated and 55 genes to be significantly down- regulated when compared to the chimp version. Some of these 116 genes have previously been linked with brain development or cognition, while others have been linked to formation of the skull, face, larynx, cartilage and connective tissue in a manner that is essential for articulation.
'Our findings may shed light on why human brains are born with the circuitry for speech and language and chimp brains are not,' said Dr Geschwind, adding: 'I'm not a person who necessarily believes that one gene is going to tell us everything, but this was really quite remarkable and does place FOXP2 in a relatively central position.' Despite FOXP2's apparent central role in the evolution of human language Geschwind does not like the idea of calling it 'the language gene' as others have done. He cautions: 'Either FOXP2 itself is pretty damn important, or it's part of a regulatory circuit - something else is regulating FOXP2 that no one else has found yet.'