Wednesday, January 29, 2014

Neanderthals gave us disease genes

Genes that cause disease in people today were picked up through interbreeding with Neanderthals, a major study in Nature journal suggests.

They passed on genes involved in type 2 diabetes, Crohn's disease and - curiously - smoking addiction.

Genome studies reveal that our species ( Homo sapiens) mated with Neanderthals shortly after leaving Africa.

But it was previously unclear what this Neanderthal DNA did and whether there were any implications for human health.

Between 2% and 4% of the genetic blueprint of present-day non-Africans came from Neanderthals.

By screening the genomes of 1,004 modern humans, Sriram Sankararaman and his colleagues identified regions bearing the Neanderthal versions of different genes.

That a gene variant associated with the inability to stop smoking should be found to be of Neanderthal origin is a surprise.

The researchers are, of course, not suggesting that our evolutionary cousins were puffing away in their caves.

Instead, they argue, this gene may have more than one function; the modern effect of this genetic marker on smoking behaviour may be one impact among several.

Pastures new

'Start Quote

When Neanderthals and modern humans met and mixed, they were at the very edge of being biological compatible'

End Quote Prof David Reich Harvard Medical School

Researchers have found that Neanderthal DNA is not distributed uniformly across the human genome, instead being commonly found in regions that affect skin and hair.

This suggests some gene variants conferred characteristics that were useful to modern humans as they moved out of Africa and into the cooler environments of Eurasia. When the populations met, Neanderthals had already been adapting to these conditions for several hundred thousand years.

The stocky hunters once covered a range stretching from Britain to Siberia, but went extinct around 30,000 years ago as Homo sapiens was expanding from an African homeland.

Neanderthal ancestry was found in regions of the genome linked to the regulation of skin pigmentation.

'We found evidence that Neanderthal skin genes made Europeans and East Asians more evolutionarily fit,' said Benjamin Vernot, co-author of a separate study in Science journal.

Genes for keratin filaments, a fibrous protein that lends toughness to skin, hair and nails, were also enriched with Neanderthal DNA. This may have helped provide the newcomers with thicker insulation against cold conditions, the scientists suggest.

'It's tempting to think that Neanderthals were already adapted to the non-African environment and provided this genetic benefit to (modern) humans,' said Prof David Reich, from Harvard Medical School, co-author of the paper in Nature.

But other genes are implicated in human illnesses, such as type 2 diabetes, long-term depression, lupus, billiary cirrhosis - an autoimmune disease of the liver - and Crohn's disease.

Asked whether Neanderthals actually suffered from these diseases too, or whether the genes in question only caused illness when transplanted to a modern human genetic background, Mr Sankararaman said: 'We don't have the fine knowledge of the genetics of Neanderthals to answer this,' but added that further study of their genomes might shed light on this question.

Desert regions

Still other regions of our genomes were discovered to be devoid of Neanderthal DNA, suggesting that certain genes had such harmful effects in the offspring of modern human-Neanderthal pairings that they have been flushed out actively through natural selection.

'We find that there are large regions of the genome where most modern humans carry little or no Neanderthal ancestry,' Mr Sankararaman told BBC News.

'This reduction in Neanderthal ancestry was probably due to selection against genes that were bad - deleterious - for us.'

The Neanderthal-deficient regions encompass genes that are specifically expressed in the testes, and on the X (female sex) chromosome.

This suggests that some Neanderthal-modern human hybrids had reduced fertility and in some cases were sterile.

'It tells us that when Neanderthals and modern humans met and mixed, they were at the very edge of being biological compatible,' said Prof Reich.

Another genome region that lacked Neanderthal genes includes a gene called FOXP2, which is thought to play an important role in human speech. and follow me on Twitter