Wednesday, 17 June 2015

Rapid Human Evolution - Update

Fore victim of kuru
Eating human brains helped Papua New Guinea tribe resist disease, research shows | Science | The Guardian

Researchers have discovered that the gene which arose in the Fore people of Papua New Guinea in response to the prion disease, kuru, also gives protection against other prion diseases.

Some time ago I reported on an unarguable case of rapid Darwinian evolution in the Fore people of Papua New Guinea. Briefly, the local religious custom of ritually eating the brain of dead relatives had produced a high risk of contracting the transmissible prion disease, kuru, an invariably fatal dementia very similar to BSE (mad cow disease) in cattle, scrapie in sheep and CJD in humans. This placed the population under intense selection pressure where any resistance to this disease had high survival value.

This is a striking example of Darwinian evolution in humans, the epidemic of prion disease selecting a single genetic change that provided complete protection against an invariably fatal dementia.

John Collinge, Prion Unit, Institute of Neurology, University College London
Subsequently, it was found that about 10% of those who had participated in the ritual but survived had a gene which was not found in the DNA of any of the victims of kuru. This gene almost certainly arose as a mutation in a single individual about 200 years earlier. When the kuru epidemic was at its height, before the brain-eating ritual was outlawed, the gene was present in just two families, so, while non-carriers died, those with this mutation survived to breed and so the frequency of the allele in the population rose dramatically in just a few generations.

Prions are deformed proteins which induce other proteins to change shape and stick together to form large polymers which damage the brain. A similar process is now known to cause Alzheimer's and Parkinson's diseases. The number of cases of these dementias are increasing and are projected to triple by 2050 to some 135.5 million people worldwide.

The team at the MRC Prion Unit, Department of Neurodegenerative Disease, UCL Institute of Neurology, London, reported in Nature a few days ago:

Mammalian prions, transmissible agents causing lethal neurodegenerative diseases, are composed of assemblies of misfolded cellular prion protein (PrP)1. A novel PrP variant, G127V, was under positive evolutionary selection during the epidemic of kuru—an acquired prion disease epidemic of the Fore population in Papua New Guinea—and appeared to provide strong protection against disease in the heterozygous state2. Here we have investigated the protective role of this variant and its interaction with the common, worldwide M129V PrP polymorphism. V127 was seen exclusively on a M129 PRNP allele. We demonstrate that transgenic mice expressing both variant and wild-type human PrP are completely resistant to both kuru and classical Creutzfeldt–Jakob disease (CJD) prions (which are closely similar) but can be infected with variant CJD prions, a human prion strain resulting from exposure to bovine spongiform encephalopathy prions to which the Fore were not exposed. Notably, mice expressing only PrP V127 were completely resistant to all prion strains, demonstrating a different molecular mechanism to M129V, which provides its relative protection against classical CJD and kuru in the heterozygous state. Indeed, this single amino acid substitution (G → V) at a residue invariant in vertebrate evolution is as protective as deletion of the protein. Further study in transgenic mice expressing different ratios of variant and wild-type PrP indicates that not only is PrP V127 completely refractory to prion conversion but acts as a potent dose-dependent inhibitor of wild-type prion propagation.

Note that this mutation was the simplest possible in the way it translates into protein formation - a substitution of one amino acid (valine) for another (glycine). This would have entailed a single substitution of the central glutamine nucleotide for cytosine in the relevant DNA triplet and it is as effective as deleting the entire protein. Exactly how this gene conveys this protection is still unknown but the team are hoping to learn more about the molecular basis for this action so it can be applied to other prion or prion-related diseases.

I don't suppose an 'Intelligent (sic) Design' creationist would like to hazard a guess as to why an intelligent designer would design these hideous prions to cause these dementias then give people who used to get them by eating dead peoples' brains a gene that completely protects them, and also protects them from prion diseases they didn't even suffer from?

How about a creationist who habitually tells people that no new information can arise by mutation because mutations destroy information and so are always harmful, explaining how this new protective mutant gene, complete with new meaning which protects against prion diseases, arose by mutation? It arose in a single DNA triplet in a single individual, with a clear pattern of inheritance and a clear pattern of increasing allele frequency under selection pressure which favoured it, exactly fitting the definition of Darwinian evolution.

If this isn't a case of rapid evolution in humans, what is it exactly?

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