Neanderthal DNA and COVID-19: Insights into Genetic Risk Factors
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Neanderthals died out around 40,000 years ago. But they did not vanish from the Earth entirely. It has become clear in the past decade that Neanderthals mated with the ancestors of modern humans, and produced viable offspring. The upshot is that almost half of the Neanderthal genome still among almost all modern people’s DNA. Such genes have been associated with everything from hairiness to fat metabolism. Many seem to be related to the immune system, and to affect the risk of developing several diseases.
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A pair of recent papers suggest covid-19 belongs on that list as well. The first paper described one Neanderthal DNA string, known as a “haplotype”, that is associated with a higher risk of serious illness. Having one copy of the haplotype, which is found on the third of the 46 chromosomes possessed by humans, doubles the chances of a trip to intensive care. Those unlucky enough to possess two copies—one from each parent—face an even higher risk. That genetic bad luck is not evenly distributed. The gene-sequence is most common among people of South Asian descent, with 63% of the population of Bangladesh carrying at least one copy; and among Europeans, where the prevalence is around 16%. It is virtually absent from Africa and in large swathes of eastern Asia. Exactly what the haplotype does is not clear. One gene within it encodes a protein that interacts with the cellular receptors that SARS-COV-2 (the virus which causes covid-19) uses to enter cells and hijack them. The haplotype is also thought to be involved in the production of signaling proteins, called cytokines, that help to regulate the immune system. An overly aggressive immune response is one mechanism by which covid-19 kills. On the other hand, some of those cytokines protect against cholera. The researchers speculate that may be why the haplotype is common in Bangladesh and India, where cholera has long been a problem.
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The second study concerns another Neanderthal haplotype, found on chromosome 12. Its effect is protective, though less potent: having a single copy is associated with a 22% lower chance of critical illness. It is present in every part of the world except Africa. Between 25% and 50% of the population of Eurasia, Vietnam, eastern China and America carry at least one copy. Scientists also have a better idea of what it does, for it was known to researchers even before the news of its Neanderthal origin. The haplotype hampers the spread of RNA viruses, of which SARS-COV-2 is one, by driving cells infected with them to self-destruct quickly. It is known to provide at least some protection against West Nile virus, hepatitis C and, intriguingly, SARS-COV-1, which caused the SARS outbreak that began in 2002. Once again, the hand of natural selection is visible. The genes in the chromosome-12 haplotype are found in other mammals, and have been lost several times in other species. That hints that carrying them comes at a significant cost, leading them to be removed if they are not being heavily used. That they are common in most human populations suggests RNA viruses have been a thorn in humanity's side for much of its evolutionary history.
