On chromosome 3, several genes play an important role in coding our immune system. However, the team of researchers noticed that many patients with severe forms of Covid-19 had a particular genetic mutation on this chromosome, which particularly concerns two genes (CCR9 and SLC6A20), which are responsible for coding some of the receptors and transporters through which our cells initiate immune responses… to a certain coronavirus for example. Clearly, carriers of this genetic mutation are less resistant to attacks by the virus.
Mutation almost absent in Africa
But where does this genetic “flaw” come from? It was by comparing genetic databases of patients around the world that Dr. Zeberg made a second surprising discovery: it would be inherited from one of our prehistoric ancestors, the Neanderthal man. Indeed, very present in Asia, a little less in Europe and America, it is almost absent among the populations of the African continent. However, the meeting of our Homo sapiens and Neanderthal ancestors took place precisely when the first began to leave Africa. Hence the deduction of a Neanderthal origin of this genetic particularity.
Why could such a genetic “flaw” have persisted for so long?
The question that then arises is why such a genetic “fault” could have persisted for so long, when, from an evolutionary perspective, one could have imagined that it would disappear over generations, under the effect of natural selection?
Better protection against AIDS
This is where another concomitant discovery comes in: a patient carrying this mutation would certainly have a greater chance of having a severe form of Covid-19, but he would also have a 27% less chance than another patient of being infected with AIDS, a disease which is precisely based on a blockage of our immune system – HIV is the acronym for human immunodeficiency virus.
There remains an enigma: why the Neanderthal organism would have needed to protect itself from AIDS, which only appeared… in the XXand century – more than 30,000 years after its extinction? The researchers’ hypothesis is that this genetic mutation was probably to protect against another disease, more widespread at the time, “like cholera or smallpox”, explains Dr. Zeberg.