A team of researchers analysed centuries-old DNA samples from plague burial pits in London identified genes that protected some against the Black Death that swept through Europe, Asia and Africa nearly 700 years ago. Their study, which showed how those aspects of our immune systems have continued to evolve since that time, is published in the journal Nature.
According to the researchers’ findings, the same genes that once conferred protection against the Black Death are today linked to an increased susceptibility to autoimmune diseases such as Crohn’s and rheumatoid arthritis.
The team focused on a 100-year window before, during and after the Black Death, which reached London in the mid-1300s. It remains the single greatest human mortality event in recorded history, killing upwards of 50% of the people in what were then some of the most densely populated parts of the world.
More than 500 ancient DNA samples were extracted and screened from the remains of individuals who had died before the plague, died from it or survived the Black Death in London, including individuals buried in the East Smithfield plague pits used for mass burials in 1348–9. Additional samples were taken from remains in Denmark.
Scientists searched for signs of genetic adaptation related to the plague, which is caused by the bacterium Yersinia pestis.
They identified four genes that were under selection, all of which are involved in the production of proteins that defend our systems from invading pathogens and found that alleles of the genes either protected or rendered one susceptible to plague.
Individuals with two identical copies of a particular gene, known as ERAP2, survived the pandemic at a much higher rates than those with the opposing set of copies, because the ‘good’ copies allowed for more efficient neutralisation of Y. pestis by immune cells.
“When a pandemic of this nature – killing 30 to 50 per cent of the population – occurs, there is bound to be selection for protective alleles in humans, which is to say people susceptible to the circulating pathogen will succumb. Even a slight advantage means the difference between surviving or passing. Of course, those survivors who are of breeding age will pass on their genes,” explained evolutionary geneticist Hendrik Poinar, an author of the Nature paper.
Europeans living at the time of the Black Death had were extremely vulnerable at first as they had no recent exposure to Yersinia pestis. Mortality rates fell in subsequent waves of the pandemic over the following centuries.
Researchers estimate that people with the ERAP2 protective allele (the good copy of the gene, or trait), were 40 to 50 per cent more likely to survive than those who did not.
“The selective advantage associated with the selected loci are among the strongest ever reported in humans showing how a single pathogen can have such a strong impact to the evolution of the immune system,” says human geneticist Luis Barreiro, an author on the paper, and professor in Genetic Medicine at the University of Chicago.
Source: McMaster University
