A geneticist had to help sort 50 boxes of bones in the basement of the Museum of London in April of 2006 on a chilly day. The remains of people who died 700 years ago were in one of the containers. The bones were from people who had been buried a year or more later in the same medieval cemetery.
She wondered what made the two groups different when she looked at the remains. Some people died during the Black Death, others didn't. Klunk is now at Daicel.
For hundreds of years, other scholars have pondered that mystery. Klunk and her colleagues found that the survivors were more likely to carry certain genes that boosted their immune response to the plague. They reported today in Nature that one variant increases the chance of surviving the plague by 40%. The authors of the study were blown away by the effect.
The findings show that the Black Death caused a jump in the proportion of people carrying a protective variant of the human genome. The variant is associated with higher risk of autoimmune diseases.
David Enard is a population geneticist at the University of Arizona. The power of natural selection in immune genes is unknown.
The Black Death is a disease that has killed many people. It killed 30% to 50% of people in Europe, the Middle East, and Africa. The catastrophe may have left a mark on the genome of survivors, giving them immunity against the plague in the future, according to researchers. It has proved difficult to identify that mark because of the rapid change in the genes involved in immunity. Anne Stone of Arizona State University is not part of the study.
New techniques for analyzing ancient DNA made it possible to search for the legacies of pathogens in the genomes of people who died long ago. Researchers studying the plague were not able to find enough samples from victims to show differences in immune genes.
The land that King Edward III bought for a plague pit was found to be an answer to the problem. Thousands of burials are indicative of a time capsule. The plague victims are buried in mass graves at the bottom, while the survivors are above them. More than 300 skeletons were found in this cemetery and two others in London, as well as more than 200 in five other locations.
Some 500 people lived during a 100 year period before, during, and after the plague.
The highest quality DNA from 206 individuals was used to examine 356 genes associated with immune responses. Before and after the Black Death in people in London, four of which were also found in samples fromDenmark, the team identified an astonishing 245 gene variant that rose or fell in frequencies.
There were changes to the code for one genes. The previous work showed that the immune cells can recognize and fight threats. The team confirmed that it can suppress Y. pestisbacteria by measuring how the genes of cultured human immune cells respond to the pathogen.
The researchers found two different variations of the same thing. They differ by a single letter. There was a big impact on immunity because of the difference between the genes. The people who had two copies of the allele were more likely to survive the plague than the people who had only one copy.
40% of Londoners carried at least one copy of the protective variant before the Black Death. Only a small percentage of plague victims carry it. Within a few generations, the protective variant was carried by more than 50% of Londoners. The proportion of people carrying the protective variant went from 45% before the Black Death to 70% afterwards.
The 10-percentage-point increase in London might not seem like much, but researchers have never before documented such a rapid surge in a human genetic variant. He says that a 10% change in allele frequencies in only three or four generations is not normal. Monty Slatkin of the University of California, Berkeley is not part of the study.
The protective variant is found in 45% of Britons in the 1000 Genomes database. The protective variant has a downside. It has been shown that it comes with a higher risk of developing an immune system disorder. The cost of the Pandemic will become apparent once it's over. The plague was endemic in Europe and Asia into the 19th century and natural selection may have favored it until recently.
The protective variant and three other potential plague-resistance variant identified by the Nature study are being looked at by researchers to see if they are still present. Tom Gilbert, an evolutionary biologist at the University of Copenhagen who co-led the study, said there were no big swings in the genes in the 54 people who lived before, during, and after the Black Death. If researchers can confirm the surge of genes in more populations, that could help rule out the possibility that the new findings were skewed, for example by the way researchers reconstruct degraded DNA.
Gilbert doesn't expect the results to fall apart. He wonders if genetic shifts explain why Y. pestis is less dangerous than it was in the 14th century. Gilbert says that they have assumed that the plague went away because they have become better at keeping rats out of their homes. It would be great if it went away because we became immune.
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