Black Death altered human genome

Imagine a rapidly spreading disease that kills millions of people with no cure in sight. Families are resigned and yet panicked, alive and yet dying inside.

This is medieval Europe during the Black Death. In the 1300s, Europeans lived in a constant state of fear. They barely fought off an invasion from the Huns, and the swath of destruction and death brought about by the war had just begun to fade when another horror struck.

Scientists have proposed numerous theories about the arrival of the Black Plague in Europe. Some state that the disease was spread when an infested boat docked in an Italian port. Others propose that the plague spread via the rats that came with the Huns. These rats were carriers of the Black Death, which was already affecting Asia.

Another problem of interest, though, is how the disease affected groups of people once it was there. The plague had profound interactions with society on the political and sociological levels, but scientists have wondered how the disease interacted with people on the genetic scale. In an analysis of the human genome, researchers at Radboud University in the Netherlands found that certain populations carried genes that coded for a particular type of immune system that could fight off the disease. The huge death toll of the plague wiped out the populations that lacked those genes. The plague served as a force of natural selection. As a result, most of the remaining populations carried a common section of the immune system genes. The research team, led by immunologist Mihai Netea, compared genomes of two similar groups.

The team gathered data from both Romanians and Rroma. Rroma immigrated from India a thousand years ago to Romania and then mixed with ethnic Romanians, producing a distinct subset of a population within the country. After the researchers contrasted both populations. The researchers then compared the Rroma with the population from northwestern parts of India.

A striking difference emerged: The Rroma which originally came from the same population as the northwestern Indians differ genetically with their ancestors. Their genomes specifically selected 20 genes that are not evident in the Indian genome. Scientists attribute these selections to the effect of the plague, which struck the European Rroma but spared the northwestern Indians.

Many of these selected genes appear to play a role in disease susceptibility. Some genes are responsible for inflammatory responses within the body while others affect the chances of contracting arthritis, an inflammatory condition. One group of genes in particular became a focus of the researchers. The genes are found on chromosome four and are responsible for coding receptor proteins within the body. These receptor proteins bind to bacteria and provoke an immune response. The researchers found that the genes code for proteins that react particularly well to Yersinia pestis, the bacteria responsible for the plague.

Although the research team established a strong case for the plague as a force for genetic selection, other scientists remain doubtful. Geneticists such as Luis Quintana-Murci at the Pasteur Institute, question whether other factors would also play a role in the selection of these particular genes.