A present-day genetic risk factor for leprosy was also associated with the disease in medieval Europeans, reports a paper published in Nature Communications this week. The study suggests that susceptibility to infection with Mycobacterium leprae (the bacterium responsible for leprosy) is, at least in part, mediated by the same genetic risk allele in populations living almost a thousand years apart, and in different parts of the world.

To better understand the underlying genetic factors responsible for the leprosy endemic in the Middle Ages in Europe, Ben Krause-Kyora and colleagues analysed ancient DNA extracted from leprosy-affected bone lesions of 69 individuals from the 12th to 14th century from the Danish St Jorgen leprosarium. The authors show that the ancient leprosy cases carried a risk allele known as DRB1*15:01 more frequently than contemporary and medieval European controls. This allele is the strongest genetic risk factor for M. leprae infections in India, China and Brazil, where the disease is still prevalent today.

The DRB1*15:01 allele is still common in contemporary Europeans, although at potentially slightly lower frequency than in the Middle Ages, and the authors speculate that this risk allele (for leprosy) may be associated with antagonistic fitness advantages that prevented it from disappearing from the European gene pool altogether.