Medieval Leprosy Genome Project

Leprosy, a devastating chronic disease caused by the bacterial pathogen Mycobacterium leprae, was prevalent in Europe until the late Middle Ages. Today, the disease is found in 91 countries worldwide with about 200,000 new infections reported annually.

Skull of Jorgen 625, side view with a genome assembly of the ancient strain of M. leprae.

To retrace the history of the disease we have reconstructed entire genome sequences of medieval and modern Mycobacterium leprae bacteria in a collaboration with Stewart Cole from EPFL Lausanne and other international researchers. The five medieval skeletons were excavated in Denmark, Sweden and the United Kingdom in addition seven leprosy genomes were reconstructed from biopsies of seven live patients.

A comparison of medieval European M. leprae genomes with modern strains from around the world revealed that all M. leprae strains share a common ancestor that existed within the last 4000 years. This is congruent with the earliest skeletal evidence for the disease in the archaeological record dated to 2000 BC from India. The genome comparisons revealed an astonishing level of genetic conservation during the past 1,000 years. We could further more show that a strain of M. leprae once present in medieval Europe is found today in the Middle East, whereas other medieval European strains are nearly identical to those now found in North America, both in leprosy patients and wild armadillos, thus suggesting a European origin of leprosy in the American armadillo population.

Excavation of the St. Mary Magdalen leprosarium in Winchester, in situ skeletons.

One skeleton from Denmark (Jorgen 625) showed extraordinary preservation of the pathogen DNA, allowing genome reconstruction without using a modern reference sequence as a scaffold, something which has never been done for an ancient organism before. We found that almost half of the DNA recovered from that particular specimen derived from M. leprae bacteria, which is orders of magnitude higher than the amount of pathogen DNA usually found in skeletons. We propose that the M. leprae DNA was far better preserved than human DNA because it was protected from degradation for centuries by the extremely thick, waxy cell wall of the leprosy bacterium.

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