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Ancient DNA Analysis Uncovers Cases of Trisomy

Trisomy 21

NEW YORK – By screening thousands of ancient DNA samples, investigators at the Max Planck Institute for Evolutionary Anthropology, the University of Adelaide, and other centers have identified several individuals with genetic syndromes, specifically trisomy 21 and trisomy 18. They found these to be consistently buried with care, suggesting they were not subject to stigmatization from their communities.

"As the [ancient DNA] record continues to grow, genetic disorders with extremely low rates of prevalence will be able to be more frequently discovered," senior and co-corresponding author Kay Prüfer, an archaeogenetics researcher with the Max Planck Institute for Evolutionary Anthropology, and his colleagues wrote in Nature Communications on Tuesday, adding that genetic, archaeological, and anthropological results "afford a perspective into the way that these disorders were viewed and treated in past communities."

Using shotgun sequencing, the researchers screened samples from 9,855 ancient individuals from historic and prehistoric sites around the world, searching for signs of trisomy based on higher-than-usual read counts across specific chromosomes.

After flagging potential trisomy cases in samples from Neolithic Ireland, Bronze Age Bulgaria, Bronze Age Greece, Iron Age Spain, and post-Medieval Finland, the team used skeletal analyses and other clues to focus in on half a dozen confirmed cases of trisomy 21, also known as Down syndrome, along with one case of trisomy 18, commonly called Edwards syndrome.

The trisomy-affected individuals ranged in age, from fetuses around 26 weeks gestation to babies 16 months old, the researchers reported, and the estimated prevalence rate of Down syndrome was roughly one in 1,643 — lower than that found in modern populations, even when focusing on children born to mothers under 20 years old.

The authors noted that the individual with Edwards syndrome and three of those with Down syndrome came from early Iron Age sites in Navarra, Spain, "potentially suggesting a higher frequency of burials of trisomy carriers in those societies."

The work builds on findings published in Communications Biology in January. There, members of another team, led by investigators at the Francis Crick Institute, described Down syndrome, mosaic Turner syndrome, and other trisomy cases found through low-coverage sequencing on Iron Age, Medieval, and post-Medieval samples from the UK.

In the current analysis, the investigators highlighted six trisomy 21 cases and a single Edwards syndrome case, which they considered in relation to burial methods, grave goods, and osteological data collected from the sampling locations, Prüfer explained in an email.

When he and his colleagues incorporated additional osteological, archaeological, contextual, and cultural clues from each of the burial sites where a trisomy-affected child was found, they found that children with trisomy 21 or trisomy 18 had been buried according to standard practices at the time — in some cases receiving special burials reserved for exceptional members of their society.

"All of this information indicates that the trisomy children were afforded a normal, and in some cases extraordinary, burial," Prüfer said, "which we interpret as showing some appreciation of the individual by their communities."