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Scientists diagnosed Down syndrome using DNA from ancient bones of seven 5,500-year-old infants. Their method, published in the journal Nature Communications, could help researchers learn more about how prehistoric societies treated people with Down syndrome and other rare diseases.
Down syndrome, which now occurs in 1 in 700 babies, is caused by an extra copy of chromosome 21. Extra chromosomes produce extra proteins, which can lead to many changes such as heart defects and learning disabilities.
Scientists have struggled to understand the history of this condition. Older mothers are now most likely to have children with this disease. However, in the past, Down syndrome may have become rare because women were more likely to die young, and without the heart surgeries and other treatments that extend lifespans today, fewer people would be born with Down syndrome. The children who died were unlikely to survive.
Archaeologists can identify some rare conditions, such as dwarfism, from bones alone. But Down syndrome, also known as trisomy 21, is a highly variable disease.
People with this disease exhibit a variety of combinations of symptoms and may have severe or mild symptoms. For example, a person with the characteristic almond-shaped eyes caused by Down syndrome may have a relatively normal bone structure.
As a result, it is difficult for archaeologists to confidently diagnose Down syndrome in ancient human remains. “You can’t say, ‘Oh, you have this change, so you have trisomy 21,'” says anthropologist Dr. Julia Gresky of the German Archaeological Institute in Berlin.
In contrast, Down syndrome is not difficult to genetically identify, at least in living humans. In recent years, geneticists have been testing methods that target DNA preserved in ancient bones.
However, this has been difficult because scientists cannot simply count entire chromosomes, as chromosomes break apart into fragments after death.
In 2020, Lara Cassidy, then a geneticist at Trinity College Dublin, and her colleagues used ancient DNA to diagnose a baby with Down syndrome for the first time. They were examining genes from human bones buried in a 5,500-year-old tomb in western Ireland. The bones of a 6-month-old boy contained an abnormally large amount of DNA from chromosome 21.
Since then, Adam Rohrlach, a statistician at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and his colleagues have found genetic signatures that can be used to quickly examine thousands of bones. We have developed a new method for
Dr. Rohrlach came up with the idea while talking with scientists at the institute about procedures for searching for ancient DNA. Because high-quality DNA sequencing is very expensive, the researchers screened the bones with an inexpensive test called shotgun sequencing before selecting some bones for further study. There was found.
If DNA was still stored in the bones, the test detected many small gene fragments. Very often they originate from microorganisms that multiply in the bones after death. However, some bones contained recognizable human DNA, and those with a high proportion were subject to additional testing.
Dr. Rohrlach learned that the institute had examined nearly 10,000 human bones in this way and that all shotgun sequencing results were stored in a database. Dr. Rohrlach and his colleagues had the idea that they might be able to scan databases for extra chromosomes.
“We thought, ‘Nobody’s ever tested for something like this,'” Dr. Rohrlach said.
He and his colleagues wrote a program to sort the recovered DNA fragments by chromosome. The program compared the DNA in each bone to the entire set of samples. They then identified specific bones with an unusual number of sequences derived from specific chromosomes.
Two days after the first conversation, the computer received the results. “Our hunch turned out to be correct,” said Dr. Rohrlach, now an associate lecturer at the University of Adelaide in Australia.
Researchers discovered that the institute’s collection contained six bones with extra DNA on chromosome 21, a hallmark of Down syndrome. Three of the cases involved babies who were about a year old, and the remaining three cases involved fetuses that died before birth.
Dr. Rohrlach also followed up on Dr. Cassidy’s 2020 study. He used a program to analyze the shotgun sequence of the Irish skeleton and discovered that she also had an extra chromosome 21, confirming her initial diagnosis.
In addition, Dr. Rohrlach discovered another skeleton with an extra copy of chromosome 18. The mutation causes a condition called Edwards syndrome, which usually leads to death before birth. The bones were from a fetus that died at 40 weeks and were severely deformed.
With the new study, Dr. Rohrlach and his colleagues cannot determine how common Down syndrome was in the past. Many children with this disease probably die before adulthood, and their fragile bones are less likely to be preserved.
“There’s so much uncertainty in sampling, what we’re finding and what we’re not finding,” Dr. Rohrlach said. “I think it would be a very brave statistician to try to make a big use of these numbers.”
But Dr. Rohrlach made the point that between 2,800 and 2,400 years ago, three children with Down syndrome and one child with Edward syndrome were all buried in two adjacent cities in northern Spain.
Usually in that culture people were cremated after death, but these children were buried inside buildings, sometimes with jewelry. “There were special babies buried in these houses, and we still don’t know why,” Dr. Rohrlach speculated.
Dr. Gresky did not believe that the evidence could rule out the possibility that the series of cases was a coincidence.
“The bones that were there were probably very well preserved,” she said. “Perhaps the archaeologists were so good and well-trained that they carried all the remains away. Perhaps they were buried to make them easier to discover.”
Still, Dr. Gresky considered the new study an important advance. First, archaeologists could compare genetically identified remains with Down syndrome and discover a hidden set of traits common to all skeletons.
And Gresky hoped other researchers would use ancient DNA to uncover the hidden history of other rare diseases. Otherwise, they will remain invisible. ”
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