"We know that people vary a lot in their rates for common diseases, but we know very little about what is actually involved," researcher David Altshuler, at the Whitehead-Massachusetts Institute of Technology Center for Genome Research in Cambridge, told United Press International.
The double-stranded DNA that makes up the human genetic code consists of roughly 3 billion pairs of biochemicals called nucleotides. Since DNA is made of four kinds of nucleotides, there are a daunting number of possible DNA combinations.
This multitude of variations is what make everyone unique. It also results in a vulnerability to certain diseases and may explain why people sometimes respond to therapies differently.
Altshuler and colleagues in Nigeria and the United States analyzed the DNA of 280 volunteers of African, Asian and European descent to look for widespread patterns in genetic variation. Their search, which encompassed more than 50 regions in the human genetic code, spanning more than 13 million nucleotides, yielded what appears to be a mosaic structure.
It turns out, single nucleotide pairs tend to vary together in big clusters and the clusters usually differ from one another in only a few ways.
"Instead of looking at the human genome and seeing no patterns in all the variability, we now see this mosaic structure of all these little blocks of variation, and each block appears to come in roughly four to five flavors," said David Valle, director of the Center for Medical Genetics at Johns Hopkins University in Baltimore, Md.
So far, the differences in DNA variations among the race found by the investigators tend to be shorter in DNA from Africans than from elsewhere, which "is strong support for the out-of-Africa hypothesis" for the origins of all humans, said Ellen Clayton, director of the Genetics and Health Policy Center at Vanderbilt University in Nashville, Tenn.
"It suggests that populations that exist in Africa are more ancient, so there's been more opportunity for DNA recombination than relatively more recent populations, namely those everywhere else," Clayton said.
The discovery of patterns also suggests in the future scientists will have to look at only a few nucleotides at a time to find information on dozens of neighboring clusters, thereby saving a lot of time in genetically analyzing complex diseases.
"You could cut the costs down to a third or a tenth of what they might otherwise be," said geneticist Lynn Jorde of the University of Utah in Salt Lake City. "That could save many, many millions of dollars."
Clayton noted this research still has a long way to ago, however, the roughly 13 million nucleotides analyzed still only constitute about 0.4 percent of the human genome.
"Our work is at the beginning of a chain in trying to understand what's fundamentally wrong. Of course, to be actually useful to patients will take many years after that," Altshuler said.
"We hope this helps us understand what's actually wrong in complex disorders such as diabetes, psychiatric illness, cancer and high blood pressure. That will help focus new diagnostic strategies and new drugs," he added.
The scientists describe their findings in the May 24 issue of the journal Science.
(Reported by Charles Choi in New York.)
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