GENET archive


8-Humans: Surprising variation in human genes

-------------------------------- GENET-news -------------------------------

TITLE:  Surprising variation in human genes
SOURCE:, USA, by Zerah Lurie,0,6716992
DATE:   23 Jul 2004 

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Surprising variation in human genes

Healthy people possess surprising variations in the number of copies of
specific genes, an unexpected finding that Cold Spring Harbor researchers
say may contribute to human diversity and may someday help identify genes
involved in certain diseases.

Using a new DNA profiling technique, the researchers found that many
people are missing large chunks of DNA, while others have extra copies.
While such genetic differences were typically associated with diseases,
this new research, appearing in today's issue of the journal Science,
found that they are also very common among healthy people.

"Because it was previously associated with disease, we thought it was
only associated with rare events," Jonathan Sebat, a postdoctoral fellow
at Cold Spring Harbor and lead author of the research, said of the gene
variation, "but this study has shown that it's pretty ubiquitous in the
normal population."

Typically, there are two copies of most genes, one from each parent. This
research means that some people have three, four or even one copy.

Of the 20 individuals studied, researchers found 221 instances of these
"Copy Number Variations." They occurred at 76 different spots and
involved 70 different genes associated with everything from neurological
function to regulation of metabolism.

Dr. David Altshuler, the director of the program for medical and
population genetics at the Broad Institute of Harvard and MIT, says that
most genetic research focuses on much smaller differences in DNA.

What's important about this research is that it examines how "not just
one letter was swapped, but whole sentences," Altshuler said.

Bruce Stillman, president and chief executive of Cold Spring Harbor
Laboratory, suggested that this research may explain the mechanism for "a
lot of normal variation we see in the human population."

Research has shown that any two people are 99.9 percent genetically
identical. But that remaining 0.1 percent is important, because it
explains why any two individuals are different. While the Humane Genome
project - completed in 2003 - mapped the entire human genetic sequence,
it could not account for these differences between individuals.

"They have characterized a different form of genetic variation that
exists in the population," said Mark Daly, a computational biologist at
the Cambridge-based Whitehead Institute. He added that "It's a genetic
variation that is a lot more likely to affect a single individual."

"There are many genetic diseases that are caused by changes in gene
dosage," Sebat said. The classic example is Down syndrome, where the
affected individual has an extra copy of all the genes on Chromosome 21.

In this study, researchers found that some people had an extra copy of
the neuropeptide y-4 receptor. Removing a copy of this gene in mice makes
them leaner, but scientists have no idea what an extra copy means in
humans. "The person who was carrying the extra copy of the neuropeptide
gene might be more prone to obesity," Sebat suggested.

Sebat also said scientists should be able to isolate certain genetic
components of diseases by comparing people with certain diseases such as
Parkinson's or autism to unaffected family members and the general
population to determine whether multiple or missing copies of certain
genes are contributing to the condition.


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