Nov. 22, 2006 -- "We are more alike, my friends, than we are unalike."
While the words of the poet Maya Angelou still ring true, researchers have evidence that we may not be nearly as alike as we think.
At least 10 percent of the genes in the human population can vary in the number of copies of DNA sequences they contain, according to new research published in the current issues of Nature and Genome Research.
While scientists once believed that the genetic blueprints of any two humans were 99.9 percent similar, this thinking may have to change.
So what's the big difference? To understand the discovery, one must first realize that the sum total of an individual's DNA is a bit like files in a filing cabinet.
In some cases, there may be more than one copy of a specific file. In other cases, one person's filing cabinet may be missing a file or two altogether.
Despite this, it was assumed that everyone had pretty much the same number of copies of certain files in their cabinets.
That was before researchers used a new technique to take a closer look at differences in the number of these copies, called copy number variations, or CNVs.
They learned that large differences existed between individuals -- differences significant enough to affect gene activity and, ultimately, an organism's physical makeup.
"While we already knew that some genes were inactive quite commonly without causing problems -- for example, all blonds are mutant in a receptor that regulates how hair gets colored -- the extent of deletions and, thus, inactivation of genes is much larger than previously thought," said Margit Burmeister, professor of psychiatry and human genetics at the University of Michigan.
"We are all mutants!"
Chasing Disease Using CNVs
The findings may have applications when it comes to predicting genetic diseases.
The research group found nearly 16 percent of known disease-related genes within the CNVs, including those associated with rare genetic disorders.
Other genes in the CNVs were associated with more common conditions, including schizophrenia, cataracts and the artery-hardening condition atherosclerosis.
"The fact that so many disease-related genes are part of detected CNVs indicates that CNVs are likely to be important new players both in causing human genetic disease and in influencing an individual's risk for genetic disease," said James Sikela of the human medical genetics program at the University of Colorado in Aurora, Colo.
"It can be expected that searches for the causes of genetic diseases will be taking into account the possibility that CNVs may be involved," Sikela said.
"It may be that a significant proportion of human diseases may be related to very small regions of our genomes in which we share close to identical sequence of our DNA, but for some reason, some of us carry in different numbers," said Dr. Julie Korenberg, of the molecular genetics division at Cedars-Sinai Medical Center in Los Angeles.
"One can expect that testing for CNVs will become an increasingly important component of genetic screening programs aimed at estimating disease risk," Korenberg said.
The study also points out that multiple copies or complete deletions in the code do not necessarily lead to known genetic diseases.
"We'd find missing pieces of DNA, some a million or so nucleotides long," according to a media release by lead researcher Stephen Scherer of the Howard Hughes Medical Institute in Chevy Chase, Md. "We used to think that if you had big changes like this, then they must be involved in disease. But we are showing that we can all have these changes."
Findings Have Implications Beyond Disease
The findings could also uncover some of the genes that determine why we are different from one another.
"It should be pointed out that CNVs can be expected to not only be important to disease but can also be expected to be potentially important to any genetically influenced difference between humans," Sikela said.
Even though the study highlights the differences that may exist between one person and the next, experts still say our genetic codes are overwhelmingly similar to one another.
"While there may be more common deletions than previously thought, human beings are still much more similar to each other than two oak trees or two dogs," Burmeister said.
"I think that it is an important way of thinking about the origins, not only of what we call disease, but also what we recognize as human variation," Korenberg said.
"We scientists find a new way in which the genome commits errors, and then we ask how these errors may account for where we came from, where we're going, and why we look and act like our families, including sharing a certain risk of their diseases," Korenberg said.