Scientists finish genetic map of chimpanzee

Work eventually may lead to better understanding of diseases, evolution

September 01, 2005|By Dennis O'Brien | Dennis O'Brien,SUN STAFF

Researchers have sequenced the genome of mankind's closest living relative, creating a molecular map of the chimpanzee that could hold keys to understanding what makes us human.

The map shows that chimpanzees share at least 96 percent of our genetic makeup - and by some accounts 99 percent. The findings also could help establish genetic links to a variety of human diseases and unravel some of the mysteries of human evolution.

Researchers hope that by comparing chimp genes with our own, they can decipher which genes are linked to capabilities that separate us from the apes, such as cognition, speech and brain development.

They also hope the information will help them find out why chimps are immune to certain diseases, such as malaria, HIV infection and Alzheimer's, that bedevil humans.

"I see this as a major event," said Dr. Francis Collins, director of the National Human Genome Research Institute, which funded the project. Several reports based on its findings appeared today in the journal Nature and in the online edition of Science.

The effort took three years, cost up to $30 million and involved 67 researchers in the U.S., Spain, Israel and Germany. Most of the sequencing was conducted at Washington University School of Medicine in St. Louis and the Broad Institute of the Massachusetts Institute of Technology and Harvard.

The researchers analyzed DNA in blood drawn from a West African chimp named Clint, born at the Yerkes National Primate Research Center in Atlanta. Teams of international researchers helped analyze the chimp genome, and results were released periodically during the project. About 90 percent of Clint's genome was released in the journal Science in 2003.

Clint died of heart failure last year at the relatively young age of 24. A New Jersey research lab is preserving his blood and tissue cells.

Experts say analysis of Clint's genes should help pinpoint the date when humans and chimps diverged from a common ancestor, believed to be 6 million years ago. The data should also make it easier to track the evolutionary paths both species have taken since that fork in the road.

"I think it's of broad scientific interest because it gets to the issue of what makes us human," said Thomas J. White, who was not involved in the study but has analyzed human and chimp genes as chief scientist at Celera Diagnostics.

Scientists liken DNA to a computer program, written in the form of long chains of chemicals called "bases" that are referred to by their initials: A, T, C and G. Different strings of these letters form different genes.

Of the 3 billion base pairs found in the chimp and human genomes, both have experienced 40 million "evolutionary events" that have changed the order of the letters in some way, said Robert Waterston, a senior author and chairman of genetics at the University of Washington medical school.

The differences range from the most minuscule changes in base pairs to changes in large-scale segments of the DNA, he said.

"This is an extremely detailed view of the evolutionary events that make us human," Waterston said. The types of genes that changed most rapidly in both humans and chimps are those linked to immune responses, researchers said.

Genes linked to the inflammation that humans produce in response to injuries also appear to be missing in the chimps. But humans have lost the function of a gene that helps protect chimps and other animals from Alzheimer's disease.

In the short term, sequencing the chimpanzee genome is more likely to shed light on when and how humans evolved, experts say. Medical therapies based on the research are likely to be years if not decades away.

A major obstacle is that roughly 90 percent of all diseases are rooted in more than one gene, said Akhilesh Pandey, a molecular biologist and biochemist at the Johns Hopkins University School of Medicine.

"It's almost like saying: Why did a person grow tall? It's not just one reason. There are many reasons," Pandey said.

Even when genes that cause specific diseases are identified, developing drugs to alter those genetic functions will be difficult.

The findings add support to the theory of evolution spelled out by Charles Darwin in 1859, researchers say. "I couldn't imagine Darwin hoping for a stronger confirmation of his ideas," Waterston said.

Despite the similarities between chimps and humans, there is 10 times more genetic difference between the species than between any two humans.

Researchers who study aging, genetics and human evolution will likely use the findings to detect genes related to characteristics that separate us from chimps: the ability to walk upright and an enlarged brain that gives us language skills and abstract thoughts.

In recent years, researchers have sequenced the genome of a dog, a cat, a cow, mice and rats. U.S. and British researchers also are working on an orangutan, a rhesus monkey and a gorilla.

But because the chimp is our closest genetic relative, some experts say that sequencing its genome is a milestone comparable to the sequencing of human DNA that was completed four years ago.

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