One Person, but Multiple Genomes ; Scientists' Discoveries of Many Gene Sequences Are Changing Medicine

By Zimmer, Carl | International Herald Tribune, September 18, 2013 | Go to article overview

One Person, but Multiple Genomes ; Scientists' Discoveries of Many Gene Sequences Are Changing Medicine


Zimmer, Carl, International Herald Tribune


Scientists' discoveries of multiple gene sequences within a single individual is changing the practice of medicine.

From biology class to the television show "CSI," we are told again and again that our genome is at the heart of our identity. Read the sequences in the chromosomes of a single cell, and learn everything about a person's genetic information -- or, as 23andme, a prominent genetic testing company, says on its Web site, "The more you know about your DNA, the more you know about yourself."

But scientists are discovering that -- to a surprising degree -- we contain genetic multitudes. Not long ago, researchers thought it was rare for the cells in a single healthy person to differ genetically. But researchers are finding that it's quite common for a person to have multiple genomes. Some people have groups of cells with mutations that are not found in the rest of the body, for example. Some have genomes that came from other people.

"There have been whispers in the matrix about this for years, even decades, but only in a very hypothetical sense," said Alexander Urban, a geneticist at Stanford University. Even three years ago, suggesting that there was widespread genetic variation in a single body would have been met with skepticism, he said. "You would have just run against the wall."

A series of recent papers by Dr. Urban and others has demonstrated that those whispers were not just hypothetical. The variation in the genomes found in a single person are too large to be ignored. "We now know it's there," Dr. Urban said. "Now we're mapping this new continent."

James Lupski, a leading expert on the human genome at Baylor College of Medicine, wrote in a recent review in Science that the existence of multiple genomes could have a tremendous impact on the practice of medicine. "It's changed the way I think," he said in an interview.

Already, scientists have found links to some rare diseases within a person's varied genetic material, and now they're beginning to investigate genetic variations to shed light on more common disorders.

Science's changing view is also raising questions about how forensic scientists and genetic counselors should use DNA.

When a human egg and sperm combine their DNA, the genome they produce contains all the necessary information for building a new human. As the egg divides to form an embryo, it produces new copies of that original genome.

For decades, geneticists have explored how an embryo can use the instructions in a single genome to develop muscles, nerves and the many other parts of the human body. They also use sequencing to understand genetic variations that can raise the risk of various diseases. Genetic counselors can look at the results of genetic screenings to help patients and their families cope with these diseases -- altering their diet, for example, if they lack a gene for a crucial enzyme.

The cost of sequencing an entire genome has fallen so drastically in 20 years -- just a few thousand dollars, down from an estimated $3 billion for the public-private partnership that sequenced the first human genome -- that doctors are beginning to sequence the entire genomes of some patients. (Sequencing can be done in as little as 50 hours.) They're identifying links between mutations and diseases that have never been seen before.

Yet all these powerful tests are based on the assumption that, inside our body, a genome is a genome is a genome. Scientists believed they could look at the genome from cells taken in a cheek swab and learn about the genomes of cells in the brain or the liver or anywhere else in the body.

In the mid-1900s, scientists began to get clues that this was not always true. In 1953, for example, a British woman donated a pint of blood. It turned out that some of her blood was Type O and some was Type A. …

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