Tuesday, September 17, 2013

Mixtures of DNA in One Person

Carol Zimmer, DNA Double Take NYTimes (Sept. 12, 2013)


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

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Dr. James R. Lupski, a leading expert on the human genome at Baylor College of Medicine, wrote in a recent review in the journal Science that the existence of multiple genomes in an individual could have a tremendous impact on the practice of medicine. ...

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Science’s changing view is also raising questions about how forensic scientists should use DNA evidence to identify people. ...

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... 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. The scientists who studied her concluded thatshe had acquired some of her blood from her twin brother in the womb, including his genomes in his blood cells.

Chimerism, as such conditions came to be known, seemed for many years to be a rarity. But “it can be commoner than we realized,” said Dr. Linda Randolph, a pediatrician at Children’s Hospital in Los Angeles who is an author of a review of chimerism published in The American Journal of Medical Genetics in July.

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A century ago, geneticists discovered one way in which people might acquire new genomes. They were studying “mosaic animals,” rare creatures with oddly-colored patches of fur. The animals didn’t inherit the genes for these patches from their parents. Instead, while embryos, they acquired a mutation in a skin cell that divided to produce a colored patch.

Mosaicism, as this condition came to be known, was difficult to study in humans before the age of DNA sequencing. Scientists could only discover instances in which the mutations and the effects were big.

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The latest findings make it clear that mosaicism is quite common — even in healthy cells.

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Medical researchers aren’t the only scientists interested in our multitudes of personal genomes. So are forensic scientists. When they attempt to identify criminals or murder victims by matching DNA, they want to avoid being misled by the variety of genomes inside a single person.

Last year, for example, forensic scientists at the Washington State Patrol Crime Laboratory Division described how a saliva sample and a sperm sample from the same suspect in a sexual assault case didn’t match.

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While the risk of confusion is real, it is manageable, experts said. “This should not be much of a concern for forensics,” said Manfred Kayser, a professor of Forensic Molecular Biology at Erasmus University in Rotterdam. In the cases where mosaicism or chimerism causes confusion, forensic scientists can clear it up by other means. In the Austrian study, for example, the scientists found no marrow donor genomes in the hair of the recipients.

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