We contain Genetic multitudes: Evidence for large diversity in the human transcriptome 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 “The more you know about your DNA, the more you know about yourself.” 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. 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. “It’s changed the way I think,” he said in an interview. Scientists are finding links from multiple genomes to certain rare diseases, 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 should use DNA evidence to identify people. It’s also posing challenges for genetic counselors, who can’t assume that the genetic information from one cell can tell them about the DNA throughout a person’s body. Human Blueprint When an 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 certain 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 the past 20 years — now 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.) And they’re identifying links between mutations and diseases that have never been seen before. Scientists believed that they could look at the genome from cells taken in a cheek swab and be able to 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. The scientists who studied her concluded that she 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. Twins can end up with a mixed supply of blood when they get nutrients in the womb through the same set of blood vessels. In other cases, two fertilized eggs may fuse together. These so-called embryonic chimeras may go through life blissfully unaware of their origins. One woman discovered she was a chimera as late as age 52. In need of a kidney transplant, she was tested so that she might find a match. The results indicated that she was not the mother of two of her three biological children. It turned out that she had originated from two genomes. One genome gave rise to her blood and some of her eggs; other eggs carried a separate genome. After a baby is born, it may leave some fetal cells behind in its mother’s body, where they can travel to different organs and be absorbed into those tissues. “It’s pretty likely that any woman who has been pregnant is a chimera,” Dr. Randolph said. The genomes of your kids may be driving cells all over your body. In a study that analyzed the cells of women’s brains, says Zimmer, scientists “found neurons with Y chromosomes in 63 percent of them. The neurons likely developed from cells originating in their sons. ” But more than just collecting strange DNA from fetal cells, other people’s genomes can be picked up and incorporated into your own body. Organ transplant recipients can, in some sense, become their donor, with some of their cells carrying their donor’s DNA. Some people, says Zimmer, are even born with multiple genetic identities: “two fertilized eggs may fuse together” making one person with two genomes.
Posted on: Fri, 20 Sep 2013 14:46:56 +0000
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