Psst, the human genome was never completely sequenced. Some scientists say it should be
By SHARON BEGLEY @sxbegle JUNE 20, 2017
The feat made headlines around the world: “Scientists Say Human Genome is Complete,” the New York Times announced in 2003. “The Human Genome,” the journals Science and Nature said in identical ta -dah cover lines unveiling the historic achievement.
There was one little problem.
“As a matter of truth in advertising, the ‘finished’ sequence isn’t finished,” said Eric Lander, who led the lab at the Whitehead Institute that deciphered more of the genome for the government-funded Human Genome Project than any other. “I always say ‘finished’ is a term of art.”
“It’s very fair to say the human genome was never fully sequenced,” Craig Venter, another genomics luminary, told STAT.
“The human genome has not been completely sequenced and neither has any other mammalian genome as far as I’m aware,” said Harvard Medical School bioengineer George Church, who made key early advances in sequencing technology.
What insiders know, however, is not well-understood by the rest of us, who take for granted that each A, T, C, and G that makes up the DNA of all 23 pairs of human chromosomes has been completely worked out. When scientists finished the first draft of the human genome, in 2001, and again when they had the final version in 2003, no one lied, exactly. FAQs from the National Institutes of Health refer to the sequence’s “essential completion,” and to the question, “Is the human genome completely sequenced?” they answer, “Yes,” with the caveat — that it’s “as complete as it can be” given available technology .
Perhaps nobody paid much attention because the missing sequences didn’t seem to matter. But now it appears they may play a role in conditions such as cancer and autism.
“A lot of people in the 1980s and 1990s [when the Human Genome Project was getting started] thought of these regions as nonfunctional,” said Karen Miga, a molecular biologist at the University of California, Santa Cruz. “But that’s no longer the case.” Some of them, called satellite regions, misbehave in some forms of cancer, she said, “so something is going on in these regions that’s important.”
Church, too, has been making that point, mentioning it at both the May meeting of an effort to synthesize genomes, and at last weekend’s meeting of the International Society for Stem Cell Research. Most of the unsequenced regions, he said, “have some connection to aging and aneuploidy ” (an abnormal number of chromosomes such as what occurs in Down syndrome). Church estimates 4 percent to 9 percent of the human genome hasn’t been sequenced. Miga thinks it’s 8 percent.
The reason for these gaps is that DNA sequencing machines don’t read genomes like humans read books, from the first word to the last. Instead, they first randomly chop up copies of the 23 pairs of chromosomes, which total some 3 billion “letters,” so the machines aren’t overwhelmed. The resulting chunks contain from 1,000 letters (during the Human Genome Project) to a few hundred (in today’s more advanced sequencing machines). The chunks overlap. Computers match up the overlaps, assembling the chunks into the correct sequence.
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Read more here/Leia mais aqui : STAT
