But news accounts mostly ignored the flip side of the experiment: Being tethered to an old mouse made young mice age faster. One substance in the blood of old mice, a protein called Beta?2?microglobulin,
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, or B2M, seemed to prematurely age the young ones,
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, Villeda and colleagues reported last year in Nature Medicine (
SN: 8/8/15, p. 10). Parabiosis experiments don’t last very long, so no one knows whether youth or decrepitude will win in the end?—?or if the two mice would have settled into middle age together.
UCLA’s Horvath has evidence that the mice may never totally sync. He monitors aging by examining molecular tags called methyl groups, which attach to various locations on DNA in a process called methylation. Methylation is an epigenetic modification of DNA. Such modifications work something like flagging passages in a book with sticky notes. Attaching a tag doesn’t change the information in the book — it just draws attention to some passages and signals that others should be ignored.
Horvath measures DNA methylation changes at 353 different spots in the human genetic instruction book, or genome. As people age,
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, 193 locations accumulate tags,
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, like playbills plastered on urban buildings. At 160 others,
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, methylation is gradually stripped away with age. Knowing how much methylation is normally found at each spot at a given chronological age allows Horvath to
calculate biological age,
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.
Taking their time
Semi-supercentenarians (people who live to be 105 to 109,
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, yellow) tend to be biologically younger than their chronological age. So do their children (teal) compared with other people (gray). The rate of aging of the semi-supercentenarians and their children (teal line) is slower than normal (black line).
Source: S,
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. Horvath et al/Aging 2015
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