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Fighting sleep

作者:仓杯    发布时间:2019-03-07 09:12:13    

By Jonathan Knight in San Francisco A BODY clock that confuses day and night causes some symptoms of a rare genetic defect, scientists say. Their discovery marks the first time circadian rhythm genes have been implicated in a behavioural disorder. One child in 25 000 is born with Smith-Magenis syndrome, a congenital disease that causes severe learning problems, hyperactivity and unusual sleep patterns. These children are missing a chunk of one of their two copies of chromosome 17, but no one knows which genes are lost. Paediatrician Helene De Leersnyder and her colleagues at the Necker Hospital in Paris tried to learn more by following the daily activity of 20 children with Smith-Magenis syndrome between the ages of 4 and 17. Each child wore a wrist monitor that tracked motion, and parents were asked to keep a detailed record of when the child napped, went to bed and woke up. The pattern that emerged resembled permanent jet lag. Despite several naps during the day, all the children went to sleep by 9 pm and were up before dawn the next morning. By early evening, most could barely keep their eyes open. “They sometimes fell asleep during dinner with food in their mouth,” says De Leersnyder. The researchers studied eight of the children more closely for 24 hours, monitoring their levels of melatonin, thought to be the body’s primary sleep-inducing hormone. Melatonin levels normally rise from 9 pm, peaking at midnight then tapering off until dawn. But the children’s melatonin levels rose during the morning to peak at noon. “The child struggles against sleep all day,” says De Leersnyder. “I believe the hyperactivity comes from the fight against sleep.” The researchers hoped to find out which body clock gene had been deleted. They suspected it might be per1, which lurks somewhere around the part of chromosome 17 that is missing in people with Smith-Magenis. The activity of this gene has a daily cycle within the brain’s master clock, the suprachiasmatic nucleus (SCN), which responds to environmental light cues from the retina and helps control the pineal gland, the brain structure that makes melatonin. The researchers found that, in fact, per1 lies just outside the deleted part of the chromosome 17, not within it. But Mike Young, a circadian rhythm researcher at Rockefeller University in New York, says it might still be involved. A nearby deletion could change the way per1 is expressed and disrupt its cycle, he says. That could profoundly alter circadian rhythms. “Misexpression can really mess things up,” he says. De Leersnyder adds that other symptoms of Smith-Magenis syndrome, such as severe learning difficulties, are probably not linked. They may be caused by the deletion of other genes that are unrelated to the body clock. More on these topics:

 

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