Narcolepsy may be rooted in the immune system: assault on brain cells appears to induce sleep disorder.
The victims of this cross fire are neurons that make a peptide called orexin, a neurotransmitter that is crucial for staying awake. The researchers say this could explain the lack of orexin neurons in people with narcolepsy, as shown previously in patient autopsies. A lack of orexin, also called hypocretin, leaves a person with disordered sleep, daytime drowsiness and the risk of nodding off abruptly--the hallmarks of narcolepsy.
The new findings, along with previous work, indicate that narcolepsy needs three conditions to occur. A person must harbor an immune system gene variation that shows up in about one-fourth of people but in 98 percent of narcolepsy patients. Second, the individual needs to encounter a pathogen in the environment that provokes an immune reaction against a foreign peptide that resembles orexin. Finally, the immune system must then destroy the brain's orexin-making neurons.
It remains unclear precisely how the immune system kills the neurons. But immunologist Elizabeth Mellins of Stanford says the new results point to narcolepsy as an autoimmune disease.
Evidence of an immune connection arose in 2009 when Stanford researchers reported that narcolepsy patients are more likely than other people to carry certain genetic variants related to immunity in addition to the well-known, common variant. Now the team has discovered an environmental trigger to go with these genetic hints. Writing in the Dec. 18 Science Translational Medicine, the Stanford researchers show that a peptide on the H1N1 flu virus that caused a pandemic in 2009 can generate an immune reaction that hits orexin neurons.
Mellins and others had noticed a spate of narcolepsy in China in 2010 in the wake of the H1N1 flu pandemic. There were also reports of narcolepsy in Northern Europe tied to a vaccine called Pandemrix that contained protein fragments from the H1N1 flu virus. About al million Europeans got the vaccine, and about 1 in 15,000 developed narcolepsy. That's far higher than narcolepsy's usual rate of less than 1 per 100,000 per year in Finland, where the link was first noted, says study coauthor Emmanuel Mignot, a Stanford sleep researcher. The vaccine, never given in the United States, is no longer in use, Mignot says.
Mignot, Mellins and colleagues screened HIN1 flu virus proteins and found a peptide in them that mimics the shape of orexin. They also tested in a lab dish immune cells called T cells from narcolepsy patients, most of whom hadn't gotten the vaccine, and found the cells reacted potently to orexin, whereas T cells from healthy people didn't. In another test, the researchers examined blood samples from four pairs of identical twins, each with one twin who had narcolepsy. These narcolepsy patients' T cells also reacted to orexin; the healthy twins' T cells didn't.
Something in the environment must have influenced the reactivity of T cells in the narcolepsy patients, says Mellins. "Most likely that explains why one twin was unlucky."
A separate test of blood from 17 Irish children who had gotten the Pandemfix vaccine showed that those who later developed narcolepsy had T cells that reacted strongly to orexin, while T cells from their vaccinated siblings who didn't develop narcolepsy were nonreactive.
"The T cell findings really provide one of the most robust clues we have that there is an autoimmune mechanism here," says sleep researcher Thomas Scammell, a Harvard neurologist. "Orexin neurons' death is like a murder mystery. We didn't know who did it or why it happened." In the new report, he says, the immune reaction sheds light on why.
There could be a range of triggers in the environment tripping this reaction, Mellins says. She surmises that, in addition to the flu peptide, streptococcus or other pathogens might play a role. The new findings also pose a parallel between narcolepsy and type 1 diabetes, an autoimmune disease in which a rogue immune attack kills pancreatic cells that make the hormone insulin. In type 1 diabetes, symptoms show up before all the cells are destroyed, a "honeymoon period" that scientists are targeting with experimental treatments, Mellins says. A similar approach might someday be applied in narcolepsy, she says, if the condition can be caught early.
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|Title Annotation:||BODY & BRAIN|
|Date:||Jan 25, 2014|
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