* GSK's Pandemrix shot linked to spike in narcolepsy cases
* Incurable disorder causes sleep attacks and nightmares
* Study finds link in flu protein found in H1N1 flu strain By Kate Kelland, Health and Science Correspondent
LONDON, July 1 (Reuters) - Scientists investigating why a GlaxoSmithKline flu vaccine triggered narcolepsy in some people say they have the first solid evidence the rare sleep disorder may be a so-called "hit-and-run" autoimmune disease.
The researchers were trying to find out why, of two different flu vaccines widely deployed during the 2009/2010 swine flu pandemic, only one -- GSK's Pandemrix -- was linked with a spike in cases of narcolepsy.
In a study published in the journal Science Translational Medicine, they said the answer could lie in a protein in the H1N1 flu strain found in high amounts in the GSK shot but at much lower levels in the other vaccine, Novartis' Focetria.
"It was a really exciting moment," Lawrence Steinman, a professor of neurology and neurological sciences who led the work at Stanford University, said of the finding.
A spokeswoman for GSK, whose Pandemrix vaccine was withdrawn from the market after the 2009/2010 pandemic, said the company was aware of the study and would "review it carefully".
"We are actively conducting research into the observed association between Pandemrix and narcolepsy and the interaction this vaccine might have had with other risk factors in those affected," she said in an emailed comment.
Narcolepsy is an incurable, lifelong brain disorder that disrupts normal sleep-wake cycles and causes severe nightmares and daytime sleep attacks that can strike at any time.
Scientists are not sure exactly what causes it, but the latest research suggests it is a type of auto-immune disease, where the immune system misfires and mistakenly attacks the body's own functions and organs.
Studies in countries where GSK's Pandemrix vaccine was used in the 2009/2010 flu pandemic -- including Britain, Finland, Sweden and Ireland -- have found a significant rise in cases of narcolepsy in children.
Narcolepsy patients have been shown to have a loss of function in "wakefulness" cells called hypocretin cells in one of the brain's sleep centres.
In their study, Steinman's team found that H1N1 pandemic flu contains a protein whose structure partly mimics a portion of a hypocretin receptor in the brain. This H1N1 protein was contained in both vaccines studied, but at much higher levels in GSK's Pandemrix than in Novartis' Focetria.
The scientists said they now believe the narcolepsy in people vaccinated with Pandemrix was caused by a so-called "hit and run" mechanism, in which high levels of the H1N1 protein stimulated the production of large amounts of antibodies to both the virus and the hypocretin receptor. (Editing by Larry King)