The sleep disorder narcolepsy is widely thought to be a chronic and incurable condition, one caused by an irreversible autoimmune assault on a cluster of very important neurons in the centre of the brain. But what if those neurons have not been destroyed after all but have just gone quiet? “Our results suggest that hypocretin neurons are alive…, opening the possibility to reverse or cure narcolepsy,” report the authors of a new study. For people with narcolepsy like me, there can be few more exciting sentences. So what have these scientists found and how upbeat should I actually be?
First, a bit of background. Almost 25 years ago, researchers identified a very important signalling molecule in the brain, a neuropeptide they called hypocretin (independently named orexin by another group). Some clever work on Doberman dogs at Stanford University revealed that the cause of narcolepsy was a defect in hypocretin signalling. In a follow-up paper, published in 2000, researchers used stains to look for hypocretin in the post-mortem brain of a person with narcolepsy. In the hypothalamus, where the hypocretin cells should be, there is nothing. Nada. When I look at this stark image, I feel about as empty as the narcoleptic specimen appears. Subsequent work confirms that narcolepsy is almost always characterised by little or no hypocretin in the brain.
What if my hypocretin neurons have not been destroyed after all, but have just gone quiet?
There are two alternative hypotheses to account for the lack of hypocretin in the brains of people with narcolepsy: either the hypocretin cells are present but just aren’t making any hypocretin; or the hypocretin cells have been completely and irreversibly destroyed. In wrapping up their brain-staining paper, its authors lumped for the second hypothesis, concluding that their findings suggested “a loss of neurons that may be irreversible once the disease is fully established.” This is the narrative that I – and most people with narcolepsy – have gone along with. But it turns out that our pessimism may have been misplaced.
In September, I booted up my computer in the comfort of my London home to join the European Narcolepsy Network’s annual meeting being held in Berlin. From the programme of talks, there was one in particular that jumped out at me, a talk by Ali Seifinejad entitled “Hypocretin neurons in narcolepsy: dead or silenced?” The science was pretty tough to follow, but I had a hunch that I was listening to something important. I have now had the opportunity to scrutinise the paper that Seifinejad and his colleagues have prepared for publication.
Over the last 20 years, scientists have used narcoleptic mice to further our understanding of the condition. There are two main ways to make a narcoleptic mouse: route A involves leaving the hypocretin cells intact but “knocking out” their ability to make the hypocretin protein; route B involves destroying the hypocretin cells completely. In both these approaches, the mice have no hypocretin and show the pathological sleepiness typical of narcolepsy. Interestingly, however, the two different mouse models have very different levels of QRFP, another neuropeptide discovered a few years after hypocretin. In route A mice, the levels of QRFP are completely normal, whereas in route B mice the levels of QRFP are dramatically reduced. The most likely explanation for this observation, confirmed by careful staining of post-mortem tissue, is that the hypocretin cells are responsible for the synthesis of much of the QRFP found in the brain.
What this means is that studying QRFP levels in the brains of people with narcolepsy should allow us to distinguish between the two alternative hypotheses that can explain why there is little or no hypocretin in the narcoleptic brain. If people with narcolepsy still have hypocretin cells but they just don’t make hypocretin any more, they should have normal levels of QRFP. If all their hypocretin cells have vanished as most people have assumed, there should be unusually low levels of QRFP.
“Strikingly,” report Seifinejad and co., “QRFP expression was not lost in [the] patients’ hypothalamus.” In fact, they found more QRFP than expected in the brains of people with narcolepsy. Given that hypocretin and QRFP are rather similar in structure and possibly also function, the increase in QRFP in narcolepsy might be the brain’s way of trying to compensate for the loss of hypocretin. “Altogether, these results suggest that hypocretin neurons might be present in narcolepsy brains but the [hypocretin] gene is inactive.”
The researchers do not stop there, but go on to investigate whether and how the hypocretin gene may be silenced in narcolepsy. All genes have promoters, a short, specific sequence of DNA that is used as a docking site for the enzyme that transcribes DNA into RNA. If there’s a problem with a promoter, the cell is going to find it difficult to make that particular RNA and without RNA there will be no protein. One way to mess with a promoter is to stick molecules onto it, a so-called “epigenetic” change that affects the way the DNA is transcribed. For people with narcolepsy, there appears to be a high level of methylation at the hypocretin promoter, exactly the kind of clingy molecules that could prevent the expression of the hypocretin gene.
The paper goes on to propose a mechanism through which an immune response to a virus could result in the methylation of the hypocretin promoter in narcolepsy with cataplexy. “Partial or transient epigenetic modulation of hypocretin expression might also be involved in narcolepsy without cataplexy, idiopathic hypersomnia or Kleine-Levin syndrome,” the authors write. And here is the question you’ve all been wondering: If the hypocretin cells are present but the hypocretin gene is just silenced, is there a way to unsilence them? Is there a way to reverse methylation?
On this, Seifinejad and colleagues are quite clear. “Methylation being reversible, our findings open the avenue for therapeutic interventions based on DNA demethylation, leading to treatment or even cure of narcolepsy.” This is not something that is going to emerge overnight or even in the next five years, but the thought that people with narcolepsy might not have lost their hypocretin cells after all and could, one day, have them reawakened is exciting beyond words.
Seifinejad, A. et al. (in press) Narcolepsy with cataplexy is caused by epigenetic silencing of hypocretin neurons