Epigenetic silencing of selected hypothalamic neuropeptides in narcolepsy with cataplexy

Abstract

Narcolepsy with cataplexy is a sleep disorder caused by deficiency in the hypothalamic neuropeptide hypocretin/orexin (HCRT), unanimously believed to result from autoimmune destruction of hypocretin-producing neurons. HCRT deficiency can also occur in secondary forms of narcolepsy and be only temporary, suggesting it can occur without irreversible neuronal loss. The recent discovery that narcolepsy patients also show loss of hypothalamic (corticotropin-releasing hormone) CRH-producing neurons suggests that other mechanisms than cell-specific autoimmune attack, are involved. Here, we identify the HCRT cell-colocalized neuropeptide QRFP as the best marker of HCRT neurons. We show that if HCRT neurons are ablated in mice, in addition to <i>Hcrt,</i> <i>Qrfp</i> transcript is also lost in the lateral hypothalamus, while in mice where only the <i>Hcrt</i> gene is inactivated <i>Qrfp</i> is unchanged. Similarly, postmortem hypothalamic tissues of narcolepsy patients show preserved <i>QRFP</i> expression, suggesting the neurons are present but fail to actively produce HCRT. We show that the promoter of the <i>HCRT</i> gene of patients exhibits hypermethylation at a methylation-sensitive and evolutionary-conserved PAX5:ETS1 transcription factor-binding site, suggesting the gene is subject to transcriptional silencing. We show also that in addition to HCRT, <i>CRH</i> and Dynorphin (<i>PDYN</i>) gene promoters, exhibit hypermethylation in the hypothalamus of patients. Altogether, we propose that <i>HCRT</i>, <i>PDYN</i>, and <i>CRH</i> are epigenetically silenced by a hypothalamic assault (inflammation) in narcolepsy patients, without concurrent cell death. Since methylation is reversible, our findings open the prospect of reversing or curing narcolepsy.