Molecular genetics and treatment of narcolepsy

Genetic Markers of Differential Vulnerability to Sleep Loss in Adults

In this review, we discuss reports of genotype-dependent interindividual differences in phenotypic neurobehavioral responses to total sleep deprivation or sleep restriction. We highlight the importance of using the candidate gene approach to further elucidate differential resilience and vulnerability to sleep deprivation in humans, although we acknowledge that other omics techniques and genome-wide association studies can also offer insights into biomarkers of such vulnerability. Specifically, we discuss polymorphisms in adenosinergic genes (ADA and ADORA2A), core circadian clock genes (BHLHE41/DEC2 and PER3), genes related to cognitive development and functioning (BDNF and COMT), dopaminergic genes (DRD2 and DAT), and immune and clearance genes (AQP4, DQB1*0602, and TNFα) as potential genetic indicators of differential vulnerability to deficits induced by sleep loss. Additionally, we review the efficacy of several countermeasures for the neurobehavioral impairments induced by sleep loss, including banking sleep, recovery sleep, caffeine, and naps. The discovery of reliable, novel genetic markers of differential vulnerability to sleep loss has critical implications for future research involving predictors, countermeasures, and treatments in the field of sleep and circadian science.

Treatment Options for Narcolepsy

Narcolepsy type 1 and narcolepsy type 2 are central disorders of hypersomnolence. Narcolepsy type 1 is characterized by excessive daytime sleepiness and cataplexy and is associated with hypocretin-1 deficiency. On the other hand, in narcolepsy type 2, cerebrospinal fluid hypocretin-1 levels are normal and cataplexy absent. Despite major advances in our understanding of narcolepsy mechanisms, its current management is only symptomatic. Treatment options may vary from a single drug that targets several symptoms, or multiple medications that each treats a specific symptom. In recent years, narcolepsy treatment has changed with the widespread use of modafinil/armodafinil for daytime sleepiness, antidepressants (selective serotonin and dual serotonin and noradrenalin reuptake inhibitors) for cataplexy, and sodium oxybate for both symptoms. Other psychostimulants can also be used, such as methylphenidate, pitolisant and rarely amphetamines, as third-line therapy. Importantly, clinically relevant subjective and objective measures of daytime sleepiness are required to monitor the treatment efficacy and to provide guidance on whether the treatment goals are met. Associated symptoms and comorbid conditions, such as hypnagogic/hypnopompic hallucinations, sleep paralysis, disturbed nighttime sleep, unpleasant dreams, REM- and non REM-related parasomnias, depressive symptoms, overweight/obesity, and obstructive sleep apnea, should also be taken into account and managed, if required. In the near future, the efficacy of new wake-promoting drugs, anticataplectic agents, hypocretin replacement therapy and immunotherapy at the early stages of the disease should also be evaluated.

Predicting Risk in Space: Genetic Markers for Differential Vulnerability to Sleep Restriction

Several laboratories have found large, highly reliable individual differences in the magnitude of cognitive performance, fatigue and sleepiness, and sleep homeostatic vulnerability to acute total sleep deprivation and to chronic sleep restriction in healthy adults. Such individual differences in neurobehavioral performance are also observed in space flight as a result of sleep loss. The reasons for these stable phenotypic differential vulnerabilities are unknown: such differences are not yet accounted for by demographic factors, IQ or sleep need, and moreover, psychometric scales do not predict those individuals cognitively vulnerable to sleep loss. The stable, trait-like (phenotypic) inter-individual differences observed in response to sleep loss-with intraclass correlation coefficients accounting for 58%-92% of the variance in neurobehavioral measures- point to an underlying genetic component. To this end, we utilized multi-day highly controlled laboratory studies to investigate the role of various common candidate gene variants-each independently-in relation to cumulative neurobehavioral and sleep homeostatic responses to sleep restriction. These data suggest that common genetic variations (polymorphisms) involved in sleep-wake, circadian, and cognitive regulation may serve as markers for prediction of inter-individual differences in sleep homeostatic and neurobehavioral vulnerability to sleep restriction in healthy adults. Identification of genetic predictors of differential vulnerability to sleep restriction-as determined from candidate gene studies-will help identify astronauts most in need of fatigue countermeasures in space flight and inform medical standards for obtaining adequate sleep in space. This review summarizes individual differences in neurobehavioral vulnerability to sleep deprivation and ongoing genetic efforts to identify markers of such differences.

Behavioral and genetic markers of sleepiness

Neurobehavioral responses to acute total and chronic partial sleep deprivation occur in healthy adults and are particularly evident in vigilant attention performance. There are large inter-individual differences in the degree of cognitive deficits--such differences are manifested in proportionality between the mean and variance as sleep loss progresses. It has recently been demonstrated via laboratory experiments that differential neurobehavioral vulnerability to sleep deprivation is not random--but rather is stable and trait-like--strongly suggesting a phenotypic response with possible genotypic involvement. These experiments also showed that vulnerability was not explained by subjects' baseline functioning or a number of other potential predictors. Differential vulnerability has been shown to extend to chronic partial sleep deprivation. One potential genetic biomarker for such differential vulnerability is the human leukocyte antigen (HLA) DQB1*0602, an allele which we recently demonstrated predicts interindividual differences in sleepiness, physiological sleep, and fatigue to chronic partial sleep deprivation in healthy adults. Determination of biomarkers of individual differences to sleep loss will help identify those individuals in the general population who are most in need of prevention of sleep debt and in need of effective countermeasures for sleep loss; will further understanding and management of vulnerability to excessive sleepiness due to common sleep and medical disorders; and will inform public policies pertaining to the need for adequate sleep.

DQB1*0602 predicts interindividual differences in physiologic sleep, sleepiness, and fatigue

OBJECTIVE

The human leukocyte antigen (HLA) DQB1*0602 allele is closely associated with narcolepsy, a neurologic disorder characterized by excessive daytime sleepiness, fragmented sleep, and shortened REM sleep latency. We evaluated whether DQB1*0602 was a novel marker of interindividual differences by determining its relationship to sleep homeostatic, sleepiness, and cognitive responses to baseline and chronic partial sleep deprivation (PSD) conditions.

METHODS

Ninety-two DQB1*0602-negative and 37 DQB1*0602-positive healthy adults participated in a protocol of 2 baseline 10 hours time in bed (TIB) nights followed by 5 consecutive 4 hours TIB nights. DQB1*0602 allelic frequencies did not differ significantly between Caucasians and African Americans.

RESULTS

During baseline, although DQB1*0602-positive subjects were subjectively sleepier and more fatigued, they showed greater sleep fragmentation, and decreased sleep homeostatic pressure and differentially sharper declines during the night (measured by non-REM EEG slow-wave energy [SWE]). During PSD, DQB1*0602-positive subjects were sleepier and showed more fragmented sleep, despite SWE elevation comparable to negative subjects. Moreover, they showed differentially greater REM sleep latency reductions and smaller stage 2 reductions, along with differentially greater increases in fatigue. Both groups demonstrated comparable cumulative decreases in cognitive performance.

CONCLUSIONS

DQB1*0602 positivity in a healthy population may represent a continuum of some sleep-wake features of narcolepsy. DQB1*0602 was associated with interindividual differences in sleep homeostasis, physiologic sleep, sleepiness, and fatigue-but not in cognitive measures-during baseline and chronic PSD. Thus, DQB1*0602 may represent a genetic biomarker for predicting such individual differences in basal and sleep loss conditions.

The complex etiology of multiple sclerosis

Multiple sclerosis is a demyelinating disease which is presumed to be a consequence of infiltrating lymphocytes autoreactive to myelin proteins. This is substantiated by several lines of clinical evidence and supported by correlative studies in preclinical models. The development of new therapeutics for MS has been guided by this perspective; however, the pathogenesis of MS has proven to be quite complex as observations exist which question the role of autoreactive lymphocytes in the etiology of MS. In addition the current immunomodulatory therapeutics do not prevent most patients from progressing into more serious forms of the disease. The development of truly transformational therapeutics for MS will likely require a broad assault that expands beyond the concept of MS being an autoimmune disease.

Nitric Oxide Signaling Triggered by the Rheumatoid Arthritis Shared Epitope: A New Paradigm for MHC Disease Association?

Many immune-mediated diseases are associated with particular MHC class I or class II alleles. In rheumatoid arthritis (RA-shared), the vast majority of patients possess HLA-DRB1 alleles encoding a shared epitope, which is a five-amino acid sequence motif in positions 70-74 of the HLA-DRbeta chain. The mechanistic basis for this association is unknown. Here we discuss recent evidence suggesting that the shared epitope may act as an allele-specific ligand that triggers increased nitric oxide (NO) production in opposite cells with resultant immune dysregulation. We propose that by doing that, the RA-shared shared epitope may form an unintended bridge between the innate and adaptive immune systems, thereby allowing aberrant signaling events that could trigger disease.

Understanding hereditary diseases using the dog and human as companion model systems

Animal models are requisite for genetic dissection of, and improved treatment regimens for, human hereditary diseases. While several animals have been used in academic and industrial research, the primary model for dissection of hereditary diseases has been the many strains of the laboratory mouse. However, given its greater (than the mouse) genetic similarity to the human, high number of naturally occurring hereditary diseases, unique population structure, and the availability of the complete genome sequence, the purebred dog has emerged as a powerful model for study of diseases. The major advantage the dog provides is that it is afflicted with approximately 450 hereditary diseases, about half of which have remarkable clinical similarities to corresponding diseases of the human. In addition, humankind has a strong desire to cure diseases of the dog so these two facts make the dog an ideal clinical and genetic model. This review highlights several of these shared hereditary diseases. Specifically, the canine models discussed herein have played important roles in identification of causative genes and/or have been utilized in novel therapeutic approaches of interest to the dog and human.

Antibodies in narcolepsy–cataplexy patient serum bind to rat hypocretin neurons

Autoimmunity is considered the most likely cause of human narcolepsy-cataplexy, but no specific autoantibodies or antigen(s) have yet been identified. By means of indirect avidin-biotin immunohistochemical method, we searched for antibodies in serum from narcolepsy-cataplexy patients and controls that bind to rat hypocretin neurons. No staining was found in eight out of nine narcolepsy-cataplexy patients or controls. The serum from one narcolepsy-cataplexy patient, however, strongly produced staining of the membrane and superficial cytoplasm of neurons in the lateral hypothalamus. Dual staining revealed that the vast majority of the hypocretin-positive neurons were positive, but nonhypocretin neurons in the same area were binding antibodies from the patient's serum. These results show that antibodies bind to specific hypocretin- and nonhypocretin-containing neurons in the hypothalamus and indicate the presence of autoantibodies in narcolepsy patients.