Narcolepsy and familial advanced sleep-phase syndrome: molecular genetics of sleep disorders

Health risks associated with genetic alterations in internal clock system by external factors

The circadian system maintains the main aspects of physiological and behavioral processes. Both circadian rhythm and sleep impact an organism's health through interaction with environmental factors. Variations in external factors influence the functionality of clock genes and disrupt 24-hour rhythmic cycle. The disrupted circadian rhythm and disregulated sleep affect an organism's health, thereby causing several disorders including cancer, depression and cardiac disorders. Considering the role of clock genes and environmental factors, extensive investigation is required focusing on pathways involved in development of life-threatening disorders. This review identifies the major risks and associated factors related with disruption in circadian system and sleep.

Genetic insights on sleep schedules: this time, it's PERsonal

The study of circadian rhythms is emerging as a fruitful opportunity for understanding cellular mechanisms that govern human physiology and behavior, fueled by evidence directly linking sleep disorders to genetic mutations affecting circadian molecular pathways. Familial advanced sleep-phase disorder (FASPD) is the first recognized Mendelian circadian rhythm trait, and affected individuals exhibit exceptionally early sleep-wake onset due to altered post-translational regulation of period homolog 2 (PER2). Behavioral and cellular circadian rhythms are analogously affected because the circadian period length of behavior is reduced in the absence of environmental time cues, and cycle duration of the molecular clock is likewise shortened. In light of these findings, we review the PER2 dynamics in the context of circadian regulation to reveal the mechanism of sleep-schedule modulation. Understanding PER2 regulation and functionality may shed new light on how our genetic composition can influence our sleep-wake behaviors.

Genome-wide association study identifies new HLA class II haplotypes strongly protective against narcolepsy

Narcolepsy is a rare sleep disorder with the strongest human leukocyte antigen (HLA) association ever reported. Since the associated HLA-DRB1*1501-DQB1*0602 haplotype is common in the general population (15-25%), it has been suggested that it is almost necessary but not sufficient for developing narcolepsy. To further define the genetic basis of narcolepsy risk, we performed a genome-wide association study (GWAS) in 562 European individuals with narcolepsy (cases) and 702 ethnically matched controls, with independent replication in 370 cases and 495 controls, all heterozygous for DRB1*1501-DQB1*0602. We found association with a protective variant near HLA-DQA2 (rs2858884; P < 3 x 10(-8)). Further analysis revealed that rs2858884 is strongly linked to DRB1*03-DQB1*02 (P < 4 x 10(-43)) and DRB1*1301-DQB1*0603 (P < 3 x 10(-7)). Cases almost never carried a trans DRB1*1301-DQB1*0603 haplotype (odds ratio = 0.02; P < 6 x 10(-14)). This unexpected protective HLA haplotype suggests a virtually causal involvement of the HLA region in narcolepsy susceptibility.

Chronobiological disorders: current and prevalent conditions

In recent decades, the hectic lifestyle of industrialized societies has wrought its effects on the quality of sleep, and these effects are evidenced by a profusion of sleep-related disorders. Regular exposure to artificial light, coupled with social and economic pressures have shortened the time spent asleep. Otherwise, Circadian Rhythm Sleep Disorders are characterized by desynchronization between the intrinsic circadian clock and the extrinsic cycles of light/dark and social activities. This desynchronization produces excessive sleepiness and insomnia. The International Classification of Sleep Disorders describes nine sleep disorders under the category of Circadian Rhythm Sleep Disorders. Currently, this diagnosis is made based on the patient's history, a sleep log alone, or the sleep logs and actigraphy conducted for at least 7 days. This review contains an overview of current treatment options, including chronotherapy, timed bright light exposure, and administration of exogenous melatonin.

Quantitative genetics of sleep in inbred mice

The timing and the organization of sleep architecture are mainly controlled by the circadian system, while sleep need and intensity are regulated by a homeostatic process. How independent these two systems are in regulating sleep is not well understood. In contrast to the impressive progress in the molecular genetics of circadian rhythms, little is known about the molecular basis of sleep. Nevertheless, as summarized here, phenotypic dissection of sleep into its most basic aspects can be used to identify both the single major genes and small effect quantitative trait loci involved. Although experimental models such as the mouse are more readily amenable to genetic analysis of sleep, similar approaches can be applied to humans.

Molecular analysis of sleep

Rest or sleep in all animal species constitutes a period of quiescence necessary for recovery from activity. Whether rest and activity observed in all organisms share a similar fundamental molecular basis with sleep and wakefulness in mammals has not yet been established. In addition and in contrast to the circadian system, strong evidence that sleep is regulated at the transcriptional level is lacking. Nevertheless, several studies indicate that single genesmay regulate some specific aspects of sleep. Efforts to better understand or confirm the role of known neurotransmission pathways in sleep-wake regulation using transgenic approaches resulted so far in only limited new insights. Recent gene expression profiling efforts in rats, mice, and fruit flies are promising and suggest that only a few gene categories are differentially regulated by behavioral state. How molecular analysis can help us to understand sleep is the focus of this chapter.