Quantitative trait locus analysis of abnormal circadian period in CS mice

Genetic variation of clock genes and cancer risk: a field synopsis and meta-analysis

BACKGROUND

The number of studies on the association between clock genes’ polymorphisms and cancer susceptibility has increased over the last years but the results are often conflicting and no comprehensive overview and quantitative summary of the evidence in this field is available.

RESULTS

Literature search identified 27 eligible studies comprising 96756 subjects (cases: 38231) and investigating 687 polymorphisms involving 14 clock genes. Overall, 1025 primary and subgroup meta-analyses on 366 gene variants were performed. Study distribution by tumor was as follows: breast cancer (n=15), prostate cancer (n=3), pancreatic cancer (n=2), non-Hodgkin's lymphoma (n=2), glioma (n=1), chronic lymphocytic leukemia (n=1), colorectal cancer (n=1), non-small cell lung cancer (n=1) and ovarian cancer (n=1).

We identified 10 single nucleotide polymorphisms (SNPs) significantly associated with cancer risk: NPAS2 rs10165970 (mixed and breast cancer shiftworkers), rs895520 (mixed), rs17024869 (breast) and rs7581886 (breast); CLOCK rs3749474 (breast) and rs11943456 (breast); RORA rs7164773 (breast and breast cancer postmenopausal), rs10519097 (breast); RORB rs7867494 (breast cancer postmenopausal), PER3 rs1012477 (breast cancer subgroups) and assessed the level of quality evidence to be intermediate. We also identified polymorphisms with lower quality statistically significant associations (n=30).

CONCLUSIONS

Our work supports the hypothesis that genetic variation of clock genes might affect cancer risk. These findings also highlight the need for more efforts in this research field in order to fully establish the contribution of clock gene variants to the risk of developing cancer.

METHODS

We conducted a systematic review and meta-analysis of the evidence on the association between clock genes’ germline variants and the risk of developing cancer. To assess result credibility, summary evidence was graded according to the Venice criteria and false positive report probability (FPRP) was calculated to further validate result noteworthiness. Subgroup meta-analysis was also performed based on participant features and tumor type. The breast cancer subgroup was further stratified by work conditions, estrogen receptor/progesterone receptor status and menopausal status, conditions associated with the risk of breast cancer in different studies.

Light and the laboratory mouse

Light exerts widespread effects on physiology and behaviour. As well as the widely-appreciated role of light in vision, light also plays a critical role in many non-visual responses, including regulating circadian rhythms, sleep, pupil constriction, heart rate, hormone release and learning and memory. In mammals, responses to light are all mediated via retinal photoreceptors, including the classical rods and cones involved in vision as well as the recently identified melanopsin-expressing photoreceptive retinal ganglion cells (pRGCs). Understanding the effects of light on the laboratory mouse therefore depends upon an appreciation of the physiology of these retinal photoreceptors, including their differing sens itivities to absolute light levels and wavelengths. The signals from these photoreceptors are often integrated, with different responses involving distinct retinal projections, making generalisations challenging. Furthermore, many commonly used laboratory mouse strains carry mutations that affect visual or non-visual physiology, ranging from inherited retinal degeneration to genetic differences in sleep and circadian rhythms. Here we provide an overview of the visual and non-visual systems before discussing practical considerations for the use of light for researchers and animal facility staff working with laboratory mice.

New Quantitative Trait Loci for the Genetic Variance in Circadian Period of Locomotor Activity between Inbred Strains of Mice

Provisional quantitativetrait loci (QTL) for circadian locomotor period and wheel-running period have been identified in recombinant inbred (RI) mouse strains. To confirm thoseQTLand identify newones, the geneticcomponent of variance of the circadian period was partitionedamongan F2 intercross of RI mouse strains (BXD19 and CXB07). First, a genomic survey using 108 SSLP markers with an average spacing of 15 cM was carried out in a population of 259 (BXD19 · CXB07)F2 animals. The genome-wide survey identified two significant QTLfor period of locomotor activity measured by infrared photobeam crossings on mousechromosomes 1 (lod score5.66) and 14 (lod score4.33). TheQTL on distal chromosome 1 confirmed a previous report based on congenic B6.D2-Mtv7a/ Ty mice. Lod scores greater than 2.0 were found on chromosomes 1, 2, 6, 12, 13, and 14. In a targeted extension study, additional genotyping was performed on these chromosomes in the full sample of 341 F2 progeny. The 6 chromosome-wide surveys identified 3 additional QTL on mouse chromosomes 6, 12, and 13. The QTLon chromosome 12 overlaps with circadian period QTLidentified in several prior studies. For wheel-running period, the chromosome-wide surveys identified QTLon chromosomes 2 and 13 and one highly suggestive QTLon proximal chromosome 1. The results are compared to other published studies of QTL of circadian period.

Clean Thoughts about Dirty Genes

The mouse has joined the fruit fly, bread mold, and cyanobacteria as a tractable genetic system for studying mechanisms of circadian rhythms. The circadian rhythms of “knock-out” mice for specific clock genes, however, have demonstrated significant variability between laboratories. In this brief review, the authors discuss possible sources of this variability, focusing particularly on questions of modifier loci of circadian rhythms that vary between inbred mouse strains. They conclude with a short list of recommendations for researchers working on circadian rhythms in mixed-strain mice.

Electric light, particularly at night, disrupts human circadian rhythmicity: is that a problem?

Over the past 3 billion years, an endogenous circadian rhythmicity has developed in almost all life forms in which daily oscillations in physiology occur. This allows for anticipation of sunrise and sunset. This physiological rhythmicity is kept at precisely 24 h by the daily cycle of sunlight and dark. However, since the introduction of electric lighting, there has been inadequate light during the day inside buildings for a robust resetting of the human endogenous circadian rhythmicity, and too much light at night for a true dark to be detected; this results in circadian disruption and alters sleep/wake cycle, core body temperature, hormone regulation and release, and patterns of gene expression throughout the body. The question is the extent to which circadian disruption compromises human health, and can account for a portion of the modern pandemics of breast and prostate cancers, obesity, diabetes and depression. As societies modernize (i.e. electrify) these conditions increase in prevalence. There are a number of promising leads on putative mechanisms, and epidemiological findings supporting an aetiologic role for electric lighting in disease causation. These include melatonin suppression, circadian gene expression, and connection of circadian rhythmicity to metabolism in part affected by haem iron intake and distribution.

Advanced sleep schedules affect circadian gene expression in young adults with delayed sleep schedules

BACKGROUND

Human circadian rhythms are regulated by the interplay between circadian genes and environmental stimuli. The influence of altered sleep-wake schedules or light on human circadian gene expression patterns is not well characterized.

METHODS

Twenty-one young adults were asked to keep to their usual sleep schedules and two blood samples were drawn at the end of the first week from each subject based on estimated time of dim light melatonin onset (DLMO); the first sample was obtained one and a half hours before the estimated DLMO and the second three hours later, at one and a half hours after the estimated DLMO. During the second week, participants were randomized into two groups, one that received a one hour blue-light (λmax=470 nm) exposure in the morning and one that received a comparable morning dim-light exposure. Two blood samples were obtained at the same clock times as the previous week at the end of the second week.

RESULTS

We measured the expression of 10 circadian genes in response to sleep-wake schedule advancement and morning blue-light stimulation in the peripheral blood of 21 participants during a two-week field study. We found that nine of the 10 circadian genes showed significant expression changes from the first to the second week for participants in both the blue-light and dim-light groups, likely reflecting significant advances in circadian phase.

CONCLUSIONS

This wholesale change in circadian gene expression may reflect considerable advances in circadian phase (i.e., advance in DLMO) from the first to the second week resulting from the advanced, daily personal light exposures.

Ubiquitin-specific peptidase 46 (Usp46) regulates mouse immobile behavior in the tail suspension test through the GABAergic system

The tail suspension test (TST) is widely recognized as a useful experimental paradigm for assessing antidepressant activity and depression-like behavior. We have previously identified ubiquitin-specific peptidase 46 (Usp46) as a quantitative trait gene responsible for decreasing immobility time in the TST in mice. This Usp46 mutation has a 3-bp deletion coding for lysine in the open reading frame, and we indicated that Usp46 is implicated in the regulation of the GABAergic system. However, it is not known precisely how the immobile behavior is regulated by the GABAergic system. Therefore, in the present study, we examined whether the immobility time is influenced by drugs affecting the action mediated by GABA_A receptor using both 3-bp deleted (the Usp46 mutant) and null Usp46 (Usp46 KO) mice. Nitrazepam, an agonist at the benzodiazepine-binding site of the GABA_A receptor, which potentiates the action of GABA, produced a dose-dependent increase in TST immobility time in the Usp46 mutant mice without affecting general behaviors. The Usp46 KO mice exhibited short immobility times comparable to the Usp46 mutant mice, which was also increased by nitrazepam administration. The effects of nitrazepam in the Usp46 mutant and KO mice were antagonized by flumazenil. These results indicate that the 3-bp deleted Usp46 mutation causes a loss-of-function phenotype, and that the GABA_A receptor might participate in the regulation of TST immobility time.

Genetic and molecular analysis of wild-derived arrhythmic mice

A new circadian variant was isolated by screening the intercross offspring of wild-caught mice (Mus musculus castaneus). This variant was characterized by an initial maintenance of damped oscillations and subsequent loss of rhythmicity after being transferred from light-dark (LD) cycles to constant darkness (DD). To map the genes responsible for the persistence of rhythmicity (circadian ratio) and the length of free-running period (tau), quantitative trait locus (QTL) analysis was performed using F(2) mice obtained from an F(1) cross between the circadian variant and C57BL/6J mice. As a result, a significant QTL with a main effect for circadian ratio (Arrhythmicity; Arrh-1) was mapped on Chromosome (Chr) 8. For tau, four significant QTLs, Short free-running period (Sfp-1) (Chr 1), Sfp-2 (Chr 6), Sfp-3 (Chr 8), Sfp-4 (Chr 11) were determined. An epistatic interaction was detected between Chr 3 (Arrh-2) and Chr 5 (Arrh-3). An in situ hybridization study of clock genes and mouse Period1::luciferase (mPer1::luc) real-time monitoring analysis in the suprachiasmatic nucleus (SCN) suggested that arrhythmicity in this variant might not be attributed to core circadian mechanisms in the SCN neurons. Our strategy using wild-derived variant mice may provide a novel opportunity to evaluate circadian and its related disorders in human that arise from the interaction between multiple variant genes.

A novel quantitative trait locus on mouse chromosome 18, "era1," modifies the entrainment of circadian rhythms

STUDY OBJECTIVE

The mammalian circadian clock in the suprachiasmatic nuclei (SCN) of the hypothalamus conveys 24-h rhythmicity to sleep-wake cycles, locomotor activity, and other behavioral and physiological processes. The timing of rhythms relative to the light/dark (LD12:12) cycle is influenced in part by the endogenous circadian period and the time of day specific sensitivity of the clock to light. We now describe a novel circadian rhythm phenotype, and a locus influencing that phenotype, in a segregating population of mice.

METHODS

By crossbreeding 2 genetically distinct nocturnal strains of mice (Cast/Ei and C57BL/6J) and backcrossing the resulting progeny to Cast/Ei, we have produced a novel circadian phenotype, called early runner mice.

RESULTS

Early runner mice entrain to a light/dark cycle at an advanced phase, up to 9 hours before dark onset. This phenotype is not significantly correlated with circadian period in constant darkness and is not associated with disruption of molecular circadian rhythms in the SCN, as assessed by analysis of period gene expression. We have identified a genomic region that regulates this phenotype-a major quantitative trait locus on chromosome 18 (near D18Mit184) that we have named era1 for Early Runner Activity locus one. Phase delays caused by light exposure early in the subjective night were of smaller magnitude in backcross offspring that were homozygous Cast/Ei at D18Mit184 than in those that were heterozygous at this locus.

CONCLUSION

Genetic variability in the circadian response to light may, in part, explain the variance in phase angle of entrainment in this segregating mouse population.

A QTL on mouse chromosome 12 for the genetic variance in free-running circadian period between inbred strains of mice

Background

Many genes control circadian period in mice. Prior studies suggested a quantitative trait locus (QTL) on proximal mouse chromosome 12 for interstrain differences in circadian period. Since the B6.D2NAhr^d/J strain has DBA/2 alleles for a portion of proximal chromosome 12 introgressed onto its C57BL/6J background, we hypothesized that these mice would have a shorter circadian period than C57BL/6J mice.

Methods

We compared circadian phenotypes of B6.D2NAhr^d/J and C57BL/6 mice: period of general locomotor activity in constant dark and rest/activity pattern in alternating light and dark. We genotyped the B6.D2NAhr^d/J mice to characterize the size of the genomic insert. To aid in identifying candidate quantitative trait genes we queried databases about the resident SNPs, whole brain gene expression in C57BL/6J versus DBA/2J mice, and circadian patterns of gene expression.

Results

The B6.D2NAhr^d/J inbred mice have a shorter circadian period of locomotor activity than the C57BL/6J strain. Furthermore, the genomic insert is associated with another phenotype: the mean phase of activity minimum in the dark part of a light-dark lighting cycle. It was one hour later than in the background strain. The B6.D2NAhr^d/J mice have a DBA/2J genomic insert spanning 35.4 to 41.0 megabase pairs on Chromosome 12. The insert contains 15 genes and 12 predicted genes. In this region Ahr (arylhydrocarbon receptor) and Zfp277 (zinc finger protein 277) both contain non-synonymous SNPs. Zfp277 also showed differential expression in whole brain and was cis-regulated. Three genes and one predicted gene showed a circadian pattern of expression in liver, including Zfp277.

Conclusion

We not only fine-mapped the QTL for circadian period on chromosome 12 but found a new QTL there as well: an association with the timing of the nocturnal activity-minimum. Candidate quantitative trait genes in this QTL are zinc finger protein 277 and arylhydrocarbon receptor. Arylhydrocarbon receptor is structurally related to Bmal1, a canonical clock gene.

Quantitative trait loci for the circadian clock in Neurospora crassa

Neurospora crassa has been a model organism for the study of circadian clocks for the past four decades. Among natural accessions of Neurospora crassa, there is significant variation in clock phenotypes. In an attempt to investigate natural allelic variants contributing to quantitative variation, we used a quantitative trait loci mapping approach to analyze three independent mapping populations whose progenitors were collected from geographically isolated locations. Two circadian clock phenotypes, free-running period and entrained phase, were evaluated in the 188 F(1) progeny of each mapping population. To identify the clock QTL, we applied two QTL mapping analyses: composite interval mapping (CIM) and Bayesian multiple QTL analysis (BMQ). When controlling false positive rates < or =0.05, BMQ appears to be the more sensitive of the two approaches. BMQ confirmed most of the QTL from CIM (18 QTL) and identified 23 additional QTL. While 13 QTL colocalize with previously identified clock genes, we identified 30 QTL that were not linked with any previously characterized clock genes. These are candidate regions where clock genes may be located and are expected to lead to new insights in clock regulation.

Molecular circadian rhythms in central and peripheral clocks in mammals

The last decade has seen tremendous progress in our understanding of the organization and function of the circadian clock. A number of so-called clock genes were discovered, and these genes and their protein products were shown to organize into feedback loops to give a near 24 h rhythmicity. However, the mechanism is much more complicated. First, many new clock components have been identified, increasing both our understanding and the overall complexity of the mechanism. Second, there is now evidence that transcription may not play a central role in determining the functioning of the clock: the identification of post-translational modifications of the clock proteins has revealed new levels of control. Finally, chromatin remodeling seems to be crucial in the regulation of the expression of major clock components. This review describes the recent advances in our knowledge of the molecular clockwork in mammals; in particular, the contribution of new clock components and of post-transcriptional and post-translational events to circadian timekeeping are discussed.

Catalase activity and rhythmic patterns in mouse brain, kidney and liver

To protect tissues from damaging effects of reactive oxygen species (ROS), organisms possess enzymatic and non-enzymatic antioxidant systems. Cytosolic-enzyme catalase (CAT) is a component of the antioxidant defence system that reduces hydrogen peroxide (H(2)O(2)) to water (H(2)O). The aim of this study was to assess the variation of antioxidant enzyme CAT activity in brain, kidney and liver of adult male mice according to tissue-specific and temporal patterns within a 24-h period (12:12 L/D). The CAT activity was assayed at 4-h intervals. The Cosinor test programme was used to detect and confirm the best corresponding rhythm. In liver, the circadian rhythm of CAT was associated with ultradian components. The prominent circadian rhythm (with a period tau=24 h) showed a peak located at the middle of the dark phase, more precisely congruent with 17 HALO (Hours After Light Onset). In kidney, only a circadian rhythm of CAT was validated with a peak time located at congruent with 17 HALO. However, in brain, the time pattern of CAT activity showed two peak times at congruent with 1 and congruent with 17 HALO, illustrating the existence of an ultradian rhythm (with a period tau=12 h). The results showed significant organ differences with the highest activity in liver, compared with kidney (-89%) and brain (-98%). This might be related to several factors such as their respective physiological function, the risk of exposure to oxidative damage and the balance between synthesis and degradation of proteins during "normal metabolism". Moreover, CAT activity revealed differences in time-related changes across a 24-h period that were more obvious in peak levels between the three tissues.

A genetic study of the suppressors of the Engrailed-1 cerebellar phenotype

The mouse Engrailed genes, En1 and En2, play an important role in the development of the cerebellum from its inception at the mid/hindbrain boundary in early embryonic development through cell type specification events and beyond. In the absence of En1, the cerebellum and caudal midbrain fail to develop normally--a phenotype that we have previously reported to be strain dependent. On the 129/S1 strain background, En1 null alleles lead to mid/hindbrain failure, whereas on the C57BL/6 background, En1 deficiency is compatible with near normal cerebellar development. We have pursued this dramatic effect of genetic background by performing a genetic modifier screen through F1 backcross and F1 intercross matings. The backcross has yielded two strong candidate intervals with suggestive linkage to a third region. Moreover, variations in rescue frequency among subgroups within the backcross indicate gender and parent of origin influences on rescue penetrance. The intercross data reveal locus heterogeneity of the En1 modifiers, with more than one compliment of C57BL/6 and 129/S1 alleles capable of mediating the rescue phenotype. These findings highlight the complexity and plasticity of gene networks involved in brain development.

A single copy of carbonic anhydrase 2 restores wild-type circadian period to carbonic anhydrase II-deficient mice

Carbonic anhydrase II (CA-II)-deficient mice have long circadian periods compared to their siblings with normal CA-II levels. The CA-II-deficient mice differ genetically from their siblings at proximal chromosome three, where the mutated carbonic anhydrase 2 gene sits on a small insert of DNA from the DBA/2J strain. The rest of the genome is that of the C57BL/6J strain. The goal of this study was to test the hypothesis that the null mutation in carbonic anhydrase 2 and the long circadian period phenotype were linked. In order to separate the effect of the null mutation in carbonic anhydrase 2 from the effect of DBA/2J alleles of other genes on the insert, two new lines of mice were studied. The first line, Kar, was developed from a CA-II-deficient mouse that had a fortuitous recombination restoring functional CA-II without affecting the rest of the DBA/2J insert. The second line was generated by breeding DBA/2J mice and C57BL/6J mice until they had the genomic composition of CA-II-deficient mice without the null mutation. Both lines of mice had circadian periods not different from C57BL/6J mice and shorter than CA-II-deficient mice. The phenotype of the new lines showed that the long circadian period characteristic of the CA-II-deficient mice arises when functional CA-II is absent, not when DBA/2J alleles are present on proximal chromosome three.

Effect of Robertsonian Translocations on the Motor Activity Rhythm in the House Mouse

Here we studied the circadian rhythm of motor activity in two groups of wild house mice from the chromosomal polymorphic zone of Barcelona, which differed in diploid number (2n): standard (2n = 40), with all acrocentric chromosomes, and Robertsonian (2n = 29-32), with several Robertsonian translocations. Motor activity under three lighting conditions, light-dark cycle, constant darkness, and constant light, was recorded for each mouse. The motor activity rhythm was examined by Fourier analysis and the daily power spectra were obtained. On the basis of the mean power spectrum of each animal and under each lighting condition, stepwise discriminant analyses were performed to classify the two chromosomal groups. This method allowed the correct classification of a large number of animals, the rhythms of about 2-2.6 hour periods being the most significant, with higher values in Robertsonian than in standard mice. Our results indicate that the daily motor activity pattern differs between the two chromosomal groups and its analysis may have a valuable interest for behavioral investigations on Robertsonian polymorphic zones of this species.

Loss of circadian photoentrainment and abnormal retinal electrophysiology in Math5 mutant mice

PURPOSE

To determine how the absence of retinal ganglion cells (RGCs) in Math5 (Atoh7) mutant mice affects circadian behavior and retinal function.

METHODS

The wheel-running behavior of wild-type and Math5 mutant mice was measured under various light-dark cycle conditions. To evaluate retinal input to the suprachiasmatic nuclei (SCN) anatomically, the retinohypothalamic tracts were labeled in vivo. To assess changes in retinal function, corneal flash electroretinograms (ERGs) from mutant and wild-type mice were compared under dark- and light-adapted conditions. Alterations in retinal neuron populations were evaluated quantitatively and with cell-type-specific markers.

RESULTS

The Math5-null mice did not entrain to light and exhibited free-running circadian behavior with a mean period (23.6 +/- 0.15 hours) that was indistinguishable from that of wild-type mice (23.4 +/- 0.19 hours). The SCN showed no anterograde labeling with a horseradish peroxidase-conjugated cholera toxin B (CT-HRP) tracer. ERGs recorded from mutant mice had diminished scotopic a- and b-wave and photopic b-wave amplitudes. The scotopic b-wave was more severely affected than the a-wave. The oscillatory potentials (OPs) and scotopic threshold response (STR) were also reduced. Consistent with these ERG findings, a pan-specific reduction in the number of bipolar cells and a smaller relative decrease in the number of rods in mutant mice were observed.

CONCLUSIONS

Math5-null mice are clock-blind and have no RGC projections to the SCN. RGCs are thus essential for photoentrainment in mice, but are not necessary for the development or intrinsic function of the SCN clock. RGCs are not required to generate any of the major ERG waveforms in mice, including the STR, which is produced by ganglion cells in some other species. The diminished amplitude of b-wave, OPs, and STR components in Math5 mutants is most likely caused by the decreased abundance of retinal interneurons.

Mammalian circadian biology: elucidating genome-wide levels of temporal organization

During the past decade, the molecular mechanisms underlying the mammalian circadian clock have been defined. A core set of circadian clock genes common to most cells throughout the body code for proteins that feed back to regulate not only their own expression, but also that of clock output genes and pathways throughout the genome. The circadian system represents a complex multioscillatory temporal network in which an ensemble of coupled neurons comprising the principal circadian pacemaker in the suprachiasmatic nucleus of the hypothalamus is entrained to the daily light/dark cycle and subsequently transmits synchronizing signals to local circadian oscillators in peripheral tissues. Only recently has the importance of this system to the regulation of such fundamental biological processes as the cell cycle and metabolism become apparent. A convergence of data from microarray studies, quantitative trait locus analysis, and mutagenesis screens demonstrates the pervasiveness of circadian regulation in biological systems. The importance of maintaining the internal temporal homeostasis conferred by the circadian system is revealed by animal models in which mutations in genes coding for core components of the clock result in disease, including cancer and disturbances to the sleep/wake cycle.

Expression of IKKα mRNA in the suprachiasmatic nucleus and circadian rhythms of mice lacking IKKα

In our previous study, quantitative trait locus (QTL) analysis using mice with abnormal circadian rhythm detected a suggestive QTL, which affects the length of free-running period, on the distal region of Chromosome 19. Among the candidate genes, we have focused on Ikkalpha gene and found that Ikkalpha mRNA is expressed in the mammalian circadian pacemaker, the suprachiasmatic nucleus (SCN) in the present study. Expression of Ikkalpha mRNA in the SCN indicated the possibility that IKKalpha is involved in the regulation of circadian clock. Therefore, to examine the role of IKKalpha in the regulation of circadian rhythms, we have further examined wheel-running activity rhythms under light-dark cycle and constant darkness, and circadian response to light. However, we could not detect any statistically significant difference between IKKalpha(+/-) mice and wild type mice. Roles of IKKalpha in the regulation of circadian system remains to be clarified in the future study.

Sleep properties of CS mice with spontaneous rhythm splitting in constant darkness

In mice, genetic differences between inbred strains have been shown for several parameters of sleep and circadian activity rhythms. Our previous studies have demonstrated that CS mice have three remarkable characteristics in the circadian rhythm of locomotor activity: (1) high activity both during the day and night, (2) unstable freerunning period and (3) spontaneous rhythm splitting. In order to characterize sleep properties of CS mice, we compared circadian sleep patterns of CS with those of C57BL/6J and C3H/He mice which have normal circadian activity rhythms. Although C57BL/6J and C3H/He mice exhibited clear daily sleep-wake rhythms in the amount of each sleep parameter (Awake, SWS, PS), CS mice did not show clear rhythms in these parameters. The differences were particularly conspicuous in PS; no apparent day-night differences in the amount of PS, PS counts and PS interval (the interval between successive PS episodes) in CS mice. In addition, the ratio of PS to total sleep time was significantly larger in CS mice than other strains. Of these parameters, the most considerable was PS latency which was extremely short and direct transition from Awake to PS without appearance of SWS frequently occurred in these mice. These results indicate that CS mice may be useful for the understanding of sleep mechanisms and its dysfunction.

A molecular perspective of human circadian rhythm disorders

A large number of physiological variables display 24-h or circadian rhythms. Genes dedicated to the generation and regulation of physiological circadian rhythms have now been identified in several species, including humans. These clock genes are involved in transcriptional regulatory feedback loops. The mutation of these genes in animals leads to abnormal rhythms or even to arrhythmicity in constant conditions. In this view, and given the similarities between the circadian system of humans and rodents, it is expected that mutations of clock genes in humans may give rise to health problems, in particular sleep and mood disorders. Here we first review the present knowledge of molecular mechanisms underlying circadian rhythmicity, and we then revisit human circadian rhythm syndromes in light of the molecular data.