Cloning, tissue expression, and mapping of a human photolyase homolog with similarity to plant blue-light receptors

A novel method for measurements of sleep/wake states, feeding and drinking behaviors using the tracking technique of 3D positions in freely moving mice

We have previously developed a 3D video tracking system which enables us to analyze long-term quantitative analysis of gene expression in freely moving mice. In the present study, we improved 3D video tracking and developed a system that analyzes more detailed behavioral data. We succeeded in simultaneously analyzing sleep-wake, feeding, and drinking behavior rhythms in the same individual using our tracking system. This system will make it possible to measure gene expression in each tissue in vivo in real time in relation to the various behavioral rhythms mentioned above.

Cryptochromes in mammals: a magnetoreception misconception?

Cryptochromes are flavoproteins related to photolyases that are widespread throughout the plant and animal kingdom. They govern blue light-dependent growth in plants, control circadian rhythms in a light-dependent manner in invertebrates, and play a central part in the circadian clock in vertebrates. In addition, cryptochromes might function as receptors that allow animals to sense the Earth's magnetic field. As cryptochromes are also present in mammals including humans, the possibility of a magnetosensitive protein is exciting. Here we attempt to provide a concise overview of cryptochromes in mammals. We briefly review their canonical role in the circadian rhythm from the molecular level to physiology, behaviour and diseases. We then discuss their disputed light sensitivity and proposed role in the magnetic sense in mammals, providing three mechanistic hypotheses. Specifically, mammalian cryptochromes could form light-induced radical pairs in particular cellular milieus, act as magnetoreceptors in darkness, or as secondary players in a magnetoreception signalling cascade. Future research can test these hypotheses to investigate if the role of mammalian cryptochromes extends beyond the circadian clock.

Short-Term UVB Irradiation Leads to Persistent DNA Damage in Limbal Epithelial Stem Cells, Partially Reversed by DNA Repairing Enzymes

The cornea is frequently exposed to ultraviolet (UV) radiation and absorbs a portion of this radiation. UVB in particular is absorbed by the cornea and will principally damage the topmost layer of the cornea, the epithelium. Epidemiological research shows that the UV damage of DNA is a contributing factor to corneal diseases such as pterygium. There are two main DNA photolesions of UV: cyclobutane pyrimidine dimers (CPDs) and pyrimidine-pyrimidone (6-4) photoproducts (6-4PPs). Both involve the abnormal linking of adjacent pyrimide bases. In particular, CPD lesions, which account for the vast majority of UV-induced lesions, are inefficiently repaired by nucleotide excision repair (NER) and are thus mutagenic and linked to cancer development in humans. Here, we apply two exogenous enzymes: CPD photolyase (CPDPL) and T4 endonuclease V (T4N5). The efficacy of these enzymes was assayed by the proteomic and immunofluorescence measurements of UVB-induced CPDs before and after treatment. The results showed that CPDs can be rapidly repaired by T4N5 in cell cultures. The usage of CPDPL and T4N5 in ex vivo eyes revealed that CPD lesions persist in the corneal limbus. The proteomic analysis of the T4N5-treated cells shows increases in the components of the angiogenic and inflammatory systems. We conclude that T4N5 and CPDPL show great promise in the treatment of CPD lesions, but the complete clearance of CPDs from the limbus remains a challenge.

UV Protection in the Cornea: Failure and Rescue

Simple Summary

The sun is a deadly laser, and its damaging rays harm exposed tissues such as our skin and eyes. The skin’s protection and repair mechanisms are well understood and utilized in therapeutic approaches while the eye lacks such complete understanding of its defenses and therefore often lacks therapeutic support in most cases. The aim here was to document the similarities and differences between the two tissues as well as understand where current research stands on ocular, particularly corneal, ultraviolet protection. The objective is to identify what mechanisms may be best suited for future investigation and valuable therapeutic approaches.

Abstract

Ultraviolet (UV) irradiation induces DNA lesions in all directly exposed tissues. In the human body, two tissues are chronically exposed to UV: the skin and the cornea. The most frequent UV-induced DNA lesions are cyclobutane pyrimidine dimers (CPDs) that can lead to apoptosis or induce tumorigenesis. Lacking the protective pigmentation of the skin, the transparent cornea is particularly dependent on nucleotide excision repair (NER) to remove UV-induced DNA lesions. The DNA damage response also triggers intracellular autophagy mechanisms to remove damaged material in the cornea; these mechanisms are poorly understood despite their noted involvement in UV-related diseases. Therapeutic solutions involving xenogenic DNA-repair enzymes such as T4 endonuclease V or photolyases exist and are widely distributed for dermatological use. The corneal field lacks a similar set of tools to address DNA-lesions in photovulnerable patients, such as those with genetic disorders or recently transplanted tissue.

Interconnections between circadian clocks and metabolism

Circadian rhythms evolved through adaptation to daily light/dark changes in the environment; they are believed to be regulated by the core circadian clock interlocking feedback loop. Recent studies indicate that each core component executes general and specific functions in metabolism. Here, we review the current understanding of the role of these core circadian clock genes in the regulation of metabolism using various genetically modified animal models. Additionally, emerging evidence shows that exposure to environmental stimuli, such as artificial light, unbalanced diet, mistimed eating, and exercise, remodels the circadian physiological processes and causes metabolic disorders. This Review summarizes the reciprocal regulation between the circadian clock and metabolism, highlights remaining gaps in knowledge about the regulation of circadian rhythms and metabolism, and examines potential applications to human health and disease.

Forward genetic approach for behavioral neuroscience using animal models

Forward genetics is a powerful approach to understand the molecular basis of animal behaviors. Fruit flies were the first animal to which this genetic approach was applied systematically and have provided major discoveries on behaviors including sexual, learning, circadian, and sleep-like behaviors. The development of different classes of model organism such as nematodes, zebrafish, and mice has enabled genetic research to be conducted using more-suitable organisms. The unprecedented success of forward genetic approaches was the identification of the transcription-translation negative feedback loop composed of clock genes as a fundamental and conserved mechanism of circadian rhythm. This approach has now expanded to sleep/wakefulness in mice. A conventional strategy such as dominant and recessive screenings can be modified with advances in DNA sequencing and genome editing technologies.

The evolution of eyes: major steps. The Keeler lecture 2017: centenary of Keeler Ltd

Ocular evolution is an immense topic, and I do not expect to cover all the details of this process in this manuscript. I will present some concepts about some of the major steps in the evolutionary process to stimulate your thinking about this interesting and complex topic. In the prebiotic soup, vision was not inevitable. Eyes were not preordained. Nor were their shapes, sizes, or current physiology. Sight is an evolutionary gift but it was not ineluctable. The existence of eyes is so basic to our profession that we often do not consider how and why vision appeared or evolved on earth at all. Although vision is a principal sensory modality for at least three major phyla and is present in three or four more phyla, there are other sensory mechanisms that could have been and were occasionally selected instead. Some animals rely on other sensory mechanisms such as audition, echolocation, or olfaction that are much more effective in their particular niche than would be vision. We may not believe those sensory mechanisms to be as robust as vision, but the creatures using those skills would argue otherwise. Why does vision exist at all? And why is it so dominant at least in the number of species that rely upon it for their principal sensory mechanism? How did vision begin? What were the important steps in the evolution of eyes? How did eyes differentiate along their various paths, and why?

Repair of (6-4) Lesions in DNA by (6-4) Photolyase: 20 Years of Quest for the Photoreaction Mechanism

Exposure of DNA to ultraviolet (UV) light from the Sun or from other sources causes the formation of harmful and carcinogenic crosslinks between adjacent pyrimidine nucleobases, namely cyclobutane pyrimidine dimers and pyrimidine(6-4)pyrimidone photoproducts. Nature has developed unique flavoenzymes, called DNA photolyases, that utilize blue light, that is photons of lower energy than those of the damaging light, to repair these lesions. In this review, we focus on the chemically challenging repair of the (6-4) photoproducts by (6-4) photolyase and describe the major events along the quest for the reaction mechanisms, over the 20 years since the discovery of (6-4) photolyase.

Unique Aspects of Cryptochrome in Chronobiology and Metabolism, Pancreatic β-Cell Dysfunction, and Regeneration: Research into Cysteine414-Alanine Mutant CRY1

Cryptochrome proteins (CRYs), which can bind noncovalently to cofactor (chromophore) flavin adenine dinucleotide (FAD), occur widely among organisms. CRYs play indispensable roles in the generation of circadian rhythm in mammals. Transgenic mice (Tg mice), ubiquitously expressing mouse CRY1 having a mutation in which cysteine414 (the zinc-binding site of CRY1) being replaced with alanine, display unique phenotypes in their circadian rhythms. Moreover, male Tg mice exhibit symptoms of diabetes characterized by beta-cell dysfunction, resembling human maturity onset diabetes of the young (MODY). The lowered proliferation of β-cells is a primary cause of age-dependent β-cell loss. Furthermore, unusually enlarged duct-like structures developed prominently in the Tg mice pancreases. The duct-like structures contained insulin-positive cells, suggesting neogenesis of β-cells in the Tg mice. This review, based mainly on the author's investigation of the unique features of Tg mice, presents reported results and recent findings related to molecular processes associated with mammalian cryptochromes, especially their involvement in the regulation of metabolism. New information is described with emphasis on the aspects of islet architecture, pancreatic β-cell dysfunction, and regeneration.

The circadian gene CRY2 is associated with breast cancer aggressiveness possibly via epigenomic modifications

Although the role of core circadian gene cryptochrome 2 (CRY2) in breast tumorigenesis has been demonstrated, the correlations of CRY2 with clinical parameters in breast cancer patients and its involvement in epigenetic processes such as DNA methylation remain relatively unexplored. In the current study, we first queried the Oncomine database and the Gene Expression-Based Outcome for Breast Cancer Online (GOBO) database to identify associations between CRY2 expression levels and clinical parameters in breast cancer patients. We then silenced CRY2 in vitro and performed a genome-wide methylation array to determine the epigenetic impact of CRY2 silencing. The Ingenuity Pathway Analysis software was used to further explore the genes exhibiting altered methylation identified using the array. We found that CRY2 was frequently down-regulated in breast cancer tissue compared to adjacent normal tissue or breast tissue from healthy controls. Lower CRY2 expression was associated with estrogen receptor (ER)-negativity (P < 0.0001), higher tumor grade (P < 0.0001), and shorter overall survival time in breast cancer patients (HR = 1.44, 95 % confidence interval (CI) 1.09-1.91). Genome-wide methylation analysis showed that a total of 515 CpG sites were hypermethylated following CRY2 knockdown, while 730 sites were hypomethylated. The pathway analysis revealed several cancer-relevant networks with genes exhibiting significantly altered methylation following CRY2 silencing. These findings suggest that the core circadian gene CRY2 is associated with breast cancer progression and prognosis, and that knockdown of CRY2 causes the epigenetic dysregulation of genes involved in cancer-relevant pathways, which provide further evidence supporting a role of the circadian system in breast tumorigenesis.

Role of type II protein arginine methyltransferase 5 in the regulation of Circadian Per1 gene

Circadian clocks are the endogenous oscillators that regulate rhythmic physiological and behavioral changes to correspond to daily light-dark cycles. Molecular dissections have revealed that transcriptional feedback loops of the circadian clock genes drive the molecular oscillation, in which PER/CRY complexes inhibit the transcriptional activity of the CLOCK/BMAL1 heterodimer to constitute a negative feedback loop. In this study, we identified the type II protein arginine methyltransferase 5 (PRMT5) as an interacting molecule of CRY1. Although the Prmt5 gene was constitutively expressed, increased interaction of PRMT5 with CRY1 was observed when the Per1 gene was repressed both in synchronized mouse liver and NIH3T3 cells. Moreover, rhythmic recruitment of PRMT5 and CRY1 to the Per1 gene promoter was found to be associated with an increased level of histone H4R3 dimethylation and Per1 gene repression. Consistently, decreased histone H4R3 dimethylation and altered rhythmic Per1 gene expression were observed in Prmt5-depleted cells. Taken together, these findings provide an insight into the link between histone arginine methylation by PRMT5 and transcriptional regulation of the circadian Per1 gene.

The Potorous CPD photolyase rescues a cryptochrome-deficient mammalian circadian clock

Despite the sequence and structural conservation between cryptochromes and photolyases, members of the cryptochrome/photolyase (flavo)protein family, their functions are divergent. Whereas photolyases are DNA repair enzymes that use visible light to lesion-specifically remove UV-induced DNA damage, cryptochromes act as photoreceptors and circadian clock proteins. To address the functional diversity of cryptochromes and photolyases, we investigated the effect of ectopically expressed Arabidopsis thaliana (6-4)PP photolyase and Potorous tridactylus CPD-photolyase (close and distant relatives of mammalian cryptochromes, respectively), on the performance of the mammalian cryptochromes in the mammalian circadian clock. Using photolyase transgenic mice, we show that Potorous CPD-photolyase affects the clock by shortening the period of behavioral rhythms. Furthermore, constitutively expressed CPD-photolyase is shown to reduce the amplitude of circadian oscillations in cultured cells and to inhibit CLOCK/BMAL1 driven transcription by interacting with CLOCK. Importantly, we show that Potorous CPD-photolyase can restore the molecular oscillator in the liver of (clock-deficient) Cry1/Cry2 double knockout mice. These data demonstrate that a photolyase can act as a true cryptochrome. These findings shed new light on the importance of the core structure of mammalian cryptochromes in relation to its function in the circadian clock and contribute to our further understanding of the evolution of the cryptochrome/photolyase protein family.

Moléculas que marcam o tempo: implicações para os fenótipos circadianos

OBJETIVO: Revisar resumidamente a literatura dos últimos 36 anos de pesquisa em cronobiologia molecular a fim de informar aos profissionais de saúde os avanços obtidos nesta área e os potenciais para aplicação na clínica médica. MÉTODO: Buscas na literatura foram realizadas utilizando as bases de dados PubMed e Scopus usando como palavras-chave "clock genes, circadian rhythms, diurnal preference, delayed sleep phase syndrome, advanced sleep phase syndrome, photoperiod and mood disorder". DISCUSSÃO: Atualmente, o mecanismo molecular da regulação da ritmicidade circadiana é compreendido em grande detalhe. Muitos estudos publicados mostram associações de polimorfismos nos genes relógio com transtornos do ritmo circadiano e com transtornos do humor. CONCLUSÕES: De maneira geral, o progresso obtido na área de cronobiologia molecular traz um melhor entendimento da regulação do sistema de temporização biológico. O desenvolvimento de estudos nesta área tem o potencial de ser aplicável ao tratamento dos transtornos dos ritmos circadianos e certos transtornos do humor, além de prevenir riscos à saúde causados por viagens intercontinentais (Jet Lag) e por trabalhos noturnos e por turnos.

Molecular characterization of Mybbp1a as a co-repressor on the Period2 promoter

The circadian clock comprises transcriptional feedback loops of clock genes. Cryptochromes are essential components of the negative feedback loop in mammals as they inhibit CLOCK-BMAL1-mediated transcription. We purified mouse CRY1 (mCRY1) protein complexes from Sarcoma 180 cells to determine their roles in circadian gene expression and discovered that Myb-binding protein 1a (Mybbp1a) interacts with mCRY1. Mybbp1a regulates various transcription factors, but its role in circadian gene expression is unknown. We found that Mybbp1a functions as a co-repressor of Per2 expression and repressed Per2 promoter activity in reporter assays. Chromatin immunoprecipitation (ChIP) assays revealed endogenous Mybbp1a binding to the Per2 promoter that temporally matched that of mCRY1. Furthermore, Mybbp1a binding to the Per2 promoter correlated with the start of the down-regulation of Per2 expression and with the dimethylation of histone H3 Lys9, to which it could also bind. These findings suggest that Mybbp1a and mCRY1 can form complexes on the Per2 promoter that function as negative regulators of Per2 expression.

Structure function analysis of mammalian cryptochromes

Members of the photolyase/cryptochrome family are flavoproteins that share an extraordinary conserved core structure (photolyase homology region, PHR), but the presence of a carboxy-terminal extension is limited to the cryptochromes. Photolyases are DNA-repair enzymes that remove UV-light-induced lesions. Cryptochromes of plants and Drosophila act as circadian photoreceptors, involved in light entrainment of the biological clock. Using knockout mouse models, mammalian cryptochromes (mCRY1 and mCRY2) were identified as essential components of the clock machinery. Within the mammalian transcription-translation feedback loop generating rhythmic gene expression, mCRYs potently inhibit the transcription activity of the CLOCK/BMAL1 heterodimer and protect mPER2 from 26S-protesome-mediated degradation. By analyzing a set of mutant mCRY1 proteins and photolyase/mCRY1 chimeric proteins, we found that the carboxyl terminus has a determinant role in mCRY1 function by harboring distinguished domains involved in nuclear import and interactions with other clock proteins. Moreover, the carboxyl terminus must cross-talk with the PHR to establish full transcription repression capacity in mCRY1. We propose that the presence of the carboxyl terminus in cryptochromes, which varies in sequence composition among mammalian, Drosophila, and plant CRYs, is critical for their different functions and possibly contributed to shape the different architecture and biochemistry of the clock machineries in these organisms.

Cryptochrome 1 from Brassica napus is up-regulated by blue light and controls hypocotyl/stem growth and anthocyanin accumulation

Cryptochromes are blue/ultraviolet-A light sensing photoreceptors involved in regulating various growth and developmental responses in plants. Investigations on the structure and functions of cryptochromes in plants have been largely confined to Arabidopsis (Arabidopsis thaliana), tomato (Lycopersicon esculentum), and pea (Pisum sativum). We report here the characterization of the cryptochrome 1 gene from Brassica napus (BnCRY1), an oilseed crop, and its functional validation in transgenics. The predicted BnCRY1 protein sequence shows a high degree of sequence identity (94%) to Arabidopsis CRY1. A semiquantitative reverse transcription-polymerase chain reaction and the western-blot analysis revealed that blue light up-regulates its transcript and protein levels in young seedlings. The BnCRY1 promoter harbors conventional light-responsive cis-acting elements, which presumably impart light activation to the GUS (beta-glucuronidase) reporter gene expressed in Arabidopsis. Although the BnCRY1 transcript could be detected in all the tissues examined, its protein was virtually undetectable in mature leaves and the root, indicating a tissue-specific translational control or protein turnover. The antisense-BnCRY1 Brassica transgenic seedlings accumulated negligible levels of CRY1 protein and displayed an elongated hypocotyl when grown under continuous white or blue light (but not under red or far-red light); the accumulation of anthocyanins was also reduced significantly. The adult transformants were also found to be tall when grown under natural light environment in a containment facility without any artificial illumination. These data provide functional evidence for a role of blue light up-regulated cry1 in controlling photomorphogenesis in Brassica species.

Functional evolution of the photolyase/cryptochrome protein family: importance of the C terminus of mammalian CRY1 for circadian core oscillator performance

Cryptochromes (CRYs) are composed of a core domain with structural similarity to photolyase and a distinguishing C-terminal extension. While plant and fly CRYs act as circadian photoreceptors, using the C terminus for light signaling, mammalian CRY1 and CRY2 are integral components of the circadian oscillator. However, the function of their C terminus remains to be resolved. Here, we show that the C-terminal extension of mCRY1 harbors a nuclear localization signal and a putative coiled-coil domain that drive nuclear localization via two independent mechanisms and shift the equilibrium of shuttling mammalian CRY1 (mCRY1)/mammalian PER2 (mPER2) complexes towards the nucleus. Importantly, deletion of the complete C terminus prevents mCRY1 from repressing CLOCK/BMAL1-mediated transcription, whereas a plant photolyase gains this key clock function upon fusion to the last 100 amino acids of the mCRY1 core and its C terminus. Thus, the acquirement of different (species-specific) C termini during evolution not only functionally separated cryptochromes from photolyase but also caused diversity within the cryptochrome family.

Light-driven enzymatic catalysis of DNA repair: a review of recent biophysical studies on photolyase

More than 50 years ago, initial experiments on enzymatic photorepair of ultraviolet (UV)-damaged DNA were reported [Proc. Natl. Acad. Sci. U. S. A. 35 (1949) 73]. Soon after this discovery, it was recognized that one enzyme, photolyase, is able to repair UV-induced DNA lesions by effectively reversing their formation using blue light. The enzymatic process named DNA photoreactivation depends on a non-covalently bound cofactor, flavin adenine dinucleotide (FAD). Flavins are ubiquitous redox-active catalysts in one- and two-electron transfer reactions of numerous biological processes. However, in the case of photolyase, not only the ground-state redox properties of the FAD cofactor are exploited but also, and perhaps more importantly, its excited-state properties. In the catalytically active, fully reduced redox form, the FAD absorbs in the blue and near-UV ranges of visible light. Although there is no direct experimental evidence, it appears generally accepted that starting from the excited singlet state, the chromophore initiates a reductive cleavage of the two major DNA photodamages, cyclobutane pyrimidine dimers and (6-4) photoproducts, by short-distance electron transfer to the DNA lesion. Back electron transfer from the repaired DNA segment is believed to eventually restore the initial redox states of the cofactor and the DNA nucleobases, resulting in an overall reaction with net-zero exchanged electrons. Thus, the entire process represents a true catalytic cycle. Many biochemical and biophysical studies have been carried out to unravel the fundamentals of this unique mode of action. The work has culminated in the elucidation of the three-dimensional structure of the enzyme in 1995 that revealed remarkable details, such as the FAD-cofactor arrangement in an unusual U-shaped configuration. With the crystal structure of the enzyme at hand, research on photolyases did not come to an end but, for good reason, intensified: the geometrical structure of the enzyme alone is not sufficient to fully understand the enzyme's action on UV-damaged DNA. Much effort has therefore been invested to learn more about, for example, the geometry of the enzyme-substrate complex, and the mechanism and pathways of intra-enzyme and enzyme <-->DNA electron transfer. Many of the key results from biochemical and molecular biology characterizations of the enzyme or the enzyme-substrate complex have been summarized in a number of reviews. Complementary to these articles, this review focuses on recent biophysical studies of photoreactivation comprising work performed from the early 1990s until the present.

Powerful skin cancer protection by a CPD-photolyase transgene

BACKGROUND

The high and steadily increasing incidence of ultraviolet-B (UV-B)-induced skin cancer is a problem recognized worldwide. UV introduces different types of damage into the DNA, notably cyclobutane pyrimidine dimers (CPDs) and (6-4) photoproducts (6-4PPs). If unrepaired, these photolesions can give rise to cell death, mutation induction, and onset of carcinogenic events, but the relative contribution of CPDs and 6-4PPs to these biological consequences of UV exposure is hardly known. Because placental mammals have undergone an evolutionary loss of photolyases, repair enzymes that directly split CPDs and 6-4PPs into the respective monomers in a light-dependent and lesion-specific manner, they can only repair UV-induced DNA damage by the elaborate nucleotide excision repair pathway.

RESULTS

To assess the relative contribution of CPDs and 6-4PPs to the detrimental effects of UV light, we generated transgenic mice that ubiquitously express CPD-photolyase, 6-4PP-photolyase, or both, thereby allowing rapid light-dependent repair of CPDs and/or 6-4PPs in the skin. We show that the vast majority of (semi)acute responses in the UV-exposed skin (i.e., sunburn, apoptosis, hyperplasia, and mutation induction) can be ascribed to CPDs. Moreover, CPD-photolyase mice, in contrast to 6-4PP-photolyase mice, exhibit superior resistance to sunlight-induced tumorigenesis.

CONCLUSIONS

Our data unequivocally identify CPDs as the principal cause of nonmelanoma skin cancer and provide genetic evidence that CPD-photolyase enzymes can be employed as effective tools to combat skin cancer.

The CPH1 gene of Chlamydomonas reinhardtii encodes two forms of cryptochrome whose levels are controlled by light-induced proteolysis

Cryptochromes are proteins related to DNA photolyases and have been shown to function as blue-light photoreceptors and to play important roles in circadian rhythms in both plants and animals. The CPH1 gene from Chlamydomonas reinhardtii was originally predicted to encode a putative cryptochrome protein of 867 amino acids with a predicted molecular mass of 91 kD (Small et al., 1995). However, western blotting with antibodies specific to the CPH1 protein revealed the presence of two proteins that migrate at apparent molecular mass of approximately 126 and 143 kD. A reexamination of the assigned intron-exon boundaries has shown that the previously assigned intron 7 is in fact part of exon 7 which leads to a predicted protein of 1,007 amino acids corresponding to a size of 104.6 kD. The two forms of CPH1 that migrate slower on SDS-PAGE presumably result from unknown posttranslational modifications. In C. reinhardtii cells synchronized by light to dark cycles, the two slow migrating forms of CPH1 protein accumulate in the dark and disappear rapidly in the light. Both red and blue light are effective at inducing the degradation of the CPH1 proteins. Proteasomes are implicated because degradation is inhibited by MG132, a proteasome inhibitor. Studies with deletion mutants indicate that the C-terminal region is important for both the posttranslational modification and the protein's stability under both light and dark conditions.

Non-cyclic and developmental stage-specific expression of circadian clock proteins during murine spermatogenesis

The central circadian clock in mammals is housed in the brain and is based on cyclic transcription and translation of clock proteins. How the central clock regulates peripheral organ function is unclear. However, cyclic expression of circadian genes in peripheral tissues is well established, suggesting that these tissues have their own endogenous oscillators. Reproduction is a process influenced by circadian rhythms in many organisms, thus making the testis an attractive model for studying clock function in peripheral organs. However, results addressing cyclic expression of clock genes in the mammalian testis are inconsistent. To resolve this issue, RNA was extracted from testes of mice at various times of day. Expression of the circadian clock genes mPer1, mPer2, Bmal1, Clock, mCry1, and Npas2 was constant at all times. Immunohistochemical localization of mPER1 and CLOCK proteins revealed restricted expression only in cells at specific developmental stages of spermatogenesis. For mPER1, these stages are the spermatogonia and the condensing spermatids. In contrast, CLOCK expression was restricted to round spermatids, specifically within the developing acrosome. Expression of mPER1 and CLOCK was constant at all times of day. These results suggest that clock proteins have noncircadian functions in spermatogenesis. Noncircadian expression of clock genes was also found in the thymus, which, like the testis, is composed primarily of differentiating cells. We propose that cyclic expression of clock genes is suspended during cellular differentiation.

The circadian clock: pacemaker and tumour suppressor

The circadian rhythms are daily oscillations in various biological processes that are regulated by an endogenous clock. Disruption of these rhythms has been associated with cancer in humans. One of the cellular processes that is regulated by circadian rhythm is cell proliferation, which often shows asynchrony between normal and malignant tissues. This asynchrony highlights the importance of the circadian clock in tumour suppression in vivo and is one of the theoretical foundations for cancer chronotherapy. Investigation of the mechanisms by which the circadian clock controls cell proliferation and other cellular functions might lead to new therapeutic targets.

UV-induced DNA damage and repair: a review

Increases in ultraviolet radiation at the Earth's surface due to the depletion of the stratospheric ozone layer have recently fuelled interest in the mechanisms of various effects it might have on organisms. DNA is certainly one of the key targets for UV-induced damage in a variety of organisms ranging from bacteria to humans. UV radiation induces two of the most abundant mutagenic and cytotoxic DNA lesions such as cyclobutane-pyrimidine dimers (CPDs) and 6-4 photoproducts (6-4PPs) and their Dewar valence Isomers. However, cells have developed a number of repair or tolerance mechanism to counteract the DNA damage caused by UV or any other stressors. Photoreactivation with the help of the enzyme photolyase is one of the most important and frequently occurring repair mechanisms in a variety of organisms. Excision repair, which can be distinguished into base excision repair (BER) and nucleotide excision repair (NER), also plays an important role in DNA repair in several organisms with the help of a number of glycosylases and polymerases, respectively. In addition, mechanisms such as mutagenic repair or dimer bypass, recombinational repair, cell-cycle checkpoints, apoptosis and certain alternative repair pathways are also operative in various organisms. This review deals with UV-induced DNA damage and the associated repair mechanisms as well as methods of detecting DNA damage and its future perspectives.

Tales from the crypt(ochromes)

Cryptochromes are a family of flavoproteins found in organisms ranging from Arabidopsis to man. Across phylogeny, these proteins have been used for pleiotropic functions ranging from blue-light-dependent development in plants and blue-light-mediated phase shifting of the circadian clock in insects to a core circadian clock component in mammals. Review of the roles of cryptochromes in model organisms reveals several common themes: Multiple cryptochrome family members within individual organisms have redundant functions; cryptochromes used in photic entrainment pathways of the circadian clock are partially redundant with other photopigments; and cryptochromes may function in circadian phototransduction and core clock mechanisms in the same organism, with different functions in different tissues. The present review summarizes recent research on the functions of cryptochrome in the circadian timekeeping and photic entrainment pathways.

Molecular cloning and circadian regulation of cryptochrome genes in Japanese quail (Coturnix coturnix japonica)

The circadian system is thought to have three components: input, pacemaker (internal clock), and output. Cryptochromes (Cry) are important clock genes, and recent findings indicate that these genes not only act as circadian photoreceptors but are also essential components in the negative feedback of the circadian system. As a first step toward understanding the avian circadian system, the authors tried to clone Japanese quail homologs of mammalian Crys and analyze their expression patterns in different circumstances. Partial cDNAs of qCry1 and qCry2, which are homologs of mammalian Cry1 and Cry2, respectively, were obtained and their gene expressions were analyzed. Both qCry1 and qCry2 mRNAs were present in all the tissues examined. The oscillation patterns of the qCry1 transcripts were tissue specific and generally showed robust changes between daytime and nighttime; except for lung and testis tissues (which showed no detectable changes between daytime and nighttime), daytime levels were higher in all of the tissues examined. This rapid oscillation in qCry1 persisted through constant darkness or constant illumination, indicating that an endogenous clock controls these changes. In contrast, the expression of qCry2 did not oscillate in any tissue examined. In addition, in tissues of the pineal gland and eye, unexpected light exposure in the dark period was able to block the decrease in qCry1 transcripts or induce its expression. These findings, in conjunction with the established roles of CRYs in other species, led the authors to propose that in the circadian system, qCRYs may play important roles similar to the known roles of CRYs of other species, such as acting as circadian photoreceptors and as components of the circadian system.

Circadian and photic regulation of cryptochrome mRNAs in the rat pineal gland

Expressions of Cry1 and Cry2 mRNA in the rat pineal gland were examined by Northern blot. The levels of Cry1 and Cry2 transcript had a marked circadian rhythm with peaks at circadian time (CT) 20 in constant darkness. But the amplitude of the Cry1 rhythm was higher than that of the Cry2 rhythm. Furthermore, a significant increase in Cry1 mRNA levels was caused by light pulse given at CT 16 but not at CT 4, but the expression of Cry2 was not significantly induced by light pulses given at either CT 4 or CT 16. These results suggest that Cry1 in the pineal gland is regulated by photic and circadian information but Cry2 is only regulated by circadian information.

Time zones: a comparative genetics of circadian clocks

The circadian clock is a widespread cellular mechanism that underlies diverse rhythmic functions in organisms from bacteria and fungi, to plants and animals. Intense genetic analysis during recent years has uncovered many of the components and molecular mechanisms comprising these clocks. Although autoregulatory genetic networks are a consistent feature in the design of all clocks, the weight of evidence favours their independent evolutionary origins in different kingdoms.

Circadian photoperception

The circadian clock is intrinsically linked to the daily cycle of day and night. A capacity for entrainment to light-dark cycles has proven to be a universal feature of the clock in all organisms examined. Here we review a wealth of recent advances that reveal more about the light input mechanisms by which the circadian clock is set to the correct time in a range of different systems. Now that we are identifying more of the molecular components of both the light input pathway and the clock mechanism itself, we are becoming increasingly less able to distinguish between the two.

Genetics of the mammalian circadian system: Photic entrainment, circadian pacemaker mechanisms, and posttranslational regulation

During the past four years, significant progress has been made in identifying the molecular components of the mammalian circadian clock system. An autoregulatory transcriptional feedback loop similar to that described in Drosophila appears to form the core circadian rhythm generating mechanism in mammals. Two basic helix-loop-helix (bHLH) PAS (PER-ARNT-SIM) transcription factors, CLOCK and BMAL1, form the positive elements of the system and drive transcription of three Period and two Cryptochrome genes. The protein products of these genes are components of a negative feedback complex that inhibits CLOCK and BMAL1 to close the circadian loop. In this review, we focus on three aspects of the circadian story in mammals: the genetics of the photic entrainment pathway; the molecular components of the circadian pacemaker in the hypothalamic suprachiasmatic nucleus; and the role of posttranslational regulation of circadian elements. A molecular description of the mammalian circadian system has revealed that circadian oscillations may be a fundamental property of many cells in the body and that a circadian hierarchy underlies the temporal organization of animals.

Cryptochrome: the second photoactive pigment in the eye and its role in circadian photoreception

Circadian rhythms are oscillations in the biochemical, physiological, and behavioral functions of organisms that occur with a periodicity of approximately 24 h. They are generated by a molecular clock that is synchronized with the solar day by environmental photic input. The cryptochromes are the mammalian circadian photoreceptors. They absorb light and transmit the electromagnetic signal to the molecular clock using a pterin and flavin adenine dinucleotide (FAD) as chromophore/cofactors, and are evolutionarily conserved and structurally related to the DNA repair enzyme photolyase. Humans and mice have two cryptochrome genes, CRY1 and CRY2, that are differentially expressed in the retina relative to the opsin-based visual photoreceptors. CRY1 is highly expressed with circadian periodicity in the mammalian circadian pacemaker, the suprachiasmatic nucleus (SCN). Mutant mice lacking either Cry1 or Cry2 have impaired light induction of the clock gene mPer1 and have abnormally short or long intrinsic periods, respectively. The double mutant has normal vision but is defective in mPer1 induction by light and lacks molecular and behavioral rhythmicity in constant darkness. Thus, cryptochromes are photoreceptors and central components of the molecular clock. Genetic evidence also shows that cryptochromes are circadian photoreceptors in Drosophila and Arabidopsis, raising the possibility that they may be universal circadian photoreceptors. Research on cryptochromes may provide new understanding of human diseases such as seasonal affective disorder and delayed sleep phase syndrome.

From xeroderma pigmentosum to the biological clock contributions of Dirk Bootsma to human genetics

This paper commemorates the multiple contributions of Dirk Bootsma to human genetics. During a scientific 'Bootsma' cruise on his sailing-boat 'de Losbol', we visit a variety of scenery locations along the lakes and canals in Friesland, passing the highlights of Dirk Bootsma's scientific oeuvre. Departing from 'de Fluessen', his homeport, with his PhD work on the effect of X-rays and UV on cell cycle progression, we head for the pioneering endeavours of his team on mapping genes on human chromosomes by cell hybridization. Next we explore the use of cell hybrids by the Bootsma team culminating in the molecular cloning of one of the first chromosomal breakpoints involved in oncogenesis: the bcr-abl fusion gene responsible for chronic myelocytic leukemia. This seminal achievement enabled later development of new methods for early detection and very promising therapeutic intervention. A series of highlights at the horizon constitute the contributions of his team to the field of DNA repair, beginning with the discovery of genetic heterogeneity in the repair syndrome xeroderma pigmentosum (XP) followed later by the cloning of a large number of human repair genes. This led to the discovery that DNA repair is strongly conserved in evolution rendering knowledge from yeast relevant for mammals and vice versa. In addition, it resolved the molecular basis of several repair syndromes and permitted functional analysis of the encoded proteins. Another milestone is the discovery of the surprising connection between DNA repair and transcription initiation via the dual functional TFIIH complex in collaboration with Jean-Marc Egly et al. in Strasbourg. This provided an explanation for many puzzling clinical features and triggered a novel concept in human genetics: the existence of repair/transcription syndromes. The generation of many mouse mutants carrying defects in repair pathways yielded valuable models for assessing the clinical relevance of DNA repair including carcinogenesis and the identification of a link between DNA damage and premature aging. His team also opened a fascinating area of cell biology with the analysis of repair and transcription in living cells. A final surprising evolutionary twist was the discovery that photolyases designed for the light-dependent repair of UV-induced DNA lesions appeared to be adopted for driving the mammalian biological clock. The latter indicates that it is time to return to 'de Fluessen', where we will consider briefly the merits of Dirk Bootsma for Dutch science in general.

Human DNA repair genes

DNA repair systems are essential for the maintenance of genome integrity. Consequently, the disregulation of repair genes can be expected to be associated with significant, detrimental health effects, which can include an increased prevalence of birth defects, an enhancement of cancer risk, and an accelerated rate of aging. Although original insights into DNA repair and the genes responsible were largely derived from studies in bacteria and yeast, well over 125 genes directly involved in DNA repair have now been identified in humans, and their cDNA sequence established. These genes function in a diverse set of pathways that involve the recognition and removal of DNA lesions, tolerance to DNA damage, and protection from errors of incorporation made during DNA replication or DNA repair. Additional genes indirectly affect DNA repair, by regulating the cell cycle, ostensibly to provide an opportunity for repair or to direct the cell to apoptosis. For about 70 of the DNA repair genes listed in Table I, both the genomic DNA sequence and the cDNA sequence and chromosomal location have been elucidated. In 45 cases single-nucleotide polymorphisms have been identified and, in some cases, genetic variants have been associated with specific disorders. With the accelerating rate of gene discovery, the number of identified DNA repair genes and sequence variants is quickly rising. This report tabulates the current status of what is known about these genes. The report is limited to genes whose function is directly related to DNA repair.

Molecular analysis of zebrafish photolyase/cryptochrome family: two types of cryptochromes present in zebrafish

BACKGROUND

Cryptochromes (CRY), members of the DNA photolyase/cryptochrome protein family, regulate the circadian clock in animals and plants. Two types of animal CRYs are known, mammalian CRY and Drosophila CRY. Both CRYs participate in the regulation of circadian rhythm, but they have different light dependencies for their reactions and have different effects on the negative feedback loop which generates a circadian oscillation of gene expression. Mammalian CRYs act as a potent inhibitor of transcriptional activator whose reactions do not depend on light, but Drosophila CRY functions as a light-dependent suppressor of transcriptional inhibitor.

RESULTS

We cloned seven zebrafish genes that carry members of the DNA photolyase/cryptochrome protein family; one (6-4)photolyase and six cry genes. A sequence analysis and determination of their in vitro functions showed that these zebrafish cry genes constitute two groups. One has a high sequence similarity to mammalian cry genes and inhibits CLOCK:BMAL1 mediated transcription. The other, which has a higher sequence similarity to the Drosophila cry gene rather than the mammalian cry genes, does not carry transcription inhibitor activity. The expressions of these cry genes oscillate in a circadian manner, but their patterns differ.

CONCLUSIONS

These findings suggest that functionally diverse cry genes are present in zebrafish and each gene has different role in the molecular clock.

[Mechanisms of repair and radiation-induced mutagenesis in higher eukaryotes]

Cells of higher eukaryotes possess several very efficient systems for the repair of radiation-induced lesions in DNA. Different strategies have been adopted at the cellular level to remove or even tolerate various types of lesions in order to assure survival and limit the mutagenic consequences. In mammalian cells, the main DNA repair systems comprise direct reversion of damage, excision of damage and exchange mechanisms with intact DNA. Among these, the direct ligation of single strand breaks (SSB) by a DNA ligase and the multi-enzymatic repair systems of mismatch repair, base and nucleotide excision repair as well as the repair of double strand breaks (DSB) by homologous recombination or non homologous end-joining are the most important systems. Most of these processes are error-free except the non homologous end-joining pathway used mainly for the repair of DSB. Moreover, certain lesions can be tolerated by more or less accurately acting polymerases capable of performing translesional DNA syntheses. The DNA repair systems are intimately integrated in the network of cellular regulation. Some of their components are DNA damage inducible. Radiation-induced mutagenesis is largely due to unrepaired DNA damage but also involves error-prone repair processes like the repair of DSB by non-homologous end-joining. Generally, mammalian cells are well prepared to repair radiation-induced lesions. However, some questions remain to be asked about mechanistic details and efficiencies of the systems for removing certain types of radiation-damage and about their order and timing of action. The answers to these questions would be important for radioprotection as well as radiotherapy.

Circadian rhythms in a nutshell

Living organisms on this planet have adapted to the daily rotation of the earth on its axis. By means of endogenous circadian clocks that can be synchronized to the daily and seasonal changes in external time cues, most notably light and temperature, life forms anticipate environmental transitions, perform activities at biologically advantageous times during the day, and undergo characteristic seasonal responses. The effects of transmeridian flight and shift work are stark reminders that although modern technologies can create "cities that never sleep" we cannot escape the recalcitrance of endogenous clocks that regulate much of our physiology and behavior. Moreover, malfunctions in the human circadian timing system are implicated in several disorders, including chronic sleep disorders in the elderly, manic-depression, and seasonal affective disorders (SAD or winter depression). Recent progress in understanding the molecular mechanisms underlying circadian rhythms has been remarkable. In its most basic form, circadian clocks are comprised of a set of proteins that, by virtue of the design principles involved, generate a self-sustaining transcriptional-translational feedback loop with a free-running period of about 24 h. One or more of the clock components is acutely sensitive to light, resulting in an oscillator that can be synchronized to local time. This review provides an overview of the roles circadian clocks play in nature, how they might have arisen, human health concerns related to clock dysfunction, and mainly focuses on the clockworks found in Drosophila and mice, the two best studied animal model systems for understanding the biochemical and cellular bases of circadian rhythms.

Dimerization and nuclear entry of mPER proteins in mammalian cells

Nuclear entry of circadian oscillatory gene products is a key step for the generation of a 24-hr cycle of the biological clock. We have examined nuclear import of clock proteins of the mammalianperiod gene family and the effect of serum shock, which induces a synchronous clock in cultured cells. Previously, mCRY1 and mCRY2 have been found to complex with PER proteins leading to nuclear import. Here we report that nuclear translocation of mPER1 and mPER2 (1) involves physical interactions with mPER3, (2) is accelerated by serum treatment, and (3) still occurs in mCry1/mCry2double-deficient cells lacking a functional biological clock. Moreover, nuclear localization of endogenous mPER1 was observed in culturedmCry1/mCry2 double-deficient cells as well as in the liver and the suprachiasmatic nuclei (SCN) ofmCry1/mCry2 double-mutant mice. This indicates that nuclear translocation of at least mPER1 also can occur under physiological conditions (i.e., in the intact mouse) in the absence of any CRY protein. The mPER3 amino acid sequence predicts the presence of a cytoplasmic localization domain (CLD) and a nuclear localization signal (NLS). Deletion analysis suggests that the interplay of the CLD and NLS proposed to regulate nuclear entry of PER in Drosophilais conserved in mammals, but with the novel twist that mPER3 can act as the dimerizing partner.

Blue-light photoreceptors in higher plants

In the past few years great progress has been made in identifying and characterizing plant photoreceptors active in the blue/UV-A regions of the spectrum. These photoreceptors include cryptochrome 1 and cryptochrome 2, which are similar in structure and chromophore composition to the prokaryotic DNA photolyases. However, they have a C-terminal extension that is not present in photolyases and lack photolyase activity. They are involved in regulation of cell elongation and in many other processes, including interfacing with circadian rhythms and activating gene transcription. Animal cryptochromes that play a photoreceptor role in circadian rhythms have also been characterized. Phototropin, the protein product of the NPH1 gene in Arabidopsis, likely serves as the photoreceptor for phototropism and appears to have no other role. A plasma membrane protein, it serves as photoreceptor, kinase, and substrate for light-activated phosphorylation. The carotenoid zeaxanthin may serve as the chromophore for a photoreceptor involved in blue-light-activated stomatal opening. The properties of these photoreceptors and some of the downstream events they are known to activate are discussed.

Photic induction of mPer1 and mPer2 in cry-deficient mice lacking a biological clock

Mice lacking mCry1 and mCry2 are behaviorally arrhythmic. As shown here, cyclic expression of the clock genes mPer1 and mPer2 (mammalian Period genes 1 and 2) in the suprachiasmatic nucleus and peripheral tissues is abolished and mPer1 and mPer2 mRNA levels are constitutively high. These findings indicate that the biological clock is eliminated in the absence of both mCRY1 and mCRY2 (mammalian cryptochromes 1 and 2) and support the idea that mammalian CRY proteins act in the negative limb of the circadian feedback loop. The mCry double-mutant mice retain the ability to have mPer1 and mPer2 expression induced by a brief light stimulus known to phase-shift the biological clock in wild-type animals. Thus, mCRY1 and mCRY2 are dispensable for light-induced phase shifting of the biological clock.

Differential regulation of mammalian period genes and circadian rhythmicity by cryptochromes 1 and 2

Cryptochromes regulate the circadian clock in animals and plants. Humans and mice have two cryptochrome (Cry) genes. A previous study showed that mice lacking the Cry2 gene had reduced sensitivity to acute light induction of the circadian gene mPer1 in the suprachiasmatic nucleus (SCN) and had an intrinsic period 1 hr longer than normal. In this study, Cry1(-/-) and Cry1(-/-)Cry2(-/-) mice were generated and their circadian clocks were analyzed at behavioral and molecular levels. Behaviorally, the Cry1(-/-) mice had a circadian period 1 hr shorter than wild type and the Cry1(-/-)Cry2(-/-) mice were arrhythmic in constant darkness (DD). Biochemically, acute light induction of mPer1 mRNA in the SCN was blunted in Cry1(-/-) and abolished in Cry1(-/-)Cry2(-/-) mice. In contrast, the acute light induction of mPer2 in the SCN was intact in Cry1(-/-) and Cry1(-/-)Cry2(-/-) animals. Importantly, in double mutants, mPer1 expression was constitutively elevated and no rhythmicity was detected in either 12-hr light/12-hr dark or DD, whereas mPer2 expression appeared rhythmic in 12-hr light/12-hr dark, but nonrhythmic in DD with intermediate levels. These results demonstrate that Cry1 and Cry2 are required for the normal expression of circadian behavioral rhythms, as well as circadian rhythms of mPer1 and mPer2 in the SCN. The differential regulation of mPer1 and mPer2 by light in Cry double mutants reveals a surprising complexity in the role of cryptochromes in mammals.

Nuclear localization of the Arabidopsis blue light receptor cryptochrome 2

The cryptochrome blue light photoreceptor family of Arabidopsis thaliana consists of two members, CRY1 and CRY2 (PHH1). CRY2 contains a putative nuclear localization signal (NLS) within its C-terminal region. We examined whether CRY2 is localized in the nucleus and whether the C-terminal region of CRY2 is involved in nuclear targeting. Total cellular and nuclear protein extracts from Arabidopsis were subjected to immunoblot analysis with CRY2-specific antibodies. Strong CRY2 signals were obtained in the nuclear fraction. Fusion proteins consisting of the green fluorescent protein (GFP) and different fragments of CRY2 were expressed in parsley protoplasts and the localization of the fusion proteins was determined by fluorescence and confocal laser scanning microscopy. GFP-fusions containing the entire CRY2 protein or its C-terminal region were found exclusively in the nucleus. We conclude from these results that CRY2 is localized in the nucleus and that nuclear localization is mediated by the C-terminal region of CRY2.

Cryptochromes--bringing the blues to circadian rhythms

Cryptochromes are blue/UV-A-absorbing photoreceptor proteins discovered originally in plants and so named because their nature proved elusive in over a century of research. Now we know that the photoreceptor essential for proper seedling establishment in blue light has homologues in the animal kingdom - in insects, in mice and in humans. In recent months, evidence has emerged pointing to a common role for cryptochromes in all of these organisms in entraining the circadian clock, a biochemical timing mechanism running within cells, synchronizing metabolism to the daily light-dark cycle and having consequences on a much larger scale in the regulation of behaviour such as the sleep-wake cycle.

Seeing the world in red and blue: insight into plant vision and photoreceptors

Plants see light through multiple photoreceptors, including phytochromes and cryptochromes. Cryptochromes are flavoproteins that participate in many blue-light responses, including phototropism in plants and entrainment of circadian rhythms in plants and animals. A novel flavoprotein, NPH1, is also implicated in plant phototropism. Phytochromes function as serine/threonine kinases whose potential interacting partners include cryptochrome (CRY1 and CRY2).

Mammalian Cry1 and Cry2 are essential for maintenance of circadian rhythms

Many biochemical, physiological and behavioural processes show circadian rhythms which are generated by an internal time-keeping mechanism referred to as the biological clock. According to rapidly developing models, the core oscillator driving this clock is composed of an autoregulatory transcription-(post) translation-based feedback loop involving a set of 'dock' genes. Molecular clocks do not oscillate with an exact 24-hour rhythmicity but are entrained to solar day/night rhythms by light. The mammalian proteins Cryl and Cry2, which are members of the family of plant blue-light receptors (cryptochromes) and photolyases, have been proposed as candidate light receptors for photoentrainment of the biological clock. Here we show that mice lacking the Cryl or Cry2 protein display accelerated and delayed free-running periodicity of locomotor activity, respectively. Strikingly, in the absence of both proteins, an instantaneous and complete loss of free-running rhythmicity is observed. This suggests that, in addition to a possible photoreceptor and antagonistic clock-adjusting function, both proteins are essential for the maintenance of circadian rhythmicity.

Human DNA repair systems: an overview

DNA repair systems act to maintain genome integrity in the face of replication errors, environmental insults, and the cumulative effects of age. More than 70 human genes directly involved in the five major pathways of DNA repair have been described, including chromosomal location and cDNA sequence. However, a great deal of information as to the precise functions of these genes and their role in human health is still lacking. Hence, we summarize what is known about these genes and their contra part in bacterial, yeast, and rodent systems and discuss their involvement in human disease. While some associations are already well understood, it is clear that additional diseases will be found which are linked to DNA repair defects or deficiencies.

Role of mouse cryptochrome blue-light photoreceptor in circadian photoresponses

Cryptochromes are photoactive pigments in the eye that have been proposed to function as circadian photopigments. Mice lacking the cryptochrome 2 blue-light photoreceptor gene (mCry2) were tested for circadian clock-related functions. The mutant mice had a lower sensitivity to acute light induction of mPer1 in the suprachiasmatic nucleus (SCN) but exhibited normal circadian oscillations of mPer1 and mCry1 messenger RNA in the SCN. Behaviorally, the mutants had an intrinsic circadian period about 1 hour longer than normal and exhibited high-amplitude phase shifts in response to light pulses administered at circadian time 17. These data are consistent with the hypothesis that CRY2 protein modulates circadian responses in mice and suggest that cryptochromes have a role in circadian photoreception in mammals.