CTG triplet repeat in mouse growth inhibitory factor/metallothionein III gene promoter represses the transcriptional activity of the heterologous promoters

Metallothionein: A Multifunctional Protein from Toxicity to Cancer

The metallothionein (MT) family is a class of low molecular weight, intracellular and cysteine-rich proteins presenting high affinity for metal ions. Although the members of this family were discovered nearly 40 years ago, their functional significance remains obscure. Four major MT isoforms, MT-1, MT-2, MT-3 and MT-4, have been identified in mammals. MTs are involved in many pathophysiological processes such as metal ion homeostasis and detoxification, protection against oxidative damage, cell proliferation and apoptosis, chemoresistance and radiotherapy resistance. MT isoforms have been shown to be involved in several aspects of the carcinogenic process, cancer development and progression. MT expression has been implicated as a transient response to any form of stress or injury providing cytoprotective action. Although MT participates in the carcinogenic process, its use as a potential marker of tumor differentiation or cell proliferation, or as a predictor of poor prognosis remains unclear. In the present review the involvement of MT in defense mechanisms to toxicity and in carcinogenicity is discussed.

Genetic Correlates of Individual Differences in Sleep Behavior of Free-Living Great Tits (Parus major)

Within populations, free-living birds display considerable variation in observable sleep behaviors, reflecting dynamic interactions between individuals and their environment. Genes are expected to contribute to repeatable between-individual differences in sleep behaviors, which may be associated with individual fitness. We identified and genotyped polymorphisms in nine candidate genes for sleep, and measured five repeatable sleep behaviors in free-living great tits (Parus major), partly replicating a previous study in blue tits (Cyanistes caeruleus). Microsatellites in the CLOCK and NPAS2 clock genes exhibited an association with sleep duration relative to night length, and morning latency to exit the nest box, respectively. Furthermore, microsatellites in the NPSR1 and PCSK2 genes associated with relative sleep duration and proportion of time spent awake at night, respectively. Given the detection rate of associations in the same models run with random markers instead of candidate genes, we expected two associations to arise by chance. The detection of four associations between candidate genes and sleep, however, suggests that clock genes, a clock-related gene, or a gene involved in the melanocortin system, could play key roles in maintaining phenotypic variation in sleep behavior in avian populations. Knowledge of the genetic architecture underlying sleep behavior in the wild is important because it will enable ecologists to assess the evolution of sleep in response to selection.

Testing for associations between candidate genes for circadian rhythms and individual variation in sleep behaviour in blue tits

The regulation of sleep in animals is controlled by environmental factors, homeostatic mechanisms and endogenous circadian oscillators. The molecular mechanisms underlying such circadian oscillators have been described in detail and a variety of genes that are components of these molecular clocks have been reported. In addition to inter-specific variation in the temporal organization of sleep, there is significant intra-specific variation in different organisms. From numerous studies in humans it is known that polymorphisms in the regulatory clock genes are causing such variation but knowledge about associations between naturally occurring polymorphisms and sleep patterns in wild animals is scarce. In this study, we investigated the phenotypic sleep correlates of eleven previously described polymorphisms in seven candidate genes within a free-living blue tit Cyanistes caeruleus population. We detected associations between four single nucleotide polymorphisms and three of the nine tested sleep parameters representing temporal organization. Awakening time was associated with polymorphisms in AANAT and PERIOD2, morning latency with a polymorphism in CKIε and the duration of the longest sleep bout with a second polymorphism in AANAT. However, by a permutation procedure we showed that the number of significant results and the most significant association has a study-wide likelihood of 46.7 and 5.9 % respectively. Further replication studies are needed to evaluate the potential associations.

Search for informative polymorphisms in candidate genes: clock genes and circadian behaviour in blue tits

The identification of functional polymorphisms in genes that underlie behavioural trait variation is a challenging but intriguing task in evolutionary biology. Given the wealth of genomic data and the increasing number of genotype-phenotype association studies in model organisms, one can ask whether and how this information can be used for non-model organisms. Here we describe two strategies to search for likely functional polymorphisms in candidate genes in a bird species that has been intensively studied by behavioural and population ecologists, the blue tit Cyanistes caeruleus. In the first approach we searched for repeating elements in coding regions of the genome using information about repeats in Gallus gallus genes. The rationale is that tandem-repeat elements have a high potential to be polymorphic and functional. The second strategy aimed to replicate reported genotype-phenotype association studies by extrapolating results from model organisms to our study species. Both strategies showed high success rates with respect to finding homologous gene regions and potentially informative genetic variants in the genes AANAT, ADCYAP1, CKIepsilon, CLOCK, CREB1, NPAS2 and PERIOD2.

Metallothionein-3, Zinc, and Copper in the Central Nervous System

Metallothionein-3 (MT-3), also known as the neuronal growth inhibitory factor, has been discovered by Uchida and coworkers in 1991 in their search for a cellular component responsible for antagonizing aberrant neuritic sprouting and increased survival of cultured neurons stimulated by Alzheimer's disease (AD) brain extract. Since this initial discovery further studies showed that MT-3 possesses peculiar structural and functional properties not shared by other members of the mammalian MT family. Several lines of evidence suggest that the metal-binding protein MT-3 plays a vital role in zinc and copper homeostasis in the brain. Although far from being understood, the unusual structural properties of MT-3 are responsible for its neuronal growth inhibitory activity, involvement in trafficking of zinc vesicles in the central nervous system, protection against copper-mediated toxicity in AD and in controlling abnormal metal-protein interactions in other neurodegenerative disorders.

Molecular characterization and function analysis of MT-10 and MT-20 metallothionein isoforms from Mytilus galloprovincialis

Structure and function of molluscan metallothioneins (MTs) are still poorly understood. The sea mussel Mytilus galloprovincialis displays two MT isoforms which differ in both primary sequences and physiological functions. MT-10 is the constitutive isoform, whereas MT-20 is mainly induced by cadmium (Cd). Both MTs were produced as recombinant proteins and showed identical Cd content and similar Cd-binding properties. Conversely, circular dichroism disclosed marked differences in the secondary conformations of the two Cd(7)-MTs. The possible relapses of these structural differences on protein stability and function were assessed. MT-10 presented a higher thermal stability and a more compact structure than MT-20, as it was inferred by absorption and emission spectroscopy studies. Moreover, the kinetics of Cd-release clearly indicated that MT-10 is much more sensitive to oxidation than is MT-20. The observed differences between MT-10 and MT-20 are discussed in terms of the different physiological roles exerted by the two isoforms in mussel.

Metallothionein expression in human neoplasia

The metallothionein family is a class of low-molecular-weight, cysteine-rich proteins with high affinity for metal ions. Four major isoforms (metallothionein-1, -2, -3, and -4) have been identified in mammals, involved in many pathophysiological processes, including metal ion homeostasis and detoxification, protection against oxidative damage, cell proliferation and apoptosis, drug and radiotherapy resistance and several aspects of the carcinogenic process. In the present review we examine the expression of metallothionein in different human tumours and its correlation with histopathological variables, tumour cell proliferation or apoptosis, resistance to radiation or chemotherapy, patient survival and prognosis. A variable profile of metallothionein and its isoforms' expression has been observed in different cancer types. Although metallothionein expression has been implicated in carcinogenic evolution, its use as a marker of tumour differentiation, cell proliferation and prognosis predictor remains unclear. Detailed studies focused on the expression of metallothionein isoforms and isotypes in different tumour types could elucidate the role of this group of proteins in the carcinogenic process, delineating its possible clinical significance for the management of patients.

Zinc or copper deficiency-induced impaired inflammatory response to brain trauma may be caused by the concomitant metallothionein changes

The role of zinc- and copper-deficient diets on the inflammatory response to traumatic brain injury (TBI) has been evaluated in adult rats. As expected, zinc deficiency decreased food intake and body weight gain, and the latter effect was higher than that observed in pair-fed rats. In noninjured brains, zinc deficiency only affected significantly lectin (increasing) and glial fibrillary acidic protein (GFAP) and Cu,Zn-superoxide dismutase (Cu,Zn-SOD) (decreasing) immunoreactivities (irs). In injured brains, a profound gliosis was observed in the area surrounding the lesion, along with severe damage to neurons as indicated by neuron specific enolase (NSE) ir, and the number of cells undergoing apoptosis (measured by TUNEL) was dramatically increased. Zinc deficiency significantly altered brain response to TBI, potentiating the microgliosis and reducing the astrogliosis, while increasing the number of apoptotic cells. Metallothioneins (MTs) are important zinc- and copper-binding proteins in the CNS, which could influence significantly the brain response to TBI because of their putative roles in metal homeostasis and antioxidant defenses. MT-I+II expression was dramatically increased by TBI, and this response was significantly blunted by zinc deficiency. The MT-III isoform was moderately increased by both TBI and zinc deficiency. TBI strongly increased oxidative stress levels, as demonstrated by malondialdehyde (MDA), protein tyrosine nitration (NITT), and nuclear factor kappaB (NF-kappaB) levels irs, all of which were potentiated by zinc deficiency. Further analysis revealed unbalanced expression of prooxidant and antioxidant proteins besides MT, since the levels of inducible nitric oxide synthase (iNOS) and Cu,Zn-SOD were increased and decreased, respectively, by zinc deficiency. All these effects were attributable to zinc deficiency, since pair-fed rats did not differ from normally fed rats. In general, copper deficiency caused a similar pattern of responses, albeit more moderate. Results obtained in mice with a null mutation for the MT-I+II isoforms strongly suggest that most of the effects observed in the rat brain after zinc and copper deficiencies are attributable to the concomitant changes in the MT expression.

Regulation of metallothionein gene expression

The rapid and robust induction of metallothioneins (MT)-I and II by a variety of inducers that include heavy toxic metals, reactive oxygen species, and different types of stress provide a useful system to study the molecular mechanisms of this unique induction process. The specific expression of MT-III in the brain and of MT-IV in the squamous epithelium of skin and tongue offers a unique opportunity to identify and characterize the tissue-specific factors involved in their expression. Studies using transgenic mice that overexpress MTs or MT null mice have revealed the role of MT in the protection of cells against numerous tissue-damaging agents such as reactive oxygen species. The primary physiological function of these proteins, however, remains an enigma. Considerable advances have been made in the identification of the cis-acting elements that are involved in the constitutive and induced expression of MT-I and MT-II. By contrast, only one key trans-activating factor, namely MTF-1, has been extensively characterized. Studies on the epigenetic silencing of MT-I and MT-II by promoter hypermethylation in some cancer cells have posed interesting questions concerning the functional relevance of MT gene silencing, the molecular mechanisms of MT suppression in these cells, particularly chromatin modifications, and the characteristics of the repressors.

Expression of metallothionein-III mRNA and its regulation by levodopa in the basal ganglia of hemi-parkinsonian rats

In the brain, metallothionein (MT)-III exhibits a free radical scavenging activity. Here we examined the expression of MT-III mRNA in the basal ganglia of 6-hydroxydopamine (6-OHDA)-lesioned hemi-parkinsonian rats and its regulation by levodopa. The level of MT-III mRNA was significantly decreased in the striatum of 6-OHDA-lesioned side. Levodopa treatment significantly increased the expression of striatal MT-III mRNA in the non-lesioned side, but showed no significant effect in the 6-OHDA-lesioned side. These results suggest that the regulation of MT-III mRNA may be related to the progressive degeneration in parkinsonism.

Reduced expression of DMAHP/SIX5 gene in myotonic dystrophy muscle

In myotonic dystrophy (DM), the expansion of CTG triplet repeats in the 3'-untranslated region of DM-protein kinase (DMPK) is a causal gene mutation. However, the pathogenic molecular mechanism of CTG repeat expansion for DM phenotypic expression is unclear. To investigate this issue, we examined the influence of CTG repeat expansion on the expression levels of DMPK gene and 3'-flanking DM locus-associated homeodomain protein (DMAHP)/SIX5 gene in the muscles of DM patients. We isolated RNA from muscle tissues of six DM patients and six controls, and performed a competitive reverse transcriptional polymerase chain reaction (RT-PCR) assay. The total mRNA level of DMAHP/SIX5 was significantly lower in DM than in controls, but the DMPK mRNA level was unchanged. Our results suggest that CTG repeat expansion influences the expression of genes other than DMPK to cause the DM phenotype.

Characterization of cis-acting elements in the promoter of the mouse metallothionein-3 gene. Activation of gene expression during neuronal differentiation of P19 embryonal carcinoma cells

The metallothionein (MT)3 gene is expressed predominantly in the brain and the organs of the reproductive system, and fails to respond to metal ions in vivo. A CTG repeat was proposed to function as a potential repressor element in nonpermissive cells, and a sequence similar to the JC virus silencer element was found to function as a negative element in permissive primary astrocytes. The objective of this study was to characterize further the mechanisms governing cell-type specific MT-3 gene transcription. We searched for a suitable cell line expressing the MT-3 gene to be used for determination of MT-3 promoter tissue specificity, and showed that MT-3 expression is activated during neuroectodermal differentiation of P19 cells induced by retinoic acid to levels similar to those found in whole brain. Deletion of the CTG repeat or of the JC virus silencer did not promote MT-3 promoter activity in nonpermissive cells, or enhance expression in permissive cells. We identified MT-3 promoter sequences interacting with liver and brain nuclear proteins, as assayed by DNase I footprinting analyses and electrophoretic mobility shift assay, and assessed the role of these sequences in the regulation of MT-3 expression by cotransfection experiments. We generated stable transfectants in permissive C6 and nonpermissive NIH-3T3 cells, and analysed the methylation status of the MT-3 gene. These studies show that regulation of tissue-specific MT-3 gene expression does not appear to involve a repressor, and suggest that other mechanisms such as chromatin organization and epigenetic modifications could account for the absence of MT-3 gene transcription in nonpermissive cells.

Regulation of metallothionein gene transcription. Identification of upstream regulatory elements and transcription factors responsible for cell-specific expression of the metallothionein genes from Caenorhabditis elegans

Metallothioneins are small, cysteine-rich proteins that function in metal detoxification and homeostasis. Metallothionein transcription is controlled by cell-specific factors, as well as developmentally modulated and metal-responsive pathways. By using the nematode Caenorhabditis elegans as a model system, the mechanism that controls cell-specific metallothionein transcription in vivo was investigated. The inducible expression of the C. elegans metallothionein genes, mtl-1 and mtl-2, occurs exclusively in intestinal cells. Sequence comparisons of these genes with other C. elegans intestinal cell-specific genes identified multiple repeats of GATA transcription factor-binding sites (i.e. GATA elements). In vivo deletion and site-directed mutation analyses confirm that one GATA element in mtl-1 and two in mtl-2 are required for transcription. Electrophoretic mobility shift assays show that the C. elegans GATA transcription factor ELT-2 specifically binds to these elements. Ectopic expression of ELT-2 in non-intestinal cells of C. elegans activates mtl-2 transcription in these cells. Likewise, mtl-2 is not expressed in nematodes in which elt-2 has been disrupted. These results indicate that cell-specific transcription of the C. elegans metallothionein genes is regulated by the binding of ELT-2 to GATA elements in these promoters. Furthermore, a model is proposed where ELT-2 constitutively activates metallothionein expression; however, a second metal-responsive factor prevents transcription in the absence of metals.

Structural and functional analysis of the rat metallothionein III genomic locus

Metallothionein III (MT III) has been reported to suppress neuronal growth in a rat in vitro model system. The protein and its specific mRNA are detected predominantly in the brain, differentiating MT III from the well-characterised archetypal metallothioneins. Isolation, sequencing and functional analysis of the rat MT III genomic locus indicated that, although the organisation of the gene was conserved between MT III and the more conventional metallothioneins, the 5' flanking region of the MT III gene was distinct. Within this region, a number of putative regulatory elements were identified, including the metal regulatory elements (MREs) characteristic of metallothionein promoters. However, despite their conservation in sequence with active elements, the MREs of MT III were unresponsive to zinc. A 'silencing element' was revealed within a 250 bp section of the MT III promoter which suppressed gene expression in two brain cell lines. The operation of this silencing region in conjunction with the inactive MREs may explain the distinct expression profile observed for MT III within the central nervous system and during neuronal development.

Localization of metallothionein-I and -III expression in the CNS of transgenic mice with astrocyte-targeted expression of interleukin 6

The effect of interleukin-6 (IL-6) on metallothionein-I (MT-I) and MT-III expression in the brain has been studied in transgenic mice expressing IL-6 under the regulatory control of the glial fibrillary acidic protein gene promoter (GFAP-IL6 mice), which develop chronic progressive neurodegenerative disease. In situ hybridization analysis revealed that GFAP-IL6 (G16-low expressor line, and G36-high expressor line) mice had strongly increased MT-I mRNA levels in the cerebellum (Purkinje and granular layers of the cerebellar cortex and basal nuclei) and, to a lesser degree, in thalamus (only G36 line) and hypothalamus, whereas no significant alterations were observed in other brain areas studied. Microautoradiography and immunocytochemistry studies suggest that the MT-I expression is predominantly localized to astrocytes throughout the cerebrum and especially in Bergman glia in the cerebellum. However, a significant expression was also observed in microglia of the GFAP-IL6 mice. MT-III expression was significantly increased in the Purkinje cell layer and basal nuclei of the cerebellum, which was confirmed by Northern blot analysis of poly(A)+ mRNA and by ELISA of the MT-III protein. In contrast, in the G36 but not G16 mice, transgene expression of IL-6 was associated with significantly decreased MT-III RNA levels in the dentate gyrus and CA3 pyramidal neuron layer of the hippocampus and, in both G36 and G16 mice, in the occipital but not frontal cortex and in ependymal cells. Thus, both the widely expressed MT-I isoform and the CNS specific MT-III isoform are significantly affected in a MT isoform- and CNS area-specific manner in the GFAP-IL6 mice, a chronic model of brain damage.

Brain injury and growth inhibitory factor (GIF)--a minireview

Growth inhibitory factor (GIF) is a small (7 kDa), heat-stable, acidic, hydrophilic metallothionein (MT)-like protein. GIF inhibits the neurotrophic activity in Alzheimer's disease (AD) brain extracts on neonatal rat cortical neurons in culture. GIF has been shown to be drastically reduced and down-regulated in AD brains. In neurodegenerative diseases in humans, GIF expression levels are reduced whereas GFAP expression levels are markedly induced in reactive astrocytes. Both GIF and GIF mRNA are present at high levels in reactive astrocytes following acute experimental brain injury. In chronological observations the level of GIF was found to increase more slowly and remain elevated for longer periods than that of glial fibrillary acidic protein (GFAP). These differential patterns and distribution of GIF and GFAP seem to be important in understanding the mechanism of brain tissue repair. The most important point concerning GIF in AD is not simply the decrease in the level of expression throughout the brain, but the drastic decrease in the level of expression in reactive astrocytes around senile plaques in AD. Although what makes the level of GIF decrease drastically in reactive astrocytes in AD is still unknown, supplements of GIF may be effective for AD, based on a review of current evidence. The processes of tissue repair following acute brain injury are considered to be different from those in AD from the viewpoint of reactive astrocytes.

Transgenic expression of interleukin 6 in the central nervous system regulates brain metallothionein-I and -III expression in mice

The metallothionein (MT) gene family consists of several members (MT-I-IV) that are tightly regulated during development. MT-I and MT-II are expressed in many tissues, including the brain, whereas MT-III is expressed mainly in the central nervous system. However, the physiological roles of these isoforms in the brain and their regulation are poorly characterized. In this report, we have studied the putative role of IL-6 in the regulation of brain MT. The present results demonstrated that transgenic mice expressing IL-6 under the regulatory control of the glial fibrillary acidic protein gene promoter (GFAP-IL6 mice), and which develop chronic progressive neurodegenerative disease, show significantly increased MT-I + II protein levels in specific brain areas. Thus, the MT-I + II levels of 1- and 3-month-old GFAP-IL6 mice (G16 and/or G36 lines) were not altered in hippocampus but they were elevated in the cerebellum (highest induction), medulla plus pons, hypothalamus and remaining brain (lowest induction). The effect of the transgenic expression of IL-6 was more dramatic for MT-I + II protein than for MT-I mRNA levels, with the latter only marginally elevated in the G16 line at 3 months but not at 6 months of age where there was a tendency to decreased levels. Brain MT-I mRNA levels also tended to decrease in the higher expressor G36 line in 3-month-old mice despite the strongly elevated MT-I + II protein levels at this age. Therefore, in addition to increasing MT gene transcription, these results suggest a post-transcriptional effect of IL-6 or of a IL-6-dependent factor, in this chronic situation. The up-regulated brain MT-I + II protein levels in the GFAP-IL6 mice was comparable to the expression of the acute-phase response gene EB22/5, suggesting that these MT isoforms could be considered acute-phase response proteins in the brain. Brain MT-III mRNA levels followed a somewhat similar pattern that those of MT-I mRNA but the decreasing effect of IL-6 transgene production with age was more dramatic for the former, suggesting differential regulation of these MT isoforms by IL-6. The results indicate that these transgenic mice might be a valuable tool for further examining the role of the MT isoforms in brain physiology and pathobiology.

Rapid protein sequencing by tandem mass spectrometry and cDNA cloning of p20-CGGBP. A novel protein that binds to the unstable triplet repeat 5'-d(CGG)n-3' in the human FMR1 gene

The autonomous expansion of the unstable 5'-d(CGG)n-3' repeat in the 5'-untranslated region of the human FMR1 gene leads to the fragile X syndrome, one of the most frequent causes of mental retardation in human males. We have recently described the isolation of a protein p20-CGGBP that binds sequence-specifically to the double-stranded trinucleotide repeat 5'-d(CGG)-3' (Deissler, H., Behn-Krappa, A., and Doerfler, W. (1996) J. Biol. Chem. 271, 4327-4334). We demonstrate now that the p20-CGGBP can also bind to an interrupted repeat sequence. Peptide sequence tags of p20-CGGBP obtained by nanoelectrospray mass spectrometry were screened against an expressed sequence tag data base, retrieving a clone that contained the full-length coding sequence for p20-CGGBP. A bacterially expressed fusion protein p20-CGGBP-6xHis exhibits a binding pattern to the double-stranded 5'-d(CGG)n-3' repeat similar to that of the authentic p20-CGGBP. This novel protein lacks any overall homology to other known proteins but carries a putative nuclear localization signal. The p20-CGGBP gene is conserved among mammals but shows no homology to non-vertebrate species. The gene encoding the sequence for the new protein has been mapped to human chromosome 3.

Characterisation of expression of mDMAHP, a homeodomain-encoding gene at the murine DM locus

We examined the expression of the murine homologue of myotonic dystrophy associated homeodomain protein (mDMAHP) using two different strategies. The first approach, RT-PCR, detected spliced transcripts in a wide range of embryonic and adult tissues, in a pattern overlapping substantially with the expression of mDMPK. A second approach, the generation of transgenic mice expressing the lacZ reporter gene from a 4.3 kb promoter fragment, also demonstrated expression in a range of tissues with potential links to the phenotype in myotonic dystrophy. We conclude that murine DMAHP has a similar pattern of expression to human DMAHP and will serve as a useful model for functional studies of this gene, although species differences, such as the reduced CpG island (1.8 kb compared with 3.5 kb) must be carefully considered.

Characterization of an expanded glutamine repeat androgen receptor in a neuronal cell culture system

Spinal and bulbar muscular atrophy (SBMA) is an inherited form of lower motor neuron degeneration caused by expansion of a CAG repeat in the androgen receptor (AR) gene. To study the mechanism by which this mutation causes neuronal pathology, we stably transfected a motor neuron hybrid cell line with human AR cDNAs containing either 24 or 65 repeats (AR24 and AR65, respectively). Both forms of receptor were able to bind ligand and activate transcription of a reporter construct equally well. Likewise, the subcellular localizations of AR24 and AR65 were similar, in both the presence and the absence of ligand. AR24- and AR65-expressing clones were phenotypically indistinguishable. They survived equally well after differentiation and were equally susceptible to damage by oxidative stress. Our studies thus demonstrate that, in a neuronal system, the expanded repeat AR functions like the normal repeat AR in several important ways. Because levels of AR65 expression were consistently lower than levels of AR24 expression, we propose that the loss of function of AR seen in SBMA may be due to decreased levels of receptor expression rather than to a difference in intrinsic properties. The postulated gain of function responsible for neuronal degeneration remains to be determined.

Negative regulation of gene expression in eukaryotes

Gene expression is mainly regulated at the transcription level. For the specific regulation of gene expression, two components are required: one is the cis-element that is the short DNA sequence in the regulatory region of the gene, and the other is the trans-acting factor that binds to the cis-element. This complex then interacts with the initiation complex, including RNA polymerase II, and regulates the gene expression. Although many elements and factors are reported as involved in the gene activation, little is known about the negative regulation of gene expression. In this study, analyses of the regulatory regions in glutathione transferase P and growth inhibitory factor genes are presented, and the mechanisms of the negative regulation are discussed.

Nucleosome assembly on CTG triplet repeats

Expansion of CTG repeat sequences is associated with several human genetic diseases. We have examined the consequences of CTG repeat expansion for nucleosome assembly and positioning. Short CTG repeats are found within the most favored DNA sequences yet defined for nucleosome assembly. We find that as few as six CTG repeats will facilitate nucleosome assembly to a similar extent as the 50 or more repeats found in disease genes. Thus an increase in nucleosome stability on expansion of existing triplet repeats is unlikely to explain the acquisition of the disease phenotype. However, the CTG repeat sequence is efficiently wrapped around the histone octamer, preferring to associate with histones at the nucleosomal dyad. Thus short segments CTG repeat sequence will facilitate the assembly of a stable positioned nucleosome which might contribute to the expansion phenomenon and the functional organization of chromatin.