Skip Navigation

Human Molecular Genetics 2005 14(Review Issue 1):R19-R26; doi:10.1093/hmg/ddi102
This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow A corrigendum has been published
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Add to My Personal Archive
Right arrow Download to citation manager
Right arrowRequest Permissions
Google Scholar
Right arrow Articles by Caballero, I. M.
Right arrow Articles by Hendrich, B.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Caballero, I. M.
Right arrow Articles by Hendrich, B.
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?

© The Author 2005. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oupjournals.org

MeCP2 in neurons: closing in on the causes of Rett syndrome

Isabel Martín Caballero and Brian Hendrich*

Institute for Stem Cell Research, Centre Development in Stem Cell Biology, School of Biological Sciences, University of Edinburgh, Edinburgh EH9 3JQ, UK

* To whom correspondence should be addressed. Tel: +44 131 650 5867; Fax: +44 131 650 7773; Email: brian.hendrich{at}ed.ac.uk

Received January 3, 2005; Revised January 25, 2005; Accepted February 23, 2005


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MeCP2
 TWO ISOFORMS OF MECP2
 MOUSE MODELS
 X-CHROMOSOME INACTIVATION
 TOO MUCH OF A...
 TARGETS OF MeCP2-MEDIATED...
 REFERENCES
 
The discovery in 1999 that Rett syndrome (RTT) is caused by mutations in a gene encoding the methyl-CpG-binding repressor protein MECP2 provided a significant breakthrough in the understanding of this devastating disease. The subsequent production of Mecp2 knockout mice 2 years later provided an experimental resource to better understand how mutations in the MECP2 gene result in RTT. This paper reviews the recent progress in understanding when and where MeCP2 function becomes important in the developing brain, why MeCP2 protein levels are crucial, which genes are normally silenced by MeCP2, and how misexpression of these targets might lead to the clinical manifestations of RTT.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MeCP2
 TWO ISOFORMS OF MECP2
 MOUSE MODELS
 X-CHROMOSOME INACTIVATION
 TOO MUCH OF A...
 TARGETS OF MeCP2-MEDIATED...
 REFERENCES
 
Rett syndrome (RTT; OMIM #321750) is a neurological disorder that affects primarily girls, with an incidence of 1/10 000 to 1/15 000 live births (1Go–4Go). The affected individuals are born asymptomatic and develop normally during the first few months of life, achieving the expected milestones in motor language and social areas. After 6–18 months of age, however, acquired speech and motor skills begin to be lost (3Go). Purposeful hand movements are replaced by characteristic hand-wringing motions and gait apraxia, followed by growth retardation and deceleration of head growth. In addition, half of affected individuals develop seizures and autistic behaviour, and by 4–7 years display gross cognitive and motor impairment together with profound hypoactivity which remains throughout their lives (3Go).

Histopathological studies on post-mortem RTT brains have revealed that the observed microcephaly correlates with reduced brain size, particularly of the prefrontal, posterior frontal and anterior temporal regions (5Go). This characteristic reduction in size roughly correlates with both a decrease in the size of individual neurons and increased neuronal packing densitiy in these regions (6Go,7Go). Additionally a reduction of dendritic arborization in cerebral cortical layers II and IV in the frontal, motor and inferior temporal regions has been observed (8Go).


   MeCP2
TOP
 ABSTRACT
 INTRODUCTION
 MeCP2
TWO ISOFORMS OF MECP2
 MOUSE MODELS
 X-CHROMOSOME INACTIVATION
 TOO MUCH OF A...
 TARGETS OF MeCP2-MEDIATED...
 REFERENCES
 
RTT is caused by mutations in the MECP2 gene (9Go). MeCP2 is a member of a family of proteins capable of binding to methylated DNA and recruiting chromatin modifying activities to silence transcription (10Go–14Go). MECP2 is an X-linked gene (15Go), and girls with RTT are normally heterozygous for a mutation in MECP2. Random X-chromosome inactivation (XCI) in these girls means that, on average, half of their cells have inactivated the X chromosome bearing the mutant MECP2 allele and are effectively normal. The other half of their cells have inactivated the X chromosome bearing the wild-type allele and are effectively null for MECP2. Thus, RTT patients are chimeras for wild-type cells and MECP2-null cells. Although favourable XCI ratios have been reported in some normal or mildly affected obligate mutation carriers (16Go–18Go), RTT is unusual among X-linked diseases in that profound skewing of XCI patterns does not normally occur (19Go,20Go). The vast majority of RTT-causing mutations are sporadic and inherited from the paternal germ line (21Go), hence males very rarely inherit mutations in MECP2. However, in the rare case when a boy does inherit a RTT-causing mutation the condition is very severe, usually presenting within days of birth and resulting in death within ~2 years (22Go,23Go).

A large body of evidence indicates that MeCP2 is predominantly expressed in differentiated neurons, with strongest expression in the more mature neurons (24Go–26Go). Studies in both rat (24Go) and mouse (27Go) identify the thalamus, medulla and basal ganglia as the first brain regions to express MeCP2 in neonatal animals, but both show reduced expression after 1–2 weeks of age. At birth, Cajal–Retzius neurons in the marginal zone and subplate cells, which are the first neurons to be born in the cortex, are positive for MeCP2 staining. Within days cortical plate cells, which later generate neurons, begin expressing high levels of MeCP2. This pattern of expression correlates roughly with maturation of the cortex (25Go,26Go). Increased MeCP2 expression similarly parallels maturation in the hippocampus, initiating in the pyramidal cell layer and continuing in the granule cell layer of the dentate gyrus (24Go,28Go). Similarly, in the cerebellum MeCP2 is expressed only in the purkinje and golgi cells (neurons born during embryonic development) before cerebellar development is complete at ~3 weeks of age (29Go). It is not until after the third week when granule cells and stellate neurons, from the granule cell and molecular layers, respectively, start to show expression of MeCP2.


   TWO ISOFORMS OF MECP2
TOP
 ABSTRACT
 INTRODUCTION
 MeCP2
 TWO ISOFORMS OF MECP2
MOUSE MODELS
 X-CHROMOSOME INACTIVATION
 TOO MUCH OF A...
 TARGETS OF MeCP2-MEDIATED...
 REFERENCES
 
It has recently been discovered that exon 2 of the four-exon MECP2 gene is not included in all forms of the MECP2 transcript (30Go,31Go) (Fig. 1). The result of this alternate splicing is that two different isoforms of MECP2 are produced using different initiator methionines, one in exon 1 and the other in exon 2. The two isoforms differ only at their very N-termini, whereas the remainder of the protein, including the methyl-CpG-binding domain (MBD) and transcriptional repression domain (TRD), are not affected by this alternate splicing (Fig. 1). This newly identified form of the MECP2 transcript, called MECP2B in humans or Mecp2{alpha} in mice, is more abundant than the previously identified MECP2A/Mecp2ß transcript in most tissues, including brain (30Go,31Go). Furthermore, using an ES-cell-based differentiation system, Kriaucionis and Bird (31Go) found that while Mecp2ß is more abundant in ES cells, the proportion of Mecp2{alpha} mRNA increases during differentiation.



View larger version (10K):
[in this window]
[in a new window]
  		Figure 1. Alternate transcripts from the MECP2 locus. The MECP2 gene is represented on the middle line with exons indicated as boxes. The predominant transcript in somatic tissues, called MECP2B in humans or Mecp2{alpha} in mice, is indicated above the line and does not include exon 2. The minor form (which predominates in murine embryonic stem cells) is called MECP2A in humans or Mecp2ß in mice, and is indicated below the gene. The ORFs in each transcript are shown in blue; the MBD is indicated in red and the TRD is shown in green. Note that the MECP2B/Mecp2{alpha} ORF initiates in exon 1, whereas the MECP2A/Mecp2ß ORF initiates in exon 2.

 
As a percentage of patients with RTT do not have mutations in exon 3 or 4 of MECP2, Mnatzakanian et al. (30Go) screened some of these patients for mutations that might affect the exon 1-encoded open reading frame (ORF). In one patient, a deletion was identified which results in a frameshift in the MECP2B ORF, but does not affect the MECP2A ORF. Although this deletion removes part of the MECP2A 5'untranslated region and could affect its transcription, quantification of mRNA levels showed no alteration of transcript ratios in the patient's whole blood (30Go). It remains possible that in addition to causing a frameshift in MECP2B, this mutation alters the translation efficiency of MECP2A or affects transcription in brain. If this mutation does only affect MECP2B, it provides evidence that its presence in humans is required to prevent RTT, irrespective of the presence of the MECP2A protein [notably this is not the case in mice; discussed subsequently and (32Go)].


   MOUSE MODELS
TOP
 ABSTRACT
 INTRODUCTION
 MeCP2
 TWO ISOFORMS OF MECP2
 MOUSE MODELS
X-CHROMOSOME INACTIVATION
 TOO MUCH OF A...
 TARGETS OF MeCP2-MEDIATED...
 REFERENCES
 
A variety of mouse models have been produced which have provided significant advances in our understanding of RTT and of MeCP2 function (Table 1). Mecp2 heterozygous female mice are viable, fertile, and appear normal well into adulthood (33Go,34Go) despite having the same genotype as RTT girls. At ~6 months of age, however, these females begin to show neurological symptoms reminiscent of RTT (34Go), indicating that the onset of RTT is unlikely to depend upon the developmental stage of affected individuals, as humans and mice develop a similar disease after the same amount of time, despite being at completely different developmental stages. Hemizygous null male or homozygous null female mice appear healthy at birth, but began to display RTT-like phenotypes after ~6 weeks and die at an average age of ~8 weeks of age (33Go,34Go). Brain architecture in null mice is grossly normal, although a slight decrease in the size of neurons and an increased packing density in the hippocampus, cerebral cortex, and cerebellum could be identified (25Go,33Go). This finding correlates with disease pathology in humans where a decrease in the size of individual neurons and increased packing density are likely contributory factors to the smaller brain size often observed in RTT patients (6Go–8Go).


View this table:
[in this window]
[in a new window]
  		Table 1. Mecp2 mouse models
 
In 2002, Shahbazian et al. (35Go) produced mice expressing a truncated form of MeCP2 often seen in RTT patients. These Mecp2 mutant males (MeCP2308/Y) were aphenotypic until 4–5 months of age, after which time they began to display tremors, kyphosis, and motor dysfunction. Heterozygous females (MeCP2308/X) have impaired motor features at 35–39 weeks and show phenotypic variability ascribed to differences in patterns of XCI. Results of learning and memory tests were reported to be normal (35Go).

Mice in which MeCP2 was deleted in nestin-positive neural precursors (and their progeny, i.e. all neurons and glia) displayed a phenotype indistinguishable from that seen in MeCP2-null mice (33Go,34Go), indicating that the primary site of action for MeCP2 is in the brain. Furthermore, Chen et al. (33Go) produced mice in which the Mecp2 gene was only deleted in cells expressing {alpha}CamKII, a kinase specifically present in postmitotic neurons. The {alpha}CamKII promoter is normally active in postnatal excitatory neurons of the forebrain, hippocampus and brainstem, and to a much lesser degree in the cerebellum (36Go), although the exact distribution and strength of its activity within the forebrain can vary from one transgenic line to another (33Go,37Go). These mice appeared normal until ~3 months of age, at which time they began to display ataxic gait and reduced nocturnal activity. Histological analysis revealed reduced brain weights and smaller neuronal cell bodies in cortex and hippocampus (33Go). Interestingly, these phenotypes were not seen in the cerebellum where {alpha}CamKII is not expressed.

Together with the growing body of protein expression data, these murine studies leave little doubt that the primary (if not the only) site of action for MECP2 function is in neurons. Two recently published studies have taken this conclusion one step further by delimiting the stage of development at which defects in MeCP2-deficient neurons first appear (25Go,38Go). By isolating neuronal precursors from murine embryos and then inducing them to differentiate in culture, Kishi and Macklis (25Go) found no evidence for defects in the proliferation or fate decisions of neuronal precursors in the absence of MeCP2, a finding that contrasts with the situation in the frog Xenopus laevis where xMeCP2 is required for embryonic development (39Go). Rather, these authors found evidence that MeCP2 is important for maturation and maintenance of postmitotic neurons in murine brains (25Go). Similarly, Matarazzo et al. (38Go) identified defects in neuronal maturation in the olfactory system of Mecp2-null mice, which are ascribed to an observed delay in neuronal maturation.


   X-CHROMOSOME INACTIVATION
TOP
 ABSTRACT
 INTRODUCTION
 MeCP2
 TWO ISOFORMS OF MECP2
 MOUSE MODELS
 X-CHROMOSOME INACTIVATION
TOO MUCH OF A...
 TARGETS OF MeCP2-MEDIATED...
 REFERENCES
 
Although it was initially thought that MeCP2 played no role in survival of neurons (20Go), recent careful studies of XCI patterns in Mecp2(+/–) mice have provided evidence to the contrary. Braunschweig et al. (27Go) found that although there was a uniform regional distribution of MeCP2-negative cells in brains of Mecp2(+/–) female mice (i.e. those cells in which the X chromosome bearing the wild-type Mecp2 allele is the inactive X), as a whole the XCI patterns were skewed in favour of inactivating the mutant allele (27Go). A similar conclusion was reached by Young and Zoghbi (40Go), who found that the X chromosome bearing the mutant Mecp2 allele is predominantly the inactive X in the cerebellum and to a lesser extent in the midbrain and cortex of Mecp2308/X female mice. In an attempt to provide an explanation for this observation, these authors monitored the survival of hippocampal cultures derived from E18 Mecp2308/X embryos. When wild-type and mutant cells were plated out at a 1 : 1 ratio, it was found that after 7 days the culture consisted of predominantly wild-type neurons. At present it is not clear whether this effect is because of a defect in plating efficiency, survival, or proliferation of MeCP2-deficient neurons and/or neuronal precursors. In any case, however, these studies provide evidence for a previously undetected role for MeCP2 in ex vivo hippocampal cultures.


   TOO MUCH OF A GOOD THING
TOP
 ABSTRACT
 INTRODUCTION
 MeCP2
 TWO ISOFORMS OF MECP2
 MOUSE MODELS
 X-CHROMOSOME INACTIVATION
 TOO MUCH OF A...
TARGETS OF MeCP2-MEDIATED...
 REFERENCES
 
As RTT-like symptoms are caused in mice by a deficiency of MeCP2 in neurons, it is important to find out whether re-introducing MeCP2 will rescue these phenotypes. Two different groups have addressed this issue by producing Mecp2/MECP2-transgenic animals (Table 1). In the first study, Mecp2ß (Fig. 1) was expressed under the control of the Tau promoter (32Go). Tau is a major neuronal microtubule-associated protein (41Go) which is expressed in postmitotic neurons at early stages of differentiation (42Go). Expression initiates around day 9 of embryonic development, and by 10.75 days Tau is expressed throughout the entire nervous system (42Go). Mecp2-deficient males harbouring one allele of the tau–Mecp2ß transgene were indistinguishable from wild-type mice (32Go). Thus, expression of the MeCP2ß isoform from day 9 of development in neurons is sufficient to fully rescue the Mecp2 deletion pheontype in mice. This study resulted in another insight into MeCP2 function when the transgene was crossed onto mice also carrying a wild-type Mecp2 allele. This genetic combination produces elevated levels of Mecp2ß in neurons and resulted in severe motor dysfunction, indicating that too much MeCP2 interferes with normal neuronal function.

The other group developed a mouse model in which (MeCP2308/Y) mice were made transgenic for the human MECP2 gene (43Go). [This transgene presumably expresses both MECP2A and MECP2B (Fig. 1), although this was not specifically shown in this study.] Transgenic mice developed a progressive neurological phenotype, the onset and lethality of which was correlated with the levels of MECP2 expression. When motor coordination and cerebellar learning were assessed, moderately affected transgenic mice performed better than their wild type littermates. Further, these mice scored higher in hippocampal learning tests than wild-type littermates, and both short-term synaptic plasticity and long-term potentiation (LTP) were found to be enhanced in MeCP2 transgenic mice (43Go). Deciphering exactly why these transgenic mice display increased learning abilities may provide further clues as to the normal functions of MeCP2 in neurons.

Interestingly, increased MECP2 expression levels have been identified an autistic patient and one pervasive developmental disorder patient (44Go). This observation raises the possibility that both deficiency and elevated expression of MECP2 may influence expression of the same set of downstream target genes, the expression of which could also be altered in other neurodevelopmental disorders, such as autism. Indeed, decreased expression of two genes has recently been demonstrated in brains from RTT, autism and Angelman syndrome patients (45Go)(and see below).


   TARGETS OF MeCP2-MEDIATED REPRESSION
TOP
 ABSTRACT
 INTRODUCTION
 MeCP2
 TWO ISOFORMS OF MECP2
 MOUSE MODELS
 X-CHROMOSOME INACTIVATION
 TOO MUCH OF A...
 TARGETS OF MeCP2-MEDIATED...
REFERENCES
 
MeCP2 possesses strong transcriptional repressor activity, binds to chromosomes and associates with chromatin modifying enzymes (13Go,14Go,46Go,47Go). It is therefore reasonable to hypothesize that loss of MECP2 in a cell would result in inappropriate expression of a large number of genes (48Go). Consistent with this assumption are reports in which a number of genes were found to be misexpressed in lymphoblast cell lines derived from RTT patients (49Go,50Go). In some cases, the authors were able to demonstrate binding of MECP2 to the relevant promoters in normal cell lines, indicating that MeCP2 does indeed contribute to silencing of these genes in cells (49Go). However cell lines are known to have high levels of CpG island methylation that is not seen in primary tissues (51Go), so the relevance of these target genes to the pathology of RTT remains to be determined.

A number of studies have been published in which global gene expression changes were identified in brain or neuronal tissue (52Go–54Go), but these have failed to produce a unified molecular picture of the RTT or Mecp2-null brain. What these studies collectively illustrate is that the problem of identifying direct targets or MECP2-mediated repression is not necessarily going to be an easy one to solve using global gene expression (52Go,54Go) or proteomics (53Go) approaches.

One likely class of genes to be direct targets of MeCP2-mediated repression are the imprinted genes (55Go). Indeed, MeCP2 has been shown to physically associate with the silent alleles of various imprinted genes (56Go–58Go). Surprisingly, no loss of imprinting could be found at these loci in Mecp2-deficient mice (33Go,34Go) casting doubt on a role for MeCP2 in the maintenance of imprinting. Early in 2005, however, convincing evidence finally emerged that MeCP2 is important for imprinting at the Dlx5 locus (59Go) (Fig. 2).



View larger version (20K):
[in this window]
[in a new window]
  		Figure 2. MeCP2 regulation of the Dlx5 imprinted locus in mice. (A) In wild-type neurons MeCP2 binds to sites on the paternal allele of the Dlx5/6 locus and exerts a repressive influence upon the region, probably through the recruitment of a histone-modifying co-repressor complex (green circles). Transcription of the Dlx5 gene is reduced from the paternal allele in mouse brain, and is said to show partial imprinting. The Dlx6 gene is not imprinted. Downstream effects of Dlx5 and Dlx6 proteins in neurons include stimulation of GABA neurotransmitter production. (B) In Mecp2-null neurons, expression from the paternal Dlx5 allele is no longer repressed, resulting in Dlx5 overproduction. Dlx6 transcription is also increased, although this may be due to transcriptional activation by Dlx5, rather than due to a direct result of the absence of MeCP2-mediated repression. Increased levels of Dlx5 and Dlx6 may result in increased or aberrant neurotransmitter production, in turn resulting in neuronal dysfunction.

 
Using chromatin immunoprecipitation to identify Mecp2-bound sequences in mouse brain, Horike et al. (59Go) identified a sequence located within an imprinted gene cluster on mouse chromosome 6 as a bona fide Mecp2-binding site. Subsequent analysis of genes located in this cluster revealed an increase in expression of the imprinted Dlx5 gene and the neighboring non-imprinted Dlx6 gene in brains of Mecp2-deficient animals which correlated with changes in local and regional chromatin organisation (59Go) (Fig. 2). Importantly, loss of imprinting at the DLX5 locus was also found in lymphocyte cell lines derived from three out of four RTT patients. Dlx5 and Dlx6 encode homeobox proteins known to regulate the expression of genes involved in neurotransmitter production (60Go) and are important in various aspects of embryonic development, including brain patterning (61Go). While it is tempting to speculate that elevated expression of DLX genes contributes significantly to the neuronal dysfunction that results in RTT (see ‘Models we would like to see’ in Table 1), it is important to determine the percentage of RTT patients that show loss of DLX5 imprinting and how general a phenomenon this is in RTT. It will also be very interesting to determine whether there are any phenotypic differences between those patients who show loss of imprinting at DLX5 and those who do not.

Phenotypic similarities between RTT and the imprinting disorder Angelman syndrome (OMIM #105830) led another group of researchers to investigate expression levels of another imprinted gene, Ube3a (ubiquitin protein ligase E3A, OMIM #601623), in Mecp2-deficient animals (45Go). Angelman syndrome is caused by deficiency or deletion of the maternally expressed UBE3A gene (62Go,63Go). Using a combination of methods to analyze protein and mRNA levels, Samaco et al. (45Go) found a small but significant decrease in expression of Ube3a/UBE3A in brains of Mecp2-deficient mice and RTT patients. Further, expression of a nearby non-imprinted gene implicated in autism, Gabrb3/GABRB3, was reduced in the same samples. A similar reduction in UBE3A and GABRB3 expression was also found in brain samples from autism and Angelman syndrome patients, indicating that the similarities in phenotype between these syndromes may have a common molecular aetiology (45Go). In this case, no evidence for loss of imprinting or for direct MeCP2 binding to either gene was found, although it remains possible that MeCP2-mediated modulation of local chromatin structure (59Go) may indirectly facilitate expression of UBE3A and/and GABRB3.

Recently, a candidate gene approach was used to identify the brain-derived neurotrophic factor (BDNF) gene as a bona fide MeCP2 target (64Go,65Go) (Fig. 3). BDNF is a neurotrophin required for survival, growth, and maintenance of neurons during development (66Go). BDNF is known to be involved in learning and memory and has the ability to modulate synaptic plasticity by regulating axonal and dendritic branching and remodelling (67Go–71Go). MeCP2 was found to associate with and maintain repression of Bdnf in resting neuronal cultures (Fig. 3A). Following neuronal depolarization, MeCP2 becomes phosphorylated and disassociates from the Bdnf promoter, allowing for full transcription of the gene (Fig. 3C). In the absence of MeCP2, Bdnf repression becomes leaky in unstimulated neurons, resulting in a two-fold increase of Bdnf mRNA levels (64Go) (Fig. 3B). This two-fold increase in mRNA levels does not constitute ‘activation’ of the Bdnf gene, as transcript levels in resting Mecp2-deficient neurons are still approximately 100-fold lower than those found in activated neurons (64Go). Rather absence of MeCP2 results in incomplete repression of Bdnf (Fig. 3B). No difference in Bdnf expression was detectable in depolarised neuronal cultures derived from Mecp2-deficient or wild-type mice (Fig. 3C).



View larger version (22K):
[in this window]
[in a new window]
  		Figure 3. MeCP2 regulation of the neurotrophic factor Bdnf. (A) In resting neurons, MECP2 (orange) and its associated co-repressor (here indicated simply as HDAC, in green) represses transcription of the Bdnf promoter III. The USF transcription factor (blue) is associated with the promoter irrespective of transcriptional status (64Go). (B) In the absence of MeCP2, repression of Bdnf is leaky, resulting in low-level transcription and, presumably, Bdnf protein production that may cause a variety of downstream effects leading to dysfunction at different levels such as reduced LTP, altered learning and memory and decreased dendritic arborization. (C) Upon membrane depolarisation (i.e. neuronal stimulation), MeCP2 is displaced from promoter III and Bdnf transcription is activated, resulting in Bdnf protein production. Note that Bdnf activation is not affected by the presence or absence of MeCP2, and that transcription of Bdnf is 100-fold higher in stimulated neurons than in MeCP2-null, unstimulated neurons. Source: Adapted from Kandel et al. (74Go).

 
Misexpression of Bdnf in Mecp2-null brains occurs only in certain cell types, and only to a level of about two-fold. This slight increase in Bdnf expression, although not enough to be detected using microarray experiments (54Go), could be sufficient to cause small changes in synaptic plasticity in MeCP2-deficient neurons that may contribute to the overall RTT phenotype (Fig. 3). Interestingly, restricted overexpression of Bdnf in the postnatal forebrain of transgenic mice does not result in RTT-like phenotypes (72Go). Clearly, further studies of Bdnf overexpression in mice will be important to determining what part is played by BDNF misexpression in the RTT phenotype (see ‘Models we would like to see’ in Table 1).

It is now clear that any changes in gene expression resulting from deficiency of MeCP2 vary between cell types (53Go,64Go) and between individuals (59Go) and are at a level that is much lower than was initially expected (48Go). The lack of a dramatic, global de-repression in the absence of MeCP2 may be because of the fact that it is one of the four known methyl-CpG-binding transcriptional repressors in mammalian cells (10Go,12Go,73Go) and the degree of functional redundancy between them remains to be determined. It now appears that in the absence of MECP2 its target genes are simply not repressed completely, rather than being fully expressed (e.g. Figs 2 and 3). In order to find these genes, we need to be able to distinguish between ‘off’ and ‘not quite off’ rather than between ‘off’ and ‘on.’ Although this potentially very subtle difference in gene expression may be difficult to detect using standard techniques, its effects upon neurons within a human brain, in which approximately half of the cells are completely normal, are clearly devastating. Nevertheless it is clear that MECP2 and RTT are, reluctantly, revealing their secrets as long as we in the research community are asking the right questions.


   ACKNOWLEDGEMENTS
 
We are grateful to Nicola Reynolds, Meng Li and Xinsheng Nan for critical reading of the manuscript. I.M.C. is the recipient of a University of Edinburgh School of Biological Sciences PhD Studentship. Our work is funded by the Wellcome Trust and the MRC.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MeCP2
 TWO ISOFORMS OF MECP2
 MOUSE MODELS
 X-CHROMOSOME INACTIVATION
 TOO MUCH OF A...
 TARGETS OF MeCP2-MEDIATED...
 REFERENCES
 

  1. Rett, A. (1966) Über ein eigenartiges hirnatrophisches Syndrom bei Hyperammonamie im Kindersalter. Wien Med. Wochenschr., 116, 723–726.[Medline]

  2. Kerr, A.M. and Stephenson, J.B. (1985) Rett's syndrome in the west of Scotland. Br. Med. J. (Clin. Res. Ed.), 291, 579–582.

  3. Hagberg, B., Aicardi, J., Dias, K. and Ramos, O. (1983) A progressive syndrome of autism, dementia, ataxia, and loss of purposeful hand use in girls: Rett's syndrome: report of 35 cases. Ann. Neurol., 14, 471–479.[CrossRef][Web of Science][Medline]

  4. Hagberg, B. (1985) Rett's syndrome: prevalence and impact on progressive severe mental retardation in girls. Acta Paediatr. Scand., 74, 405–408.[Web of Science][Medline]

  5. Subramaniam, B., Naidu, S. and Reiss, A.L. (1997) Neuroanatomy in Rett syndrome: cerebral cortex and posterior fossa. Neurology, 48, 399–407.[Abstract/Free Full Text]

  6. Bauman, M.L., Kemper, T.L. and Arin, D.M. (1995) Microscopic observations of the brain in Rett syndrome. Neuropediatrics, 26, 105–108.[Web of Science][Medline]

  7. Bauman, M.L., Kemper, T.L. and Arin, D.M. (1995) Pervasive neuroanatomic abnormalities of the brain in three cases of Rett's syndrome. Neurology, 45, 1581–1586.[Abstract/Free Full Text]

  8. Armstrong, D., Dunn, J.K., Antalffy, B. and Trivedi, R. (1995) Selective dendritic alterations in the cortex of Rett syndrome. J. Neuropathol. Exp. Neurol., 54, 195–201.[Web of Science][Medline]

  9. Amir, R.E., Van Den Veyver, I.B., Wan, M., Tran, C.Q., Francke, U. and Zoghbi, H.Y. (1999) Rett syndrome is caused by mutations in X-linked MECP2, encoding methyl-CpG-binding protein 2. Nat. Genet., 23, 185–188.[CrossRef][Web of Science][Medline]

  10. Hendrich, B. and Bird, A. (1998) Identification and characterization of a family of mammalian methyl-CpG binding proteins. Mol. Cell. Biol., 18, 6538–6547.[Abstract/Free Full Text]

  11. Bird, A.P. and Wolffe, A.P. (1999) Methylation-induced repression—belts, braces, and chromatin. Cell, 99, 451–454.[CrossRef][Web of Science][Medline]

  12. Jorgensen, H.F. and Bird, A. (2002) MeCP2 and other methyl-CpG binding proteins. Ment. Retard. Dev. Disabil. Res. Rev., 8, 87–93.[CrossRef][Web of Science][Medline]

  13. Nan, X., Ng, H.-H., Johnson, C.A., Laherty, C.D., Turner, B.M., Eisenmann, R.N. and Bird, A. (1998) Transcriptional repression by the methyl-CpG-binding protein MeCP2 involves a histone deacetylase complex. Nature, 393, 386–389.[CrossRef][Medline]

  14. Jones, P.L., Veenstra, G.J.C., Wade, P.A., Vermaak, D., Kass, S.U., Landsberger, N., Strouboulis, J. and Wolffe, A.P. (1998) Methylated DNA and MeCP2 recruit histone deacetylase to repress transcription. Nat. Genet., 19, 187–191.[CrossRef][Web of Science][Medline]

  15. D'Esposito, M., Quaderi, N.A., Ciccodicola, A., Bruni, P., Esposito, T., D'Urso, M. and Brown, S.D. (1996) Isolation, physical mapping, and northern analysis of the X-linked human gene encoding methyl CpG-binding protein, MECP2. Mamm. Genome, 7, 533–535.[CrossRef][Web of Science][Medline]

  16. Amir, R.E., Van den Veyver, I.B., Schultz, R., Malicki, D.M., Tran, C.Q., Dahle, E.J., Philippi, A., Timar, L., Percy, A.K., Motil, K.J. et al. (2000) Influence of mutation type and X chromosome inactivation on Rett syndrome phenotypes. Ann. Neurol., 47, 670–679.[CrossRef][Web of Science][Medline]

  17. Villard, L., Cardoso, A.K., Chelly, P.J., Tardieu, P.M. and Fontes, M. (2000) Two affected boys in a rett syndrome family: clinical and molecular findings. Neurology, 55, 1188–1193.[Abstract/Free Full Text]

  18. Hoffbuhr, K., Devaney, J.M., LaFleur, B., Sirianni, N., Scacheri, C., Giron, J., Schuette, J., Innis, J., Marino, M., Philippart, M. et al. (2001) MeCP2 mutations in children with and without the phenotype of Rett syndrome. Neurology, 56, 1486–1495.[Abstract/Free Full Text]

  19. Plenge, R.M., Stevenson, R.A., Lubs, H.A., Schwartz, C.E. and Willard, H.F. (2002) Skewed X-chromosome inactivation is a common feature of X-linked mental retardation disorders. Am. J. Hum. Genet., 71, 168–173.[CrossRef][Web of Science][Medline]

  20. Zoghbi, H.Y., Percy, A.K., Schultz, R.J. and Fill, C. (1990) Patterns of X chromosome inactivation in the Rett syndrome. Brain Dev., 12, 131–135.[Web of Science][Medline]

  21. Trappe, R., Laccone, F., Cobilanschi, J., Meins, M., Huppke, P., Hanefeld, F. and Engel, W. (2001) MECP2 mutations in sporadic cases of Rett syndrome are almost exclusively of paternal origin. Am. J. Hum. Genet., 68, 1093–1101.[CrossRef][Web of Science][Medline]

  22. Schanen, C. (2001) Rethinking the fate of males with mutations in the gene that causes Rett syndrome. Brain Dev., 23(Suppl. 1), S144–S146.

  23. Hendrich, B. and Bickmore, W. (2001) Human diseases with underlying defects in chromatin structure and modification. Hum. Mol. Genet., 10, 2233–2242.[Abstract/Free Full Text]

  24. Mullaney, B.C., Johnston, M.V. and Blue, M.E. (2004) Developmental expression of methyl-CpG binding protein 2 is dynamically regulated in the rodent brain. Neuroscience, 123, 939–949.[CrossRef][Web of Science][Medline]

  25. Kishi, N. and Macklis, J.D. (2004) MECP2 is progressively expressed in post-migratory neurons and is involved in neuronal maturation rather than cell fate decisions. Mol. Cell. Neurosci., 27, 306–321.[Web of Science][Medline]

  26. Shahbazian, M.D., Antalffy, B., Armstrong, D.L. and Zoghbi, H.Y. (2002) Insight into Rett syndrome: MeCP2 levels display tissue- and cell-specific differences and correlate with neuronal maturation. Hum. Mol. Genet., 11, 115–124.[Abstract/Free Full Text]

  27. Braunschweig, D., Simcox, T., Samaco, R.C. and LaSalle, J.M. (2004) X-Chromosome inactivation ratios affect wild-type MeCP2 expression within mosaic Rett syndrome and Mecp2–/+ mouse brain. Hum. Mol. Genet., 13, 1275–1286.[Abstract/Free Full Text]

  28. Altman, J. and Bayer, S.A. (1990) Mosaic organization of the hippocampal neuroepithelium and the multiple germinal sources of dentate granule cells. J. Comp. Neurol., 301, 325–342.[CrossRef][Web of Science][Medline]

  29. Altman, J. and Bayer, S.A. (1978) Prenatal development of the cerebellar system in the rat. I. Cytogenesis and histogenesis of the deep nuclei and the cortex of the cerebellum. J. Comp. Neurol., 179, 23–48.[CrossRef][Web of Science][Medline]

  30. Mnatzakanian, G.N., Lohi, H., Munteanu, I., Alfred, S.E., Yamada, T., MacLeod, P.J., Jones, J.R., Scherer, S.W., Schanen, N.C., Friez, M.J. et al. (2004) A previously unidentified MECP2 open reading frame defines a new protein isoform relevant to Rett syndrome. Nat. Genet., 36, 339–341.[CrossRef][Web of Science][Medline]

  31. Kriaucionis, S. and Bird, A. (2004) The major form of MeCP2 has a novel N-terminus generated by alternative splicing. Nucleic Acids Res., 32, 1818–1823.[Abstract/Free Full Text]

  32. Luikenhuis, S., Giacometti, E., Beard, C.F. and Jaenisch, R. (2004) Expression of MeCP2 in postmitotic neurons rescues Rett syndrome in mice. Proc. Natl Acad. Sci. USA, 101, 6033–6038.[Abstract/Free Full Text]

  33. Chen, R.Z., Akbarian, S., Tudor, M. and Jaenisch, R. (2001) Deficiency of methyl-CpG binding protein-2 in CNS neurons results in a Rett-like phenotype in mice. Nat. Genet., 27, 327–331.[CrossRef][Web of Science][Medline]

  34. Guy, J., Hendrich, B., Holmes, M., Martin, J.E. and Bird, A. (2001) A mouse Mecp2-null mutation causes neurological symptoms that mimic Rett syndrome. Nat. Genet., 27, 322–326.[CrossRef][Web of Science][Medline]

  35. Shahbazian, M., Young, J., Yuva-Paylor, L., Spencer, C., Antalffy, B., Noebels, J., Armstrong, D., Paylor, R. and Zoghbi, H. (2002) Mice with truncated MeCP2 recapitulate many Rett syndrome features and display hyperacetylation of histone H3. Neuron, 35, 243–254.[CrossRef][Web of Science][Medline]

  36. Silva, A.J. (2003) Molecular and cellular cognitive studies of the role of synaptic plasticity in memory. J Neurobiol., 54, 224–237.[CrossRef][Web of Science][Medline]

  37. Tsien, J.Z., Chen, D.F., Gerber, D., Tom, C., Mercer, E.H., Anderson, D.J., Mayford, M., Kandel, E.R. and Tonegawa, S. (1996) Subregion- and cell type-restricted gene knockout in mouse brain. Cell, 87, 1317–1326.[CrossRef][Web of Science][Medline]

  38. Matarazzo, V., Cohen, D., Palmer, A.M., Simpson, P.J., Khokhar, B., Pan, S.J. and Ronnett, G.V. (2004) The transcriptional repressor Mecp2 regulates terminal neuronal differentiation. Mol. Cell. Neurosci., 27, 44–58.[CrossRef][Web of Science][Medline]

  39. Stancheva, I., Collins, A.L., Van den Veyver, I.B., Zoghbi, H. and Meehan, R.R. (2003) A mutant form of MeCP2 protein associated with human Rett syndrome cannot be displaced from methylated DNA by notch in Xenopus embryos. Mol. Cell., 12, 425–435.[CrossRef][Web of Science][Medline]

  40. Young, J.I. and Zoghbi, H.Y. (2004) X-chromosome inactivation patterns are unbalanced and affect the phenotypic outcome in a mouse model of rett syndrome. Am. J. Hum. Genet., 74, 511–520.[CrossRef][Web of Science][Medline]

  41. Binder, L.I., Frankfurter, A. and Rebhun, L.I. (1985) The distribution of tau in the mammalian central nervous system. J. Cell. Biol., 101, 1371–1378.[Abstract/Free Full Text]

  42. Tucker, K.L., Meyer, M. and Barde, Y.A. (2001) Neurotrophins are required for nerve growth during development. Nat. Neurosci., 4, 29–37.[CrossRef][Web of Science][Medline]

  43. Collins, A.L., Levenson, J.M., Vilaythong, A.P., Richman, R., Armstrong, D.L., Noebels, J.L., David Sweatt, J. and Zoghbi, H.Y. (2004) Mild overexpression of MeCP2 causes a progressive neurological disorder in mice. Hum. Mol. Genet., 13, 2679–2689.[Abstract/Free Full Text]

  44. Samaco, R.C., Nagarajan, R.P., Braunschweig, D. and LaSalle, J.M. (2004) Multiple pathways regulate MeCP2 expression in normal brain development and exhibit defects in autism-spectrum disorders. Hum. Mol. Genet., 13, 629–639.[Abstract/Free Full Text]

  45. Samaco, R.C., Hogart, A. and Lasalle, J.M. (2005) Epigenetic overlap in autism-spectrum neurodevelopmental disorders: MECP2 deficiency causes reduced expression of UBE3A and GABRB3. Hum. Mol. Genet., 14, 483–492.[Abstract/Free Full Text]

  46. Nan, X., Campoy, F. and Bird, A. (1997) MeCP2 is a transcriptional repressor with abundant binding sites in genomic chromatin. Cell, 88, 471–481.[CrossRef][Web of Science][Medline]

  47. Fuks, F., Hurd, P.J., Wolf, D., Nan, X., Bird, A.P. and Kouzarides, T. (2003) The Methyl-CpG-binding Protein MeCP2 Links DNA Methylation to Histone Methylation. J. Biol. Chem., 278, 4035–4040.[Abstract/Free Full Text]

  48. Willard, H.F. and Hendrich, B.D. (1999) Breaking the silence in Rett syndrome. Nat. Genet., 23, 127–128.[CrossRef][Web of Science][Medline]

  49. Ballestar, E., Ropero, S., Alaminos, M., Armstrong, J., Setien, F., Agrelo, R., Fraga, M.F., Herranz, M., Avila, S., Pineda, M. et al. (2005) The impact of MECP2 mutations in the expression patterns of Rett syndrome patients. Hum. Genet., 116, 91–104.[CrossRef][Web of Science][Medline]

  50. Traynor, J., Agarwal, P., Lazzeroni, L. and Francke, U. (2002) Gene expression patterns vary in clonal cell cultures from Rett syndrome females with eight different MECP2 mutations. BMC Med. Genet., 3, 12.[CrossRef][Medline]

  51. Antequera, F., Boyes, J. and Bird, A. (1990) High levels of de novo methylation and altered chromatin structure at CpG islands in cell lines. Cell, 62, 503–514.[CrossRef][Web of Science][Medline]

  52. Colantuoni, C., Jeon, O.H., Hyder, K., Chenchik, A., Khimani, A.H., Narayanan, V., Hoffman, E.P., Kaufmann, W.E., Naidu, S. and Pevsner, J. (2001) Gene expression profiling in postmortem Rett Syndrome brain: differential gene expression and patient classification. Neurobiol. Dis., 8, 847–865.[CrossRef][Web of Science][Medline]

  53. Matarazzo, V. and Ronnett, G.V. (2004) Temporal and regional differences in the olfactory proteome as a consequence of MeCP2 deficiency. Proc. Natl Acad. Sci. USA, 101, 7763–7768.[Abstract/Free Full Text]

  54. Tudor, M., Akbarian, S., Chen, R.Z. and Jaenisch, R. (2002) Transcriptional profiling of a mouse model for Rett syndrome reveals subtle transcriptional changes in the brain. Proc. Natl Acad. Sci. USA, 99, 15536–15541.[Abstract/Free Full Text]

  55. da Rocha, S.T. and Ferguson-Smith, A.C. (2004) Genomic imprinting. Curr. Biol., 14, R646–R649.[CrossRef][Web of Science][Medline]

  56. Drewell, R.A., Goddard, C.J., Thomas, J.O. and Surani, M.A. (2002) Methylation-dependent silencing at the H19 imprinting control region by MeCP2. Nucleic Acids Res., 30, 1139–1144.[Abstract/Free Full Text]

  57. Fournier, C., Goto, Y., Ballestar, E., Delaval, K., Hever, A.M., Esteller, M. and Feil, R. (2002) Allele-specific histone lysine methylation marks regulatory regions at imprinted mouse genes. EMBO J., 21, 6560–6570.[CrossRef][Web of Science][Medline]

  58. Gregory, R.I., Randall, T.E., Johnson, C.A., Khosla, S., Hatada, I., O'Neill, L.P., Turner, B.M. and Feil, R. (2001) DNA methylation is linked to deacetylation of histone H3, but not H4, on the imprinted genes Snrpn and U2af1-rs1. Mol. Cell. Biol., 21, 5426–5436.[Abstract/Free Full Text]

  59. Horike, S.I., Cai, S., Miyano, M., Cheng, J.F. and Kohwi-Shigematsu, T. (2005) Loss of silent-chromatin looping and impaired imprinting of DLX5 in Rett syndrome. Nat. Genet., 37, 31–40.[CrossRef][Web of Science][Medline]

  60. Stuhmer, T., Anderson, S.A., Ekker, M. and Rubenstein, J.L. (2002) Ectopic expression of the Dlx genes induces glutamic acid decarboxylase and Dlx expression. Development, 129, 245–252.[Abstract/Free Full Text]

  61. McLarren, K.W., Litsiou, A. and Streit, A. (2003) DLX5 positions the neural crest and preplacode region at the border of the neural plate. Dev. Biol., 259, 34–47.[CrossRef][Web of Science][Medline]

  62. Matsuura, T., Sutcliffe, J.S., Fang, P., Galjaard, R.J., Jiang, Y.H., Benton, C.S., Rommens, J.M. and Beaudet, A.L. (1997) De novo truncating mutations in E6-AP ubiquitin-protein ligase gene (UBE3A) in Angelman syndrome. Nat. Genet., 15, 74–77.[CrossRef][Web of Science][Medline]

  63. Kishino, T., Lalande, M. and Wagstaff, J. (1997) UBE3A/E6-AP mutations cause Angelman syndrome. Nat. Genet., 15, 70–73.[CrossRef][Web of Science][Medline]

  64. Chen, W.G., Chang, Q., Lin, Y., Meissner, A., West, A.E., Griffith, E.C., Jaenisch, R. and Greenberg, M.E. (2003) Derepression of BDNF transcription involves calcium-dependent phosphorylation of MeCP2. Science, 302, 885–889.[Abstract/Free Full Text]

  65. Martinowich, K., Hattori, D., Wu, H., Fouse, S., He, F., Hu, Y., Fan, G. and Sun, Y.E. (2003) DNA methylation-related chromatin remodeling in activity-dependent BDNF gene regulation. Science, 302, 890–893.[Abstract/Free Full Text]

  66. Barde, Y.A. (1994) Neurotrophins: a family of proteins supporting the survival of neurons. Prog. Clin. Biol. Res., 390, 45–56.[Medline]

  67. Lo, D.C. (1995) Neurotrophic factors and synaptic plasticity. Neuron, 15, 979–981.[CrossRef][Web of Science][Medline]

  68. Shimada, A., Mason, C.A. and Morrison, M.E. (1998) TrkB signaling modulates spine density and morphology independent of dendrite structure in cultured neonatal Purkinje cells. J. Neurosci., 18, 8559–8570.[Abstract/Free Full Text]

  69. Yacoubian, T.A. and Lo, D.C. (2000) Truncated and full-length TrkB receptors regulate distinct modes of dendritic growth. Nat. Neurosci., 3, 342–349.[CrossRef][Web of Science][Medline]

  70. Lom, B. and Cohen-Cory, S. (1999) Brain-derived neurotrophic factor differentially regulates retinal ganglion cell dendritic and axonal arborization in vivo. J. Neurosci., 19, 9928–9938.[Abstract/Free Full Text]

  71. Alsina, B., Vu, T. and Cohen-Cory, S. (2001) Visualizing synapse formation in arborizing optic axons in vivo: dynamics and modulation by BDNF. Nat. Neurosci., 4, 1093–1101.[CrossRef][Web of Science][Medline]

  72. Huang, Z.J., Kirkwood, A., Pizzorusso, T., Porciatti, V., Morales, B., Bear, M.F., Maffei, L. and Tonegawa, S. (1999) BDNF regulates the maturation of inhibition and the critical period of plasticity in mouse visual cortex. Cell, 98, 739–755.[CrossRef][Web of Science][Medline]

  73. Prokhortchouk, A., Hendrich, B., Jorgensen, H., Ruzov, A., Wilm, M., Georgiev, G., Bird, A. and Prokhortchouk, E. (2001) The p120 catenin partner Kaiso is a DNA methylation-dependent transcriptional repressor. Genes Dev., 15, 1613–1618.[Abstract/Free Full Text]

  74. Kandel, E., Schwartz, J. and Jessell, T. (2000) Principles of Neural Science. McGraw-Hill, Stamford, CT.


Add to CiteULike CiteULike   Add to Connotea Connotea   Add to Del.icio.us Del.icio.us    What's this?


This article has been cited by other articles:


Home page
 Phil Trans R Soc BHome page
N. D Allen
Temporal and epigenetic regulation of neurodevelopmental plasticity
Phil Trans R Soc B, January 12, 2008; 363(1489): 23 - 38.
[Abstract] [Full Text] [PDF]

Home page
 J. Biol. Chem.Home page
A. Rogaeva, X.-M. Ou, H. Jafar-Nejad, S. Lemonde, and P. R. Albert
Differential Repression by Freud-1/CC2D1A at a Polymorphic Site in the Dopamine-D2 Receptor Gene
J. Biol. Chem., July 20, 2007; 282(29): 20897 - 20905.
[Abstract] [Full Text] [PDF]

Home page
 Proc. Natl. Acad. Sci. USAHome page
B. E. McGill, S. F. Bundle, M. B. Yaylaoglu, J. P. Carson, C. Thaller, and H. Y. Zoghbi
From the Cover: Enhanced anxiety and stress-induced corticosterone release are associated with increased Crh expression in a mouse model of Rett syndrome
PNAS, November 28, 2006; 103(48): 18267 - 18272.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow FREE Full Text (PDF) Freely available
Right arrow A corrigendum has been published
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Add to My Personal Archive
Right arrow Download to citation manager
Right arrowRequest Permissions
Google Scholar
Right arrow Articles by Caballero, I. M.
Right arrow Articles by Hendrich, B.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Caballero, I. M.
Right arrow Articles by Hendrich, B.
Social Bookmarking
Add to CiteULike   Add to Connotea   Add to Del.icio.us  
What's this?