MECP2 mutation in non-fatal, non-progressive encephalopathy in a male

Unusual double mutation in MECP2 and CDKL5 genes in Rett-like syndrome: Correlation with phenotype and genes expression

INTRODUCTION

Rett syndrome (RTT) is a neuro-developmental disorder affecting almost exclusively females and it divided into classical and atypical forms of the disease. RTT-like syndrome was also described and presents an overlapping phenotype of RTT. RTT-like syndrome has been associated with several genes including MECP2 and CDKL5 having common biological pathways and regulatory interactions especially during neural maturation and synaptogenesis.

METHODS

We report patient with Rett-like syndrome for whom clinical features and their progression guided toward the screening of two candidate genes MECP2 and CDKL5 by sequencing. Severity score was evaluated by "Rett Assessment Rating Scale" (R.A.R.S.). Predictions of pahogenicity and functional effects used several bioinformatic tools and qRT-PCR was conducted to evaluate gene expression.

RESULTS

Mutational screening revealed two mutations c.1065 C > A (p.S355R) in MECP2 gene and c.616 G > A (p.D206N) mutation in CDKL5 gene in the patient with a high R.A.R.S. Bioinformatic investigations predicted a moderate effect of p.S355R in MECP2 gene but a more pathogenic one of p.D206N mutation in CDKL5. Effect of c.616 G > A mutation on structure and stability of CDKL5 mRNA was confirmed by qRT-PCR. Additionally, analysis of gene expression revealed a drastic effect of CDKL5 mutant on its MeCP2 and Dnmt1 substrates and also on its MYCN regulator.

CONCLUSIONS

The co-existence of the two mutations in CDKL5 and MECP2 genes could explain the severe phenotype in our patient with RTT-Like and is consistent with the data related to the interactions of CDKL5 with MeCP2 and Dnmt1 proteins.

Epilepsy and genetic in Rett syndrome: A review

INTRODUCTION

Rett syndrome (RTT) is a severe X-linked neurodevelopmental disorder that primarily affects girls, with an incidence of 1:10,000-20,000. The diagnosis is based on clinical features: an initial period of apparently normal development (ages 6-12 months) followed by a rapid decline with regression of acquired motor skills, loss of spoken language and purposeful hand use, onset of hand stereotypes, abnormal gait, and growth failure. The course of the disease, in its classical form, is characterized by four stages. Three different atypical variants of the disease have been defined. Epilepsy has been reported in 60%-80% of patients with RTT; it differs among the various phenotypes and genotypes and its severity is an important contributor to the clinical severity of the disease.

METHODS

In this manuscript we reviewed literature on RTT, focusing on the different genetic entities, the correlation genotype-phenotype, and the peculiar epileptic phenotype associated to each of them.

RESULTS

Mutations in MECP2 gene, located on Xq28, account for 95% of typical RTT cases and 73.2% of atypical RTT. CDKL5 and FOXG1 are other genes identified as causative genes in atypical forms of RTT. In the last few years, a lot of new genes have been identified as causative genes for RTT phenotype.

CONCLUSIONS

Recognizing clinical and EEG patterns in different RTT variants may be useful in diagnosis and management of these patients.

Next-Generation Sequencing Reveals Novel Mutations in X-linked Intellectual Disability

Robust diagnostics for many human genetic disorders are much needed in the pursuit of global personalized medicine. Next-generation sequencing now offers new promise for biomarker and diagnostic discovery, in developed as well as resource-limited countries. In this broader global health context, X-linked intellectual disability (XLID) is an inherited genetic disorder that is associated with a range of phenotypes impacting societies in both developed and developing countries. Although intellectual disability arises due to diverse causes, a substantial proportion is caused by genomic alterations. Studies have identified causal XLID genomic alterations in more than 100 protein-coding genes located on the X-chromosome. However, the causes for a substantial number of intellectual disability and associated phenotypes still remain unknown. Identification of causative genes and novel mutations will help in early diagnosis as well as genetic counseling of families. Advent of next-generation sequencing methods has accelerated the discovery of new genes involved in mental health disorders. In this study, we analyzed the exomes of three families from India with nonsyndromic XLID comprising seven affected individuals. The affected individuals had varying degrees of intellectual disability, microcephaly, and delayed motor and language milestones. We identified potential causal variants in three XLID genes, including PAK3 (V294M), CASK (complex structural variant), and MECP2 (P354T). Our findings reported in this study extend the spectrum of mutations and phenotypes associated with XLID, and calls for further studies of intellectual disability and mental health disorders with use of next-generation sequencing technologies.

Rett syndrome: a neurological disorder with metabolic components

Rett syndrome (RTT) is a neurological disorder caused by mutations in the X-linked gene methyl-CpG-binding protein 2 (MECP2), a ubiquitously expressed transcriptional regulator. Despite remarkable scientific progress since its discovery, the mechanism by which MECP2 mutations cause RTT symptoms is largely unknown. Consequently, treatment options for patients are currently limited and centred on symptom relief. Thought to be an entirely neurological disorder, RTT research has focused on the role of MECP2 in the central nervous system. However, the variety of phenotypes identified in Mecp2 mutant mouse models and RTT patients implicate important roles for MeCP2 in peripheral systems. Here, we review the history of RTT, highlighting breakthroughs in the field that have led us to present day. We explore the current evidence supporting metabolic dysfunction as a component of RTT, presenting recent studies that have revealed perturbed lipid metabolism in the brain and peripheral tissues of mouse models and patients. Such findings may have an impact on the quality of life of RTT patients as both dietary and drug intervention can alter lipid metabolism. Ultimately, we conclude that a thorough knowledge of MeCP2's varied functional targets in the brain and body will be required to treat this complex syndrome.

Functional Profiling of Human MeCP2 by Automated Data Comparison Analysis and Computerized Expression Pathway Modeling

OBJECTIVES

Methyl-CpG binding protein 2 (MeCP2) is a ubiquitous epigenetic factor that represses gene expression by modifying chromatin. Mutations in the MeCP2 gene cause Rett syndrome, a progressive neurodevelopmental disorder. Recent studies also have shown that MeCP2 plays a role in carcinogenesis. Specifically, functional ablation of MeCP2 suppresses cell growth and leads to the proliferation of cancer cells. However, MeCP2's function in adult tissues remains poorly understood. We utilized a weight matrix-based comparison software to identify transcription factor binding site (TFBS) of MeCP2-regulated genes, which were recognized by cDNA microarray analysis.

METHODS

MeCP2 expression was silenced using annealed siRNA in HEK293 cells, and then a cDNA microarray analysis was performed. Functional analysis was carried out, and transcriptional levels in target genes regulated by MeCP2 were investigated. TFBS analysis was done within genes selected by the cDNA microarray analysis, using a weight matrix-based program and the TRANSFAC 6.0 database.

RESULTS

Among the differentially expressed genes with a change in expression greater than two-fold, 189 genes were up-regulated and 91 genes were down-regulated. Genes related to apoptosis and cell proliferation (JUN, FOSL2, CYR61, SKIL, ATF3, BMABI, BMPR2, RERE, and FALZ) were highly up-regulated. Genes with anti-apoptotic and anti-proliferative functions (HNRPA0, HIS1, and FOXC1) were down-regulated. Using TFBS analysis within putative promoters of novel candidate target genes of MeCP2, disease-related transcription factors were identified.

CONCLUSIONS

The present results provide insights into the new target genes regulated by MeCP2 under epigenetic control. This information will be valuable for further studies aimed at clarifying the pathogenesis of Rett syndrome and neoplastic diseases.

Regulatory functions and pathological relevance of the MECP2 3'UTR in the central nervous system

Methyl-CpG-binding protein 2 (MeCP2), encoded by the gene MECP2, is a transcriptional regulator and chromatin-remodeling protein, which is ubiquitously expressed and plays an essential role in the development and maintenance of the central nervous system (CNS). Highly enriched in post-migratory neurons, MeCP2 is needed for neuronal maturation, including dendritic arborization and the development of synapses. Loss-of-function mutations in MECP2 cause Rett syndrome (RTT), a debilitating neurodevelopmental disorder characterized by a phase of normal development, followed by the progressive loss of milestones and cognitive disability. While a great deal has been discovered about the structure, function, and regulation of MeCP2 in the time since its discovery as the genetic cause of RTT, including its involvement in a number of RTT-related syndromes that have come to be known as MeCP2-spectrum disorders, much about this multifunctional protein remains enigmatic. One unequivocal fact that has become apparent is the importance of maintaining MeCP2 protein levels within a narrow range, the limits of which may depend upon the cell type and developmental time point. As such, MeCP2 is amenable to complex, multifactorial regulation. Here, we summarize the role of the MECP2 3' untranslated region (UTR) in the regulation of MeCP2 protein levels and how mutations in this region contribute to autism and other non-RTT neuropsychiatric disorders.

NUDT21-spanning CNVs lead to neuropsychiatric disease and altered MeCP2 abundance via alternative polyadenylation

The brain is sensitive to the dose of MeCP2 such that small fluctuations in protein quantity lead to neuropsychiatric disease. Despite the importance of MeCP2 levels to brain function, little is known about its regulation. In this study, we report eleven individuals with neuropsychiatric disease and copy-number variations spanning NUDT21, which encodes a subunit of pre-mRNA cleavage factor Im. Investigations of MECP2 mRNA and protein abundance in patient-derived lymphoblastoid cells from one NUDT21 deletion and three duplication cases show that NUDT21 regulates MeCP2 protein quantity. Elevated NUDT21 increases usage of the distal polyadenylation site in the MECP2 3′ UTR, resulting in an enrichment of inefficiently translated long mRNA isoforms. Furthermore, normalization of NUDT21 via siRNA-mediated knockdown in duplication patient lymphoblasts restores MeCP2 to normal levels. Ultimately, we identify NUDT21 as a novel candidate for intellectual disability and neuropsychiatric disease, and elucidate a mechanism of pathogenesis by MeCP2 dysregulation via altered alternative polyadenylation.

DOI:http://dx.doi.org/10.7554/eLife.10782.001

The X-chromosome carries a number of genes that are involved in a child's intellectual development. One of these genes encodes a protein called MeCP2, which is important for brain function after birth. Mutations in the MECP2 gene cause a disorder known as Rett syndrome. At around 18 months of age, affected children begin to lose the cognitive and motor skills that they had previously acquired. Individuals with extra copies of this gene also show cognitive impairments. For both diseases, individuals with levels of the MeCP2 protein that are the most different from those found in healthy individuals also show the most severe symptoms.

To produce the protein that is encoded by a particular gene, enzymes inside the cell must first make a copy of that gene using a molecule called messenger ribonucleic acid (or mRNA). This mRNA is then used as a template to assemble the protein itself. In the case of MECP2, two different mRNA templates are produced: a long version and a short version. A gene called NUDT21 makes a protein that regulates whether the long or short version of MECP2 mRNA is made.

Gennarino, Alcott et al. have now discovered that people with too many, or too few, copies of the NUDT21 gene have intellectual disabilities and altered levels of MeCP2 protein. Specifically, individuals with extra copies of NUDT21—and thus higher levels of the corresponding protein—produce more of the long MECP2 mRNA. The production of proteins from this long mRNA is less efficient than from the short mRNA; therefore, these individuals have lower levels of MeCP2 protein. The opposite is true for individuals who lack a copy of the NUDT21 gene.

To confirm these data, Gennarino, Alcott et al. grew cells in the laboratory from patients with extra copies of the NUDT21 gene and found that reducing the production of its protein returned the levels of the MeCP2 protein back to normal. These findings show that alterations in the NUDT21 gene cause changes in the level of MeCP2 protein in cells and leads to neuropsychiatric diseases.

DOI:http://dx.doi.org/10.7554/eLife.10782.002

Rett syndrome: disruption of epigenetic control of postnatal neurological functions

Loss-of-function mutations in the X-linked gene Methyl-CpG-binding protein 2 (MECP2) cause a devastating pediatric neurological disorder called Rett syndrome. In males, these mutations typically result in severe neonatal encephalopathy and early lethality. On the other hand, owing to expression of the normal allele in ∼50% of cells, females do not suffer encephalopathy but instead develop Rett syndrome. Typically females with Rett syndrome exhibit a delayed onset of neurologic dysfunction that manifests around the child's first birthday and progresses over the next few years. Features of this disorder include loss of acquired language and motor skills, intellectual impairment and hand stereotypies. The developmental regression observed in patients with Rett syndrome arises from altered neuronal function and is not the result of neurodegeneration. Maintenance of an appropriate level of MeCP2 appears integral to the function of healthy neurons as patients with increased levels of MeCP2, owing to duplication of the Xq28 region encompassing the MECP2 locus, also present with intellectual disability and progressive neurologic symptoms. Despite major efforts over the past two decades to elucidate the molecular functions of MeCP2, the mechanisms underlying the delayed appearance of symptoms remain unclear. In this review, we will highlight recent findings that have expanded our knowledge of MeCP2's functions, and we will discuss how epigenetic regulation, chromatin organization and circuit dynamics may contribute to the postnatal onset of Rett syndrome.

The role of MeCP2 in the brain

Methyl-CpG binding protein 2 (MeCP2) was first identified in 1992 as a protein that binds specifically to methylated DNA. Mutations in the MECP2 gene were later found to be the cause of an autism spectrum disorder, Rett syndrome. Despite almost 20 years of research into the molecular mechanisms of MeCP2 function, many questions are yet to be answered conclusively. This review considers several key questions and attempts to evaluate the current state of evidence. For example, is MeCP2 just a methyl-CpG binding protein? Is it a multifunctional protein or primarily a transcriptional repressor? We also consider whether MeCP2, as a chromosome-binding protein, acts at specific sites within the genome or more globally, and in which cell types it is functionally important. Finally, we consider two alternative views of MeCP2 in the brain: as a regulator of brain development or as a factor that helps maintain neuronal/glial function.

A novel MECP2 change in an indian boy with variant rett phenotype and congenital blindness: implications for genetic counseling and prenatal diagnosis

Mutations in MECP2 gene are the primary cause of Rett syndrome, a neurodevelopmental disorder that primarily affects girls, and affect 90% to 95% patients with classical Rett syndrome. MECP2 mutations, once thought to be lethal in males, now present a broad spectrum of clinical manifestations in males. This article reports a family with a 9-year-old boy with Rett-like phenotype and congenital blindness, who inherited a novel MECP2 variant (p.P430S) from his asymptomatic mother. The variant was also identified in the asymptomatic maternal grandfather and maternal aunts of the proband, ruling out the possibility that the p.P430S was involved in the phenotype. Findings of the study suggest that a careful evaluation of the pathogenic nature of MECP2 variants identified in males be conducted before proposing genetic counseling or prenatal diagnosis to the family and that the interference of other factors like modifier genes, environment, epigenetics, and mosaicism be taken into account.

Identification and characterization of novel sequence variations in MECP2 gene in Rett syndrome patients

Rett syndrome (RS) is a neurodevelopmental disorder caused by mutations in MECP2 gene. Exons 2, 3, and 4, in addition to intronic and 3'UTR adjacent regions, were sequenced in 80 patients with RS. Twenty-nine sequence variations were detected in 49 patients, 34 (69.4%) patients with the classic form of RS, and 15 (30.6%) patients with atypical forms of RS. Thirteen of the 29 detected mutations represent novel sequence variations. Missense mutation T158M was the most commonly observed mutation, detected in nine patients (11.2%). Six hotspot pathogenic mutations (R133C, T158M, R168X, R255X, R270X, and R294X) were responsible for the phenotype in 26/80 patients (32.5%).

Disruption of the epigenetic code: an emerging mechanism in mental retardation

Mental retardation (MR) is a highly diverse group of cognitive disorders. Gene defects account for about half of all patients and mutations causative for impaired cognition have been identified in more than 400 genes. While there are numerous genetic defects underlying MR, a more limited number of pathways is emerging whose disruption appears to be shared by groups of MR genes. One of these common pathways is composed of MR genes that encode regulators of chromatin structure and of chromatin-mediated transcription regulation. Already more than 20 "epigenetic MR genes" have been identified and this number is likely to increase in the coming years when deep sequencing of exomes and genomes will become commonplace. Prominent examples of epigenetic MR genes include the methyl CpG-binding protein MECP2 and the CREB binding protein, CBP. Interestingly, several epigenetic MR proteins have been found to interact directly with one another or act together in complexes that regulate the local chromatin structure at target genes. Thus, it appears that the functions of individual epigenetic MR proteins converge onto similar biological processes that are crucial to neuronal processes. The next challenge will be to gain more insight into patterns of altered DNA methylation and histone modifications that are caused by epigenetic gene mutations and how these will disrupt the brain-specific expression of target genes. Such research may reveal that a wide variety of mutations in the genetic code result in a more limited number of disruptions to the epigenetic code. If so, this will provide a rationale for therapeutic strategies.

Abnormalities of cell packing density and dendritic complexity in the MeCP2 A140V mouse model of Rett syndrome/X-linked mental retardation

Background

Rett syndrome (RTT), a common cause of mental retardation in girls, is associated with mutations in the MECP2 gene. Most human cases of MECP2 mutation in girls result in classical or variant forms of RTT. When these same mutations occur in males, they often present as severe neonatal encephalopathy. However, some MECP2 mutations can also lead to diseases characterized as mental retardation syndromes, particularly in boys. One of these mutations, A140V, is a common, recurring missense mutation accounting for about 0.6% of all MeCP2 mutations and ranking 21^st by frequency. It has been described in familial X-linked mental retardation (XLMR), PPM- X syndrome (Parkinsonism, Pyramidal signs, Macroorchidism, X-linked mental retardation) and in other neuropsychiatric syndromes. Interestingly, this mutation has been reported to preserve the methyl-CpG binding function of the MeCP2 protein while compromising its ability to bind to the mental retardation associated protein ATRX.

Results

We report the construction and initial characterization of a mouse model expressing the A140V MeCP2 mutation. These initial descriptive studies in male hemizygous mice have revealed brain abnormalities seen in both RTT and mental retardation. The abnormalities found include increases in cell packing density in the brain and a significant reduction in the complexity of neuronal dendritic branching. In contrast to some MeCP2 mutation mouse models, the A140V mouse has an apparently normal lifespan and normal weight gain patterns with no obvious seizures, tremors, breathing difficulties or kyphosis.

Conclusion

We have identified various neurological abnormalities in this mouse model of Rett syndrome/X-linked mental retardation which may help to elucidate the manner in which MECP2 mutations cause neuronal changes resulting in mental retardation without the confounding effects of seizures, chronic hypoventilation, or other Rett syndrome associated symptoms.

Neurologic aspects of MECP2 gene duplication in male patients

Duplications in Xq28 involving the methyl CpG binding protein 2 gene (MECP2) have been described in male patients with severe mental disability, delayed milestones, absence of language, hypotonia replaced by spasticity and retractions, and recurrent and often severe infections. In a study involving five patients in two families, multiplex ligation-dependent probe amplification was used to screen the Xq28 region that includes MECP2, focusing on the presence of gene duplications. Some manifestations of the disease observed in these patients may occur less regularly than the classical abnormalities. Epilepsy with frequent seizures of the myoclonic-astatic type was observed in these patients and was associated with a slowing of the background electroencephalographic activity, rather than the generalized spike-waves or polyspike-waves usually observed in this type of seizure. In addition, cerebral abnormalities were observed with magnetic resonance imaging that were inconstant and nonspecific but that could nonetheless assist in diagnosis of this genetic pathology.

A novel MECP2 mutation in a boy with neonatal encephalopathy and facial dysmorphism

Methly-CpG-binding protein 2 (MECP2) mutations cause Rett syndrome in females. Here we report on a male infant with neonatal encephalopathy, myoclonic jerks, and irregular breathing patterns caused by a novel frameshift mutation in the MECP2 gene. In addition he has facial dysmorphisms previously not described in these patients.

A multiplexed ARMS-PCR approach for the detection of common MECP2 mutations

Rett syndrome (RTT), an X-linked dominant neurodevelopmental disorder, is caused mainly by de novo mutations in the methyl-CpG-binding protein 2 gene (MECP2). Although more than 200 different MECP2 mutations have been identified throughout the gene, 7 of those (p.R133C, p.T158M, p.R168X, p.R255X, p.R270X, p.R294X, and p.R306C) account for up to two-thirds of pathogenic mutations in RTT patients. A rapid and efficient screening strategy for these mutations can be used as a preliminary step for genetic diagnosis of RTT. The current protocols used for this purpose are of high cost and require special equipment. We have designed a simpler multiplex amplification refractory mutation system (ARMS)-PCR strategy that allows identification of these common MECP2 mutant alleles in four PCR reactions. The assay was tested in 14 RTT patients who were previously genotyped using PCR-restriction fragment length polymorphism and DNA sequencing. A complete concordance was observed between the results of the two methods. The multiplex ARMS-PCR does not require any special equipment, and it provides rapid, reproducible, and cost-effective detection of common MECP2 mutations. The assay can be carried out efficiently in a standard molecular genetics laboratory and suitable as a preliminary screen for all patients with RTT diagnosis.

MeCP2 involvement in the regulation of neuronal alpha-tubulin production

Rett syndrome (RTT) is a severe neurodevelopmental disorder caused by a dominant mutation in the X-linked methyl CpG binding protein 2 (MeCP2) gene. Neuroanatomically, RTT is characterized by a reduction in dendritic arborization and perikaryal size in the brain. MECP2 binds methylated promoters and facilitates assembly of a multiprotein repressor complex that includes Sin3A and the histone deacetylases HDAC1/HDAC2. MeCP2 has recently been found to be downregulated in autistic spectrum disorders such as Angelman syndrome (AS) and RTT, which share some phenotypic manifestations. We have conducted expression analysis of cytoskeleton-related genes in brain tissue of RTT and AS patients. Striking examples of genes with reduced expression were TUBA1B and TUBA3 that encode the ubiquitous alpha-tubulin and the neuronal specific alpha-tubulin, respectively. In accordance with the downregulation of expression of these genes, we have observed a reduction in the level of the corresponding protein product-tyrosinated alpha-tubulin. Low levels of alpha-tubulin and deteriorated cell morphology were also observed in MeCP2(-/y) MEF cells. The effects of MeCP2 deficiency in these cells were completely reversed by introducing and expressing the human MeCP2 gene. These results imply that MeCP2 is involved in the regulation of neuronal alpha-tubulin and add molecular evidence that reversal of the effects of MeCP2 deficiency is achievable. This raises hopes for a cure of Rett syndrome and related MeCP2 deficiency disorders of the autistic spectrum.

Mutations in the MECP2 gene are not a major cause of Rett syndrome-like or related neurodevelopmental phenotype in male patients

Rett syndrome is a genetic neurodevelopmental disorder that affects mainly girls, but mutations in the causative MECP2 gene have also been identified in boys with classic Rett syndrome and Rett syndrome-like phenotypes. We have studied a group of 28 boys with a neurodevelopmental disorder, 13 of which with a Rett syndrome-like phenotype; the patients had diverse clinical presentations that included perturbations of the autistic spectrum, microcephaly, mental retardation, manual stereotypies, and epilepsy. We analyzed the complete coding region of the MECP2 gene, including the detection of large rearrangements, and we did not detect any pathogenic mutations in the MECP2 gene in these patients, in whom the genetic basis of disease remained unidentified. Thus, additional genes should be screened in this group of patients.

A partial loss of function allele of methyl-CpG-binding protein 2 predicts a human neurodevelopmental syndrome

Rett Syndrome, an X-linked dominant neurodevelopmental disorder characterized by regression of language and hand use, is primarily caused by mutations in methyl-CpG-binding protein 2 (MECP2). Loss of function mutations in MECP2 are also found in other neurodevelopmental disorders such as autism, Angelman-like syndrome and non-specific mental retardation. Furthermore, duplication of the MECP2 genomic region results in mental retardation with speech and social problems. The common features of human neurodevelopmental disorders caused by the loss or increase of MeCP2 function suggest that even modest alterations of MeCP2 protein levels result in neurodevelopmental problems. To determine whether a small reduction in MeCP2 level has phenotypic consequences, we characterized a conditional mouse allele of Mecp2 that expresses 50% of the wild-type level of MeCP2. Upon careful behavioral analysis, mice that harbor this allele display a spectrum of abnormalities such as learning and motor deficits, decreased anxiety, altered social behavior and nest building, decreased pain recognition and disrupted breathing patterns. These results indicate that precise control of MeCP2 is critical for normal behavior and predict that human neurodevelopmental disorders will result from a subtle reduction in MeCP2 expression.

Mechanisms of disease: neurogenetics of MeCP2 deficiency

Rett syndrome (RTT) is unique among genetic, chromosomal and other developmental disorders because of its extreme female gender bias, early normal development, and subsequent developmental regression with loss of motor and language skills. RTT is caused by heterozygosity for mutations in the X-linked gene MECP2, which encodes methyl-CpG binding protein 2. MeCP2 is a multifunctional protein that can act as an architectural chromatin-binding protein, a function that is unrelated to its ability to bind methyl-CpG and to attract chromatin modification complexes. Inactivating mutations that cause RTT in females are not prenatally lethal in males, but lead to profound congenital encephalopathy. Molecular diagnoses of RTT, through demonstration of a MECP2 mutation, made at an early stage of the disorder, usually confirm the sporadic nature and very low recurrence risk of the condition. A positive DNA test result, however, also predicts the inevitable clinical course, given the lack of effective intervention. Initial hypotheses indicating that the MeCP2 protein acts as a genome-wide transcriptional repressor were not confirmed by global gene expression studies in various tissues of individuals with RTT and mouse models of MeCP2 deficiency. Rather, recent evidence points to low-magnitude effects of a small number of genes--including the brain--derived neurotrophic factor pathway and glucocorticoid response genes-that might affect formation and maturation of synapses or synaptic function in postmitotic neurons.

MECP2 mutations in males

Rett syndrome (RS; MIM 312750) is a severe neurological disorder affecting exclusively females. Its prevalence is about 1 in 10,000 female births, and it is a prominent cause of profound mental handicap in women. RS is caused by mutations in the X-linked methyl CpG-binding protein 2 (MECP2) gene. These mutations were initially thought to be lethal in males. However, MECP2 mutations are now frequently identified in mentally retarded male patients. The frequency of disease-causing MECP2 mutations in this population is between 1.3% and 1.7%. Surprisingly, MECP2 mutations in males are responsible for a wide spectrum of neurological disorders, ranging from mild mental retardation to severe neonatal encephalopathy. The aim of this review is to describe the nature of the MECP2 mutations identified in male patients to date and their associated phenotypes.

Rett syndrome: new clinical and molecular insights

In this review, we give a clinical overview of Rett syndrome (RTT), and provide a framework for clinical and molecular approaches to the diagnosis of this severe neurodevelopmental disorder. We also discuss issues that need to be considered in the management of RTT patients, and raise some of the challenges associated with genetic counselling.

MECP2 mutations or polymorphisms in mentally retarded boys: diagnostic implications

BACKGROUND

Among the well characterized X-linked conditions causing mental retardation, mutations in the methyl-CpG-binding protein 2 gene (MECP2) in Xq28 have been found in up to 85% of patients with Rett syndrome, a neurologic disorder which, in addition to other symptoms, severely affects higher cognitive functions in females. Mutations in the MECP2 gene are involved in a broad spectrum of phenotypes from classical Rett syndrome to mild intellectual difficulties in females and neonatal encephalopathy in males. Recently, mutations in the MECP2 gene were reported in males with non-specific mental retardation suggesting that defects in MECP2 could be responsible for up to 2% of X-linked mental retardation.

METHODS

We screened by denaturing high-pressure liquid chromatography the entire coding region and flanking intronic sequences of the MECP2 gene in a cohort of 354 mentally retarded males found negative for an expansion across the FRAXA CGG repeat and in a family in which a boy and his sister were mentally retarded.

RESULTS

We identified mainly silent polymorphisms within the MECP2 gene, together with four sequence alterations of unknown significance, i.e. three missense mutations (T197M, T228S, and P376S) and one substitution at position -19 in intron 3 (378-19delT). Further familial investigations allowed us to ruled out a pathogenic effect for the intronic variant, the T228S and the P376S missense mutations.

CONCLUSIONS

These results confirm that MECP2 mutations in males are far more rare than initially thought and call for a careful evaluation of the pathogenicity of the MECP2 missense mutations identified in mentally retarded males before genetic counseling is proposed to the relatives.

MECP2 abnormality phenotypes: clinicopathologic area with broad variability

Rett syndrome is a neurodevelopmental disorder that occurs worldwide and predominantly affects girls. The MECP2 gene has been put forward as the underlying gene. Interestingly, other clinical presentations in addition to Rett syndrome have been reported to be the results of deviations in MECP2. This prompted us to outline a working hypothesis of how these diverse phenotypes are connected. Our aim was to summarize the clinical picture of deviations in MECP2 at this moment to obtain a comprehensive overview. Thus, we have attempted to create a gradient, starting at the left with the most severely affected MECP2-deviant subgroups, represented by boys who are diseased in the intrauterine phase or as neonates, and at the right, the most mildly affected subgroup, female asymptomatic carriers. In the center, with dominant numbers, we have placed classic Rett syndrome presentations, together with the late-onset Rett syndrome variant and preserved speech variant. In conclusion, we feel that it is important to emphasize that Rett syndrome is a strictly clinical diagnosis that is not identical to the far broader concept of MECP2 deviations.

Does genotype predict phenotype in Rett syndrome?

Mutations in the X-linked gene encoding the methyl-CpG binding protein MeCP2 are the primary cause of classic and atypical Rett syndrome and have recently been shown to contribute to other neurodevelopmental disorders of varying severity. To determine whether there are molecular correlates to the phenotypic heterogeneity, numerous groups have performed genotype-phenotype correlation studies. These studies have yielded conflicting results, in part because they used different criteria for determining severity and classifying mutations. Evolution of the phenotype with age and variable expressivity arising from individual variability in X-chromosome inactivation patterns are among other reasons the findings varied. Nonetheless, evidence of differences in the phenotypic consequences of specific types of mutations is emerging. This review analyzes the available literature and makes recommendations for future studies.

Molecular diagnosis of Rett syndrome

In 1999, mutations in the MECP2 gene were identified as the primary cause of Rett syndrome. MECP2 mutations can be found in 70% to 80% of all clinically defined Rett syndrome cases; in classic Rett syndrome, this frequency is even higher. In most cases, missense and nonsense mutations affecting functionally important domains can be found. Additionally, a hot spot for small deletions has been defined, and several gross rearrangements have also been described. Among female individuals with Rett syndrome, the spectrum of clinical phenotypes is broad, but most fulfill the diagnostic criteria. In contrast, male individuals with mutations in the MECP2 gene are rare, and only a minority have clinical symptoms resembling Rett syndrome.

MeCP2 dysfunction in humans and mice

Rett syndrome is a leading cause of postnatal neurodevelopmental regression. Rett syndrome is caused by mutations in MECP2, the gene encoding methyl-CpG binding protein 2. In up to 96% of all classic cases, Rett syndrome cases are caused by mutations or deletions in MECP2. The phenotypic spectrum of MECP2 mutations is broad and includes mental retardation with or without seizures, Angelman syndrome-like phenotype, and autism. Mecp308/Y mice carry a truncating mutation and display many of the features seen in Rett syndrome. Social behavior abnormalities and impaired social interactions in Mecp308/Y mice suggest that MeCP2 plays a role in modulating the activity of genes and neurons important for social interactions. Mice that overexpress MeCP2 at twice the endogenous levels develop a progressive neurologic disorder, demonstrating that MeCP2 levels are tightly regulated and raising the possibility that duplications or gain-of-function mutations of MECP2 might underlie some cases of neurodevelopmental X-linked disorders.

Delayed Maturation of Neuronal Architecture and Synaptogenesis in Cerebral Cortex ofMecp2-Deficient Mice

We detected morphologic abnormalities in the cerebral cortex of Mecp2-hemizygous (Mecp2(-/y)) mice. The cortical thickness of both somatosensory and motor cortices in mutants did not increase after 4 weeks of age, as compared with that in wild-type male mice. The density of neurons in those areas was significantly higher in layers II/III and V of Mecp2(-/y) mice than in wild-type mice, particularly in layers II/ III after 4 weeks of age. In layer II/III of the somatosensory cortex of Mecp2(-/y) mice, the diameter of the apical dendrite was thin and the number of dendritic spines was small. Electron microscopy revealed that two-week-old mutants already had numerous premature postsynaptic densities. These results indicate that Mecp2(-/y) mice suffered delayed neuronal maturation of the cerebral cortex and that the initial neuronal changes were caused by premature synaptogenesis. Rett syndrome patients with a heterozygous mutation of Mecp2 display developmental disorders including cortical malfunctions such as mental retardation, autism, and epilepsy. Our results provide evidence of the similarity with Rett syndrome brains in some respects and suggest that MeCP2/Mecp2 plays some role in synaptogenesis.

Reduced proportion of Purkinje cells expressing paternally derived mutant Mecp2308 allele in female mouse cerebellum is not due to a skewed primary pattern of X-chromosome inactivation

Rett syndrome (RTT) is an X-linked disorder caused by mutations in the methyl CpG binding protein 2 (MECP2) gene. The pattern of X-chromosome inactivation (XCI) is thought to play a role in phenotypic severity. In the present study, patterns of XCI were assessed by lacZ staining of embryos and adult brains of mice heterozygous for a X-linked Hmgcr-nls-lacZ transgene on a mutant mouse model of RTT. We found that there was no difference between the lacZ staining patterns in the brain of wild-type and heterozygous mutant embryos at embryonic day 9.5 (E9.5) suggesting that Mecp2 has no effect on the primary pattern of XCI. At 20 weeks of age, there was no significant difference between XCI patterns in the Purkinje cells in the cerebellum of heterozygous mutant and wild-type mice when the mutant allele was inherited from the mother. However, when the mutant allele was paternally inherited, a significant difference was detected. Thus, parental origin of the mutation may have a bearing on phenotype through XCI patterns. An estimation of the Purkinje cell precursor number based on XCI mosaicism revealed that, when the mutation was paternally inherited, the precursor number was less than that in the wild-type mice. Therefore, it is likely that the number of precursor cells allocated to the Purkinje cell lineage is affected by a paternally inherited mutation in Mecp2. We also observed that the pattern of XCI in cultured fibroblasts was significantly correlated with patterns in the Purkinje cells in mutant animals but not in wild-type mice.

Methyl-CpG binding protein 2 gene (MECP2) variations in Japanese patients with Rett syndrome: pathological mutations and polymorphisms

A total of 45 different mutations of methyl-CpG-binding protein 2 gene (MECP2) were identified in 145 of 219 Japanese patients with typical or atypical Rett syndrome (RTT) (66.2%). A missense mutation, T158M was the most common mutation of MECP2, identified in 22 (19.1%) patients, followed by four nonsense mutations, R168X (14.8%), R270X (13.0%), R255X (9.6%), and R294X (6.1%) in 115 patients with classical RTT. Two missense mutations, R133C (33.3%) and R306C (23.3%), and a nonsense mutation, R294X (13.3%), were common in 30 patients with atypical RTT, including the preserved speech variant (PSV). Frameshift mutations due to nucleotide deletion or insertion were identified in 22 patients with MECP2 mutations, and one of them had a 3.6 kb deletion encompassing exons 3 and 4. Three patients with classical RTT had a splicing anomaly. The wide spectrum of phenotypic variability in patients with RTT has been considered to be correlated with the mutation type and location in MECP2, and X-inactivation. However, most patients showed a random X-inactivation pattern evaluated by an androgen receptor gene polymorphism in this study, suggesting that a skewed X-inactivation might not be a main modification factor on clinical phenotypes of RTT. In addition, three new missense mutations, P176R, A378V and T479M, were identified in patients with RTT, but also in healthy Japanese, indicating that these mutations are non-pathogenic in Japanese. Information about rare polymorphic variations is very important for the molecular diagnosis of RTT, although rare polymorphic variants might differ among ethnic groups.

X-linked mental retardation: further lumping, splitting and emerging phenotypes

X-linked mental retardation (XLMR) is a very heterogeneous condition, subdivided in two categories mainly based on clinical features: syndromic XLMR (MRXS) and non-syndromic XLMR (MRX). Although it was thought that 20-25% of mental retardation (MR) in males was caused by monogenetic X-linked factors, recent estimations are lower: in the range of 10-12%. The number of identified genes involved in XLMR has been rapidly growing in the past years. Subsequently, an increasing number of patients and families have been reported in which mutations in XLMR genes have been identified. It was observed previously, that mutations in several of XLMR genes can result in syndromic and in non-syndromic phenotypes. This observation has been confirmed for the more recently identified genes. Therefore, in this review, focus has been given on the clinical data and on phenotype-genotype correlations for those genes implicated in both non-syndromic and syndromic XLMR.

MeCP2 deficiency in Rett syndrome causes epigenetic aberrations at the PWS/AS imprinting center that affects UBE3A expression

Rett syndrome (RS) is a severe and progressive neurodevelopmental disorder caused by heterozygous mutations in the X-linked methyl CpG binding protein 2 (MeCP2) gene. MeCP2 is a nuclear protein that binds specifically to methylated DNA and functions as a general transcription repressor in the context of chromatin remodeling complexes. RS shares clinical features with those of Angelman syndrome (AS), an imprinting neurodevelopmental disorder. In AS patients, the maternally expressed copy of UBE3A that codes for the ubiquitin protein ligase 3A (E6-AP) is repressed. The similar phenotype of these two syndromes led us to hypothesize that part of the RS phenotype is due to MeCP2-associated silencing of UBE3A. Indeed, UBE3A mRNA and protein are shown here to be significantly reduced in human and mouse MECP2 deficient brains. This reduced UBE3A level was associated with biallelic production of the UBE3A antisense RNA. In addition, MeCP2 deficiency resulted in elevated histone H3 acetylation and H3(K4) methylation and reduced H3(K9) methylation at the PWS/AS imprinting center, with no effect on DNA methylation or SNRPN expression. We conclude, therefore, that MeCP2 deficiency causes epigenetic aberrations at the PWS imprinting center. These changes in histone modifications result in loss of imprinting of the UBE3A antisense gene in the brain, increase in UBE3A antisense RNA level and, consequently reduction in UBE3A production.

Specific Genetic Disorders and Autism: Clinical Contribution Towards their Identification

Autism is a heterogeneous disorder that can reveal a specific genetic disease. This paper describes several genetic diseases consistently associated with autism (fragile X, tuberous sclerosis, Angelman syndrome, duplication of 15q11-q13, Down syndrome, San Filippo syndrome, MECP2 related disorders, phenylketonuria, Smith-Magenis syndrome, 22q13 deletion, adenylosuccinate lyase deficiency, Cohen syndrome, and Smith-Lemli-Opitz syndrome) and proposes a consensual and economic diagnostic strategy to help practitioners to identify them. A rigorous initial clinical screening is presented to avoid unnecessary laboratory and imaging studies. Regarding psychiatric nosography, the concept of "syndromal autism"--autism associated with other clinical signs should be promoted because it may help to distinguish patients who warrant a multidisciplinary approach and further investigation.

Mutations in the X-linked cyclin-dependent kinase-like 5 (CDKL5/STK9) gene are associated with severe neurodevelopmental retardation

Recently, we showed that truncation of the X-linked cyclin-dependent kinase-like 5 (CDKL5/STK9) gene caused mental retardation and severe neurological symptoms in two female patients. Here, we report that de novo missense mutations in CDKL5 are associated with a severe phenotype of early-onset infantile spasms and clinical features that overlap those of other neurodevelopmental disorders, such as Rett syndrome and Angelman syndrome. The mutations are located within the protein kinase domain and affect highly conserved amino acids; this strongly suggests that impaired CDKL5 catalytic activity plays an important role in the pathogenesis of this neurodevelopmental disorder. In view of the overlapping phenotypic spectrum of CDKL5 and MECP2 mutations, it is tempting to speculate that these two genes play a role in a common pathogenic process.

Trisomy 21 and Rett syndrome: a double burden

Rett syndrome is a severe neurodevelopmental disorder generally affecting girls. Affected individuals are apparently normal at birth but later pass through a period of regression with loss of hand and communication skills and the development of hand stereotypies and dyspraxia. Mutations in the methyl-CpG binding protein 2 (MECP2) gene, have now been found to cause Rett syndrome in up to 80% of classical cases. We report a girl with Down syndrome, one of three children with birth defects in a family of five. From the age of 18 months she developed symptomatology considered by her primary physician to be very characteristic of Rett syndrome. However, this remained a clinical diagnosis till the age of 12 years. Laboratory confirmation of the dual diagnosis, which includes a R168X mutation in the MECP2 gene in addition to trisomy 21, has now been possible. The presence of one neurological or developmental disorder does not necessarily preclude a diagnosis of Rett syndrome.

Rett syndrome: a prototypical neurodevelopmental disorder

Rett syndrome, one of the leading causes of mental retardation and developmental regression in girls, is the first pervasive developmental disorder with a known genetic cause. The majority of cases of sporadic Rett syndrome are caused by mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2). MeCP2 binds methylated DNA and likely regulates gene expression and chromatin structure. Genotype/phenotype analysis revealed that the phenotypic spectrum of MECP2 mutations in humans is broader than initially suspected: Mutations have been discovered in Rett syndrome variants, mentally retarded males, and autistic children. A variety of in vivo and in vitro models has been developed that allow analysis of MeCP2 function and pathogenic studies of Rett syndrome. Because the neuropathology of Rett syndrome shares certain features with other neurodevelopmental disorders, a common pathogenic process may underlie these disorders. Thus, Rett syndrome is a prototype for the genetic, molecular, and neurobiological analysis of neurodevelopmental disorders.

Phenotypic manifestations of MECP2 mutations in classical and atypical Rett syndrome

Since the identification of mutations in MECP2 in girls and women with apparent Rett syndrome, numerous efforts have been made to develop phenotype-genotype correlations. These studies have produced conflicting results in part related to use of different clinical severity scales, different diagnostic criteria, and different stratification by age and mutation group as well as the possible effects of unbalanced X-chromosome inactivation. The present study applied a revised ordinal scoring system that allowed for correction for patient ages. We analyzed 85 patients with mutation in MECP2. Sixty-five (76%) had one of eight common mutations. Patients with missense mutations had lower total severity scores and better language performance than those with nonsense mutations. No difference was noted between severity scores for mutations in the methyl-binding domain (MBD) and the transcriptional repression domain (TRD). However, patients with missense mutations in TRD had the best overall scores and better preservation of head growth and language skills. Analysis of specific mutation groups demonstrated a striking difference for patients with the R306C mutation including better overall score, later regression, and better language with less motoric impairment. Indeed, these patients as a group accounted for the differences in overall scores between the missense and nonsense groups. Thus, the impact of specific mutations coupled with possible variation in X-chromosome inactivation must be considered carefully in the derivation of phenotype-genotype correlations. These results emphasize the limitations of such analyses in larger mutation groups, either by type or position.

Investigation ofUBE3A andMECP2 in Angelman syndrome (AS) and patients with features of AS

Angelman syndrome (AS) is an imprinted neurobehavioral disorder characterized by mental retardation, absent speech, excessive laughter, seizures, ataxia, and a characteristic EEG pattern. Classical lesions, including deletion, paternal disomy, or epigenetic mutation, are confirmatory of AS diagnoses in 80% of cases. Loss-of-function mutations of the UBE3A gene have been identified in approximately 8% of AS cases, failing to account for the remaining patient population, and there appears to be a higher prevalence of mutations in familial than sporadic cases. We screened UBE3A in 45 index cases of AS without obvious 15q11-13 abnormalities. Pathological mutations were identified in 3/6 (50%) familial and 4/39 (>10%) sporadic cases. By combining our data with those of the literature, we demonstrate statistically that the frequency of UBE3A mutations is significantly higher in the familial than sporadic subsets of AS. This indicates that an independent molecular mechanism or 'phenocopy' exists for the sporadic group. Rett syndrome (RS), caused by mutations of the MECP2 gene, and patients with deletions of 22q13.3 --> qter, have overlapping clinical features with AS. We screened 24 of the sporadic AS cases without detectable UBE3A mutations for mutations of MECP2, but found none. A separate cohort of 43 atypical patients with features common to AS and RS, in whom 15q11-13 lesions and 22q13.3 --> qter deletion had been ruled out, were also screened for MECP2 mutations. One male patient was mosaic for a frameshift mutation of this gene (previously reported). While MECP2 mutations can cause a phenotype reminiscent of AS in rare cases, they fail to account for the excess of sporadic patients with a definitive clinical diagnosis of AS.

MECP2 gene mutations in non-syndromic X-linked mental retardation: Phenotype-genotype correlation

Non-syndromic X-linked mental retardation (MRX) is a frequent cause of inherited mental retardation. It is a heterogeneous condition in which the first 12 genes discovered to date explain no more than 15% of the MRX situations ascertained by recurrence in multiplex families. In Rett syndrome (RTT), an X-linked dominant condition mostly sporadic and usually lethal in males, most affected females have been shown to be mutated in the Methyl-CpG binding protein 2 gene (MECP2) that maps at Xq28. Some mentally retarded males related to RTT females carry the same mutation. Several MRX families mapping to Xq28 were subsequently tested for MECP2 and a causative mutation was discovered in three families, suggesting that it could be one of the main genes involved in MRX. We report here the corresponding phenotypes in these three families of increasing severity. In family 1, an in-frame deletion DeltaP387-M466 was found in the 3' region. The patients had severe to mild non-progressive MR, with better motor skills than verbal abilities. In family 2, an Arg to Trp substitution (R167W) was found between the transcription repression domain (TRD) and the methyl binding domain (MBD). The patients had brisk reflexes and essential tremor with mild and non-progressive MR, poor motor co-ordination and written language difficulties. In the third family (MRX16), a Glu to Gly substitution (E137G) was found in the MBD. The patients had manifestations similar to those of family 2, but MR was mild to moderate, speech articulation was poor and some had verbal stereotypies. Regression of language skills was suspected in three patients. Phenotype-genotype correlation could thus be suspected and is discussed in these three families.

MECP2 analysis in mentally retarded patients: implications for routine DNA diagnostics

Rett syndrome (RTT) is one of the most common neurodevelopmental disorders in females. The disease is caused by mutations in the methyl-CpG-binding protein 2 gene (MECP2), and various mutations have been reported. The phenotypic spectrum in both female and male patients is diverse, ranging from very mild to congenital encephalopathy and prenatal lethality. In this study, the question was addressed as to whether implementation of systematic screening of MECP2 in patients with an unexplained mental retardation in DNA diagnostics would be reasonable, and the spectrum of phenotypes resulting from mutations in this gene was further explored. Mutational analysis of MECP2 was performed in mentally retarded female patients who were negative for FMR1 CGG repeat expansion, in male and female patients with clinical features suggestive of either Angelman or Prader-Willi syndrome without methylation defects on chromosome 15q11-q13. In the cohort of females negative for the molecular Fragile-X studies (N=92), one nonsense mutation (p.Q406X) was found. In the cohort of Angelman-negative patients (N=63), two missense mutations (p.R133C in a female patient and a mosaic p.T158M in a male patient) were found, which have been reported many times in patients with classical RTT syndrome. In the Prader-Willi-negative group (N=98), no pathogenic mutations were found. The results support testing of patients with features suggestive of Angelman syndrome, but without methylation defects on chromosome 15q11-q13 for mutations in MECP2. In the remaining patients with unexplained mental retardation, additional clinical features should determine whether analysis of MECP2 is indicated.

Retinochoroidal atrophy in two adult patients with Angelman syndrome

We describe a new ocular finding, retinochoroidal atrophy (RCA), associated with optic disk paleness in two adult patients with Angelman syndrome (AS) due to maternal 15q11-13 deletion. The ocular involvement described in children with AS consists iris and choroids hypopigmentation due to loss of function of one copy of P gene involved in maternal deletion. The loss of one copy of the same gene of paternal origin leads to a similar ocular phenotype as in Prader-Willi syndrome (PWS). However to our knowledge, RCA has never been described before in PWS, suggesting that other maternally expressed genes, particularly UBE3A, could be responsible for the retinal changes observed in the adult AS phenotype. Although, further investigations would be necessary to better understand the role of the UBE3A in the retina, the findings reported here should prompt a systematic ophthalmologic evaluation adult patients with AS in order to establish the real incidence of RCA and prevent further disability in these patients.

MECP2 and beyond: phenotype-genotype correlations in Rett syndrome

The association of Rett syndrome with pathogenic mutations of the methyl-CpG binding protein 2 (MECP2) gene was first made in 1999. Since that time, it has been found that the clinical phenotype can, at least in part, be explained in terms of the type and location of the MECP2 mutation and epigenetic factors such as skewing of X-chromosome inactivation. In addition, MECP2 mutations may be associated with non-Rett syndrome clinical phenotypes, including nonsyndromic and syndromic X-linked mental retardation and Angelman-like phenotypes. Intense research efforts are currently focused on understanding the pathogenesis of Rett syndrome, using sophisticated techniques such as microarray analysis, and the development of mouse models, with an ultimate aim being the development of targeted therapies that could ameliorate or even prevent the devastating consequences of this enigmatic neurodevelopmental disorder.

Mutations and polymorphisms in the human methyl CpG-binding protein MECP2

Rett syndrome (RTT or RS) is a neurodevelopmental disorder and one of the most frequent genetic diseases in girls. Mutations of the MECP2 gene have been found in a variety of different RTT phenotypes. The MECP2 gene (Xq28) has been described in 1992. Up to now, 218 different mutations have been reported in a total group, of more than 2,100 patients. Mutations in the MECP2 gene are responsible for up to 75% of the classical RTT cases. The mutations, are distributed along the whole gene and are comprised of all types of mutations. Several polymorphisms and benign genetic variants have also been described. Apart from spared reported familial cases, almost all cases are sporadic. RTT syndrome has been considered to be a lethal trait in males. Studying the parental origin of the mutations, however, we and others have found a very high prevalence of de novo mutations on the paternal chromosome. In this work we summarize the mutational reports published until now. One of our aims was to check the mutations' descriptions for consistency and particularly to rename them according to the recommended mutation nomenclature. The increasing number of investigations on the functions of the MeCP2 can help to gain more information about the neuropathogenetic mechanisms causing RTT. Hum Mutat 22:107-115, 2003.

Methyl-CpG-binding protein 2 is localized in the postsynaptic compartment: an immunochemical study of subcellular fractions

Methyl-CpG-binding protein 2 is a characteristic member of the methyl-CpG-binding protein family of transcription regulators. In conjunction with Sin3, MeCP2 recruits class I histone deacetylases to methyl-CpG regions to suppress transcription. Rett syndrome, a disorder characterized by mental retardation and autistic features, is associated in a majority of cases with mutations within the coding region of the MeCP2 gene. Considering that defective MeCP2 has mainly been related to Rett syndrome and other neurologic manifestations, we examined methyl-CpG-binding protein 2 cellular and subcellular compartmentalization in normal brain by immunochemical methods. Methyl-CpG-binding protein 2 immunoreactivity is present mainly in neurons; while the few immunostained glia show label confined to nuclei, many neurons also show slight perikaryal staining. Using well-characterized tissue fractions, we found that methyl-CpG-binding protein 2 but not Sin3 is found in both nuclear and postsynaptic compartments. This novel extranuclear localization is not unique to methyl-CpG-binding protein 2, since it has been previously reported for other transcription regulators such as c-Fos. These findings support the concept that methyl-CpG-binding protein 2 may link synaptic activity and transcriptional regulation in neurons.

Rett syndrome: the complex nature of a monogenic disease

Rett syndrome (RTT) is a severe neurodevelopmental disorder affecting almost exclusively girls. It is currently considered a monogenic X-linked dominant disorder due to mutations in MECP2 gene, encoding the methyl-CpG binding protein 2. A few RTT male cases, resulting from mosaicism for MECP2 mutations, have been reported. Male germline MECP2 mutations cause either severe encephalopathy with death at birth (usually in brothers of classical RTT females) or X-linked recessive mental retardation (XLMR). To date the wide phenotypic heterogeneity associated with MECP2 mutations in females (from classical RTT to healthy carriers) has been explained by differences in X chromosome inactivation. However, conflicting results have been obtained in different studies, with both random and highly skewed X-inactivation reported in healthy carrier females. Consequently it is possible that mechanisms other than X-inactivation play a role in the expressivity of MECP2 mutations. To explain the phenotypic heterogeneity associated with MECP2 mutations we propose a digenic model in which the presence of a "mutated" allele in a second gene, leading to a less functional protein, determines the clinical severity of the MECP2 mutation. The model is supported by the identification of the same mutation in XLMR and RTT cases. The carrier mothers of XLMR families are clinically asymptomatic and present balanced X chromosome inactivation. Therefore the same mutation arising in different genetic backgrounds can cause XLMR in males, remain silent in the carrier females and cause classic RTT in females. MECP2 mutations account for approximately 70-80% of classic RTT cases. MECP2 negative cases might result from mutations in noncoding regions of MECP2 gene. Alternatively, these cases might be due to mutations in other genes (locus heterogeneity). This hypothesis is supported by the identification of several chromosomal rearrangements in MECP2 negative patients with RTT and RTT-like phenotypes. MeCP2 is considered a general transcriptional repressor. However, conditional mouse mutants with selective loss of Mecp2 in the brain develop clinical manifestations similar to RTT, indicating that MECP2 is exclusively required for central nervous system function. The involvement of MeCP2 in methylation-specific transcriptional repression suggests that MECP2 related disorders result from dysregulated gene expression. Studies on gene expression have been performed in mouse and human brains. A relatively small number of gene expression changes were identified. It is possible that MeCP2 causes dysregulation of a very small subset of genes that are not detected with this method of analysis, or that very subtle changes in many genes cause the neuronal phenotype.

Effects of MECP2 mutation type, location and X-inactivation in modulating Rett syndrome phenotype

Rett syndrome (RTT) is a clinically defined disorder that describes a subset of patients with mutations in the X-linked MECP2 gene. However, there is a high degree of variability in the clinical phenotypes produced by mutations in MECP2, even amongst classical RTT patients. In a large-scale screening project, this variability has been examined by looking at the effects of mutation type, functional domain affected and X-inactivation. Mutations have been identified in 60% of RTT patients in this study (25% of whom were atypical), including 23 novel mutations and polymorphisms. More mutations were found in classical patients (63%) compared to atypical patients (44%). All of the pathogenic mutations were de novo in patients for whom parent DNA was available for screening. A composite phenotype score was developed, based on the recommendations for reporting clinical features in RTT of an international collaborative group. This score proved useful for summarising phenotypic severity, but did not correlate with mutation type, domain affected or X-inactivation, probably due to complex interactions between all three. Other correlations suggested that truncating mutations and mutations affecting the methyl-CpG-binding domain tend to lead to a more severe phenotype. Skewed X-inactivation was found in a large proportion (43%) of our patients, particularly in those with truncating mutations and mutations affecting the MBD. It is therefore likely that X-inactivation does modulate the phenotype in RTT.

Diagnostic evaluation of developmental delay/mental retardation: An overview

Mental retardation (MR) is one of the few clinically important disorders for which the etiopathogenesis is still poorly understood. It is a condition of great concern for public health and society. MR is currently defined as a significant impairment of cognitive and adaptive functions, with onset before age 18 years. It may become evident during infancy or early childhood as developmental delay (DD), but it is best diagnosed during the school years. MR is estimated to occur in 1-10% of the population, and research on its etiology has always been a challenge in medicine. The etiopathogenesis encompasses so many different entities that the attending physician can sometimes feel a "virtual panic," starting a wide-range diagnostic evaluation. The Consensus Conference of the American College of Medical Genetics has recently established guidelines regarding the evaluation of patients with MR [Curry et al., 1997], emphasizing the high diagnostic utility of cytogenetic studies and neuroimaging in certain clinical settings. However, since then there has been substantial progress in molecular cytogenetics and neuroimaging techniques, the use of which has allowed recognition and definition of new disorders, thus increasing the diagnostic yield. This review will focus on the most appropriate investigations shown to be, at present, necessary to define the etiology of DD/MR, in the context of recommendations for the clinical evaluation of the patient with undiagnosed MR.

Angelman syndrome: a review of the clinical and genetic aspects

Angelman syndrome (AS) is a neurodevelopmental disorder characterised by severe learning difficulties, ataxia, a seizure disorder with a characteristic EEG, subtle dysmorphic facial features, and a happy, sociable disposition. Most children present with delay in developmental milestones and slowing of head growth during the first year of life. In the majority of cases speech does not develop. Patients with AS have a characteristic behavioural phenotype with jerky movements, frequent and sometimes inappropriate laughter, a love of water, and sleep disorder. The facial features are subtle and include a wide, smiling mouth, prominent chin, and deep set eyes. It is caused by a variety of genetic abnormalities involving the chromosome 15q11-13 region, which is subject to genomic imprinting. These include maternal deletion, paternal uniparental disomy, imprinting defects, and point mutations or small deletions within the UBE3A gene, which lies within this region. UBE3A shows tissue specific imprinting, being expressed exclusively from the maternal allele in brain. The genetic mechanisms identified so far in AS are found in 85-90% of those with the clinical phenotype and all interfere with UBE3A expression.

Absence of MeCP2 mutations in patients from the South Carolina autism project

The methyl-CpG binding protein 2 (MeCP2) gene has recently been identified as the gene responsible for Rett syndrome (RS), a pervasive developmental disorder considered by many to be one of the autism spectrum disorders. Most female patients with MeCP2 mutations exhibit the classic features of RS, including autistic behaviors. Most male patients with MeCP2 mutations exhibit moderate to severe developmental delay/mental retardation. Ninety nine patients from the South Carolina autism project (SCAP) were screened for MeCP2 mutations, including all 41 female patients from whom DNA samples were available plus the 58 male patients with the lowest scores on standard IQ tests and/or the Vineland Adaptive Behavior Scale. No pathogenic mutations were observed in these patients. One patient had the C582T variant, previously reported in the unaffected father of an RS patient. Two other patients had single nucleotide polymorphisms in the 3' UTR of the gene, G1470A and C1516G. These variants were seen in 12/82 and 1/178 phenotypically normal male controls, respectively. The findings from this and other studies suggest that mutations in the coding sequence of the MeCP2 gene are not a significant etiological factor in autism.