iPS cells to model CDKL5-related disorders

Rett syndrome (RTT) is a progressive neurologic disorder representing one of the most common causes of mental retardation in females. To date mutations in three genes have been associated with this condition. Classic RTT is caused by mutations in the MECP2 gene, whereas variants can be due to mutations in either MECP2 or FOXG1 or CDKL5. Mutations in CDKL5 have been identified both in females with the early onset seizure variant of RTT and in males with X-linked epileptic encephalopathy. CDKL5 is a kinase protein highly expressed in neurons, but its exact function inside the cell is unknown. To address this issue we established a human cellular model for CDKL5-related disease using the recently developed technology of induced pluripotent stem cells (iPSCs). iPSCs can be expanded indefinitely and differentiated in vitro into many different cell types, including neurons. These features make them the ideal tool to study disease mechanisms directly on the primarily affected neuronal cells. We derived iPSCs from fibroblasts of one female with p.Q347X and one male with p.T288I mutation, affected by early onset seizure variant and X-linked epileptic encephalopathy, respectively. We demonstrated that female CDKL5-mutated iPSCs maintain X-chromosome inactivation and clones express either the mutant CDKL5 allele or the wild-type allele that serve as an ideal experimental control. Array CGH indicates normal isogenic molecular karyotypes without detection of de novo CNVs in the CDKL5-mutated iPSCs. Furthermore, the iPS cells can be differentiated into neurons and are thus suitable to model disease pathogenesis in vitro.     

Novel de novo nonsense mutation of MECP2 in a patient with Rett syndrome

Because of the recent identification of several mutations of methyl-CpG-binding protein 2 (MECP2) in patients with Rett syndrome (RTT), a patient with suspected RTT from an autism clinic was screened for mutations. She was found to have a novel heterozygous nonsense mutation, 129C>T (Q19X), which leads to the most severely truncated MECP2 protein reported to date. Sequencing of parental DNA revealed the mutation was de novo. The patient was not affected with microcephaly or hyperventilation, but had other features of Rett syndrome including severe mental retardation and symptoms of autistic disorder. Moderately skewed X-chromosome inactivation (XCI) may have contributed to her relatively mild phenotype.     

Quantitative localization of heterogeneous methyl-CpG-binding protein 2 (MeCP2) expression phenotypes in normal and Rett syndrome brain by laser scanning cytometry

Rett syndrome (RTT) is an X-linked, dominant neurodevelopmental disorder caused by mutations in MECP2, encoding the methyl-CpG-binding protein 2 (MeCP2). A major paradox in the pathogenesis of RTT is how mutations in ubiquitously transcribed MECP2 result in a phenotype specific to the central nervous system (CNS) during postnatal development. To address this question, we have used a novel approach for quantitating the level and distribution of wild-type and mutant MeCP2 in situ by immunofluorescence and laser scanning cytometry. Surprisingly, cellular heterogeneity in MeCP2 expression level was observed in normal brain with a subpopulation of cells exhibiting high expression (MeCP2(hi)) and the remainder exhibiting low expression (MeCP2(lo)). MeCP2 expression was significantly higher in CNS compared with non-CNS tissues of human and mouse by automated quantitation of MeCP2 on multiple tissue arrays. Quantitative localization of MeCP2 expression phenotypes in normal human brain showed a mosaic, but distinct, distribution pattern, with MeCP2(hi) neurons highest in layer IV of the cerebrum and MeCP2(lo )neurons highest in the granular layer of the cerebellum. In female RTT brains, MECP2 mutant-expressing cells were identified as cells negative for the MeCP2 C-terminal epitope. MECP2 mutant-expressing cells were randomly localized in Rett cerebrum and cerebellum and showed normal MeCP2 expression with N-terminal-specific anti-MeCP2. These results demonstrate a CNS-specific cellular phenotype of MeCP2 high expression and suggest that MECP2 mutations in RTT are only manifested in MeCP2(hi) cells. In addition, our results demonstrate the power of laser scanning cytometry in examining complex cellular phenotypes in disease pathogenesis.     

<Emphasis Type="Italic">MECP2 </Emphasis>and <Emphasis Type="Italic">CDKL5</Emphasis> gene mutation analysis in Chinese patients with Rett syndrome

Rett syndrome (RTT) is a progressive neurodevelopmental disorder that is caused by mutations in the X-linked methyl-CpG-binding protein2 (MECP2) gene. In this study, the MECP2 sequences in 121 unrelated Chinese patients with classical or atypical RTT were screened for deletions and mutations. In all, we identified 45 different MECP2 mutations in 102 of these RTT patients. The p. T158M mutation (15.7%) was the most common, followed in order of frequency by p. R168X (11.8%), p. R133C (6.9%), p. R270X (6.9%), p. G269fs (6.9%), p. R255X (4.9%), and p. R306C (3.9%). In addition, we identified five novel MECP2 mutations: three missense (p. K305E, p. V122M, p. A358T), one insertion (c.45-46insGGAGGA), and one 22 bp deletion (c.881-902del22). Large deletions represented 10.5% of all identified MECP2 mutations. Conversely, mutations in exon 1 appeared to be rare (0.9%). The remaining cases without MECP2 mutations were screened for the cyclin-dependent kinase-like 5 (CDKL5) gene using denaturing high-performance liquid chromatography (DHPLC). One synonymous mutation (p. I72I) was found in exon 5, suggesting that CDKL5 is a rare cause of RTT. The overall MECP2 mutation detection rate for this patient series was 84.3:87.9% in 107 classical RTT cases and 57.1% in 14 atypical RTT cases. Moreover, there were two patients with homozygous mutations and normal female karyotypes. However, we did not pinpoint a significant relationship between genotype and phenotype in these cases.     

Real-time quantitative PCR as a routine method for screening large rearrangements in Rett syndrome: Report of one case of MECP2 deletion and one case of MECP2 duplication

Mutations in the X-linked methyl-CpG-binding protein 2 gene (MECP2) are found in 70-80% of cases of classical Rett syndrome (RTT) and in about 50% of cases of preserved speech variant (PSV). This high percentage of MECP2 mutations, especially in classical RTT cases, suggests that another major RTT locus is unlikely. Missed mutations may be due to the limited sensitivity of the methodology used for mutation scanning and/or the presence of intronic mutations. In a double-copy gene, such as MECP2 in females, current methodologies (e.g., DGGE, SSCP, DHPLC, direct sequencing) are prone to miss gross rearrangements. Three previous reports during 2001-2003 have shown the presence of large deletions in a fraction of MECP2-negative classical RTT patients. We developed a reliable, single tube, quantitative PCR assay for rapid determination of MECP2 gene dosage. This method involves a multiplex reaction using a FAM labeled TaqMan probe with a TAMRA quencher derived from MECP2 exon 4 and two primers derived from the same exon and RNAaseP as an internal reference. The copy number of the MECP2 gene was determined by the comparative threshold cycle method (ddCt). Each sample was run in quadruplicate. We validated this assay through the analysis of 30 healthy controls (15 female and 15 male) and we then applied this method to eight classical RTT and six PSV patients, all negative for MECP2 mutations. We identified gross rearrangements in two patients: a deletion in a classical RTT patient and a duplication in a PSV patient. Our results confirm that a fraction of MECP2-negative RTT cases have MECP2 gross rearrangements and we propose real-time quantitative PCR as a simple and reliable method for routine screening of MECP2 in addition to DHPLC analysis.     

X-Chromosome inactivation ratios affect wild-type MeCP2 expression within mosaic Rett syndrome and Mecp2-/+ mouse brain

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder caused by mutations in MECP2, encoding methyl-CpG-binding protein 2 (MeCP2). The onset of symptoms in RTT is delayed until 6-18 months and 4-6 months in the Mecp2(-/+) mouse model, corresponding to a dynamic and gradual accumulation of MeCP2 expression in individual neurons of the postnatal brain. Because of X chromosome inactivation (XCI), cells within RTT females are mosaic for expression of the heterozygous MECP2 mutation. Using the targeted Mecp2 mouse model, we investigated the effect of Mecp2 mutation on XCI and developmental MeCP2 expression in wild-type (wt)-expressing neurons by quantitative laser scanning cytometry. Mecp2(-/+) female mice exhibited uniform regional distribution of Mecp2 mutant-expressing cells in brain, but unbalanced XCI in the population, favoring expression of the Mecp2 wt allele. Interestingly, MeCP2 expression in Mecp2 wt-expressing cells from Mecp2(-/+) mice was significantly lower than those from Mecp2(+/+) age-matched controls. The negative effect of Mecp2 mutation on wt Mecp2 expression correlated with the percentage of Mecp2 mutant-expressing cells in the cortex. Similar results were observed in two RTT females with identical MECP2 mutations but different XCI ratios. These results demonstrate that Mecp2-mutant neurons affect the development of surrounding neurons in a non-cell-autonomous manner and suggest that environmental influences affect the level of MeCP2 expression in wt neurons. These results help in explaining the role of XCI in the pathogenesis of RTT and have important implications in designing therapies for female RTT patients.     

Molecular analysis of Japanese patients with Rett syndrome: Identification of five novel mutations and genotype-phenotype correlation

Rett syndrome is an X-linked dominant neurodevelopmental disorder that affects females almost exclusively. The recent identification of mutations of the methyl-CpG-binding protein 2 gene (MECP2) in patients with RTT, encouraged us to analyze the gene in 37 Japanese patients divided into classical RTT (14 cases), variant RTT (13 cases), and mentally retarded patients with Rett-like features (10 cases). Mutations in MECP2 were identified from most of the patients with classical and variant RTT (25 of 27 cases). Six reported common mutations were detected in 17 cases, and rare single nucleotide substitutions were found in 3 patients. In addition, one insertion mutation (1189insA) and four deletion mutations including one double deletion mutant (451delG, 100del4, 1124del53 and 881del289 plus 1187del8) were newly identified. In the 10 mentally retarded patients with Rett-like features, however, no mutation was detected in the coding region of MECP2. The finding of MECP2 mutations in 92.5% of patients with RTT indicates that RTT fulfilling the diagnostic criteria are due to genetic alteration.     

Rett syndrome: analysis of MECP2 and clinical characterization of 31 patients

Only recently have mutations in MECP2 been found to be a cause of Rett Syndrome (RTT), a neuro-developmental disorder characterized by mental retardation, loss of expressive speech, deceleration of head growth and loss of acquired skills that almost exclusively affects females. We analysed the MECP2 gene in 31 patients diagnosed with RTT. Sequencing of the coding region and the splice sites revealed mutations in 24 females (77.40%). However, no abnormalities were detected in any of the parents that were available for investigation. Eleven mutations have not been described previously. Confirming two earlier studies, we found that most mutations are truncating and only a few of them are missense mutations. Several females carrying the same mutation display different phenotypes indicating that factors other than the type or position of mutations influence the severity of RTT. Four females with RTT variants were included in the study. Three of these presented with preserved speech while the fourth patient with congenital RTT lacked the initial period of normal development. Detection of mutations in these cases reveals that they are indeed variants of RTT. They represent the mild and the severe extremes of RTT. Conclusions: mutations in MECP2 seem to be the main cause for RTT and can be expected to be found in approximately 77% of patients that fulfil the criteria for RTT. Therefore analysis of MECP2 should be performed if RTT is suspected. Three mutation hotspots (T158M, R168X and R255X) were confirmed and a further one (R270X) newly identified. We recommend screening for these mutations before analysing the coding region.     

Rett syndrome in adolescent and adult females: clinical and molecular genetic findings

Rett syndrome (RTT) is a neurodevelopmental disorder which is diagnosed clinically. We report on 30 adolescent and adult females with classical or atypical RTT of whom 24 have a MECP2 mutation. In these 24 females, the clinical manifestations, degree of severity, and disorder profiles are discussed as well as the genotype phenotype correlation. After X-chromosome inactivation (XCI) study in these cases, we found no correlation between skewing and milder phenotype. Three large deletions were found after additional Southern blot analysis in three classical RTT cases. We confirm that early truncating mutations in MECP2 are responsible for a more severe course of the disorder. Three disorder profiles related to the missense mutations R133C and R306C, and to deletions in the C terminal segment are described and are of interest for further clinical study on larger numbers of cases. The R133C genotype has a predominantly autistic presentation while the R306C genotype is associated with a slower disease progression. The phenotype of the "hotspot" deletions in the C terminal segment is predominantly characterized by rapid progressive neurogenic scoliosis. Older women with RTT are underdiagnosed: seven adults were first diagnosed as having RTT between 29 and 60 years of age, and confirmed on finding a MECP2 mutation. Knowledge of the clinical phenotype of RTT at an adult age is important for all involved in the care of these individuals. The involvement of the parent support group is very important in this matter.     

The array of clinical phenotypes of males with mutations in Methyl‐CpG binding protein 2

Mutations in the X‐linked gene MECP2 are associated with a severe neurodevelopmental disorder, Rett syndrome (RTT), primarily in girls. It had been suspected that mutations in Methyl‐CpG‐binding protein 2 (MECP2) led to embryonic lethality in males, however such males have been reported. To enhance understanding of the phenotypic spectrum present in these individuals, we identified 30 males with MECP2 mutations in the RTT Natural History Study databases. A wide phenotypic spectrum was observed, ranging from severe neonatal encephalopathy to cognitive impairment. Two males with a somatic mutation in MECP2 had classic RTT. Of the remaining 28 subjects, 16 had RTT‐causing MECP2 mutations, 9 with mutations that are not seen in females with RTT but are likely pathogenic, and 3 with uncertain variants. Two subjects with RTT‐causing mutations were previously diagnosed as having atypical RTT; however, careful review of the clinical history determined that an additional 12/28 subjects met criteria for atypical RTT, but with more severe clinical presentation and course, and less distinctive RTT features, than females with RTT, leading to the designation of a new diagnostic entity, male RTT encephalopathy. Increased awareness of the clinical spectrum and widespread comprehensive genomic testing in boys with neurodevelopmental problems will lead to improved identification.     

Analysis of Hungarian patients with Rett syndrome phenotype for MECP2, CDKL5 and FOXG1 gene mutations

Rett syndrome (RTT) is characterized by a relatively specific clinical phenotype. We screened 152 individuals with RTT phenotype. A total of 22 different known MECP2 mutations were identified in 42 subjects (27.6%). Of the 22 mutations, we identified 7 (31.8%) frameshift-causing deletions, 4 (18.2%) nonsense, 10 (45.5%) missense mutations and one insertion (4.5%). The most frequent pathologic changes were: p.Thr158Met (14.2%) and p.Arg133Cys (11.9%) missense, and p.Arg255Stop (9.5%) and p.Arg294Stop (9.5%) nonsense mutations. We also detected the c.925C >T (p.Arg309Trp) mutation in an affected patient, whose role in RTT pathogenesis is still unknown. Patients without detectable MECP2 defects were screened for mutations of cyclin-dependent kinase-like 5 (CDKL5) gene, responsible for the early-onset variant of RTT. We discovered two novel mutations: c.607G >T resulting in a termination codon at aa203, disrupting the catalytic domain, and c.1708G >T leading to a stop at aa570 of the C terminus. Both patients with CDKL5 mutation presented therapy-resistant epilepsy and a phenotype fitting with the diagnosis of early-onset variant of RTT. No FOXG1 mutation was detected in any of the remaining patients. A total of 110 (72.5%) patients remained without molecular genetic diagnosis that necessitates further search for novel gene mutations in this phenotype. Our results also suggest the need of screening for CDKL5 mutations in patients with Rett phenotype tested negative for MECP2 mutations.     

Balanced X chromosome inactivation patterns in the Rett syndrome brain

In Rett syndrome (RTT), an X-linked disorder essentially limited to females, neurological development goes awry. Causing this disarray in neuronal function is a mutated form of a protein known as methyl-CpG-binding protein 2 (MeCP2). Because the MECP2 gene is subject to X chromosome inactivation (XCI) in females, a number of studies have addressed whether the percentage of cells inactivating the normal vs. mutant chromosome in heterozygous females influences the phenotypic outcome of MECP2 mutations. Because most of these studies measured XCI in peripheral blood, however, interpretation of the results requires the assumption that XCI patterns in blood are representative of those in the brain, the primarily affected tissue. Here, we have analyzed the MECP2 sequence and XCI status in 13 brains of RTT patients. Mutations were identified in nine of the cases, with eight of these representing C to T transitions at CpG dinucleotides, and one being a novel frameshift mutation (765delA). Patterns of XCI were balanced in 10 of 10 cases for which the assay was informative. As previous studies have shown that a majority of RTT patients have balanced XCI patterns in peripheral blood, our results suggest that the pattern in blood is an accurate indicator of XCI patterns in the brain for a majority of cases, but there are some notable exceptions that this study may help explain. Given the correlation between balanced XCI and classic RTT, these results suggest that a certain percentage of neurons expressing the mutant MECP2 gene may be required for RTT to become manifest.     

Rett syndrome mutation MeCP2 T158A disrupts DNA binding, protein stability and ERP responses

Mutations in the MECP2 gene cause the autism spectrum disorder Rett syndrome (RTT). One of the most common MeCP2 mutations associated with RTT occurs at threonine 158, converting it to methionine (T158M) or alanine (T158A). To understand the role of T158 mutations in the pathogenesis of RTT, we generated knockin mice that recapitulate the MeCP2 T158A mutation. We found a causal role for T158A mutation in the development of RTT-like phenotypes, including developmental regression, motor dysfunction, and learning and memory deficits. These phenotypes resemble those present in Mecp2 null mice and manifest through a reduction in MeCP2 binding to methylated DNA and a decrease in MeCP2 protein stability. The age-dependent development of event-related neuronal responses was disrupted by MeCP2 mutation, suggesting that impaired neuronal circuitry underlies the pathogenesis of RTT and that assessment of event-related potentials (ERPs) may serve as a biomarker for RTT and treatment evaluation.     

Age-dependent expression of MeCP2 in a heterozygous mosaic mouse model

Functional deficiency of the X-linked methyl-CPG binding protein 2 (MeCP2) leads to the neurodevelopmental disorder Rett syndrome (RTT). Due to random X-chromosome inactivation (XCI), most RTT patients are females who are heterozygous for the MECP2 mutation and therefore mosaic in MeCP2 deficiency. Some MECP2 heterozygote females are found to have unbalanced XCI, which may affect the severity of neurological symptoms seen in these patients; however, whether MeCP2 deficiency affects XCI in the postnatal and adult brain is unclear. Here we developed a novel MeCP2 mosaic mouse model in which the X chromosome containing the wild-type Mecp2 expresses a green fluorescent protein (GFP) transgene, while the X chromosome harboring the mutant Mecp2 does not. Due to random XCI, the neurons in the female MeCP2 mosaic mice express either wild-type MeCP2 (GFP+) or mutant MeCP2 (GFP-), and the two can be distinguished by GFP fluorescence. Using this mouse model, we evaluated XCI in female heterozygote mice from 3 to 9 months after birth. We found that MeCP2 deficiency does not affect XCI at 3 months of age, but does alter the proportion of wild-type MeCP2-expressing neurons at later ages, suggesting that MeCP2 impacts XCI patterns in an age-dependent manner. Given the important function of MeCP2 in neuronal development, our data could shed light on how MeCP2 deficiency affects postnatal brain functions and the dynamic changes in the neurological symptoms of RTT.     

Spectrum and distribution of MECP2 mutations in 424 Rett syndrome patients: a molecular update

Mutations in the MECP2 (Methyl-CpG-binding protein) gene have been reported to cause Rett syndrome (RTT), an X-linked progressive encephalopathy. Recent studies have identified large gene rearrangements that escape the common PCR-based mutation screening strategy and mutations in a novel MeCP2 isoform (named MECP2B). We have collected the results of MECP2 mutational analysis concerning 424 RTT patients conducted in eight laboratories in France. In total, 121 different MECP2 mutations were identified. R168X (11.5%) is the most common of MECP2 mutations, followed by R270X (9%), R255X (8.7%), T158 M (8.3%) and R306C (6.8%). Only eight mutations had relative frequency>3%. Large and complex rearrangements not previously detected using only a PCR-based strategy represent 5.8% of MECP2 mutations. On the contrary, mutation in exon 1 appears to be rare (less than 0.5%). These data demonstrate the high allelic heterogeneity of RTT in France and suggest that routine mutation screening in MECP2 should include quantitative analysis of the MECP2 gene. This study represents an important instrument for molecular diagnosis strategy and genetic counseling in RTT families.     

Molecular characteristics of Chinese patients with Rett syndrome

Objective

Rett syndrome (RTT) is a neurodevelopmental disorder which affects 1/10,000 girls. The aim of this study is to delineate the molecular characteristics of Rett syndrome in China based on the largest group of Chinese patients ever studied.

Methods

In all, 365 Chinese patients with Rett syndrome were recruited. Clinical information including the family reproductive history was collected through interviewing patients and their parents as well as questionnaires. MECP2, CDKL5, FOXG1 mutational analysis was performed using polymerase chain reaction (PCR), direct sequencing and multiplex ligation-dependent probe amplification (MLPA). The parental origin of mutated MECP2 gene, the MECP2 gene mutation rate in the patients' mothers, and the X-chromosome inactivation pattern of the mothers who carry the mutation were also analyzed.

Results

Almost all of the patients were sporadic cases except one pair of twins. The pregnancy loss in probands' mothers and sex ratio of offspring in probands^' generation were available in 352 families and were comparable to the general population. Out of the 365 cases, 315 had MECP2 gene mutations and 3 had de novo CDKL5 gene mutations. No patients had FOXG1 mutation. Among the 315 cases with MECP2 mutations, 274 were typical cases and 41 were atypical cases. All the 3 cases with CDKL5 gene mutations were atypical RTT with early-onset seizures. The analysis of parental origin of mutated MECP2 gene were performed on 139 cases, 90 (64.7%) cases were informative for the study. The result showed 94.4% cases with mutations from paternal origin and 5.6% from maternal origin. Among the cases with paternal mutation, 90.6% had point mutations. C > T was the most common one, accounting for 85.7% of the point mutations. Only one normal phenotype mother (0.41%) carried the same p.R133C mutation of MECP2 gene as her daughter with mild phenotype. The different patterns of X-chromosome inactivation in the mother and the daughter may explain their different phenotypes.

Conclusion

The high rate of paternal origin of the mutated MECP2 gene may explain the high occurrence of RTT in female gender. The family cases of RTT are rare and the recurrence risk of RTT is very low in China. Only 0.41% (1/244) mothers carry the pathogenic gene. FOXG1 mutations were not found in this group of Chinese patients.     

Mutation analysis of the HDAC 1, 2, 8 and CDKL5 genes in Rett syndrome patients without mutations in MECP2

Mutations in the MECP2 gene are found in only 80% of patients with Rett syndrome (RTT). Therefore other genes have to be involved in the pathogenesis of RTT. By using our defined diagnostic criteria we first re-evaluated 50 girls with possible RTT in whom the sequencing of the MECP2 gene had not revealed any mutations. Only 15 of theses patients fulfilled all criteria for RTT and could be considered to have classical RTT. In eight of these, further molecular analyses revealed large deletions of the MECP2 gene. In the remaining seven girls we then analyzed the genes HDAC1, HDAC2, and HDAC8 that encode for the histone deacetylases 1, 2, and 8 which interact with MeCP2 and are essential for its function. Although these histone deacetylase genes have been considered as good candidate genes for RTT our molecular analysis of these genes did not detect any mutations. Because recently mutations in CDKL5 were reported in patients with RTT, we included this gene in our analysis but failed to detect any mutations. We conclude that only a subgroup of girls with possible RTT and no detectable mutation in the sequencing of the MECP2 gene do really have classical RTT. In many of those large MECP2 gene deletions can be detected by further analysis. The genes HDAC1, HDAC2, and HDAC8 do not seem to play a role in the pathogenesis of RTT and at least in our subgroup no mutations in the CDKL5 gene were detected.     

Screening for MECP2 mutations in Spanish patients with an unexplained mental retardation

Rett syndrome (RTT) is an X-linked progressive encephalopathy. Mutations in the MECP2 (methyl-CpG-binding protein) gene have been found to cause RTT. In the past few years, the role of MECP2 mutations in patients with mental disorders other than RTT has been studied, finding that mutations in MECP2 also contribute to non-syndromic entities. More recently, it has been demonstrated that RTT shares clinical features with those of Angelman syndrome, another neurodevelopmental disorder. These observations must be confirmed in a large series, to better understand the criteria needed for justifying a molecular test. Consequently, we have searched for MECP2 mutations in 294 patients (43 Angelman and Prader-Willi like included) with mental retardation (MR) of unknown aetiology. We found six polymorphisms (three novel, three previously reported) in 10 patients, one novel unclassified silent change (p.V222V) in a man, and one causative mutation in a girl with MR. Once this case was clinically reviewed, the girl presented symptoms of atypical RTT. The mutation (p.Y141C) lies within the methyl-binding domain, and has only been reported once in another atypical RTT. Our results show that the MECP2 mutations account for a low frequency (1/416 chromosomes = 0.24%) among mentally retarded individuals, which imply that it is necessary to perform an exhaustive clinical examination of patients before determining whether analysis of MECP2 is required or not.