Chromosome mapping of Rett syndrome: a likely candidate region on the telomere of Xq

Rett syndrome: the Brazilian contribution to the gene discovery

OBJECTIVE

A brief history of the syndrome discovered by Andreas Rett is reported in this paper.

METHODS

Although having been described in 1966, the syndrome was only recognized by the international community after a report by Hagberg et al. in 1983. Soon, its importance was evident as a relatively frequent cause of severe encephalopathy among girls.

CONCLUSION

From the beginning it was difficult to explain the absence of male patients and the almost total predominance of sporadic cases (99%), with very few familial cases. For these reasons, it was particularly difficult to investigate this condition until 1997, when a particular Brazilian family greatly helped in the final discovery of the gene, and in the clarification of its genetic mechanism.

RESULTS

Brief references are made to the importance of the MECP2 gene, 18 years later, as well as to its role in synaptogenesis and future prospects.

Mechanisms of sex difference: a historical perspective

OBJECTIVE

The history of the discovery of mechanisms contributing to sex difference helps to better appreciate gender factors in a variety of disease states. The objective of this article is to illustrate four mechanisms of sex differences in disease incidence: X-linkage (including inactivation, escape from inactivating, skewed inactivation), sex-specific exposure to disease-producing pathogens, fetal microchimerism, and iron depletion.

METHODS

This is a historic review.

RESULTS

An emphasis on sex difference led to the uncovering of four different mechanisms by which illness rates differ in men and women.

CONCLUSIONS

Research into many disease states can benefit from a focus on potential mechanisms that yield sex differences in illness susceptibility, progression, and outcome.

Autism genetics: strategies, challenges, and opportunities

Although genes have long been appreciated to play a critical role in determining the risk for pervasive developmental disorders, the specific transcripts contributing to autism spectrum disorders (ASD) have been quite difficult to characterize. However, recent findings are now providing the first insights into the molecular mechanisms underlying these syndromes and have begun to shed light on the allelic architecture of ASD. In this article, we address what is known about the relative contributions of various types of genetic variation to ASD, consider the obstacles facing gene discovery in this complex disorder, and evaluate the common methodologies employed to address these issues, including linkage, molecular and array-based cytogenetics, and association strategies. We review the current literature, highlighting recent findings implicating both rare mutations and common genetic polymorphisms in the etiology of autism. Finally, we describe key advances in genomic technologies that are transforming all areas of human genetics and consider both the opportunities and challenges for autism research posed by these rapid changes.

Identifying patterns of communicative behaviors in girls with Rett syndrome

The purpose of this study was to investigate behaviors of girls with Rett syndrome under different conditions and to determine if there were patterns of functional communicative behaviors. Six communication protocols portraying familiar and non-familiar activities were created to assist in identifying specified behaviors. The protocols incorporated activities and pre-scheduled interruptions designed to enhance communicative behaviors. Eight girls in the third and fourth stages of Rett syndrome were videotaped during the protocols. Results indicated that the girls' alternating eye gazing increased and persistence decreased when activities were interrupted. Additional behaviors varied by participant across different protocols. Several patterns of effective and ineffective communication were observed. Implications for practice and future research are discussed.

Clinical variability in Rett syndrome

The clinical variability of Rett syndrome, associated with mutations in the MECP2 gene, varies from classically symptomatic female patients to asymptomatic female patients, and male patients who have none of the diagnostic features considered pathognomonic of this disease. Multiple factors contribute to this variability. In our studies, mutations closer to the amino-terminus, prior to amino acid 255, led to severe clinical manifestations, such as inability to walk, severe dysphagia, and urinary organic acid abnormalities, compared with mutations toward the carboxyl-terminus. However, we found no correlation between severity and mutation type (missense versus nonsense). Despite the importance of mutation location to clinical severity, the widely varying severity within the same mutation suggests that in females, X-chromosome inactivation or other epigenetic phenomena also have roles in determining severity. We propose that stages 1 and 2 of the disease are a consequence of failed, time-linked, postnatal expression of MeCP2 in cerebellar neurons. This, in association with glutamate N-methyl-D-aspartate receptor-mediated neuroexcitotoxic injury to the differentiating neurons, results in the transient age-specific autistic-like behavior, motor, and cognitive dysfunction associated with these stages.

Associations between MeCP2 mutations, X-chromosome inactivation, and phenotype

Rett syndrome is a neurodevelopmental disorder of early postnatal brain growth in girls. Patients show a normal neonatal period with subsequent developmental regression and a loss of acquired skills (communication and motor skills), deceleration of head growth, and development of typical hand stereotypies. Recent studies have shown that mutations in the X-linked methyl CpG binding protein 2 gene (MeCP2) cause most typical cases of Rett syndrome. The MeCP2 gene encodes a protein that binds methylated cytosine residues of CpG dinucleotides and mediates, with histone deacetylases and transcriptional repressors, the transcription "silencing" of other genes. Girls with Rett syndrome exhibit mosaic expression for the MeCP2 defect at the cellular level, with most patients showing random X-inactivation and approximately equal numbers of cells expressing the normal MeCP2 gene and the mutated MeCP2 gene. In rare cases, females with a MeCP2 mutation escape phenotypic expression of the disorder because of nonrandom X-inactivation and the preferential inactivation of the mutated MeCP2 allele. Nonrandom patterns of X-inactivation may also contribute to the clinical variability often seen in girls with Rett syndrome. The spectrum of clinical phenotype caused by MeCP2 mutations is wide, including milder "preserved speech" variants, the severe congenital Rett variant, and a subset of X-linked recessive mental retardation in boys. Studies have shown that atypical and classical Rett syndrome can caused by the same MeCP2 mutations, indicating clinical phenotype is variable even among girls with the same MeCP2 mutation. The relationship between type of MeCP2 mutation, X-inactivation status, and clinical phenotype of Rett syndrome is complex and likely involves other environmental and polygenic modifiers.

Importance of Rett syndrome in child neurology

The syndrome of brain atrophy in girls described by Andreas Rett in 1966 [Rett, Wien Klin Wochenschr, 1966;116:723-726] was brought to the attention of the English-speaking world by Hagberg et al. in 1983 [Hagberg et al., Ann Neurol, 1983;14:471-479]. Four clinical stages after the age of 6 months were described in classical cases of Rett syndrome (RS), namely early onset stagnation at 6 months to 1(1/2) years, the rapid destructive stage at 1-3 years, the pseudo-stationary stage from pre-school to school years, and the late motor deterioration stage at 15-30 or more years. The rapid destructive stage causes profound dementia with loss of speech and hand skills, stereotypic movements, ataxia, apraxia, irregular breathing with hyperventilation while awake, and frequently seizures. Most cases are isolated in their families, apart from identical twins. However, linkage studies in rare familial cases suggested a critical region at Xq28. In 1999 American investigators found several mutations in the X-linked gene MECP2 encoding Methyl-CpG-binding protein 2 in a proportion of Rett patients. The protein MeCP2 can bind methylated DNA and when mutated may interfere with transcriptional silencing of other genes and result in abnormal chromatin assembly. Many different mutations of the protein are being studied in humans and in mice. Neuropathological studies have shown decreased brain growth and decreased size of individual neurons, with thinned dendrites in some cortical layers, and abnormalities in substantia nigra, suggestive of deficient synaptogenic development, probably starting before birth. Electrophysiology demonstrates progressively abnormal electroencephalograms (EEG) in the first three stages of the syndrome, with some subsequent improvement and occurrence of pseudoseizures. Neurometabolic factors are discussed in detail, particularly reduced levels of dopamine, serotonin, noradrenaline and choline acetyltransferase (ChAT) in brain, also estimation of nerve growth factors, endorphin, substance P, glutamate and other amino acids and their receptor levels. Autonomic dysfunction is described, particularly reduced vagal and overactive sympathetic activity. Neuro-imaging may be required for further investigation, as shown in the differential diagnosis.

The biological functions of the methyl-CpG-binding protein MeCP2 and its implication in Rett syndrome

Methylation of DNA is essential for development in the mouse and plays an important role in inactivation of the X-chromosome, genomic imprinting and gene silencing. The properties of the methyl-CpG binding proteins (MeCPs) are being proved to be the key to interpreting the connection between DNA methylation and transcriptional repression. The founder member of the family, MeCP2, consists of a single polypeptide that contains both a methyl-CpG binding domain (MBD) and transcriptional repression domain (TRD). MBD binds to a single symmetrically methylated CpG site and is responsible for chromatin localization of the protein. NMR studies have revealed that the MBD adopts a wedge-shaped molecular structure. The TRD interacts with Sin3, which is known to form complexes with histone deacetylases. MeCP2-mediated transcriptional repression may involve two distinct mechanisms, one being dependent on chromatin modification by histone deacetylation and the other being chromatin independent. Mutations in MeCP2 gene cause the X-linked neurodevelopmental disease Rett syndrome. The spectrum of mutations reflects the importance of the MBD and TRD domains. We speculate that abnormal gene expression in Rett patients leads to dysfunction of the central nervous system. We propose a genetic therapeutic approach based on activation of the wild type copy of the MeCP2 gene located in the inactive X chromosome.

DHPLC analysis of the MECP2 gene in Italian Rett patients

Rett Syndrome (RTT) is an X-linked dominant neurodevelopmental disorder, which almost exclusively affects girls, with an estimated prevalence of one in 10,000-15,000 female births. Mutations in the methyl CpG binding protein 2 gene (MECP2) have been identified in roughly 75% of classical Rett girls. The vast majority of Rett cases (99%) are sporadic in origin, and are due to de novo mutations. We collected DNA samples from 50 Italian classical Rett girls, and screened the MECP2 coding region for mutations by denaturing high-performance liquid chromatography (DHPLC) and subsequent direct sequencing. DHPLC is a recently developed method for mutation screening which identifies heteroduplexes formed in DNA samples containing mismatches between wild type and mutant DNA strands, combining high sensitivity, reduced cost per run, and high throughput. In our series, 19 different de novo MECP2 mutations, eight of which were previously unreported, were found in 35 out of 50 Rett girls (70%). Seven recurrent mutations were characterized in a total of 22 unrelated cases. Initial DHPLC screening allowed the identification of 17 out of 19 different mutations (90%); after optimal conditions were established, this figure increased to 100%, with all recurrent MECP2 mutations generating a characteristic chromatographic profile. Detailed clinical data were available for 27 out of 35 mutation carrying Rett girls. Milder disease was detectable in patients carrying nonsense mutation as compared to patients carrying missense mutations, although this difference was not statistically significant (P = 0.077).

Segregation of a totally skewed pattern of X chromosome inactivation in four familial cases of Rett syndrome without MECP2 mutation: implications for the disease

BACKGROUND

Rett syndrome is a neurodevelopmental disorder affecting only girls; 99.5% of Rett syndrome cases are sporadic, although several familial cases have been reported. Mutations in the MECP2 gene were identified in approximately 70-80% of sporadic Rett syndrome cases.

METHODS

We have screened the MECP2 gene coding region for mutations in five familial cases of Rett syndrome and studied the patterns of X chromosome inactivation (XCI) in each girl.

RESULTS

We found a mutation in MECP2 in only one family. In the four families without mutation in MECP2, we found that (1) all mothers exhibit a totally skewed pattern of XCI; (2) six out of eight affected girls also have a totally skewed pattern of XCI; and (3) it is the paternally inherited X chromosome which is active in the patients with a skewed pattern of XCI. Given that the skewing of XCI is inherited in our families, we genotyped the whole X chromosome using 32 polymorphic markers and we show that a locus potentially responsible for the skewed XCI in these families could be located on the short arm of the X chromosome.

CONCLUSION

These data led us to propose a model for familial Rett syndrome transmission in which two traits are inherited, an X linked locus abnormally escaping X chromosome inactivation and the presence of a skewed XCI in carrier women.

Molecular genetics of Rett syndrome

Rett syndrome is a neurodevelopmental disorder affecting almost exclusively females. It affects approximately one in 15000 females and is characterized by a loss of purposeful hand use, autism, ataxia and seizure. The disorder is usually sporadic, but rare familial cases have also been reported. Recently it has been shown that familial cases are an X-linked dominant disorder and the disease locus maps to Xq28. A candidate gene called methyl-CpG-binding protein 2 was identified from the Xq28 region and was shown to contain mutations in about 77% of Rett syndrome patients. Since the encoded protein was previously shown to be a global transcriptional repressor, undesired expression of yet unidentified genes that are normally repressed is considered to be pathogenic in Rett syndrome.

Molecular genetics of Rett syndrome and clinical spectrum of MECP2 mutations

Rett syndrome, a neurodevelopmental disorder that is a leading cause of mental retardation in females, is caused by mutations in the X-linked gene encoding methyl-CpG-binding protein 2 (MeCP2). MECP2 mutations have subsequently been identified in patients with a variety of clinical syndromes ranging from mild learning disability in females to severe mental retardation, seizures, ataxia, and sometimes neonatal encephalopathy in males. In classic Rett syndrome, genotype-phenotype correlation studies suggest that X chromosome inactivation patterns have a more prominent effect on clinical severity than the type of mutation. When the full range of phenotypes associated with MECP2 mutations is considered, however, the mutation type strongly affects disease severity. MeCP2 is a transcriptional repressor that binds to methylated CpG dinucleotides throughout the genome, and mutations in Rett syndrome patients are thought to result in at least a partial loss of function. Abnormal gene expression may thus underlie the phenotype. Discovering which genes are misregulated in the absence of functional MeCP2 is crucial for understanding the pathogenesis of this disorder and related syndromes.

A Rett syndrome patient with a ring X chromosome: further evidence for skewing of X inactivation and heterogeneity in the aetiology of the disease

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder, characterised by regression of development in young females. Recently, mutations in the MECP2 gene were found to be present in 80% of sporadic cases, but in much lower frequency (< 30%) among familial cases. Several reports claim that the pattern of X chromosome inactivation (XCI) relates to the penetrance of RTT; in some cases skewed XCI is seen in Rett patients, and in others it is observed among normal carriers. We present here a case of RTT with a 46,X,r(X) in which complete skewed inactivation of the ring was demonstrated. Further, no mutations were found in the MECP2 gene present on the intact X. Our data, in conjunction with two previously published cases of X chromosome abnormalities in RTT, indicate that X chromosome rearrangements are sporadically associated with RTT in conjunction with extreme skewing of X inactivation. Based on our case and reported data, we discuss the evidence for a second X-linked locus for RTT associated with lower penetrance, and a different pattern of XCI, than for MECP2. This would result in a larger proportion of phenotypically normal carrier women transmitting the mutation for this putative second locus, and account for the minority of sporadic and majority of familial cases that are negative for MECP2 mutations.

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

To study the clinical overlap between Rett (RTT) and Angelman syndromes (AS), we screened the MECP2 gene in a cohort of 78 patients diagnosed as possible AS but who showed a normal methylation pattern at the UBE3A locus. MECP2 missense (R106W, G428S), nonsense (R255X, R270X), and frameshift mutations (803 delG) were identified in 6/78 patients including 4/6 female cases consistent with RTT, one female case with progressive encephalopathy of neonatal onset, and one isolated male case with non-fatal, non-progressive encephalopathy of neonatal onset. This study shows that MECP2 mutations can account for a broad spectrum of clinical presentations and raises the difficult issue of the screening of the MECP2 gene in severe encephalopathy in both males and females.

MECP2 mutations in Danish patients with Rett syndrome: high frequency of mutations but no consistent correlations with clinical severity or with the X chromosome inactivation pattern

Rett syndrome (RTT) is a neurodevelopmental disorder, which almost exclusively affects girls, who, after an initial period of apparently normal development, display gradual loss of speech and purposeful hand use, gait abnormalities and stereotypical hand movements. In the year 2000, mutations in the gene for the methyl CpG binding protein 2, MECP2, have been identified in 35-80% of the patients in three different studies. We have identified 15 different MECP2 mutations in 26 of 30 Danish RTT patients. The mutations included five novel mutations (one point mutation, three smaller deletions involving identical regions in the gene, and one duplication). In contrast to the point mutations and the duplication, which all affect the methyl binding domain or the transcriptional repressing domain, the three overlapping deletions are clustered in the 3' end of the gene. We found no consistent correlation between the type of mutation and the clinical presentation of the patient or the X-inactivation pattern in peripheral blood. Our high mutation detection rate, compared to two of the previous studies, underscores the importance of the inclusion criteria of the patients and supports that MECP2 is the most important, if not the only, gene responsible for RTT.

Rett syndrome: review of biological abnormalities

The Rett syndrome (RS) is a peculiar, sporadic, atrophic disorder, almost entirely confined to females. After the first six months of life there is developmental slowing with reduced communication and head growth for about one year. This is followed by a rapid destructive stage with severe dementia and loss of hand skills (with frequent hand wringing), apraxia and ataxia, autistic features and irregular breathing with hyperventilation. Seizures often supervene. Subsequently there is some stabilization in a pseudo-stationary stage during the preschool to school years, associated with more emotional contact but also abnormalities of the autonomic and skeletal systems. After the age of 15-20 years, a late motor deterioration occurs with dystonia and frequent spasticity but seizures become milder. RS has generally been considered an X-linked disorder in which affected females represent a new mutation, with male lethality. Linkage studies suggested a critical region at Xq28. In 1999, mutations in the gene MECP2 encoding X-linked methyl cytosine-binding protein 2 (MeCP2) were found in a proportion of Rett girls. This protein can bind methylated DNA. Analyses are leading to much further investigation of mutants and their effects on genes. Neuropathological and electrophysiological studies of RS are described. Description of neurometabolic factors includes reduced levels of dopamine, serotonin, noradrenaline and choline acetyltransferase (ChAT) in brain, also estimation of nerve growth factors, endorphin, substance P, glutamate and other amino acids and their receptor levels. The results of neuroimaging are surveyed, including volumetric magnetic resonance imaging (MRI) and positron emission tomography (PET).

Reduced nerve growth factor in Rett syndrome postmortem brain tissue

To determine whether reduced nerve growth factor (NGF) and/or its high affinity receptor, trkA, play a role in the pathophysiology of Rett syndrome (RS), we used immunohistochemistry in paraffin-embedded human autopsy brain tissue, to quantify NGF and trkA levels within the frontal cortex of 9 RS females and 10 female controls of similar age. The results showed a significant reduction of NGF expression in RS patients (p < 0.001). Specifically, all RS brains exhibited NGF levels at or below the minimum level observed in controls. In 3 RS brains there was no NGF detected. TrkA expression was also reduced in the RS group (p = 0.035). Interestingly, the expression of NGF in the RS group was significantly related to the presence of cortical astrogliosis (r = 0.91) as indicated by immunostaining for glial fibrillary acidic protein (GFAP). This suggests that while the signals for NGF production during injury remain intact, the critical developmental signals required for early NGF production are impaired.

Family data in Rett syndrome: association with other genetic disorders

BACKGROUND

Rett syndrome is a neurological disorder, almost exclusively affecting girls.

METHODOLOGY

Between 1993 and 1995 pedigree data were obtained from families of girls registered with the Australian Rett syndrome database.

RESULTS

Although 21 individual disorders were reported to be present in family members of affected girls, there was no apparent clustering of the same disorder in different families. However it was certain that a geneticist had been involved in only 10.9% of cases.

CONCLUSIONS

Mutations in the MECP2 gene have now been reported in a proportion of sporadic cases. Thus, it will be important to examine this phenotype-genotype correlation in the Australian cohort. Where a mutation is found, prenatal diagnosis in a subsequent pregnancy will be a possibility. Using the Australian population database and in conjunction with the clinical genetic services in each state it is planned to contact families with an affected girl to offer testing and counselling.

Mutation screening in Rett syndrome patients

Rett syndrome (RTT) was first described in 1966. Its biological and genetic foundations were not clear until recently when Amir et al reported that mutations in the MECP2 gene were detected in around 50% of RTT patients. In this study, we have screened the MECP2 gene for mutations in our RTT material, including nine familial cases (19 Rett girls) and 59 sporadic cases. A total of 27 sporadic RTT patients were found to have mutations in the MECP2 gene, but no mutations were identified in our RTT families. In order to address the possibility of further X chromosomal or autosomal genetic factors in RTT, we evaluated six candidate genes for RTT selected on clinical, pathological, and genetic grounds: UBE1 (human ubiquitin activating enzyme E1, located in chromosome Xp11.23), UBE2I (ubiquitin conjugating enzyme E2I, homologous to yeast UBC9, chromosome 16p13.3), GdX (ubiquitin-like protein, chromosome Xq28), SOX3 (SRY related HMG box gene 3, chromosome Xq26-q27), GABRA3 (gamma-aminobutyric acid type A receptor alpha3 subunit, chromosome Xq28), and CDR2 (cerebellar degeneration related autoantigen 2, chromosome 16p12-p13.1). No mutations were detected in the coding regions of these six genes in 10 affected subjects and, therefore, alterations in the amino acid sequences of the encoded proteins can be excluded as having a causative role in RTT. Furthermore, gene expression of MECP2, GdX, GABRA3, and L1CAM (L1 cell adhesion molecule) was also investigated by in situ hybridisation. No gross differences were observed in neurones of several brain regions between normal controls and Rett patients.

Molecular approaches to the Rett syndrome gene

Rett syndrome is a neurodevelopmental disorder affecting 1 in 10,000 to 15,000 females worldwide. Apparently normal at birth, girls with Rett syndrome undergo developmental regression and acquire a neurologic and behavioral profile that has been used to define diagnostic criteria for the disorder. Neurochemical and anatomic alterations indicate that Rett syndrome appears to result from an arrest of normal neuronal maturation. Although Rett syndrome generally occurs sporadically, rare familial recurrences indicate a genetic basis for the disorder. Data from familial recurrences are consistent with an X-linked dominant locus causing the classic phenotype in female patients and a distinct, more severe phenotype in hemizygous male patients. Exclusion mapping data from rare kindreds with recurrent Rett syndrome localize the gene to the distal long arm of the X chromosome (Xq27.3-Xqter).

Metacarpophalangeal pattern profile and bone age in Rett syndrome: further radiological clues to the diagnosis

Hand radiographs of 100 girls representing 73% of the known Australian population of girls with Rett syndrome, age 20 years or less, were available for this study. Control radiographs were matched for age, sex, and laterality. Bone age was assessed against standard radiographs in Greulich and Pyle [1959: Radiographic Atlas of Skeletal Development of the Hands and Wrist, 2nd ed.]. A metacarpophalangeal pattern (MCPP) profile comparing the relative lengths of the hand bones with mean population norms by age was produced by converting the length of each of the 19 metacarpal and phalangeal bones into a Z score. In girls less than 15 years old, bone age was more advanced in Rett syndrome than in age-matched control girls (left hand P = 0.03, right hand 0.004), but was most advanced in the younger group and normalized with age. In Rett syndrome, the mean Z score for the 19 metacarpal and phalangeal bones was 1.0 in children under 5 years, -0.27 in those aged 5-11 years, and -1.7 in those aged 12 years and over. This variation between age groups was much greater than in the controls. The dips in the MCPP profile occurred at MC2 and D1, and the peaks at M5, P5, and M4. An MCPP profile may provide an additional aid to diagnosis in cases of Rett syndrome where all the criteria are not met, but in children under age 5 years, advanced bone age may be more helpful as a marker.

Linkage analysis in Rett syndrome families suggests that there may be a critical region at Xq28

A whole X chromosome study of families in which Rett syndrome had been diagnosed in more than one member indicated that the region between Xq27 and Xqter was the most likely region to harbour a gene which may be involved in the aetiology of the disease. Further, more detailed studies of Xq28 detected weak linkage and a higher than expected sharing of maternally inherited alleles. It is suggested that there may be more than one gene involved in the aetiology of this syndrome, particularly as the very rare families in which more than one girl is affected often show variable clinical symptoms.