Neither uniparental disomy nor skewed X-inactivation explains Rett syndrome

Rett syndrome: clinical review and genetic update

Rett syndrome (RS) is a severe neurodevelopmental disorder that contributes significantly to severe intellectual disability in females worldwide. It is caused by mutations in MECP2 in the majority of cases, but a proportion of atypical cases may result from mutations in CDKL5, particularly the early onset seizure variant. The relationship between MECP2 and CDKL5, and whether they cause RS through the same or different mechanisms is unknown, but is worthy of investigation. Mutations in MECP2 appear to give a growth disadvantage to both neuronal and lymphoblast cells, often resulting in skewing of X inactivation that may contribute to the large degree of phenotypic variation. MeCP2 was originally thought to be a global transcriptional repressor, but recent evidence suggests that it may have a role in regulating neuronal activity dependent expression of specific genes such as Hairy2a in Xenopus and Bdnf in mouse and rat.

Genetic basis of Rett syndrome

The origin of Rett syndrome has long been debated, but several observations have suggested an X-linked dominant inheritance pattern. We and others have pursued an exclusion-mapping strategy using DNA from a small number of familial Rett syndrome cases. This work resulted in the narrowing of the region likely to harbor the mutated gene to Xq27.3-Xqter. After systematic exclusion of several candidate genes, we discovered mutations in MECP2, the gene that encodes the transcriptional repressor, methyl-CpG-binding protein 2. Since then, nonsense, missense, or frameshift mutations have been found in at least 80% of girls affected with classic Rett syndrome. Sixty-four percent of mutations are recurrent C > T transitions at eight CpG dinucleotides mutation hotspots, while the C-terminal region of the gene is prone to recurrent multinucleotide deletions (11%). Most mutations are predicted to result in total or partial loss of function of MeCP2. There is no clear correlation between the type and position of the mutation and the phenotypic features of classic and variant Rett syndrome patients, and XCI appears to be a major determinant of phenotypic severity. Further research focuses on the pathogenic consequences of these mutations along the hypothesis of loss of transcriptional repression of a small number of genes that are essential for neuronal function in the maturing brain.

Review and meta-analysis of systematic searches for uniparental disomy (UPD) other than UPD 15

All systematic searches for uniparental disomy (UPD) so far published and comprising clinically defined populations (Silver-Russell syndrome/primordial growth retardation (SRS/PGR) (n = 14), multiple malformations (n = 2), or rare syndromes (n = 12)) or situations at risk (confined placental mosaicism (CPM) (n = 13), spontaneous abortions (n = 6), additional marker chromosomes (n = 15), balanced non-Robertsonian translocations (n = 3), or balanced Robertsonian translocations (n = 15)) were reviewed. In many studies clinical and/or cytogenetic information on fluorescent in situ hybridization (FISH) results was very scarce. Meta-analysis concerning an adequate number of cases was possible for SRS/PGR, CPM, additional marker chromosomes, and balanced Robertsonian translocations only. As expected, the highest risk for UPD was found in cases with translocations between homologous acrocentric chromosomes (11 cases with UPD of 15 investigated) and in CPM due to a meiotic error (25 of 51 cases). In prenatal investigations or in cases with a normal phenotype, translocations between nonhomologous acrocentric chromosomes implied a risk for UPD of less than 0.5%. The risks for maternal UPD 7 in cases with SRS/PGR, for UPD 15 in cases with an additional inv dup(15) marker chromosome, and for UPD of any chromosome in cases with multiple malformation/mental retardation were approximately 5.5%, and approximately 1.3%, respectively. Searches for UPD in well-defined syndromes (Brachmann-De Lange syndrome, Sotos syndrome, Rett syndrome, Weaver syndrome, or XX true hermaphroditism) were disappointing. Not a single case was found.

Molecular-cytogenetic investigation of skewed chromosome X inactivation in Rett syndrome

We have developed an approach to differentiate homologous X chromosomes in metaphase chromosomes and interphase nuclei by a fluorescence in situ hybridization (FISH) technique with chromosome X-specific alpha-satellite DNA probe. FISH analysis of metaphase chromosomes in a cohort of 33 girls with Rett syndrome (RTT) allowed us to detect eight girls with structurally different X chromosomes, one X chromosome with a large and another one with a small centromeric heterochromatin (so-called chromosomal heteromorphism). Step-wise application of differential replication staining and the FISH technique to identify the inactivation status of paternal and maternal chromosome X in RTT girls was applied. Skewed X inactivation in seven RTT girls with preferential inactivation of one X chromosome over the other X chromosome was detected in 62-93% of cells. Therefore, non-random or skewed X inactivation with variable penetrance in blood cells could take place in RTT.

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 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).

A new Rett syndrome family consistent with X-linked inheritance expands the X chromosome exclusion map

Although familial recurrences of Rett syndrome (RTT) comprise only approximately 1% of the reported cases, it is these cases that hold the key for the understanding of the genetic basis of the disorder. Families in which RTT occurs in mother and daughter, aunt and niece, and half sisters are consistent with dominant inheritance and variable expressivity of the phenotype. Recurrence of RTT in sisters is likely due to germ-line mosaicism in one of the parents, rather than to recessive inheritance. The exclusive occurrence of classic RTT in females led to the hypothesis that it is X-linked and may be lethal in males. In an X-linked dominant disorder, unaffected obligate-carrier females would be expected to show nonrandom or skewed inactivation of the X chromosome bearing the mutant allele. We investigated the X chromosome inactivation (XCI) patterns in the female members of a newly identified family with recurrence of RTT in a maternal aunt and a niece. Skewing of XCI is present in the obligate carrier in this family, supporting the hypothesis that RTT is an X-linked disorder. However, evaluation of the XCI pattern in the mother of affected half sisters shows random XCI, suggesting germ-line mosaicism as the cause of repeated transmission in this family. To determine which regions of the X chromosome were inherited concordantly/discordantly by the probands, we genotyped the individuals in the aunt-niece family and two previously reported pairs of half sisters. These combined exclusion-mapping data allow us to exclude the RTT locus from the interval between DXS1053 in Xp22.2 and DXS1222 in Xq22.3. This represents an extension of the previous exclusion map.

Rett syndrome

Rett syndrome (RS) is a neurological disorder that mainly, and possibly exclusively, affects girls. After its description in 1966 by Andreas Rett in the Wiener Klinische Wochenschrift, awareness and interest in RS were enhanced by the 1983 report of Hagberg et al. in the Annals of Neurology. Diagnosis, and indeed the hypothesis that it exists, continue to be based upon a consistent constellation of clinical features observed in thousands of female patients world-wide. A diagnostic marker has not been identified. Notwithstanding this serious limitation, it is generally agreed that RS is a distinct entity and that it is genetically determined. Although it is associated with devastating loss of function between infancy and the fifth year of life, its course becomes relatively static thereafter, setting it apart from most of the genetic neurodegenerative disorders of childhood. Neuropathological and neurochemical studies call attention to RS as a neurodevelopmental disorder. Clarification of its pathogenesis may provide new insight into normal brain development. This report summarizes existing information and concepts about RS, and presents recent advances.

Earlier finishing of Xp21.2 subband replication of the inactive X chromosome in Rett syndrome girl but not in her 47,XXX mother

X-inactivation mosaicism has been proposed to explain the origin of Rett syndrome. We present the results of the cytogenetic analysis, including RBG dynamic replication pattern, in a girl with Rett syndrome. The late replicating X chromosome (LRX) showed the earlier replication of subband Xp21.2 in 36% of analysed cells. Unexpectedly the maternal karyotype 47,XXX was found. Replication timing of both maternal LRX chromosomes was normal. The critical region of Xp essential for RS is proposed.

Rett syndrome

Images

A comparative study of X-inactivation in Rett syndrome probands and control subjects

X-inactivation has been studied in a series of monozygotic female twins and their female relatives by a PCR method which detects methylation at the androgen receptor locus (HUMARA). The results obtained are compared to those from an earlier study employing probe M27 beta which detects locus DXS255. Analysis of X-inactivation in girls with Rett syndrome and their mothers by four different methods did not indicate a direct relationship between non-random inactivation of the X-chromosome and the presence of the disease with the exception that any skewing detected in the probands tended to favour the preferential inactivation of the paternally inherited X-chromosome. No evidence for the involvement of uniparental disomy in the etiology of the disease was found.

X chromosome inactivation in 30 girls with Rett syndrome: analysis using the probe

Rett syndrome (RS) is a neurologic disorder with an exclusive incidence in females. A nonrandom X-inactivation could provide insight into the understanding of this disease. We performed molecular analysis based on the differential methylation of the active and inactive X with probe M27 beta, taking into account the parental origin of the two Xs, in 30 control girls, 8 sisters, and 30 RS girls. In 27 control an 31 RS mothers, the inactivation status of the X transmitted to their daughters was also analyzed. The results showed a significantly increased frequency of partial paternal X inactivation (> 65%) in lymphocytes from 16/30 RS compared with 4/30 controls (P = 0.001). These results do not support the hypothesis of a monogenic X-linked mutation but should be taken into account when researching the etiology of this disease.