A de novo X; 3 translocation in Rett syndrome

Rett syndrome is a neurodegenerative disorder that occurs exclusively in females. The syndrome is sporadic in most cases with the exception of a few familial cases with an inheritance pattern through maternal lines. These observations raised the possibility that Rett syndrome may be due to an X-linked dominant mutation which is lethal in the male. To evaluate this hypothesis, we have systematically performed high-resolution chromosome analysis on 28 patients with Rett syndrome searching for deletions and/or translocations. In one patient, a de novo balanced translocation was observed with the chromosome constitution of 46, X, t (X;3) (p22.11;q13.31). This finding supports the hypothesis of an X-linked dominant mutation and suggests that the Rett gene might map to distal Xp21 or proximal Xp22.     

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.     

Father-to-daughter transmission of focal dermal hypoplasia associated with nonrandom X-inactivation: support for X-linked inheritance and paternal X chromosome mosaicism

Focal dermal hypoplasia (FDH) is a rare syndrome of severe developmental anomalies of the tissues and organs derived from ectoderm and mesoderm. Though data have suggested that FDH is an X-linked dominant trait associated with male hemizygote lethality, a hypothesis supported by the observation of three unrelated infants with FDH manifestations and de novo chromosome rearrangements involving Xp22, observations of father-to-daughter transmission have suggested possible genetic heterogeneity and autosomal dominant inheritance with sex limitation. We hypothesize that, if FDH is an X-linked disorder, cells expressing an active disease locus might experience a selective disadvantage resulting in a nonrandom pattern of X-inactivation in patient tissue. To test this hypothesis, we studied one of the two previously described families demonstrating father-to-daughter inheritance of FDH. To determine if the affected daughter had a skewed pattern of X-inactivation consistent with X-linked inheritance of FDH, somatic cell hybrids were constructed by fusing hypoxanthine phosphoribosyl transferase (HPRT)-deficient rodent fibroblasts with either patient dermal fibroblasts or peripheral white blood cells (WBCs); hybrid clones retaining an active X chromosome were analyzed to determine the parental origin of the active X chromosome. Analyses of resulting hybrid clones showed that while hybrids constructed from skin fibroblasts contained an active X chromosome inherited from either of the patient's parents, hybrids constructed from WBCs showed a skewed pattern of X-inactivation; 11 of 11 hybrids contained an active maternal X chromosome (chi 2 = 12.2, P = .001).(ABSTRACT TRUNCATED AT 250 WORDS)     

Muscular dystrophy in an X; 1 translocation female suggests that Duchenne locus is on X chromosome short arm.

A unique combination of a Duchenne-like muscular dystrophy in a girl with a translocation-inversion rearrangement involving an X chromosome and a no 1 chromosome appeared as a result of both gene mutation and chromosome mutation in the mother. The X-autosome rearrangement would permit full expression of an X-linked recessive gene, such as that for Duchenne muscular dystrophy, in a female, and this would satisfactorily explain the characteristic Duchenne-like course of our patient's illness. The simultaneous de novo appearance of the Duchenne mutation and the X;1 rearrange suggests possible sites for the Duchenne locus on the X chromosome short arm (at Xp1106 or Xp2107).     

Microphthalmia and chorioretinal lesions in a girl with an Xp22.2-pter deletion and partial 3p trisomy: clinical observations relevant to Aicardi syndrome gene localization

We present a 4-year-old girl with a maternally derived, unbalanced X;3 translocation resulting in partial Xp monosomy and partial 3p trisomy. She had chorioretinal defects, developmental delay, infantile seizures, and microphthalmia. These findings initially suggested a diagnosis of Aicardi syndrome. However, she had a normal-appearing corpus callosum on CT and magnetic resonance imaging scans of the brain and her retinal findings were not typical for Aicardi syndrome. This represents the 6th reported example of microphthalmia associated with an Xp22 chromosome abnormality. Four of these individuals also had features suggestive of focal dermal hypoplasia (FDH), which was not evident in our patient. The available evidence supports the hypothesis that gene disruption at Xp22 may lead to findings similar to those seen in Aicardi syndrome and FDH, both of which are believed to be X-linked dominant male lethal conditions.     

Increased skewing of X chromosome inactivation in Rett syndrome patients and their mothers

Rett syndrome is a largely sporadic, X-linked neurological disorder with a characteristic phenotype, but which exhibits substantial phenotypic variability. This variability has been partly attributed to an effect of X chromosome inactivation (XCI). There have been conflicting reports regarding incidence of skewed X inactivation in Rett syndrome. In rare familial cases of Rett syndrome, favourably skewed X inactivation has been found in phenotypically normal carrier mothers. We have investigated the X inactivation pattern in DNA from blood and buccal cells of sporadic Rett patients (n=96) and their mothers (n=84). The mean degree of skewing in blood was higher in patients (70.7%) than controls (64.9%). Unexpectedly, the mothers of these patients also had a higher mean degree of skewing in blood (70.8%) than controls. In accordance with these findings, the frequency of skewed (XCI > or =80%) X inactivation in blood was also higher in both patients (25%) and mothers (30%) than in controls (11%). To test whether the Rett patients with skewed X inactivation were daughters of skewed mothers, 49 mother-daughter pairs were analysed. Of 14 patients with skewed X inactivation, only three had a mother with skewed X inactivation. Among patients, mildly affected cases were shown to be more skewed than more severely affected cases, and there was a trend towards preferential inactivation of the paternally inherited X chromosome in skewed cases. These findings, particularly the greater degree of X inactivation skewing in Rett syndrome patients, are of potential significance in the analysis of genotype-phenotype correlations in Rett syndrome.     

Mother and daughter with a terminal Xp deletion: implication of chromosomal mosaicism and X-inactivation in the high clinical variability of the microphthalmia with linear skin defects (MLS) syndrome

The microphthalmia with linear skin defects (MLS or MIDAS) syndrome is a rare X-linked dominant inherited disorder with male lethality, associated with segmental aneuploidy of the Xp22.2 region in most of the cases. However, we recently described heterozygous sequence alterations in a single gene, HCCS, in females with MLS. Beside the classical MLS phenotype, occasional features such as sclerocornea, agenesis of the corpus callosum, and congenital heart defects can occur. Although the majority of cases are sporadic, mother-to-daughter transmission has been observed and a high intra- and interfamilial phenotypic variability exists. We describe an asymptomatic mother and her daughter presenting with the typical features of MLS syndrome. By cytogenetic analysis both females were found to have a terminal Xp deletion with the breakpoint in Xp22.2, mapping near to or within the MSL3L1 gene which is located centromeric to HCCS. FISH analysis revealed that the mother is a mosaic with 45,X[11]/46,X,del(X)(p22.2)[89], while in all cells of the MLS-affected daughter a hybridization pattern consistent with a 46,X,del(X)(p22.2) karyotype was detected. By haplotype analysis we identified the paternal X chromosome of the mother to carry the terminal Xp deletion. X-inactivation studies showed a completely skewed pattern in mother and daughter with the deleted X chromosome to be preferentially inactivated in their peripheral blood cells. We suggest that both chromosomal mosaicism as well as functional X chromosome mosaicism could contribute to the lack of any typical MLS feature in individuals with a heterozygous MLS-associated mutation. The 45,X cell population, that most likely is also present in other tissues of the mother, might have protected her from developing MLS. Nonetheless, a non-random X-inactivation pattern in favor of activity of the wild-type X chromosome in the early blastocyte could also account for the apparent lack of any disease sign in this female.     

Disruption of genes in the retinoid cascade may explain the microscopic neuroblastoma in a fetus with de novo unbalanced translocation (3;10)(q21;q26)

BBB syndrome and G syndrome were originally reported as distinct X-linked disorders. Clinical studies indicated that BBB and G syndromes were likely to represent variant expression of the same disorder, now referred to as “Optiz” GBBB syndrome. Several occurrences of male-to-male transmission in both syndromes led to the hypothesis that GBBB syndrome was a single autosomal dominant, sec influenced disorder, now tentatively mapped to 5p12-13. We report on a large pedigree in which GBBB syndrome appears to cosegregate with a pericentric inversion of the X chromosome inv(X)(p22.3q26). It indicates the possible existence of a true X-linked form of GBBB syndrome, which does not appear phenotypically different from its autosomal counterpart. The gene could map in the vicinity of the breakpoints, in Xp or Xq. The existence of two genes affecting a common pathogenetic pathway could explain the gender-dependent expressivity of GBBB phenotype. © 1995 Wiley-Liss, Inc.     

Skewed X chromosome inactivation failed to explain the normal phenotype of a carrier female with MECP2 mutation resulting in Rett syndrome

Mutations in the X-linked MECP2 gene cause Rett syndrome, a neurodevelopmental disorder that exclusively affects girls. Females with the MECP2 mutations exhibit a broad spectrum of clinical presentations ranging from classical Rett syndrome to asymptomatic carriers, which can be explained by differences in X chromosome inactivation (XCI). Here, we report a family with a girl with Rett syndrome in whom a novel missense mutation in the MECP2 gene was transmitted through the maternal germ line. The carrier mother was asymptomatic and presented non-random XCI in the peripheral blood cells, which resulted in the X chromosome harboring the mutant allele that was predominantly active. Thus, the presence of non-random XCI in the peripheral blood cells did not provide an explanation for the normal phenotype of the carrier mother. This result suggests that mechanisms other than XCI may contribute to the phenotypic heterogeneity associated with MECP2 mutations.     

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.


Keywords: Rett syndrome; skewed X chromosome inactivation; X chromosome; MECP2     

p.R270X MECP2 mutation and mortality in Rett syndrome

Among cases in the Australian Rett Syndrome Database, the nonsense mutation p.R270X is one of the most commonly occurring single pathogenic MECP2 mutations. In two recent published reports of the MECP2 mutational spectrum the p.R270X appeared to be under represented. We hypothesised that increased mortality arising from this mutation may underlie this apparent discrepancy. We investigated our hypothesis in two independent study groups from Australia and the UK with prospective data collections (total n=524). Only females with Rett syndrome and an identified MECP2 mutation were included. Significant differences in survival were detected among Rett syndrome cases grouped for the eight most frequent mutations (log-rank chi(2) (7)=15.71, P=0.03). Moreover, survival among cases with p.R270X, when compared with survival among cases with all the other mutations was reduced (log-rank chi(2) (2)=6.94, P=0.01). Our observation of a reduced survival associated with the p.R270X mutation offers an explanation for the under representation of p.R270X in older subjects with Rett syndrome.     

Isolation of a yeast artificial chromosome contig spanning the X chromosomal translocation breakpoint in a patient with Rett syndrome

Rett syndrome is a neurodevelopmental disorder observed exclusively in females. A de novo X;3 translocation was detected in a patient (TH) with Rett syndrome. The X chromosomal breakpoint maps to Xp21.3 between the distal end of the Duchenne muscular dystrophy (DMD) gene and the DXS28 (C7) locus. To determine if this translocation caused the Rett syndrome in this patient, our efforts focused on mapping and cloning of the X chromosomal breakpoint in this patient. Toward these goals, we generated a set of radiation-reduced hybrid cell lines for the short arm of the X chromosome to use as a source for region-specific markers. Using Alu-PCR, 13 new DNA markers were isolated from a radiation-reduced hybrid, which retained both DMD and DXS28. These markers were localized within Xp21 using DNA from males with various interstitial deletions in this region. Two new markers, K23-2p and K23b-1, were found to be closer flanking markers to the X chromosomal breakpoint than DMD and DXS28. Long range restriction mapping using K23-2p and K23b-1 determined that the maximum distance between them was 800 kb. Several of the new markers were developed into sequence tagged-sites and were used to isolate yeast artificial chromosome (YAC) clones. A total of 22 YAC clones was isolated and characterized; these YACs were then developed into 3 large contigs in the Xp21.3 region. This effort resulted in the cloning of the region containing the X chromosomal translocation breakpoint of the Rett syndrome patient in a 170-kb YAC clone. © 1993 Wiley-Liss, Inc.     

An explanation for another familial case of Rett syndrome: maternal germline mosaicism

Rett syndrome (RTT; OMIM#312750) is a severe neurodevelopmental disorder that affects mainly girls. It has an estimated incidence of 1:10,000-15,000 females. Mutations in the X-linked gene methyl CpG-binding protein 2 (MECP2) have been found in most patients. The most accepted explanation for the sex bias is that the Rett mutation in sporadic cases has its origin in the paternal germline X chromosome and can thus only be transmitted to females. The majority of cases are sporadic (99.5%) but some familial cases have been described. These cases can either be explained by germline mosaicism or by asymptomatic carrier mothers with skewing of X-inactivation towards the wild-type MECP2 allele. We describe one of the few familial cases of RTT in which a maternal germline mosaicism is the most likely explanation. The mutation p.Arg270fs (c.808delC) was identified in both a girl with classical RTT and her brother who had the severe neurological phenotype usually described in males. The mutation was absent in DNA extracted from blood of both parents. These type of events must be taken into consideration in the genetic counselling of families after the diagnosis of a first case of RTT in a female or a MECP2 mutation in a male.     

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.     

The oral-facial-digital syndrome type 1 (OFD1), a cause of polycystic kidney disease and associated malformations, maps to Xp22.2-Xp22.3

Key features of the oral-facial-digital syndrome type 1 (OFD1) include malformations of the face, oral cavity and digits. In addition, the clinical phenotype often includes mental retardation and renal functional impairment. Approximately 75% of cases of OFD1 are sporadic, and the condition occurs almost exclusively in females. In familial cases, the most likely mode of inheritance is considered to be X-linked dominant with prenatal lethality in affected males. Therefore, the OFD1 gene product appears to have widespread importance in organogenesis and is essential for fetal survival. We have studied two kindreds in which the clinical course was dominated by polycystic kidney disease requiring dialysis and transplantation. Using polymorphic chromosome markers spaced at approximately 10 cM intervals along the X chromosome, we mapped the disease to a region on the short arm of the X chromosome (Xp22.2-Xp22.3) spanning 19.8 cM and flanked by crossovers with the markers DXS996 and DX7S105. There was a maximum lod score of 3.32 in an 'affecteds only' analysis using a marker within the KAL gene (theta = 0.0 ), thereby confirming the location of the gene for OFD1 on the X chromosome. The remainder of the X chromosome was excluded by recombinants in affected individuals. The importance of our findings includes the definitive assignment of this male-lethal disease to the X chromosome and the mapping of a further locus for a human polycystic kidney disease. Furthermore, this mapping study suggests a possible mouse model for OFD1 as the X-linked dominant Xpl mutant, in which polydactyly and renal cystic disease occurs, maps to the homologous region of the mouse X chromosome.      

Another observation of microphthalmia in an XX male: microphthalmia with linear skin defects syndrome without linear skin lesions

A case of microphthalmia with Xp microdeletion is reported. The patient was a boy who showed bilateral microphthalmia with corneal opacities, hypospadias without evidence of hypogonadism, and a conduction disturbance of the heart (Wenckebach conduction). No skin lesion was discerned. High-resolution chromosome analysis revealed the karyotype of 46,X,del(X)(p22). The phenotype was considered to be microphthalmia with linear skin defects (MLS) syndrome without skin lesions. Polymerase chain reaction and fluorescence in-situ hybridization analyses revealed that the chromosome aberration resulted from an X;Y translocation: the presence of pseudoautosomal boundary Y and the sex-determining region of Y was confirmed, while Xp deletion involving the region distal to DXS1129 was ascertained. Thus the chromosome designation using the ISCN 1995 nomenclature is 46,X,der(X),t(X;Y)(p22.13;q11.2). Despite the absence of skin lesions, the Xp deletion of our patient corresponded to those of previously reported typical cases of MLS syndrome. Our observation further supports the current hypothesis that the phenotypic variation of MLS syndrome represents tissue-different X inactivation rather than different genetic effects of two contiguous genes. Received: August 3, 1998 / Accepted: August 31, 1998     

Variation at the fragile X locus does not influence susceptibility to bipolar disorder

A infant girl had red stellate skin lesions on the cheeks and neck, and mildly short palpebral fissures. Her skin abnormality was typical of microphthalmia with linear skin defects (MLS), a newly recognized syndrome consisting of congenital linear skin defects and ocular abnormalities in females monosomic for Xp22. She died suddenly and unexpectedly at age 4 months; the cause of death was ascribed to oncocytic cardiomyopathy. Oncocytic cardiomyopathy occurs only in young children, who present with refractory arrhythmias leading to cardiac arrest. The coexistence of two rare conditions, one of which is mapped to the X chromosome, and an excess of affected females with oncocytic cardiomyopathy, make it likely that oncocytic cardiomyopathy is also X-linked, with Xp22 being a candidate region. Overlapping manifestations in the two conditions (ocular abnormalities in cases of oncocytic cardiomyopathy and arrhythmias in MLS) offer additional support for this hypothesis. © 1994 Wiley-Liss, Inc.     

Parental origin of de novo MECP2 mutations in Rett syndrome

Rett syndrome (RTT) is a neurodevelopmental disorder occurring almost exclusively in females as sporadic cases. Recently, DNA mutations in the MECP2 gene have been detected in approximately 70% of patients with RTT. To explain the sex-limited expression of RTT, it has been suggested that de novo X-linked mutations occur exclusively in male germ cells resulting therefore only in affected daughters. To test this hypothesis, we have analysed 19 families with RTT syndrome due to MECP2 molecular defects. In seven informative families we have found by DHPLC a nucleotide variant which could be used to differentiate between the maternal and the paternal allele. In each subject investigated from these families, we have amplified specifically each allele and sequenced allele-specific PCR products to identify the allele bearing the mutation as well as the parental origin of each X chromosome. This approach allowed us to determine the parental origin of de novo mutations in all informative families. In five cases, the de novo MECP2 mutations have a paternal origin and in the two other cases a maternal origin. In all transitions at CpG, the de novo mutation observed was of paternal origin. The high frequency of male germ-line transmission of the mutation (71% of RTT informative cases) is consistent with a predominant occurrence of the disease in females.     

MECP2 mutations account for most cases of typical forms of Rett syndrome

Rett syndrome (RTT) is a severe progressive neurological disorder that affects almost exclusively females, with an estimated prevalence of approximately one in 10 000-15 000 female births. Most cases are sporadic, but several reports about familial recurrence support X-linked dominant inheritance with male lethality. The gene responsible for this disorder, MECP2, was recently identified by candidate gene strategy. Mutations were detected in <25% of RTT cases in this first report. To characterize the spectrum of mutations in the MECP2 gene in RTT patients, we selected 46 typical RTT patients and performed mutation screening by denaturing gradient gel electrophoresis combined with direct sequencing. We identified 30 mutations, accounting for 65% of RTT patients. They include 12 novel mutations (11 located in exon 3 and one in exon 2). Mutations, such as R270X and frameshift deletions in a (CCACC) (n) rich region, have been found with multiple recurrences. Most of the mutations were de novo, except in one family where the non-affected transmitter mother exhibited a bias of X inactivation. Although this study showed that MECP2 mutations account for most cases of typical forms of RTT (65%) and mutations in non-coding regions cannot be excluded for the remaining cases, an alternative hypothesis that takes into account the homogeneous phenotype and exclusive involvement of females, could be the implication in RTT of a putative second X-linked gene.