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.     

Skewed X chromosome inactivation in carriers is not a constant finding in FG syndrome

Genetic heterogeneity has been demonstrated in FG syndrome. We report a systematic study of the X-inactivation profile of obligate carriers and other females in FG pedigrees. It was expected that the characterization of particular X-inactivation profiles in carriers in some families might be related to the same mutated gene. Analysis of the X-inactivation profiles in carriers demonstrated different profiles but no correlation was found with the results of the linkage study.     

Skewed X-chromosome inactivation is associated with primary but not secondary ovarian failure

Premature ovarian failure (POF) is the occurrence of menopause before the age of 40, and may present with either primary or secondary amenorrhea. Numerous cases of POF in women with X-chromosome deletions or translocations have been reported; thus, it is possible that smaller rearrangements undetectable by conventional cytogenetics may contribute to POF in some patients. In females with an abnormal X chromosome, cells with inactivation of the normal X may be selected against, causing skewed X-chromosome inactivation (XCI). We therefore assessed XCI by methylation sensitive restriction digestion and PCR amplification at the androgen receptor (AR) locus, in 4 primary and 55 secondary POF patients and 109 control women. In samples heterozygous at AR and therefore informative for the skewing assay, the frequency of skewed XCI among the women with secondary amenorrhea was identical to that in control women, with 4 out of 48 (8.3%) secondary ovarian failure patients and 8 out of 97 (8.2%) control women having > or =90% skewing. Notably, all three primary amenorrhea patients that were informative at AR had skewed XCI > or =90% (P = 0.001 vs. control women; Fisher's exact test). To investigate whether X-chromosome copy number alterations were responsible, DNA from selected patients with skewed XCI was examined by high resolution DNA microarray, however no potential regions of DNA addition or deletion were confirmed by FISH or PCR. X-chromosome abnormalities undetectable by array, or reduced follicular pool due to an early trisomic rescue event, may explain the skewed XCI observed in POF patients presenting with primary amenorrhea.     

Skewed X chromosome inactivation and early-onset breast cancer

Background

Skewed X chromosome inactivation may be more common in women with epithelial ovarian cancer and early‐onset breast cancer. We tested this hypothesis in a group of 235 breast cancer patients and 253 controls (mean age 45.8 years) from a larger population based case control study.

Methods

We measured X chromosome inactivation with the AR gene assay in lymphocyte DNA digested with the methylation specific enzyme HpaII. We judged skewness using an adjusted measure (relative to the undigested sample) with a cut point of 75%, and an unadjusted measure where skewed was defined as >90% of the signal from one allele in the HpaII digested sample.

Results

There were no significant differences in any of the skewing measures between cases and controls. Using the adjusted skewing measure among pre‐menopausal subjects under the age of 50, 14% of cases versus 11% of controls were skewed, OR = 1.2, 95% CI 0.6 to 2.3; using the unadjusted measure, OR = 0.9, 95% CI 0.4 to 2.0.

Conclusions

While we cannot rule out a subtle difference of approximately twofold or less, we have failed to find a significant difference in the prevalence of skewed X chromosome inactivation in younger women with breast cancer compared to controls.     

Molecular cytogenetic analysis of a duplication Xp in a female with an abnormal phenotype and random X inactivation

We describe a female infant with severe abnormal phenotype with a de novo partial duplication of the short arm of the X chromosome. Chromosome painting confirmed the origin of this X duplication. Molecular cytogenetic analysis with fluorescence in situ hybridization (FISH) was performed with YAC probes, further delineating the breakpoints. The karyotype was 46, X dup(X)(p11-p21.2).Cytogenetic replication studies showed that the normal and duplicated X chromosomes were randomly inactivated in lymphocytes. In most females with structurally abnormal X chromosomes, the abnormal chromosome is inactivated and they are phenotypically apparently normal relatives of phenotypically abnormal males having dupX. Therefore, in this case, there is functional disomy of Xp11-p21.2 in the cells with an active dup(X), most likely resulting in abnormal clinical findings in the patient.     

Skewed X inactivation in manifesting carriers of Duchenne muscular dystrophy

We studied X inactivation patterns in manifesting carriers of familial and sporadic Duchenne muscular dystrophy (DMD) or unaffected carriers of DMD by analysis of the methylation of Hpall sites in the first exon of the human androgen-receptor gene (HUMARA) from peripheral blood samples. Three of the four manifesting carriers, four of the five asymptomatic carriers, and 31 of the 32 female controls were heterozygous for the CAG repeat of HUMARA. All manifesting carriers showed skewed X inactivation, while all unaffected carriers showed almost symmetrical inactivation. One family studied over three generations is noteworthy because it includes two mother/daughter pairs, one an affected pair with skewed X inactivation, and the other a phenotypically normal carrier pair with random X inactivation. On the other hand, the extent of X inactivation for each X chromosome in 31 female controls was widely distributed. These data suggest that in carriers of DMD, both affected and unaffected, it is valuable to analyze the pattern of skewed X inactivation because it provides important prognostic information. Carriers of DMD with skewed X inactivation might show slowly progressive myopathy with advancing age.     

Novel mutation of the DAX1 gene in a patient with X-linked adrenal hypoplasia congenita and hypogonadotropic hypogonadism

X-linked adrenal hypoplasia congenita (AHC) is characterized by primary adrenal insufficiency and is frequently associated with hypogonadotropic hypogonadism (HHG). Mutations of the DAX1 gene have been reported in patients with AHC and HHG. We found a novel DAX1 mutation in our patient. Sequence analysis of the patient's DAX1 demonstrated a 1-bp (G) deletion at codon 49 in exon 1. The mutation shifts the reading frame, resulting in completely different amino acid sequences from codon 49 to the premature stop codon at 84. The G was present at this position in the sequences of the father and 2 younger brothers. Direct sequence and single-strand conformation polymorphism analyses of polymerase chain reaction fragments revealed that the mutation at codon 49 was heterozygously present in the mother's DAX1 gene. The codon 84 is located in the first half of the DNA binding domain, and the mutation site is closer to the N-terminus than those in previously reported cases. The onset of adrenal insufficiency in the neonatal period as seen in our patient has also been reported in other patients with different DAX1 mutations, especially in a patient with DAX1 protein lacking 11 amino acids at the C-terminus. Thereore, it is less likely that position of termination codons correlate to clinical manifestations. Am. J. Med. Genet. 76:62–66, 1998. © 1998 Wiley-Liss, Inc.     

X inactivation analysis in a female with hypomelanosis of Ito associated with a balanced X;17 translocation: evidence for functional disomy of Xp.

X inactivation analysis was performed on normal and hypopigmented skin samples obtained from a female with hypomelanosis of Ito associated with a balanced whole arm X;17 translocation. Severe skewing of X inactivation resulting in inactivity of the intact X was found in blood and cultures of both types of skin, but analysis of DNA prepared directly from hypopigmented skin showed significant inactivation of the translocated X, inconsistent with the usual mechanism of phenotypic expression in X;autosome translocations. In addition, dual colour FISH analysis using centromere specific probes for chromosomes X and 17 showed that the breakpoints on both chromosomes lie within the alphoid arrays, making interruption of a locus on either chromosome unlikely. While partial variable monosomy of loci on chromosome 17p cannot be excluded as contributing to the phenotype in this patient, it is argued that the major likely factor is partial functional disomy of sequences on Xp in cell lineages that have failed to inactivate the intact X chromosome.     

Nine novel mutations in NR0B1 (DAX1) causing adrenal hypoplasia congenita

X‐linked adrenal hypoplasia congenita (AHC) is caused by mutations in the NR0B1 gene. This gene encodes an orphan member of the nuclear receptor superfamily, DAX1. Ongoing efforts in our laboratory have identified nine novel NR0B1 mutations in X‐linked AHC patients (Y81X, 343delG, 457delT, 629delG, L295P, 926‐927delTG, 1130delA, 1141‐1155del15, and E428X). Two additional families segregate previously identified NR0B1 mutations (501delA and R425T). Sequence analysis of the mitochondrial D‐loop indicates that the 501delA family is unrelated through matrilineal descent to our previously analyzed 501delA family. © 2001 Wiley‐Liss, Inc.     

Skewed X inactivation and survival: a 13-year follow-up study of elderly twins and singletons

In mammalian females, one of the two X chromosomes is inactivated in early embryonic life. Females are therefore mosaics for two cell populations, one with the maternal and one with the paternal X as the active X chromosome. A skewed X inactivation is a marked deviation from a 50:50 ratio. In populations of women past 55-60 years of age, an increased degree of skewing (DS) is found. Here the association between age-related skewing and mortality is analyzed in a 13-year follow-up study of 500 women from three cohorts (73-100 years of age at intake). Women with low DS had significantly higher mortality than the majority of women who had a more skewed DS (hazard ratio: 1.30; 95% CI: 1.04-1.64). The association between X inactivation and mortality was replicated in dizygotic twin pairs for which the co-twin with the lowest DS also had a statistically significant tendency to die first in the twin pairs with the highest intra-pair differences in DS (proportion: 0.71; 95% CI: 0.52-0.86). Both results suggest that lower DS is associated with higher mortality. We therefore propose that age-related skewing may be partly due to a population selection with lower mortality among those with higher DS.     

X chromosome inactivation pattern in female carriers of X linked hypophosphataemic rickets.

X linked hypophosphataemia (XLH) results from an abnormality of renal tubular phosphate reabsorption. The disorder is inherited as an X linked dominant trait and the gene has been mapped to Xp22.1-p22.2. A candidate gene (PEX) has recently been isolated. The most striking clinical features are growth retardation and skeletal abnormalities. As expected for X linked dominant disorders, females are less affected. However, such a gene dosage effect does not exist for renal phosphate reabsorption. Preferential X chromosome inactivation has been proposed as a possible explanation for this lack of gene dosage. We have examined the X inactivation pattern in peripheral blood cells from 12 females belonging to seven families with XLH using PCR analysis at the androgen receptor locus. The X inactivation pattern in these patients did not differ significantly from the pattern in 30 healthy females. The X inactivation pattern in peripheral blood cells does not necessarily reflect the X inactivation pattern in renal cells. However, the finding of a normal distribution of X inactivation in peripheral blood cells indicates that the similarity in the renal handling of phosphate in male and female patients is not related to a ubiquitous preferential X inactivation.     

Cytomegalic adrenal hypoplasia with pituitary cytomegaly

Summary A case of cytomegalic congenital adrenal hypoplasia (C.C.A.H.) is reported with similar cytomegalic changes in the hypophysis. In this case the pituitary gland was small as were the thyroid and testes. The various groups of patients with C.A.H. are presented. Associated pituitary cytomegaly does not appear to have been previously described, and this association between the adrenal and pituitary changes is discussed. It is suggested that the adrenal cytomegaly is the result of overstimulation by the pituitary and the similar change in the latter may be the result of overstimulation of a hypoplastic organ.