X-chromosome inactivation and human genetic disease

The inactivation of one X-chromosome in females in early development is the process by which the effective dosage of X-linked genes is equalized between XX females and XY males. The mechanism that brings this about is the subject of intense research. The X-linked gene Xist is a key player, which is necessary but not sufficient for the initiation of X-inactivation. It codes for an untranslated RNA that coats the inactive X-chromosome, which takes on properties characteristic of heterochromatin, but how this change in chromatin is brought about remains unknown. Because of X-inactivation, females heterozygous for X-linked genes are mixtures of two types of cells and show a variable phenotype. The proportions of the two types of cells can depart from equality due to cell selection either at the tissue or whole organism level. In rare cases, changes in the Xist gene can cause skewing of X-inactivation. A few genes escape from X-inactivation either wholly or partially. CONCLUSION: X-chromosome inactivation is a physiological mechanism that equalizes gene-dosage effects on the sex chromosomes. The occurrence of this normal process affects the phenotype seen in females carrying X-linked mutant genes or chromosome anomalies.     

Skewed X inactivation in healthy individuals and in different diseases

In female mammalian cells, one of the two X chromosomes is inactivated in early embryonic life. Females are mosaics for two cell populations, one with the maternal and one with the paternal X as the active chromosome. Skewed X inactivation is arbitrarily defined, often as a pattern where 80% or more of the cells show a preferential inactivation of one X chromosome. Inactivation is presumed to be permanent for all descendants of a cell; however, after about 55 years of age, the frequency of skewed X inactivation in peripheral blood cells increases, probably through selection. Unfavourable skewing of X inactivation, where the X chromosome carrying a mutant allele is the predominantly active X, has been found in affected female carriers of several X-linked disorders; however, for many X-linked disorders, a consistent relationship between the pattern of X inactivation and clinical phenotype has been difficult to demonstrate. One reason for this may be that peripheral blood cells are not a representative or relevant tissue in many disorders. In some severe X-linked disorders, post-inactivation selection takes place against the X chromosome carrying the mutant allele, leading to a completely skewed X-inactivation pattern. Skewed X inactivation has also been reported in young females with breast cancer, and may indicate an effect of X-linked genes on the development of this condition.
Conclusion: The process of X inactivation and the resultant degree of skewing is clearly important for the expression of genetic diseases. It is also important to consider, however, that under normal conditions the frequency of skewed X inactivation increases with age in peripheral blood cells. Analysis of the expression of a large proportion of the genes on the X chromosome has revealed that X-chromosome inactivation is more heterogeneous than previously thought.     

Skewed X inactivation in healthy individuals and in different diseases

In female mammalian cells, one of the two X chromosomes is inactivated in early embryonic life. Females are mosaics for two cell populations, one with the maternal and one with the paternal X as the active chromosome. Skewed X inactivation is arbitrarily defined, often as a pattern where 80% or more of the cells show a preferential inactivation of one X chromosome. Inactivation is presumed to be permanent for all descendants of a cell; however, after about 55 years of age, the frequency of skewed X inactivation in peripheral blood cells increases, probably through selection. Unfavourable skewing of X inactivation, where the X chromosome carrying a mutant allele is the predominantly active X, has been found in affected female carriers of several X-linked disorders; however, for many X-linked disorders, a consistent relationship between the pattern of X inactivation and clinical phenotype has been difficult to demonstrate. One reason for this may be that peripheral blood cells are not a representative or relevant tissue in many disorders. In some severe X-linked disorders, post-inactivation selection takes place against the X chromosome carrying the mutant allele, leading to a completely skewed X-inactivation pattern. Skewed X inactivation has also been reported in young females with breast cancer, and may indicate an effect of X-linked genes on the development of this condition.Conclusion: The process of X inactivation and the resultant degree of skewing is clearly important for the expression of genetic diseases. It is also important to consider, however, that under normal conditions the frequency of skewed X inactivation increases with age in peripheral blood cells. Analysis of the expression of a large proportion of the genes on the X chromosome has revealed that X-chromosome inactivation is more heterogeneous than previously thought.     

Imprinting, the X-chromosome, and the male brain: explaining sex differences in the liability to autism

Males are at least four times more likely to develop autism than females. Among relatives with a broader autistic phenotype, males predominate too. Autism is a highly heritable disorder, yet genome scans have not revealed any predisposing loci on the sex chromosomes. A nongenetic explanation for male vulnerability, such as exposure to prenatal androgens, is unlikely for a variety of reasons. A novel genetic mechanism that resolves many of the outstanding difficulties is outlined here. The imprinted-X liability threshold model hypothesizes that the threshold for phenotypic expression of many autistic characteristics is influenced by an imprinted X-linked gene(s) that is protective in nature. Imprinted genes are known to play an important role in normal fetal and behavioral development. The gene is expressed only on the X-chromosome that is inherited from the father and raises the threshold for phenotypic expression. It is normally silenced when transmitted maternally. Because only females have a paternal X-chromosome, the threshold for phenotypic expression is higher in them than in males. Evidence for the existence of the genetic locus was found in a study of females with X-monosomy (Turner's syndrome) in which females had either a single paternal or maternal X-chromosome. Identifying the sites of action of this X-linked gene could lead to the discovery of autosomal loci that confer more directly a predisposition to autism.     

X-chromosome inactivation: role in skin disease expression

The occurrence of X inactivation in mammals has the consequence that all women are functional mosaics. In X-linked skin disorders, Lyonization usually gives rise to a mosaic pattern, as manifest by the appearance of the lines of Blaschko. This arrangement of lesions is observed in male-lethal X-linked traits, such as incontinentia pigmenti, focal dermal hypoplasia, Conradi-Hünermann-Happle syndrome, oral-facial-digital syndrome type 1 and MIDAS (microphthalmia, dermal aplasia and sclerocornea) syndrome, as well as in various X-linked non-lethal phenotypes, such as hypohidrotic ectodermal dysplasia of Christ-Siemens-Touraine, IFAP (ichthyosis follicularis-alopecia-photophobia) syndrome and X-linked dyskeratosis congenita. Analogous X-inactivation patterns have been documented in human bones, teeth, eyes and, possibly, the brain. Patterns that are distinct from the lines of Blaschko are also seen, such as the lateralization observed in CHILD (congenital hemidysplasia with ichthyosiform nevus and limb defects) syndrome, and the chequerboard pattern seen in women heterozygous for X-linked congenital hypertrichosis. Exceptional cases of either severe or absent involvement in a woman heterozygous for an X-linked trait can be explained by skewing of X inactivation. Some X-linked skin disorders are caused by genes that escape inactivation, which is why heterozygous female 'carriers' of these disorders do not show mosaicism. A well-known example is X-linked recessive ichthyosis due to steroid sulphatase deficiency, the locus for which is situated at the tip of the short arm of the X chromosome and does not undergo Lyonization. On the other hand, in the case of Fabry disease, the gene encoding alpha-galactosidase A is subject to inactivation. Remarkably, however, the skin lesions of women do not show a mosaic pattern.Conclusion: In the various X-linked skin disorders, affected women show quite dissimilar degrees of involvement and forms of manifestation because X inactivation may give rise to different patterns of functional mosaicism. Paradoxically, no such pattern is observed in women with Fabry disease. Like many X-linked diseases, Fabry disease should neither be called recessive nor dominant, because these dichotomous terms are obscured by the mechanism of X inactivation.     

Carrier Detection in Agammaglobulinemia by X Chromosome Inactivation Analysis

Using a recently developed strategy to analyze patterns of X chromosome inactivation in cell populations, we found that two mothers and a sister were carriers in three atypical or sporadic cases of patients with agammaglobulinemia, two of whom were brothers. In this study, a phosphogiycerate kinase 1 (PGK1) gene probe was used to detect patterns of methylation of X-chromosome genes. A random pattern of X inactivation was observed in isolated peripheral blood granulocytes. In contrast, one of the two X chromosomes was preferentially active in the Epstein-Barr virus (EBV)-transformed peripheral B cells of the family members of these patients. The volume of the blood specimen could be significantly reduced using EBV-transfomed B cell lines which contained multiple clones. The analysis described here can be used to distinguish between X-linked agammaglobulinemia (XLA) and other forms of a- or hypogammaglobulinemia as well as to detect the carrier state.     

Duchenne muscular dystrophy in a 4-year-old girl due to heterozygous frame shift deletion of the dystrophin gene and skewed X-inactivation

X-linked recessive diseases affect males, whereas female carriers are generally asymptomatic.We report on a 4-year-old girl who presented with a classical phenotype of Duchenne Muscular Dystrophy (DMD), a severe X-linked recessive type of muscular dystrophy affecting boys in early childhood.A thorough diagnostic work-up revealed that this resulted from a heterozygous out-of frame deletion in the DMD-gene in combination with an X-inactivation ratio of <10:90 in blood leukocytes and muscle.The case exemplifies that a skewed X-inactivation pattern has to be taken into account as mechanism causing clinical symptoms in female carriers of X-linked recessive disorders.     

Biology of the X chromosome.

The biology of the X chromosome is unique, as there are two Xs in females and only a single X in males, whereas the autosomes are present in duplicate in both sexes. The presence of only a single autosome, which can occur as a result of an error in meiotic segregation, is invariably an embryonic lethal event. Monosomy for the X chromosome is viable because of dosage compensation, a system found in all organisms with an X:Y form of sex determination, which brings about equality of expression of most X-linked genes in females and males. In mammals, the dosage compensation system involves silencing of most of the genes on one X chromosome; it is called X chromosome inactivation. In this review, we focus first on recent advances in our understanding of the molecular basis of the X inactivation mechanism. Then we consider an unusual feature of X inactivation, the mosaic nature of the female and subsequent exposure to somatic cell selection.     

Molecular analysis of X-linked agammaglobulinemia with growth hormone deficiency

To address the relationship between the gene (or genes) that causes the syndrome of X-linked hypogammaglobulinemia with isolated growth hormone deficiency and the gene responsible for typical X-linked agammaglobulinemia (XLA), we have used cytogenetics, examination of X chromosome inactivation patterns in potential carriers of the defect, and linkage analysis to study two unrelated families in which the affected males had isolated growth hormone deficiency and immunologic findings indistinguishable from those of typical XLA. A deletion could not be demonstrated in either family by G-banded karyotypes or flow cytometric analysis of metaphase chromosomes. Studies of X inactivation showed that mothers of affected boys from both families exhibited selective use of a single X chromosome as the active X chromosome in B cells but not T cells. This pattern is the same as that seen in obligate carriers of typical XLA. Linkage analysis demonstrated the most likely location for this gene (or genes) to be the midportion of the long arm of the X chromosome between DXS3 and DXS94. This segment of the X chromosome, which constitutes approximately 5% of the total X chromosome, encompasses the gene for XLA. These findings are consistent with the combination of XLA and growth hormone deficiency being caused by a small, contiguous, gene deletion syndrome involving the gene for XLA or an allelic variant of the gene for typical XLA.     

年龄和组织类型对X连锁Alport综合征女性患者X染色体失活方式的影响

目的 探讨不同年龄和组织类型对X连锁Alport综合征（XLAS）女性患者X染色体失活方式的影响。方法 纳入1997年1月至2006年12月北京大学第一医院（我院）儿科肾脏病遗传门诊就诊符合XLAS诊断标准的家系。根据年龄,将XLAS家系中女性患者分为两组,年龄≤30岁为A组,年龄>30岁为B组。采集患者及其家属静脉血3 mL,盐析法提取淋巴细胞基因组DNA。对家系中的女性患者在前臂下1/3处进行皮肤活检,并进行成纤维细胞培养。通过PCR扩增雄性激素受体（AR）基因第一外显子CAG重复序列的多肽性分析X染色体失活。每一标本X染色体失活分析均检测2次,取其平均值用于两组X染色体失活的差异分析,以X染色体失活率≥80%作为X染色体失活倾斜的标准。结果 研究期间我院儿科肾脏病遗传门诊共诊断XLAS家系186个,符合纳入和排除标准的XLAS家系共32个,包括36例女性患者,32例男性患者。A组和B组分别为13和23例,平均年龄分别为（17.9±11.7）和（38.6±6.2）岁。①36例女性患者中,外周血中AR基因位点杂合者32例,异质性为88.9%;AR基因位点纯合者4例。外周血中X染色体失活倾斜比例为12.5%（4/32例）,其中A组0例,B组4/20例（20.0%）,两组差异无统计学意义（χ2=2.743,P=0.098）。②12例女性患者同时分析外周血和皮肤成纤维细胞X染色体失活,结果显示两种组织中X染色体失活方式明显不同,3例患者仅皮肤成纤维细胞显示为X染色体失活倾斜而外周血无失活倾斜;1例外周血显示X染色体失活倾斜而皮肤成纤维细胞无失活倾斜。7/12例（58.3%）患者两种组织中以同一条X染色体失活为主;5/12例（41.7%）患者则相反,两种组织间X染色体失活明显不同。结论 不同组织类型而不是年龄可影响XLAS女性患者的X染色体失活方式,这可能是有关XLAS女性患者和X染色体失活研究结果不一致的一个主要原因。     

Alpha-Thalassämie-Retardierungs-Syndrom

Alpha-thalassemia X-linked mental retardation (ATRX, MIM#301040) syndrome is associated with severe mental retardation, muscular hypotonia, facial dysmorphism, and genital anomalies in males. We present the case of a family with two affected boys. The diagnosis was made by maternal X-chromosome inactivation studies which showed marked skewing of maternal X-chromosome inactivation and ATRX analysis revealed a novel mutation in exon 34.     

Duchenne muscular dystrophy in a 46 XY female

The most common muscular dystrophy, Duchenne muscular dystrophy (DMD), is an X-linked disorder that ordinarily has full clinical expression only in males. Reports of typical clinical features in females are rare but have occurred with a phenotypically identical autosomal recessive muscular dystrophy as well as in females with X-chromosome abnormalities such as the Turner syndrome. A girl with full expression of DMD due to a 46 XY karyotype is reported, and other clinical conditions in which expression of the DMD gene occurs in females are reviewed.     

X chromosome inactivation analysis to distinguish sporadic cases of X-linked agammaglobulinaemia from common variable immunodeficiency

The X chromosome inactivation analysis of eight female relatives was performed to elucidate the X chromosome gene defect of six male hypogammaglobulinaemic individuals. The patients had diminished numbers of circulating B-cells and no relevant family history. The methylation status of three X-linked genes, phosphoglycerate kinase, hypoxanthine phosphoribosyl transferase and DXS255, was determined on DNA from Epstein-Barr virus-transformed B-cell lines established from the female relatives. The methylation pattern of at least one gene was informative in all eight females examined. While both alleles were equally methylated in four of eight females, the remaining four female relatives of three hypogammaglobulinaemia patients exhibited a non-random methylation pattern in their B-cells, suggesting that these three patients represented sporadic cases of X-linked agammaglobulinaemia (XLA). The clinical or immunological status of these three patients did not differ from the remaining two who had early onset hypogammaglobulinaemia and who were tentatively diagnosed as having common variable immunodeficiency. The sixth patient had recurrent infections after undergoing surgical removal of a brain tumour at 22 years of age, although his immunological features did not distinguish him from the other patients. X chromosome inactivation analysis can be useful in differentiating XLA from hypogammaglobulinaemia in male patients.     

Severe X-linked chronic granulomatous disease in two unrelated females

Chronic granulomatous disease (CGD) is a rare primary immunodeficiency caused by mutations of one of the subunits of phagocyte reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase leading to decreased or complete absence of neutrophil oxidative burst. We report the clinical and laboratory findings in two young unrelated females 14 and 9 years of age and natives of Tahiti and Reunion Islands, respectively, with severe X-linked granulomatous disease. In both cases, the infectious pattern was unusual, with convergent symptoms suggesting underlying mycobacterial infection. Functional analysis revealed low residual NADPH oxidase activity with about 5–10% of normal neutrophil population. De novo null mutations affecting the CYBB gene that encodes the gp91 protein were found in both cases in the heterozygous state (in patient 1, p.Arg130X in exon 5, and in patient 2, a novel insertion in exon 6, c.632_633insCATC). Methylation analysis confirmed that phenotype expression was linked to skewed X inactivation and showed that the de novo mutation arose on the maternally inherited chromosome in one case and on the paternally inherited chromosome in the other case. In conclusion, X-linked CGD carriers could therefore be at risk for severe infectious diseases depending on the skewed X inactivation pattern and the infectious context.     

Heterogeneity of clinical severity and molecular lesions in Aicardi syndrome

All patients with Aicardi syndrome are female or have a 47,XXY karyotype. This finding, along with a report of an Aicardi syndrome patient with an Xp22/autosome translocation, led to the hypothesis that Aicardi syndrome might be caused by an X-linked dominant, male-lethal mutation on the short arm of the X chromosome. To study this hypothesis, we investigated X chromosome inactivation patterns in peripheral lymphocytes from seven patients. We used two methods: methylation-sensitive restriction enzyme analysis and segregation of the active X chromosome in somatic cell hybrids. We found that three of seven cytogenetically normal girls with Aicardi syndrome had profoundly skewed X-inactivation in their lymphocytes, supporting the concept that Aicardi syndrome is X linked. Three of the five girls with the greatest degree of psychomotor retardation and the poorest seizure control had skewed X-inactivation. In contrast, the two highest-functioning children had random X-inactivation. We screened DNA using eight polymorphic probes from the Xp22 region but were unable to identify a deletion in any of the seven patients. Nonrandom X-inactivation in lymphocytes and possibly other tissues in some, but not all, patients with Aicardi syndrome may reflect heterogeneity of their molecular lesions.     

A case of juvenile myelomonocytic leukemia with concomitant cytomegalovirus infection

Infantile cytomegalovirus (CMV)-associated disease and juvenile myelomonocytic leukemia (JMML) frequently present with similar clinical features, and thus the differential diagnosis is often difficult. An early and definite diagnosis of these disorders is required because their therapeutic approaches are very different. The authors describe a 2-month-old Japanese girl with JMML and CMV infection. The CMV antigen was detected by immunologic staining of leukocytes using the peroxidase-labeled monoclonal antibody HRP-C7. To assess clonality, the X-chromosome inactivation pattern was evaluated using polymerase chain reaction analysis of the human androgen receptor gene with or without predigestion of chromosomal DNA with HhaI or HpaII. The patient showed evidence of monoclonal origin of mononuclear cells at diagnosis. Although CMV infection mimicking JMML has previously been reported in two patients, to the authors' knowledge this is the first report describing a firm and definitive diagnosis of JMML based on the study of X-chromosome inactivation patterns.     

Improved definition of carrier status in X-linked hypohidrotic ectodermal dysplasia by use of restriction fragment length polymorphism-based linkage analysis

The detection of carriers of the X-linked disorder hypohidrotic ectodermal dysplasia is problematic because of random X-inactivation; the diagnosis was previously based on the observation of subtle defects in ectodermal structures in at-risk females. Linkage studies have recently mapped hypohidrotic ectodermal dysplasia to the region Xq11-q21.1. We assessed the improvement in carrier detection by the method of linkage analysis, in which restriction fragment length polymorphisms were used as markers, in 72 at-risk female members of 29 families. Carriers analyses were based on pedigree information, dental examination of at-risk females (phenotype), and DNA analyses at seven linked marker loci. Linkage analysis based on restriction fragment length polymorphisms significantly improved risk estimates over those based on phenotype and pedigree alone. When all available information was combined, 85% (61/72) of the at-risk females had final risks of less than 5% or greater than 95%, and 68% (49/72) had risks less than 1% or greater than 99%. A diagnosis of hypohidrotic ectodermal dysplasia was also excluded (97.5% probability) by DNA and linkage analyses from a sample of cord blood from an at-risk male; a similar approach can be taken for prenatal diagnosis of the disorder.     

Disease manifestations and X inactivation in heterozygous females with Fabry disease

Aim: Fabry disease is an X-linked lysosomal storage disorder characterized by an accumulation of neutral glycosphingolipids in multiple organ systems caused by α-galactosidase A deficiency due to mutations in the GLA gene. The majority of heterozygous females show the characteristic signs and symptoms of the disease, and some of them are severely affected. The current hypothesis for the occurrence of disease manifestations in females is skewed X inactivation favouring the mutant GLA allele.
Method: We analyzed the patterns of X inactivation in the leukocytes of 28 biochemically and genetically characterized symptomatic Fabry disease heterozygotes and their correlation with clinical and biochemical disease expression.
Results: X inactivation patterns in symptomatic females who are heterozygous for Fabry disease did not differ from those of female controls of the same age ( p = 0.669). Thirteen (46%) of the 28 females with Fabry disease showed random X inactivation, ten (36%) moderate skewing, and five (18%) highly skewed X inactivation. Segregation analysis was performed in the families of six females who had highly or moderately skewed X inactivation. In four of these females, skewing favoured the wild-type GLA allele and in the other two skewing favoured the mutant allele. Patterns of X inactivation or the extent of skewing were not related to the severity of clinical manifestations or to residual enzyme activity.
Conclusion: In this study we provide evidence that heterozygous females with Fabry disease show random X inactivation. Our data do not support the hypothesis that the occurrence and severity of disease manifestations in the majority of Fabry heterozygotes are related to skewed X inactivation.     

The hyper IgM syndrome--an evolving story

The hyper IgM syndromes (HIGM) are a group of primary immune deficiency disorders characterized by defective CD40 signaling by B cells affecting class switch recombination and somatic hypermutation. As a consequence, patients with HIGM have decreased concentrations of serum IgG and IgA and normal or elevated IgM, leading to increased susceptibility to infections. The most common HIGM syndrome is X-linked and due to mutations of CD40 ligand (CD40L) expressed by activated CD4(+) T lymphocytes. Four other genes, expressed by B cells, have been associated with the HIGM phenotype. Mutations of CD40, the receptor for CD40L, cause a rare autosomal form of HIGM with a clinical phenotype similar to CD40L deficiency. Mutations of Activation-Induced Cytidine Deaminase (AICDA) and Uracil (DNA) Glycosylase (UNG), both expressed by follicular B lymphocytes, lead to defective class switch recombination and somatic hypermutation. Mutations of Nuclear Factor kappa B Essential Modulator (NEMO), an X-chromosome associated gene, result in hypohidrotic ectodermal dysplasia and immune deficiency. Thus, the molecular definition of these rare primary immune deficiency disorders has shed light on the complex events leading to the production of high-affinity, antigen-specific antibodies of different isotypes.