Genetic mapping of X-linked loci involved in skewing of X chromosome inactivation in the human

Unfavorable switching of skewed X chromosome inactivation leads to Menkes disease in a female infant

Menkes disease is an X-linked disorder of copper metabolism caused by mutations in the ATP7A gene, and female carriers are usually asymptomatic. We describe a 7-month-old female patient with severe intellectual disability, epilepsy, and low levels of serum copper and ceruloplasmin. While heterozygous deletion of exons 16 and 17 of the ATP7A gene was detected in the proband, her mother, and her grandmother, only the proband suffered from Menkes disease clinically. Intriguingly, X chromosome inactivation (XCI) analysis demonstrated that the grandmother and the mother showed skewing of XCI toward the allele with the ATP7A deletion and that the proband had extremely skewed XCI toward the normal allele, resulting in exclusive expression of the pathogenic ATP7A mRNA transcripts. Expression bias analysis and recombination mapping of the X chromosome by the combination of whole genome and RNA sequencing demonstrated that meiotic recombination occurred at Xp21-p22 and Xq26-q28. Assuming that a genetic factor on the X chromosome enhanced or suppressed XCI of its allele, the factor must be on either of the two distal regions derived from her grandfather. Although we were unable to fully uncover the molecular mechanism, we concluded that unfavorable switching of skewed XCI caused Menkes disease in the proband.

Cross-species examination of X-chromosome inactivation highlights domains of escape from silencing

Background

X-chromosome inactivation (XCI) in eutherian mammals is the epigenetic inactivation of one of the two X chromosomes in XX females in order to compensate for dosage differences with XY males. Not all genes are inactivated, and the proportion escaping from inactivation varies between human and mouse (the two species that have been extensively studied).

Results

We used DNA methylation to predict the XCI status of X-linked genes with CpG islands across 12 different species: human, chimp, bonobo, gorilla, orangutan, mouse, cow, sheep, goat, pig, horse and dog. We determined the XCI status of 342 CpG islands on average per species, with most species having 80–90% of genes subject to XCI. Mouse was an outlier, with a higher proportion of genes subject to XCI than found in other species. Sixteen genes were found to have discordant X-chromosome inactivation statuses across multiple species, with five of these showing primate-specific escape from XCI. These discordant genes tended to cluster together within the X chromosome, along with genes with similar patterns of escape from XCI. CTCF-binding, ATAC-seq signal and LTR repeats were enriched at genes escaping XCI when compared to genes subject to XCI; however, enrichment was only observed in three or four of the species tested. LINE and DNA repeats showed enrichment around subject genes, but again not in a consistent subset of species.

Conclusions

In this study, we determined XCI status across 12 species, showing mouse to be an outlier with few genes that escape inactivation. Inactivation status is largely conserved across species. The clustering of genes that change XCI status across species implicates a domain-level control. In contrast, the relatively consistent, but not universal correlation of inactivation status with enrichment of repetitive elements or CTCF binding at promoters demonstrates gene-based influences on inactivation state. This study broadens enrichment analysis of regulatory elements to species beyond human and mouse.

X-linked intellectual disability: Phenotypic expression in carrier females

To better understand the landscape of female phenotypic expression in X-linked intellectual disability (XLID), we surveyed the literature for female carriers of XLID gene alterations (n = 1098) and combined this with experience evaluating XLID kindreds at the Greenwood Genetic Center (n = 341) and at the University of Adelaide (n = 157). One-hundred forty-four XLID genes were grouped into nine categories based on the level of female phenotypic expression, ranging from no expression to female only expression. For each gene, the clinical presentation, gene expression in blood, X-inactivation (XI) pattern, biological pathway involved, and whether the gene escapes XI were noted. Among the XLID conditions, 88 (61.1%) exhibited female cognitive phenotypic expression only, while 56 (38.9%) had no female phenotypic expression (n = 45), phenotype expression with normal cognition in females (n = 8), or unknown status for female phenotypic expression (n = 3). In twenty-four (16.6%) XLID genes, XI was consistently skewed in female carriers, in 54 (37.5%) XI showed variable skewing, and in 33 (22.9%) XI was consistently random. The XI pattern was unknown in 33 (22.9%) XLID conditions. Therefore, there is evidence of a female carrier phenotype in the majority of XLID conditions although how exactly XI patterns influence the female phenotype in XLID conditions remains unclear.

Skewed X-chromosome inactivation plays a crucial role in the onset of symptoms in carriers of Becker muscular dystrophy

BACKGROUND

Becker muscular dystrophy (BMD) is an X-linked recessive disorder affecting approximately 1: 18.000 male births. Female carriers are usually asymptomatic, although 2.5-18% may present muscle or heart symptoms. In the present study, the role of the X chromosome inactivation (XCI) on the onset of symptoms in BMD carriers was analysed and compared with the pattern observed in Duchenne muscular dystrophy (DMD) carriers.

METHODS

XCI was determined on the lymphocytes of 36 BMD carriers (both symptomatic and not symptomatic) from 11 families requiring genetic advice at the Cardiomyology and Medical Genetics of the Second University of Naples, using the AR methylation-based assay. Carriers were subdivided into two groups, according to age above or below 50 years. Seven females from the same families known as noncarriers were used as controls. A Student's t-test for nonpaired data was performed to evaluate the differences observed in the XCI values between asymptomatic and symptomatic carriers, and carriers aged above or below 50 years. A Pearson correlation test was used to evaluate the inheritance of the XCI pattern in 19 mother-daughter pairs.

RESULTS

The results showed that symptomatic BMD carriers had a skewed XCI with a preferential inactivation of the X chromosome carrying the normal allele, whereas the asymptomatic carriers and controls showed a random XCI. No concordance concerning the XCI pattern was observed between mothers and related daughters.

CONCLUSIONS

The data obtained in the present study suggest that the onset of symptoms in BMD carriers is related to a skewed XCI, as observed in DMD carriers. Furthermore, they showed no concordance in the XCI pattern inheritance.

Selection of X-chromosome Inactivation Model

To address the complexity of the X-chromosome inactivation (XCI) process, we previously developed a unified approach for the association test for X-chromosomal single-nucleotide polymorphisms (SNPs) and the disease of interest, accounting for different biological possibilities of XCI: random, skewed, and escaping XCI. In the original study, we focused on the SNP-disease association test but did not provide knowledge regarding the underlying XCI models. One can use the highest likelihood ratio (LLR) to select XCI models (max-LLR approach). However, that approach does not formally compare the LLRs corresponding to different XCI models to assess whether the models are distinguishable. Therefore, we propose an LLR comparison procedure (comp-LLR approach), inspired by the Cox test, to formally compare the LLRs of different XCI models to select the most likely XCI model that describes the underlying XCI process. We conduct simulation studies to investigate the max-LLR and comp-LLR approaches. The simulation results show that compared with the max-LLR, the comp-LLR approach has higher probability of identifying the correct underlying XCI model for the scenarios when the underlying XCI process is random XCI, escaping XCI, or skewed XCI to the deleterious allele. We applied both approaches to a head and neck cancer genetic study to investigate the underlying XCI processes for the X-chromosomal genetic variants.

Determining the role of skewed X-chromosome inactivation in developing muscle symptoms in carriers of Duchenne muscular dystrophy

Duchenne and Becker dystrophinopathies (DMD and BMD) are X-linked recessive disorders caused by mutations in the dystrophin gene that lead to absent or reduced expression of dystrophin in both skeletal and heart muscles. DMD/BMD female carriers are usually asymptomatic, although about 8 % may exhibit muscle or cardiac symptoms. Several mechanisms leading to a reduced dystrophin have been hypothesized to explain the clinical manifestations and, in particular, the role of the skewed XCI is questioned. In this review, the mechanism of XCI and its involvement in the phenotype of BMD/DMD carriers with both a normal karyotype or with X;autosome translocations with breakpoints at Xp21 (locus of the DMD gene) will be analyzed. We have previously observed that DMD carriers with moderate/severe muscle involvement, exhibit a moderate or extremely skewed XCI, in particular if presenting with an early onset of symptoms, while DMD carriers with mild muscle involvement present a random XCI. Moreover, we found that among 87.1 % of the carriers with X;autosome translocations involving the locus Xp21 who developed signs and symptoms of dystrophinopathy such as proximal muscle weakness, difficulty to run, jump and climb stairs, 95.2 % had a skewed XCI pattern in lymphocytes. These data support the hypothesis that skewed XCI is involved in the onset of phenotype in DMD carriers, the X chromosome carrying the normal DMD gene being preferentially inactivated and leading to a moderate-severe muscle involvement.

X inactivation and reactivation in X-linked diseases

X chromosome inactivation (XCI) is the phenomenon by which mammals compensate for dosage of X-linked genes in females (XX) versus males (XY). XCI patterns can be random or show extreme skewing, and can modify the mode of inheritance of X-driven phenotypes, which contributes to the variability of human pathologies. Recent findings have shown reversibility of the XCI process, which has opened new avenues in the approaches used for the treatment of X-linked diseases.

X-chromosome genetic association test accounting for X-inactivation, skewed X-inactivation, and escape from X-inactivation

X-chromosome inactivation (XCI) is the process in which one of the two copies of the X-chromosome in females is randomly inactivated to achieve the dosage compensation of X-linked genes between males and females. That is, 50% of the cells have one allele inactive and the other 50% of the cells have the other allele inactive. However, studies have shown that skewed or nonrandom XCI is a biological plausibility wherein more than 75% of cells have the same allele inactive. Also, some of the X-chromosome genes escape XCI, i.e., both alleles are active in all cells. Current statistical tests for X-chromosome association studies can either account for random XCI (e.g., Clayton's approach) or escape from XCI (e.g., PLINK software). Because the true XCI process is unknown and differs across different regions on the X-chromosome, we proposed a unified approach of maximizing likelihood ratio over all biological possibilities: random XCI, skewed XCI, and escape from XCI. A permutation-based procedure was developed to assess the significance of the approach. We conducted simulation studies to compare the performance of the proposed approach with Clayton's approach and PLINK regression. The results showed that the proposed approach has higher powers in the scenarios where XCI is skewed while losing some power in scenarios where XCI is random or XCI is escaped, with well-controlled type I errors. We also applied the approach to the X-chromosomal genetic association study of head and neck cancer.

Human X-chromosome inactivation pattern distributions fit a model of genetically influenced choice better than models of completely random choice

In eutherian mammals, one X-chromosome in every XX somatic cell is transcriptionally silenced through the process of X-chromosome inactivation (XCI). Females are thus functional mosaics, where some cells express genes from the paternal X, and the others from the maternal X. The relative abundance of the two cell populations (X-inactivation pattern, XIP) can have significant medical implications for some females. In mice, the 'choice' of which X to inactivate, maternal or paternal, in each cell of the early embryo is genetically influenced. In humans, the timing of XCI choice and whether choice occurs completely randomly or under a genetic influence is debated. Here, we explore these questions by analysing the distribution of XIPs in large populations of normal females. Models were generated to predict XIP distributions resulting from completely random or genetically influenced choice. Each model describes the discrete primary distribution at the onset of XCI, and the continuous secondary distribution accounting for changes to the XIP as a result of development and ageing. Statistical methods are used to compare models with empirical data from Danish and Utah populations. A rigorous data treatment strategy maximises information content and allows for unbiased use of unphased XIP data. The Anderson-Darling goodness-of-fit statistics and likelihood ratio tests indicate that a model of genetically influenced XCI choice better fits the empirical data than models of completely random choice.

Familial skewed x chromosome inactivation in adrenoleukodystrophy manifesting heterozygotes from a Chinese pedigree

Background

X-linked adrenoleukodystrophy (X-ALD) is an inherited neurodegenerative disorder caused by mutations in the ABCD1 gene. Approximately 20% of X-ALD female carriers may develop neurological symptoms. Skewed X chromosome inactivation (XCI) has been proposed to influence the manifestation of symptoms in X-ALD carriers, but data remain conflicting so far. We identified a three generation kindred, with five heterozygous females, including two manifesting carriers. XCI pattern and the ABCD1 allele expression were assessed in order to determine if symptoms in X-ALD carriers could be related to skewed XCI and whether skewing within this family is more consistent with genetically influenced or completely random XCI.

Results

We found a high frequency of skewing in this family. Four of five females had skewed XCI, including two manifesting carriers favoring the mutant allele, one asymptomatic carrier favoring the normal allele, and one female who was not an X-ALD carrier. Known causes of skewing, such as chromosomal abnormalities, selection against deleterious alleles, XIST promoter mutations, were not consistent with our results.

Conclusions

Our data support that skewed XCI in favor of the mutant ABCD1 allele would be associated with the manifestation of heterozygous symptoms. Furthermore, XCI skewing in this family is genetically influenced. However, the underlying mechanism remains to be substantiated by further experiments.

The genetic basis of graves' disease

The presented comprehensive review of current knowledge about genetic factors predisposing to Graves’ disease (GD) put emphasis on functional significance of observed associations. In particular, we discuss recent efforts aimed at refining diseases associations found within the HLA complex and implicating HLA class I as well as HLA-DPB1 loci. We summarize data regarding non-HLA genes such as PTPN22, CTLA4, CD40, TSHR and TG which have been extensively studied in respect to their role in GD. We review recent findings implicating variants of FCRL3 (gene for FC receptor-like-3 protein), SCGB3A2 (gene for secretory uteroglobin-related protein 1- UGRP1) as well as other unverified possible candidate genes for GD selected through their documented association with type 1 diabetes mellitus: Tenr–IL2–IL21, CAPSL (encoding calcyphosine-like protein), IFIH1(gene for interferon-induced helicase C domain 1), AFF3, CD226 and PTPN2. We also review reports on association of skewed X chromosome inactivation and fetal microchimerism with GD. Finally we discuss issues of genotype-phenotype correlations in GD.

Familial skewed X-chromosome inactivation linked to a component of the cohesin complex, SA2

The gene dosage inequality between females with two X-chromosomes and males with one is compensated for by X-chromosome inactivation (XCI), which ensures the silencing of one X in every somatic cell of female mammals. XCI in humans results in a mosaic of two cell populations: those expressing the maternal X-chromosome and those expressing the paternal X-chromosome. We have previously shown that the degree of mosaicism (the X-inactivation pattern) in a Canadian family is directly related to disease severity in female carriers of the X-linked recessive bleeding disorder, haemophilia A. The distribution of X-inactivation patterns in this family was consistent with a genetic trait having a co-dominant mode of inheritance, suggesting that XCI choice may not be completely random. To identify genetic elements that could be responsible for biased XCI choice, a linkage analysis was undertaken using an approach tailored to accommodate the continuous nature of the X-inactivation pattern phenotype in the Canadian family. Several X-linked regions were identified, one of which overlaps with a region previously found to be linked to familial skewed XCI. SA2, a component of the cohesin complex is identified as a candidate gene that could participate in XCI through its association with CTCF.

Female Hunter syndrome caused by a single mutation and familial XCI skewing: implications for other X-linked disorders

Familial X-chromosome inactivation (XCI) skewing was investigated in a family in which a female mucopolysaccharidosis type II (MPS II) (Hunter syndrome, an X-linked genetic disease) occurred. Among eight related females aged under 60 years from three generations who were tested, four revealed a non-random pattern of XCI. Detailed genetic analysis failed to find mutations in genes that were previously reported as important for the XCI process. Haplotype analysis excluded linkage of non-random XCI with genes localized on the X-chromosome. We propose that analysis of the XCI pattern should be taken into consideration when assessing risk factors for X-linked recessive genetic disorders.

The pituitary-thyroid axis set point in women is uninfluenced by X chromosome inactivation pattern? A twin study

OBJECTIVE

The pituitary-thyroid axis (PTA) set point is determined by a combination of genetic and environmental factors. However, despite considerable efforts to characterize the background, the causative genes as well as environmental factors are not well established. Theoretically, as shown for autoimmune thyroid disease, the pattern of X chromosome inactivation (XCI) could offer a novel explanation for the observed variability of the PTA set point in women.

DESIGN AND PATIENTS

To examine the impact of XCI pattern on the PTA set point, we studied whether within-cohort (n = 318 subjects) and within-twin pair (n = 159 pairs) differences in XCI are correlated with serum concentrations of thyrotropin (TSH), free triiodothyronine (FT3) and free thyroxine (FT4).

METHODS

X chromosome inactivation was determined by PCR analysis of a polymorphic CAG repeat in the first exon of the androgen receptor gene. Thyroid variables were measured using a solid-phase time-resolved fluoroimmunometric assay. Zygosity was established by DNA fingerprinting.

RESULTS

In the overall study population (within cohort), no significant correlations were found between TSH [regression coefficient (β) = -0·28 (95% confidence intervals, -0·66 to 0·11), P = 0·158], FT3 [β = -0·25 (-0·85 to 0·34), P = 0·403], FT4 [β = 0·08 (-0·91 to 1·07), P = 0·876] and XCI pattern. Essentially similar results were found in the within-pair analysis. Controlling for confounders such as age, body mass index, smoking and zygosity did not change the findings.

CONCLUSIONS

In a sample of female twins, we found no evidence of a relationship between XCI pattern and PTA set point.

Inter- and intra-individual variation in allele-specific DNA methylation and gene expression in children conceived using assisted reproductive technology

Epidemiological studies have reported a higher incidence of rare disorders involving imprinted genes among children conceived using assisted reproductive technology (ART), suggesting that ART procedures may be disruptive to imprinted gene methylation patterns. We examined intra- and inter-individual variation in DNA methylation at the differentially methylated regions (DMRs) of the IGF2/H19 and IGF2R loci in a population of children conceived in vitro or in vivo. We found substantial variation in allele-specific methylation at both loci in both groups. Aberrant methylation of the maternal IGF2/H19 DMR was more common in the in vitro group, and the overall variance was also significantly greater in the in vitro group. We estimated the number of trophoblast stem cells in each group based on approximation of the variance of the binomial distribution of IGF2/H19 methylation ratios, as well as the distribution of X chromosome inactivation scores in placenta. Both of these independent measures indicated that placentas of the in vitro group were derived from fewer stem cells than the in vivo conceived group. Both IGF2 and H19 mRNAs were significantly lower in placenta from the in vitro group. Although average birth weight was lower in the in vitro group, we found no correlation between birth weight and IGF2 or IGF2R transcript levels or the ratio of IGF2/IGF2R transcript levels. Our results show that in vitro conception is associated with aberrant methylation patterns at the IGF2/H19 locus. However, very little of the inter- or intra-individual variation in H19 or IGF2 mRNA levels can be explained by differences in maternal DMR DNA methylation, in contrast to the expectations of current transcriptional imprinting models. Extraembryonic tissues of embryos cultured in vitro appear to be derived from fewer trophoblast stem cells. It is possible that this developmental difference has an effect on placental and fetal growth.

We have screened a population of children conceived in vitro for epigenetic alterations at two loci that carry parent-of-origin specific methylation marks. We made the observation that epigenetic variability was greater in extraembryonic tissues than embryonic tissues in both groups, as has also been demonstrated in the mouse. The greater level of intra-individual variation in extraembryonic tissues of the in vitro group appears to result from these embryos having fewer trophoblast stem cells. We also made the unexpected observation that variability in parental origin-dependent epigenetic marking was poorly correlated with gene expression. In fact, there is such a high level of inter-individual variation in IGF2 transcript level that the presumed half-fold reduction in IGF2 mRNA accounted for by proper transcriptional imprinting versus complete loss of imprinting would account for less than 5% of the total population variance. Given this level of variability in the expression of an imprinted gene, the presumed operation of “parental conflict” as the selective force acting to maintain imprinted gene expression at the IGF2/H19 locus in the human should be revisited.

No correlation between androgen receptor CAG and GGN repeat length and the degree of genital virilization in females with 21-hydroxylase deficiency

CONTEXT

In 21-hydroxylase (CYP21A2) deficiency (21OHD), the level of in vitro enzymatic function allows for classification of mutation groups (null, A, B, C) and prediction of disease severity. However, genital virilization in affected females correlates only weakly with CYP21A2 mutation groups, suggesting the influence of genetic modifiers.

OBJECTIVE

The objective of the study was to investigate the influence of the polymorphic CAG and GGn repeats of the androgen receptor (AR) gene on the degree of genital virilization in 21OHD females.

DESIGN AND PATIENTS

Design of the study was the determination of CYP21A2 genotype, degree of genital virilization (Prader stage), and X-weighted biallelic mean of AR CAG and GGn repeat length in 205 females with 21OHD.

OUTCOME MEASUREMENTS

Correlation of AR CAG and GGn repeat lengths with Prader stages using nested stepwise logistic regression analysis was measured.

RESULTS

CYP21A2 mutation groups null and A showed significantly higher levels of genital virilization than groups B and C (P < 0.01). However, Prader stages varied considerably within mutation groups: null, Prader I-V (median IV); A, Prader I-V (median IV); B, Prader I-V (median III); C, 0-III (median I). Mean GGn repeat length of patients was not significantly associated with Prader stages, classified as low (0-I), intermediate (II-III), or severe (IV-V) (odds ratio per repeat: 0.98, 95% confidence interval 0.71-1.35). In contrast, patients with Prader 0-I showed a trend toward longer CAG repeats without reaching statistical significance (P = 0.07, odds ratio per repeat: 0.82, 95% confidence interval 0.65-1.02).

CONCLUSION

Neither CAG nor GGn repeat lengths are statistically significant modifiers of genital virilization in females with 21OHD.

Preliminary evidence of a noncausal association between the X-chromosome inactivation pattern and thyroid autoimmunity: a twin study

An increased frequency of skewed X-chromosome inactivation (XCI) is found in clinically overt autoimmune thyroid disease (AITD) compared with controls. Whether skewed XCI is involved in the pathogenesis of autoantibodies to thyroid peroxidase (TPOAb) in euthyroid subjects is unknown. To examine the impact of XCI on the serum concentration of TPOAb, we studied whether within-cohort and within-twin-pair differences in XCI are associated with differences in serum concentrations of TPOAb. A total of 318 euthyroid female twin individuals distributed in 159 pairs were investigated. XCI was determined by PCR analysis of a polymorphic CAG repeat in the first exon of the androgen receptor gene. TPOAb concentrations were measured using a solid-phase time-resolved fluoroimmunometric assay. Overall (within cohort), there was a significant association between XCI and serum concentrations of TPOAb; regression coefficient (beta)=1.45 (95% confidence interval, 0.52-2.38), P=0.003. The association remained significant in the within-pair analysis; beta=1.74 (0.79-2.69), P<0.001. The relationship was nonsignificant within the 82 monozygotic pairs (beta=0.57 (-0.78-1.92), P=0.405), whereas the association was significant in the 77 dizygotic pairs (beta=2.17 (0.81-3.53), P=0.002). This preliminary finding of a significant association between TPOAb concentrations and XCI within cohort and within dizygotic but not within monozygotic twin pairs may indicate that XCI per se does not have a major role in the pathogenesis of TPOAb. More likely, XCI and TPOAb are influenced by shared genetic determinants.

Analysis of skewed X-chromosome inactivation in females with rheumatoid arthritis and autoimmune thyroid diseases

Introduction

The majority of autoimmune diseases such as rheumatoid arthritis (RA) and autoimmune thyroid diseases (AITDs) are characterized by a striking female predominance superimposed on a predisposing genetic background. The role of extremely skewed X-chromosome inactivation (XCI) has been questioned in the pathogenesis of several autoimmune diseases.

Methods

We examined XCI profiles of females affected with RA (n = 106), AITDs (n = 145) and age-matched healthy women (n = 257). XCI analysis was performed by enzymatic digestion of DNA with a methylation sensitive enzyme (HpaII) followed by PCR of a polymorphic CAG repeat in the androgen receptor (AR) gene. The XCI pattern was classified as skewed when 80% or more of the cells preferentially inactivated the same X-chromosome.

Results

Skewed XCI was observed in 26 of the 76 informative RA patients (34.2%), 26 of the 100 informative AITDs patients (26%), and 19 of the 170 informative controls (11.2%) (P < 0.0001; P = 0.0015, respectively). More importantly, extremely skewed XCI, defined as > 90% inactivation of one allele, was present in 17 RA patients (22.4%), 14 AITDs patients (14.0%), and in only seven controls (4.1%, P < 0.0001; P = 0.0034, respectively). Stratifying RA patients according to laboratory profiles (rheumatoid factor and anti-citrullinated protein antibodies), clinical manifestations (erosive disease and nodules) and the presence of others autoimmune diseases did not reveal any statistical significance (P > 0.05).

Conclusions

These results suggest a possible role for XCI mosaicism in the pathogenesis of RA and AITDs and may in part explain the female preponderance of these diseases.

Molecular mechanisms underlying hemophilia A phenotype in seven females

BACKGROUND

Hemophilia A (HA) in females is a rare observation. Here we describe various genetic mechanisms that result in phenotypic expression of HA in seven females.

METHODS

The F8 gene was examined in all patients and relatives by direct sequencing. Multiplex ligation-dependent probe amplification (MLPA) was performed for large deletion screening. X chromosome inactivation was studied by PCR analysis of a polymorphic CAG repeat in the first exon of the human androgen receptor (HUMARA) gene.

RESULTS

In two females sequencing of the F8 gene revealed homozygous missense mutations (Arg593Cys and Tyr1680Phe) as a consequence of consanguineous marriage. The third case was due to compound heterozygosity comprising the missense mutation Leu412Phe inherited from the carrier mother, together with a de novo large deletion spanning exon 9-22, probably originating from the germ cells of the healthy father. Three further cases shared a common mechanism representing heterozygous mutations in the F8 gene (Arg1781His, Arg327His, small deletion in exon 10) combined with non-random inactivation of the X chromosome. The final case describes a coincidental inheritance of HA and Coffin-Lowry syndrome in the same family. The HA phenotype results from a heterozygous small deletion affecting the F8 gene (c.6872 del CT leading to Thr2272fs) and a complete inactivation of the maternal X chromosome, which segregates with Coffin-Lowry syndrome in the two brothers of the proposita.

CONCLUSIONS

In conclusion, molecular genetic analysis represents an essentially valuable tool in elucidating the nature of the molecular mechanisms underlying the HA phenotype in females.

Skewed X chromosome inactivation and breast and ovarian cancer status: evidence for X-linked modifiers of BRCA1

BACKGROUND

X chromosome inactivation, which silences gene expression from one of the two X chromosomes in females, is usually random. Skewed X inactivation has been implicated in both the expression and the suppression of X-linked disease phenotypes and has been reported to occur more frequently in breast and ovarian cancer patients, including BRCA1 or BRCA2 mutation carriers, than in control subjects.

METHODS

We assessed the pattern of X chromosome inactivation using methylation-specific polymerase chain reaction amplification of the exon 1 microsatellite region of the X-linked androgen receptor (AR) gene in DNA from blood samples obtained from control subjects without a personal history of breast or ovarian cancer (n = 735), ovarian cancer patients (n = 313), familial breast cancer patients who did not carry mutations in BRCA1 or BRCA2 (n = 235), and affected and unaffected carriers of mutations in BRCA1 (n = 260) or BRCA2 (n = 63). We defined the pattern of X chromosome inactivation as skewed when the same X chromosome was active in at least 90% of cells. The association between skewed X inactivation and disease and/or BRCA mutation status was assessed by logistic regression analysis. The association between skewed X inactivation and age at cancer diagnosis was assessed by Cox proportional hazards regression analysis. All statistical tests were two-sided.

RESULTS

The age-adjusted frequency of skewed X inactivation was not statistically significantly higher in ovarian cancer or familial breast cancer case subjects compared with control subjects. Skewed X inactivation was higher in BRCA1 mutation carriers than in control subjects (odds ratio [OR] = 2.7, 95% confidence interval [CI] = 1.1 to 6.2; P = .02), particularly among unaffected women (OR = 6.1, 95% CI = 1.5 to 31.8; P = .005). Among BRCA1 mutation carriers, those with skewed X inactivation were older at diagnosis of breast or ovarian cancer than those without skewed X inactivation (hazard ratio [HR] of breast or ovarian cancer = 0.37, 95% CI = 0.14 to 0.95; P = .04). Among BRCA2 mutation carriers, skewed X inactivation also occurred more frequently in unaffected carriers than in those diagnosed with breast or ovarian cancer (OR = 5.2, 95% CI = 0.5 to 28.9; P = .08) and was associated with delayed age at onset (HR = 0.59, 95% CI = 0.37 to 0.94; P = .03).

CONCLUSIONS

Skewed X inactivation occurs at an increased frequency in BRCA1 (and possibly BRCA2) mutation carriers compared with control subjects and is associated with a statistically significant increase in age at diagnosis of breast and ovarian cancer.

Monozygotic twins reveal germline contribution to allelic expression differences

Variation in the level of gene expression is a major determinant of a cell's function and characteristics. Common allelic variants of genes can be expressed at different levels and thus contribute to phenotypic diversity. We have measured allelic expression differences at heterozygous loci in monozygotic twins and in unrelated individuals. We show that the extent of differential allelic expression is highly similar within monozygotic twin pairs for many loci, implying that allelic differences in gene expression are under genetic control. We also show that even subtle departures from equal allelic expression are often genetically determined.

No evidence that skewing of X chromosome inactivation patterns is transmitted to offspring in humans

Skewing of X chromosome inactivation (XCI) can occur in normal females and increases in tissues with age. The mechanisms underlying skewing in normal females, however, remain controversial. To better understand the phenomenon of XCI in nondisease states, we evaluated XCI patterns in epithelial and hematopoietic cells of over 500 healthy female mother-neonate pairs. The incidence of skewing observed in mothers was twice that observed in neonates, and in both cohorts, the incidence of XCI was lower in epithelial cells than hematopoietic cells. These results suggest that XCI incidence varies by tissue type and that age-dependent mechanisms can influence skewing in both epithelial and hematopoietic cells. In both cohorts, a correlation was identified in the direction of skewing in epithelial and hematopoietic cells, suggesting common underlying skewing mechanisms across tissues. However, there was no correlation between the XCI patterns of mothers and their respective neonates, and skewed mothers gave birth to skewed neonates at the same frequency as nonskewed mothers. Taken together, our data suggest that in humans, the XCI pattern observed at birth does not reflect a single heritable genetic locus, but rather corresponds to a complex trait determined, at least in part, by selection biases occurring after XCI.

Turner syndrome and the evolution of human sexual dimorphism

Turner syndrome is caused by loss of all or part of an X chromosome in females. A series of recent studies has characterized phenotypic differences between Turner females retaining the intact maternally inherited versus paternally inherited X chromosome, which have been interpreted as evidence for effects of X-linked imprinted genes. In this study I demonstrate that the differences between Turner females with a maternal X and a paternal X broadly parallel the differences between males and normal females for a large suite of traits, including lipid profile and visceral fat, response to growth hormone, sensorineural hearing loss, congenital heart and kidney malformations, neuroanatomy (sizes of the cerebellum, hippocampus, caudate nuclei and superior temporal gyrus), and aspects of cognition. This pattern indicates that diverse aspects of human sex differences are mediated in part by X-linked genes, via genomic imprinting of such genes, higher rates of mosaicism in Turner females with an intact X chromosome of paternal origin, karyotypic differences between Turner females with a maternal versus paternal X chromosome, or some combination of these phenomena. Determining the relative contributions of genomic imprinting, karyotype and mosaicism to variation in Turner syndrome phenotypes has important implications for both clinical treatment of individuals with this syndrome, and hypotheses for the evolution and development of human sexual dimorphism.

Thyroid Epigenetics: X Chromosome Inactivation in Patients with Autoimmune Thyroid Disease

The autoimmune thyroid diseases (AITDs) are female-predominant diseases with a ratio of approximately seven females to each male. X chromosome inactivation (XCI), an epigenetic phenomenon, has been suggested to be skewed in many such female patients with AITD. We analyzed female genomic DNA from 87 patients with Graves' disease (GD), 47 patients with Hashimoto's thyroiditis (HT), and 69 healthy controls. Using an XCI assay based on Hpa II digestion and PCR and DNA sequencing, we found skewed heterozygous XCI (>or=80%) in 20 of 70 GD patients (28.6%) and 11 of 43 HT patients (25.6%), giving a total of 31 of 113 AITD patients (27.4%) with skewed XCI. In contrast, only 5 of 58 healthy controls had skewed XCI (8.6%). Statistical analysis confirmed that XCI skewing was significantly associated with AITD (P = 0.004, OR = 4.0), demonstrating that the degree of XCI is an important contributor to the increased risk of females in developing AITD.

Heritable skewed X-chromosome inactivation leads to haemophilia A expression in heterozygous females

Factor VIII gene, F8, mutations cause haemophilia A (HA), an X-linked recessive disorder. Expression in heterozygous females has been ascribed to skewed X-chromosome inactivation (XCI). To investigate the cause of HA in three heterozygous females within an Atlantic Canadian kindred, the proband (severely affected girl, FVIII activity: 2%) and 17 relatives across three generations were studied. F8 genotype, FVIII activity, XCI ratio (XCIR) (paternal active X: maternal active X), karyotype, submegabase resolution tiling set array competitive genome hybridization (competitive genomic hybridization (SMRT)), and microsatellite analyses were utilized. A positive linear relationship between FVIII activity and percentage-activated normal X-chromosome was found in HA heterozygous females (R(2)=0.87). All affected, but no unaffected females, had an XCIR skewed toward activation of the mutant X-chromosome (proband 92:8, SD 2). Unexpectedly, high numbers of females have dramatically skewed XCIRs (>80:20 or <20:80) (P<0.05). The distribution of XCIR frequencies within this family was significantly different than predicted by normal population data or models of random XCI (P<0.025), with more females having higher degrees of skewing. Known causes of skewing, such as chromosomal abnormalities, selection against deleterious alleles, and X-inactive-specific transcript mutations, are not consistent with our results. This study shows that FVIII activity in HA heterozygous females can be directly related to XCI skewing, and that low FVIII activity in females in this family is due to unfavourable XCI skewing. Further, the findings suggest that these XCI ratios are genetically influenced, consistent with a novel heritable human X controlling element (XCE) functioning similarly to the mouse Xce.

A locus for familial skewed X chromosome inactivation maps to chromosome Xq25 in a family with a female manifesting Lowe syndrome

In mammals, X-linked gene products can be dosage compensated between males and females by inactivation of one of the two X chromosomes in the developing female embryos. X inactivation choice is usually random in embryo mammals, but several mechanisms can influence the choice determining skewed X inactivation. As a consequence, females heterozygous for X-linked recessive disease can manifest the full phenotype. Herein, we report a family with extremely skewed X inactivation that produced the full phenotype of Lowe syndrome, a recessive X-linked disease, in a female. The X chromosome inactivation studies detected an extremely skewed inactivation pattern with a ratio of 100:0 in the propositus as well as in five out of seven unaffected female relatives in four generations. The OCRL1 "de novo" mutation resides in the active paternally inherited X chromosome. X chromosome haplotype analysis suggests the presence of a locus for the familial skewed X inactivation in chromosome Xq25 most likely controlling X chromosome choice in X inactivation or cell proliferation. The description of this case adds Lowe syndrome to the list of X-linked disorders which may manifest the full phenotype in females because of the skewed X inactivation.

X chromosome-inactivation patterns of 1,005 phenotypically unaffected females

X-chromosome inactivation is widely believed to be random in early female development and to result in a mosaic distribution of cells, approximately half with the paternally derived X chromosome inactive and half with the maternally derived X chromosome inactive. Significant departures from such a random pattern are hallmarks of a variety of clinical states, including being carriers for severe X-linked diseases or X-chromosome cytogenetic abnormalities. To evaluate the significance of skewed patterns of X inactivation, we examined patterns of X inactivation in a population of >1,000 phenotypically unaffected females. The data demonstrate that only a very small proportion of unaffected females show significantly skewed inactivation, especially during the neonatal period. By comparison with this data set, the degree of skewed inactivation in a given individual can now be quantified and evaluated for its potential clinical significance.

Epigenetic modification of the X chromosome influences susceptibility to polycystic ovary syndrome

CONTEXT

The cause of polycystic ovary syndrome (PCOS) is unknown, although genetic and environmental influences are clearly implicated. Some genetic studies have suggested the involvement of X-linked genes in PCOS, but the influence of X chromosome inactivation (XCI) on manifestation of this disorder has not previously been examined.

OBJECTIVE

The objective of the study was to test the null hypothesis that XCI has no influence on clinical presentation of PCOS.

DESIGN

We examined patterns of XCI between sister pairs with the same genotype at a polymorphic locus on the X chromosome in families with PCOS.

SETTING

The study was conducted at a private practice.

PARTICIPANTS

PCOS was defined as hyperandrogenemia with chronic anovulation. Forty families were studied in which DNA was obtained from at least one parent, the proband, and one sister that could be accurately diagnosed as being affected or unaffected.

MAIN OUTCOME MEASURE(S)

Relative expression of two X-linked alleles was determined, and the ratio of one to the other represented the pattern of XCI.

RESULTS

The statistical odds on a different clinical presentation between sisters was approximately 29 times higher in sister pairs with different patterns of XCI, compared with sister pairs with the same pattern of XCI (odds ratio 28.9; 95% confidence interval 4.0-206; P = 0.0008).

CONCLUSIONS

This study provides evidence to refute the null hypothesis and propose a closer inspection of X-linked genes in PCOS, one in which both genotype and epigenotype are considered. Environmental determinants of PCOS may alter clinical presentation via epigenetic modifications, which currently remain undetected in traditional genetic analyses.

Age-associated skewing of X-inactivation ratios of blood cells in normal females: a candidate-gene analysis approach

X-inactivation is a random process that occurs in females early during embryogenesis. Females are mosaics with an equal proportion of cells with the paternal (Xp) or maternal X-chromosome (Xm) in the active state. However, close to 40% of healthy females aged more than 60 y.o. present a significant skewing of X-inactivation ratios (Xp:Xm >3 :1). The exact etiology of this age-associated skewing (AAS) in blood cells is unknown. We hypothesized that AAS is due to hemizygous cell selection caused by allelic variants in hematopoiesis or cell survival genes. To test this hypothesis, we recruited 700 unrelated healthy females of French Canadian ancestry aged more than 60. We determined X-inactivation ratio at the HUMARA locus. We genotyped 81 different SNPs, using TaqMan technology, in 15 different candidate genes with known role in hematopoiesis, cell cycle, or X-inactivation. Extensive statistical analyses were conducted and demonstrated that none of the 15 candidate genes investigated contribute significantly to AAS.

Twin study of genetic and aging effects on X chromosome inactivation

To investigate the genetic influence on X chromosome inactivation and on age-related skewing of X inactivation, in particular, we analysed the X inactivation pattern (XIP) in peripheral blood cells from 118 young monozygotic (MZ) twin pairs (18-53 years), 82 elderly MZ twin pairs (55-94 years), 146 young dizygotic (DZ) twin pairs (20-54 years) and 112 elderly DZ twin pairs (64-95 years). Elderly twins had a higher frequency of skewed X inactivation (34%) than young twins (15%) (P<0.001). Our data suggest that the increase in skewing occurs after age 50-60 years. The intraclass correlation was 0.61 and 0.58 in young and elderly MZ twin pairs, and 0.08 and 0.09 in young and elderly DZ twin pairs. Biometric analysis showed that dominant genetic effects accounted for 63 and 58% of the variance of XIP in the young and elderly twin pairs, respectively. The dominant genetic effect and the shared environment for monochorionic MZ twins may explain the high intraclass correlation for the MZ twin pairs compared to the DZ twin pairs. We did not observe a significant decrease in the intraclass correlation in elderly MZ twins compared to young MZ twins, which would be expected if age-related skewing were due to stochastic factors. We conclude that the increased skewing with age implies that a genetically dependent selection of blood cells take place.

Familial nonrandom inactivation linked to the X inactivation centre in heterozygotes manifesting haemophilia A

A basic tenet of the Lyon hypothesis is that X inactivation occurs randomly with respect to parental origin of the X chromosome. Yet, nonrandom patterns of X inactivation are common - often ascertained in women who manifest recessive X-linked disorders despite being heterozygous for the mutation. Usually, the cause of skewing is cell selection disfavouring one of the cell lineages created by random X inactivation. We have identified a three generation kindred, with three females who have haemophilia A because of extreme skewing of X inactivation. Although they have both normal and mutant factor VIII (FVIII) alleles, only the mutant one is transcribed; and, they share an XIST allele that is never transcribed. The skewing in this case seems to result from an abnormality in the initial choice process, which prevents the chromosome bearing the mutant FVIII allele from being an inactive X.

A longitudinal study of X-inactivation ratio in human females

We investigated the effect of aging on X chromosome inactivation by performing a longitudinal study in a population of 178 normal females. We examined X-inactivation ratios (fraction of cells with the same X chromosome active) in two sets of peripheral blood DNA samples collected about two decades apart. We observed a strong correlation between the ratios of individual females at the two time points and found no significant difference between the two sets of measurements. These observations indicate that aging, per se (as opposed to being "aged"), has little effect on X-inactivation. However, we also found that several females who were older than 60 years of age at the time of the first measurement acquired significant changes in the X-inactivation ratio. We speculate that, if X-inactivation skewing is a frequently acquired trait in older females, it is acquired as the result of a discontinuous or catastrophic process and is not the result of constant selection for or against hematopoietic stem cells with a particular X chromosome active.

A survey of genetic and epigenetic variation affecting human gene expression

The identification of human sequence polymorphisms that regulate gene expression is key to understanding human genetic diseases. We report a survey of human genes that demonstrate allelic differences in gene expression, reflecting the presence of putative allele-specific cis-acting factors of either genetic or epigenetic nature. The expression of allelic transcripts in heterozygous samples is assessed directly by relative quantitation of intragenic marker alleles in messenger or heteronuclear RNA derived from cells or tissues. This survey used 193 single-nucleotide polymorphisms (SNPs) from 129 genes expressed in lymphoblastoid cell lines, to identify 23 genes (18%) with common allele-specific transcripts whose expression deviated from the expected equimolar ratio. A subset of these deviations, or "allelic imbalances," can be observed in multiple samples derived from reference CEPH ("Centre d'Etude du Polymorphisme Humain") pedigrees and demonstrate a spectrum of patterns of transmission, including cosegregation of allelic skewing across generations compatible with Mendelian inheritance as well as random monoallelic expression for three genes (IL1A, HTR2A, and FGB). Additional studies for BTN3A2 provide evidence of SNPs and haplotypes in complete linkage disequilibrium with high- and low-expressing transcripts. The pipeline described herein offers tools for efficient identification and characterization of allelic expression allowing identification of regulatory sequence variants as well as epigenetic variation affecting human gene expression.

Functional intergenic transcription: a case study of the X-inactivation centre

Long known to be riddled with repetitive elements and regarded as 'junk', intergenic regions in the mammalian genome now appear to be more than incidental spacers between coding sequences. Here, I review the example of Xite, an intergenic region at the X-inactivation centre which was recently shown to regulate the X-chromosome choice decision. Xite contains a series of DNaseI-hypersensitive sites and harbours two intergenic transcription start sites. These intergenic transcription elements act at the onset of X-chromosome inactivation (XCI) to bias the selection of the active X. It has been proposed that Xite acts in cis on Tsix by promoting its persistence during XCI. Xite has also been proposed to be a candidate for the X-controlling element, a naturally occurring modifier of XCI ratios in mice and possibly also in humans. It seems likely that intergenic transcription will turn out to be a widespread phenomenon in mammals and that, more importantly, it will emerge as a significant regulatory mechanism for the expression of coding sequences.

Fragile-X syndrome and skewed X-chromosome inactivation within a family: a female member with complete inactivation of the functional X chromosome

Fragile X syndrome is the most common form of inherited mental retardation. It is caused by the increase in length of a stretch of CGG triplet repeats within the FMR1 gene. A full mutation (> 200 repeats) leads to methylation of the CpG island and silencing of the FMR1 gene. We present here two sisters that are compound heterozygotes for a full mutation and a 53 repeat intermediate allele, one of them showing mental retardation and clinical features of an affected male (speech delay, hyperactivity, large ears, prominent jaw, gaze aversion), while the other is borderline normal (mild delay). Southern blot and FMRP expression analysis showed that the sister with mental retardation had the normal FMR1 gene totally methylated and no detectable protein, while her sister had 70% of her cells with the normal FMR1 gene unmethylated and normal FMRP levels. We found that the observed phenotypic differences between both sisters who are cytogenetically normal, are caused by extreme skewed X-chromosome inactivation. Analysis of the extended family showed that most of the other female family members that carry a pre-mutation or a full mutation showed some degree of skewing in their X-chromosome inactivation. The presence of several family members with skewed X inactivation and the direction and degree of skewing is inconsistent with a mere selection during development, and suggests a genetic origin for this phenomenon.

X-inactivation patterns in carriers of X-linked myotubular myopathy

X-linked myotubular myopathy is a rare severe muscle disorder in affected male neonates. Most female carriers are free from symptoms. Skewed X inactivation has been proposed to be responsible for the affected phenotype seen in some carriers. We have compared the X inactivation patterns in blood DNA with the clinical phenotype in carriers of X-linked myotubular myopathy. The X-inactivation analysis was performed using HpaII predigestion of DNA followed by polymerase chain reaction of the highly polymorphic CAG repeat of the androgen receptor (AR) gene. The frequency of skewed X inactivation was similar in the X-linked myotubular myopathy carriers (22%) and in 235 controls (18%). Three overtly affected carriers had skewed X inactivation with the mutated X as the predominantly active X in at least two of them. Four females with mild symptoms had random X inactivation. The unaffected X-linked myotubular myopathy carriers had either skewed X inactivation in favour of expression from the normal X or random X-inactivation. Thus, there was a tendency for females with a more severe phenotype to have a skewed pattern of X inactivation, while females with an intermediate phenotype had a random pattern of X-inactivation.

Xite, X-inactivation intergenic transcription elements that regulate the probability of choice

Allelic expression differences contribute to phenotypic variation. In X chromosome inactivation (XCI), unfavorable XCI ratios promote X-linked disease penetrance in females. During XCI, one X is randomly silenced by Xist. X chromosome choice is determined by asymmetric expression of Tsix whose antisense action represses Xist. Here, we discover a cis element in the mouse X-inactivation center that regulates Tsix. Xite harbors intergenic transcription start sites and DNaseI hypersensitive sites with allelic differences. At the onset of XCI, deleting Xite downregulates Tsix in cis and skews XCI ratios, suggesting that Xite promotes Tsix persistence on the active X. Truncating Xite RNA is inconsequential, indicating that Xite action does not require intact transcripts. We propose that allele-specific Xite action promotes Tsix asymmetry and generates X chromosome inequality. Therefore, Xite is a candidate for the Xce, the classical modifier of XCI ratios.

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.

The causes and consequences of random and non-random X chromosome inactivation in humans

X chromosome (X) inactivation is a remarkable biological process including the choice and cis-limited inactivation of one X, as well as the stable maintenance of this silencing by epigenetic chromatin alterations. The process results in females generally being mosaic for two populations of cells--one with each parental X active. In this review, we discuss recent advances in our understanding of how inactivation works, as well as the causes and clinical implications of deviations from random inactivation.

Report of 33 novel AVPR2 mutations and analysis of 117 families with X-linked nephrogenic diabetes insipidus

X-linked nephrogenic diabetes insipidus (NDI) is a rare disease caused by mutations in the arginine vasopressin receptor 2 gene (AVPR2). Thirty-three novel AVPR2 mutations were identified in 62 families that were not included in our previous studies. This study describes the diversity of mutations observed in a total of 117 families, the number of affected people at the time of diagnosis, skewed X chromosome inactivation in severely affected females, the inferred parental origin of de novo mutations, and it provides estimates of incidence. Among 117 families, there were 82 different putative disease-causing mutations. Based on haplotype analysis, it can be inferred that when the same AVPR2 mutation is identified in different families that were not known to be related, the mutations most likely arose independently. More than half of the families had only one affected male; two families presented with a severely affected female and no family history of NDI. A de novo mutation arose during oogenesis in the mother in 20% of isolated cases. The estimate of about 8.8 per million male live births of the incidence of X-linked NDI in the province of Quebec, Canada may be representative of the general population except in Nova Scotia and New Brunswick, where the incidence is more than six times higher. Documentation of the diversity of mutations will assist in revealing the full spectrum of clinical variation. Discussion of genetic and population genetic aspects of X-linked NDI may contribute to early diagnosis and treatment.

X-linked genetic factors regulate hematopoietic stem-cell kinetics in females

X inactivation makes females mosaics for 2 cell populations, usually with an approximate 1:1 distribution. Skewing of this distribution in peripheral blood cells is more common among elderly women.1–3 The depletion of hematopoietic stem cells followed by random differentiation may explain the acquired skewing with age.4 However, an animal model suggests that selection processes based on X-linked genetic factors are involved.5 We studied peripheral blood cells from 71 monozygotic twin pairs aged 73 to 93 years and from 33 centenarians, and we found that with age, 1 of the cell populations becomes predominant for most women. We also observed a strong tendency for the same cell line to become predominant in 2 co-twins. This suggests that X-linked genetic factors influence human hematopoietic stem cell kinetics. The fact that females have 2 cell lines with different potentials could be one of the reasons women live longer than men.

X-linked genetic factors regulate hematopoietic stem-cell kinetics in females

X inactivation makes females mosaics for 2 cell populations, usually with an approximate 1:1 distribution. Skewing of this distribution in peripheral blood cells is more common among elderly women.1–3 The depletion of hematopoietic stem cells followed by random differentiation may explain the acquired skewing with age.4 However, an animal model suggests that selection processes based on X-linked genetic factors are involved.5 We studied peripheral blood cells from 71 monozygotic twin pairs aged 73 to 93 years and from 33 centenarians, and we found that with age, 1 of the cell populations becomes predominant for most women. We also observed a strong tendency for the same cell line to become predominant in 2 co-twins. This suggests that X-linked genetic factors influence human hematopoietic stem cell kinetics. The fact that females have 2 cell lines with different potentials could be one of the reasons women live longer than men.

Sex-based differences in gene transmission and gene expression

The higher prevalence of certain diseases among women suggests involvement of genetic mechanisms linked to the sex chromosomes or of sex-limited gene expression that may be developmentally or hormonally regulated. Analysis of genetic markers and gene expression patterns provides the means for testing hypotheses related to these mechanisms.

Heterogeneous X inactivation in trophoblastic cells of human full-term female placentas

In female mammalian cells, one of the two X chromosomes is inactivated to compensate for gene-dose effects, which would be otherwise doubled compared with that in male cells. In somatic lineages in mice, the inactive X chromosome can be of either paternal or maternal origin, whereas the paternal X chromosome is specifically inactivated in placental tissue. In human somatic cells, X inactivation is mainly random, but both random and preferential paternal X inactivation have been reported in placental tissue. To shed more light on this issue, we used PCR to study the methylation status of the polymorphic androgen-receptor gene in full-term human female placentas. The sites investigated are specifically methylated on the inactive X chromosome. No methylation was found in microdissected stromal tissue, whether from placenta or umbilical cord. Of nine placentas for which two closely apposed samples were studied, X inactivation was preferentially maternal in three, was preferentially paternal in one, and was heterogeneous in the remaining five. Detailed investigation of two additional placentas demonstrated regions with balanced (1:1 ratio) preferentially maternal and preferentially paternal X inactivation. No differences in ratio were observed in samples microdissected to separate trophoblast and stromal tissues. We conclude that methylation of the androgen receptor in human full-term placenta is specific for trophoblastic cells and that the X chromosome can be of either paternal or maternal origin.