Skewed X-chromosome inactivation in human miscarriages

The sex ratio in the first trimester of pregnancy shifts toward males due to increased elimination of female embryos. One reason for this phenomenon may be disruption of X chromosome inactivation. In this paper, we have analyzed the nature of the X chromosome inactivation in extraembryonic tissues of induced and spontaneous abortuses with 46,XX karyotype. Both equiprobable and asymmetric inactivation have been found in chorionic cytotrophoblast from spontaneous and induced abortuses. In the extraembryonic mesoderm of the control group of embryos, only equiprobable inactivation has been found, whereas this parameter was shifted in 15% of spontaneous abortions. The highest incidence of the selective inactivation of one of the parent homologues was found in the group with a lack of development of embryos and embryos from women with recurrent miscarriages. One of the reasons for the observed results can be compartmentalization of cells in the blastocyst leading to the nonrandom redistribution of cells and the predominance in the inner mass of cells with an active X chromosome with aberrations incompatible with normal embryonic development.     

X-chromosome inactivation: molecular mechanisms from the human perspective

X-chromosome inactivation is an epigenetic process whereby one X chromosome is silenced in mammalian female cells. Since it was first proposed by Lyon in 1961, mouse models have been valuable tools to uncover the molecular mechanisms underlying X inactivation. However, there are also inherent differences between mouse and human X inactivation, ranging from sequence content of the X inactivation center to the phenotypic outcomes of X-chromosome abnormalities. X-linked gene dosage in males, females, and individuals with X aneuploidies and X/autosome translocations has demonstrated that many human genes escape X inactivation, implicating cis-regulatory elements in the spread of silencing. We discuss the potential nature of these elements and also review the elements in the X inactivation center involved in the early events in X-chromosome inactivation.     

Polymorphic X-chromosome inactivation of the human TIMP1 gene.

X inactivation silences most but not all of the genes on one of the two X chromosomes in mammalian females. The human X chromosome preserves its activation status when isolated in rodent/human somatic-cell hybrids, and hybrids retaining either the active or inactive X chromosome have been used to assess the inactivation status of many X-linked genes. Surprisingly, the X-linked gene for human tissue inhibitor of metalloproteinases (TIMP1) is expressed in some but not all inactive X-containing somatic-cell hybrids, suggesting that this gene is either prone to reactivation or variable in its inactivation. Since many genes that escape X inactivation are clustered, we examined the expression of four genes (ARAF1, ELK1, ZNF41, and ZNF157) within approximately 100 kb of TIMP1. All four genes were expressed only from the active X chromosome, demonstrating that the factors allowing TIMP1 expression from the inactive X chromosome are specific to the TIMP1 gene. To determine if this variable inactivation of TIMP1 is a function of the hybrid-cell environment or also is observed in human cells, we developed an allele-specific assay to assess TIMP1 expression in human females. Expression of two alleles was detected in some female cells with previously demonstrated extreme skewing of X inactivation, indicating TIMP1 expression from the inactive chromosome. However, in other cells, no expression of TIMP1 was observed from the inactive X chromosome, suggesting that TIMP1 inactivation is polymorphic in human females.     

Skewed X-chromosome inactivation in human embryos with mosaic trisomy 16

The sex ratio and X-chromosome inactivation were analyzed in placental tissues of human spontaneous abortuses with pure and mosaic forms of chromosome 16 trisomy. The sex ratio value was found to decrease with an increase in the share of cells with the trisomic karyotype, which suggests differential survival of embryos belonging to different sexes. The pattern of X-chromosome inactivation in cells of extraembryonic mesoderm in the control group of embryos and in spontaneous abortuses with the level of trisomy 16 below 80% corresponded to random X-inactivation, whereas in most embryos with a frequency of trisomy 16 exceeding 80% skewed inactivation was observed. Our results support the hypothesis about the existence of an autosomal transfactor influencing the initiation of X-chromosome inactivation and suggest its possible localization on chromosome 16.     

X-chromosome inactivation and escape

X-chromosome inactivation, which was discovered by Mary Lyon in 1961 results in random silencing of one X chromosome in female mammals. This review is dedicated to Mary Lyon, who passed away last year. She predicted many of the features of X inactivation, for e.g., the existence of an X inactivation center, the role of L1 elements in spreading of silencing and the existence of genes that escape X inactivation. Starting from her published work here we summarize advances in the field.     

X-chromosome inactivation in development and cancer

X-chromosome inactivation represents an epigenetics paradigm and a powerful model system of facultative heterochromatin formation triggered by a non-coding RNA, Xist, during development. Once established, the inactive state of the Xi is highly stable in somatic cells, thanks to a combination of chromatin associated proteins, DNA methylation and nuclear organization. However, sporadic reactivation of X-linked genes has been reported during ageing and in transformed cells and disappearance of the Barr body is frequently observed in cancer cells. In this review we summarise current knowledge on the epigenetic changes that accompany X inactivation and discuss the extent to which the inactive X chromosome may be epigenetically or genetically perturbed in breast cancer.     

Ontogeny of X-chromosome inactivation in the female germ line

Data are presented on G6PD electrophoretic patterns in fetal ovarian preparations of G6PD heterozygotes. The results indicate that in the early or mitotic period of female germ cell development, only a single X chromosome is active in each oogonium just as is the case for X inactivated somatic tissue. However, in the later or meiotic stage, reactivation of the inactive X chromosome in each oocyte occurs so that two functional X chromosomes are present in each oocyte.     

Structural heterochromatin and X-chromosome inactivation]

Our previous studies on the expression of the G6PD and alpha-GAL genes from the X chromosome of inter-specific hybrids of voles of the Microtus genus have demonstrated an unusual pattern of X-inactivation in the parents. The observed phenomenon was explained as the presumable result of nonrandom inactivation of the X chromosomes with a heterochromatin block in crosses involving Microtus arvalis whose X lacks a heterochromatin region and also of random X inactivation when both parents had heterochromatin blocks on the Xs. Based on known models, we discuss here the possible mechanisms of the effect of heterochromatin on X-inactivation; we give preference to the model postulating binding of nonhistone protein to the inactivation centre as the key event. The hypothesis we offer suggests change in chromatin conformation in the inactivation centre during packaging of heterochromatic region of a chromosome; the protein molecules diffusing along the chromosome towards the heterochromatin region by the "facilitated diffusion" mechanism may happen to be in the region of the X-inactivation centre, which, being in a favorable state, binds specifically to it; as a consequence, the binding probability of protein to heterochromatin increases as compared to chromosome without heterochromatin block.     

X-chromosome inactivation and cell memory.

Mammalian X-chromosome inactivation is an excellent example of the faithful maintenance of a determined chromosomal state. As such, it may provide insight into the mechanisms for cell memory, defined as the faithful maintenance of a determined state in clonally derived progeny cells. We review here the aspects of X-chromosome inactivation that are relevant to cell memory and discuss the various molecular mechanisms that have been proposed to explain its occurrence, with emphasis on DNA methylation and a recently proposed mechanism that depends on the timing of replication.     

New lessons from random X-chromosome inactivation in the mouse

X-chromosome inactivation (XCI) ensures dosage compensation in mammals. Random XCI is a process where a single X chromosome is silenced in each cell of the epiblast of mouse female embryos. Operating at the level of an entire chromosome, XCI is a major paradigm for epigenetic processes. Here we review the most recent discoveries concerning the role of long noncoding RNAs, pluripotency factors, and chromosome structure in random XCI.     

The search for the mouse X-chromosome inactivation centre

The phenomenon of X-chromosome inactivation in female mammals, whereby one of the two X chromosome present in each cell of the female embryo is inactivated early in development, was first described by Mary Lyon in 1961. Nearly 30 years later, the mechanism of X-chromosome inactivation remains unknown. Strong evidence has accumulated over the years, however, for the involvement of a major switch or inactivation centre on the mouse X chromosome. Identification of the inactivation centre at the molecular level would be an important step in understanding the mechanism of X-inactivation. In this paper we review the evidence for the existence and location of the X-inactivation centre on the mouse X-chromosome, present data on the molecular genetic mapping of this region, and describe ongoing strategies we are using to attempt to identify the inactivation centre at the molecular level.