Extreme skewing of X chromosome inactivation in mothers of homosexual men

Human sexual preference is a sexually dimorphic trait with a substantial genetic component. Linkage of male sexual orientation to markers on the X chromosome has been reported in some families. Here, we measured X chromosome inactivation ratios in 97 mothers of homosexual men and 103 age-matched control women without gay sons. The number of women with extreme skewing of X-inactivation was significantly higher in mothers of gay men (13/97=13%) compared to controls (4/103=4%) and increased in mothers with two or more gay sons (10/44=23%). Our findings support a role for the X chromosome in regulating sexual orientation in a subgroup of gay men.     

Increased skewing of X chromosome inactivation with age in both blood and buccal cells

The X chromosome inactivation pattern in peripheral blood cells becomes more skewed after age 55, and a genetic effect on this age-related skewing has been reported. We investigated the effect of age on X inactivation phenotype in blood, buccal cells and tissue from duodenal biopsies in 80 females aged 19-90 years. The X inactivation pattern correlated positively with age in blood (r = 0.238, P = 0.034) and buccal cells (r = 0.260, P = 0.02). The mean degree of skewing was higher in the elderly (>/=55 years) than in the young (<55 years) in blood (70.1 and 63.5%, respectively, P = 0.013) and in buccal cells (64.7 and 59.0%, respectively, P = 0.004). Correlation of X inactivation between the different tissues was high in all tissues with a tendency to increase with age for blood and buccal cells (P = 0.082). None of the duodenal biopsies had a skewed X inactivation, and the mean degree of skewing was similar in the two age groups. The tendency for the same X chromosome to be the preferentially active X in both blood and buccal cells with advancing age is in agreement with a genetic effect on age-related skewing and indicates that genes other than those involved in hematopoiesis should be investigated in the search for genes contributing to age related skewing.     

Random X-chromosome inactivation: skewing lessons for mice and men

The mammalian X-chromosome exists in two flavors, active and inactive, in each cell of the adult female. This phenomenon originates from the process of random choice occurring early in development in a small number of progenitor cells in which the decision is made to inactivate either one or the other X chromosome on a cell-autonomous basis. Once made, this initial decision is irreversible, although exceptions exist in specific chromosomal territories and cell lineages. Recent findings implicate various factors, including non-coding RNAs and chromatin modification complexes, as effectors in the initiation and maintenance of X-chromosome inactivation. The functional redundancy of such factors almost certainly plays an important role in the stability of the inactive X. Studying skewing or bias opens an important opportunity for understanding facets of the random choice process.     

X inactivation Xplained

Random inactivation of one of the two female X chromosomes establishes dosage compensation between XY males and XX females in placental mammals. X inactivation is controlled by the X inactivation center (Xic). Recent advances in genome sequencing show that the Xic has evolved from an ancestral vertebrate gene cluster in placental mammals and has undergone separate rearrangements in marsupials. The Xic ensures that all but one X chromosome per diploid genome are inactivated. Which chromosome remains active is randomly chosen. Pairing of Xic loci on the two X chromosomes and alternate states of the X chromosomes before inactivation have recently been implicated in the mechanism of random choice. Chromosome-wide silencing is then initiated by the noncoding Xist RNA, which evolved with the mammalian Xic and covers the inactive X chromosome.     

Fragile X expression and X inactivation

Summary The major concept of fragile X pathogenesis postulates that the fragile site at band Xq27.3 [fra(X)] represents the primary defect. The expression of fra(X) is predicted to be an intrinsic property of the mutated chromosome and, hence, should not be suppressed by X inactivation in females or induced by X-linked trans-acting factors. We made fibroblast clones of a fra(X)-positive female. Monoclonality was demonstrated using the DNA methylation assay at DXS255. The mutated X chromosomes and their states of genetic activity in the different clones were also defined by molecular methods. Five clones were selected to induce expression of fra(X) by 10^-7 M FUdR; two carried an active mutated X chromosome, in the other three the mutated X chromosome was inactivated. Fra(X) was found expressed in both types of clones. The percentages of positive cells were as high as 7–10%, regardless of the genetic activity of the mutated X chromosomes. DNA replicating patterns, obtained by BUdR labelling, demonstrated that expression occurred only on the mutated X chromosomes previously identified by molecular methods. The concept that the fragile site represents the primary mutation is now strongly supported by experimental evidence. The expression of fra (X) in females is independent of X inactivation and other trans-acting factors.     

X chromosome inactivation: theme and variations

My contribution to this special issue on Vertebrate Sex Chromosomes deals with the theme of X chromosome inactivation and its variations. I will argue that the single active X--characteristic of mammalian X dosage compensation--is unique to mammals, and that the major underlying mechanism(s) must be the same for most of them. The variable features reflect modifications that do not interfere with the basic theme. These variations were acquired during mammalian evolution--to solve special needs for imprinting and locking in the inactive state. Some of the adaptations reinforce the basic theme, and were needed because of species differences in the timing of interacting developmental events. Elucidating the molecular basis for the single active X requires that we distinguish the mechanisms essential for the basic theme from those responsible for its variations.     

X chromosome inactivation: heterogeneity of heterochromatin

The silent X chromosome in mammalian females is a classic example of facultative heterochromatin, the term highlighting the compacted and inactive nature of the chromosome. However, it is now clear that the heterochromatin of the inactive X is not homogeneous--as indeed, not all genes on the inactive X are silenced. We summarize known features and events of X inactivation in different mouse and human model systems, and highlight the heterogeneity of chromatin along the inactive X. Characterizing this heterogeneity is likely to provide insight into the cis-acting sequences involved in X chromosome inactivation.     

Escape from X inactivation

Although the process of X inactivation in mammalian cells silences the majority of genes on the inactivated X chromosome, some genes escape this chromosome-wide silencing. Genes that escape X inactivation present a unique opportunity to study the process of silencing and the mechanisms that protect some genes from being turned off. In this review, we will discuss evolutionary aspects of escape from X inactivation, in relation to the divergence of the sex chromosomes. Molecular characteristics, expression, and epigenetic modifications of genes that escape will be presented, including their developmental regulation and the implications of chromatin domains along the X chromosome in modeling the escape process.     

X inactivation and disease

X inactivation is the mechanism by which mammals adjust the X-linked gene dosage between the sexes. The dosage difference between XX females and XY males is functionally equalized by silencing one of the two X chromosomes in female cells. This dosage-compensation mechanism is based on the long functional Xist RNA. Here, we review our understanding of dosage compensation and Xist function in the context of disease.     

The process of X inactivation in the mouse

X chromosome inactivation (XCI) is an excellent model for studying how epigenetic marks are initiated during early embryogenesis. XCI is an essential process that takes place in females, leading to dosage compensation between males and females. In mouse, it occurs in two waves: the first one is paternally imprinted, during the preimplantation period and the second one occurs in a random fashion. We provide here an update of the main molecular steps and hypothesis underlining this complex process.     

Silence of the fathers: early X inactivation

X chromosome inactivation is the mammalian answer to the dilemma of dosage compensation between males and females. The study of this fascinating form of chromosome-wide gene regulation has yielded surprising insights into early development and cellular memory. In the past few months, three papers reported unexpected findings about the paternal X chromosome (X(p)). All three studies agree that the X(p) is imprinted to become inactive earlier than ever suspected during embryonic development. Although apparently incomplete, this early form of inactivation insures dosage compensation throughout development. Silencing of the X(p) persists in cells of extraembryonic tissues, but it is erased and followed by random X inactivation in cells of the embryo proper. These findings challenge several aspects of the current view of X inactivation during early development and may have profound impact on studies of pluripotency and epigenetics.     

X-changing information on X inactivation

In female somatic cells of mammalian species one X chromosome is inactivated to ensure dosage equality of X-encoded genes between females and males, during development and adulthood. X chromosome inactivation (XCI) involves various epigenetic mechanisms, including RNA mediated gene silencing in cis, DNA methylation, and changes in chromatin modifications and composition. XCI therefore provides an attractive paradigm to study epigenetic gene regulation in a more general context. The XCI process starts with counting of the number of X chromosomes present in a nucleus, and initiation of XCI follows if this number exceeds one per diploid genome. Recently, X-encoded RNF12 has been identified as a dose-dependent activator of XCI. In addition, other factors, including the pluripotency factors OCT4, SOX2 and Nanog, have been implicated to play a role in suppression of initiation of XCI. In this review, we highlight and explain these new and old findings in the context of a stochastic model for X chromosome counting and XCI initiation.