Paternally biased cpDNA inheritance in Turnera ulmifolia (Turneraceae)

We end-labeled Hin fI restriction digests of a PCR-amplified plastid encoded gene, the large subunit of ribulose bisphosphate carboxylase, to investigate patterns of cpDNA inheritance in Turnera ulmifolia. A total of 70 progeny from crosses among plants taken from ten populations revealed varying patterns of inheritance. A majority of progeny inherited the paternal cpDNA (64%), while 19% exhibited maternal and 17% biparental inheritance. Eight variegated progeny showed biparental inheritance and were analyzed in greater detail. We extracted and analyzed the cpDNA content of light- vs. dark- green leaf sectors from these plants. The results showed that vegetative segregation of cpDNA had occurred for seven of the eight plants.     

X chromosome inactivation mosaicism in the mouse

A cytologically detectable mosaicism resulting from X-chromosome inactivation occurring in mice heterozygous for Cattanach's translocation has been used to examine the time of X chromosome inactivation, and the sizes of primordial precursor pools for lung, thymus, spleen, fascia, and melanocytes. The extent of covariance in mosaic composition among tissues within individuals suggests that, if X inactivation occurs randomly, it must occur after determination of embryoblast cells, at some time immediately before or after implantation, and that it must occur before divergence of mesoderm from ectoderm. The extent of independent variance among the various tissues is such as to suggest that none of them arise from primordial precursor pools smaller than 20 to 30 cells.     

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.     

X chromosome inactivation in human and mouse pluripotent stem cells

Since the groundbreaking hypothesis of X chromosome inactivation (XCI) proposed by Mary Lyon over 50 years ago, a great amount of knowledge has been gained regarding this essential dosage compensation mechanism in female cells. For the mammalian system, most of the mechanistic studies of XCI have so far been investigated in the mouse model system, but recently, a number of interesting XCI studies have been extended to human pluripotent stem cells, including both embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs). Emerging data indicate that XCI in hESCs and hiPSCs is much more complicated than that of their mouse counterparts. XCI in human pluripotent stem cells is not as stable and is subject to environmental influences and epigenetic regulation in vitro. This mini-review highlights the key differences in XCI between mouse and human stem cells with a greater emphasis placed on the understanding of the epigenetic regulation of XCI in human stem cells.     

X inactivation in the mouse embryo deficient for Dnmt1: distinct effect of hypomethylation on imprinted and random X inactivation

It has been suggested that DNA methylation plays a crucial role in genomic imprinting and X inactivation. Using DNA methyltransferase 1 (Dnmt1)-deficient mouse embryos carrying X-linked lacZ transgenes, we studied the effects of genomic demethylation on X inactivation. Based on the expression pattern of lacZ, the imprinted X inactivation in the visceral endoderm, a derivative of the extraembryonic lineage, was unaffected in Dnmt1 mutant embryos at the time other imprinted genes showed aberrant expression. Random X inactivation in the embryonic lineage of Dnmt1 mutant embryos, however, was unstable as a result of hypomethylation, causing reactivation of, at least, one lacZ transgene that had initially been repressed. Our results suggest that maintenance of imprinted X inactivation in the extraembryonic lineage can tolerate extensive demethylation while normal levels of methylation are required for stable maintenance of X inactivation in the embryonic lineage.     

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.     

Paternally transmitted chromosomal aberrations in mouse zygotes determine their embryonic fate

The developmental consequences of chromosomal aberrations in embryos include spontaneous abortions, morphological defects, inborn abnormalities, and genetic/chromosomal diseases. Six germ-cell mutagens with different modes of action and spermatogenic stage sensitivities were used to investigate the relationship between the types of cytogenetic damage in zygotes with their subsequent risk of postimplantation death and of birth as a translocation carrier. Independent of the mutagen used, over 98% of paternally transmitted aberrations were chromosome type, rather than chromatid type, indicating that they were formed during the period between exposure of male germ cells and initiation of the first S phase after fertilization. There were consistent one-to-one agreements between the proportions of a) zygotes with unstable aberrations and the frequencies of dead embryos after implantation (slope = 0.87, confidence interval [CI]: 0.74, 1.16) and b) zygotes with reciprocal translocations and the frequency of translocation carriers at birth (slope = 0.74, CI: 0.48, 2.11). These findings suggest that chromosomal aberrations in zygotes are highly predictive of subsequent abnormal embryonic development and that development appears to proceed to implantation regardless of the presence of chromosomal abnormalities. Our findings support the hypothesis that, for paternally transmitted chromosomal aberrations, the fate of the embryo is already set by the end of G1 of the first cell cycle of development.     

X chromosome inactivation revealed by the X-linked lacZ transgene activity in periimplantation mouse embryos

Using H253 mouse stock harboring X-linked HMG-lacZ transgene, we examined X chromosome inactivation patterns in sectioned early female embryos. X-gal staining patterns were generally consistent with the paternal X inactivation in the trophectoderm and the primitive endoderm cell lineages and random inactivation in the epiblast lineages. The occurrence of embryonic visceral endoderm cells apparently at variance with the paternal X chromosome inactivation in 7.5 dpc embryos was explained by the replacement of visceral endoderm cells with cells of epiblast origin. The frequency of cells negative for X-gal staining in 4.5-5.5 dpc XmXp* embryos fluctuated considerably especially in the extraembryonic ectoderm and the primitive endoderm, whereas it was less variable in the embryonic ectoderm. We could not, however, determine whether it is a normal phenomenon revealed for the first time by the use of HMG-lacZ transgene or an abnormality caused by the multicopy transgene.     

X chromosome inactivation in female embryos of a marsupial mouse (Antechinus stuartii)

Blastocysts and late gestation stages of the marsupial mouse, Antechinus stuartii, were examined cytologically and electrophoretically to investigate X chromosome activity during embryogenesis. A late replicating X chromosome was identified in the protoderm cells of female unilaminar blastocysts and in the cells of embryonic and extra-embryonic regions of older blastocysts. Sex chromatin bodies were also observed in female bilaminar and trilaminar blastocysts. The X linked enzyme -galactosidase showed no evidence of paternal allele expression in the extra-embryonic region of bilaminar blastocysts or in the yolk sac and embryonic tissue of known heterozygotes. It is concluded that the late replicating X chromosome is paternal in origin and that unlike the laboratory mouse, X inactivation is not correlated with cell differentiation in Antechinus.     

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.