Viability, development and incidence of chromosome anomalies of preimplantation embryos from XO mice

Reproductive outcomes after preimplantation genetic testing in mosaic Turner syndrome: a retrospective cohort study of 100 cycles

PURPOSE

The purpose of this study is to explore the reproductive outcomes of women with Turner syndrome (TS) in preimplantation genetic testing (PGT) cycles.

METHODS

A retrospective study of 100 controlled ovarian stimulating cycles, 68 TS (sixty-four mosaic Turner syndrome (MTS) and four pure Turner syndrome (PTS)) women underwent PGT was conducted from 2013 to 2018.

RESULTS

Embryo X chromosome abnormal rates of TS women were significantly higher than women with normal karyotype (7.04 vs 1.61%, P<0.01). Cumulative live birth rates (CLBR) after PGT-NGS treatment were lower in TS than control (31.15 vs 45.59%, P<0.05). Clinical pregnancy rates per transfer (CPR), miscarriage rates (MR) and live birth rates per transfer (LBR) remained comparable between TS and control group. Reproductive outcomes (X chromosome abnormal rates, CPR, MR, LBR and CLBR) among low (<10%), medium (10-50%) and high (>50%) level 45,X mosaicism groups were not statistically different.

CONCLUSIONS

To avoid high risk of embryo X chromosome abnormalities, prenatal or preimplantation genetic testing should be recommended to mosaic or pure TS patients.

Premature ovarian insufficiency in the XO female mouse on the C57BL/6J genetic background

In humans, all but 1% of monosomy 45.X embryos die in utero and those who reach term suffer from congenital abnormalities and infertility termed Turner's syndrome (TS). By contrast, XO female mice on various genetic backgrounds show much milder physical defects and normal fertility, diminishing their value as an animal model for studying the infertility of TS patients. In this article, we report that XO mice on the C57BL/6J (B6) genetic background showed early oocyte loss, infertility or subfertility and high embryonic lethality, suggesting that the effect of monosomy X in the female germline may be shared between mice and humans. First, we generated XO mice on either a mixed N2(C3H.B6) or B6 genetic background and compared the number of oocytes in neonatal ovaries; N2.XO females retained 45% of the number of oocytes in N2.XX females, whereas B6.XO females retained only 15% of that in B6.XX females. Second, while N2.XO females were as fertile as N2.XX females, both the frequency of delivery and the total number of pups delivered by B6.XO females were significantly lower than those by B6.XX females. Third, after mating with B6 males, both N2.XO and B6.XO females rarely produced XO pups carrying paternal X chromosomes, although a larger percentage of embryos was found to be XO before implantation. Furthermore, B6.XO females delivered 20% XO pups among female progeny after mating with C3H males. We conclude that the impact of monosomy X on female mouse fertility depends on the genetic background.

Sex Chromosome Effects on Male-Female Differences in Mammals

Fundamental differences exist between males and females, encompassing anatomy, physiology, behaviour, and genetics. Such differences undoubtedly play a part in the well documented, yet poorly understood, disparity in disease susceptibility between the sexes. Although traditionally attributed to gonadal sex hormone effects, recent work has begun to shed more light on the contribution of genetics - and in particular the sex chromosomes - to these sexual dimorphisms. Here, we explore the accumulating evidence for a significant genetic component to mammalian sexual dimorphism through the paradigm of sex chromosome evolution. The differences between the extant X and Y chromosomes, at both a sequence and regulatory level, arose across 166 million years. A functional result of these differences is cell autonomous sexual dimorphism. By understanding the process that changed a pair of homologous ancestral autosomes into the extant mammalian X and Y, we believe it easier to consider the mechanisms that may contribute to hormone-independent male-female differences. We highlight key roles for genes with homologues present on both sex chromosomes, where the X-linked copy escapes X chromosome inactivation. Finally, we summarise current experimental paradigms and suggest areas for developments to further increase our understanding of cell autonomous sexual dimorphism in the context of health and disease.

A primer on the use of mouse models for identifying direct sex chromosome effects that cause sex differences in non-gonadal tissues

In animals with heteromorphic sex chromosomes, all sex differences originate from the sex chromosomes, which are the only factors that are consistently different in male and female zygotes. In mammals, the imbalance in Y gene expression, specifically the presence vs. absence of Sry, initiates the differentiation of testes in males, setting up lifelong sex differences in the level of gonadal hormones, which in turn cause many sex differences in the phenotype of non-gonadal tissues. The inherent imbalance in the expression of X and Y genes, or in the epigenetic impact of X and Y chromosomes, also has the potential to contribute directly to the sexual differentiation of non-gonadal cells. Here, we review the research strategies to identify the X and Y genes or chromosomal regions that cause direct, sexually differentiating effects on non-gonadal cells. Some mouse models are useful for separating the effects of sex chromosomes from those of gonadal hormones. Once direct “sex chromosome effects” are detected in these models, further studies are required to narrow down the list of candidate X and/or Y genes and then to identify the sexually differentiating genes themselves. Logical approaches to the search for these genes are reviewed here.

Evolution of mammalian X chromosome-linked imprinting

We analyse the evolution of X chromosome-linked imprinting by modifying our previous model of imprinting of autosomal genes that influence the trade-off between maternal fecundity and offspring viability through alterations in maternal investment (Mills and Moore, 2004). Unlike previous genetic models, we analyse X-linked imprinting in the context of populations at equilibrium for either autosomal or X-linked biallelically expressed alleles at loci that influence the fecundity/viability trade-off. We show that selection under parental conflict over maternal investment in offspring can parsimoniously explain the occurrence of sex-specific gene expression patterns, without a requirement to postulate direct selection for sexual dimorphism mediated through imprinting. We note that sex chromosome imprinting causes a small distortion of the post-weaning sex ratio, providing a possible selection pressure against the evolution of X-linked imprints. We discuss our conclusions in the context of recent reports of imprinting of mouse X-linked Xlr genes.

Comparison of Gene Expression in Male and Female Mouse Blastocysts Revealed Imprinting of the X-Linked Gene, Rhox5/Pem, at Preimplantation Stages

Mammalian male preimplantation embryos develop more quickly than females . Using enhanced green fluorescent protein (EGFP)-tagged X chromosomes to identify the sex of the embryos, we compared gene expression patterns between male and female mouse blastocysts by DNA microarray. We detected nearly 600 genes with statistically significant sex-linked expression; most differed by 2-fold or less. Of 11 genes showing greater than 2.5-fold differences, four were expressed exclusively or nearly exclusively sex dependently. Two genes (Dby and Eif2s3y) were mapped to the Y chromosome and were expressed in male blastocysts. The remaining two (Rhox5/Pem and Xist) were mapped to the X chromosome and were predominantly expressed in female blastocysts. Moreover, Rhox5/Pem was expressed predominantly from the paternally inherited X chromosome, indicating sex differences in early epigenetic gene regulation.

Retarded postimplantation development of X0 mouse embryos: impact of the parental origin of the monosomic X chromosome

About 12-17% of the embryos obtained by mating mice carrying the In(X)1H or Paf mutations are of the 39,X (X0) genotype. Depending on the mutant mice used for mating, the monosomic X chromosome can be inherited from the paternal (XP) or the maternal (XM) parent. The XP0 embryos display developmental retardation at gastrulation and early organogenesis. XP0 embryos also display poor development of the ectoplacental cone, which is significantly smaller in size and contains fewer trophoblasts than XX siblings. In contrast, XM0 embryos develop normally and are indistinguishable from XX littermates. In both types of X0 embryos, an X-linked lacZ transgene is expressed in nearly all cells in both the embryonic and the extraembryonic tissues, suggesting that X inactivation does not occur when only one X is present. Of particular significance is the maintenance of an active XP chromosome in the extraembryonic tissues where normally the paternal X chromosome is preferentially inactivated in XX embryos. The differential impact of the inheritance of X chromosomes from different parents on the development of the X0 embryos raises the possibility that the XP is less capable than the XM in providing the appropriate dosage of X-linked activity that is necessary to support normal development of the embryo and the ectoplacental cone. Alternatively, the development of the XP0 embryo may be compromised by the lack of activity of one or several X-linked genes which are expressed only from the maternal X chromosome. Without the activity of these genes, embryonic development may be curtailed even though all other loci on the XP chromosome are actively transcribed.

Analysis of colchicine-induced chromosomal aberrations in embryos obtained from XO mice

A high incidence of chromosomal anomalies has been observed in the offspring of our XO mouse colony. However, the reason(s) are unknown. We hypothesized that XO dams might be more susceptible to certain chromosomal mutagens. Therefore, we assessed whether the oocytes of XO mice are more susceptible to colchicine. Pregnant XO and XX mice were intraperitoneally (i.p.) injected with colchicine, 0.2 mg/kg, 6 h after mating (between resumption of meiosis II and extrusion of the second polar body). Contrary to our expectation, the incidence of chromosomal anomalies induced by colchicine exposure did not differ between XX and XO dams. These findings suggest that the chromosomal stability of the oocytes from XO mice may not be affected by colchicine exposure at the stage of extrusion of the second polar body. The effects of chromosomal imbalance inherent in XO mice on chemical susceptibilities should be further investigated.

X-chromosome inactivation in mammals

The inactive X chromosome differs from the active X in a number of ways; some of these, such as allocyclic replication and altered histone acetylation, are associated with all types of epigenetic silencing, whereas others, such as DNA methylation, are of more restricted use. These features are acquired progressively by the inactive X after onset of initiation. Initiation of X-inactivation is controlled by the X-inactivation center (Xic) and influenced by the X chromosome controlling element (Xce), which causes primary nonrandom X-inactivation. Other examples of nonrandom X-inactivation are also presented in this review. The definition of a major role for Xist, a noncoding RNA, in X-inactivation has enabled investigation of the mechanism leading to establishment of the heterochromatinized X-chromosome and also of the interactions between X-inactivation and imprinting as well as between X-inactivation and developmental processes in the early embryo.