Primary and secondary nonrandom X chromosome inactivation in early female mouse embryos carrying Searle's translocation T(X; 16)16H

By means of a cytological technique involving 5-bromodeoxyuridine, acridine orange, and fluorescence microscopy, the asynchronously replicating, hence genetically inactivated, X chromosome was identified in 6-to 8-day embryos from female mice heterozygous for Searle's translocation T(X;16)16H (abbreviated as T16H) mated with either karyotypically normal males or males carrying Cattanach's translocation T(X;7)lCt in order to analyse the way in which the total inactivation of the normal X is achieved in adult T16H heterozygotes. Embryos examined included 9 Xⁿ/X⁽⁷⁾;16/16, 3X¹⁶/Xⁿ;16^x/16, 12X¹⁶/X⁽⁷⁾;16^x/16, 5 X¹⁶/Xⁿ; 16/16, 8 X¹⁶/X⁽⁷⁾; 16/16 and 2 Xⁿ/Y; 16^x/16/16. In these notations X¹⁶, 16^x, X⁽⁷⁾ and Xⁿ represent Searle's X with the centromeric segment of the X, Searle's X with the centomeric segment of chromosome 16, Cattanachs's X with insertion of a chromosome 7 segment, and normal X, respectively. The X⁽⁷⁾ exerted no apparent effect upon embryonic development up to the 8th day of gestation and X chromosome inactivation. — The asynchronously replicating X was the Xⁿ in X¹⁶/ Xⁿ;16^x/16 and X⁽⁷⁾ in X¹⁶/X⁽⁷⁾;16^x/16 embryos except a small number of cells on day 6 (13/493) and on day 7 (1/886) in which almost the entire 16^x replicated asynchronously. The X¹⁶, on the other hand, never showed replication asynchrony. That the X¹⁶ is indeed unable to become inactivated was indicated by the observation that the X¹⁶ as well as Xⁿ or X⁽⁷⁾ did not replicate asynchronously in Xⁿ/X¹⁶; 16/16 and X¹⁶/X⁽⁷⁾; 16/16 embryos. X¹⁶-inactive cell lines, if occurring, should have been genetically less unbalanced than any other cell line in such embryos. It is highly likely therefore that the ultimate inactivation pattern in T16H heterozygotes has been accomplished by (1) the inability of the X¹⁶ to become inactive; (2) inactivation in favor of the Xⁿ; and (3) rapid elimination of 16^x-inactive cells. Severe growth retardation and early death of X¹⁶/Xⁿ;16/16 and X¹⁶/X⁽⁷⁾; 16/16 embryos having no inactive X suggested that functional X disorny is detrimental to embryogenesis. These embryos further indicated that the concurrence of at least two X chromosomal loci separated by the T16H breakpoint is necessary for the homologous X chromosome becoming inactivated.     

X-chromosome inactivation in differentiating mouse embryonic stem cells carrying X-linked GFP and lacZ transgenes

Three new female ES cell lines (GLM1, GLP1 and GLP2) were established from mouse embryos carrying GFP (green fluorescent protein) and HMG-lacZ transgenes on one of two X chromosomes in cis. Using these cell lines, we studied the temporal relationships among three events relevant to X-chromosome inactivation: replication asynchrony of the X chromosome, and quenching of GFP fluorescence and beta-galactosidase (beta-gal) activity, during cell differentiation induced by embryoid body (EB) formation and retinoic acid (RA) treatment. In embryoid bodies adhering to the bottom of culture dishes, GFP-negative cells appeared first in the peripheral outgrowths 4 days after the initiation of EB formation, followed about 24 hours later by the appearance of cells negative for beta-gal and those having a single allocyclic X chromosome. Although the frequency of cells with an allocyclic X chromosome could reach 80% in adherent embryoid bodies, it tended to remain low and variable in embryoid bodies maintained in suspension. In spite of apparently parallel extinction of GFP and lacZ in embryoid bodies, their concurrent occurrence did not always characterize RA-induced differentiation. Moreover, an allocyclic X chromosome was identified in not more than 20 percent of informative metaphase cells up to 10 days after initiation of RA treatment. These findings suggest that RA-induced differentiation of female ES cells does not always accompany X-inactivation.     

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.     

Karyotypes and X chromosome inactivation in segregants of a murine X-autosome translocation, T(X;4)37H

Karyotypes and X chromosome inactivation were studied in embryos obtained from female mice carrying T(X;4)37H translocation on day 6 to 8 of gestation by a BrdU-acridine orange method. A total of 18 different karyotypes were found in 477 embryos examined: 90.0% embryos were products expected from 2:2 alternate or adjacent 1 disjunction. 3:1 and adjacent 2 disjunctions accounted for approximately 8.0% and 0.7% conceptuses, respectively. In the embryo proper of balanced T37H/ + conceptuses, inactivation was random with respect to the normal X and the larger translocation X (4x) chromosome. In all the cells with the 4x inactive, the late replication apparently did not spread to the attached autosomal portion, although black/brown coat variegation implies spreading of inactivation into the autosomal region. The X chromosome segment deprived of the inactivation center remained active in all the cells examined and it exerted deleterious effects on embryonic or fetal development. Observation in embryos having two maternally derived X chromosomes showed that they were indeed resistant to inactivation in early extraembryonic cell lineages, and two copies of active X chromosomes in the trophectoderm fatally affected embryonic development due to inability to form the extraembryonic ectoderm and ectoplacental cone from the polar trophectoderm. In unbalanced X aneuploids the X chromosomes with the deletion were preferentially inactivated due to strong selection against nullisomy X.     

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.     

Sex ratio and testis size in mice carrying Sxr and T(X;16)16H

Mice heterozygous for the T(X;16)16H translocation and carrying Sxr on their normal (inactive) X chromosome (ie, T16H/X Sxr individuals) may develop as males, females, or hermaphrodites. The proportion of males varied from 22% to 65% depending on the source of the normal X chromosome. A model is proposed, according to which relatively small variations in the spreading of inactivation from the X chromosome into the attached Sxr fragment produce large changes in the proportion of males. Testis weight in T16H/X Sxr males was found to be significantly smaller than in X/X Sxr males, irrespective of the source of the normal X chromosome.     

X chromosome imprinting and inactivation in preimplantation mammalian embryos

Dosage compensation for the mammalian X chromosome involves the silencing of one X chromosome to achieve equal X-linked gene expression between males and females. X chromosome inactivation (XCI) is controlled by a complex set of genetic elements located in a region known as the X chromosome inactivation center, and is regulated by a combination of genomic imprinting, cell lineage-dependent erasure of imprinting, an unidentified mechanism of X chromosome counting, an incompletely understood means of selection of one X chromosome for inactivation and developmentally regulated changes in X chromosome chromatin. A detailed understanding of when and how these components of XCI occur is essential for elucidating the operative mechanisms. A model accounting for early events related to XCI, including observations in uniparental and aneuploid embryos, is presented.     

X chromosome inactivation patterns in normal and X-linked hereditary nephropathy carrier dogs

Alport syndrome (AS) and hereditary nephropathy (HN) are glomerular nephropathies caused by mutations in the genes encoding the type IV collagens. In a mixed breed of dog, termed Navasota (NAV) dogs, X-linked hereditary nephropathy (XLHN) is caused by a 10-bp deletion in exon 9 of COL4A5. Males harboring this mutation succumb to end-stage renal disease before 18 months of age. In contrast, female carriers of this disease survive much longer, most have a normal life-span, and vary in disease progression as compared with XLHN-affected males. X chromosome inactivation (XCI) patterns have been studied in human X-linked AS carriers and some have been shown to have a high degree of skewed XCI. However, similar studies have never been reported in an animal model of this disease. Therefore, patterns of XCI were examined in XLHN-carrier NAV dogs. The variation in XCI among the 26 XLHN-carrier and seven normal female NAV dogs studied was low and only three were found to preferentially inactivate one X chromosome, all of which were XLHN-carriers. The average skewedness among all dogs was 59% and 57% among the XLHN-carriers. No significant difference in XCI was found between the two groups (P = 0.477). It is clear from these data that genotype does not seem to have an effect on inactivation; the majority of these dogs have random patterns of XCI. Highly skewed X chromosome inactivation also appears to be random, given that no difference was observed between the XLHN-carriers and normal females. Because of the apparent rarity of skewed XCI, these dogs may not be a suitable model for studying a potential correlation between this phenomenon and disease progression.     

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.     

Nonrandom chromosome changes in methylnitrosourea (MNU) induced mouse T-cell lymphomas.

Since nonrandom chromosome changes in neoplastic cells have proven to be good indicators of the site of gene alterations related to transformation, the authors examined the chromosomes of T-cell lymphomas induced in RF/J strain mice with methylnitrosourea (MNU). All treated mice developed thymic lymphomas within 10 weeks of injection. Chromosomes of the thymus cells were examined at intervals before and during lymphoma development, as well as after they were passaged in syngeneic and in nude mice for periods up to 424 days. In preparations made directly from the thymus cells nonrandom numerical and structural alterations were found that involved the X, 3, 15, 4, 8, 12, 14 and 17. (Chromosomes showing alterations are listed in decreasing order of the frequency of their occurrence). In cells passaged in nude mice the chromosomes similarly altered were the 10, X, 3, 12, 6, 1, 4, 19, 15, 18 and 14. In tumor cells passaged in syngeneic mice most of the same chromosomes were involved but the order was 15, 14, X, 1, 5, 6, 3, 11 and 12. The X, 15, 14, 3 and 12 were aberrant in both direct preparations and in those from passaged cells, suggesting that these chromosomes carry genes which, when altered, are particularly important in the multistep process of neoplastic transformation. Most of these chromosomes, or their homologs in other species, have been found to be involved frequently in several different cancers of mice and men, as for example the region on the mouse 15 carrying the Myc and Pvt-1 genes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Expression of an X-linked muscular dystrophy in a female due to translocation involving Xp21 and non-random inactivation of the normal X chromosome

Summary A young female was diagnosed as having X-linked muscular dystrophy of the Duchenne type. Chromosome studies, including trypsin-Giemsa banding, Quinacrine fluorescence, and nucleolus organizer region (NOR) silver staining revealed an X-autosome reciprocal translocation t(X;21) (p21;p12). Utilizing both [³H] thymidine autoradiography and the BrdU-Hoechst 33258-Giemsa technique, lymphocytes and fibroblasts were found to show a preferential inactivation of the normal X suggesting the presence of a single mutant gene on the translocated X. This patient is one of seven reported cases of an X-linked muscular dystrophy associated with an X-autosome translocation. In all seven cases the exchange point in the X chromosome is in band p21 at or near the site of the Duchenne gene.     

Allele‐specific expression of the MAOA gene and X chromosome inactivation in in vitro produced bovine embryos

During embryogenesis, one of the two X chromosomes is inactivated in embryos. The production of embryos in vitro may affect epigenetic mechanisms that could alter the expression of genes related to embryo development and X chromosome inactivation (XCI). The aim of this study was to understand XCI during in vitro, pre‐implantation bovine embryo development by characterizing the allele‐specific expression pattern of the X chromosome‐linked gene, monoamine oxidase A (MAOA). Two pools of ten embryos, comprised of the 4‐, 8‐ to 16‐cell, morula, blastocyst, and expanded blastocyst stages, were collected. Total RNA from embryos was isolated, and the RT‐PCR‐RFLP technique was used to observe expression of the MAOA gene. The DNA amplicons were also sequenced using the dideoxy sequencing method. MAOA mRNA was detected, and allele‐specific expression was identified in each pool of embryos. We showed the presence of both the maternal and paternal alleles in the 4‐, 8‐ to 16‐cell, blastocyst and expanded blastocyst embryos, but only the maternal allele was present in the morula stage. Therefore, we can affirm that the paternal X chromosome is totally inactivated at the morula stage and reactivated at the blastocyst stage. To our knowledge, this is the first report of allele‐specific expression of an X‐linked gene that is subject to XCI in in vitro bovine embryos from the 4‐cell to expanded blastocyst stages. We have established a pattern of XCI in our in vitro embryo production system that can be useful as a marker to assist the development of new protocols for in vitro embryo production. Mol. Reprod. Dev. 77: 615–621, 2010. © 2010 Wiley‐Liss, Inc.     

Visualization of inactive X chromosome in preimplantation embryos utilizing MacroH2A–EGFP transgenic mouse

One of the two X chromosomes is inactivated in female eutherian mammals. MacroH2A, an unusual histone variant, is known to accumulate on the inactive X chromosome (Xi) during early embryo development, and can thus be used as a marker of the Xi. In this study, we produced a transgenic mouse line expressing the mouse MacroH2A1.2–enhanced green fluorescent protein (EGFP) fusion protein (MacroH2A–EGFP) under the control of a CAG promoter and verified whether MacroH2A–EGFP would be useful for tracing the process of X chromosome inactivation by visualizing Xi noninvasively in preimplantation embryos. In transgenic female mice, MacroH2A–EGFP formed a fluorescent focus in nuclei throughout the body. In female blastocysts, the MacroH2A–EGFP focus colocalized with Xist RNA, well known as a marker of Xi. Fluorescence marking of Xi was first observed in some embryonic cells between the 4‐ and 8‐cell stages. These results demonstrate that MacroH2A can bind to the Xi by around the 8‐cell stage in female mouse embryos. These MacroH2A–EGFP transgenic mice might be useful to elucidate the process of X chromosome inactivation during the mouse life cycle. genesis 51:259–267. © 2013 Wiley Periodicals, Inc.     

Location of phosphorylase kinase (Phk) in the mouse X chromosome

The phosphorylase kinase deficiency (Phk) locus has been located in the mouse X chromosome, the order of genes being centromere-Bn-Phk-Ta-jp. Since the Phk locus of the mouse may be identical to the locus responsible for the X-linked phosphorylase kinase deficiency trait of man, and there may be a high degree of gene-order homology in the X chromosome of all mammals, the location of Phk in the mouse reported here may aid in locating the phosphorylase kinase gene on the X chromosome of man.This research was supported by grants AM 13359 (to F.H.) and AM 14461 (to D.L.C.) from the National Institute of Arthritis and Metabolic Diseases, and by an allocation (to E.M.E.) from NIH General Research Support Grant RR-05545 from the Division of Research Resources to The Jackson Laboratory. F.H. is a recipient of a Research Career Development Award (AM 46 421) of the National Institute of Arthritis and Metabolic Diseases.