scholarly journals Maternal DNMT3A-dependent de novo methylation of the zygotic paternal genome inhibits gene expression in the early embryo

2020 ◽  
Author(s):  
Julien Richard Albert ◽  
Wan Kin Au Yeung ◽  
Keisuke Toriyama ◽  
Hisato Kobayashi ◽  
Ryutaro Hirasawa ◽  
...  
Keyword(s):  
De Novo ◽  
Cpg Islands ◽  
Blastocyst Stage ◽  
Early Embryo ◽  
Mass Spectrometry Analysis ◽  
Paternal Allele ◽  
Cell Stage ◽  
Paternal Genome ◽  
Spectrometry Analysis ◽  
Genomic Regions

ABSTRACTDe novo DNA methylation (DNAme) during mammalian spermatogenesis yields a densely methylated genome, with the exception of CpG islands (CGIs), which are hypomethylated in sperm. Following fertilization, the paternal genome undergoes widespread DNAme loss before the first S-phase. Paradoxically, recent mass spectrometry analysis revealed that a low level of de novo DNAme occurs exclusively on the zygotic paternal genome. However, the loci involved and impact on genic transcription was not addressed. Here, we employ allele-specific analysis of wholegenome bisulphite sequencing (WGBS) data and show that a number of genomic regions, including several dozen CGI promoters, are de novo methylated on the paternal genome in 2-cell embryos. A subset of these promoters maintains DNAme through development to the blastocyst stage. Consistent with zygotic paternal DNAme acquisition (PDA), many of these loci are hypermethylated in androgenetic blastocysts but hypomethylated in parthenogenetic blastocysts. Strikingly, PDA is lost following maternal deletion of Dnmt3a. Furthermore, a subset of promoters showing PDA which are normally transcribed from the paternal allele in blastocysts show premature transcription at the 4-cell stage in maternal Dnmt3a knockout embryos. These observations uncover an unexpected role for maternal DNMT3A activity in postfertilization epigenetic reprogramming and transcriptional silencing of the paternal genome.

scholarly journals Maternal DNMT3A-dependent de novo methylation of the paternal genome inhibits gene expression in the early embryo

2020 ◽  
Vol 11 (1) ◽  
Author(s):  
Julien Richard Albert ◽  
Wan Kin Au Yeung ◽  
Keisuke Toriyama ◽  
Hisato Kobayashi ◽  
Ryutaro Hirasawa ◽  
...  
Keyword(s):  
De Novo ◽  
Cpg Islands ◽  
Blastocyst Stage ◽  
Early Embryo ◽  
Mass Spectrometry Analysis ◽  
Paternal Allele ◽  
Sequencing Data ◽  
Cell Stage ◽  
Paternal Genome ◽  
Spectrometry Analysis

Abstract De novo DNA methylation (DNAme) during mammalian spermatogenesis yields a densely methylated genome, with the exception of CpG islands (CGIs), which are hypomethylated in sperm. While the paternal genome undergoes widespread DNAme loss before the first S-phase following fertilization, recent mass spectrometry analysis revealed that the zygotic paternal genome is paradoxically also subject to a low level of de novo DNAme. However, the loci involved, and impact on transcription were not addressed. Here, we employ allele-specific analysis of whole-genome bisulphite sequencing data and show that a number of genomic regions, including several dozen CGI promoters, are de novo methylated on the paternal genome by the 2-cell stage. A subset of these promoters maintains DNAme through development to the blastocyst stage. Consistent with paternal DNAme acquisition, many of these loci are hypermethylated in androgenetic blastocysts but hypomethylated in parthenogenetic blastocysts. Paternal DNAme acquisition is lost following maternal deletion of Dnmt3a, with a subset of promoters, which are normally transcribed from the paternal allele in blastocysts, being prematurely transcribed at the 4-cell stage in maternal Dnmt3a knockout embryos. These observations uncover a role for maternal DNMT3A activity in post-fertilization epigenetic reprogramming and transcriptional silencing of the paternal genome.

Tight junction assembly during mouse blastocyst formation is regulated by late expression of ZO-1 alpha+ isoform

Development ◽  
1997 ◽  
Vol 124 (10) ◽  
pp. 2027-2037 ◽  
Author(s):  
B. Sheth ◽  
I. Fesenko ◽  
J.E. Collins ◽  
B. Moran ◽  
A.E. Wild ◽  
...  
Keyword(s):  
Tight Junction ◽  
De Novo ◽  
Transmembrane Protein ◽  
Blastocyst Stage ◽  
Early Embryo ◽  
Cell Junctions ◽  
Cell Stage ◽  
Membrane Assembly ◽  
Morula Stage ◽  
Late Expression

The mouse preimplantation embryo has been used to investigate the de novo synthesis of tight junctions during trophectoderm epithelial differentiation. We have shown previously that individual components of the tight junction assemble in a temporal sequence, with membrane assembly of the cytoplasmic plaque protein ZO-1 occurring 12 hours before that of cingulin. Subsequently, two alternatively spliced isoforms of ZO-1 (alpha+ and alpha-), differing in the presence or absence of an 80 residue alpha domain were reported. Here, the temporal and spatial expression of these ZO-1 isoforms has been investigated at different stages of preimplantation development. ZO-1alpha- mRNA was present in oocytes and all preimplantation stages, whilst ZO-1alpha+ transcripts were first detected in embryos at the morula stage, close to the time of blastocoele formation. mRNAs for both isoforms were detected in trophectoderm and ICM cells. Immunoprecipitation of 35S-labelled embryos also showed synthesis of ZO-1alpha- throughout cleavage, whereas synthesis of ZO-1alpha+ was only apparent from the blastocyst stage. In addition, 33P-labelling showed both isoforms to be phosphorylated at the early blastocyst stage. The pattern and timing of membrane assembly of the two isoforms was also distinct. ZO-1alpha- was initially seen as punctate sites at the cell-cell contacts of compact 8-cell embryos. These sites then coalesced laterally along the membrane until they completely surrounded each cell with a zonular belt by the late morula stage. ZO-1alpha+ however, was first seen as perinuclear foci in late morulae before assembling at the tight junction. Membrane assembly of ZO-1alpha+ first occurred during the 32-cell stage and was zonular just prior to the early blastocyst stage. Immunostaining indicative of both isoforms was restricted to the trophectoderm lineage. Membrane assembly of ZO-1alpha+ and blastocoele formation were sensitive to brefeldin A, an inhibitor of intracellular trafficking beyond the Golgi complex. In addition, the tight junction transmembrane protein occludin co-localised with ZO-1alpha+ at the perinuclear sites in late morulae and at the newly assembled cell junctions. These results provide direct evidence from a native epithelium that ZO-1 isoforms perform distinct roles in tight junction assembly. Moreover, the late expression of ZO-1alpha+ and its apparent intracellular interaction with occludin may act as a final rate-limiting step in the synthesis of the tight junction, thereby regulating the time of junction sealing and blastocoele formation in the early embryo.

scholarly journals Embryo culture in presence of oviductal fluid induces DNA methylation changes in bovine blastocysts

Reproduction ◽  
2017 ◽  
Vol 154 (1) ◽  
pp. 1-12 ◽  
Author(s):  
Antonio D Barrera ◽  
Elina V García ◽  
Meriem Hamdi ◽  
María J Sánchez-Calabuig ◽  
Ángela P López-Cardona ◽  
...  
Keyword(s):  
Dna Methylation ◽  
Blastocyst Stage ◽  
Early Embryo ◽  
Cpg Methylation ◽  
Bovine Embryos ◽  
Cell Stage ◽  
The One ◽  
Genomic Regions ◽  
Oviductal Fluid

During the transit through the oviduct, the early embryo initiates an extensive DNA methylation reprogramming of its genome. Given that these epigenetic modifications are susceptible to environmental factors, components present in the oviductal milieu could affect the DNA methylation marks of the developing embryo. The aim of this study was to examine if culture of bovine embryos with oviductal fluid (OF) can induce DNA methylation changes at specific genomic regions in the resulting blastocysts. In vitro produced zygotes were cultured in medium with 3 mg/mL bovine serum albumin (BSA) or 1.25% OF added at the one- to 16-cell stage (OF1–16), one- to 8-cell stage (OF1–8) or 8- to 16-cell stage (OF8–16), and then were cultured until Day 8 in medium with 3 mg/mL BSA. Genomic regions in four developmentally important genes (MTERF2, ABCA7, OLFM1, GMDS) and within LINE-1 retrotransposons were selected for methylation analysis by bisulfite sequencing on Day 7–8 blastocysts. Blastocysts derived from OF1–16 group showed lower CpG methylation levels in MTERF2 and ABCA7 compared with the BSA group. However, CpG sites within MTERF2, ABCA7 and OLFM1 showed higher methylation levels in groups OF1–8 and OF8–16 than in OF1–16. For LINE-1 elements, higher CpG methylation levels were observed in blastocysts from the OF1–16 group than in the other experimental groups. In correlation with the methylation changes observed, mRNA expression level of MTERF2 was increased, while LINE-1 showed a decreased expression in blastocysts from OF1–16 group. Our results suggest that embryos show transient sensitivity to OF at early stages, which is reflected by specific methylation changes at the blastocyst stage.

scholarly journals A preliminary study of Dihydroorotase from Methanococcus jannaschii

2014 ◽  
Vol 70 (a1) ◽  
pp. C481-C481
Author(s):  
Aditya Singh ◽  
Michael Colaneri ◽  
Jacqueline Vitali
Keyword(s):  
Mass Spectrometry ◽  
De Novo ◽  
Hydrophobic Interaction Chromatography ◽  
Size Exclusion ◽  
Mass Spectrometry Analysis ◽  
Yale University ◽  
Spectrometry Analysis ◽  
Bacteria And Fungi ◽  
Physics Department ◽  
Research Institute

Dihydroorotase (DHOase) catalyzes the reversible cyclization of N-carbamoyl-L-aspartate to form L-dihydroorotate in the third step of de novo pyrimidine biosynthesis. It is a Zinc metalloenzyme and a member of the aminohydrolase superfamily. There are two classes of the enzyme. Class I, typically ~45 kDa, is found in higher organisms, bacteria and yeast. Class II, typically ~38 kDa, is found in bacteria and fungi. Some organisms have multiple DHOase sequences. The M. jannaschii pyrC gene coding for DHOase was subcloned and expressed in E. coli. Protein purification consisted of ammonium sulfate precipitation, heat treatment at 850C, and phenyl-sepharose hydrophobic interaction chromatography. The protein was confirmed in the SDS gel using Liquid Chromatography-Mass Spectrometry (Proteomics Laboratory, Lerner Research Institute, Cleveland, OH). Size Exclusion Chromatography-Laser Light Scattering (Keck Biotechnology Laboratory, Yale University, New Haven, CT) indicated that the protein is a monomer in solution with a molecular weight of 47 kDa. A model constructed with the I-TASSER server (Zhang, 2008) suggested that the binding site contains only one Zn ion per monomer coordinated by the conserved His56, His58 and Asp302. Asp146 is further away and does not coordinate with the Zn ion. According to the mass spectrometry analysis, the protein does not contain a carboxylated lysine. Our progress on this study will be presented. Acknowledgements: We thank Dr. Belinda Willard (Lerner Research Institute) for the LC-MS and Dr. Ewa Folta-Stogniew (Yale University) for the SEC- LS analysis. The presentation was supported in part by a graduate faculty travel award and by a contribution from the Physics Department at Cleveland State University.

213 HOXB9 PROTEIN IS PRESENT THROUGHOUT EARLY EMBRYO DEVELOPMENT IN THE BOVINE

2013 ◽  
Vol 25 (1) ◽  
pp. 255
Author(s):  
C. Sauvegarde ◽  
D. Paul ◽  
R. Rezsohazy ◽  
I. Donnay
Keyword(s):  
Embryo Development ◽  
Inner Cell Mass ◽  
Cell Mass ◽  
Mature Oocyte ◽  
Blastocyst Stage ◽  
Early Embryo ◽  
Immature Oocyte ◽  
Cell Stage ◽  
Morula Stage ◽  
Oocyte Stage

Hox genes encode for homeodomain transcription factors well known to be involved in developmental control after gastrulation. However, the expression of some of these genes has been detected during oocyte maturation and early embryo development. An interesting expression profile has been obtained for HOXB9 in the bovine (Paul et al. 2011 Mol. Reprod. Dev. 78, 436): its relative expression increases between the immature oocyte and the zygote, further increases at the 5- to 8-cell stage to peak at the morula stage before decreasing at the blastocyst stage. The main objective of this work is to establish the HOXB9 protein profile from the immature oocyte to the blastocyst in the bovine. Bovine embryos were produced in vitro from immature oocytes obtained from slaughterhouse ovaries. Embryos were collected at the following stages: immature oocyte, mature oocyte, zygote (18 h post-insemination, hpi), 2-cell (26 hpi), 5 to 8 cell (48 hpi), 9 to 16 cell (96 hpi), morula (120 hpi), and blastocyst (180 hpi). The presence and distribution of HOXB9 proteins were detected by whole-mount immunofluorescence followed by confocal microscopy using an anti-human HOXB9 polyclonal antibody directed against a sequence showing 100% homology with the bovine protein. Its specificity to the bovine protein was controlled by Western blot on total protein extract from the bovine uterus and revealed, among a few bands of weak intensities, 2 bands of high intensity corresponding to the expected size. Oocytes or embryos were fixed and incubated overnight with rabbit anti-HOXB9 (Sigma, St. Louis, MO, USA) and mouse anti-E-cadherin (BD Biosciences, Franklin Lakes, NJ, USA) primary antibodies and then for 1 h with goat anti-rabbit Alexafluor 555 conjugated (Cell Signaling Technology, Beverly, MA, USA) and goat anti-mouse FITC-conjugated (Santa Cruz Biotechnology Inc., Santa Cruz, CA, USA) secondary antibodies. Embryos were then mounted in Vectashield containing DAPI. HOXB9 is detected from the immature oocyte to the blastocyst stage. At the immature oocyte stage, it is mainly localised in the germinal vesicle with a weak signal in the cytoplasm. At the mature oocyte stage, HOXB9 labelling is present in the cytoplasm. At the zygote stage, a stronger immunoreactivity is observed in the pronuclei than in the cytoplasm. From the 2-cell stage to the morula stage, the presence of HOXB9 is also more important in the nuclei than in the cytoplasm. HOXB9 is also observed at the blastocyst stage where it is localised in the nuclei of the trophectoderm cells, whereas an inconstant or weaker labelling is observed in the inner cell mass cells. In conclusion, we have shown for the first time the presence of the HOXB9 protein throughout early bovine embryo development. The results obtained suggest the presence of the maternal HOXB9 protein because it is already detected before the maternal to embryonic transition that occurs during the fourth cell cycle in the bovine. Finally, the pattern obtained at the blastocyst stage suggests a differential role of HOXB9 in the inner cell mass and trophectoderm cells. C. Sauvegarde holds a FRIA PhD grant from the Fonds National de la Recherche Scientifique (Belgium).

124. DNA METHYLATION IN THE 2-CELL MOUSE EMBRYO IS THE RESULT OF DNMT ACTIVITY DURING DEVELOPMENT FROM THE ZYGOTE TO THE 2-CELL STAGE

2009 ◽  
Vol 21 (9) ◽  
pp. 43
Author(s):  
Y. Li ◽  
H. D. Morgan ◽  
L. Ganeshan ◽  
C. O'Neill
Keyword(s):  
Dna Methylation ◽  
Dna Methyltransferase ◽  
Cytosine Methylation ◽  
De Novo ◽  
Culture Conditions ◽  
Early Embryo ◽  
Cleavage Stage ◽  
Cell Stage ◽  
Stage Of Development ◽  
First Time

In an accompanying abstract we show for the first time that global demethylation of both paternally- and maternally-derived genomes occurs prior to syngamy. It is commonly considered that new methylation of the genome does not commence until late in the preimplantation stage. Yet embryos during cleavage stage are known to show DNA methylation. This creates a paradox, if global demethylation occurs by the time of syngamy yet remethylation does not occur until the blastocysts stage, how can cleavage stage embryos possess methylated DNA. We examined this paradox. We examined DNA methylation in 2-cell embryos by confocal microscopy of anti-methylcytosine immunofluorescence and propidium iodide co-staining of whole mounts. We confirmed that DNA in late zygotes was substantially demethylated in both the male and female pronuclei. By the 2-cell stage, embryos collected direct from the oviduct showed high levels of cytosine methylation. We assessed whether this accumulation of cytosine methylation during the early 2-cell stage was a consequence of DNA methyltransferase (DNMT) activity. This was achieved by treating late stage zygotes with the DNMT inhibitor RG108 (5 μM) for the period of development spanning pronuclear stage 5 to early 2-cell stage. The embryos that developed in the presence of the DNA methyltransferase inhibitor showed significantly less methylcytosine staining than the embryos in the untreated culture conditions (P<0.001). Treatment of embryos during this period with RG108 significantly reduced their capacity to develop to normal blastocysts, indicating that this early DNA re-methylation reaction was important for the normal development of the embryo. Our results show for the first time that de novo methylation of the genome occurs as early as the 2-cell stage of development and that this is mediated by a RG108-sensitive DNMT activity. The results substantially change our understanding of epigenetic reprogramming in the early embryo.

scholarly journals Epigenetic asymmetry in the mammalian zygote and early embryo: relationship to lineage commitment?

2003 ◽  
Vol 358 (1436) ◽  
pp. 1403-1409 ◽  
Author(s):  
Wolf Reik ◽  
Fatima Santos ◽  
Kohzoh Mitsuya ◽  
Hugh Morgan ◽  
Wendy Dean
Keyword(s):  
Inner Cell Mass ◽  
Cell Mass ◽  
Blastocyst Stage ◽  
Early Embryo ◽  
Lineage Commitment ◽  
Imprinted Genes ◽  
Mammalian Development ◽  
Paternal Genome ◽  
Developmental Defects ◽  
Set Up

Epigenetic asymmetry between parental genomes and embryonic lineages exists at the earliest stages of mammalian development. The maternal genome in the zygote is highly methylated in both its DNA and its histones and most imprinted genes have maternal germline methylation imprints. The paternal genome is rapidly remodelled with protamine removal, addition of acetylated histones, and rapid demethylation of DNA before replication. A minority of imprinted genes have paternal germline methylation imprints. Methylation and chromatin reprogramming continues during cleavage divisions, but at the blastocyst stage lineage commitment to inner cell mass (ICM) or trophectoderm (TE) fate is accompanied by a dramatic increase in DNA and histone methylation, predominantly in the ICM. This may set up major epigenetic differences between embryonic and extraembryonic tissues, including in X–chromosome inactivation and perhaps imprinting. Maintaining epigenetic asymmetry appears important for development as asymmetry is lost in cloned embryos, most of which have developmental defects, and in particular an imbalance between extraembryonic and embryonic tissue development.

131 DYNAMICS OF HISTONE H3 METHYLATION AT POSITIONS K4 AND K9 IN MOUSE, RABBIT, AND BOVINE PRE-IMPLANTATION EMBRYOS

2006 ◽  
Vol 18 (2) ◽  
pp. 174 ◽  
Author(s):  
K. Lepikhov ◽  
F. Yang ◽  
C. Wrenzycki ◽  
V. Zakhartchenko ◽  
H. Niemann ◽  
...  
Keyword(s):  
Histone H3 ◽  
Distribution Patterns ◽  
Blastocyst Stage ◽  
Dna Demethylation ◽  
Preimplantation Embryos ◽  
Staining Procedure ◽  
Cell Stage ◽  
Paternal Genome ◽  
Histone H3 Methylation ◽  
Rabbit Embryos

In mammals, upon the penetration of sperm into the oocyte, the paternal genome undergoes dramatic epigenetic changes. Protamin packaging of DNA is replaced by histones that acquire specific modifications. In mouse zygotes, paternal DNA gets rapidly demethylated by an active mechanism. In bovine zygotes the methylation from paternal DNA is erased only partially, and in rabbit zygotes it persists at the initial level. To understand whether these reprogramming differences are also reflected in histone modifications, we examined the dynamic changes of histone H3 methylation at positions K4 and K9 in mouse, bovine, and rabbit zygotes and in preimplantation embryos using an immunofluorescence staining procedure (Lepikhov and Walter 2004 BMC Dev. Biol. 4, 12). In zygotes, maternal chromatin contains both types of histone H3 methylation. After fertilization protamines in sperm are very quickly replaced by histones. After the formation of nucleosomes, histone H3 acquires methylation at position K4 in a stepwise manner: first as mono-methylated form and later as tri-methylated. In the late zygote, both paternal and maternal pronuclei show equal levels of histone H3 methylation at position K4. Regardless of the differences in DNA reprogramming in these 3 species, H3/K9 di-methylation is not detected on paternal genomes and is only associated with maternal genomes. During the subsequent cleavage stages, H3/K9 di-methylation decreases gradually and becomes hardly detectable in 4-cell bovine and rabbit embryos. In mouse embryos, it is detectable through all the stages. Bovine embryos reacquire this type of modification at the 8-16 cell stage, and it remains at the very low levels in rabbit, embryos until the blastocyst stage. In conclusion, mouse, rabbit and bovine zygotes show similar patterns of H3/K4triMe and H3/K9diMe distribution despite the difference in paternal DNA demethylation. The dynamics of H3/K9diMe distribution patterns in cleavage stage embryos from all embryos do not correlate with embryonic genomic activation events.

90 TRANSCRIPTION OF USP9Y AND ZFY IN DEVELOPING AND ARRESTED BOVINE EMBRYOS IN VITRO

2013 ◽  
Vol 25 (1) ◽  
pp. 193
Author(s):  
J. Caudle ◽  
C. K. Hamilton ◽  
F. A. Ashkar ◽  
W. A. King
Keyword(s):  
Embryo Development ◽  
Blastocyst Stage ◽  
Early Embryo ◽  
Bovine Embryos ◽  
Cell Stage ◽  
Male Embryo ◽  
Male Development ◽  
Embryonic Genome ◽  
Genome Activation

Sexual dimorphisms such as differences in growth rate and metabolism have been observed in the early embryo, suggesting that sex chromosome-linked gene expression may play an active role in early embryo development. Furthermore, in vitro sex ratios are often skewed toward males, indicating that Y-linked genes may benefit development. While little attention has been paid to the Y chromosome, expression of some Y-linked genes such as SRY and ZFY has been identified in the early embryo, and only a few studies have systematically examined early stages. Identification of transcripts of Y-linked genes in the early embryo may provide insights into male development and provide markers of embryonic genome activation in male embryos. The objectives of this study were i) to examine the timing of transcription of 2 Y chromosome-linked genes involved with sperm production and male development, ubiquitin-specific peptidase 9 (USP9Y) and zinc finger protein (ZFY), in in vitro-produced bovine embryos from the 2-cell stage to the blastocyst stage and ii) to determine if USP9Y and ZFY transcripts are present in in vitro-produced embryos arrested at the 2- to 8-cell stages. To examine the chronology of transcription of these genes, pools of 30 embryos for each developmental stage, 2-cell, 4-cell, 8-cell, 16-cell, morula, and blastocyst, were produced by bovine standard in vitro embryo production (Ashkar et al. 2010 Hum. Reprod. 252, 334–344) using semen from a single bull. Pools of 30 were used to balance sex ratios and to account for naturally arresting embryos. Embryos for each developmental stage were harvested and snap frozen. Total RNA was extracted from each pool, reverse transcribed to cDNA and by using PCR, and transcripts of USP9Y and ZFY were detected as positive or negative. In addition pools of 30 embryos arrested at the 2- to 8-cell stage harvested 7 days after IVF were processed and analysed in the same way to determine if transcripts from the Y chromosomes are present in developmentally arrested embryos. Transcripts of USP9Y and ZFY were detected in the pooled embryos from the 8-cell stage through to the blastocyst stage, but none were detected in the 2-cell or 4-cell pools. Transcripts of ZFY were detected in the arrested 2- to 8-cell embryo pool, but transcripts of USP9Y were not detected. Given that these Y genes begin expression at the 8-cell stage, coincident with embryonic genome activation, it was concluded that these genes may be important for early male embryo development. Furthermore, the results suggest that arrested embryos that have stopped cleaving before the major activation of the embryonic genome are still capable of transcribing at least some of these genes. The absence of USP9Y transcripts in the arrested embryos suggests that it may be important for early male embryo development. Funding was provided by NSERC, the CRC program, and the OVC scholarship program.

X Chromosome Inactivation and Imprinting

1996 ◽  
Vol 45 (1-2) ◽  
pp. 85-85
Author(s):  
M.F. Lyon
Keyword(s):  
X Chromosome ◽  
Germ Cells ◽  
Blastocyst Stage ◽  
X Inactivation ◽  
Xist Gene ◽  
Paternal Allele ◽  
Xist Expression ◽  
Cell Stage ◽  
Reading Frame ◽  
Genetic Imprinting

In contrast to the random inactivation of either maternal or paternal X-chromosome in the somatic cells of eutherian mammals, in marsupials the paternal X-chromosome is preferentially inactivated in all cells. Similar exclusively paternal X-inactivation occurs in two extraembryonic cell lineages of mice and rats. Thus, genetic imprinting is an important feature of X-inactivation. In embryonic development the initiation of X-inactivation is thought to occur through the X-inactivation centre, located on the X-Chromosome, and thus imprinting probably acts through this centre. A candidate gene for a role in the inactivation centre is Xist (X inactive specific transcript) which is expressed only from the inactive X-Chromosome. The expression of Xist in the mouse embryo is appropriate for it to be a cause rather than a consequence of inactivation. It appears before inactivation, and only the paternal allele is expressed in the extraembryonic lineages. In the germ cells also changes in X-chromosome activity are accompanied by changes in Xist expression. Studies of methylation of the Xist gene have shown that in male tissues where Xist is not active it is fully methylated, whereas in the female the allele on the active X-chromosome only is methylated. In male germ cells, where Xist is expressed, it is demethylated and the demethylation persists in mature spermatozoa. Thus a methylation difference in germ cells could possibly be the imprint. In androgenotes, with paternally derived chromosomes, Xist is expressed at the 4-cell stage, whereas in gynogenotes and parthenogenotes expression does not appear until the blastocyst stage. Thus, Xist expression shows imprinting. When expression appears in parthenogenotes it is random, suggesting that the imprint has been lost. The Xist gene has no open reading frame and is thought to act through mRNA but its function is unknown.

Sign in / Sign up

Export Citation Format

Share Document