Gene imprinting during placental and embryonic development

New insights into human pre-implantation metabolism in vivo and in vitro

The metabolism of pre-implantation embryos is far from being understood. In human embryos, the two major obstacles are the scarcity of material, for obvious ethical reasons, and complete absence of a relevant in vivo control model. Over-extrapolation from animal species to human systems adds to the complexity of the problem. Removal of some metabolites from media has been proposed, such as glucose and essential amino acids, on the basis of their pseudo "toxicity". In contrast, addition of some compounds such as growth factors has been proposed in order to decrease apoptosis, which is a natural physiologic process. These suggestions reflect the absence of global knowledge, and in consequence mask reality. Some aspects of metabolism have been ignored, such as lipid metabolism. Others are seriously underestimated, such as oxidative stress and its relationship to imprinting/methylation, of paramount importance for genetic regulation and chromosomal stability. It has become increasingly obvious that more studies are essential, especially in view of the major extension of ART activities worldwide.

Expression of the imprinted IGF2 and H19 genes in the endometrium of cases with unexplained infertility

OBJECTIVE

As genomic imprinting plays a critical role in the development of the placenta, the aim of this study was to detect whether the expression levels of the imprinted genes IGF2 and H19 in the endometrium differ between infertile and fertile women.

STUDY DESIGN

Total RNA was extracted from 30 (15 unexplained infertile and 15 fertile) women's endometrial tissue. cDNA was synthesized from total RNAs of each sample. IGF2 and H19 mRNA expression levels were measured quantitatively using the Real Time PCR method. In order to determine the allelic expression of IGF2 and H19, genomic DNA was extracted from endometrial tissues.

RESULTS

When compared with the control group, increased mRNA expression of IGF2 was detected (1.5-fold change, P=0.015) in the unexplained infertility group. In contrast, H19 expression was lower in the infertility group as compared to the control group (4-fold change, P<0.0001). Restriction analysis of cDNA-derived PCR product showed that all patients and controls indicated monoallelic expression of IGF2 and H19.

CONCLUSION

Our results showed that altered expression of these imprinted genes might affect implantation and that their timely and appropriate activation is important for proper functioning. To understand the molecular epigenetic basis of implantation and placental development, genomic imprinted genes should be further investigated.

Paternal and maternal factors in preimplantation embryogenesis: interaction with the biochemical environment

Paternal effect on embryonic development occurs as early as fertilization. Incorrect formation of the spermatozoon due to centrosome defects and abnormal concentrations of any components involved in the activation process lead to failure immediately or in the subsequent cell cycles. Sperm chromosomal abnormalities result in early embryo developmental arrests. Generally poor spermatozoa lead to poor blastocyst formation. Sperm DNA fragmentation may impair even late post-implantation development. The DNA repair capacity of the oocytes is of major importance. Early preimplantation development, i.e. until maternal to zygotic transition, is maternally driven. Maternal mRNAs and proteins are of major importance, as there is an unavoidable turnover of these reserves. Polyadenylation of these mRNAs is precisely controlled, in order to avoid too early or too late transcription and translation of the housekeeping genes. An important set of maternal regulations, such as DNA stability, transcriptional regulation and protection against oxidative stress, are impaired by age. The embryo biochemical endogenous pool is very important and may depend upon the environment, i.e. the culture medium. Paternal, maternal and environmental factors are unavoidable parameters; they become evident when age impairs oocyte quality.

Changes in the structure of nuclei after transfer to oocytes

Nuclear transfer and the potential for cloning animals have refocused attention on the oocyte. This focus is not limited to the use of the oocyte as a recipient in nuclear transfer procedures, but more broadly in terms of what factors are present in the oocyte that are responsible for establishing the developmental pattern of RNA synthesis and subsequent protein production. Deviations in the pattern of RNA synthesis can result in abortions, as well as abnormalities at birth. This paper will focus on the changes to nuclear structure that result from transfer to the cytoplasm of an oocyte, as well as some of the changes in the patterns of RNA synthesis that have been described.

A mola hydatidosa coexistent with a foetus in a bovine freemartin pregnancy

Molar transformations of the bovine placenta are extremly rare phenomenona and the aetiology of this genuine placental disease is still unknown. In the present study, an uncommon case of a German Holstein Friesian foetus co-twinned with a hydatidiform mole is described. Cytogenetic and fluorescence in situ hybridization analysis of cell cultures as well as prove of the presence of the SRY gene sequence revealed a heterosexual twin pregnancy. A chimeric condition of the mole was also established. In addition, an XO cell population was detected in the co-twin as well as in the mole. Upon examination of microsatellites of the parents, the mole and the co-twin an androgenetic origin of the mole is suggested, supporting the hypothesis that molar transformation of the bovine placenta may be caused by an androgenetic origin. Furthermore, the present observation demonstrates that the freemartin condition in cattle can be induced even in cases where severe placental transformations had subsequently occurred and no foetus proper could be detected at delivery.

The sheep (Ovis aries) H19 gene: genomic structure and expression patterns, from the preimplantation embryo to adulthood

H19, which is one of the most abundantly expressed imprinted genes during mammalian embryonic and foetal development, has been cloned from a ruminant. The sheep (Ovis aries) gene contains five exons interspersed by four exceptionally small introns; only short stretches of the nucleotide sequence, particularly in exon 1, show good homology with the human gene. The size of the exons and introns and the sequences around the splice junctions however, are well conserved between the species. The gene encodes a approximately 2.6 kb transcript which contains several potential short open reading frames, none of which is conserved between the ovine and human or murine transcripts, supporting a previous hypothesis that the gene product is the untranslated RNA itself. H19 mRNA is highly abundant in most ovine embryonic and foetal tissues of mesodermal and endodermal origins but was not detected in tissues of ectodermal origin such as the trophectoderm and the foetal brain. Expression of H19 in the extraembryonic membranes was detected only after the ovine conceptus began attachment to the endometrium and the embryo itself had undergone early organogenesis. This may be regarded as the first step in implantation; thus, in comparison with the mouse, the initiation of H19 expression appears to be determined by the timing of implantation rather than by the stage of development of the embryo itself. In most tissues, H19 expression is temporally linked to IGF2, a major foetal growth factor. The exceptions were the elongated blastocyst, the trophectoderm and brain, where low levels of IGF2 were observed in the absence of detectable H19. The abundance of H19 mRNA was in general, directly correlated with IGF2 mRNA abundance in mesodermal and endodermal tissues, suggesting that the two ovine genes share common regulatory elements that co-ordinately regulate their expression. Though both are generally regarded as embryonic and foetal genes, their expression was still maintained at a fairly high level in the adult sheep liver, lung, skeletal muscle, adrenal gland and kidney, suggesting that these organs are significant sources of IGF II in the adult.

Epigenetic risks related to assisted reproductive technologies: short- and long-term consequences for the health of children conceived through assisted reproduction technology: more reason for caution?

Does the manipulation of gametes and embryos as practised in human IVF invoke perturbations in fetal and neonatal phenotype? There is increasing evidence that the answer is 'yes', although the degree of perturbation may be less acute than observed in other species. However, the long-term consequences are not known, and may prove to be considerable. There is now a substantial body of evidence from animal models suggesting that assisted reproductive technologies (ART) are associated with altered outcomes in fetal and neonatal development. Epigenetic modification of gene expression is an attractive hypothesis that accounts for these differences and is one of a number of causal pathways that may be activated by cellular stress invoked during manipulation. Here we widen the debate to propose that environment-induced cellular stress also acts to modify fetal and placental gene expression, potentially also contributing to phenotype skewing after ART.

Expression of the imprinted H19 oncofetal RNA in epithelial ovarian cancer

STUDY

To examine the expression of the imprinted maternally expressed H19 gene in benign, low malignant potential (borderline) and malignant surface epithelial ovarian tumors.

DESIGN

In situ hybridization for H19 RNA using S-labeled and digoxigenin-labeled probes was performed on paraffin sections of ovarian surface epithelial tumors. The serous tumors included nine section cystadenomas, twelve serous tumors of low malignant potential and twenty serous carcinomas, grade I-IIII (FIGO classification). A smaller group included two mucinous cystadenomas, four mucinous tumors of low malignant potential and two mucinous cystadenocarcinomas.

RESULTS

H19 expression was found to be positive in 6/9 (67%) serous cystadenomas, 9/12 (75%) of serous tumors of low malignant potential and 13/20 (65%) of invasive serous carcinomas. Expression in mucinous tumors was confined to the stroma beneath the epithelial lining.

CONCLUSION

H19 is expressed in the majority of serous epithelial tumors. Taking into consideration the high percentage of H19 expressing serous ovarian neoplasms we suggest that H19 RNA may be used as an adjuvant tumor marker for the diagnosis and mainly for staging and follow-up of patients with serous ovarian carcinoma.

Genomic imprinting and the endometrial cycle. The expression of the imprinted gene H19 in the human female reproductive organs

H19 is an imprinted maternally expressed gene, which is not translated to protein and functions as an RNA molecule. It is closely related to the oppositely imprinted paternally expressed insulin-like growth factor 2 (IGF-2). While the biological function of H19 is not understood IGF-2 is a growth factor that plays a role in human follicular and endometrial differentiation. We examined the expression of H19 in the endometrium and ovary during the menstrual cycle by in situ hybridization applied to paraffin sections of human endometrium and ovaries at different stages of differentiation. In the endometrium, H19 expression was confined to the stroma and fluctuated with endometrial dating to reach its peak in the late secretory stage. IGF-2 was also prominently expressed in late secretory endometrium, but its expression was evident both in the stroma and glandular epithelium. Expression of H19 was not found in primordial, primary, and preantral follicles of the ovary, but prominent expression was evident in the theca of antral and cystic atretic follicles, and focal expression was noted in the granulosa of corpora lutea. An association between H19 expression during the menstrual cycle and the differentiation state of the human female reproductive tract, which is under hormonal control, is suggested.

Relaxation of imprinting in carcinogenesis

The study of genes at the heat of tumorigenesis has helped unveil an elaborate biochemical circuitry that governs the proliferation and differentiation of cells. Genomic imprinting is rapidly being recognized as a fundamental process in tumor biology. Aberrant relaxation of imprinted genes have been detected in a wide variety of cancers, of both embryonal and nonembryonal origin. However, despite a vast array of experimental observation, both the purpose and pathogenic mechanism of relaxation of imprinting remain an enigma. Hypotheses are examined in this regard along with speculation for future research.

Down's syndrome as a model for the decisive role of maternal lineage in human evolution

The study of human evolution and the mechanism of this process can be approached from physical anthropology, which examines phenotypic expression and molecular evolution, which investigates genotypic change. Alternatively, we suggest that human evolutional process can also be explained using present day examples of abberations in evolution. Thus, from both genotypic and phenotypic perspectives, we address the question of whether Down's syndrome is an instructive example to look into the decisive role of maternal lineage in human evolution.

On the oncodevelopmental role of human imprinted genes

Genome imprinting has an essential role in normal embryonal mammalian development. Starting early in differentiation, the transcripts of certain human genes, e.g. the paternally-H19 and the maternally-imprinted IGF2, are expressed in specific tissues and organs during fetal life. In several malignant disorders, imprinted genes are, again, unfolded. Characteristically, expression follows the same tissue presentation as during embryogenesis. Clinical paternal disomies, i.e. trophoblastic diseases, and their maternal counterpart, i.e. ovarian teratomas, are associated with apparent relaxation of imprinting once they turn malignant. Paediatric neoplasms, like Wilm's tumor (WT) and rhabdomyosarcoma, often express IGF2 and H19. Recently, we have found H19 expression in invasive urothelial cancer. Evidently, imprinted genes display an oncodevelopmental mode of expression, very much like the classical oncofetal proteins AFP and CEA. Based on available data, including tumor preferential paternal allele retention and chromosome 11 short arm physical linkage with oncogenes like H-ras, we hypothesize that imprinted genes not only accompany cancer but may play a causative role as well.

Expression of the imprinted gene H19 in the human fetus

The H19 gene is a parenterally imprinted maternally expressed gene which has a pivotal role in embryogenesis and fetal development. It is tightly linked to the IGF-II gene on chromosome 11p15.5 which is reciprocally imprinted. We studied the expression of the human H19 by in situ hybridization in an embryo 35 days post coitus (dpc) and in a fetus from the second trimester of pregnancy. The expression pattern of H19 in the human fetal tissues was similar to its expression in the mouse, and paralleled, with some exceptions, the expression of IGF-II in human fetuses. Abundant expression was found in organs comprising the fetoplacental unit: the placenta, the fetal adrenal, and liver. The expression in the fetal adrenal cortex was most prominent in the definitive cortex and somewhat weaker in the fetal zone. Considerable expression of H19 was found in the fetal liver as early as 35 dpc and in the second trimester. Hematopoietic cells in fetal liver did not express the gene. Moderate expression of H19 was detected in the epithelium of the small intestines, in endometrial stroma and Fallopian tube. In the kidney conspicuous labeling of the metanephric blastema was noted, which was markedly reduced with differentiation to tubules. This pattern of expression is identical to that of IGF-II in the fetal kidney and is relevant to the evolution of Wilms' tumor. No expression of H19 was found in the neural tube of the first trimester embryo or in the developing fetal brain in the second trimester, nor were transcripts detected in the choroid plexus.(ABSTRACT TRUNCATED AT 250 WORDS)

Parental imprinting of autosomal mammalian genes

The molecular mechanisms introducing epigenetic modifications that lead to differential silencing of some autosomal alleles depending on their parental legacy are still largely unknown, but recent results from studies of endogenously imprinted genes and particular transgenes make DNA methylation a strong candidate. At the same time, these results have raised new questions about the details of the imprinting process.