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Gametes and Embryo Epigenetic Reprogramming Affect Developmental Outcome: Implication for Assisted Reproductive Technologies

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Gametes and Embryo Epigenetic Reprogramming Affect Developmental Outcome: Implication for Assisted Reproductive Technologies 0031-3998/05/5803-0437 PEDIATRIC RESEARCH Vol. 58, No. 3, 2005 Copyright © 2005 International Pediatric Research Foundation, Inc. Printed in U.S.A. Gametes and Embryo Epigenetic Reprogramming Affect Developmental Outcome: Implication for Assisted Reproductive Technologies SAJI JACOB AND KELLE H. MOLEY Department of Obstetrics & Gynecology [S.J., K.H.M.], Washington University School of Medicine, St. Louis, MO 63110; and Department of Ob/Gyn [S.J.], Southern Illinois Healthcare Foundation, Alton, IL 62003 ABSTRACT There is concern about the health of children who are con- Abbreviations ceived with the use assisted reproductive technologies (ART). In ART, assisted reproductive technologies addition to reports of low birth weight and chromosomal anom- AS, Angelman syndrome alies, there is evidence that ART may be associated with in- ATRX, X-linked ␣-thalassemia/mental retardation creased epigenetic disorders in the infants who are conceived BWS, Beckwith-Wiedemann syndrome using these procedures. Epigenetic reprogramming is critical DNMT, DNA methyltransferase during gametogenesis and at preimplantation stage and involves FMR1, fragile X mental retardation gene 1 DNA methylation, imprinting, RNA silencing, covalent modifi- HDAC, histone deacetylase cations of histones, and remodeling by other chromatin- ICF, immunodeficiency, centromeric instability and facial associated complexes. Epigenetic regulation is involved in early anomalies embryo development, fetal growth, and birth weight. Distur- IVF, in vitro fertilization bances in epigenetic reprogramming may lead to developmental LBW, low birth weight problems and early mortality. Recent reports suggest the in- LOI, loss of imprinting creased incidence of imprinting disorders such as Beckwith- LOS, Wiedemann syndrome, Angelman syndrome, and retinoblastoma large offspring syndrome in children who are conceived with the use of ART. These may MBD, methyl-binding domain result from an accumulation of epigenetic alterations during RB, retinoblastoma embryo culture and/or by altered embryonic developmental tim- RTS, Rubinstein-Taybi syndrome ing. Further research is urgently needed to determine whether a RTT, Rett syndrome causal relationship between ART and epigenetic disorders exists. UPD, uniparental disomy Until then, cautious review of both short-term and long-term VLBW, very low birth weight ART outcomes at a national level is recommended. (Pediatr Res 58: 437–446, 2005) Recent reports of birth defects and health problems in and ICSI singletons as compared with naturally conceived children who are conceived with assisted reproductive technol- children (3–5). It has been postulated that embryos in culture ogies (ART) has led to the initiation of prospective and retro- may acquire epigenetic defects as a result of the abnormal spective follow-up studies in these children to evaluate the environmental conditions that may lead to these aberrant phe- safety of these techniques. These investigations have deter- notypes. Several lines of evidence support this hypothesis. The mined that development is normal and that malformation rates accumulation of severe epigenetic disturbances above a certain are either similar or only slightly greater than those in the threshold may lead to early mortality. Alternatively, embryos general population (1,2). These studies, however, have also that develop to term may still have epigenetic defects, and revealed a lower birth weight in the in vitro fertilization (IVF) these may result in obvious aberrant phenotypes or in subtle changes in gene expression that can be easily overlooked. Received February 4, 2005; accepted April 18, 2005. These epigenetic reprogramming disturbances can occur at Correspondence: Kelle H. Moley, M.D., Washington University School of Medicine, Department of Obstetrics & Gynecology, Campus Box 8064, St. Louis, MO 63110; gametogenesis or at preimplantation embryo stage and could e-mail: [email protected] be influenced by in vitro culture conditions. This review ex- DOI: 10.1203/01.PDR.0000179401.17161.D3 plores epigenetic reprogramming in the gametes and embryo 437 438 JACOB AND MOLEY and how normal epigenetic modification may be influenced by EPIGENETIC REPROGRAMMING ART. Epigenetics is commonly defined as the study of heritable changes in gene function that occur without a change in the INTRODUCTION TO EPIGENETICS DNA sequence. Epigenetic modifications work in concert with the genetic information of a DNA sequence. These modifica- Epigenetic changes are defined as chemical alterations to the tions include DNA methylation, imprinting, RNA silencing, DNA or to the histone proteins associated with it that change covalent modifications of histones, and remodeling by other the structure without altering the nucleotide sequence. The chromatin-associated complexes. All of these alterations reg- DNA, complexed together with the histones, make up chroma- ulate gene expression. These are referred to as epigenetic tin. These epigenetic changes modify the structure of the patterns, and they are normally built into the genome during chromatin by making it more condensed or more open. These differentiation through genetically determined programs. Ad- changes in turn affect gene expression within the DNA by ditional epigenetic alterations to the DNA allow cells to alter either allowing or preventing accessibility to the DNA of the the expression level of the different genes in response to factors involved with transcription. environmental factors without having to change the DNA code Unintended gene silencing caused by epigenetic modifica- itself. tions has been linked to several human diseases. Specifically, Inheritance or persistence of these epigenetic modifications DNA methylation and histone deacetylation are the major is referred to as epigenetic reprogramming. The most critical forms of epigenetic modifications that occur in tumors. DNA periods at which epigenetic reprogramming occurs are those methylation is a chemical modification of the DNA molecule during gametogenesis and the preimplantation embryonic stage itself, which is regulated by an enzyme called DNA methyl- (6). Reprogramming during gametogenesis is essential for the transferase (DNMT). Methylation can directly switch off gene imprinting mechanism that regulates the differential expression expression by preventing the ability of transcription factors to of paternally and maternally derived genes. Primordial germ access DNA and binding to promoters. Another chemical cells undergo demethylation, both globally (7) and within modification of DNA is histone deacetylation. In this process, imprinted loci as they migrate along the genital ridge (7,8). enzymes called histone deacetylases (HDACs) are associated After this demethylation or erasure, which ensures genetic with methyl-binding domain (MBD) proteins and are attracted totipotency, CpG methylation of imprinted genes is reestab- to DNA. Activation of these HDACs leads to alteration of the lished during gametogenesis through de novo methylation, in histones and chromatin structure, thereby causing the chroma- both eggs (9) and sperm (10). Imprints are established differ- tin condensation, inaccessibility of the DNA promoters, and entially in sperm and oocyte and are maintained in the embryo gene silencing (see Fig. 1). and further through all somatic cell divisions. Methylation and chromatin remodeling take place with each cell division and provide the germ cell genome with molecular blueprints for oocyte activation and embryonic development. There are sig- nificant differences between mature oocytes and sperm in epigenetic organization. The sperm genome is more methyl- ated than the oocyte genome. The chromatin is well compacted with protamines in sperm but with histones in oocytes. The oocyte chromatin structure is more “closed” than that of sperm cells. Finally, imprinted loci are differentially methylated among sperm and oocytes. At fertilization, the oocyte and sperm genomes are transcriptionally silent. After fertilization, chromosomes from the sperm decondense and are remodeled. Protamines are exchanged for maternal histones as rapid de- methylation takes place (11). The oocyte genome is demeth- ylated more slowly by a passive mechanism (12). Genes that are imprinted with methylation marks, however, are protected from demethylation (13), so parental imprints are maintained. At the time of implantation, de novo methylation occurs ge- nome-wide in a lineage-specific pattern (14). Epigenetic repro- Figure 1. Epigenetic modifications that silence gene expression. (Top) Open gramming during the preimplantation period is essential for chromatin is characterized by nonmethylated DNA and histones with acety- correct development because this phenomenon regulates ex- lated tails. This permits accessibility of the DNA promoters with assembly of pression of early embryonic genes, cell cleavage, and cell transcription factors and transcription by RNA polymerase. (Middle) Activity determination. As the organism ages, the early embryonic of DNMT leads to methylation of DNA. This event may directly block binding genes are silenced and tissue-specific genes are expressed. by transcription factors and prevent transcription. In addition, this event may attract and recruit MBD proteins that have associated HDACs. (Bottom) DNA Imprinted genes maintain their methylation marks from the methylation and histone deacetylation result in the condensation of chromatin germ cell stage, and they avoid these general erasure processes into a compact state that is
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