The most common outcome of fertilization is embryonic death around the time of implantation, with estimates of loss ranging from 50 to 80%. The major cause of embryonic failure is an unbalanced chromosome constitution due to aneuploidy, polyploidy or, less frequently, chromosome deletion or duplication. There now exists the technological means to define the underlying molecular processes by which chromosome aberrations lead to loss of embryonic viability. The ability to analyse the chromosome complements of spermatozoa, oocytes and preimplanta-tion embryos combined with recombinant DNA technology applicable to single cells provides the means for such studies. It should be possible, therefore, to compare chromosomally normal and abnormal preimplantation embryos with regard to expression of specific genes and whether their expression is directly associated with embryonic death or simply a secondary consequence of a generalized genomic imbalance. Such studies may eventually contribute to better understanding of the developmental origin of congenital malformations as well as infertility and pregnancy failure.
A basic question in early human embryogenesis is the timing of gene expression. Timing of gene expression may be particularly relevant to embryonic viability, for it has been hypothesized that failure of genome activation is responsible for embryonic arrest in the preimplantation period (Braude et al., 1988, 1989). Although an attractive hypothesis, transcriptional expression in patterns of polypeptide synthesis has continued in the majority of embryos arrested between the two- and eight-cell stages (Artley et al., 1992; Bolton, 1997) and alternative explanations for periimplantation embryonic death are necessary. Transition from dependence on maternally-derived transcripts originally present in the oocyte to embryonic expression is generally agreed to occur between the four- and eight-cell stage in human preimplantation embryos (Braude et al., 1988; Tesarik et al., 1986). However, paternal transcripts for the Y-linked genes, ZFY and SRY (Ao et al., 1994; Fiddler et al., 1995), as well as the protein kinase gene associated with myotonic dystrophy (Daniels et al., 1995), have been identified in the late pronucleate one-cell stage. Such studies illustrate the inherent difficulty of molecular analyses based on single cells, as expression at the protein level for these Y-linked genes has not been demonstrated nor are their functions at this stage of development known. It may be that these purportedly tissue-specific genes, ZFY and SRY, have no functional significance in preimplantation embryos and that the presence of paternal transcripts at the one-cell stage represents either a generalized derepression of the paternal genome following fertilization or, more specifically, a derepression of transcriptional control for paternally inherited genes containing CpG islands, which characterizes both ZFY and SRY (Daniels & Monk, 1997).
The role of the Y chromosome in human development has yet to be fully defined, particularly in the preimplantation period. There is an issue as to whether ZFY and SRY are actually transcription and/or growth factors responsible not only for sex determination but for growth rate differences been XX and XY embryos beginning as early as the preimplantation period (Pergament et al., 1994; Erickson, 1997) and, in turn, for documented differences between the two sexes in incidences of various types of congenital malformations (Lubinsky, 1997; Fiddler & Pergament, 1997).
Expression studies during the preimplantation period have identified several categories of genes: housekeeping genes; transcription and growth factor genes; tissue-specific genes; sex-determining genes; novel genes; and, genes of unknown function. For housekeeping genes, ubiquitious cytoskeletal elements, beta-actin, keratin-18 and alpha tubulin, and cell adhesion molecules have been identified in the preimplantation human embryo, as well as hypoxanthine phosphoribosyl transferase (HPRT), adenosine phosphoribosyl transferase (APRT), hexokinase I, glucoses-phosphate dehydrogenase (G-6-PD), adenosine deaminase (Campbell et al., 1995; Taylor et al., 1997; Daniels & Monk, 1997). Not surprisingly, gene regulatory elements, including transcription regulators, cell cycle genes, growth factors, proto-oncogenes and receptors, comprise the largest number of genes expressed during the preimplanation period. Examples include the transcription regulators OCT 4 and OCT 6 (Abdel-Rahman et al., 1995); a cell surface glycoprotein, CD44, which may play a role in implantation (Campbell et al., 1995); the cell cycle gene, cyclin B1 (Heikiheimo et al., 1995); tumour necrosis factor and its receptor (Sharkey et al., 1995); interleukin-1 system genes involved in embryonic implantation (Krussel et al., 1998); insulin-like growth factor and its receptors (Liu et al., 1997; Lighten et al., 1997); epidermal growth factor (EGF), transforming growth factor-alpha (TGF-n) and epidermal growth factor receptor (EGF-R) (Chia et al., 1995).
The availability of cDNA libraries from single human preimplantation embryos at different stages of development now makes possible investigation of the activities of tissue-specific genes, novel genes and genes of unknown function. The initial studies have found, in addition to housekeeping genes, such tissue-specific genes as globin and interleukin-10, human transposable element, LINE-1, and expressed sequence tags (ESTs) listed in the GenBank and dbEST databases, as well as novel embryonic stage-specific transcripts of unknown function (Adjaye et al., 1998). The availability of human embryonic cDNA libraries also makes possible investigating such diverse genetic phenomena as imprinting, e.g. establishing the timing of selective silencing of either maternal or paternal alleles, as well as the timing of the expansion of trinucleotide repeats associated with specific human diseases, such as fragile X syndrome (Adjaye et al., 1998; Mailer et al., 1997). Until recently, gene expression studies in human preimplantation embryos were limited by the difficulty in obtaining experimental specimens and by a technology requiring analysis of single cells. It is equally important to be able to define the chromosome complement of preimplantation embryos used in such studies, in order to address the criticism that investigations based on chromosomally abnormal embryos, e.g. tripronucleate zygotes and their embryonic derivates, may have provided incorrect information on timing and level of gene expression.
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