DNA methylation and demethylation events during meiotic prophase in the mouse testis

DNA methylation changes at infertility genes in newborn twins conceived by in vitro fertilisation

BACKGROUND

The association of in vitro fertilisation (IVF) and DNA methylation has been studied predominantly at regulatory regions of imprinted genes and at just thousands of the ~28 million CpG sites in the human genome.

METHODS

We investigated the links between IVF and DNA methylation patterns in whole cord blood cells (n = 98) and cord blood mononuclear cells (n = 82) from newborn twins using genome-wide methylated DNA immunoprecipitation coupled with deep sequencing.

RESULTS

At a false discovery rate (FDR) of 5%, we identified one significant whole blood DNA methylation change linked to conception via IVF, which was located ~3 kb upstream of TNP1, a gene previously linked to male infertility. The 46 most strongly associated signals (FDR of 25%) included a second region in a gene also previously linked to infertility, C9orf3, suggesting that our findings may in part capture the effect of parental subfertility. Using twin modelling, we observed that individual-specific environmental factors appear to be the main overall contributors of methylation variability at the FDR 25% IVF-associated differentially methylated regions, although evidence for methylation heritability was also obtained at several of these regions. We replicated previous findings of differential methylation associated with IVF at the H19/IGF2 region in cord blood mononuclear cells, and we validated the signal at C9orf3 in monozygotic twins. We also explored the impact of intracytoplasmic sperm injection on the FDR 25% signals for potential effects specific to male or female infertility factors.

CONCLUSIONS

To our knowledge, this is the most comprehensive study of DNA methylation profiles at birth and IVF conception to date, and our results show evidence for epigenetic modifications that may in part reflect parental subfertility.

Methylation-dependent and independent regulatory regions in the Na,K-ATPase alpha4 (Atp1a4) gene may impact its testis-specific expression

The α4 Na,K-ATPase is a sperm-specific protein essential for sperm motility and fertility yet little is known about the mechanisms that regulate its expression in germ cells. Here, the potential involvement of DNA methylation in regulating the expression of this sperm-specific protein is explored. A single, intragenic CpG island (Mα4-CGI) was identified in the gene encoding the mouse α4 Na,K-ATPase (Atp1a4), which displayed reduced methylation in mouse sperm (cells that contain α4) compared to mouse kidney (tissue that lacks α4 expression). Unlike the intragenic CGI, the putative promoter (the -700 to +200 region relative to the transcriptional start site) of Atp1a4 did not show differential methylation between kidney and sperm nevertheless it did drive methylation-dependent reporter gene expression in the male germ cell line GC-1spg. Furthermore, treatment of GC-1spg cells with 5-aza2-deoxycytidine led to upregulation of the α4 transcript and decreased methylation of both the Atp1a4 promoter and the Mα4-CGI. In addition, Atp1a4 expression in mouse embryonic stem cells deficient in DNA methyltransferases suggests that both maintenance and de novo methylation are involved in regulating its expression. In an attempt to define the regulatory function of the Mα4-CGI, possible roles of the Mα4-CGI in regulating Atp1a4 expression via methylation-dependent transcriptional elongation inhibition in somatic cells and via its ability to repress promoter activity in germ cells were uncovered. In all, our data suggests that both the promoter and the intragenic CGI could combine to provide multiple modes of regulation for optimizing the Atp1a4 expression level in a cell type-specific manner.

MicroRNAs and DNA methylation as epigenetic regulators of mitosis, meiosis and spermiogenesis

Spermatogenesis is composed of three distinctive phases, which include self-renewal of spermatogonia via mitosis, spermatocytes undergoing meiosis I/II and post-meiotic development of haploid spermatids via spermiogenesis. Spermatogenesis also involves condensation of chromatin in the spermatid head before transformation of spermatids to spermatozoa. Epigenetic regulation refers to changes of heritably cellular and physiological traits not caused by modifications in the DNA sequences of the chromatin such as mutations. Major advances have been made in the epigenetic regulation of spermatogenesis. In this review, we address the roles and mechanisms of epigenetic regulators, with a focus on the role of microRNAs and DNA methylation during mitosis, meiosis and spermiogenesis. We also highlight issues that deserve attention for further investigation on the epigenetic regulation of spermatogenesis. More importantly, a thorough understanding of the epigenetic regulation in spermatogenesis will provide insightful information into the etiology of some unexplained infertility, offering new approaches for the treatment of male infertility.

Epigenetic regulation of sox30 is associated with testis development in mice

DNA methylation is involved in tissue-specific and developmentally regulated gene expression. Here, we screened a novel methylation gene Sox30, whose methylation might contribute to its regulation and testis development in mice. Sox30 is a member of Sox transcription factors, and is considered to be involved in spermatogonial differentiation and spermatogenesis. However, the precise function and regulatory expression pattern remain unclear. In the present study, we found that Sox30 is highly expressed in adult testes but not in ovaries. Sox30 expression begins in early development, and in the testes, it is specifically increased coincidentally with development until adulthood. Moreover, Sox30 is expressed not only in testis germ cells, but also in sertoli cells. Sox30 is hypo-methylated in testis, epididymis and lung of adult mice, in which Sox30 is expressed. By contrast, Sox30 is hypermethylated in ovary, heart, brain, liver, kidney, spleen, pancreas, muscle, intestine, pituitary gland, blood and hippocampus of adult mice, in which the Sox30 is absent. Importantly, decreased methylation at CpG islands of Sox30 is observed in mouse developmental testes after birth, which is associated with enhanced Sox30 expression. However, the hypermethylated status of Sox30 is maintained in ovaries that does not express Sox30 during this period. Further, following demethylation treatment using 5-aza-dC, Sox30 expression is restored in GC2, TM3 and TM4 cell lines. This observation convincingly confirms that methylation really contributes to Sox30 silencing. In summary, we show that Sox30 expression is under the control of DNA methylation status, and this expression pattern is associated with testis development in mice.

Loss of DNMT1o disrupts imprinted X chromosome inactivation and accentuates placental defects in females

The maintenance of key germline derived DNA methylation patterns during preimplantation development depends on stores of DNA cytosine methyltransferase-1o (DNMT1o) provided by the oocyte. Dnmt1o^mat−/− mouse embryos born to Dnmt1^Δ1o/Δ1o female mice lack DNMT1o protein and have disrupted genomic imprinting and associated phenotypic abnormalities. Here, we describe additional female-specific morphological abnormalities and DNA hypomethylation defects outside imprinted loci, restricted to extraembryonic tissue. Compared to male offspring, the placentae of female offspring of Dnmt1^Δ1o/Δ1o mothers displayed a higher incidence of genic and intergenic hypomethylation and more frequent and extreme placental dysmorphology. The majority of the affected loci were concentrated on the X chromosome and associated with aberrant biallelic expression, indicating that imprinted X-inactivation was perturbed. Hypomethylation of a key regulatory region of Xite within the X-inactivation center was present in female blastocysts shortly after the absence of methylation maintenance by DNMT1o at the 8-cell stage. The female preponderance of placental DNA hypomethylation associated with maternal DNMT1o deficiency provides evidence of additional roles beyond the maintenance of genomic imprints for DNA methylation events in the preimplantation embryo, including a role in imprinted X chromosome inactivation.

During oocyte growth and maturation, vital proteins and enzymes are produced to ensure that, when fertilized, a healthy embryo will arise. When this natural process is interrupted, one or more of these essential elements can fail to be produced thus compromising the health of the future embryo. We are using a mouse model, lacking an enzyme (DNMT1o) produced in the oocyte and only required post-fertilization in the early embryo for the maintenance of inherited DNA methylation marks. Here, we reveal that oocytes lacking DNMT1o, when fertilized, generated conceptuses with a wide variety of placental abnormalities. These placental abnormalities were more frequent and severe in females, and showed specific genomic regions constantly deprived of their normal methylation marks. The affected genomic regions were concentrated on the X chromosome. Interestingly, we also found that a region important for the regulation of the X chromosome inactivation process was hypomethylated in female blastocysts and was associated with sex-specific abnormalities in the placenta, relaxation of imprinted X chromosome inactivation, and disruption of DNA methylation later in development. Our findings provide a novel unanticipated role for DNA methylation events taking place within the first few days of life specifically in female preimplantation embryos.

Transcriptional and epigenetic status of protamine 1 and 2 genes following round spermatids injection into mouse oocytes

The use of round spermatids that are fully active at the transcriptional level to create zygotes (i.e. round spermatid injection; ROSI) raises the question regarding the downregulation of all specific genes that are transcribed from the paternal genome at fertilization. In this study, we show that protamine 1 and 2 mRNAs, which are specific to the round spermatid stage, are repressed at the two-pronuclei (6 h) and two-cell (30 h) stages postfertilization, respectively, in ROSI embryos, by distinct mechanisms. Both genes are fully methylated in round spermatids and sperm but unmethylated in oocytes. At 6 h postfertilization, the protamine 1 and 2 genes are actively demethylated, but the demethylation process happens more rapidly in ROSI than in sperm zygotes. Treatment of zygotes with trichostatin A, a histone deacetylase (HDAC) inhibitor, maintained the protamine 2 mRNAs expression up to 30 h postfertilization while the DNA methylation status of the gene is not affected. Thus, HDACs are involved in the clearance of protamine 2 mRNAs in ROSI two-cell embryos independently of the methylation status of the repressed gene. Contrastingly, HDACs are not directly involved in protamine 1 regulation since trichostatin A does not reverse the silencing of the gene in ROSI embryos at 6 h. The protamine 1 CpG island located in the coding region is actively demethylated in ROSI one-cell embryos where the gene is repressed and may contribute to the regulation of protamine 1 gene expression. The comparison with gene reprogramming occurring during nuclear transfer makes ROSI embryos an attractive model to study the mechanisms involved in gene silencing elicited by the oocyte.

Biology of sperm chromatin structure and relationship to male fertility and embryonic survival

Embryonic mortality in mammals is typically thought to result from 'female factor' infertility. There is growing evidence, however, that the status of sperm chromatin (DNA) at the time of fertilisation can also influence embryonic survival. During the final stages of spermatogenesis (spermiogenesis) a number of unique biochemical, morphological and physiological processes take place that are associated with marked changes in the structure of sperm chromatin. In early stages of spermatogenesis, sperm DNA is associated with histone nucleoproteins and structured into classical nucleosome core particles similar to other somatic cells. As spermiogenesis proceeds, the histone nucleoproteins are replaced by transition proteins which are subsequently replaced by protamines. At the completion of spermiogenesis the chromatin of mature sperm has a toroidal structure that is tightly compacted and resistant to denaturation. The compaction is necessary to protect sperm chromatin during transit through the epididymis and female reproductive tract. Disruption to chromatin remodelling during spermiogenesis results in chromatin that is susceptible to denaturation. Inappropriate chromatin structure has been shown in a number of mammalian species to be related to male infertility, and specifically the failure of embryonic development. A range of techniques are available to assess chromatin status in sperm but arguably the most informative is the sperm chromatin structure assay (SCSA). The SCSA is a flow cytometric assay that uses the metachromatic properties of acridine orange to measure the susceptibility of sperm chromatin to acid-induced denaturation. A relationship has been demonstrated, primarily in men, between the SCSA outcome and the probability of continued embryonic development and the establishment of pregnancy after fertilisation. The contribution of sperm chromatin instability to reproductive wastage in both natural mating and assisted reproduction warrants further investigation as it may prove valuable as a means of decreasing the incidence of embryonic mortality. In this regard, it is possible that 'male factor' infertility may emerge as an even more important component in embryonic development.

Adverse Effects of 5-Aza-2′-Deoxycytidine on Spermatogenesis Include Reduced Sperm Function and Selective Inhibition of de Novo DNA Methylation

The anticancer agent, 5-aza-2'-deoxycytidine (5-azaCdR, decitabine), causes DNA hypomethylation and a robust, dose-dependent disruption of spermatogenesis. Previously, we have shown that altered testicular histology and reduced sperm production in 5-azaCdR-treated animals is associated with decreased global sperm DNA methylation and an increase in infertility and/or a decreased ability to support preimplantation embryonic development. The goal of this study was to determine potential contributors to 5-azaCdR-mediated infertility including alterations in sperm motility, fertilization ability, early embryo development, and sequence-specific DNA methylation. We find that although 5-azaCdR-treatment adversely affected sperm motility and the survival of sired embryos to the blastocyst stage, the major contributor to infertility was a marked (56-70%) decrease in fertilization ability. Sperm DNA methylation was investigated using Southern blot, restriction landmark genomic scanning, and quantitative analysis of DNA methylation by real-time polymerase chain reaction. Interestingly, hypomethylation was restricted to genomic loci that have been previously determined to acquire methylation during spermatogenesis, demonstrating that 5-azaCdR selectively inhibits de novo methylation activity. Similar to previous studies, we show that mice that are heterozygous for a nonfunctional Dnmt1 gene are partially protected against the deleterious effects of 5-azaCdR; however, methylation levels are not restored in these mice, suggesting that adverse effects are due to another mechanism(s) in addition to DNA hypomethylation. These results demonstrate that clinically relevant doses of 5-azaCdR specifically impair de novo methylation activity in male germ cells; however, genotype-specific differences in drug responses suggest that adverse reproductive outcomes are mainly mediated by the cytotoxic properties of the drug.

Developmental acquisition of genome-wide DNA methylation occurs prior to meiosis in male germ cells

The development of germ cells is a highly ordered process that begins during fetal growth and is completed in the adult. Epigenetic modifications that occur in germ cells are important for germ cell function and for post-fertilization embryonic development. We have previously shown that male germ cells in the adult mouse have a highly distinct epigenetic state, as revealed by a unique genome-wide pattern of DNA methylation. Although it is known that these patterns begin to be established during fetal life, it is not known to what extent DNA methylation is modified during spermatogenesis. We have used restriction landmark genomic scanning (RLGS) and other techniques to examine DNA methylation at multiple sites across the genome during postnatal germ cell development in the mouse. Although a significant proportion of the distinct germ cell pattern is acquired prior to the type A spermatogonial stage, we find that both de novo methylation and demethylation occur during spermatogenesis, mainly in spermatogonia and spermatocytes in early meiotic prophase I. Alterations include predominantly non-CpG island sequences from both unique loci and repetitive elements. These modifications are progressive and are almost exclusively completed by the end of the pachytene spermatocyte stage. These studies better define the developmental timing of genome-wide DNA methylation pattern acquisition during male germ cell development.

Gene Regulation in Spermatogenesis

Mammalian spermatogenesis is a complex hormone-dependent developmental program in which a myriad of events must take place to ensure that germ cells reach their proper stage of development at the proper time. Many of these events are controlled by cell type- and stage-specific transcription factors. The regulatory mechanisms involved provide an intriguing paradigm for the field of developmental biology and may lead to the development of new contraceptives an and innovative routs to treat male infertility. In this review, we address three aspects of the genetic regulatory mechanism that drive spermatogenesis. First, we detail what is known about how steroid hormones (both androgens and estrogens) and their cognate receptors initiate and maintain mammalian spermatogenesis. Steroids act through three mechanistic routes: (i) direct activation of genes through hormone-dependent promoter elements, (ii) secondary transcriptional responses through activation of hormone-dependent transcription factors, and (iii) rapid, transcription-independent (nonclassical) events induced by steroid hormones. Second, we provide a survey of transcription factors that function in mammalian spermatogenesis, including homeobox, zinc-finger, heat-shock, and cAMP-response family members. Our survey is not intended to cover all examples but to give a flavor for the gamut of biological roles conferred by transcription factors in the testis, particularly those defined in knockout mice. Third, we address how testis-specific transcription is achieved. In particular, we cover the evidence for and against the idea that some testis-specific genes are transcriptionally silent in somatic tissues as a result of DNA methylation.

Cytological evaluation of global DNA methylation in mouse testicular genome

Global methylation of DNA from different testicular cell types has been studied by DNA end-labeling and nick translation of fixed chromatin (in situ), following digestion with cytosine methylation-sensitive restriction enzymes. Both at the level of chromatic (chromosome) and naked DNA, there is extensive methylation of the genome. Although the extent of methylation was nearly the same among different cell types in the MspI, HpaII, and HhaI digested end-labelled DNA, in the chromosome preparations the digestion patterns varied in cell type-specific manner, pachytene being the most sensitive and spermatids and sperm the most resistant. The differential sensitivity is attributable to the difference in the chromatin organisation in different testicular cell types though no specific region could be identified as particularly more sensitive or resistant to the enzymes. Pachytene bivalents do not reveal a consistent segmental pattern of digestion, but the perichiasmate regions of diplotene/diakinesis and metaphase I chromosomes show hypersensitivity to the enzymes.

Quantitation by image analysis of global DNA methylation in human spermatozoa and its prognostic value in in vitro fertilization: a preliminary study

OBJECTIVE

To determine the relationship between sperm DNA methylation level and sperm characteristics and pregnancy rates.

DESIGN

Prospective study. Quantitation by image analysis of DNA methylation in sperm nucleus.

SETTING

Department of Reproduction Biology, Edouard Herriot Hospital, Lyon, France.

PATIENT(S)

Infertile couples undergoing IVF-ET.

INTERVENTION(S)

The immunostaining of 5 methyl-cytosine was performed on the spare sperm suspension that was used for an assisted reproduction technology procedure.

MAIN OUTCOME MEASURE(S)

Sperm characteristics according to World Health Organization criteria, sperm motility parameters with computer-assisted semen analysis, sperm DNA methylation level, and heterogeneity index (HI).

RESULT(S)

Sperm DNA methylation level and HI are correlated with sperm DNA characteristics. HI is negatively correlated with fertilization rate; sperm DNA methylation level is correlated with pregnancy rate.

CONCLUSION(S)

The DNA methylation level in human spermatozoa could be a new approach to evaluating the ability of spermatozoa to fertilize and lead to normal embryo development.

Regulation of ALF gene expression in somatic and male germ line tissues involves partial and site-specific patterns of methylation

ALF (TFIIAalpha/beta-like factor) is a germ cell-specific counterpart of the large (alpha/beta) subunit of general transcription factor TFIIA. Here we isolated homologous GC-rich promoters from the mouse and human ALF genes and used promoter deletion analysis to identify sequences active in COS-7 and 293 cells. Further, bisulfite sequence analysis of the mouse ALF promoter showed that all 21 CpG dinucleotides between -179 and +207 were partially methylated in five somatic tissues, brain, heart, liver, lung, and muscle, and in epididymal spermatozoa from adult mice. In contrast, DNA from prepubertal mouse testis and from purified spermatocytes were unmethylated except at C(+19)G and C(+170)G. We also found that ALF expression correlates with a strong promoter-proximal DNase I-hypersensitive site present in nuclei from testis but not from liver. Finally we show that in vitro methylation of the ALF promoter inhibits activity and that 5-aza-2'-deoxycytidine treatment reactivates the endogenous ALF gene in a panel of seven different mouse and human somatic cell lines. Overall the results show that silencing in somatic cells is methylation-dependent and reversible and that a unique CpG-specific methylation pattern at the ALF promoter precedes expression in pachytene spermatocytes. This pattern is transient as remethylation of the ALF promoter in haploid germ cell DNA has occurred by the time spermatozoa are present in the epididymis.

Deoxyribonucleic acid hypomethylation of male germ cells by mitotic and meiotic exposure to 5-azacytidine is associated with altered testicular histology

Genomic methylation patterns originate during gametogenesis and are postulated to be involved in important developmental events, including gene regulation, embryogenesis, and genomic imprinting. In previous work, treatment of male rats with 5-azacytidine, a drug that blocks DNA methylation, resulted in abnormal embryo development when germ cells were exposed throughout spermatogenesis, encompassing mitotic, meiotic, and postmeiotic development, but not if they were only exposed postmeiotically. To explore the mechanisms underlying the effects of 5-azacytidine on sperm function, we determined the effects of the drug on testicular morphology, assessed whether exposure of meiotic spermatocytes resulted in abnormal pregnancy outcome, and examined the role of germ cell genomic demethylation in mediating the effects of 5-azacytidine on spermatogonia and spermatocytes. Male Sprague Dawley rats were treated three times a week with saline or 5-azacytidine (2.5 and 4.0 mg/kg) for 6 weeks (meiotic and postmeiotic germ cell exposure) and 11 weeks (mitotic, meiotic, and postmeiotic exposure). Six weeks of paternal treatment with the highest dose of 5-azacytidine resulted in an increase in preimplantation loss (corpora lutea minus implantation sites) without affecting testicular morphology or altering sperm DNA methylation levels. Eleven weeks of 5-azacytidine treatment at doses that cause preimplantation loss resulted in severe abnormalities of the seminiferous tubules, such as degeneration and loss of germ cells, atrophy of seminiferous tubules, presence of multinuclear giant cells, and sloughing of immature germ cells into the lumen, and a 22-29% decrease in genomic methylation levels in epididymal sperm. On closer evaluation of testicular histology using terminal deoxynucleotidyl transferase-mediated deoxy-UTP nick end-labeling detection in situ, both 6 and 11 weeks of 5-azacytidine treatment resulted in an increase over the control value in the number of apoptotic germ cells in the seminiferous tubules. Analysis of DNA methylation levels in isolated germ cells of treated males indicated that spermatogonia were more susceptible to the hypomethylating effects of 5-azacytidine than were spermatocytes. These studies provide evidence of an association between demethylation of germ cell DNA and alterations in testicular histology.

Distribution of spontaneous CpG-associated G:C --> A:T mutations in the lacZ gene of Muta mice: effects of CpG methylation, the sequence context of CpG sites, and severity of mutations on the activity of the lacZ gene product

In our previous study using transgenic Muta mice, G:C --> A:T transitions at 5'-CG-3' (CpG) sites, which are the most common mammalian spontaneous mutation, were detected in 197 of 330 spontaneous lacZ mutants. These transitions were recovered at only 27 of the 357 mutable G:C pairs within CpG sites where the transition could produce a missense or termination codon in the lacZ gene. To address the underlying mechanism for the uneven distribution of mutated CpG sites, the CpG methylation status of the Muta lacZ gene was analyzed by a bisulfite method. All the CpG sites examined in the coding region were evenly methylated at a high level, and no site-specific methylation was evident. Analysis of the sequence context around the mutated CpG sites, however, revealed that 21 of these 27 sites contained a CpG flanked by a pyrimidine on the 5' side, and that 187 of the 197 mutants resulted from substitutions at these sites. Moreover, we found five hotspots among those sites, the location of which was intimately related to the enzymatic activity of the gene product: one site produced a nonsense codon; three sites, one of which corresponded to the nucleophile at the active site, resided in the substrate-binding pocket; and the other site was located in a region conserved in the beta-galactosidase family. These results strongly suggest that recovery of lacZ mutations at each site largely depend on the adjacent sequence context and the extent to which the mutation damages the enzymatic activity of the gene product.

Recombinant human DNA (cytosine-5) methyltransferase. II. Steady-state kinetics reveal allosteric activation by methylated dna

Initial velocity determinations were conducted with human DNA (cytosine-5) methyltransferase (DNMT1) on unmethylated and hemimethylated DNA templates in order to assess the mechanism of the reaction. Initial velocity data with DNA and S-adenosylmethionine (AdoMet) as variable substrates and product inhibition studies with methylated DNA and S-adenosylhomocysteine (AdoHcy) were obtained and evaluated as double-reciprocal plots. These relationships were linear for plasmid DNA, exon-1 from the imprinted small nuclear ribonucleoprotein-associated polypeptide N, (CGG.CCG)(12), (m(5)CGG. CCG)(12), and (CGG.CCG)(73) but were not linear for (CGG. Cm(5)CG)(12). Inhibition by AdoHcy was apparently competitive versus AdoMet and uncompetitive/noncompetitive versus DNA at </=20 microM AdoMet. Addition of the product (methylated DNA) to unmethylated plasmid DNA increased V(max(app)) resulting in mixed stimulation and inhibition. Velocity equations indicated a two-step mechanism as follows: first, activation of DNMT1 by methylated DNA that bound to an allosteric site, and second, the addition of AdoMet and DNA to the catalytic site. The preference of DNMT1 for hemimethylated DNA may be the result of positive cooperativity of AdoMet binding mediated by allosteric activation by the methylated CG steps. We propose that this activation plays a role in vivo in the regulation of maintenance methylation.

Testes-specific transgene expression in insulin-like growth factor-I transgenic mice

Insulin-like growth factor-I (IGF-I) is a low molecular weight peptide that mediates the cell proliferating actions of growth hormone. Evidence exists indicating that IGF-I is produced by various cell types and this growth factor has been implicated in a variety of reproductive processes. To investigate the effect of IGF-I over-expression on reproductive systems, we generated three independent lines of transgenic mice harbouring a human IGF-I cDNA (hIGF-I) under the control of a Cytomegalovirus immediate early (CMV) promoter. The CMV promoter was used in an attempt to direct expression of IGF-I into a variety of tissues both reproductive and non-reproductive. Yet expression of the foreign hIGF-I gene, determined by Northern blot, was found to occur only in the testicular tissues of the male mice, apparently due to methylation of the transgene in all the tissues tested except the testes, which demonstrate transgene hypomethylation. Evaluation of the transgene expression during testicular development revealed that expression begins between 10 and 15 days of development, coinciding with the appearance of the zygotene and pachytene primary spermatocytes during early spermatogenesis, therefore indicating germ line expression of the transgene. Extensive study of the CMV-hIGF-I transgenic lines of mice has revealed that the effects of the transgene expression do not extend beyond the testicular tissues. No significant differences (P > 0.05) in the IGF-I serum levels, growth rates, or testicular histology have been observed between transgenic and non-transgenic male siblings. The ability of transgenic males to produce offspring also appears unaffected. Evaluation of the IGF binding protein (IGFBP) levels in the testicular tissues of CMV-hIGF-I transgenic mice by Western ligand blot revealed an increase in the concentration of testicular proteins with molecular weights corresponding to IGFBP-2 and IGFBP-3. These results suggest that the testicular over-expression of IGF-I induces increased IGFBP localization in this tissue. Inhibition of IGF activity by the IGFBPs would explain the lack of a dramatic physiological effect in the CMV-hIGF-I transgenic mice, despite the presence of elevated testicular IGF-I. The observation that testis specific IGF-I overexpression induces localization of IGFBPs in this tissue confirms the existence of a well regulated testicular IGF system and supports the convention that this growth factor plays an important role in testicular function.

Myopia, intelligence, and the expanding human neocortex: behavioral influences and evolutionary implications

The first two parts of this monograph document that areas of the human neocortex heavily used to cope with a complex, language-driven society have been expanding rapidly and suggest strongly that this is linked with the huge upsurge that's occurred in myopia, and with the large gradual 20th-century increase in measured intelligence. Part III proposes mechanisms capable of supporting such rapid changes, without violating the basic precepts of Darwin's thinking. Part IV discusses the social and evolutionary ramifications of our apparent proclivity for rapid, progressive, adaptive neocortical change, and suggests areas for productive research.

Molecular mechanisms of male germ cell differentiation

During spermatogenesis, diploid stem cells differentiate, undergo meiosis, and transform into haploid spermatozoa. As this precisely timed series of events proceeds, chromosomal ploidy is reduced and the nucleosomes of the chromatin are replaced by a transcriptionally quiescent protamine-containing nucleus. The premature termination of transcription during the haploid phase of spermatogenesis necessitates an especially prominent role for posttranscriptional regulation in the temporal and spatial expression of many testis-specific proteins and isozymes. In this review article, discussion will focus on novel mechanisms regulating gene expression in mammalian male germ cells from genome to protein.

Molecular mechanisms of male germ cell differentiation

During spermatogenesis, diploid stem cells differentiate, undergo meiosis, and transform into haploid spermatozoa. As this precisely timed series of events proceeds, chromosomal ploidy is reduced and the nucleosomes of the chromatin are replaced by a transcriptionally quiescent protamine-containing nucleus. The premature termination of transcription during the haploid phase of spermatogenesis necessitates an especially prominent role for posttranscriptional regulation in the temporal and spatial expression of many testis-specific proteins and isozymes. In this review article, discussion will focus on novel mechanisms regulating gene expression in mammalian male germ cells from genome to protein.

Sex-specific exons control DNA methyltransferase in mammalian germ cells

The spermatozoon and oocyte genomes bear sex-specific methylation patterns that are established during gametogenesis and are required for the allele-specific expression of imprinted genes in somatic tissues. The mRNA for Dnmt1, the predominant maintenance and de novo DNA (cytosine-5)-methyl transferase in mammals, is present at high levels in postmitotic murine germ cells but undergoes alternative splicing of sex-specific 5′ exons, which controls the production and localization of enzyme during specific stages of gametogenesis. An oocyte-specific 5′ exon is associated with the production of very large amounts of active Dnmt1 protein, which is truncated at the N terminus and sequestered in the cytoplasm during the later stages of oocyte growth, while a spermatocyte-specific 5′ exon interferes with translation and prevents production of Dnmt1 during the prolonged crossing-over stage of male meiosis. During the course of postnatal oogenesis, Dnmt1 is present at high levels in nuclei only in growing dictyate oocytes, a stage during which gynogenetic developmental potential is lost and biparental developmental potential is gained.

Genomic analysis of the mouse protamine 1, protamine 2, and transition protein 2 gene cluster reveals hypermethylation in expressing cells

To understand the role of chromatin structure in the expression of the mouse protamine 1, protamine 2, and transition protein 2 genes during spermatogenesis, we have examined the genomic organization of this cluster of "haploid-specific" genes. As seen in the human genome, protamine 2, transition protein 2, and approximately 2.8 kb of a CpG island, hereafter called CpG island-dTP2, were clustered in a small region. Methylation analyses of this region have demonstrated that i) unlike most other tissue-specific genes, the protamine 1, protamine 2, and transition protein 2 genes were located in a large methylated domain in round spermatids, the cell type where they are transcribed, ii) the protamine 1 gene was only partially methylated in somatic cells and in testes from 7-day-old mice, and iii) the approximately 2 kb upstream and downstream of the CpG island-dTP2 were only partially methylated in somatic tissues. DNase I analysis revealed the presence of at least five strong DNase I hypersensitive sites over the CpG island-dTP2 in somatic tissues, but not in germ cells, and sequence analysis indicated that the CpG island-dTP2 is homologous to a CpG island located approximately 10.6 kb downstream of the human transition protein 2 gene. Although the nature of a CpG island-dTP2 and the function of a CpG island-dTP2-containing somatic tissue-specific DNase I hypersensitive sites in close proximity to the germ cell-specific gene cluster are unclear, the "open" chromatin structure of the CpG island-dTP2 may be responsible for the partial methylation pattern of the flanking sequences including the transition protein 2 gene in somatic tissues.

The role of DNA methylation in cancer genetic and epigenetics

The past few years have seen a wider acceptance of a role for DNA methylation in cancer. This can be attributed to three developments. First, the documentation of the over-representation of mutations at CpG dinucleotides has convincingly implicated DNA methylation in the generation of oncogenic point mutations. The second important advance has been the demonstration of epigenetic silencing of tumor suppressor genes by DNA methylation. The third development has been the utilization of experimental methods to manipulate DNA methylation levels. These studies demonstrate that DNA methylation changes in cancer cells are not mere by-products of malignant transformation, but can play an instrumental role in the cancer process. It seems clear that DNA methylation plays a variety of roles in different cancer types and probably at different stages of oncogenesis. DNA methylation is intricately involved in a wide diversity of cellular processes. Likewise, it appears to exert its influence on the cancer process through a diverse array of mechanisms. It is our task not only to identify these mechanisms, but to determine their relative importance for each stage and type of cancer. Our hope then will be to translate that knowledge into clinical applications.

Genomic imprinting in unstable DNA diseases

Evidence for recombination suppression has been identified in linkage studies of several unstable DNA diseases. Also sex-specific changes in recombination frequency have been detected at the loci of Huntington's disease and myotonic dystrophy. It can be hypothesized that meiotic recombination is regulated by genome-wide genomic imprinting and that changes in meiotic recombination imply the presence of the genomic imprinting defect. If aberrant recombination at the locus of trinucleotide repeat expansion is verified, new theoretical and experimental opportunities will arise in studies on the role of genomic imprinting in the unstable DNA disease.

DNA methyltransferase in normal and Dnmtn/Dnmtn mouse embryos

The mouse genome experiences a large decrease in net 5-methylcytosine between fertilization and implantation; de novo methylation brings 5-methylcytosine to adult somatic cell levels between implantation and gastrulation. Very little is known of the regulation of demethylation or de novo methylation. Levels of the one known form of DNA methyltransferase are very high in early embryos, but the enzyme is localized to the cytoplasm during most of preimplantation development. We show here that DNA methyltransferase is found exclusively in nuclei of the conceptus after implantation, and that nuclei of proximal decidual cells are free of detectable DNA methyltransferase. High levels of DNA methyltransferase were seen in all tissues, including the developing nervous system, of 9.5- to 12.5-day embryos. The large maternal stores of DNA methyltransferase become limiting prior to embryonic day 9.5, as shown by barely detectable immunostaining in 9.5-day embryos homozygous for a loss-of-function mutation (Dnmtn) in the DNA methyltransferase gene. These mutant embryos failed to develop past the 25-somite stage and showed evidence of developmental delay and some developmental asynchrony. Normal embryonic and extraembryonic tissues contained similar levels of DNA methyltransferase, even though severely reduced methylation levels and a loss of imprinting have previously been observed in extraembryonic tissues. These findings suggest that methylation patterns are not a simple function of the concentration of DNA methyltransferase, and that unidentified factors must be involved in the regulation of de novo methylation during early development of the mouse.

DNA methylation patterns in human tissues of uniparental origin using a zinc-finger gene (ZNF127) from the Angelman/Prader-Willi region

In order to further our understanding of the epigenetic modifications of DNA and its role in imprinting, we examined DNA methylation patterns of human tissues of uniparental origin. We used complete hydatidiform moles (CHM), which are totally androgenetic conceptions, to examine the paternal methylation pattern in the absence of a maternal contribution and we used ovarian teratomas to represent the maternal counterpart. We carried out an analysis of DNA methylation of a gene which has been shown to contain sites which are differentially methylated in a parent-specific fashion. The gene, ZNF127, is located on chromosome 15q11-q13 in the region associated with Prader-Willi and Angelman syndromes. The parent-of-origin DNA methylation has been postulated to reflect the presence of an imprint and recent studies have confirmed that ZNF127 is differentially expressed only from the paternal chromosome. We identified a unique pattern of hyper- and hypomethylated sites in androgenetic conceptions which was nearly identical to the paternal pattern found in sperm. This may represent the paternal germ-line methylation imprint. We also studied partial hydatidiform moles, non-molar triploid conceptions, normal chorionic villi, and somatic tissue. These all demonstrated a modified DNA methylation pattern characteristic of normal chorionic villi with only limited findings of the imprint. Our results suggest that human androgenetic conceptions may provide an excellent model to analyze epigenetic DNA modifications, such as methylation, in imprinted genes. The paternal allele-specific methylation imprint will also be useful clinically to confirm the androgenetic nature of suspected molar conceptions in which parental blood samples may not be available.

Organization and analysis of the complete rat calmodulin-dependent protein kinase IV gene

A 42-kilobase pair region of rat DNA containing the Ca2+/calmodulin-dependent protein kinase IV (CaM kinase IV) gene has been cloned and characterized. The gene consists of 12 exons and 11 introns and is predicted to encode both beta and alpha forms of CaM kinase IV as well as the testis-specific calmodulin-binding protein calspermin. The promoter utilized to generate the alpha-kinase isoform is located in intron 1, whereas the promoter utilized to produce the calspermin transcript is contained in intron 10. The calspermin promoter region which extends from -200 to +321 relative to the calspermin transcription initiation site that contains two cyclic AMP response elements (CRE) at -70 and -50 and has been shown previously to be inactive in NIH3T3 cells (Sun, Z., Sassone-Corsi, P., and Means, A. R. (1995) Mol. Cell. Biol. 15, 561-571) was ligated to the lacZ reporter gene and used to generate transgenic mice. The promoter was expressed exclusively in postmeiotic testis where beta-galactosidase was found predominantly in elongating spermatids. The cell and developmental specificity of transgene expression was very similar to the pattern shown by the endogenous gene. Although the transgene promoter was silent in somatic tissues, beta-galactosidase expression could be restored in primary cultures of skin fibroblasts by introduction of vectors encoding CREM tau and CaM kinase IV.

Isolation of a novel cDNA that encodes a protein localized to the pre-acrosome region of spermatids

We have identified a novel cDNA clone, named AZ1, obtained from a cDNA library of mRNA prepared from C3H10T1/2 cells that had been transiently exposed to 5-azacytidine, a potent demethylating reagent. The amount of transcript increased with 5-azacytidine treatment of C3H10T1/2 cells and the transcript was highly expressed in mouse testis. As the mutant mouse jsd/jsd, which has a defect in germ cell maturation, barely expressed the transcript, the message was expected to be expressed specifically in spermatocytes. The mRNA was detected at significant levels in the testes from mice aged 16 days after birth, suggesting that its expression started at the pachytene spermatocyte stage. The elucidated nucleotide sequence contained a 2841-nucleotide open reading frame, and the expected amino acid sequence had a molecular mass of 107,254 Da. Specific antibodies raised against the fusion protein including glutathione S-transferase revealed an approximately 130-kDa band of a translation product in testis and in cultured cells transfected with AZ1 cDNA in the expression vector on Western-blot analysis. The protein was localized to the pre-acrosome region of round and elongated spermatids. However, it was not detected at a more advanced stage of spermatids, i.e. just before their release from Sertoli cells. This protein may play an important role in spermatogenesis.

5-Aza deoxyCytidine-induced inhibition of differentiation of spermatogonia into spermatocytes in the mouse

In order to explore the significance of DNA methylation in proliferation and differentiation of germ cells in testis, 5-aza,2'-deoxyCytidine (5-azaCdR), a hypomethylating agent, was administered in vivo to neonatal mice having only spermatogonial (premeiotic) cells. End-labeling of the MspI, HpaII, and HhaI digested DNA revealed considerable loss of methylation following the treatment. Cellular and histological preparations of the testis showed complete inhibition of differentiation into spermatocytic stage. Analysis of protein synthesis in the treated and control testis by growing the cells in 35S-Methionine medium and resolving the lysate by SDS-PAGE revealed that the programme of expression of at least 5 polypeptides (35.0, 31.5, 27.0, 22.5, and 18.0 KD) was altered as a result of 5-azaCdR incorporation. It appears that DNA methylation plays a critical role in the differentiation of gonia into primary spermatocytes.

Germ line specific factors in chemical mutagenesis

Chemical mutagenesis test results have not revealed evidence of germ line specific mutagens. However, conventional assays have indicated that there are male-female differences in mutagenic response, as well as quantitative/qualitative differences in induced mutations which depend upon the particular cell stage exposed. Many factors inherent in the germ line can be speculated to influence chemical transport to, and interaction with, target cell populations to result in mutagenic outcomes. The level of uncertainty regarding the general operation of such factors, in combination with the limited availability of chemical test data designed to address comparative somatic and germ cell mutagenesis, leaves open the question of whether there are mutagens specifically affecting germ cells. This argues for a conservative approach to interpreting germ cell risk from somatic cell mutation analysis.

Mechanism of active DNA demethylation during embryonic development and cellular differentiation in vertebrates

Incubation of hemimethylated and labelled oligodeoxynucleotides with nuclear extracts from differentiating chicken embryos and mouse myoblasts resulted in the replacement of m5C by C. One of the enzymes involved is m5CpG endonuclease. It cleaves only m5CpG and not, m5CpT, m5CpA, m5CpC or m6ApT. The enzyme is not sequence specific and catalyses the reaction in the presence of high concentrations of EDTA or EGTA.

The CpG-rich promoter of human LDH-C is differentially methylated in expressing and nonexpressing tissues

A comparison of nucleotide sequences of murine Ldh-a and Ldh-c genes and human LDH-A, LDH-B, and LDH-C reveals that mouse Ldh-c has lost the CpG "island" present in the genes for the somatic isozymes. However, the human LDH-C gene has a CpG-rich region of 230 bp surrounding its promoter. Endonuclease sensitivity coupled with polymerase chain reaction (PCR) demonstrate the presence of nine heavily methylated sites in this region in different somatic cells. The same sites are specifically hypomethylated in expressing tissues. 3' sites bordering the CpG-rich region appear to be methylated in both expressing and nonexpressing tissues. Furthermore, the methylated promoter forms a specific complex in vitro with a methyl-DNA binding protein. Evolutionary and functional implications of these observations are discussed.

Genomic imprinting: lessons from mouse transgenes

Genomic imprinting is a non-Mendelian form of inheritance that results in an expression difference between the two parental alleles of an autosomal locus. The study of mouse transgenes has provided us with descriptions of a variety of imprinting or parent-of-origin effects, thereby anticipating similar inheritance phenomena in non-transgenic mice. Many mouse transgenes exhibit parent-of-origin behavior only on mixed strain backgrounds, whereas others are imprinted on inbred strain backgrounds. In the former cases, the parent-of-origin effects are due to strain-specific modifiers of DNA methylation and expression. These are inherited in a parent-specific fashion and exert their effects after fertilization. In the latter cases, true germline transgene imprinting, the creation of an imprinted locus occurs in a series of sequential steps. First, there is an erasure of the imprint from the previous generation in both male and female fetal germ cells. Second, upon completion of gametogenesis, distinctive methylation patterns have been placed on the transgene sequences of the two mature gametes. Third, only one of these inherited patterns is maintained in the early, pre-implantation embryo. The pattern of the other parental allele is erased. Finally, the methylation pattern of the alleles evolve in the later stages of development, but nonetheless the methylation difference (imprint) of the locus persists. Transgene imprinting behaviors, either on mixed strain backgrounds and on inbred genetic backgrounds, have counterparts in endogenous genetic phenomena.

DNA methylation changes during mouse spermatogenesis

Genomic imprinting in mammals is thought to be mediated by differences in the methylation level of cytosine residues in the genome. These differences in DNA methylation are thought to be generated during the development of the germ line. To characterize the profile of global methylation of the mouse genome during male gametogenesis, we have quantified the relative level of methylation in individual cells during meiosis and spermatogenesis. A decrease in the level of DNA methylation is observed from meiotic cells to elongated spermatids. The erasure of the somatic pattern of methylation during spermatogenesis suggests the existence of a subsequent mechanism generating the parental specific methylation patterns leading to genomic imprinting of specific alleles.

Differences between human sperm and somatic cell DNA in CpG methylation within the HLA class I chromosomal region

PROBLEM

We investigated the possible negative regulatory mechanisms that repress classical human leukocyte antigen (HLA) class I gene expression in human spermatozoa and searched for novel testis-specific coding sequences that might be present in MHC class I chromosomal region.

METHOD

We performed a comparative DNA methylation analysis of this genomic region in both purified human spermatozoa and mononuclear blood cells from the same donors, using methylation-sensitive restriction enzymes followed by classical or pulsed field gel electrophoresis and hybridization with HLA class I locus-specific probes.

RESULTS

Unmethylated CpG sites were detected in the 3' part of HpaII tiny fragments of the HLA-F and HLA-G genes in spermatozoal DNA. In contrast, no difference was observed in the methylation status of the HLA-B, HLA-C, and HLA-E genes between germ and somatic cells. CpG unmethylation events were also detected in several parts of this chromosomal region (outside the known loci) in spermatozoal DNA.

CONCLUSIONS

These results suggest that this genomic region undergoes changes in its DNA methylation pattern during the developmental process. We hypothesize that these dynamic changes have functional importance, including a possible transcriptional activity of nonclassical class I genes and/or as yet undescribed testis-specific coding sequences.

DNA methylation of the fragile X locus in somatic and germ cells during fetal development: relevance to the fragile X syndrome and X inactivation

To obtain insights into mechanisms responsible for methylation of CpG islands on the inactive X chromosome of normal females, we examined methylation of the fragile X (FraX) locus in a variety of tissues from normal fetuses and adults, and from males with the FraX syndrome. We identified 20 CCGG sites (MspI-HpaII sites M1-M20) within a 12-kb BglII fragment that includes the CpG island. Sites M3-M18, within the 1.2-kb CpG island are unmethylated on the active X in normal males and females at all ages and in all tissues studied. In contrast, these sites are at least partially methylated on the inactive X chromosome in a variety of tissues from normal females by six weeks from conception. The exceptional tissues are chorionic villi and gonads, which are significantly undermethylated. In addition, fetal germ cells are unmethylated at site M3, which is methylated on the inactive X in other tissues; thus, the methylation imprint of the inactive X has been erased. Methylation of the locus on the fragile X chromosome is similar to that of the normal inactive X but is more extensive and less heterogeneous. This suggests that the expansion of the island and the greater number of CpGs that result from amplification of the CGG repeat enhance the methylatibility of the island. Additional studies show that the chromatin of the CpG island is nuclease hypersensitive on the active X but insensitive on both inactive and FraX.

Gene expression, X-inactivation, and methylation during spermatogenesis: the case of Zfa, Zfx, and Zfy in mice

While it has become clear that X-inactivation in the female soma is complete in mouse (in contrast to being "patchy" in man), the degree of X-inactivation in the testes has not been ascertained. We have compared autosomal and X-linked zinc finger homolog expression and X-linked and Y-linked zinc finger homolog methylation in an attempt to elucidate this question. Using RTPCR, we have extended earlier studies of Zfx and Zfa expression in developing testes and find that Zfa expression starts at the time of X-inactivation while Zfx expression is continuous. Cell separation studies did not preclude continued expression of Zfx in adult germ cells. The methylation status of four CCGG residues in the Zfx promoter was studied using PCR bridging this region before and after DNA digestion with the isoschizomers Msp I and Hpa II, the latter being methylation sensitive. Hpa II resistant Zfx promoter DNA was found in all female tissues, but not in male tissues, including the testes. Previous studies have shown that Zfy is expressed at meiosis (like Zfa and unlike Zfx). Despite its expression, the Zfy gene is adjacent to, or contains, highly methylated CCGG sites since hybridization after Msp I digestion detected multiple small fragments that were not released after DNA digestion with Hpa II. Thus, Zfx is not methylated in sperm, while Zfy is, in contrast to their apparent patterns of expression.

Molecular analysis of human chromosome 16 cosmid clones containing NotI sites

To test the feasibility of using cloned NotI sites as markers for physical mapping, we have screened for cosmid clones spanning the NotI sites on human Chromosome (Chr) 16. Fluorescence in situ hybridization analysis of these clones confirms the previously reported cluster of NotI sites on 16p13.3. Methylation status of the cloned NotI sites on genomic DNA was established by hybridization of the cosmids to Southern blots containing EcoRI and EcoRI/NotI digest of genomic DNA. These results indicated that four of six clones included in our study can be used as linking clones for physical mapping. Two clones have NotI sites which are not cleavable in the cell lines tested. In one clone, the NotI site exists as an isolated rare-cutting restriction enzyme site, whereas in the other clone the NotI site appears to be island-related.

A cytochemical and immunocytochemical study of DNA distribution in spermatid nuclei of mouse, rabbit, and bull

DNA distribution in mouse, rabbit and bull spermatids was analyzed by electron microscopy, after using a Feulgen-like HCl-osmium ammine procedure, and after immunocytochemistry with anti-DNA antibodies. In addition, nucleic acids were visualized with the intercalating dye ethidium bromide and phosphotungstic acid. The parts of DNA displaying a beta helix configuration (possibly A-T rich parts) were identified by epifluorescence microscopy after staining with Hoechst 33258. In all 3 species, young spermatid nuclei were seen to have large areas poor in DNA, as well as DNA-rich areas, which were mostly concentrated into a peripheral layer close to the acrosome and into one or several masses, displaying species-specific locations. These DNA-rich areas were stained with Hoechst 33258. Elongating spermatid nucleic contained homogeneously distributed DNA, and this was evident following both immunocytochemistry and nucleic acid histochemistry in all 3 species. However, the distribution appeared more heterogeneous after the Feulgen-like procedure, and was accompanied by a disappearance of Hoechst-fluorescence. In fully elongated spermatids, all nuclear areas stained with Hoechst 33258, while the 3 other techniques labeled either all or species-specific parts of the condensed chromatin. The reasons for these variable reactions are discussed in terms of technique specificities, DNA configuration and nucleoprotein moiety replacements.

Reversible inactivation of a transgene in Arabidopsis thaliana

Fifty percent of Arabidopsis thaliana plants transgenic for a hygromycin resistance gene failed to transmit the resistance phenotype to the progeny. The complete transgene was, however, inherited in all cases according to Mendelian laws as observed by Southern analysis. This discrepancy between genotype and phenotype was the result of a reduced level of transcript in the sensitive transformants. The gene inactivation occurred in plants with multicopy integration of the foreign DNA. No definite correlation was found between gene inactivity and methylation of cytidine residues in the transgene sequence. Explants from several sensitive transformed plants regained a low level of hygromycin resistance on callus induction medium. Subsequent generations obtained by self-pollination were sensitive. In contrast, spontaneous restoration of hygromycin tolerance was observed in seedlings originating from out-crosses with wild-type plants or a different sensitive transformant. A reduction of the copy number was not a prerequisite for spontaneous reactivation. The resistance was often lost again in the next generation. Inactivation and reactivation of the transgene are therefore reversible.

Programmed demethylation in CpG islands during human fetal development

The mechanism for establishing the DNA methylation patterns observed in adult mammalian tissues is not well understood. To determine when adult patterns are established for housekeeping genes, we examined the clustered CpGs in genes on the human active X chromosome (PGK, G6PD, P3, GdX, HPRT) and the autosomal gene, DHFR. We find unique methylation patterns present at the P3 locus in all tissues analyzed from 6- to 9-week fetal specimens, and at the HPRT locus in adrenal gland DNA at this stage of development. Adult patterns are established subsequently by demethylating specific CpGs. Our results show that demethylating events affecting CpG islands are programmed during mammalian fetal development. They suggest that the process of de novo methylation in the fetus methylates at least some sites in the 3' region of the CpG islands in active genes and that adult patterns are established at 6-14 weeks developmental age by sequence-specific demethylation.