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Biomedical subjects

H Cedar

Publications and source records attributed to H Cedar.

At least 37 records · Page 2Linked to original sources
PubMed1996

A role for nuclear NF-kappaB in B-cell-specific demethylation of the Igkappa locus.

The immunoglobulin kappa gene is specifically demethylated during B-cell maturation in a process which utilizes discrete cis-acting modules such as the intronic kappa enhancer element and the matrix attachment region (MAR). While any MAR sequence is sufficient for this reaction, mutation analysis indicates that tissue specificity is mediated by kappaB binding sequences within the kappa intronic enhancer. The plasmacytoma cell line S107 lacks kappaB binding activity and fails to demethylate the kappa locus. However, B-cell specific demethylation is restored by the introduction of an active kappaB binding protein gene relB. This represents the first demonstration of a trans-acting factor involved in cell-type-specific demethylation, and suggests that the same protein-DNA recognition system used for transcription may also contribute to the earlier developmental events that bring about activation of the kappa locus.

B-Lymphocytes↗
PubMed1995

Gamete-specific methylation correlates with imprinting of the murine Xist gene.

We have investigated the potential role of DNA methylation as a regulator of imprinted Xist expression in mouse preimplantation embryos. The active paternal allele was found to be unmodified in sperm at CpG loci near the 5' end of the gene transcription unit. In contrast, on the inactive maternal allele, these same sites are initially methylated in the oocyte and then remain modified in the early embryo. In the male germ line, these methyl moieties are removed during spermatogenesis, and this occurs before the programmed reactivation of Xist in the testis. This represents a clear-cut example of a potential methylation imprinting signal that is reprogrammable and gamete derived.

Alleles↗
PubMed1994

Sp1 elements protect a CpG island from de novo methylation.

Animal somatic cell DNA is characterized by a bimodal pattern of methylation: tissue-specific genes are methylated in most cell types whereas housekeeping genes have 5' CpG islands which are constitutively unmethylated. Because methyl moieties derived from the gametes are erased in the morula and early blastula, this profile must be re-established in every generation; this is apparently accomplished by a wave of non-CpG island de novo methylation that occurs at implantation. Using transfection into embryonic stem cells and transgenic mice as a model system, we now show that Sp1 elements play a key role in protecting a CpG island in the adenine phosphoribosyltransferase (APRT) gene from de novo methylation. This recognition mechanism represents a critical step in embryogenesis, as it is responsible for setting up the correct genome methylation pattern which, in turn, is involved in regulating basal gene expression in the organism.

Adenine Phosphoribosyltransferase↗
PubMed1994

Allelic inactivation regulates olfactory receptor gene expression.

We suggest a model in which a hierarchy of controls is exerted on the family of odorant receptor genes to assure that a sensory neuron expresses a single receptor from a family of 1000 genes. We propose that a cis-regulatory element directs the stochastic expression of only one gene from a large array of linked receptor genes. Moreover, only one allelic array encoding multiple receptor genes is active in an individual neuron. We demonstrate that in a neuron expressing a given receptor, expression derives exclusively from one allele. In addition, we observe that alleles encoding the odorant receptors are replicated asynchronously, a phenomenon consistently associated with allelic inactivation. This model, involving inactivation of one allelic array and cis control of the active array, provides a mechanism such that individual neurons express one or a small number of receptors.

Alleles↗
PubMed1994

B cell-specific demethylation: a novel role for the intronic kappa chain enhancer sequence.

We studied the molecular mechanism of demethylation and its role in kappa chain gene regulation. Following transfection into B cell cultures, this gene undergoes regional demethylation in a process that is developmentally regulated in a lineage- and stage-specific manner. Although a germline V kappa promoter is not required for the demodification activity, a fragment containing the intronic kappa chain transcriptional enhancer and the nearby matrix attachment region is essential. In its natural location downstream to the J kappa 5 sequence, this element induces bidirectional demodification of plasmid constructs in a distance- and orientation-independent manner. When this enhancer is placed in an upstream position, however, the kappa gene remains modified and transcriptionally inactive, demonstrating that demethylation is required for kappa chain activation. These studies suggest that the kappa enhancer plays a dual role in regulating B cell differentiation by inducing demethylation and by promoting tissue-specific transcription.

Animals↗
PubMed1994

Role of DNA methylation in the regulation of transcription.

DNA methylation plays an important role in the regulation of gene expression during development. Methyl moieties at CpG residues suppress transcription by affecting DNA-protein interactions, thus altering the accessibility of genes to trans-acting factors in the cell. Because it works in cis, this mechanism is important in the control of X inactivation and genomic imprinting.

Animals↗
PubMed1994

Developmental changes in methylation of spermatogenesis-specific genes include reprogramming in the epididymis.

We have determined the status of DNA methylation at specific sites in three spermatogenesis-specific genes, Pgk-2, ApoA1 and Oct-3/4, throughout the development and differentiation of male germ cells in the mouse. We observed a specific demethylation event in the Pgk-2 gene in prospermatogonia at about the time of birth, about 10 days before the onset of transcription which first occurs in primary spermatocytes. All three genes were unmethylated in adult spermatogenic cells in the testis, but were remethylated in mature spermatozoa in the vas deferens. Surprisingly, we found that this remethylation is part of the process of sperm maturation which occurs in the epididymis.

5-Methylcytosine↗
PubMed1993

Replication structure of the human beta-globin gene domain.

The animal cell genome is organized into a series of replicons with an average size of 50-300 kilobases; each of these units is characterized by its own origin of replication which serves as the point of initiation for DNA synthesis. In animal viruses, origin usage can be regulated by cis-acting elements, and in some cases, replication may be cell-type specific. Little is known, however, about the organization and control of endogenous tissue-specific gene replication. To understand this process, we have used a replication direction assay to examine DNA fragments covering more than 200 kilobases of the human beta-like globin domain, and have identified a single bidirectional origin located upstream of the beta-globin itself. This locus is used to initiate DNA synthesis in expressing cells, where the globin domain replicates early, and in non-expressing cells, which are characterized by late replication of the same region. Deletion of this origin sequence, as occurs in the haemoglobin Lepore syndrome, cancels bidirectional DNA synthesis at this site and leads to a striking reversal of replication direction upstream to the locus. This represents the first genetic proof of the existence of specific, discrete origins of replication in animal cells.

Cell Line↗
PubMed1993

Allele-specific replication timing of imprinted gene regions.

Several lines of evidence suggest that the paternal and maternal genomes may have different expression patterns in the developing organism and this has been confirmed by the identification of endogenous genes that are parentally imprinted in the mouse. Little is known about the precise mechanisms involved in the process, but structural differences between the two alleles must somehow provide cis-acting signals for directing parental-specific transcription. Cell-cycle replication time is one parameter that has been shown to be associated with both tissue-specific gene expression and the allele-specific transcription patterns of the X chromosomes in female cells. For this reason we have examined the replication timing patterns for the chromosomal regions containing the imprinted genes Igf2, Igf2r, H19 and Snrpn in the mouse. At all of these sites, and their corresponding positions in the human genome, the two homologous alleles replicate asynchronously and it is always the paternal allele that is early-replicating. Thus imprinted genes appear to be embedded in large DNA domains with differential replication patterns, which may provide a structural imprint for parental identity.

Alleles↗
PubMed1993

Maternal-specific methylation of the imprinted mouse Igf2r locus identifies the expressed locus as carrying the imprinting signal.

The mouse insulin-like growth factor type 2 receptor (Igf2r) is imprinted and expressed exclusively from the maternally inherited chromosome. To investigate whether methylation could function as the imprinting signal, we have cloned 130 kb from the Igf2r locus and searched for sequences methylated in a parental-specific manner. Two regions have been identified: region 1 contains the start of transcription and is methylated only on the silent paternal chromosome; region 2 is contained in an intron and is methylated only on the expressed maternal chromosome. Methylation of region 1 is acquired after fertilization, in contrast with the methylation of region 2, which is inherited from the female gamete. Methylation of region 2 may mark the maternal Igf2r locus in a manner that could act as an imprinting signal. These data suggest that the expressed locus carries a potential imprinting signal and imply that methylation is necessary for expression of the Igf2r gene.

Amino Acid Sequence↗
PubMed1993

The role of XbaI polymorphism of the apolipoprotein B gene in determining levels and covariability of lipid and lipoprotein variables in a sample of Israeli offspring with family history of myocardial infarction.

We have determined the frequency of DNA polymorphism of the gene for human apolipoprotein B detected with XbaI in 525 Israeli offspring whose parents experienced a myocardial infarction. The relative frequencies of the X1 (8.6 kb) and X2 (5.0 kb) alleles were 0.67 and 0.33, respectively, with no significant differences between males and females and across the different origin groups. Significant variation in sex, age and body mass adjusted plasma levels of cholesterol (P = 0.02), LDL-C (P = 0.02) and apo B (P = 0.03) were associated with the XbaI polymorphism. An interaction with age was demonstrated. For young individuals a simple codominant association of the XbaI site with cholesterol and LDL-C was evident and the differences between the two homozygote groups ranged between 22 and 25 mg/dl. For individuals above age 25 these differences were about 12 mg/dl with no significant difference between the X1X2 and the X2X2 genotype groups. In our study sample the apo B XbaI polymorphism accounted for 1% of the variability of plasma cholesterol, LDL-C and apo B levels. The XbaI polymorphism also had an effect on the associations among lipid and lipoprotein variables. In conclusion, we have demonstrated an association of the apo B XbaI polymorphism with the metabolism of the apo B-containing lipoprotein particles in a sample of Israeli offspring with a family history of myocardial infarction.

Adult↗
PubMed1993

Dynamics of DNA methylation during development.

DNA methylation plays a role in the repression of gene expression in animal cells. In the mouse preimplantation embryo, most genes are unmethylated but a wave of de novo methylation prior to gastrulation generates a bimodal pattern characterized by unmethylated CpG island-containing housekeeping genes and fully modified tissue-specific genes. Demethylation of individual genes then takes place during cell type specific differentiation, and this demodification may be a required step in the process of transcriptional activation. DNA modification is also involved in the maintenance of gene repression on the inactive X chromosome in female somatic cells and the marking of parental alleles at genomically imprinted gene loci.

Animals↗
PubMed1993

The ontogeny of allele-specific methylation associated with imprinted genes in the mouse.

We have investigated the DNA methylation patterns in genomically imprinted genes of the mouse. Both Igf2 and H19 are associated with clear-cut regions of allele-specific paternal modification in late embryonic and adult tissues. By using a sensitive PCR assay, it was possible to follow the methylation state of individual HpaII sites in these genes through gametogenesis and embryogenesis. Most of these CpG moieties are not differentially modified in the mature gametes and also become totally demethylated in the early embryo in a manner similar to non-imprinted endogenous genes. Thus, the overall allele-specific methylation pattern at these sites must be established later during embryogenesis after the blastula stage. In contrast, sites in an Igf2r gene intron and one CpG residue in the Igf2 upstream region have allele-specific modification patterns which are established either in the gametes or shortly after fertilization and are preserved throughout pre-implantation embryogenesis. These studies suggest that only a few DNA modifications at selective positions in imprinted genes may be candidates for playing a role in the maintenance of parental identity during development.

Alleles↗
PubMed1993

XbaI polymorphism of the apolipoprotein B gene and plasma lipid and lipoprotein response to dietary fat and cholesterol: a clinical trial.

A dietary trial was carried out on a group of offspring whose parents were hospitalized for an acute myocardial infarction. The XbaI Restriction Fragment Length Polymorphism (RFLP) was used to examine the genetic contribution of variation at this apo B locus to the response of lipids and lipoproteins to dietary manipulations. Twenty participants were homozygotes for the 8.0 kb fragment (X1X1), two were homozygotes for the 5.0 kb fragment (X2X2), and 15 were heterozygotes (X1X2). Subjects were randomized to a 5-week crossover study. Half began on a low SFA--cholesterol (LSC) diet for 5 weeks and, after a washout period of 4 weeks, they were placed on a high SFA--cholesterol (HSC) diet for a second period of 5 weeks. This order was reversed in the second group of participants. Significant changes in total cholesterol, LDL-C, and apo B were observed when subjects were moved from the LSC to the HSF diet. The corresponding average change induced by the dietary manipulations in X1X1 subjects compared with subjects with X2 allele were: 18.1 +/- 17.6 mg/dl and 9.5 +/- 19.6 mg/dl for total cholesterol and 15.8 +/- 15.3 mg/dl and 4.8 +/- 20.9 mg/dl for LDL-C, respectively. Our observation indicated that variation at the apo B XbaI locus may interact with baseline levels to determine individual dietary response in LDL-C level. However, the differences between the genotypic classes were not statistically significant, suggesting that the apo B XbaI locus is not a major determinant of interindividual differences in lipid and lipoprotein response to diet in this population.

Adult↗