Modulation of tk expression in mouse pericentromeric heterochromatin

Nuclear topography of the c-myc gene in human leukemic cells

The c-myc gene plays an essential role in the regulation of the cell cycle and differentiation. Therefore, changes of the c-myc positioning during differentiation are of great interest. As a model system of cell differentiation, the HL-60 and U-937 human leukemic cell lines were used in our experiments. These cells can be induced to differentiation into granulocytes that represent one of the pathways of blood cell maturation. In this study, changes of the topographic characteristics of the c-myc gene (8q24), centromeric region of chromosome 8 and chromosome 8 domain during differentiation of HL-60 and U-937 cells were detected using fluorescence in-situ hybridisation (FISH). FISH techniques and fluorescence microscopy combined with image acquisition and analysis (high-resolution cytometry) were used in order to detect the topographic features of nuclear chromatin. Increased centre of nucleus-to-gene and gene-to-gene distances of c-myc genes, centromeric region of chromosome 8 and chromosome 8 domains were found early after the induction of granulocytic differentiation by dimethyl sulfoxide (DMSO) or retinoic acid (RA); the size of the chromosome 8 domains was rapidly reduced. In differentiated cells, c-myc is located at greater distances from the centromeric regions of chromosome 8. These results support the idea that relocation of the c-myc gene to the nuclear periphery and the condensation of the chromosome 8 domain might be associated with the c-myc gene expression due to common kinetics during granulocytic differentiation.

Nuclear organization and transcriptional silencing in yeast

Transcriptional repression at the yeast silent mating type loci requires the formation of a nucleoprotein complex at specific cis-acting elements called silencers, which in turn promotes the binding of a histone-associated Sir-protein complex to adjacent chromatin. A similar mechanism of long-range transcriptional repression appears to function near telomeric repeat sequences, where it has been demonstrated that Sir3p is a limiting factor for the propagation of silencing. A combined immunofluorescence/in situ hybridization method for budding yeast was developed that maintains the three-dimensional structure of the nucleus. In wild-type cells the immunostaining of Sir3p, Sir4p and Rap1 colocalizes with Y' subtelomeric sequences detected by in situ hybridization. All three antigens and the subtelomeric in situ hybridization signals are clustered in foci, which are often adjacent to, but not coincident with, nuclear pores. This colocalization of Rap1, Sir3p and Sir4p with telomeres is lost in sir mutants, and also when Sir4p is overexpressed. To test whether the natural positioning of the two HM loci, located roughly 10 and 25 kb from the ends of chromosome III, is important for silencer function, a reporter gene flanked by wild-type silencer elements was integrated at various internal sites on other yeast chromosomes. We find that integration at internal loci situated far from telomeres abrogates the ability of silencers to repress the reporter gene. Silencing can be restored by creation of a telomere at 13 kb from the reporter construct, or by insertion of 340 bp of yeast telomeric repeat sequence at this site without chromosomal truncation. Elevation of the internal nuclear pools of Sir1p, Sir3p and Sir4p can relieve the lack of repression at the LYS2 locus in an additive manner, suggesting that in wild-type cells silencer function is facilitated by its juxtaposition to a pool of highly concentrated Sir proteins, such as those created by telomere clustering.

Chromatin organization in the homogeneously staining regions of a methotrexate-resistant mouse cell line: interspersion of inactive and active chromatin domains distinguished by acetylation of histone H4

We have analyzed the organization of the homogeneously staining regions (HSRs) in chromosomes from a methotrexate-resistant mouse melanoma cell line. Fluorescence in situ hybridization techniques were used to localize satellite DNA sequences and the amplified copies of the dihydrofolate reductase (DHFR) gene that confer drug-resistance, in combination with immunofluorescence using antibody probes to differentiate chromatin structure. We show that the major DNA species contained in the HSRs is mouse major satellite, confirming previous reports, and that this is interspersed with DHFR DNA in an alternating tandem array that can be resolved at the cytological level. Mouse minor satellite DNA, which is normally located at centromeres, is also distributed along the HSRs, but does not appear to interfere with centromere function. The blocks of major satellite DNA are coincident with chromatin domains that are labelled by an autoantibody that recognizes a mammalian homologue of Drosophila heterochromatin-associated protein 1, shown previously to be confined to centric heterochromatin in mouse. An antiserum that specifically recognizes acetylated histone H4, a marker for active chromatin, fails to bind to the satellite DNA domains, but labels the intervening segments containing DHFR DNA. We can find no evidence for the spreading of the inactive chromatin domains into adjacent active chromatin, even after extended passaging of cells in the absence of methotrexate selection.

Effect of cis-located human satellite DNA on the electroporation efficiency of neo and HSV-1 tk containing plasmids

Highly repetitive satellite DNAs comprise a significant portion of higher eukaryotic genomes and have been implicated in a variety of chromosome processes, such as centromere structure and function, that are related to their presence in heterochromatin. In addition, heterochromatin can induce metastable expression of adjacent genes. However, the role of highly repetitive satellite DNAs in these effects remains to be elucidated. In an effort to address this question, plasmids containing a human 1797-bp EcoRI satellite II DNA, plus the neo and the HSV-1 tk genes, were electroporated into a TK-/NEO- human cell line. The presence of the satellite DNA sequences within the electroporated plasmids was found to interfere with the generation of stable TK+, but not NEO+, transfectants depending on the location and/or orientation of the cloned satellite DNA.

A view of interphase chromosomes

Metaphase chromosomes are dynamically modified in interphase. This review focuses on how these structures can be modified, and explores the functional mechanisms and significance of these changes. Current analyses of genes often focus on relatively short stretches of DNA and consider chromatin conformations that incorporate only a few kilobases of DNA. In interphase nuclei, however, orderly transcription and replication can involve highly folded chromosomal domains containing hundreds of kilobases of DNA. Specific "junk" DNA sequences within selected chromosome domains may participate in more complex levels of chromosome folding, and may index different genetic compartments for orderly transcription and replication. Three-dimensional chromosome positions within the nucleus may also contribute to phenotypic expression. Entire chromosomes are maintained as discrete, reasonably compact entities in the nucleus, and heterochromatic coiled domains of several thousand kilobases can acquire unique three-dimensional positions in differentiated cell types. Some aspects of neoplasia may relate to alterations in chromosome structure at several higher levels of organization.

The effects of chromosome rearrangements on the expression of heterochromatic genes in chromosome 2L of Drosophila melanogaster

The light (lt) gene of Drosophila melanogaster is located at the base of the left arm of chromosome 2, within or very near centromeric heterochromatin (2Lh). Chromosome rearrangements that move the lt+ gene from its normal proximal position and place the gene in distal euchromatin result in mosaic or variegated expression of the gene. The cytogenetic and genetic properties of 17 lt-variegated rearrangements are described in this report. We show that five of the heterochromatic genes adjacent to lt are subject to inactivation by these rearrangements and that the euchromatic loci in proximal 2L are not detectably affected. The properties of the rearrangements suggest that proximity to heterochromatin is an important regulatory requirement for at least six 2Lh genes. We discuss how the properties of the position effects on heterochromatic genes relate to other proximity-dependent phenomena such as transvection.

Chromosomal recombination and breakage associated with instability in mouse centrometric satellite DNA

A mouse L cell line containing the centromeric insertion of herpes thymidine kinase genes (tk) was previously shown to undergo a high frequency of DNA rearrangement at the site of tk insertion. Analysis of TK- revertants had demonstrated that DNA rearrangements were usually associated with DNA deletion and were always mediated by intrachromosomal recombinations. In this study, we further analyzed several TK+ subclones to examine the mode of DNA rearrangements in the absence of negative selection pressure. In two clones, LC2-3F and LC2-3E17, rearrangements were accompanied by DNA amplification and were mediated by intrachromosomal recombination. In subclone LC2-3E17-19, we further detected perturbations in the pattern of centromeric heterochromatization. This was associated with chromosome instability, as evidenced by chromosome breakage at the centromere. The analysis of three other sibling clones, LC2-3, LC2-6 and LC2-15, further suggests that reciprocal recombination events may play a role in such centromeric rearrangements. These results suggest that DNA rearrangements in the centromere may be mediated by a number of different mechanisms, and generally do not affect chromosome stability except when accompanied by changes in the pattern of heterochromatization.

Transgenes as probes for active chromosomal domains in mouse development

Embryonic development entails a well defined temporal and spatial programme of gene expression, which may be influenced by active chromosomal domains. These chromosomal domains can be detected using transgenes which integrate randomly throughout the genome, as their expression can be affected by chromosomal position. Position effects are probably exerted most strongly on transgenes that do not contain strong promoters, enhancers or other modulating sequences. Here we have systematically explored position effects using a transgene with the weak herpes-simplex-virus thymidine-kinase promoter, linked to the readily visualized lacZ indicator gene (HSV-TK-lacZ). Each transgenic fetus with detectable expression displayed a unique lacZ staining pattern. Thus expression of this construct is apparently dictated entirely by its chromosomal position, without any construct specificity. Furthermore the transgene is faithfully transmitted to subsequent generations, allowing for systematic mapping of changes in expression during development and in adult life. These results demonstrate that transgenes can indeed be powerful tools to probe the genome for active chromosomal regions, with the potential for identifying endogenous genes involved in organogenesis and pattern formation.