Biomedical subjects

Susan Gilchrist

Publications and source records attributed to Susan Gilchrist.

6 recordsLinked to original sources

PubMed2005

Does radial nuclear organisation influence DNA damage?

It has been suggested that chromatin at the nuclear periphery could act to shield DNA sequences in the nuclear interior from damage. To test this hypothesis, we have examined the nuclear distribution of sites of DNA repair induced by oxidation or UV-C. We do not detect more damage (repair) at the nuclear periphery than in the nuclear interior. In fact, contrary to the body guard hypothesis, there is an excess of damage detected in the nuclear interior. This is further supported by sequence comparison between genes on human chromosomes 18 or 19, and their counterparts in the chimpanzee. The synonymous substitution rate for genes on chromosome 19, which is located towards the centre of the human nucleus, was higher than that for genes on chromosome 18, which is located at the nuclear periphery. We conclude that chromatin at the periphery of the human nucleus is not able to protect more internally located sequences from damage and mutation. We suggest that features of the chromatin structure, or base composition, of sequences in the nuclear centre make them more susceptible to damage.

Cell Line, Tumor↗

PubMed2005

Chromatin organization in the mammalian nucleus.

Mammalian cells package their DNA into chromatin and arrange it in the nucleus as chromosomes. In interphase cells chromosomes are organized in a radial distribution with the most gene-dense chromosomes toward the center of the nucleus. Gene transcription, replication, and repair are influenced by the underlying chromatin architecture, which in turn is affected by the formation of chromosome territories. This arrangement in the nucleus presumably facilitates cellular functions to occur in an efficient and ordered fashion and exploring the link between transcription and nuclear organization will be an exciting area of further research.

Animals↗

PubMed2004

Altered protein dynamics of disease-associated lamin A mutants.

BACKGROUND: Recent interest in the function of the nuclear lamina has been provoked by the discovery of lamin A/C mutations in the laminopathy diseases. However, it is not understood why mutations in lamin A give such a range of tissue-specific phenotypes. Part of the problem in rationalising genotype-phenotype correlations in the laminopathies is our lack of understanding of the function of normal and mutant lamin A. To investigate this we have used photobleaching in human cells to analyse the dynamics of wild-type and mutant lamin A protein at the nuclear periphery. RESULTS: We have found that a large proportion of wild-type lamin A at the nuclear periphery is immobile, but that there is some slow movement of lamin A within the nuclear lamina. The mobility of an R482W mutant lamin A was indistinguishable from wild-type, but increased mobility of L85R and L530P mutant proteins within the nuclear lamina was found. However, the N195K mutant shows the most enhanced protein mobility, both within the nucleoplasm and within the lamina. CONCLUSION: The slow kinetics of lamin A movement is compatible with its incorporation into a stable polymer that only exchanges subunits very slowly. All of the myopathy-associated lamin A mutants that we have studied show increased protein movement compared with wild-type. In contrast, the dynamic behaviour of the lipodystrophy-associated lamin A mutant was indistinguishable from wild-type. This supports the hypothesis that the underlying defect in lamin A function is quite distinct in the laminopathies that affect striated muscle, compared to the diseases that affect adipose tissue. Our data are consistent with an alteration in the stability of the lamin A molecules within the higher-order polymer at the nuclear lamina in myopathies.

Cell Line, Tumor↗

PubMed2004

The radial positioning of chromatin is not inherited through mitosis but is established de novo in early G1.

The organization of chromatin in the nucleus is nonrandom. Different genomic regions tend to reside in preferred nuclear locations, relative to radial position and nuclear compartments. Several lines of evidence support a role for chromatin localization in the regulation of gene expression. Therefore, a key problem is how the organization of chromatin is established and maintained in dividing cell populations. There is controversy about the extent to which chromatin organization is inherited from mother to daughter nucleus. We have used time-lapse microscopy to track specific human loci after exit from mitosis. In comparison to later stages of interphase, we detect increased chromatin mobility during the first 2 hr of G1, and during this period association of loci with nuclear compartments is both gained and lost. Although chromatin in daughter nuclei has a rough symmetry in its spatial distribution, we show, for the first time, that the association of loci with nuclear compartments displays significant asymmetry between daughter nuclei and therefore cannot be inherited from the mother nucleus. We conclude that the organization of chromatin in the nucleus is not passed down precisely from one cell to its descendents but is more plastic and becomes refined during early G1.

Cell Line↗

PubMed2004

Nuclear organization of centromeric domains is not perturbed by inhibition of histone deacetylases.

It is well established that modification of lysines in histone molecules correlates with gene expression and chromatin structure. It is not known whether this operates entirely at a local level, e.g. through the recruitment of specific proteins, or whether histone modifications might impact on more long-range aspects of chromatin organization. There is a distinctive organization of chromatin within the nucleus and the chromatin at the nuclear periphery of mammalian cells appears to be hypoacetylated. Previously it had been suggested that inhibition of histone deacetylases by TSA causes a gross remodeling of nuclear structure, specifically the recruitment of centromeric heterochromatin to the nuclear periphery. Here, we have quantified the nuclear organization of histone modifications and the localization of centromeric domains in human cells before and after TSA treatment. TSA alters the nuclear distribution of histone acetylation, but not that of histone methylation. TSA elevates levels of histone acetylation at the nuclear periphery but we see no alteration in the position of centromeric domains in the nuclei of treated cells. We conclude that the distinctive nuclear localization of centromeric domains is independent of histone acetylation.

Acetylation↗

PubMed2002

Spatial organization of active and inactive genes and noncoding DNA within chromosome territories.

The position of genes within the nucleus has been correlated with their transcriptional activity. The interchromosome domain model of nuclear organization suggests that genes preferentially locate at the surface of chromosome territories. Conversely, high resolution analysis of chromatin fibers suggests that chromosome territories do not present accessibility barriers to transcription machinery. To clarify the relationship between the organization of chromosome territories and gene expression, we have used fluorescence in situ hybridization to analyze the spatial organization of a contiguous approximately 1 Mb stretch of the Wilms' tumor, aniridia, genitourinary anomalies, mental retardation syndrome region of the human genome and the syntenic region in the mouse. These regions contain constitutively expressed genes, genes with tissue-restricted patterns of expression, and substantial regions of intergenic DNA. We find that there is a spatial organization within territories that is conserved between mouse and humans: certain sequences do preferentially locate at the periphery of the chromosome territories in both species. However, we do not detect genes necessarily at the periphery of chromosome territories or at the surface of subchromosomal domains. Intraterritory organization is not different among cell types that express different combinations of the genes under study. Our data demonstrate that transcription of both ubiquitous and tissue-restricted genes is not confined to the periphery of chromosome territories, suggesting that the basal transcription machinery and transcription factors can readily gain access to the chromosome interior.

Animals↗