Nuclear compartmentalization and gene activity during cellular differentiation

C. Francastel, PhD,1 and M. Groudine MD.PhD

Fred Hutchinson Cancer Research Center, Division of Basic Sciences, Seattle, WA, USA.

Gene silencing in mammalian cells may be mediated by positioning of a gene in proximity to the heterochromatic compartment in interphase nuclei, suggesting that the eukaryotic nucleus is divided into heterochromatin compartments that repress transcription, and compartments in which transcription is favored. We believe this nuclear compartmentalization may help to establish a tissue-specific pattern of gene expression required for the onset and progression of cellular differentiation. During erythroid maturation the entire genome is progressively silenced and packaged into heterochromatin, whereas the b-globin locus is among the last to be silenced. The tissue-specific activation and maintenance of its expression in the repressive environment of a terminally differentiating red cell are due, at least in part, to the Locus Control Region (LCR) comprised of several DNaseI hypersensitive sites (HS) that contain numerous binding sites for erythroid and ubiquitous transcription factors. We previously demonstrated that stable gene expression and open chromatin configuration require both a functional enhancer and positioning away from centromeric heterochromatin, and revealed that enhancers can mediate the localization of genes to nuclear compartments that favor gene activation, away from the repressive compartment of heterochromatin. This led to the hypothesis that activators bound to tissue-specific LCRs/enhancers may act to establish and maintain gene expression in differentiated cells by ensuring that a linked gene resides in a nuclear compartment permissive for transcription, thereby preventing its inclusion in facultative heterochromatin that forms during cell differentiation, and permitting it to be active in the appropriate lineage. As a model to gain insights into the relationship between nuclear compartmentalization and tissue-specific transcription, we investigated the location of various factors known as transcriptional activators or repressors, and followed changes of their location during the process of erythroid maturation, using the murine erythroleukemia cell line (MEL) system. Data suggesting that the activity of the erythroid-specific transcription factor NF-E2, required for MEL cell differentiation and globin gene expression, may be regulated through the nuclear compartmentalization of its subunits will be presented. We also found that NF-E2 may be involved in positioning of the globin locus away from heterochromatin to a nuclear compartment permissive for transcription, upon induction of erythroid differentiation. These results complement our previous observation that the NF-E2 site in an erythroid specific enhancer is essential for the maintenance of expression and nuclear location of a transgene away from heterochromatin in erythroid cellsErreur! Signet non dfini.. In addition to demonstrating the relocation of NF-E2 after erythroid differentiation, we have also shown that a number of repressors (e.g., HDAC-1, MeCP2, etc) relocate away from centromeric sequences upon MEL differentiation. Interestingly, as red cells mature, the majority of DNA is heterochromatinized at non-centromeric sites in the nucleus. Thus, this observation that repressors move from centromeres during differentiation provides a possible basis for this maturation associated increase in heterochromatin formation.

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Paper presented at the International Symposium on Predictive Oncology and Intervention Strategies; Paris, France; February 9 - 12, 2002; in the section on Predictive Markers.