The role of chromatin condensation during granulopoiesis in the regulation of gene cluster expression.

Changes in nuclear architecture play an important role in the regulation of gene expression. The importance of epigenetic changes is observed during granulopoiesis, when changes in the nuclear architecture are considered a major factor that influences the downregulation of genes. We aimed to assess the influence of chromatin condensation on the regulation of gene expression during granulopoiesis. Based on a previously published microarray analysis, we chose loci with different levels of transcriptional activity during granulopoiesis. Fluorescent in situ hybridisation (FISH) and immunofluorescent labelling of RNA polymerase II were used to determine the relationship between the transcriptional activity of gene clusters and their localisation within areas with different levels of chromatin condensation. Although active loci were positioned outside of areas of condensed chromatin, downregulation of genes during granulopoiesis was not accompanied by a shift of the downregulated loci to condensed areas. Only the beta-globin cluster was subjected to chromatin condensation and localised to condensed areas. Our results indicate that granulopoiesis is accompanied by a non-random, tissue-specific pattern of chromatin condensation. Furthermore, we observed that the decrease in the quantity of RNA polymerase II correlates with the differentiation process and likely acts in synergy with chromatin condensation to downregulate total gene expression.