Interchromosomal Contacts of rDNA Clusters with DUX Genes in Human Chromosome 4 Are Very Sensitive to Heat Shock Treatment

Induction of the Erythroid Differentiation of K562 Cells Is Coupled with Changes in the Inter-Chromosomal Contacts of rDNA Clusters

The expression of clusters of rDNA genes influences pluripotency; however, the underlying mechanisms are not yet known. These clusters shape inter-chromosomal contacts with numerous genes controlling differentiation in human and Drosophila cells. This suggests a possible role of these contacts in the formation of 3D chromosomal structures and the regulation of gene expression in development. However, it has not yet been demonstrated whether inter-chromosomal rDNA contacts are changed during differentiation. In this study, we used human leukemia K562 cells and induced their erythroid differentiation in order to study both the changes in rDNA contacts and the expression of genes. We observed that approximately 200 sets of rDNA-contacting genes are co-expressed in different combinations in both untreated and differentiated K562 cells. rDNA contacts are changed during differentiation and coupled with the upregulation of genes whose products are mainly located in the nucleus and are highly associated with DNA- and RNA-binding, along with the downregulation of genes whose products mainly reside in the cytoplasm or intra- or extracellular vesicles. The most downregulated gene is ID3, which is known as an inhibitor of differentiation, and thus should be switched off to allow for differentiation. Our data suggest that the differentiation of K562 cells leads to alterations in the inter-chromosomal contacts of rDNA clusters and 3D structures in particular chromosomal regions as well as to changes in the expression of genes located in the corresponding chromosomal domains. We conclude that approximately half of the rDNA-contacting genes are co-expressed in human cells and that rDNA clusters are involved in the global regulation of gene expression.

The Role of rDNA Clusters in Global Epigenetic Gene Regulation

The regulation of gene expression has been studied for decades, but the underlying mechanisms are still not fully understood. As well as local and distant regulation, there are specific mechanisms of regulation during development and physiological modulation of gene activity in differentiated cells. Current research strongly supports a role for the 3D chromosomal structure in the regulation of gene expression. However, it is not known whether the genome structure reflects the formation of active or repressed chromosomal domains or if these structures play a primary role in the regulation of gene expression. During early development, heterochromatinization of ribosomal DNA (rDNA) is coupled with silencing or activation of the expression of different sets of genes. Although the mechanisms behind this type of regulation are not known, rDNA clusters shape frequent inter-chromosomal contacts with a large group of genes controlling development. This review aims to shed light on the involvement of clusters of ribosomal genes in the global regulation of gene expression. We also discuss the possible role of RNA-mediated and phase-separation mechanisms in the global regulation of gene expression by nucleoli.

Chromosomal Translocations in NK-Cell Lymphomas Originate from Inter-Chromosomal Contacts of Active rDNA Clusters Possessing Hot Spots of DSBs

Simple Summary

There are nine DSB hot spots located in the non-transcribed spacer of human rDNA units. Circular chromosome conformation capture data indicate that the rDNA clusters often shape contact with a specific set of chromosomal regions containing genes controlling differentiation and cancer, and often possessing the DSB hot spots. The data suggest a mechanism for rDNA-mediated translocation, and some of them could lead to tumorigenesis. Here, we searched for translocations in which rDNA clusters are involved. WGS data of normal T cells and NK-cell lymphomas from the same individuals were used. We revealed numerous translocations in which rDNA units are involved. The sites of these translocations in normal T cells and in the lymphomas were mostly different, but occurred at about the same frequency in both cell types. We conclude that oncogenic translocations lead to dysregulation of a specific set of genes controlling development.

Abstract

Endogenous hot spots of DNA double-strand breaks (DSBs) are tightly linked with transcription patterns and cancer. There are nine hot spots of DSBs (denoted Pleiades) in human rDNA units that are located exclusively inside the intergenic spacer (IGS). Profiles of Pleiades coincide with the profiles of γ-H2AX, suggesting a high level of in vivo breakage inside rDNA genes. The data were confirmed by microscopic observation of the largest γ-H2AX foci inside nucleoli in interphase chromosomes. Circular chromosome conformation capture (4C) data indicate that the rDNA units often make contact with a specific set of chromosomal regions containing genes that are involved in differentiation and cancer. Interestingly, these regions also often possess hot spots of DSBs that provide the potential for Robertsonian and oncogenic translocations. In this study, we searched for translocations in which rDNA clusters are involved. The whole genome sequence (WGS) data of normal T cells and NK-cell lymphomas from the same individuals revealed numerous translocations in which Pleiades were involved. The sites of these translocations in normal T cells and in the lymphomas were mostly different, although there were also some common sites. The genes at translocations in normal cells and in lymphomas are associated with predominantly non-overlapping lists of genes that are depleted with silenced genes. Our data indicate that rDNA-mediated translocations occur at about the same frequency in the normal T cells and NK-lymphoma cells but differ at particular sites that correspond to open chromatin. We conclude that oncogenic translocations lead to dysregulation of a specific set of genes controlling development. In normal T cells and in NK cells, there are hot spots of translocations at sites possessing strong H3K27ac marks. The data indicate that Pleiades are involved in rDNA-mediated translocation.

Interchromosomal Contacts of rDNA Clusters in Three Human Cell Lines Are Associated with Silencing of Genes Controlling Morphogenesis

To study the rDNA contacts with genes in three human cell lines of different origin, we used 4C approach. Our data indicate that the same set of about five hundred genes frequently shape contacts with rDNA clusters in HEK293T, K652, and hESM01 cells. Gene ontology search suggests that the genes are involved in development and morphogenesis. Approximately one hundred of these genes are highly associated with silencing by H3K27me3 mark in different normal cells, including bronchial epithelial cells, keratinocytes, myoblasts, monocytes, endothelial cells, kidney epithelial cells, and some others. We conclude that the concerted silencing of specific group of rDNA-contacting genes controlling development occurs during differentiation. We assume that the phase separation mechanisms may be involved in the rDNA-mediated silencing of a set of genes via the contacts with inactive rDNA clusters.

Dynamics of Whole-Genome Contacts of Nucleoli in Drosophila Cells Suggests a Role for rDNA Genes in Global Epigenetic Regulation

Chromosomes are organized into 3D structures that are important for the regulation of gene expression and differentiation. Important role in formation of inter-chromosome contacts play rDNA clusters that make up nucleoli. In the course of differentiation, heterochromatization of rDNA units in mouse cells is coupled with the repression or activation of different genes. Furthermore, the nucleoli of human cells shape the direct contacts with genes that are involved in differentiation and cancer. Here, we identified and categorized the genes located in the regions where rDNA clusters make frequent contacts. Using a 4C approach, we demonstrate that in Drosophila S2 cells, rDNA clusters form contacts with genes that are involved in chromosome organization and differentiation. Heat shock treatment induces changes in the contacts between nucleoli and hundreds of genes controlling morphogenesis. We show that nucleoli form contacts with regions that are enriched with active or repressive histone marks and where small non-coding RNAs are mapped. These data indicate that rDNA contacts are involved in the repression and activation of gene expression and that rDNA clusters orchestrate large groups of Drosophila genes involved in differentiation.