NORs on human acrocentric chromosome p-arms are active by default and can associate with nucleoli independently of rDNA

Abstract

Nucleoli, the sites of ribosome biogenesis and the largest structures in human nuclei, form around nucleolar organizer regions (NORs) comprising ribosomal DNA (rDNA) arrays. NORs are located on the p-arms of the five human acrocentric chromosomes. Defining the rules of engagement between these p-arms and nucleoli takes on added significance as describing the three-dimensional organization of the human genome represents a major research goal. Here we used fluorescent in situ hybridization (FISH) and immuno-FISH on metaphase chromosomes from karyotypically normal primary and hTERT-immortalized human cell lines to catalog NORs in terms of their relative rDNA content and activity status. We demonstrate that a proportion of acrocentric p-arms in cell lines and from normal human donors have no detectable rDNA. Surprisingly, we found that all NORs with detectable rDNA are active, as defined by upstream binding factor loading. We determined the nucleolar association status of all NORs during interphase, and found that nucleolar association of acrocentric p-arms can occur independently of rDNA content, suggesting that sequences elsewhere on these chromosome arms drive nucleolar association. In established cancer lines, we characterize a variety of chromosomal rearrangements involving acrocentric p-arms and observe silent, rDNA-containing NORs that are dissociated from nucleoli. In conclusion, our findings indicate that within human nuclei, positioning of all 10 acrocentric chromosomes is dictated by nucleolar association. Furthermore, these nucleolar associations are buffered against interindividual variation in the distribution of rDNA.

Prevalence and Phenotypic Impact of Robertsonian Translocations

“…Mol Syndromol 2021;12:1-11 DOI: 10.1159/000512676 the 47S rDNA arrays located on the short arms clustering in the nucleolus are transcriptionally active, this clustering is independent of the number of rDNA blocks [van Sluis et al, 2020]. The rDNA operons are transcribed by RNA polymerase I [Yuan et al, 2007].…”

“…Yet, the pseudodiploid male fibrosarcoma cell line HT1080, the highly aneuploid female cervical carcinoma cell line HeLa, and the female bone osteosarcoma epithelial cell line U2OS all carry RTs [van Sluis et al, 2020]. HeLa cells are defective for a TP53-encoded 3′→5′ exonuclease and U2OS cells are defective for the alpha thalassemia/mental retardation X-linked (ATRX) gene.…”

“…In the genomes of healthy individuals, the total rDNA copy numbers range between 120 and 670 as measured in lymphocytes from peripheral blood [Parks et al, 2018;Porokhovnik and Lyapunova, 2019]. In addition, the number of rDNA clusters and distal junction blocks vary considerably between acrocentric chromosomes and between individuals [van Sluis et al, 2019[van Sluis et al, , 2020. This wide range of rDNA copy numbers found in both mouse and healthy humans represent differences of approximately 66 million nucleotides of rDNA between individuals at the extremes.…”

The Utility of Nucleolar Organizer Regions Quantitation in Early Prediction of Lung Neoplastic Transformation

“…Higher NORs count usually an indication for cellular proliferation, which is considered as an initial step towards neoplastic progression [17]. These nuclear regions are associated with rDNA which usually indicates cellular activity, mostly related to cell division [18].…”

“…AgNORs count method was widely used in oral exfoliative cytology particularly among cigarette smokers to predict early cellular proliferative activity indicating neoplastic change [ 19 ], but its use for lung cytology is scarce [ 20 ]. It was reported that cellular proliferative quantified by AgNORs might play a substantial role in the early detection and diagnosis of premalignant and malignant lesions in the absence of clinical manifestations [ 21 ].…”