Essay on the nucleoli survey by the alpha- and beta-satellite DNA probes of the acrocentric chromosomes in mitogen-stimulated human lymphocytes

AgNOR status in Down's syndrome infants and a plausible phenotype formation hypothesis

Down's syndrome (DS) or trisomy 21 is the most frequent genetic birth defect associated with mental retardation. Although DS has been known for more than a 100 years and its chromosomal basis recognized for half a century (1959), the underlying patho-mechanisms for the phenotype formation remain elusive and cannot be fully explained by simple gene dosage effect. The general consensus is that the extra chromosome 21 genes perturb the global metabolism of the body cells. Our experiments show that the most prominent metabolic perturbation occurs during ribosome biogenesis in the cells of DS babies/infants. In humans, ribosomal RNA (rRNA) gene families or nucleolar organizer regions (NORs) are localized at the secondary constriction (on the satellite stalks) of five pairs of acrocentric chromosomes (13, 14, 15, 21 and 22) and their activities are evaluated specifically either in metaphase or interphase through a procedure known as AgNOR or silver staining. Our successive AgNOR studies, supported by RNA and nuclear protein measurement, show that cells from DS infants produce more ribosomes than expected, accounting for the extra set of active rRNA gene family (1/6-1/11) situated on the extra chromosome 21. Thus, the presence of an extra chromosome 21 stimulates a global increase in ribosome biogenesis in cooperation with other NOR-bearing chromosomes, causing unnecessary rRNA and ribosomal proteins synthesis compared to controls. Following the description of NORs, AgNOR, AgNOR-proteins, AgNOR measurement and our experimental results, we propose that the extra RNA and protein synthesis can cause a fundamental handicap to DS infants, contributing to the formation of DS phenotypes, due to the wasted energy in producing unnecessary macromolecules, including energy (GTP)-dependent transport of the excessive ribosomes from the nucleus to the cytoplasm.

How transcription proceeds in a large artificial heterochromatin in human cells

Heterochromatin is critical for genome integrity, and recent studies have suggested the importance of transcription in heterochromatin for maintaining its silent state. We previously developed a method to generate a large homogeneously staining region (HSR) composed of tandem plasmid sequences in human cells that showed typical heterochromatin characteristics. In this study, we examined transcription in the HSR. We found that transcription of genes downstream to no-inducible SRα promoter was restricted to a few specific points inside the large HSR domain. Furthermore, the HSR localized to either to the surface or to the interior of the nucleolus, where it was more actively transcribed. The perinucleolar or intranucleolar locations were biased to late or early S-phase, and the location depended on either RNA polymerase II/III or I transcription, respectively. Strong activation of the inducible TRE promoter resulted in the reversible loosening of the HSR domain and the appearance of transcripts downstream of not only the TRE promoters, but also the SRα promoters. During this process, detection of HP1α or H3K9Me3 suggested that transcription was activated at many specific points dispersed inside large heterochromatin. The transcriptional rules obtained from studying artificial heterochromatin should be useful for understanding natural heterochromatin.

NOR Expression increases in interphase lymphocytes of Down syndrome babies/children as AgNORs surface, according to the mitogen concentration in the culture medium

The extra chromosome 21 of Down syndrome (DS) or trisomy 21 patients contains an average of 40 extra copies of rRNA genes and the in vivo/in vitro regulation of the activity of these genes is not fully understood. The objective of this work was to compare the NORs expression pattern in interphase lymphocytes of DS patients with regular trisomy 21 and control individuals according to phytohemagglutinin (PHA) concentration (0.37, 0.75, 1.48 and 2.21 ml) per 100 ml of medium. Because the AgNOR staining is an indicator of the active rRNA genes, comparison of the image analysis values of the AgNOR area in 72 h cultivated lymphocytes for each concentration of PHA between DS patients (N=30) and controls (N=24) provided a plausible conclusion on the regulation of the extra rRNA genes in DS lymphocytes. The nucleolus organizer regions area/total nuclear area (NORa/TNa) was calculated using an in-house computer program. Fifty consecutive interphases per PHA concentration were analysed for each individual, for determination of the NORa/TNa. In contrast to healthy controls, NORa/TNa of lymphocytes from DS patient babies/children (0-8 years old) increased gradually in parallel with the PHA concentration in the culture medium: 10.44+/-1.72% for 0.37 ml of PHA, 11.74+/-1.93% for 0.75 ml of PHA, 13.25+/-2.03% for 1.48 ml of PHA and 13.43+/-2.08% for 2.21 ml of PHA per 100 ml of medium. Contrary to control cells (in which the NORa/TNa ratio according to PHA concentration in the culture medium remains constant), DS interphase lymphocytes in culture do not down-regulate their NOR expression. These results obtained from interphase NORs are consistent with the previous results obtained by evaluating the mean of AgNOR+ chromosome number in metaphase cells, also in relation to the mitogen concentration in the culture medium.

NOR expression increases on metaphase chromosomes of down syndrome lymphocytes in concordance with mitogen concentration in culture medium

BACKGROUND

Regulation of nucleolus organizer region (NOR) expression in trisomy 21 (Down syndrome [DS]) cells is not fully explained. This work compared NOR expression on metaphase chromosomes in gradiently stimulated lymphocytes from DS patients with those from healthy controls.

METHOD

Conventional peripheral blood culture (72 h) and chromosomal preparation procedures were used except that blood samples from each individual were cultivated in the same but gradiently increasing concentrations (0.37, 0.75, 1.48, and 2.21 ml) of phytohemagglutinin (PHA) per 100 ml of medium. One hundred consecutive metaphases per concentration were analyzed for scoring the means of the active NORs bearing chromosomes (AgNOR+ chromosome) per individual and per concentration.

RESULTS

In contrast to healthy controls (n=24), AgNOR+ chromosomal number in lymphocytes from 30 DS patients increased in concordance to the gradient of PHA concentration in the culture medium.

CONCLUSION

DS lymphocytes do not downregulate their NOR expression in the limit of control cells. This in vitro result may serve as a clue for the explanation of the DS phenotype due to the wasted energy in producing unnecessary rRNA transcripts and AgNOR proteins in utero during organogenesis. These results also indicate that precautions must be used in routine work of NOR evaluation/interpretation in DS lymphocytes.

Structure-specific DNA-binding proteins as the foundation for three-dimensional chromatin organization

Any functions of tandem repetitive sequences need proteins that specifically bind to them. Telomere-binding TRF2/MTBP attaches telomeres to the nuclear envelope in interphase due to its rod-domain-like motif. Interphase nuclei organized as a number of sponge-like ruffly round chromosome territories that could be rotated from outside. SAF-A/hnRNP-U and p68-helicase are proteins suitable to do that. Their location in the interchromosome territory space, ATPase domains, and the ability to be bound by satellite DNAs (satDNA) make them part of the wires used to help chromosome territory rotates. In case of active transcription p68-helicase can be involved in the formation of local "gene expression matrices" and due to its satDNA-binding specificity cause the rearrangement of the local chromosome territory. The marks of chromatin rearrangement, which have to be heritable, could be provided by SAF-A/hnRNP-U. During telophase unfolding the proper chromatin arrangement is restored according to these marks. The structural specificity of both proteins to the satDNAs provides a regulative but relatively stable mode of binding. The structural specificity of protein binding could help to find the "magic" centromeric sequence. With future investigations of proteins with the structural specificity of binding during early embryogenesis, when heterochromatin formation goes on, the molecular mechanisms of the "gene gating" hypothesis (Blobel, 1985) will be confirmed.

Condensed chromatin surface and NORs surface enhancement in mitogen-stimulated lymphocytes of Down syndrome patients

Mitogen-stimulated lymphocytes of 20 Down syndrome (DS) patients with regular trisomy 21 contain more condensed chromatin surface (11.28 +/- 2.64 % of the total nuclear surface: mean +/- SD) and more nucleolus organiser regions surface (13.21 +/- 3.45 %) than that of 12 healthy controls: (8.84 +/- 2.23 and 9.12 +/- 2.33 %, reciprocally). The source of this peculiarity has been investigated. A computer program was designed for the planimetric measurement of the condensed chromatin surface (CCs)/ total nuclear surface(TNs) and the nucleolus organiser regions surface (NORss) /TNs proportions in interphase nuclei. CCs/TNs and NORss/TNs of 100 maximally activated nuclei (MANs) were measured for each patient and control case. The difference was found highly significant (P<0.01). Nuclei with a diameter of >/= 17 micrometer measured on the slide (in flattened state) were considered as maximally activated nuclei (MANs). NORss/TNs enhancement and fluorescent in situ hybridisation (FISH) studies in MANs of DS patients indicate that this phenomenon is due to the over-expression (or lack of downregulative mechanism) of NORs (rDNA) to some extent, including the NOR of the supernumerary chromosome 21. No statistical difference was observed between 12 healthy controls and 5 Robertsonian translocation type of DS Patients (where the two involved NORs are missing) when the two parameters were considered.