Higher NORs-expression in lymphocyte of trisomy 21 babies/children: in vivo evaluation

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 regulation of these genes is not known. The objective of this work is to compare the NORs expression in interphase nuclei in non-stimulated lymphocytes of DS patients and healthy controls. Because the AgNOR staining is the indicator of the active rRNA genes, comparison of the image analysis values of AgNORs area between DS's and healthy controls' interphase lymphocytes is considered to be sufficient to evaluate the level of rDNA activities in the two groups. The Nucleolus Organizer Regions area/Total Nuclear area (NORa/TNa) was calculated using a computer program designed by us. 100 consecutive NORa/TNa per individual were evaluated. We report that 24 DS children's peripheral lymphocytes show significantly higher NORa/TNa mean value (6.32 +/- 1.77%) than that of the 20 healthy controls' cells (5.31 +/- 1.34%) (2-tailed Mann-Whitney U test, z = 19.4, P = 0.000). The same is true for the nucleolus (AgNOR spot) number per nucleus. The mean value of nucleoli number per nucleus in DS lymphocytes was significantly higher than that of the controls: z = 14.6, P = 0.000. In conclusion, extra rRNA genes on the chromosome 21 are not down-regulated in DS patients' lymphocytes. Rather, extra NORs expressions in 'in vivo' condition contribute to the increase of AgNORs area and AgNOR spots number per nucleus. This is the first work on the comparison of NORs activities in resting (non-stimulated) interphase lymphocytes between DS and healthy controls.