Molecular and cytogenetic studies on nucleolar cistrons (rDNA) in mouse leukemia cells

The gene dosage change of nucleolar cistrons (rDNA) in tumor cells has not been extensively studied. The present studies showed that increased dosage, as well as abnormal distribution of rDNA, was frequently associated with leukemia cells of SL/Ni and AKR mice. In normal SL cells, 37%, 39%, and 25% of rDNA was located in nucleolar organizer regions (NOR) of chromosomes #12, #18, and #19, respectively. Increase of rDNA/DNA was shown by hybridization on filter membranes in SL1, SL2, SL3, and M1 leukemia cells. Direct measurement of rDNA/DNA in G1 cells revealed an 11% increase in synchronized M1 cells. The increased rDNA dosage was explained by trisomy 12 in SL1 and SL2, the ectopic NOR of #9 in SL3, and the double t(X;19) marker chromosomes in M1. On the other hand, in normal AKR cells, 27%, 29%, and 45% of rDNA was assigned to NORs of chromosomes #15, #16, and #18, respectively. The relative rDNA distribution among NORs estimated by autoradiographic grain counting was suggested to be abnormal in AKR leukemia cells despite their normal karyotype; 36% rDNA was shown to be in chromosomes #15 and #16, respectively, by relative reduction in chromosome #18 in AKR1; the trisomy 15 explained the increased rDNA in AKR2; a relative increase was found in chromosome #15 in AKR3. These results were discussed with reference to the reported NOR involvement in chromosome translocation and amplification in tumor cells.