Yoshiki Horita

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.

Amplification and abnormal chromosomal distribution of ribosomal genes (rDNA) in rat erythroleukemia cells

The ribosomal cistrons (rDNA)/genome ratio was measured in five cell lines derived from three chemically induced erythroblastic leukemias (D-1, D-2, and NE26) in the Long-Evans (LE) rat and compared with values in the normal liver, bone marrow, and fetus. The ratio was 20-42% higher in the leukemias than in normal tissues. The number of autoradiographic silver grains of 125I-labeled rRNA hybridized in situ over three nucleolus organizer regions (NORs) of leukemia cells was determined and compared with that of the normal cells. Although the distribution of silver grains of normal cells averaged 44.6%, 25.9%, and 29.5% in NORs of chromosomes #3, #11, and #12, respectively, their distribution was abnormal in two of the leukemias examined; rDNA was amplified in chromosomes #12 of two sublines (K1DA and K1DB) of one leukemias (D-1), and in one chromosome #3 of two sublines (K2D and K3D) of another leukemia (D-2). We consider the possibility that these abnormal patterns of rDNA distribution are related to the increase in rDNA in leukemia cells.