Sequence complexity and diversity of polyadenylated RNA molecules transcribed in human myeloid cells. Leukemic myeloblasts and HL60 promyelocytes uninduced and induced to terminal differentiation with retinoic acid.

The kinetic composition of the polyadenylated RNAs of leukemic myeloblasts and HL60 promyelocytes was examined by the cDNA-poly (A) +RNA reassociation technique before and after the induction of differentiation with retinoic acid. The data obtained in the homologous hybridizations show the following main features: the sequence complexity of the total poly(A) +RNA molecules is 72,000 different sequences in leukemic myeloblasts, 64,000 in HL60 promyelocytes before induction and 26,000 after treatment of HL60 with retinoic acid; the number of sequences reacting as abundant is clearly higher in HL60 promyelocytes (5,000) than in leukemic myeloblasts (1,000), and the number decreases sharply after induction (300); the repetition frequency of the abundant and rare components is almost unchanged between leukemic myeloblasts and HL60 promyelocytes, while it is markedly increased after induction with retinoic acid. The heterologous reactions show that the observed differences in complexity are mainly related to the missing of rare sequences. Moreover, an important portion of sequences already present in leukemic myeloblasts has a definitely higher repetition frequency in HL60 promyelocytes. Finally, the most abundant sequences in HL60 cells after induction are already present before treatment. The data presented here suggest that, during human myeloid differentiation, important, possibly transcriptional, regulatory mechanisms of gene expression are active long after the first commitment event of the hemopoietic undifferentiated stem cell.