Dynamics of Ribosomal RNA Transcription and Abundance in Normal and Leukemic Hematopoiesis.

Transcription of ribosomal RNAs (rRNAs) from rDNA repeats is the first step of ribosome biogenesis, accounting for a major portion of all cellular transcription. Often regarded as a housekeeping process, its cell-type-specific regulation in complex organ systems is largely neglected. We used rRNA FISH-Flow to profile nascent and mature rRNA levels in detail across mouse hematopoiesis, and observed that rRNA abundance is a cell-type-specific property, largely uncoupled from cell cycling or protein synthesis rates. Absolute quantification of rRNA molecules unexpectedly revealed that 28S rRNA is in excess in all cell types, most prominently in the normal myeloid lineage. In acute myeloid leukemia (AML), leukemic progenitors showed notably higher nascent and mature rRNA levels than matched normal counterparts. Across contexts of hematopoiesis, broad trends in rRNA transcription paralleled changes in accessibility but not methylation of rDNA repeats. Collectively, our work provides a detailed map of the complex dynamics of rRNAs within and between normal and leukemic hematopoiesis.