EPIGENETIC ALTERATIONS INDUCED BY THE PML-RAR ONCOGENE DURING THE TRANSFORMATION PROCESS OF ITS TARGET CELLS

Acute promyelocytic leukaemia (APL) is a subtype of acute myeloid leukaemia, caused by the t(15;17) translocation that generates an aberrant transcription factor PML-RARα. Owing to its ability to affect gene expression through chromatin modifications, PML-RARα is potentially capable of a multitude of alterations in the chromatin landscape, most of which are still poorly characterized. To investigate PML-RARα mechanistical activity at early stages of tumor initiation, first we started elucidating the target cell(s) in which the leukemogenesis takes place. Our results showed that phenotypically defined common myeloid progenitor hematopoietic (CMPs), are capable of initiating leukemogenesis. We found that PML-RARα can induce an adult stem cell signature in normal myeloid progenitors, distinct from that observed in frankly established leukemic stem cells. Among the transcriptional targets identified, the cell cycle inhibitor p21, required for self-renewal of normal and leukemic stem cells, was shown to be indispensable for this phenomenon. To mechanistically dissect the pre-leukemic epigenome of the identified hematopoietic subpopulation, we developed ad hoc technological approaches to study small population of cells (miniChIP-sequencing, Nuclease Accessible Site Sequencing and DNA methylation sequencing). The analysis of the changes between chromatin states show that the oncogene in the pre-leukemic state does not impose a global alteration of chromatin but, rather, it functions as a local modulator of chromatin accessibility involving genomic regions enriched in important regulators of normal and aberrant hematopoiesis.