Abstract :

Myelodysplastic syndromes (MDS) are clonal hematopoietic disorders characterized by ineffective hematopoiesis, peripheral cytopenia, and a risk of leukemic transformation. Among them, MDS with mutation of the gene encoding the SF3B1 splicing factor constitute a distinct clinical and biological entity, associated with the presence of ringed sideroblasts and ineffective erythropoiesis. The aim of this thesis was to elucidate the epigenetic and transcriptional mechanisms underlying erythroid differentiation defects in this context and to explore new therapeutic options.SF3B1 gene mutations are initiating genetic events that confer a selective advantage to clonal cells. They generate splicing abnormalities, in particular the use of a cryptic splicing site 3' of the intron and inappropriate intron retention. Since intron retention is a physiological co-transcriptional mechanism for erythroblast specification, this results in a profound modification of the transcriptome with an over-representation of pathways regulating the cell cycle, ribosome biogenesis, or DNA repair, and an alteration of terminal erythroid differentiation. In connection with the loss of intron retention, our work first revealed an overall loss of DNA-RNA hybrids or R-loops, which form during transcription through hybridization of nascent RNA with template DNA. This structural defect contributes to transcriptional reprogramming affecting DNA repair, cell cycle regulation, and splicing pathways. We then showed that inhibition of histone deacetylases (HDACs) by hydroxamic acid suberoylanilide or vorinostat restores R-loop generation and reorganizes the transcriptional landscape by decreasing the expression of genes involved in DNA replication or repair, the cell cycle, and ribosome biogenesis. The chromatin state is modified, notably with an increase in H3K27ac activation marks at promoters and H3K36me3 on gene bodies, and increased chromatin accessibility at the promoters of genes targeted by the erythroid transcription factors KLF1/EKLF and GATA1. These changes correlate with improved terminal erythroid differentiation.Our results suggest that SF3B1 mutations associate epigenetic abnormalities with RNA splicing defects, contributing to inefficient erythropoiesis. Targeting HDACs appears to be a promising therapeutic strategy for restoring the erythroid differentiation program. Thus, this thesis highlights the importance of interactions between R-loops, histone modifications, and transcription in the pathophysiology of SF3B1-MDS and opens up new perspectives for targeted epigenetic modulation of erythropoiesis.

Keywords : myelodysplastic syndromes, SF3B1, R-loops, histone deacetylase inhibitor, anemia, chromatin accessibility.