AbstractBackgroundThe epigenetic machinery is frequently altered in acute myeloid leukemia. Focusing on cytogenetically normal (CN) AML, we previously described an abnormal H3K27me3 enrichment covering 70 kb on theHIST1cluster (6.p22) in CN-AML patient blasts. Here, we further investigate the molecular, functional, and prognosis significance of this epigenetic alteration named H3K27me3HIST1inNPM1-mutated (NPM1mut) CN-AML.ResultsWe found that three quarter of theNPM1mut CN-AML patients were H3K27me3HIST1^high. H3K27me3HIST1^highgroup of patients was associated with a favorable outcome independently of known molecular risk factors. In gene expression profiling, the H3K27me3HIST1^highmark was associated with lower expression of the histone genesHIST1H1D,HIST1H2BG,HIST1H2AE, andHIST1H3Fand an upregulation of genes involved in myelomonocytic differentiation. Mass spectrometry analyses confirmed that the linker histone protein H1d, but not the other histone H1 subtypes, was downregulated in the H3K27me3HIST1^highgroup of patients. H1d knockdown primed ATRA-mediated differentiation of OCI-AML3 and U937 AML cell lines, as assessed on CD11b/CD11c markers, morphological and gene expression analyses.ConclusionsOur data suggest thatNPM1mut AML prognosis depends on the epigenetic silencing of theHIST1cluster and that, among the H3K27me3 silenced histone genes,HIST1H1Dplays a role in AML blast differentiation.Graphical abstract