16Background 17 During cellular differentiation, the chromatin landscape changes dynamically and contributes to 18 the activation of cell-type specific transcriptional programs. Disruptor of telomeric silencing 1-19 like (DOT1L) is a histone methyltransferase that mediates mono-, di-and trimethylation of lysine 20 79 of histone H3 (H3K79me1, 2, 3). Its enzymatic activity is critical for driving cellular 21 differentiation into cardiomyocytes, chondrocytes and neurons, from embryonic or other type of 22 stem cells in physiological settings. Ectopic localization of DOT1L in MLL-rearranged leukemias 23 is causative for leukemogenesis and relapse. Little is known about the causal relevance of DOT1L 24 2 methyltransferase activity in the global chromatin context and how its enzymatic function affects 25 transcriptional and global chromatin states. Recent reports conducted in leukemia cell models have 26 suggested that deposition of H3K79me2 may be critical to preserve histone H3K27 acetylation 27 (ac) and enhancer activity, and to sustain expression of highly transcribed genes. If and to what 28 extent DOT1L affects chromatin states and enhancer activity during physiological differentiation 29 processes is currently unknown.30 31Results 32 We measure global changes of seven histone modifications during the differentiation process via 33 high-throughput and quantitative ChIP-seq in an in-vitro neuronal differentiation model of mouse 34 embryonic stem cells (mESC). We observe that H3K27ac globally decreases, whereas H3K79me2 35 globally increases during differentiation, while other modifications remain globally unaltered. 36 Pharmacological inhibition of DOT1L in mESC and mESC-derived neural progenitors results in 37 decreased expression of highly transcribed genes and increased expression of normally repressed 38 genes. Acute DOT1L inhibition primes neural progenitors towards a mature differentiation state. 39 Transcriptional downregulation associates with decreased accessibility of enhancers specifically 40 bound by the master regulator SOX2.
42Conclusions 43 In-vitro neuronal differentiation couples with a genome-wide accumulation of H3K79me2, never 44 described previously in mammalian cells. Acute inhibition of DOT1L is sufficient to initiate a 45 defined transcriptional program, which biases the transcriptome of neural progenitor cells towards 46 neuronal differentiation. H3K79me2 is not generally causative for maintaining transcriptional 47 3 levels at a genome-wide scale. In contrast, DOT1L inactivation reduces the chromatin accessibility 48 of enhancers bound by SOX2 in-vivo, thereby reducing the expression level of a restricted number 49 of genes. Our work establishes that DOT1L activity gates differentiation of progenitors by 50 allowing SOX2-dependent transcription of stemness programs. 51 52 INTRODUCTION 53 54In eukaryotes, nuclear DNA is wrapped around histones, which constitute the building blocks of 55 chromatin. Histones are subject to a variety of covalent and reversible modifications, mo...