Genome binding/occupancy profiling by high throughput sequencing
Summary
Nucleosome positioning is critical to chromatin accessibility, and is associated with gene expression programs in cells. Previous nucleosome mapping methods assemble profiles from cell populations and reveal a cell-averaged pattern: nucleosomes are positioned and form a phased array surrounding the transcription start sites (TSSs ) of active genes and DNase I hypersensitive sites (DHSs). However, cells exhibit remarkable expression heterogeneity in response to active signaling even in a homogenous population of cells, which may be related to the heterogeneity in chromatin accessibility. Here, we report a technique, termed single-cell MNase-seq (scMNase-seq), to measure genome-wide nucleosome positioning and chromatin accessibility simultaneously in single cells. Application of scMNase-seq to NIH3T3, mouse primary naïve CD4 T and embryonic stem cells (mESC) reveals two novel principles of nucleosome organization: (1) nucleosomes surrounding TSSs of silent genes or in heterochromatin regions show large positioning variation across different cells but are highly uniformly spaced along the nucleosome array and, (2) In contrast, nucleosomes surrounding TSSs of active genes and DHSs show small positioning variation across different cells but show relatively low spacing uniformness along the nucleosome array. We found a bimodal distribution of nucleosome spacing at DHSs, which corresponds to inaccessible and accessible states and is associated with nucleosome variation and accessibility variation across cells. Nucleosome variation within single cells is smaller than that across cells and variation within the same cell type is smaller than that across cell types. A large fraction of naïve CD4 T cells and mESCs show depleted nucleosome occupancy at the de novo enhancers detected in their respectively differentiated lineages, revealing the existence of cells primed for differentiation to specific lineages in undifferentiated cell populations.
Overall design
Measuring chromatin accessibility and nucleosome positioning in single-cells by applying single-cell MNase-seq to 48 NIH3T3 cells, 198 mouse embryonic stem cells (mESCs), and 278 mouse naïve CD4 T cells.
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