Transcriptome-wide noise controls lineage choice in mammalian progenitor cells

Abstract

Even in clonal populations of cells, there is significant phenotypic variation from cell to cell. This could reflect the 'noise' inherent in gene expression: or the various cell states could represent stable phenotypic variants. Chang et al. analysed the behaviour of an 'outlier' in clonal populations of mouse haematoipoietic stem cells that had very high expressions of the stem cell marker Sca-1 and found that outliers possessed distinct transcriptomes. Though the transcriptomes eventually reverted back to that of the median cells, while they differed they could drive the cells to express characteristics of distinct cell fates. Thus clonal heterogeneity of gene expression may not be due to noise in the expression of individual genes, but rather is a manifestation of metastable states of a slowly fluctuating transcriptome. These fluctuations may govern the reversible, stochastic priming of multipotent progenitor cells in cell fate decision. Even in clonal populations of cells, there is significant phenotypic variation from cell to cell: Huang and colleagues analyse the behaviour of an 'outlier', which had very high expressions of the stem cell marker Sca-1, and conclude that clonal heterogeneity of gene expression level is a manifestation of metastable states of a slowly fluctuating transcriptome. These fluctuations may govern the reversible, stochastic priming of multipotent progenitor cells in cell fate decision. (See Nature Reports Stem Cells). Phenotypic cell-to-cell variability within clonal populations may be a manifestation of ‘gene expression noise’1,2,3,4,5,6, or it may reflect stable phenotypic variants7. Such ‘non-genetic cell individuality’7 can arise from the slow fluctuations of protein levels8 in mammalian cells. These fluctuations produce persistent cell individuality, thereby rendering a clonal population heterogeneous. However, it remains unknown whether this heterogeneity may account for the stochasticity of cell fate decisions in stem cells. Here we show that in clonal populations of mouse haematopoietic progenitor cells, spontaneous ‘outlier’ cells with either extremely high or low expression levels of the stem cell marker Sca-1 (also known as Ly6a; ref. 9) reconstitute the parental distribution of Sca-1 but do so only after more than one week. This slow relaxation is described by a gaussian mixture model that incorporates noise-driven transitions between discrete subpopulations, suggesting hidden multi-stability within one cell type. Despite clonality, the Sca-1 outliers had distinct transcriptomes. Although their unique gene expression profiles eventually reverted to that of the median cells, revealing an attractor state, they lasted long enough to confer a greatly different proclivity for choosing either the erythroid or the myeloid lineage. Preference in lineage choice was associated with increased expression of lineage-specific transcription factors, such as a >200-fold increase in Gata1 (ref. 10) among the erythroid-prone cells, or a >15-fold increased PU.1 (Sfpi1) (ref. 11) expression among myeloid-prone cells. Thus, clonal heterogeneity of gene expression level is not due to independent noise in the expression of individual genes, but reflects metastable states of a slowly fluctuating transcriptome that is distinct in individual cells and may govern the reversible, stochastic priming of multipotent progenitor cells in cell fate decision.