Origins and implications of pluripotent stem cell variability and heterogeneity

Abstract

Pluripotent stem cells constitute a platform to model disease and developmental processes and can potentially be used in regenerative medicine. However, not all pluripotent cell lines are equal in their capacity to differentiate into desired cell types in vitro. Genetic and epigenetic variations contribute to functional variability between cell lines and heterogeneity within clones. These genetic and epigenetic variations could 'lock' the pluripotency network resulting in residual pluripotent cells or alter the signalling response of developmental pathways leading to lineage bias. The molecular contributors to functional variability and heterogeneity in both embryonic stem (ES) cells and induced pluripotent stem (iPS) cells are only beginning to emerge, yet they are crucial to the future of the stem cell field.

Figure 1. Model of the mouse pluripotency network

A model representing our current understanding of the maintenance of the pluripotent state in mouse pluripotent stem cells. The transcription factors OCT4, SOX2 and NANOG (commonly referred to as OSN) form an autoregulated core that upregulates other pluripotency factors, as well as signalling pathways of ectoderm or mesendoderm fates and the pro-differentiation MAPK1 pathway. At the same time, OSN factors repress lineage-specific programmes (in concert with Polycomb group (PcG) chromatin regulators). When cultured in the presence of LIF (leukaemia inhibitory factor; which acts through STAT3 (signal transducer and activator of transcription 3)) and BMP4 (bone morphogenetic protein 4; which acts through ID (inhibitor of differentiation) and SMAD1), MAPK1-enabled differentiation is blocked in mouse embryonic stem (ES) cells. Perturbations to the pluripotency network are likely to have distinct functional consequences for in vitro differentiation depending on which part of the network is affected. Disruptions to the differentiation signalling cascade infrastructures (red boxes) are more likely to be apparent at the expression level in the pluripotent state, whereas disruptions to either lineage commitment and differentiation programmes or genetic programmes responsible for rapidly repressing the pluripotency network upon differentiation (blue boxes) are more likely to remain silent until directed to differentiate. FGF, fibroblast growth factor; MEP, mesendodermal progenitor; miR, micro RNA; EP, ectodermal progenitor; OLIG2, oligodendrocyte transcription factor 2; OTX1, orthodenticle homolog 1; RBL2, retinoblastoma-like 2; TBX3, T box 3; ZIC2, zinc-finger protein of the cerebellum 2.