(601g) Direct Reprogramming as a Random Drift in Cell State

Direct reprogramming of somatic cells into induced pluripotent stem cells (iPSCs) can be achieved by overexpression of Oct4, Sox2, Klf4 and c-Myc (OSKM) transcription factors, but quantifying the efficiency and timescales of the crucial events occurring during direct reprogramming has been problematic due to the cellular and genetic heterogeneity of de novo infected somatic cells. To circumvent these issues, we have characterized the reprogramming efficiency and kinetics of over 1000 somatic cell derived monoclonal populations similarly expressing OSKM over an extended period of time. Upon OSKM induction, most if not all monocytes and lineage-committed B cells harboring genetic rearrangements have the potential to generate iPSCs albeit with very different latencies. The observed dynamics are consistent with a continuous stochastic process, in which the conversion from a somatic cell to an iPSC upon continued growth and OSKM expression can be thought of as a drift in cell state. This cell state can be defined by a gene expression or epigenetic pattern, whose fluctuations drive the conversion to an iPSC. Such fluctuations are likely due to cell-to-cell extrinsic heterogeneity or the inherent stochastic nature of gene expression or regulatory signaling processes. Computational simulations and quantitative analyses of these transitions suggest that the number of cell divisions is a key parameter driving epigenetic reprogramming to pluripotency.