Epigenetic memory in induced pluripotent stem cells

Somatic cell nuclear transfer and transcription factor-based reprogramming revert adult cells to an embryonic state, and yield pluripotent stem cells that can generate all tissues. These two reprogramming methods reset genomic methylation, an epigenetic modification of DNA that influences gene expression, by different mechanisms and kinetics, leading us to hypothesize that the resulting pluripotent stem cells might have different properties. Here we observe that low passage induced pluripotent stem cells (iPSC) derived by factor-based reprogramming harbor residual DNA methylation signatures characteristic of their somatic tissue of origin, which favors their differentiation along lineages related to the donor cell, while restricting alternative cell fates. Such an “epigenetic memory” of the donor tissue could be reset by differentiation and serial reprogramming, or by treatment of iPSC with chromatin-modifying drugs. In contrast, the differentiation and methylation of nuclear transfer-derived pluripotent stem cells were more similar to classical embryonic stem cells than were iPSC, consistent with more effective reprogramming. Our data demonstrate that factor-based reprogramming can leave an epigenetic memory of the tissue of origin that may influence efforts at directed differentiation for applications in disease modeling or treatment.