Classic epigenetic modifications include DNA methylation and histone modifications as well as more broadly chromatin accessibility and non-coding RNAs. Epigenetic mechanisms enable heritable changes in gene expression that occur without altering the underlying DNA sequence. The various epigenetic layers act to control DNA accessibility and chromatin structure, which defines transcription factor binding and downstream gene regulation. Epigenetic regulators are essential in mammalian development and play a crucial role in both the reprogramming of somatic cells into induced pluripotent stem cells (iPSCs) and the subsequent differentiation of iPSCs into specific cell lineages 1.
iPSC Reprogramming Efficiency
Reprogramming somatic cells into iPSCs involves the introduction of specific transcription factors—such as the original set of Oct4, Sox2, Klf4, and c-Myc. However, the efficiency of the reprogramming process although depending on the starting cell type, is generally low, with only a small proportion of cells successfully reprogrammed. The transcription factors exert their effects by binding to enhancer and promoter regions in the genome. The accessibility of these binding sites is heavily influenced by epigenetic modifications and the local chromatin structure 2.
If the transcription factor binding sites are epigenetically restricted and therefore inaccessible, the factors may fail to bind properly, leading to incomplete or unstable reprogramming. In newly generated iPSCs incomplete resetting of epigenetic modifications can result in variability in the pluripotency as well as subsequent differentiation capacity of the iPSCs and be accompanied by genomic instability and elevated tumorigenicity. Methods such as genome-wide DNA methylation profiling, histone modification analysis, and chromatin accessibility assays can be used to assess the epigenetic landscape of newly generated iPSCs and may be a useful tool when selecting clones in the future.
Differentiation Potential of iPSCs
Epigenetic mechanisms are central to guiding and maintaining cell identity. Consequently, the epigenetic state of the original somatic cell prior to reprogramming may influence the differentiation potential of the resulting iPSCs 3. A potential residual "epigenetic memory" could lead to the continued expression of lineage-specific markers from the starting cell type, which may facilitate differentiation of the iPSC towards its somatic cell of origin. For therapeutic or research purposes that require broad differentiation potential, iPSC lines with minimal epigenetic memory are preferable. Interestingly, this preferential differentiation tendency appears to diminish with the continued passage of iPSCs, particularly in murine models, suggesting that epigenetic marks may be gradually reset or corrected over time in culture. The potential role and significance of retained epigenetic memory in differentiation is still under debate, but it does appear that donor-related differences are just as influential as the underlying epigenetic modifications 4.
Improving iPSC Epigenetic Stability
To enhance reprogramming efficiency and the functional reliability of iPSCs, various strategies have been proposed. These include selecting optimal donor cell types, refining culture conditions, and incorporating targeted genetic modifications. Such approaches aim to improve epigenetic reprogramming fidelity, reduce variability, and ensure greater stability and safety of iPSC-derived cells for downstream applications 5, 6.
Conclusion
Proper resetting of epigenetic modifications is key to successful iPSC reprogramming and differentiation. By understanding and controlling these mechanisms, researchers can improve iPSC quality, consistency, and applicability in both clinical and research settings.
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References
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- Hochedlinger, K. and Jaenisch R. Induced Pluripotency and Epigenetic Reprogramming. Cold Spring Harb Perspect Biol. 7,
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- Scesa, G., Adami, R. and Bottai, D. iPSC Preparation and Epigenetic Memory: Does the Tissue Origin Matter? 10:1470. 2021.
- Poetsch, M. S., Strano, A. and Guan, K. Human Induced Pluripotent Stem Cells: From Cell Origin, Genomic Stability, and Epigenetic Memory to Translational Medicine. Stem Cells, 40, 2022.
- Buckberry, S., Liu, X., Poppe, D., Tan, J.P., Sun, G., Chen, J., Nguyen, T.V., de Mendoza, A., Pflueger, J., Frazer, T., Vargas-Landín, D.B., Paynter, J.M., Smits, N., Liu, N., Ouyang, J.F., Rossello, F.J., Chy, H.S., Rackham, O.J.L., Laslett, A.L., Breen, J., Faulkner, G.J., Nefzger, C.M., Polo, J.M. and Lister, R. Transient naive reprogramming corrects hiPS cells functionally and epigenetically. Nature. 2023
