Stem cells with baggage

Induced pluripotent stem (iPS) cells are produced by reprogramming differentiated adult cells using a cocktail of transcription factors. They share many properties that are characteristic of embryonic stem (ES) cells generated by somatic-cell nuclear transfer (SCNT), and of ES cells from naturally fertilized embryos. The three cell types are not identical, however, and an interesting difference has now been discovered: iPS cells retain an ‘epigenetic memory’ of the donor tissue from which they derive, whereas SCNT-based reprogramming resets the DNA-methylation state of adult cells so it is closer to the ES cell-like state. The epigenetic memory retained in iPS cells may influence efforts at directed differentiation for disease models or therapeutics. [Article p. 285; News & Views p. 280] In a separate study, Ji et al. examine the role of specific DNA-methylation marks in the developmental progression of particular cell lineages. They present a genome-wide DNA-methylation analysis of haematopoietic cell populations that reveals remarkable epigenetic plasticity. Changes in DNA methylation emerge as perhaps a principal factor directing cell-fate choices such as commitment to myeloid or lymphoid development. [Letter p. 338]