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Human embryonic stem cells and genomic instability Owing to their original properties, pluripotent human embryonic stem cells (hESCs) and their progenies are highly valuable not only for regenerative medicine, but also as tools to study development and pathologies or as cellular substrates to screen and test new drugs. However, ensuring their genomic integrity is one important prerequisite for both research and therapeutic applications. Until recently, several studies about the genomic stability of cultured hESCs had described chromosomal or else large genomic alterations detectable with conventional karyotypic methods. In the past year, several laboratories have reported many small genomic alterations, in the megabase-sized range, using more sensitive karyotyping methods, showing that hESCs are prone to acquire focal genomic abnormalities in culture. As these alterations were found to be nonrandom, these findings strongly advocate for high-resolution monitoring of human pluripotent stem cell lines, especially when intended to be used for clinical applications KEYWORDS: culture adaptation n genomic alteration n human pluripotent stem cells n karyotype n regenerative medicine
Human pluripotent stem cells Human embryonic stem cells (hESCs) are derived from the inner cell mass of a human blastocyst (stage 5.5–7.5 days postfertilization). hESCs are pluripotent and self-renewable. These two essential properties make hESCs capable of virtually endless division while maintaining their capacity to differentiate, under specific conditions, into all cell types of the organism. Undifferentiated hESCs are characterized by the expression of three key transcription factors implicated in stemness regulation: POU5F1 (formerly known as OCT‑4), SOX2 and NANOG [1] . In addition, hESCs express a panel of surface markers, including tumor rejection antigens (TRA‑1–60 and TRA‑1–81), stage-specific embryonic antigens (SSEA‑3 and SSEA‑4 but not SSEA‑1, in contrast to mouse ESCs) and alkaline phosphatase [1] . Telomerase is highly activated after fertilization and its expression is maintained in hESCs. The first ESC lines were isolated from mouse embryos in the early 1980s [2,3] , but the first report on the derivation of hESCs was only published nearly 20 years later in 1998 [4] . To date, ESCs from at least three additional species have been isolated: monkey [5] , rat [6,7] and dog [8] . Pre-implantation genetic diagnosis procedure is a screening procedure that allows embryos produced by in vitro fertilization to be tested for the presence of a specific genetic defect prior to uterine implantation. Since 2005, several hESC lines carrying specific gene mutations related to genetic diseases have been derived from
pre-implantation genetic diagnosed embryos [9–12] . These cell lines represent an extremely powerful model to unravel the mechanisms of pathogenesis and uncover new therapeutic targets. More recently, a new type of human pluripotent stem cells, induced pluripotent stem cells (iPSCs), has become accessible to the scientific community. Human iPSCs exhibit the same phenotypical characteristics as the hESC. iPSCs were first produced in 2006 from mouse cells [13] and in 2007 from human cells [14,15] . Human iPSCs are derived from somatic cells such as adult fibroblasts [14] or, more recently, adult blood cells [16] that have been reprogrammed by forcing the transgenic expression of defined transcription factors. In the first study [13] , carried out on mouse fibroblasts, a cocktail of four transcription factors (Oct4, Sox2, c‑Myc and Klf4) was successfully used to fully reprogram the somatic cells into pluripotent stem cells. The same four factors were