Etoposide induces MLL rearrangements and other chromosomal abnormalities in human embryonic stem cells

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Abstract
MLL rearrangements are hallmark genetic abnormalities in infant leukemia known to arise in utero. They can be induced during human prenatal development upon exposure to etoposide. We also hypothesize that chronic exposure to etoposide might render cells more susceptible to other genomic insults. Here, for the first time, human embryonic stem cells (hESCs) were used as a model to test the effects of etoposide on human early embryonic development. We addressed whether: (i) low doses of etoposide promote MLL rearrangements in hESCs and hESCs-derived hematopoietic cells; (ii) MLL rearrangements are sufficient to confer hESCs with a selective growth advantage and (iii) continuous exposure to low doses of etoposide induces hESCs to acquire other chromosomal abnormalities. In contrast to cord blood-derived CD34+ and hESC-derived hematopoietic cells, exposure of undifferentiated hESCs to a single low dose of etoposide induced a pronounced cell death. Etoposide induced MLL rearrangements in hESCs and their hematopoietic derivatives. After long-term culture, the proportion of hESCs harboring MLL rearrangements diminished and neither cell cycle variations nor genomic abnormalities were observed in the etoposide-treated hESCs, suggesting that MLL rearrangements are insufficient to confer hESCs with a selective proliferation/survival advantage. However, continuous exposure to etoposide induced MLL breaks and primed hESCs to acquire other major karyotypic abnormalities. These data show that chronic exposure of developmentally early stem cells to etoposide induces MLL rearrangements and make hESCs more prone to acquire other chromosomal abnormalities than postnatal CD34+ cells, linking embryonic genotoxic exposure to genomic instability.