LINE-1-mediated retrotransposition in human pluripotent stem cells: Consequences for genomic stability of hES and hiPS cells and its derivatives

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The use of pluripotent stem cells derived from the inner cell mass of developing blastocysts (human embryonic stem cells, hESCs) or generated by reprogramming of fibroblasts (human induced pluripo-tent stem cells, hiPSCs) holds great therapeutic promise for regenerative medicine. However, a vital unanswered question is whether these cells or their derivatives are truly safe for administration. Spe-cifically, it is unclear whether the integrity of the genome of pluripotent stem cells is maintained during their generation, expansion and differentiation. The appearance of genetic mutations during their ex-pansion or differentiation could undermine stem cell therapies. Such mutations could be induced by human non-LTR retrotransposons (LINE1 or L1, Alu, SVA) which represent the currently mobilized group of human endogenous retroelements. About 35% of the human genome are the consequence of the mobilization of non-LTR retrotransposons which is executed by the L1-encoded protein machin-ery. We will investigate whether hESCs and hiPSCs differ in their support for L1 retrotransposition and analyze the extent of genomic destabilization of these cells by L1- retrotransposition. We want to determine both the role of L1 activity in the formation of karyotypic abnormalities that are frequently observed in pluripotent stem cells in tissue culture, and the effect of differentiation on L1 mobilization rates. We will evaluate if there are any L1 integration preferences for specific genomic regions in hESC and hiPSC lines that could potentially affect neighbouring gene expression. The proposed ap-proaches will help to elucidate whether L1-mediated retrotransposition can contribute to genome fluid-ity and variability of hESCs and hiPSCs, and will be useful to assess the activity of endogenous mobile elements in pluripotent stem cells in order to evaluate their safety for therapy.