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Achieving transgene integration into preselected genomic sites is currently one of the central tasks in stem cell gene therapy. A strategy to mediate such targeted integration involves site-specific endonucleases. Two genomic sites within the MBS85 and chemokine (C-C motif) receptor 5 (CCR5) genes (AAVS1 and CCR5 zinc-finger nuclease (CCR5-ZFN) sites, respectively) have recently been suggested as potential target regions for integration as their disruption has no functional consequence. We hypothesized that efficient transgene integration maybe affected by DNA accessibility of endonucleases and therefore studied the transcriptional and chromatin status of the AAVS1 and CCR5 sites in eight human induced pluripotent stem (iPS) cell lines and pooled CD34+ hematopoietic stem cells (HSCs). Matrix chromatin immunoprecipitation (ChIP) assays demonstrated that the CCR5 site and surrounding regions possessed a predominantly closed chromatin configuration consistent with its transcriptional inactivity in these cell types. In contrast, the AAVS1 site was located within a transcriptionally active region and exhibited an open chromatin configuration in both iPS cells and HSCs. To show that the AAVS1 site is readily amendable to genome modification, we expressed Rep78, an AAV2-derived protein with AAVS1-specific endonuclease activity, in iPS cells after adenoviral gene transfer. We showed that Rep78 efficiently associated with the AAVS1 site and triggered genome modifications within this site. On the other hand, binding to and modification of the CCR5-ZFN site by a ZFN was relatively inefficient. Our data suggest a critical influence of chromatin structure on efficacy of site-specific endonucleases used for genome editing.

Chromatin structure of two genomic sites for targeted transgene integration in induced pluripotent stem cells and hematopoietic stem cells