Enhancing the direct reprogramming towards human naïve and expanded pluripotency
Naïve pluripotent and expanded potential stem cells (EPSCs) represent the pre-implantation epiblast which are molecularly, epigenetically and developmentally distinct from conventional primed pluripotent stem cells representing the epiblast of post-implantation embryo. These stem cell types offer expanded developmental potential towards extraembryonic tissues, clonality at the single cell level, high homogeneity and are more amenable for genome engineering. Here we use a polycistronic cassette to directly reprogram fibroblasts into induced EPSCs without a detour to primed pluripotency. When we replace SOX2 with engineered SOX17 factor (eSOX17),  we obtain 7 to 10 -fold more iEPSC colonies within shorter time periods. We next tested our reprogramming regimen in four media supporting naïve pluripotency reprogramming and could reproducibly obtain large numbers of clonally expandable colonies with eSOX17 whilst SOX2 occasionally fails. The resultant naïve pluripotent and expanded potential stem cells molecularly and functionally resemble their counterparts derived from embryonic stem cells. In sum, the use of engineered SOX17 factor enables a direct, fast, efficient and reproducible reprogramming of somatic cells into cells representative of the pre-implantation epiblast from somatic cells. Overall design: We analyzed the gene expression profiles of induced expanded potential stem cells (iEPSCs) reprogrammed from human dermal firoblasts with OCT4/KLF4/C-MYC/SOX17FNV or OCT4/KLF4/C-MYC/SOX2.