Enhancing reprogramming towards induced human expanded pluripotency through substitution of SOX2 with engineered SOX17 transcription factors

Expanded potential stem cells (EPSCs) represent a distinct and developmentally primitive stem cell population characterized by their broad developmental potential, which encompasses both embryonic and extra-embryonic lineages. In this study, we employed a polycistronic cassette to directly reprogram human fibroblasts into induced Expanded Potential Stem Cells (iEPSCs). Substituting SOX2 with engineered SOX17 transcription factors resulted in an approximately five-fold increase in the average yield of iEPSC colonies, while maintaining the molecular and functional integrity of the resulting clonal lines. Notably, under feeder-free conditions, SOX2 occasionally failed to reprogram and yielded inconsistent colony numbers, whereas engineered SOX17 and miniaturized SOX17 reproducibly produced feeder-free iEPSCs. In summary, the use of engineered SOX17 enables efficient and robust reprogramming of human fibroblasts into EPSCs, allowing for modeling of early human pre-implantation development, investigating placental disorders, and expanding the toolkit for drug development with a versatile model of pluripotent stem cells that exhibit broader developmental capabilities. Engineered SOX17 factors, including a miniaturized variant, replace SOX2 to drive efficient direct reprogramming to induced human expanded potential stem cells under feeder-based and feeder-free culture conditions.