Yolk Sac Elements in Tumors Derived from Pluripotent Stem Cells: Borrowing Knowledge from Human Germ Cell Tumors.

Pluripotent stem cell (PSC)-based therapies are currently in clinical trials. However, one of the main safety concerns includes the potential for cancer formation of the PSC-derived products. Currently, the teratoma in vivo assay is accepted by regulatory agencies for identifying whether PSCs have the potential to become malignant. Yolk sac elements (YSE) are one of the elements that could arise from PSC. Whereas the other malignant element, embryonal carcinoma, is thoroughly studied, this is not the case for YSE. Therefore, more research is needed to assess the nature of YSE. We propose that it is imperative to include the formation of YSE in the safety assessment of PSC due to their close resemblance to the clinical entity of yolk sac tumor (YST), a human malignant germ cell tumor (hGCT). In this review, we extrapolate knowledge from YST to better understand YSE derived from PSC. We demonstrate that both share a similar morphology and that the same characteristic immunohistochemical markers can be used for their identification. We discuss the risk these tumors pose, thereby touching upon genetic abnormalities and gene expression that characterize them, as well as possible disease mechanisms. Integrating the molecular and immunohistochemical markers identified in this review into future research will help to better address the potential malignancy associated with PSC.