Induction of <scp>SOX17</scp> with stimulation of <scp>WNT</scp>, <scp>TGF</scp>‐beta, and <scp>FGF</scp> signaling drives embryonal carcinomas into the yolk‐sac tumor lineage resulting in increased cisplatin resistance

Relapsing germ cell tumor (GCT) patients often harbor components of the aggressive subtype yolk-sac tumor (YST), suggesting that YST formation is an escape mechanism under therapy. Nevertheless, the molecular mechanisms inducing YST development from its stem cell-like precursor embryonal carcinoma (EC) are largely unexplored. We demonstrated that the induction of the transcription factor SOX17 together with the stimulation of WNT, TGF-beta / Activin, and FGF signaling drives EC cells into the YST lineage. Single cell RNA sequencing revealed that this cell fate switch was accompanied by the upregulation of the typical YST factors AFP, ANKRD1, APOA1, CST1, FOXA2, GATA6, and GPC3 and microRNAs, while pluripotency-related genes NANOG, POU5F1, and SOX2 were downregulated. Chromatin immunoprecipitation followed by sequencing analysis revealed that SOX17 may act in concert with FOXA2 and GATA factors to initiate YST formation. Xenografting of the YST-like cells into nude mice led to the growth of mixed GCT with YST components, confirming that these cells are able to form a YST in vivo. Moreover, the expression of cisplatin resistance factors was induced in a subpopulation of YST-like cells, suggesting that the formation of a YST is accompanied by the acquisition of cisplatin resistance. Indeed, the YST-like cells presented as less sensitive to cisplatin than their parental cells. Our study deciphered the molecular mechanisms forcing EC to differentiate into the YST lineage, which is accompanied by the acquisition of cisplatin resistance, confirming that YST formation is an escape mechanism for GCT under therapy. Thus, GCT patients should be screened for YST elements under therapy to identify patients at risk of developing therapy resistance.