GCT-19. MODELING GERM CELL TUMORS WITH KIT MUTANT hiPSCs

Central Nervous System Germ Cell Tumor (CNS GCT) is the second most common pediatric brain tumor in Japan, and within CNS GCT, germinoma is the most common subtype, accounting for 62.3%. Recent reports of transcriptome and methylome analysis suggested that germinoma highly resemble the state of gonocytes, the germ cells at around 5th to 7th week of human embryo development. It is also identified that 60% of germinoma harbored somatic mutations in KIT/RAS pathway. As the protocol to derive gonocytes from human iPSCs have been reported, we aimed to recapitulate tumorigenesis by generating human iPSCs bearing common genetic mutations and derive gonocytes from them. We first introduced the most common mutation KITD816V to human iPSCs using CRISPR/Cas9, and confirmed in iPSCs that mutated KIT was phosphorylated in the absence of ligand stimulation, and also found that KIT activation contribute to the phosphorylation of AKT but not of ERK. Upon differentiation towards primordial germ cell -like cells (PGCLCs), KIT mutant lines were efficiently induced into PGCLCs, however, by comparing conditions with or without KIT ligand (SCF), mutant lines exhibited less dependency to SCF compared to wildtype cells. Mutant cells were further differentiated to gonocytes following published protocol and the cells were collected for transcriptome analysis. By comparing with the transcriptome of germinoma, we confirmed that germinoma samples express germ cell genes similar to gonocytes. We are attempting to identify the molecular mechanism of tumorigenesis in relation to KIT activation using this system.