GDF15 and glioblastoma: Compared with primary GBM, secondary GBM showed higher expression of ITGB1, ACVR1, MYC, STAT3 and BMP2 but significantly decreased expression of genes related to germ cell mature (Nanos3, DND1, DDX4, SOX17, SYCP3, DMC1, ZP3 and GDF15) and genes related to inhibition of PGC-EGC conversion (BMP4 and SOX17) (Fig. 3A, D, and Additional file 1: Fig. S4A, Table S3), further indicating that the activation of PGC-EGC/ES-like conversion was one of driving events to malignant prognosis of gliomas.