SPP1 and glioblastoma: Notably, PDPN was included in a 9 gene prognostic signature defined by Colman et al. [6], the inflammatory cytokine IL6 has been linked to poor prognosis in GBM [52] and high serum levels of the secreted glycoprotein SPP1 (osteopontin) correlate with poor survival in GBM [53].