Alternative definition : KEGG: Glioblastoma multiforme (GBM) formation is either de novo (primary GBMs) or
due to the progression of a lower grade glioma to a higher grade one through the acquisition
of additional mutations (secondary GBMs). In primary GBM, disruption of the p53 pathway
often occurs through loss of ARF, or less frequently through amplification of MDM2.
Disruption of the RB pathway occurs through loss of INK4A. Amplification and/or mutation
of the epidermal growth factor receptor (EGFR) is the most frequently detected genetic
defect that is associated with primary GBM. In secondary GBM, loss of p53 and activation
of the growth-factor-receptor-tyrosine-kinase signaling pathway (such as through overexpression
of PDGF/PDGFR) initiates tumour formation, whereas disruption of the retinoblastoma
(RB) pathway contributes to the progression of tumour development. Loss of PTEN has
been implicated in both pathways, although it is much more common in the pathogenesis
of primary GBM.;