Selection pressures of TP53 mutation and microenvironmental location influence epidermal growth factor receptor gene amplification in human glioblastomas.

Epidermal growth factor receptor (EGFR) gene amplification occurs in glioblastomas as so-called double minutes. Because double minutes are extrachromosomal fragments, selection pressures must operate to maintain high EGFR copy number over multiple cell divisions. In analyses of glioblastoma lysates, EGFR amplification has been observed almost exclusively in glioblastomas harboring wild-type TP53 genes, which raises the alternative hypotheses that TP53 mutation either prevents amplification or selects against maintenance of EGFR-amplified cells. To address these possibilities at the cellular level, we studied 14 glioblastomas for TP53 mutation and EGFR gene amplification status, using fluorescence in situ hybridization (FISH) for the latter. Remarkably, four of the six cases with TP53 mutation had isolated EGFR-amplified cells in different regions, demonstrating that EGFR amplification occurs frequently at the cellular level in TP53-mutant glioblastomas. Thus, TP53 mutation does not prevent EGFR amplification but does not facilitate selection of EGFR-amplified cells. Of the eight cases without TP53 mutation, five had widespread EGFR amplification. In four of these five cases, multiple regions of the tumor were available for examination; FISH demonstrated a gradation of EGFR amplification, with highly amplified cells, primarily at the invading edges rather than the relatively solid tumor centers, suggesting that EGFR overexpression, when selected for in vivo, may be related to tumor invasion.