Next-generation repeat-free FISH probes for DNA amplification in glioblastoma in vivo: Improving patient selection to MDM2-targeted inhibitors

A next-generation FISH probe mapping to the MDM2 locus-specific region has recently been designed. The level of MDM2 gene amplification (high versus low) may allow selection of patients for cancer treatment with MDM2 inhibitors and may predict their responsiveness. We investigated the spectrum of MDM2 gene alterations using the new probes in vivo after visualizing single neoplastic cells in situ from a series of glioblastomas. Signals from next-generation repeat-free FISH interphase probes were identified in tissue microarrays that included 3 spots for each of the 48 cases. The murine double minutes (MDM2)-specific DNA probe and the satellite enumeration probe for chromosome 12 were used. Three cases (6%) showed more than 25 signals (high gene amplification), and 7 (15%) showed 3-10 signals (gains); among these, 4 cases (8%) had an equal number of MDM2 and centromeric signals on chromosome 12 (polyploidy). Genomic heterogeneity was observed only in 3 cases with low gene amplification. In our series, 6% of glioblastomas exhibited high MDM2 amplification (in vivo) with a pattern related to the known double minutes/chromothripsis phenomenon (in situ), and only cases with low amplification showed genomic heterogeneity. We concluded that the rate of MDM2 gene amplification can be a useful predictive biomarker to improve patient selection.