HGG-21. Oncogenic tyrosine kinase gene fusions in infant-type hemispheric gliomas - comparison of RNA- and DNA-based methods for their reliable detection

High-grade diffuse gliomas (HGG) in early childhood are characterized by a more favorable outcome compared to older children. We demonstrated in previous studies that these tumors have stable genomes. Activating tyrosine kinase gene fusions in infant-type hemispheric gliomas represent therapeutic targets. 50 supratentorial HGG occurring in children younger than four years were retrieved from the archives of the Brain Tumor Reference Center, Institute of Neuropathology, Bonn University. DNA and RNA were extracted from FFPE tumor samples. Gene fusions were identified on the DNA level by FISH using break-apart probes for ALK, NTRK1, -2, -3, ROS1 and MET and Molecular Inversion Probe (MIP) methodology. On the RNA level, fusion transcripts were detected by targeted RNA sequencing as well as Nanostring assay with fusion-specific probes. 37 supratentorial HGG occurred in the first year of life, 13 HGG between one and four years. 18 cases showed fusions of ALK to different partners; all occurred in the first year of life (18/37, 48.6%). Fusions of ROS1 were found in 5, MET in 3, NTRK1, -2, -3 in 10 cases. 12 cases showed no and two cases novel fusions. The different methods led to comparable results. Only recurrent fusions with known fusion partners were detectable with fusion sequence-specific Nanostring probes and library construction for targeted RNA sequencing failed in a fraction of cases. Break-apart FISH led to reliable results on the next day, and MIP technology represented the most sensitive method for analysis of FFPE samples. Gene fusions involving the tyrosine kinase genes ALK, MET, ROS1 and NTRK1, -2, -3 occurred in 72% of HGG of young children; most frequent were ALK fusions occurring in tumors of infants. DNA-based MIP technology represented the most robust and sensitive assay. A combination of RNA- and DNA-based methods to detect these fusions with high reliability is recommended.