P17.08.a clinical and molecular characterization of extracranial metastases in glioblastomas

Abstract BACKGROUND Glioblastoma (GBM) is the most common and malignant primary brain tumor in adults, but in contrast to many other malignancies they rarely metastasize. Metastasizing GBMs cause both diagnostic and therapeutic challenges and are generally poorly investigated. Therefore, our aim was to characterize these tumors clinically and molecularly. MATERIAL AND METHODS We collected and examined the largest cohort to date of tissue from 16 glioma patients (14 GBMs and 2 lower-grade gliomas) with extracranial metastases, including metastases to scalp (7) lymph nodes (5), bone (2), liver (1), and upper neck (1). After histopathological re-evaluation of the tumors, we assessed the associated magnetic resonance imaging (MRI) scans. Paired samples from the primary tumors, recurrences and metastases were investigated by next-generation sequencing (NGS) analysis with the TSO500 panel, genome-wide 850 K methylation profiling and immunohistochemistry followed by artificial intelligence-based quantification of markers of stemness and immune cells in the tumor microenvironment (TME). RESULTS Clinically, the patients were comparable to other GBM patients (gender, age, treatment, and survival). 12/16 patients developed one or more intracranial recurrence(s). MRI scans revealed proximity of the brain tumors to dura, large vessels, and ventricles in a high proportion of the patients. Moreover, we found that (6/7) scalp lesions were in fact extracranial extensions of the brain tumors. NGS data showed that the metastases were clonally derived from the primary tumors and not from the brain recurrences. Methylation- and copy number analysis overall confirmed the initial diagnoses and revealed a patient-specific signature across paired samples in a majority of the patients. Additionally, we observed methylation subclass switch during tumor progression and identified differentially methylated regions enriched in the olfactory transduction pathway when comparing metastases to primary tumors. Levels of tumor stemness were generally preserved according to methylation-based stemness index and OLIG2 and SOX2 protein expression. Similarly, the composition of immune cells in the TME did not change significantly, as shown by cell-type deconvolution and immune cell quantification. CONCLUSION Extracranial metastases occurred as early events and overall preserved the tumor- and TME characteristics of the primary brain tumors. Factor such as brain tumor location, neurosurgery and epigenetic regulation of genes associated with olfactory transduction could be involved in glioblastoma metastasis.