Midline pediatric high-grade astrocytomas (pHGAs) are incurable with few treatment targets identified. Most tumors harbor K27M mutations on histone 3 variants. In 40 treatment-naïve midline pHGAs, 39 analyzed by whole-exome sequencing, we find additional somatic mutations specific to tumor location. Gain-of-function mutations in ACVR1 occur in tumors of the pons in conjunction with H3.1 K27M, while FGFR1 mutations/fusions occur in thalamic tumors associated with H3.3 K27M. Hyper-activation of the bone morphogenetic protein (BMP)/ACVR1 developmental pathway in pHGAs harbouring ACVR1 mutations led to increased phospho-SMAD1/5/8 expression and up-regulation of BMP downstream early response genes in tumour cells. Global DNA methylation profiles were significantly associated with the K27M mutation regardless of the mutant H3 variant and irrespective of tumor location, supporting its role in driving the epigenetic phenotype. This significantly expands the potential treatment targets and further justifies pre-treatment biopsy in pHGA as a means to orient therapeutic efforts in this disease.

The release of EAAs is closely linked to the release of structural amino acids and may thus reflect nonspecific development of membrane micropores, rather than presynaptic neuronal vesicular exocytosis. The magnitude of EAA release in patients with focal contusions and ischemic events may be sufficient to exacerbate neuronal damage, and these patients may be the best candidates for treatment with glutamate antagonists in the future.

Purpose: Pediatric DMGs are highly malignant tumors with poor clinical outcomes. Over 70% of DMG patients harbor the histone 3 p.K27M (H3K27M) mutation, which correlates with a poorer clinical outcome, and is also used as a criterion for enrollment in clinical trials. Because complete surgical resection of DMG is not an option, biopsy at presentation is feasible, but re-biopsy at time of progression is rare. While imaging and clinical-based disease monitoring is the standard of care, molecular-based longitudinal characterization of these tumors is almost non-existent. To overcome these hurdles we examined if liquid biopsy allows measurement of disease response to precision therapy. Experimental Design: We established a sensitive and specific methodology which detects major driver mutations associated with pediatric DMGs using droplet digital PCR (n=48 subjects, n=110 specimens). Quantification of ctDNA for H3K27M was used for longitudinal assessment of disease response compared to centrally reviewed MRI data. Results: H3K27M was identified in cerebrospinal fluid (CSF) and plasma in 88% of patients with DMG, with CSF being the most enriched for ctDNA. We demonstrated the feasibility of multiplexing for detection of H3K27M, and additional driver mutations in patient’s tumor and matched CSF, maximizing the utility of a single source of liquid biome. A significant decrease in H3K27M plasma ctDNA agreed with MRI assessment of tumor response to radiotherapy in 83% (10/12) of patients. Conclusions: Our liquid biopsy approach provides a molecularly-based tool for tumor characterization, and is the first to indicate clinical utility of ctDNA for longitudinal surveillance of DMGs.

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Diagnosis of CNS germ cell tumor is often delayed, and presentation may include movement disorders or mimic psychiatric disease. MRI interpretation can be challenging and may require serum/CSF markers and biopsy for diagnosis.