LAUSR

Diffuse midline glioma (DMG) is the most aggressive and lethal pediatric brain tumor, with no effective therapies to date. Immune checkpoint blockade (ICB) has failed clinically in DMG, largely due to limited T cell infiltration, low immune checkpoint (IC) expression, and a low tumor mutational burden. Adenosine deaminase acting on RNA (ADAR) is an RNA-editing enzyme that suppresses innate immune activation by editing endogenous double-stranded RNA (dsRNA), preventing its sensing. In DMG, the oncohistone mutation H3K27M induces epigenetic dysregulation and derepression of retroelements—major sources of endogenous dsRNA. We demonstrate that ADAR is significantly overexpressed at both the transcript and protein levels in H3K27M+ patient samples and cell lines. Functional depletion of ADAR via siRNA or CRISPR-Cas9 in patient-derived DMG cells results in marked proliferation deficits compared to controls and to non-malignant astrocytes (Mann-Whitney test, P < 0.0001). Loss of ADAR leads to increased retroelement expression (RNA-seq, qPCR), activation of type I interferon (IFN) signaling (RNA-seq, ELISA), and upregulation of ICs on the tumor cell surface (RNA-seq, western blot, flow cytometry). To pharmacologically target ADAR, we identified that high-dose all-trans retinoic acid (ATRA) induces ADAR degradation in glioma cells. ATRA treatment phenocopies ADAR depletion—reducing proliferation, activating IFN responses, and increasing IC expression. Preliminary in vivo studies in an immunocompetent orthotopic DMG model show that ATRA significantly suppresses tumor growth and prolongs survival. These findings suggest that ADAR plays a central role in immune evasion in DMG and is a promising therapeutic target. ATRA-mediated ADAR degradation offers a novel, translationally relevant strategy to enhance tumor immunogenicity. Ongoing studies aim to define mechanisms of ATRA action and identify optimal immunotherapy combinations for clinical translation.