DIPG-17. TREATMENT OF DIFFUSE INTRINSIC PONTINE GLIOMA USING ALLELE-SPECIFIC SIRNA TARGETING H3.3 K27M MUTANT HISTONE

Abstract In recent years CNS tumors have surpassed leukemia and lymphoma as the deadliest forms of pediatric cancer, yet many of these tumors remain resistant to treatment. Understanding of the molecular pathology of a particularly fatal pediatric glioma, diffuse intrinsic pontine glioma (DIPG), has improved dramatically over the past decade. However, this knowledge has not yet yielded new targeted therapeutics or advances in treatment. The majority of DIPG tumors (~70-80%) harbor a K27M mutation in the histone H3.3 gene resulting in significant epigenetic changes which drive tumorigenesis. We are utilizing cutting edge siRNA chemistries to develop a therapeutic specifically targeting this oncohistone H3.3 K27M. It has been shown by others that ex vivo disruption of the H3.3 K27M mutant significantly delays tumor development in orthotopic xenograft models and could be a promising target for treatment of disease. New siRNA chemistries allow broad distribution and silencing throughout the brain with durable silencing for months. Utilizing this technology, we have engineered siRNA compounds that can discriminate between wild-type (WT) and K27M mutant H3.3 alleles. Our first-generation compounds achieved >95% silencing of K27M mutant with only 40% silencing of WT H3.3. We have developed second-generation compounds to improve the discrimination between the two alleles. These compounds allow us to selectively knockdown the mutant driver of DIPG tumorigenesis and we hypothesize that silencing of this oncohistone will revert the epigenetic landscape of DIPG to a more wild-type form detrimentally impacting tumor growth and progression. Together the in vivo potency of the new siRNA chemistries combined with the precision allele-specific targeting of mutant K27M oncohistone may be a promising path towards development of an effective and safe therapy for DIPG.