Abstract
Oncoproteins with direct mitogenic functions (e.g., tyrosine protein kinases, RAS-family G proteins, MYC family transcription factors) trigger a hypertranscription state that induces replication stress (RS). Sustained RS leads to genomic instability that can prove lethal. Accordingly, cells transformed by mitogenic oncoproteins become dependent on genomic repair mechanisms--many of which have druggable components with important therapeutic potential since (i) suppression of genomic repair can be selectively lethal to transformed cells, and (ii) many mitogenic oncoproteins (e.g., MYC and RAS family members) are challenging targets for drug development. Unfortunately, some oncogenic drivers (especially those of children and young adults) have no direct mitogenic function. For example, a recurrent K27M amino acid substitution in histone 3 (H3K27M) is the most prevalent oncogenic driver in diffuse midline gliomas (DMGs), a uniformly fatal pediatric cancer. The H3K27M oncoprotein--a challenging target for drug development--acts broadly at an epigenetic level to permit neoplastic transformation but has no direct mitogenic function. We show here that H3K27M oncoprotein triggers hypertranscription leading to pronounced RS as measured by the expression of intracellular markers (e.g., phosphorylation of RPA and CHK1). A key player in genomic repair that opposes RS is the ATR protein kinase. We show that H3K27M expression results in high basal activation of ATR kinase. A clinical-stage, brain-penetrant inhibitor of ATR kinase (ART0380) exacerbates genomic damage in patient-derived H3K27M-mutated DMG cells, and these cells are selectively vulnerable to ART0380 (relative to H3K27 WT cells) in vitro and in DMG xenografts. It is noteworthy that ART0380 synergizes with radiation to kill H3K27M-mutant DMG cells in vitro and (in preliminary studies) in DMG xenografts. Our findings warrant ART0380 evaluation in pediatric DMGs and the cooperative relationship with radiotherapy, standard-of-care for DMG treatment, may facilitate such future clinical trials.