A review of current therapeutics targeting the mitochondrial protease ClpP in diffuse midline glioma, H3 K27-altered

Abstract

Diffuse midline gliomas (DMGs) are devastating pediatric brain tumors recognized as the leading cause of cancer-related death in children. DMGs are high-grade gliomas (HGGs) diagnosed along the brain's midline. Euchromatin is the hallmark feature of DMG, caused by global hypomethylation of H3K27 either through point mutations in histone H3 genes (H3K27M), or by overexpression of the enhancer of zeste homolog inhibitory protein. In a clinical trial for adults with progressive HGGs, a 22-year-old patient with a thalamic DMG, H3 K27-altered, showed a remarkable clinical and radiological response to dordaviprone (ONC201). This response in an H3 K27-altered HGG patient, coupled with the lack of response of patients harboring wildtype-H3 tumors, has increased the clinical interest in dordaviprone for the treatment of DMG. Additional reports of clinical benefit have emerged, but research defining mechanisms of action (MOA) fall behind dordaviprone's clinical use, with biomarkers of response unresolved. Here, we summarize dordaviprone's safety, interrogate its preclinical MOA identifying the mitochondrial protease "ClpP" as a biomarker of response, and discuss other ClpP agonists, expanding the arsenal of potential weapons in the fight against DMG. Finally, we discuss combination strategies including ClpP agonists, and their immunomodulatory effects suggestive of a role for the tumor microenvironment in DMG patient response.

Keywords: CLPP; CLPP agonist; DMG; ONC201; dordaviprone.

Figure 1.

Expression of dordaviprone cellular targets across pediatric cancers, compared with diffuse gliomas. (A) DRD2 and (B) CLPP mRNA expression in diffuse glioma (including diffuse midline glioma, diffuse pontine glioma, and other high-grade gliomas), low-grade glioma, medulloblastoma, ependymoma, neuroblastoma, osteosarcoma, kidney, adrenal gland, AML (acute myeloid leukemia), ALL (acute lymphoid leukemia), and NBM (normal bone marrow). Diseases are grouped into BT (brain tumor), ST (solid tumor), and HM (hematological malignancies). RNA expression data were analyzed from publicly available data (normalized fragments per kilobase of transcript per million mapped reads [FPKM]). (C) CLPP protein expression (z-score) across pediatric central nervous system cancers analyzed from publicly available data. The dotted line is the median expression level for diffuse glioma, while turquoise dots are less than the dotted line and orange have the median above the dotted line. Statistical significance determined compared only to diffuse gliomas (one-way ANOVA, *P < .05,**P < .01,***P < .001,****P < .0001).

Figure 2.

Normal mitochondrial function in cells. ATP production in mitochondria is produced by a cascade of reaction including glycolysis, tricarboxylic acid (TCA) cycle, or electron transport chain (ETC). The ClpXP complex, comprised of ClpP and ClpX, degrades misfolded proteins in the mitochondria, to maintain its functional integrity.

Figure 3.

Hyperactivation of ClpP by dordaviprone. During respiration, misfolded proteins are identified by a degradation tag and degraded by the ClpXP complex, passing through the axial pore. Dordaviprone replaces ClpX and binds ClpP, in H sites, removing protein selectivity and hyperactivating its protease activity. Additionally, dordaviprone increases axial pore size, resulting in more efficient degradation of mitochondrial respiratory complex proteins, regardless of their folding.

Figure 4.

Dordaviprone influences the immune microenvironment. (A) Dordaviprone increases levels of circulating inflammatory cytokines and effector molecules. (B) Proposed mechanism of dordaviprone-induced release of sequestered T cells in the bone marrow. Dordaviprone antagonizes DRD2 signaling, preventing β-arrestin from internalizing S1PR1. S1PR1 surface localization promotes T-cell trafficking. (C) Dordaviprone increases CD4+ and CD8+ T cell levels, as well as NK cells both in the peripheral blood and in the tumor microenvironment of solid tumors, (D) to reprogram macrophage metabolism driving the proinflammatory M1 phenotype.