Identification of Novel RAS Signaling Therapeutic Vulnerabilities in Diffuse Intrinsic Pontine Gliomas

Abstract

Diffuse intrinsic pontine gliomas (DIPG) are incurable brain tumors with an aggressive onset. Apart from irradiation, there are currently no effective therapies available for patients with DIPG, who have a median survival time of less than one year. Most DIPG cells harbor mutations in genes encoding histone H3 (H3K27M) proteins, resulting in a global reduction of H3K27 trimethylation and activation of oncogenic signaling pathways. Here we show that the H3K27M mutations contribute to RAS pathway signaling, which is augmented by additional RAS activators including PDGFRA. H3K27M mutation led to increased expression of receptor tyrosine kinases (RTK). A RAS pathway functional screen identified ERK5, but not ERK1/2, as a RAS pathway effector important for DIPG growth. Suppression of ERK5 decreased DIPG cell proliferation and induced apoptosis in vitro and in vivo. In addition, depletion or inhibition of ERK5 significantly increased survival of mice intracranially engrafted with DIPG cells. Mechanistically, ERK5 directly stabilized the proto-oncogene MYC at the protein level. Collectively, our data demonstrate an underappreciated role of H3K27M in RAS activation and reveal novel therapeutic targets for treating DIPG tumors. Significance: These findings identify the H3K27M mutation as an enhancer of RAS activation in DIPG and ERK5 as a novel, immediately actionable molecular target.

Epigenetic activation of a RAS/MYC axis in H3.3K27M-driven cancer

“…We found that, both in vitro and in vivo, these pathways were epigenetically activated in an H3.3K27M-dependent fashion, including through upregulation of PDGFRA and MYC. This is supported by recent work showing that H3.3K27M leads to in vitro activation of the RAS/MAPK cascade 66 and the finding that a MYC superenhancer was among the most highly activated by the presence of H3K27M 67 . Together, this suggests that H3.3K27M-mediated epigenetic changes activate RAS/ MAPK and MYC, with H3.3K27M epigenetically activating multiple members of the RAS/MAPK cascade as well as its downstream transcriptional targets.…”

Copper: An Intracellular Achilles’ Heel Allowing the Targeting of Epigenetics, Kinase Pathways, and Cell Metabolism in Cancer Therapeutics

“…A recent study transfected the H3K27M mutation into neural stem cells (NSCs) resulting in the activation of Ras and as upregulation of RNA and proteins of RTKs platelet derived growth factor (PDGFR) and EGFR, their respective growth factors, PDGFAA and EGF and initiated increased downstream signaling of the MEKK2‐MEK5‐ERK5 signaling cascade. Targeting of ERK5 inhibited tumor growth and promoted apoptosis [119] …”

Utilizing preclinical models to develop targeted therapies for rare central nervous system cancers

“… 59 Other groups have taken focused RNAi screening approaches to identify novel targets for DMG growth, including using a kinome-wide shRNA screen in DMG stem cell cultures, 60 an epigenomic/chromatin-associated shRNA screen in patient-derived DMG cultured cells, 61 and a RAS signaling pathway-targeted siRNA screen in both DMG stem cell and patient-derived cultures. 62 The latter two studies validated their findings in vivo using orthotopic xenograft models. 61 , 62 This technology, however, has been limited to assessing the molecular targets intrinsic to tumor cells and does not account for the diverse and complex extrinsic interactions of cancer cells with the tumor microenvironment.…”

“… 62 The latter two studies validated their findings in vivo using orthotopic xenograft models. 61 , 62 This technology, however, has been limited to assessing the molecular targets intrinsic to tumor cells and does not account for the diverse and complex extrinsic interactions of cancer cells with the tumor microenvironment. Additionally, there are technical challenges to RNAi screens, including the transfection process, resulting in incomplete penetrance and potentially only partial knockdown.…”