René Bernards

A single gene mutation predicts response to immune checkpoint blockade in ovarian clear cell carcinoma

There is a lack of genetic biomarkers for predicting response to immune checkpoint blockade (ICB) therapy in cancer. The recent discovery that loss‐of‐function mutations in the gene encoding the protein phosphatase 2A (PP2A) scaffold protein PPP2R1A confer sensitivity to immune checkpoint blockade in ovarian clear cell carcinoma, therefore represents a breakthrough. Mechanistically, mutations in the PPP2R1A gene induce a strong interferon gamma response in tumor cells, which enhances infiltration of activated CD8+ T cells into the tumor. The activity of these T cells is then fortified by ICB. Furthermore, preclinical studies have shown that PP2A inhibition leads to the generation of neoantigens by disrupting RNA splicing, and PP2A inhibition can remodel the immune microenvironment of tumors to enhance responses to ICB. The finding that loss‐of‐function PPP2R1A mutations predict benefit from immunotherapy also suggests that pharmacological inhibition of PP2A may act synergistically with ICB therapy.

NOTCH Signaling Limits the Response of Low-Grade Serous Ovarian Cancers to MEK Inhibition

Abstract Low-grade serous ovarian cancer (LGSOC) is a rare subtype of epithelial ovarian cancer with high fatality rates in advanced stages due to its chemoresistant properties. LGSOC is characterized by activation of MAPK signaling, and recent clinical trials indicate that the MEK inhibitor (MEKi) trametinib may be a good treatment option for a subset of patients. Understanding MEKi-resistance mechanisms and subsequent identification of rational drug combinations to suppress resistance may greatly improve LGSOC treatment strategies. Both gain-of-function and loss-of-function CRISPR-Cas9 genome-wide libraries were used to screen LGSOC cell lines to identify genes that modulate the response to MEKi. Overexpression of MAML2 and loss of MAP3K1 were identified, both leading to overexpression of the NOTCH target HES1, which has a causal role in this process as its knockdown reversed MEKi resistance. Interestingly, increased HES1 expression was also observed in selected spontaneous trametinib-resistant clones, next to activating MAP2K1 (MEK1) mutations. Subsequent trametinib synthetic lethality screens identified SHOC2 downregulation as being synthetic lethal with MEKis. Targeting SHOC2 with pan-RAF inhibitors (pan-RAFis) in combination with MEKi was effective in parental LGSOC cell lines, in MEKi-resistant derivatives, in primary ascites cultures from patients with LGSOC, and in LGSOC (cell line–derived and patient-derived) xenograft mouse models. We found that the combination of pan-RAFi with MEKi downregulated HES1 levels in trametinib-resistant cells, providing an explanation for the synergy that was observed. Combining MEKis with pan-RAFis may provide a promising treatment strategy for patients with LGSOC, which warrants further clinical validation.