Nelson K.Y. Wong

FOLR1 as a therapeutic target in platinum-resistant ovarian carcinoma: unique expression patterns across ovarian carcinoma histotypes and molecular subtypes of low-grade serous carcinoma

With the development of novel antibody-drug conjugates (ADCs), folate receptor alpha (FOLR1) is a promising therapeutic target for the treatment of platinum-resistant tubo-ovarian carcinomas. The main aims of this study were to assess FOLR1 protein expression in a large cohort of ovarian carcinoma histotypes. To inform future clinical trial design we identified molecular correlates of FOLR1 expression in low-grade serous carcinoma (LGSC). One thousand five hundred forty-seven ovarian carcinoma samples from 5 different Canadian cohorts were successfully evaluated by immunohistochemistry for FOLR1 expression using the PS2+ system. Statistical analyses with clinicopathological parameters, LGSC molecular subtypes, and overall survival (OS) were performed. High FOLR1 expression was detected in 44% of high-grade serous carcinomas, and in 30% LGSC, 8% clear cell, 6% endometrioid, and 0% mucinous and/or mesonephric-type adenocarcinomas. In 160 LGSC cases, FOLR1 expression was more frequent in cases with normal MAPK pathway status (37% MAPK wild type vs. 14% canonical MAPK pathway mutations; p=0.002), low progesterone receptor (PR) expression (41%) vs. 23% (Allred score >2; p A significant proportion of LGSC express high FOLR1 levels supporting the development of clinical trials to investigate ADCs targeting FOLR1 as novel agents for treating this disease. In LGSC, high FOLR1 expression was associated with fewer MAPK pathway alterations, low PR expression, and p16 loss.

Differential Preclinical Efficacy of Combined CDK4/6 and MEK Inhibition in Low-Grade Serous Ovarian Carcinoma Based on KRAS/NF1 Mutational Status

Low-grade serous ovarian carcinoma (LGSOC) usually presents in advanced stages and is associated with a high mortality rate. Clinical trials targeting the MAPK and cell cycle pathways in LGSOC have shown promising results for its treatment, however there is a need to improve efficacy and define predictive biomarkers to guide patient selection for treatment using these agents. We therefore evaluated cell cycle protein expression by immunohistochemistry (IHC) in 186 LGSOC cases, and evaluated the efficacy of the MEK inhibitor, trametinib, in combination with the CDK4/6 inhibitor, palbociclib, in preclinical models of LGSOC. Abnormal p16 expression was observed in 20% of primary and 46% of recurrent tumors, and it was associated with poorer survival (log-rank p = 0.005). Notably, cell lines with increased sensitivity to trametinib were more likely to harbor mutations in KRAS or NF1 and displayed low pRb levels. Palbociclib showed limited efficacy in vitro; however, the combination of palbociclib and trametinib treatment produced synergistic antiproliferative effects in KRAS/NF1-wild-type cell lines, which displayed higher pRb levels. Acquired drug resistance was linked to increased cyclin D1/E1 expression. This study confirms abnormal p16 IHC as a negative prognostic marker in LGSOC and establishes key determinants of sensitivity to CDK4/6 inhibitor-based therapy.

Multiomics Characterization of Low-Grade Serous Ovarian Carcinoma Identifies Potential Biomarkers of MEK Inhibitor Sensitivity and Therapeutic Vulnerability

Abstract Low-grade serous ovarian carcinoma (LGSOC) is a rare tumor subtype with high case fatality rates in patients with metastatic disease. There is a pressing need to develop effective treatments using newly available preclinical models for therapeutic discovery and drug evaluation. Here, we use multiomics integration of whole-exome sequencing, RNA sequencing, and mass spectrometry–based proteomics on 14 LGSOC cell lines to elucidate novel biomarkers and therapeutic vulnerabilities. Comparison of LGSOC cell line data with LGSOC tumor data enabled predictive biomarker identification of MEK inhibitor (MEKi) efficacy, with KRAS mutations found exclusively in MEKi-sensitive cell lines and NRAS mutations found mostly in MEKi-resistant cell lines. Distinct patterns of Catalogue of Somatic Mutations in Cancer mutational signatures were identified in MEKi-sensitive and MEKi-resistant cell lines. Deletions of CDKN2A/B and MTAP genes were more frequent in cell lines than tumor samples and possibly represent key driver events in the absence of KRAS/NRAS/BRAF mutations. These LGSOC cell lines were representative models of the molecular aberrations found in LGSOC tumors. For prediction of in vitro MEKi efficacy, proteomic data provided better discrimination than gene expression data. Condensin, minichromosome maintenance, and replication factor C protein complexes were identified as potential treatment targets in MEKi-resistant cell lines. This study suggests that CDKN2A/B or MTAP deficiency may be exploited using synthetically lethal treatment strategies, highlighting the importance of using proteomic data as a tool for molecular drug prediction. Multiomics approaches are crucial to improving our understanding of the molecular underpinnings of LGSOC and applying this information to develop new therapies. Significance: These findings highlight the utility of global multiomics to characterize LGSOC cell lines as research models, to determine biomarkers of MEKi resistance, and to identify potential novel therapeutic targets.

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.