Next-generation genomic sequencing has identified multiple novel molecular alterations in cancer. Since the identification of DNA methylation and histone modification, it has become evident that genes encoding epigenetic modifiers that locally and globally regulate gene expression play a crucial role in normal development and cancer progression. The histone methyltransferase enhancer of zeste homolog 2 (EZH2) is the enzymatic catalytic subunit of the polycomb-repressive complex 2 (PRC2) that can alter gene expression by trimethylating lysine 27 on histone 3 (H3K27). EZH2 is involved in global transcriptional repression, mainly targeting tumor-suppressor genes. EZH2 is commonly overexpressed in cancer and shows activating mutations in subtypes of lymphoma. Extensive studies have uncovered an important role for EZH2 in cancer progression and have suggested that it may be a useful therapeutic target. In addition, tumors harboring mutations in other epigenetic genes such as ARID1A, KDM6, and BAP1 are highly sensitive to EZH2 inhibition, thus increasing its potential as a therapeutic target. Recent studies also suggest that inhibition of EZH2 enhances the response to tumor immunotherapy. Many small-molecule inhibitors have been developed to target EZH2 or the PRC2 complex, with some of these inhibitors now in early clinical trials reporting clinical responses with acceptable tolerability. In this review, we highlight the recent advances in targeting EZH2, its successes, and potential limitations, and we discuss the future directions of this therapeutic subclass.

Metastatic urothelial carcinoma is generally incurable with current systemic therapies. Chromatin modifiers are frequently mutated in bladder cancer, with ARID1A -inactivating mutations present in about 20% of tumors. EZH2, a histone methyltransferase, acts as an oncogene that functionally opposes ARID1A. In addition, PI3K signaling is activated in more than 20% of bladder cancers. Using a combination of in vitro and in vivo data, including patient-derived xenografts, we show that ARID1A-mutant tumors were more sensitive to EZH2 inhibition than ARID1A WT tumors. Mechanistic studies revealed that (a) ARID1A deficiency results in a dependency on PI3K/AKT/mTOR signaling via upregulation of a noncanonical PI3K regulatory subunit, PIK3R3, and downregulation of MAPK signaling and (b) EZH2 inhibitor sensitivity is due to upregulation of PIK3IP1, a protein inhibitor of PI3K signaling. We show that PIK3IP1 inhibited PI3K signaling by inducing proteasomal degradation of PIK3R3. Furthermore, ARID1A-deficient bladder cancer was sensitive to combination therapies with EZH2 and PI3K inhibitors in a synergistic manner. Thus, our studies suggest that bladder cancers with ARID1A mutations can be treated with inhibitors of EZH2 and/or PI3K and revealed mechanistic insights into the role of noncanonical PI3K constituents in bladder cancer biology.

Cancer cells utilize vitamin folate to fulfill their excessive demand for nucleotides and amino acids. Dihydrofolate reductase (DHFR), an enzyme involved in folate metabolism converts dihydrofolate into tetrahydrofolate, which is required for the de novo synthesis of purines, and certain amino acids. DHFR inhibitors are used as a chemotherapeutic agent. Cancer sequencing analysis has identified additional enzymes in folate metabolism that are dysregulated in cancer. Methylenetetrahydrofolate dehydrogenase 1 like (MTHFD1L), one such enzyme is overexpressed in bladder cancer. MTHFD1L is a mitochondrial enzyme involved in the folate cycle by catalyzing the reaction of formyl-tetrahydrofolate to formate and tetrahydrofolate (THF). THF is crucial for de novo purine and thymidylate synthesis and is also involved in the regeneration of methionine. Cancer cells rely on purines derived from the de novo pathway for the nucleotides whereas normal cells favor the salvage pathway. In this study we examined MTHFD1L expression in bladder cancer. By using publicly available cancer transcriptome data analysis web-portal UALCAN, we found overexpression of MTHFD1L in bladder cancer and expression was associated with overall survival. RT-PCR and immunoblot analysis confirmed the overexpression of MTHFD1L in muscle invasive bladder cancer tissues compared to normal urothelium. Furthermore, our investigations suggested a critical role for MTHFD1L in bladder cancer cell proliferation, colony formation and invasion. Thus, in this study, we show the significance of the folate metabolic enzyme MTHFD1L in aggressive bladder cancers and suggest that being an enzyme, MTHFD1L serves as a valuable therapeutic target.