Time for arginine methylation: PRMT5 inhibition to advance cholangiocarcinoma treatment

Cholangiocarcinoma (CCA) is a highly aggressive adenocarcinoma of the biliary tract system with unsatisfactory therapeutic options (1). Standard frontline treatment for unresectable or metastatic CCA consisting of cisplatin and gemcitabine combined with checkpoint inhibitors targeting programmed cell death ligand 1 (PD-L1) or programmed cell death 1 (PD-1), offer objective response rates of less than 30% and a median survival of approximately a year (1). Targeted therapies against FGFR2 fusions and IDH1 mutations have gained regulatory approval in CCA, but these are applicable only in a minority of patients (1). Disease-agnostic approvals of therapies targeting HER2 overexpression, NTRK fusions, RET fusions, and microsatellite-unstable tumors also benefit patients with CCA, but again, only a small minority. Therefore, novel strategies to treat CCA are urgently needed.

Molecular heterogeneity stands as a major barrier to improving outcomes in CCA. Genetic alterations in DNA only explain a part of this heterogeneity. A rising number of studies suggest a major role for epigenetic perturbations in controlling CCA fate (2). Indeed, epigenetic vulnerabilities including histone modifications have been suggested as novel CCA targets (1). An example for a histone regulator is the protein arginine methyltransferase 5 (PRMT5). PRMT5 forms a homo-tetramer that associates with methylosome protein 50 (MEP50) in a highly active complex which exhibits high affinity for arginine residues. Via histone methylation, PRMT5 functions as a transcriptional co-repressor supporting gene expression of oncogenic signaling via regulation of genes such as p53, NFκB or p21 (2). In addition, PRMT5 regulates splicing via its role as the enzymatic component of the methylosome, a multi-subunit complex containing MEP50, facilitating small nuclear ribonucleoprotein assembly (Figure 1). PRMT5 and MEP50 function have been shown to be important in regulating genome stability and DNA repair (3). Preclinical studies of PRMT5 inhibitors have shown antitumor activity across several tumor types including lymphoma, breast cancer, head and neck or esophageal cancer (2), but the potential of PRMT5 as a therapeutic target in CCA has been largely unexplored to date.

Figure 1. Mechanism of action model for PRMT5 as a therapeutic target in CCA.

In CCA, PRMT5 and MEP50 are overexpressed and play an important role in regulating the genome stability, as part of the methylosome complex, which controls the assembly and stability of splicing complexes. Pharmacological PRMT5 inhibition induces DNA damage and subsequent apoptosis in CCA cell lines with inhibition of tumor growth as shown in in vivo models by Elurbide et al. (4). Drug-induced PRMT5 inhibition is further enhanced in cancer cells carrying a MTAP deletion by modulating MTA levels. MTAP loss occurs in 25% of intrahepatic CCAs as shown by Elurbide et al. in CCA tissues (4). MTA accumulation potently and selectively inhibits PRMT5, acting as ‘double-edged sword’ in the context of PRMT5 inhibition. Abbreviations: MEP50, Methylosome Protein 50; MTAP, Methyl-thioadenosine Phosphorylase; MTA, Methyl-thioadenosine; PRMT5, Protein Arginine Methyltransferase 5.

Elurbide et al. (4) investigated the potential of PRMT5 as a novel therapeutic target in CCA by evaluating expression of the target in patient tissues and inhibition of the target in preclinical models. First, using publicly available transcriptomic datasets from human CCA tissues, they showed overexpression of PRMT5 and MEP50 in intrahepatic CCA but not extrahepatic CCA. However, by immunohistochemistry, Elurbide, et al. showed overexpression of both proteins in both anatomic subtypes. Using several preclinical models, the authors convincingly showed proof-of-concept for PRMT5 as a key mediator of DNA stability in CCA. They further showed that the PRMT5 inhibitors, GSK3326595 and JNJ64619178, significantly reduced both proliferation of CCA cell lines and organoid growth and showed synergistic effects with cisplatin and gemcitabine. PRMT5 inhibition induced apoptosis and led to decreased expression of genes involved in chromatin remodeling and DNA repair. Notably, in two immunocompetent mouse models including an orthotopic model based on the hydrodynamic tail vein injection technique (HDTV), PRMT5 inhibition significantly reduced tumor burden without significant adverse effects. PRMT5 inhibition also led to increased expression of genes involved in antigen presentation and immune response, shown by increased infiltration of CD4+ and CD8+ T cells, suggesting a potential effect in promoting antitumor immunity as seen in preclinical models of other tumor types (5). The use of two complementary immunocompetent mouse models in the study provides robust in vivo proof-of-concept. Future studies may evaluate the functional impact on the tumor microenvironment in other HDTV-based models mimicking the immune variability found in patients (6) and investigate the combination of PRMT5 inhibitors with checkpoint inhibitors. Collectively, the study of Elurbide et al. (4) provides evidence for the important role in PRMT5 in maintaining the genomic stability in CCA, extending similar findings in other cancer types (3), and identifies PRMT5 as a promising therapeutic target in CCA.

A clinical application of this work includes the opportunity to treat patients with tumors bearing methyl-thio-adenosine phosphorylase (MTAP) deletion. MTAP cleaves methyl-thioadenosine (MTA) to generate precursor substrates for methionine and adenine salvage pathways. Hence, MTAP deletion causes MTA to accumulate leading to PRMT5 inhibition, creating a state whereby MTAP-deleted cells are particularly vulnerable to further inhibition of PRMT5 (7) (Figure 1). This was functionally confirmed for CCA by Elurbide et al. (4) by showing that MTAP-deleted cell lines were far more sensitive to PRMT5 inhibitors than cell lines with preserved MTAP expression. MTAP is often co-deleted with CDKN2A, a tumor suppressor gene, due to their close proximity on chromosome 9p21.3 (8). Homozygous deletions in this region, encompassing both CDKN2A and MTAP, are frequently observed in various cancers (40% of glioblastomas; 25% of melanomas, urothelial carcinomas, and pancreatic adenocarcinomas; and 15% of non−small cell lung carcinomas) (8). For CCA, Elurbide et al. (4), showed that MTAP protein expression was undetectable in ∼25% of intrahepatic CCA suggesting that MTAP deletion is also a frequent event in CCA. By genomic sequencing, the frequency of MTAP deletion was found to be 15% in a large cohort of patients with intrahepatic CCA (9). Overall, these studies identify an expanded population of patients that could benefit from PRMT5 inhibitors (1). Notably, multiple tumor profiling companies report out CDKN2A loss but not MTAP loss, so improved reporting or further education of the oncology community will be important as this class of drugs emerges in the clinic.

Recent advances in small molecule-based epigenetic therapies have sparked significant hope in the field of cancer treatment. A variety of inhibitors, including those targeting histone deacetylase (HDAC), DNA N-methyltransferase (DNMT), enhancer of zeste homolog 2 (EZH2), bromodomain and extra-terminal domain (BET), and notably PRMT5, have entered clinical trials (10). PRMT5 inhibitors have shown encouraging antitumor activity across a variety of MTAP-deleted solid tumors including CCA and gallbladder cancer (11), but the full potential of this therapy remains yet to be established. Although these inhibitors can cause gastrointestinal side effects and cytopenias, their adverse event profile is considered manageable (2, 10). If their full therapeutic potential, either as monotherapy or in combination with immune checkpoint inhibitors, can be realized, they stand to significantly improve patient outcomes. The study by Elurbide et al. (4) offers compelling preclinical evidence for pursuing evaluation of PRMT5 inhibitors in patients with cholangiocarcinoma, one of the most lethal and challenging cancers to treat.

Abbreviations

CCA

Cholangiocarcinoma

HDAC

histone deacetylase

HDTV

Hydrodynamic Tail Vein Injection

mOS

median overall survival

MEP50

Methylosome Protein 50

MTAP

Methylthioadenosine Phosphorylase

MTA

Methylthioadenosine

PRMT5

Protein Arginine Methyltransferase 5

SoC

standard-of-care