EZH2 Gene Editing

Enhancer of zeste homolog 2 (EZH2) is a member of the polycomb group genes (PcGs) family, which is a group of important epigenetic regulators that can inhibit transcription. Polycomb repressive complex 2 (PRC2) is one of the two core complexes of PcG protein, which mediates gene silencing mostly by regulating chromatin structure. EZH2 is an enzymatic catalytic subunit of PRC2 that can change gene expression by trimethylation of Lys-27 in histone 3 (H3K27me3). H3K27me3 is related to the repression of gene expression and is regarded as a key epigenetic event during tissue development and stem cell fate determination. A large amount of evidence suggests that EZH2 is closely related to the aberrant transcriptome in cancer cells. In fact, EZH2 and the product of its enzymatic action H3K27me3 have been associated with poor prognosis in various human malignancies.

EZH2 signaling

As a catalytic subunit of PRC2, EZH2 inhibits the transcription of target genes by triggering the trimethylation (Lysine-27) of H3K27me. Through regulating gene transcription, EZH2 participates in biological mechanisms including cell fate, cell lineage specification and tumorigenesis. EZH2 can also affect cell cycle progression, apoptosis, autophagy, DNA damage repair, and cellular senescence. In addition, EZH2 can interact with non-histone targets or directly interact with proteins to influence downstream targets in a PRC2-independent manner. In conclusion, EZH2 has three different functions, including PRC2-dependent H3K27m3, PRC2-dependent non-histone protein methylation, and PRC2-independent gene transactivation.

Figure 1. EZH2 signaling and its role in regulating downstream signaling pathways. (Mirzaei S, et al., 2022)

The roles of EZH2 in cancer

Some evidence shows that EZH2 is involved in the development and progression of various cancers. The early evidence of EZH2's role in prostate cancer came from the observation that EZH2 overexpression is related to the worse progression of prostate cancer. Similar findings have emerged in other human cancers including bladder cancer, breast cancer, endometrial cancer and melanoma, because high levels of EZH2 were shown to correlate with aggressiveness and advanced disease in each of these cancer types. EZH2 has been proven to be essential for the proliferation of cancer cell lines, and it has been independently found that ectopic EZH2 expression has a proliferative advantage upon noncancerous cells. Forced expression of EZH2 results in the development of myeloproliferative disorder in mice. In an immortalized human epithelial cell line, expression of EZH2 leads to neoplastic transformation of breast epithelial cells, a phenotype that depends on the protein's methyltransferase domain.

EZH2 as a therapeutic target

According to the key role of EZH2 signaling in important biological mechanisms, regulating cancer progression and affecting the immune system, significant efforts have been made to develop novel EZH2 inhibitors. Different types of EZH2 inhibitors have been developed, some of which are being used in clinical trials to treat patients. The S-adenosylmethionine (SAM)-competitive inhibitors are among the common EZH2 inhibitors in cancer therapy. The GSK126 is such compound that inhibits methyltransferase activity of EZH2 and reduces the level of H3K27me3. These activities pave the way for stimulating silent PRC2 downstream targets and preventing cancer growth in xenografts. The EED226 is another EZH2 inhibitor that can inhibit the activity of PRC2. There is a binding site on the H3K27me3, called EED, with which EED226 can bind, thus triggering conformational alterations and inhibiting PRC2 activity. In addition to pre-clinical experiments, clinical studies also evaluated the role of EZH2 inhibitors in the treatment of cancer patients. The intravenous administration of GSK1816126 showed moderate anti-tumor activity in solid tumors and lymphoma. The previous clinical trial was in phase I, emphasizing that the low half-life survival of EZH2 inhibitors limited their tumor-suppressor activity. Thus, targeted delivery methods of EZH2 inhibitors such as nanotechnological approaches can be developed in near future.