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Polycomb repressive complexes as therapeutic targets in hematologic malignancies
Makoto Yamagishi1, Atsushi Iwama2, Issay Kitabayashi3
1Laboratory of Viral Oncology and Genomics, Department of Computational Biology and Medical Sciences, Graduate School of Frontier Sciences, The University of Tokyo, Tokyo, Japan.
Abstract:
Polycomb-group proteins form PRC1 and PRC2 complexes that mediate heritable transcriptional repression via H2AK119 ubiquitination and H3K27 trimethylation. Canonical PRC1 catalyzes H2AK119 monoubiquitination through RING1A/B, whereas PRC2 deposits H3K27me3 via EZH1/2. Variant PRC1 (vPRC1) and PRC2 subtypes use distinct recruitment mechanisms, enabling both PRC2-dependent and -independent silencing. In hematopoiesis, components of PRC1, such as BMI1, and those of PRC1.1, including BCOR and PCGF1, have been well characterized for their roles in governing hematopoietic stem cell (HSC) self-renewal and lineage specification. Similarly, PRC2 components, particularly EZH1/2, SUZ12, and EED, are essential for the maintenance of HSCs. Aberrations in PRC1/2, such as gain- or loss-of-function mutations in EZH2 and loss-of-function mutations in BCOR, drive hematologic malignancies, including acute myeloid leukemia (AML), myelodysplastic syndrome (MDS), and lymphomas. In malignant lymphoma, mutations in chromatin regulators (e.g., EZH2, KMT2D, and CREBBP) reshape the epigenetic landscape, disrupting differentiation and immune recognition. Elevated H3K27me3 represents an early and shared epigenetic feature across diverse subclones in lymphoid neoplasms, including adult T-cell leukemia/lymphoma (ATL). Therapeutic targeting of H3K27me3 with EZH2-selective inhibitors such as tazemetostat has shown clinical benefit in lymphoma; however, their efficacy is limited by functional redundancy with EZH1. The dual EZH1/2 inhibitor valemetostat overcomes this limitation by reactivating tumor-suppressor genes, achieving durable responses in ATL and peripheral T-cell lymphoma (PTCL). Nonetheless, therapeutic resistance can emerge through PRC2 gatekeeper mutations and compensatory DNA methylation. These findings underscored the value of targeting the dysregulated epigenome and support the continued clinical development of dual EZH1/2 inhibitors.
Insights
Polycomb repressive complexes (PRC1 and PRC2) regulate gene expression and are crucial for hematopoietic stem cell maintenance. Aberrant PRC1/2 activity drives hematologic malignancies, but dual EZH1/2 inhibitors show promise.
Area of Science:
- Epigenetics and transcriptional regulation
- Hematopoiesis and stem cell biology
- Cancer epigenetics and therapeutics
Background:
- Polycomb group (PcG) proteins, organized into PRC1 and PRC2 complexes, control heritable gene silencing through histone modifications.
- PRC1 and PRC2 play critical roles in hematopoietic stem cell (HSC) self-renewal and lineage specification.
- Dysregulation of PRC1/2 components, including mutations in EZH2 and BCOR, is implicated in hematologic malignancies like AML, MDS, and lymphomas.
Purpose of the Study:
- To review the roles of PRC1 and PRC2 in normal hematopoiesis and their involvement in hematologic malignancies.
- To discuss the therapeutic targeting of PRC2, including EZH2-selective and dual EZH1/2 inhibitors, in lymphoid neoplasms.
- To highlight challenges in PRC2-targeted therapy, such as resistance mechanisms, and the potential of dual inhibitors.
Main Methods:
- Literature review and synthesis of existing research on Polycomb group proteins in hematopoiesis and cancer.
- Analysis of the mechanisms of action for PRC1 and PRC2 in transcriptional repression and disease.
- Evaluation of clinical data and therapeutic strategies involving EZH2 and dual EZH1/2 inhibitors.
Main Results:
- PRC1 and PRC2 are essential for HSC maintenance, and their aberrant function drives various hematologic cancers.
- Elevated H3K27me3 is a common epigenetic feature in lymphoid neoplasms, making PRC2 a viable therapeutic target.
- Dual EZH1/2 inhibitors, like valemetostat, demonstrate efficacy in treating ATL and PTCL by overcoming resistance to EZH2-specific agents.
Conclusions:
- Targeting the epigenome, particularly PRC2, is a promising strategy for treating hematologic malignancies.
- Dual EZH1/2 inhibitors offer a potential therapeutic advantage over EZH2-selective inhibitors due to broader target engagement.
- Continued development of dual EZH1/2 inhibitors is warranted to overcome therapeutic resistance and improve patient outcomes in lymphoid neoplasms.
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