GLP overexpression is associated with poor prognosis in Chronic Lymphocytic Leukemia and its inhibition induces leukemic cell death

CBX7 inhibitors affect H3K9 methyltransferase-regulated gene repression in leukemic cells

The epigenome of leukemic cells is dysregulated, and genes required for cell cycle arrest and differentiation may become repressed, which contributes to the accumulation of undifferentiated malignant blood cells. Here, we show that the Polycomb group protein CBX7 can interact with H3K9 methyltransferases EHMT1/2 and SETDB1. We aimed to assess whether combined interfering with these H3K9 methyltransferases and CBX7 could derepress target genes and thereby induce growth arrest of leukemic cells. We found that pharmacologic inhibition of CBX7 abolishes the interaction of CBX7 with EHMT1/2 and SETDB1 and subsequently reduces H3K9 methylation levels which reactivates target gene expression. Reversely, upon pharmacologic inhibition of H3K9 methyltransferases, CBX7 can take over gene repression. Finally, we found that combined inhibition of CBX7 and EHMT1/2 or SETDB1 had additive effects on reducing cell growth and inducing differentiation. However, we did not detect changes in epigenetic modifications, nor target gene derepression, after combination treatment. In contrast, CBX7 inhibitors alone did affect both Polycomb-associated H2Aub-mediated gene repression as well as H3K9 methyltransferase activity. Therefore, we suggest that CBX7 is a promising therapeutic target in leukemia, as its inhibition can reactivate Polycomb and H3K9 methyltransferase target gene expression.

How CBX proteins regulate normal and leukemic blood cells

Hematopoietic stem cell (HSC) fate decisions are dictated by epigenetic landscapes. The Polycomb Repressive Complex 1 (PRC1) represses genes that induce differentiation, thereby maintaining HSC self-renewal. Depending on which chromobox (CBX) protein (CBX2, CBX4, CBX6, CBX7, or CBX8) is part of the PRC1 complex, HSC fate decisions differ. Here, we review how this occurs. We describe how CBX proteins dictate age-related changes in HSCs and stimulate oncogenic HSC fate decisions, either as canonical PRC1 members or by alternative interactions, including non-epigenetic regulation. CBX2, CBX7, and CBX8 enhance leukemia progression. To target, reprogram, and kill leukemic cells, we suggest and describe multiple therapeutic strategies to interfere with the epigenetic functions of oncogenic CBX proteins. Future studies should clarify to what extent the non-epigenetic function of cytoplasmic CBX proteins is important for normal, aged, and leukemic blood cells.

GLP and G9a histone methyltransferases as potential therapeutic targets for lymphoid neoplasms

Histone methyltransferases (HMTs) are enzymes that regulate histone methylation and play an important role in controlling transcription by altering the chromatin structure. Aberrant activation of HMTs has been widely reported in certain types of neoplastic cells. Among them, G9a/EHMT2 and GLP/EHMT1 are crucial for H3K9 methylation, and their dysregulation has been associated with tumor initiation and progression in different types of cancer. More recently, it has been shown that G9a and GLP appear to play a critical role in several lymphoid hematologic malignancies. Importantly, the key roles played by both enzymes in various diseases made them attractive targets for drug development. In fact, in recent years, several groups have tried to develop small molecule inhibitors targeting their epigenetic activities as potential anticancer therapeutic tools. In this review, we discuss the physiological role of GLP and G9a, their oncogenic functions in hematologic malignancies of the lymphoid lineage, and the therapeutic potential of epigenetic drugs targeting G9a/GLP for cancer treatment.

Molecular and functional anticancer effects of GLP/G9a inhibition by UNC0646 in MeWo melanoma cells

In recent years, histone methyltransferases (HMTs) have emerged as important therapeutic targets in cancer due to their oncogenic role. Herein, we used the GLP/G9a inhibitor UNC0646 to assess whether the inhibition of such HMTs could induce cell death in MeWo melanoma cells. Furthermore, we investigated the cellular and molecular mechanisms involved in the observed cell death events. Finally, we performed a functional genomics analysis of 480 melanoma samples to characterize G9a/GLP involvement in melanoma. Interestingly, after UNC0646 treatment, MeWo cells underwent apoptosis, followed by loss of mitochondrial membrane potential and the generation of reactive oxygen species (ROS). Furthermore, MeWo cells treated with UNC0646 showed cell cycle arrest and inhibition of proliferation. At the molecular level, UNC0646 treatment increased the transcriptional levels of CDK1 and BAX, and decreased BCL-2 mRNA levels. Finally, we performed a functional enrichment analysis, which demonstrated that dozens of biological pathways were enriched in melanoma samples according to GLP and G9a expression, including apoptosis and necrosis. Taken together, our data show that inhibition of GLP/G9a using UNC0646 exerts anticancer effects on melanoma cells by controlling their proliferation and inducing apoptosis.

Structure, Activity, and Function of the Protein Lysine Methyltransferase G9a

G9a is a lysine methyltransferase catalyzing the majority of histone H3 mono- and dimethylation at Lys-9 (H3K9), responsible for transcriptional repression events in euchromatin. G9a has been shown to methylate various lysine residues of non-histone proteins and acts as a coactivator for several transcription factors. This review will provide an overview of the structural features of G9a and its paralog called G9a-like protein (GLP), explore the biochemical features of G9a, and describe its post-translational modifications and the specific inhibitors available to target its catalytic activity. Aside from its role on histone substrates, the review will highlight some non-histone targets of G9a, in order gain insight into their role in specific cellular mechanisms. Indeed, G9a was largely described to be involved in embryonic development, hypoxia, and DNA repair. Finally, the involvement of G9a in cancer biology will be presented.

Efficient and flexible Integration of variant characteristics in rare variant association studies using integrated nested Laplace approximation

Rare variants are thought to play an important role in the etiology of complex diseases and may explain a significant fraction of the missing heritability in genetic disease studies. Next-generation sequencing facilitates the association of rare variants in coding or regulatory regions with complex diseases in large cohorts at genome-wide scale. However, rare variant association studies (RVAS) still lack power when cohorts are small to medium-sized and if genetic variation explains a small fraction of phenotypic variance. Here we present a novel Bayesian rare variant Association Test using Integrated Nested Laplace Approximation (BATI). Unlike existing RVAS tests, BATI allows integration of individual or variant-specific features as covariates, while efficiently performing inference based on full model estimation. We demonstrate that BATI outperforms established RVAS methods on realistic, semi-synthetic whole-exome sequencing cohorts, especially when using meaningful biological context, such as functional annotation. We show that BATI achieves power above 70% in scenarios in which competing tests fail to identify risk genes, e.g. when risk variants in sum explain less than 0.5% of phenotypic variance. We have integrated BATI, together with five existing RVAS tests in the 'Rare Variant Genome Wide Association Study' (rvGWAS) framework for data analyzed by whole-exome or whole genome sequencing. rvGWAS supports rare variant association for genes or any other biological unit such as promoters, while allowing the analysis of essential functionalities like quality control or filtering. Applying rvGWAS to a Chronic Lymphocytic Leukemia study we identified eight candidate predisposition genes, including EHMT2 and COPS7A.

Insight into the multi-faceted role of the SUV family of H3K9 methyltransferases in carcinogenesis and cancer progression

Growing evidence implicates histone H3 lysine 9 methylation in tumorigenesis. The SUV family of H3K9 methyltransferases, which include G9a, GLP, SETDB1, SETDB2, SUV39H1 and SUV39H2 deposit H3K9me1/2/3 marks at euchromatic and heterochromatic regions, catalyzed by their conserved SET domain. In cancer, this family of enzymes can be deregulated by genomic alterations and transcriptional mis-expression leading to alteration of transcriptional programs. In solid and hematological malignancies, studies have uncovered pro-oncogenic roles for several H3K9 methyltransferases and accordingly, small molecule inhibitors are being tested as potential therapies. However, emerging evidence demonstrate onco-suppressive roles for these enzymes in cancer development as well. Here, we review the role H3K9 methyltransferases play in tumorigenesis focusing on gene targets and biological pathways affected due to misregulation of these enzymes. We also discuss molecular mechanisms regulating H3K9 methyltransferases and their influence on cancer. Finally, we describe the impact of H3K9 methylation on therapy induced resistance in carcinoma. Converging evidence point to multi-faceted roles for H3K9 methyltransferases in development and cancer that encourages a deeper understanding of these enzymes to inform novel therapy.

Epigenetic priming by EHMT1/EHMT2 in acute lymphoblastic leukemia induces TP53 and TP73 overexpression and promotes cell death

Euchromatic histone-lysine N-methyltransferase 1 (EHMT1) and EHMT2 are upregulated in various human cancers, and their deregulation is associated with tumor development and progression. In this paper, we investigated the expression level of EHMT1/EHMT2 in acute lymphoblastic leukemia (ALL) and whether the modulation of these enzymes could have any cellular or molecular impact on ALL cells. For this, we used UNC0646 as a priming strategy to target EHMT1/EHMT2 and investigated its effect on proliferation and cell viability of Jurkat cells by MTT assay. Then, considering the IC50 and IC75, cellular death was determined by Annexin V/PI staining using flow cytometry. Finally, we investigated by RT-PCR the molecular bases that could be involved in the observed effects. Interestingly, accessing the International Microarray Innovations in Leukemia (MILE) study group, we detected that both EHMT1 and EHMT2 are overexpressed in ALL. More important, we determined that inhibition of EHMT1/EHMT2 significantly decreased Jurkat cell viability in a dose-dependent manner. Accordingly, we observed that inhibition of EHMT1/EHMT2 promoted Jurkat cell death, which was accompanied by increased expression of P53, TP73, BAX, and MDM4. These results clearly indicate that inhibition of EHMT1/EHMT2 induces pro-apoptotic gene expression in ALL and promotes cell death. More importantly, the modulation of these histone methyltransferases may be a promising epigenetic target for ALL treatment.