MOLECULAR & CELLULAR ONCOLOGY 2016, VOL. 3, NO. 3, e1152345 (2 pages) http://dx.doi.org/10.1080/23723556.2016.1152345
AUTHOR’S VIEW
Polycomb and lung cancer: When the dosage makes the (kind of) poison Gaetano Gargiulo, Elisabetta Citterio, and Michela Serresi Division of Molecular Genetics, The Netherlands Cancer Institute, Amsterdam, The Netherlands
ABSTRACT
ARTICLE HISTORY
The Polycomb transcriptional repressors regulate normal tissue homeostasis and their function is often hijacked during oncogenesis. We recently uncovered the Polycomb repressive complex-2 (PRC2) genes Ezh2 and Eed as oncogenotype-dependent cancer genes. Notably, within the same oncogenotype, PRC2 dosage modulates lung tumor homeostasis and critically impacts non-tumor tissue function.
Received 3 February 2016 Revised 3 February 2016 Accepted 4 February 2016 KEYWORDS
Epigenetics; Ezh2; inflammation; Kras; Lung Cancer; p53; Polycomb
The Polycomb transcriptional repressors play a critical role in vertebrate organism development and adult tissue regeneration. The Polycomb repressive complex 2 (PRC2) is a holoenzyme composed of several proteins including the SET-domain histone methyltransferase (HMT) Enhancer of Zeste-2 (Ezh2), and the WD-repeat protein Embryonic ectoderm development (Eed).1 PRC2 contributes to gene silencing through Ezh2 HMT activity toward lysine 27 of histone H3 (H3K27me1/2/3). Eed is required to maintain PRC2 stability and subsequent global and local H3K27me3 levels.2 Ezh2 overexpression is believed to provide a PRC2 gain of function. Compared to Ezh2 depletion, loss of Eed more effectively abrogates PRC2 function by preventing the compensatory effect exerted by the Ezh2 paralog Ezh1.1 Both Ezh2 and Eed have been implicated as context-dependent cancer genes. Until now, this was largely based on the discovery of both PRC2 activating and inactivating mutations3,4 in different types of cancers, each harboring a diverse spectrum of oncogenotypes. We employed genetically engineered mouse models (GEMMs) with mutations in the Kras driver oncogene to address the consequences of both gain and loss of Ezh2/Eed function on tumor development. Kras is frequently mutated in non-small cell lung cancers (NSCLCs), which account for 80% of all lung cancer cases.5 Using Kras-driven GEMMs that faithfully recapitulate human NSCLC biology, we provide direct evidence supporting context-dependent functions for the essential PRC2 component Eed.6 Moreover, our data suggest that—within the same context/ oncogenotype—PRC2 dosage imbalance in either direction contributes to lung cancer homeostasis by different mechanisms.6 When the overall survival of mice bearing a Kras-driven tumor was evaluated, conditional overexpression of Ezh2 in the lung epithelium appeared to promote oncogenic Ras signaling toward adenocarcinoma formation. Conversely, PRC2 inactivation by loss of Eed delayed tumor initiation. These findings support a pro-oncogenic function for PRC2 in Kras-driven tumor initiation. Strikingly, however, simultaneous inactivation of Trp53 (best known as tumor suppressor p53) reverted the delay
CONTACT Gaetano Gargiulo © 2016 Taylor & Francis Group, LLC
[email protected]
in tumor formation in Eed conditional knockout mice. In fact, KrasG12D;Trp53¡/¡;Eed¡/¡-driven tumors killed the animals significantly earlier than the KrasG12D;Trp53¡/¡ control tumors did. Such accelerated tumorigenesis is indicative of a strong tumor suppressive role for PRC2 in Kras-driven NSCLC in which p53 is lost (Fig. 1A). In line with these results, the prooncogenic effect of Ezh2 in cellular transformation was completely abolished in the KrasG12D;Trp53¡/¡ oncogenotype. Consistent with the results in mouse models, retrospective gene expression analysis of lung cancer patient biopsies indicated that both Ezh2-high and Eed-low expressing human tumors exist, and that both confer poor prognosis.6 Overall, these results support 2 major conclusions regarding the role of PRC2 in Krasdriven lung cancer: (1) Eed is a context-dependent tumor suppressor gene; (2) Trp53 inactivation constitutes the switch turning Eed function from pro-oncogenic into pro-tumor suppressive, thereby modulating the impact of Eed loss on overall animal survival (Fig. 1A). The mechanistic bases for the effect of Trp53 on the pro-tumor suppressive switch of Eed and the pro-oncogenic role of Ezh2 in cellular transformation both remain to be clarified. This will be important in order to identify prognostic, if not predictive, biomarkers for response to PRC2 inhibition, as evidence in other tissues is suggestive of a widespread tumor suppressive function for PRC2 in solid tumors.7 Interestingly, although Eed deletion in Kras-driven tumor cells extended animal survival compared to Ezh2 overexpression, it promoted degeneration of the normal tissue surrounding the tumor. In fact, the tumor cells exploited long-term PRC2 depletion to affect tissue integrity by promoting a strong inflammatory phenotype,6 with features reminiscent of a macrophage activation syndrome.8 Such activation of sterile inflammation by Eed loss severely affected the ability of the alveolar tissue to perform gas exchange, thereby featuring the fifth cardinal sign of inflammation, the functio laesa. Thus, on one hand PRC2 gain of function by means of increased Ezh2 levels supported Krasdriven cellular transformation, ultimately resulting in efficient
e1152345-2
G. GARGIULO ET AL.
Figure 1. A role for Polycomb Repressive Complex-2 (PRC2) in Kras-driven lung cancer. (A) The Polycomb context-dependence concept. If Trp53 (the mouse gene encoding p53) is present, loss of the essential PRC2 component Eed in Kras-driven lung cancer extends animal survival, thereby featuring an oncogenic function for PRC2. If p53 is deleted, animal survival is drastically reduced, featuring a tumor suppressive function for PRC2. Thus, p53 function constitutes the switch between opposite functions. (B) The Polycomb dosage imbalance concept. Compared to Eed loss, when Ezh2 is overexpressed in epithelial cells carrying an oncogenic Kras mutation, tumor formation is accelerated and animal survival is shortened. Thus, excess Ezh2 affects normal lung tissue by promoting tumor formation. When Eed is deleted, tumor inflammation strongly impairs lung tissue function. Hence, in Kras-driven lung cancer, PRC2 dosage imbalance in either direction has negative consequences.
lung adenocarcinoma formation whereas, on the other hand, impairing PRC2 function by means of Eed deletion also severely affected animal survival, in this case by promoting a marked decline in key organismal functions. These findings support the view that a change in PRC2 gene dosage—either upregulation or downregulation—can contribute to tumor progression and normal tissue erosion through different mechanisms (Fig. 1B). Tumor self-propagation at the expense of the normal surrounding tissues—otherwise known as tumor homeostasis— involves several mechanisms collectively referred to as the hallmarks of cancer.9 In homeostatic systems, critical elements have to be maintained at an appropriate concentration to prevent potentially catastrophic consequences of their excess or deficiency. Our study indicates that PRC2 function is a critical element in Kras-dependent lung tumor tissue; in fact, an alteration in either Ezh2 or Eed gene dosage affects tumor homeostasis. This effect involves sustained proliferative signaling, evasion of growth suppression, and/or regulation of tumor-promoting inflammation, all of which represent major hallmarks of cancer. As our studies did not reveal oncogene-induced senescence upon PRC2 depletion, future research should address how PRC2 loss mechanistically promotes inflammation and whether PRC2-regulated inflammation drives a non-cell autonomous senescence program,10 with potential implications for anticancer therapy. As PRC2 inhibitors are moving into the clinic for anticancer therapy (https://clinicaltrials.gov/ct2/results? term DEzh2Cinhibitors), a detailed understanding of the mechanisms through which PRC2 operates in preclinical cancer models becomes paramount.
Disclosure of potential conflicts of interest No potential conflicts of interest were disclosed.
Acknowledgments We thank Maarten van Lohuizen for critical reading of this commentary.
References 1. Margueron R, Reinberg D. The Polycomb complex PRC2 and its mark in life. Nature 2011; 469:343-9; PMID:21248841; http://dx.doi.org/ 10.1038/nature09784 2. Montgomery ND, Yee D, Chen A, Kalantry S, Chamberlain SJ, Otte AP, Magnuson T. The murine polycomb group protein Eed is required for global histone H3 lysine-27 methylation. Curr Biol 2005; 15:942-7; PMID:15916951; http://dx.doi.org/10.1016/j.cub.2005.04.051 3. Koppens M, Van Lohuizen M. Context-dependent actions of Polycomb repressors in cancer. Oncogene (2016); 35; 1341–1352; PMID:26050622; http://dx.doi.org/10.1038/onc.2015.195 4. Conway E, Healy E, Bracken AP. ScienceDirectPRC2 mediated H3K27 methylations in cellular identity and cancer. Curr Opin Genet Dev 2015; 37:42-8; PMID:26497635; http://dx.doi.org/10.1016/j.ceb.2015.10.003 5. Cancer Genome Atlas Research Network. Comprehensive molecular profiling of lung adenocarcinoma. Nature 2014; 511:543-50; PMID:25079552; http://dx.doi.org/10.1038/nature13385 6. Serresi M, Gargiulo G, Proost N, Siteur B, Cesaroni M, Koppens M, Xie H, Sutherland KD, Hulsman D, Citterio E, et al. Polycomb Repressive Complex 2 Is a Barrier to KRAS-Driven Inflammation and Epithelial-Mesenchymal Transition in Non-Small-Cell Lung Cancer. Cancer Cell 2016; 29:17-31; PMID:26766588; http://dx.doi.org/10.1016/j.ccell.2015.12.006 7. Wassef M, Rodilla V, Teissandier A, Zeitouni B, Gruel N, Sadacca B, Irondelle M, Charruel M, Ducos B, Michaud A, et al. Impaired PRC2 activity promotes transcriptional instability and favors breast tumorigenesis. Genes & Dev. 2015; 29:2547–2562; PMID:25561492 8. Ravelli A. Macrophage activation syndrome. Curr Opin Rheumatol 2002; 14:548-52; PMID:12192253; http://dx.doi.org/10.1097/00002281200209000-00012 9. Hanahan D, Weinberg RA. Hallmarks of cancer: the next generation. Cell 2011; 144:646-74; PMID:21376230; http://dx.doi.org/10.1016/j. cell.2011.02.013 10. Di Mitri D, Alimonti A. Non-Cell-Autonomous Regulation of Cellular Senescence in Cancer. Trends Cell Biol 2015; PMID:26564316; http:// dx.doi.org/10.1016/j.tcb.2015.10.005
AUTHOR’S VIEW
Polycomb and lung cancer: When the dosage makes the (kind of) poison Gaetano Gargiulo, Elisabetta Citterio, and Michela Serresi Division of Molecular Genetics, The Netherlands Cancer Institute, Amsterdam, The Netherlands
ABSTRACT
ARTICLE HISTORY
The Polycomb transcriptional repressors regulate normal tissue homeostasis and their function is often hijacked during oncogenesis. We recently uncovered the Polycomb repressive complex-2 (PRC2) genes Ezh2 and Eed as oncogenotype-dependent cancer genes. Notably, within the same oncogenotype, PRC2 dosage modulates lung tumor homeostasis and critically impacts non-tumor tissue function.
Received 3 February 2016 Revised 3 February 2016 Accepted 4 February 2016 KEYWORDS
Epigenetics; Ezh2; inflammation; Kras; Lung Cancer; p53; Polycomb
The Polycomb transcriptional repressors play a critical role in vertebrate organism development and adult tissue regeneration. The Polycomb repressive complex 2 (PRC2) is a holoenzyme composed of several proteins including the SET-domain histone methyltransferase (HMT) Enhancer of Zeste-2 (Ezh2), and the WD-repeat protein Embryonic ectoderm development (Eed).1 PRC2 contributes to gene silencing through Ezh2 HMT activity toward lysine 27 of histone H3 (H3K27me1/2/3). Eed is required to maintain PRC2 stability and subsequent global and local H3K27me3 levels.2 Ezh2 overexpression is believed to provide a PRC2 gain of function. Compared to Ezh2 depletion, loss of Eed more effectively abrogates PRC2 function by preventing the compensatory effect exerted by the Ezh2 paralog Ezh1.1 Both Ezh2 and Eed have been implicated as context-dependent cancer genes. Until now, this was largely based on the discovery of both PRC2 activating and inactivating mutations3,4 in different types of cancers, each harboring a diverse spectrum of oncogenotypes. We employed genetically engineered mouse models (GEMMs) with mutations in the Kras driver oncogene to address the consequences of both gain and loss of Ezh2/Eed function on tumor development. Kras is frequently mutated in non-small cell lung cancers (NSCLCs), which account for 80% of all lung cancer cases.5 Using Kras-driven GEMMs that faithfully recapitulate human NSCLC biology, we provide direct evidence supporting context-dependent functions for the essential PRC2 component Eed.6 Moreover, our data suggest that—within the same context/ oncogenotype—PRC2 dosage imbalance in either direction contributes to lung cancer homeostasis by different mechanisms.6 When the overall survival of mice bearing a Kras-driven tumor was evaluated, conditional overexpression of Ezh2 in the lung epithelium appeared to promote oncogenic Ras signaling toward adenocarcinoma formation. Conversely, PRC2 inactivation by loss of Eed delayed tumor initiation. These findings support a pro-oncogenic function for PRC2 in Kras-driven tumor initiation. Strikingly, however, simultaneous inactivation of Trp53 (best known as tumor suppressor p53) reverted the delay
CONTACT Gaetano Gargiulo © 2016 Taylor & Francis Group, LLC
[email protected]
in tumor formation in Eed conditional knockout mice. In fact, KrasG12D;Trp53¡/¡;Eed¡/¡-driven tumors killed the animals significantly earlier than the KrasG12D;Trp53¡/¡ control tumors did. Such accelerated tumorigenesis is indicative of a strong tumor suppressive role for PRC2 in Kras-driven NSCLC in which p53 is lost (Fig. 1A). In line with these results, the prooncogenic effect of Ezh2 in cellular transformation was completely abolished in the KrasG12D;Trp53¡/¡ oncogenotype. Consistent with the results in mouse models, retrospective gene expression analysis of lung cancer patient biopsies indicated that both Ezh2-high and Eed-low expressing human tumors exist, and that both confer poor prognosis.6 Overall, these results support 2 major conclusions regarding the role of PRC2 in Krasdriven lung cancer: (1) Eed is a context-dependent tumor suppressor gene; (2) Trp53 inactivation constitutes the switch turning Eed function from pro-oncogenic into pro-tumor suppressive, thereby modulating the impact of Eed loss on overall animal survival (Fig. 1A). The mechanistic bases for the effect of Trp53 on the pro-tumor suppressive switch of Eed and the pro-oncogenic role of Ezh2 in cellular transformation both remain to be clarified. This will be important in order to identify prognostic, if not predictive, biomarkers for response to PRC2 inhibition, as evidence in other tissues is suggestive of a widespread tumor suppressive function for PRC2 in solid tumors.7 Interestingly, although Eed deletion in Kras-driven tumor cells extended animal survival compared to Ezh2 overexpression, it promoted degeneration of the normal tissue surrounding the tumor. In fact, the tumor cells exploited long-term PRC2 depletion to affect tissue integrity by promoting a strong inflammatory phenotype,6 with features reminiscent of a macrophage activation syndrome.8 Such activation of sterile inflammation by Eed loss severely affected the ability of the alveolar tissue to perform gas exchange, thereby featuring the fifth cardinal sign of inflammation, the functio laesa. Thus, on one hand PRC2 gain of function by means of increased Ezh2 levels supported Krasdriven cellular transformation, ultimately resulting in efficient
e1152345-2
G. GARGIULO ET AL.
Figure 1. A role for Polycomb Repressive Complex-2 (PRC2) in Kras-driven lung cancer. (A) The Polycomb context-dependence concept. If Trp53 (the mouse gene encoding p53) is present, loss of the essential PRC2 component Eed in Kras-driven lung cancer extends animal survival, thereby featuring an oncogenic function for PRC2. If p53 is deleted, animal survival is drastically reduced, featuring a tumor suppressive function for PRC2. Thus, p53 function constitutes the switch between opposite functions. (B) The Polycomb dosage imbalance concept. Compared to Eed loss, when Ezh2 is overexpressed in epithelial cells carrying an oncogenic Kras mutation, tumor formation is accelerated and animal survival is shortened. Thus, excess Ezh2 affects normal lung tissue by promoting tumor formation. When Eed is deleted, tumor inflammation strongly impairs lung tissue function. Hence, in Kras-driven lung cancer, PRC2 dosage imbalance in either direction has negative consequences.
lung adenocarcinoma formation whereas, on the other hand, impairing PRC2 function by means of Eed deletion also severely affected animal survival, in this case by promoting a marked decline in key organismal functions. These findings support the view that a change in PRC2 gene dosage—either upregulation or downregulation—can contribute to tumor progression and normal tissue erosion through different mechanisms (Fig. 1B). Tumor self-propagation at the expense of the normal surrounding tissues—otherwise known as tumor homeostasis— involves several mechanisms collectively referred to as the hallmarks of cancer.9 In homeostatic systems, critical elements have to be maintained at an appropriate concentration to prevent potentially catastrophic consequences of their excess or deficiency. Our study indicates that PRC2 function is a critical element in Kras-dependent lung tumor tissue; in fact, an alteration in either Ezh2 or Eed gene dosage affects tumor homeostasis. This effect involves sustained proliferative signaling, evasion of growth suppression, and/or regulation of tumor-promoting inflammation, all of which represent major hallmarks of cancer. As our studies did not reveal oncogene-induced senescence upon PRC2 depletion, future research should address how PRC2 loss mechanistically promotes inflammation and whether PRC2-regulated inflammation drives a non-cell autonomous senescence program,10 with potential implications for anticancer therapy. As PRC2 inhibitors are moving into the clinic for anticancer therapy (https://clinicaltrials.gov/ct2/results? term DEzh2Cinhibitors), a detailed understanding of the mechanisms through which PRC2 operates in preclinical cancer models becomes paramount.
Disclosure of potential conflicts of interest No potential conflicts of interest were disclosed.
Acknowledgments We thank Maarten van Lohuizen for critical reading of this commentary.
References 1. Margueron R, Reinberg D. The Polycomb complex PRC2 and its mark in life. Nature 2011; 469:343-9; PMID:21248841; http://dx.doi.org/ 10.1038/nature09784 2. Montgomery ND, Yee D, Chen A, Kalantry S, Chamberlain SJ, Otte AP, Magnuson T. The murine polycomb group protein Eed is required for global histone H3 lysine-27 methylation. Curr Biol 2005; 15:942-7; PMID:15916951; http://dx.doi.org/10.1016/j.cub.2005.04.051 3. Koppens M, Van Lohuizen M. Context-dependent actions of Polycomb repressors in cancer. Oncogene (2016); 35; 1341–1352; PMID:26050622; http://dx.doi.org/10.1038/onc.2015.195 4. Conway E, Healy E, Bracken AP. ScienceDirectPRC2 mediated H3K27 methylations in cellular identity and cancer. Curr Opin Genet Dev 2015; 37:42-8; PMID:26497635; http://dx.doi.org/10.1016/j.ceb.2015.10.003 5. Cancer Genome Atlas Research Network. Comprehensive molecular profiling of lung adenocarcinoma. Nature 2014; 511:543-50; PMID:25079552; http://dx.doi.org/10.1038/nature13385 6. Serresi M, Gargiulo G, Proost N, Siteur B, Cesaroni M, Koppens M, Xie H, Sutherland KD, Hulsman D, Citterio E, et al. Polycomb Repressive Complex 2 Is a Barrier to KRAS-Driven Inflammation and Epithelial-Mesenchymal Transition in Non-Small-Cell Lung Cancer. Cancer Cell 2016; 29:17-31; PMID:26766588; http://dx.doi.org/10.1016/j.ccell.2015.12.006 7. Wassef M, Rodilla V, Teissandier A, Zeitouni B, Gruel N, Sadacca B, Irondelle M, Charruel M, Ducos B, Michaud A, et al. Impaired PRC2 activity promotes transcriptional instability and favors breast tumorigenesis. Genes & Dev. 2015; 29:2547–2562; PMID:25561492 8. Ravelli A. Macrophage activation syndrome. Curr Opin Rheumatol 2002; 14:548-52; PMID:12192253; http://dx.doi.org/10.1097/00002281200209000-00012 9. Hanahan D, Weinberg RA. Hallmarks of cancer: the next generation. Cell 2011; 144:646-74; PMID:21376230; http://dx.doi.org/10.1016/j. cell.2011.02.013 10. Di Mitri D, Alimonti A. Non-Cell-Autonomous Regulation of Cellular Senescence in Cancer. Trends Cell Biol 2015; PMID:26564316; http:// dx.doi.org/10.1016/j.tcb.2015.10.005
