- Email: [email protected]
Succinate: A New Epigenetic Hacker
Cancer Cell
Previews Snyder, E.L., Watanabe, H., Magendantz, M., Hoersch, S., Chen, T.A., Wang, D.G., Crowley, D., Whittaker, C.A., Meyerson, M., Kimura, S., and Jacks, T. (2013). Mol. Cell 50, 185–199. Winslow, M.M., Dayton, T.L., Verhaak, R.G.W., Kim-Kiselak, C., Snyder, E.L., Feldser, D.M., Hub-
bard, D.D., DuPage, M.J., Whittaker, C.A., Hoersch, S., et al. (2011). Nature 473, 101–104.
Yoshizawa, A., Motoi, N., Riely, G.J., Sima, C.S., Gerald, W.L., Kris, M.G., Park, B.J., Rusch, V.W., and Travis, W.D. (2011). Mod. Pathol. 24, 653–664.
Yin, Z., Gonzales, L., Kolla, V., Rath, N., Zhang, Y., Lu, M.M., Kimura, S., Ballard, P.L., Beers, M.F., Epstein, J.A., and Morrisey, E.E. (2006). Am. J. Physiol. Lung Cell. Mol. Physiol. 291, L191–L199.
Zhang, Y., Goss, A.M., Cohen, E.D., Kadzik, R., Lepore, J.J., Muthukumaraswamy, K., Yang, J., DeMayo, F.J., Whitsett, J.A., Parmacek, M.S., and Morrisey, E.E. (2008). Nat. Genet. 40, 862–870.
Succinate: A New Epigenetic Hacker Ming Yang1 and Patrick J. Pollard1,* 1Cancer Biology and Metabolism Group, Nuffield Department of Medicine, Henry Wellcome Building for Molecular Physiology, University of Oxford, Oxford OX3 7BN, UK *Correspondence: [email protected] http://dx.doi.org/10.1016/j.ccr.2013.05.015
Epigenetic reprogramming is a feature of many human cancers. In this issue of Cancer Cell, Letouze´ and colleagues describe DNA hypermethylation in paragangliomas harboring mutations in succinate dehydrogenase genes. These tumors accumulate succinate, which inhibits 2-oxoglutarate-dependent histone and DNA demethylase enzymes, resulting in epigenetic silencing that affects neuroendocrine differentiation. Metabolic rewiring has long been regarded as a consequence of malignant transformation driven by abnormal signal transduction mediated by oncogenes and tumor suppressors. The identification that the metabolite (R)-2-hydroxyglutarate [(R)-2HG] plays an oncogenic role in subsets of gliomas and acute myelogenous leukemia with mutations in isoforms of isocitrate dehydrogenase (IDH) has provided direct evidence linking altered metabolism and cancer and founded the notion of the ‘‘oncometabolite’’ (Ward et al., 2010). (R)-2HG is a product of the neomorphic activities of mutant IDH1/2 and accumulates to millimolar concentrations in IDH-associated gliomas (Dang et al., 2010). Due to the structural similarity to 2-oxoglutarate (2OG), (R)-2HG competitively inhibits the 2OG-utilizing Jumonji C (JmjC)-domain containing histone lysine demethylases and the TET family of 5-methylcytosine (5mC) hydroxylases, leading to DNA hypermethylation at CpG islands in promoter regions of genes involved in cell differentiation (Kim and DeBerardinis, 2013). The Krebs cycle enzymes succinate dehydrogenase (SDH) and fumarate hydratase (FH) are tumor suppressors whose loss-of-function mutations predispose to familial cancer syndromes (Frezza
et al., 2011). SDH and FH inactivation results in a blockade of Krebs cycle, impaired respiration, and abnormal accumulation of their substrates succinate and fumarate, respectively. Both metabolites are also inhibitors of 2OG-dependent dioxygenases, particularly the hydroxylases for the transcription factor hypoxiainducible factor (HIF), resulting in HIF-dependent activation of a ‘‘pseudohypoxic’’ response characterized by enhanced angiogenesis and increased anaerobic metabolism (Frezza et al., 2011). In the case of fumarate, the metabolite is also an endogenous electrophile and can chemically modify cysteine residues in Kelch-like ECH-associated protein 1, leading to constitutive (and potentially oncogenic) activation of transcription factor nuclear factor erythroid 2-related factor 2 (Adam et al., 2011; Ooi et al., 2011). Until recently, HIF activation by fumarate and succinate has been the only mechanistic link to oncogenesis that has been described in FH and SDH mutant cancer models (Pollard et al., 2007; Selak et al., 2005). However, at least in the context of FH-deficiency, a causal role for HIF in tumorigenesis is questionable (Adam et al., 2011), and the quest for other oncogenic drivers in SDH- and FH-associated malignancy continues. Recent re-
ports by Killian et al. (2013) and Letouze´ et al. (2013; in this issue of Cancer Cell) both demonstrate that, similarly to (R)2HG, succinate can also remodel the epigenome and alter gene expression. SDH mutations occur frequently in paraganglioma (PGL), pheochromocytoma (PCC), gastrointestinal stromal tumor (GIST), as well as in renal carcinoma (Killian et al., 2013). GIST tumors can alternatively be driven by mutations in signal transduction kinases. Killian et al. (2013) compared the methylation profiles of GISTs with mutations in either SDH or KIT tyrosine kinase and uncovered marked global hypermethylation in the SDH versus KIT mutant subgroup. Further examination of a PGL/PCC cohort showed a similar hypermethylation phenotype in SDH mutant tumors compared to SDH-wild-type reference tissues. Bioinformatic analyses of methylomes from developmentally distinct tumors including GIST, PGL, PCC, and gliomas revealed comparable methylation signatures in those harboring SDH and IDH mutations (Killian et al., 2013). Letouze´ et al. (2013) classified a large PGL/PCC cohort based on DNA methylation and obtained three stable clusters. Strikingly, a distinct subgroup characterized by the presence of a
Cancer Cell 23, June 10, 2013 ª2013 Elsevier Inc. 709
Cancer Cell
Previews
Figure 1. Epigenetic Reprogramming by Oncometabolites Mutations in the metabolic enzymes isocitrate dehydrogenase (IDH)-1 and -2, succinate dehydrogenase (SDH), and fumarate hydratase (FH) lead to abnormal accumulation of (R)-2-hydroxyglutarate [(R)-2HG], succinate, and fumarate, respectively. Their accumulation inhibits the activities of 2-oxoglutarate (2OG)-dependent dioxygenases, including the TET family of DNA modifying enzymes and the JmjC domain-containing histone lysine demethylases (KDMs). Subsequent epigenetic alterations result in cell differentiation arrest and promote malignant transformation. Thus, epigenetic modification, through the action of oncometabolites, is a shared feature among IDH-, SDH-, and FH-associated cancers. SUCC, succinate; FUM, fumarate; Mal, malate; mIDH1/2, mutant IDH1/2; Me, methyl group.
hypermethylator phenotype contained 16 of 17 samples that harbor SDH mutations. The prevailing model is that accumulation of succinate in these tumors inhibits 2OGdependent oxidative demethylation in a manner analogous to the action of (R)2HG, giving rise to DNA hypermethylation. Integration of methylation and gene expression data revealed, whereas most hypermethylated CpG sites had no functional consequences, there were 191 genes that showed correlated levels of hypermethylation and downregulation (Letouze´ et al., 2013). SDH- and VHL-related tumors share some similar features in gene expression profiles, possibly due to activation of HIF-orchestrated hypoxia-related genes. Nonetheless, SDH- and VHL-associated PGL/PCC from the cohort studied were clustered as two distinct gene-expression subgroups, suggesting that epigenetic remodelling plays a major role in the transcriptional divergence of these two tumor subclasses. Intriguingly, the only tumor sample in the hypermethylated PGL/PCC subgroup without SDH mutations was shown to harbor germline inactivating FH mutations. This finding corroborates recent evidence demonstrating that fumarate-mediated inhibition of TET and KDM enzymes alters histone and DNA methylation pat-
terns (Xiao et al., 2012) and raises the necessity to test FH-associated neoplasms for epigenetic abnormalities in future studies. Consistent with the findings by Killian et al. (2013), Letouze´ et al. (2013) compared the CpG methylator phenotypes between SDH/FH mutant PGL/ PCC and IDH mutant gliomas and identified significant overlap in hypermethylated CpG sites. Particularly, 17 genes were concordantly hypermethylated and downregulated in both tumor groups. The connection in methylation profiles between the SDH, FH, and IDH mutant tumors may reflect a similar inhibitory role for succinate, fumarate, and (R)2HG (see Figure 1), whereas the differences may partly be attributed to the variable susceptibilities of the panoply of 2OG-dependent oxygenases to the three oncometabolites. All of the SDH mutant PGL/PCC in this study exhibited low 5-hydroxymethylcytosine (5hmC) levels by immunohistochemistry analyses, and the majority also showed elevated histone methylation at H3K27, supporting the hypothesis that succinate inhibits TET- and KDMmediated demethylations. Consistently, Sdhb / mouse chromaffin cells displayed higher 5mC/5hmC ratios compared to wild-type cells, which could be
710 Cancer Cell 23, June 10, 2013 ª2013 Elsevier Inc.
reversed by the addition of exogenous 2OG to the culture medium. Importantly, the hypermethylated genes detected in these cells significantly overlapped with those identified in SDH mutant PGL/PCC. The SDH enzyme complex comprises four subunits (SDHA, SDHB, SDHC, and SDHD). Although mutations in all subunits occur in cancer, tumors harboring mutations in the catalytic subunit SDHB are particularly malignant and associated with a higher risk of metastasis (Frezza et al., 2011). Letouze´ et al. (2013) showed that the mean methylation levels across all CpG sites are higher in SDHB-mutated PGL/PCC than other SDH mutant PGL/ PCC. It is possible that the loss of SDHB function results in bona fide inactivation of the SDH complex, whereas reductions of enzyme activity through mutations in other subunits are not as complete. Consequently, there might be higher succinate accumulation and stronger inhibition of demethylation in SDHB mutant tumors. Although the differences in succinate levels could not be demonstrated from the SDH mutant PGL/PCC cohort, the heterogeneity of tumor samples may preclude accurate analyses and further investigations employing cell culture models could be more informative. Two of the most significantly silenced genes in the hypermethylated subgroup,
Cancer Cell
Previews PNMT and KRT19, are respectively involved in neuroendocrine differentiation and epithelial-to-mesenchymal transition (EMT). Interestingly, despite exhibiting a decreased growth rate similar to the Fh1 / mouse embryonic fibroblasts, the Sdhb / chromaffin cells showed markedly increased migratory capacities, potentially reflecting epigenetic deregulation of EMT genes and the aggressive nature of SDHB-related PCC/PGL. Recently, inhibitors that specifically target mutant IDH1 and IDH2 have been reported. Such inhibitors could reverse (R)-2HG-mediated epigenetic deregulation, induce cell differentiation, and suppress the growth of IDH mutant tumor cells (Kim and DeBerardinis, 2013). Letouze´ et al. (2013) showed that transient treatment with a DNA methyltransferase inhibitor repressed the migration capacities of Sdhb / cells, implicating the possibility of epigenetic targeting in SDH-related tumors. However, succinate and fumarate accumulation exert various
nonepigenetic cellular impacts, and, to achieve therapeutic efficacy, a synergistic lethality approach is probably desired. Nonetheless, the emergence of epigenetically silenced gene sets may serve as useful biomarkers for early detection of cancers characterized by Krebs cycle defects.
Kim, J., and DeBerardinis, R.J. (2013). Science 340, 558–559. Letouze´, E., Martinelli, C., Loriot, C., Burnichon, N., Abermil, N., Ottolenghi, C., Janin, M., Menara, M., Nguyen, A.T., Benit, P., et al. (2013). Cancer Cell 23, this issue, 739–752. Ooi, A., Wong, J.C., Petillo, D., Roossien, D., Perrier-Trudova, V., Whitten, D., Min, B.W., Tan, M.H., Zhang, Z., Yang, X.J., et al. (2011). Cancer Cell 20, 511–523.
REFERENCES Adam, J., Hatipoglu, E., O’Flaherty, L., Ternette, N., Sahgal, N., Lockstone, H., Baban, D., Nye, E., Stamp, G.W., Wolhuter, K., et al. (2011). Cancer Cell 20, 524–537. Dang, L., White, D.W., Gross, S., Bennett, B.D., Bittinger, M.A., Driggers, E.M., Fantin, V.R., Jang, H.G., Jin, S., Keenan, M.C., et al. (2010). Nature 465, 966. Frezza, C., Pollard, P.J., and Gottlieb, E. (2011). J. Mol. Med. 89, 213–220. Killian, J.K., Kim, S.Y., Miettinen, M., Smith, C., Merino, M., Tsokos, M., Quezado, M., Smith, W.I., Jr., Jahromi, M.S., Xekouki, P., et al. (2013). Cancer Discov. Published online May 21, 2013. http://dx. doi.org/10.1158/2159-8290.CD-13-0092.
Pollard, P.J., Spencer-Dene, B., Shukla, D., Howarth, K., Nye, E., El-Bahrawy, M., Deheragoda, M., Joannou, M., McDonald, S., Martin, A., et al. (2007). Cancer Cell 11, 311–319. Selak, M.A., Armour, S.M., MacKenzie, E.D., Boulahbel, H., Watson, D.G., Mansfield, K.D., Pan, Y., Simon, M.C., Thompson, C.B., and Gottlieb, E. (2005). Cancer Cell 7, 77–85. Ward, P.S., Patel, J., Wise, D.R., Abdel-Wahab, O., Bennett, B.D., Coller, H.A., Cross, J.R., Fantin, V.R., Hedvat, C.V., Perl, A.E., et al. (2010). Cancer Cell 17, 225–234. Xiao, M., Yang, H., Xu, W., Ma, S., Lin, H., Zhu, H., Liu, L., Liu, Y., Yang, C., Xu, Y., et al. (2012). Genes Dev. 26, 1326–1338.
Cancer Stem Cells Activate STAT3 the EZ Way Shaun D. Fouse1 and Joseph F. Costello1,* 1Brain Tumor Research Center, Department of Neurosurgery, University of California, San Francisco, San Francisco, CA 94158, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.ccr.2013.05.016
Activated STAT3 and increased expression of the histone methyltransferase EZH2 are independently associated with the most malignant subset of gliomas. In this issue of Cancer Cell, Kim and colleagues discover that EZH2 enhances STAT3 activation by trimethylating lysine180 in STAT3 and does so preferentially in glioma stem-like cells. Presuming that cancer stem cells are responsible for therapeutic resistance in cancer patients, the discovery of the molecular differences that drive cancer stem cell phenotypes holds significant value for improving therapy. More valuable still would be the ability to selectively inhibit these molecular drivers of stemness. In this issue of Cancer Cell, Kim et al. (2013) employ cultures of glioma stem-like cells (GSCs) and their isogenic bulk tumor cells along with intracranial xenografts and come one step closer to achieving this ambitious goal.
Mutation or overexpression of enhancer of Zeste homolog 2 (EZH2) can drive the clonal expansion of leukemias and the growth of solid cancers, though individual EZH2 alterations follow divergent paths toward malignancy. EZH2 is most known as an enzymatically active component of the Polycomb repressive complex 2 (PRC2), which is responsible for depositing methyl groups onto lysine 27 (K27) of histone H3 (H3K27). EZH2 plays an important role in maintaining stem cell function. The most common EZH2 mutations in cancer occur within
the SET domain at Y641, which result in increased trimethylation activity, leading to an increase in global H3K27me3 levels in B cell lymphomas (Yap et al., 2011). In contrast, in breast cancer cells, AKTmediated phosphorylation of S21 reduces EZH2 interaction with histone H3, leading to a decrease in global H3K27me3 levels (Cha et al., 2005). In addition, in pediatric brain tumors, recurrent somatic mutation of K27 on histone H3 variant H3F3A may deplete H3K27me3 on canonical H3 indirectly because of increased interaction between mutant H3F3A and EZH2.
Cancer Cell 23, June 10, 2013 ª2013 Elsevier Inc. 711
Previews Snyder, E.L., Watanabe, H., Magendantz, M., Hoersch, S., Chen, T.A., Wang, D.G., Crowley, D., Whittaker, C.A., Meyerson, M., Kimura, S., and Jacks, T. (2013). Mol. Cell 50, 185–199. Winslow, M.M., Dayton, T.L., Verhaak, R.G.W., Kim-Kiselak, C., Snyder, E.L., Feldser, D.M., Hub-
bard, D.D., DuPage, M.J., Whittaker, C.A., Hoersch, S., et al. (2011). Nature 473, 101–104.
Yoshizawa, A., Motoi, N., Riely, G.J., Sima, C.S., Gerald, W.L., Kris, M.G., Park, B.J., Rusch, V.W., and Travis, W.D. (2011). Mod. Pathol. 24, 653–664.
Yin, Z., Gonzales, L., Kolla, V., Rath, N., Zhang, Y., Lu, M.M., Kimura, S., Ballard, P.L., Beers, M.F., Epstein, J.A., and Morrisey, E.E. (2006). Am. J. Physiol. Lung Cell. Mol. Physiol. 291, L191–L199.
Zhang, Y., Goss, A.M., Cohen, E.D., Kadzik, R., Lepore, J.J., Muthukumaraswamy, K., Yang, J., DeMayo, F.J., Whitsett, J.A., Parmacek, M.S., and Morrisey, E.E. (2008). Nat. Genet. 40, 862–870.
Succinate: A New Epigenetic Hacker Ming Yang1 and Patrick J. Pollard1,* 1Cancer Biology and Metabolism Group, Nuffield Department of Medicine, Henry Wellcome Building for Molecular Physiology, University of Oxford, Oxford OX3 7BN, UK *Correspondence: [email protected] http://dx.doi.org/10.1016/j.ccr.2013.05.015
Epigenetic reprogramming is a feature of many human cancers. In this issue of Cancer Cell, Letouze´ and colleagues describe DNA hypermethylation in paragangliomas harboring mutations in succinate dehydrogenase genes. These tumors accumulate succinate, which inhibits 2-oxoglutarate-dependent histone and DNA demethylase enzymes, resulting in epigenetic silencing that affects neuroendocrine differentiation. Metabolic rewiring has long been regarded as a consequence of malignant transformation driven by abnormal signal transduction mediated by oncogenes and tumor suppressors. The identification that the metabolite (R)-2-hydroxyglutarate [(R)-2HG] plays an oncogenic role in subsets of gliomas and acute myelogenous leukemia with mutations in isoforms of isocitrate dehydrogenase (IDH) has provided direct evidence linking altered metabolism and cancer and founded the notion of the ‘‘oncometabolite’’ (Ward et al., 2010). (R)-2HG is a product of the neomorphic activities of mutant IDH1/2 and accumulates to millimolar concentrations in IDH-associated gliomas (Dang et al., 2010). Due to the structural similarity to 2-oxoglutarate (2OG), (R)-2HG competitively inhibits the 2OG-utilizing Jumonji C (JmjC)-domain containing histone lysine demethylases and the TET family of 5-methylcytosine (5mC) hydroxylases, leading to DNA hypermethylation at CpG islands in promoter regions of genes involved in cell differentiation (Kim and DeBerardinis, 2013). The Krebs cycle enzymes succinate dehydrogenase (SDH) and fumarate hydratase (FH) are tumor suppressors whose loss-of-function mutations predispose to familial cancer syndromes (Frezza
et al., 2011). SDH and FH inactivation results in a blockade of Krebs cycle, impaired respiration, and abnormal accumulation of their substrates succinate and fumarate, respectively. Both metabolites are also inhibitors of 2OG-dependent dioxygenases, particularly the hydroxylases for the transcription factor hypoxiainducible factor (HIF), resulting in HIF-dependent activation of a ‘‘pseudohypoxic’’ response characterized by enhanced angiogenesis and increased anaerobic metabolism (Frezza et al., 2011). In the case of fumarate, the metabolite is also an endogenous electrophile and can chemically modify cysteine residues in Kelch-like ECH-associated protein 1, leading to constitutive (and potentially oncogenic) activation of transcription factor nuclear factor erythroid 2-related factor 2 (Adam et al., 2011; Ooi et al., 2011). Until recently, HIF activation by fumarate and succinate has been the only mechanistic link to oncogenesis that has been described in FH and SDH mutant cancer models (Pollard et al., 2007; Selak et al., 2005). However, at least in the context of FH-deficiency, a causal role for HIF in tumorigenesis is questionable (Adam et al., 2011), and the quest for other oncogenic drivers in SDH- and FH-associated malignancy continues. Recent re-
ports by Killian et al. (2013) and Letouze´ et al. (2013; in this issue of Cancer Cell) both demonstrate that, similarly to (R)2HG, succinate can also remodel the epigenome and alter gene expression. SDH mutations occur frequently in paraganglioma (PGL), pheochromocytoma (PCC), gastrointestinal stromal tumor (GIST), as well as in renal carcinoma (Killian et al., 2013). GIST tumors can alternatively be driven by mutations in signal transduction kinases. Killian et al. (2013) compared the methylation profiles of GISTs with mutations in either SDH or KIT tyrosine kinase and uncovered marked global hypermethylation in the SDH versus KIT mutant subgroup. Further examination of a PGL/PCC cohort showed a similar hypermethylation phenotype in SDH mutant tumors compared to SDH-wild-type reference tissues. Bioinformatic analyses of methylomes from developmentally distinct tumors including GIST, PGL, PCC, and gliomas revealed comparable methylation signatures in those harboring SDH and IDH mutations (Killian et al., 2013). Letouze´ et al. (2013) classified a large PGL/PCC cohort based on DNA methylation and obtained three stable clusters. Strikingly, a distinct subgroup characterized by the presence of a
Cancer Cell 23, June 10, 2013 ª2013 Elsevier Inc. 709
Cancer Cell
Previews
Figure 1. Epigenetic Reprogramming by Oncometabolites Mutations in the metabolic enzymes isocitrate dehydrogenase (IDH)-1 and -2, succinate dehydrogenase (SDH), and fumarate hydratase (FH) lead to abnormal accumulation of (R)-2-hydroxyglutarate [(R)-2HG], succinate, and fumarate, respectively. Their accumulation inhibits the activities of 2-oxoglutarate (2OG)-dependent dioxygenases, including the TET family of DNA modifying enzymes and the JmjC domain-containing histone lysine demethylases (KDMs). Subsequent epigenetic alterations result in cell differentiation arrest and promote malignant transformation. Thus, epigenetic modification, through the action of oncometabolites, is a shared feature among IDH-, SDH-, and FH-associated cancers. SUCC, succinate; FUM, fumarate; Mal, malate; mIDH1/2, mutant IDH1/2; Me, methyl group.
hypermethylator phenotype contained 16 of 17 samples that harbor SDH mutations. The prevailing model is that accumulation of succinate in these tumors inhibits 2OGdependent oxidative demethylation in a manner analogous to the action of (R)2HG, giving rise to DNA hypermethylation. Integration of methylation and gene expression data revealed, whereas most hypermethylated CpG sites had no functional consequences, there were 191 genes that showed correlated levels of hypermethylation and downregulation (Letouze´ et al., 2013). SDH- and VHL-related tumors share some similar features in gene expression profiles, possibly due to activation of HIF-orchestrated hypoxia-related genes. Nonetheless, SDH- and VHL-associated PGL/PCC from the cohort studied were clustered as two distinct gene-expression subgroups, suggesting that epigenetic remodelling plays a major role in the transcriptional divergence of these two tumor subclasses. Intriguingly, the only tumor sample in the hypermethylated PGL/PCC subgroup without SDH mutations was shown to harbor germline inactivating FH mutations. This finding corroborates recent evidence demonstrating that fumarate-mediated inhibition of TET and KDM enzymes alters histone and DNA methylation pat-
terns (Xiao et al., 2012) and raises the necessity to test FH-associated neoplasms for epigenetic abnormalities in future studies. Consistent with the findings by Killian et al. (2013), Letouze´ et al. (2013) compared the CpG methylator phenotypes between SDH/FH mutant PGL/ PCC and IDH mutant gliomas and identified significant overlap in hypermethylated CpG sites. Particularly, 17 genes were concordantly hypermethylated and downregulated in both tumor groups. The connection in methylation profiles between the SDH, FH, and IDH mutant tumors may reflect a similar inhibitory role for succinate, fumarate, and (R)2HG (see Figure 1), whereas the differences may partly be attributed to the variable susceptibilities of the panoply of 2OG-dependent oxygenases to the three oncometabolites. All of the SDH mutant PGL/PCC in this study exhibited low 5-hydroxymethylcytosine (5hmC) levels by immunohistochemistry analyses, and the majority also showed elevated histone methylation at H3K27, supporting the hypothesis that succinate inhibits TET- and KDMmediated demethylations. Consistently, Sdhb / mouse chromaffin cells displayed higher 5mC/5hmC ratios compared to wild-type cells, which could be
710 Cancer Cell 23, June 10, 2013 ª2013 Elsevier Inc.
reversed by the addition of exogenous 2OG to the culture medium. Importantly, the hypermethylated genes detected in these cells significantly overlapped with those identified in SDH mutant PGL/PCC. The SDH enzyme complex comprises four subunits (SDHA, SDHB, SDHC, and SDHD). Although mutations in all subunits occur in cancer, tumors harboring mutations in the catalytic subunit SDHB are particularly malignant and associated with a higher risk of metastasis (Frezza et al., 2011). Letouze´ et al. (2013) showed that the mean methylation levels across all CpG sites are higher in SDHB-mutated PGL/PCC than other SDH mutant PGL/ PCC. It is possible that the loss of SDHB function results in bona fide inactivation of the SDH complex, whereas reductions of enzyme activity through mutations in other subunits are not as complete. Consequently, there might be higher succinate accumulation and stronger inhibition of demethylation in SDHB mutant tumors. Although the differences in succinate levels could not be demonstrated from the SDH mutant PGL/PCC cohort, the heterogeneity of tumor samples may preclude accurate analyses and further investigations employing cell culture models could be more informative. Two of the most significantly silenced genes in the hypermethylated subgroup,
Cancer Cell
Previews PNMT and KRT19, are respectively involved in neuroendocrine differentiation and epithelial-to-mesenchymal transition (EMT). Interestingly, despite exhibiting a decreased growth rate similar to the Fh1 / mouse embryonic fibroblasts, the Sdhb / chromaffin cells showed markedly increased migratory capacities, potentially reflecting epigenetic deregulation of EMT genes and the aggressive nature of SDHB-related PCC/PGL. Recently, inhibitors that specifically target mutant IDH1 and IDH2 have been reported. Such inhibitors could reverse (R)-2HG-mediated epigenetic deregulation, induce cell differentiation, and suppress the growth of IDH mutant tumor cells (Kim and DeBerardinis, 2013). Letouze´ et al. (2013) showed that transient treatment with a DNA methyltransferase inhibitor repressed the migration capacities of Sdhb / cells, implicating the possibility of epigenetic targeting in SDH-related tumors. However, succinate and fumarate accumulation exert various
nonepigenetic cellular impacts, and, to achieve therapeutic efficacy, a synergistic lethality approach is probably desired. Nonetheless, the emergence of epigenetically silenced gene sets may serve as useful biomarkers for early detection of cancers characterized by Krebs cycle defects.
Kim, J., and DeBerardinis, R.J. (2013). Science 340, 558–559. Letouze´, E., Martinelli, C., Loriot, C., Burnichon, N., Abermil, N., Ottolenghi, C., Janin, M., Menara, M., Nguyen, A.T., Benit, P., et al. (2013). Cancer Cell 23, this issue, 739–752. Ooi, A., Wong, J.C., Petillo, D., Roossien, D., Perrier-Trudova, V., Whitten, D., Min, B.W., Tan, M.H., Zhang, Z., Yang, X.J., et al. (2011). Cancer Cell 20, 511–523.
REFERENCES Adam, J., Hatipoglu, E., O’Flaherty, L., Ternette, N., Sahgal, N., Lockstone, H., Baban, D., Nye, E., Stamp, G.W., Wolhuter, K., et al. (2011). Cancer Cell 20, 524–537. Dang, L., White, D.W., Gross, S., Bennett, B.D., Bittinger, M.A., Driggers, E.M., Fantin, V.R., Jang, H.G., Jin, S., Keenan, M.C., et al. (2010). Nature 465, 966. Frezza, C., Pollard, P.J., and Gottlieb, E. (2011). J. Mol. Med. 89, 213–220. Killian, J.K., Kim, S.Y., Miettinen, M., Smith, C., Merino, M., Tsokos, M., Quezado, M., Smith, W.I., Jr., Jahromi, M.S., Xekouki, P., et al. (2013). Cancer Discov. Published online May 21, 2013. http://dx. doi.org/10.1158/2159-8290.CD-13-0092.
Pollard, P.J., Spencer-Dene, B., Shukla, D., Howarth, K., Nye, E., El-Bahrawy, M., Deheragoda, M., Joannou, M., McDonald, S., Martin, A., et al. (2007). Cancer Cell 11, 311–319. Selak, M.A., Armour, S.M., MacKenzie, E.D., Boulahbel, H., Watson, D.G., Mansfield, K.D., Pan, Y., Simon, M.C., Thompson, C.B., and Gottlieb, E. (2005). Cancer Cell 7, 77–85. Ward, P.S., Patel, J., Wise, D.R., Abdel-Wahab, O., Bennett, B.D., Coller, H.A., Cross, J.R., Fantin, V.R., Hedvat, C.V., Perl, A.E., et al. (2010). Cancer Cell 17, 225–234. Xiao, M., Yang, H., Xu, W., Ma, S., Lin, H., Zhu, H., Liu, L., Liu, Y., Yang, C., Xu, Y., et al. (2012). Genes Dev. 26, 1326–1338.
Cancer Stem Cells Activate STAT3 the EZ Way Shaun D. Fouse1 and Joseph F. Costello1,* 1Brain Tumor Research Center, Department of Neurosurgery, University of California, San Francisco, San Francisco, CA 94158, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.ccr.2013.05.016
Activated STAT3 and increased expression of the histone methyltransferase EZH2 are independently associated with the most malignant subset of gliomas. In this issue of Cancer Cell, Kim and colleagues discover that EZH2 enhances STAT3 activation by trimethylating lysine180 in STAT3 and does so preferentially in glioma stem-like cells. Presuming that cancer stem cells are responsible for therapeutic resistance in cancer patients, the discovery of the molecular differences that drive cancer stem cell phenotypes holds significant value for improving therapy. More valuable still would be the ability to selectively inhibit these molecular drivers of stemness. In this issue of Cancer Cell, Kim et al. (2013) employ cultures of glioma stem-like cells (GSCs) and their isogenic bulk tumor cells along with intracranial xenografts and come one step closer to achieving this ambitious goal.
Mutation or overexpression of enhancer of Zeste homolog 2 (EZH2) can drive the clonal expansion of leukemias and the growth of solid cancers, though individual EZH2 alterations follow divergent paths toward malignancy. EZH2 is most known as an enzymatically active component of the Polycomb repressive complex 2 (PRC2), which is responsible for depositing methyl groups onto lysine 27 (K27) of histone H3 (H3K27). EZH2 plays an important role in maintaining stem cell function. The most common EZH2 mutations in cancer occur within
the SET domain at Y641, which result in increased trimethylation activity, leading to an increase in global H3K27me3 levels in B cell lymphomas (Yap et al., 2011). In contrast, in breast cancer cells, AKTmediated phosphorylation of S21 reduces EZH2 interaction with histone H3, leading to a decrease in global H3K27me3 levels (Cha et al., 2005). In addition, in pediatric brain tumors, recurrent somatic mutation of K27 on histone H3 variant H3F3A may deplete H3K27me3 on canonical H3 indirectly because of increased interaction between mutant H3F3A and EZH2.
Cancer Cell 23, June 10, 2013 ª2013 Elsevier Inc. 711