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Identification of the Glioma Cell of Origin
WORLD NEUROSURGERY NEWS
A recent report by Schmitz et al. in the journal Science shows that in the plant arabidopsis, alterations in cytosine methylation in DNA can arise spontaneously and affect the expression of the genome, resulting in a range of phenotypic variation in genetically identical individuals (5). Entire epialleles—alleles that are epigenetically expressed or silenced— can therefore be brought into play in some individuals without invoking gene mutation. Another study reported in Science, by Johnson et al., showed that a maternal RNA transcript in C. elegans is required for expression of the corresponding gene in the offspring, which, in turn, allows it to become phenotypically male, whether it is genetically male or hermaphroditic (2). The transcript is theorized to bind antisense inhibitors of gene expression, which allow normal expression. Clearly, the content of the genome itself is only a small part of the story that is eventually written. The generation of the very first cell, formed by the union of the egg and sperm, leads to the formation of diverse cell types with essentially identical genetic information (with very few exceptions). Research has identified epigenetic alterations in the form of DNA methylation and acetylation as primary mechanisms underlying the diverse genetic structures and cellular phenotypes. Methylation and acetylation at distinct locations in the genome allow each cell type to have the same genetic information but the ability to express only certain segments of the genome, thereby generating cellular identity. Recent excitement over the conversion of fibroblasts into a pseudo-embryonic state and generation of induced pluripotent stem cells (iPS) has made use of this simple observation (3, 4, 6). By altering the expression of key transcription factors, researchers have
REFERENCES 1. Adachi JI, Mishima K, Wakiya K, Suzuki T, Fukuoka K, Yanagisawa T, Matsutani M, Sasaki A, Nishikawa R: O (6)-methylguanine-DNA methyltransferase promoter methylation in 45 primary central nervous system lymphomas: quantitative assessment of methylation and response to temozolomide treatment. J Neurooncol 107:147-153, 2012. 2. Johnson CL, Spence AM: Epigenetic licensing of germline gene expression by maternal RNA in C. elegans. Science 333:1311-1314, 2011.
been able to unlock masked segments of DNA in somatic cells and to revert them to an “embryonic” stage where the cell can be coaxed into another cell type by providing it with the necessary signals. This advance has led to the recent demonstration, for example, of the formation of functional neurons from iPS cells (4). Heritable information can be passed outside the nuclear DNA in multiple ways. Extrachromosomal DNA can be transmitted through the mitochondria. Modifications of the histones can promote or suppress the expression of the chromatin wrapped around it. Posttranscriptional and posttranslational activity can affect the phenotypic expression of cells despite the same genome. One piece of cellular machinery whose prominence we are increasingly understanding is the activity of small RNA molecules. Yet all of these examples represent just a fraction of the possible epigenetic changes. Epigenetics is stirring up the field of genetics because it confounds the traditional notion that environmental factors pass changes transgenerationally only through natural selection. Of interest to clinicians is the idea that epigenetic changes can influence the behavior of lines of cells within a single individual—the patient—and potentially could be manipulated to the benefit of an individual, for instance, to kill cancer cells or to activate a gene that can overcome disease. Many of these mechanisms have not yet been shown to be present in humans or in human cancer. But clearly, there are many ways for epigenetic mechanisms to influence the expression of neurosurgical disease. Neurosurgeons cannot ignore this rapidly expanding field. Although we can now sequence DNA at an astounding rate, making sense of the text of life will require much more than just reading the script.
3. Meissner A, Wernig M, Jaenisch R: Direct reprogramming of genetically unmodified fibroblasts into pluripotent stem cells. Nat Biotechnol 25:1177-1181, 2007. 4. Pang ZP, Yang N, Vierbuchen T, Ostermeier A, Fuentes DR, Yang TQ, Citri A, Sebastiano V, Marro S, Sudhof TC, Wernig M: Induction of human neuronal cells by defined transcription factors. Nature 476: 220-223, 2011. 5. Schmitz RJ, Schultz MD, Lewsey MG, O’Malley RC, Urich MA, Libiger O, Schork NJ, Ecker JR: Transgen-
erational epigenetic instability is a source of novel methylation variants. Science 334:369-373, 2011. 6. Takahashi K, Yamanaka S: Induction of pluripotent stem cells from mouse embryonic and adult fibroblast cultures by defined factors. Cell 126:663-676, 2006.
1878-8750/$ - see front matter © 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.wneu.2012.07.003
Identification of the Glioma Cell of Origin Jay D. Turner, Adib A. Abla, Nader Sanai
Data continue to mount that implicate a subpopulation of self-renewing cells, termed cancer stem cells, in the pathogenesis of glioma. In a recent report, Liu et al. (1) used an elegant approach to identify oligodendrocyte precursor cells (OPCs) as a cell of origin for glioma. A mouse-model system known as mosaic analysis with double markers (MADM) was used to sporadically initiate p53 and NF1 mutations (common in human glioma) in neural stem cells (NSCs)
200
www.SCIENCEDIRECT.com
and their progeny. Mutant cells labeled with green fluorescent protein (GFP) and wild type (WT) cells labeled with red fluorescent protein (RFP) were tracked throughout the process of tumorigenesis. In this model, all gliomas were GFP-positive, indicating that they originated from MADM-induced p53/NF1 mutated cells. In order to identify the cell of origin for glioma in this model, a proliferative index was calculated using a ratio of GFP to RFP posi-
78 [3/4]: 194-201, SEPTEMBER/OCTOBER 2012 WORLD NEUROSURGERY
WORLD NEUROSURGERY NEWS
Figure 1. Oligodendrocyte precursor cells (OPCs) serve as the cell of origin in the pretransforming stage of gliomagenesis. (A) Green fluorescent protein (GFP) to red fluorescent protein (RFP) ratios for neural stem cells, astrocytes, oligodendrocytes, and oligodendrocyte progenitors cells demonstrates the OPC’s significantly higher proliferative ratio. (B) At 7 days, all BrdU-labeled cells were also positive for an OPC marker, Olig2. (C, D) All BrdU-positive cells were mutant cells (GFP positive) and
tive cells (G/R ratio) in OPCs, neurons, astrocytes, and oligodendrocytes in pretransforming MADM brains. OPCs were the only lineage to demonstrate marked expansion of mutated cells at this stage (Figure 1). Next, the authors analyzed the expression pattern of cell-specific markers in fully developed tumors. Cellular markers of NSCs, astrocytes, and OPCs were found within the tumor; however, proliferating cells only demonstrated OPC markers, suggesting that NSCs and astrocytes were bystanders within the tumor. All of the data up to this point demonstrated that p53/NF1 mutations in NSCs can give rise to tumorigenic OPCs. For their next series of experiments, p53/NF1 mutations were generated in OPCs themselves using the MADM model. Using this system, the authors found GFPpositive gliomas at nearly full penetrance, as they did with the NSC model. This study adds additional evidence in support of OPCs as the cell of origin in this glioma mouse model. Interestingly, the gene expres-
colocalized with Olig2. A small percentage of dividing cells were PDGFR␣ (OPC marker) negative but still stained for Olig2, suggesting that these cells were newly differentiated oligodendrocytes from initially BrdU-labeled mutant OPCs. As, astrocytes; OLi, oligodendrocytes; MADM, mosaic analysis with double markers; Olig 2, oligodendrocyte transcription factor 2; BrdU, bromodeoxyuridine; PDGFR␣, platelet-derived growth factor receptor-␣.
sion pattern of the tumors most directly correlated with the proneural subtype in human glioblastoma. With diverse molecular profiles, it is possible that other subtypes of glioblastoma may be driven by different cells of origin. Nonetheless, this work identifies OPCs as a potential cellular target for the development of new and effective treatment strategies for malignant glioma.
REFERENCE 1. Liu C, Sage JC, Miller MR, Verhaak RG, Hippenmeyer S, Vogel H, Foreman O, Bronson RT, Nishiyama A, Luo L, Zong H: Mosaic analysis with double markers reveals tumor cell of origin in glioma. Cell 146:209-221, 2011.
1878-8750/$ - see front matter © 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.wneu.2012.07.004
WORLD NEUROSURGERY 78 [3/4]: 194-201, SEPTEMBER/OCTOBER 2012
www.WORLDNEUROSURGERY.org
201
A recent report by Schmitz et al. in the journal Science shows that in the plant arabidopsis, alterations in cytosine methylation in DNA can arise spontaneously and affect the expression of the genome, resulting in a range of phenotypic variation in genetically identical individuals (5). Entire epialleles—alleles that are epigenetically expressed or silenced— can therefore be brought into play in some individuals without invoking gene mutation. Another study reported in Science, by Johnson et al., showed that a maternal RNA transcript in C. elegans is required for expression of the corresponding gene in the offspring, which, in turn, allows it to become phenotypically male, whether it is genetically male or hermaphroditic (2). The transcript is theorized to bind antisense inhibitors of gene expression, which allow normal expression. Clearly, the content of the genome itself is only a small part of the story that is eventually written. The generation of the very first cell, formed by the union of the egg and sperm, leads to the formation of diverse cell types with essentially identical genetic information (with very few exceptions). Research has identified epigenetic alterations in the form of DNA methylation and acetylation as primary mechanisms underlying the diverse genetic structures and cellular phenotypes. Methylation and acetylation at distinct locations in the genome allow each cell type to have the same genetic information but the ability to express only certain segments of the genome, thereby generating cellular identity. Recent excitement over the conversion of fibroblasts into a pseudo-embryonic state and generation of induced pluripotent stem cells (iPS) has made use of this simple observation (3, 4, 6). By altering the expression of key transcription factors, researchers have
REFERENCES 1. Adachi JI, Mishima K, Wakiya K, Suzuki T, Fukuoka K, Yanagisawa T, Matsutani M, Sasaki A, Nishikawa R: O (6)-methylguanine-DNA methyltransferase promoter methylation in 45 primary central nervous system lymphomas: quantitative assessment of methylation and response to temozolomide treatment. J Neurooncol 107:147-153, 2012. 2. Johnson CL, Spence AM: Epigenetic licensing of germline gene expression by maternal RNA in C. elegans. Science 333:1311-1314, 2011.
been able to unlock masked segments of DNA in somatic cells and to revert them to an “embryonic” stage where the cell can be coaxed into another cell type by providing it with the necessary signals. This advance has led to the recent demonstration, for example, of the formation of functional neurons from iPS cells (4). Heritable information can be passed outside the nuclear DNA in multiple ways. Extrachromosomal DNA can be transmitted through the mitochondria. Modifications of the histones can promote or suppress the expression of the chromatin wrapped around it. Posttranscriptional and posttranslational activity can affect the phenotypic expression of cells despite the same genome. One piece of cellular machinery whose prominence we are increasingly understanding is the activity of small RNA molecules. Yet all of these examples represent just a fraction of the possible epigenetic changes. Epigenetics is stirring up the field of genetics because it confounds the traditional notion that environmental factors pass changes transgenerationally only through natural selection. Of interest to clinicians is the idea that epigenetic changes can influence the behavior of lines of cells within a single individual—the patient—and potentially could be manipulated to the benefit of an individual, for instance, to kill cancer cells or to activate a gene that can overcome disease. Many of these mechanisms have not yet been shown to be present in humans or in human cancer. But clearly, there are many ways for epigenetic mechanisms to influence the expression of neurosurgical disease. Neurosurgeons cannot ignore this rapidly expanding field. Although we can now sequence DNA at an astounding rate, making sense of the text of life will require much more than just reading the script.
3. Meissner A, Wernig M, Jaenisch R: Direct reprogramming of genetically unmodified fibroblasts into pluripotent stem cells. Nat Biotechnol 25:1177-1181, 2007. 4. Pang ZP, Yang N, Vierbuchen T, Ostermeier A, Fuentes DR, Yang TQ, Citri A, Sebastiano V, Marro S, Sudhof TC, Wernig M: Induction of human neuronal cells by defined transcription factors. Nature 476: 220-223, 2011. 5. Schmitz RJ, Schultz MD, Lewsey MG, O’Malley RC, Urich MA, Libiger O, Schork NJ, Ecker JR: Transgen-
erational epigenetic instability is a source of novel methylation variants. Science 334:369-373, 2011. 6. Takahashi K, Yamanaka S: Induction of pluripotent stem cells from mouse embryonic and adult fibroblast cultures by defined factors. Cell 126:663-676, 2006.
1878-8750/$ - see front matter © 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.wneu.2012.07.003
Identification of the Glioma Cell of Origin Jay D. Turner, Adib A. Abla, Nader Sanai
Data continue to mount that implicate a subpopulation of self-renewing cells, termed cancer stem cells, in the pathogenesis of glioma. In a recent report, Liu et al. (1) used an elegant approach to identify oligodendrocyte precursor cells (OPCs) as a cell of origin for glioma. A mouse-model system known as mosaic analysis with double markers (MADM) was used to sporadically initiate p53 and NF1 mutations (common in human glioma) in neural stem cells (NSCs)
200
www.SCIENCEDIRECT.com
and their progeny. Mutant cells labeled with green fluorescent protein (GFP) and wild type (WT) cells labeled with red fluorescent protein (RFP) were tracked throughout the process of tumorigenesis. In this model, all gliomas were GFP-positive, indicating that they originated from MADM-induced p53/NF1 mutated cells. In order to identify the cell of origin for glioma in this model, a proliferative index was calculated using a ratio of GFP to RFP posi-
78 [3/4]: 194-201, SEPTEMBER/OCTOBER 2012 WORLD NEUROSURGERY
WORLD NEUROSURGERY NEWS
Figure 1. Oligodendrocyte precursor cells (OPCs) serve as the cell of origin in the pretransforming stage of gliomagenesis. (A) Green fluorescent protein (GFP) to red fluorescent protein (RFP) ratios for neural stem cells, astrocytes, oligodendrocytes, and oligodendrocyte progenitors cells demonstrates the OPC’s significantly higher proliferative ratio. (B) At 7 days, all BrdU-labeled cells were also positive for an OPC marker, Olig2. (C, D) All BrdU-positive cells were mutant cells (GFP positive) and
tive cells (G/R ratio) in OPCs, neurons, astrocytes, and oligodendrocytes in pretransforming MADM brains. OPCs were the only lineage to demonstrate marked expansion of mutated cells at this stage (Figure 1). Next, the authors analyzed the expression pattern of cell-specific markers in fully developed tumors. Cellular markers of NSCs, astrocytes, and OPCs were found within the tumor; however, proliferating cells only demonstrated OPC markers, suggesting that NSCs and astrocytes were bystanders within the tumor. All of the data up to this point demonstrated that p53/NF1 mutations in NSCs can give rise to tumorigenic OPCs. For their next series of experiments, p53/NF1 mutations were generated in OPCs themselves using the MADM model. Using this system, the authors found GFPpositive gliomas at nearly full penetrance, as they did with the NSC model. This study adds additional evidence in support of OPCs as the cell of origin in this glioma mouse model. Interestingly, the gene expres-
colocalized with Olig2. A small percentage of dividing cells were PDGFR␣ (OPC marker) negative but still stained for Olig2, suggesting that these cells were newly differentiated oligodendrocytes from initially BrdU-labeled mutant OPCs. As, astrocytes; OLi, oligodendrocytes; MADM, mosaic analysis with double markers; Olig 2, oligodendrocyte transcription factor 2; BrdU, bromodeoxyuridine; PDGFR␣, platelet-derived growth factor receptor-␣.
sion pattern of the tumors most directly correlated with the proneural subtype in human glioblastoma. With diverse molecular profiles, it is possible that other subtypes of glioblastoma may be driven by different cells of origin. Nonetheless, this work identifies OPCs as a potential cellular target for the development of new and effective treatment strategies for malignant glioma.
REFERENCE 1. Liu C, Sage JC, Miller MR, Verhaak RG, Hippenmeyer S, Vogel H, Foreman O, Bronson RT, Nishiyama A, Luo L, Zong H: Mosaic analysis with double markers reveals tumor cell of origin in glioma. Cell 146:209-221, 2011.
1878-8750/$ - see front matter © 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.wneu.2012.07.004
WORLD NEUROSURGERY 78 [3/4]: 194-201, SEPTEMBER/OCTOBER 2012
www.WORLDNEUROSURGERY.org
201