Epigenetic Mechanisms
of Carcinogenesis: Commentary
Jay I. Goodman, Ph.D.
Department of Pharmacology and Toxicology, Michigan State University, B-440
Life Sciences Building
East Lansing, MI 48824 U.S.A.
Tel: (517)-353-9346
Fax: (517)-353-8915
E-mail: goodman3@pilot.msu.edu
There is, particularly among many toxicologists, an excessive focus upon mutagenesis as the (read the one and only) mechanism underlying carcinogenesis. Therefore, I am delighted to see this provocative, timely review of epigenetic mechanisms of carcinogenesis by Klaunig et al. I would like to take this opportunity to respond, in a succinct fashion, by amplifying and extending some of the points made. Specifically, I will 1) expand upon the definition of the term "epigenetic" and its role in carcinogenesis, 2) discuss the role that increased gene expression, without mutation, may play in transformation, 3) provide an example of a dose-response relationship between a tumor promoter and its effect on DNA methylation, 4) indicate that the initiation stage of carcinogenesis may have an epigenetic rather than a mutagenic basis, and 5) discuss the possible interrelationships between a genotoxic event and altered DNA methylation, an epigenetic event.
Epigenetic regulation of gene expression is based upon a modulation of gene expression by heritable mechanisms superimposed on that conferred by the primary DNA sequence, e.g. DNA methylation, 5-methylcytosine content of DNA (Holiday, 1994). In contrast to mutation, this involves a heritable alteration of gene expression that is not based upon a change in DNA base sequence. Altered DNA methylation leading to aberrant gene expression due, in part, to affecting the ability of methylated DNA-binding proteins to interact with their cognate cis elements, may underlie some of the crucial changes in gene expression involved in carcinogenesis (Samiec and Goodman, 1999). The key role that altered DNA methylation may play in carcinogenesis, as an epigenetic, nongenotoxic mechanism, has been the subject of several reviews (Holiday, 1987; Counts and Goodman, 1995a; Counts and Goodman, 1995b; Counts and Goodman, 1995c; Baylin, 1997; Trosko et al., 1998; and Jones and Laird, 1999).
The role that mutation plays in carcinogenesis by activating proto-oncogenes to oncogenes and silencing tumor suppresser genes is appreciated widely. It is axiomatic that, in order to affect the phenotype of a cell, a mutated oncogene must be expressed. However, aberrant increased expression of nonmutated genes has been shown to play a key roll too (Shastry, 1995). Proto-oncogene overexpression may be a mechanism of activation of the ras pathway, alternative to point mutation (Mangues et a., 1994; Clark et al., 1996). Overexpression of myc as well as K-ras can contribute to transformation (Schwab, 1983; Lee, 1991). Furthermore, overexpression of HER2/c-erbB2 receptor tyrosine kinase induces the transformed phenotype of NIH3T3 cells and is required for tumor formation and progression in nude mice (Baasner et al., 1996). In this context, it is important to note that the U.S. Environmental Protection Agency's proposed Cancer Risk Assessment Guidelines include a section (section 2.3.5.3) entitled "Nonmutagenic and Other Effects" which refers explicitly to a role for altered DNA methylation as a basis for the altered gene expression involved in carcinogenesis (U.S. Environmental Protection agency, 1996).
The view of carcinogenesis as consisting of three biological stages, initiation, promotion and progression provides a very useful conceptual framework for understanding the cancer problem and approaching it experimentally (Dragan et al., 1993). The first stage, initiation, involves a heritable alteration to the genome that facilitates the clonal expansion of initiated cells in response to a promotion stimulus. It is usually assumed that mutagenesis provides the basis for initiation. However, there could be an epigenetic basis too. Despite the fact that many initiators of carcinogenesis have been shown to be capable of acting as mutagens under certain experimental circumstances, one does not have to assume that all of their biological effects stem from mutagenesis. Hypermethylation-induced silencing of a tumor suppresser gene(s) (Counts and Goodman, 1995; Baylin, 1997; Jones and Laird, 1999) and/or hypomethylation-facilitated aberrant increase in expression of an oncogene(s) (Counts and Goodman, 1995) are plausible mechanisms that could underlie initiation. The involvement of epigenetics in initiation is not mutually incompatible with a role for mutation. Indeed, one or the other, or perhaps both, may play key roles depending upon the particular circumstances, e.g., causative agent, dose, target organ and species.
Carcinogen adducts in DNA involving chemical carcinogens (Wilson and Jones, 1983; Wilson et al., 1987; Hepburn et al., 1991) or free radical adducts (Weitzman et al., 1994) have been shown to result in decreased DNA methylation. Additionally, depending upon the location of a particular alkylated (O6-methyl) guanine in relation to a potentially methylatable cytosine residue, DNA methylation may be either increased or decreased (Tan and Li, 1990). Thus, a genotoxic compound may produce an epigenetic change that can persist, even though the DNA damage may be repaired.
It is now quite clear that carcinogenesis involves more than mutagenesis and an increased focus on epigenetic events underlying the transformation of a normal cell into a frank malignancy is appropriate.
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