Downloaded from genesdev.cshlp.org on September 24, 2021 - Published by Cold Spring Harbor Laboratory Press PERSPECTIVE Deconstructing Myc Robert N. Eisenman1 Division of Basic Sciences, Fred Hutchinson Cancer Research Center, Seattle, Washington 98109-1024, USA Background Max, determines cell behavior by activation of a specific set of target genes. However, on second glance a number Although myc was among the very earliest oncogenes of complicating issues arise. First, Myc–Max transcrip- identified and the subject of intense study, it has none- tional activity on either synthetic promoters or putative theless proven to be an enduring enigma. To a large ex- cellular target genes is disappointingly weak and variable tent the problem derives from the apparent gap between (compared with other activators)—on the order of 2–5 Myc’s biological role and what is surmised to be its mo- fold. This makes detailed dissection of transcriptional lecular function. Myc family proteins (comprising c-, N-, function difficult and, even worse, makes validation of and L-Myc) promote proliferation, growth, and apopto- putative target genes both awkward and contentious (for sis; inhibit terminal differentiation; and, when deregu- review, see Grandori and Eisenman 1997). Second, al- lated, are profoundly involved in the genesis of an ex- though the transcriptional activity of Myc–Max was traordinarily wide range of cancers (for recent review, see widely confirmed, there were also reports that Myc Grandori et al. 2000). Alongside this veritable mountain could repress transcription of a number of genes depen- of biological effects, the molecular characterization of dent on a specific subclass of initiator elements (INRs) Myc—as a relatively weak transcriptional regulator of located at the transcriptional initiation sites of certain uncertain target genes—looks like a molehill. Indeed genes (Li et al. 1994). Because the repression experiments some have wondered whether the transcriptional activi- did not demonstrate Myc binding to INRs, they left open ties of Myc might be more apparent than real. the possibility that repression might be indirect (e.g. The notion that Myc proteins might function in tran- through activation of a repressor by Myc–Max). Third, scription arose over a decade ago, after Myc was shown mutational analyses of Myc Box II, a highly conserved to be a nuclear protein. An N-terminal fragment of Myc region present in the N-terminal transactivation region stimulated transcription when fused to a heterologous of all Myc proteins, produced confounding results. Sev- DNA binding domain, and the C-terminal basic-helix- eral reports indicated that mutation of Myc Box II had loop-helix-zipper (bHLHZ) of Myc resembled those little effect on transcription activation but abrogated found in certain families of transcription factors. But Myc repression activity. Yet other studies indicated that full-length Myc did not dimerize or bind DNA and there- loss of Myc Box II primarily affected activation. Because fore could not be demonstrated to behave as a transcrip- Myc Box II is crucial for the ability of Myc to transform tion factor. This picture changed with the discovery of cells, these results raised the issue of the relative impor- Max, another bHLHZ protein which heterodimerizes tance of activation and repression (for review, see Gran- with Myc to form a sequence-specific DNA binding dori et al. 2000). complex. Myc–Max heterodimers recognize the E-box In principle, the importance of Myc transcriptional ac- sequence CACGTG (as well as related non-canonical tivity could be settled by identification of specific target sites) and activate transcription from synthetic reporter genes. The hope was that Myc–Max might modulate ex- genes containing multimerized binding sites in mamma- pression of a small number of obviously critical genes lian cells as well as yeast (Blackwood and Eisenman whose known functions would explain the various bio- 1991; Amati et al. 1992; Kretzner et al. 1992). The effects logical effects of Myc. However, over the last eight years of Myc on cell proliferation and apoptosis are negated by numerous putative target genes have been identified and, mutations in the Myc transactivation region, or in the with the advent of global expression arrays, the number bHLHZ domain required for association with Max and that are activated or repressed by Myc continues to grow. DNA, indicating a tight linkage between the transcrip- Whereas a few of these targets are cell cycle regulators, tional and biological activities of Myc. and therefore might help explain the ability of Myc to At first glance these findings would be expected to drive proliferation, many other candidate target genes settle the issue of Myc function in favor of the idea that have functions relating to metabolism, ribosome biogen- it is a transcription factor which, as a heterodimer with esis, or translation, and do not immediately suggest a unitary mechanism for how Myc works. To make mat- ters worse, there has been controversy over the validity 1 E-MAIL [email protected]; FAX (206) 667-6522. of a number of the candidate target genes. For example, Article and publication are at http://www.genesdev.org/cgi/doi/10.1101/ gad928101. in a rat cell line in which c-myc has been deleted, the GENES & DEVELOPMENT 15:2023–2030 © 2001 by Cold Spring Harbor Laboratory Press ISSN 0890-9369/01 $5.00; www.genesdev.org 2023 Downloaded from genesdev.cshlp.org on September 24, 2021 - Published by Cold Spring Harbor Laboratory Press Eisenman introduction and activation of exogenous c-myc results large complex containing histone acetyltransferase in rescue of its slow proliferation phenotype (Mateyak et (HAT) activity. TRRAP was purified as a 400 kD protein al. 1997). However, an analysis of Myc target genes in associated with the controversial Myc Box II region of these cells indicated that only one of nine appeared de- c-Myc, mentioned above (McMahon et al. 1998). TRRAP pendent on Myc for activation (Bush et al. 1998). Given was also identified as a subunit common to several HAT- the relatively weak transcriptional activity of Myc, this containing complexes including hGCN5/PCAF and and other studies raised the possibility that many pre- Tip60/NuA4(Vassilev et al. 1998; Ikura et al. 2000). A sumed targets are either unimportant, incorrect, or indi- yeast TRRAP ortholog, Tra1p, is a component of the rect, and that activation of transcription by Myc may yeast SAGA transcription complex which also possesses likewise be irrelevant (for review, see Cole and McMa- HAT activity as well as other factors that are thought to hon 1999). influence positioning of the basal transcription machin- Although the Myc morass seemed intractable, work ery (Saleh et al. 1998). Putative dominant negative mu- from several different areas has recently converged to tants of TRRAP inhibit Myc transforming activity, indi- confirm and extend the notion that Myc, and other Max cating that interaction with TRRAP is important for at interacting proteins such as the Mad repressor (thought least this aspect of Myc biology. The implication that to be an antagonist of Myc), function through modula- TRRAP binding to Myc results in HAT recruitment has tion of target gene expression. First, both Myc and Mad recently been confirmed by coimmunoprecipitation ex- have been found in complexes which possess histone periments showing that endogenous Myc–Max het- modifying activities (for review, see Knoepfler and Eisen- erodimers are associated with hGCN5 and HAT activity man 1999; Amati et al. 2001). Second, in two papers in (McMahon et al. 2000). this issue (Bouchard et al. 2001; Frank et al. 2001) the Two other functions relating to chromatin-mediated effects of Myc and Mad on the chromatin structure of a transcriptional activity are associated with Myc. The number of target genes are shown to be consistent with TIP48/TIP49 proteins, which possess ATPase/helicase the functions of the transcription complexes which they activities, are detected in association with the Myc N- recruit. In addition, other recent work has begun to re- terminal transactivation region, and a Tip49 mutation in veal both the mechanism and importance of Myc repres- its ATPase region dominantly interferes with Myc onco- sion (Seoane et al. 2001; Staller et al. 2001). Finally, a genicity (Wood et al. 2000). As Tip48 and Tip49 require deeper understanding of the biology of Myc function has Myc Box II for interaction with Myc (Wood et al. 2000), furnished a rationale for the role and number of Myc and have also been found to be subunits of a TRRAP– target genes. Tip60 HAT complex (Ikura et al. 2000), it is plausible that TRAPP mediates binding of the TIP60 complex with Myc, although this remains to be examined in more SINful TRRAPing and TIPing of transcriptional detail. A third chromatin modifying activity reported to activities be associated with Myc is INI1 (Cheng et al. 1999), an Delineating the underlying mechanism through which ortholog of yeast SNF5, which is a subunit of the SWI/ Myc and Mad modulate transcription would bolster the SNF ATP-dependent chromatin remodeling complex argument that their transcriptional activities are an in- (Wang et al. 1996). trinsic part of their function rather than secondary ef- fects or assay artifacts. This was first shown for the Mad Myc and Mad mediate histone acetylation proteins which contain a clearly defined N-terminal re- and deacetylation pression domain that associates directly with the highly conserved corepressor Sin3 (Ayer et al. 1995; Schreiber- The association of Mad proteins with HDACs, and of Agus et al. 1995; Brubaker et al. 2000). That Sin3 in turn Myc proteins with HATs, leads to the prediction that binds the class I histone deacetylases HDAC1 and these histone modifying activities will be manifested at HDAC2 immediately suggests that Mad–Max recruits Mad and Myc binding sites in chromatin.