Myc proteins belong to the basic helix-loop-helix zipper (bHLHz) superfamily of DNA binding proteins that dimerize on CANNTG E-box sequences to function as transcription factors (TF). Myc forms dimers with Max on the CACGTG sequence that activate transcription (1
), while Myc also has a repressive function as well through association with Miz-1 (4
). Myc proteins regulate many aspects of embryonic development as well as normal cell biology including cell cycle, metabolism, differentiation, senescence, apoptosis, and DNA replication (reviewed in (5
)). Deregulation of Myc genes, particularly N-Myc, has been strongly linked with many human neural cancers including neuroblastoma and medulloblastoma (6
). In medulloblastoma, Myc can cooperate with REST to drive tumorigenesis by blocking differentiation (9
), which may be a key function in normal stem cells as well. Given the roles of both Myc and REST in ESC biology (10
), they may also cooperate in that context. N-Myc amplification is known to correlate with poor prognosis in neuroblastoma (11
), but the molecular mechanism by which N-Myc contributes to tumorigenesis is still largely an open question. Myc proteins are atypical bHLHz factors in that they are relatively weak transcriptional activators of potentially thousands of genes (12
). This weak, but apparently widespread transcriptional function is a long-standing puzzle.
Intense interest in Myc function has been stimulated more recently by studies linking both c- and N-Myc to the generation of induced pluripotent stem (iPS) cells (reviewed in (13
)) as well as to murine embryonic stem cell (mESC) biology (15
), together suggesting that Myc has important roles in regulating stem cell self-renewal and pluripotency. The molecular mechanisms by which Myc influences iPS cell formation and ESC biology are open questions. One possibility is that Myc contributes to the process through global chromatin reprogramming, suggested by studies (16
) implicating Myc in regulation of widespread histone modifications. Consistent with this idea, a very large Myc-regulated transcriptional program is operative in ESC that may have significance for Myc’s normal and neoplastic functions (18
) as well as iPS formation, but how Myc regulates this program at the chromatin level and its impact on cell biology remain unknown.
In neural stem cells, loss of N-Myc is sufficient to cause nuclear condensation most likely due to a global spread of heterochromatin (20
). Myc’s recruitment of histone acetyltransferases such as GCN5 (21
) and TIP60 (22
) as well as its regulation of histone acetylation at a number of genic loci (17
), suggests regulation of euchromatin through histone acetylation is involved. Unlike most TF, Myc binds to tens of thousands of genomic sites, both near and far from core promoters (24
) indicating the euchromatic program may be quite expansive. Also at least at genes Myc may directly impact transcription via binding to PTEF-b (30
). Additional evidence suggests the Myc regulated chromatin program involves both histone acetylation but also methylation of lysine 4 of histone H3 (triMeK4) (16
) possibly through the demethylase LID (32
Since Myc’s normal and neoplastic functions depend on its ability to bind DNA and likely in turn to influence chromatin, the specific nature of its activity on cellular chromatin is a critically important open question. To address this key gap we conducted a functional genomics study in which we mapped Myc binding and chromatin function in a global, unbiased manner in the human genome using a conditional N-Myc transgene in neuroblastoma cells, finding that Myc has a global role in euchromatin maintenance. These findings support a new model in which Myc proteins act not only as classical TF but also more broadly to maintain widespread euchromatin.