To address the role of N-myc in neurogenesis and in nervous system tumors, it was conditionally disrupted in neuronal progenitor cells (NPCs) with a nestin-Cre transgene. Null mice display ataxia, behavioral abnormalities, and tremors that correlate with a twofold decrease in brain mass that disproportionately affects the cerebellum (sixfold reduced in mass) and the cerebral cortex, both of which show signs of disorganization. In control mice at E12.5, we observe a domain of high N-Myc protein expression in the rapidly proliferating cerebellar primordium. Targeted deletion of N-myc results in severely compromised proliferation as shown by a striking decrease in S phase and mitotic cells as well as in cells expressing the Myc target gene cyclin D2, whereas apoptosis is unaffected. Null progenitor cells also have comparatively high levels of the cdk inhibitors p27 Kip1 and p18 Ink4c , whereas p15 Ink4b , p21 Cip1 , and p19 Ink4d levels are unaffected. Many null progenitors also exhibit altered nuclear morphology and size. In addition, loss of N-myc disrupts neuronal differentiation as evidenced by ectopic staining of the neuron specific marker ␤TUBIII in the cerebrum. Furthermore, in progenitor cell cultures derived from null embryonic brain, we observe a dramatic increase in neuronal differentiation compared with controls. Thus, N-myc is essential for normal neurogenesis, regulating NPC proliferation, differentiation, and nuclear size. Its effects on proliferation and differentiation appear due, at least in part, to down-regulation of a specific subset of cyclin-dependent kinase inhibitors.

Skeletal muscle differentiation is initiated by the transcription factor MyoD, which binds directly to the regulatory regions of genes expressed during skeletal muscle differentiation and initiates chromatin remodeling at specific promoters. It is not known, however, how MyoD initially recognizes its binding site in a chromatin context. Here we show that the H/C and helix III domains, two domains of MyoD that are necessary for the initiation of chromatin remodeling at the myogenin locus, together regulate a restricted subset of genes, including myogenin. These domains are necessary for the stable binding of MyoD to the myogenin promoter through an interaction with an adjacent protein complex containing the homeodomain protein Pbx, which appears to be constitutively bound at this site. This demonstrates a specific mechanism of targeting MyoD to loci in inactive chromatin and reveals a critical role of homeodomain proteins in marking specific genes for activation in the muscle lineage.

The family of myc proto-oncogenes encodes transcription factors (c-, N-, and L-Myc) that regulate cell growth and proliferation and are involved in the etiology of diverse cancers. Myc proteins are thought to function by binding and regulating specific target genes. Here we report that Myc proteins are required for the widespread maintenance of active chromatin. Disruption of N-myc in neuronal progenitors and other cell types leads to nuclear condensation accompanied by large-scale changes in histone modifications associated with chromatin inactivation, including hypoacetylation and altered methylation. These effects are largely reversed by exogenous Myc as well as by differentiation and are mimicked by the Myc antagonist Mad1. The first chromatin changes are evident within 6 h of Myc loss and lead to changes in chromatin structure. Myc widely influences chromatin in part through upregulation of the histone acetyltransferase GCN5. This study provides the first evidence for regulation of global chromatin structure by an oncoprotein and may explain the broad effects of Myc on cell behavior and tumorigenesis.

Many of the earliest stem cell studies were conducted on cells isolated from tumors rather than from embryos. Of particular interest was research on embryonic carcinoma cells (EC), a type of stem cell derived from teratocarcinoma. The EC research laid the foundation for the later discovery of and subsequent work on embryonic stem cells (ESC). Both ESC isolated from the mouse (mESC) and then later from humans (hESC) shared not only pluripotency with their EC cousins, but also robust tumorigenicity as each readily form teratoma. Surprisingly, decades after the discovery of mESC, the question of what drives ESC to form tumors remains largely an open one. This gap in the field is particularly serious as stem cell tumorigenicity represents the key obstacle to the safe use of stem cell-based regenerative medicine therapies. Although some adult stem cell therapies appear to be safe, they have only a very narrow range of uses in human disease. Our understanding of the tumorigenicity of human induced pluripotent stem cells (IPSC), perhaps the most promising modality for future patient-specific regenerative medicine therapies, is rudimentary. However, IPSC are predicted to possess tumorigenic potential equal to or greater than that of ESC. Here, the links between pluripotency and tumorigenicity are explored. New methods for more accurately testing the tumorigenic potential of IPSC and of other stem cells applicable to regenerative medicine are proposed. Finally, the most promising emerging approaches for overcoming the challenges of stem cell tumorigenicity are highlighted.