Germinal centers (GC) are sites of intense B cell proliferation, central for T cell dependent antibody responses. However, the role of MYC, a key cell cycle regulator, in this process has been questioned. Here, we identified MYC positive B cell subpopulations in immature and mature GCs, and show through genetic ablation of Myc that they play indispensable roles in GC formation and maintenance. The identification of these functionally critical cellular subsets has important implications for human B cell lymphomagenesis, which mostly originates from GC B cells and frequently involves MYC chromosomal translocations. As these translocations are generally dependent on transcription of the recombining partner loci, the MYC positive GC subpopulations may be at a particularly high risk for malignant transformation.

Germline inactivation of c-myc in mice causes embryonic lethality. Therefore, we developed a LoxP/Cre-based conditional mutation approach to test the role of c-myc in mouse embryonic fibroblasts (MEFs) and mature B lymphocytes. Cre expression resulted in reduced proliferation of wild-type MEFs, but c-Myc-deficient MEFs showed a further reduction. In contrast to fibroblasts, Cre expression had no apparent affect on wild-type B cell proliferation. Deletion of both c-Myc genes in B cells led to severely impaired proliferation in response to anti-CD40 plus IL-4. However, treated cells did upregulate several early activation markers but not CD95 or CD95 ligand. We discuss these findings with respect to potential c-Myc functions in proliferation and apoptosis and also discuss potential limitations in the Cre-mediated gene inactivation approach.

Inhibition of the mutationally activated Wnt cascade in colorectal cancer cell lines induces a rapid G1 arrest and subsequent differentiation. This arrest can be overcome by maintaining expression of a single Tcf4 target gene, the proto-oncogene c-Myc. Since colorectal cancer cells share many molecular characteristics with proliferative crypt progenitors, we have assessed the physiological role of c-Myc in adult crypts by conditional gene deletion. c-Myc-deficient crypts are lost within weeks and replaced by c-Myc-proficient crypts through a fission process of crypts that have escaped gene deletion. Although c-Myc − / − crypt cells remain in the cell cycle, they are on average much smaller than wild-type cells, cycle slower, and divide at a smaller cell size. c-Myc appears essential for crypt progenitor cells to provide the necessary biosynthetic capacity to successfully progress through the cell cycle.

Cell proliferation is strictly controlled during differentiation. In T cell development, the cell cycle is normally arrested at the CD4+CD8+ stage, but the mechanism underlying such differentiation-specific exit from the cell cycle has been unclear. Fbxw7 (also known as Fbw7, Sel-10, hCdc4, or hAgo), an F-box protein subunit of an SCF-type ubiquitin ligase complex, induces the degradation of positive regulators of the cell cycle, such as c-Myc, c-Jun, cyclin E, and Notch. FBXW7 is often mutated in a subset of human cancers. We have now achieved conditional inactivation of Fbxw7 in the T cell lineage of mice and found that the cell cycle is not arrested at the CD4+CD8+ stage in the homozygous mutant animals. The mutant mice manifested thymic hyperplasia as a result of c-Myc accumulation and eventually developed thymic lymphoma. In contrast, mature T cells of the mutant mice failed to proliferate in response to mitogenic stimulation and underwent apoptosis in association with accumulation of c-Myc and p53. These latter abnormalities were corrected by deletion of p53. Our results suggest that Fbxw7 regulates the cell cycle in a differentiation-dependent manner, with its loss resulting in c-Myc accumulation that leads to hyperproliferation in immature T cells but to p53-dependent cell-cycle arrest and apoptosis in mature T cells.

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We examined the genetic requirements for the Myc family of oncogenes in normal Sonic hedgehog (Shh)-mediated cerebellar granule neuronal precursor (GNP) expansion and in Shh pathway-induced medulloblastoma formation. In GNPenriched cultures derived from N-myc Fl/Fl and c-myc Fl/Fl mice, disruption of N-myc, but not c-myc, inhibited the proliferative response to Shh. Conditional deletion of c-myc revealed that, although it is necessary for the general regulation of brain growth, it is less important for cerebellar development and GNP expansion than N-myc. In vivo analysis of compound mutants carrying the conditional N-myc null and the activated Smoothened (ND2:SmoA1) alleles showed, that although granule cells expressing the ND2:SmoA1 transgene are present in the N-myc null cerebellum, no hyperproliferation or tumor formation was detected. Taken together, these findings provide in vivo evidence that N-myc acts downstream of Shh/Smo signaling during GNP proliferation and that N-myc is required for medulloblastoma genesis even in the presence of constitutively active signaling from the Shh pathway. (Cancer Res 2006; 66(17): 8655-61)

c‐Myc (Myc) is highly expressed in developing embryos where it regulates body size by controlling proliferation but not cell size. However, Myc is also induced in many postmitotic tissues, including adult myocardium, in response to stress where the predominant form of growth is an increase in cell size (hypertrophy) and not number. The function of Myc induction in this setting is unproven. Therefore, to explore Myc's role in hypertrophic growth, we created mice where Myc can be inducibly inactivated, specifically in adult myocardium. Myc‐deficient hearts demonstrated attenuated stress‐induced hypertrophic growth, secondary to a reduction in cell growth of individual myocytes. To explore the dependence of Myc‐induced cell growth on CycD2, we created bigenic mice where Myc can be selectively activated in CycD2‐null adult myocardium. Myc‐dependent hypertrophic growth and cell cycle reentry is blocked in CycD2‐deficient hearts. However, in contrast to Myc‐induced DNA synthesis, hypertrophic growth is independent of CycD2‐induced Cdk2 activity. These data suggest that Myc is required for a normal hypertrophic response and that its growth‐promoting effects are also mediated through a CycD2‐dependent pathway.

The transcription factor Myc (c-Myc) plays an important role in cell growth and cell death, yet its physiological function remains unclear. Ectopic activation of Myc has been recently suggested to regulate cell mass, and Drosophila dmyc controls cellular growth and size independently of cell division. By contrast, it has been proposed that in mammals Myc controls cell division and cell number. To gain insights into this debate we have specifically knocked out Myc in epidermis. Myc epidermal knockout mice are viable and their keratinocytes continue to cycle, but they display severe skin defects. The skin is tight and fragile, tears off in areas of mechanical friction and displays impaired wound healing. Steady-state epidermis is thinner, with loss of the proliferative compartment and premature differentiation. Remarkably, keratinocyte cell size, growth and endoreplication are reduced, and stem cell amplification is compromised. The results provide new and direct evidence for a role for endogenous Myc in cellular growth that is required for hyperproliferative cycles and tissue homeostasis.