Endothelial c-Myc knockout enhances diet-induced liver inflammation and fibrosis.

Endothelial cells play an essential role in inflammation through synthesis and secretion of chemoattractant cytokines and expression of adhesion molecules required for inflammatory cell attachment and infiltration. The mechanisms by which endothelial cells control the pro-inflammatory response depend on the type of inflammatory stimuli, endothelial cell origin, and tissue involved. In the present study, we investigated the role of the transcription factor c-Myc in inflammation using a conditional knockout mouse model in which Myc is specifically deleted in the endothelium. At a systemic level, circulating monocytes, the chemokine CCL7, and the extracellular-matrix protein osteopontin were significantly increased in endothelial c-Myc knockout (EC-Myc KO) mice, whereas the cytokine TNFSF11 was downregulated. Using an experimental model of steatohepatitis, we investigated the involvement of endothelial c-Myc in diet-induced inflammation. EC-Myc KO animals displayed enhanced pro-inflammatory response, characterized by increased expression of pro-inflammatory cytokines and leukocyte infiltration, and worsened liver fibrosis. Transcriptome analysis identified enhanced expression of genes associated with inflammation, fibrosis, and hepatocellular carcinoma in EC-Myc KO mice relative to control (CT) animals after short-exposure to high-fat diet. Analysis of a single-cell RNA-sequencing dataset of human cirrhotic livers indicated downregulation of MYC in endothelial cells relative to healthy controls. In summary, our results suggest a protective role of endothelial c-Myc in diet-induced liver inflammation and fibrosis. Targeting c-Myc and its downstream pathways in the endothelium may constitute a potential strategy for the treatment of inflammatory disease.

MYC-mediated early glycolysis negatively regulates proinflammatory responses by controlling IRF4 in inflammatory macrophages.

MYC activates different metabolic programs in a cell-type- and cell-status-dependent manner. However, the role of MYC in inflammatory macrophages has not yet been determined. Metabolic and molecular analyses reveal that MYC, but not hypoxia inducible factor 1 (HIF1), is involved in enhancing early glycolytic flux during inflammatory macrophage polarization. Ablation of MYC decreases lactate production by regulating lactate dehydrogenase (LDH) activity and causes increased inflammatory cytokines by regulating interferon regulatory factor 4 (IRF4) in response to lipopolysaccharide. Moreover, myeloid-specific deletion of MYC and pharmacological inhibition of the MYC/LDH axis enhance inflammation and the bacterial clearance in vivo. These results elucidate the potential role of the MYC/LDH/IRF4 axis in inflammatory macrophages by connecting early glycolysis with inflammatory responses and suggest that modulating early glycolytic flux mediated by the MYC/LDH axis can be used to open avenues for the therapeutic modulation of macrophage polarization to fight against bacterial infection.

Arterialization requires the timely suppression of cell growth.

The formation of arteries is thought to occur by the induction of a highly conserved arterial genetic programme in a subset of vessels that will later experience an increase in oxygenated blood flow1,2. The initial steps of arterial specification require both the VEGF and Notch signalling pathways3-5. Here, we combine inducible genetic mosaics and transcriptomics to modulate and define the function of these signalling pathways in cell proliferation, arteriovenous differentiation and mobilization. We show that endothelial cells with high levels of VEGF or Notch signalling are intrinsically biased to mobilize and form arteries; however, they are not genetically pre-determined, and can also form veins. Mechanistically, we found that increased levels of VEGF and Notch signalling in pre-arterial capillaries suppresses MYC-dependent metabolic and cell-cycle activities, and promotes the incorporation of endothelial cells into arteries. Mosaic lineage-tracing studies showed that endothelial cells that lack the Notch-RBPJ transcriptional activator complex rarely form arteries; however, these cells regained the ability to form arteries when the function of MYC was suppressed. Thus, the development of arteries does not require the direct induction of a Notch-dependent arterial differentiation programme, but instead depends on the timely suppression of endothelial cell-cycle progression and metabolism, a process that precedes arterial mobilization and complete differentiation.

Targeted silencing of MYCL1 by RNA interference inhibits migration and invasion of MGC-803 gastric cancer cells.

MYCL1 protein expression encoded by a proto-oncogene MYCL1, a member of the MYC family, is correlated with poor prognosis in gastric cancer patients. Nevertheless, the role of MYCL1 in gastric cancer cells remains unknown. In this study, the expression levels of MYCL1 mRNA and protein were downregulated by lentiviral-mediated RNA interference (RNAi) in the MGC-803 gastric cancer cell line. Then, the influence of MYCL1 on the biological behaviour of gastric cancer cells was investigated. Finally, a stable animal model of the MGC-803 human gastric cancer tumour model in nude mice was made successfully. Functionally, silencing of MYCL1 inhibited migration and invasion of the MGC-803 line in vitro and was accompanied with some ultrastructural changes. These results provide some evidences that lentiviral-mediated MYCL1 silencing may be a novel therapeutic strategy for the treatment of gastric cancer. SIGNIFICANCE OF THE STUDY: Gastric cancer is one of the most common malignant tumours worldwide and the second leading cause of cancer-related death in China. Our previous study revealed that expression of MYCL1 in gastric cancer tissue was associated with poor prognosis of patients. However, the potential underlying mechanism is still unclear. In the current study, we displayed the influence of MYCL1 gene on invasion and migration phenotype of gastric cancer cells and provided a possible explanation from the aspect of structural alteration. Our results suggested that downregulation of MYCL1 may be a potential therapeutic strategy for gastric cancer.

Oncogenic zinc finger protein ZNF322A promotes stem cell-like properties in lung cancer through transcriptional suppression of c-Myc expression.

ZNF322A, a C2H2 zinc finger transcription factor, is an oncoprotein in lung cancer. However, the transcription mechanisms of ZNF322A in lung cancer stem cell-like reprogramming remain elusive. By integrating our chromatin immunoprecipitation-sequencing and RNA-sequencing datasets, we identified and validated the transcriptional targets of ZNF322A, which were significantly enriched in tumorigenic functions and developmental processes. Indeed, overexpression of ZNF322A promoted self-renewal ability and increased stemness-related gene expressions in vitro and in vivo. Importantly, ZNF322A bound directly to c-Myc promoter and recruited histone deacetylase 3 to transcriptionally suppress c-Myc expression, which in turn increased mitochondrial oxidative phosphorylation and promoted cell motility, thus maintaining stem cell-like properties of lung cancer. Clinically, ZNF322AHigh/c-MycLow expression profile was revealed as an independent indicator of poor prognosis in lung cancer patients. Our study provides the first evidence that ZNF322A-centered transcriptome promotes lung tumorigenesis and ZNF322A acts as a transcription suppressor of c-Myc to maintain lung cancer stem cell-like properties by shifting metabolism towards oxidative phosphorylation.

Dual targeting of p53 and c-MYC selectively eliminates leukaemic stem cells.

Chronic myeloid leukaemia (CML) arises after transformation of a haemopoietic stem cell (HSC) by the protein-tyrosine kinase BCR-ABL. Direct inhibition of BCR-ABL kinase has revolutionized disease management, but fails to eradicate leukaemic stem cells (LSCs), which maintain CML. LSCs are independent of BCR-ABL for survival, providing a rationale for identifying and targeting kinase-independent pathways. Here we show--using proteomics, transcriptomics and network analyses--that in human LSCs, aberrantly expressed proteins, in both imatinib-responder and non-responder patients, are modulated in concert with p53 (also known as TP53) and c-MYC regulation. Perturbation of both p53 and c-MYC, and not BCR-ABL itself, leads to synergistic cell kill, differentiation, and near elimination of transplantable human LSCs in mice, while sparing normal HSCs. This unbiased systems approach targeting connected nodes exemplifies a novel precision medicine strategy providing evidence that LSCs can be eradicated.

c-Myc Sustains Transformed Phenotype and Promotes Radioresistance of Embryonal Rhabdomyosarcoma Cell Lines.

We have previously reported that the MEK/ERK pathway sustains in vitro and in vivo transformed phenotype and radioresistance of embryonal rhabdomyosarcoma (ERMS) cell lines. Furthermore, we found that aberrant MEK/ERK signaling activation promotes c-Myc oncoprotein accumulation. In this study, the role of c-Myc in sustaining the ERMS transformed and radioresistant phenotype is characterized. RD and TE671 cell lines conditionally expressing MadMyc chimera protein, c-Myc-dominant negative and shRNA directed to c-Myc were used. Targeting c-Myc counteracted in vitro ERMS adherence and in suspension, growth motility and the expression of pro-angiogenic factors. c-Myc depletion decreased MMP-9, MMP-2, u-PA gelatinolytic activity, neural cell adhesion molecule sialylation status, HIF-1α, VEGF and increased TSP-1 protein expression levels. Rapid but not sustained targeting c-Myc radiosensitized ERMS cells by radiation-induced apoptosis, DNA damage and impairing the expression of DNA repair proteins RAD51 and DNA-PKcs, thereby silencing affected ERMS radioresistance. c-Myc sustains ERMS transformed phenotype and radioresistance by protecting cancer cells from radiation-induced apoptosis and DNA damage, while promoting radiation-induced DNA repair. This data suggest that c-Myc targeting can be tested as a promising treatment in cancer therapy.

Management of Patients with MYC-Altered Lymphomas.

Patients diagnosed with non-Burkitt high-grade B cell non-Hodgkin lymphomas demonstrating rearrangement in MYC, an oncogene promoting cellular proliferation, frequently do not achieve long-term disease-free survival due to a suboptimal response to standard front-line and salvage therapies. Double-hit lymphomas, harboring rearrangements in MYC as well as BCL2 and/or BCL6, appear to carry a particularly poor prognosis, although patients with this disease appear to achieve better survival outcomes when treated with intensified chemotherapy. Increased expression of MYC protein by immunohistochemistry as well as increased copy number or amplification of MYC may also be adverse pathologic features of non-Burkitt high-grade B cell non-Hodgkin lymphomas, although the benefit of treating these patients with intensified as opposed to standard dose chemotherapy remains unclear. Recognition and proper management of patients with MYC-altered lymphomas is crucial to improving patient outcomes.

Transcription-associated processes cause DNA double-strand breaks and translocations in neural stem/progenitor cells.

High-throughput, genome-wide translocation sequencing (HTGTS) studies of activated B cells have revealed that DNA double-strand breaks (DSBs) capable of translocating to defined bait DSBs are enriched around the transcription start sites (TSSs) of active genes. We used the HTGTS approach to investigate whether a similar phenomenon occurs in primary neural stem/progenitor cells (NSPCs). We report that breakpoint junctions indeed are enriched around TSSs that were determined to be active by global run-on sequencing analyses of NSPCs. Comparative analyses of transcription profiles in NSPCs and B cells revealed that the great majority of TSS-proximal junctions occurred in genes commonly expressed in both cell types, possibly because this common set has higher transcription levels on average than genes transcribed in only one or the other cell type. In the latter context, among all actively transcribed genes containing translocation junctions in NSPCs, those with junctions located within 2 kb of the TSS show a significantly higher transcription rate on average than genes with junctions in the gene body located at distances greater than 2 kb from the TSS. Finally, analysis of repair junction signatures of TSS-associated translocations in wild-type versus classical nonhomologous end-joining (C-NHEJ)-deficient NSPCs reveals that both C-NHEJ and alternative end-joining pathways can generate translocations by joining TSS-proximal DSBs to DSBs on other chromosomes. Our studies show that the generation of transcription-associated DSBs is conserved across divergent cell types.

FOXO1 couples metabolic activity and growth state in the vascular endothelium.

Endothelial cells (ECs) are plastic cells that can switch between growth states with different bioenergetic and biosynthetic requirements. Although quiescent in most healthy tissues, ECs divide and migrate rapidly upon proangiogenic stimulation. Adjusting endothelial metabolism to the growth state is central to normal vessel growth and function, yet it is poorly understood at the molecular level. Here we report that the forkhead box O (FOXO) transcription factor FOXO1 is an essential regulator of vascular growth that couples metabolic and proliferative activities in ECs. Endothelial-restricted deletion of FOXO1 in mice induces a profound increase in EC proliferation that interferes with coordinated sprouting, thereby causing hyperplasia and vessel enlargement. Conversely, forced expression of FOXO1 restricts vascular expansion and leads to vessel thinning and hypobranching. We find that FOXO1 acts as a gatekeeper of endothelial quiescence, which decelerates metabolic activity by reducing glycolysis and mitochondrial respiration. Mechanistically, FOXO1 suppresses signalling by MYC (also known as c-MYC), a powerful driver of anabolic metabolism and growth. MYC ablation impairs glycolysis, mitochondrial function and proliferation of ECs while its EC-specific overexpression fuels these processes. Moreover, restoration of MYC signalling in FOXO1-overexpressing endothelium normalizes metabolic activity and branching behaviour. Our findings identify FOXO1 as a critical rheostat of vascular expansion and define the FOXO1-MYC transcriptional network as a novel metabolic checkpoint during endothelial growth and proliferation.

Regulation of c-Myc expression by the histone demethylase JMJD1A is essential for prostate cancer cell growth and survival.

The histone demethylase JMJD1A, which controls gene expression by epigenetic regulation of H3K9 methylation marks, functions in diverse activities, including spermatogenesis, metabolism and stem cell self-renewal and differentiation. Here, we found that JMJD1A knockdown in prostate cancer cells antagonizes their proliferation and survival. Profiling array analyses revealed that JMJD1A-dependent genes function in cellular growth, proliferation and survival, and implicated that the c-Myc transcriptional network is deregulated following JMJD1A inhibition. Biochemical analyses confirmed that JMJD1A enhances c-Myc transcriptional activity by upregulating c-Myc expression levels. Mechanistically, JMJD1A activity promoted recruitment of androgen receptor (AR) to the c-Myc gene enhancer and induced H3K9 demethylation, increasing AR-dependent transcription of c-Myc mRNA. In parallel, we found that JMJD1A regulated c-Myc stability, likely by inhibiting HUWE1, an E3 ubiquitin ligase known to target degradation of several substrates including c-Myc. JMJD1A (wild type or mutant lacking histone demethylase activity) bound to HUWE1, attenuated HUWE1-dependent ubiquitination and subsequent degradation of c-Myc, increasing c-Myc protein levels. Furthermore, c-Myc knockdown in prostate cancer cells phenocopied effects of JMJD1A knockdown, and c-Myc re-expression in JMJD1A-knockdown cells partially rescued prostate cancer cell growth in vitro and in vivo. c-Myc protein levels were positively correlated with those of JMJD1A in a subset of human prostate cancer specimens. Collectively, our findings identify a critical role for JMJD1A in regulating proliferation and survival of prostate cancer cells by controlling c-Myc expression at transcriptional and post-translational levels.

N-myc is a key switch regulating the proliferation cycle of postnatal cerebellar granule cell progenitors.

N-myc plays an important role in early cerebellar development; however, the role of N-myc in postnatal cerebellar development is still unknown. In this study, inducible and reversible N-myc mouse models (Nmyc(TRE/TRE):tTS and Nmyc(EGFP/TRE):tTS) are used to regulate and track the expression of endogenous N-myc in vivo. Loss of N-myc at the neonatal stage results in reduced proliferation of granule cell precursors (GCPs) and reduced cerebellar volume/mass. Restoration of N-myc expression no later than postnatal day 4 can rescue the cerebellar developmental defect caused by the absence of N-myc after birth. During cerebellar postnatal development, N-myc acts as a key switch, regulating the proliferation cycle of postnatal granule cell progenitors. Loss of N-myc significantly impairs the Sonic hedgehog signalling pathway, and disrupts the expression of cell cycle effectors with a significant reduction of Ccnd2. More importantly, N-myc negatively regulates the expression of microRNA-9 during postnatal cerebellar development. Our findings demonstrate that over-expression of miR-9 can inhibit the proliferation of GCPs. The regulation of these factors by N-myc is at least partly responsible for the switch role of N-myc in the proliferation cycle of GCPs.

Redeployment of Myc and E2f1-3 drives Rb-deficient cell cycles.

Robust mechanisms to control cell proliferation have evolved to maintain the integrity of organ architecture. Here, we investigated how two critical proliferative pathways, Myc and E2f, are integrated to control cell cycles in normal and Rb-deficient cells using a murine intestinal model. We show that Myc and E2f1-3 have little impact on normal G1-S transitions. Instead, they synergistically control an S-G2 transcriptional program required for normal cell divisions and maintaining crypt-villus integrity. Surprisingly, Rb deficiency results in the Myc-dependent accumulation of E2f3 protein and chromatin repositioning of both Myc and E2f3, leading to the 'super activation' of a G1-S transcriptional program, ectopic S phase entry and rampant cell proliferation. These findings reveal that Rb-deficient cells hijack and redeploy Myc and E2f3 from an S-G2 program essential for normal cell cycles to a G1-S program that re-engages ectopic cell cycles, exposing an unanticipated addiction of Rb-null cells on Myc.

Structurally diverse c-Myc inhibitors share a common mechanism of action involving ATP depletion.

The c-Myc (Myc) oncoprotein is deregulated in a large proportion of diverse human cancers. Considerable effort has therefore been directed at identifying pharmacologic inhibitors as potential anti-neoplastic agents. Three such groups of small molecule inhibitors have been described. The first is comprised of so-called "direct" inhibitors, which perturb Myc's ability to form productive DNA-binding heterodimers in association with its partner, Max. The second group is comprised of indirect inhibitors, which largely function by targeting the BET-domain protein BRD4 to prevent the proper formation of transcriptional complexes that assemble in response to Myc-Max DNA binding. Thirdly, synthetic lethal inhibitors cause the selective apoptosis of Myc over-expressing either by promoting mitotic catastrophe or altering Myc protein stability. We report here a common mechanism by which all Myc inhibitors, irrespective of class, lead to eventual cellular demise. This involves the depletion of ATP stores due to mitochondrial dysfunction and the eventual down-regulation of Myc protein. The accompanying metabolic de-regulation causes neutral lipid accumulation, cell cycle arrest, and an attempt to rectify the ATP deficit by up-regulating AMP-activated protein kinase (AMPK). These responses are ultimately futile due to the lack of functional Myc to support the requisite anabolic response. Finally, the effects of Myc depletion on ATP levels, cell cycle arrest, differentiation and AMPK activation can be mimicked by pharmacologic inhibition of the mitochondrial electron transport chain without affecting Myc levels. Thus, all Myc inhibitors promote a global energy collapse that appears to underlie many of their phenotypic consequences.

Direct and indirect targeting of MYC to treat acute myeloid leukemia.

PURPOSE: Acute myeloid leukemia (AML) is the most common acute leukemia in adults and is often resistant to conventional therapies. The MYC oncogene is commonly overexpressed in AML but has remained an elusive target. We aimed to examine the consequences of targeting MYC both directly and indirectly in AML overexpressing MYC/Myc due to trisomy 8/15 (human/mouse), FLT3-ITD mutation, or gene amplification. METHODS: We performed in vivo knockdown of Myc (shRNAs) and both in vitro and in vivo experiments using four drugs with indirect anti-MYC activity: VX-680, GDC-0941, artemisinin, and JQ1. RESULTS: shRNA knockdown of Myc in mice prolonged survival, regardless of the mechanism underlying MYC overexpression. VX-680, an aurora kinase inhibitor, demonstrated in vitro efficacy against human MYC-overexpressing AMLs regardless of the mechanism of MYC overexpression, but was weakest against a MYC-amplified cell line. GDC-0941, a PI3-kinase inhibitor, demonstrated efficacy against several MYC-overexpressing AMLs, although only in vitro. Artemisinin, an antimalarial, did not demonstrate consistent efficacy against any of the human AMLs tested. JQ1, a bromodomain and extra-terminal bromodomain inhibitor, demonstrated both in vitro and in vivo efficacy against several MYC-overexpressing AMLs. We also confirmed a decrease in MYC levels at growth inhibitory doses for JQ1, and importantly, sensitivity of AML cell lines to JQ1 appeared independent of the mechanism of MYC overexpression. CONCLUSIONS: Our data support growing evidence that JQ1 and related compounds may have clinical efficacy in AML treatment regardless of the genetic abnormalities underlying MYC deregulation.

Nitric oxide prevents aortic neointimal hyperplasia by controlling macrophage polarization.

OBJECTIVE: Nitric oxide synthase 3 (NOS3) prevents neointima hyperplasia by still unknown mechanisms. To demonstrate the significance of endothelial nitric oxide in the polarization of infiltrated macrophages through the expression of matrix metalloproteinase (MMP)-13 in neointima formation. APPROACH AND RESULTS: After aortic endothelial denudation, NOS3 null mice show elevated neointima formation, detecting increased mobilization of LSK (lineage-negative [Lin]-stem-cell antigen 1 [SCA1]+KIT+) progenitor cells, and high ratios of M1 (proinflammatory) to M2 (resolving) macrophages, accompanied by high expression of interleukin-5, interleukin-6, MCP-1 (monocyte chemoattractant protein), VEGF (vascular endothelial growth factor), GM-CSF (granulocyte-macrophage colony stimulating factor), interleukin-1ß, and interferon-γ. In conditional c-Myc knockout mice, in which M2 polarization is defective, denuded aortas showed extensive wall thickening as well. Conditioned medium from NOS3-deficient endothelium induced extensive repolarization of M2 macrophages to an M1 phenotype, and vascular smooth muscle cells proliferated and migrated faster in conditioned medium from M1 macrophages. Among the different proteins participating in cell migration, MMP-13 was preferentially expressed by M1 macrophages. M1-mediated vascular smooth muscle cell migration was inhibited when macrophages were isolated from MMP-13-deficient mice, whereas exogenous administration of MMP-13 to vascular smooth muscle cell fully restored migration. Excess vessel wall thickening in mice lacking NOS3 was partially reversed by simultaneous deletion of MMP-13, indicating that NOS3 prevents neointimal hyperplasia by preventing MMP-13 activity. An excess of M1-polarized macrophages that coexpress MMP-13 was also detected in human carotid samples from endarterectomized patients. CONCLUSIONS: These findings indicate that at least M1 macrophage-mediated expression of MMP-13 in NOS3 null mice induces neointima formation after vascular injury, suggesting that MMP-13 may represent a new promising target in vascular disease.

L-Myc expression by dendritic cells is required for optimal T-cell priming.

The transcription factors c-Myc and N-Myc--encoded by Myc and Mycn, respectively--regulate cellular growth and are required for embryonic development. A third paralogue, Mycl1, is dispensable for normal embryonic development but its biological function has remained unclear. To examine the in vivo function of Mycl1 in mice, we generated an inactivating Mycl1(gfp) allele that also reports Mycl1 expression. We find that Mycl1 is selectively expressed in dendritic cells (DCs) of the immune system and controlled by IRF8, and that during DC development, Mycl1 expression is initiated in the common DC progenitor concurrent with reduction in c-Myc expression. Mature DCs lack expression of c-Myc and N-Myc but maintain L-Myc expression even in the presence of inflammatory signals such as granulocyte-macrophage colony-stimulating factor. All DC subsets develop in Mycl1-deficient mice, but some subsets such as migratory CD103(+) conventional DCs in the lung and liver are greatly reduced at steady state. Importantly, loss of L-Myc by DCs causes a significant decrease in in vivo T-cell priming during infection by Listeria monocytogenes and vesicular stomatitis virus. The replacement of c-Myc by L-Myc in immature DCs may provide for Myc transcriptional activity in the setting of inflammation that is required for optimal T-cell priming.

c-Myc regulates cell proliferation during lens development.

Myc protooncogenes play important roles in the regulation of cell proliferation, growth, differentiation and survival during development. In various developing organs, c-myc has been shown to control the expression of cell cycle regulators and its misregulated expression is detected in many human tumors. Here, we show that c-myc gene (Myc) is highly expressed in developing mouse lens. Targeted deletion of c-myc gene from head surface ectoderm dramatically impaired ocular organogenesis, resulting in severe microphtalmia, defective anterior segment development, formation of a lens stalk and/or aphakia. In particular, lenses lacking c-myc presented thinner epithelial cell layer and growth impairment that was detectable soon after its inactivation. Defective development of c-myc-null lens was not caused by increased cell death of lens progenitor cells. Instead, c-myc loss reduced cell proliferation, what was associated with an ectopic expression of Prox1 and p27(Kip1) proteins within epithelial cells. Interestingly, a sharp decrease in the expression of the forkhead box transcription factor Foxe3 was also observed following c-myc inactivation. These data represent the first description of the physiological roles played by a Myc family member in mouse lens development. Our findings support the conclusion that c-myc regulates the proliferation of lens epithelial cells in vivo and may, directly or indirectly, modulate the expression of classical cell cycle regulators in developing mouse lens.

Progressive effects of N-myc deficiency on proliferation, neurogenesis, and morphogenesis in the olfactory epithelium.

N-myc belongs to the myc proto-oncogene family, which is involved in numerous cellular processes such as proliferation, growth, apoptosis, and differentiation. Conditional deletion of N-myc in the mouse nervous system disrupted brain development, indicating that N-myc plays an essential role during neural development. How the development of the olfactory epithelium and neurogenesis within are affected by the loss of N-myc has, however, not been determined. To address these issues, we examined an N-myc(Foxg1Cre) conditional mouse line, in which N-myc is depleted in the olfactory epithelium. First changes in N-myc mutants were detected at E11.5, with reduced proliferation and neurogenesis in a slightly smaller olfactory epithelium. The phenotype was more pronounced at E13.5, with a complete lack of Hes5-positive progenitor cells, decreased proliferation, and neurogenesis. In addition, stereological analyses revealed reduced cell size of post-mitotic neurons in the olfactory epithelium, which contributed to a smaller olfactory pit. Furthermore, we observed diminished proliferation and neurogenesis also in the vomeronasal organ, which likewise was reduced in size. In addition, the generation of gonadotropin-releasing hormone neurons was severely reduced in N-myc mutants. Thus, diminished neurogenesis and proliferation in combination with smaller neurons might explain the morphological defects in the N-myc depleted olfactory structures. Moreover, our results suggest an important role for N-myc in regulating ongoing neurogenesis, in part by maintaining the Hes5-positive progenitor pool. In summary, our results provide evidence that N-myc deficiency in the olfactory epithelium progressively diminishes proliferation and neurogenesis with negative consequences at structural and cellular levels.