We show here that inducible expression of Braf(V600E) off the endogenous Braf gene in mouse melanocytes stimulates skin hyperpigmentation and the appearance of nevi harboring senescent melanocytes. Additionally, approximately 70% of Braf(V600E) mice develop melanomas that reproduce many of the cardinal histological and molecular features of human melanoma and whose cells can colonize the lungs of nude mice. We show that the tumor suppressor p16(INK4a) is not required to induce melanocyte senescence and that its loss is not required for tumor progression, although it does regulate tumor penetrance and latency. Thus, we have developed a mouse model of melanoma driven by Braf(V600E) expressed at physiological levels that reflects the genetics and pathology of the human disease.

Tumor progression is a multistep process in which proproliferation mutations must be accompanied by suppression of senescence. In melanoma, proproliferative signals are provided by activating mutations in NRAS and BRAF, whereas senescence is bypassed by inactivation of the p16 Ink4a gene. Melanomas also frequently exhibit constitutive activation of the Wnt/␤-catenin pathway that is presumed to induce proliferation, as it does in carcinomas. We show here that, contrary to expectations, stabilized ␤-catenin reduces the number of melanoblasts in vivo and immortalizes primary skin melanocytes by silencing the p16 Ink4a promoter. Significantly, in a novel mouse model for melanoma, stabilized ␤-catenin bypasses the requirement for p16 Ink4a mutations and, together with an activated N-Ras oncogene, leads to melanoma with high penetrance and short latency. The results reveal that synergy between the Wnt and mitogen-activated protein (MAP) kinase pathways may represent an important mechanism underpinning the genesis of melanoma, a highly aggressive and increasingly common disease.[Keywords: Mitf; Wnt; senescence; development; tumor suppressor; oncogene] Supplemental material is available at http://www.genesdev.org.

The Ca(++)-dependent cell adhesion molecule E-cadherin is expressed throughout mouse development and in adult tissues. Classical gene targeting has demonstrated that E-cadherin-deficient embryos die at the blastocyst stage. To study the involvement of E-cadherin in organogenesis, a conditional gene inactivation scheme was undertaken using the bacteriophage P1 recombinase Cre/loxP system. Mice with homozygous loxP sites in both alleles of the E-cadherin (Cdh1) gene were generated and these mice were crossed with transgenic mice with the Cre recombinase under the control of the hormone-inducible MMTV promoter. This resulted in deletion of the E-cadherin gene in the differentiating alveolar epithelial cells of the mammary gland. The mutant mammary gland developed normally up to 16-18 days of pregnancy but exhibited a dramatic phenotype around parturition. The production of milk proteins was so drastically reduced that adult mutant mothers could not suckle their offspring. Thus, the lack of E-cadherin affected the terminal differentiation program of the lactating mammary gland. In concordance with this finding, the prolactin-dependent activation of the transcription factor Stat5a was initiated but not maintained in the mutant gland. Instead, without E-cadherin massive cell death was observed at parturition and the mutant mammary gland at this stage resembled that of the involuted gland normally seen after weaning. These results demonstrate an essential role for E-cadherin in the function of differentiated alveolar epithelial cells. No tumors were detected in mutant glands lacking E-cadherin.

Melanoblasts (Mbs) are thought to be strictly regulated by cell–cell interactions with epidermal keratinocytes, although the precise molecular mechanism of the regulation has been elusive. Notch signaling, whose activation is mediated by cell–cell interactions, is implicated in a broad range of developmental processes. We demonstrate the vital role of Notch signaling in the maintenance of Mbs, as well as melanocyte stem cells (MSCs). Conditional ablation of Notch signaling in the melanocyte lineage leads to a severe defect in hair pigmentation, followed by intensive hair graying. The defect is caused by a dramatic elimination of Mbs and MSCs. Furthermore, targeted overexpression of Hes1 is sufficient to protect Mbs from the elimination by apoptosis. Thus, these data provide evidence that Notch signaling, acting through Hes1, plays a crucial role in the survival of immature Mbs by preventing initiation of apoptosis.