Conditional Cre-mediated recombination has emerged as a robust method of introducing somatic genetic alterations in an organ-specific manner in the mouse. Here, we generated and characterized mice harboring a 4-hydroxytamoxifen (OHT)-inducible Cre recombinase-estrogen receptor fusion transgene under the control of the melanocyte-specific tyrosinase promoter, designated Tyr::CreER(T2). Cre-mediated recombination was induced in melanocytes in a spatially and temporally controlled manner upon administration of OHT and was documented in embryonic melanoblasts, follicular bulb melanocytes, dermal dendritic melanocytes, epidermal melanocytes of tail skin, and in putative melanocyte stem cells located within the follicular bulge. Functional evidence suggestive of recombination in follicular melanocyte stem cells included the presence of Cre-mediated recombination in follicular bulb melanocytes 1 year after topical OHT administration, by which time several hair cycles have elapsed and the melanocytes residing in this location have undergone multiple rounds of apoptosis and replenishment. These Tyr:: CreER(T2) transgenic mice represent a useful resource for the evaluation of melanocyte developmental genetics, the characterization of melanocyte stem cell function and dynamics, and the construction of refined mouse models of malignant melanoma.

Malignant melanoma is a common and frequently lethal disease. Current therapeutic interventions have little effect on survival, emphasizing the need for a better understanding of the genetic, epigenetic, and phenotypic changes in melanoma formation and progression. We identified 17 genes that were not previously known to be silenced by methylation in melanoma using a microarray-based screen following treatment of melanoma cell lines with the DNA methylation inhibitor 5-Aza-2 ¶-deoxycytidine. Eight of these genes have not been previously shown to undergo DNA methylation in any form of cancer. Three of the genes, QPCT, CYP1B1, and LXN, are densely methylated in >95% of uncultured melanoma tumor samples. Reexpression of either of two of the silenced genes, HOXB13 and SYK, resulted in reduced colony formation in vitro and diminished tumor formation in vivo, indicating that these genes function as tumor suppressors in melanoma.

SUMMARY BrafV600E induces benign, growth-arrested melanocytic nevus development, but also drives melanoma formation. Cdkn2a loss in BrafV600E melanocytes in mice results in rare progression to melanoma, but only after stable growth arrest as nevi. Immediate progression to melanoma is prevented by upregulation of miR-99/100 which downregulates mTOR and IGF1R signaling. mTORC1 activation through Stk11 (Lkb1) loss abrogates growth-arrest of BrafV600E melanocytic nevi, but is insufficient for complete progression to melanoma. Cdkn2a loss is associated with mTORC2 and Akt activation in human and murine melanocytic neoplasms. Simultaneous Cdkn2a and Lkb1 inactivation in BrafV600E melanocytes results in activation of both mTORC1 and mTORC2/Akt, inducing rapid melanoma formation in mice. In this model, activation of both mTORC1/2 is required for Braf-induced melanomagenesis.

Aberrant promoter hypermethylation of tumor-associated genes leading to their inactivation is a common event in many cancer types. Using a sensitive restriction-multiplex PCR method, we studied the promoter hypermethylation profile of the p16, p15, hMLH1, MGMT and E-cad genes in oral squamous cell carcinoma (OSCC) of Indians. We analyzed a total of 51 samples for the p15 tumor-suppressor gene and 99 samples for each of the remaining genes. Our studies indicate an incidence of promoter hypermethylation of 23% each for p16 and p15, 8% for hMLH1, 41% for MGMT and 35% for E-cad. We observed aberrant hypermethylation of the promoter region of at least 1 of these genes in 74.5% of cases (n ‫؍‬ 51) for which all the 5 genes were studied. Abnormal methylation was detected in tumors irrespective of stage and location in the oral cavity, whereas no abnormal methylation was detectable in normal oral squamous tissues obtained from 25 OSCC patients. Oral squamous cell carcinoma (OSCC) is one of the most widely prevalent cancer types in developing countries, including India, and is associated with tobacco and alcohol abuse. 1 Although recent research has given deeper insight into the etiology of the disease, its occurrence as a potentially fatal disease continues unabated. It is therefore essential to identify and develop newer risk markers for diagnosis and therapy of OSCC.Methylation profiling of promoter regions is essential for understanding the regulation of imprinted genes, X-chromosome inactivation and the role of various tumor-suppressor genes in the genesis of different types of cancer. 2,3 Methylation profiling of tumor-suppressor genes is a potentially powerful diagnostic tool for early detection of various types of cancer. 3-5 Assessment of methylation inactivation is important in assigning functional roles of genes and may have implications in the function of other interacting proteins. 3,6 Inactivation of several tumor-associated genes, especially tumor-suppressor genes, has been attributed to aberrant hypermethylation of their promoter regions. 2,3,6 We included in our study 5 tumor-suppressor genes: p16, a CDK inhibitor involved in regulation of the cell cycle by the cyclin D-Rb pathway, control of which is lost in virtually all tumor types; 7 p15, another CDK inhibitor involved in cell-cycle regulation; 8 hMLH1, the human homolog of bacterial MutL, involved in mismatch repair; 9 MGMT, the gene involved in repair of methylated guanosine residues formed due to alkylated carcinogens; and E-cadherin, involved in homotypic epithelial cell-cell adhesion. 10 All of these genes are known to be inactivated by methylation in various cancers. 3,11 Methylation profiling of CpG sites originally involved digestion of sample DNA using a methylation-sensitive restriction enzyme followed by Southern hybridization 7,12 or PCR using primers flanking the restriction site. [13][14][15] Using these assays, methylation patterns could not be determined from very small amounts of DNA and the occurrence of false-negatives due to degra...

Questions persist about the nature and number of cells with tumor-propagating capability in different types of cancer, including melanoma. In part, this is because identification and characterization of purified tumorigenic subsets of cancer cells has not been achieved to date. Here, we report tumor formation after injection of single purified melanoma cells derived from three novel mouse models. Tumor formation occurred after every injection of individual CD34+p75− melanoma cells, with intermediate rates using CD34−p75− cells, and rarely with CD34−p75+ cells. These findings suggest that tumorigenic melanoma cells may be more common than previously thought and establish that multiple distinct populations of melanoma-propagating cells (MPC) can exist within a single tumor. Interestingly, individual CD34−p75− MPCs could regenerate cellular heterogeneity after tumor formation in mice or multiple passages in vitro, whereas CD34+p75− MPCs underwent self-renewal only, showing that reestablishment of tumor heterogeneity is not always a characteristic of individual cells capable of forming tumors. Functionally, single purified MPCs were more resistant to chemotherapy than non-MPCs. We anticipate that purification of these MPCs may allow a more comprehensive evaluation of the molecular features that define tumor-forming capability and chemotherapeutic resistance in melanoma.