In this month's Nature Genetics, Stanford scientists report on the development of new experimental models of melanoma that may facilitate the identification of cooperating transforming mutations.

Cancer is the end product of the accumulation of multiple genetic lesions. Most epithelial human cancers harbor complex karyotypes that reflect cycles of genomic instability. Although much progress has been made in identifying and characterizing cancer-associated mutations, the complexity of the cancer genome often makes it difficult to connect a cancer genotype to specific malignant phenotypes.

Over the past few years, several groups have created cell and animal models of cancer with the goal of deciphering the roles of specific pathways implicated in oncogenesis. Human cell models have proven quite useful in this regard.1 However, these experimental systems have typically assayed tumorigenicity via subcutaneous implants in immunodeficient mice. Reliance upon these protocols raises the concern that such systems fail to incorporate critical features of cancer such as the input of tissue-specific stromal cells. Moreover, since such cell-based models require some period of culturing ex vivo, an additional concern is that the process of culturing imposes artificial requirements for transformation.

A recent report from Chudnovsky et al2 describes the development of melanocyte transformation models that not only provide insight into genes that cooperate to induce tumorigenicity but also recapitulate key features of human malignant melanoma. By using genetically engineered human melanocytes and primary human keratinocytes, these investigators generated a facsimile of the human dermis in immunodeficient mice. By introducing mutations associated with patient-derived tumor samples into this model system, they were able to create tumors that displayed both clinical and histological features of invasive melanoma. Importantly, the histopathologic features of the tumors correlated with the particular combination of genes introduced into the engineered cells.

Specifically, Chudnovsky et al2 perturbed the p53 and RB pathways that govern cell-cycle control and survival, the Ras signaling pathway that regulates several pathways involved in growth regulation, and the pathway(s) regulated by the catalytic subunit of human telomerase (hTERT). The authors show that the simultaneous disruption of these pathways in melanocytes drives the formation of invasive melanoma. Since they also generated parallel cultures in which only some of these pathways were disrupted, they were able to investigate the contributions of individual cancer-associated mutations to melanoma formation. These experiments yielded several interesting observations that highlight the utility of this model system. For instance, it has long been appreciated that hTERT and telomerase mediate cellular immortality, but recent observations suggest that the hTERT protein may have additional functions in tumorigenesis.3, 4 Indeed, Chudnovsky et al2 found that hTERT overexpression is required to drive invasive melanoma even though this expression of hTERT did not appear to be required for immortality. Therefore, these observations suggest that telomerase may have more complex roles in melanoma than previously appreciated.

A key element of these experiments is the use of untransformed human keratinocytes to create an environment closer to that which surrounds premalignant melanocytes in vivo. Although many studies have demonstrated the importance of orthotopic implantation in tumor formation,5 this system provides a foundation with which to study melanoma mutations in the appropriate microenvironment. Similar studies in human keratinocytes6 and prostate epithelial cells7 reinforce the notion that reassembling the tumor environment permits the development of improved cancer models. Since recent work suggests that stromal cells contribute to tumor development,8 such systems promise to allow one to dissect the interactions between tumor cells and stromal cells in transformation. Taken together, these types of model systems will allow us to understand how individual genes influence the behavior of specific human tumors.

This new model system represents one tool in a growing arsenal of experimental systems that will facilitate our understanding of melanoma initiation and progression. Since anatomical as well as cell biological differences between murine and human skin tissue architecture complicate direct comparison between murine and human model systems of melanoma, these orthotopic models enable the rapid generation of matched human melanocytes, while avoiding the laborious and expensive procedures required to generate a similar cohort of genetically engineered mice. Indeed, such cell-based studies should help identify specific combinations of genetic alterations around which new murine models can be developed. Such murine systems will then make it possible to analyze the effects of development and the immune system on tumorigenesis in undisturbed tissue.

The findings of Chudnovsky et al2 support the notion that it may one day be possible to screen groups of mutations efficiently and rapidly to determine if they suffice to induce melanoma. Importantly, this coculture and grafting protocol does not appear to require genetic instability, facilitating the study of particular mutations without the potential confounding effects of other mutations. Although further experiments are necessary to determine the widespread utility of this system in assaying the contribution of newly identified mutations to melanoma, this experimental model provides an interesting new platform that will not only help to delineate critical pathways in transformation but may also be leveraged for drug discovery and validation efforts that target these pathways.