The researchers began by introducing several mutant genes commonly found in human melanomas. These included genes that interfere with the retinoblastoma (Rb) and p53 tumor suppressor pathways, which normally act to keep cell growth in check, as well as human telomerase reverse transcriptase (hTERT), the enzyme that protects the ends of chromosomes during cell division. This enzyme is associated with the progression of many cancers. None of these mutant genes, when introduced individually or in combination, led to cancerous growth. But when a mutant form of Ras--produced by an oncogene that sends growth signals to a cell--was added to this combination, it produced clinical features of invasive human melanoma: darkly pigmented skin that progressed to ulcerated tumor nodules.
Melanoma was observed as early as one month after the oncogenic combination was introduced. The tumors demonstrated aggressive local invasion but did not metastasize.
Next, Chudnovsky and colleagues investigated whether interfering with either the Rb or p53 pathways could trigger melanoma. They found that in combination with Ras and hTERT, the expression of cyclin-dependent kinase 4 (CDK4),which promotes cell growth and inhibits Rb function, induced invasive melanoma in human tissue. Similarly, inhibition of p53, when combined with Ras and hTERT, resulted in invasive melanoma.
The scientists also investigated the role of elevated telomerase activity. In combination with other cancer genes, hTERT, the active protein in the enzyme telomerase, caused progressively invasive melanoma. In contrast, melanocytes that did not receive hTERT remained in an early stage of benign growth.
Finally, the team looked at the PI3K and Raf pathways, frequently implicated in the development of melanoma. They found the exp