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Cancer is a multistep genetic process that includes mutational activation of oncogenes and inactivation of tumor suppressor genes. The Ras oncogenes are the most frequently mutated oncogenes in human cancers (30%), with a high frequency associated with cancers of the lung, colon, and pancreas. Mutational activation of Ras is commonly an early event in the development of these cancers. Thus, whether mutated Ras is required for tumor maintenance and what aspects of the complex malignant phenotype might be promoted by mutated Ras are issues that remain unresolved for these and other human cancers. The recent development of interfering RNA to selectively impair expression of mutated Ras provides a powerful approach to begin to resolve these issues. In this chapter, we describe the use of retrovirus-based RNA interference approaches to study the functions of Ras and Ras effectors (Raf, RalA, RalB, and Tiam1) in the growth of pancreatic carcinoma and other human tumor cell lines. Finally, we also compare the use of constitutive and inducible shRNA expression vectors for analyses of mutant Ras function.

In conclusion, RDA provides a fast, technically simple, and inexpensive way to characterize genes aberrantly expressed due to Ras transformation. The identification and characterization of these genes may provide insight not only into the mechanism by which Ras causes transformation, but also may identify novel targets for rational drug design and development of anticancer drugs.