Oncogenic KRAS provides a uniquely powerful and variable oncogenic contribution among RAS family members in the colonic epithelium

Activating mutations of the RAS family of small GTPases are among the most common genetic events in human tumorigenesis. Constitutive activation of the three canonical family members, KRAS, NRAS, and HRAS segregate strongly by tissue type. Of these, KRAS mutations predominate in human tumors, including those arising from the colon and lung. We sought to compare the oncogenic contributions of different RAS isoforms in a comparable genetic setting and to explore downstream molecular changes that may explain the apparent differential oncogenic effects of the various RAS family members. We utilized colorectal cancer cell linescharacterizedbyoncogenicKRASinparallelwithisogenicallyderivedlinesinwhichthemutantallelehasbeendisrupted.We additionallyattemptedtoreconstitutetheisogenicderivativeswithoncogenicformsofotherRASfamilymembersandanalyzethem in parallel. Pairwise analysis of HCT 116 and DLD-1 cell lines as well as their isogenic derivatives reveals distinct K-RAS G13D signaturesdespitethegeneticsimilaritiesofthesecelllines.InDLD-1,forexample,oncogenicK-RASenhancesthemotilityofthese cellsbydownregulationofRap1activity,yetisnotassociatedwithincreasedERK1/2phosphorylation.InHCT116,however,ERK1/ 2phosphorylationiselevatedrelativetotheisogenicderivative,butRap1activityisunchanged.K-RASisuniquelyoncogenicinthe colonic epithelium, though the molecular aspects of its oncogenic contribution are not necessarily conserved across cell lines. We therefore conclude that the oncogenic contribution of K-RAS is a function of its multifaceted functionality and is highly contextdependent. J. Cell. Physiol. 210: 740–749, 2007. 2006 Wiley-Liss, Inc. Activating mutations of the members of the RAS family of small GTPases are among the most common genetic events in human tumorigenesis and characterize approximately 30% of all human cancers (Bos, 1989). Constitutive RAS signaling contributes to the transformed phenotype by promoting proliferation, enhanced motility, survival and loss of anchorage dependence. Activating mutations of RAS family members do not occur with equal frequency across tumor types, but rather segregate strongly by tissue of origin, with one isoform predominating in any particular RAS-driven tumor. For example, mutations to KRAS are the most frequent RAS family mutations and occur in 30% of nonsmall cell lung cancers, 50% of colorectal adenocarcinomas and 90% of pancreatic adenocarcinomas. Fifteen percent of melanomas and 30% of acute myelogenous leukemias display activatingNRAS mutations. Activating mutations of HRAS are far less common but contribute to squamous cell carcinomas, bladder carcinomas, and to renal cancers (Downward, 2003). Isoform-specific functions may explain the