Some non-small cell lung cancers (NSCLCs) harbor a single specific mutated oncogene that is thought to be the primary genetic "driver" leading to cancer. The two most commonly mutated oncogenes in lung cancer encode for the epidermal growth factor receptor (EGFR) and KRAS. EGFR kinase domain mutations were only recently identified, but they have already been established in the clinic as valid predictors of increased sensitivity to EGFR kinase inhibitors (gefitinib and erlotinib). By contrast, even though KRAS mutations were identified in NSCLC tumors more than 20 years ago, we have only just begun to appreciate the clinical value of KRAS tumor status. Recent studies indicate that patients with mutant KRAS tumors fail to benefit from adjuvant chemotherapy, and their disease does not respond to EGFR inhibitors. There is a dire need for therapies specifically for patients with KRAS mutant NSCLC. In this review, we summarize the initial discovery of RAS mutations in NSCLC, describe work exploring associations with clinical factors and outcomes, and provide an overview of current approaches to targeting KRAS mutant NSCLC.

Purpose: KRAS mutations are found in f25% of lung adenocarcinomas inWestern countries and, as a group, have been strongly associated with cigarette smoking.These mutations are predictive of poor prognosis in resected disease as well as resistance to treatment with erlotinib or gefitinib. Experimental Design: We determined the frequency and type of KRAS codon 12 and 13 mutations and characterized their association with cigarette smoking history in patients with lung adenocarcinomas. Results: KRAS mutational analysis was done on 482 lung adenocarcinomas, 81 (17%) of which were obtained from patients who had never smoked cigarettes. KRAS mutations were found in 15% (12 of 81; 95% confidence intervals, 8-24%) of tumors from never smokers. Similarly, 22% (69 of 316; 95% confidence intervals, 17-27%) of tumors from former smokers, and 25% (21 of 85; 95% confidence intervals, 16-35%) of tumors from current smokers had KRAS mutations. The frequency of KRAS mutation was not associated with age, gender, or smoking history. The number of pack years of cigarette smoking did not predict an increased likelihood of KRAS mutations. Never smokers were significantly more likely than former or current smokers to have a transition mutation (G!A) rather than the transversion mutations known to be smokingrelated (G!Tor G!C; P < 0.0001).Conclusions: Based on our data, KRAS mutations are not rare among never smokers with lung adenocarcinoma and such patients have a distinct KRAS mutation profile.The etiologic and biological heterogeneity of KRAS mutant lung adenocarcinomas is worthy of further study.Since the identification of somatic epidermal growth factor receptor (EGFR) mutations, there has been heightened interest in the molecular basis of lung cancer in patients who never smoked cigarettes (1 -3). Somatic mutations in EGFR have been identified in f15% of all patients with lung adenocarcinoma, with the proportion increasing to 50% in patients who never smoked cigarettes. There is an inverse relationship between cigarette smoking history and frequency of EGFR mutations, with the frequency of EGFR mutations decreasing significantly among patients who smoked more than 15 pack years (4). Such refined understanding of the relationship between smoking history and presence of EGFR mutations has allowed the design of clinical trials which use smoking history to enrich the number of patients with somatic EGFR mutations (5-7).In contrast to EGFR mutations, KRAS mutations were initially identified in patients with lung adenocarcinoma who had a history of heavy cigarette smoking and were thought to be uncommon in patients without a history of smoking cigarettes (8). These mutations are found in f25% of lung adenocarcinomas in western countries but are less common in Asian populations (9, 10). KRAS mutations have been associated with poor prognosis in resected non -small cell lung cancer (NSCLC; refs. 11-13), lack of survival benefit from adjuvant chemotherapy (14), and resistance to erlotinib or gefitinib (15). More than 95% of KRAS ...

EGFR and KRAS mutations define distinct molecular subsets of resected lung adenocarcinoma. Because EGFR and KRAS mutations also predict whether tumors are sensitive or resistant, respectively, to EGFR tyrosine kinase inhibitors, they can readily be used in clinical trials to help guide the administration of specific types of adjuvant therapy.

Genetic lesions affecting a number of kinases and other elements within the epidermal growth factor receptor (EGFR) signaling pathway have been implicated in the pathogenesis of human non-small-cell lung cancer (NSCLC). We performed mutational profiling of a large cohort of lung adenocarcinomas to uncover other potential somatic mutations in genes of this pathway that could contribute to lung tumorigenesis. We have identified in 2 of 207 primary lung tumors a somatic activating mutation in exon 2 of MEK1 (i.e., mitogen-activated protein kinase kinase 1 or MAP2K1) that substitutes asparagine for lysine at amino acid 57 (K57N) in the nonkinase portion of the kinase. Neither of these two tumors harbored known mutations in other genes encoding components of the EGFR signaling pathway (i.e., EGFR, HER2, KRAS, PIK3CA, and BRAF). Expression of mutant, but not wild-type, MEK1 leads to constitutive activity of extracellular signal-regulated kinase (ERK)-1/2 in human 293T cells and to growth factorindependent proliferation of murine Ba/F3 cells. A selective MEK inhibitor, AZD6244, inhibits mutant-induced ERK activity in 293T cells and growth of mutant-bearing Ba/F3 cells. We also screened 85 NSCLC cell lines for MEK1 exon 2 mutations; one line (NCI-H1437) harbors a Q56P substitution, a known transformation-competent allele of MEK1 originally identified in rat fibroblasts, and is sensitive to treatment with AZD6244.

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