Mouse model for lung tumorigenesis through Cre/lox controlled sporadic activation of the K-Ras oncogene

RAS Mutations and Oncogenesis: Not all RAS Mutations are Created Equally

Mutation in RAS proteins is one of the most common genetic alterations observed in human and experimentally induced rodent cancers. In vivo, oncogenic mutations have been shown to occur at exons 12, 13, and 61, resulting in any 1 of 19 possible point mutations in a given tumor for a specific RAS isoform. While some studies have suggested a possible role of different mutant alleles in determining tumor severity and phenotype, no general consensus has emerged on the oncogenicity of different mutant alleles in tumor formation and progression. Part of this may be due to a lack of a single, signature pathway that shows significant alterations between different mutations. Rather, it is likely that subtle differences in the activation, or lack thereof, of downstream effectors by different RAS mutant alleles may determine the eventual outcome in terms of tumor phenotype. This paper reviews our current understanding of the potential role of different RAS mutations on tumorigenesis, highlights studies in model cell culture and in vivo systems, and discusses the potential of expression array and computational network modeling to dissect out differences in activated RAS genes in conferring a transforming phenotype.

Molecular biology of lung cancer: clinical implications

Lung cancer is a heterogeneous disease clinically, biologically, histologically, and molecularly. Understanding the molecular causes of this heterogeneity, which might reflect changes occurring in different classes of epithelial cells or different molecular changes occurring in the same target lung epithelial cells, is the focus of current research. Identifying the genes and pathways involved, determining how they relate to the biological behavior of lung cancer, and their utility as diagnostic and therapeutic targets are important basic and translational research issues. This article reviews current information on the key molecular steps in lung cancer pathogenesis, their timing, and clinical implications.

Functional specificity of ras isoforms: so similar but so different

H-ras, N-ras, and K-ras are canonical ras gene family members frequently activated by point mutation in human cancers and coding for 4 different, highly related protein isoforms (H-Ras, N-Ras, K-Ras4A, and K-Ras4B). Their expression is nearly ubiquitous and broadly conserved across eukaryotic species, although there are quantitative and qualitative differences of expression depending on the tissue and/or developmental stage under consideration. Extensive functional studies have determined during the last quarter century that these Ras gene products are critical components of signaling pathways that control eukaryotic cell proliferation, survival, and differentiation. However, because of their homology and frequent coexpression in various cellular contexts, it remained unclear whether the different Ras proteins play specific or overlapping functional roles in physiological and pathological processes. Initially, their high degree of sequence homology and the observation that all Ras isoforms share common sets of downstream effectors and upstream activators suggested that they were mostly redundant functionally. In contrast, the notion of functional specificity for each of the different Ras isoforms is supported at present by an increasing body of experimental observations, including 1) the fact that different ras isoforms are preferentially mutated in specific types of tumors or developmental disorders; 2) the different transforming potential of transfected ras genes in different cell contexts; 3) the distinct sensitivities exhibited by the various Ras family members for modulation by different GAPs or GEFs; 4) the demonstration that different Ras isoforms follow distinct intracellular processing pathways and localize to different membrane microdomains or subcellular compartments; 5) the different phenotypes displayed by genetically modified animal strains for each of the 3 ras loci; and 6) the specific transcriptional networks controlled by each isoform in different cellular settings.

Bioluminescent orthotopic mouse models of human localized non-small cell lung cancer: feasibility and identification of circulating tumour cells

Background

Preclinical models of non-small cell lung cancer (NSCLC) require better clinical relevance to study disease mechanisms and innovative therapeutics. We sought to compare and refine bioluminescent orthotopic mouse models of human localized NSCLC.

Methods

Athymic nude mice underwent subcutaneous injection (group 1-SC, n = 15, control), percutaneous orthotopic injection (group 2-POI, n = 30), surgical orthotopic implantation of subcutaneously grown tumours (group 3-SOI, n = 25), or transpleural orthotopic injection (group 4-TOI, n = 30) of A549-luciferase cells. Bioluminescent in vivo imaging was then performed weekly. Circulating tumour cells (CTCs) were searched using Cellsearch® system in SC and TOI models.

Results

Group 2-POI was associated with unexpected direct pleural spreading of the cellular solution in 53% of the cases, forbidding further evaluation of any localized lung tumour. Group 3-SOI was characterized by high perioperative mortality, initially localized lung tumours, and local evolution. Group 4-TOI was associated with low perioperative mortality, initially localized lung tumours, loco regional extension, and distant metastasis. CTCs were detected in 83% of nude mice bearing subcutaneous or orthotopic NSCLC tumours.

Conclusions

Transpleural orthotopic injection of A549-luc cells in nude mouse lung induces localized tumour, followed by lymphatic extension and specific mortality, and allowed the first time identification of CTCs in a NSCLC mice model.

Rapid validation of cancer genes in chimeras derived from established genetically engineered mouse models

Recent technological advances have opened the door for the fast and cost-effective generation of genetically engineered mouse models (GEMMs) to study cancer. We describe here a conceptually novel approach for the generation of chimeric GEMMs based on the controlled introduction of various genetic elements in embryonic stem cells (ESCs) that are derived from existing mouse strains with a predisposition for cancer. The isolation of GEMM-derived ESC lines is greatly facilitated by the availability of the newly defined culture media containing inhibitors that effectively preserve ESC pluripotency. The feasibility of the GEMM-ESC approach is discussed in light of current literature and placed into the context of existing models. This approach will allow for fast and flexible validation of candidate cancer genes and drug targets and will result in a repository of GEMM-ESC lines and corresponding vector collections that enable easy distribution and use of preclinical models to the wider scientific community.

Acute sensitivity of the oral mucosa to oncogenic K-ras

Mouse models of cancer represent powerful tools for analysing the role of genetic alterations in carcinogenesis. Using a mouse model that allows tamoxifen-inducible somatic activation (by Cre-mediated recombination) of oncogenic K-ras^G12D in a wide range of tissues, we observed hyperplasia of squamous epithelium located in moist or frequently abraded mucosa, with the most dramatic effects in the oral mucosa. This epithelium showed a sequence of squamous hyperplasia followed by squamous papilloma with dysplasia, in which some areas progressed to early invasive squamous cell carcinoma, within 14 days of widespread oncogenic K-ras activation. The marked proliferative response of the oral mucosa to K-ras^G12D was most evident in the basal layers of the squamous epithelium of the outer lip with hair follicles and wet mucosal surface, with these cells staining positively for pAKT and cyclin D1, showing Ras/AKT pathway activation and increased proliferation with Ki-67 and EdU positivity. The stromal cells also showed gene activation by recombination and immunopositivity for pERK indicating K-Ras/ERK pathway activation, but without Ki-67 positivity or increase in stromal proliferation. The oral neoplasms showed changes in the expression pattern of cytokeratins (CK6 and CK13), similar to those observed in human oral tumours. Sporadic activation of the K-ras^G12D allele (due to background spontaneous recombination in occasional cells) resulted in the development of benign oral squamous papillomas only showing a mild degree of dysplasia with no invasion. In summary, we show that oral mucosa is acutely sensitive to oncogenic K-ras, as widespread expression of activated K-ras in the murine oral mucosal squamous epithelium and underlying stroma can drive the oral squamous papilloma–carcinoma sequence. Copyright © 2011 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Integrative genomic analyses identify BRF2 as a novel lineage-specific oncogene in lung squamous cell carcinoma

BACKGROUND

Traditionally, non-small cell lung cancer is treated as a single disease entity in terms of systemic therapy. Emerging evidence suggests the major subtypes--adenocarcinoma (AC) and squamous cell carcinoma (SqCC)--respond differently to therapy. Identification of the molecular differences between these tumor types will have a significant impact in designing novel therapies that can improve the treatment outcome.

METHODS AND FINDINGS

We used an integrative genomics approach, combing high-resolution comparative genomic hybridization and gene expression microarray profiles, to compare AC and SqCC tumors in order to uncover alterations at the DNA level, with corresponding gene transcription changes, which are selected for during development of lung cancer subtypes. Through the analysis of multiple independent cohorts of clinical tumor samples (>330), normal lung tissues and bronchial epithelial cells obtained by bronchial brushing in smokers without lung cancer, we identified the overexpression of BRF2, a gene on Chromosome 8p12, which is specific for development of SqCC of lung. Genetic activation of BRF2, which encodes a RNA polymerase III (Pol III) transcription initiation factor, was found to be associated with increased expression of small nuclear RNAs (snRNAs) that are involved in processes essential for cell growth, such as RNA splicing. Ectopic expression of BRF2 in human bronchial epithelial cells induced a transformed phenotype and demonstrates downstream oncogenic effects, whereas RNA interference (RNAi)-mediated knockdown suppressed growth and colony formation of SqCC cells overexpressing BRF2, but not AC cells. Frequent activation of BRF2 in >35% preinvasive bronchial carcinoma in situ, as well as in dysplastic lesions, provides evidence that BRF2 expression is an early event in cancer development of this cell lineage.

CONCLUSIONS

This is the first study, to our knowledge, to show that the focal amplification of a gene in Chromosome 8p12, plays a key role in squamous cell lineage specificity of the disease. Our data suggest that genetic activation of BRF2 represents a unique mechanism of SqCC lung tumorigenesis through the increase of Pol III-mediated transcription. It can serve as a marker for lung SqCC and may provide a novel target for therapy. Please see later in the article for the Editors' Summary.

Non-germline genetically engineered mouse models for translational cancer research

Genetically engineered mouse models (GEMMs) of cancer have affected virtually all areas of cancer research. However, the accelerated discovery of new cancer genes emerging from large-scale cancer genomics and new chemical entities pouring from the drug discovery pipeline have strained the capacity of traditional germline mouse models to provide crucial insights. This Review introduces new approaches to modelling cancer, with emphasis on a growing collection of non-germline GEMMs (nGEMMs). These offer flexibility, speed and uniformity at reduced costs, thus paving the way for much needed throughput and practical preclinical therapeutic testing models.

Cell of origin of lung cancer

Lung cancer is a devastating disease and a major therapeutic burden with poor survival rates. The discovery of rare cells with stem cell-like properties in solid tumours is emerging as an important area of cancer research and may help explain the resistance of these tumours to current therapeutics. Despite rapid developments in cancer stem cell research in other solid tumours, progress in the lung has been hampered by an incomplete understanding of the epithelial stem cell hierarchy, the heterogeneity of disease and the lack of a suitable in vivo transplantation model to assess stem cell behaviour. In this review we critically discuss what is currently known about the role of normal stem cells and cancer-initiating cells in lung tumour development, and briefly discuss strategies aimed at advancing the field of lung stem cell biology, with an emphasis on the design and manipulation of state-of-art mouse models.

A mouse model of melanoma driven by oncogenic KRAS

The small G-protein NRAS is mutated in 22% of human melanomas, whereas the related proteins KRAS and HRAS are mutated in only 2% and 1% of melanomas, respectively. We have developed a mouse model of melanoma in which Cre recombinase/LoxP technology is used to drive inducible expression of (G12V)KRAS in the melanocytic lineage. The mice develop skin hyperpigmentation, nevi, and tumors that bear many of the cardinal histopathology features and molecular characteristics of human melanoma. These tumors invade and destroy the underlying muscles and cells derived from them can grow as subcutaneous tumors and colonize the lungs of nude mice. These data establish that oncogenic KRAS can be a founder event in melanomagenesis.

Rapid and robust transgenic high-grade glioma mouse models for therapy intervention studies

PURPOSE

To develop a transgenic mouse model of glioma that can be conveniently used for testing therapy intervention strategies. High-grade glioma is a devastating and uniformly fatal disease for which better therapy is urgently needed. Typical for high-grade glioma is that glioma cells infiltrate extensively into surrounding pivotal brain structures, thereby rendering current treatments largely ineffective. Evaluation of novel therapies requires the availability of appropriate glioma mouse models.

EXPERIMENTAL DESIGN

High-grade gliomas were induced by stereotactic intracranial injection of lentiviral GFAP-Cre or CMV-Cre vectors into compound LoxP-conditional mice, resulting in K-Ras(v12) expression and loss of p16(Ink4a)/p19(Arf) with or without concomitant loss of p53 or Pten.

RESULTS

Tumors reproduced many of the features that are characteristic for human high-grade gliomas, including invasiveness and blood-brain barrier functionality. Especially, CMV-Cre injection into p53;Ink4a/Arf;K-Ras(v12) mice resulted in high-grade glioma with a short tumor latency (2-3 weeks) and full penetrance. Early detection and follow-up was accomplished by noninvasive bioluminescence imaging, and the practical utility for therapy intervention was shown in a study with temozolomide.

CONCLUSION

We have developed a realistic high-grade glioma model that can be used with almost the same convenience as traditional xenograft models, thus allowing its implementation at the forefront of preclinical evaluation of new treatments.

Kinase-dead BRAF and oncogenic RAS cooperate to drive tumor progression through CRAF

We describe a mechanism of tumorigenesis mediated by kinase-dead BRAF in the presence of oncogenic RAS. We show that drugs that selectively inhibit BRAF drive RAS-dependent BRAF binding to CRAF, CRAF activation, and MEK-ERK signaling. This does not occur when oncogenic BRAF is inhibited, demonstrating that BRAF inhibition per se does not drive pathway activation; it only occurs when BRAF is inhibited in the presence of oncogenic RAS. Kinase-dead BRAF mimics the effects of the BRAF-selective drugs and kinase-dead Braf and oncogenic Ras cooperate to induce melanoma in mice. Our data reveal another paradigm of BRAF-mediated signaling that promotes tumor progression. They highlight the importance of understanding pathway signaling in clinical practice and of genotyping tumors prior to administering BRAF-selective drugs, to identify patients who are likely to respond and also to identify patients who may experience adverse effects.

Development of a mouse model for sporadic and metastatic colon tumors and its use in assessing drug treatment

Most genetically engineered mouse (GEM) models for colon cancer are based on tissuewide or germline gene modification, resulting in tumors predominantly of the small intestine. Several of these models involve modification of the adenomatous polyposis coli (Apc) gene and are excellent models for familial cancer predisposition syndromes. We have developed a stochastic somatic mutation model for sporadic colon cancer that presents with isolated primary tumors in the distal colon and recapitulates the entire adenoma-carcinoma-metastasis axis seen in human colon cancer. Using this model, we have analyzed tumors that are either solely mutant in the Apc gene or in combination with another colon cancer-associated mutant gene, the Kras G12D allele. Because of the restricted location in the distal colon, the natural history of the tumors can be analyzed by serial colonoscopy. As the mammalian target of rapamycin (mTOR) pathway is a critical component of the complex signaling network in colon cancer, we used this model to assess the efficacy of mTOR blockade through rapamycin treatment of mice with established tumors. After treatment, Apc mutant tumors were more than 80% smaller than control tumors. However, tumors that possessed both Apc and Kras mutations did not respond to rapamycin treatment. These studies suggest that mTOR inhibitors should be further explored as potential colorectal cancer therapies in patients whose tumors do not have activating mutations in KRAS.

Drug efficacy testing in mice

The traditional path of drug development passes from in vitro screening and response assessment to validation of drug efficacy in cell line xenografts. While xenografts have their merits, historically, more often than not, they have not served as an accurate predictor of drug efficacy in humans. The refinement and increased availability of genetically engineered mouse models (GEMMs) of cancer has made GEMMs an attractive avenue for the preclinical testing of therapeutic agents. The histopathologic and genetic resemblance of GEMMs to human cancer are an important measure to evaluate their suitability for pre-clinical studies and a number of studies using kinase inhibitors have now been performed in GEMMs. We have highlighted several of the salient advantages and challenges associated with GEMM studies. Well-characterized GEM models of human cancer should aide in the prioritization of both established and novel therapeutics.

Cre/loxP-regulated transgenic zebrafish model for neural progenitor-specific oncogenic Kras expression

Ras proteins regulate signaling pathways that control many cellular responses, such as proliferation, survival, and differentiation. However, there are intriguing questions about the relationship between the developmental timing of specific mutations and the resultant phenotypes in individual cells. In this study, we used the Cre/loxP system for maintaining transgenic zebrafish lines harboring oncogenic Kras(V12) under the nestin promoter, and investigated the developmental effects of Ras activation in neural progenitor cells. Activated human Kras(V12) was induced within pDSNesLCherryLEGFPKRas(V12) transgenic fish by Cre mRNA injection. Cre-mediated gene excision was confirmed by polymerase chain reaction, and the injected embryos were investigated for Kras(V12) effects using the hemotoxylin-eosin staining, terminal deoxynucleotidyl transferase-mediated digoxigenin-dUTP nick-end labeling assay, and in situ hybridization. pDSNesLCherryLEGFPKRas(V12) transgenic embryos normally expressed mCherry in their central nervous system throughout the developmental stage. However, Cre mRNA injection efficiently excised the flanking stop sequence, and the injected embryos expressed enhanced green fluorescent protein in their brain with severe edema. Brain histology showed that neuronal cell differentiation could occur in spite of oncogenic Kras(V12) overexpression, but massive apoptosis and brain edema caused early embryonal death. In summary, the overexpression of Kras(V12) induces extensive apoptosis of neural progenitor cells followed by severe edema of the brain. However, some neural progenitor cells are resistant to Kras(V12) and can retain their ability to differentiate into neurons. Finally, our transgenic model demonstrates the inability of Kras(V12) alone to induce brain tumors at the early stage of development.

Preventive effects of bexarotene and budesonide in a genetically engineered mouse model of small cell lung cancer

In the present study, we examined the effect of bexarotene (Targretin) and budesonide in the chemoprevention of small cell lung carcinoma using a lung-specific knockout model of Rb1 and p53. Upon treatment with bexarotene, tumor incidence, number, and load were significantly reduced (P < 0.05). Budesonide treatment trended to inhibition, but the effect was not statistically significant (P > 0.05). Immunohistochemical staining indicated that bexarotene treatment decreased cell proliferation and increased apoptosis in tumors. The Rb1/p53 gene-targeted mouse seems to be a valuable model for chemopreventive studies on human small cell lung cancer. Our results indicate that the retinoid X receptor agonist bexarotene may be a potent chemopreventive agent in this cancer type.

Validation of intratracheal instillation of lung tumour cells in mice using single photon emission computed tomography/computed tomography imaging

In search of better predictive animal models for evaluating treatment response in lung cancer, orthotopic lung tumour models are a great step forward over traditional subcutaneous models. Crucial in the development of such orthotopic models is a reliable and reproducible instillation method. Because cells are instilled inside the thorax, the accuracy of the instillation and visualization of tumour growth demands the use of non-invasive imaging methods. We used a minimally invasive intratracheal intubation method to instill bioluminescent lung tumour cells in the lung parenchyma. Adaptation of the cell containing medium provides the possibility of tracing the exact location of the injection by means of single photon emission computed tomography/computed tomography (CT) imaging. The transplantation medium was also optimized to prevent migration of the injected substance. This results in the outgrowth of single and well-defined lung tumours at the instillation site. Finally, tumour growth was validated and longitudinally monitored with a combination of CT and bioluminescence imaging. The reported transplantation procedure enables the assessment of injection accuracy and provides a good approach for the generation of orthotopic lung tumour models for future response imaging studies.

The hypervariable region of K-Ras4B is responsible for its specific interactions with calmodulin

K-Ras4B belongs to the family of p21 Ras GTPases, which play an important role in cell proliferation, survival, and motility. The p21 Ras proteins, such as K-Ras4B, K-Ras4A, H-Ras, and N-Ras, share 85% sequence homology and activate very similar signaling pathways. Only the C-terminal hypervariable regions differ significantly. A growing body of literature demonstrates that each Ras isoform possesses unique functions in normal physiological processes as well as in pathogenesis. One of the central questions in the field of Ras biology is how these very similar proteins achieve such remarkable specificity in protein-protein interactions that regulate signal transduction pathways. Here we explore specific binding of K-Ras4B to calmodulin. Using NMR techniques and isothermal titration calorimetry, we demonstrate that the hypervariable region of K-Ras4B contributes in a major way to the interaction with calmodulin, while the catalytic domain of K-Ras4B provides a way to control the interaction by nucleotide binding. The hypervariable region of K-Ras4B binds specifically to the C-terminal domain of Ca(2+)-loaded calmodulin with micromolar affinity, while the GTP-gamma-S-loaded catalytic domain of K-Ras4B may interact with the N-terminal domain of calmodulin.

Immune response in lung cancer mouse model mimics human anti-Hu reactivity

Most patients with paraneoplastic encephalomyelitis/sensory neuronopathy PEM/SN have small-cell lung cancer (SCLC) and develop antibodies against neuronal-specific Hu proteins, which are abnormally expressed in the tumor. Anti-Hu reactivity is present in ~16% of SCLC patients without PEM/SN. Here we test the hypothesis that engineered SCLC-prone mice may exhibit anti-Hu reactivity. We show that tumors from SCLC-prone mice misexpress Hu proteins, and 14% of mice harbor anti-Hu antibodies. Mice appear to show reactivity prior to clinical diagnosis of SCLC. This mouse model system will be useful to study SCLC-associated autoimmunity, its diagnostic value, and the potential protective role of oncoantigen-directed autoantibodies.

Atypical protein kinase C{iota} is required for bronchioalveolar stem cell expansion and lung tumorigenesis

Protein kinase Ciota (PKCiota) is an oncogene required for maintenance of the transformed phenotype of non-small cell lung cancer cells. However, the role of PKCiota in lung tumor development has not been investigated. To address this question, we established a mouse model in which oncogenic Kras(G12D) is activated by Cre-mediated recombination in the lung with or without simultaneous genetic loss of the mouse PKCiota gene, Prkci. Genetic loss of Prkci dramatically inhibits Kras-initiated hyperplasia and subsequent lung tumor formation in vivo. This effect correlates with a defect in the ability of Prkci-deficient bronchioalveolar stem cells to undergo Kras-mediated expansion and morphologic transformation in vitro and in vivo. Furthermore, the small molecule PKCiota inhibitor aurothiomalate inhibits Kras-mediated bronchioalveolar stem cell expansion and lung tumor growth in vivo. Thus, Prkci is required for oncogene-induced expansion and transformation of tumor-initiating lung stem cells. Furthermore, aurothiomalate is an effective antitumor agent that targets the tumor-initiating stem cell niche in vivo. These data have important implications for PKCiota as a therapeutic target and for the clinical use of aurothiomalate for lung cancer treatment.

HIF2alpha cooperates with RAS to promote lung tumorigenesis in mice

Members of the hypoxia-inducible factor (HIF) family of transcription factors regulate the cellular response to hypoxia. In non-small cell lung cancer (NSCLC), high HIF2alpha levels correlate with decreased overall survival, and inhibition of either the protein encoded by the canonical HIF target gene VEGF or VEGFR2 improves clinical outcomes. However, whether HIF2alpha is causal in imparting this poor prognosis is unknown. Here, we generated mice that conditionally express both a nondegradable variant of HIF2alpha and a mutant form of Kras (KrasG12D) that induces lung tumors. Mice expressing both Hif2a and KrasG12D in the lungs developed larger tumors and had an increased tumor burden and decreased survival compared with mice expressing only KrasG12D. Additionally, tumors expressing both KrasG12D and Hif2a were more invasive, demonstrated features of epithelial- mesenchymal transition (EMT), and exhibited increased angiogenesis associated with mobilization of circulating endothelial progenitor cells. These results implicate HIF2alpha causally in the pathogenesis of lung cancer in mice, demonstrate in vivo that HIF2alpha can promote expression of markers of EMT, and define HIF2alpha as a promoter of tumor growth and progression in a solid tumor other than renal cell carcinoma. They further suggest a possible causal relationship between HIF2alpha and prognosis in patients with NSCLC.

Reactive Oxygen Species And Lung Tumorigenesis By Mutant K-ras: A Working Hypothesis

Wild-type K-ras is tumor suppressive in mouse lung, but mutant K-ras is actively oncogenic. Thus, the mutant protein must acquire new, dominant protumorigenic properties. Generation of reactive oxygen species could be one such property. The authors demonstrate increased peroxides in lung epithelial cells (E10)-transfected with mutant hK-ras(va112). An associated increase in DNA damage (comet assay) correlates with increased cyclooxygenase-2 protein. This DNA damage is completely abrogated by a specific cyclooxygenase-2 inhibitor (SC58125) or by a cell-permeable modified catalase. Literature is reviewed regarding generation of reactive oxygen and cyclooxygenase-2 induction by ras, cyclooxygenase-2 release of DNA-damaging reactive oxygen, and involvement of cyclooxygenase-2 and reactive oxygen in lung cancer

Conditional mouse lung cancer models using adenoviral or lentiviral delivery of Cre recombinase

The development of animal models of lung cancer is critical to our understanding and treatment of the human disease. Conditional mouse models provide new opportunities for testing novel chemopreventatives, therapeutics and screening methods that are not possible with cultured cell lines or xenograft models. This protocol describes how to initiate tumors in two conditional genetic models of human non-small cell lung cancer (NSCLC) using the activation of oncogenic K-ras alone or in combination with the loss of function of p53. We discuss methods for sporadic expression of Cre in the lungs through engineered adenovirus or lentivirus, and provide a detailed protocol for the administration of the virus by intranasal inhalation or intratracheal intubation. The protocol requires 1-5 min per mouse with an additional 30-45 min to set-up and allow for the recovery of mice from anesthesia. Mice may be analyzed for tumor formation and progression starting 2-3 weeks after infection.

Stem cells are dispensable for lung homeostasis but restore airways after injury

Local tissue stem cells have been described in airways of the lung but their contribution to normal epithelial maintenance is currently unknown. We therefore developed aggregation chimera mice and a whole-lung imaging method to determine the relative contributions of progenitor (Clara) and bronchiolar stem cells to epithelial maintenance and repair. In normal and moderately injured airways chimeric patches were small in size and not associated with previously described stem cell niches. This finding suggested that single, randomly distributed progenitor cells maintain normal epithelial homeostasis. In contrast we found that repair following severe lung injury resulted in the generation of rare, large clonal cell patches that were associated with stem cell niches. This study provides evidence that epithelial stem cells are dispensable for normal airway homeostasis. We also demonstrate that stem cell activation and robust clonal cellular expansion occur only during repair from severe lung injury.

A quantitative volumetric micro-computed tomography method to analyze lung tumors in genetically engineered mouse models

Two genetically engineered, conditional mouse models of lung tumor formation, K-ras(LSL-G12D) and K-ras(LSL-G12D)/p53(LSL-R270H), are commonly used to model human lung cancer. Developed by Tyler Jacks and colleagues, these models have been invaluable to study in vivo lung cancer initiation and progression in a genetically and physiologically relevant context. However, heterogeneity, multiplicity and complexity of tumor formation in these models make it challenging to monitor tumor growth in vivo and have limited the application of these models in oncology drug discovery. Here, we describe a novel analytical method to quantitatively measure total lung tumor burden in live animals using micro-computed tomography imaging. Applying this methodology, we studied the kinetics of tumor development and response to targeted therapy in vivo in K-ras and K-ras/p53 mice. Consistent with previous reports, lung tumors in both models developed in a time- and dose (Cre recombinase)-dependent manner. Furthermore, the compound K-ras(LSL-G12D)/p53(LSL-R270H) mice developed tumors faster and more robustly than mice harboring a single K-ras(LSL-G12D) oncogene, as expected. Erlotinib, a small molecule inhibitor of the epidermal growth factor receptor, significantly inhibited tumor growth in K-ras(LSL-G12D)/p53(LSL-R270H) mice. These results demonstrate that this novel imaging technique can be used to monitor both tumor progression and response to treatment and therefore supports a broader application of these genetically engineered mouse models in oncology drug discovery and development.

The role of cancer stem cells in neoplasia of the lung: past, present and future

Through the identification and subsequent targeting of an exquisitely unique and phenotypically defined cancer stem-cell population exhibiting discrete therapeutic vulnerabilities (a potential source of tumor recurrence) better survival rates for these patients may be achieved. It is this impetus that is making the field of pulmonary stem cell biology a growing field in biomedicine. These efforts are leading to the steady identification of multi-potent, self-renewing and proliferative progenitor cell populations throughout the bronchopulmonary tree. These cells give rise to both transiently amplifying (TA) and terminally differentiated (TD) cells, which (like in many other organs) are crucial for tissue homeostasis. In leukemia, it has been shown that partially committed cells, which are normally responsible for tissue maintenance after trauma, may undergo transformation via mutations resulting in the selective expression of genes that accentuate and perpetuate these cells' self-renewal capabilities. It is therefore perhaps legitimate to consider stem cells as protumorigenic. It is when these cells undergo genetic mutations which make them acquire the ability to metastasize, that cancer occurs, rendering the concept of "cancer stem cells" a rather attractive one indeed.

Polycomb group protein Bmi1 is required for growth of RAF driven non-small-cell lung cancer

BACKGROUND

We have previously described a RAF oncogene driven transgenic mouse model for non small cell lung cancer (NSCLC). Here we examine whether tumor initiation and growth requires the stem cell self-renewal factor Bmi1.

PRINCIPAL FINDINGS

In order to evaluate Bmi1 function in NSCLC two founder lines that differ in incidence and latency of tumor formation were compared. Ablation of Bmi1 expression in both lines had a dramatically decreased tumor growth. As the line with shorter latency matched the life span of Bmi1 knock out mice, these mice were chosen for further study. The absence of Bmi1 did not decrease the number of tumor initiation in these mice as only the size and not the number of tumors decreased. Reduction in tumor growth resulted from an increase in cell death and decrease in cell cycle progression that corresponded with up-regulation of the p16(INK4a) and p19(ARF).

SIGNIFICANCE

The data identifies Bmi1 as an important factor for expansion but not initiation of RAF driven NSCLC.

Promotion of lung carcinogenesis by chronic obstructive pulmonary disease-like airway inflammation in a K-ras-induced mouse model

Lung cancer is the leading cause of cancer deaths in the United States. In addition to genetic abnormalities induced by cigarette smoke, several epidemiologic studies have found that smokers with chronic obstructive pulmonary disease (COPD), an inflammatory disease of the lungs, have an increased risk of lung cancer (1.3- to 4.9-fold) compared to smokers without COPD. This suggests a link between chronic airway inflammation and lung carcinogenesis, independent of tobacco smoke exposure. We studied this association by assaying the inflammatory impact of products of nontypeable Haemophilus influenzae, which colonizes the airways of patients with COPD, on lung cancer promotion in mice with an activated K-ras mutation in their airway epithelium. Two new mouse models of lung cancer were generated by crossing mice harboring the LSL-K-ras(G12D) allele with mice containing Cre recombinase inserted into the Clara cell secretory protein (CCSP) locus, with or without the neomycin cassette excised (CCSP(Cre) and CCSP(Cre-Neo), respectively). Lung lesions in CCSP(Cre-Neo)/LSL-K-ras(G12D) and CCSP(Cre)/LSL-K-ras(G12D) mice appeared at 4 and 1 month of age, respectively, and were classified as epithelial hyperplasia of the bronchioles, adenoma, and adenocarcinoma. Weekly exposure of CCSP(Cre)/LSL-K-ras(G12D) mice to aerosolized nontypeable Haemophilus influenzae lysate from age 6-14 weeks resulted in neutrophil/macrophage/CD8 T-cell-associated COPD-like airway inflammation, a 3.2-fold increase in lung surface tumor number (156 +/- 9 versus 45 +/- 7), and an increase in total lung tumor burden. We conclude that COPD-like airway inflammation promotes lung carcinogenesis in a background of a G12D-activated K-ras allele in airway secretory cells.

Immunogenicity of premalignant lesions is the primary cause of general cytotoxic T lymphocyte unresponsiveness

Cancer is sporadic in nature, characterized by an initial clonal oncogenic event and usually a long latency. When and how it subverts the immune system is unknown. We show, in a model of sporadic immunogenic cancer, that tumor-specific tolerance closely coincides with the first tumor antigen recognition by B cells. During the subsequent latency period until tumors progress, the mice acquire general cytotoxic T lymphocyte (CTL) unresponsiveness, which is associated with high transforming growth factor (TGF) beta1 levels and expansion of immature myeloid cells (iMCs). In mice with large nonimmunogenic tumors, iMCs expand but TGF-beta1 serum levels are normal, and unrelated CTL responses are undiminished. We conclude that (a) tolerance to the tumor antigen occurs at the premalignant stage, (b) tumor latency is unlikely caused by CTL control, and (c) a persistent immunogenic tumor antigen causes general CTL unresponsiveness but tumor burden and iMCs per se do not.

Cis-acting genomic elements of the Pas1 locus control Kras mutability in lung tumors

The Pas1 locus is the major tumor modifier of lung tumorigenesis in mouse inbred strains. Of six genes contained in a conserved haplotype, three (Casc1, Kras and Ifltd1) have been proposed as Pas1 candidates, but mechanistic evidence is sparse. Herein, we examined urethane-induced lung tumorigenesis in a new mouse model developed by replacing the Kras gene with an Hras gene in the susceptible A/J-type Pas1 locus and crossing these mice with either C57BL/6J or A/J mice. Heterozygous mice carrying the Hras-replacement gene were more susceptible than wild-type mice to lung carcinogenesis, indicating that Hras replacement not only compensates for Kras functions, but is more active. Indeed, most lung tumors carried a Gln61Leu mutation in the Hras-replacement gene, whereas no mutations were observed in the endogenous Hras gene. Thus, the context of the Kras locus determined mutability of ras genes. In mice carrying the Hras-replacement gene, the mutation frequency affecting the wild-type Kras gene was much higher when this gene was located in the A/J type than in the C57BL/6J-type Pas1 locus (12 versus 0%, -log P=5.0). These findings identify cis-acting elements in the Pas1 locus as the functional components controlling genetic susceptibility to lung tumorigenesis by modulating mutability of the Kras gene.

Inter-cellular adhesion disruption and the RAS/RAF and beta-catenin signalling in lung cancer progression

Cadherin cell adhesion molecules play an essential role in creating tight intercellular association and their loss has been correlated with poor prognosis in human cancer. Mutational activation of protein kinases and loss of cell adhesion occur together in human lung adenocarcinoma but how these two pathways interconnect is only poorly understood. Mouse models of human lung adenocarcinoma with oncogene expression targeted to subtypes of lung epithelial cells led to formation of adenomas or adenocarcinomas that lacked metastatic potential. Conditional genetic abrogation of epithelial tumour cell adhesion in mice with benign lung tumours induced by oncogenic RAF kinase has been demonstrated to induce intratumourous vascularization (angiogenic switch), progression to invasive adenocarcinoma and micrometastasis. Importantly, breaking cell adhesion in benign oncogene-driven lung tumour cells activated beta-catenin signalling and induced the expression of several genes that are normally expressed in intestine rather than the lung. I will discuss potential routes to nuclear beta-catenin signalling in cancer and how nuclear beta-catenin may epigenetically alter the plasticity of tumour cells during malignant progression.

Anti-tumor activity of ESX1 on cancer cells harboring oncogenic K-ras mutation

Human ESX1 is a 65-kilodalton (kDa) paired-like homeoprotein that is proteolytically processed into N-terminal 45-kDa and C-terminal 20-kDa fragments. The N-terminal ESX1 fragment, which contains the homeodomain, localizes to the nucleus and represses mRNA transcription from the K-ras gene. When we inoculated human colorectal carcinoma HCT116 constitutive expressing N-terminal region of ESX1 (N-ESX1) into nude mice, transfectant cells uniformly showed decreased tumor-forming activity compared with that of the parental cells. Furthermore, pretreatment of HCT116 carcinoma cells with a fusion protein consisting of N-ESX1 and the protein-transduction domain derived from the human immunodeficiency virus type-1 TAT protein gave rise to a dramatic reduction in the tumorigenicity of HCT116 cells in nude mice. Our results provide first in vivo evidence for the molecular targeting therapeutic application of the K-ras repressor ESX1, especially TAT-mediated transduction of N-ESX1, in the treatment of human cancers having oncogenic K-ras mutations.

The adaptive immune response to sporadic cancer

Most of the current experimental cancer models do not reflect the pathophysiology of real-life cancer. Cancer usually occurs sporadically and is clonal in origin. Between tumor initiation and progression, clinically unapparent pre-malignant cells may persist for years or decades in humans. Recently, mouse models of sporadic cancer have been developed. The mouse germ-line can be engineered with high precision so that defined genes can be switched on and off in the adult organism in a targeted manner. Analysis of the immune response against sporadic tumors requires the knowledge of a tumor antigen. Ideally, a silent oncogene, for which the mice are not tolerant, is stochastically activated in individual cells. This approach offers the opportunity to analyze the adaptive immune response throughout the long process of malignant transformation and most closely resembles cancer in humans. In such a model with the highly immunogenic SV40 large T antigen as a dormant oncogene, we discovered that sporadic cancer is recognized by the adaptive immune system at the pre-malignant stage, concomitant with the induction of tumor antigen-specific tolerance. These results demonstrated that even highly immunogenic sporadic tumors are unable to induce functional cytotoxic T lymphocytes. Based on this model, we conclude that immunosurveillance plays little or no role against sporadic cancer and that tumors must not escape immune recognition or destruction.

Ras proteins: paradigms for compartmentalised and isoform-specific signalling

Ras GTPases mediate a wide variety of cellular processes by converting a multitude of extracellular stimuli into specific biological responses including proliferation, differentiation and survival. In mammalian cells, three ras genes encode four Ras isoforms (H-Ras, K-Ras4A, K-Ras4B and N-Ras) that are highly homologous but functionally distinct. Differences between the isoforms, including their post-translational modifications and intracellular sorting, mean that Ras has emerged as an important model system of compartmentalised signalling and membrane biology. Ras isoforms in different subcellular locations are proposed to recruit distinct upstream and downstream accessory proteins and activate multiple signalling pathways. Here, we summarise data relating to isoform-specific signalling, its role in disease and the mechanisms promoting compartmentalised signalling. Further understanding of this field will reveal the role of Ras signalling in development, cellular homeostasis and cancer and may suggest new therapeutic approaches.

LKB1 modulates lung cancer differentiation and metastasis

Germline mutation in serine/threonine kinase 11 (STK11, also called LKB1) results in Peutz-Jeghers syndrome, characterized by intestinal hamartomas and increased incidence of epithelial cancers. Although uncommon in most sporadic cancers, inactivating somatic mutations of LKB1 have been reported in primary human lung adenocarcinomas and derivative cell lines. Here we used a somatically activatable mutant Kras-driven model of mouse lung cancer to compare the role of Lkb1 to other tumour suppressors in lung cancer. Although Kras mutation cooperated with loss of p53 or Ink4a/Arf (also known as Cdkn2a) in this system, the strongest cooperation was seen with homozygous inactivation of Lkb1. Lkb1-deficient tumours demonstrated shorter latency, an expanded histological spectrum (adeno-, squamous and large-cell carcinoma) and more frequent metastasis compared to tumours lacking p53 or Ink4a/Arf. Pulmonary tumorigenesis was also accelerated by hemizygous inactivation of Lkb1. Consistent with these findings, inactivation of LKB1 was found in 34% and 19% of 144 analysed human lung adenocarcinomas and squamous cell carcinomas, respectively. Expression profiling in human lung cancer cell lines and mouse lung tumours identified a variety of metastasis-promoting genes, such as NEDD9, VEGFC and CD24, as targets of LKB1 repression in lung cancer. These studies establish LKB1 as a critical barrier to pulmonary tumorigenesis, controlling initiation, differentiation and metastasis.

p38alpha MAP kinase is essential in lung stem and progenitor cell proliferation and differentiation

Stem cell function is central for the maintenance of normal tissue homeostasis. Here we show that deletion of p38alpha mitogen-activated protein (MAP) kinase in adult mice results in increased proliferation and defective differentiation of lung stem and progenitor cells both in vivo and in vitro. We found that p38alpha positively regulates factors such as CCAAT/enhancer-binding protein that are required for lung cell differentiation. In addition, p38alpha controls self-renewal of the lung stem and progenitor cell population by inhibiting proliferation-inducing signals, most notably epidermal growth factor receptor. As a consequence, the inactivation of p38alpha leads to an immature and hyperproliferative lung epithelium that is highly sensitized to K-Ras(G12V)-induced tumorigenesis. Our results indicate that by coordinating proliferation and differentiation signals in lung stem and progenitor cells, p38alpha has a key role in the regulation of lung cell renewal and tumorigenesis.

Toxicity of ligand-dependent Cre recombinases and generation of a conditional Cre deleter mouse allowing mosaic recombination in peripheral tissues

Ligand-activated Cre recombinases are widely used for studying gene function in vitro and in conditional mouse models. To compare ligand-dependent Cre recombinases, different Cre estrogen receptor fusions were introduced into the ROSA26 locus of embryonic stem (ES) cells and assayed for genotoxicity and recombination efficiency. Of the tested recombinases, the CreERT2 variant showed no toxicity and was highly responsive to ligand induction. To constitutively express CreERT2 in mice and also to clarify whether the CreERT2 system displays background activity, we generated a knock-in mouse line harboring the CreERT2 coding region under the control of the ROSA26 locus. Analysis of this ROSA26-CreERT2 deleter mouse with different reporter strains revealed ubiquitous recombination in the embryo and partial recombination in peripheral and hematopoietic tissues but no effective CreERT2 expression in the brain. Furthermore, using flow cytometry, we found low-level background recombination in noninduced bitransgenic ROSA26-CreERT2/EGFP reporter mice. To determine whether background activity poses a general problem for conducting conditional in vivo experiments with the ROSA26-CreERT2 deleter, we used a sensitive conditional skin cancer model. In this assay, cancer induction was completely restricted to induced bitransgenic CreERT2/K-Ras(V12) mice, whereas noninduced control animals did not show any sign of cancer, indicating the usefulness of the ROSA-CreERT2 system for regulating conditional gene expression in vivo. The ROSA26-CreERT2 deleter strain will be a convenient experimental tool for studying gene function under circumstances requiring partial induction of recombination in peripheral tissues and will be useful for uncovering previously unknown or unsuspected phenotypes.

Conditional expression of mutated K-ras accelerates intestinal tumorigenesis in Msh2-deficient mice

K-ras mutation occurs in 40-50% of human colorectal adenomas and carcinomas, but its contribution to intestinal tumorigenesis in vivo is unclear. We developed K-ras(V12) transgenic mice that were crossed with Ah-Cre mice to generate K-ras(V12)/Cre mice, which showed beta-naphthoflavone-induction of Cre-mediated LoxP recombination that activated intestinal expression of K-ras(V12) 4A and 4B transcripts and proteins. Only very occasional intestinal adenomas were observed in beta-naphthoflavone-treated K-ras(V12)/Cre mice aged up to 2 years, suggesting that mutated K-ras expression alone does not significantly initiate intestinal tumourigenesis. To investigate the effects of mutated K-ras on DNA mismatch repair (MMR)-deficient intestinal tumour formation, these mice were crossed with Msh2(-/-) mice to generate K-ras(V12)/Cre/Msh2(-/-) offspring. After beta-naphthoflavone treatment, K-ras(V12)/Cre/Msh2(-/-) mice showed reduced average lifespan of 17.3+/-5.0 weeks from 26.9+/-6.8 (control Msh2(-/-) mice) (P<0.01). They demonstrated increased adenomas in the small intestine from 1.41 (Msh2(-/-) controls) to 7.75 per mouse (increased fivefold, P<0.01). In the large intestine, very few adenomas were found in Msh2(-/-) mice (0.13 per mouse) whereas K-ras(V12)/Cre/Msh2(-/-) mice produced 2.70 adenomas per mouse (increased 20-fold, P<0.01). Over 80% adenomas from K-ras(V12)/Cre/Msh2(-/-) mice showed transgene recombination with expression of K-ras(V12) 4A and 4B transcripts and proteins. Sequencing of endogenous murine K-ras showed mutations in two out of 10 tumours examined from Msh2(-/-) mice, but no mutations in 17 tumours from K-ras(V12)/Cre/Msh2(-/-) mice. Expression of K-ras(V12) in tumours caused activation of the mitogen-activated protein kinase and Akt/protein kinase B signaling pathways, demonstrated by phosphorylation of p44MAPK, Akt and GSK3beta, as well as transcriptional upregulation of Pem, Tcl-1 and Trap1a genes (known targets of K-ras(V12) expression in stem cells). Thus, mutated K-ras cooperates synergistically with MMR deficiency to accelerate intestinal tumorigenesis, particularly in the large intestine.

Lung cancer and lung stem cells: strange bedfellows?

Lung cancer is a significant disease with survival rates remaining poor despite numerous therapeutic advances during the last 30 years. Understanding lung cancer pathogenesis through murine modeling may improve future human therapies, and new data indicate that mutations within different endogenous stem cells situated throughout airways can drive cancer formation. Airway stem cells maintain prototumorigenic characteristics, including high proliferative capacity, multipotent differentiation, and a long lifespan relative to other cells. These cells localize to proximal airway submucosal glands/intercartilagenous rings, neuroepithelial bodies, and terminal bronchioles/bronchoalveolar duct junctions. Recent studies suggest that endogenous stem cell signaling and differentiation pathways are maintained within distinct cancer types, and that destabilization of this signaling machinery may initiate region-specific lung cancers. A better understanding of this relationship among stem cell regulation, cellular mutation, and lung cancer oncogenesis is critical for developing the next wave of lung cancer therapies.

Using genetically engineered mouse models of cancer to aid drug development: an industry perspective

Recent developments in the generation and characterization of genetically engineered mouse models of human cancer have resulted in notable improvements in these models as platforms for preclinical target validation and experimental therapeutics. In this review, we enumerate the criteria used to assess the accuracy of various models with respect to human disease and provide some examples of their prognostic and therapeutic utility, focusing on models for cancers that affect the largest populations. Technological advancements that allow greater exploitation of genetically engineered mouse models, such as RNA interference in vivo, are described in the context of target and drug validation. Finally, this review discusses stratagems for, and obstacles to, the application of these models in the drug development process.

Building a better model of cancer

The 2006 Cold Spring Harbor Laboratory meeting on the Mechanisms and Models of Cancer was held August 16–20. The meeting featured several hundred presentations of many short talks (mostly selected from the abstracts) and posters, with the airing of a number of exciting new discoveries. We will focus this meeting review on models of cancer (primarily mouse models), highlighting recent advances in new mouse models that better recapitulate sporadic tumorigenesis, demonstrations of tumor addiction to tumor suppressor inactivation, new insight into senescence as a tumor barrier, improved understanding of the evolutionary paths of cancer development, and environmental/immunological influences on cancer.

Sustained leukaemic phenotype after inactivation of BCR-ABLp190 in mice

Pharmacological inactivation of cancer genes or products is being used as a strategy for therapy in oncology. To investigate the potential role of BCR-ABLp190 cessation in leukaemia development, we generated mice carrying a tetracycline-repressible BCR-ABLp190 transgene. These mice were morphologically normal at birth, and developed leukaemias. Disease was characterized by the presence of B-cell blasts co-expressing myeloid markers, reminiscent of the human counterpart. BCR-ABLp190 activation can initiate leukaemia in both young and adult mice. Transitory expression of BCR-ABLp190 is enough to develop leukaemia. Suppression of the BCR-ABLp190 transgene in leukaemic CombitTA-p190 mice did not rescue the malignant phenotype, indicating that BCR-ABLp190 is not required to maintain the disease in mice. Similar results were obtained by inactivation of BCR-ABLp190 with STI571 (Gleevec; Novartis, East Hanover, NJ, USA) in leukaemic CombitTA-p190 mice. However, gradual suppression of BCR-ABLp190 in leukaemic CombitTA-p190 mice identified a minimum level of BCR-ABLp190 expression necessary to revert the specific block in B-cell differentiation in the leukaemic cells. Overall, the findings indicate that BCR-ABLp190 appears to cause epigenetic and/or genetic changes in tumour-maintaining cells that render them insensitive to BCR-ABLp190 inactivation.

Genetic and epigenetic alterations in lung tumors from bitransgenic Ki-rasG12C expressing mice

Mutations in Ki-ras occur in approximately 30-50% of patients with adenocarcinoma (AC) of the lung. We previously reported the development of a bitransgenic mouse model that expressed the human Ki-ras(G12C) allele in a lung-specific, tetracycline-inducible manner and gave rise to benign lung tumors. In the current study, these benign tumors, which represent relatively early lesions in neoplastic progression, were analyzed for molecular alterations secondary to mutant Ki-ras expression to determine the gene(s) that contribute to adenoma (AD) development. Tumors were removed following doxycycline (DOX) treatment for 9 and 12 mo and examined for alterations in cell-cycle regulatory genes. Quantification of mRNA expression for cyclin D1, retinoblastoma, p16(Ink4a), p19(Arf), and survivin was carried out by real-time PCR. All of the tumors examined exhibited a mean reduction of approximately fivefold for the retinoblastoma gene (P < 0.02). Increased expression of both p19(Arf) and survivin were detected in a majority of the tumors examined (P < 0.01 and 0.001, respectively), but no change in cyclin D1 RNA expression was observed. A subset of the lung tumors (8/28) displayed reduced levels of p16(Ink4a) expression (P = 0.02). Immunohistochemical analysis confirmed the upregulation of p19(Arf) and survivin in all 10 of the lung tumors examined. However, increased staining for cyclin D1 was observed in the tumor tissue. In addition, increased levels of activated p53 were found in lung tumor tissues stained with an anti-phospho-p53 antibody, while an absence of staining was observed with an anti-phospho-pRb antibody in both normal control and tumor tissue. Analysis of the methylation status of p16(Ink4a) by methylation-specific PCR (MSP) demonstrated that seven of eight tumors exhibiting decreased expression of p16(Ink4a) had at least partial methylation of the promoter region. Single stranded conformational polymorphism (SSCP) analysis demonstrated that neither exons 1 or 2 of p16(Ink4a) nor exons 5-8 of p53 exhibited mutations. These data thus identify alterations in specific genes and pathways that combine with the mutation in Ki-ras to promote the formation of benign lung tumors and suggest potential targets for the development of novel chemotherapeutic and chemopreventive agents during the early stages of lung tumor progression.