Adenocarcinoma of the lung is the most common form of lung cancer, but the cell of origin and the stages of progression of this tumor type are not well understood. We have developed a new model of lung adenocarcinoma in mice harboring a conditionally activatable allele of oncogenic K-ras. Here we show that the use of a recombinant adenovirus expressing Cre recombinase (AdenoCre) to induce K-ras G12D expression in the lungs of mice allows control of the timing and multiplicity of tumor initiation. Through the ability to synchronize tumor initiation in these mice, we have been able to characterize the stages of tumor progression. Of particular significance, this system has led to the identification of a new cell type contributing to the development of pulmonary adenocarcinoma.

MicroRNAs (miRNAs) are a new class of small noncoding RNAs that post-transcriptionally regulate the expression of target mRNA transcripts. Many of these target mRNA transcripts are involved in proliferation, differentiation and apoptosis, processes commonly altered during tumorigenesis. Recent work has shown a global decrease of mature miRNA expression in human cancers. However, it is unclear whether this global repression of miRNAs reflects the undifferentiated state of tumors or causally contributes to the transformed phenotype. Here we show that global repression of miRNA maturation promotes cellular transformation and tumorigenesis. Cancer cells expressing short hairpin RNAs (shRNAs) targeting three different components of the miRNA processing machinery showed a substantial decrease in steady-state miRNA levels and a more pronounced transformed phenotype. In animals, miRNA processing-impaired cells formed tumors with accelerated kinetics. These tumors were more invasive than control tumors, suggesting that global miRNA loss enhances tumorigenesis. Furthermore, conditional deletion of Dicer1 enhanced tumor development in a K-Ras-induced mouse model of lung cancer. Overall, these studies indicate that abrogation of global miRNA processing promotes tumorigenesis.

About 30% of human tumours carry ras gene mutations. Of the three genes in this family (composed of K-ras, N-ras and H-ras), K-ras is the most frequently mutated member in human tumours, including adenocarcinomas of the pancreas ( approximately 70-90% incidence), colon ( approximately 50%) and lung ( approximately 25-50%). To construct mouse tumour models involving K-ras, we used a new gene targeting procedure to create mouse strains carrying oncogenic alleles of K-ras that can be activated only on a spontaneous recombination event in the whole animal. Here we show that mice carrying these mutations were highly predisposed to a range of tumour types, predominantly early onset lung cancer. This model was further characterized by examining the effects of germline mutations in the tumour suppressor gene p53, which is known to be mutated along with K-ras in human tumours. This approach has several advantages over traditional transgenic strategies, including that it more closely recapitulates spontaneous oncogene activation as seen in human cancers.