Mouse models for human lung cancer

Mouse models of lung cancer

Human lung cancer is responsible for approximately 30% of all cancer deaths worldwide with >160,000 deaths in the United States alone annually. Recent advances in the identification of novel mutations relevant to lung cancer from a myriad of genomic studies might translate into meaningful diagnostic and therapeutic progress. Towards this end, a genetic model animal system that can validate the oncogenic roles of these mutations in vivo would facilitate the understanding of the pathogenesis of lung cancer as well as provide ideal preclinical models for targeted therapy testing. The mouse is a promising model system, as complex human genetic traits causal to lung cancer, from inherited polymorphisms to somatic mutations, can be recapitulated in its genome via genetic manipulation. We present here a brief overview of the existing mouse models of lung cancers and the challenges and opportunities for building the next generation of lung cancer mouse models.

Harnessing preclinical mouse models to inform human clinical cancer trials

The urgent need for better cancer treatments has stimulated interest in employing small-animal models to evaluate potential drug therapies. Robust mouse models of many human cancers have been generated using sophisticated technologies for engineering germ-line mutations. As we enter into an age of targeted therapeutics, these strains provide novel platforms for validating new anticancer drugs, assessing therapeutic index, identifying surrogate markers of tumor progression, and defining epigenetic and environmental influences on tumorigenesis.

Functional facets of the pulmonary neuroendocrine system

Pulmonary neuroendocrine cells (PNECs) have been around for 60 years in the scientific literature, although phylogenetically they are ancient. Their traditionally ascribed functions include chemoreception and regulation of lung maturation and growth. There is recent evidence that neuroendocrine (NE) differentiation in the lung is regulated by genes and pathways that are conserved in the development of the nervous system from Drosophila to humans (such as achaete-scute homolog-1), or implicated in the carcinogenesis of the nervous or NE system (such as the retinoblastoma tumor suppressor gene). In addition, complex neural networks are in place to regulate chemosensory and other functions. Even solitary PNECs appear to be innervated. For the first time ever, we have mouse models for lung NE carcinomas, including the most common and virulent small cell lung carcinoma. Moreover, PNECs may be important for inflammatory responses, and pivotal for lung stem cell niches. These discoveries signify an exciting new era for PNECs and are likely to have therapeutic and diagnostic applications.

Analysis of orthologous gene expression between human pulmonary adenocarcinoma and a carcinogen-induced murine model

Human adenocarcinoma (AC) is the most frequently diagnosed human lung cancer, and its absolute incidence is increasing dramatically. Compared to human lung AC, the A/J mouse-urethane model exhibits similar histological appearance and molecular changes. We examined the gene expression profiles of human and murine lung tissues (normal or AC) and compared the two species' datasets after aligning approximately 7500 orthologous genes. A list of 409 gene classifiers (P value <0.0001), common to both species (joint classifiers), showed significant, positive correlation in expression levels between the two species. A number of previously reported expression changes were recapitulated in both species, such as changes in glycolytic enzymes and cell-cycle proteins. Unexpectedly, joint classifiers in angiogenesis were uniformly down-regulated in tumor tissues. The eicosanoid pathway enzymes prostacyclin synthase (PGIS) and inducible prostaglandin E(2) synthase (PGES) were joint classifiers that showed opposite effects in lung AC (PGIS down-regulated; PGES up-regulated). Finally, tissue microarrays identified the same protein expression pattern for PGIS and PGES in 108 different non-small cell lung cancer biopsies, and the detection of PGIS had statistically significant prognostic value in patient survival. Thus, the A/J mouse-urethane model reflects significant molecular details of human lung AC, and comparison of changes in orthologous gene expression may provide novel insights into lung carcinogenesis.

Epigenetic regulation of the tumor suppressor gene TCF21 on 6q23-q24 in lung and head and neck cancer

The identification of tumor suppressor genes has classically depended on their localization within recurrent regions of loss of heterozygosity. According to Knudson's two-hit hypothesis, the remaining allele is lost, either genetically or, more recently identified, through epigenetic events. To date, retrospective analyses have determined promoter methylation as a common alternative alteration in cancer cells to silence cancer-related genes. Here we report an application of restriction landmark genomic scanning that allows for DNA methylation profiling along a region of recurrent loss of heterozygosity at chromosome 6q23-q24. This approach resulted in the identification of a tumor suppressor gene, TCF21, which is frequently lost in human malignancies. We demonstrate that TCF21 is expressed in normal lung airway epithelial cells and aberrantly methylated and silenced in the majority of head and neck squamous cell carcinomas and non-small-cell lung cancers analyzed. TCF21 is known to regulate mesenchymal cell transition into epithelial cells, a property that has been shown to be deficient in carcinomas. We further demonstrate that exogenous expression of TCF21 in cells that have silenced the endogenous TCF21 locus resulted in a reduction of tumor properties in vitro and in vivo.

A pathway in quiescent cells that controls p27Kip1 stability, subcellular localization, and tumor suppression

We have created two knock-in mouse models to study the mechanisms that regulate p27 in normal cells and cause misregulation of p27 in tumors: p27(S10A), in which Ser10 is mutated to Ala; and p27(CK-), in which point mutations abrogate the ability of p27 to bind cyclins and CDKs. These two mutant alleles identify steps in a pathway that controls the proteasomal degradation of p27 uniquely in quiescent cells: Dephosphorylation of p27 on Ser10 inhibits p27 nuclear export and promotes its assembly into cyclin-CDK complexes, which is, in turn, necessary for p27 turnover. We further show that Ras-dependent lung tumorigenesis is associated with increased phosphorylation on Ser10 and cytoplasmic mislocalization of p27. Indeed, we find that p27(S10A) is refractory to Ras-induced cytoplasmic translocation and that p27(S10A) mice are tumor resistant. Thus, phosphorylation of p27 on Ser10 is an important event in the regulation of the tumor suppressor function of p27.

Three-dimensional magnetic resonance microscopy of pulmonary solitary tumors in transgenic mice

We attempted to accurately detect pulmonary solitary tumors and other complicated pulmonary disorders in aging inbred transgenic mice by cardiac- and respiratory-gated MR microscopy at 4.7 Tesla. A comparison of in vivo MR images with histological results demonstrated that submillimeter lung tumors and most of the nontumor lesions could be detected by screening with two-dimensional (2D) gradient echo (GRE) imaging. Subsequently performed 2D spin-echo (SE) imaging provided higher image contrast, which distinguished the tumors from the surrounding complications. On the 3D GRE images and the generated maximum intensity projection (MIP) and volume-rendered (VR) images, proper spatial localization of solitary tumors relative to the orientation of the pulmonary vessels was exhibited, and the tumor volume could also be measured. This promising method is noninvasive and has the potential to eventually replace invasive histopathology because it obviates the need to kill groups of animals at multiple time points.

Quantitative analysis of early chemically-induced pulmonary lesions in mice of varying susceptibilities to lung tumorigenesis

Inbred mice vary in their susceptibility to develop macroscopic, chemically-induced, pulmonary neoplasias. It is not known, however, whether microscopic lesions appear in resistant strains but do not grow or if no early lesions arise at all. We show herein that resistant C57BL/6J (B6) and intermediately resistant BALB/cByJ (BALB) mice form very few urethane-induced early microadenomas (i.e. adenomas larger than hyperplasic foci, but detectable only by light microscopy). Additionally, while all urethane-induced microadenomas in sensitive A/J mice gave rise to adenomas, most microscopic tumors induced in BALB mice by 2-stage, 3-methylcholanthrene/butylated hydroxytoluene carcinogenesis spontaneously regressed. The formation of microscopic lesions is thus genetically dependent, but whether they continue to grow or regress depends on how they were induced.

Deletion of XPC leads to lung tumors in mice and is associated with early events in human lung carcinogenesis

Chromosome 3p and 1p deletions are among the most frequent genetic changes in human lung cancer and although candidate tumor suppressor genes have been identified in these regions, no causative correlations have been drawn between deletion or mutation of these and lung carcinogenesis. We identify XPC and Gadd45a as genes within each of these regions involved in lung tumor initiation and progression, respectively. One hundred percent of XPC-/- mice develop multiple spontaneous lung tumors with a minority progressing to non-small cell lung adenocarcinoma, occasionally with metastasis to adjacent lymph nodes. Deletion of Gadd45a alone does not lead to increased lung tumors in mice, but coupled with an XPC deletion, it results in lung tumor progression. Analysis of published data indicated allelic loss of XPC in most human lung tumors and allelic loss of Gadd45a in some human lung and other cancer types. Because DNA repair capacity is compromised in XPC+/- cells, it is possible that the loss of a single XPC allele in the human lung might confer a mutator phenotype. Coupled with cigarette carcinogens, decreased DNA repair would lead to additional mutations in genes such as p53 that are frequent targets in lung cancer.

Genotype-phenotype relationships in a mouse model for human small-cell lung cancer

Lung tumors are usually classified into small-cell lung cancer (SCLC) or non-SCLC (NSCLC) depending on their pathological and histological characteristics. SCLC is defined not only by its characteristic neuroendocrine differentiation, aggressiveness, and metastatic potential, but also by a specific set of genetic aberrations, including the loss of the tumor suppressor genes p53 and Rb1 and the amplification of any member of the Myc family of oncogenes. We have previously described a mouse model of SCLC by somatic conditional disruption of Trp53 and Rb1 genes that closely resembles the human condition. Based on the possibility to study early tumor lesions and to culture and subclone progressed tumors and metastases, we discuss here a strategy to define genotype-phenotype relationships that can explain the underlying biology of lung neuroendocrine tumors. We have found that tumors may be constituted by genetically variant cell populations, which might represent different progression stages. Interestingly, we observed L-myc amplification and Ascl-1 expression in those populations showing neuroendocrine differentiation. Non-neuroendocrine cell populations from the same tumors did not show L-myc amplification nor Ascl-1 expression. We propose that this genetic divergence can play a relevant role in the definition of some phenotypic characteristics like metastasis potential or chemoresistance.