During the past two decades scientists have developed a variety of lung cancer treatments which have proved to be efficient in combating disease manifestations and enabling further research on gene alterations and their effect on lung cancer development . One option is EGFR tyrosine kinase inhibitor. When the tyrosine kinase receptor EGFR experiences a spontaneous mutation, the mutant EGFR protein leads to uncontrolled cell proliferation . Studies on EGFR mutation has led to the development and approval of several drugs by the FDA which block EGFR receptor specifically such as erlotinib and gefitinib . Another molecular target in treating lung cancer is the anaplastic lymphoma kinase (ALK) fusion gene . ALK rearrangements occurring in the ALK kinase domain along with EML4, NPM and TFG have been identified to exhibit oncogenic activity by hyperactivating ALK, thus creating inversions or translocations on chromosome 2 that fuse variable regions of a partner gene with exon 20 of the ALK gene [42–44]. This discovery has led to an increased understanding of ALK’s role in disease metastasis, and subsequently, the development of targeted drugs. The incidence of tumor associated EGFR mutation and anaplastic lymphoma kinase (ALK) rearrangement varies from 10% (in the USA) to 35% (in East Asia) and 5–7%, respectively, in patients with NSCLC [45–48]. The use of tyrosine kinase inhibitors targeting EGFR and ALK subpopulations have resulted in significant patterns of clinical practice [49–51].
In the last few years, treatment of patients with non-small cell lung cancer (NSCLC) has impressively benefitted from immunotherapy, in particular from the inhibition of immune checkpoints such as programmed cell death-1 (PD-1) and its corresponding cell death ligand-1 (PD-L1) [52–57]. Subsequently, immune checkpoint inhibitors (ICI) on T-cell stimulation facilitate immune mediated elimination of tumor cells . These antibody mediated therapies have then shown to produce beneficial effects against many malignancies and now play a major role in advanced lung cancer management . Early clinical trials with drugs such as nivolumab, pembrolizumab or avelumab have shown rapid and durable responses in about 14–20% of pre-treated patients with advanced NSCLC [59–66]. However, concrete evidence suggests that only a small portion of lung cancer patients benefit from this treatment and some patients showed severe immune-related adverse events and systemic autoimmune responses . Unfortunately, very little is known regarding the mechanisms underlying acquired resistance to immune checkpoint inhibitor therapy . It is clear that additional studies are needed to explore the mechanisms behind the resistance to both immune checkpoint inhibitor therapy and targeted therapies, as well as to develop robust pre-clinical in vivo models to evaluate novel treatments with better prediction of their effects in humans.
Our spontaneous non-small cell lung cancer models reported here would provide a valuable tool for evaluating personalized therapeutic strategies. Different from other lung tumor animal models, our lung tumor model limits the damage to the lung. Further, mutations in the KRAS gene, which are acquired mutations, closely mimic the events that lead to spontaneous lung cancer development in humans. More importantly, our model is a “treatable” model, as these mice develop a single lung tumor that is easy to follow up, in contrast to other engineered lung tumor models (e.g. KRAS) which develop multiple lung tumors. In addition, our lung cancer model serves as a treatable model because these tumor bearing mice survive more than 8 weeks after initial detection of lung cancer with a micro CT. Thus, our models provide a sufficient window for evaluating new treatment strategies.
From a histopathological perspective the lung tumors in our mice model resemble human adenocarcinoma (Fig. 5), a major type of non-small cell lung cancer in humans. These lung tumors exhibited areas of variant histology, including areas of clear secretory change, areas of high nuclear grade, areas of oncolytic change and areas of solid proliferation. These variant histological patterns are evidence of dedifferentiation, a phenomenon which human lung tumors readily exhibit .
Our results further demonstrated that the human mutant TP53-273H has a similar oncogenic potential that essentially initiates lung cancer formation in both FVB/N and A/J stains. First, we found that a single spontaneous lung adenocarcinoma developed in both FVB/N and A/J mice. After comparing lung tumor rates between the FVB/N-NT (wild type) mice and the A/J-NT (wild type) mice, we deduced that overall the wild type FVB/N-NT mice are less sensitive to develop a spontaneous tumor (Fig. 2b, Table 1, Table 2). Furthermore, by comparing lung tumor rates between FVB/N-SPC-TP53-273H and A/J-SPC-TP53-273H transgenic mice, we found that the A/J-SPC-TP53-273H transgenic mice have a higher lung cancer rate (Fig. 2A), which may be due to an increased sensitivity in this strain. However, when we compared the oncogenic potential (tumor rate difference between the transgenic mice and non-transgenic mice within a strain and age range), we found the lung tumor rates caused by the human mutant TP53-273H gene were similar between FVB/N and A/J mice (Table 3). For example, at an age of 13–15 months the oncogenic potential of the mutant TP53-273H gene in A/J strain was 0.26 (26%). In the same age range, the oncogenic potential of the mutant TP53-273H gene in FVB/N strain was 0.22 (22%). This indicates that the oncogenic potential observed due to the mutant TP53-273H gene is unique regardless of the fact that A/J mice exhibit higher susceptibility to spontaneous and chemically induced lung cancer [68, 69]. Since FVB/N mice are larger in litter size when directly compared to A/J stain, we think the FVB/N transgenic mice would provide a better platform for anti-cancer treatment evaluations. As the mutant TP53 gene is under the control of the surfactant protein C promoter, these mice develop tumors only in lung tissue. Additionally, these mice have sufficient immune components that resemble the human immune system and deliver a good platform for evaluating immune checkpoint inhibitors in treatment of spontaneous lung cancer.