Autochthonous murine models for the study of smoker and never-smoker associated lung cancers

Abstract

Lung cancer accounts for the greatest number of cancer deaths in the world. Tobacco smoke-associated cancers constitute the majority of lung cancer cases but never-smoker cancers comprise a significant and increasing fraction of cases. Recent genomic and transcriptomic sequencing efforts of lung cancers have revealed distinct sets of genetic aberrations of smoker and never-smoker lung cancers that implicate disparate biology and therapeutic strategies. Autochthonous mouse models have contributed greatly to our understanding of lung cancer biology and identified novel therapeutic targets and strategies in the era of targeted therapy. With the emergence of immuno-oncology, mouse models may continue to serve as valuable platforms for novel biological insights and therapeutic strategies. Here, we will review the variety of available autochthonous mouse models of lung cancer, their relation to human smoker and never-smoker lung cancers, and their application to immuno-oncology and immune checkpoint blockade that is revolutionizing lung cancer therapy.

Keywords: Lung cancer; mouse models; never-smoker; smoker.

Figure 1

Methods of autochthonous lung cancer induction. (A) Carcinogens are applied to mice through intraperitoneal injections, tracheal delivery, or cutaneous deposition; (B) oncogenes are expressed under a tissue-specific or a ubiquitous promoter, often during development. Expression of oncogenes under lung specific-promoters leads to development of tumors primarily in the lungs whereas as expression under ubiquitous promoters may generate tumors in ectopic sites; (C) in conditional knock-in mice, a Loxp or Frt (blue triangle)–STOP–Loxp (LSL) or Frt (FSF) cassette prevents expression of an oncogene in the absence of Cre or Flp recombinase, respectively. Cre or Flp can be expressed via (I) delivery of adenovirus- or lentivirus-expressing recombinase, (II) expression of Cre under a tissue specific promoter or (III) expression of a tamoxifen-inducible Cre-mutated estrogen receptor fusion protein (CreERT) under a tissue-specific promoter. CreERT is expressed in a tissue specific manner but is sequestered from the nucleus until tamoxifen engages the fused estrogen receptor leading to DNA recombination; (D) loxp or Frt sites are placed in genes such that one or several exons will be deleted by Cre or Flp recombination. Cre or Flp is delivered in analogous methods for oncogene expression as in (C); (E) a tissue-specific or ubiquitous promoter regulates the expression of tTA (Tet-Off system) or rtTA (Tet-On system) transcriptional activators. Tetracycline or its analogue, doxycycline, binds to tTA (Tet-Off) to inhibit transcription and its withdrawal is required for oncogene transcription. In contrast, tetracycline or doxycycline binds to rtTA (Tet-On) transcriptional activator to induce oncogene transcription; (F) lentiviruses encoding for Cre recombinase, Cas9 nuclease, and sgRNA against targeted gene are administered to mice intratracheally. Cre deletes the stop cassette to induce oncogene expression and delete other target genes (usually tumor suppressors). Cas9, guided by target sgRNA, induces indels in the target gene to induce knock-outs of the target gene. Alternatively, mice with LSL-Cas9 under a ubiquitous promoter (in brackets) can be infected with lentiviruses expressing constructs (in brackets) with Cre recombinase, sgRNA against targeted gene, an identification sequence for the sgRNA (ID), and a unique barcode (BC) for multiplex knock-out of distinct genes. The CRISPR/Cas9 system is not completely efficient. Thus, the tumors will be heterogeneous with some expressing the targeted gene(s) (green tumors) and others with the targeted gene(s) knocked-out (pink tumors).