These mice conditionally express an L858R mutation of the Egfr (epidermal growth factor receptor) gene associated with a subset of non-small cell lung cancers in humans. A floxed Stop cassette blocks expression of Egfr until it is excised with Cre recombinase.
Dr. Tyler Jacks, Massachusetts Institute of Technology
Egfr (epidermal growth factor receptor) is a transmembrane protein in the erbB family of tyrosine kinase receptors. Extracellular ligands such as epidermal growth factor (EGF) and transforming growth factor-α (TGF-α) bind EGFR and induce receptor dimerization, autophosphorylation and activation of signal transduction pathways. Mutations in the kinase domain of EGFR, among them L858R (leucine to arginine), are associated with approximately 10% of non-small cell lung cancers (NSCLCs) in the United States and with approximately 35% of cases in Asia.
In this mutant mouse strain, a loxP-flanked Stop cassette(LSL) located in intron 1 of Egfr blocks expression of the gene. Homozygous floxed mice express no EGFR protein are not viable, mimicking the phenotype of the Egfr knockout mice. Upon Cre-mediated excision of the floxed stop cassette, the L858R mutant form of the protein is expressed, albeit at levels lower than those found in wildtype mice. This reduced expression is thought to be the result of the mutant protein being constitutively active and engaging a negative feedback loop which results in its degradation.
When mutant protein expression is directed to the lung (via adeno-cre or lentiviral-cre excision of the LSL-cassette), mice fail to develop the anticipated lung tumors. Most human patients with EGFR mutations have amplification of the gene in their tumors, suggesting that both mutation and overexpression are required for tumor development. If the stop cassette is deleted in the germline, mice develop a wavy coat characteristic of other Egfr mutations and are also reduced in body size. These mice also develop low grade liver lesions at a late time point. Mutant mice lacking the stop cassette are viable as homozygotes.
This strain was constructed with a two-step targeting approach. A construct containing an FRT-neo-FRT cassette in intron 20 and a T->G point mutation in exon 21 was targeted to the Egfr locus in C57BL/6-derived v26.2 embryonic stem (ES) cells. The ES cells were then re-targeted to place a lox-Stop-lox (LSL) cassette in intron 1 of Egfr. ES cells carrying both targeting events in cis were then selected for embryo injection. Germline mice were crossed to a C57BL/6 background Flpe deleter line to remove the FRT-neo-FRT cassette. This strain was maintained on a mixed genetic backgound that involves 129 and C57BL/6 by the donating laboratory.
|Allele Name||targeted mutation 1.1, Tyler Jacks|
|Gene Symbol and Name||Egfr, epidermal growth factor receptor|
|Strain of Origin||C57BL/6|
|Molecular Note||The targeting construct introduced an FRT-neo-FRT cassette into intron 20 and a T->G point mutation in exon 21. The point mutation resulted in the amino acid substitution of arginine for lysine at position 858 (L858R). The locus was re-targeted to place a lox-Stop-lox (LSL) cassette in intron 1. Flp-mediated recombination removed the neo cassette.|
Homozygous floxed stop (LSL) animals are not viable as they are in essence null for Egfr which is known to be incompatible with development. Mutant mice lacking the stop cassette can be bred to homozygosity and are viable.
When using the STOCK Egfrtm1.1Tyj/J mouse strain in a publication, please cite the originating article(s) and include JAX stock #018473 in your Materials and Methods section.
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