Figure 3. p53 constrains lung tumour progression in the Kras^LA2-MADM model.

(a) Schematic of MADM-mediated LOH of p53 in Kras^LA2,Rosa26-Cre^ERT2/Kras^WT; MADM-p53 mice. Stochastic recombination results in removal of one of two duplicate copies of mutant Kras exon1 (Kras^G12D) and an intervening neo cassette permitting expression of mutant Kras expression and tumour initiation²⁵. G2-X MADM recombination, resulting in p53^KO/KO (green, GFP+/tdTomato−) and p53^WT/WT (red, GFP−/tdTomato+) cells, is initiated through tamoxifen activation of Cre^ERT2, permitting localization of Cre to the nucleus. This diagram was adapted with permission from the original MADM schematic²¹. (b) Two green tumours (black arrows) were observed on whole-mount analysis of lungs from Kras^LA2, Rosa26-Cre^ERT2/Kras^WT; MADM-p53 mice (n=8), whereas none were observed in Kras^LA2,Rosa26-Cre^ERT2/Kras^WT; MADM mice (not harbouring p53 mutation, n=10). White arrows denote tumors without fluorescence labelling. We did not detect any red or yellow tumours in either cohort of mice by whole-mount analysis. Merged fluorescence images of green and red filters are shown. (c) Histologic section of a tumour in b showed green adenocarcinoma cells adjacent to colourless adenoma cells (predominantly to the right of the line). Some green adenocarcinoma cells (arrow) are intercalating in the adenoma area. Blue, DAPI-stained nuclei. Scale bar, 100 μm. (d) Kras^LA2,Rosa26-Cre^ERT2/Kras^WT; MADM-p53 adenoma harbouring rare yellow cells. Blue, DAPI-stained nuclei. Scale bar, 100 μm.