Fig. 5. Metronomic imposition of Myc hypomorphism protects against KRasG12D-driven lung tumourigenesis without triggering associated haematological pathologies.
a Schematic of metronomic prevention study. Endogenous Myc was maintained in MRK mice at normal physiological wt levels throughout development and into adult life by continuous administration of tetracycline. At 5 weeks of age tetracycline was withdrawn to hypomorph endogenous Myc and 3 weeks later KRasG12D sporadically activated in lung epithelium by Adv-Cre inhalation. Two weeks later, mice were put on a metronomic regimen of 1 week’s relaxation from hypomorphism followed by 4 weeks’ re-imposition of Myc hypomorphism, and this was then repeated for a total of 4 cycles. Peripheral blood was withdrawn for analysis just prior to each 1 week’s relaxation period. Normal levels of endogenous Myc were maintained in control mice by continuous administration of tetracycline throughout. Mice were euthanized 22 weeks post Adv-Cre activation of KRasG12D and tissues harvested. b Metronomic imposition of Myc hypomorphism spares mice from haematological pathology. Blood counts of total leukocytes, erythrocytes and platelets taken at various times of the metronomic regimen described (10, 15 and 20 weeks) showing substantial amelioration of the pathological changes caused by sustained Myc hypomorphism. Control mice were maintained on tetracycline throughout. Results represent mean ± SD in each treatment group. Multiple Unpaired t test with Welch correction, single pooled variance, Holm-Šídák method, shows non-significant difference between groups. For leukocytes, at 10 weeks n = 11 and n = 4 for non-Myc hypomorphed and metronomically Myc hypomorphed mice, respectively, with adjusted p = 0.368391; at 15 weeks n = 7 for both groups with adjusted p = 0.974878; at 20 weeks n = 3 and n = 7, respectively, with adjusted p = 0.966531. For erythrocytes, at 10 weeks n = 4 for both non-Myc hypomorphed and metronomically Myc hypomorphed mice with adjusted p = 0.978717; at 15 weeks n = 11 and n = 8, respectively, with adjusted p = 0.978717; at 20 weeks n = 3 and n = 4, respectively, with adjusted p = 0.707513. For Platelets, at 10 weeks n = 4 and n = 5 for non-Myc hypomorphed and metronomically Myc hypomorphed mice, respectively, with adjusted p = 0.286349; at 15 weeks n = 3 and p = 9, respectively, with adjusted p = 0.928754; at 20 weeks n = 3 and n = 5, respectively with adjusted p = 0.928754. c Left: percentage lung tumour burden relative to total lung in control (normal Myc level – blue) versus metronomically Myc hypomorphed MRK mice (black) 22 weeks post Adv-Cre activation of KRasG12D. Results depict mean ± SD in each treatment group. The unpaired t-test Welch’s correction and two-tailed analysis was used to analyse tumour burden. ***p < 0.001. SD = standard deviation. n = 11 for non-Myc hypomorphed and n = 8 for metronomically Myc hypomorphed mice with p = 0.0004. Right: Representative H&E-stained sections of lungs from control (normal Myc level) versus metronomically Myc hypomorphed MRK mice 22 weeks post Adv-Cre activation of KRasG12D. Arrows mark regions shown at higher magnification below. Source data are provided as a Source Data file.