Variability of test parameters from mice of different age groups in published data sets

The use of mice as animal models in biomedical research allows the standardization of genetic background, housing conditions as well as experimental protocols, which all affect phenotypic variability. In this study, the phenotypic variability of test parameters was analyzed in genetically identical mice of different age groups, i.e., early adults versus late adults. Therefore, published data sets of genetically identical mice of different age groups collected from the same investigator/ project were retrospectively analyzed. Morphological parameters, blood parameters and behavioral tests were analyzed which are predominantly used in biomedical research. The JaxKOMP project examined C57BL/6NJ mice with an age of 7-20 weeks and 66-81 weeks. Further substrains of C57BL/6N mice with an age of 8-16 weeks and 49-63 weeks were examined as wild-type controls from various investigators of the International Mouse Phenotyping Consortium (IMPC). Additional data sets of young and old groups of genetically identical mice were derived from the Mouse Phenome Database (MPD) and the RIKEN BioResource Research Center (RBRC). The phenotypic variability of the chosen traits and parameters was measured by calculating the coefficient of variation (CV = standard deviation/ mean) of the animals with the same sex of a given mouse strain. Subsequently, the CVs of the young and the old mouse group were compared. The comparison of the phenotypic variability of the late adults versus early adults revealed the appearance of unpredictable interactions between genotype, environment and experiment. Overall, a higher phenotypic variability of the late adults appeared almost consistently for body weight including lean mass and fat mass for females as well as for hematology and immunology parameters, particularly for females. Clinical chemistry often appeared inconspicuous. No noticeable differences were detected for the traits echocardiography and electrocardiogram, whereas late adults also often showed a higher phenotypic variability for behavioral tests.