Neoplastic transformation: the contrasting stability of human and mouse cells.

The probability of a mouse cell becoming fully transformed in vivo or in vitro is enormously greater than that of a human cell. The number of events in tumour progression is similar in rodent and human cells, and it is unlikely that the difference in neoplastic transformation frequency can be explained on the basis of gene mutation in oncogenes and tumour suppressor genes. Instead, it is proposed that mouse cells may be (a) more subject to destabilization of the karyotype, (b) have less efficient check point cell cycle controls after DNA is damaged and/or (c) have less stringent epigenetic controls of gene activity, based on DNA methylation. Much evidence exists that mouse or rat cells are less efficient in DNA repair, maintenance of DNA methylation and other aspects of DNA metabolism. These relate to the difference in longevity in these and other mammalian species. Ageing is likely to be due to the failure of cell and tissue maintenance. Long lived species invest more in various somatic maintenance mechanisms than do short lived ones, and this includes protection against neoplastic transformation. The future study of the basis of the difference between human and mouse or rat cells in resistance to transformation is likely to yield important insights into the sequential events in tumour progression.