Evidence for an increase in presumed somatic mutation during the ageing of human cells in culture.

A method has been developed for the direct measurement of genetic variants in mammalian cells in culture which does not require cell division and is therefore suitable for ageing studies. Previous evidence suggests that these variants are mutations. The variant frequency was measured during the lifespan of populations of MRC-5 human embryo lung fibroblasts. The frequency increased substantially during the lifespan of the culture and regression analysis shows that the increase is exponential. No increase invariant frequency was found in a immortal transformed line cultured for over 100 passages. Evidence is presented that the variants are regulatory mutations. This is the first direct evidence for the involvement of presumed somatic mutations in the ageing of mammalian cells. 5-Fluorouracil but not p-fluorophenylalanine shortened the life of the cells and increase the variant frequency. The data lend support to a model in which mutations are an expected consequence of errors in protein synthesis.