Risk estimation from somatic mutation assays

The ability to quantify somatic mutations in vivo provides a new source of toxicological information that is relevant to the assessment of cancer risk. The major experimental factors that influence the mutant frequency are age, time after treatment, treatment protocol, and tissue analyzed. In untreated mice, the mutant frequency increases very rapidly with age from conception to birth, more slowly from birth to adulthood, and very slowly thereafter. All somatic tissues studied so far in adults have similar mutant frequencies. The time after treatment (expression time) is the most important experimental variable. The minimum time for expression varies from one tissue to another. To be valid, comparisons between tissues and treatments must be made after complete expression of the mutations. Unfortunately, the minimum expression time has not been characterized in most tissues. Since carcinogens are tissue specific, and many chemicals are distributed in the body in complex patterns, it is to be expected that there will be differences in the frequency of mutation induced in different tissues. As yet this has not been extensively studied. Since the mutations detected by the transgenic assays are neutral, the mutants should accumulate as the integral of the mutation rate. Hence chronic treatment protocols should be more effective than acute and subacute protocols whenever they permit substantially larger doses to be delivered. Such protocols are more relevant to human exposure and are preferable for dose extrapolations. The importance of transcription in determining mutation rates is not yet known, but it is noteworthy that the transgenes are not transcribed whereas the Ioci involved in carcinogenesis are. The mutation spectrum is important for quantitative risk estimation. Risk estimation must also take into account the spectrum of mutations that are involved in the carcinogenic process in the tissue and the spectrum of mutations that are detectable by the assay. New assays are being used to quantify mutations in vivo in order to understand the carcinogenic process, to search for the environmental factors involved in human cancer, and to evaluate the carcinogenic hazard qualitatively.