Factors affecting somatic mutation frequencies in vivo

The factors that influence the spontaneous mutant frequencies in mammalian tissues have been ranked on the basis of data from our laboratory together with published data. Some of the data come from the endogenous hprt and Dlb-1 loci, but most come from transgenic mice carrying the bacterial lacI and lacZ genes in recoverable lambda phage vectors. Since there is evidence that these bacterial loci are selectively neutral, the mutant frequency observed is the integral of the mutation rates from the formation of the zygote. The factors that affect the inferred mutation rate, in decreasing order of importance are: site of integration of the transgene, age, tissue, and strain. Insufficient data exist to determine the influence of gender (probably small) and inter-laboratory variables (probably at least as important as age). The two most surprising results are (1) that about half of all mutations arise during development (and half of these in utero) and (2) that most somatic tissues, whether queiscent or actively proliferating, have similar mutant frequencies and similar increases during adult life.