Is there a difference among human populations in the rate with which mutation produces electrophoretic variants?

A model of weak selection in the infinite alleles framework

Ewens (1972) proposed a model in the infinite allele framework for populations with neutrality of all alleles at a particular locus. This paper proposes a generalisation of Ewens' result for situations where there is a form of weak selection. The models considered here are continuous time, discrete state space Markov processes.

A revised indirect estimate of mutation rates in Amerindians

We have previously raised the possibility that the mutation rate resulting in rare electrophoretic variants is higher in tribal/tropical-dwelling/nonindustrialized societies than in civilized/temperate-dwelling/industrialized societies. Here, we report the results of examining 11 additional proteins for the occurrence of rare electrophoretic variants in 10 Amerindian tribes, for a total of 8,968 determinations and a total of 17,648 locus tests. When these data are combined with the results of all our previous similar studies of Amerindians, a total of 272,298 polypeptides, the products of 43 different loci, have been examined for the occurrence of rare electrophoretic variants. On the assumption that these variants are maintained by mutation pressure and are essentially neutral in their phenotypic effects, we have calculated by three different approaches that it requires an average mutation rate of 1.3 X 10(-5)/locus per generation to maintain the observed variant frequency. Concurrently, we are reporting elsewhere that a direct estimate of the mutation rate resulting in electromorphs in various studies of civilized industrialized populations is 0.3 X 10(-5)/locus per generation. Although this difference appears to have statistical significance, the nonquantifiable uncertainties to both approaches are such that our enthusiasm for a true difference in mutation rates between the two types of populations has diminished. However, even the lower of these estimates, when corrected for all the types of genetic variation that electrophoresis does not detect, implies total locus and gametic mutation rates well above those which in the past have dominated genetic thinking.

On three methods for estimating mutation rates indirectly

Methods for estimating a mutation rate mu has been proposed by Kimura and Ohta; Nei; and Rothman and Adams. It is shown here that all three methods are best applied to rare alleles and that they are all based upon the assumption that all alleles ultimately become extinct. If there is a neutral allele in a growing population, there is conditioning on ultimate extinction, which implies that the underlying stochastic process can be approximated by a branching process for which the mean number of offspring is less than 1. The low numerical values of t0, the mean time to extinction of a line descended from a single mutant, found in two simulation studies, can be attributed to two features. First, the data on which these studies were based came from a fairly rapidly growing population. In such a population, we would expect that extinction, if it does occur, takes place quickly. A second factor is that the effective population size is somewhat lower than the actual number of adults. Population subdivision and migration does not seem to play a significant role. Conservative high and low estimates of mutation rates are computed, and an estimate is obtained for the standard deviation of the estimate of mu. These allow a rough estimate of a 95% confidence interval, which contains estimates of mu found by Neel and Rothman.

General human health risks associated with the use of chemicals

Health risks to man associated with the use of chemicals include carcinogenesis, mutagenesis, and systems damage. Adverse effects from chemical exposures are determined by the nature and amount of the chemical, the type and length of exposure, and individual susceptibility to the chemical. A series of short- or long-term tests have been devised to predict human risk from suspect chemicals. These tests have provided enough information to establish guidelines for human safety, but they are not capable of providing sufficient information for unequivocal, scientifically valid standards for exposure limits. New methods now under development promise to provide more detailed information on early effects and cumulative damage to individuals. Developing countries should use existing data and regulatory experiences to the greatest possible extent to establish exposure limits according to local needs. When more ideal methods for the detection of chemical damage to man are available, these approaches and the data derived from their use can be incorporated into the programs in developing countries.

A "disproportion" between the frequency of rare electropmorphs and enzyme deficiency variants in Amerindians

Our previous studies have revealed a higher frequency of nonpolymorphic electrophoretic variants in blood samples from Amerindians than in similar samples from Caucasians and Japanese. Our present study finds, by contrast, that the frequency of deficiency variants of 11 erythrocyte enzymes, sampled in nine Amerindian tribes of Central and South America, is essentially the same (1.5/1,000 determinations) as in Caucasians or Japanese. Possible explanations of the elevated frequency of mobility variants in the tropical-zone/ unacculturated populations include: higher mutation rates resulting in both electrophoretic and activity variants in Amerindians but increased selection against deficiency variants in the Amerindians, or comparable mutation rates in both populations coupled with a greater probability of a mobility variant attaining a relatively high frequency among the Amerindians.

Virus-induced gene mutations of eukaryotic cells

Most animal viruses studied so far induce chromosomal aberrations. In addition, adenoviruses, papovaviruses, and retroviruses are known to induce gene mutations like mutagenic bacteriophages. At least in one case studied retrovirus induced mutagenesis involves gene and/or scripton splitting analogous to the mutagenic mechanism of action of mutatorphage Mu and other movable DNA elements. On the contrary, several results obtained by independent means indicate that Simian virus 40, a papovavirus, does not act by splitting the affected gene but presumably by generation of base pair substitutions or of other minor DNA damages leading to amino acid substitutions. The mechanisms involved are still unknown. There a some hints, however, that these mechanisms might have some step(s) in common with processes leading to malignancy. In fact those viruses proved unequivocally so far to be capable of inducing gene mutations are oncogenic viruses.

Frequency of thermostability variants: estimation of total "rare" variant frequency in human populations

Eight erythrocyte enzymes were examined for thermostability in an unselected sample of 100 newborn infants. Three thermolabile variants, one each of lactate dehydrogenase, glucosephosphate isomerase, and glucose-6-phosphate dehydrogenase, were identified, none of which was detectable as a variant by standard electrophoretic techniques. All were inherited. This frequency of 3.8 heritable thermostabiliy variants per 1000 determinations is to be compared with a frequency of electrophoretically detectable variants of 1.1 per 1000 determinations, a frequency of 2.4 enzyme-deficiency variants per 1000 determinations, and a frequency of 1.1 hypo/hyperactive enzyme-activity variants per 1000 determinations in this human newborn population. The total measured frequency of individuals with rare enzyme deficiency or electrophoretic or thermostability (or both) variants at these loci is 8.3 per 1000 determinations. A similar distribution and frequency is seen when the comparison is limited to the seven loci studied by all techniques. It is clear that not all of the electrophoretic and thermostability variants present in the population are detected by the techniques used in this study. Accordingly, it is estimated that the true frequency of carriers of a rare variant for each of these enzyme-coding loci averages greater than 10/1000. Some implications of these frequencies for human disease are discussed.