Towards an understanding of the nature and fitness of induced mutations in germ cells of mice: homozygous viability and heterozygous fitness effects of induced specific-locus, dominant cataract and enzyme-activity mutations

Studies on mutations in male germ cells of transgenic mice following exposure to isopropyl methanesulfonate, ethylnitrosourea or X-ray

Transgenic mice have recently been used for mutagenesis assays in vivo. The present study was undertaken to clarify whether such assays can detect mutations induced after treatment of male germ cells in mouse with isopropyl methanesulfonate (iPMS), ethylnitrosourea (ENU) or X-ray irradiation. The transgenic mice used for assay are Muta Mouse (MM) strain, which carries 80 copies of the bacterial lacZ gene per cell as targets for mutagenesis. Male MM animals were given a single intraperitoneal injection of 200 mg/kg iPMS, 150 mg/kg ENU or were irradiated with 500 rads of X-rays. Vasa deferential sperm, caudal epididymal sperm and/or whole testes were extracted at various times after treatment with each agent. After the genomic DNA was extracted from each tissue, mutation analysis at the lacZ locus was carried out by the method of Myhr et al. The spontaneous lacZ- mutant frequencies were on the order of 10(-5)-10(-6). The lacZ- mutant frequencies in all treatment groups were increased over the control animals. The iPMS-induced mutant frequency in postmeiotic stages was low. However, ENU induced relatively high mutant frequencies in the spermatogonia. X-rays induced mutant frequencies in the late spermatid and early spermatid stages that were higher than the mutant frequencies in spermatogonia. Mutant frequencies in MM detected after treatment of male germ cells with ENU or X-rays were lower than mutant frequencies detected by the mouse specific-locus test in previous reports. Hence, considering the lower resolution power of the transgenic animal mutagenesis assays using the target lacZ gene compared with the specific locus test, to detect mutations induced in male germ cells, it is not clear whether this assay is a practical alternative to the specific locus test.

Allelism tests of 15 dominant cataract mutations in mice

Fifteen autosomal dominant mutations that cause cataract of lenses in mice were tested for allelism. The outcrosses of double mutants revealed three allelism groups, consisting of 5, 4 and 2 mutations as well as 4 mutations which segregated independently. The results indicated 7 different cataract loci in the sample of 15 mutations. The biomicroscopic examination of the eyes showed that phenotypically similar as well as very distinct cataract mutations can be alleles of the same gene. Conversely, phenotypically similar mutations were shown to be non-allelic.

Induction of specific-locus and dominant lethal mutations in male mice by 1-methyl-1-nitrosourea (MNU)

1-Methyl-1-nitrosourea (MNU) induced specific-locus mutations in mice in all spermatogenic stages except spermatozoa. After intraperitoneal injection of 70 mg/kg body weight of MNU a high yield of specific-locus mutations was observed in spermatids (21.8 x 10(-5) mutations per locus per gamete). The highest mutational yield was induced in differentiating spermatogonia. In 1594 offspring we observed 5 specific-locus mutants (44.8 x 10(-5) mutations per locus per gamete). In addition, 2 mosaics were recovered, which gave a combined mutation rate of 62.7 x 10(-5). In As spermatogonia the mutation rate was 3.9 x 10(-5). The same dose of 70 mg/kg of MNU induced dominant lethal mutations 5-48 days post treatment, mainly due to post-implantation loss in spermatids and spermatocytes. It is interesting to compare the induction pattern of mutations by MNU with methyl methanesulfonate (MMS), ethyl methanesulfonate (EMS) and ethylnitrosourea (ENU). Based on the different spermatogenic response of the induction of specific-locus mutations we can characterize the 4 mutagens in the following way: EMS = MMS not equal to MNU not equal to ENU.

The induction of forward and reverse specific-locus mutations and dominant cataract mutations in spermatogonia of treated strain DBA/2 mice by ethylnitrosourea

The mutagenic effectiveness of ethylnitrosurea (ENU) was assessed in treated spermatogonia of DBA/2 mice. In a total of 17,515 offspring examined following 160 mg ENU/kg body weight treatment of parental males, 26 forward specific-locus mutations, 2 reverse specific-locus mutations and 9 dominant cataract mutations were recovered. ENU increased the mutation rate to all 3 genetic endpoints. However, ENU was less effective in treated DBA/2 mice than in the standard experimental protocol employing treated hybrid (102 X C3H)F1 male mice. This observed difference for a direct-acting mutagen such as ENU may result from differences in the detoxification of ENU or from differences in the DNA-repair capabilities of strain DBA/2. The first documented reverse mutation of the b allele is reported. The reversion was shown to be due to an AT to GC transition. To date, in addition to the reverse mutation of the b allele, 5 independent ENU-induced mutations recovered in germ cells of the mouse have been molecularly characterized and all have been shown to be base substitutions at an AT site. This is in contrast to the expected mechanism of ENU mutation induction due to O6-ethylguanine adduct formation which results in a GC to AT base-pair substitution and emphasizes the complexities of mutagenesis in germ cells of mammals.

Ionizing radiation and genetic risks. II. Nature of radiation-induced mutations in experimental mammalian in vivo systems

This paper reviews data on the nature of spontaneous and radiation-induced mutations in the mouse. The data are from studies using a variety of endpoints scorable at the morphological or the biochemical level and include pre-selected as well as unselected loci at which mutations can lead to recessive or dominant phenotypes. The loci used in the morphological recessive specific-locus tests permit the recovery of a wide spectrum of induced changes. Important variables that affect the nature of radiation-induced mutations (assessed primarily using tests for viability of homozygotes) include: germ cell stage, type of irradiation and the locus. Most of the results pertain to irradiated stem cell spermatogonia. The data on morphological specific-locus mutations show that overall, more than two-thirds of the X- or gamma-ray-induced mutations are lethal when homozygous. This proportion may be lower for those that occur spontaneously, but the numbers of tested mutants are small. For spontaneous mutations, there is evidence for the occurrence of mosaics and for proviral insertions. Most or all tested induced enzyme activity variants, dominant visibles (recovered in specific-locus experiments) and dominant skeletal mutations are lethal when homozygous and this is true of 50% of dominant cataract mutations, but again, the numbers of tested mutants are small. Electrophoretic mobility variants, which are known to be due to base-pair changes, are seldom induced by irradiation. At the histocompatibility loci, no radiation-induced mutations have been recovered, presumably because deletions are incompatible with survival even in heterozygotes. All these findings are consistent with the view that in mouse germ cells, most radiation-induced mutations are DNA deletions. Some mutations (in the morphological specific-locus tests) which had previously been inferred to be deletions on the basis of genetic analyses have now been shown to be DNA deletions by molecular methods. However, the possibility cannot be excluded that at least a small proportion of induced mutations may be intragenic changes. The data on the rates of induction of recessive lethals and of dominant skeletal and dominant cataract mutations (and proportions of the latter two which are homozygous lethal) can be used to estimate the proportions of recessive lethals which are expressed as skeletal abnormalities or cataracts. These calculations show that about 10% of recessive lethals manifest themselves as skeletal and less than 0.2% as cataract mutations.(ABSTRACT TRUNCATED AT 400 WORDS)

Ionizing radiation and genetic risks. IV. Current methods, estimates of risk of Mendelian disease, human data and lessons from biochemical and molecular studies of mutations

This paper is aimed at a synthesis of conclusions and concepts from the first three papers of this series and an inquiry of their relevance to the estimation of the risk of autosomal dominant and X-linked diseases in man, due to exposure to ionizing radiation. For a population under conditions of continuous irradiation, the doubling-dose method (DD method) enables the prediction of the excess risk of dominant and X-linked diseases at equilibrium. Per unit dose, this quantity is the product of the natural prevalence of these diseases (assumed to be 10,000/10(6) livebirths) and the reciprocal of the DD. The DD currently used is 1 Gy and is based primarily on data on the induction of recessive specific-locus mutations in male mice. The estimate of risk to the first generation is derived from that at equilibrium; the figure is about 15% of the equilibrium value (i.e., 15 cases/10(6) livebirths/cGy). With the direct method, the first-generation risk of dominant disease is estimated using data on the induction of dominant skeletal and cataract mutations in male mice and a number of correction factors. The estimates are about 10-20 cases and 0-9 cases, respectively, for irradiation of males and females, per 10(6) livebirths/cGy. In the Japanese studies, no significant adverse genetic effects, attributable to exposure of the parents to the atomic bombs, could be demonstrated with respect to any of the endpoints used. Most of the latter are clinically and socially relevant but mutationally insensitive. On the basis of these data, Neel and colleagues have estimated that the gametic DD for genetic effects of radiation in man is at least about 4-5 times the 1 Gy value thus far used. The concepts, assumptions, and the data-base used with the DD method have been re-examined. Arguments are advanced to support the thesis that ionizing radiation is probably not very efficient in inducing the very specific molecular changes that are known to underlie spontaneous mutations which cause naturally occurring dominant genetic diseases. It is suggested that (i) the DD estimate of 1 Gy that is used to estimate risk for autosomal dominant and X-linked diseases is conservative and (ii) the 1% prevalence figure for these diseases that is used for this purpose may be too high. If these suggestions are correct, then the estimate of risk for the dominant and X-linked diseases may need to be revised downwards.(ABSTRACT TRUNCATED AT 400 WORDS)

International Commission for Protection Against Environmental Mutagens and Carcinogens. ICPEMC publication No. 18. Review of the genotoxicity and carcinogenicity of antischistosomal drugs; is there a case for a study of mutation epidemiology? Report of a task group on mutagenic antischistosomals

One of the interests of ICPEMC is to identify situations in which the possible induction of inherited defects in man by mutagen exposure could actually be studied. The large-scale use of mutagenic drugs in field programmes against schistosomiasis, mainly during the 1970's, was considered a possible case. An ICPEMC task group approached the problem by (1) updating the genetic toxicology data base for antischistosomal drugs, and (2) reviewing possible study areas. Expertise was combined from genetic toxicology, mutation epidemiology and tropical medicine. It was considered that: (a) if any, hycanthone would be the most appropriate candidate drug for study; (b) it would be virtually impossible to meet the basic requirements of an appropriate mutation epidemiology study, in endemic countries; (c) as more defined genetic endpoints would be selected (e.g. sentinel phenotypes) the required large sample sizes would seem prohibitive, since documentation on past programmes is limited and local demography would render the reliable tracking of substantial numbers of offspring of treated persons an almost impossible task; (d) in most endemic countries proper diagnosis and registration of inherited defects is largely lacking; (e) the problems encountered in demonstrating inherited effects in humans after heavy or chronic exposure to established animal mutagens such as ionizing radiation and cancer chemotherapy, in combination with the ambiguous nature of the animal germ cell data with hycanthone, do not particularly warrant large expectations; (f) since non-mutagenic antischistosomal drugs are now in use, the problem is academic and of low priority in the endemic countries whose medical and research resources are often limited. Thus, studying offspring of hycanthone-treated people to demonstrate the mutagenic potential of the drug in man is not a viable enterprise.

The effect of multi-locus deletions in heterozygotes; a model study using Drosophila

In a model study using Drosophila, we examined the heterozygous effects of a number of well-defined X-chromosomal deletions (i.e., those differing in location, but of about the same length, and those differing in length but located in the same general region), using relative viability and/or fertility as indicators. Most of the deletions were originally isolated in radiation or chemical mutagenesis experiments and maintained since then in stocks using appropriate balancer chromosomes. The results show that (i) most of the deletions have pronounced deleterious effects in heterozygotes; (ii) the size of the deletion per se is not a critical factor in determining relative heterozygous viability, but its location is and (iii) it is possible to tentatively identify, with respect to the deletions, putative genes that affect viability in Drosophila.