Significance of the perigametic interval as a major source of spontaneous mutations that result in mosaics

The Selby-Russell Dispute Regarding the Nonreporting of Critical Data in the Mega-Mouse Experiments of Drs William and Liane Russell That Spanned Many Decades: What Happened, Current Status, and Some Ramifications

The Russells began their studies of the hereditary effects of radiation in the late 1940s, and their experiments contributed much to what is known about the induction of gene mutations in mice. I had a close association with them for about 26 years, and they relied on me considerably for database management and statistical support. In 1994, I was shocked to discover that, in experiments on males, they had failed to report numerous spontaneous mutations that arose during the perigametic interval and were detected as clusters of mutations. I realized that their nondisclosure of this information meant that the decades-long application of their data to estimate hereditary risks of radiation to humans using the doubling-dose approach had resulted in a several-fold overestimation of risk. I accordingly reported the situation to funding agencies. The resulting complicated situation is referred to here as the Selby-Russell Dispute. Highlights of the resulting investigation, as well as what occurred afterward, are described, and reasons will be provided to show why, in my opinion, the hereditary risk from radiation in humans was likely overestimated by at least 10-fold because the Russells decided not to report critical information from their massive experiments.

Similarities and differences in patterns of germline mutation between mice and humans

Whole genome sequencing (WGS) studies have estimated the human germline mutation rate per basepair per generation (~1.2 × 10⁻⁸) to be higher than in mice (3.5–5.4 × 10⁻⁹). In humans, most germline mutations are paternal in origin and numbers of mutations per offspring increase with paternal and maternal age. Here we estimate germline mutation rates and spectra in six multi-sibling mouse pedigrees and compare to three multi-sibling human pedigrees. In both species we observe a paternal mutation bias, a parental age effect, and a highly mutagenic first cell division contributing to the embryo. We also observe differences between species in mutation spectra, in mutation rates per cell division, and in the parental bias of mutations in early embryogenesis. These differences between species likely result from both species-specific differences in cellular genealogies of the germline, as well as biological differences within the same stage of embryogenesis or gametogenesis.

Estimates of mutation rates differ between species. Here, Lindsay et al. perform side-by-side analyses of germline mutation rates using multi-sibling mouse and human pedigrees and find different mutation rates between species, also stratified by sex and temporal stage of mutation acquisition.

In Utero Exposure to Benzo[a]Pyrene Increases Mutation Burden in the Soma and Sperm of Adult Mice

BACKGROUND

Mosaicism, the presence of genetically distinct cell populations within an organism, has emerged as an important contributor to disease. Mutational events occurring during embryonic development can cause mosaicism in any tissue, but the influence of environmental factors on levels of mosaicism is unclear.

OBJECTIVES

We investigated whether in utero exposure to the widespread environmental mutagen benzo[a]pyrene (BaP) has an impact on the burden and distribution of mutations in adult mice.

METHODS

We used the Muta™Mouse transgenic rodent model to quantify and characterize mutations in the offspring of pregnant mice exposed to BaP during postconception days 7 through 16, covering the major period of organogenesis in mice. Next-generation DNA sequencing was then used to determine the spectrum of mutations induced in adult mice that were exposed to BaP during fetal development.

RESULTS

Mutation frequency was significantly increased in the bone marrow, liver, brain, and sperm of first filial generation (F1) males. Developing embryos accumulated more mutations and exhibited higher proportions of mosaicism than exposed adults, particularly in the brain. Decreased sperm count and motility revealed additional negative impacts on the reproductive function of F1 males.

CONCLUSION

In utero exposure to environmental mutagens contributes to somatic and germline mosaicism, permanently affecting both the genetic health of the F1 and the population gene pool. Citation: Meier MJ, O'Brien JM, Beal MA, Allan B, Yauk CL, Marchetti F. 2017. In utero exposure to benzo[a]pyrene increases mutation burden in the soma and sperm of adult mice. Environ Health Perspect 125:82-88; http://dx.doi.org/10.1289/EHP211.

Meiotic interstrand DNA damage escapes paternal repair and causes chromosomal aberrations in the zygote by maternal misrepair

De novo point mutations and chromosomal structural aberrations (CSA) detected in offspring of unaffected parents show a preferential paternal origin with higher risk for older fathers. Studies in rodents suggest that heritable mutations transmitted from the father can arise from either paternal or maternal misrepair of damaged paternal DNA, and that the entire spermatogenic cycle can be at risk after mutagenic exposure. Understanding the susceptibility and mechanisms of transmission of paternal mutations is important in family planning after chemotherapy and donor selection for assisted reproduction. We report that treatment of male mice with melphalan (MLP), a bifunctional alkylating agent widely used in chemotherapy, induces DNA lesions during male mouse meiosis that persist unrepaired as germ cells progress through DNA repair-competent phases of spermatogenic development. After fertilization, unrepaired sperm DNA lesions are mis-repaired into CSA by the egg's DNA repair machinery producing chromosomally abnormal offspring. These findings highlight the importance of both pre- and post-fertilization DNA repair in assuring the genomic integrity of the conceptus.

Comparison of the genetic effects of equimolar doses of ENU and MNU: while the chemicals differ dramatically in their mutagenicity in stem-cell spermatogonia, both elicit very high mutation rates in differentiating spermatogonia

Mutagenic, reproductive, and toxicity effects of two closely related chemicals, ethylnitrosourea (ENU) and methylnitrosourea (MNU), were compared at equimolar and near-equimolar doses in the mouse specific-locus test in a screen of all stages of spermatogenesis and spermiogenesis. In stem-cell spermatogonia (SG), ENU is more than an order of magnitude more mutagenic than MNU. During post-SG stages, both chemicals exhibit high peaks in mutation yield when differentiating spermatogonia (DG) and preleptotene spermatocytes are exposed. The mutation frequency induced by 75mgMNU/kg during this peak interval is, to date, the highest induced by any single-exposure mutagenic treatment - chemical or radiation - that allows survival of the exposed animal and its germ cells, producing an estimated 10 new mutations per genome. There is thus a vast difference between stem cell and differentiating spermatogonia in their sensitivity to MNU, but little difference between these stages in their sensitivity to ENU. During stages following meiotic metaphase, the highest mutation yield is obtained from exposed spermatids, but for both chemicals, that yield is less than one-quarter that obtained from the peak interval. Large-lesion (LL) mutations were induced only in spermatids. Although only a few of the remaining mutations were analyzed molecularly, there is considerable evidence from recent molecular characterizations of the marker genes and their flanking chromosomal regions that most, if not all, mutations induced during the peak-sensitive period did not involve lesions outside the marked loci. Both ENU and MNU treatments of post-SG stages yielded significant numbers of mutants that were recovered as mosaics, with the proportion being higher for ENU than for MNU. Comparing the chemicals for the endpoints studied and additional ones (e.g., chromosome aberrations, toxicity to germ cells and to animals, teratogenicity) revealed that while MNU is generally more effective, the opposite is true when the target cells are SG.

Effects of Male Germ-Cell Stage on the Frequency, Nature, and Spectrum of Induced Specific-Locus Mutations in the Mouse

By means of the mouse specific-locus test (SLT) with visible markers, which is capable of detecting intragenic mutations as well as larger lesions, about 20 mutagens have been studied comparatively across arrays of male germ-cell stages. In addition, a very large historical control, accumulated over decades, provides data on spontaneous mutations in males. Each mutagen has a characteristic germ-cell-stage sensitivity pattern. Although most chemicals yield their maximum numbers of mutations following exposure of spermatozoa and late spermatids, mutagens have now been identified that peak in each of the major stages of spermatogenesis and spermiogenesis, including those in which effects on recombination can also be induced. Stem-cell spermatogonia have yielded positive results with only five of 15 mutagenic chemicals. In postspermatogonial stages, all chemicals, as well as radiations, induce primarily large lesions (LL). By contrast, in spermatogonia (either stem-cell or differentiating) all chemicals except one (bleomycin) produce very few such lesions. The spectrum of relative mutation frequencies at the seven loci of the SLT is characteristic for treated germ-cell stage and mutagen. Treatments that induce primarily LL are characterized by a great preponderance of s (Ednrb)-locus mutations (possibly due to a paucity of haplo-insufficient genes in the surrounding region); and those that induce very few, if any, LL by a great preponderance of p-locus mutations. Spontaneous locus-spectra differ from both types of treatment-induced spectra; moreover, there are two distinct types of spontaneous spectra, depending on whether mutations occurred in mitotic cells or during the perigametic interval.

Mortality of pollen grains may result from errors of meiosis: study of pollen tetrads in Typha latifolia L

In the cattail Typha latifolia the four haploid products of meiosis remain attached and form the flat tetrad of pollen grains. Gametophytic lethals arisen de novo in diploid cells of sporophyte must manifest themselves as pollen tetrads with two dead grains. This could allow to estimate the rate of recessive lethals arresting pollen grain development. We studied pollen samples collected from 44 sprouts in two populations in the vicinity of Novosibirsk. The anomalous tetrads T1, T2, T3, and T4 carrying one, two, three, and four dead grains, respectively, were detected in each sampled individual. The mean frequency of all anomalous tetrads in the two populations was 3.4% and 8.7%. The frequencies of tetrad classes varied widely among the individuals with correlation coefficient up to 0.94, but their ratios remained nearly constant. The majority of anomalous tetrads were presented by T1 and T2 classes (their sum comprising 72.7 and 74.0% in two populations), T1 being a little more abundant. The observed pattern of frequencies of tetrads with dead grains can be explained by errors of male meiosis such as chromosome non-disjunction in both meiotic divisions. The tetrads with two dead pollen grains may result mostly from non-disjunction in anaphase I, and those with one pollen grain from non-disjunction in anaphase II, thus making tetrad analysis ineffective for estimating the rate of gametophytic lethals.

Molecular and phenotypic analysis of 25 recessive, homozygous-viable alleles at the mouse agouti locus

Agouti is a paracrine-acting, transient antagonist of melanocortin 1 receptors that specifies the subapical band of yellow on otherwise black hairs of the wild-type coat. To better understand both agouti structure/function and the germline damage caused by chemicals and radiation, an allelic series of 25 recessive, homozygous-viable agouti mutations generated in specific-locus tests were characterized. Visual inspection of fur, augmented by quantifiable chemical analysis of hair melanins, suggested four phenotypic categories (mild, moderate, umbrous-like, severe) for the 18 hypomorphs and a single category for the 7 amorphs (null). Molecular analysis indicated protein-coding alterations in 8 hypomorphs and 6 amorphs, with mild-moderate phenotypes correlating with signal peptide or basic domain mutations, and more devastating phenotypes resulting from C-terminal lesions. Ten hypomorphs and one null demonstrated wild-type coding potential, suggesting that they contain mutations elsewhere in the > or = 125-kb agouti locus that either reduce the level or alter the temporal/spatial distribution of agouti transcripts. Beyond the notable contributions to the field of mouse germ cell mutagenesis, analysis of this allelic series illustrates that complete abrogation of agouti function in vivo occurs most often through protein-coding lesions, whereas partial loss of function occurs slightly more frequently at the level of gene expression control.

High incidence of mosaic mutations induced by irradiating paternal germ cells of the medaka fish, Oryzias latipes

Delayed-type mutations induced by radiation have recently been demonstrated in various somatic-cell systems. Such mutations are thought to result from the transmission of genetic instability through many cell divisions subsequent to a single exposure to ionizing radiation. Here, we have examined whether 'transgenerational' delayed-type mutations can arise during embryonic development of the medaka fish as a result of exposing the sperm and spermatids of live fish to 137Cs gamma-radiation. To do this, we made use of a sensitive specific-locus test (SLT) for the medaka that we have recently developed. Because the medaka has a transparent egg membrane and embryo body, both visible mosaics and whole-body mutations can be detected during development at an early-expressed pigmentation locus. When wild-type +/+ males were gamma-irradiated and then mated with wl/wl females, the frequency of F1 embryos with both wild-type orange leucophores (wl/+) and mutant-type white leucophores (wl/wl*) (mosaic mutants) was about 5.7x10(-3)/Gy. The frequency of embryos with only white leucophores (whole-body mutants) was about 1.3x10(-3)/Gy. These results suggest that delayed mutations frequently arise in medaka fish embryos that have been fertilized with irradiated sperm. Some possible mechanisms involved in the generation of these delayed mutational events (including genomic instability in the early embryos) are discussed.

Molecular nature of 11 spontaneous de novo mutations in Drosophila melanogaster

To investigate the molecular nature and rate of spontaneous mutation in Drosophila melanogaster, we screened 887,000 individuals for de novo recessive loss-of-function mutations at eight loci that affect eye color. In total, 28 mutants were found in 16 independent events (13 singletons and three clusters). The molecular nature of the 13 events was analyzed. Coding exons of the locus were affected by insertions or deletions >100 nucleotides long (6 events), short frameshift insertions or deletions (4 events), and replacement nucleotide substitutions (1 event). In the case of 2 mutant alleles, coding regions were not affected. Because approximately 70% of spontaneous de novo loss-of-function mutations in Homo sapiens are due to nucleotide substitutions within coding regions, insertions and deletions appear to play a much larger role in spontaneous mutation in D. melanogaster than in H. sapiens. If so, the per nucleotide mutation rate in D. melanogaster may be lower than in H. sapiens, even if their per locus mutation rates are similar.

Effect of mutagen-induced cell lethality on the dose response of germline mutations

Molecular tests for mutations require a sample of tissue from which DNA is extracted, to determine the presence or absence of one or more mutations per sample. To ensure mutation fixation each sample must consist of an equal number of cells that have had one or more DNA replications. In an in vivo test, surviving stem cells compensate to give the same number of cells per sample, leaving as the only evidence for stem cell lethality the increase in mutants of clonal origin because the mutant clone developed from a population of fewer stem cells. A problem is that an increase in mutagen dose increases stem cell death, resulting in a decreased number of surviving target cells, thus giving a downward bias of samples with one or more mutations per sample. To compare in vivo tests with molecular tests we will use as a model system the sex-linked recessive lethal (SLRL) test for germ cell mutations in Drosophila melanogaster. Spermatogonia cells in male larvae were exposed to ENU and mutations detected in sperm cells from adults. The same SLRL data were analyzed by two methods: (1) The conventional analysis of SLRL data, in which each mutation of a cluster of mutations of common origin was counted. (2) An analysis was used to simulate a sample for molecular analysis by determining mutations per male with an equal size sample of progeny per male. With this second analysis a correction factor is required based on the change in cluster size of mutants of common origin.

Evolutionary genetics: what is driving male mutation?

In mammals, most new mutations occur in males. But a study of the evolution of a human X to Y chromosomal translocation has revealed a sex bias much lower than previous estimates. Patterns of substitution suggest that differential methylation between male and female germ lines is a key determinant of the mutation rate.

Ionizing radiation and genetic risks. XIII. Summary and synthesis of papers VI to XII and estimates of genetic risks in the year 2000

This paper recapitulates the advances in the field of genetic risk estimation that have occurred during the past decade and using them as a basis, presents revised estimates of genetic risks of exposure to radiation. The advances include: (i) an upward revision of the estimates of incidence for Mendelian diseases (2.4% now versus 1.25% in 1993); (ii) the introduction of a conceptual change for calculating doubling doses; (iii) the elaboration of methods to estimate the mutation component (i.e. the relative increase in disease frequency per unit relative increase in mutation rate) and the use of the estimates obtained through these methods for assessing the impact of induced mutations on the incidence of Mendelian and chronic multifactorial diseases; (iv) the introduction of an additional factor called the "potential recoverability correction factor" in the risk equation to bridge the gap between radiation-induced mutations that have been recovered in mice and the risk of radiation-inducible genetic disease in human live births and (v) the introduction of the concept that the adverse effects of radiation-induced genetic damage are likely to be manifest predominantly as multi-system developmental abnormalities in the progeny. For all classes of genetic disease (except congenital abnormalities), the estimates of risk have been obtained using a doubling dose of 1 Gy. For a population exposed to low LET, chronic/ low dose irradiation, the current estimates for the first generation progeny are the following (all estimates per million live born progeny per Gy of parental irradiation): autosomal dominant and X-linked diseases, approximately 750-1500 cases; autosomal recessive, nearly zero and chronic multifactorial diseases, approximately 250-1200 cases. For congenital abnormalities, the estimate is approximately 2000 cases and is based on mouse data on developmental abnormalities. The total risk per Gy is of the order of approximately 3000-4700 cases which represent approximately 0.4-0.6% of the baseline frequency of these diseases (738,000 per million) in the population.

Ionizing radiation and genetic risks. XI. The doubling dose estimates from the mid-1950s to the present and the conceptual change to the use of human data on spontaneous mutation rates and mouse data on induced mutation rates for doubling dose calculations

This paper provides an overview of the concept of doubling dose, changes in the database employed for calculating it over the past 30 years and recent advances in this area. The doubling dose is estimated as a ratio of the average rates of spontaneous and induced mutations in a defined set of genes. The reciprocal of the doubling dose is the relative mutation risk per unit dose and is one of the quantities used in estimating genetic risks of radiation exposures. Most of the doubling dose estimates used thus far have been based on mouse data on spontaneous and induced rates of mutations. Initially restricted to mutations in defined genes (with particular focus on the seven genes at which induced recessive mutations were studied in different laboratories), the doubling dose concept was subsequently expanded to include other endpoints of genetic damage. At least during the past 20 years, the magnitude of the doubling dose has remained unchanged at approximately 1 Gy for chronic low LET radiation exposures. One of the assumptions underlying the use of the doubling dose based on mouse data for predicting genetic risks in humans, namely, that the spontaneous rates of mutations in mouse and human genes are similar, is incorrect; this is because of the fact that, unlike in the mouse, the mutation rate in humans differs between the two sexes (being higher in males than in females) and increases with paternal age. Further, an additional source of uncertainty in spontaneous mutation rate estimates in mice has been uncovered. This is related to the non-inclusion of mutations which arise as germinal mosaics and which result in clusters of identical mutations in the following generation. In view of these reasons, it is suggested that a prudent way forward is to revert to the use of human data on spontaneous mutation rates and mouse data on induced mutation rates for doubling dose calculations as was first done in the 1972 BEIR report of the US National Academy of Sciences. The advantages of this procedure are the following: (i) estimates of spontaneous mutation rates in humans, which are usually presented as sex-averaged rates, automatically include sex differences and paternal age-effects; (ii) since human geneticists count all mutations that arise anew irrespective of whether they are part of a cluster or not, had clusters occurred, they would have been included in mutation rate calculations and (iii) one stays close to the aim of risk estimation, namely, estimation of the risk of genetic diseases in humans. On the basis of detailed analyses of the pertinent data, it is now estimated that the average spontaneous mutation rate of human genes (n=135 genes) is: (2.95+/-0.64)x10(-6) per gene and the average induced mutation rate of mouse genes (n=34) is: (0.36+/-0.10)x10(-5) per gene per Gy for chronic low LET radiation. The resultant doubling dose is (0.82+/-0.29) Gy. The standard error of the doubling dose estimate incorporates sampling variability across loci for estimates of spontaneous and induced mutation rates as well as variability in induced mutation rates in individual mouse experiments on radiation-induced mutations. We suggest the use of a rounded doubling dose value of 1 Gy for estimating genetic risks of radiation. Although this value is the same as that used previously, its conceptual basis is different and the present estimate is based on more extensive data than has so far been the case.

The evolution of haploid, diploid and polymorphic haploid-diploid life cycles: the role of meiotic mutation

Here I present a simple population genetic model to investigate the evolution of polymorphic haploid-diploid life cycles. The key feature of the model is the assumption of mutation occurring during meiosis. I show that, in addition to regions favoring haploid or diploid life cycles, there are substantial regions of the parameter space under which polymorphic haploid-diploid life cycles are expected to evolve.