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Relationship: 202

Title

A descriptive phrase which clearly defines the two KEs being considered and the sequential relationship between them (i.e., which is upstream, and which is downstream). More help

Increase, Mutations leads to Increase, Heritable mutations in offspring

Upstream event

The causing Key Event (KE) in a Key Event Relationship (KER). More help

Increase, Mutations

Downstream event

The responding Key Event (KE) in a Key Event Relationship (KER). More help

Increase, Heritable mutations in offspring

Key Event Relationship Overview

The utility of AOPs for regulatory application is defined, to a large extent, by the confidence and precision with which they facilitate extrapolation of data measured at low levels of biological organisation to predicted outcomes at higher levels of organisation and the extent to which they can link biological effect measurements to their specific causes. Within the AOP framework, the predictive relationships that facilitate extrapolation are represented by the KERs. Consequently, the overall WoE for an AOP is a reflection in part, of the level of confidence in the underlying series of KERs it encompasses. Therefore, describing the KERs in an AOP involves assembling and organising the types of information and evidence that defines the scientific basis for inferring the probable change in, or state of, a downstream KE from the known or measured state of an upstream KE. More help

AOPs Referencing Relationship

AOP Name	Adjacency	Weight of Evidence	Quantitative Understanding	Point of Contact	Author Status	OECD Status
Alkylation of DNA in male pre-meiotic germ cells leading to heritable mutations	adjacent	High	Moderate	Evgeniia Kazymova (send email)	Open for citation & comment	WPHA/WNT Endorsed

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) that help to define the biological applicability domain of the KER.In general, this will be dictated by the more restrictive of the two KEs being linked together by the KER. More help

Term	Scientific Term	Evidence	Link
mouse	Mus musculus	High	NCBI
human	Homo sapiens	High	NCBI

Sex Applicability

An indication of the the relevant sex for this KER. More help

Life Stage Applicability

An indication of the the relevant life stage(s) for this KER. More help

Key Event Relationship Description

Provides a concise overview of the information given below as well as addressing details that aren’t inherent in the description of the KEs themselves. More help

If a mutation arises in spermatogonial stem cells and does not influence cellular function, the mutation can become fixed and/or propagated within the stem cell population. Mutations that do not affect sperm maturation will persist through the succeeding stages of spermatogenesis and will be found in the mature sperm of the organism throughout its reproductive lifespan. Mutations can also occur in differentiating spermatogonia; however, once the sperm generated by these dividing spermatogonia are ejaculated there will be no ‘record’ of the induced mutation. Mutations that impair spermatogenesis or the viability of the cell will be lost via apoptosis and cell death, potentially contributing to decreased fertility. Mutations that do not impact sperm motility, morphology or ability to penetrate the zona pellucida (or other important sperm parameters), and that are present in mature sperm, may be transmitted to the egg resulting in the development of an offspring with a mutation. Thus, increased incidence of mutations in germ cells leads to increased incidence of mutations in the offspring. There is a great deal of evidence demonstrating that exposure to a variety of DNA alkylating agents induces mutations in male spermatogenic cells.

Evidence Collection Strategy

Include a description of the approach for identification and assembly of the evidence base for the KER. For evidence identification, include, for example, a description of the sources and dates of information consulted including expert knowledge, databases searched and associated search terms/strings. Include also a description of study screening criteria and methodology, study quality assessment considerations, the data extraction strategy and links to any repositories/databases of relevant references.Tabular summaries and links to relevant supporting documentation are encouraged, wherever possible. More help

Evidence Supporting this KER

Addresses the scientific evidence supporting KERs in an AOP setting the stage for overall assessment of the AOP. More help

Biological Plausibility

Addresses the biological rationale for a connection between KEupstream and KEdownstream. This field can also incorporate additional mechanistic details that help inform the relationship between KEs, this is useful when it is not practical/pragmatic to represent these details as separate KEs due to the difficulty or relative infrequency with which it is likely to be measured. More help

Evolution requires heritable mutations that are transmitted to offspring via parental gametes. This is the primary mechanism by which an offspring would have a sequence variant in every single one of its cells that is not found in its parents. Indeed, as stated in a recent review in Science by Shendura and Aikey "Germline mutations are the principal cause of heritable disease and the ultimate source of evolutionary change." Thus, this KER is supported by substantive understanding of genetics and evolution, with heritable germ cell mutations forming the basis for the selective evolution of species.

In addition, in toxicology, a large body of literature demonstrates that exposure to specific genotoxic chemicals during pre-meiotic stages of spermatogenesis leads to mutations in both the sperm and the offspring, supporting that mutations occurring in sperm fertilize the egg and result in offspring with mutations (reviewed in Demarini 2012; Marchetti and Wyrobek 2005; Yauk et al. 2012). Indeed, ENU is used as a tool in genetics to create offspring carrying mutations for the purposes of understanding gene function ( e.g., http://www.riken.jp/en/research/labs/brc/mutagen_genom). In these studies, male mice are mutagenized by exposure to ENU and mated to females. The offspring of these males have a much higher incidence of mutation; the function of new mutations occurring in genes in these offspring is used to study gene function.

Thus, overall this KER is biologically plausible and widely understood.

Empirical Evidence

Provides specific (citable) evidence that supports the idea of a change in the upstream KE (KEupstream) leading to, or being associated with, a subsequent change in the downstream KE (KEdownstream), assuming the perturbation of KEupstream is sufficient. More help

Identification of mutations in sperm requires the destruction of the sperm. Thus, tracking a mutated spermatogonial stem cell through to fertilization and characterization of the mutation in the offspring is not possible, and the empirical evidence to support this KER is weak. No single study has looked at the dose-response relationship of the same mutation endpoint in germ cells and offspring because technologies are not currently available to do this. We caution that comparing mutation rates across different genes or genetic loci is imprecise, because factors intrinsic to specific loci govern mutation rates (e.g., length, GC content, transcribed versus non-transcribed, coding versus non-coding, chromatin structure, DNA methylation, sequence, etc.).

Increased numbers of germ cell mutations occur in mature sperm 42+ days post-exposure in mice (indicating that mutations arose in pre-meiotic male germ cells). Mating in this time interval to produce offspring also results in increased incidence of mutation in the descendants of exposed males, indicating temporal concordance. By virtue of the required experimental designs, mutations measured in the offspring occur after the mutations in germ cells. For example, mutations identified in proteins via electrophoresis (a variation of the SLT test) are found in the offspring of male mice mated 10+ weeks post-exposure to ENU (Lewis et al. 1991). These inherited changes are the result of mutations in stem cells that persist through spermatogenesis and are transmitted to offspring.

The only assay that presently can measure mutations in both sperm and offspring is the tandem repeat mutation assay. A single study on one dose of radiation (1 Gy X-ray) against matched controls has shown that increases in mutation frequencies in exposed sperm are similar to the increases observed in the offspring of exposed males for tandem repeats (Yauk et al. 2002), suggesting that tandem repeat mutations in sperm are transmitted to offspring. Alkylating agents cause similar increases in tandem repeat mutations in both sperm and in the offspring through comparison across studies (Dubrova et al. 2008; Swayne et al. 2012; Vilarino-Guell et al. 2003), but dose-response studies have not been conducted. It is advisable that dose-response experiments in sperm for tandem repeats be conducted in the future to address this gap.

Many studies have shown the induction of transgene mutations recovered in mature sperm derived from toxicant-exposed pre-meiotic male germ cells in transgenic mutation reporter mice (e.g., Brooks and Dean 1997; Liegibel and Schmezer 1997; Mattison et al. 1997). One study measured transgene mutations in the offspring of mice exposed to three single i.p. doses of 100 mg/kg ENU (in 7 day intervals) (Barnett et al. 2002). Four inherited transgene mutations were found from 280 mice (confirmed in multiple somatic tissues), for a mutant frequency of 35.7 x 10E-5 per locus. There is no comparable study on the sperm of lacI transgenic mice, or a similar exposure in another transgenic strain for comparison. However, conversion of the per locus lacI offspring mutant frequency to per nucleotide reveals a mutant frequency of 3.31 x 10-7 per nucleotide. LacZ mutant frequencies for 150 mg/kg (half the exposure level of the lacI transgenic mice) exposures of MutaMouse males to ENU results in a per nucleotide mutation frequency ranging from 0.37 to 2.21 x 10E-7 (Liegibel and Schmezer 1997; van Delft and Baan 1995). Thus, the induced mutation frequency in sperm and offspring are within the same range, despite higher doses in the offspring study, supporting incidence concordance for these events.

Finally, it has been documented that DNA damage and mutation accumulates as human males age (reviewed in Paul and Robaire 2013), which is concordant with increased incidence of mutation in the offspring of ageing fathers (Kong et al. 2012; Sun et al. 2012). Comparison of the dose-response characteristics of this relationship is not possible because of differences in the mutagenic endpoints measured in sperm versus offspring.

Uncertainties and Inconsistencies

Addresses inconsistencies or uncertainties in the relationship including the identification of experimental details that may explain apparent deviations from the expected patterns of concordance. More help

There are no inconsistencies in the data for this KER, although the data are limited. There is a possibility that mutations can arise in the early embryo instead of in the spermatogenic cells. However, given the study designs for this type of work (where > 42 days pass prior to sperm collection or mating – see OECD TG488 for the time-series required for transgene mutation analysis in sperm), it is unlikely that this contributes significantly. Limitations in technology currently prevent the analyses required to describe the incidence of mutations in sperm versus offspring, but this is a future research direction. It should be noted that the locations and types of mutations would influence the probably of transmission; this relationship has not been confirmed empirically and limits extrapolation across studies applying different endpoints.

Known modulating factors

This table captures specific information on the MF, its properties, how it affects the KER and respective references.1.) What is the modulating factor? Name the factor for which solid evidence exists that it influences this KER. Examples: age, sex, genotype, diet 2.) Details of this modulating factor. Specify which features of this MF are relevant for this KER. Examples: a specific age range or a specific biological age (defined by...); a specific gene mutation or variant, a specific nutrient (deficit or surplus); a sex-specific homone; a certain threshold value (e.g. serum levels of a chemical above...) 3.) Description of how this modulating factor affects this KER. Describe the provable modification of the KER (also quantitatively, if known). Examples: increase or decrease of the magnitude of effect (by a factor of...); change of the time-course of the effect (onset delay by...); alteration of the probability of the effect; increase or decrease of the sensitivity of the downstream effect (by a factor of...) 4.) Provision of supporting scientific evidence for an effect of this MF on this KER. Give a list of references. More help

Quantitative Understanding of the Linkage

Captures information that helps to define how much change in the upstream KE, and/or for how long, is needed to elicit a detectable and defined change in the downstream KE. More help

Mutations conferring a selective disadvantage to sperm or to the embryo will not be measured in live born offspring and will be eliminated. Thus, mutation frequency in sperm should be equal to mutation rate derived by measuring mutations in the offspring for non-selective loci (as is seen in the rodent tandem repeat and transgene mutation examples described above); or, sperm mutation frequency should be greater than mutation rate measured by identifying mutations in the offspring. However, quantitative data to demonstrate this are lacking because of current technical limitations to study this. It is anticipated that improved models will be developed to predict the likely outcome of increased rates of heritable mutation from sperm mutation frequency data when more data are available from studies applying next generation sequencing technologies in sperm and pedigrees.

Response-response Relationship

Provides sources of data that define the response-response relationships between the KEs. More help

Time-scale

Information regarding the approximate time-scale of the changes in KEdownstream relative to changes in KEupstream (i.e., do effects on KEdownstream lag those on KEupstream by seconds, minutes, hours, or days?). More help

Known Feedforward/Feedback loops influencing this KER

Define whether there are known positive or negative feedback mechanisms involved and what is understood about their time-course and homeostatic limits. More help

Domain of Applicability

A free-text section of the KER description that the developers can use to explain their rationale for the taxonomic, life stage, or sex applicability structured terms. More help

Mutation is the underlying source of evolution and occurs in every species. Theoretically, any sexually reproducing organism (i.e., producing gametes) can acquire mutations in their gametes and transmit these to descendants. Thus, the present KER is relevant to any species producing sperm.

References

List of the literature that was cited for this KER description. More help

Barnett, L.B., R.W. Tyl, B.S. Shane, M.D. Shelby and S.E. Lewis (2002), "Transmission of mutations in the lacI transgene to the offspring of ENU-treated Big Blue male mice", Environ. Mol. Mutagen., 40(4): 251-257.

Brooks, T.M. and S.W. Dean (1997), "The detection of gene mutation in the tubular sperm of Muta Mice following a single intraperitoneal treatment with methyl methanesulphonate or ethylnitrosourea", Mutat. Res., 388(2-3): 219-222.

Demarini, D.M. (2012), "Declaring the existence of human germ-cell mutagens", Environ. Mol. Mutagen., 53(3): 166-172.

Dubrova, Y.E., P. Hickenbotham, C.D. Glen, K. Monger, H.P. Wong and R.C. Barber (2008), "Paternal exposure to ethylnitrosourea results in transgenerational genomic instability in mice", Environ. Mol. Mutagen., 49(4): 308-311.

Kong, A., M.L. Frigge, G. Masson, S. Besenbacher, P. Sulem, G. Magnusson, S.A. Gudjonsson, A. Sigurdsson, A. Jonasdottir, W.S. Wong, G. Sigurdsson, G.B. Walters, S. Steinberg, H. Helgason, G. Thorleifsson, D.F. Gudbjartsson, A. Helgason, O.T. Magnusson, U. Thorsteinsdottir and K. Stefansson K. (2012), "Rate of de novo mutations and the importance of father's age to disease risk", Nature, 488(7412): 471-475.

Lewis, S.E., L.B. Barnett, B.M. Sadler and M.D. Shelby (1991), "ENU mutagenesis in the mouse electrophoretic specific-locus test, 1. Dose-response relationship of electrophoretically-detected mutations arising from mouse spermatogonia treated with ethylnitrosourea", 'Mutat. Res., 249(2): 311-315.

Liegibel, U.M. and P. Schmezer (1994), "Detection of the two germ cell mutagens ENU and iPMS using the LacZ/transgenic mouse mutation assay" Mutat. Res., 341(1):17-28.

Marchetti, F. and A.J. Wyrobek (2005), "Mechanisms and consequences of paternally-transmitted chromosomal abnormalities", Birth Defects Res C Embryo Today, 75(2): 112-129.

Mattison, J.D., L.B. Penrose and B. Burlinson (1997), "Preliminary results of ethylnitrosourea, isopropyl methanesulphonate and methyl methanesulphonate activity in the testis and epididymal spermatozoa of Muta Mice", Mutat. Res. 388(2-3): 123-7.

O'Brien, J.M., A. Williams, J. Gingerich, G.R. Douglas, F. Marchetti and C.L. Yauk (2013), "No evidence for transgenerational genomic instability in the F1 or F2 descendants of Muta™Mouse males exposed to N-ethyl-N-nitrosourea", Mutat Res., 741-742:11-7

Paul,C. and B. Robaire (2013), "Ageing of the male germ line", Nat. Rev. Urol., 10(4): 227-234.

Shendura, J. and M. Akey (2015), "The origins, determinants, and consequences of human mutations", Science, 349(6255): 1478-1483.

Sun, J.X., A. Helgason, G. Masson, S.S. Ebenesersdottir, H. Li, S. Mallick, S. Gnerre, N. Patterson, A. Kong, D. Reich and K. Stefansson (2012), "A direct characterization of human mutation based on microsatellites", Nat. Genet., 44(10): 1161-1165.

Swayne, B.G., A. Kawata, N.A. Behan, A. Williams, M.G. Wade, A.J. Macfarlane and C.L. Yauk (2012), "Investigating the effects of dietary folic acid on sperm count, DNA damage and mutation in Balb/c mice", Mutat. Res., 737(1-2): 1-7.

Vilarino-Guell, C., A.G. Smith and Y.E. Dubrova (2003), "Germline mutation induction at mouse repeat DNA loci by chemical mutagens", 'Mutat. Res., 526(1-2): 63-73.

Yauk, C.L., Y.E. Dubrova, G.R. Grant and A.J. Jeffreys (2002), "A novel single molecule analysis of spontaneous and radiation-induced mutation at a mouse tandem repeat locus", Mutat Res., 500(1-2): 147-156.

Yauk, C.L., L.J. Argueso, S.S. Auerbach, P. Awadalla, S.R. Davis, D.M. Demarini, G.R. Douglas, Y.E. Dubrova, R.K. Elespuru, T.M. Glover, B.F. Hales , M.E. Hurles, C.B. Klein, J.R. Lupski, D.K. Manchester, F. Marchetti, A. Montpetit, J.J. Mulvihill, B. Robaire, W.A. Robbins, G.A. Rouleau, D.T. Shaughnessy, C.M. Somers, J.G. Taylor 6th, J. Trasler, M.D. Waters, T.E. Wilson, K.L. Witt and J.B. Bishop (2013), "Harnessing genomics to identify environmental determinants of heritable disease" Mutation Research, 752(1): 6-9.