Increase, Aneuploid offspring

An aneuploid offspring is an organism born with an incorrect number of chromosomes (which is present in all of its cells) [reviewed in Marchetti et al., 2016]. In most cases, the aneuploid condition will result in the death of the conceptus at different stages of embryo-fetal development depending on the chromosome involved in the aneuploidy. In humans, most

aneuploid embryos survive until the blastocyst stage and are lost around the time of implantation [Fragouli et al., 2013]; however, a decline in the rate of aneuploidy is already observed between early cleavage stage and the blastocyst stage [Fragouli et al., 2014]. When aneuploid fetuses survive to birth, they will originate offspring affected by aneuploid syndromes, characterized by variable symptoms depending on the specific chromosome involved.

The health consequences of a trisomic condition are well established in both humans and mice. Each of the 19 autosomal trisomies of the mouse has been produced and the survival and phenotype of each trisomy characterized [Epstein, 1988]. Growth retardation is almost invariably present and congenital malformations are frequently detected. Trisomic fetuses generally survive until at least mid-gestation. However, with the exception of trisomy 19 and to a lesser extent trisomy 16 and 18, all die prior to parturition. The precise cause of death of the trisomic embryos is not known. In some instances, it appears to be related to extremely poor embryonic growth and development. Aneuploid mouse zygotes are karyotypically unstable during preimplantation development leading to a state of chaotic mosaic aneuploidy within the blastocyst [Lightfoot et al., 2006]. In contrast to the survival of trisomic embryos and fetuses until at least mid-gestation, mouse autosomal monosomies are lethal in the pre- or peri-implantation period, with only rare survivors until day 6 of gestation [Magnuson et al., 1985]. Due to dosage compensation mechanisms, aneuploidies of the sex chromosomes in the mouse are viable [Russell, 1976].

Survival data of aneuploidies in humans generally match those in mice: aneuploidies of the sex chromosomes are viable, all autosomal monosomies and most trisomies die before birth, with the exception of trisomy 13, 18 and 21 that, in some cases, survive until shortly after birth or much longer (as in the case of Down syndrome). Even in the case of trisomy 21, the most viable of the human trisomies, an estimated 80% or more fetuses die in utero [Hecht and Hecht, 1987]. Aneuploid conditions compatible with life present a range of adverse health effects from infertility (e.g., Klinefelter syndrome due to XXY karyotype) to severe mental and physical impairment and reduced life span (e.g., Edwards Syndrome due to trisomy 18).

Diagnostic laboratories around the world use both phenotypic and molecular approaches to determine whether an individual is aneuploid. Most commonly, tests during pregnancy are used to determine whether a pregnancy is aneuploid [Rink and Norton, 2016]. These include screening tests such as ultrasound examinations [Benacerraf, 2005; Rao and Plat, 2016]; or diagnostic tests during the first or second trimesters, such as chorionic villus sampling [Hogge et al., 1985; Jenkins and Wapner, 1999], amniocentesis [Crandall and Lebher, 1976; Dacu and Wilroy, 1985], and serum markers [Canick et al., 2006]. These are well-established methods that have been used for decades. Recent developments in genomics approaches allow now the diagnosis of an aneuploid pregnancy by detecting fetal cell-free DNA in the blood of the mother [Bianchi et al., 2014; Gil et al., 2017; Sehnert et al., 2011; Valderramos et al., 2016]. When the diagnosis is done after birth, it may be based on the results of a physical exam. For example, children with Down syndrome have distinct facial features that include a flat face, slanting eyes and a small mouth [Fink et al., 1975; Farkas et al., 2002]. A karyotypical analysis of peripheral blood lymphocytes to confirm the presence of the extra chromosome is also conducted.

Aneuploid offspring have been measured in mouse and humans, but can occur in any sexually reproducing species.

Various international regulatory agencies have established policies and practices for the assessment and management of heritable mutagenic hazards. Indeed, heritable effects are an important regulatory endpoint noted by agencies around the world [Yauk et al., 2015a].

The World Health Organization (WHO)/International Programme on Chemical Safety (IPCS) developed a harmonized scheme for mutagenicity testing. In this document the relationship between somatic cell mutagenicity and germ cell risk is summarized as: “For substances that give positive results for mutagenic effects in somatic cells in vivo, their potential to affect germ cells should be considered. If there is toxicokinetic or toxicodynamic evidence that germ cells are actually exposed to the somatic mutagen or its bioactive metabolites, it is reasonable to assume that the substance may also pose a mutagenic hazard to germ cells and thus a risk to future generations.” [Eastmond et al., 2009].

Thus, assessment of heritable mutagenic hazards such as aneuploidy, are an important regulatory endpoint. During drug and chemical development, agents that induce aneuploidy would not be developed further. There is currently not a specific example that can be referenced of a regulatory decision based on this adverse outcome. However, the UK Committee on Mutagenicity of Chemicals in Foods, Consumer Products and the Environment in its 2007 annual report (https://www.gov.uk/government/collections/com-guidance-statements) did recommend that the risk assessment of certain benzimidazoles be conducted solely on the aneugenic properties of these compounds.

The development of AOPs related to mutagenicity in germ cells [Yauk et al., 2015b; 2016] is expected to aid the identification of potential hazards to germ cell genomic integrity and support regulatory efforts to protect population health.

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