The rubella virus (RV) was the first virus shown to be teratogenic in humans. The wealth of data on the clinical symptoms associated with congenital rubella syndrome is in stark contrast to an incomplete understanding of the forces leading to the teratogenic alterations in humans. This applies not only to RV, but also to congenital viral infections in general and includes (1) the mode of vertical transmission, even at early gestation, (2) the possible involvement of inflammation as a consequence of an activated innate immune response, and (3) the underlying molecular and cellular alterations. With the progress made in the development of pluripotent stem cell-based models including organoids and embryoids, it is now possible to assess congenital virus infections on a mechanistic level. Moreover, antiviral treatment options can be validated, and newly emerging viruses with a potential impact on human embryonal development, such as that recently reflected by the Zika virus (ZIKV), can be characterized. Here, we discuss human cytomegalovirus (HCMV) and ZIKV in comparison to RV as viruses with well-known congenital pathologies and highlight their analysis on current models for the early phase of human development. This includes the implications of their genetic variability and, as such, virus strain-specific properties for their use as archetype models for congenital virus infections. In this review, we will discuss the use of induced pluripotent stem cells (iPSC) and derived organoid systems for the study of congenital virus infections with a focus on their prominent aetiologies, HCMV, ZIKV, and RV. Their assessment on these models will provide valuable information on how human development is impaired by virus infections; it will also add new insights into the normal progression of human development through the analysis of developmental pathways in the context of virus-induced alterations. These are exciting perspectives for both developmental biology and congenital virology.

The expanding therapeutic use of thalidomide (TD) remains limited by its species-specific teratogenicity in humans and rabbits, but not rodents. The R and S isomers of TD may be selectively responsible for its respective therapeutic and teratogenic effects, but rapid in vivo racemization makes this impossible to confirm. Fluorothalidomide (FTD), a fluorinated TD analogue with stable, non-racemizing isomers, may serve as a model compound for determining stereoselective effects. In vivo, FTD was undetectable in plasma, suggesting rapid breakdown, as confirmed in vitro, where FTD hydrolyzed up to 22-fold faster than TD. Unlike TD, FTD in pregnant rabbits and mice was highly toxic and lethal to both dams and fetuses. In rabbit embryo culture, FTD initiated optic (eye) vesicle and hindbrain but not classic limb bud embryopathies. Chemical instability, potent general toxicity and absence of limb bud embryopathies make FTD an unsuitable stereoselective model for TD teratogenesis. TD teratogenesis may involve its bioactivation by embryonic prostaglandin H synthases (PHSs) to a free radical intermediate that increases embryopathic reactive oxygen species (ROS) formation. However, the teratogenic potential of rapidly formed TD hydrolysis products and the determinants of species-specific teratogenesis are unclear. For some teratogens, mouse strains that are resistant in vivo are susceptible in embryo culture, suggesting maternal and/or placental determinants of risk. However, TD and two hydrolysis products, 2-phthalimidoglutaramic acid (PGMA) and 2-phthalimidoglutaraic acid (PGA), were non-embryopathic in CD-1 mouse embryo culture. Also, mice deficient in oxoguanine glycosylase 1 (OGG1), which repairs oxidatively damaged DNA, were resistant to TD embryopathies in culture and in vivo. Therefore, murine resistance to TD teratogenesis is dependent on embryonic factors, rather than maternal/placental determinants or increased DNA repair. In contrast, rabbit embryos exposed in culture to TD, PGMA and PGA exhibited head/brain, otic (ear) vesicle and classic limb bud embryopathies, validating the first mammalian embryo culture model for TD teratogenesis and providing the first evidence of a teratogenic role for TD hydrolysis products. Pretreatment with eicosatetraynoic acid (ETYA), a dual PHS/lipoxygenase inhibitor, or phenylbutylnitrone (PBN), a free radical spin trapping agent, completely blocked TD, PGMA and PGA-initiated embryopathies, implicating a PHS-dependent, ROS-mediated embryopathic mechanism.

The expanding therapeutic use of thalidomide (TD) remains limited by its species-specific teratogenicity in humans and rabbits, but not rodents. The R and S isomers of TD may be selectively responsible for its respective therapeutic and teratogenic effects, but rapid in vivo racemization makes this impossible to confirm. Fluorothalidomide (FTD), a fluorinated TD analogue with stable, non-racemizing isomers, may serve as a model compound for determining stereoselective effects. In vivo, FTD was undetectable in plasma, suggesting rapid breakdown, as confirmed in vitro, where FTD hydrolyzed up to 22-fold faster than TD. Unlike TD, FTD in pregnant rabbits and mice was highly toxic and lethal to both dams and fetuses. In rabbit embryo culture, FTD initiated optic (eye) vesicle and hindbrain but not classic limb bud embryopathies. Chemical instability, potent general toxicity and absence of limb bud embryopathies make FTD an unsuitable stereoselective model for TD teratogenesis. TD teratogenesis may involve its bioactivation by embryonic prostaglandin H synthases (PHSs) to a free radical intermediate that increases embryopathic reactive oxygen species (ROS) formation. However, the teratogenic potential of rapidly formed TD hydrolysis products and the determinants of species-specific teratogenesis are unclear. For some teratogens, mouse strains that are resistant in vivo are susceptible in embryo culture, suggesting maternal and/or placental determinants of risk. However, TD and two hydrolysis products, 2-phthalimidoglutaramic acid (PGMA) and 2-phthalimidoglutaraic acid (PGA), were non-embryopathic in CD-1 mouse embryo culture. Also, mice deficient in oxoguanine glycosylase 1 (OGG1), which repairs oxidatively damaged DNA, were resistant to TD embryopathies in culture and in vivo. Therefore, murine resistance to TD teratogenesis is dependent on embryonic factors, rather than maternal/placental determinants or increased DNA repair. In contrast, rabbit embryos exposed in culture to TD, PGMA and PGA exhibited head/brain, otic (ear) vesicle and classic limb bud embryopathies, validating the first mammalian embryo culture model for TD teratogenesis and providing the first evidence of a teratogenic role for TD hydrolysis products. Pretreatment with eicosatetraynoic acid (ETYA), a dual PHS/lipoxygenase inhibitor, or phenylbutylnitrone (PBN), a free radical spin trapping agent, completely blocked TD, PGMA and PGA-initiated embryopathies, implicating a PHS-dependent, ROS-mediated embryopathic mechanism.