Environmental perturbation can lead to non-genetic, non-Mendelian transmission of acquired traits to descendants. Such inheritance may be intergenerational or transgenerational, the latter describing effects that persist in the absence of any exposure (either directly or of parental germ cells) to the triggering environment. Numerous reports of non-genetic inheritance ascribe it to epigenetic mechanisms, but behavioural, microbiotic, cultural, and/or ecological factors more likely play a part in many instances.

During my doctoral studies, I explored inter- and transgenerational effects of developmental exposure to three groups of endocrine insults – endocrine disrupting chemicals (EDCs), synthetic glucocorticoids (sGCs), and hyperthyroidism – in two different species, humans and mice.

EDCs are exogenous substances that interfere with endocrine axes to cause adverse health effects. Humans are ubiquitously exposed to EDCs and recent animal studies have suggested a role for them in non-genetic inheritance. For the first part of my thesis, I investigated prospective relationships between maternal serum concentrations of three groups of EDCs (phthalate metabolites, phenols, and parabens) in early pregnancy and male infant genital development in a human cohort study. I found associations between bisphenol A (BPA) level and offspring cryptorchidism, and between detection of n-propyl paraben and shorter anogenital distance (a marker of intrauterine androgen activity). Next, I investigated inter- and transgenerational effects of EDCs in mice by exposing pregnant/lactating females to BPA and phenotyping their patrilineal F1-F3 offspring. I observed metabolic changes in F1 and F2, but not F3, males, which instead had increased body weight; conclusions about mechanism could not be drawn.

sGCs are a group of drugs that bind with high potency to glucocorticoid receptors (GRs), with pleiotropic effects. Therapeutic indications include inflammatory and autoimmune diseases, and threatened preterm birth. The latter usage results in significant fetal exposure to sGCs, which may affect the germline and impact on future generations. I therefore used the sGC dexamethasone and two novel genetic models to investigate inter- and transgenerational effects of GR activation in mice. In a patrilineal experiment, antenatal dexamethasone treatment caused impaired glucose tolerance in F1 and F2 males, and heavier internal genitalia in F2 females, but these findings were not replicated in F2 mice generated by in vitro fertilisation (IVF), suggesting a non-germline effect. The F3 phenotype was of increased body weight (males) and altered thyroid hormone levels and body composition (females). Dexamethasone treatment of oocyte donors prior to IVF led to altered body composition and heavier testes in male offspring, consistent with epigenetic inheritance. Mice expressing a constitutively active GR (ΔGR) under a germ cell- or epididymis-specific promoter had no robust phenotype, but their wild-type male offspring had improved glycaemia, with sexually dimorphic effects on body composition also observed.

Intrauterine exposure to hyperthyroidism due to a maternal thyroid hormone receptor mutation has recently been associated with non-genetic patrilineal transgenerational transmission of a thyroid hormone resistance phenotype in humans. In the final part of my thesis, I used the sperm of four F2-generation men from this pedigree to determine the feasibility of assaying various epigenetic marks in cryopreserved human samples, with success for long RNAs, small RNAs, and DNA methylation; and to test the hypothesis that non-genetic transmission is associated with hypermethylation at a specific locus, for which I found no evidence.

My results confirm and extend previous reports of inter- and transgenerational effects of endocrine insults, including novel findings in both humans and mice. Further studies are required to delineate underlying mechanisms and develop protective strategies.