Mechanisms responsible for the transgenerational inheritance of intrauterine growth restriction phenotypes

Abstract

The in utero developmental environment is known to play an important role in shaping future health and susceptibility to chronic diseases. Exposure to environmental factors during pregnancy, including but not limited to uteroplacental insufficiency (UPI), suboptimal diets, drugs, caffeine, or pathogen-induced immune activation results in the intrauterine growth restriction (IUGR) of the developing fetus, otherwise known as “small for gestational age” (SGA). Most IUGR babies are born with low birth weight and signs of aberrant cardiorenal or metabolic function, predisposing them to late-onset health problems. Due to the difficulties in obtaining the appropriate tissues for sampling, as well as carrying out both longitudinal and generational studies in humans, rodent studies are more advantageous for investigating the effects of IUGR on offspring phenotypes, as well as the mechanisms behind the increased disease risk. This Thesis is comprised of 5 chapters. Chapter 1 is the Introduction, which provides the general rationale for this PhD project and the layout of the Thesis. Chapter 2 is a published review (2022) on health outcomes of the growth restricted offspring in both human and rodent studies. The associations between IUGR and each specific chronic disease risk, including hypertension, kidney disease, and diabetes were discussed in detail. Using tissues extracted from offspring across different generations, epigenetic mechanisms such as DNA methylation, histone modifications and long non-coding RNAs were investigated and suggested to play a role in the sex-specific and multigenerational transmission of IUGR phenotypes in these studies. One of the IUGR models mentioned in this chapter was a UPI-induced IUGR model in rats, established by Professor Mary Wlodek (The University of Melbourne). This model has been shown to reflect metabolic characteristics in humans. We previously reported* changes to expression of a DNA methyltransferase and two imprinted genes (Cdkn1c and Kcnq1) known to be important in both human and rodent kidney development in kidneys of offspring from this model. The mentioned review and this article provide the rationale for the remainder of the thesis, highlighting that the mechanisms involved in the sex-specific differences and transgenerational transmission of IUGR phenotypes requires further studies. *Doan TNA, et al. Epigenetic mechanisms involved in intrauterine growth restriction and aberrant kidney development and function. Journal of Developmental Origins of Health and Disease. 2021;12(6):952-962. doi:10.1017/S2040174420001257. In Chapter 3 (research article, recently published, 2024), we examined the epigenetic alterations that may explain the changes to imprinted gene expression we had previously reported in this IUGR rat model. DNA methylation of an imprinting control region (KvDMR1) known to regulate expression of Cdkn1c and Kcnq1 was studied using region-specific DNA methylation analysis. Additionally, expression of the antisense long non-coding Kcnq1ot1 was investigated as well as additional neighbouring imprinted and non-imprinted genes in this cluster. Besides investigating the molecular mechanisms that may be involved in the IUGR induced phenotypes, another focus of this Thesis was whether there was transmission of IUGR phenotypes across generations down the paternal line of the UPI model, similar to the published maternal line. In Chapters 4 and 5 (research manuscripts, unsubmitted), data of growth profiles, cardiovascular function, metabolic function (Chapter 4) and renal function (Chapter 5) of growth restricted offspring in the first (F1), second (F2), and third (F3) generations from the paternal line were analysed. Results from Chapters 4 and 5 suggest that there are changes to the metabolic and renal function of offspring in the paternal line, even in the F2 and F3 generations which are not directly affected by the in utero insult. These offspring are potentially at a higher risk of developing chronic diseases later in life, especially if exposed to another environmental stress postnatally. For instance, the extensive vasculature and haemodynamic changes occurring during a normal pregnancy (discussed in Appendix A (review, published 2020)) might act as a “second hit” in the growth restricted females, magnifying the early symptoms into clinical conditions. The final chapter, Chapter 6, presents a general discussion of findings from this research, the significance of these findings, and future directions.