Epigenetic Risk Factors for Diseases: A Transgenerational Perspective

Abstract

Each individual is determined by a combination of genetic and non-genetic factors that together shape physiological and biological functions during development and adulthood. While genetic features are embedded in the DNA sequence inherited from parents, non-genetic features (which include epigenetic modifications) are acquired through experiences and environmental exposure across life. However, it is now acknowledged that non-genetic features can also be inherited from parents and propagate across generations. This chapter discusses the concept of non-genetic germline inheritance in mammals and examines possible routes of transmission of non-genetic information involving germline-dependent and germline-independent modes of transfer. It reviews current evidence that environmental factors can induce non-genetic alterations in the germline that can impact behavioral and physiological features in the offspring. This chapter also addresses the underlying molecular mechanisms, provides initial insight into the implication of epigenetic marks and non-coding RNAs in male germ cells, and questions the way non-genetic modifications can be induced and maintained in germ cells. It highlights promising areas of current research and reflects on evolutionary perspectives and future challenges.

Appendix 1: Experimental Methods to Examine Different Routes of Non-genetic Inheritance of Acquired Information in Rodents

Acquired information can be transmitted to the offspring via several routes that do not involve germ cells, and these factors can operate during mating, in utero, or peri- or postnatally in mammals. (1) During mating, seminal fluid is transmitted from male to female in addition to sperm. The composition of seminal fluid can change in response to environmental factors and influence the offspring independently from sperm (Perry et al. 2013). In flies, it was shown that seminal fluid can transfer acquired features from a male which mated but did not fertilize the female, to the offspring sired by a subsequent male (Crean et al. 2014). (2) In mammals, in utero components such as hormones, immune factors, nutrients, or toxins can influence the fetus and elicit persistent phenotypic traits later in life. Stress in gestating females can alter the offspring’s emotional, social, and cognitive abilities (Weinstock 2008; Moisiadis and Matthews 2014), in part by altering plasma glucocorticoids or androgens. Exposure to specific flavors or odors can also modulate the olfactory system and taste preference in the offspring (Schaal et al. 2000; Ong and Muhlhausler 2011; Todrank et al. 2011). In certain bird species, females can load antibodies against encountered pathogens into the eggs’ yolk, providing protection to their progeny (Frésard et al. 2013). (3) Peri- and postnatally, the amount and quality of maternal care strongly influence the offspring’s behavior in adulthood. Female pups receiving poor maternal care will also show reduced maternal behaviors themselves, which is associated with epigenetic dysregulation in the brain (Weaver et al. 2004; Champagne 2008). Additionally, milk composition during lactation (Liu et al. 2014), microbiota (Stilling et al. 2014), or odor cues (Debiec and Sullivan 2014) can also transfer environmental experiences from mother to offspring.

Distinguishing these germline-independent forms of inheritance from non-genetic germline-dependent components is challenging and requires specific experimental strategies. Using patrilines helps control the influence of some in utero and postnatal factors, but the presence of the male during mating may still exert an influence on the embryo. Additionally, rodent females have been reported to adjust maternal investment based on the sire’s fitness (Curley et al. 2011; Mashoodh et al. 2012). The following strategies can be used to control some of these confounding factors: (1) Cross-fostering of pups to a control dam can be used to exclude the contribution of peri-/postnatal maternal influences (Bohacek et al. 2015). (2) Artificial insemination or in vitro fertilization (IVF) can directly test transmission through the germline and avoid confounding effects of seminal fluid and interactions during mating (Dietz et al. 2011; Dias and Ressler 2014). IVF – routinely available in many facilities – involves in vitro culture conditions which, in addition to superovulation, can create further uncontrollable epigenetic confounds (Denomme and Mann 2012). Artificial insemination has rarely been used in mice, but new simplified protocols are now available and should be used more routinely in studies of non-genetic germline inheritance (Bohacek et al. 2016). (3) Embryo transfer can be particularly useful in female line studies to avoid in utero and postnatal maternal effects, but also involves in vitro biases. (4) Direct injection of molecules such as sperm RNAs into fertilized wild-type oocytes followed by transplantation in a pseudo-pregnant mother is another elegant approach to directly test the contribution of specific germline components (Rassoulzadegan et al. 2006; Gapp et al. 2014a). These procedures can help determine the germline dependence of non-genetic inheritance and should be employed more systematically but with caution since some can affect epigenetic reprogramming.