Abstract PL01-03: Developmental reprogramming of cancer susceptibility
Authors: Walker Cheryl Lyn 
Published: January 2014

Gene-environment (GxE) interactions are traditionally understood as promoting the acquisition of mutations that drive multi-stage carcinogenesis, and in the case of inherited tumor suppressor gene defects, additional mutations required for cancer development. GxE interactions are traditionally understood to describe mechanisms whereby genetic risk factors modify the effects of environmental exposures to increase (or decrease) cancer risk, and are thought to be a major determinant of individual risk for this disease. Typically, GxE refers to an increased (or decreased) sensitivity to an environmental carcinogen owing to a specific germline alteration carried by exposed individuals, often as a single nucleotide polymorphism (SNP). This is best illustrated in colon cancer where hereditary defects in DNA mismatch repair genes such as MSH2 decrease repair efficiency, enhancing the efficacy of genotoxic carcinogens, inducing microsatellite instability and predisposing to adenomatous polyposis coli (APC).

The ‘developmental origins of health and disease’ (DOHaD) hypothesis provides an alternative model whereby environmental exposures during development increase susceptibility to disease in adulthood, not by inducing mutations, but by reprogramming the epigenome. We have recently found that epigenetic reprogramming contributes to DOHaD to function as a new type of GxE interaction where environmental exposures target the epigenome to increase cancer susceptibility (1). Studies in Eker rats with a defect in the Tsc2 gene first pointed to the fact that exposure to environmental estrogens during development could cooperate with a tumor suppressor gene defect to increase tumor suppressor gene penetrance (2). In this model, brief neonatal exposure to environmental xenoestrogens during uterine development significantly increased tumor incidence (i.e. gene penetrance), as well as tumor multiplicity and size. Neonatal xenoestrogen exposure also resulted in a reprogramming of estrogen-responsive gene expression, manifest as an increased expression of estrogen-responsive genes in the adult myometrium at 5 months of age, many months prior to tumor development (3).

Thus, the interaction between increased estrogen responsiveness (reprogramming) and the defect in Tsc2 promoted the development of hormone-dependent leiomyoma, effectively increasing tumor suppressor gene penetrance. Importantly, in the absence of the tumor suppressor gene defect, environmental estrogen exposure alone failed to induce tumors even though gene expression was reprogrammed. Conversely, ovariectomy virtually ablated tumor development in genetically predisposed animals, indicating that in the absence of ovarian hormones, the tumor suppressor gene defect was not sufficient to induce tumorigenesis. This indicates it is the interaction of developmental reprogramming with the tumor suppressor gene defect that promotes tumorigenesis and increases tumor suppressor gene penetrance.

Activation of PI3K/AKT signaling and phosphorylation of epigenomic “readers, writers and erasers” provides the mechanism by which environmental xenoestrogens disrupt the epigenetic machinery of a cell during developmental reprogramming (4). We showed that environmental estrogens bind to membrane-associated estrogen receptor (ER) to activate rapid, non-genomic (or more appropriately pre-genomic) ER signaling. Pre-genomic signaling by xenoestrogens activates PI3K signaling and the kinase AKT. Among the targets for this non-genomic signaling was the epigenomic “writer” enhancer of zeste homologue 2 (EZH2), a histone methyltransferase and member of the polycomb repressive complex. Phosphorylation of serine 21 of EZH2 by AKT inactivates EZH2, reducing the repressive H3K27me3 epigenetic methyl mark laid down by this “writer” in the developing uterus. These data identified activation of rapid non-genomic ER signaling as a direct mechanism by which environmental estrogens inappropriately activate kinases to modulate the activity of EZH2 and other epigenomic “readers, writers and erasers” to disrupt the cell's epigenetic machinery during developmental reprogramming (5).

As a result of this developmental reprogramming, estrogen responsive genes in the uterus become hyper-sensitive to hormone, promoting development of hormone-dependent tumors. In Eker rats exposed neonatally to xenoestrogens, we found that over half of the estrogen-responsive genes examined in the adult uterus displayed an inappropriate, exaggerated response to steroid hormones: reprogrammed genes were over-expressed during the estrus cycle when estrogen levels were high, and remained elevated even during periods of the estrous cycle when hormones were at their nadir. As a result, the uterus acquired a “hyper-estrogenized phenotype” that cooperated with the Tsc2 tumor suppressor gene defect to promote tumor development and effectively increase tumor suppressor gene penetrance. Together, these data identify developmental reprogramming as a new type of GxE interaction that can cooperate with a genetic predisposition, not by inducing mutations, but by reprogramming the epigenome to modulate gene expression and promote tumor development.

What does this mean for human health? In genetically susceptible individuals, developmental reprogramming represents a new type of GxE interaction that can promote tumor development by reprogramming the epigenome in a way that cooperates with a genetic predisposition to promote tumorigenesis. While existing data provide proof-of-concept that reprogramming of the epigenome can interact with a tumor suppressor gene defect to increase risk, it is likely that alterations in the epigenome can cooperate with other less penetrant, more prevalent genetic defects, such as SNPs, to similarly increase cancer risk. Importantly, the impact of developmental reprogramming on cancer susceptibility may not be evident for many years or even decades after the initial environmental exposure. As a result, our ability to recognize (and measure) specific environmental exposures that contribute to adult cancer risk is severely limited by studies that focus only on adult exposures. Identification of the epigenetic alterations induced by developmental reprogramming that cooperate with inherited genetic defects will open new avenues for developing biomarkers to identify individuals at increased cancer risk as a result of early life environmental exposures.

Finally, additional insights into how developmental reprogramming increases cancer risk holds promise for interventions to reverse the impact of this type of GxE interaction. In contrast to germline alterations in tumor suppressor genes and cancer-associated SNPs, which are largely irreversible, epigenetic alterations induced by developmental reprogramming are potentially reversible with epigenetic therapies. By increasing our knowledge regarding how epigenetic reprogramming increases cancer risk, we may not only be able to better identify who is at increased risk from early life environmental exposures, but be able to develop interventions that can reverse the epigenetic effects of developmental reprogramming to decrease cancer risk associated with this type of GxE interaction.

Citation Format: Cheryl Lyn Walker. Developmental reprogramming of cancer susceptibility. [abstract]. In: Proceedings of the Eleventh Annual AACR International Conference on Frontiers in Cancer Prevention Research; 2012 Oct 16-19; Anaheim, CA. Philadelphia (PA): AACR; Cancer Prev Res 2012;5(11 Suppl):Abstract nr PL01-03.