2012 Study Abstracts
Gene–environment interactions have been traditionally understood to promote the acquisition of mutations that drive multistage carcinogenesis, and, in the case of inherited defects in tumour suppressor genes, additional mutations are required for cancer development. However, the developmental origins of health and disease (DOHAD) hypothesis provides an alternative model whereby environmental exposures during development increase susceptibility to cancer in adulthood, not by inducing genetic mutations, but by reprogramming the epigenome. We hypothesize that this epigenetic reprogramming functions as a new type of gene–environment interaction by which environmental exposures target the epigenome to increase cancer susceptibility.
The developmental origins of health and disease (DOHAD) hypothesis
The developmental origins of health and disease (DOHAD) hypothesis posits that increased risk of disease in adulthood can result from an adverse developmental environment that reprogrammes cellular and tissue responses to normal physiological signals in a manner that increases disease susceptibility.
An example is altered DNA methylation that is observed in the uterus in response to in utero exposure to the xenoestrogen diethylstilbestrol (DES). The adult uterus of an animal exposed to this xenoestrogen in utero exhibits alterations in the methylation of the lactotransferrin (Ltf) gene after puberty. If ovariectomized, the aberrant DNA methylation that is observed in intact animals is not observed in the castrated female uterus. Similarly, early life exposure to DES or to the phytoestrogen genistein can reprogramme high-mobility group nucleosome-binding domain 5 (Hmgn5; also known as Nsbp1) gene methylation, causing the promoter of this gene to become hypomethylated in the adult uterus, which increases gene expression.
Mechanisms of epigenetic reprogramming
If developmental exposures are reprogramming the epigenome to increase susceptibility to diseases such as cancer, the effects of this reprogramming should be apparent in the ‘normal’ tissue prior to the development of disease. Ample evidence exists that developmental reprogramming induces epigenetic changes that can be detected in at-risk tissues prior to the development of tumours. In the reproductive tract, examples of genes in which developmental reprogramming induces persistent alterations in DNA methylation include FOS, LTF , homeobox A10 (HOXA10), HMGN5 and phosphodiesterase 4D variant 4 (PDE4D4). Early studies demonstrated that neonatal DES exposure induced aberrant methylation of specific CpG sites in Ltf and Fos in the mouse uterus. More recently, developmental exposure to several endocrine-disrupting compounds (EDCs), including xenoestrogens, was found to modulate the expression and promoter methylation of HOX genes, which have key roles in uterine development. Exposure to DES in utero results in increased promoter methylation and decreased HOXA10 expression, whereas bisphenol A (BPA) exposure results in decreased methylation of HOXA10, increased oestrogen receptor (ER) binding by HOXA10 and increased HOXA10 expression in the adult uterus.
Similarly, neonatal exposure of mice to DES or genistein also induces persistent hypomethylation of the Hmgn5 promoter and aberrant overexpression of this gene in the uterus throughout life, and exposure is associated with an increased risk of developing uterine tumours in adult female mice.
Early life environmental exposures and reproductive tract cancer
The xenoestrogen DES, a synthetic stilbene oestrogen, was administered to pregnant women from the 1940s to the 1970s to prevent complications of pregnancy. In the early 1970s, the daughters of women who took DES during the first trimester (so-called ‘DES daughters’) were frequently diagnosed with congenital reproductive tract abnormalities (specifically, a ‘T-shaped’ uterus), dysplasia and cervical intraepithelial neoplasia, and with a significantly increased rate of an otherwise rare type of vaginal clear cell adenocarcinoma. Continued follow-up of DES daughters has now revealed that they also have an increased relative risk for developing breast cancer (~twofold to threefold that of unexposed women) and uterine leiomyoma, although not all data on this point are concordant. DES sons may also be at a higher risk of developing prostate cancer; however, the final verdict on this point requires additional investigation.
Animal studies that simulate the human DES experience show that exposure of the developing reproductive tract to DES and other xenoestrogens imparts a permanent oestrogen imprint that alters reproductive tract morphology, induces persistent expression of oestrogen-responsive genes and induces a high incidence of uterine adenocarcinoma. Uterine leiomyoma, sometimes referred to as fibroids, arise from the smooth muscle layer of the uterus and are the most common benign tumour in women. In rats carrying a genetic defect in the tuberous sclerosis 2 (Tsc2) tumour suppressor gene, exposure to DES during uterine development causes the tumour suppressor defect to become fully penetrant, increasing tumour incidence from 65% to 100% (discussed below). This increased penetrance is associated with the reprogramming of oestrogen-responsive genes, which become hyper-responsive to hormone and promote the development of hormone-dependent uterine leiomyomas. DES exposure during this crucial perinatal period of development also modulates IGF signalling in the adult endometrium, decreasing negative feedback to insulin receptor substrate 1 (IRS1) and increasing the incidence of endometrial hyperplasia in rats that are genetically predisposed to develop these preneoplastic lesions.
Inappropriate exposure to environmental oestrogens, such as DES, the plasticizor BPA or the soy phytoestrogen genistein, during mammary gland development alters the susceptibility of the mammary gland to chemical carcinogenesis in adulthood. The mammary gland begins developing in utero and is not fully mature until after pregnancy and lactation. Importantly, the effect of xenoestrogen exposure during mammary gland development differs throughout life. Xenoestrogen exposure in utero can increase the risk for mammary tumorigenesis, in part by altering mammary gland architecture and increasing the number of target cells for transformation in terminal end buds (TEBs) of the mammary gland. Conversely, xenoestrogen exposure during the postnatal period can decrease susceptibility to mammary gland tumorigenesis by inducing a programme of differentiation in mammary epithelial cells that mimics the protective effects of pregnancy. In addition to xenoestrogen exposure, there is a suggestion that prenatal famine may increase breast cancer incidence in exposed individuals.
Testicular cancer, one of the features of testicular dysgenesis syndrome (TDS), which includes poor semen quality, undescended testis and hypospadias, has also been linked to early life environmental exposures. Experimental studies from animal models and human epidemiology support a causal association between TDS and exposure to compounds that disrupt the endocrine system, such as DES, and anti-androgens, such as vinclozolin, during male reproductive tract development. In humans, the majority of testicular cancers are derived from germ cells, and epidemiology data suggest in utero DES exposure may be a risk factor. Testicular cancer is preceded by carcinoma in situ, and recent studies suggest that these lesions have a gene expression pattern that is similar to gonocytes (spermatogonia) and exhibit DNA hypomethylation and altered methylation of histones H3K9, H3K27 and H3K4, which is analogous to the epigenetic pattern that is seen in fetal germ cells. These data have led to the suggestion that germ cells in the developing testis are the target for the developmental reprogramming that is associated with TDS.
The maternal environment might also play a part in susceptibility to testicular cancer. Low birth weight is a risk factor for testicular germ-cell cancer (which comprises more than 95% of testicular cancer), and a recent meta-analysis of 18 epidemiological studies indicated that low birth weight is also a risk factor for testicular cancer. Although germ-cell testicular cancer is very rare in experimental animal models, by contrast, interstitial cancers, benign tumours and adenocarcinoma of the rete testis are frequently observed in rodent testes that have been prenatally exposed to DES. In the prostate, early life exposure of rodents to xenoestrogens such as methoxychlor, BPA or genistein leads to prostate hypertrophy and increased inflammation with age in the prostate of adult animals and can also predispose to prostate carcinogenesis.
A new gene–environment interaction?
Studies in Eker rats with a defect in Tsc2 first pointed to the fact that exposure to environmental oestrogens during development could cooperate with a tumour suppressor gene defect to increase the penetrance of the defective tumour suppressor gene. In this model, brief neonatal exposure to environmental oestrogens, such as DES, during uterine development significantly increased tumour incidence (that is, penetrance), as well as tumour multiplicity and size. Neonatal xenoestrogen exposure also resulted in the reprogramming of oestrogen-responsive gene expression, which manifested as an increased expression of oestrogen-responsive genes in the adult myometrium at 5 months of age, many months prior to tumour development. This suggests that the combined increase in oestrogen responsiveness (reprogramming) and the defect in Tsc2 promoted the development of hormone-dependent leiomyoma, effectively increasing tumour suppressor gene penetrance. Importantly, in the absence of the tumour suppressor gene defect, environmental oestrogen exposure alone failed to induce tumours, even though gene expression was reprogrammed (K. L. Greathouse and C.L.W., unpublished observations). Conversely, ovariectomy almost completely ablated tumour development in genetically predisposed animals, indicating that, in the absence of ovarian hormones, the tumour suppressor gene defect was not sufficient to induce tumorigenesis. Thus, developmental reprogramming can cooperate with a tumour suppressor gene defect to increase its penetrance, thereby functioning as a new type of gene–environment interaction.
As a result of this developmental reprogramming, oestrogen-responsive genes in the uterus become hyper-sensitive to hormone, which promotes tumour development in adult animals that are neonatally exposed to environmental oestrogens. In Eker rats that are neonatally exposed to DES, more than 50% of the oestrogen-responsive genes that have been examined in the adult uterus displayed an inappropriate, exaggerated response to steroid hormones: reprogrammed genes were overexpressed during the oestrus cycle when oestrogen levels were high, and remained elevated even during periods of the oestrous cycle when hormones were at their lowest. As a result, the uterus displayed an ‘oestrogenized phenotype’ that cooperated with the tumour suppressor gene defect to increase tumour suppressor gene penetrance. Together, these data lead us to propose that developmental reprogramming is a type of gene–environment interaction that can cooperate with a genetic predisposition, not by inducing mutations, but by reprogramming the epigenome to modulate gene expression in order to promote tumour development.
- Full study (free access) : Developmental reprogramming of cancer susceptibility, Nat Rev Cancer, NCBI PubMed, PMC3820510, 2012 Jun 14.
- Featured image Ken Treloar.