Mechanisms Linking Environmental Exposures to Cancer

by Maryann Donovan, PhD, MPH, Scientific Director, Center for Environmental Oncology of UPCI

For more than half a century, scientists have understood that most cancer happens in humans when the healthy cells we are born with lose their ability to stay under control. Importantly, most cancer is not inherited but rather comes about because of changes that accumulate over our lifetimes. This cancer process can take several decades to occur, reflecting many different exposures that happen over long periods of time. The multiple steps through which the disease arises is referred to as carcinogenesis.

Sometimes the basic building blocks of all living material—our DNA—becomes damaged and as a result, cells lose the ability to control cell growth. This process of cancer initiation can come about either as a result of an inborn defect, a random cellular event, or exposure to genotoxic insults like sunlight, radiation, or chemicals. Now damaged, these initiated cells have lost their signals to stay in balance. Through a process called promotion, these abnormal cells continue to divide and in the process they pass on their mistakes and acquire new ones. Although damage to the DNA of critical cellular genes can start cancer, typically other events will determine whether or not the disease occurs.

What is going on in the cells that develop into cancer? The center or nucleus of each cell in our body contains 23 pair of chromosomes containing 3 billion base pairs of DNA that encode 30,000 to 40,000 genes. Housekeeping genes, repair genes, even genes that tell cells when to die, are all part of what keeps us alive and well by setting directions for the production of proteins that are needed for us to live. Fortunately for most of us most of the time, when these genes are damaged, they get repaired. But when the damage is not repaired, cancer can occur. Oncogenes, for example, are genes that are typically silent, but when they are turned on they can take over and cause unregulated cells to grow. In contrast, tumor suppressor genes have the job of keeping cells in place. When tumor suppressor genes become inactive, they can no longer do their job of keeping things in order, and cell growth can go awry with cancer as a possible outcome.

For some genes, even one small change in the DNA can alter the protein’s amino acid sequence and profoundly alter the way the protein works. In this way, genetic changes that alter the coding sequence of cellular genes can set the stage for cancer initiation. When genetic changes are passed on from parents to children, they are referred to as inherited genetic damage.

Although some exposures may cause genetic damage to our DNA, over the last 10 years, a fundamentally new direction of research has emerged defining an additional mechanism, called epigenetic change, by which exposures throughout a lifetime may affect the chance the cancer will occur. In the nucleus, DNA is tightly wound around proteins called histones into a DNA-protein complex called chromatin. It looks like beads on a string and is shown in Figure 1.

Figure 1. Chromatin in the nucleus looks like beads on a string.

The expression of genes in the nucleus relies on regulatory proteins in the nucleus that recognize certain signals. Control regions (gene promoters) of genes include characteristic nucleotide sequences consisting of strings of CpG dinucleotides (CpG islands). If the cell wants to disguise this region, it can mark it with a chemical stop sign know as a methyl group. This stop sign attaches to the Cp residues and blocks the site. In other words, when Cp becomes methylated in the regulatory region of a gene this sends a signal to shut down gene expression. Alternatively, if the methyl group is removed, this may send a signal that the gene should be turned on. In addition to DNA changes, proteins in the nucleus can also be modified. It turns out that chromatin histone proteins have tails that can be also modified by methylation and acetylation. These additional changes can also determine whether or not specific genes are expressed. This modification to DNA and proteins in the nucleus is at the core of epigenetics and has revolutionized scientific thinking about the mechanisms that link health and the environment.

Although the role of diet and other environmental exposures on marking nuclear DNA and proteins is a pretty recent discovery, not all epigenetic changes are acquired from post-birth exposures. In fact, prenatal imprinting, which has been understood for many years, is a form of epigenetic change that is inherited from Mom or Dad. Research into the function of imprinted genes has provided important clues linking “epigenetic change” with chronic diseases including cancer (1 and 2a). Several imprinting examples illustrate the functional importance of epigenetic changes. Patients with Beckwith-Wiedemann syndrome have small deletions on chromosome 11 that cause defective imprinting leading to prenatal overgrowth, birth defects, and cancer. In Rett syndrome, methylation occurs normally but the cells do not recognize the mark and in later childhood, neurodevelopment is affected. Finally, loss of imprinting (LOI) of the insulin-like growth factor-II gene (IGF2) has been shown to increase the risk of colon cancer in some families with the disease.

More recently, in studies with the agouti mouse model, Dr. Randy Jirtle and colleagues have shown that “environment,” for example, mice fed a diet that contains Bisphenol A—widely used in polycarbonate plastics—are born with altered promoter methylation and gene expression. (2b) Shown in Figure 2A and 2B, Agouti mice, fed a diet high in folic acid, vitamin B12, choline, betaine, or genistein (soy), all chemicals that provide methyl groups, increases DNA methylation and produces healthier offspring that are brown colored, lean and healthy (promoter methylated and gene off). Alternatively, a diet depleted of methyl donors, results in overexpression of the agouti locus and unhealthy offspring that are yellow colored, obese and prone to cancer (promoter unmethylated and gene on (2). In other studies (1), exposure of rats to vinclozolin, a fungicide that disrupts hormone function, or methoxychlor, an estrogenic insecticide, produces sperm defects and male infertility. In other studies, breast cancer, kidney disease, prostate disease, and immune abnormalities were also reported. These effects, which are transmitted by male rats but affect male and female offspring, are seen 4 generations out and correlated with an altered DNA methylation pattern. In other words, the researchers interpret their results to suggest that epigenetic modification was responsible for the diseases that were caused. (4)

Figure 2. A). Dr. Randy Jirtle with Agouti mice in his laboratory. Pictures courtesy of Dr. Randy Jirtle, Duke University.

Figure 2. B). The yellow mouse on the left is the offspring of a pregnant dam fed a standard diet and the Agouti locus is unmethylated. The brown mouse on the right is the offspring of a pregnant dam fed a diet high in methyl donors. In this case, the agouti locus became methylated shutting off expression of the gene.

More evidence for the accumulation of epigenetic changes over the course of a lifetime was published in 2005 by Dr. Mario Fraga and colleagues. The group compared epigenetic differences (both DNA methylation and histone acetylation) in pairs of identical twins ranging in age from 3 to 74 years of age. Although the chromosomes of 3 year old twins were virtually indistinguishable, the chromosomes of 50 year old twins who had different lifestyles looked very different and suggested that “epigenetic drift” had occurred over time. (5)

How might epigenetic change contribute to cancer? In a recent review, Dr. Feinberg suggests that genetic and epigenetic factors may complement one another in many or most tumors (6). In the “epigenetic hypothesis,” Dr. Feinberg suggests that epigenetic modification can play a role in both cancer initiation and cancer progression by providing a background that helps to determine if cancer will develop or not. In the early stage of cancer, epigenetic changes may determine the effect of genetic changes, increasing the likelihood that a genetic change will lead to cancer. During later stages, epigenetic changes may ensure that growth factor stimulation leads to cell growth rather than cell death. Mutation is necessary for cancer to develop, but epigenetic changes may increase the probability that cancer will occur once the genetic lesion becomes fixed.

References

• 1 Feinberg AP. 2007.  Phenotypic plasticity and the epigenetics of human disease.  Nature, 447(24)433-440.
• 2a Waterland RA, Jirtle RL. 2004. Early nutrition, epigenetic changes at transposons and imprinted genes, and enhanced susceptibility to adult chronic diseases.  Nutrition 20:63-68.
• 2b.  Dolinoy DC, Huang D, Jirtle RL. 2007. Maternal nutrient supplementation counteracts bisphenol A-induced DNA hypomethylation in early development. PNAS 104(32):13056-13061.
• 3. Jirtle RL, Skinner MK. Environmental epigenomics and disease susceptibility. 2007.  Nature Reviews (Genetics) 8:253-262.
• 4. Anway MD, Cupp AS, Uzumcu M, Skinner MK.  2005. Epigenetic transgenerational actions of endocrine disruptors and male fertility.  Science 308:1466-1469.
• 5. Fraga MF, Ballestar E, Paz MF, et al.  2005.  Epigenetic differences arise during the lifetime of monozygotic twins.  PNAS 102(30): 10604-10609.
• 6. Feinberg AP. 2004.  The epigenetics of cancer etiology.  Seminars in Cancer Biology, 14: 427-432.