A Critical Review of Nutrition-Based Epigenetic Oncogenesis

Photo taken by Madeline Chan

Written by: Mahnoor Shahab and Lana Amoudi

Abstract

Epigenetics has become a widely researched topic and is used to understand the development of complex diseases. Epigenetic modifications can be a result of various exposures, including an individual’s diet, ultimately leading to the development of cancer. In this critical review, we explore the association between cancer and the impact of nutrition on specific epigenetic mechanisms such as DNA methylation, histone modifications, and microRNA. Current research suggests that nutritional compounds such as EGCG, genistein, and folate can modulate these changes as well as impact prenatal development upon early exposure. However, the underlying mechanisms by which these compounds work remain unclear and studies about early-life nutritional exposure are limited. This review identifies existing research, as well as areas in which further studies need to be conducted to fully understand the association between cancer epigenetics and diet. 

Introduction

In 1942, embryologist Conrad Waddington first introduced the term “epigenetics”, defining it as changes that affect gene expression but do not alter the DNA sequence.1 Many studies have since investigated this topic, fueled by the idea that epigenetic changes are associated with a variety of diseases. Epigenetic modifications occur naturally but can also be influenced by lifestyle factors such as diet. Through a study of the impact of nutrition on human embryos, it became evident that nutritional-induced epigenetic variations can occur throughout life and can directly impact disease risk. Furthermore, many types of cancer are a result of epigenetic changes such as DNA methylation, histone modifications, and microRNA. Since epigenetic changes are strongly linked to diet, it has also been shown that certain plant compounds such as epigallocatechin gallate (EGCG), genistein, folate, and many others can significantly impact cancer development. Through understanding the effect of nutritional components of cancer epigenetics, researchers can begin to develop treatment and prevention strategies. 

Cancer Epigenetics

Advancements in research have shown that human cancer cells have numerous epigenetic abnormalities. One of the most important epigenetic modifications is DNA methylation, which is the addition of a methyl group to cytosine in a CpG dinucleotide sequence in the promoter region.2 In a study of lung cancer, more than 40 genes displayed some type of alteration in DNA methylation patterns1. Hypermethylation of promoters of cancer-related genes can lead to their inactivation and genetic instability, a process often involving tumour suppressor genes.1 Inversely, hypomethylation can result in the activation of oncogenes which contribute to the formation of tumors. However, genes involved in cell cycle regulation and DNA repair are generally hypermethylated in the majority of cancers. 

Histone modifications primarily involve acetylation and methylation. Histones have positively charged lysine residues which get attracted to the DNA backbone. Histone acetyltransferase (HAT) adds acetyl groups to reduce the positive charge of histones, thereby reducing attraction to negatively charged DNA and producing a loose chromatin structure. This allows transcription factors to access the DNA. Inversely, histone deacetylase (HDAC) removes acetyl groups and produces a tightly packed chromatin structure. In a study evaluating gastric cancer, it was shown that HDAC expression was upregulated and was associated with tumor aggressiveness.3 Further, in a study by Chen et al., it was found that there was a correlation between histone modification and survival rate in patients with a specific type of oral cancer.4 More specifically, those with low levels of acetylation exhibited advanced tumor progression and a poorer prognosis. However, further studies with a larger patient cohort are needed to support the accuracy of these findings. 

Small, non-coding pieces of RNA known as microRNA (miRNA) are an important epigenetic modification since they can suppress gene expression. They bind a complementary sequence on mRNA, thereby inhibiting its translation.  Numerous studies have shown that abnormal expression of miRNAs is linked to carcinogenesis. To illustrate this, a study found that a decreased expression of the let-7 family of miRNAs was associated with increased tumor formation.5 Another study has demonstrated that increasing the expression of this family of miRNAs by normalizing its levels inhibits the growth of animal tumour cells, both in vivo and in vitro.6 The let-7 family of miRNAs are known as tumor suppressors and regulators of apoptosis. However, as these studies have only recently started to investigate these mechanisms in animal models, more research needs to be done prior to applying this to clinical cancer treatment. Therefore, since a wide variety of epigenetic changes and dietary factors can lead to increased risk of cancer development, the interplay between nutrition and cancer epigenetics is an important area of investigation. 

EGCG

Epigallocatechin gallate (EGCG), a compound found in green tea, has been extensively studied for its anticarcinogenic properties. EGCG has been shown to be involved in epigenetic changes by lowering DNA methyltransferase (DNMT) activity.1 This can result in the reversal of hypermethylation in tumor suppressor genes. EGCG can also modulate miRNA expression. In a study by Tsang and Kwok, human cancer cells treated with EGCG showed significant miRNA regulatory changes.7 However, these observations were seen with 100 μmol/L of the compound, which is not physiologically achievable. Other studies have also found correlations between EGCG treatment and miRNA expression, through testing on mice tumours. These findings suggest that supplementation of EGCG at nutritionally achievable levels can help regulate cellular functions through modulating miRNA expression or DNMT activity. 

Another study demonstrated that repeated applications of 5mg EGCG before treating with okadaic acid (a tumor promoting compound) completely prevented tumor growth in mouse skin.8 As a result, the researchers believed that EGCG has a ‘sealing effect’ as it interrupts the interaction of tumor promoter ligands with their receptors on the cell membrane. A Japanese 10 year prospective cohort study involved investigating the daily consumption of green tea of 8,522 people. Findings showed that patients who consumed over 10 cups (120 mL each) of green tea per day developed cancer 7.3 years later than those who consume less than 3 cups a day. It was also shown that stage I and II breast cancer patients who had more than 5 cups of green tea per day had a lower recurrence rate and longer disease-free period compared to those who consumed less. However, green tea showed no effects in stage III breast cancer because there are many more accumulated genetic changes in the cancer cells which are hard to reverse. Therefore, it has been demonstrated that EGCG has a positive impact and can reduce tumor volume or recurrence rate. The cancer preventive properties of this compound are well known and many have incorporated it as part of diet therapy. However, physicians currently don’t highly recommend it to cancer patients in case EGCG interacts with other molecular pathways or interferes with cancer treatment drugs being taken. 

Genistein

Genistein is a plant based human estrogen-mimicking compound that is primarily found in soybeans. It is known to exhibit anti-cancer properties in animals and in human tumor cells. However, in vitro studies using non-transformed human cell lines still need to be conducted to determine the impacts of genistein on lowering cancer incidence. In a study using esophageal cancer cells, genistein was found to inhibit DNMT activity, leading to partial reversal of DNA hypermethylation.9 Another study using brain tumour cells demonstrated that genistein can also suppress telomerase activity, thereby inhibiting the growth of cancer cells.10 Similar to EGCG, genistein can also alter miRNA expression to help regulate cellular functions. Furthermore, epidemiological studies revealed that a higher intake of soy-rich diets is linked to lower incidence of colorectal cancer in China and Japan.1 Although several studies have demonstrated how genistein can prevent cancer development on a cellular level, there are very few in vivo studies. Using animal or human models to determine the effects of genistein is very important as the compound may function differently. This is an area that requires further investigation to evaluate if this compound could potentially be used in a clinical setting. 

Folate

Another compound that significantly impacts the epigenetics of cancer is folate, a water-soluble B vitamin present in green leafy vegetables. Folate is involved in 1-carbon metabolism, and is the essential source of the 1-carbon group required to methylate DNA. However, in order for DNA methylation to occur, folate must be reduced to tetrahydrofolate (THF) via dihydrofolate reductase action within the liver. Once THF is formed, the single-carbon groups can then be transported to other molecules and methylate DNA.11 Studies have shown that folate deficiencies are linked to the development of anemia, cardiovascular disease, neural tube defects, as well as various cancers.12 Although the mechanisms by which low folate contributes to cancer remain unclear, several studies have been conducted to understand this connection. In 2 observational studies (cohort and case-control), the intake of folic acid, a synthetic form of folate, was measured to assess the association between folate status and DNA methylation.12 Another controlled feeding trial of folate conducted in healthy adults showed significant increases in DNA methylation at day 69 post-treatment with a dosage of 111μg/d of folic acid.13 These studies concluded DNA hypomethylation was associated with cancer, however, associations between DNA methylation and folate status were inconsistent. This lack of consistency could be due to a variety of reasons including the imprecision and variations in the methodological techniques used to detect DNA methylation. The limited sample size of both cohorts in these studies could also contribute to the inconsistency of the results. To mitigate this, more research with larger cohorts and consistent assays is required to accurately determine whether a stronger association exists between folate status and DNA methylation. 

Nutritional Impact on Cancer Risk in Child Development

Research has also shown that early-life nutritional exposure (including those occurring in utero) can induce epigenetic changes that modify an individual’s cancer development risk later in life. One of the earliest evidence of this relationship comes from the Dutch Hunger Winter Study conducted in the mid-20th century. The study explored the impact of prenatal exposure to famine in human offspring. When compared to unexposed individuals 60 years later, participants in this study had lower DNA methylation of the insulin-like growth factor 2 (IGF2) gene with a significant increase in disease risk being observed.14 The same cohort has been further investigated and recent findings from a genome-scale analysis confirmed the association present between DNA methylation and prenatal famine exposure in growth related pathways.15 

The Dutch Hunger Winter Study drove other researchers to further investigate diet’s ability to modify epigenetic profiles and impact the health of adults as a result of early-life exposure. Studies assessing genistein exposure in early-life showed a strong association with reduction in cancer risk. Specifically, in a study conducted by Pei RJ et. al., the effects of genistein/soy were assessed using pregnant rodent models. The results of this study suggested that administration of genistein in the prenatal period has protective effects against N-methyl-N-nitrosourea (MNU)-induced carcinoma via reduction in the levels of cells necessary for cancer cell proliferation.16 Similar animal studies also suggested that exposure to this compound during the period preceding puberty reduces later susceptibility to developing breast cancer.17 Furthermore, these studies suggest that if exposure to this compound occurs during the fetal period, then no protection was seen. Unfortunately, these studies do not propose a convincing explanation as to why breast cancer risk-reducing effects of this compound are strongest during early childhood, thus, the underlying mechanisms must be further investigated. In addition, there’s minimal research assessing these associations in human models suggesting a future area of research.

Along with genistein, there has been an increasing interest in in utero effects of folate exposure on long-term health outcomes and future risks of cancers. Many studies have been conducted to assess folate’s impact on early life and have proven the possibility that folate/folic acid, in maternal diet, can change DNA methylation levels in the offspring.18 One study, utilizing agouti mouse models, suggested the presence of a clear association between maternal dietary changes in 1-carbon availability and DNA methylation patterns.19 Another study, conducted by Steegers-Theunissen et al, also examined this association and found that maternal use of folic acid was associated with a 4.5% increase in IGF2 DMR methylation.20 Although these studies propose significant associations between the compound and risk of developing cancer, these relationships remain complex and involve other components of the 1-carbon metabolism. This suggests that there may not be a definitive association between folate intake in the mother and an increase in DNA methylation in the child. To further understand these complicated mechanisms as well as the effects of possible confounders, further research is necessary.

Unfortunately, minimal research is done on EGCG’s nutritional exposure and impact on early life. Due to the lack of information in this area as well as green tea’s known effects on a variety of health related issues, this might be an area worth investigating in the future. 

Conclusion

Epigenetics is a rapidly growing field of interest to many researchers. This growing interest is a result of the many epigenetic modifications that can occur naturally or through the influence of environmental and lifestyle factors such as nutrition. To this date, research has shown that compounds such as EGCG, genistein, and folate have an association with epigenetic modifications as well as an impact on prenatal development upon early exposure. However, the underlying mechanisms through which such compounds manage to do so remain unclear and require further investigation. Furthermore, the understanding of the associations between these early-life induced epigenetic modifications and risk of cancer development is also limited. Although there are many studies assessing the impacts of these compounds and their association with early-life exposure in in utero animal models, there is not a wide body of research exploring these connections in studies with larger sample sizes and in humans. In addition, due to the various types of cancers and types of assays used to detect the presence of epigenetic modifications, there is a lack of consistency amongst research, making it that much more difficult to assess if these associations are definitive. However, nonetheless, there has been an increasing interest in assessing in utero effects of nutritional exposures and more research will be conducted in the near future to further explore these associations. 

References

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