The concept of field cancerization (also known as field effect or field defect) was introduced in 1953 by Slaughter et al. to explain the development of multiple primary tumors, local recurrence, abnormal tissue surrounding the cancer and multifocal areas of precancerous change Citation[1]. Field cancerization reflected the susceptibility of normal tissue to undergo early genetic changes as a result of exposure to carcinogens, leading to the development of cancer. Even though these initial findings made were based on histological observations, with the advances in modern molecular techniques, it is now evident that field cancerization occurs at the molecular level. It has now been established that precancerous cells that are adjacent to the tumor cells harbor tumor-specific genetic alterations in various organs, including the lung Citation[2], esophagus Citation[3], stomach Citation[4], colon Citation[5], prostate Citation[6], breast Citation[7], cervix Citation[8] and so on. More recently, studies have shown that aberrant DNA methylation patterns are potential biomarkers of field cancerization in several cancers.
How do environmental factors lead to field cancerization? In a recent study, Lee et al. evaluated the use of DNA methylation markers for risk assessment of cancer of the upper aerodigestive tract (UADT) Citation[9]. The study population comprised a training set of 255 patients and a validation set of 224 patients, in whom lifestyle risk factors, such as alcohol consumption, betel quid chewing and cigarette smoking (abbreviated together as ABC) were recorded along with other demographic characteristics. Interestingly, they observed a step-wise increase in methylation of four genes: healthy subjects without exposure to ABC had the lowest level of methylation in the normal esophageal mucosae, followed by normal mucosae from healthy patients with ABC exposure, then normal mucosae from cancer patients, and the highest methylation levels were observed in cancerous mucosae. Correlation of increased methylation of a single gene to alcohol consumption and betel quid chewing was also observed. Smokers had increased methylation of all four markers. All four epigenetic markers were found to be useful as a quantitative measure for field cancerization along the UADT and methylation of HOXA9 was the best discriminative marker in predicting UADT cancer, although the effect of methylation on gene-expression levels was not determined.
A recent publication by Nanjo et al. identified seven novel epigenetic markers for gastric cancer risk Citation[10]. They compared the levels of methylation of seven CpG islands (CGI) among healthy volunteers who had never been infected with Helicobacter pylori (group 1), healthy volunteers with current H. pylori infection (group 2), healthy volunteers with past infection (group 3), gastric cancer patients with current infection (group 4) and gastric cancer patients with past infection (group 5). Among the healthy individuals, methylation levels of the seven CGIs were significantly greater in group 2 than in group 1; however, methylation levels in group 3 were lower than those in group 2, but significantly higher than that of group 1. The authors propose that H. pylori infection leads to aberrant methylation in stem cells, and that remains even after infection has been eradicated. Increased methylation of the seven CGIs was observed in group 5 compared with group 3, suggesting that it might be possible to estimate gastric cancer risk in individuals with past H. pylori infection, although prospective studies may be required to prove this hypothesis. It is not clear whether methylation levels/patterns are different in gastric cancer patients not infected with H. pylori. Hence, it would have been of interest to compare the methylation levels with a sixth group, viz gastric cancer patients not infected with H. pylori. Interestingly, methylation of these seven CGIs was not associated with changes in gene expression, and the authors suggest that methylation of the seven CGIs may reflect the overall epigenomic damage in gastric stem cells and that the degree of epigenomic damage is more likely to be linked to epigenetic field defects in gastric cancer than change of expression of individual genes. Although this is an interesting proposition and may be true for some CGIs, it is unlikely that epigenetic field defects in cancer are not associated with changes in gene expression. In addition, one needs to consider that epigenetic regulation of genes is also linked to CpG shore methylation Citation[11].
Currently, the molecular mechanisms of epigenetic field defects are unclear. Ushijima and Hattori have recently reviewed the contribution of H. pylori-triggered inflammation in the formation of an epigenetic field effect and the use of these epigenetic markers as biomarkers of gastric cancer risk Citation[12]. The authors have provided an insight into the possible mechanisms that lead to epigenetic field effect in gastric cancer by H. pylori infection. Based on the study by Katsurano et al. in a mice colitis model, it was hypothesized that the expression of Il1β and Nos2 might be involved in methylation induction in gastric mucosae by H. pylori infection, and that IL1β signaling from chronic inflammation and elevation of nitric oxide may lead to aberrant DNA methylation as a result of deregulated DNA methyltransferases Citation[13].
Epigenetic changes in field cancerization not only involve hypermethylation, but also hypomethylation. Pavicic et al. developed methylation-specific multiplex ligation-dependent probe amplification assays to study the methylation status of LINE-1 methylation (a marker of global hypomethylation associated with tumorigenesis) in cases of colorectal, gastric and endometrial cancers, which were grouped by patient category (sporadic, Lynch syndrome and familial colorectal cancer type X) and microsatellite instability status Citation[14]. They observed significantly lower methylation levels in tumor DNA relative to paired normal DNA. Differences in LINE-1 methylation levels among normal colorectal mucosae samples correlated with differences in the respective tumor tissues. Patient category-specific variation in LINE-1 methylation was observed rather than a tissue-specic difference; LINE-1 methylation showed a decreasing trend from patients with sporadic colorectal cancer to Lynch syndrome to familial colorectal cancer type X. The results show that epigenetic field defects arise not only due to environmental risk factors, but can also be caused by genetic factors.
The above studies, while providing novel insights into epigenetics and field cancerization also leave behind unanswered questions. One of the questions is ‘when do epigenetic changes become important for cancer development?‘ To answer this, epigenetic changes need to be evaluated in patients who are stratified, based on duration of carcinogen exposure. For example, in patients with a history of smoking, it would be interesting to study whether there is reversal of epigenetic changes when they stop smoking. In the case of epigenetic changes induced by H. pylori infection, it would be of interest to determine the contributing factors leading to epigenetic changes in stem cells postinfection, since not all patients with H. pylori infection develop gastric cancer. Another important aspect of determining the clinical significance of epigenetic markers of field cancerization is to validate the candidate gene methylation markers in a prospective study with long term follow-up of healthy individuals positive for the markers. This longitudinal study will establish whether the aberrant epigenetic alterations are truly representative of a preneoplastic state. Blood-based DNA methylation markers offer a noninvasive tool for tumor detection and risk assessment. This is particularly useful for routine monitoring of patients with a history of carcinogen exposure, and will reduce patient discomfort and reduce healthcare costs. However, so far, research has focused on tissue-based studies for identifying candidate methylation markers of field cancerization. It may be of interest to examine whether epigenetic changes in normal mucosae representative of field cancerization are reflected in blood DNA and in bodily fluids, such as salivary rinse, sputum and urine. The results of the above discussed papers suggest that field changes in methylation may be potential targets of risk assessment and chemoprevention. In their review, Ushijima and Hattori disclose interesting unpublished results on studies conducted in gerbils showing that administration of a 5-azadeoxycytidine decreased the incidence of gastric cancers induced by H. pylori infection and by exposure to the mutagen, N-methyl-N-nitrosurea Citation[12]. This finding reveals that infection induced aberrant epigenetic field defects play a key role in the progression to gastric cancer. In addition, it suggests that reversal of methylation in precancerous cells may prevent the development of tumors. Although the use of demethylating agents, such as 5-azacitidine and 5-azadeoxycytidine seems an exciting possibility, the use of a less toxic agent may be favored for use in the clinic as a chemopreventive agent. Genistein, a natural, nontoxic dietary isoflavone has been shown to reverse hypermethylation, and has been implicated as a potential therapeutic agent in prostate cancer Citation[15]. It may be interesting to examine whether use of such natural methylation inhibitors alters the development of tumors in healthy individuals with an epigenetic predisposition to cancer.
In conclusion, the recent advances in epigenetic field cancerization hold promise for the use of methylation markers in risk stratification of cancer. These markers may also be useful in the future for chemoprevention studies.
Financial & competing interests disclosure
The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.
No writing assistance was utilized in the production of this manuscript.
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