A field of defective tissue (field defect) appears to be an early stage in the progression to adenocarcinoma of the colon or esophagus, the most common type of cancer in these organs. The defective field is characterized by reduced ability to undergo "apoptosis" (programmed cell death). There is also aberrant increased proliferation of cells. Genomic instability, resulting in loss of chromosomes or parts of chromosomes and/or improper attachments of parts of chromosomes to other chromosomes often occurs. Mutations and epimutations in genes predisposing to cancer occur within these fields. A mutation is a change in the nucleotide sequence of DNA which can be replicated and thus inherited from one cell generation to the next. Epimutations usually consist of addition of a small methyl group to the dinucleotide sequence Cytosine-Guanine that occurs in the areas of DNA that regulate whether a gene will make its RNA copy. Epimutations are replicated when a cell divides, just as mutations are replicated. Also there is evidence for aberrant over-expression or under-expression of specific genes. Most of these early molecular alterations in gene expression appear to promote either enhanced apoptosis resistance, enhanced proliferation, or genomic instability. The molecular alterations that are present in the defective field are often, but not always, also present to an even greater degree in the cancer itself. Many of the important events in the development of colonic and esophageal adenocarcinoma first occur in tissues that appear to be normal by microscopic examination.

How changes in the predisposing field come about

If, in a normal population of dividing cells within the colon or esophagus, a cell acquires a growth advantage through a mutation or an epimutation, it will tend to expand as a clone at the expense of neighboring cells. Thus a patch of abnormal tissue will arise. Within this patch, a second genetic or epigenetic change may occur so that another cell acquires a growth advantage compared to cells within the patch, and this cell will expand clonally, forming a secondary patch within the first patch. This process may be repeated several more times until a malignant cell arises which clonally expands into a cancer (Figure 1). If this is the general process by which solid tumors arise, then tumors generally should be associated with, and be preceded by, a field of abnormality reflecting the succession of premalignant events.

Mutations or epimutations in colon or esophageal cells which contribute to malignant progression may arise by the action of chemicals from the environment on DNA (e.g. cigarette smoke components or dietary carcinogens), or by excessive production of chemicals normally produced in the body, which cause stress when produced in excess. For example, excess bile acid secretion into the intestine in response to a high fat diet can cause cellular stress to the intestinal epithelial cells. Although bile acids are needed to emulsify fats and thus aid in their digestion, bile acids can also cause DNA damage and other cellular damages.

In general, a growth advantage may be acquired by a mutation or epimutation that causes either a decreased ability to undergo apoptosis (less suicide) or by increased cell division (proliferation). Both of these types of change will not only lead to clonal expansion, but can also increase the likelihood of further mutation. A reduced ability to undergo apoptosis is frequently observed in colon cancers. Normal cells which have received DNA damage, and which are unable to sufficiently repair this damage, ordinarily undergo apoptosis. This process of altruistic cell suicide is advantageous to humans since it avoids the potential of replicating DNA with a damaged template, likely to cause a mutation in the daughter DNA. Such mutations can lead to cancer. Thus a cell which is defective in the ability to undergo apoptosis in response to unrepaired DNA damage may not only have a growth advantage, but is also more likely to have excessive DNA damage and undergo mutation. In the case of a clone that divides more rapidly (increased proliferation), mutations are more likely to arise over time, because, to a first approximation, mutation rate per unit time is proportional to number of cycles of DNA replication (since mutations arise as errors of replication). Given these considerations, a field of defective cells is likely to contain mutations and/or epigenetic alterations that promote either reduced apoptosis capability or more frequent cell division, or both.

Progression to adenocarcinoma of the colon

Sporadic adenocarcinoma of the colon appears to arise by a pathway involving the following stages: normal flat mucosa, formation of a field of defective flat mucosa, aberrant crypt foci, adenoma (polyp) with low grade dysplasia, adenoma with high grade dysplasia, and adenocarcinoma. Current research has identified 15 specific alterations (described in our longer article in Electronic Journal of Biotechnology ) that occur in the field of defective flat mucosa. These specific alterations include changes in function (such as apoptosis resistance or chromosome instability), specific mutations or epimutations, and changes in expression of proteins, etc., before any microscopic or gross tissue alteration is apparent.

Bile acids have been implicated as important causative factors in colon cancer. Bile acids induce apoptosis when present at concentrations comparable to those found in fecal water after high-fat meals. This apoptosis occurs in goblet cells (mucin-secreting cells) which are common in the lining of the inside surface of the colon. Increased apoptosis caused by bile acids may select for cells with an apoptosis resistant phenotype as shown in Figure 2. In this figure, level (1) indicates normal colonic epithelial cells. These cells encounter high physiologic levels of bile acids after a high fat meal. Bile acids are not carcinogens, but they cause various cellular damages including DNA damage, as indicated in level (2). Cells with higher levels of DNA damage or other cellular damages undergo apoptosis, allowing selective survival of cells with apoptosis-resistant mutations or apoptosis-resistant epigenetic changes as shown in level (3). Upon further DNA damages, some caused by dietary carcinogens, as shown in level (4), this population of apoptosis-resistant cells will be blocked in apoptosis. The apoptosis-resistant cells will allow error-prone replication past the DNA damages caused by dietary carcinogens, resulting in cells with further mutations and colon cancer as shown in level (5).

This model is supported by our findings and those of others. Colon cancer patients who have had their occurrence of tumor plus surrounding area removed are at higher than average risk for subsequent colon cancer. Within the normal-appearing (non-tumorous) portion of the colonic epithelium of such colon cancer patients, we have found that goblet cells are, on average, more resistant to bile acid-induced apoptosis than are colonic goblet cells from individuals with cancer-free colons. We also found that these apoptosis-resistant cells appear to occur in patches, rather than being evenly distributed along the colon.

Progression to adenocarcinoma of the esophagus

Barrett's esophagus (BE) is a lesion resulting from chronic gastroesophageal reflux disease (GERD) in which the normal stratified squamous epithelium of the esophagus is replaced by a different appearing tissue, called metaplastic columnar epithelium, that predisposes to the development of esophageal adenocarcinoma. At the gross level, Barrett's mucosa usually appears as a well-defined area, with irregular margins, consisting of salmon-pink, velvety mucosa similar to the adjacent gastric mucosa. When examined microscopically, BE is seen to have goblet cells, similar to those of the epithelium of the small intestine and colon. The sequence of events in the progression from BE to adenocarcinoma, is considered to be (1) Barrett's metaplasia; (2) low grade dysplasia (cells growing in irregular patterns); (3) high grade dysplasia and (4) invasive adenocarcinoma. About 6% to 15% of individuals with chronic GERD develop BE. GERD exposes the lower esophagus to bile acids from the duodenum and acid/pepsin from the stomach which, in combination, may be the cause of the development of BE. Approximately 0.5% to 1.0% of patients with Barrett's metaplasia develop adenocarcinoma of the esophagus, one of the most lethal of all cancers.

Current research has identified 14 specific alterations (described in our longer article in Electronic Journal of Biotechnology) that occur in the defective field, the Barrett's metaplasia area. Some of the alterations in BE are the same as those found in the defective field preceding a colon adenocarcinoma. The similar alterations include apoptosis resistance, chromosome instability, aberrant proliferation, specific types of mutation and epimutation, and comparable changes in expression of genes.

Conclusions

Field defects appear to be precursors to adenocarcinoma of the colon and esophagus. Study of field defects gives insight into the early stages of carcinogenesis, and may provide early biomarkers for identifying individuals at risk for cancer.