The pre-malignant lesion: the adenomatous polyp

There is abundant evidence that virtually all colorectal carcinomas begin as adenomatous polyps. 

In a number of studies, colonic polyps have been left in situ and the follow up of these patients showed variable natural history from complete regression, increase in size or progression to carcinoma.

Histological studies have revealed a spectrum of dysplasia within adenomatous polyps up to carcinoma-in-situ and it is not uncommon for true invasive carcinomas to have associated adenomatous tissue.  In a study of post mortem examinations of the colon, the population with the highest proportion of adenomas was observed in the area with the highest incidence of colonic cancer. Also, the segmental distribution of adenomas within the colon was found to be similar to the site distribution of cancer.

The transition from benign adenoma to colorectal cancer is thought to have a long natural history of between 10 and 35 years.  It is estimated that the annual conversion rate of a polyp to a cancer is approximately 0.25%.

The adenoma / carcinoma sequence

Colorectal carcinogenesis has long been thought to be due to a stepwise accumulation of cellular mutations.  Studies of adenoma and carcinoma cell types have revealed that there is a monoclonal expansion of a single or small number of colonic epithelial cells.  Much research has been done to identify mutations in oncogenes, tumour suppressor genes or DNA repair genes that confer a growth advantage in these neoplastic cells.

A genetic model for the sequence of adenoma transformation to carcinoma was proposed by Vogelstein which in light of recent advances in molecular genetics has become modified to include additional genes that contribute (Figure 1).

  Figure 1. Genetic model of colorectal carcinogenesis

It is thought to be the accumulation rather than the order of genetic mutations that is critical to the development of colorectal cancer.

Cellular hyperproliferation is a preceding step to adenoma formation and is associated with loss or mutation of both alleles of the adenomatous polyposis coli (APC) gene on chromosome 5q.  Inherited mutation in this gene gives rise to familial adenomatous polyposis (FAP) which is typified by hundreds and occasionally thousands of adenomatous polyps.  The APC gene is a tumour suppressor gene which prevents uncontrolled epithelial cell proliferation.  Mutation results in a truncated APC protein that fails to bind to ß-catenin, which in turn fails to promote cell adhesion via the calcium dependent cell surface adhesion molecule E cadherin.

The over expression of the cyclo-oxygenases (COX 1 and COX2) catalyses the conversion of arachidonic acid to prostaglandin H2 and other derivatives, e.g. malondialdehyde, which is itself mutagenic, promoting further polyp proliferation.  This explains why the non-steroidal anti-inflammatory drugs, which inhibit the cyclo-oxygenases, have been postulated as possessing a potential therapeutic role in the prophylaxis of colorectal cancer.

A significant loss of DNA methyl groups has been shown to occur early in colorectal tumourogenesis.  Hypomethylation interferes with chromosome precipitation and mitotic separation resulting in increased frequency of  genetic alterations.

Approximately 50% of colorectal cancers and adenomas larger than 1cm have been found to exhibit mutation in the K-ras oncogene.  This mutation is present from the intermediate stage (>1cm, mild & moderate dysplasia) of adenoma formation and therefore probably represents an initiating stage for adenoma progression.

Allelic and chromosomal losses occur frequently in colorectal cancer.  This can result in loss of regions that code for tumour suppressor genes.  Allelic loss of chromosome 18q occurs in more than 70% of carcinomas and 50% of late adenomas.  In this area, the tumour suppression gene DCC (deleted in colorectal cancer) is located.  This gene encodes a cell surface-localized protein and is thought to be involved in cell adhesion and its preservation is associated with the majority of mucinous colorectal tumours suggesting a function in cellular differentiation or phenotype modulation.

Similarly, the p53 gene located on chromosome 17p frequently exhibits allelic loss (also termed loss of heterozygosity).  This occurs in approximately 75% of cases of colorectal cancers but infrequently at the adenoma stage.  In addition, somatic mutations in the remaining p53 allele have been regularly observed in keeping with the proposed function of p53 as a tumour suppressor gene.  Thus, mutation in one allele coupled with loss of the other appears to be tumourogenic and an important step in the transformation of adenoma to carcinoma.

Cancers are amongst the commonest causes of death in this country, accounting for about one in every four deaths - almost 130,000 per annum¹.  The majority of cancer deaths are from tumours found in four principal sites: lung, colorectal, breast and prostate (Figure 1).

Figure 1. Cancer deaths in England and Wales by sex, 1996.

These cancers are common in Western countries but there is a much lower incidence in third world countries (Figure 2).  It has been observed that immigrants moving from a low risk area to one of high risk acquire the same risk as the indigenous population within one or two generations suggesting that environmental factors are responsible².

Figure 2. Geographic variation in the incidence of colon and rectal cancer in men ².

Tobacco smoking, electromagnetic radiation, environmental chemicals, hormones, bacterial or viral infection, level of physical activity, reproductive and sexual behaviour are thought to be important in the aetiology of cancer at certain sites³.  However, it is thought that diet remains the most important factor and it has been estimated that dietary change could result in a reduction of fatal cancers of between 35 and 70%⁴.

Colorectal cancer is the second most common cause of death from cancer in the Western world.  In 1985, there was estimated to be 677,500 new cases and 394,100 deaths worldwide^5,6.  In the United Kingdom there is approximately a 4% lifetime risk, 31,000 new cases annually and 16,000 deaths⁷.

The prevalence increases with age with over 90% of cases occurring after the age of 55 (Table 1).  The  incidence of colon cancer varies little between the sexes^8,9, whereas rectal cancer is twice as common in men as in women².

Causes of Bowel Cancer

Environmental Factors

The role of diet in tumourogenesis

Diet might influence the risk of cancer in a number of ways, such as the direct ingestion of pro-carcinogenic or carcinogenic compounds, induction of protective mechanisms such as apoptosis, suppression of DNA damage through antioxidants in food or modification of cell proliferation and methylation of DNA.

Dietary factors in colorectal neoplasia

The geographic variation in colorectal cancer corresponds to a high per capita consumption of red meat and dietary fat and to a lesser degree is inversely associated with the amount of dietary fibre.  Epidemiological evidence has implicated these as principal risk factors and various other less consistent dietary factors with the risk of colorectal cancer.  Specific components within the diet are obviously hard to establish because of the heterogeneity of the food we eat.  The recognised risks in colorectal cancer are listed:

Total Energy Intake

Total energy intake has been found consistently to correlate with an increase in the risk of colorectal cancer.  However, because energy-contributing nutrients such as dietary fat, protein and carbohydrate are highly correlated with total energy, the association with colon cancer from energy per se as opposed to other components of the energy-providing nutrients is often not clear.  Slattery et al. demonstrated that once total energy and physical activity had been corrected for, there was no significant association seen for dietary fat, protein and carbohydrate.  This contradicts many hypotheses and studies that have shown that dietary fat correlates with the risk of colorectal cancer (below) and it may be that these factors are inextricable.

Dietary Fat

Dietary fat has been associated with cancers of the breast, colon, rectum, endometrium, ovary, prostate and gall bladder.  Obviously, dietary fat correlates with total energy intake which has been shown to be a risk factor.

The risk of colorectal cancer correlates with the consumption of animal fat but not vegetable fat. There is an inverse correlation with fish and fish oil consumption, when expressed as a proportion of total or animal fat, and this correlation was significant for both male and female colorectal cancer and for female breast cancer.

Prentice et al. summarised the evidence from the literature in 1990 and presented 8 studies.

Relative risks for below- to above-median fat consumption

⁺males and females, colon and rectum

*males and females colon                          **males and females rectum

Most of the studies demonstrated a lower risk with a below-median fat consumption, although 2 studies found relative risks above unity these were not statistically significant. 

Red Meat

Willet et al. demonstrated in a cohort study of 88,751 women that the relative risk of colon cancer in women who ate beef, pork, or lamb as a main dish every day was 2.49 (95 percent confidence interval, 1.24 to 5.03), as compared with those reporting consumption less than once a month.  Similarly, Hsing et al. in a cohort study of 17,633 men with 20 years follow up found an increased risk of colon cancer for those who consumed red meat more than twice a day (RR = 1.8, 95% CI 0.8 - 4.4).  There are now numerous studies that have demonstrated a similar effect in the aetiology of colorectal adenomas.

Exposure of meats to high temperatures can result in the formation of heterocyclic amines and aromatic hydrocarbons that are carcinogenic in animals.  Some studies have observed relatively strong associations of colorectal neoplasia with consumption of broiled or grilled meats and browning of the meat surface and in a more recent study, Sinha et al. concluded that it was the high temperature cooking methods that contributed more to the increased risk than the absolute red meat intake itself.

Protein

The association of protein with colorectal cancer has been little investigated because the emphasis has been on red meat as the protein source most often associated.  Potter et al. demonstrated that the most consistent risk factor for colorectal cancer was dietary protein, which was associated with two to three times a relative risk of colon and rectal cancer in women for all levels of consumption above the lowest quintile. For male colon cancer the corresponding relative risk was similar; but for male rectal cancer, risk was elevated only at old ages.  The actual protein source was not analysed and so how much the observation is due to meat proteins is uncertain.

Alcohol

Although ethanol has generally not been found to induce cancer in experimental animals, there is evidence that alcohol consumption increases the risk of cancer in humans.  The exact mechanism for this increase risk is not clear but studies have shown a dose response relationship between the alcohol intake and relative risk of cancer.

It is postulated that alcohol may:

• contain carcinogenic contaminants
• generate carcinogenic metabolites such as acetaldehyde
• act as a solvent to increase penetration of target tissues by carcinogens
• reduce nutrients necessary for health
• inhibit hepatic detoxification of carcinogens
• catalyse activation of pro-carcinogens
• affect levels of hormones e.g. oestrogens
• increase cellular exposure to oxidants
• suppress immune function

Alcohol and colorectal cancer

Methylation of DNA is thought to have a role in the regulation of gene expression. A high consumption of alcohol, an antagonist of methyl-group metabolism, therefore may give rise to an increase risk of colon neoplasia through DNA hypomethylation.

Furthermore, a diet high in folate and methionine can counteract the poor methyl group availability that alcohol causes.

The International Agency for Research on Cancer in a review in 1988 found that 4 of the 9 cohort studies and 6 of 9 case control studies demonstrated significant increase in rectal cancer particularly in beer drinkers.

Klatsky et al. showed that when daily alcohol intake of three or more drinks was compared with abstainers, relative risk for rectal cancer was 3.17 (95% CI: 1.05 - 9.57) and relative risk for colon cancer was 1.71 (95% CI: 0.92 - 3.19).  This association was stronger in women who demonstrated a relative risk for colon cancer of 2.56 (95% CI: 1.03-6.40) compared with a relative risk of 1.16 (95% CI: 0.46-2.90) for men with colon cancer.

Sugar

In a small study by Bristol et al., colorectal cancer patients were found to consume 16% more energy than controls mainly in the form of carbohydrate (21%) and fat (14%).  Furthermore, the carbohydrate was mainly ingested as refined sugars.  This study, however, was retrospective and therefore could be liable to recall bias, and the findings for carbohydrate intake were not corrected for total energy and fat intake.

Tuyns et al^. also showed a detrimental effect of high sugar (oligosaccharides) as opposed to polysaccharides but with no association with confounding risk factors such as fat and protein intake.

Dietary Fibre, Fruit & Vegetables

Dietary fibre, principally derived from cereals, fruit and vegetables, has been found consistently to be protective in colorectal cancer in many epidemiological studies By increasing faecal weight, diluting large intestinal contents, and speeding up transit time, fibre is thought to change the milieu within the colon to reduce interaction between faecal mutagens and the mucosa.

However, a recent large prospective study of 88,757 women by Fuchs et al. refuted this hypothesis following adjustment for age, established risk factors, and total energy intake.

Obviously, fruit and vegetables contribute to the total amount of dietary fibre we consume, but as a factor independent to total dietary fibre, cruciferous vegetables (such as broccoli) have been shown to be protective against colorectal neoplasia.  More specifically high carotenoid vegetables, cruciferae, high vitamin C fruits show the greatest reduction in incidence of colorectal adenomas suggesting that micronutrients within these food items play a part in addition to the fibre they contain.

Micronutrients

There are many papers written on micronutrients and colorectal neoplasia few have significant statistical power and, as with investigation of any dietary variable, there are many confounding genetic and co-environmental factors.  Certainly, the overall impression is that these factors may make a small difference in the aetiology of colorectal neoplasia and that much more work is required to define their role in carcinogenesis.

Calcium

In the United States in 1985, Garland et al. investigated an observation that mortality rates from colorectal cancer were higher in populations exposed to the least amounts of natural sunlight, suggesting that differences in vitamin D production and calcium absorption could be responsible.  Risk of colorectal cancer was inversely correlated with dietary vitamin D and calcium and this remained true after adjustment for age, daily cigarette consumption, body mass index, ethanol consumption, and percentage of calories obtained from fat.

More recent studies have been divided as to whether low calcium intake was a risk factor for colorectal neoplasia or no association was seen.

It has been postulated that calcium acts to reduce the incidence of colorectal neoplasia by reducing lipid damage in the colon by complexing with fat to form mineral-fat complexes or soaps.

Vitamins

There has been great interest in the anti-oxidant vitamins A(and its provitamin beta-carotene), C and E as protective agents in colorectal carcinogenesis.  In addition to dietary intake, these compounds are common supplemental dietary products taken either separately or as "multivitamins" popularised by health promotion.

Vitamin A and its pro-vitamin beta-carotene act as anti-oxidants, are found in many fruits and vegetables and have been shown to be protective against colorectal neoplasia in the majority of studies, whilst a few have shown no significant effect.

Vitamin C has been found in humans to reduce colonic crypt cell proliferation and therefore postulated to reduce neoplasia.  Again, some studies have demonstrated a preventative effect though a few showed no significant difference.

In 1980 Cook et al. investigated the effect of dietary vitamin E levels on colonic neoplasia in mice.  The results demonstrated a lower rate of adenoma and cancer formation in the high dose vitamin E group.  This result has been reproduced in several centres but most reports have shown no significant difference in relative risk.

Selenium

The trace element selenium has been associated with a decreased risk of colorectal neoplasia.  In a rat model, Feng et al. demonstrated the ability of selenium to reduce aromatic amine-induced colon carcinogenesis.

Smoking

Tobacco smoke is a major source of a multitude of carcinogens including nitrosamines, polycyclic hydrocarbons and heterocyclic amines. Cigarette smoking has been strongly associated with colorectal adenomas but this association has been less strong with colorectal cancer although Giovannucci et al. in a large cohort study produce convincing data for a lead time between smoking and cancer formation of some 35 years. 

Terry et al. postulated that this was because subjects with colorectal adenomas were included in the control group of cancer case-control studies.  Analysis of a "manufactured" control group compared to a pure adenoma-free control group showed a trend in keeping with this hypothesis but did not reach statistical significance.

In addition, it may be that the effect of cigarette smoking on the colorectal adenoma-carcinoma sequence occurs in the earlier stages of the formation of adenoma.

Physical Activity and Body Mass

Most studies that have investigated the relationship between physical exercise and colorectal neoplasia have found an inverse relationship.

One hypothesis for the role of physical activity in colorectal neoplasia is that exercise stimulates colonic peristalsis and thereby reduces colonic transit time.

The combination of high physical activity and lower body mass effects many of the bodies homeostatic mechanisms - decreased insulin, glucose, triglycerides, prostaglandins and possibly growth factors. 

Alternatively, there is an obvious link between physical activity and obesity.  However being overweight is likely to be a surrogate. As such, risk factors including a high-fat, high energy diet, with inadequate consumption of fruit and vegetables; and lack of physical activity are likely to contribute to a high incidence of colon cancer as well as obesity.  Therefore body mass per se has only been weakly associated with colorectal neoplasia.

Pharmaceuticals

There is a very wide variety of pharmaceutical products that are in use - probably in excess of 100,000.  It is hardly surprising then that some groups of products have been found to influence colorectal neoplasia.

Hormone Replacement Therapy

Hormone replacement therapy has been consistently shown to decrease the risk of colorectal neoplasia and there is an inverse relationship with duration of treatment (odds ratios compared with no hormone replacement therapy range from 0.39 to 0.74).

It has been postulated that the reason for this is that with time the oestrogen receptor gene is silenced by methylation and hormone replacement therapy reverses this trend.  Experimental data has shown that endogenous oestrogens protect against Apc-associated tumour formation and is associated with an increase in oestrogen receptor beta and a decrease in oestrogen receptor alpha expression in the target tissue.  Alternatively, oestrogens may act on the colonic vitamin D receptor to decrease neoplasia.

Non-steroidal anti-inflammatory drugs (NSAIDs)

As can be seen from Vogelstein's genetic model for colorectal carcinogenesis (Figure 3), over expression of the cyclo-oxygenases COX1 and COX2 is thought to be an early step in the aetiology of colorectal adenomas and carcinomas.  The Non-Steroidal Anti-Inflammatory Drugs (NSAIDs) are potent inhibitors of cyclo-oxygenase and have been shown to inhibit polyp and cancer formation even in patients with FAP.

Genetics of Bowel Cancer

As can be deduced from the adenoma - carcinoma sequence (polyp / carcinoma sequence), besides factors critical only to adenoma progression, the aetiology of colorectal adenomatous polyps should be the same as that for colorectal cancer - the adenoma just being an intermediate step in the development of cancer.

Indeed, there are well recognised inherited syndromes characterised by multiple colorectal adenomas that invariably give rise to colorectal cancer.  These cases are uncommon and only constitute about 5% of reported cancers.

There is increasing evidence however that many adenoma cases have some previously unrecognised genetic predisposition with absolute adenoma risk being modified by environmental factors.

Genetic Syndromes

Familial Adenomatous Polyposis

Familial adenomatous polyposis (FAP) is a rare autosomal dominant syndrome characterised by hundreds or even thousands of adenomatous colorectal polyps affecting approximately 1 in 10,000 individuals.  Untreated these polyps almost invariably progress to carcinoma by a mean age of 44.  Mutations are found in the APC (adenomatous polyposis  coli) gene which is located on chromosome 5q and penetrance is thought to approach 100%.  Mutation at the APC locus appears to be a common event in colorectal cancer induction, the only difference there being that in FAP-associated lesions there is an inherited genetic defect.

The designation Gardner Syndrome is used for phenotypic variants of FAP with additional extra colonic manifestations e.g. osteomas, epidermoid cysts and fibromas and Turcot's syndrome for the association with medulloblastoma.

The APC gene on chromosome 5

Phenotypical variation in FAP is now known to depend on the location of the APC mutation.  Attenuated FAP, in which there are relatively few polyps, is associated with mutations in codons at the 5' end of the APC gene whereas mutation between codon 1285 and 1465 result in profuse polyposis syndrome.  The best example of this variation giving rise to extra colonic features is the presence of Congenital Hypertrophy of the Retinal Pigment Epithelium (CHRPE) which is associated with mutations in codons 542 to 1309.

HNPCC is an autosomal dominant syndrome that is estimated to be responsible for approximately 2% of colorectal cancers.  The syndrome is characterised by familial clustering of colorectal cancers along with other cancers such as endometrium, ovary, gastric, hepatobiliary and renal tract.  In the absence of a clear phenotype an absolute definition of HNPCC is therefore obviously difficult - the Amsterdam Criteria and Bethesda Criteria attempt to classify on clinical grounds those families most at risk of having mutation in the mismatch repair genes that give rise to HNPCC

Non-coding regions of DNA exhibit variation which principally consists of highly repetitive segments of DNA consisting of several iterations of a specific sequence known at "DNA repeats".  Such sequences are unique to each person and are the basis for the precise DNA fingerprinting used in forensics.  These repeats of 2-5 nucleotide segments are known as microsatellite DNA.  A single pair of PCR oligonucleotide primers that surround such sequences produce variably-sized DNA fragments depending upon the number of repeats.

The mismatch repair pathway is responsible for detecting and repairing short segments of mismatched base pairs (such as a mutation from C to T on one strand T opposite G, or the addition of extra nucleotides, resulting in unpaired bases within the helix)⁵⁵. Since microsatellite repeats are susceptible to such mutation, disorders of the mismatch repair pathway lead to errors in these polymorphic segments - termed microsatellite instability (MSI).

In HNPCC, microsatellite instability was discovered to be the result of germline mutations in the genes that encode the components of the DNA proofreading complex. These genes are hMSH2, hMLH1, hPMS1, and hPMS2.  hMSH2 and hMLH1 are the most commonly mutated in HNPCC, whereas mutation in the genes hPMS1 and hPMS2 contribute only to a small proportion of cases.  The resultant colorectal cell lines show a higher accumulation of other mutations and deletions presumably reflecting the in vivo accumulation of errors resulting in an increased risk of cancer.