Information for Health Professionals

Colorectal Cancer Aetiology


 

Epidemiology

 

Colorectal cancer is diagnosed in approximately 35,000 people in the United Kingdom each year, the third most commonly diagnosed cancer after breast and lung cancer (NICR 2005; ONS 2005, ISD_Online, 2005 #540).  With 16,148 deaths in 2004 it was the second most common-cause of cancer death.  This is also true in other Caucasian populations (Jemal et al. 2007).

Age-standardised death rates from Colon and re...

Age-standardised death rates from Colon and rectum cancers by country (per 100,000 inhabitants). (Photo credit: Wikipedia)

Colorectal cancer, however,is not equally common throughout the world (Boyle and Langman 2000).  In the ‘westernised’countries colorectal cancerrepresents ~13.4% of all incident cases compared to ~7.8% elsewhere.

The occurrence of colorectal cancer is strongly related to age, with 93% occurring after the age of 50 years (ONS 2005).  Before this age there are equal numbers of males and females diagnosed, although after males predominate, with an eventual overall male to female ratio of 1.2:1.  Using data for England and Wales the overall lifetime risk of colorectal cancer is 1 in 18 for men and 1 in 20 for women (CRUK. 2006).  Sixty two percent of cancers are situated in the colon and the remainder in the rectum (ONS 2005).  Seventy percent are left-sided and the remainder right-sided (Toms 2004).

English: Gross appearance of an opened colecto...

Gross appearance of an opened colectomy specimen containing an invasive colorectal carcinoma and two adenomatous polyps. (Photo credit: Wikipedia)

More than 90% of colorectal cancers are adenocarcinomas arising from benign precursor adenomas.  A small proportion may arise from other types of polyps including hyperplastic and serrated polyps (Young et al. 2007).  The remainder of colorectal malignant tumours include carcinoids and lymphomas.  The National Polyp Study showed that endoscopic removal of adenomas prevents colorectal cancer (Winawer et al. 1993).  Endoscopic surveillance and polyp removal is a vital tool making colorectal cancer also one of the most preventable cancers as polyps removed at an early stage can prevent the cancer ever occurring.   Of course the endoscopic procedure performed must be technically perfect, performed at the correct stage in cancer development and in appropriate groups, e.g. for age, risk etc.  The resources are not available to perform colonoscopy on whole populations.  Identifying individuals at high risk due to genetic or environmental factors will help us undertake these preventative measures with the highest levels of efficacy and efficiency.

The concordance rates of cancer in monozygous and dizygous twins suggest that about one-third of the variation in cancer risk might typically be ascribed to genetic factors (Lichtenstein et al. 2000).  Highly penetrant (Mendelian) genetic syndromes account for 5–10% (Cannon-Albright et al. 1988) (Houlston et al. 1992) (Johns and Houlston 2001).  These include Familial Adenomatous Polyposis (FAP), due to germline mutations in the tumour suppressor gene APC, and Hereditary Non-Polyposis Colorectal cancer (HNPCC), due to germline mutations DNA mismatch repair enzymes MLH1, MSH2, PMS2 and MSH6. HNPCC accounts for 1-4% of colorectal cancer and is characterised by cancers with DNA microsatellite instability (MSI).  There is good evidence that endoscopic surveillance of patients with high-risk genetic predisposition also prevents cancer (Jarvinen et al. 2000; Dove-Edwin et al. 2005; Dove-Edwin et al. 2006).

Proportion of colorectal cancer due to environmental factors is about 65%, with 5-10% caused by cancer syndromes and the remainder due to low penetrance heritability

Environmental factors such as physical activity, obesity, diet and other exogenous factors also contribute to the aetiology of colorectal cancer (Linseisen et al. 2002; Bingham et al. 2003; Ferrari et al. 2007).    Diet and lifestyle has been found to modify risk across the cancer spectrum e.g., smoking and lung cancer (Doll and Hill 1950).  However, there are difficulties studying the effects of ‘five pieces of fruit a day’ in population studies due to analytical challenges with accurate food monitoring, variation in the fruit types and other confounding variables such as smoking, BMI, etc, and results of studies of environmental factors often vary widely.  Large international studies such as the European Prospective Investigation into Cancer and Nutrition (EPIC) are attempting to unravel the evidence of sometimes contradictory epidemiological studies (Gonzalez 2006).  Genetic predisposition alleles with a low penetrance may be more susceptible to environmental influences which modify an individual’s risk of colorectal neoplasia. There have been very few studies that clearly address the mechanisms of mutational induction by the environment in colorectal cancer.  Although the contribution of individual environmental factors is uncertain, they play an important role in the multifactorial aetiology of colorectal cancer.

 

The normal large bowel

 

The large bowel is embryologically derived from midgut and hindgut which in have their junction at the splenic flexure.  The vascular supply and lymphatic drainage of the large bowel is similarly divided, and this junction is sometimes taken to separate the ‘right’ and ‘left’ colorectum.  In adults, the large bowel lumen is continuous with nine anatomical regions: The caecum, ascending colon, hepatic flexure, transverse colon, splenic flexure, descending colon, sigmoid colon and the rectum.  The mucosa lies over a basement membrane, underneath is the muscularis mucosa, lamina propria and muscularis propria (Figure 1‑1).

 

 

 

The Wnt signalling pathway and colonic crypt homeostasis

 

The Wnt, and to a lesser extent the TGF-ß, BMP, Notch, and Par polarity pathways are the major players in homeostatic control of the adult epithelium (Sancho et al. 2004).  The surface of the colon is covered by an epithelium composed of four differentiated cell types (enterocytes, enteroendocrine,goblet and Paneth cells) that invaginates at regular intervalsto form crypts.  The bottom of the crypts is occupied by a few stem cells that give rise to actively dividing precursorcells that populate the bottom two-thirds of the crypts. The epithelial cell precursors migrate upward in an ordered fashion, which isalso controlled by Wnt factors, and they stop proliferating when they reach the top third of the crypt, possibly because they are too far from the Wnt source. Meanwhile, they continue their migration and colonise the surface of the colon.After about a week, epithelial cells undergo apoptosis and areshed in the lumen of the gut.  The Paneth cells constitute anexception, as they migrate downward and occupy the very bottomof the crypt (van Es et al. 2005).

Thus, the epithelium of the colon is under perpetual renewal.  Wnt growth factors activate a cascade of intracellular events, which is known as the canonical Wnt pathway that ultimately leads to the expression of a genetic program controlling the co-ordinated expansion, fate and sorting of the epithelial cell population (Figure 1‑2).

The Wnt signalling pathway (McDonald SAC et al. (2006) Mechanisms of Disease: from stem cells to colorectal cancer)

The canonical Wnt signalling cascade controls cell behaviour by activating the transcriptional properties of DNA-binding proteins of the T-cell factor(TCF)/lymphoid enhancer factor-1 family (Clevers 2006). Wnt induces stabilisation of cytosolic ß-catenin, which is transported to the nucleus and associates with TCFs, leading to the expression of specific target genes (McDonald et al. 2006). In the absence of Wnt signalling, ß-catenin is targeted for degradation in proteosomes, and TCFs block expression of Wnt target genes.  Thus activation of the pathway may be demonstrated by nuclear localisation of ß-catenin.  In colorectal cancer mutations in components of the Wnt pathway mimic the effect of a permanent Wnt stimulation, causing epithelial cells to proliferate inappropriately.

Wnt factors can also trigger the activation of at least twoother, ß-catenin-independent, non-canonical pathways. They are referredto as non-canonical signalling and their implication in colorectal cancer, if any, is not yet elucidated.  The list of genes targeted by the Wnt pathway is very long (http://www.stanford.edu/%7ernusse/pathways/targets.html), indicating its wide range of roles in cellular homeostasis.

The transforming growth factor-ß pathway

The transforming growth factor-ß (TGF-ß) superfamily comprises a large number of structurally related polypeptide growth factors and each of these is capable of regulating a complex array of cellular processes (Hata et al. 1998).  In normal epithelial cells including intestinal epithelium, TGF-ß has a major inhibitory role and a substantial body of evidence suggests a role as tumour suppressor.  Neoplastic transformation results in loss of normal growth inhibitory responses to TGF-ß (Kurokowa et al. 1987).  Inhibition of intestinal cell proliferation by TGF-ß treatment results in cell cycle arrest from mid-to-late G1 (Ko et al. 1998).   To date, some of the most important contributors to the growth response seen on TGF-ß exposure appear to involve cell cycle regulatory genes, including C-MYC, CDC25A, and the cyclin-dependent kinase inhibitors (CDKIs) p15INK4B, p21WAF1/CIP1, and p27KIP1 (Ko et al. 1994; Ko et al. 1995).  Other targets of this pathway include the SMAD (Small mothers against decapentaplegic) and BMP genes (bone morphogenetic proteins), important mediators of signal transduction.  In vivo, there is increased expression of both TGF-ß1 and the TGF-ß type II receptor in intestinal epithelial cells as they migrate from the proliferative compartment toward the lumen in both the small intestine and the colon (Winesett et al. 1996).  This pattern of expression is inversely correlated with the mitotic activity in the gut epithelium.  Reduced expression of the TGF-ß type I receptor was been strongly linked to colorectal cancer predisposition (Valle et al. 2008).  However a larger study published subsequently did not demonstrate allele-specific expression or an association polymorphisms within this gene with colorectal neoplasia (Carvajal-Carmona et al 2010).

Colorectal Cancer Development

Pathway from normal colorectal epithelium to cancer

 

Colorectal cancer develops via an adenoma to carcinoma sequence with the accumulation of a number of genetic and epigenetic mutations (Figure 1‑3) (Morson 1968; Fearon and Vogelstein 1990).  The mutations accumulated vary in hereditary cancer depending on the initiating mutation.  In their normal state, tumour suppressor genes inhibit cell proliferation.  Growth inhibition is lost when both alleles are inactivated by mutation and/or epigenetic changes, such as promoter methylation which stifles expression of the gene.  Tumour suppressor genes broadly conform to Knudson’s classic two-hit hypothesis, where inactivation of both alleles is required for tumour suppressor genes to lose their normal function (Knudson 1971).  In contrast, proto-oncogenes act by promoting cell proliferation.  Mutation of these genes leads to abnormal oncogenic over-expression or increased activity of the protein.

The adenoma-to-carcinoma sequence for colorectal cancer is probably most commonly initiated by bi-allelic mutation of the APC tumour suppressor gene.  APC mutations have been found in microadenomas (Otori et al. 1998), the earliest lesion on the pathway (also called aberrant crypt foci (Roncucci et al. 1991)), and in ~60-80% of early sporadic adenomas and carcinomas (Cottrell et al. 1992; Miyoshi et al. 1992; Nakamura et al. 1992).  APC is a key member of the canonical Wnt signalling pathway, and the key mechanism by which mutation of this gene contributes to carcinogenesis is by activation of this pathway.  However, further accumulated mutations in additional genes are required for progression of the early lesions to cancer.

The adenoma-to-carcinoma sequence: A: Aberrant crypt foci are seen using chromoendoscopy with x200 magnification (centre and 2 o’clock) surrounded by normal crypts. An early initiating mutation occurs here, usually a tumour suppressor gene such as APC. B: Adenomatous Polyp: Mutations in proto-oncogenes such as KRAS and BRAF lead to adenomatous polyp formation. C: Other genetic and epigenetic alterations such as promoter hypo-/hyper-methylation cause progression. There are hyperchromatic nuclei with prominent nucleoli indicate highly dysplastic crypts on the left side of this image. D; Adenocarcinoma, with invasion through the muscularis layer, and mucin producing glands with abnormal polarity. This is often associated with genomic copy number variation of regions such as 18q and 17p (P53).

Activating mutations of the oncogenes KRAS (Kirsten rat sarcoma viral oncogene homolog) and BRAF (v-raf murine sarcoma viral oncogene homolog B1), both members of the MAPK (mitogen activated protein kinase) signalling pathway, are found in the transition from early to an intermediate lesion in approximately 50% and 10% of cases respectively (Bos et al. 1987; Rajagopalan et al. 2002; Yuen et al. 2002).  Mutations of codons 12 and 13 in exon 2 of KRAS tend to occur in 30-60% of colorectal carcinomas (Kressner et al. 1998). The KRAS gene product,a 21 kDa protein located at the inner plasma membrane, is involved in the transduction of mitogenic signals.  The Ras protein is activated transiently as a response to extracellular signals such as growth factors, cytokines and hormones that stimulate cell surface receptors (Campbell et al. 1998), and mutations in KRAS constitutively activate the Ras protein.

BRAF V600E substitution mutation

The substitution mutation of BRAF V600E is present in 4-12% of unselected colorectal tumours, and it is associated with sporadic MSI tumours but not HNPCC tumours (Vandrovcova et al. 2006).  MSI tumours outside the context of HNPCC are usually caused by methylation of the MLH1 promoter.  Indeed, there is a hypothesis that some tumours might develop through a separate hyperplastic polyp-serrated adenoma pathway (Spring et al. 2006).

Epigenetic changes such as promoter hypo-/hyper-methylation can cause disregulation of expression of many genes important in colorectal cancer (Hitchins et al. 2005; Hitchins et al. 2006).  Further progression to late type adenoma is associated with loss of 18q in 50% of large adenomas and 75% of carcinomas (Vogelstein et al. 1988; Fearon et al. 1990).  This causes loss of SMAD2 and SMAD4, members of the TGF-ß signalling pathway.  Point mutations of these genes have also been identified in colorectal cancer (Eppert et al. 1996; Hahn et al. 1996; Thiagalingam et al. 1996).  The adenoma to carcinoma transition appears to be associated with loss of 17p (Fearon et al. 1987; Rodrigues et al. 1990; Akiyama et al. 1998).  The 17p locus contains the P53 gene (Baker et al. 1990), the so-called gatekeeper of the cell which has important roles in the regulation of the cell cycle and apoptosis.  Loss of heterozygosity of 17p correlates with missense and truncating mutations in P53 (Baker et al. 1989; Baker, Preisinger et al. 1990).  Tumour invasion and metastasis are associated with loss of 8p (Hughes et al. 2006), and loss of E-cadherin function, a component of adherens-junctions (Hao et al. 1997; Christofori and Semb 1999).

The progression from adenoma to invasive carcinoma probably takes 10-40 years (Ilyas et al. 1999).  However, not all lesions will undergo malignant transformation, the reason for which is unclear.  It may be that the necessary mutations do not accumulate because of death, or because of environmental influences such as diet.

Genetic instability and colorectal cancer

 

Colorectal cancer may be subdivided genetically by the types of mutations which accumulate genome-wide during carcinogenesis.  It had been observed nearly a century ago that most cancers were aneuploid, and it has been noted that the degree of aneuploidy in colorectal cancer correlates with the severity of the neoplastic behaviour (Heim and Mitelman 1989).  A series of deletions, duplications, and rearrangements occur.  Allelic losses appear to be important in the progression from premalignant to malignant neoplasia in the colorectum.  This process is called chromosomal instability (CIN) and accounts for approximately 75% of colorectal cancers.  Ten to 15% are not CIN but do have smaller mutational events which are caused by loss of DNA mismatch repair (Fishel et al. 1993) and are referred to as MSI tumours.  The CpG island mutator phenotype (CIMP) is associated with methylation of promoter regions, CpG rich regions, which causes silencing of genes.  Tumours are often both MIN and CIMP, as methylation of mismatch repair gene promoters usually occurs in sporadic MSI tumours (Kane et al. 1997).

The role of genomic instability in causing and promoting tumour growth remains controversial (Lengauer et al. 1998; Tomlinson and Bodmer 1999).  Some argue that instabilityis necessary for tumourigenesis (Loeb 1991), while others take the viewthat Darwinian selection is the driving force. It is becoming clear that many cancers harbour multiple mutations, the great majority ofwhich probably have no significant effect on tumour growth.  It may well be that some tumours with an inherited DNA repair defect accumulate more mutations than others.

 

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About kjmonahan

Service lead for Family History of Bowel Cancer Clinic

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