The Adenomatosis Polyposis Coli gene is found mutated in a group of colorectal cancer syndrome known as Familial Adenomatosis Polyposis (FAP) syndrome and in some sporadic variety of cancers (Weinberg 2007).
This gene occupies the 5p21 region of the human genome and its protein is about 2843 amino acid residue long. It is made up of ten repeated sequence motifs, seven repeats of 20 amino acid residues and a basic domain. These include Î²-catenin binding region occupying 1020-1169 region and the glycogen synthase kinase (GSK-3Î²) binding region occupying 1324-2075. There are many isoform of the protein and most of these have amino acid 312-412 deletion via an alternative splicing mechanism. Many of these sites are involved in the phosphorylation, glycosylation and myristylation of many proteins (Souhami et al 2002).
Approximately 5% of colorectal cancers tend to have a mutated allele of the gene in their germline resulting in the familial variant of the disease while 95% of mutation occurs in the sporadic fashion. In the western population, 1% of all cancers have this mutation, and this manifests the first step to the progression of colon carcinogenesis. The aberration of this gene causes hyperplasia of epithelial cell leading to adenoma and later progress toward carcinoma and ultimately results in metastases (Weinberg 2007).
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A group of researchers in John Hopkins Medical School found that the first stage of the pathogenesis of colorectal cancer almost always involve the loss of heterozygosity (LOH) of the long arm of chromosome 5 gene. This was later known as "Adenomatous Polyposis Coli" gene because it was shown to be associated with identification of multiple polyps on the luminal wall of the colon (Weinberg 2007).
The APC gene codes for a protein that is relevant in the normal regulation of cell adhesion via E-cadherin, the signal transduction via the B-catenin/Wnt pathway, and the transcription of cyclin D1 and myc via the Tcf/Lef (Alfred et al 2006). This is illustrated in the following discussion.
a) The biology of the colonic crypt
The intestinal epithelial cells which are renewed every 3-4 days, consist of limitless replicative cells (known as stem cell) which are undifferentiated at the crypt, and the limited replicative cells (which are in various stages of differentiation) at villous undergoing apoptosis (don't change this part, I think you change slightly the meaning). This homeostasis is maintained by the interaction between APC protein, Î²-catenin and WNT signal. At the bottom of the crypt, the APC gene is lacking and the nearby stroma cells stimulate continuously WNT signal causing high concentration of Î²-catenin in the cytosol due to (link between this 2 sentences : WNT causes high concentration of b-catenin because of the lack of APC the lack of APC that allows Î²-catenin to escape degradation. As a result, the Î²-catenin is translocated to the nucleus to activate DNA binding proteins Lef-1 (Lymphoid enhancer factor) and Tcf-1 (T-cell factor) (detail but I think lef and Tcf-1 are the same protein, and italic). This forms a heterodimeric transcription factor, which further attract other nuclear protein and enhances the transcription gene such as (myc and cyclin D are not the only one transcript by this TF) of myc and cyclin D-1(Martin et al 2004). The net effect is to increase proliferation of the cell by interfering with the cell cycle at the S phase and the G2-M phase. It also exerts a positive effect in the prevention of apoptosis at the G2-M together with myc to promote proliferation and undifferentiation. The high concentration of Î²-catenin also associates with E-cadherin modulating the cell-cell adhesion signal (Olmeda et al 2004). )I don't know if you understand this part of George work but I don't so I think you shouldn't change it; it's too risky to change the meaning!)
As the cell migrates toward the villus, the level of Î²-catenin is kept low because of increased APC expression and the fading WNT signal as a result of the migration away from stroma cells. This allows the APC to degrade Î²-catenin and the subsequent differentiation and apoptosis to occur. APC promotes Î²-catenin degradation by putting together the protein useful to target to Î²-catenin to the proteosome pathway. The degradation complex constitued of APC/ Î²-catenin/Axin/GSK-3Î². The close prosimity of GSK-3Î² and Î²-catenin allows the former to phosphorylate the later. Phosphorylated Î²-catenin is target by the ubiquitination. Axin is also important for Î²-catenin ubiquitination. The binding of WNT to his receptor causes the phosphorylation of GSK-3Î². As a result GSK-3Î² can't bind the APC anymore. (Not sure about the two last sentences, will check)
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This homeostasis is important because it keeps the stem cell in a relative non-hostil environment and it prevents the accumulation of DNA damage by killing after few days the cells exposed to the mutagenic environment of the colon.
This is relatively a new discovery. It is observed that the APC associate with the component of micro-tubule spindles and to the centromere of the cell during anaphase and telophase regulating the normal segregation of the chromosome during mitosis (Weinberg 2007) (Olmeda et al 2004).
Disruption in Cancer
The mutation of APC tumour suppressor gene plays a primordial role in the tumorigenic process of intestinal epithelial cells, which are illustrated in the characteristic features of both Familial Adenomatous Polyposis (FAP) syndrome and sporadic case of colorectal cancer. The majority of both sporadic and inherited allelle have frameshift or nonsense-type mutation which results in truncated protein that has lost many important domains (Weinberg 2007). The following discussion explains the central role of APC in colon carcinogenesis by demonstrating how the malfunction of APC protein provides "door" to the multi step process of invasive colon carcinogenesis and the disruption it caused in the colon homeostasis.
The genetic analysis of family members with FAP syndrome leads to the discovery of APC tumour suppressor gene. Study by Bisgaard et al (1994) showed that almost 100% of these patients have the potential to develop colorectal cancer if no early active treatment is undertaken. This study implicates that inherited mutations of APC has almost 100% penetrance. Up to 90% of sporadic cases of colorectal cancer seem to have mutation in the APC gene. Hypermethylation of APC promoter or Î²-catenin mutation has also been reported in certain cases of somatic colorectal cancer that do not have APC mutation (Weinberg 2007). This suggests that the APC mutation is almost always the first step in the tumorigenesis of both sporadic and familial cancer and underpins the key factor it plays in the prevention of uncontrolled intestinal growth.
The continuous proliferation as a result from the mutation of APC proves that the effect must have a direct role to the disruption of Wnt signalling pathway. Indeed, it is well understood that APC acts as a down regulator of Î²-catenin; and the mutant APC protein would therefore leads to the accumulation of cytoplasmic Î²-catenin. This subsequently leads to an increased level of Î²-catenin/Tcf complex which alters the expression of genes such as c-myc and cyclin D. The ultimate aim is to prime the cell to initiate uncontrollable growth.
The role of APC in Wnt signalling pathway and its importance in the tumorigenesis have been extensively studied and are well recognised in many literature reviews. However, the APC may have contributed to the development of polyposis by other less well-characterised mechanisms. For example, it appears that the APC is involved in cell migration as well as by direct or indirect interaction with the actins and/or the microtubules network at the leading edge of the cell. Sansom et al (2003) demonstrated a decreased speed of migration of the mice intestinal epithelial cell from the crypt to the villi as a result from the effect of mutant APC product. In addition, it has been reported that the APC may also play an important role in apoptosis. Studies by Morin et al (1996) showed that the restoration of APC gene expression in human cell culture decreases the cell growth by inducing the apoptosis cascade.
Previous discussion has explained the function of APC in the homeostasis of the intestinal epithelial through the stabilisation of microtubule spindle and the regulation of cell cycle. Many cases of colorectal cancer in FAP patients exhibit chromosomal instability (CIN) at an early stage of pathogenesis leading to aneuploidy (Shih et al 2001). The APC mutation in mice cell impairs its function to bind to microtubule, which seems to enhance the instability state (Fodde et al 2001). Mutation of only one allele of APC gene may perturb the microtubule function during mitosis via a dominant negative effect (Green and Kaplan 2003). The mutation of one allele consequently favours genetic alteration. Baeg et al (1995) discovered that the insertion of APC gene in NIH 3T3 culture cells blocks the incorporation of BrdU during DNA synthesis, and subsequently stop the cell cycle progression prior to S phase. On the other hand, similar experiment showed that the mutant APC reverse the above process; which hence promotes cell proliferation. This experiment clearly supports the role of APC as tumour suppressor gene.
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The genetic analysis of both FAP and sporadic colorectal cancer patients highlights the crucial role of APC mutation in the uncontrolled growth rate of intestinal epithelial cells. Although the evidence of deregulation of Wnt signalling pathway leading to polyps growth are well established, the importance of disruptions in the non-traditional role of APC are not well known. The role of APC and the effect of its mutation in chromosomal segregation leading to genetic instability need to be further investigated.
Early detection of colon cancer in people with FAP can improve the chances of a cure and its overall survival. Surgery has always been the treatment option for this group of patients depending on age, family history and the number of polyps in the rectum. Many literatures have recently focused on the possible beneficial effects of non-steroidal anti-inflammatory drugs (NSAIDs), particularly highly selective COX-2 inhibitors, which have markedly shown to inhibit uncontrolled proliferation of APC knockout cells in experimental animal models. The traditional NSAIDs such as Aspirin, Naproxen, Ibuprofen and Diclofenac have been widely used clinically to reduce fever, inhibit the inflammatory process in acute or chronic illness and acts as analgesics, depending on its selectivity to the type of cyclooxygenases (COX-1 and COX-2). This seems to be an exciting area for future research in colorectal cancer prevention and treatment.
COX-2 is an inducible enzyme that participates in inflammatory response and intestinal tumorigenesis, in contrast with COX-1 which is constitutively expressed for various normal physiological functions. The COX-2 enzyme, which is expressed by the stromal and epithelial cells, produces a series of prostaglandins from arachidonic acid, and it is thought that the accumulation of one of its intermediate, prostaglandin E2 (PGE2), has been shown to contribute to cell transformation, such as loss of contact inhibition, increased anchorage-independent growth, down modulation of expression of the cell surface protein E-cadherin, reduced apoptosis and increased rate of proliferation by activating epidermal growth factor receptor (EGFR) (Weinberg 2007). Moreover, it makes reasonable sense that the positive outcome of polyp regression in FAP by COX-2 inhibitors lies from the fact that COX-2 are overexpressed in 40% of human adenomas and in 80% of adenocarcinomas relative to normal mucosa (Eisinger et al 2006). However, Azumaya et al (2002) revealed that the frequency and the intensity of neoplastic epithelial COX-2 expression are significantly reduced in small adenomas (less than 5mm in diameter) of both sporadic colorectal cancer and from FAP.
The incidence of colorectal cancer is reduced among chronic low dose NSAIDS users as demonstrated by several epidemiological studies. Many of these studies supports a "causal" relationship between signalling through COX-2 and colorectal carcinogenesis. Eisinger et al (2006) claimed that inhibition of COX-2 in FAP patients with Sulindac, a metabolite of NSAID sulindac sulfoxide caused significant polyp regression by decreasing the mean diameter and number of intestinal polyp. Although the wild type APC gene product down regulates level of cytoplasmic Î²-catenin, many literatures suggest that it also play a role in down regulating the expression level of COX-2 protein. However, there is still uncertainty as to whether the mechanism by which the APC controls the production of COX-2 depends on the direct interaction with Î²-catenin/Tcf/Lef transcription factor signalling pathway. Haertel-Wiesmann et al (2000) concluded that, despite cell's signalling through Wnt pathway increased the level of expression of COX-2 mRNA, the overexpression of B-catenin per se did not show to affect COX-2 expression. Hence, these studies demonstrated that COX-2 could be regulated by Wnt signalling via unknown Î²-catenin-independent mechanism.
Although the overexpression of COX-2 in FAP patients confers some advantage for NSAIDS to act upon, there are, however, several reports indicating that certain types of selective COX-2 inhibitors can suppress tumour cell growth independent of COX inhibition. Maier et al (2005) investigated using Caco-2 cells by Western Blot analysis showed that Celecoxib affected cellular distribution of Î²-catenin by degradation through both proteosomal and caspases pathway. Meanwhile, experiment by Eisinger et al (2006) showed that Sulindac treatment of APCMin mice decreased B-Catenin level by more than 50% in intestinal tumours compared to untreated control.
Despite many evidence show the positive outcome of NSAID in the regression of the size and number of polyps in FAP as well as in the prevention of recurrence adenomas in these patients, the use of this drug in clinical setting for various indication has been limited by their side effects profiles which are potentially catastrophic and cause significant morbidity. Inhibition of COX-1 enzyme disrupts its normal homeostatic function for mucosal protection in the intestine which can lead to potentially serious haemorrhage. Rofecoxib, an example of the selective Cox-2 inhibitor, has been withdrawn from the market in 2004 due to its significant increased risk of acute myocardial infarction and stroke despite its lowered gastrointestinal side effect profile (Juni et al 2004). Therefore, analysis of the risks and benefits of the use of NSAID as colorectal cancer chemoprevention need to be addressed further.
In conclusion, knowledge about the role of APC tumour suppressor gene in the homeostasis of colorectal epithelial cell proves to be fundamental to consolidate our understanding of how the effect of its mutation contributes to the early pathogenesis of colorectal cancer in FAP patients. Disruption of the normal physiological process by the mutation in APC tumour suppressor gene triggers the initial pathogenesis by allowing this cell to divide uncontrollably to form polyps and provide a platform to allow further mutations to occur within those clonal epithelial cells. Indeed, the successful regression of polyps in many experimental studies showed by many type of NSAIDS, particularly COX-2 inhibitors, provides future therapeutic opportunities despite its pitfall in clinical setting.
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