Adenomatous Polyposis Coli and Colorectal Cancer
By: Ines Belza Garcia
Introduction
Adenomatous Polyposis Coli, abbreviated APC , is a tumor suppressor gene that is commonly mutated in cancer. Particularly, APC mutations are commonly found in colorectal cancers. In fact, 90% of colorectal cancers have an APC mutation, making it a recurring theme in cancer research. The first time I encountered the APC gene was at an internship in the oncology department of my city's hospital. I met a patient who, predictably, had an APC mutated colorectal cancer. Now, I have the opportunity to investigate this condition through a more theoretical viewpoint.
APC is a gene located on chromosome 5, cytoband q22.2. A total of three isoforms have been found in humans as a result of alternating promoters and splicing. Isoform 1A is most highly expressed in the brain , while isoform 1B is expressed highly in all tissues except for the brain. The APC protein is usually located in the nucleoplasm , the Golgi apparatus, and the plasma membrane of a cell. The main function that APC has in the cell is as a negative regulator in the Wnt signaling pathway, which will be further discussed below.
The stage (extent of spread) of a colorectal cancer depends on how deeply it grows into the wall and if it has spread outside the colon or rectum. (source)
Wnt Signaling Pathway
As a modulator of cell proliferation, the Wnt signaling pathway plays a central role in cancer. Wnt molecules are growth stimulating factors that have a palmitoleic acid attached to themselves. They are thought to be transported throughout the body in exosomes. The Wnt signaling pathway influences cellular proliferation through β-catenin. When β-catenin is not present, the transcription factor known as 'TCF' , which has the ability to increase transcription of genes that encourage growth and proliferation, is inhibited by a protein called 'Groucho'. On the other hand, when β-catenin is present, it dislodges Groucho from TCF, allowing increased transcription of genes that increase proliferation. These genes are known as Wnt Target Genes because the Wnt signaling pathway can influence the levels of β-catenin in the cell, thus regulating the transcription of such genes.
The Wnt signaling pathway relies on a group of proteins known to form the 'destruction complex', which involves Dvl, Axin, Ckl, GSK3, β TrCP, β-catenin, and APC. In its inactivated state, β-catenin is phosphorylated, signaling β TrCP to ubiquinate the β-catenin in the complex. Thus, β-catenin is tagged for proteasomal degradation. It is in this way that the cell controls and lowers β-catenin levels in the cell.
To initiate the signaling pathway, Wnt arrives at a receptor called 'frizzled'. The attachment of Wnt activates the frizzled receptor, which in turn causes the phosphorylation of a membrane-bound protein called LRP. This induces the translocation of the destruction complex toward LRP and frizzled. When Dvl binds to LRP, it becomes activated, inhibiting the destruction complex by preventing the phosphorylation of β-catenin and the subsequent degradation of β-catenin. In response, the β-catenin levels in the cell increase along with the transcription of Wnt Target Genes that increase growth and proliferation.
As APC forms an integral part of the destruction complex, its absence caused by an APC mutation prevents the cell from regulating β-catenin levels, allowing the uncontrolled proliferation characteristic of tumor cells.
Schematic representation of Wnt secretion. In the endoplasmic reticulum, Wnt gets palmitoylated by porcupine. Acylated Wnt is then transported to the Golgi body through cargo protein. (source)
Furthermore, the Wnt signaling pathway is also related to other processes characteristic of cancer cells. The activated Wnt signaling pathway upregulates expression of EMT-promoting genes, including SNAI1, ZEB1, ZEB2, TCF3, and KLF8. Thus, it is thought to encourage cell metastasis. It also increases expression of cyclooxygenase 2, which is known to play a role in angiogenesis. Similarly, Wnt also activates many genes that support oxidative phosphorylation and aerobic glycolysis. This results in a cell with increased glycolysis and excessive lactate production, which is also known as the Warburg effect. This provides the cell with a lot more energy while creating byproducts that support metastasis, proliferation, and inflammation.
Familial Adenomatous Polyposis
FAP is a rare condition concerning a mutated APC gene that is inherited in an autosomal dominant manner. Although any mutation of the APC gene can classify as FAP, most mutations result in a truncated APC protein. FAP is a cancer predisposition syndrome which is characterized by hundreds upon thousands of precancerous colorectal polyps that usually appear at the age of 16. Individuals with this condition require frequent colectomies in order to monitor the polyps and prevent malignant tumor
formation. FAP is also associated with a series of extra-colonic manifestations. Patients may exhibit Gardner syndrome – colonic polyposis in conjunction with an osteoma or a soft tissue tumor, or Turcot syndrome – a colonic polyposis and a medulloblastoma. While these are the most frequent conditions, FAP comes in conjunction with many other tumor types. FAP is also distinguished from attenuated FAP, which is a variant condition of FAP. Attenuated FAP consists of less frequency of polyposis (averaging 30 polyps) and a much later age of onset. It is also known to have a lower risk of colorectal cancer than FAP.
Almost all colon cancers begin with polyps which then develop into a cancer. However, not all polyps will become cancerous , and there are a few criteria which can help identify which polyps are more propense to carcinogenesis. There are three main types of polyps: adenomatous polyps (adenomas) which sometimes turn into cancer, hyperplastic or inflammatory polyps which are very common but often not pre-cancerous, and serrated adenomas which have a high risk of colorectal cancer. Higher risk of cancer is also found in polyps greater than 1cm, in clusters of more than three polyps, and if dysplasia is seen after polyp removal.
APC Mutations
APC mutations can be divided into two subcategories: genetic and epigenetic mutations. Genetic mutations are those with an amino acid substitution, deletion, or insertion. More than 90% of these mutations result in a stable truncated protein that is nonfunctional in the destruction complex. In APC, most epigenetic mutations are found in the hypermethylation of the 1A promoter. Hypermethylation of the promoter impedes transcription of APC. As was explained by the previously discussed Wnt signaling pathway, the absence of a properly functioning APC allows TCF to act as a transcription factor that allows for increased and uncontrolled proliferation of the cell, even inducing metastasis and angiogenesis.
The fractions of colon cancer cases that arise in various family risk settings. (source)
Diagnosis
Often, diagnosis of FAP is done through colonoscopies to identify polyps. A patient may take a colonoscopy if there is known family history of FAP, or if there is polyp-induced discomfort. However, 30% of FAP sufferers have no family history of FAP. This means that they are unlikely to have regular colonoscopy checkups and often results on colorectal cancer. More recently, some laboratories have been using fecal samples to identify APC mutations originating from the colorectal endothelium , though this technique still needs to be warranted by a medical professional.
Current Treatment
As of today, there is no targeted treatment for APC aberrant tumors. Thus, they are treated in the same way that other colorectal cancers are, using a combination of chemotherapy and surgery. As in many other conditions, chemotherapy can be single agent (using mainly fluoropyrimidine), or it can be a multiple agent regiment (using a combination of oxaliplatin (OX), irinotecan (IRI), and capecitabine (CAP)). There are a few commonly used combinations of these chemotherapeutic agents – FOLFOX (5FU+OX), FOXFIRI (5FU+IRI), CAPOX (CAP+OX). Cetuximab (an anti-EGFR drug) and Bevacizumab (an anti-angiogenesis drug) are also frequently used in colorectal cancers.
APC mutations and Diet
Colorectal cancer has been found to be one of the cancers with most association to lifestyle and diet. In 2008, Diergaarde et al. found colorectal cancer incidence to be correlated with red meat consumption. Then, a 2012 study conducted by Gaye et al. found that truncated APC mutations and hypermethylation of the APC 1A promoter was correlated to red meat and processed meat consumption. It was particularly strongly correlated to GC** → **AT mutations. Although many recent studies support the
idea that diet plays a strong role in the development of colorectal cancer, it should still be considered that 70% of colorectal cancers are familial, and diet does not play an equal role in each individual patient. Furthermore, diet studies are almost never causative due to the difficulty in monitoring controlled variables. Thus, all conclusions found are merely correlational.
Schematic representation of Wnt pathway modulators. The Wnt signaling pathway is targeted at multiple steps for cancer therapy. (source)
Future of Targeted Therapy
Though there may not be any target therapies for APC mutant cancers on the market, there are many currently being researched. APC is a moderately druggable protein, with a draggability score of 0.504. This means that many of the therapies being researched do not target APC, but rather elements of the Wnt pathway. Most studies also focus on downstream targeting because upstream targeting of Wnt would have a minimal effect on APC aberrant tumors.
One path being taken by scientists is to decrease β-catenin transcription through antisense mediated suppression. However, this is a non-tumor specific approach that can affect any normal functional cell in the body. Also, β-catenin has other functions outside of Wnt signaling. For example, β catenin can also be found in the cell membrane for cell-cell signaling functions. In a similar way, some scientists have also thought to inhibit TCF transcription by disrupting the bond between β-catenin and TCF.
Another indirect approach of targeted therapy is through the inhibition of COX2 (cyclooxygenase 2). COX2 is a regulator of cell proliferation, differentiation, and tumorigenesis. It is also a downstream target of Wnt. Some drugs have been found to inhibit COX2. These include MF-tricyclic, celecoxib, and cecum.However, none have passed the testing stage and like many other cancer therapies are highly toxic.
Targeted therapy has also been found in the burgeoning field of gene therapy. Using aminoglycosides and macrolides, which induces readthrough of premature stop codons, nonsense APC mutations of truncated proteins can be read into normal functional proteins. However, this therapy is highly toxic as it may also encourage the readthrough of normal stop codons.
Unlike many other cancers, colorectal cancer also has a unique environment created by the gut microbiota. It has been found that the microbiome plays a role in FOLFOX resistance by mediating autophagy. The EGFR signaling pathway is also regulated by primary-secondary bile acid transformation as well as microbial metabolism, making the gut microbiome heavily involved in cancerous signaling pathways. However, the microbiome remains an important part of healthy patients, and antibiotic use has been correlated with poor chance of survival in colorectal cancer patients.
_ By: Ines Belza Garcia _ (External student, guest post on the request of Mentors)
References
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The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of Elio Academy.
