What is Colon Cancer?
Colon cancer (also known as colorectal cancer (CRC), bowel cancer, or rectal cancer) is a type of cancer that affects the stomach and bowels and is the third most diagnosed cancer in both men and women alike. Colon cancer is primarily caused by lifestyle, being more common in developed countries, and age, since 90% of all diagnoses are in individuals over the age of 50. Nonetheless, it is estimated that heredity is responsible for a third of the susceptibility to colon cancer. This emphasizes the importance of considering family history and genetic risk, with the hope that this knowledge can lead to proactive choices that may prevent the development of colon cancer altogether. The diagnosis of colon cancer (CRC) is complicated, since the development of this cancer depends on many heritable and environmental factors. Thus, colon cancer diagnostics rely on a variety of tools including a colonoscopy and genetic testing.
Colon Cancer Causes
Progressive Development of Colon cancer
Colon cancer is associated with symptoms that include vomiting, bloating, changes in bowel movements, bleeding, cramps and unexplained weight loss. It is most often diagnosed using a colonoscopy, where an endoscopic examination of the bowels is performed and may also include a biopsy. Colon cancers are predominantly derived from epithelial cells and present with a broad spectrum, ranging from benign growths to highly invasive cancer. This progression from a benign growth to a malignant cancer does not occur simultaneously. Instead, it involves multiple mutations, whereby the cells acquire new characteristics such as increased growth and ability to migrate, leading to a more malignant state. It is estimated that at least 5-7 mutational events occur in a normal epithelial cell in order for a malignant colon tumor to develop and that each tumor has a unique and distinct mutational gene signature, arguing against a one-size-fits-all approach to colon cancer therapies.
Hereditary Forms of Colon Cancer
The risk for developing colon cancer is higher in those that have a first-degree relative that has been affected. The magnitude of this risk is dependent on the following factors:
- The age of diagnosis of the affected relative,
- The degree of relatedness,
- The number of affected relatives.
The highest relative risk is for those with an affected relative that was diagnosed before the age of 45, since cancers in individuals with a hereditary predisposition typically occur earlier than in sporadic cases.
The two major forms of hereditary colon cancer are familial adenomatous polyposis (FAP) and Lynch syndrome, also referred to as hereditary nonpolyposis colon cancer (HNPCC). FAP is caused by mutations in the APC gene, a gene that is classified as a tumor suppressor gene because it prevents the uncontrolled growth of cells. Individuals with APC mutations develop growths called polyps in their colon, which in their current state are not serious but if untreated can develop into cancer. Lynch syndrome is caused by mutations in genes responsible for DNA quality control and repair (mismatch repair genes), which are required to fix damaged DNA. These major genes that have been implicated in hereditary colon cancer are both necessary and sufficient for disease causation. They are inherited in an autosomal dominant fashion, which means that each descendant of that individual has a 50% chance of inheriting the predisposition to colon cancer. Moreover, inheriting a mutation in these genes also increases an individual’s risk for developing other cancers including endometrial cancer.
However, these hereditary mutations are believed to only account for a small fraction of all colon cancer cases and many families exhibit clustering of colon cancer or of adenomas without the presence of these specific mutations (Figure 1). As many as 69 genes have so far been associated with colon cancer and many of these genes have been discovered through genome association studies and DNA sequencing. While individually, variations in these genes are not sufficient to cause disease, their interaction with other genes and the environment may ultimately lead to the development of colon cancer.
Sporadic Cases of Colon Cancer
Most of the cases of colon cancer are considered sporadic because they occur in isolated patients and involve newly arising mutations (Figure 1). The study of inherited cancers can also provide important clues about the genes mutated in sporadic cases. For example, one of the earliest events in sporadic cases is the mutation of the APC gene, with it being mutated in 85% of sporadic colon cancer tumors. Genetic information can also be used for molecular profiling of individual colon cancer patients, which can have a significant impact on prognosis since genetic heterogeneity can impact response to therapeutics.
Figure 1. The fraction of colon cancer cases that arise in various family risk settings .
Lifestyle and diet
Lifestyle and diet also affect the risk of colon cancer, with obesity, smoking and alcohol use increasing the risk. Conversely, maintaining a healthy diet rich in antioxidants with regular physical activity is believed to lower the risk of colon cancer. Regular use of low doses of aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDS) lowers risk of colon cancer, as well as post-menopausal hormone use. Regular screening and removal of adenomatous polyps greatly improves survival rates for people with a genetic risk of colon cancer. Screening for colon cancer is another important factor to consider since colon cancer is very treatable if it has not spread to other parts of the body.
Colon Cancer Treatment
Colonoscopy is the gold standard for colon cancer screening. Stool blood tests are not as effective as a colonoscopy because they are not as specific and may lead to false-positive results, and they fail to detect polyps in early stages of disease. In addition to colonoscopy, a number of non-invasive molecular diagnostic tests are available that can help in monitoring the risk of colon cancer.
Colonsentry – is a blood test that analyzes a number of genes related to colon cancer. The Colonsentry test provides a numerical risk score that provides risk assessment at a given point in time. Patients with low test scores can skip the colonoscopy, while patients with higher risk estimates may have to undergo colonoscopy to remove polyps.
Cologic test is the detection of methylated DNA biomarkers that are also highly specific to colon cancer and allow disease detection at early stages, making the treatment much easier and effective.
What you can do to lower your risk of developing colon cancer
Broad genetic screening for individuals with a family history of cancer can provide an opportunity for early disease detection and diagnostics. A colonoscopy every 5 years after the age of 45 is recommended for the general population, however for patients with higher risk of colon cancer, more frequent assessment is recommended. Please remember – early screening and monitoring is the key for successful mitigation of cancer risks.
Randall W. Burt. Gastroenterology 2000; 119 (3): 837-853. Colon Cancer Screening.