Colorectal cancer is the fourth leading cause of cancer-associated deaths after lung, liver, and stomach cancer.
According to World Cancer Research Fund International (WCRFI), there were 1.4 million new cases diagnosed in 2012. In 2010, there was an estimated 1.23 million cases of colorectal cancer that resulted in an estimated 715,000 deaths worldwide, an increase of 46% from 490,000 in 1990. The Cancer Research Fund International further predicts 2.4 million cases will be diagnosed annually worldwide by 2035.
Colorectal cancer is more common in developed countries, with overall incidence varying ten-fold. A diet high in highly processed foods (common in developed countries) is also thought to be associated with an increased risk of colorectal cancer. Given the relatively high and rapidly rising incidence of colorectal cancer, and its prevalence in the developed world, colorectal cancer is an attractive target for pharmaceutical development.1,2
The fact that radiation therapy is rarely used due to the sensitivity of the intestinal tract to radiation means that chemotherapy is often the treatment of choice after therapy. Additionally, there are a number of syndromes are associated with an increased risk of colorectal cancer, most notably familial adenomatous polyposis (FAP) syndrome, in which virtually all affected individuals will develop colorectal cancer and which accounts for almost 1% of all colorectal cancer.3
FAP results from mutations in the APC (adenomatous polyposis coli) gene that are inherited in an autosomally dominant pattern. The protein produced by the APC gene is involved in the organization of other proteins during cell division and is responsible for ensuring that the correct number of chromosomes in each daughter cell is correct. APC is a large protein and mutations that lead to FAP inactivate the protein, causing it to become non-functional, most often through frameshift mutations that lead to premature stop codons, and truncated protein.4
Lynch syndrome (HPNCC or hereditary nonpolyposis colorectal cancer) is also associated with an increased risk of colorectal cancer and a variety of other cancers (most commonly endometrial, but also ovarian, stomach, small intestine, hepatobiliary tract, urinary tract, brain and skin).5 HPNCC is due to mutations in the APC gene that impairs DNA mismatch repair, and is inherited in an autosomally dominant pattern.
Another syndrome that is associated with mutations in the APC gene is Gardner syndrome or familial colorectal polyposis. Gardner syndrome results in colorectal polyps that develop into colorectal cancer, which is general fatal by age 35-45. However Gardner syndrome can also cause polyps to develop in the stomach, duodenum, spleen, kidneys, liver, and mesentery. These polyps may also develop into tumors, and tumors may also be found in the skull (osteomas), epidermoid and sebaceous cysts, and fibromas.6 Thyroid cancer is also associated with Gardner syndrome, in addition to desmoid tumors that form at sites where surgery was performed to remove parts of the colon due to the number of polyps present.
In the non-familial cases of colorectal cancer, research has shown that mutations caused by external factors that reduce the DNA repair capability of the cells and result in genomic instability play a significant role in the development of these cases of colorectal cancer. Detailed analysis of the mutations present in these forms of colorectal cancer have shown that the mutated genes fall into the regulatory pathways associated with the WNT and TGF-b (transforming growth factor-b) signaling pathways, and ultimately to increased activity of MYC. Increased MYC activity leads to unregulated expression of many genes involved in cell proliferation, and is associated with an increase in cell division.7
Symptoms of colorectal cancer vary according to the position of the tumor within the colon, but generally involve changes in the function of the GI tract – constipation, changes in stool caliber, loss of appetite, as well as rectal bleeding and anemia. Colorectal cancer is usually diagnosed by colonoscopy, where a biopsy is taken and evaluated histologically. The extent of disease is then usually determined by CT scan (or other imaging modalities).
Treatment of colorectal cancer generally begins with surgery to remove the tumor and surrounding colon. Radiation therapy is rarely used for colorectal cancer due to the sensitivity of the surrounding normal intestines to radiation damage, consequently, surgery is generally followed by chemotherapy typically with regimes that include 5-fluorouracil and leucovorin. Newer therapeutic strategies incorporate compounds like irinotecan and oxaliplatin.