Colorectal cancer (CRC) is the third most common cancer with an estimated global incidence of 1.8 million. In India, the incidence and prevalence of the disease is 27,605 and 53,700 respectively with a mortality of 19,548 (https://www.uicc.org/new-global-cancer-data-globocan-2018). Although the incidence rates of CRCs are lower in India compared to the Western countries, the mortality rates are higher due to their late-stage presentation and lack of effective therapies. The risk factors for developing CRC, include age, gender, race, family history, inheritance, and inflammatory bowel disease. Lifestyle factors which attribute to this burden include physical inactivity, obesity, a diet low in fruits and fibres, smoking and alcohol consumption. More than 90% of the CRCs occur in people who are 50 years older in Western countries, but in India, there is a mounting incidence of younger people presenting with late-stage disease.
In general, about 75% of the CRCs are sporadic while 25% are familial. Among the familial, the vast majority is familial adenomatous polyposis (FAP), an autosomal dominant disease caused by the loss-of-function (LOF) mutation in the adenomatous polyposis coli (APC) gene on chromosome 5q21. A less prevalent familial CRC is Lynch syndrome (LS), an autosomal dominant disease also known as Hereditary non-polyposis CRC caused by mutations in one of the DNA mismatch repair (MMR) genes, such as MSH2, MSH6, MLH1, and PMS2. In India, the prevalence of familial colorectal cancer is 10-15% (Maharaj, Shukla et al. 2014). This could be a gross underestimation because of lack of systematic genetic screening, cost of the test and strong social stigma. In a recent study, we identified several FAP and Lynch syndrome families who would normally not undergo genetic testing but for our research initiative (Majumder, Shah et al. 2018, Majumder, Shah et al. 2018).
The common symptoms of CRC are altered bowel habits, rectal bleeding, constipation, diarrhea, unexplained weight loss, etc. with rectal bleeding being the most important symptom (Loh, Majid et al. 2013). Colorectal cancer is treated based on the presentation stage of the disease and the age of the patient. Surgery is still the mainstay of treatment for a locally advanced tumourr that is resectable. Adjuvant chemotherapy or radiation therapy is decided depending on the stage and clearance of resection margins. In the case of nodes positive stage III and IV cancers, where surgery is not feasible, chemotherapy is used as the first line of treatment. Improvements in surgery and combination chemotherapies in the last 20 years have doubled the 5-year survival of CRCs, yet the global mortality remains at ~50%. As discussed below, colorectal cancer serves as a poster-child of our understanding of the molecular basis of carcinogenesis, which is applicable to all cancers in general.
Accumulation of mutations
Bert Vogelstein and his team from John Hopkins University studied familial colorectal cancer in the 80s and 90s to reveal for the first time the mechanism of cancer progression occurring by stepwise accumulation of mutations in oncogenes and tumour suppressor genes. Oncogenes, such as RAS, BRAF, PI3K endow cancer cells to divide incessantly and survive indefinitely creating a dependence of cancer cells to their presence, referred to as genetic addiction. Mutations resulting in the activation of oncogenes (gain-of-function) are therefore a hallmark of cancer. By contrast, tumour suppressor genes, such as APC (adenomatous polyposis coli), TP53, TGF-β (transforming growth factor-β) prevent cells from growing aberrantly and typically lose their function (loss-of-function), which is the second hallmark. Vogelstein’s work demonstrated how mutations in oncogenes and tumour suppressor genes occur in a sequential pattern to transform normal colon epithelial cells into colorectal cancer (see figure). The mechanistic basis for the two-hit model of cancer initiation gained scientific acceptance through these studies, in which the transformation of healthy epithelial cells requires at least two hits, one that takes out the function of a tumour suppressor gene, and the other that activates oncogenes. Over 80% of sporadic CRC is caused by the inactivation of APC resulting in colon cancer cell survival. The second hit is the activation of RAS oncogene that results in cellular transformation and formation of adenoma. Additional mutations in TP53 pushes the cells to become an adenocarcinoma.
The genetic foundation emerging out of these studies has a profound impact on cancer treatment and the development of novel therapies. Successful targeted therapies inhibiting the function of overexpressed genes, such as HER2 in breast cancer, or EGFR in colorectal cancer or gain of function mutations in EGFR (epidermal growth factor receptor) in non-small-cell lung cancer, or BRAF in melanoma are grounded on the concept of driver mutations and genetic addiction. Metastatic CRCs are treated with monoclonal antibodies that starve the tumour of nutrients and oxygen by preventing the growth of blood vessels, such as Avastin or Zaltrap that bind VEGF (vascular endothelial growth factor), or Cyramza, which binds VEGF receptor-2 (VEGFR2). A subset of colorectal cancer patients overexpressing EGFR protein is treated with monoclonal antibodies Erbitux or Vectibix that block EGFR signalling. In 2018, a novel inhibitor targeting neurotrophin receptor (NTRK) fusions, Vitrakvi was approved for all solid tumours that express these fusions including CRC. Overall, NTRK fusions are rare in cancers; however, the presence of this fusion in colon cancer can be treated. The use of EGFR monoclonal antibodies Erbitux or Vectibix to treat colon cancer in India is limited, however, published reports indicate superior overall survival of Vectibix compared to Erbitux in combination with Folfox in metastatic colorectal cancer with wild-type KRAS (Pathak, S et al. 2018). The anti-angiogenic therapies or targeted therapies have extended limited benefit and new therapies are needed to treat this deadly disease.
Cancer immunotherapy has produced long-term benefit and has given a new lease of life to 15-20% of patients with terminal disease, who had exhausted all therapies. The therapy boosts patients’ immune system by targeting a group of receptors called checkpoint control proteins. The three checkpoint control proteins currently targeted by monoclonal antibodies are cytotoxic T-lymphocyte associated protein-4, CTLA-4 (targeted by Yervoy), programmed cell-death protein-1, PD-1 (targeted by Opdivo and Keytruda) and programmed cell-death protein-ligand-1, PD-L1 (targeted by Tecentriq and Imfinzi). In colorectal cancer, Opdivo and Keytruda have been approved to treat 10-15% of tumours with microsatellite instability (MSI) phenotype. These tumours carry a high number of mutations and show an overall response of over 70% to checkpoint inhibition. The correlation between high tumour mutation burden (TMB) first reported in colon cancer has been reproduced in other cancer types qualifying this feature as a critical biomarker of patient selection (Le, Durham et al. 2017). In India, the incidence of MSI ranges from 5-10% (Maharaj, Shukla et al. 2014). However, the prohibitively high cost of checkpoint inhibitor antibodies will have limited utility in India.
Use of high TMB as a biomarker of patient selection has left over 75% of colorectal cancers and 70% of all cancers untreatable with checkpoint blockade therapy. To expand the utility of these powerful therapies into other cancers, the field is actively pursuing discovery of novel biomarkers for patient selection, combining checkpoint inhibitors with other therapies to make tumours more immunogenic and sensitizing them to immune-mediated attack. There is also a concerted effort from the industry to develop vaccines against tumour-specific neoantigens to induce an immunological response. Limited results from ongoing clinical trials in melanoma and ovarian cancer suggest that vaccines can skew the immunological balance towards a more inflammatory phenotype permissive to T cell-mediated killing of tumour cells (Ott, Hu et al. 2017, Tanyi, Bobisse et al. 2018).
We have shown in two recent studies that in familial colorectal cancer – familial adenomatous coli (FAP) and Lynch syndrome, cancer-specific mutations are immunogenic and an interventional approach with vaccines may be feasible (Majumder, Shah et al. 2018, Majumder, Shah et al. 2018). The progression of colon cancer through an intermediate polyp-stage is an ideal intervention point for vaccine therapy since the immune system is not tolerized to the neoantigens that appear during polyp development. Further research is required to identify whether neoantigens in polyps are private, or shared between patients, whether multiple polyps in the same individual have a similar mutational profile, and what fraction of the mutations are immunogenic. In addition, clinical use of vaccines will rely on good delivery mechanisms and use of optimal adjuvants if the delivery route is through peptides.
In conclusion, our understanding of the molecular mechanisms of colon cancer carcinogenesis has exploded in recent years with the use of next generations sequencing technology and advanced data analysis tools. A range of novel targetable driver mutations, including gene fusions have been reported in these studies (Seshagiri, Stawiski et al. 2012, Dienstmann, Vermeulen et al. 2017). In India, colorectal cancer is usually detected at an advanced stage with limited treatment options. Identifying high-risk patients carrying known risk alleles through genetic testing should be part of colorectal cancer screening awareness programmes. MSIhi tumours stand to benefit the most by checkpoint blockade therapy. Finally, vaccines should be considered in neoadjuvant setting to delay recurrence or in familial cancer after surgical removal of polyps.