Some of the cells of our body can grow uncontrollably and can spread to the other parts of the body, this condition is called cancer. Human body consists of trillions of cells with their own structure and function. A recent estimate by the scientists tells the average number of cells in the human body is around 37 trillion; written as 37,000,000,000,000.
Among all these, any cell can develop into cancerous cells, infiltrate the normal human tissue and has the capacity to spread all over the body.
It is the most dreadful disease with a very high incidence in the 21st century, every 1 person among 4 has the chance of being diagnosed with cancer. There are 200 different types of cancer. 1/3rd of the people diagnosed with cancer are estimated to die because of cancer. Prostate cancer is the leading site of cancer in male and breast cancer accounts for most cancers in females. The most deaths are caused by respiratory (lungs and bronchus) cancer, followed by prostate and breast cancer in male and female respectively.
Together with the morbidity, mortality and the quality of life; Cancer leads to economic impact to the patient, healthcare system and the country causing an economic burden. The medical care costs for cancer in the U.S. were $183 billion in 2015 and are projected to increase to $246 billion in 2030.
Causes of Cancer
- Genetic cause
- Behavior / Lifestyle
- Environmental Cause
- Virus and Bacterial Infections
Gene mutations leading to cancer can either be inherited or acquired. The mutated gene from sperm or ova leasing to zygote formation leading to Germline/ hereditary mutation accounts for only a very less fraction of total cancer cases. Acquired mutation in the gene is developed in some other time later in life and cannot be passed to offspring as they are not present in ova or sperm. A Healthy individual can acquire mutations through exposure to radiation, cigarette smoke, hormones, diet etc. However, mutations can occur randomly. Every time a cell divides into two, it has to go through replication of DNA during which sometimes mistakes can happen while making a new copy and not rectifying that ultimately leads to mutations which might cause cancer.
Epigenetic changes like DNA methylation and Histone modifications can also lead to cancer. DNA methylation can cause gene silencing which turns off a gene making it unable to go through transcription. Similarly, the DNA is wrapped around the histone proteins, the modifications in the histone protein can also lead to cancer. Addition of acetyl group (acetylation) to the histone proteins can turn on the gene whereas removal of acetyl group (deacetylation) can turn off the gene. Drugs altering the histone modifications can be used to treat the cancer as they can turn on or off the genes.
Behavior / Lifestyle
Tobacco use is declining but it still is a leading preventable cause of cancer. In the US, tobacco use is the cause of 30% of cancer deaths. Tobacco use is responsible for 17 types of cancer including lungs. Cigarette smoking is the cause for almost 80% of respiratory tract cancers, however, it also causes digestive tract cancer, prostate cancer, myeloid leukemia and ovarian cancer. A recent rise in e-cigarette or vaping devices is also a responsible cause for cancer as it has been convicted to 1000s of deaths in the US alone. Moreover, second hand smoking contains 69 different chemicals that are related to causing cancer and is one of the tobacco related behaviors causing cancer.
20% of cancer is due to poor diet, lack of physical activity and obesity. Obesity arises due to intake of calories more than metabolic needs over a long period of time. Obesity increases the risk of breast, gastrointestinal, prostrate and hematological cancers. Obesity is not mutagenic and it doesn’t initiate cancer, it can trigger cell signaling and metabolism helping in spread and influencing the tumorigenesis.
Red meat and processed meat consumption increase the risk of cancer, especially colorectal cancer. The red meats and processed meats produce DNA damaging carcinogens N-nitroso compounds (NOCs) while cooking.
Heavy Alcohol consumption is responsible for multiple types of cancer. 6% of cancer deaths worldwide are due to alcohol consumption. Meta Analyses have shown the relation between various somatic mutations reflecting DNA damaging activity to alcohol use.
Environmental Cause (Environmental, Occupational and Industrial pollutants)
Exposure to UV radiation from sunlight or artificial sources has been shown to be a cause for melanoma in youths. The transversion of nucleotides leading to mutation due to UV exposure has a potential effect on expression of genes.
Exposure to novel chemicals has been a prominent cause in different cancers. Asbestos, arsenic, and radon has been shown to be the causes of various cancers. These different carcinogenic chemicals can alter DNA repair, induce epigenetic changes, induce oxidative stress, induce chronic inflammation, alter cell proliferation, death, ultimately leading to cancer.
Virus and Bacterial Infections
Infections with certain bacteria, viruses and parasites have been associated as the risk factors for various types of cancers.
Some viruses can alter the genetic machinery of the cell to disrupt the signaling pathway that affects cell growth and proliferation. Some viruses insert their DNA/RNA into the host cell pushing the cell towards cancer. Human papillomavirus (HPV) is the main cause of cervical cancer. Epstein Barr Virus infects B-cells leading to lymphoma.
Infection of Helicobacter pylori can complicate stomach cancer. It can inflame and infect the inner layer of the stomach which later can develop into cancer. Chlamydia trachomatis is bacteria that can infect the female reproductive system as well as other parts of the body in both men and women which spread through sex.
Some parasitic worms are also the risk factors for certain cancers. Opisthorchis viverrini and Clonorchis sinensis (flatworms) can cause bile duct cancer. Schistosoma haematobium has been linked to bladder cancer.