Genesis of Cancer

Cancer is not a single disease rather it is a general term used to describe various malignant tumours that affect all forms of higher organisms including plants and animals. About 270 types of cancer are known to affect human beings. Every living cell in the human body has the potential to become cancerous (malignant), after which it proliferates indiscriminately usually forming a mass, known as neoplasm or tumour. Cancer can be defined as an abnormal, excessive, uncoordinated, autonomous and purposeless clonal proliferation of cells in any tissue or organ of the body. Cancerous cells have a tendency to spread and grow (metastasise) in other parts of the body. Cancer preys on the host and continues to grow indefinitely, competing with normal tissues for nutrition. Ultimately, a stage comes when the cancerous cells engulf almost all the available nutrients, thus starving the host to death. Our knowledge of cancer goes back to the dawn of civilisation. The evidence of cancer has been found in the prehistoric animals and the Egyptian mummies. The earliest records on cancer have been traced to the ancient Egyptian, Greek and Indian writings. Sushruta, the Indian surgeon, described various types of tumours in his text 'Sushruta Samhita' written in circa 600 BC. The word "cancer" is derived from the Latin word cancrum (Greek: karkinos), which means crab. The Roman physician, Galen, hypothesised in AD 200 that cancer is caused by an excess of black bile in the blood. This hypothesis was accepted by almost all well-known physicians until the 18th century. Today, it is believed that cancer is caused by mutations in the genes. Ancient Egyptians used to cauterise tumours with red-hot iron rods. It is believed that the Greek surgeon, Leonides, was the first to operate upon cancer in 180 BC. Cancer has its origin since the evolution of multicellular organisms. To understand the genesis of human cancer one should know that an adult human body is made up of approximately 70 trillion cells, which are structural and functional units of the body. The cells are organised in specialised tissues to form various organs and systems of the body. The origin of all these cells can be traced to a single cell, called the zygote, which is formed by the fusion of ovum and sperm during the event of fertilisation. The zygote initially passes through a phase of rapid cell division by mitosis, after which some cells undergo changes in their size, shape and contents depending on the specialised work they would undertake later. This phase of specialisation, known as the phase of cell differentiation, enables the cells to form various tissues of the body. The process of cell division and differentiation is essential for the growth and development of the body. In an adult human being, more than a thousand billion cells are created every day. At the same time, an equal number of cells die through a controlled suicide process, known as apoptosis (programmed cell death). For example, an adult human body contains about 5 litres of blood and each millilitre of the blood contains about 5 million red blood cells. Keeping in mind that the average lifespan of a red blood cell is 120 days, it is estimated that approximately 2.5 million cells must divide every second in the bone marrow to replace the dying red blood cells. Similarly, the human skin gets replaced by the new cells within 2 to 3 months. Likewise, the injured tissues in the body are repaired by replacing the damaged cells with the new cells produced as a result of rapid cell division. The process of cell division in the human body is a well-regulated phenomenon controlled by genes (made of deoxyribonucleic acid commonly known as DNA). If the DNA is damaged by some external or internal factors, there may occur mutations in the genes. The mutated genes may lose control over the normal cell division resulting in an unregulated proliferation of the cell that leads to the genesis of cancer. The process of conversion of a normal cell to a malignant cell is known as malignant transformation. The risk of cancer increases manifold when a person is exposed to certain physical, chemical and biological agents, collectively known as carcinogens, which cause mutations in the genes by damaging the DNA. When a carcinogen enters a cell, the natural cellular response is to convert it into a harmless substance and eliminate it from the body. This job is carried out by the detoxifying enzymes, however, the rate of detoxification varies in different individuals. If the process of detoxification occurs slowly due to lack of detoxifying enzymes, the carcinogen will remain in the cell for a longer period, thus enhancing the risk of cancer. Carcinogenesis is a multi-step process. The sub-optimal dose of a carcinogen may only alter the affected cell. This altered cell is known as an initiated cell, which possesses the highest risk of transforming into a malignant cell on further exposure to the same carcinogen or some other substance, which may or may not be a carcinogen. The substance that transforms the initiated cell to the cancerous cell is known as a tumour promoter. Although every living cell in the human body has the potential to become malignant, but the process of malignant transformation is an extremely rare event because the DNA chromosomal strands are replicated in each cell with incredible precision and the proofreading process repairs the damaged DNA strands before the mitotic process is allowed to proceed. In spite of all these precautions taken by Nature, one newly formed cell among every few millions may still become malignant, but the immune system of the body acts as a scavenger by destroying these abnormal cells. Ultimately, only a fraction of the mutated cells ever grow to form a cancer. The malignant transformation of a cell is probably not a critical event in the development of cancer rather it is the body's inability to destroy the newly formed cancerous cells when they are few in number that leads to the genesis of cancer. It has been observed that most of the newly formed cancerous cells never grow beyond microscopic stage because immune cells of the body normally recognise and destroy the cancerous cells by producing certain proteins, called cytokines, which include interleukins (IL-1 to IL-15), interferons (alpha, beta and gamma), tumour necrosis factors (TNF) and colony-stimulating factors (CSF). It has been observed that the risk of developing cancer is multiplied manifold in those persons, whose immune system is suppressed due to any reason such as undernutrition, old age, HIV infection, abuse of drugs and chronic stress.