The word ‘cancer’ comes from Latin, meaning a crab. A tumour was called a cancer because of swollen veins around the area resembling a crab’s limbs. The study of cancer is called Oncology. Oncology is a word derived from the Greek, onchos, a lump, or tumour. The abnormal tissue growth is called neoplasm. If a neoplasm can cause harm by spreading, it is said to be malignant.
Cancer was known to ancient civilizations. However the disease as it would be defined today was established as an entity by German pathologistsof 19th century. They described cellular nature of cancer and classified cancer. At the beginning of the 20th century, most major forms of cancer had been described. Further, attention was focused on finding the cause and introducing treatment. In 1775 Pott recognised cancer in chimney sweeps. He associated soot with cancer. From this time onwards environmental and occupational hazards were recognised as follows :
shale oil skin cancer in workers
radio active ores lung cancer in miners
beta-naphthylamine bladder cancer in rubber industry workers
cigarettes lung cancer
Later it was discovered that certain viruses can also cause cancer.One of the earliest virus, causing cancer, described was Rous sarcoma virus. Recently, human T-cell leukaemia has been found to be due to the virus HTLV-1.
Some forms of cancer can also be inherited. A rare eye tumour, retinoblastoma is inherited. It is a dominant character showing Mendelian inheritance.
Cancer In Biology :
The knowledge of cancer biology is growing rapidly. Researches are being conducted to fully understand the development of cancer at the cellular or molecular level. The available information is not sufficient for satisfactory
treatment of cancer. During normal development and growth the cells in our body divide mitotically and get differentiated to specialized cells of the tissues. The processes of cell mitosis, growth and differentiation are controlled by cellular
genes. Cancer is caused due to mutation or abnormal activation of such genes. such a mutation can happen in a single cell. Thus it may be monoclonal in origin. With further growth of cancer, additional mutations may occur in the daughter cells giving rise to subclones. The mutated cells may remain as heterogeneous cancer cells. Among these subclones some may have greater capacity and metastasize to distant tissues. They may also remain more resistant to damage from various anticancer treatments. The cancer cells have characteristic properties. They can be differentiated from normal cells under microscopic observation. These cells have
large nuclei. In each cancer cell, the ratio of nucleus to cytoplasm is high. They have prominent nucleoli. The cells can grow indefinitely in culture medium. As component cell of a tissue they remain less differentiated. Even after getting organised into tissues unlike other cells they do not lose their replicative capacity. Cancer cells have the ability to invade surrounding
tissues. The sequence of events that convert a normal cell into a cancer cell is called carcinogenesis. The process of carcinogenesis includes, intiation, growth, promotion, conversion, propagation and progression. Progression
includes the processes of invasion and metastasis. Mature cancers have relatively uncontrolled growth, behaviour. As other normal cells they do not show any of the normal intracellular and extracellular growth control mechanisms. Initially the cancer cells have an
exponential growth. Gradually their growth surpasses blood vascular supply. This results in slowing down of growth.
Molecular Biology of Cancer :
Techniques in molecular biology have helped in understanding the most intimate structure of the cancer cell. It has been found that at molecular level two mechanisms operate.
1. The parts of genome involved in cell growth become activated. These are called the ‘proto-oncogenes’. These strands of DNA induce malignant growthtranformation in the cells. The conversion of proto-oncogenes into oncogenes can happen due to ‘point mutations’ on DNA. Further such cancer cells dis play chromosomal abnormalites such as duplication, deletions and translocations. Thus such alterations in gene arrangement can lead to generation of oncogenes. Oncogenes of human tumours gr hox11 Acute T-cell leukemia Translocation erbB-2 Breast and ovarian carcinomas Amplification L-myc Lung carcinoma Amplification ret Thyroid carcinoma DNA rearrangement. A distinct mechanism by which oncogenes are activated in human tumors is amplification. It results in elevated gene expression. Gene amplification is very common in cancer cells. It occurs a thousand times more frequently than in normal cells. Molecular biologists are now working on the products of oncogenes.
2. The growth of normal cells is controlled by suppressor genes. In cancer, parts of the genome functioning as the suppressor gene are either lost or inactivated. Hence, negative regulators of cell proliferation are removed. It contributes to the abnormal proliferation of cells.
Gene Type of cancer
APC Colon / rectum carcinoma
BRCA 1 Breast and ovarian carcinoma
1 NK 4 Melanoma, lung carcinoma, brain tumors, leukemias, lymphoma
Rb Retinoblastoma
PTEN Brain tumors, kidney and lung carcinomas.
The protein products of the tumor suppressor genes normally inhibit cell proliferation. Inactivation of such genes therefore leads to tumor development. The complete sequence of events required for the development of any human cancer is not yet known. But it is clear that both the activation of oncogenes and the inactivation of tumor suppressor genes are critical steps in tumor initiation and progression. Simultaneous effect on both the genes will result in multiple genetic defect. It results in the increased proliferation, invasiveness and metastatic potential of cancer cells.
Types of cancer:
Cancers are named according to the tissues from which they arise.
1. Sarcoma - Malignancy in structural tissues Ex: Osteosarcoma (bones), liposarcoma (fa).
2. Carcinoma - Epithelial cancers. Ex : Lung carcinoma, breast carcinioma.
3. Lymphoma - Lymphatic tissues
4. Leukemia - White blood cells.
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