o Cancer is the second most common cause of death
in the world after cardiovascular diseases. Human of all ages develop cancer
and wide variety of organs are affected.
o Cancer cells are characterized by three
properties: 1) diminished or unrestrained control of growth and cell division;
2) invasion of local tissue; and 3) spread or metastasis to other parts of the
body. Cell growth and cell division are regulated by finely tuned control
mechanisms.When these processes lose their control, a cell begins to grow
uncontrollably. The result is the formation of a mass of cells called a tumour.
The overall processes is called neoplasm.
o The primary event in neoplasm appears to be the
alteration in DNA . Most of the tumours are localized without spread and so
without risk to the host, they are called benign tumours. Sometimes they start
interfering with normal tissues and secreting excessive amount of some hormones
or other biologically active substances. These substances are called as tumour
markers.
o Changes in the levels of tumour markers in serum
are useful in detecting tumours and cancers. Increase in Ca++ level is seen in
multiple myeloma and bone cancer. Elevated levels of alkaline phosphatase is
observed in malignancy of bone, liver and carcinoma of bronchus.
o Tumours become life threatening , when their
cells instead of being localized spread throught the body. They become
malignant and cause cancer. These cells can spread and invade surrounding
tissues or can travel through the blood stream. Development of secondary areas
of growth away from the original site of growth is called metastases. Changes
in growth properties of cells and their subsequent ability to form malignant
tumours are collectively referred to as transformation.
o Cancers are classified according to the tissue
and cell type from which they originate. Those arising from epithelial cells
are called carcinomas . Those arising from connective tissue or muscle cells
are called as sarcomas. Cancers that do not fit into either of these broad
categories include leukemias , lymphomas and cancer of the cells of central
nervous system. Nearly 90% of human cancers are carcinomas
Agents causing cancer fall into 3 categories: a)
radiant energy, b) chemical compounds and c) viruses.
a. Radiant
energy: Ultraviolet rays ,
X-rays andg-rays are mutagenic andcarcinogenic. These rays damage DNA which is
presumed to be the basic mechanism of carcinogenicity.
Apart from direct effects on DNA, X - rays and
gamma rays cause free radicals to form in tissues which may act as carcinogens.
b. Chemical
compounds: A wide variety
of organic and inorganic molecules may becarcinogenic.
Examples of organic carcinogens are Benzopyrene,
Dimethylbenzanthracene, Dimethylnitrosamine and Aflatoxin B1.
Examples of inorganic carcinogens are arsenic,
asbestos, beryllium, cadmium and chromium.
Very few carcinogens interact with DNA directly
without further metabolism. These carcinogens are called as direct carcinogens.
But many of the carcinogens need prior metabolic activation and the activated
ultimate carcinogen can interact with DNA.The ultimate carcinogens are usually
electrophiles which readily attack nucleophilic groups in DNA,RNA and proteins.
c. Viruses:
Viruses that contain
either DNA or RNA as their genome which inducecancer are called as oncogenic
viruses. For example Epstein - Barr virus cause Burkitt’s lymphoma and
nasopharyngeal carcinoma and Herpes simplex virus cause cancer in the cervix in
humans.
The following changes are found in tumour cells
·
Increased
synthesis of RNA and DNA.
·
Decreased
catabolism of pyrimidines
·
Increased
rates of aerobic and anaerobic glycolysis.
Some changes that have been detected at the
surface of malignant cells are alteration in transport properties, permeability,
surface charge, diminished adhesion, appearance of new antigens, changes of
glycolipid constituents and alterations of the oligosaccharide chains of
glycoproteins.
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