Cancer

• Cancer is one of the most dreaded diseases of human beings and is a major cause of death all over theglobe.
• More than a million Indians suffer from cancer and a large number of them die from it annually. 
• The mechanisms that underlie development of cancer or oncogenic transformation of cells, its treatment and control have been some of the most intense areas of research in biology and medicine.
• Uncontrolled, abnormal and excessive mitotic division of cells is called cancer (Crab = cancer).
• These abnormal and undifferentiated cells are called cancerous cells. Study of cancer is called oncology.
• In our body, cell growth and differentiation is highly controlled and regulated. In cancer cells, there is breakdown of these regulatory mechanisms. Normal cells show a property called contact inhibition by virtue of which contact with other cells inhibits their uncontrolled growth. Cancer cells appears to have lost his property.

As a result of this, cancerous cells just continue to divide giving rise to masses of cells called tumors.

Tumors are of two types :

(i) Benign (ii) Malignant.

• Benign tumors normally remain confined to their original location and do not spread to other parts of the body and cause little damage.
• The malignant tumors, on the other hand are a mass of proliferating cells called neoplastic or tumor cells. These cells grow very rapidly, invading and damaging the surrounding normal tissues. As these cells actively divide and grow they also starve the normal cells by competing for vital nutrients. Cells sloughed from such tumors reach distant sites through blood, and wherever they get lodged in the body, they start a new tumor there. This property called metastasis is the most feared property of malignant tumors.

TYPES OF CANCER

• Cancers are classified on the basis of the tissue of origin from where they arose. Most of the cancers fall into one of the following categories :

(i) Carcinomas : Cancers of this type arise in epithelial tissues, such as skin or the epithelial lining of internal organs or glands (about 85 per cent of all tumours).

eg. (a) Brain carcinoma (b) Oral carcinoma

(c) Gastric carcinoma (d) Colon carcinoma

(e) Lung carcinoma (f) Cervical carcinoma

(g) Adeno carcinona (gland tumour) (h) Breast carcinoma

• Breast cancer in female and prostate cancer in males are common incidence in world.
• Cervical (Uterine Cervix) cancer in females and oral cancer in males are common in India.
• Lung cancer accounts for 31.1% of all cancer death in men and 25% in women.
• Melanomas or Melanocarcinoma : These are cancerous growths of melanocytes (a type of skin cells).

(ii) Sarcomas : These are derived from tissues of mesodermal origin, eg. bone, fat and cartilage. They are rare in humans (about 1 per cent of all tumours).

eg. Bone cancer – Osteosarcoma

Muscle cancer – Myosarcoma

Lymph node cancer – Lymphosarcoma.

(iii) Leukaemias and lymphomas : These are tumours of haematopoietic cells.

1. Chronic Myelogenous Leukaemia (CML): This fatal cancer occurs mainly due to reciprocal translocation between chromosome-22 and chromosome 9 which produces abnormal chromosome 22 (Philadelphia chromosome).
2. Burkitt's Lymphoma : This is produced due to reciprocal translocation between chromosome-8 and chromosome-14.

 Normal mechanism of body growth :-

• Normal cell growth is under the control of some critical regulatory genes, which regulate cell proliferation, differentiation and survival.
• Alterations in these genes lead to oncogenic trasformation.

These genes can be divided into the following three categories :

1. Genes that induce cellular proliferation, eg. genes encoding growth factors, growth factor receptors, transcription factors etc. (Proto-oncogenes). Furthermore, several genes called cellular oncogenes (c-onc) or proto oncogenes have been identified in normal cells which, when activated under certain conditions, could lead to oncogenic transformation of the cells.
2. Genes that inhibit cellular proliferation (Tumour Suppressor Genes).
3. Genes that regulate programmed cell death. (Suicide genes)

• All these genes are involved in normal growth.
• Cancer is caused by mutant alleles of these genes, whose products do not respond to normal regulatory signals.
• As a result, the mutated cell proliferates uncontrollably.
• Cancer cells don't show contact inhibition phenomenon which is shown by normal cells (when normal cells come in contact with other cells they inhibit uncontrolled growth by activation of tumour suppressor genes of cells).

CAUSES OF CANCER

Transformation of normal cells into cancerous neoplastic cells may be induced by physical, chemical or biologicalagents. These agents are called carcinogens. Ionising radiations like X-rays and gamma rays and non-ionizin gradiations like UV cause DNA damage leading to neoplastic transformation. The chemical carcinogens present in tobacco smoke have been identified as a major cause of lung cancer. Cancer causing viruses called oncogenic.

• viruses have genes called viral oncogenes.
• Chemical or physical agents that can cause cancer are known as Carcinogens.

Depending on their mode of action, carcinogens fall into the following main categories :

1. Agents that can cause alternations in the genetic material (DNA), resulting in oncogenic transformation that can lead to cancer eg. various types of radiations and chemicals.
2. Agents that promote the proliferation of cells, which have already undergone genetic alternations responsible for oncogenic transformation. These agents are called tumor promoters, eg. some growth factors and hormones.
3. Cancer causing DNA and RNA viruses (tumour viruses) have been shown to be associated with oncogenic transformation.

CARCINOGENS

• Transformation of normal cells into cancerous neoplastic cells may be induced by physical, chemical or biological agents. These agents are called carcinogens.

(1) Radiations : Ionising radiations like X-rays and gamma rays and non-ionizing radiations like UV cause DNA damage leading to neoplastic transformation. There are 5 times more incidence of leukemia in Hiroshima & Nagasaki due to radiation effect of nuclear bombing.

(2) Chemical factor : The chemical carcinogens present in tobacco smoke have been identified as a major cause of lung cancer. Tobacco and betal chewing cause oral cancer. Heavy smoking (N–Nitrosodimethyl amine) causes oral cancer, cancer of larynx and lungs. Combustion product of coal and pesticides, artificial flavour, sweetners, synthetic food, add flavour, hormonal imbalance in body cause cancer.

Hormonal imbalance or estrogen excess causes breast cancer.

Urinary bladder cancer is common in dye workers.

(3) Physical factors ; Sharp teeth cause Tongue cancer.

Kashmiri people keep 'Kangri' close to skin that cause skin cancer & this skin cancer is called kangri cancer.

(4) Biological factors : Cancer causing viruses called oncogenic viruses have genes called viral oncogenes.

CANCER DETECTION AND DIAGNOSIS

Early detection of cancers is essential as it allows the disease to be treated successfully in many cases.

Following are the techniques for cancer detection and diagnosis

(i) Blood examination :

• This is the test for increased WBC counts in the case of leukemias.
• Detection of the tumor markers like - a-feto protein (AFP) for liver cancer, PSA (Prostate specific antigen) for prostate cancer, alkaline phosphatase for bone metastasis.

(ii) Biopsy : A piece of the suspected tissue is cut into thin sections, stained and is examined under microscope. This is generally a histopathological study by a pathologists (Cancerous cells have a very high value of karyoplasmic index, nucleus large and irregular, nucleolus large, number of mitochondria and ribosomes increase).

(iii) F.N.A.C. : (Fine needle aspiration cytology) - eg. breast cyst / tumour. Fluid is collected from tumors and examined for presence of cancer cells.

(iv) Pap smear : It is used for cervical carcinoma. Slide is prepared from cervical fluid.

(v) X-Ray, C-T Scan and M.R.I : These techniques are very useful to detect cancers of the internal organs.

• In C.T. Scan, X-rays are used to generate a three dimensional image (3-D) of the internals of an object.
• In MRI we use strong magnetic fields and non-ionising radiations to accurately detect pathological and physiological changes in the living tissue

(vi) Modern techniques :

• Antibodies (Monoclonal) against cancer-specific antigens are also used for detection of certain cancers.
• Techniques of molecular biology can be applied to detect genes in individuals with inherited susceptibility to certain cancers. Identification of such genes, which predispose an individual to certain cancers, may be very helpful in prevention of cancers. Such individuals may be advised to avoid exposure to particular carcinogens to which they are susceptible (e.g., tobacco smoke in case of lung cancer).

TREATMENT OF CANCER

Following are the methods :

(a) Surgery : By removing the entire cancerous tissue and involved lymph nodes.

(b) Radiotherapy : Tumour cells are irradiated lethally taking proper care of the normal cells and tissues surroundings the tumour mass. Cobalt therapy (Co - 60), Iodine therapy (I-131), X-ray radiations are given. These radiations destroy the rapidly dividing cells.

(c) Chemotherapy : Several chemotherapeutic drugs are used to kill the cancerous cells. Anti-cancer drugs inhibit synthesis of DNA in cancer cells stopping their cell cycle. some eg. :

• Vincristine (Weed - Cantharanthus roseus = Vinca rosea)
• Vinblastin (Weed - Cantharanthus roseus = Vinca rosea)
• Taxol - Obtained from Taxus baccata

Majority of drugs have side effects like hair loss, anemia etc.

Most of cancer are treated by combination therapy of surgery, radiation and anti cancerous drug.

(d) Immunotherapy :

• Monoclonal antibodies : They are designed to attach specific proteins in cancer cells. Their uses :  (i) To allow the immune system itself to destroy the cancer cells. (ii) To deliver radiation directly to cancer cells. (iii) Carry drugs directly to cancer cells.
• Cancer vaccine : Research is in progress to develop cancer vaccines.

(e) Tumor cells have been shown to avoid detection and destruction by immune system. Therefore, the patients are given substances called biological response modifiers such as a-interferon which activate their immune system and help in destroying the tumor.

GOLDEN KEY POINTS

• Capsid of HIV is formed by – P24, P17
• Envelope of HIV is formed by lipid bilayer, GP-41 & GP-120
• Carcinogen leads to oncogenic transformation.
• Cancer cells do not show contact inhibition.