Most people know DNA but many are unsure about genes. One can explain the gene structure by looking at a gold chain. A gold chain is made up of a series of individual interlocking loops. If you start taking out loops, one at a time, there will be no chain left when the last loop is out . The genes are like interlocking loops and the chain is one strand of chromosome. A normal chromosome has two strands of genes, twisted anticlockwise fashion like a spiral staircase. The chemically the genes are made of Deoxyribose Nucleic Acids (DNA).There are four different nucleic acids; they are known by A, T, G, C. AT and GC always appear in pairs and are called Base Pairs. The three of these base pairs make a word of the coded instructions that each gene carries for the multitude of functions the chromosomes have to perform through out the life. We have 23 pairs of chromosomes and we inherit on strand of chromosome from each parent.
Believe it or not, every cell of our body has a define time of existence: there is a time to be born, time to achieve maturity, time to die and be replaced by a new cell. Different cell lines have different lifespan e.g. white blood cells live 5 days; red blood cells live 120 days and so on. Deaths of cells are programmed into the genes. If and when programmed cell death fails in one cell, this cell continues to live and multiply. This is the beginning of a tumor growth and ultimately ends up in a cancer formation.
23 pairs of human chromosomes carry 3 billion of base pairs and so far functions of 30,000 genes are known.
In an individual the cells are dividing regularly throughout the life. During cell division the chromosomes duplicate themselves and then separate and move to new cells. This is the critical time when things can go wrong. Errors may appear in spelling of coded words (mutation), words may be dropped (deletion), or attached to a different place (translocation). Cells of an older person have divided many more times than a younger one and elderly population are at higher risk of acquiring abnormal genes (Somatic Mutation). It explains the reason for increased cancer rate in old age. Increasing concentration of cancer causing chemicals in the environment, food additives, hormones, pesticides, cigarettes, ultraviolet rays of sun, microwaves, x-rays and virus infections etc. either individually or collectively are constantly influencing the normal cell division at this critical juncture. Any minor or major dislocation of this process will lead to somatic mutations. Derailed repair of DNA is an additional cause of mutation.
To guard against these mishaps and to find the defective genes, other genes are endowed with surveillance function and are called Tumor- Suppresser Genes. However, there is no guarantee that mutation will not happen to this very tumor-suppresser genes. And when it happens it is transmitted to the next generation of children.
A class of genes have the role of caretaker function-Caretaker Genes.It looks after the entire population of chromosomes. The deficiency of these genes increases rate of mutation in all genes. Another class of genes, called Gatekeeper gene, restrains growth of individual cells and promotes cell differentiation.
A specific group of cells in our body are constantly keeping a vigil for such mutant cells (Surveillance cells).Once they detect a mutant cells they mark them with a protein; and Killer cells then move in and promptly remove these mutant cells from the body.
Somatic mutation or inherited defective gene/chromosome is the immediate cause of cancer. The development of cancer, however, is a failed multi-step process. A mutant cell has growth advantage over his neighbors but it must escape from the surveillance cells before it can divide again. When mutant cells have gone through 5 to 10 generation of cumulative mutations then mutant cells have a chance to establish as a Malignant Phenotype (potential to cause cancer in this individual).
These are cancer causing genes. They were first discovered in certain retrovirus induced cancer in chicken. Similar genes (homologous genes) are also present in human cells. But they are very tightly controlled by tumor-suppresser and caretaker genes. To escape from the scrutiny of tumor-suppressed-genes and caretaker-genes the ochogenes have to mutate first. There are 3 such mechanisms.
1. Translocation: A piece of chromosome containing specific genes breaks away from its normal location and attaches to a different chromosome e.g. In chronic myelogenous leukemia. a piece chromosome 9 is grafted to chromosome 22. This abnormality is better known as Philadelphia chromosome of CML.
2. DNA Amplification: A certain part of base pairs of genes appears repeatedly over and over again. This is often seen in Sarcoma and aggressive breast cancer.
3. Point mutation: One base pair mutation appears in several gene locations; often present in pancreatic cancer and colon cancer.
When one copy of mutated onchogenes overrides the effect of its other normal copy and disease is manifested in the person carrying the mutated genes. This mode of inheritance is known as Autosomal dominant inheritance. Mutation of one copy of the tumor-suppresser genes does not adversely affect the individual; to have an increased risk of cancer both copies of tumor-suppresser genes must be defective. This mode of inheritance is called Autosomal recessive inheritance. One mutated copy is usually inherited; the other may be due to somatic mutation.
Every normal person has many mutated genes and these mutations are generally harmless. Somatic mutations of genes for the most part are not transmitted to the next generation. Hereditary cause of cancer is not more than 5 to 10 % of all cancers. Only about 100 hereditary cancers/ syndromes are known. We read in the newspaper about finding of a new gene for a particular cancer; it does not mean that gene itself is the cause of cancer; the abnormal gene should be considered as one of the risk factors.
Genes are named according to their functions or association with a specific disease.
To specify the location of genetic mutations on chromosomes, numbers and letters are used like astronomers assign letters and numbers to designate points of light on the dark night sky. To give an example: a 11q 13.1 number signifies chromosome # 11, q is the long arm of this chromosome and 13.1 is the band location. Bands are detected when chromosomes are stained with dyes for microscopic examination. (p stands for short arm of chromosomes and q for the long arm).
Here are some of the well known hereditary cancers:-
1. Hereditary Adenomatous Polyposis Colon Cancer: The disease is due to APC gene mutation, located on 5q21, dominant mode of inheritance. Patients develop multiple colon polyps, some or many of them eventually turn cancerous.
2. Hereditary Non-adenomatous Colon Cancer: Also known as Lynch syndrome due to MHS2,MHL1,PMS2 gene mutations, located on any of these positions – 3p21.3, 7p12.2p16, dominant mode of inheritance.
3. Multiple Endocrine Neoplasia 2a - MEN 2a: Due to RET gene mutation, located on 10q11.2. Patients have medullar thyroid cancer, pheochromocytoma and parathyroid over activity.
4. Hereditary Breast/ Ovarian Cancer 1: Due to onchogene mutation of BRCA1gene, located on 17q21; dominant mode of inheritance. Carrier of BRCA1gene has 80% risk of breast cancer and 60% risk of ovarian cancer; and has some increased risk of colon, pancreatic, uterine cancers.
5. Hereditary Breast/Ovarian Cancer 2: Due to onchogene mutation of BRCA2 gene, located on 13q12.3, dominant mode of inheritance. Female carriers in addition to the risk of breast and ovarian cancers also are at risk of cancer of colon, stomach, gall bladder, bile duct and melanoma. Male members have risk of breast, colon and prostate cancers.
5. Hereditary Prostrate Cancer: Due to mutation of onchogene HPC1, location 1q24-25, dominant mode of inheritance. Carriers of this mutation develop cancer in age 30s - 40s, this cancer is aggressive in nature.
6. Cancers due to Onchogene Translocation:
Cancers of blood cell lines and lymphatic tissues are strongly associated with such defects. Examples are- Chronic myelogenous leukemia, Mantel cell lymphoma, Follicular lymphoma, Ewing’s tumor, T-cell leukemia, Burkett’s lymphoma.
Cells carry out functions according to instructions coded in the genes, like applications run on computer software programs. When software programs are corrupted by hackers or computer viruses they create havocs. Similar processes are at work in cells having cancer enhancing mutations.
Every day scientists and researchers are adding more knowledge in genetics and cancer research; and progress in recent years has been astounding. Your interest in this subject will encourage more research.