Bioinformatics Genetics

Analysis of Mutations in Cancer

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With the realization that cancer is a genetic disease, detection of mutations in genomic DNA has become an important discipline in many areas of cancer research. 

Cells become cancer cells largely because of mutations in the genes. Often many mutations are needed before a cell becomes a cancer cell. The mutations may affect different genes that control cell growth and division. 

It is now clear that cancer is a genetic disease. Cancer has many causes, but ultimately all these causes exert their effects on a special class of genes called cancer genes or proto-oncogenes. Oncogenes normally carry out basic cellular functions, generally related to the regulation of cell division. However, certain types of events can change a proto-oncogene into an oncogene that is, into a state in which it promotes the two main characteristics of cancer: (1) uncontrolled cell division leading to an overgrown group of cells called a tumor

(2) the spread of tumor cells throughout the body to form new tumors, a process called metastasis

One of the main ways in which proto-oncogenes can be changed into their cancer-causing (oncogenic) state is by mutation. Spontaneous or environmentally induced mutation occurs in a proto-oncogene of a single cell, which then undergoes multiple cell divisions to form a tumor. Because all the cells of the tumor carry the mutated oncogene, you can see that a tumor is a mutant clone. This, then, is the sense in which cancer is a genetic disease caused by somatic mutation.

Cells become cancer cells largely because of mutations in their genes. Often many mutations are needed before a cell becomes a cancer cell. The mutations may affect different genes that control cell growth and division. Some of these genes are called tumor suppressor genes. Mutations may also cause some normal genes to become cancer-causing genes known as oncogenes (oncogenes and tumor suppressor genes are discussed in more detail later).

We have 2 copies of most genes, one from each chromosome in a pair. In order for a gene to stop working completely and potentially lead to cancer, both copies have to be “knocked out” with mutations. That means for most genes, it takes 2 mutations to make that gene stop working completely.

An inherited gene mutation is present in the egg or sperm that formed the child. After the egg is fertilized by the sperm, it creates one cell called a zygote that is then divided to create a fetus (which became a baby). Since all the cells in the body came from this first cell, this kind of mutation is in every cell in the body (including some eggs or sperm) and so can be passed on to the next generation. This type of mutation is also called germline (because the cells that develop into eggs and sperm are called germ cells) or hereditary. Inherited mutations are thought to be a direct cause of only a small fraction of cancers. An acquired mutation is not present in the zygote, but is acquired some time later in life. It occurs in one cell, and then is passed on to any new cells that are the offspring of that cell. This kind of mutation is not present in the egg or sperm that formed the fetus, so it cannot be passed on to the next generation. Acquired mutations are much more common than inherited mutations. Most cancers are caused by acquired mutations. This type of mutation is also called sporadic, or somatic.

Experts agree that it takes more than one mutation in a cell for cancer to occur. When someone has inherited an abnormal copy of a gene, though, their cells already start out with one mutation. This makes it all the easier (and quicker) for enough mutations to build up for a cell to become cancer. That is why cancers that are inherited tend to occur earlier in life than cancers of the same type that are not inherited. Some people have a high risk of developing cancer because they have inherited mutations in certain genes. To learn more about this, see Family Cancer Syndromes. Gene variants can also play a role in diseases that impact cancer risk like diabetes and obesity.

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