The most common human genetic disease is cancer. The conversion from a normal cell to a malignant cancer is due to the changes in cell’s DNA, also known as mutations.
The cells in the human body work in an organized manner, dividing and reproducing in a severely controlled and harmonized fashion. The essential part of cell division is DNA replication, whereby the cell makes an identical copy of its genome before it divides.
During the process of DNA replication mistakes or ‘mutations’ can occur. This can cause changes in the genome of the new cell. However, the cell’s machinery can repair most of these mistakes. But sometimes the change in the DNA remains due to missing of some mistakes.
Types of genes linked to cancer
If an individual possesses an error in a DNA repair gene, mistakes may remain uncorrected. Then, these mistakes can become mutations. These mutations may finally lead to cancer, mostly mutations in tumor suppressor genes or oncogenes. Various genes which contribute to the development of cancer fall into extensive categories:
Tumor suppressor genes; these are the protective genes and normally they limit the cell growth by;
- Monitoring how speedily cells divide into new cells
- Repairing the mismatched DNA
- Controlling when a cell dies
When a mutation occurs in tumor suppressor gene, cells can grow uncontrollably. Therefore, they may ultimately form a tumor. Some examples of the tumor suppressor genes include BRCA1, BRCA2, and p53 or TP53.
The most commonly altered gene in people with cancer is p53 or TP53. More than 50% of cancers comprise a damaged or missing p53 gene. Germline mutations are rare. But patients who carry them are at a greater risk of developing several different types of cancer.
Oncogenes; these are the genes which turn a healthy cell into a cancerous cell. The mutations occurring in these genes may not be hereditary.
Two common oncogenes are;
- HER2 is a specialized protein which controls the cancer growth and range. It is present in some cancer cells such as ovarian and breast cancer cells.
- The RAS family of genes makes proteins which take part in cell growth, cell communication pathways, and cell death.
DNA repair genes; when DNA is copied these genes fix the mistakes made. Many of them can also function as tumor suppressor genes. BRCA1, BRCA2, and p53 are DNA repair genes. Mutations in DNA repair genes may be acquired or inherited. The example of an inherited kind is Lynch syndrome.
If these mutations occur in the genes which control growth such as proto-oncogenes and tumor suppressor genes, uncontrolled growth of cell can occur. This can lead to tumor formation and cancer. Proto-oncogenes can encourage the cell to proliferate, thus causing mutations in these genes. Consequently, these genes are called as oncogenes as they cause the cell to multiply all the time.
If tumor suppressor genes present in our body don’t stop the cell from multiplying, the cells may carry on multiplying. Thus, these genes become mutated and stop working. A cell must attain mutations in numerous growth-controlling genes to become cancerous.
Mutations generally accumulate over several years before a cell becomes cancerous. That is why most of the cancer types are more common in older people than younger ones.
The cancer cycle
Uncontrolled cell growth is the hallmark of cancer. There is an extreme regulation in timing and rate of the cell division in our body. However, this regulation is lost due to the mutations in genes which control the cell cycle. This causes an uncontrolled cell division.
- Several mutations in genes controlling the cell cycle allow the cells to multiply out of control unevenly.
- Hence, this leads to a mass of cells continues to grow and grow, finally developing into a tumor.
- Primarily the tumor limits to the tissue in which it is present, for instance, the breast tissue. However, as the tumor grows bigger it needs more nutrients. So, in angiogenesis, the tumor starts to develop its own network of blood vessels.
- Metastasis is the process in which the cancer cells begin to migrate around the body through the lymphatic vessels or bloodstream. The following illustration shows how uncontrolled cell growth leads to cancer.
Some individuals can inherit a predisposition to certain cancers. For example, individuals with hereditary non-polyposis colorectal cancer or familial adenomatous polyposis have mutated genes which predispose them to the colon cancer.
Due to which, patients are frequently diagnosed with cancer at a younger age than those who are not predisposed. Thus, cancer will incline to ‘run’ in the family. Predisposing mutations mostly affect the genes involved in DNA repair and the regulation of cell growth and division.
Usually, multiple tumors are developing in individuals who are predisposed to cancer. In general, rarer new mutations are needed for cancer to develop in individuals who are not predisposed.
Cancer is a much complex disease. This means that lifestyle and environmental factors, as well as genetic features, influence it. Environmental exposures usually include different factors like UV light, chemicals, and radiation.
Lifestyle factors include excessive alcohol consumption, cigarette smoking, and diet. Some cancers are also linked to certain types of exposures. For instance, long exposure to carcinogens from tobacco is related to numerous cancers, including lung, mouth, bladder, and throat cancers. The links between cancer and environmental factors are very complex and may differ depending on the individual.
Challenges in understanding cancer genetics
Modern researchers have learned many things about how cancerous genes work. Cancer probably involves mutations in multiple genes. Moreover, some evidence also suggests that these genes can interact with their environment. This further confounds our understanding of the role which genes play in cancer.
Researchers are trying to study how genetic changes can affect cancer development. This knowledge has led to many improvements in cancer care, including early detection, risk reduction, the use of targeted therapy, and survival.
Further learning cancer genetics may also help doctors find better ways to:
- Predict a person’s risk of cancer
- Diagnose cancer
- Treat cancer