Carcinogenesis Health Article

Definition

Also called tumorigenesis, carcinogenesis is the molecular process by which cancer develops.

Description

The development of cancer is a complicated process in which a large number of factors interact to disrupt normal cell growth and division. Cancer can be caused by a number of internal factors such as heredity, immunology, and hormones as well as external factors such as chemicals, viruses, diet, and radiation. Although attention is often focused on environmental chemicals (such as asbestos and coal tar) as a cause of cancer, only 5% of cancers can be linked to chemical exposure. We now know that the chief causes of cancer are lifestyle factors such as diet, cigarette smoke, alcohol, and sun exposure. In fact, dietary factors are associated with 35% of all human cancers and cigarette smoke for another 30%.

Whatever the cause of cancer, its development is a multi-stage process involving damage to the genetic material of cells (deoxyribonucleic acid, or DNA). This damage occurs in genes regulating normal cell growth and division. Because several stages or several mutations are required for cancer to develop, there is usually a long latent period before cancer appears.

Carcinogens

Agents that cause cancer (carcinogens) can be classified as genotoxic or nongenotoxic (also referred to as epigenetic). Genotoxins cause irreversible genetic damage or mutations by binding to DNA. Genotoxins include chemical agents like N-methyl-N-nitrosourea (MNU) or non-chemical agents such as ultraviolet light and ionizing radiation. After the carcinogen enters the body, the body makes an attempt to eliminate it through a process called biotransformation (a series of reactions in which the chemical structure of a compound is altered). The purpose of these reactions is to make the carcinogen more water-soluble so that it can be removed from the body. But these reactions can also convert a less toxic carcinogen into a more toxic one. Certain viruses can also act as carcinogens by interacting with DNA.

Nongenotoxins do not directly affect DNA but act in other ways to promote growth. These include hormones and some organic (carbon-based) compounds.

Stages of carcinogenesis

Cancer develops through four definable stages: initiation, promotion, progression and malignant conversion. These stages may progress over many years. The first stage, initiation, involves a change in the genetic makeup of a cell. This may occur randomly or when a carcinogen interacts with DNA causing damage. This initial damage rarely results in cancer because the cell has in place many mechanisms to repair damaged DNA. However, if repair does not occur and the damage to DNA is in the location of a gene that regulates cell growth and proliferation, DNA repair, or a function of the immune system, then the cell is more prone to becoming cancerous.

During promotion, the mutated cell is stimulated to grow and divide faster and becomes a population of cells. Eventually a benign tumor becomes evident. In human cancers, hormones, cigarette smoke, or bile acids are substances that are involved in promotion. This stage is usually reversible as evidenced by the fact that lung damage can often be reversed after smoking stops.

The progression phase is less well understood. During progression, there is further growth and expansion of the tumor cells over normal cells. The genetic material of the tumor is more fragile and prone to additional mutations. These mutations occur in genes that regulate growth and cell function such as oncogenes, tumor suppressor genes, and DNA mismatch-repair genes. These changes contribute to tumor growth until conversion occurs, when the growing tumor becomes malignant and possibly metastatic. Many of these genetic changes have been identified in the development of colon cancer and thus it has become a model for studying multi-stage carcinogenesis.