1.1 World Health Organization’s definition of cancer
The World Health Organization (WHO) defines cancer as a generic term for a large group of diseases characterized by the growth of abnormal cells beyond their usual boundaries that can then invade adjoining parts of the body and/or spread to other organs.
1.2 Hallmarks of cancer
All types of cancer share common features or “Hallmarks”. Sustaining proliferative signaling, evading growth suppression, activating invasion and metastasis, enabling replicative immortality, inducing angiogenesis, and last but not least resisting apoptosis are all common features of cancer. The Hallmarks of cancer are summarized below in Figure 1.
Fig 1. The Hallmarks of Cancer (data from (Hanahan and Weinberg, 2011))
Under normal conditions, cell growth and division are regulated by growth-promoting signals, that instruct the cell to grow and divide in a well-controlled manner. On the other hand, cancerous cells do not depend on these signals, thus deregulating these signals.
Evading growth suppression is another trait shared by all types of cancers. Normally cellular growth is inhibited by tumor suppressor genes. The RB (retinoblastoma-associated) and TP53 proteins are two well-known examples of tumor suppressor genes. The RB is a tumor suppressor protein. While the TP53 has the ability to stop the cell cycle, check a damaged DNA strand and repair it. Thus, a mutation any of these tumor suppressor genes will lead to indefinite growth.
Metastasis of cancerous cells can be summarized into two major steps. The first step is the ability to spread from the primary tumor site to distant tissues. The second step is the successful colonization of these cells after adapting to their new microenvironment of the foreign tissue,
Another feature of cancer is the ability to enable replicative immortality. Normal cells undergo a limited amount of cell cycles enabling them to grow and divide. However cancerous cells possess the ability to replicate indefinitely.
Angiogenesis is the process of forming new blood vessels from pre-existing vessels. Like any normal cell, tumors require oxygen, nutrients, and the ability to evacuate metabolic wastes and CO2 to survive. Angiogenic factors released by tumors stimulate the formation of new blood vessels to supply the tumor.
Last but not least, tumors have the ability to resist cell death. Apoptosis is defined as programmed cell death. Apoptosis is a well-regulated process, playing an essential role in maintaining the health of the body by eliminating dysfunctional cells. The P53 gene has a pro-apoptotic function. Over 50% of cancers in humans carry a loss of function mutation in the P53 gene (Ozaki and Nakagawara, 2011). Thus, this will allow tumors to resist cell death.