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Initiation is where the first genetic mutation occurs in a cell. Promotion is the clonal expansion (repeated division) of this transformed cell into a visible tumor that is usually benign. Following promotion, progression may take place where more genetic mutations are acquired in a sub-population of tumor cells.
A transmissible cancer is a cancer cell or cluster of cancer cells that can be transferred between individuals without the involvement of an infectious agent, such as an oncovirus. [1] [2] The evolution of transmissible cancer has occurred naturally in other animal species, but human cancer transmission is rare. [2]
One of the most significant tumor suppressors is known as p53. It plays such a critical role in regulation of cell division and cell death that in 70% of cancer cells p53 is found either mutated or functionally inactivated. Often times tumors can not form successfully without deactivating critical tumor suppressors like p53. [6]
The classical view of cancer is a set of diseases driven by progressive genetic abnormalities that include mutations in tumor-suppressor genes and oncogenes, and in chromosomal abnormalities. A role for epigenetic alterations was identified in the early 21st century. [100]
In modern English, tumor (non-US spelling: tumour) is used as a synonym for a neoplasm (a solid or fluid-filled cystic lesion that may or may not be formed by an abnormal growth of neoplastic cells) that appears enlarged in size. [12] [13] Some neoplasms do not form a tumor; these include leukemia and most forms of carcinoma in situ.
Carcinogenesis is caused by mutation and epimutation of the genetic material of normal cells, which upsets the normal balance between proliferation and cell death. This results in uncontrolled cell division in the body. The uncontrolled and often rapid proliferation of cells can lead to benign or malignant tumours (cancer).
The tumor types are typical for each type of tumor suppressor gene mutation, with some mutations causing particular cancers, and other mutations causing others. The mode of inheritance of mutant tumor suppressors is that an affected member inherits a defective copy from one parent, and a normal copy from the other.
The first mouse mutant in the Apc gene came from a colony of randomly mutagenized mice. [5] This mouse model is called Min (multiple intestinal neoplasia) mouse. It was found to carry a truncation mutation at codon 850 of the Apc gene. The Min mouse can develop up to 100 polyps in the small intestine in addition to colon tumors.