HOME Introduction to Veterinary Oncology

What is Cancer?

Neoplasias are "new cell growths" that develop faster than normal adjacent tissue and in an uncoordinated and persistent manner. They may be benign or malignant, but the word "cancer" is often associated with malignant growth. Neoplasic cells are different from normal ones in many aspects:

 

• Uncontrolled proliferation independent of new cells
• Not clearly differentiated (associated with the capacity to take a special morphology and do specific functions)

• Cell adherence and communications altered


What causes cancer?


Cancer development is a multiple step process that involves changes and "errors" in cell DNA. The ways in which a cell can turn into a neoplasic one is not fully understood, but the most important change is the alteration of genes in charge of growth and cell differentiation control. Specific genes may:


• Be active (known as oncogenes) or,
• Be inactive (known as tumor suppressor genes) or,
• Have their expression levels altered.



Sometimes oncogenes or tumor suppressor genes are altered in an indirect way, under the influence of genetic changes in DNA repair genes. Failure of their functions causes increase of abnormal DNA sections, being some of them important for proper cell development



Transition from a normal cell to a malignant one requires many mutations. Researches conducted on human colon cancer show that illness progression from benign adenoma (polyps) to invasive carcinoma is accompanied by an increased expression of genes, predominantly tumor suppressor gene that have undergone mutations. At least 4 or 5  mutated tumor suppressor genes are needed for the appearance of a carcinoma, but less changes are required for adenomas. In a similar way, as human glioma increase its histopatologic degree, hence becoming more aggressive, the number of mutated genes increases from 2 or 3 (grade II) up to 6 to 8 (grade III). In both examples, number of mutations is more important that the order in which they have occurred.



Genetic changes can occur in stem cells DNA, hence, they will be present in every body cell since birth. Much more common is when mutations happen spontaneously in somatic cells, as a result of ageing process. Spontaneous mutation accumulation happens slowly, but external factors accelerates the process. Hence, cancer development may be discussed under the following aspects:



Spontaneous genetic events
 

External stimuli
- Biological (virus, parasites, hormones)
- Physical (UV, radiation, trauma)
- Chemical

Hereditary genetic events (family cancer)