Whereas the oncogenes of DNA tumor viruses are viral genes, oncogenes activated by RNA retroviruses turned out to be cellular in origin. Oncogene products fall into at least five functional classes, all related to growth control. The erbB oncogene is derived from an EGF receptor gene. The ras family oncogenes are derived from small G protein genes. The fos and jun oncogenes are derived from genes for common bZIP class transcription factors.
Mutant oncogenes release cells from normal growth controls.
Constitutive activity can result from inability to be normally regulated, from altered substrate or binding specificity, or simply from overproduction. This realization led to understanding the molecular basis of many cancers. However, dominant oncogene mutations do not account for all cancers. They are often genes that must normally be turned off by a growth factor response, in order for cell proliferation to occur. We will discuss tumor suppressor genes and cell death genes in subsequent lectures.