Oncogenes and growth factors Mechanisms of growth factor responses are being clarified by the study of oncogenes . Oncogenes are mutant viral or cellular genes whose expression can transform normal cells into neoplastic or cancer cells. They are of interest for developmental biologists because they cause breakdown of normal growth controls, and therefore were suspected to code for altered forms of normal components of the growth control system regulated by growth factors. As you have learned in other courses, oncogenes were discovered in studying tumor viruses. 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.