An
oncogene is a
gene that causes a
cell to develop into a
tumor cell.
A
protooncogene is a gene that is involved in
signal transduction and execution of
mitogenic signals, usually through its
protein product. Upon
activation, it (or its product) becomes a tumor inducing agent, an oncogene.
The protooncogene can become an oncogene by a relatively small modification of its original function. There are two basic activation types:
- A mutation within a protooncogene can cause a change in the protein structure, caused by
- An increase in protein concentration, caused by
- an increase of protein expression (through misregulation)
- an increase of protein stability, prolonging its existence and thus its activity in the cell
- a gene duplication[?], resulting in a doubled amount of protein in the cell
Growth factors are usually
secreted[?] by few special cells to induce cell proliferation in other cells. If a cell that usually does
not produce growth factors suddenly starts to do so (because it developed an oncogene), it will thereby induce its own uncontrolled proliferation (
autocrine loop[?]), as well as the proliferation of neighbouring cells.
There are six known classes of
tyrosine kinases that can become an oncogene:
- Receptor tyrosine kinases that become constitutive (permanently) active.
- Cytoplasmic tyrosine kinases, often products of viral oncogenes.
- Regulatory GTPases, for example, the Ras protein[?].
- Cytoplasmic Serine/Threonine kinases and their regulatory subunits, for example, the Raf kinase[?], and cyclines[?] (through overexpression).
- Adaptor proteins[?] in signal transduction.
- Transcription factors.