Oncogenes are found in
three forms: cellular proto-oncogenes that have been mutated, cellular proto-oncogenes
that have been captured by retroviruses, and virus-specific genes that behave like
cellular proto-oncogenes that have been mutated. When a mutation event is involved,
it is said that the proto-oncogene has been "activated." The next several
slides (slides 6-12) look at these three classes of oncogenes, starting with the viral
genes (DNA tumor viruses and retroviruses). The rest of the lecture goes into a
discussion of the cellular proto-oncogenes and how they are activated.
Oncogenes are said to be "dominant" in their action.
This is because they result from a "gain of function" mutation that
results in their overexpression or unregulated activity (i.e., they remain constitutively
active). Since they promote cell growth and proliferation, their unregulated
activity would provide a continuous stimulation of cell division. Because of this
dominant action, it is only necessary that one of the two alleles be activated for the
effects on cell growth to be felt.
Tumor suppressor genes are cellular genes that normally suppress
cell growth and proliferation. They are said to be "recessive" in their
action. This is because they result from a "loss of function" mutation,
such as a deletion or inactivating point mutation. Usually, loss of only one allele
of a tumor suppressor gene is not in itself sufficient to cause a problem, because the
product of the normal allele remains to suppress cell growth. Because tumor
suppressor genes are recessive, it is necessary that both alleles be inactivated for the
effects on cell growth to be felt.