Oncogene

Oncogenes are genes that can cause malignant
transformation of normal cells. They were first recognised as
viral oncogenes (v-onc) carried by RNA viruses. These
retroviruses incorporate a DNA copy of their genomic RNA
into host DNA and cause neoplasia in animals. Sequences
homologous to those of viral oncogenes were subsequently
detected in the human genome and called cellular oncogenes
(c-onc). Numerous proto-oncogenes have now been identified,
whose normal function is to promote cell growth and
differentiation. Mutation in a proto-oncogene results in altered,
enhanced, or inappropriate expression of the gene product
leading to neoplasia. Oncogenes act in a dominant fashion in
tumour cells, i.e. mutation in one copy of the gene is sufficient
to cause neoplasia. Proto-oncogenes may be activated by point
mutations, but also by mutations that do not alter the coding
sequence, such as gene amplification or chromosomal
translocation. Most proto-oncogene mutations occur at a
somatic level, causing sporadic cancers. Exceptions include the
germline mutation in the RET oncogene responsible for
dominantly inherited multiple endocrine neoplasia type II.