Cancer-Critical Genes I: Proto-oncogenes

Cancer-critical genes are a group of genes that, upon mutation or alteration, contribute to the development of cancer. 

These genes can be categorized into two major classes based on how the mutation affects gene activity - proto-oncogenes and tumor-suppressor genes.

Under normal conditions, proto-oncogenes encode for proteins that act as signals for cell division. A gain-of-function mutation in the proto-oncogenes converts them into an overactive form called oncogenes.

The activation of oncogenes leads to overexpression of the cell-growth proteins resulting in uncontrolled cell proliferation and, hence, the formation of a tumor.

Gain-of-function mutations in proto-oncogenes are usually dominant, and even a single mutation can transform them into an oncogene.

For example, the proto-oncogene Ras encodes for an intracellular signal-transduction protein, Ras. Upon activation, Ras switches ON the cell growth signaling cascades. Upon mutation, the Ras gene turns into an oncogene and produces a mutated protein. This mutated Ras switches ON the signaling pathways, even in the absence of any activation signals, leading to the uncontrolled growth of cells and, ultimately, tumor formation.