1.7.1 Oncogene Activation in Cancer Cells Definition

Oncogene Activation in Cancer Cells Definition is the description of the process by which a normal growth-promoting gene, referred to in its unaltered state as a proto-oncogene, acquires an alteration that converts it into an abnormally active oncogene within a cancer cell, resulting in a growth-promoting signal that is stronger, more persistent, or less dependent on normal regulatory control than the signal generated by the gene's unaltered counterpart. Oncogene activation is one of the two principal categories of driver alteration recognized in cancer cell biology, alongside the loss of function of genes that would otherwise restrain cellular growth.

Defining Features of Oncogene Activation

Conversion of a Normal Regulatory Component Into an Abnormal Driver

Oncogene activation describes a transformation in which a gene that ordinarily participates as a regulated component of normal growth-signaling pathways is converted into a component that operates independent of, or in excess of, its normal regulatory constraints, so that it continues to promote cell growth even under conditions in which the normal, unaltered gene would remain quiescent or would respond only to appropriate external signals.

Dominant Functional Consequence

Oncogene activation typically affects only a single allele of the gene in question, and because the abnormal product or abnormal quantity generated by that single altered allele is sufficient to drive abnormal signaling, oncogene activation behaves as a dominant alteration at the cellular level, distinguishing it from alterations that require inactivation of both gene copies to produce a functional consequence.

Routes by Which Oncogene Activation Occurs

Alteration of the Coding Sequence

Oncogene activation can occur through a change to the coding sequence of the proto-oncogene itself, producing an altered protein that retains its basic functional role but that operates in a constitutively active state, no longer requiring the upstream signal that would normally be needed to trigger its activity.

Increase in Gene Copy Number

Oncogene activation can occur through an increase in the number of copies of the proto-oncogene present within the cancer cell's genome, resulting in production of a correspondingly increased quantity of the gene's normal protein product and an overall amplification of the growth-promoting signal that protein contributes.

Relocation to an Abnormal Regulatory Context

Oncogene activation can occur through a chromosomal rearrangement that repositions the proto-oncogene under the control of a different gene's regulatory elements, resulting in expression of the gene at an inappropriate level, at an inappropriate time, or within a cell type in which it would not normally be active.

Fusion With a Partner Gene

Oncogene activation can occur through the joining of the proto-oncogene's coding sequence with that of a separate gene, producing a chimeric protein that combines functional elements from both contributing genes and that frequently displays constitutive activity not present in either original protein individually.

Loss of Epigenetic Silencing

Oncogene activation can occur through removal of the epigenetic marks that would normally maintain the proto-oncogene in a silenced or low-expression state, resulting in inappropriate transcriptional activation of the gene without any accompanying change to its coding sequence.

Consequences of Oncogene Activation for Cell Behavior

Sustained Proliferative Signaling

The immediate functional consequence of oncogene activation is the generation of a sustained signal promoting cell growth and division, allowing the affected cell to continue proliferating under conditions that would normally restrain the proliferation of a cell carrying only unaltered proto-oncogenes.

Contribution to a Growing Population of Abnormal Cells

Because the abnormal signaling produced by an activated oncogene confers a proliferative advantage upon the cell that carries it, that cell and its descendants tend to increase in number relative to surrounding normal cells, contributing to the expansion of an abnormal cell population over time.

Significance of Oncogene Activation Within Cancer Cell Biology

A Recurrent and Identifiable Category of Driver Event

Oncogene activation affecting specific, recurrently identified genes is observed across many independent cancers, indicating that activation of these particular genes confers a substantial and reproducible selective advantage to the cells that acquire it, distinguishing such events from the broader background of alterations that accumulate in cancer cells without conferring any comparable advantage.