1.6.13 Epigenetic Gene Activation Definition

Epigenetic Gene Activation Definition is the description of a stable increase in a gene's expression achieved through removal of repressive chemical modifications or establishment of permissive chemical modifications at its regulatory DNA or surrounding chromatin, rather than through any alteration of the gene's own nucleotide sequence, resulting in a gene that becomes accessible to and actively engaged by the transcriptional machinery even though its underlying sequence has not changed. Epigenetic gene activation is heritable across cell divisions, allowing an activated state established in one cell to be transmitted faithfully to that cell's descendants.

Conceptual Basis of Epigenetic Gene Activation

Activation Without Sequence Alteration

The defining feature of epigenetic gene activation is that the coding sequence of the affected gene remains completely unchanged. What changes instead is the accessibility of the gene's regulatory region, brought about by a shift in the chemical modifications and chromatin configuration surrounding the gene rather than by any change to the genetic instructions the gene itself encodes.

Distinction From Ordinary Transcriptional Induction

Epigenetic gene activation is distinguished from the ordinary, transient induction of gene expression that occurs as part of normal cellular signaling, in that epigenetic activation establishes a stable, self-reinforcing accessible state that persists across cell divisions independent of the continued presence of the signal that originally triggered it.

Mechanisms of Epigenetic Gene Activation

Activation Through Loss of DNA Methylation

A gene can be epigenetically activated through the removal of methylation marks previously present at its regulatory region, restoring the ability of transcription-initiating proteins to bind that region and removing the proteins that would otherwise recognize methylated DNA and recruit chromatin-compacting factors.

Activation Through Permissive Histone Modification

A gene can be epigenetically activated through the establishment of histone modifications associated with open, transcriptionally accessible chromatin at its surrounding nucleosomes, increasing the physical accessibility of the gene's regulatory region to the transcriptional machinery.

Activation Through Chromatin Opening

Beyond specific chemical marks, a gene can be epigenetically activated through active remodeling that repositions or ejects nucleosomes from its regulatory region, working alongside loss of repressive methylation and gain of permissive histone modification to produce an overall open chromatin configuration at that locus.

Stability and Self-Reinforcement of Epigenetic Gene Activation

Reinforcing Nature of an Established Active State

Once epigenetic gene activation has been established at a locus, the contributing mechanisms tend to reinforce one another, with open chromatin favoring further loss of repressive marks, and reduced methylation favoring continued binding of activating transcriptional proteins, producing a stable active state that resists spontaneous reversion to a silenced condition.

Capacity for Reversal

Because epigenetic gene activation depends on chemical modifications and chromatin configuration rather than on the underlying DNA sequence, an activated locus retains, in principle, the capacity to be returned to a silenced state through reestablishment of repressive marks, distinguishing epigenetic activation from a permanent genetic alteration such as a mutation that directly increases a gene's inherent activity.

Epigenetic Gene Activation in Cancer Cell Biology

Activation of Genes That Promote Cellular Proliferation

A recurrent event in cancer cell biology is the epigenetic activation of a gene that promotes cellular proliferation or survival and that would not normally be expressed in the cell type from which the cancer arose, producing an abnormal source of growth-promoting activity that contributes to malignant cellular behavior without requiring any genetic alteration of the activated gene itself.

Cooperation With Genetic Alteration

Epigenetic gene activation frequently occurs alongside genetic alterations that independently increase the activity of growth-promoting genes within a cancer cell, such that the overall pattern of abnormally active growth-promoting genes within a tumor reflects contributions from both direct genetic alteration and epigenetic activation mechanisms.

Significance of Epigenetic Gene Activation Within Cancer Cell Biology

An Alternative Pathway to Gain of Gene Function

Epigenetic gene activation provides cancer cells with an alternative pathway to inappropriately increase the activity of a growth-promoting gene without requiring a mutation or amplification of that gene's sequence, expanding the range of mechanisms available to a cancer cell and complicating any assessment of a tumor's gene activity that considers genetic sequence alone.