1.6.10 Chromatin Accessibility Definition

Chromatin Accessibility Definition is the description of the degree to which a given region of DNA, packaged within chromatin, is physically available to be bound by the proteins responsible for transcription, replication, and DNA repair, as determined by the density of nucleosome packaging and the surrounding chemical modifications present at that region. Chromatin accessibility exists along a continuum ranging from highly accessible regions in which DNA is loosely associated with histone proteins and readily bound by regulatory proteins, to poorly accessible regions in which DNA is tightly wound and largely shielded from such binding, and this accessibility is the immediate physical property that permits or prevents the transcription of a given gene.

Conceptual Basis of Chromatin Accessibility

Accessibility as a Physical Property

Chromatin accessibility describes a physical, structural characteristic of a genomic region rather than a chemical modification in itself. While DNA methylation and histone modification are chemical marks that influence accessibility, accessibility itself refers to the practical consequence of those marks, namely whether the DNA at a given location is physically exposed and available for protein binding.

Relationship to Nucleosome Positioning

Because DNA is wound around histone proteins to form repeating nucleosome units, the precise positioning and spacing of these nucleosomes along a stretch of DNA directly determines how much of that DNA is exposed at the surface of the chromatin fiber versus how much remains wrapped around the histone core, making nucleosome positioning a central determinant of local chromatin accessibility.

Regulation of Chromatin Accessibility

Influence of Chemical Modifications

Both DNA methylation and histone modification contribute to establishing chromatin accessibility, with methylation of regulatory regions and certain histone modifications favoring dense nucleosome packaging and reduced accessibility, while other histone modifications favor loosened packaging and increased accessibility.

Active Remodeling by Dedicated Protein Complexes

Chromatin accessibility is also directly regulated by dedicated protein complexes that use chemical energy to slide, eject, or reposition nucleosomes along the DNA, actively increasing or decreasing accessibility at specific genomic locations independent of the chemical modifications present.

Binding of Regulatory Proteins

The binding of specific regulatory proteins to a genomic region can itself displace or reposition nucleosomes, creating a locally accessible region that permits subsequent binding of additional regulatory factors required for transcription, illustrating that accessibility and protein binding can reinforce one another.

Measurement of Chromatin Accessibility

Assays Based on Differential Susceptibility to Cleavage or Insertion

Chromatin accessibility is commonly assessed using laboratory methods that exploit the differential susceptibility of accessible versus inaccessible DNA to enzymatic cleavage or to insertion of foreign genetic material, with accessible regions showing substantially higher rates of cleavage or insertion than regions shielded by dense nucleosome packaging.

Significance of Chromatin Accessibility Within Cancer Cell Biology

A Direct Predictor of Gene Expression Potential

Chromatin accessibility at the regulatory region of a gene is one of the most direct physical predictors of whether that gene has the potential to be actively transcribed, making genome-wide maps of chromatin accessibility a powerful tool for inferring which regulatory genes are active or silenced within a cancer cell without requiring direct measurement of gene expression itself.

Redistribution of Accessibility in Cancer Cells

Cancer cells frequently display genome-wide redistribution of chromatin accessibility relative to their normal cell of origin, with newly accessible regions often corresponding to genes and regulatory elements that promote proliferation, and newly inaccessible regions often corresponding to genes that would otherwise restrain cellular growth.