Chromatin Accessibility Alteration

Chromatin Accessibility Alteration is a change in the degree to which specific regions of the genome are physically open and available for binding by transcription factors and other regulatory proteins, reflecting an underlying shift in nucleosome positioning and chromatin compaction that distinguishes cancer cells from their normal tissue counterparts.

Chromatin Accessibility in Normal Cells

Open versus Closed Chromatin

Regions of the genome exist along a spectrum from tightly compacted, transcriptionally inert heterochromatin to loosely packaged, transcriptionally permissive euchromatin. Active promoters, enhancers, and other regulatory elements typically reside in accessible, nucleosome-depleted regions that allow the transcriptional machinery physical access to the underlying DNA sequence.

Determinants of Accessibility

Local chromatin accessibility is shaped by the combined influence of DNA methylation, histone modifications, nucleosome positioning, and the binding of specific regulatory proteins, together forming a dynamic and cell-type-specific accessibility landscape that defines which genes a given cell is poised to express.

Patterns of Alteration in Cancer

Gain of Accessibility at Oncogenic Regulatory Elements

Regions that are normally closed in the cell of origin can become newly accessible in cancer cells, often corresponding to activation of enhancers or promoters that drive expression of oncogenes or genes supporting proliferative and survival pathways.

Loss of Accessibility at Regulatory Elements

Conversely, regions that are normally open and functionally active in healthy tissue can become closed in cancer cells, frequently corresponding to silencing of tumor suppressor genes or genes required for normal cellular differentiation.

Redistribution of Regulatory Activity

Beyond simple gains and losses, cancer cells often display a broader redistribution of accessible chromatin, with entirely new patterns of open regulatory elements emerging that do not correspond to any accessibility state observed in the normal tissue of origin, suggesting the activation of aberrant or ectopic regulatory programs.

Mechanisms Underlying Accessibility Changes

Chromatin Remodeling Complex Dysfunction

Multi-protein complexes that use energy to reposition or evict nucleosomes play a central role in establishing accessibility, and mutations affecting the genes encoding subunits of these complexes are recurrently found across many cancer types, directly disrupting normal chromatin remodeling activity.

Transcription Factor Dysregulation

Because many transcription factors help maintain accessible chromatin at their binding sites through recruitment of remodeling machinery, abnormal activity or expression of specific transcription factors in cancer cells can drive new patterns of chromatin opening or closing at their target regulatory elements.

Coupling with DNA Methylation and Histone Modification

Chromatin accessibility does not change in isolation but is closely intertwined with alterations in DNA methylation and histone modification, since these marks directly influence nucleosome stability and positioning, meaning accessibility changes often accompany or result from these other epigenetic alterations.

Functional Consequences

Altered Enhancer Activity

Because enhancers must be accessible to function, gains or losses of accessibility at these distal regulatory elements can dramatically change the expression of the genes they control, even when the target gene's own promoter and coding sequence remain unaltered.

Rewired Gene Regulatory Networks

Widespread accessibility changes can collectively rewire the gene regulatory network of a cell, shifting it away from the balanced regulatory state of normal tissue toward a network configuration that favors continuous proliferation and resistance to normal growth constraints.

Detection and Mapping

Nuclease-Based Accessibility Assays

Techniques that use enzymes capable of preferentially cutting DNA in open chromatin regions, followed by sequencing, allow genome-wide mapping of accessible regions and comparison of accessibility profiles between normal and malignant tissue.

Single-Cell Accessibility Profiling

Because tumors are composed of heterogeneous cell populations, single-cell approaches to measuring chromatin accessibility allow researchers to resolve accessibility differences between distinct subpopulations within the same tumor, revealing regulatory heterogeneity that bulk measurements would average out and obscure.

Clinical and Biological Significance

Mapping chromatin accessibility alterations provides insight into the regulatory elements driving abnormal gene expression in a given tumor, complementing information obtained from DNA sequence mutations and offering a route to identify regulatory vulnerabilities that might not be apparent from genetic analysis alone. Accessible regulatory regions unique to cancer cells are also being explored as candidate biomarkers and as targets for therapies aimed at disrupting the abnormal transcriptional programs that these regions support.