Copy Number Alterations

Copy Number Alterations is the category of genetic alteration in which the normal number of copies of a gene or larger chromosomal segment is increased or decreased within a cancer cell's genome, representing a large-scale structural change distinct from small sequence-level alterations, and capable of dramatically shifting the overall dosage, and consequently the functional output, of the affected genes without necessarily altering their underlying DNA sequence at all.

Conceptual Basis

Normal Cells Maintain a Standard Copy Number for Most Genes

Normal human cells typically carry exactly two copies of most genes, one inherited from each parent, meaning the standard baseline copy number against which cancer-associated copy number alterations are measured is generally two, with deviations from this baseline representing the defining feature of a copy number alteration.

Gene Dosage Directly Influences Functional Output

Because the overall amount of a given protein produced by a cell is generally influenced by how many copies of the corresponding gene are present, a copy number alteration can meaningfully change a gene's functional output even without any change to the underlying DNA sequence of the gene itself, simply by altering how many working copies of that sequence are present.

Categories of Copy Number Alterations

Amplifications

An amplification refers to an increase in the copy number of a gene or chromosomal segment beyond the normal baseline, sometimes involving many additional copies of the affected region, and is a particularly common mechanism by which cancer-promoting oncogenes achieve the excessive functional activity that contributes to malignant behavior.

Deletions

A deletion refers to a decrease in the copy number of a gene or chromosomal segment below the normal baseline, ranging from loss of a single copy to complete loss of both copies, and is a particularly common mechanism by which protective tumor suppressor genes lose sufficient functional activity to contribute to cancer development.

Scale of Copy Number Alterations

Focal Copy Number Alterations

Some copy number alterations affect only a small, focal genomic region, often encompassing just a single gene or a small number of neighboring genes, suggesting that the specific selective advantage driving that particular alteration is closely tied to the dosage change of that specific, limited set of genes.

Broad or Whole-Chromosome Copy Number Alterations

Other copy number alterations affect much larger regions, encompassing an entire chromosome arm or an entire chromosome, simultaneously altering the dosage of a very large number of genes at once, reflecting a distinct underlying mechanism, typically related to errors during chromosome segregation, from that responsible for smaller, more focal alterations.

Mechanisms Generating Copy Number Alterations

Errors During DNA Replication

Certain copy number alterations arise from specific errors occurring during the DNA replication process itself, in which particular genomic regions become duplicated or deleted due to disruptions in the normal replication process at that location.

Errors During Chromosome Segregation

Broader copy number alterations, particularly those affecting entire chromosomes or chromosome arms, commonly arise from errors during the process of chromosome segregation at cell division, in which chromosomes fail to be distributed correctly and evenly between the two resulting daughter cells.

Significance of Copy Number Alterations for Cancer Biology

A Major Mechanism of Oncogene Activation and Tumor Suppressor Loss

Copy number alterations represent one of the most significant and frequently observed mechanisms by which cancer cells achieve both excessive oncogene activity, through amplification, and insufficient tumor suppressor activity, through deletion, complementing the alternative mechanisms provided by sequence-level alterations such as single nucleotide variants and small insertions and deletions.

Contribution to Overall Genomic Instability

Because the same underlying cellular defects that produce elevated rates of copy number alteration frequently also contribute to broader chromosomal instability, the presence of extensive copy number alterations within a tumor genome often reflects and further compounds a more general underlying instability in that tumor's genome maintenance mechanisms.

Summary

Copy Number Alterations describes increases, termed amplifications, or decreases, termed deletions, in the number of copies of a gene or larger chromosomal segment present within a cancer cell's genome, ranging in scale from small, focal alterations affecting individual genes to broad alterations affecting entire chromosomes, and representing a major mechanism by which cancer cells achieve both excessive oncogene activity and insufficient tumor suppressor activity, distinct from sequence-level alterations and closely connected to broader genomic instability.