Copy Number Variants and Structural Variants

Copy number variants and structural variants are large-scale changes to the genome — deletions, duplications, insertions, inversions, and translocations — that alter the number of copies of a DNA segment or its arrangement, rather than a single base. Because they can encompass whole genes or regulatory regions, they require detection and interpretation methods distinct from those used for point mutations.

Definition

A copy number variant is a gain or loss of a DNA segment that changes its number of copies relative to a reference; structural variants are the broader class of large genomic rearrangements — including deletions, duplications, insertions, inversions, and translocations — that alter genome content or organisation.

Scope

The entry covers the main classes of structural change, how they are detected and described, the special interpretive question of dosage sensitivity, and why these variants are handled differently from small sequence variants. It is a methodological reference topic, not clinical guidance.

Core questions

• What types of structural change occur in the genome, and how are they classified?
• How are copy number and structural variants detected and described?
• Why is dosage sensitivity central to interpreting copy number change?
• How does interpretation of large variants differ from that of single-nucleotide variants?

Key concepts

• Copy number variant (deletion / duplication)
• Structural variant (inversion, translocation, insertion)
• Dosage sensitivity (haploinsufficiency / triplosensitivity)
• Breakpoint and gene content
• Microarray and sequencing-based detection
• Benign copy number polymorphism

Mechanisms

Structural variants arise through mechanisms such as non-allelic homologous recombination and replication errors that delete, duplicate, invert, or relocate DNA segments. They are detected by approaches including chromosomal microarray and short- and long-read sequencing, each with characteristic resolution and breakpoint accuracy (Alkan et al., 2011). Interpretation hinges on which genes or regulatory elements a variant spans and on dosage sensitivity — whether losing a copy (haploinsufficiency) or gaining one (triplosensitivity) of the affected genes is deleterious. Early genome surveys showed that copy number variation is common and includes much benign polymorphism (Sebat et al., 2004; Sharp et al., 2006), so distinguishing pathogenic from benign change requires population reference data such as that from large sequencing consortia (1000 Genomes Project Consortium, 2015).

Clinical relevance

Copy number and structural variants account for a substantial share of genomic disease and are reported by cytogenomic and sequencing laboratories, so understanding their classes and dosage logic is part of appraising such reports. The entry describes detection and interpretation concepts and is not a basis for individual diagnosis or treatment.

Epidemiology

Copy number variation is a normal and abundant feature of human genomes, with many variants representing benign polymorphism; this background was established by early genome-wide surveys and refined by large population projects (Sebat et al., 2004; Sharp et al., 2006; 1000 Genomes Project Consortium, 2015). The pathogenic fraction is a minority that must be separated from this common variation.

History

Large chromosomal abnormalities were visible by karyotyping for decades, but submicroscopic copy number variation was recognised as widespread only in the mid-2000s through array-based surveys (Sebat et al., 2004; Sharp et al., 2006). Sequencing then enabled base-pair resolution of breakpoints and systematic discovery and genotyping of structural variants (Alkan et al., 2011; 1000 Genomes Project Consortium, 2015).

Debates

How should the clinical significance of a copy number variant be judged?

Because copy number variation includes abundant benign polymorphism, judging a variant pathogenic depends on gene content, dosage sensitivity, size, and population frequency rather than presence alone, and borderline calls remain interpretively difficult.

Seminal works

• sebat-2004
• sharp-2006
• alkan-2011

Frequently asked questions

How do copy number variants differ from point mutations?

Copy number variants change the number of copies of a whole DNA segment, which can span entire genes, whereas point mutations alter a single base; their detection and interpretation methods therefore differ.

Are all copy number variants harmful?

No. Copy number variation is common in healthy genomes and much of it is benign polymorphism; only a minority is pathogenic, and distinguishing the two relies on gene content, dosage sensitivity, and population frequency.