Structural Variants: Types and Mechanisms

Structural variants are the catalogue of ways a genome can be rearranged at the segmental scale—deletions, duplications, insertions, inversions, and translocations—together with the mutational processes that generate them. Distinguishing these types and understanding their mechanistic origins is what allows a rearrangement to be named, its breakpoints interpreted, and its likely consequences reasoned about.

Definition

Structural variants are genomic alterations affecting DNA segments—including deletions, duplications, insertions, inversions, and inter- or intrachromosomal translocations—classified as balanced when total sequence content is preserved and unbalanced when it is gained or lost.

Scope

This topic lays out the principal structural-variant classes and the contrast between balanced rearrangements (which preserve total DNA content) and unbalanced ones (which change it), then connects each class to the DNA repair and recombination mechanisms that create it. It is a reference treatment of variant taxonomy and mechanism, not clinical guidance.

Core questions

• What are the major classes of structural variant and how do they differ?
• Which variants are balanced versus unbalanced, and why does that distinction matter?
• By what mechanisms—recombination, replication error, repair, retrotransposition—do they arise?
• What distinguishes recurrent variants with shared breakpoints from non-recurrent ones?

Key concepts

• Deletion, duplication, insertion, inversion, translocation
• Balanced vs. unbalanced rearrangement
• Recurrent vs. non-recurrent variant
• Breakpoint and junction sequence
• Non-homologous end joining (NHEJ)
• Microhomology
• Complex structural variant

Key theories

Non-allelic homologous recombination (NAHR)

Recombination between highly similar but non-allelic repeat copies, such as segmental duplications flanking a region, produces recurrent deletions and duplications whose breakpoints cluster within the shared repeats.

Replication-based rearrangement (FoSTeS / MMBIR)

Fork stalling and template switching and microhomology-mediated break-induced replication explain non-recurrent and complex rearrangements, in which the replication machinery switches templates using short stretches of microhomology and joins distant sequences.

Mechanisms

Structural variants are produced by a small set of well-characterized processes. Non-allelic homologous recombination uses long stretches of near-identical sequence—frequently segmental duplications—as substrates, yielding recurrent deletions and duplications with reproducible breakpoints. Non-homologous end joining and microhomology-mediated end joining repair double-strand breaks by rejoining ends, often with small deletions or insertions and little or no homology. Replication-based mechanisms (fork stalling and template switching, and microhomology-mediated break-induced replication) generate non-recurrent and complex events by template switching during replication. Retrotransposition inserts new copies of mobile elements. Each mechanism leaves characteristic junction signatures that paired-end and split-read sequencing can resolve.

Clinical relevance

The class and mechanism of a structural variant shape how it is interpreted in the health sciences—balanced rearrangements may disrupt a gene at a breakpoint while leaving dosage intact, whereas unbalanced events change gene copy number. This entry explains variant types and their mechanistic origins as a conceptual framework and is not a basis for individual diagnosis or treatment.

Epidemiology

Paired-end and read-depth sequencing studies have shown that deletions are the most frequently catalogued unbalanced variant, with duplications, insertions, and inversions also abundant, and that mechanism leaves a population signature: regions flanked by long homologous repeats show recurrent variants, while much of the remaining structural variation is non-recurrent. Integrated maps across thousands of genomes have quantified the relative contribution of each class.

History

As genome-wide surveys revealed that structural variation was pervasive, attention turned to how it arises. Lupski's work on recurrent genomic disorders established non-allelic homologous recombination as a mechanism tied to genome architecture, and the 2009 Hastings, Lupski, Rosenberg, and Ira synthesis organized recombination-, replication-, and repair-based routes into a unified framework. Paired-end sequencing then allowed breakpoint junctions to be read directly and mechanisms to be inferred from junction signatures.

Debates

How common are complex, multi-breakpoint rearrangements?

Replication-based and catastrophic processes can produce variants with several breakpoints that resist simple classification; their true prevalence and mechanistic boundaries remain under investigation as long-read methods resolve more junctions.

Key figures

• James R. Lupski
• P. J. Hastings
• Jan O. Korbel
• Stephen W. Scherer
• Evan E. Eichler

Seminal works

• feuk-2006
• hastings-2009
• korbel-2007

Frequently asked questions

What is the difference between a balanced and an unbalanced structural variant?

A balanced variant, such as an inversion or a reciprocal translocation, rearranges DNA without changing the total amount of sequence, whereas an unbalanced variant, such as a deletion or duplication, gains or loses genomic material.

Why do some structural variants recur at the same breakpoints?

Recurrent variants typically arise by non-allelic homologous recombination between long, near-identical repeat copies that flank a region, so the breakpoints fall within those shared repeats and reappear across unrelated individuals.