Chromosomal Structural Variations

Chromosomal structural variations are large-scale rearrangements of the genome — deletions, duplications, insertions, inversions, and translocations — that change the order, orientation, or amount of DNA over segments ranging from kilobases to whole chromosome arms. They are a major component of genetic diversity and an important cause of disease, sometimes acting by altering gene dosage and sometimes by disrupting genes or their regulatory context at rearrangement breakpoints.

Definition

Chromosomal structural variation is a class of genomic change involving segments typically larger than a kilobase, including deletions, duplications, insertions, inversions, and translocations, which may be balanced (no net gain or loss of material) or unbalanced (with gain or loss).

Scope

This topic covers structural rearrangements of chromosomes: balanced changes such as inversions and reciprocal translocations that preserve total genetic content, and unbalanced changes such as deletions and duplications that alter it. It addresses how such variants arise, how they are detected, and the mechanisms by which they cause disease. Dosage changes specifically defined by copy number are treated more fully in the sibling topic on copy number variation; this entry is reference material, not clinical guidance.

Key concepts

• Deletion and duplication
• Inversion
• Translocation (reciprocal and Robertsonian)
• Balanced versus unbalanced rearrangement
• Breakpoint and gene disruption
• Non-allelic homologous recombination
• Genomic disorders
• Position effect

Mechanisms

Structural variants arise through several mechanisms, including non-allelic homologous recombination between repeated sequences (favoured at regions flanked by segmental duplications), non-homologous end joining, and replication-based errors. The disease consequence depends on the type and location of the rearrangement: unbalanced deletions and duplications change the dosage of the genes they span; balanced inversions and translocations may be harmless if they spare genes but can disrupt a gene at a breakpoint or separate a gene from its regulatory elements (a position effect). Recurrent rearrangements mediated by flanking repeats give rise to recognised genomic disorders (Stankiewicz & Lupski, 2010; Feuk et al., 2006). Large population studies have catalogued thousands of structural variants per genome, most of them benign (Sudmant et al., 2015).

Clinical relevance

Structural variants underlie many chromosomal and genomic disorders and contribute to developmental and reproductive conditions; balanced rearrangements may be clinically silent in a carrier yet cause unbalanced outcomes in offspring. Different assays (karyotyping, chromosomal microarray, and genome sequencing) detect different size ranges and types of structural change. This topic describes how such variants arise and are characterised and is not a basis for individual diagnostic or treatment decisions.

Epidemiology

Population-scale sequencing shows that structural variants, though far fewer in number than single-nucleotide variants, affect more total base pairs per genome; the integrated map of 2,504 genomes catalogued tens of thousands of structural variant sites, most common and benign (Sudmant et al., 2015).

Debates

Can a balanced rearrangement cause disease?

A balanced translocation or inversion preserves total genetic material and may be benign, but it can still cause disease if a breakpoint disrupts a gene or its regulatory region, and it can produce unbalanced gametes leading to affected offspring, so balance at the chromosomal level does not guarantee a neutral effect.

Seminal works

• feuk-2006
• stankiewicz-2010
• sudmant-2015

Frequently asked questions

What is the difference between a balanced and an unbalanced rearrangement?

A balanced rearrangement (such as a reciprocal translocation or inversion) reorganises DNA without net gain or loss of genetic material, whereas an unbalanced rearrangement (such as a deletion or duplication) changes the total amount of DNA present.

How do structural variants differ from point mutations?

Point mutations change one or a few nucleotides, while structural variants involve larger segments — typically a kilobase or more — that are deleted, duplicated, inverted, or moved, often affecting many genes or their regulation at once.