Structural Chromosomal Rearrangements

Structural chromosomal rearrangements are changes in the arrangement, orientation, or copy number of chromosomal segments that arise when chromosomes break and rejoin abnormally. Unlike numerical abnormalities, which alter whole-chromosome counts, structural rearrangements reshape the internal organization of chromosomes — deleting, duplicating, inverting, or relocating blocks of DNA — and they underlie a large share of congenital, developmental, and oncologic genetic findings.

Definition

A structural chromosomal rearrangement is an alteration of chromosome structure produced by one or more breaks followed by abnormal reunion, resulting in a segment that is deleted, duplicated, inverted, or translocated relative to the normal chromosome complement.

Scope

This area orients the reader to the principal categories of structural change — deletions and duplications (gains and losses of segments), inversions and translocations (reorientation and exchange of segments), and the distinction between balanced and unbalanced rearrangements. It frames how such rearrangements form, how they are detected by karyotyping and chromosomal microarray, and why some are clinically silent in carriers while others produce phenotypes. It is a reference overview within cytogenetics, not clinical guidance.

Sub-topics

• Balanced Versus Unbalanced Rearrangement
• Deletion and Duplication
• Inversion and Translocation

Core questions

• Which segment of which chromosome is altered, and by how much?
• Is the rearrangement balanced (no net gain or loss of material) or unbalanced (net gain or loss)?
• What molecular mechanism generated the rearrangement?
• Does the rearrangement disrupt or dysregulate dosage-sensitive genes?

Key concepts

• Breakpoint and reunion
• Copy-number gain and loss
• Dosage sensitivity (haploinsufficiency and triplosensitivity)
• Balanced versus unbalanced rearrangement
• Karyotype and ISCN nomenclature
• Chromosomal microarray and copy-number variation
• Non-allelic homologous recombination

Mechanisms

Structural rearrangements originate from DNA double-strand breaks that are repaired incorrectly, or from recombination between similar (non-allelic homologous) sequences such as low-copy repeats. Hastings and colleagues describe the main routes to copy-number change — non-allelic homologous recombination, non-homologous end joining, and replication-based mechanisms — which together account for deletions, duplications, inversions, and translocations. The clinical consequence of a rearrangement depends largely on whether genetic material is gained or lost and whether dosage-sensitive genes lie within or near the affected segment; balanced rearrangements with intact breakpoints are often phenotypically silent, whereas unbalanced changes alter gene dosage.

Clinical relevance

Structural rearrangements are detected across prenatal diagnosis, evaluation of developmental disability and congenital anomalies, and cancer cytogenetics, and they inform recurrence-risk counselling for carriers of balanced rearrangements. Professional consensus positions chromosomal microarray as a first-tier test for unexplained developmental disability or congenital anomalies because it detects submicroscopic gains and losses that karyotyping misses. This entry describes how these findings are categorized and detected and is not a basis for individual diagnostic or treatment decisions.

Epidemiology

Balanced reciprocal translocations and Robertsonian translocations are among the more common structural findings in the general population, while submicroscopic copy-number variants are detectable in a substantial fraction of individuals evaluated for developmental disorders. Precise frequencies depend on the detection method, since microarray resolves changes far below the limit of conventional karyotyping.

Evidence & guidelines

Miller and colleagues (2010) issued a consensus statement, endorsed by clinical genetics organizations, recommending chromosomal microarray as a first-tier diagnostic test for individuals with unexplained developmental disabilities or congenital anomalies, reflecting its higher diagnostic yield for submicroscopic structural variants than conventional karyotyping.

History

Recognition of structural chromosome change followed the establishment of accurate human chromosome counting and banding techniques in the mid-twentieth century, which allowed deletions, duplications, inversions, and translocations to be visualized in the karyotype. The later advent of molecular and array-based methods, reviewed by Alkan and colleagues, revealed a far larger landscape of submicroscopic structural variation than light microscopy could resolve.

Key figures

• James R. Lupski
• Evan E. Eichler
• P. J. Hastings

Seminal works

• hastings-2009
• alkan-2011
• miller-2010

Frequently asked questions

How do structural rearrangements differ from numerical chromosomal abnormalities?

Numerical abnormalities change the number of whole chromosomes (for example, an extra chromosome), whereas structural rearrangements alter the internal organization of one or more chromosomes by deleting, duplicating, inverting, or relocating segments.

Why can some structural rearrangements cause no symptoms?

Balanced rearrangements that neither add nor remove genetic material, and that do not disrupt important genes at their breakpoints, often leave gene dosage intact, so a carrier may have no clinical features even though the chromosome structure is abnormal.