Structural Variants in the Human Genome: Beyond SNPs and Small Mutations
When scientists first catalogued human genetic variation, the focus was heavily on single nucleotide polymorphisms — the simplest and most easily detected form of genetic difference. But SNPs represent only a fraction of the genomic variation between individuals. Structural variants — large-scale changes in the structure or copy number of genomic segments — affect far more base pairs across any two genomes and play important roles in human biology, evolution, and disease that are only now being fully appreciated thanks to advances in long-read sequencing technology.
Types of Structural Variants
Structural variants (SVs) encompass several distinct classes of genomic change. Copy number variants (CNVs) are regions where individuals carry more or fewer copies of a genomic segment than the standard two — deletions remove one or both copies, while duplications create additional copies. Inversions flip a segment of DNA, reversing its orientation within the chromosome. Insertions add sequence at a specific genomic location, often from mobile elements such as transposons that have copied and pasted themselves elsewhere in the genome. Translocations move segments between chromosomes. Each class can range from a few hundred base pairs to megabases in length and can have dramatically different biological consequences depending on where in the genome the change falls.
Structural Variants in Disease
Structural variants are among the most powerful drivers of genetic disease in both developmental and acquired conditions. Large chromosomal deletions and duplications are a major cause of intellectual disability, autism spectrum disorder, and congenital malformations — detectable by chromosomal microarray analysis, which identifies CNVs across the genome. Some structural variants have been clinically characterised in detail: deletions of chromosome 22q11.2 cause DiGeorge syndrome, while specific duplications at chromosome 17p11.2 cause Charcot-Marie-Tooth disease. In cancer, structural rearrangements are frequently driver events — the BCR-ABL fusion gene in chronic myeloid leukaemia, for instance, results from a translocation between chromosomes 9 and 22.
The Challenge of Detecting Structural Variants
Structural variants are harder to detect than SNPs, which is one reason they were systematically understudied for so long. Short-read sequencing platforms, which produce reads of 100 to 300 base pairs, struggle with large and repetitive SVs because the short reads cannot uniquely map across the breakpoints of large deletions or duplications. Long-read sequencing — producing reads of tens of thousands of base pairs — spans these breakpoints far more reliably and has revealed a landscape of structural variation substantially richer than short-read data suggested. Population-scale long-read sequencing projects are now building reference catalogues of SVs comparable to the SNP catalogues developed over the past two decades.
Structural Variants and Human Evolution
Beyond disease, structural variation has played a major role in human evolution. Some of the most significant genetic differences between humans and our closest relatives, chimpanzees, are structural rather than sequence-level changes. Copy number variation in genes like AMY1 (salivary amylase) may have been adaptive as human diets shifted to include more starch. Segmental duplications — large blocks of near-identical sequence — are hotspots for further rearrangement and have generated new gene families in the human lineage. Understanding structural variation is therefore not just medically relevant; it is central to understanding how the human genome works and how it has evolved.
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