Copy Number Variation and Structural Variants

Core Idea

Copy number variations (CNVs) are segments ≥1 kb that differ in copy number between individuals, representing the most common form of structural variation in human genomes (~12% of the genome is CNV). CNVs can be benign or cause disease through gene dosage imbalance; recurrent CNVs associated with neurodevelopmental disorders often involve duplications and deletions of genomic regions with segmental duplications. CNVs contribute to both genetic disease and human evolution.

Explainer

From your study of genomics and unequal crossing over, you know that genomes are not static blueprints — they can gain or lose segments through errors in recombination. Copy number variations (CNVs) are the result: stretches of DNA, typically 1 kilobase or larger, that exist in different numbers of copies in different individuals. Where you might carry two copies of a particular genomic region (one per chromosome), another person might carry one, three, or even ten copies. These are not rare mutations — CNVs collectively affect more base pairs of the human genome than single-nucleotide polymorphisms (SNPs) do.

The primary mechanism generating CNVs is nonallelic homologous recombination (NAHR), which is essentially the unequal crossing over you have already studied, applied at a genomic scale. When two stretches of highly similar sequence (segmental duplications) flank a region, the recombination machinery can misalign them during meiosis. Crossover between misaligned copies produces one chromosome with a duplication and another with a deletion. This is why CNV "hotspots" cluster in regions rich in segmental duplications. Other mechanisms include nonhomologous end joining (NHEJ) and replication-based errors like fork stalling and template switching.

Many CNVs are benign — they fall in non-coding regions or involve genes that are dosage-insensitive. Some are even adaptive: populations with historically high-starch diets tend to carry more copies of the *AMY1* gene (encoding salivary amylase), enhancing starch digestion. But when a CNV deletes or duplicates a dosage-sensitive gene, the consequences can be severe. The classic example is the 22q11.2 deletion syndrome (DiGeorge syndrome): a 3-megabase deletion on chromosome 22 removes about 30 genes and causes heart defects, immune deficiency, and learning difficulties. Similarly, duplications of the 17p12 region cause Charcot-Marie-Tooth disease type 1A by increasing the dosage of the *PMP22* gene, which encodes a peripheral nerve myelin protein.

Detecting CNVs requires methods that go beyond standard SNP genotyping. Array comparative genomic hybridization (array CGH) compares fluorescence intensity between a test and reference genome across thousands of genomic probes, revealing regions of gain or loss. Modern whole-genome sequencing detects CNVs through read-depth analysis (regions with more copies produce more sequencing reads) and by identifying discordant paired-end reads that span deletion or duplication breakpoints. Understanding CNVs has transformed clinical genetics — they are now routinely assessed in patients with intellectual disability, autism spectrum disorder, and congenital anomalies, and they explain a significant fraction of cases that were previously considered idiopathic.